Session B1: Atom Interferometry and New Techniques with Ultracold Atoms

Low-temperature, high-density magneto-optical trapping of potassium using the open 4S $\rightarrow$ 5P transition at 405\,nm

We report the laser cooling and trapping of neutral potassium on an open transition. Fermionic $^{40}$K is captured using a magneto-optical trap (MOT) on the closed $\mathrm{4S_{1/2}} \rightarrow \mathrm{4P_{3/2}} $ transition at 767\,nm and then transferred, with unit efficiency, to a MOT on the open $\mathrm{4S_{1/2}} \rightarrow \mathrm{5P_{3/2} }$ transition at 405\,nm. Because the $\mathrm{5P_{3/2} }$ state has a smaller line width than the $\mathrm{4P_{3/2}}$ state, the Doppler limit is reduced. We observe temperatures as low as 63(6)\,$\mu$K, the coldest potassium MOT reported to date. The density of trapped atoms also increases, due to reduced temperature and reduced expulsive light forces. We measure a two-body loss coefficient of $\beta = 2 \times 10^{-10}$\,cm$^3\,$s$^{-1}$, and estimate an upper bound of $8 \times 10^{-18}$\,cm$^2$ for the ionization cross section of the 5P state at 405\,nm. The combined temperature and density improvement in the 405\,nm MOT is a twenty-fold increase in phase space density over our 767\,nm MOT, showing enhanced pre-cooling for quantum gas experiments. A qualitatively similar enhancement is observed in a 405\,nm MOT of bosonic $^{41}$K.   

Four techniques to achieve deeper Fermi degeneracy in Fermi-Bose mixtures

The study of exotic superfluid phases of ultracold atoms requires the achievement of deeper Fermi degeneracy with respect to the one already available. I will describe four techniques for efficient sympathetic cooling of Fermi gases with a different species Bose gas: bichromatic optical dipole [1] and light-assisted magnetic trapping [2], quasi one-dimensional Fermi-Bose mixtures [3], and fast adiabatic cooling [4]. \\[4pt] [1] R. Onofrio and C. Presilla, Phys. Rev. Lett. 89, 100401 (2002);\\[0pt] [2] M. Brown-Hayes and R. Onofrio, Phys. Rev. A 70, 063614 (2004); [3] M. Brown-Hayes et al., Phys. Rev. A 78, 013617 (2008);\\[0pt] [4] S. Choi, R. Onofrio, and B. Sundaram, arXiv 1109.4908.   

Optimized sympathetic cooling of atomic mixtures via fast adiabatic strategies

The talk will explore the extent to which frictionless cooling techniques may be useful in sympathetic cooling of Fermi gases. It is argued that optimal cooling of an atomic species may be obtained by means of sympathetic cooling with another species whose trapping frequency is dynamically changed to maintain constancy of the Lewis-Riesenfeld adiabatic invariant, which in turn determines the temporal-profile of the changing frequency. An important motivating factor is that an usually undesired feature of these techniques, i.e., the fact that the atomic cloud does not increase its phase-space density and therefore its degeneracy, turns into a crucial asset when viewed from the perspective of maintaining the gas in the nondegenerate regime, thus making it an optimal coolant. Advantages and limitations of this cooling strategy are discussed, with particular regard to the possibility of cooling Fermi gases to a deeper degenerate regime. We also show that the links between the suggested method and quantum squeezing.   

Quantum degenerate Bose-Fermi mixture of chemically different atomic species with widely tunable interactions

We have created a quantum degenerate Bose-Fermi mixture of 23Na and 40K with widely tunable interactions via broad interspecies Feshbach resonances. Twenty Feshbach resonances between 23Na and 40K were identified. The large and negative triplet background scattering length between 23Na and 40K causes a sharp enhancement of the fermion density in the presence of a Bose condensate. As explained via the asymptotic bound-state model (ABM), this strong background scattering leads to a series of wide Feshbach resonances observed at low magnetic fields. Our work opens up the prospect to create chemically stable, fermionic ground state molecules of 23Na-40K where strong, long-range dipolar interactions will set the dominant energy scale.   

Condensate Fraction in a BEC Dimer

Recent experiments studying a Bose Einstein Condensate (BEC) in a two-mode system, equivalent to a ``dimer,'' have shown that many qualitative dynamical features of the BEC can be understood from motions in the underlying classical (two-dimensional) phase space (phi, z). Using a Bose-Hubbard model for the dimer, we focus on quantum deviations from motions in the classical phase space. We introduce a ``quantum'' phase space (QPS), which we define as the minimum condensate fraction c(tau;phi,z) of initial coherent states (phi,z) in the time interval [0,tau]. We find that lines of equal condensate fraction in the QPS do mimic the classical trajectories of constant energy in many respects, such that the QPS clearly reflects Josephson oscillations and self-trapping. However, novel quantum features beyond the classical description appear at finite time tau. These include symmetry breaking and enhanced c(tau; phi, z) near the classical hyperbolic fixed point and along a ridge near the classical separatrix. These features of the QPS can be readily studied in current experiments.   

Resonant Hawking radiation in Bose-Einstein condensates

We study double-barrier interfaces separating regions of asymptotically subsonic and supersonic flow of Bose-condensed atoms [1]. These setups contain at least one black hole sonic horizon from which the analogue of Hawking radiation should be generated and emitted against the flow in the subsonic region. Multiple coherent scattering by the double-barrier structure strongly modulates the transmission probability of phonons, rendering it very sensitive to their frequency. As a result, resonant tunneling occurs with high probability within a few narrow frequency intervals. This gives rise to highly non-thermal spectra with sharp peaks. We find that these peaks are mostly associated with decaying resonances and only occasionally with dynamical instabilities. Even at achievable non-zero temperatures, the radiation peaks can be dominated by spontaneous emission, i.e. enhanced zero-point fluctuations, and not, as is often the case in analogue models, by stimulated emission.\\[4pt] [1] I. Zapata, M. Albert, R. Parentani, F. Sols, New J. Phys. 13, 063048 (2011).   

Momentum Resolved Optical Lattice Modulation Spectroscopy for Bosons in Optical Lattice

We propose a new method of optical lattice modulation spectroscopy for studying the spectral function of ultracold bosons in an optical lattice. We show that different features of the single particle spectral function in different quantum phases can be obtained by measuring the change in momentum distribution after the modulation. In the Mott phase, this gives information about the momentum dependent gap to particle-hole excitations as well as their spectral weight. In the superfluid phase, one can obtain the spectrum of the gapless Bogoliubov quasiparticles as well as the gapped amolitude fluctuations. The distinct evolution of the response with modulation frequency in the two phases can be used to identify these phases and the quantum phase transition separating them.   

Population of atoms in the output of an atom Michelson interferometer

A cloud of Bose-Einstein condensate (BEC) sitting at the center of a weakly confining trap in an atom Michelson interferometer is split into two clouds that travel along different paths. The two clouds evolve and accumulate relative phase between them due to field of interest, confining potential and interatomic interactions. At the end of the interferometric cycle, the two clouds are recombined and the population of atoms found in the cloud at rest and the moving clouds depends on the relative phase. We derive an expression for the probability of counting any number of atoms within each cloud after recombination, study the dependence of the probability on the relative phase and relate our findings with experiments.   

Prototyping method for Bragg-type atom interferometers

We present a method for rapid modeling of new Bragg ultracold atom-interferometer (AI) designs useful for assessing the performance of such interferometers. The method simulates the overall effect on the condensate wave function in a given AI design using two separate elements. These are (1) modeling the effect of a Bragg pulse on the wave function and (2) approximating its evolution during the intervals between the pulses. The actual sequence of these pulses and intervals is then followed to determine the approximate final wave function from which the interference pattern can be calculated. The exact evolution between pulses is assumed to be governed by the Gross-Pitaevskii equation (GPE). We have developed both 1D and 3D versions of this method and have determined their validity by comparing their predicted interference patterns with those obtained by numerical integration of the 1D GPE and with the results of an experiment performed at NIST. We find good agreement between the 1D interference patterns predicted by this method and those found by the GPE. We show that we can reproduce the results of the NIST experiment and that this method provides estimates of 1D interference patterns 10,000 times faster than direct integration of the GPE.   

Applications of chirped Raman adiabatic rapid passage to atom interferometry

We present robust atom optics, based on chirped Raman adiabatic rapid passage (ARP), in the context of atom interferometry. Such ARP light pulses drive coherent population transfer between two hyperfine ground states by sweeping the frequency difference of two fixed-intensity optical fields with large single photon detunings. Since adiabatic transfer is less sensitive to atom temperature and non-uniform Raman beam intensity than standard Raman pulses, this approach should improve the stability of atom interferometers operating in dynamic environments. In such applications, chirped Raman ARP may also provide advantages over the previously demonstrated stimulated Raman adiabatic passage (STIRAP) technique, which requires precise modulation of beam intensity and zeroing of the single photon detuning. We demonstrate a clock interferometer with chirped Raman ARP pulses, and compare its stability to that of a conventional Raman pulse interferometer. We also discuss potential improvements to inertially sensitive atom interferometers. Copyright {\copyright} 2011 by The Charles Stark Draper Laboratory, Inc. All rights reserved.   

Berry-gauge tuned Bose-Einstein condensate gyroscope

If stable, the many-body ground state of a dilute gas of ultra-cold, bosonic atoms occupying a superposition of two internal (hyperfine) states is a Bose-Einstein condensate (BEC) of effective spin 1/2 bosons. The superfluid BEC dynamics admits long-lived quantized vortex states in which the complex phase of the superfluid order parameter, which we call the charge phase, undergoes an integer number of $2\pi$ windings along a multiply connected path - a closed trajectory that encloses a region in which the superfluid density vanishes. In response to an overall rotation of the ring, a quantization event can occur that can be used to sense rotation. Unfortunately, the sensitivity of the ring BEC gyroscope would be limited as the quantization event sets in at a rotation frequency that is not as low as the frequencies measured by other devices such as ring laser gyroscopes. We show that the recently realized synthetic magnetic fields, in which the controlled position dependence of the spin results in an effective gauge field, can tune the BEC ring gyroscope to trigger a quantization event at much smaller rotation frequency. In addition, the effective gauge field can undergo its own quantization events in which the spin vector undergoes an integer number of $2\pi$ or $4\pi$ windings.   

Frustration and glassiness in spin models with cavity-mediated interactions

Optical cavity photons can mediate interatomic interactions that are both long-ranged and sign-changing, thus enabling the realization of models and phases (such as supersolids [1]) that are inaccessible in conventional optical lattices. Here, we introduce a general scheme for realizing frustrated magnetic models possessing spin-glass states, using three-level atoms trapped at fixed positions inside a transversely pumped multimode cavity. We show that the effective Hamiltonian for such a system is analogous to the Hopfield associative-memory model [2], and, like the Hopfield model, possesses a spin glass phase for a sufficiently large ratio of the number of cavity modes to atoms. We argue that the spin glass phase should be accessible with realistic experimental parameters, discuss experimental signatures of the spin glass phase, and address the impact of dissipative processes such as cavity photon decay on the realizable phases and phase transitions. \\[4pt] [1] K. Baumann et al., \textit{Nature} 464, 1301 (2010); S. Gopalakrishnan et al., \textit{Nat. Phys.} 5, 845 (2009). \\[0pt] [2] J.J. Hopfield, \textit{Proc. Nat. Acad. Sci} 79, 2554 (1982).   

Manipulation of ultracold rubidium atoms using a single linearly chirped laser pulse

At ultracold temperatures, atoms are free from thermal motion, which makes them ideal objects of investigations in the fields of high precision spectroscopy, metrology, quantum computation, producing Bose condensates, etc. The quantum state of ultracold atoms may be created and manipulated by optical pulses of rather low intensity and by making use of quantum control methods. Here, we discuss how the field phase variation, pulse duration and intensity, when perform in conformity, may yield a desired population distribution within the hyperfine structure of alkali atoms. We theoretically investigate ultracold Rb vapor using picosecond chirped pulses of $kW/cm^2$ beam intensity and show a possibility of controllable population transfer between hyperfine levels of $5^{2}S_{1/2}$ state through Raman resonances. Satisfying the one-photon resonance condition with the $5^{2}P_{1/2}$ or $5^{2}P_{3/2}$ state allows us to enter the adiabatic region of population transfer at very low field intensities, such that corresponding Rabi frequencies are less or equal to the hyperfine split. This methodology provides with a robust way to create a specifically designed superposition state in Rb in the basis of the hyperfine levels and perform state manipulation.   

Optical response of dark exciton Bose-Einstein condensate

We study the optical response of Bose-Einstein condensate (BEC) formed by dark excitons in semiconductors. We consider the example of GaAs, where dark excitons are formed by heavy-holes and electrons with opposite signs of projections of their spins resulting in natural fragmentation of the condensate. Direct coupling of such excitons is dipole prohibited and, therefore, the optical response of semiconductor with dark condensate is provided through interaction of light with bright exciton states. We show that the Coulomb interaction of optically excited bright excitons with dark excitons residing in the BEC leads to effective renormalization of characteristics of bright excitons --- e.g. their mass decreases --- dependening on the density of the condensate. As a result the exciton resonances experiences red-shift with decreasing amplitude. This provides an opportunity for indirect spectroscopy of dark exciton BEC.   

Electron field noise sensing with a scanning ion

Controlled shuttling of ions in a planar ion trap can serve as highly sensitive method for probing the electric field.\footnote{G. Huber et al. A trapped-ion local field probe. \textit{Appl Phys B} \textbf{100}, 725-730 (2010).} We apply this idea to the measurement of field noise effects on a microfabricated planar trap as a function of ion position. In particular, by shuttling an ion between different trapping zones, the field noise can be determined near different material surfaces, in particular dielectric materials. Surface effects on these materials are suspected to cause field fluctuations that decohere the motional state of trapped ions. By shuttling the ion to a determined location and measuring the electric field noise, a measure of the electric field noise can be determined as a function of proximity to dielectrics. A systematic examination of field noise due to dielectric versus metallic surfaces is valuable in, for example, a trapped-ion cavity QED system, where the high-reflectivity mirrors used in the optical cavities are large, exposed dielectric regions.\footnote{P. Herskind, S. Wang, M. Shi, Y. Ge, M. Cetina and I. Chuang, \textit{Optics Letters}, Vol. 36, Issue 16, 3045-3047 (2011).}   

Session B2: Invited Session: Science Diplomacy: Africa and the Middle East

ASP2012: Fundamental Physics and Accelerator Sciences in Africa

Much remains to be done to improve education and scientific research in Africa. Supported by the international scientific community, our initiative has been to contribute to fostering science in sub-Saharan Africa by establishing a biennial school on fundamental subatomic physics and its applications. The school is based on a close interplay between theoretical, experimental, and applied physics. The lectures are addressed to students or young researchers with at least a background of 4 years of university formation. The aim of the school is to develop capacity, interpret, and capitalize on the results of current and future physics experiments with particle accelerators; thereby spreading education for innovation in related applications and technologies, such as medicine and information science. Following the worldwide success of the first school edition, which gathered 65 students for 3-week in Stellenbosch (South Africa) in August 2010, the second edition will be hosted in Ghana from July 15 to August 4, 2012. The school is a non-profit organization, which provides partial or full financial support to 50 of the selected students, with priority to Sub-Saharan African students.   

The African Laser Centre: Transforming the Laser Community in Africa

We describe the genesis and programs of the African Laser Centre (ALC), which is an African nonprofit network of laser users that is based in Pretoria, South Africa. Composed of over thirty laboratories from countries throughout the continent of Africa, the ALC has the mission of enhancing the application of lasers in research and education. Its programs include grants for research and training, equipment loans and donations, student scholarships, faculty grants for visits to collaborators' institutions, conferences, and technician training. A long-term goal of the ALC is to bring a synchrotron light source to Africa, most probably to South Africa. One highly popular program is the biennial conference series called the US-Africa Advanced Studies Institute, which is funded by the ALC in collaboration with the U.S. National Science Foundation and the International Center for Theoretical Physics in Trieste. The Institutes typically bring about thirty faculty and graduate students from the U.S. to venues in Africa in order to introduce U.S. and African graduate students to major breakthroughs in targeted areas that utilize lasers. In this presentation, we will summarize the ALC achievements to date and comment on the path forward.   

SESAME -- A light source for the Middle East

Developed under UNESCO and modelled on CERN, SESAME (Synchrotron-light for Experimental Science and Applications in the Middle East) is an international research centre in construction in Jordan, enabling world-class research while promoting peace through scientific cooperation. Its centerpiece, a new 2.5 GeV 3rd Generation Electron Storage Ring (133m circumference, 26nm-rad emittance, 12 places for insertion devices), will provide intense light from infra-red to hard X-rays. The Council (Bahrain, Cyprus, Egypt, Iran, Israel, Jordan, Pakistan, Palestinian Authority, Turkey), provides the annual budget. Concrete shielding is complete, and a staff of 21 is installing the refurbished 0.8 GeV BESS Y I injector system, a gift from Germany. The facility can serve 25 simultaneous experiments. Beamline equipment has been provided by Daresbury (UK), the Helmholtz Assoc. (Germany), the Swiss Light Source, LURE (France), the Univ. of Liverpool, Elettra (Italy) and US labs. Jordan has contributed {\$}3.3M, in addition to a building and land. The EU has contributed {\$}4.8M. Commitments confirmed by Members look set to provide most of {\$}35M needed to complete construction of the ring and 3 beamlines. A training program has been underway since 2000. See www.sesame.org.jo   

Marshak Lectureship: The Turkish Accelerator Center, TAC

The Turkish Accelerator Center (TAC) project is comprised of five different electron and proton accelerator complexes, to be built over 15 years, with a phased approach. The Turkish Government funds the project. Currently there are 23 Universities in Turkey associated with the TAC project. The current funded project, which is to run until 2013 aims \begin{enumerate} \item To establish a superconducting linac based infra-red free electron laser and Bremsstrahlung Facility (TARLA) at the Golbasi Campus of Ankara University, \begin{enumerate} \item To establish the Institute of Accelerator Technologies in Ankara University, and \end{enumerate} \item To complete the Technical Design Report of TAC. The proposed facilities are a 3$^{rd}$ generation Synchrotron Radiation facility, SASE-FEL facility, a GeV scale Proton Accelerator facility and an electron-positron collider as a super charm factory. \end{enumerate} In this talk, an overview on the general status and road map of TAC project will be given. National and regional importance of TAC will be expressed and the structure of national and internatonal collaborations will be explained.   

Andrei Sakharov Prize Lecture: Physics and Freedom in Ethiopia

I will highlight my forty years involvement in physics education and research in Ethiopia and draw lessons that could be learnt from it.   

Session B3: Invited Session: Superconducting Coherence, Fluctuations & Inhomogeneity in Mesoscopic & Low-Dimensions

Emergence of h/e-period oscillations in the critical temperature of small superconducting rings and critical velocity in one-dimensional superconductors

When a large ring of superconductor is thread by a magnetic flux, the resistance and the critical temperature exhibit oscillations in a flux quantum of h/2e. The flux quantum of an electron circling a thread flux on a clean metallic ring is on the contrary h/e. When the radius starts to shrink, electrons that compose of Cooper pairs may be able to roam around the ring individually without costing too much energy. An h/e period should thus arise. We discuss the emergence of h/e-period oscillations in the critical temperature of small superconducting rings and a few scenarios of superconducting-normal metal transitions. Interestingly, a threading flux is equivalent to a momentum boost in the circumferential direction of the ring. We also discuss a related issue as to how high a flow velocity one-dimensional superconductors can sustain before superconductivity gives way to this instability and the system becomes normal.   

Manipulating superconducting fluctuations in ultrasmall loops and quasi one-dimensional wires of Al

Superconducting fluctuations and the control of these fluctuations have been a problem of long-standing interest, with recent impetus provided by its relevance to the pursuit of very high temperature superconductivity through the engineering of global phase coherence. In quasi one-dimensional superconductors, fluctuations due to thermal or quantum processes lead to phase slips, and the appearance of a finite electrical resistance. We found that the critical current in mesoscopic quasi one-dimensional wires of Al is influenced by the bulk measurement electrodes, and in fact increases with magnetic field at low fields, suggesting that the phase slips are suppressed by the loss of superconductivity in the bulk electrodes. Manipulation of superconducting fluctuations is also possible in ultrasmall loops, where the strength of the fluctuations is controlled by the loop's size in comparison with $\xi$ and the enclosed flux. For ultrasmall loops with a circumference $\sim$ $\pi$$\xi$(0), de Gennes predicted more than three decades ago that superconductivity could be completely destroyed near half-integer-flux quanta even to zero temperature. Furthermore, the resulting destructive regime, likely dominated by quantum fluctuations at low temperatures, was predicted to be suppressed with the addition of a superconducting side branch. We observed this Little-Parks-de Gennes effect in ultrasmall Al loops prepared by e-beam lithography and we found that the addition of a superconducting side branch restores the lost phase coherence. We will present our most recent data and discuss the implications of our experimental observations.   

Magnetic-field-induced superconductivity and damping of phase slips in Zn nanowires

We report an observation that Zn nanowires connected with Zn electrodes, after being driven resistive by the current, re-entered their superconducting state upon the application of a small magnetic field [1, 2]. A detailed experimental investigation was carried out, with variation of parameters such as magnetic field orientation, wire length, etc.. The results provide solid evidence that this is a nonequilibrium effect associated the coupling with the boundary electrodes. There are two characteristic length scales involved, approaching either of which weakens the effect. Most importantly, we demonstrated that it is more appropriate to consider the effect to be a stabilization of superconductivity that has been suppressed by an applied current. Although we do not present a formal theory to explain all of our results here, the effect is most likely a consequence of the dampening of phase fluctuations by quasiparticles which are created in the electrodes by small magnetic fields.\\[4pt] [1] Yu Chen, S. D. Snyder, and A. M. Goldman, Phys. Rev. Lett. 103, 127002(2009).\\[0pt] [2] Yu Chen, Yen-Hsiang Lin, S. D. Snyder, and A. M. Goldman, Phys. Rev. B 83, 054505(2011)   

Quantum liquid crystals: spontaneously modulated Fermi superfluids

What happens to a BCS superconductor when the numbers of up and down fermions are unequal? This is a fascinating and difficult problem in many-body physics. The most exciting possibility is that pairing and polarization compete to produce a modulated superfluid state, known as an Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase. This is a quantum liquid crystal exhibiting microscale phase separation, in which the excess fermions self-organize into domain walls where the pairing amplitude changes sign. There is a growing body of indirect evidence for the existence of FFLO states. As an example, I present our recent work on Al thin film superconductors in parallel magnetic fields [1]. Just below the Chandrasekhar-Clogston spin paramagnetic transition, our tunneling density of states measurements reveal a significant population of subgap states. These excess states cannot be explained in terms of conventional superconductivity, but they are a natural consequence of disordered FFLO physics. There is also hope for realizing FFLO in ultracold gases of neutral fermionic atoms. Continuum FFLO states are very fragile, but our calculations suggest that FFLO states are greatly stabilized by an optical lattice. For a cubic lattice with suitable parameters, up to 80\% of the fermions participate in the FFLO phase [2]. Furthermore, we propose an interferometric technique to detect pairing amplitude modulations, which may provide the first \emph{direct} evidence of FFLO [3]. \\[4pt] [1] Y. L. Loh, N. Trivedi, Y. M. Xiong, P. W. Adams, and G. Catelani, ``Origin of Excess Low-Energy States in a Disordered Superconductor in a Zeeman Field,'' PRL 107, 067003 (2011) \\[0pt] [2] Y. L. Loh and N. Trivedi, ``Detecting the Elusive Larkin-Ovchinnikov Modulated Superfluid Phases in Imbalanced Fermi Gases in Optical Lattices,'' PRL 165302 (2010) \\[0pt] [3] M. Swanson, Y. L. Loh, and N. Trivedi, ``Proposal for interferometric detection of topological defects in modulated superfluids,'' arXiv:1106.3908   

Session B4: Hybrid Systems, Optomechanics and Macroscopic Systems at the Quantum Limit I

Observing quantum phenomena in cavity optomechanics

Recent efforts have produced optomechanical systems whose mechanical elements are prepared at or near their quantum ground state. But what manifestly quantum effects can be measured with these new systems? Here we present results from our experiment, using the collective motion of an ultracold atom ensemble as a mechanical oscillator. The motion is driven by shot noise in the light's radiation pressure, allowing us to observe the production of nonclassical states of light by optomechanics -- here, quadrature-squeezed light. Notably, this nonlinear optical effect occurs with only 40 pW of pump power. We also measure the quantization of the oscillator, by observing a 3:1 asymmetry in the light it scatters to low- and high-energy optical sidebands. Analyzing the light emitted from the cavity moreover provides a spectroscopic record of the energy exchanged between light and motion, allowing us to directly quantify the necessary diffusive heating of a quantum backaction-limited position measurement.   

Cryogenic optomechanics with a 261kHz mechanical oscillator

Mechanical motion can interact with light via radiation pressure force. With recent experimental advances over the last few years, such optomechanical coupling has been used to reach quantum ground state of mechanical oscillators, which opens interesting new regime of observing quantum mechanics in macroscopic objects. The optomechanical devices used in this talk consist of a dielectric SiN membrane located inside a high finesse optical cavity. Combining cryogenic cooling in He3 refrigerator and resolved sideband laser cooling enables us to cool the membrane's mechanical mode (whose mechanical frequency is 261kHz) to less than 60 phonons. We will describe some technical challenges in our experiments such as the role of classical phase noise of the cooling laser at the mechanical frequency and our efforts to significantly reduce it via a filter cavity.~   

Ultraefficient Cooling of Resonators: Beating Sideband Cooling with Quantum Control

There is presently a great deal of interest in cooling high-frequency micro- and nano-mechanical oscillators to their ground states. The present state of the art in cooling mechanical resonators is a version of sideband cooling, which was originally developed in the context of cooling trapped ions. Here we present a method based on quantum control that uses the same configuration as sideband cooling--coupling the resonator to be cooled to a second microwave (or optical) auxiliary resonator--but will cool significantly colder. This is achieved by applying optimal control and varying the strength of the coupling between the two resonators over a time on the order of the period of the mechanical resonator. As part of our analysis, we also obtain a method for fast, high-fidelity quantum information transfer between resonators.   

Postselected optomechanical superpositions

We present a scheme for achieving macroscopic quantum superpositions in weakly coupled optomechanical systems by using single photon postselection. This method allows the creation of macroscopic superpositions with currently achievable device parameters, and allows observation of decoherence on a timescale unconstrained by the system's optical decay time. This method relieves many of the challenges associated with previous optical schemes for measuring macroscopic superpositions, and only requires the devices to be in the weak coupling regime. Prospects for observing novel decoherence mechanisms are also discussed.   

Coupling a single spin in diamond to the quantum motion of a mechanical cantilever

We present theoretical considerations for a magnetized mechanical cantilever coupled to a single electronic spin associated with a nitrogen-vacancy (NV) defect center in diamond. This coupled system has recently been implemented in an experiment where the NV spin was used to detect the thermal motion of a magnetic force microscope cantilever at room temperature, reading out the spin state optically using the spin-selective fluorescence of the NV. The possibility to extend this system to the quantum regime opens the door to applications such as readout and transfer of quantum information, as well as interesting theoretical questions. For example, it should be possible to reach the regime of strong coupling between the spin and the motion of the cantilever, in analogy to cavity quantum electrodynamics. We discuss the prospects for reaching the strong coupling regime and the conditions for measuring the onset of quantum effects, such as measuring the zero point motion of the cantilever using the spin as a detector.   

Thermally induced parametric instability in back-action evading measurement of micromechanical quadrature near the zero-point level

Back-action evading (BAE) measurement of mechanical resonators allows, in principle, detection of a single quadrature of motion with sensitivity far below the standard quantum limit, limited in practice only by the non-idealities in the measurement. We report the results of experiments utilizing two-tone BAE in a tightly coupled cavity quantum electro-mechanical system ($\omega_c$=7.1GHz, $\omega_m$=10MHz, g=14MHz/nm). Due to excess dissipation in the microwave cavity, we observe a parametric instability induced by the thermal shift of mechanical resonance frequency. This bounds the minimum position imprecision on one quadrature and we measure the imprecision reaching twice the zero-point motion. We discuss the device requirements to avoid this thermal mechanism and perform measurements below the zero-point level.   

Mechanical squeezing via parametric amplification and feedback control

We discuss the mechanical squeezing that can result from position measurement and feedback applied to a parametrically driven mechanical oscillator. If the parametric drive is optimally detuned from resonance, correlations between the quadratures of motion allow unlimited steady-state squeezing. This contrasts to a parametric drive alone, which is limited to 3dB of squeezing. Compared to back-action evasion, we demonstrate that the measurement strength, temperature and efficiency requirements for quantum squeezing are significantly relaxed.   

Asymmetric absorption and emission of energy by a macroscopic mechanical oscillator in a microwave circuit optomechanical system

We measure the asymmetry in rates for emission and absorption of mechanical energy in an electromechanical system composed of a macroscopic suspended membrane coupled to a high-Q, superconducting microwave resonant circuit. This asymmetry is inherently quantum mechanical because it arises from the inability to annihilate the mechanical ground state. As such, it is only appreciable when the average mechanical occupancy approaches one. This measurement is now possible due to the recent achievement of ground state cooling of macroscopic mechanical oscillators [1,2]. Crucially, we measure the thermal cavity photon occupancy and account for it in our analysis. Failure to correctly account for the interference of these thermal photons with the mechanical signal can lead to a misinterpretation of the data and an overestimate of the emission/absorption asymmetry. \\[4pt] [1] J. D. Teufel, T. Donner, Dale Li, J. W. Harlow, M. S. Allman, K. Cicak, A. J. Sirois, J. D. Whittaker, K. W. Lehnert, R. W. Simmonds, ``Sideband Cooling Micromechanical Motion to the Quantum Ground State,'' Nature, 475, 359–363 (2011).\\[0pt] [2] Jasper Chan, et al, ``Laser cooling of a nanomechanical oscillator into its quantum ground state,'' Nature, 478, 89-92 (2011).   

Quantum many-body system based on phonons and donors in silicon

Cold atoms in optical lattices have become an indispensable tool for the study of many-body physics. Here, we introduce a novel many-body quantum system based on phonons with potentially useful properties. Theoretical results will be presented on the possibility of interacting systems based on phonitons, hybrid composite objects of a matter excitation and a cavity phonon. We discuss experimentally accessible regimes in silicon phoniton systems involving Mott insulator and superfluid phases. We consider experimental tools to probe these many-body states and give explicit designs for devices where they can be observed.   

Development of a dispersive read-out technique for quantum measurements of nanomechanical resonators

The development of techniques to observe non-classical behavior of micro- and nano- scale mechanical structures has received considerable attention in recent years because of the potential to use these systems for fundamental studies of quantum mechanics as well as their possible role as new technologies for applications ranging from the sensing of weak forces to quantum communication. One important route for observing such behavior is the coupling of micro- and nanomechanical resonators with superconducting qubits. Under certain conditions, qubit-coupled mechanical devices are formally analogous to Jaynes-Cummings systems which have been used in fields such as cavity QED for explorations of matter-radiation interactions and the quantum nature of light. Correspondingly, experiments in the last couple of years have begun to develop superconducting qubits as tools to manipulate and measure quantum states of mechanics. In this talk, we will discuss our efforts to integrate charge-type superconducting qubits as elements for dispersive (non-resonant) read-out and control of nanomechanical resonators, including preliminary system design and the prospects of implementing this system for read-out of the number-state statistics of nanomechanical modes.   

Measuring Quantum Optomechanical Self-induced Oscillations: Photon Correlation and Homodyne Tomography

Motivated by recent experimental advances in fabricating systems with large optomechanical couplings, we study the self-induced mechanical oscillations in the strong quantum regime for a single cell optomechanical system. We show that, under strong optomechanical coupling $g_M\ge\kappa$, the \emph{persistent }state of the mechanical oscillator can have non-classical, \emph{strongly negative} Wigner density, which can be measured by non-destructive homodyne tomography. We further propose to detect the onset of the quantum self-induced oscillation using the easier-to-measure photon two-point correlation functions $g^{(2)}(t)$. We show that there are two distinct signatures in the long-term time-average and the line-shape of $g^{(2)}(t)$ at the onset of self-induced oscillations. We show that $g^{(2)}(t)$ exhibits long-term coherence extending much beyond the optical decay time $1/\kappa$, the decay of which in the red- and blue-detune regime we explain using models of optomechanical cooling and phase noise.   

Controllable Coherent Transfer between a Superconducting Resonator and a Mechanical Oscillator

We report experimental results of controllable coupling between a 7.5 GHz superconducting resonator and a 10 MHz mechanical oscillator. Through time domain measurements, we demonstrate controlled coherent energy transfer between these two systems. Furthermore, by utilizing a Josephson parametric amplifier we have been able to verify coherent transfer of small amplitude states. We compare these results to frequency domain measurements and discuss experimental limitations.   

Using interference for high fidelity quantum state transfer in optomechanics

We present a theoretical study of a two-cavity optomechanical system (e.g. a single mechanical resonator coupled to both a microwave and an optical cavity), investigating how interference can be used to perform mechanically-mediated quantum state transfer between the two cavities. We show that this optomechanical system possesses an effective ``mechanically-dark'' mode which is immune to mechanical dissipation; utilizing this feature allows highly efficient transfer of intra-cavity states, as well as of itinerant photon states. Simple analytic expressions for the fidelity of transferring both Gaussian and non-Gaussian states are provided. Our work has relevance to ongoing experimental efforts in quantum optomechanics (e.g., C.~A.~Regal and K.~W.~Lehnert, J. Phys.: Conf. Ser. 264, 012025 (2011); A.~H.~Safavi-Naeini and O.~Painter, New J. Phys. 13, 013017 (2011)).   

Adiabatic State Conversion and Photon Transmission in Optomechanical Systems

Light-matter interaction in optomechanical systems in the strong coupling regime can be explored as a tool to transfer cavity states and to transmit photon pulses. Here, we show that quantum state conversion between cavity modes with different wavelengths can be realized with high fidelity by adiabatically varying the effective optomechanical couplings. During this adiabatic process, the quantum state is preserved in the dark mode of the cavities, similar to the adiabatic transfer schemes in EIT systems. The fidelity for gaussian states is derived by solving the Langevin equation in the adiabatic limit and shows negligible dependence on the mechanical noise. We also show that an input pulse can be transmitted to an output channel with a different wavelength via the effective optomechanical couplings. The condition for optimal transmission is derived in the frequency domain. Input pulses with a narrow spectral width can be transmitted with high fidelity. For input pulses with a large spectral width, the shape of the output pulses can be manipulated by applying time-dependent effective couplings. (1) L. Tian, arXiv:1111.2119. (2) L. Tian and H. L. Wang, Phys. Rev. A 82, 053806 (2010).   

Single atom array to form a Rydberg ring

Single atom arrays are ideal quantum systems for studying few-body quantum simulation and quantum computation [1]. Towards realizing a fully controllable array we did a lot of experimental efforts, which include rotating single atoms in a ring optical lattice generated by a spatial light modulator [2], high efficient loading of two atoms into a microscopic optical trap by dynamically reshaping the trap with a spatial light modulator [3], and trapping a single atom in a blue detuned optical bottle beam trap [4]. Recently, we succeeded in trapping up to 6 atoms in a ring optical lattice with one atom in each site. Further laser cooling the array and manipulation of the inner states will provide chance to form Ryberg rings for quantum simulation. \\[4pt] [1] M. Saffman et al., Rev. Mod. Phys. 82, 2313 (2010)\\[0pt] [2] X.D. He et al., Opt. Express 17, 21014 (2009)\\[0pt] [3] X.D. He et al., Opt. Express 18, 13586 (2010)\\[0pt] [4] P. Xu et al., Opt. Lett. 35, 2164 (2010)   

Session B5: Surface Electronic & Lattice Properties: Interfaces, Spin Effects, Etc.

Spatially Resolved Nano-Scale Characterization of Electronic States in SrTiO$_3$(001) Surfaces by STM/STS

We have performed low temperature scanning tunneling microscopy/spectroscopy (STM/STS) measurements on TiO$_2$-terminated SrTiO$_3$(001) thin film surfaces. The conductance map exhibited electronic modulations that were completely different from the surface structure. We also found that the electronic modulations were strongly dependent on temperature and the density of atomic defects associated with oxygen vacancies. These results suggest the existence of strongly correlated two-dimensional electronic states near the SrTiO$_3$ surface, implying the importance of electron correlation at the interfaces of SrTiO$_3$-related heterostructures.   

The Structure of High Polarization Surface of the Antiferromagnet Cr$_{2}$O$_{3}$

Manipulation of magnetically ordered states by electrical means is among the most promising approaches towards novel spintronic devices. Electric control of the exchange bias can be realized when the passive antiferromagnetic pinning layer is replaced by a magneto-electric antiferromagnet, like the prototypical magneto-electric Cr$_{2}$O$_{3}$(0001), so long as there is also a finite remanent magnetization at the surface or boundary. We have demonstrated that a very unusual high polarization surface magnetic order exists at the surface of the Cr2O3 (0001) surface and is robust against surface roughness from spin polarized inverse photoemission, and X-ray magnetic circular dichroism. We have also performed LEED (low energy electron diffraction) I(V) analysis to explore the surface structure above and below Neel Temperature (308 K). Temperature dependent LEED was also carried out at several different electron kinetic energies and Debye temperature was extracted. The surface and bulk Debye temperatures were obtained by fitting Debye temperature as a function of electron kinetic energy.   

Resonance Photoemission and Spin Polarization of Fe$_{x}$Co$_{1-x}$S$_{2}$

The electronic structure in the region of the Fe/Co 3d band for Fe$_{x}$Co$_{1-x}$S$_{2}$ has been investigated by photoemission and spin polarized photoemission. The comparison between the results for different content of doped Fe was made, specifically x=0, 0.05, 0.10, 0.15 and 1. While the surface spin polarization of Fe$_{x}$Co$_{1-x}$S$_{2}$, measured by spin polarized ultraviolet photoemission, was reduced compared with the bulk value, we see that the spin polarization increases with Fe doping level for textured thin films. The resonance photoemission spectroscopy shows that sulfur bands have strong resonance at the photon energy of the Co 2p core level, indicating strong hybridization between Co and sulfur bands in Fe$_{x}$Co$_{1-x}$S$_{2}$ (small x) however, the ultraviolet photoelectron spectroscopy (UPS) of FeS$_{2}$ exhibits a slightly different d-band density of states than Fe$_{x}$Co$_{1-x}$S$_{2}$.   

Theoretical Band Offsets in c-Si/Si-XII Heterojunctions

Many different phases of silicon can be formed under pressure, with some being metastable at standard temperatures and pressures. For one such phase, Si-XII, experiments have recently suggested it to be a semiconductor, confirming theoretical predictions that it has a narrow gap in its electronic band structure. Current-voltage measurements show rectifying behavior in \mbox{c-Si/Si-XII} heterojunctions, indicative of a band discontinuity at the interface. We present computations that quantify this band discontinuity using bulk band structures obtained with Density Functional Theory within the Local Density Approximation. In particular, we demonstrate the use of a semiconductor's intrinsic charge neutrality level to determine band lineups.   

Band Alignment in Transparent Conducting Oxide Schottky Junctions

Better understanding and control of band alignment in oxide-semiconductor heterostructures is essential for improving the performance of devices such as sensitized solar cells and quantum dot based light emitting devices. We will present studies of Schottky junctions formed between Al-doped ZnO (AZO) conducting oxide thin films and lightly doped silicon. AZO films with varying oxygen content have been synthesized by control of oxygen pressure during growth. Transport measurements (I-V and C-V) on devices are used to illustrate the degree to which the oxide stoichiometry can be used to engineer the junction characteristics.   

Interface Band Structure Effects upon Hot Electron Transport Across Non-Epitaxial Metal-Semiconductor Interfaces

The interface band structure of a semiconductor is shown to affect the transport of hot electrons across a non epitaxial metal-semiconductor interface. This is observed by measuring the hot electron attenuation length of Ag utilizing ballistic electron emission microscopy (BEEM). The attenuation length is observed to increase sharply for energies approaching the Schottky barrier height when deposited upon Si(001) substrates and decrease slightly when deposited upon Si(111) substrates. A theoretical description demonstrates that this is due to conservation of parallel momentum and differences in interface band structure of the two silicon orientations. At higher tip biases the attenuation lengths converge allowing extraction of the inelastic and elastic scattering lengths in the silver.   

A numerical simulation study of inverse doped surface layer in Schottky barrier modification

The Poisson's and continuity equations are solved by iterative method to obtain the potential and electron and hole concentrations inside the semiconductor near the metal semiconductor interface for different inverse layer thickness and doping concentrations. The barrier height (BH) and ideality factor (IF) obtained by fitting of simulated current voltage data into thermionic emission diffusion current equation. The derived BH increases with increase in inverse layer thickness as well as with increase in the inverse layer doing concentration and then saturates at maximum value. The IF first rises with increase in inverse layer thickness and then attaining a maximum value at a particular thickness it decreases approaching unity value for large inverse layer thickness. It is observed that for large inverse layer thickness the BH attains a maximum value with unity IF. Thus, there are two regimes, namely, non-ideal regime corresponding to less inverse layer thickness where the BH has increased less and IF is more than unity and ideal regime corresponding to large inverse layer thickness where the BH attains maximum value with unity IF.   

Density functional calculations of the Schottky barrier height and effective work function in Ni/oxide interfaces

In high-k/metal gate stacks of complementary metal-oxide semiconductor devices, it is important to control the effective work functions of metals such that they should match to the doping levels of poly-Si gates. However, it is known that metal work functions are strongly affected by interface dipoles and defects. In this work, we perform first-principles density-functional calculations to study the Schottky barrier heights and the effective metal work functions in Ni/SiO$_2$ and Ni/HfO$_2$ interface structures. We use the advanced approaches such as hybrid density functional and quasi-particle \textit{GW} calculations for the exchange-correlation potential and discuss the limitations of GGA calculations. We also examine the effects of O-vacancy defects introduced at the interface on the Schottky barrier height and the effective work function. We find that, in the Ni/HfO$_2$ interface, the $p$-type Schottky barrier height tends to increase with increasing of the defect density due to the charge transfer at the interface, whereas it is little affected in the Ni/SiO$_2$ interface.   

Models for (001) epitaxial interfaces between CdTe and ZnO

Epitaxial interfaces between ZnO and CdTe appear difficult to achieve given their different crystal structures (CdTe is zinc blende with conventional lattice constant $a$ = 6.482 {\AA}, ZnO is hexagonal wurtzite with $a$ = 3.253 {\AA} and $c$ = 5.213 {\AA}.) However, ZnO also occurs in a metastable zinc-blende structure with an fcc primitive lattice constant close to the hexagonal $a$ value. Since this value is close to half of the CdTe conventional (simple cubic) lattice constant, (001)-oriented cubic ZnO films might grow epitaxially on a CdTe (001) surface in an R45\r{ } $\surd $2$\times \surd $2 configuration. Many alignments of the interfacial layers are possible, and we describe density-functional calculations on several of these to identify the most likely, and to predict valence-band offsets between CdTe and ZnO for each. Growth of ZnO on Te-terminated CdTe (001) is predicted to produce small or even negative (CdTe below ZnO) valence band offsets, resulting in a Type I band alignment. Growth on Cd-terminated CdTe is predicted to produce large positive offsets for a type II alignment as needed, for example, in solar cells. Calculations with the GGA + U method (with U = 7.5 eV for Zn 3d states) gave a valence band offset of +1.8 eV while a hybrid HSE06 + U calculation gave +2.6 eV. An experimental measurement on a ZnO film grown on CdTe (001) yielded a value of +2.2 eV for the valence band offset.   

The crystalline Si3N4/Si interface; the electronic structure of defects

A semiconducting beta-Si3N4(0001)/Si(111) interface model without dangling bonds is presented, and its geometric and electronic structure is compared to previous models based on calculations in a density functional theory framework. Furthermore nitrogen and phosphorus defects in the silicon layer are investigated, in particular how these defects modify the electronic structure and the electronic properties of the interface as a function of their distance to it. The local geometric structure of the nitrogen and phosphorus defects is also investigated close to the interface.   

Epitaxial n-ZnO on p-Si with native SiOx reduced by Al buffer

RF sputtering was employed to deposit n-type zinc oxide epitaxial thin films on p-type silicon substrates to form p-i-n diodes. A buffer layer of crystalline metal oxide was introduced by redox reaction between an aluminum layer and the native SiO$_{2}$. The aluminum layer was sputtered to various thicknesses and then annealed in situ for different times. The epitaxial relations follow (111) $_{Si}$//(0001) $_{ZnO}$ and [110] $_{Si}$//[11$\bar{2}$0] $_{ZnO}$, though certain degree of mosaicity was observed wavering around the [11$\bar{2}$] $_{Si}$ axis. Cross-sectional TEM observations of the interfaces, x-ray crystallography via $\omega$-2$\theta$ and rocking scans in regards to the perfection of the structures and orientations are agreeable. The current-voltage characteristics of the p-i-n diodes show promising outlooks for light emitting and photovoltaic applications.   

Emergence of orbital angular momentum due to broken inversion symmetry and its contribution to Rashba-type splitting

We demonstrate that the chiral orbital angular momentum (OAM) structure can emerge as a result of broken inversion symmetry especially at the metal surfaces. The surface-normal electric field is responsible for chiral OAM states even if spin-orbit interaction is negligible. Such chiral OAM structure can be measured by a circular dichroism (CD) in angle-resolved photoemission spectroscopy (ARPES). To confirm the existence of OAM and its detection by CD-ARPES, we perform simulation of CD-ARPES for Cu surface states by first-principles calculation and the results agree well with our CD-ARPES experiment. Addition of the spin-orbit interaction to the chiral OAM structure produces a chiral spin angular momentum (SAM) pattern and the corresponding Rashba-type band splitting. We assert that OAM polarization should be a more widespread feature than the chiral spin structure which requires strong spin-orbit coupling.   

Chiral Orbital Angular Momentum and Circular Dichroism ARPES in p- and d-orbital Bands

We derive explicit formulas relating the circular dichroism angle-resolved photoemission (CD-ARPES) signal to the existence of nonzero chiral orbital angular momentum (OAM) in the band structure. The existence of nonzero chiral OAM is a generic feature of surface states that break inversion symmetry, as pointed out in several recent articles [1-3]. We propose that CD-ARPES setup is an effective probe of the OAM of quasi-particles occupying the surface states. Explicit formulas for the $p$- and $d$-orbital bands are derived to show that the CD-ARPES signal is proportional to the OAM in the momentum space.\\[4pt] [1] S. R. Park, C. H. Kim, J. Yu, J. H. Han and C. Kim, Phys. Rev. Lett. \textbf{107}, 156803 (2011).\\[0pt] [2] S. R. Park \textit{et al}., arXiv:1103.0805 (2011).\\[0pt] [3] Choong H. Kim \textit{et al}., arXiv:1107.3285 (2011).   

A first principles study of water adsorption on $\alpha $-Pu (020) surface

Adsorptions of water in molecular (H$_{2}$O) and dissociative (OH+H, H+O+H) configurations on the $\alpha $-Pu (020) surface have been studied using \textit{ab initio }methods. The full-potential FP/LAPW+lo method has been used to calculate the adsorption energies at the scalar relativistic with no spin-orbit coupling (NSOC) and fully relativistic with spin-orbit coupling (SOC) theoretical levels. It is found that the SOC effect increases the adsorption energies by $\sim $0.30 eV for the two dissociative adsorptions. Weak physisoprtions have been observed for the molecule H$_{2}$O on the $\alpha $-Pu (020) surface with primarily a covalent bonding, while the two dissociative adsorptions are chemisorptive with ionic bonding. At the SOC level, the most stable adsorption energy is 0.58eV, the corresponding values being 5.44 eV and 5.73 eV for the partial dissociation and complete dissociation cases, respectively. Completely dissociative adsorption at a long bridge site for the dissociated O atom and two short bridge sites for the two dissociated H atoms is the most stable adsorption site. Hybridizations of O(2p)-H(1s)-Pu(5f)-Pu(6d) are observed for the two dissociative adsorptions, implying that some of the Pu-5f electrons become further delocalized and participate in chemical bonding.   

Characterization of Hydrogen Interactions with $\delta $-Pu using Electronic Structure Theory

The generalized gradient approximation to density functional theory was used to study surface, bulk, defect, and reaction states of hydrogen in $\delta $-Pu. The quasi-disordered anti-ferromagnetic arrangement gave a volume of 24.1 {\AA}$^{3}$ and a bulk modulus of 48.1 GPa for $\delta $-Pu, in reasonable agreement with the experimental values of 24.9 {\AA}$^{3}$ and 30-35 GPa. This arrangement was thus subsequently used for all calculations. We have determined that hydrogen interactions with $\delta $-Pu are exothermic in character at all levels ranging from dissociative chemisorption to interstitial absorption, the formation of hydrogen-vacancy complexes, and generation of a hydride phase. The exothermic character of these interactions appears to be the reason for the rapid hydriding reaction, which has been determined experimentally to be essentially a barrierless process. The anionic character is observed to be retained. Our studies also indicate that vacancies do not appear to be strong traps for hydrogen, since the interstitial absorption sites are exothermic in nature. We will propose a scheme by which hydrogen interacts with Pu. Results will be compared with previous studies in the literature where available.   

Session B6: Thermal Properties of Graphene and Graphene Derivatives

Nanoscale Thermal Transport in Graphene Interfaces

We have investigated nanoscale thermal transport in epitaxial graphene systems using first-principles calculations and the Landauer formalism for phonon transport. Two types of interfaces are investigated: graphene-dielectric and graphene-metal heterojunctions. Hexagonal boron nitride (h-BN), SiC and SiC with hydrogen passivation (SiC-H) are studied as potential dielectric substrate materials for graphene devices. As for graphene-metal contacts, we have considered Au and Ti as prototypical systems for physisorbed and chemisorbed metal contacts, respectively. The interfacial thermal resistances of h-BN/G system is 5.3 10$^{-9}$ Km$^{2}$/W at room temperature, which is approximately one order of magnitude smaller than that of SiC/G system (55-79 10$^{-9}$ Km$^{2}$/W). Further analysis shows that heat conduction at the graphene interfaces is dominated by low-lying acoustic phonons and the thermal resistances strongly depend on atomic details at the interface such as lattice mismatch, disorder and surface reconstruction. Our work demonstrates the importance of developing a microscopic description of phonon dynamics at heterogeneous interfaces to engineer and design devices with optimal thermal management.   

Isotope impurity doping in graphene

Isotope impurities provide a powerful technique to study phonon related properties of graphene. The advantage of this approach is that phonon frequencies or thermal properties can be modified so that we can identify the origin of the interaction in the unknown optical spectra without changing either the electrical or chemical properties. In the presence of a $^{13}$C isotope impurity in a normally $^{12}$C lattice, the phonon wavefunction of graphene becomes localized. When we discuss the electron-phonon interaction of graphene, we treat phonons in terms of a delocalized wavefunction. However, in real graphene, we know that 2\% of the atoms are $^{13}$C and thus phonons have a finite lifetime, which results in a natural width in the Raman spectra. Many experimental results were obtained recently related to isotopic doping of graphene and carbon nanotubes and its effect on Raman spectra. In this work, we calculate the localization length of the phonons as well as the power law dependence for this effect. At the same time, we calculate the phonon lifetime of the different phonon modes within first order perturbation theory to elucidate the physical mechanisms behind isotope doping and to provide further insight into the experimental results.   

Polarization and electric field dependent electron-phonon interaction in graphene and graphene oxide

Raman spectroscopy has been emerged as one of the important tool to understand various physical properties of graphene and graphene oxide. In this study, polarized Raman scattering has been performed to understand the electron-phonon interaction in connection with the second order Raman scattering process (2D band) in monolayer, bilayer and suspended bilayer graphene. The 2D Raman band shows strong polarization dependent irrespective of number of graphene layers. This effect has been explained on the basis of anisotropic photon scattering through the nodes at the~K-point of the Brillouin zone in graphene during optical absorption. We also explored the electron-phonon interaction in graphene oxide in presence of electric field. The in-situ Raman studies show significant changes in the D and G Raman bands. In particular, the splitting of G band was observed with increase in voltage which indicates the electric field can alter the deformation-potential mediated electron-phonon interaction in graphene oxide. The electrical conductivity of graphene oxide was also found to increase dramatically with increase in voltage.   

High-fidelity phonon transport simulation in graphene devices using efficient, variance-reduced Monte Carlo methods

In this talk we consider thermal transport in graphene at sufficiently small scales that diffusive transport (Fourier's law) is no longer valid. In this regime, the Boltzmann transport equation may be used to describe thermal transport, and Monte Carlo is the typical solution method, especially in the context of device simulation. Unfortunately, due to the slow rate of convergence of Monte Carlo solutions with the number of samples, such simulations can be prohibitively costly. By employing a recently developed variance-reduced Monte Carlo method, we are able to efficiently simulate the Boltzmann equation for phonon transport without resorting to approximations for reducing the problem complexity or dimensionality, such as neglecting phonon-phonon scattering events, or modeling boundary scattering as a homogeneous relaxation (scattering) process. We use our simulations to characterize the error associated with these approximations in the context of thermal transport in graphene devices, but also to study thermal transport in novel two-dimensional geometries.   

Thermal transport in graphene-based nanostructures

Thermal conductivity of graphene and graphene-based nanostructures, such as graphene nanoribbons (GNRs), CVD-grown polycrystalline graphene (PCG), and nano-patterned single-layer graphene (SLG), is of great interest due to their potential applications as logic devices, high-frequency amplifiers, and heat spreaders in future nanoelectronic circuits. Both line edge roughness (LER) and substrate scattering have been shown to impact thermal transport and reduce graphene's record thermal conductivity; however, the combination of these two mechanisms, which will both be present simultaneously in supported nanostructures, has not been explored previously, and their combined impact on thermal transport has not be assessed. We solve the phonon Boltzmann transport equation and calculate the thermal conductivity tensor in a variety of suspended and supported graphene nanostructures. We show that there is a competition between LER and substrate scattering processes, leading to a high degree of directional anisotropy of thermal conductivity. We demonstrate that transport in narrow ribbons (W$<$100 nm) and PCG or nano-SLG with small feature sizes is dominated by roughness scattering and highly anisotropic. We discuss different avenues of controlling thermal transport in graphene-based devices.   

Measurements of Thermal Conductivity and Thermopower in Suspended Single and Few Layer Graphene

Detailed study on thermal conductivity and thermopower in suspended single and few layer graphene has been hampered by experimental challenges mainly due to the difficulty of obtaining suspended samples larger than a few micrometers. Here, we report on a high-yield process for the fabrication of suspended graphene samples of one to few atomic layers on micro-thermometer devices, which allow us to investigate electrical and thermal conductivity, and thermopower from 4 K to 500 K. We find that the thermal conductivity values in suspended graphene are largely limited by scattering with surface residues in as-made devices, and they can reach values comparable to those of bulk graphite after thorough cleaning of the sample surfaces. In addition, temperature-dependent thermopower results in suspended graphene are reported.   

Thermal Transport in Graphene from Large-scale Molecular Dynamics Simulations

Carbon-based materials display exceptional thermal properties. The thermal conductivity of carbon allotropes can range five orders of magnitude. In the bulk, amorphous carbon is a very poor heat conductor, with $\kappa \approx 0.01$ W/m/K, whereas diamond has the highest thermal conductivity among elemental solids, $\kappa \approx 2000$ W/m/K at room temperature. Even broader ranges can be achieved by considering carbon nanostructures. Thermal conductivities as large as $5000$ W/m/K have been measured for suspended graphene and carbon nanotubes. In spite of intense investigations, there is much controversy over the actual value of the thermal conductivity of graphene, both experimentally and theoretically. Here, we present results from equilibrium and non-equilibrium molecular dynamics simulations aimed at understanding the mechanism of heat transport in graphene. In particular, we investigate the influence of finite-size and uniaxial strain on the thermal conductivity of graphene, performing large scale molecular dynamics simulations of micrometer-size models containing up to $10^6$ atoms.   

Tunable thermal transport and negative differential thermal conductance in graphene nanoribbons

We have studied thermal transport in graphene nanoribbons (GNRs) using classical nonequilibrium molecular dynamics simulations. We show that the calculated thermal conductivity of GNRs can be dramatically tuned by the shape, isotope composition, defects, and chirality and hydrogen passivation of the edges [1-2]. For GNRs under large temperature bias beyond the linear response, we have studied negative differential thermal conductance (NDTC) [3]. The NDTC is found to vanish for a sufficiently long GNR whose temperature is fixed at one end. Furthermore, for diffusive thermal transport obeying differential Fourier's law in a generic one-dimensional system, we analytically show that NDTC requires temperature dependent thermal conductivity and simultaneously varying temperatures at both ends. We have also studied asymmetrical GNRs and observed thermal rectification [1] and direction dependent NDTC [3]. Our studies may be useful for nanoscale thermal managements and thermal signal processing using GNRs and understanding low dimensional thermal transport in general. \newline [1] J. Hu et. al., \textbf{Nano Letters 9}, 2730 (2009) \newline [2] J. Hu et. al., \textbf{Applied Physics Letters 97}, 133107 (2010) \newline [3] J. Hu et. al., \textbf{Applied Physics Letters 99}, 113101 (2011)   

Heat Transport in Graphene

We fabricated suspended graphene devices and measured their thermal conductivity, $\kappa$, as a function of both temperature, T, and charge carrier density, n. Heat transport is a powerful tool to obtain information about both the phononic and electronic properties of graphene. Recent heat transport experiments in graphene have shown a high $\kappa$, but a detailed mapping of graphene's heat conductivity versus T and n is not yet available. The measurement technique we developed is a two-point method which uses graphene as its own heat source (Joule heating) and thermometer (resistivity). We report $\kappa$ at temperatures ranging from 6 to 350 Kelvin, and at charge carrier densities close to the Dirac point up to about 1.5 $\times 10^{11}/cm^{2}$, in graphene crystals whose length varies from 250 nm up to one micron. We observed that the thermal conductivity increases by over two orders of magnitude over the temperature range, and that it also increases with the crystal's length. $\kappa$ can be tuned by an order of magnitude with gate voltage, opening the possibility of creating room temperature heat transistors.   

Raman Measurements of Thermal Transport in Suspended Monolayer Graphene of Variable Sizes in Vacuum and Gaseous Environments

Using micro-Raman spectroscopy, the thermal conductivity of a graphene monolayer grown by chemical vapor deposition and suspended over holes with different diameters ranging from 2.9 to 9.7 $\mu $m was measured in vacuum, thereby eliminating errors caused by heat loss to the surrounding gas. The obtained thermal conductivity values of the suspended graphene range from (2.6$\pm $0.9) to (3.1$\pm $1.0)$\times $10$^{3}$Wm$^{-1}$K$^{-1}$ near 350 K without showing the sample size dependence predicted for suspended, clean, and flat graphene crystal. The lack of sample size dependence is attributed to the relatively large measurement uncertainty as well as grain boundaries, wrinkles, defects, or polymeric residue that are possibly present in the measured samples. Moreover, from Raman measurements performed in air and CO$_{2}$ gas environments near atmospheric pressure, the heat transfer coefficient for air and CO$_{2}$ was determined and found to be (2.9+5.1/-2.9) and (1.5+4.2/-1.5)$\times $10$^{4}$Wm$^{-2}$K$^{-1}$, respectively, when the graphene temperature was heated by the Raman laser to about 510 K.   

Effect of electron-electron interactions in thermoelectric power in graphene

Thermoelectric power (TEP) of graphene is previously measured in the disorder limited transport regime where the semiclassical Mott relation agrees with experimental data. In this presentation, we report the TEP measurement on graphene samples deposited on hexa boron nitride substrates where drastic suppression of disorder is achieved. Our results show that at high temperatures where the inelastic scattering rate due to electron-electron (e-e) interactions is higher than the elastic scattering rate by disorders, the measured TEP exhibit a large enhancement compared to the expected TEP from the Mott relation. We also investigated TEP in the quantum Hall regime at a high magnetic fields, where we observed symmetry broken integer quantum Hall and fractional quantum Hall states due to the strong e-e interactions.   

Derivative relations between electrical and thermoelectric quantum transport coefficients in graphene

We find that the empirical relation between the longitudinal and Hall resistivities (i.e. $R_{xx}$ and $R_{xy})$ and its counterpart between the Seebeck and Nernst coefficients (i.e. $S_{xx}$ and $S_{xy})$, both originally discovered in conventional two-dimensional electron gases [1,2], hold surprisingly well for graphene in the quantum transport regime except near the Dirac point. These empirical relations can be described by the following equations: \[ R_{xx} =\alpha _r \cdot \frac{B}{n}\frac{dR_{xy} }{dB}, \quad S_{yx} =\alpha _s \cdot \frac{B}{n}\frac{dS_{xx} }{dB} \] Here R and S are electrical resistivity and thermoelectric conductivity tensor respectively. The validity of the relations is cross-examined by independently varying the magnetic field and the carrier density in graphene. We demonstrate that the pre-factor, \textit{$\alpha $}$_{s}$, does not depend on carrier density in graphene. By tuning the carrier mobility therefore the degree of disorders, we find that the pre-factor stays unchanged. Our experimental results validate both derivative relations for massless Dirac fermions except near the Dirac point. \\[4pt] [1] A. M. Chang and D. C. Tsui, Solid State Commun. \textbf{56}, 153 (1985).\\[0pt] [2] B. Tieke et al, Phys. Rev. Lett. \textbf{78}, 4621 (1997).   

Ultra-sensitive thermal measurements of graphene: pathway to single microwave photon detector

As a result of the linear energy spectrum and low dimensionality, electrons in graphene have astoundingly small thermal conductance and heat capacitance. By developing a large bandwidth, high sensitivity Johnson noise measurement technique at microwave frequency, we can measure the temperature of graphene electrons down to a precision of 0.1 mK within 1 second. This enables us to study the thermal properties of this wonder material: for instance, we have measured the energy transfer from electrons to phonons at 1 pW/K $\mu$m$^2$ level and a record-low heat capacity, only about 100 kB/$\mu$m$^2$, at low temperature. We have applied these exotic thermal quantities to make a graphene bolometer and mixer. Our measurement data agree well with the theory that the proposed graphene caloriometer should have single photon sensitivity from IR down to microwave regime at ultra-low temperature.   

Nano Peltier cooling device from geometric effects using a single graphene nanoribbon

Based on the phenomenon of curvature-induced doping in graphene we propose a class of Peltier cooling devices, produced by geometrical effects, without gating. We show how a graphene nanoribbon laid on an array of curved nano cylinders can be used to create a targeted cooling device. Using theoretical calculations and experimental inputs, we predict that the cooling power of such a device can approach $1kW/cm^2$, on par with the best known techniques using standard lithography methods. The structure proposed here helps pave the way toward designing graphene electronics which use geometry rather than gating to control devices.   

Electrical breakdown of graphene and few-layer graphene structures

The electrical breakdown of graphene and few-layer graphene (FLG) structures are investigated. To better understand the dynamics of these nano-scale thermal effects, we investigate graphene and FLG structures of various dimensions and find that significant joule heating occurs inducing the structures to evolve. A distinct change in the behavior during electrical stressing indicates that different mechanisms and geometrical effects occur at the various stages of evolution. The results could have implications on the development of high current carrying nanoscale graphene devices. Supported in part by NSF Award No. DMR-0805136, the Kentucky NSF EPSCoR program through award EPS-0814194, and the University of Kentucky Center for Advanced Materials.   

Session B7: Focus Session: Computational Design of Materials - Nanostructured and Energy Materials

The Harvard Clean Energy Project: High-throughput screening of organic photovoltaic materials using first-principles electronic structure theory

We present the Harvard Clean Energy Project (CEP) which is concerned with the computational screening and design of new organic photovoltaic materials. CEP has established an automated, high-throughput, in silico framework to study millions of potential candidate structures. This presentation discusses the CEP branch which employs first-principles computational quantum chemistry for the characterization of molecular motifs and the assessment of their quality with respect to applications as electronic materials. In addition to finding specific structures with certain properties, it is the goal of CEP to illuminate and understand the structure-property relations in the domain of organic electronics. Such insights can open the door to a rational, systematic, and accelerated development of future high-performance materials. CEP is a large-scale investigation which utilizes the massive computational resource of IBM's World Community Grid. In this context, it is deployed as a screensaver application harvesting idle computing time on donor machines. This cyberinfrastructure paradigm has already allowed us to characterize 3.5 million molecules of interest in about 50 million DFT calculations.   

The Harvard Clean Energy Project: High-throughput screening of organic photovoltaic materials using cheminformatics, machine learning, and pattern recognition

Organic photovoltaic devices have emerged as competitors to silicon-based solar cells, currently reaching efficiencies of over 9\% and offering desirable properties for manufacturing and installation. We study conjugated donor polymers for high-efficiency bulk-heterojunction photovoltaic devices with a molecular library motivated by experimental feasibility. We use quantum mechanics and a distributed computing approach to explore this vast molecular space. We will detail the screening approach starting from the generation of the molecular library, which can be easily extended to other kinds of molecular systems. We will describe the screening method for these materials which ranges from descriptor models, ubiquitous in the drug discovery community, to eventually reaching first principles quantum chemistry methods. We will present results on the statistical analysis, based principally on machine learning, specifically partial least squares and Gaussian processes. Alongside, clustering methods and the use of the hypergeometric distribution reveal moieties important for the donor materials and allow us to quantify structure-property relationships. These efforts enable us to accelerate materials discovery in organic photovoltaics through our collaboration with experimental groups.   

Thermodynamic Stability of Semiconductors for Photocatalytic Water Splitting

Band structure engineering design of the light-absorbing semiconductors for water splitting has attracted wide attention recently. One of such design is to use the Z-scheme where a photocathode is connected with photoanode to reduce (generate H2) and oxidize (generate O2) water respectively. This requires the conduction band of photocathode and valence band of photoanode to straddle the redox levels of water. However, equally important in this design is the thermodynamic stability of the semiconductors in the aqueous solution upon illumination, i.e., the semiconductors may be oxidized (or reduced) before the water is oxidized (or reduced), causing the corrosion of the photoanode (photocathode). We will present our theoretical study on the thermodynamic stability of a series of photocatalytic semiconductors, including metal oxides, sulfides and nitrides, through the combination of phenomenological models for the semiconductor corrosion and the first-principles total energy and band alignment calculations. We find that almost all sulfides and nitrides are unstable as photoanode, while most of oxides are stable. This limits the choice of the photoanode materials for oxygen evolution. In contrast, for photocathode, most of the considered semiconductors are stable and resistant to reduction, indicating a much wider choice of the photocathode materials for hydrogen evolution.   

Rational design of competitive electrocatalysts for the oxygen reduction reaction in hydrogen fuel cells

The large-scale application of one of the most promising clean and renewable sources of energy, hydrogen fuel cells, still awaits efficient \textit{and} cost-effective~ electrocatalysts for the oxygen reduction reaction (ORR) occurring on the cathode. We demonstrate that truly rational design renders electrocatalysts possessing both qualities. By unifying the knowledge on surface morphology, composition, electronic structure and reactivity, we solve that sandwich-like structures are an excellent choice for optimization. Their constituting species couple synergistically yielding reaction-environment stability, cost-effectiveness and tunable reactivity. This cooperative-action concept enabled us to predict two advantageous ORR electrocatalysts. Density functional theory calculations of the reaction free-energy diagrams confirm that these materials are more active toward ORR than the so far best Pt-based catalysts. Our designing concept advances also a general approach for engineering materials in heterogeneous catalysis.   

First principles design of the Pd/Co/Pd sandwich-like structure as a promising electrocatalyst for the oxygen reduction reaction on hydrogen fuel cell cathode

In the search of Pt free catalytic materials, experiments have shown an enhancement in the catalytic activity of Pd-Co alloys over Pd surfaces, comparable to the one obtained with Pt, as well as a Pd enrichment of the topmost layers. In this work we present the rational design of a new catalyst material towards the oxygen reduction reaction (ORR) consisting on Pd(111) surface sandwiched with Co as the second layer (Pd/Co/Pd). The calculated reaction free energy diagrams confirm that the proposed sandwich-like structure is highly active towards ORR. The higher catalytic activity of the Pd/Co/Pd system is traced to the change in the electronic local density of states of the Pd surface atoms. Namely the hybridization of the Pd d-states with the Co majority-spin band causes a low-energy shift of the Pd d-band. This results in a reduction of surface reactivity which is favorable for ORR. We have also studied form first principles the stability of the system and evaluated the dissolution potential of Pd in Pd/Co/Pd. The results suggest that the system will be stable in the reaction environment.   

Computational Design of Co-Doping Method for Indium-Reduced Chalcopyrite-Type Photovoltaic Materials

Chalcopyrite-type semiconductor CuInSe$_{2}$ (CIS) is one of the most promising materials for low cost photovoltaic solar-cells. However, from the point of resource security, high concentration of In in CIS is serious disadvantage. In this paper, we propose co-doping method to reduce the concentration of In in CIS-based photovoltaic materials, i.e., 2In are replaced by Zn and Sn. According to the electronic structure calculations by the KKR-CPA-LDA [1] with the self-interaction correction [2], the substitution of Zn and Sn for In does not alter the electronic structure of CIS so much. We extend our co-doping method to enhance the efficiency of solar energy conversion. In addition to Zn+Sn co-doping, we introduce S impurities at Se sites. Due to the phase separation it is found that nano-structures with high concentration of S are self-organized under the layer-by-layer crystal growth condition. Since type-II band alignment is expected between Cu(Zn, Sn)Se$_{2}$ and Cu(Zn, Sn)S$_{2}$, we can expect efficient electron-hole separation in decomposed Cu(Zn, Sn)(Se, S)$_{2}$ [3]. \\[4pt] [1] H. Akai, http://sham.phys.sci.osaka-u.ac.jp/kkr/ \\[0pt] [2] A. Filippetti and N. A. Spaldin, Phys. Rev. B 67 (2003) 125109.\\[0pt] [3] Y. Tani et al., Appl. Phys. Express 3 (2010) 101201.   

Effect of carbon and nitrogen doping on the structure of amorphous GeTe phase change material

Carbon and Nitrogen-doped GeTe are promising materials for use in phase change memories since the addition of C or N increases the stability of the amorphous phase. By combining ab initio molecular dynamics and X-ray scattering experiments, we show that carbon deeply modifies the structure of the amorphous phase through long carbon chains, tetrahedral and triangular units centred on carbon. A clear signature of these units is the appearance of an additional interatomic distance around 3.3 A in the pair correlation function. Besides, the first Ge-Ge and Ge-Te distances are almost not affected by doping. The implications for the vibrational and thermal properties are finally discussed.   

Towards Efficient Solar Cells: Optimizing Non-Equilibrium Hyperdoping Methods

Electrical and optical properties of semiconductors are mainly controlled by the concentration of dopants. While the highest dopant concentrations reachable in most traditional doping methods are limited by the equilibrium solubility of the dopant in the pure semiconductor material, much higher concentrations are observed after femtosecond laser treatment of silicon in a sulfur containing atmosphere. Due to altered optical properties \emph{laser-hyperdoped} silicon is considered a promising material for next generation photovoltaic cells. To control the dopant concentration distribution and thereby tune physical properties of the material, we apply advanced adjoint based optimization techniques to the models describing the laser induced melting and diffusion processes. This allows to determine optimal process protocols generating a desired concentration distribution. Applying advanced PDE-contrained optimization techniques to laser hyperdoping thereby opens new avenues for improving the efficiency of photovoltaic cells.   

First-Principles Materials Design of Chalcopyrite-Type Photovoltaic Materials with Self-Organized Nano-Structures

Cu(In, Ga)Se$_{2}$ (CIGS) is a chalcopyrite-type semiconductor and one of the most promising materials for low cost photovoltaic solar-cells. In this paper, based on first-principles calculations, we propose that spinodal decomposition will enhance the conversion efficiency in CIGS. Our calculations are based on the KKR-CPA-LDA [1] with the self-interaction correction [2]. From the calculated mixing energy of CIGS, it is found that the system favors the spinodal decomposition. We also perform Monte Carlo simulations and find that quasi-one-dimensional nano-structures with high concentration of impurities are formed under the layer-by-layer crystal growth condition in CIGS [3]. It is expected that the photo-generated electron-hole pairs are efficiently separated by the type-II interface and then effectively transferred along the quasi-one-dimensional structures in CIGS. Moreover, we can expect multiplication of generated carriers due to the multi-exciton effects in nano-structures [3]. \\[4pt] [1] H. Akai, http://sham.phys.sci.osaka-u.ac.jp/kkr/ \\[0pt] [2] A. Filippetti and N. A. Spaldin, Phys. Rev. B 67 (2003) 125109.\\[0pt] [3] Y. Tani et al., Appl. Phys. Express 3 (2010) 101201.   

Computational Nano-materials Design of Self-Organized Cd(Te,S) and Cd(Te,Se)Type II Nanowire (Konbu-Phase) by Spinodal Nano-Decomposition for High-Efficiency Photovoltaic Solar-Cells

Based on multi-scale simulations combined ab initio electronic structure calculation (KKR-CPA) and Monte Carlo simulation (MCS) of the two-dimensional layer-by-layer crystal growth, we have designed the self-organized quasi-one-dimensional nano-structures (Konbu-Phase) fabricated by two-dimensional spinodal nano-decomposition for high-efficiency photovoltaic solar cells (PVSCs) in Cd(Te$_{1-x}$S$_{x}$), and Cd(Te$_{1-x}$Se$_{x}$). The Konbu-Phase enhances the nano-scale electron-hole separation in PVSCs due to their Type II band alignment. The Konbu-Phase also increases the efficiency of PVSCs by multi-exciton formation using the inverse Auger effect in the self-organized quasi-one-dimensional nanostructures. We also discuss how to fabricate Konbu-Phase starting from the uniform nano-particles made by the photo-chemical reactions. Reference: M. Oshitani, K. Sato, H. Katayama-Yoshida, Applied Physics Express 4 (2011) 022302.   

Half-Heusler semiconductors as piezoelectrics

We use a first-principles rational-design approach to demonstrate the potential of semiconducting half-Heusler compounds as a previously-unrecognized class of piezoelectric materials. We scan a large number of compounds, testing for insulating character and calculating structural, dielectric, and piezoelectric properties. Of the 792 compounds considered, 234 are found to be nonmetallic, of which 189 are further found to be elastically stable. We compare the computed structural parameters to available experimental values for the half-Heusler compounds considered that have been experimentally studied, as reported in the Inorganic Crystal Structure Database. Calculated piezoelectric coefficients ($d_{14}$) and electromechanical coupling factors ($k_{14}$) are often high enough to compare favorably with those of piezoelectrics currently in use. We analyze how factors such as electronegativity and ionic radius influence the piezoelectricity of the compound. Moreover, we show that even if toxic or expensive elements are excluded, we are still left with many combinations having reasonably high piezoelectric response. Our results provide guidance for the experimental realization and characterization of high-performance materials of this class that may find practical applications.   

Oxygen transport in ceria: a first-principles study

Ceria (CeO2) is an important material for environmentally benign applications, ranging from solid-oxide fuel cells (SOFC) to oxygen storage [1-2]. The key characteristic needed to be improved is the mobility of oxygen ions. Optimization of ionic transport in ceria has been the topic of many studies. In particular, it has been discovered how the ionic conductivity in ceria might be improved by choosing the proper kind and concentration of dopants [3]. In this presentation we will approach the problem from a different direction by adjusting structural parameters of ceria via the change of external conditions. A systematic first-principles study of the energy landscape and kinetics of reduced ceria as a function of external parameters reveals a physically transparent way to improve oxygen transport in ceria. \\[4pt] [1] N. Skorodumova, S. Simak, B. Lundqvist, I. Abrikosov, and B. Johansson, Physical Review Letters 89, 14 (2002). \\[0pt] [2] A. Trovarelli, in Catalysis by Ceria and related materials (Imperial College Press, London, 2002). \\[0pt] [3] D. A. Andersson, S. I. Simak, N. V. Skorodumova, I. A.Abrikosov, and B. Johansson, Proceedings of the National Academy of Sciences of the United States of America 103, 3518 (2006).   

First Principles Optical Absorption Spectra of Organic Molecules Adsorbed on Titania Nanoparticles

We present results from first principles computations on passivated rutile TiO$_2$ nanoparticles in both free-standing and dye-sensitized configurations to investigate the size dependence of their optical absorption spectra. The computations are performed using time-dependent density functional theory (TDDFT) as well as GW-Bethe-Salpeter-Equation (GWBSE) methods and compared with each other. We interpret the first principles spectra for free-standing TiO$_2$ nanoparticles within the framework of the classical Mie-Gans theory using the bulk dielectric function of TiO$_2$. We investigate the effects of the titania support on the absorption spectra of a particular set of perylene-diimide (PDI) derived dye molecules, namely brominated PDI (Br$_2$C$_{24}$H$_8$N$_2$O$_4$) and its glycine and aspartine derivatives.   

Stoichiometric anti-phase boundaries in monolayer boron-nitride

We propose a stoichiometric structure for domain boundaries in monolayer boron nitride, on the basis of the seeds of extended topological defect lines that have been observed in graphene under electronic irradiation [1]. The structure consists of a periodic extended line defect with an atomic structure consisting of alternating fourfold and eightfold BN rings. The structure is shown to be lower in energy than non-stoichiometric counterparts, and to lead to the formation of shallow electron and hole bands inside the band gap of the BN bulk matrix. We suggest that the existence of such defect bands may be experimentally observed in optical experiments. \\[4pt] [1] J. Kotakoski, A. V. Krasheninnikov, and J. C. Mayer, Physical Review Letters, 106, 105505 (2011).   

Understanding Heat Dissipation in Carbon Nanotube Resonators: New Insights from Molecular Dynamics Simulation

Dissipation in carbon nanotube (CNT) resonators under conditions of steady state continuous driving has been simulated within classical molecular dynamics (MD) making use of a newly developed ``Phonostat'' algorithm. A previous study on heat flow in CNT resonators using MD simulation showed an anomalous heat dissipation and gateway kind of behavior was proposed. Here, we focus on characterizing the ``gateway'' modes and identify the pathways of heat dissipation by clamping such modes using our phonostat algorithm. In this present study we see three or four different sets of heat flow behavior when different sets of gateway modes are clamped as against a single heat flow behavior presented in the previous study. Our new results are explained in terms of filling of different subsets of phonon modes and the magnitude of friction between them. Our simulation results show that controlling these gateway modes is the key to improve the quality factor (Q) of CNT resonators for sensing applications, which has been a major challenge till now.   

Session B8: Focus Session: Frustrated magnetism - Spin ice

Spin dynamics in the frozen state of the dipolar spin ice material Dy$_2$Ti$_2$O$_7$

Low temperature magnetic ac susceptibility measurements of single crystal dipolar spin ice Dy$_2$Ti$_2$O$_7$ are presented. The measured dynamics qualitatively agree with simulations based on current magnetic monopole theory, but not with thermal relaxation measurements, whose dynamics freeze out at a slower rate. The relaxation is found to exhibit thermally activated Arrhenius behavior with an activation energy of 9.79\,K. A comparison between the measurement results of Ho$_2$Ti$_2$O$_7$ and Dy$_2$Ti$_2$O$_7$ will also be made.   

Anderson-Higgs transition in quantum spin ice Yb$_{2}$Ti$_{2}$O$_{7}$

We have carried out polarized elastic neutron-scattering experiments on single crystals Yb$_{2}$Ti$_{2}$O$_{7}$ from 1 K to 0.04 K. The results reveal that the diffuse [111]-rod scattering [1] is suppressed below T$_{c}$ $\sim $ 0.21 K, where magnetic Bragg peaks and a full depolarization of neutron spins are observed with the thermal hysteresis, indicating a first-order ferromagnetic transition. Theoretically, a quantum spin ice state [2] above T$_{c }$ has been realized from an effective classical model where $<$111$>$ Ising moments, i.e., pseudospin-1/2 interact mainly through a magnetic dipolar interaction [3], and a transition from magnetic Coulomb phase to Higgs phase has emerged at T$_{c }$ in Yb$_{2}$Ti$_{2}$O$_{7}$. [1] K. A. Ross \textit{et al.}, Phys. Rev. Lett. \textbf{103}, 227202 (2009). [2] S. Onoda \textit{et al.}, J. Phys.: Conf. Series, in press. [3] S. T. Bramwell and M. J. P. Gingras, Science \textbf{294}, 1495 (2001).   

Schwinger-boson approach to spin-liquid and Higgs phases in quantum spin ice: Yb$_2$Ti$_2$O$_7$ and Pr$_2$Zr$_2$O$_7$

The most generic pseudospin-$1/2$ quantum spin ice model for rare-earth magnetic pyrochlore oxides, that include Yb$_2$Ti$_2$O$_7$ and Pr$_2TM_2$O$_7$ (TM=Sn, Zr, Hf, and Ir), is studied by means of a Schwinger-boson approach. From a projective symmetry group and a mean-field analysis on this magnetically anisotropic model, we classify and analyze quantum spin liquid phases in the space of four coupling constants, including both U(1) and $Z_2$ spin liquids that are characterized by power-law decaying and exponentially decaying spin correlations, respectively, as well as Higgs phases showing long-range orders. We apply the analysis to cases of the model parameters extracted from both microscopic arguments and the fitting to neutron-scattering experiments of some of the above materials and clarify the dynamical magnetic excitation spectra that take the continuum form due to the deconfined monopolar spinons. The relevance to recent experimental results is discussed.   

Dynamical strings in quantum spin ice

Spin ice is a highly frustrated ferromagnet displaying rich emergent phenomena. Recently, new spin ice materials such as Yb$_2$Ti$_2$O$_7$ and Pr$_2$Zr$_2$O$_7$ have stood out as possible candidates for quantum spin ice, in which quantum fluctuations could play a major role. In this talk, we discuss new emergent phenomena in quantum spin ice in an external magnetic field applied along the (100) lattice direction. When quantum monopoles are confined by the external field, the open string binding a monopole pair (Dirac string) becomes a dynamical object with a field-dependent tension. The motion of an open string includes longitudinal expansion and contraction and transverse fluctuations. The emergent quantum string theory in this context allows for simple analytical solution and straightforward numerical simulation. Moreover, vibrational modes of the string can be detected by experimental techniques such as neutron scattering and THz spectroscopy.   

Higgs transitions of spin ice

Spin ice is a frustrated magnetic material displaying a variety of interesting phenomena. At low temperature, it exhibits a ``Coulomb phase'', in which there is no magnetic order and correlations have power-law forms at long distances. In this talk, I will describe the effects of applying perturbations that favor ordered states and show that the resulting phase transitions cannot be described by the standard Landau paradigm. They are instead naturally viewed as Higgs transitions of an emergent gauge theory; this perspective leads to long-wavelength theories for the critical properties. I will present specific examples of transitions described by this approach that result from perturbations such as an applied magnetic field or additional spin--spin interactions.   

Loop statistics in the Coulomb phase

The Coulomb phase is a classical gauge field theory arising in frustrated systems with ``divergence free'' constraints, such as spin ice [1]. In this talk, we show how this phase can be understood as a loop model, and characterized by their loop length distribution and fractal dimensions [2]. Comparing similar models in 2- and 3-dimensions allows us to extract insights from connections to Stochastic-Loewner Evolution (SLE) processes, percolation and polymer physics. We mention implications of these results for related models and experiments (Heisenberg magnets, itinerant electrons [3]). \\[4pt] [1] Henley, Annual Review of Condensed Matter Physics {\bf 1}, 179 (2010).\\[0pt] [2] Jaubert, Haque, Moessner, Phys. Rev. Lett. {\bf 107}, 177202 (2011)\\[0pt] [3] Jaubert, Pitaecki, Haque \& Moessner, in preparation (2012).   

Similarities and differences between spin and water ice models

In the present contribution we provide a brief survey of the amazing analogy between spin and water ice basically from the water ice physics point of view. Special attention is paid to the following question: which of the theoretical concepts developed in ice physics could be applied to the study of spin ice and other frustrated systems. We show that the analogy between ground states of these systems can be extended other properties. In Section 1 we outline the history and present state of the analogy, and thoroughly investigate the similarity between quasi-particle excitations in ordinary ice (point defects in the ice proton system) and in spin ice (magnetic monopoles). The magnetic monopole concentration is shown to have a break in its temperature dependence arising due the magnetic Coulomb interaction (melting of the Coulomb phase). In Section 2 we develop a theory of magnetic charge transport, study the magnetic relaxation as well as the screening of magnetic field in spin ice. The transport analogy between spin and water ice is shown to be of limited nature: it is impossible to produce a stationary magnetic current in the commonly accepted model of spin ice. An extended spin ice model is suggested which is free from this disadvantage. In Section 3 we discuss the problem of how the magnetic ordering is modified near the free surface of spin ice or near its interface with other magnets. The study is based on the concepts previously used in water ice physics. In concluding Section 4 we discuss the differences between water and spin ice models (differences in numerical relations between appropriate constants, limited nature of transport analogy) and indicate some other problems which are not considered in this contribution (thermal effects of charge currents, quantum properties of the models, effect of frustrations on electron energy spectrum).   

Spin Ice correlations in a macroscopic system

We report the realization of spin ice like correlations in a macroscopic array of ferromagnetic rods arranged in a honeycomb lattice. We found that this system has a rich dynamics that can be rationalized as the result of the interplay between the viscous rotation of each rod and Coulomb like interactions between magnetic charges located at the ends of the magnets. The dynamical response of this system has also been explored by using an external magnetic dipole moving at a distance $h$ from the lattice. A clear separation between the interaction strengths permitted us to observe localized as well as a collective dynamics depending on the value of two dimensionless numbers; one associated with the two relevant time scales of the system and the other related with the strength of internal and external magnetic forces. This new spin-ice realization will allow the manipulation of parameters almost impossible to control on its microscopic relatives, such as inertia, vacancies, and quenched geometrical disorder between others.   

Internal field distribution in spin ice materials

At low temperatures, spin ice is populated by a finite density of magnetic monopoles --- point like topological defects with a mutual magnetic Coulomb interaction. Here we study the distribution of magnetic fields inside spin ice. This is of conceptual importance as it reflects the monopolar fields set up by defects in a spin ice configuration. We discuss its manifestations in experiments involving local field probes, such as NMR or muon spin rotation. Averaged over the bulk of the sample, this distribution resembles one set up by a random spin arrangement. However, somewhat counter intuitively, the density of low-field locations decreases as the local ferromagnetic correlations imposed by the ice rules develop. The $1/r^2$ Coulomb field of a single monopole is visible in (magnetic) voids of the lattice where lattice-scale effects due to the immediate proximity of other spins are supressed.   

Probing the spin ice state in the cubic pyrochlore Ho$_{2}$Ge$_{2}$O$_{7}$

Spin ices are a remarkable magnetic ground state that can arise in geometrically frustrated pyrochlores, A$_{2}$B$_{2}$O$_{7}$, when magnetic rare earth ions are situated on the vertices of a lattice of corner sharing tetrahedra. Competing nearest-neighbor and long-range dipolar interactions result in a short-range ordered ground state for each tetrahedron in which two spins point in and two spins point out [1]. The excitations in spin ices are equally remarkable; spin ices are the only known hosts of magnetic monopoles, emergent quasiparticles with a net magnetic charge. The cubic pyrochlore Ho$_{2}$Ge$_{2}$O$_{7}$ was prepared with a high temperature and high pressure technique. Preliminary DC susceptibility, heat capacity and X-ray diffraction experiments confirmed that Ho$_{2}$Ge$_{2}$O$_{7}$ has the bulk properties of a spin ice including residual entropy equal to the Pauling value for water ice [2]. The results of a polarized neutron scattering experiment performed at ILL as well as AC susceptibility and heat capacity measurements will be presented, and compared to the canonical spin ices.\\[4pt] [1] S. T. Bramwell \textit{et al.}, Phys. Rev. Lett. \textbf{87}, 047205 (2001). \newline [2] H. Zhou \textit{et al.}, Nature Communications \textbf{2}, 478 (2011).   

Far infrared time domain terahertz spectroscopic study of Ho2Ti2O7

We report a far infrared time domain terahertz spectroscopic study of the spin ice material holmium titanate. The complex dielectric constant was obtained in the terahertz frequency range. Several low energy excitations were identified from the optical spectra. We will discuss the possible nature of those excitations and their relevance to the spin ice physics.   

Calorimetric Study of Diluted Spin Ice Materials

Spin ice materials Dy$_2$Ti$_2$O$_7$ and Ho$_2$Ti$_2$O$_7$ have been the subject of ongoing interest for over ten years. The cooperative magnetic ground state can be mapped onto the proton disordered ground state in water ice, and its residual entropy follows the same Pauling's estimate. Interestingly it was found in a previous study that, upon dilution of the magnetic rare earth ions Dy$^{3+}$ and Ho$^{3+}$ by non-magnetic substitutes Y$^{3+}$, the residual entropy depends non-monotonically on the dilution level. In this work we investigate through Monte Carlo simulations microscopic models to account quantitatively for the calorimetric experimental measurements, and thus also the residual entropies as a function of dilution. Features of the dilution physics in the specific heat are captured quantitatively by the microscopic models and the interplay between dilution and frustration is understood on the basis of a Bethe lattice calculation. The effect of the dipolar interactions between magnetic spins are exposed numerically for various dilution concentrations. Our work explains the previous discrepancy of the residual entropy between different species of rare earth ions and the generalized Pauling's estimate.   

Spin Ice: Magnetic Excitations without Monopole Signatures using Muon Spin Rotation

Theory predicts the low-temperature magnetic excitations in spin ices consist of deconfined magnetic charges, or monopoles. A recent transverse-field (TF) muon spin rotation ($\mu$SR) experiment [S T Bramwell {\it et al}, Nature {\bf 461}, 956 (2009)] reports results claiming to be consistent with the temperature and magnetic field dependence anticipated for monopole nucleation $-$ the so-called second Wien effect. We demonstrate via a new series of $\mu$SR experiments in Dy$_2$Ti$_2$O$_7$ that such an effect is not observable in TF $\mu$SR. Rather, as found in many highly frustrated magnetic materials, we observe spin fluctuations which become temperature independent at low temperatures, behavior which dominates over any possible signature of thermally nucleated monopole excitations.   

Session B9: Focus session: Complex Bulk Oxides: Iridates

Dipoles on monopoles in spin ice

Close connection of electricity and magnetism is one of the cornerstones of modern physics. This connection plays crucial role both from the fundamental point of view and in practical applications, including recent advance in spintronics and in the study and development of multiferroic materials. A new breakthrough was the recent proposal of Castelnovo, Moessner and Sondhi that in spin ice systems, e.g. in some pyrochlores, one can model the magnetic monopoles -- the objects displaying the properties of isolated magnetic charges. Such monopoles are a hot topic nowadays. Usually one discuss mainly their thermodynamic and magnetic properties. I will show that every \textit{magnetic monopole} in spin ice should have an \textit{electric dipole} attached to it. This can be seen from the results obtained for frustrated Hubbard system [1]. Both the electronic mechanism discussed in [1] and the lattice effects (magnetostriction) lead to the conclusion that for 3in/1out and 3out/1in tetrahedra there should appear an electric dipole directed from the center of tetrahedron to the ``special'' spin. This will lead to electric activity of monopoles, and to possibility to address and influence them not only by magnetic, but also by an electric field. Several consequences of this effect will be discussed. In general, the analogy between electricity and magnetism goes even further than usually assumed: whereas electrons have \textit{electric charge} and spin, i.e. \textit{magnetic dipole}, magnetic monopoles in spin ice have both \textit{magnetic charge} and \textit{electric dipole}. \\[4pt] [1] L.N.Bulaevskii, C.D.Batista, M.V.Mostovoy and D.I.Khomskii, Phys.Rev. B\textbf{78}, 028402 (2008)   

Raman scattering study of the pressure- and field-dependent phases of Sr$_{2}$IrO$_{4}$

Transition metal oxides (TMOs) with a perovskite structure are of interest due to the many fascinating phenomena they exhibit. Among TMOs, iridates with 5d orbitals exhibit an unexpected insulating state due to large spin-orbit coupling. With the extended nature of the 5d orbital giving rise to a strong crystal field, the competition between the onsite Coulomb interaction and spin-orbit coupling--which have comparable energy scales-opens up the possibility for studying novel phases that develop when tuning this competition with using an external perturbation with magnetic field or pressure. In this talk we present temperature, magnetic field and pressure dependent Raman study of Sr$_{2}$IrO$_{4}$, which provides an opportunity Pressure- and field-tuned Raman spectroscopy provides an opportunity to explore the novel phases of this material under extreme conditions.   

Quantum Hall effects in a Weyl semimetal: Possible application in pyrochlore iridates

There has been much interest in pyrochlore iridates A2Ir2O7 where both strong spin-orbital coupling and strong correlation are present. A recent local density approximation calculation [X. Wan, A. M. Turner, A. Vishwanath, and S. Y. Savrasov, Phys. Rev. B 83, 205101 (2011)] suggests that the system is likely in a three-dimensional topological semimetallic phase: a Weyl semimetal. Such a system has zero carrier density and arrives at the quantum limit even in a weak magnetic field. In this talk, I will discuss two quantum effects of this system in a magnetic field: a pressure-induced anomalous Hall effect and a magnetic-field-induced charge density wave at the pinned wave vector connecting Weyl nodes with opposite chiralities. A general formula of the anomalous Hall coefficients in a Weyl semimetal is also given. Both proposed effects can be probed by experiments in the near future and can be used to detect the Weyl semimetal phase.   

Electronic ground state properties of Iridate oxides from x-ray absorption spectroscopy

Element (Ir)- and orbital (5d)-specific L$_{2,3}$ edge x-ray absorption and magnetic circular dichroism measurements are used to probe the nature of the electronic ground state in magnetic insulators BaIrO$_3$ [1] and Sr$_2$IrO$_4$ [2,3]. A spin-only description of the magnetic ground state is directly ruled out by the measurements. Instead, the measurements show spin-orbit entanglement in 5$d$ states resulting in a larger orbital ($L_z$) than spin ($S_z$) contribution to the magnetic moment, even in the presence of strong crystal field and band effects. Measured x-ray absorption cross sections at spin-orbit split L$_{2,3}$ edges impose constraints on the nature of the ground state [3]. Experiments under chemical- (doping) and applied-pressure conditions provide evidence for a delicate interplay between electronic bandwidth and Coulomb interactions leading to the gapped, spin-orbit coupled ground state of these complex oxides. \\[4pt] [1] M. A. Laguna Marco et al., Phys. Rev. Lett. 105, 216407 (2010).\\[0pt] [2] B. J. Kim et al., Phys. Rev. Lett. 101, 076402 (2008).\\[0pt] [3] L. C. Chapon and S. W. Lovesey, J. Phys. Condens. Matter 23, 252201 (2011).   

Lattice-driven magnetoresistivity and metal-insulator transition in single-layered iridates

{Sr$_{2}$IrO$_{4}$} exhibits a novel insulating state driven by spin-orbit interactions. Here we report two novel phenomena, namely a large magnetoresistivity that is extremely sensitive to the orientation of magnetic field but exhibits no apparent correlation with the magnetization, and a robust metallic state that is induced by dilute electron {(La$^{3+}$)} or hole (K$^{+}$) doping on Sr$^{2+}$ ions in {Sr$_{2}$IrO$_{4}$}. This study reveals that a strong spin-orbit interaction alters the balance between the competing energies so profoundly that (1) the spin degree of freedom alone is no longer a dominant force; (2) underlying transport properties delicately hinge on the {Ir-O-Ir} bond angle via a strong magnetoelastic coupling; and (3) a highly insulating state in {Sr$_{2}$IrO$_{4}$} is proximate to a metallic state, and the transition is governed by lattice distortions that can be controlled via either magnetic field or chemical doping.   

Time-resolved optical study of magnetism in Sr2IrO4

We report a time-resolved optical pump-probe study of the Jeff=1/2 Mott insulator Sr2IrO4. The temperature dependence of the electronic relaxation rate exhibits clear anomalies at magnetic ordering temperatures of 240K and 100K, which are consistent with the development of bulk decay channels via emission of magnetic excitations. We will then discuss time-resolved second harmonic generation studies and contrast the magnetic properties of the surface and the bulk.   

Unusual ferromagnetism and strong spin-orbit coupling in Post-Perovskite $CaIrO_3 $

Strong spin-orbit coupling (SOC) and strong correlations have been considered essential in understanding the unusual physical properties of the 4d and 5d transition-metal oxides, such as the SOC driven Mott insulating state in $Sr_2 IrO_4$. Recently, an unusual atomic-like orbital moment and strong SOC have been confirmed experimentally in $9R-BaIrO_3$ through analysis of the branching ratio at the Ir $L_{2,3}$ absorption edges as obtained from x-ray absorption and x-ray magnetic circular dichroism (XMCD) measurements. We have applied the same techniques to probe unusual ferromagnetism and SOC in the post-perovskite (pPv) $CaIrO_3$, which is an insulator and exhibits weak ferromagnetism below $T_C \approx 110K$. The branching ratio at the Ir $L_{2,3}$ absorption edges, which is close to unity in pPv $CaIrO_3$, appears to indicate an even stronger spin-orbit interaction in the pPv $CaIrO_3$ than in $9R-BaIrO_3$. However, it has been challenging to model the Ir 5d orbital moment, as probed by the XMCD measurements, due to the understood local octahedral-site distortions.   

Exploring Magnetic Ordering in Sr3Ir2O7

Iridium oxide members of the Ruddlesden-Popper series have generated a great deal of interest recently due to their novel Mott insulating phases within these 5$d$-electron systems. These surprising Mott phases have been proposed to form due to a delicate interplay between spin orbit coupling effects and electronic correlation [1]. Here we present measurements probing the nature of the spin correlations and charge behavior in the bilayer variant of the Ruddlesden-Popper series, Sr$_{3}$Ir$_{2}$O$_{7}$. Our neutron scattering results reveal an antiferromagnetic spin structure and will be discussed in parallel with transport and bulk magnetization measurements detailing the electronic behavior in this material. \\[4pt] [1] B. J. Kim et al., Phys. Rev. Lett. 101, 076402 (2008).   

Doping a Spin-Orbit Mott Insulator: Topological Superconductivity from the Kitaev-Heisenberg Model and Possible Application to (Na$_2$/Li$_2$)IrO$_3$

We study the effects of doping a Mott insulator on the honeycomb lattice where spins interact via direction dependent Kitaev couplings $J_{\rm K}$, and weak antiferromagnetic Heisenberg couplings $J$. This model is known to have a spin liquid ground state and may potentially be realized in correlated insulators with strong spin orbit coupling. The effect of hole doping is studied within a $t$-$J$-$J_{\rm K}$ model, treated using the SU(2) slave boson formulation, which correctly captures the parent spin liquid. We find superconductor ground states with spin triplet pairing that spontaneously break time reversal symmetry. Interestingly, the pairing is qualitatively different at low and high dopings, and undergoes a first order transition with doping. At high dopings, it is smoothly connected to a paired state of electrons propagating with the underlying free particle dispersion. However, at low dopings the dispersion is strongly influenced by the magnetic exchange, and is entirely different from the free particle band structure. Here the superconductivity is fully gapped and topological, analogous to spin polarized electrons with $p_x+ip_y$ pairing. These results may be relevant to honeycomb lattice iridates such as A$_2$IrO$_3$ (A\,=\,Li or Na) on doping.   

Correlated Itinerant Electrons in Pyrochlore Bi$_{2}$Ir$_{2}$O$_{7}$

Strong spin-orbit coupling in the $5d$-based iridates rigorously competes with other relevant energies, and motivates novel insulating states. Therefore, a metallic state does not commonly occur in the iridates, but the unusual balance between different degrees of freedom in the iridates almost guarantee that it will exhibit extraordinary properties when it does occur. Here we report anomalous transport and thermodynamic properties including Hall effect of single-crystal Bi$_{2}$Ir$_{2}$O$_{7}$ along with our electronic structure calculations utilizing a LSDA+U scheme. The results will be discussed along with comparisons drawn with other pyrochlore iridates and related materials.   

Measurement of the Mott insulating gap of Sr$_{2}$IrO$_{4}$ with a scanning tunneling microscope

Recently the 5d transition metal oxide Sr$_{2}$IrO$_{4}$ has become a material of interest. This is due to comparable interaction strengths between crystal field splitting, the Coulomb interaction, and spin-orbit coupling, resulting in a Mott insulating ground state that has a finite resistance even at cryogenic temperatures. In order to fully understand this material it is important to measure the Mott insulating gap. A scanning tunneling microscope is an excellent tool for studying this material for its ability to directly measure this gap. Our preliminary results show this gap measured on single crystals to be $\sim $50 meV which is comparable to the activation energy of this sample. We will discuss our current STM results and compare our results with other optical conductivity data obtained from this material.   

Mapping the fluctuating Ir$^{4+}$ dimers across the phase diagram of Cu(Ir$_{1-x}$Cr$_{x}$)$_{2}$S$_{4}$ (0$\leq$x$\leq$0.6)

Elucidating the role of fluctuations in systems with competing interactions, such as Cu(Ir$_{1-x}$Cr$_{x}$)$_{2}$S$_{4}$, enables comprehensive understanding of their physical properties. While CuIr$_{2}$S$_{4}$ exhibits complex behavior with metal-insulator transition accompanied by charge ordering and Ir$^{4+}$-spin-dimerization at 226~K, CuCr$_{2}$S$_{4}$ displays ferromagnetic metallic behavior (T$_{C}$$\sim$377~K) understood within the double exchange model. Intermediate composition range sees suppression of end-member properties, with broad features observed in susceptibility around 180~K attributed to Cr$^{3+}$ low spin to high-spin crossover [1]. Robust fluctuating Ir$^{4+}$ dimers are detected and their evolution examined across the phase diagram by the X-ray atomic pair distribution function method. Although their long range order is destroyed already by x$\approx$0.05, Ir$^{4+}$ dimers exist locally at low temperature at all compositions studied. We provide detailed account of the Cr-doping and temperature dependence of the local dimers, and estimate characteristic length-scale on which they are observable. Fluctuating dimers disappear on heating, for intermediate compositions at temperatures above 180~K. \\[4pt] [1] R. Endoh et al., Phys. Rev. B $\bf{68}$, 115106 (2003).   

Coupling of Orbital and Magnetic Orders to Colossal Negative Thermal Expansion in Novel Mott Insulators

Ca$_{2}$RuO$_{4}$ is intimately associated with both \textbf{\textit{negative volume thermal expansion (NVTE)}} and \textbf{\textit{negative linear thermal expansion (NLTV)}} when doped by a 3d transition metal ion M for Ru. The NVTE and NLTE observed in this system constitutes a compelling and extraordinary example in that (1) the coefficient of NVTE and NLTE reaches -213 $\times $ 10$^{-6}$ K$^{-1}$ and -148 $\times $ 10$^{-6}$ K$^{-1}$, respectively, constituting \textbf{\textit{colossal negative thermal expansion (NTE)}}; (2) the NTE anomalies closely track the onset temperatures of orbital and magnetic orders, in sharp contrast to classic NTE that shows no relevance to physical properties; (3) the NTE and physical properties can be effectively tuned via varying M and x in Ca$_{2}$Ru$_{1-x}$M$_{x}$O$_{4}$; (4) the NTE occurs near room temperature and extends over a wide temperature interval ranging from 100 K to 350 K. Moreover, NTE and Invar effect commonly exist in these 4d-based ruthenates and 5d-based iridates, e.g. Sr$_{n+1}$Ir$_{n}$O$_{3n+1}$ and BaIrO$_{3}$. These novel NTE materials provide a much-needed paradigm for functional materials with anomalous thermal expansion and electronic characteristics.   

Session B10: Invited Session: Equilibration and Relaxation in Cold Atoms

Timescales for equilibration and adiabaticity in optical lattices

What are the timescales governing local and global dynamics in strongly correlated systems? How do we probe this dynamics in an isolated quantum system without coupling the system to leads? High in-situ resolved experiments on bosons in optical lattices are answering precisely these questions by imaging the gas following a sudden change of the lattice potential. The results are striking. Experiments have revealed a disparity as large as two orders of magnitude between fast equilibration of local number fluctuations and slow global mass redistribution. In this talk, I will provide a simple model which captures all the relevant physics. Additionally, I will show that the fast timescales for local dynamics challenge the accepted notions of adiabaticity times, invalidating routinely used techniques such as band-mapping as useful probes of quantum many body systems. References: S. S. Natu, K. R. A. Hazzard and E. J. Mueller, Phys. Rev. Lett. 106 125301 (2011).   

Pomeranchuk effect and spin-gradient cooling of Bose-Bose mixtures in an optical lattice

We theoretically investigate finite-temperature thermodynamics and demagnetization cooling of two-component Bose-Bose mixtures in a cubic optical lattice, by using bosonic dynamical mean-field theory (BDMFT). We calculate the finite-temperature phase diagram, and remarkably find that the system can be heated from the superfluid into the Mott insulator at low temperature, analogous to the Pomeranchuk effect in $^3$He. This provides a promising many-body cooling technique. We examine the entropy distribution in the trapped system and discuss its dependence on temperature and an applied magnetic field gradient. Our numerical simulations quantitatively validate the spin- gradient demagnetization cooling scheme proposed in recent experiments.   

The amplitude mode at the superfluid-mott insulator transition

We study a two dimensional gas of repulsively interacting bosons in the presence of both an optical lattice and a trap using optical lattice modulation spectroscopy. The strongly interacting superfluid supports two types of low energy modes associated with the symmetry breaking at the phase transition: gapless phase (Goldstone) modes and gapped amplitude (Anderson-Higgs) modes. Both experimentally and in theoretical simulations lattice modulation spectroscopy shows an onset of absorption at a frequency associated with the amplitude mode gap, followed by a broad absorption peak at higher frequencies. From the simulations, we learn that energy is being absorbed by various amplitude modes, which inside a trap resemble the modes of a (gapped) drum. Our main results are: (1) despite coupling to the phase modes, modulation spectroscopy shows a sharp absorption onset at the frequency associated with the amplitude mode gap; (2) as we approach the Mott transition the gap softens and finally disappears at the transition point; (3) in the weak coupling regime, deep in the superfluid phase, the amplitude mode disappears.   

Two-dimensional Fermi gases

Pairing of fermions is ubiquitous in nature and it is responsible for a large variety of fascinating phenomena like superconductivity, superfluidity of 3He, the anomalous rotation of neutron stars, and the BEC-BCS crossover in strongly interacting Fermi gases. When confined to two dimensions, interacting many-body systems bear even more subtle effects, many of which lack understanding at a fundamental level. Most striking is the, yet unexplained, effect of high-temperature superconductivity in cuprates, which is intimately related to the two-dimensional geometry of the crystal structure. In particular, the question how many-body pairing is established at high temperature and whether it precedes superconductivity are crucial questions to be answered. We will report on recent experiments of pairing in a two-dimensional atomic Fermi gas in the regime of strong coupling. We perform angle-resolved photoemission spectroscopy to measure the spectral function of the gas and we observe a many-body pairing gap even above the predicted superfluid transition temperature.   

Relaxation Dynamics and Pre-thermalization in an isolated Quantum System

Understanding non-equilibrium dynamics of many-body quantum systems is crucial for understanding many fundamental and applied physics problems ranging from decoherence and equilibration to the development of future quantum technologies such as quantum computers which are inherently non-equilibrium quantum systems. One of the biggest challenges is that there is no general approach to characterize the resulting quantum states. In this talk I will present how to use the full distribution functions of a quantum observable to study the relaxation dynamics in one-dimensional quantum systems and to characterize the underlying many body states. Interfering two 1 dimensional quantum gases allows to study how the coherence created between the two many body systems by the splitting process [1] slowly dies by coupling to the many internal degrees of freedom available [2]. To reveal the nature of the quantum states behind this de-coherence we analyze the interference of the two evolving quantum systems. The full distribution function of the shot to shot variations of the interference patterns [3,4], especially its higher moments, allows characterizing the underlying physical processes [5]. Two distinct regimes are clearly visible in the experiment: for short length scales the system is characterized by spin diffusion, for long length scales by spin decay [6]. After a rapid evolution the distributions approach a steady state which can be characterized by thermal distribution functions. Interestingly, its (effective) temperature is over five times lower than the kinetic temperature of the initial system. Our system, being a weakly-interacting Bosons in one dimension, is nearly integrable and the dynamics is constrained by constants of motion which leads to the establishment of a generalized Gibbs ensemble and pre-thermalization. We therefore interpret our observations as an illustration of the fast relaxation of a nearly integrable many-body system to a quasi-steady state through de-phasing. The observation of an effective temperature significant different from the expected kinetic temperature supports the observation of the generalized Gibbs state [6]. \\[4pt] [1] T. Schumm \emph{et al.} Nature Physics, {\bf 1}, 57 (2005).\\[0pt] [2] S. Hofferberth \emph{et al.} Nature {\bf 449}, 324 (2007).\\[0pt] [3] A. Polkovnikov, \emph{et al.} Proc. Natl. Acad. Sci. {\bf 103}, 6125 (2006); V. Gritsev, \emph{et al.}, Nature Phys. {\bf 2}, 705 (2006); \\[0pt] [4] S. Hofferberth \emph{et al.} Nature Physics {\bf 4}, 489 (2008); \\[0pt] [5] T. Kitagawa, \emph{et al.}, Phys. Rev. Lett. {\bf 104}, 255302 (2010); New Journal of Physcs, {\bf 13} 073018 (2011)\\[0pt] [6] Gring \emph{et al.}, to be published   

Session B11: Focus Session: Graphene/BN

Tunneling spectroscopy of graphene boron nitride heterostructures

We report on the fabrication and measurement of a graphene tunnel junction using hexagonal boron nitride as a tunnel barrier between graphene and a metal gate. The tunneling behavior into graphene is altered by the interactions with phonons and the presence of disorder. We extract properties of graphene and observe multiple phonon-enhanced tunneling thresholds. Finally, differences in the measured properties of two devices are used to shed light on mutually-contrasting previous results of scanning tunneling microscopy in graphene.   

Spin and valley quantum Hall ferromagnetism in graphene on hexa-Boron nitride substrates

In graphene subjected to a quantizing magnetic field, the strong Coulomb interactions and fourfold combined spin/valley degeneracy lead to an approximate SU(4) isospin symmetry within individual Landau levels). At partial filling, exchange interactions can drive the ground state to polarize ferromagnetically within this expanded isospin space, manifesting experimentally as additional integer quantum Hall plateaus outside the normal sequence. Here we report the observation of a wide number of these quantum Hall isospin ferromagnetic states. Using tilted field magnetotransport, we classify the states appearing at different Landau Level filling factors by their real spin structure. We find evidence for real spin polarized states supporting Skyrmionic excitations, charge- or spin- density order, and valley textured excitations at different filling factors. We also observe unexpected reentrant behavior in tilted field in the higher Landau levels. Our results confirm graphene as a highly isotropic SU(4) ferromagnet, in which symmetry breaking is dictated by the interplay between the Zeeman effect, lattice scale interactions, and disorder.   

Atomic Scale Observation of Electron-Electron Interactions in Single-Layer Graphene Devices on Boron Nitride Dielectrics

We have performed scanning tunneling spectroscopy measurements on gated-graphene devices in the quantum Hall regime under varying disorder potential landscapes. Relatively thin hexagonal boron-nitride (h-BN) crystals are mechanically exfoliated on SiO2/Si substrates and single-layer graphene films are later transferred on pre-located h-BN crystals. In this device scheme, we can investigate the interactions of Dirac particles with local impurities ranging from strongly disordered to weakly perturbed environments by adjusting the thickness of h-BN crystals, while varying both the Fermi-energy with respect to a Dirac point and magnetic field. In the h-BN devices, we have observed that the electron-hole puddles are larger in lateral size than those observed on SiO2 devices, and resonance scatterings are significantly reduced due to weakened disorder potentials. Accordingly, we start observing well-defined Landau levels (LLs) as early as 0.5 T and the width of individual LLs, broadened by the scattering of charged carriers, is much narrower than those from graphene on SiO2. In high magnetic fields, we observe the electronic structure of graphene devices is significantly altered by the electron-electron interactions and the formation of large interaction energy gaps. We will discuss the spatial, orbital quantum number, and magnetic field dependence of the observed interaction gaps.   

Landau Level Lifetimes and Residual Disorder in Single Layer Graphene on Boron-Nitride Substrates

Realization of the intrinsic electronic properties of graphene devices has been limited by charge scattering and surface roughness found when graphene is placed on SiO2 substrates. Recently, graphene devices fabricated on hexagonal boron-nitride (h-BN) dielectrics have shown superior device performance compared with graphene placed directly on SiO2 substrates. We have performed scanning tunneling microscopy / spectroscopy (STM / STS) measurements to investigate the local electronic structure of graphene devices on h-BN substrates as a function of charge density and magnetic field. The disorder potential is significantly reduced compared with graphene in direct contact with SiO2. Correspondingly, the widths of Landau levels (LLs) are much narrower becoming comparable to those measured in epitaxial graphene on SiC. The energy and the spatial dispersion of LLs is used to analyze the Fermi velocity of the Dirac particles at different charge densities, an electron-hole asymmetry, and discrete splittings of LLs due to residual spatially varying disorder potential.   

STM of graphene on boron nitride

Graphene on hexagonal boron nitride (hBN) has been shown to have significantly improved mobility and charge inhomogeneity based on electrical transport measurements. Using scanning tunneling microscopy, we have observed that the surface roughness is reduced by one order of magnitude as compared to graphene on SiO$_2$ devices. Near the Dirac point, graphene breaks up into a series of electron and hole puddles due to potential fluctuations. Using scanning tunneling spectroscopy, we have shown that the potential fluctuations are also reduced by an order of magnitude on hBN. The ultraflat and clean nature of graphene on hBN devices allows for the observation of scattering from buried step edges. The energy and spatial dependence of the scattering gives information about the dispersion relation of graphene and the chiral nature of the quasiparticles. In this talk, I will also discuss our recent spectroscopy measurements on hBN.   

Thickness Dependence of Electrical Breakdown in h-BN dielectric using C-AFM microscopy

For nano-scale graphene transistor applications, hexagonal boron nitride (h-BN) is a highly desirable dielectric material that is being investigated not only because of its intrinsic properties but also because of its low lattice mismatch with hexagonal graphene. Currently, SiO$_{2}$ limits the carrier mobility of graphene due to substrate phonon coupling. Therefore, h-BN can be employed for mobility enhancement beyond the values achievable on standard dielectric. Decreasing the device dimensionality however, requires a more detailed understanding of electrical breakdown at the nanoscale. We will present on the intrinsic breakdown electric field (E$_{BF})$ of h-BN thin films in a metal-insulator-metal (MIM) configuration. This nanoscaled MIM structure is measured using conductive-atomic-force microscopy (C-AFM). Here, C-AFM is used to extract breakdown voltage for various thicknesses of mechanically exfoliated h-BN flakes. We measure the dielectric properties of h-BN flakes that vary from 2nm to 25nm and determine the ultimate scalability of hBN dielectrics.   

Transport in graphene-boron nitride heterostructures

Transferring graphene on hexagonal boron nitride permits the fabrication of high mobility graphene devices. We report on in-plane transport measurements on dual gated graphene systems using boron nitride as a substrate. The low amount of disorder allows for ballistic effects to be probed, as we can gate-define regions narrower than the mean free path. This work is supported by the Center on Functional Engineered Nano-Architectonics.   

Tunable and Sizable Band Gap of Single Layer Graphene Sandwiched between Hexagonal Boron Nitride

It is a big challenge to open a tunable and sizable band gap of single layer graphene without big loss in structural integrity and carrier mobility. By using density functional theory calculations, we show that the band gap of single layer graphene can be opened to 0.16 (without electrical field) and 0.34 eV (with a strong electrical field) when sandwiched between two hexagonal boron nitride single layers in a proper way. The zero-field band gaps are increased by about 50{\%} when many-body effects are included. Ab initio quantum transport simulation of a dual-gated FET out of such a sandwich structure further confirms an electrical field-enhanced transport gap. The tunable and sizeable band gap and structural integrity render this sandwich structure a promising candidate for high-performance single layer graphene field effect transistors.   

High quality charge- and spin transport in graphene on commercially available boron nitride

In order to overcome the limitations that a silicon oxide substrate imposes on the electronic transport properties of graphene, hexagonal boron nitride (h-BN) has proven to be an excellent alternative. We present a fast, simple and accurate transfer technique of graphene, which yields atomically flat graphene flakes on h-BN that are almost completely free of bubbles or wrinkles. Using this transfer technique we prepared single- and bilayer graphene electronic devices on commercially available hexagonal boron nitride and extract mobilities as high as 125 000 cm$^{2}$/Vs at room temperature and 275 000 cm$^{2}$/Vs at 4.2 K. The high electronic quality is further confirmed by magnetotransport measurements, which show the development of the 1e$^{2}$/h Landau level already at 5T (P. J. Zomer et al. arXiv:1110.1045v1). Finally, we present very recent results of spin transport in high mobility h-BN supported graphene flakes (P. J. Zomer et al. in preparation). In conclusion, the potential of commercially available boron nitride combined with our transfer technique makes high mobility graphene devices more accessible.   

Examining CVD graphene quality using hexagonal boron nitride substrates

Chemical vapor deposition (CVD) of graphene has proven to be the most inexpensive and scalable synthesis technique for continuous graphene monolayers. However, CVD graphene typically has a lower mobility than that from exfoliation. This is likely due to a combination of intrinsic (defect and grain boundary) and extrinsic (substrate and contamination) effects. By fabricating CVD graphene transistors on hexagonal boron nitride (h-BN) substrates, we are able to reduce the extrinsic substrate interactions that otherwise occur with silicon dioxide layers. This greatly improves the mobility in our devices (up to 29000 cm$^2$/Vs). While such improvements from h-BN have been previously observed in exfoliated graphene devices, its success with CVD graphene is particularly notable because it shows that the low mobilities observed in CVD graphene are not from intrinsic effects, and that current synthesis techniques are more than sufficient to consistently produce graphene with $>$10000 cm$^2$/Vs. Furthermore, our research reveals that characterization of CVD growth recipes by measuring mobilities on silicon dioxide is insufficient, as scattering from the oxide will dominate, revealing little information about intrinsic graphene quality.   

Session B12: Graphene: Electronic Structure and Interactions - Bilayer Graphene

Berry phase and pseudospin winding number in bilayer graphene

In 2006, two seminal studies on the novel quantum Hall effect of bilayer graphene [K. S. Novoselov et al., Nat. Phys. {\bf 2}, 177 (2006); E. McCann and V. I. Fal'ko, Phys. Rev. Lett. {\bf 96}, 086805 (2006)] appeared. Those papers claim that a non-trivial Berry phase of $2\pi$ in bilayer graphene is responsible for the novel quantum Hall effect described. Since then, it has become widely accepted by people working on the novel physics of graphene nanostructures that bilayer graphene has a non-trivial Berry phase of $2\pi$ (different from 0, as for conventional two-dimensional electron gas). In this talk, we show that (i) the relevant Berry phase for bilayer graphene is the same as that for a conventional two-dimensional electron gas and especially that (ii) what is actually obtained in the quantum Hall measurements is not the absolute value of the Berry phase of graphene multilayers but the pseudospin winding number. The results of our study ask for a re-interpretation of the numerous works related to the Berry phase in graphene multilayers.   

Electron-hole asymmetry and band mass renormalization in bilayer graphene: elucidating the role of electron-electron interactions with first-principles GW calculations

The electron-hole asymmetry and the band masses of bilayer graphene are fundamental quantities in various phenomena and in the applications of this material. \textit{A priori}, both of these quantities can depend on a number of microscopic mechanisms, including single-particle effects such as next-nearest neighbor hopping amplitude, as well as many-body effects such as the electron-electron interaction. We calculate the low energy electronic structure of bilayer graphene from first-principles, within the GW approximation. Our results indicate that both the electron-hole asymmetry and the band mass are strongly renormalized by electron-electron interactions. Our results are in good agreement with recent Shubnikov-de Haas experiments on bilayer graphene.   

Spontaneous broken time reversal symmetry and persistent current states in bilayer graphene

We report on the possibility of electron interaction driven spontaneously broken time reversal symmetry states in bilayer graphene through Fermi surface instability (aka Pomeranchuk instability) in both $\ell =0$ and $\ell =1$ channels. The conditions for spontaneous current carrying equilibrium states are most favorable in the regime of high interlayer potential difference and finite doping. These states are accompanied by valley polarization, which leads to a finite Hall conductivity that is approximately proportional to carrier density. Ref. cond-mat arXiv:1111.1765   

Time-reversal symmetry breaking states in bilayer graphene

The time-reversal symmetry breaking states in bilayer graphene which do not break translational symmetry can be classified by the spatial symmetry of the spontaneous current patterns. Among the four possible states, only one has anomalous Hall effect. In a simple model with near-neighbor interactions, we compare their energies in the mean-field approximation. We also compare which of the states is compatible with the observed gapped state in bilayer graphene.   

Spontaneous symmetry breaking in bilayer graphene

Recent experiments [1-4] provided compelling evidence for the correlated electron behavior in undoped bilayer graphene at both zero and finite magnetic field. The key question concerns the nature of the broken-symmetry phases realized experimentally. I will present the phase diagram for the zero-density state in the quantum Hall regime ($\nu=0$ state) obtained within the framerwork of quantum Hall ferromagnetism. Comparing these results with the experimental data of Refs. [1,4], I will argue that the $\nu=0$ insulating state realized in bilayer graphene is the canted antiferromagnetic phase. I will also show that the (canted) antiferromagnetic phase can persist at all magnetic fields down to zero and argue that this is the most likely scenario for the insulating state observed in Ref. [4]. \\[4pt] [1] R. T. Weitz {\em et al.}, Science 330, 812 (2010). \\[0pt] [2] F. Freitag {\em et al.}, arXiv:1104.3816 (2011). \\[0pt] [3] A. S. Mayorov, {\em et al.}, Science 333, 860 (2011). \\[0pt] [4] J. Velasco Jr. {\em et al.}, arXiv:1108.1609 (2011). \\[0pt] [5] M. Kharitonov, arXiv:1103.6285, arXiv:1105.5386, arxiv:1109.1553 (2011).   

Transport Spectroscopy of Symmetry-Broken Insulating States in Bilayer Graphene

The flat bands in bilayer graphene (BLG) are sensitive to electric fields $E $directed between the layers, and magnify the electron-electron interaction effects, thus making BLG an attractive platform for new two-dimensional (2D) electron physics. Theories have suggested the possibility of a variety of interesting broken symmetry states, some characterized by spontaneous mass gaps, when the electron-density is at the carrier neutrality point (CNP). The theoretically proposed gaps in bilayer graphene are analogous to the masses generated by broken symmetries in particle physics and give rise to large momentum-space Berry curvatures accompanied by spontaneous quantum Hall effects. Though recent experiments have provided convincing evidence of strong electronic correlations near the CNP in BLG, the presence of gaps is controversial. Here we present transport measurements in ultra-clean double-gated BLG, using source-drain bias as a spectroscopic tool to resolve a gap of $\sim $2 meV at the CNP. The gap can be closed by an electric field $E\sim $ 15 mV/nm but increases monotonically with a magnetic field $B$, with an apparent particle-hole asymmetry above the gap, thus providing a spectroscopic mapping of the ground states in BLG.   

Transport in Bilayer Graphene at the Charge Neutrality Point

Bilayer graphene (BLG) at the charge neutrality point is strongly susceptible to electronic interactions, and expected to undergo a phase transition into a state with spontaneous broken symmetries. We experimentally investigate transport properties of a large number ultra-clean BLG devices as functions of temperature, disorder, out-of-plane electric field, and charge density. We will discuss the results in terms of various theoretical models and possible phase transitions ~arising from the rich many body physics in this 2D system.   

Unconventional Landau levels in biased bilayer graphene

We utilize a generalized tight-binding model to study how the bias electric field impacts the magneto-electronic properties of graphene bilayer. With the availability of Landau wave function, the distribution among its sublattices enables the detailed observation of the Landau levels. The external electric field induces different electric potential on respective layers, which in turn lifts the inter-valley degeneracy. In addition, in a certain field range, Landau levels are coupled with each other and reveal the anomalous behavior: despite the serious hybridization of wave functions, these states in energy are still well-behaved Landau levels. These significant changes are directly reflected in the magneto-optical spectra, including the splitting of absorption peaks as well as the enhancement or quenching in response to field strength.   

Interaction-Enhanced Coherence in Graphene and Topological Insulator Bilayers

We analyze the interlayer coherence properties of graphene and topological insulator electron-hole bilayers by solving imaginary-axis Dirac-model gap equations in the random phase approximation. By accounting self-consistently for the dynamical screening of Coulomb interactions in the gapped phase, we find that the gap can rise to values of the order of the Fermi energy in the strong interactions regime. For graphene, we comment on the supportive role of remote bands not included in our two-band Dirac model.   

Analysis of fermions on a honeycomb bilayer lattice with finite-range interactions in the weak-coupling limit

We extend previous analyses of fermions on a honeycomb bilayer lattice via weak-coupling renormalization group (RG) methods with extremely short-range and extremely long-range interactions to the case of finite-range interactions. In particular, we consider different types of interactions including screened Coulomb interactions, much like those produced by a point charge placed either above a single infinite conducting plate or exactly halfway between two parallel infinite conducting plates. We map out the phases that the system enters as a function of the range of the interaction. For spin-$\frac{1}{2}$ fermions, we discover that the system enters an antiferromagnetic phase for short ranges of the interaction and a nematic phase at long ranges, in agreement with the previous work. Our results can help reconcile the recent results of two seemingly contradictory experiments. We also consider the effects of an applied magnetic field on the system in the antiferromagnetic phase via variational mean field theory, obtaining results in qualitative agreement with the experimental data. We find that the antiferromagnetic order parameter increases with field, at first quadratically at low fields, then as $B/\ln(B/B_0)$ at higher fields.   

Wave-packet dynamics in twisted bilayer graphene

It has been shown recently that high-quality epitaxial graphene (EPG) can be grown on the SiC substrate that exhibits interesting physical properties. In particular, the multilayer graphene films grown on the C-face show rotational disorder. It is expected that the twisted layers exhibit unique new physics that is distinct from that of either single layer graphene or graphite. In this work, the time-dependent wave-packet propagation in twisted bilayer graphene (TBG) is studied based on a tight-binding model with parameters derived from density functional theory. Our results demonstrate intriguing dynamical behavior of electrons in TBG, which is related to the specific interlayer coupling. By varying the twist angle and the initial wave packet, we can effectively control the propagation of electrons in TBG.   

Fermi-velocity reduction in twisted bilayer graphene: large-scale density-functional calculations

Twisted bilayer graphene (BLG) in which two graphene layers are rotated with a certain angle has been observed experimentally and its electronic structure has been studied using mainly tight-binding models [1]. We here report large-scale electronic-structure calculations with local-density approximation in the density-functional theory that clarify salient nature of the twisted BLG with the small rotation angles. The calculations have been done using the real-space density-functional theory (RSDFT) scheme which we have developed for the next-generation massively parallel computers. Implementation of the ultrasoft pseudopotential scheme in RSDFT allows us to treat thousands -of-carbon-atom systems in moderate-size computers. We have found that the Fermi velocity of the Dirac electron in the twisted BLG is substantially reduced compared with that in single layer graphene when the twist angle is small enough. This corroborates the observed reduction in the tight-binding models. We have also found that the wave functions at the Fermi level is strongly localized at AA-stacked region in the BLGs, opening a possibility of new magnetic functions.\\[4pt] [1] A. Luican et al., PRL 106, 126802 (2011); G. Trambly et al., Nano letters 10, 804 (2010); S. Shallcross et al., PRB 81, 165105 (2010).   

A Full electric-field tuning of thermoelectric power in a dual-gated Bi-layer graphene device

By using high quality microcrystals of hexagonal boron nitride as top gate dielectric, we fabricated dual-gated bilayer graphene devices. We demonstrate a full electric field tuning of thermoelectric power resulting from the opening of a band-gap by applying a perpendicular electric field on bilayer graphene. We uncover a large enhancement in thermoelectric power at low temperature. At 15 K, the thermoelectric power can be amplified by more than four-fold attaining a value of $\sim$ 50$\mu$V/K at a displacement field of 0.8 V/nm. Our result may open up a new possibility in thermoelectric application using graphene-based device.   

Phase diagram of the honeycomb bilayer from functional renormalization

The phase diagram for interacting electrons on the honeycomb bilayer with Bernal stacking is explored by means of the functional renormalization group. For half-filling and including a range of repulsive onsite, nearest-neighbor and next-to-nearest neighbor interactions we analyze the emergent instabilities and find antiferromagnetic, two types of charge-density-waves and quantum spin Hall order. The presented phase diagram covers the relevant region for the bilayer graphene parameters which overlaps with the phase boundary between the antiferromagnetic state and the quantum spin Hall state. We comment on the effect of small dopings.   

The Hubbard model on the bilayer honeycomb lattice with Bernal stacking

Using a combination of quantum Monte Carlo, the functional renormalization group and mean-field theory we study the Hubbard model on the bilayer honeycomb as a model for interacting electrons on bilayer graphene. The free bands consisting of two Fermi points with quadratic dispersions lead to a finite density of states, which triggers the antiferromagnetic instability and spontaneously breaks sublattice and spin rotational symmetry once a local Coulomb repulsion is introduced. We show that the antiferromagnetic instability is insensitive to the inclusion of extended Coulomb interactions and discuss effects on the sublattice magnetization and of finite size systems in numerical approaches.   

Session B13: Focus Session: Low-Dimensional and Molecular Magnetism - 1D Magnetism/single-chain magnets - Theory

Ferromagnetic versus helical order in edge sharing CuO$_2$ chains - a computational study

The magnetic ground state of edge sharing CuO$_2$ spin 1/2 Heisenberg chains with nearest neighbor exchange $J_1$ and second neighbor exchange $J_2$ depends delicately on structural details of the crystal structure, like Cu-O-Cu bond angles, Cu-O distances and the position of the cations. Without taking into account a renormalization by the interchain coupling, a critical ratio $\alpha=-J_2/J_1$ separates a ferromagnetic from a helical ground state (FM for $\alpha < 1/4$, helical for $\alpha > 1/4$). Here, we present a density functional based band structure study that investigates the different influences of various structural parameters for Li$_2$ CuO$_2$ as example compound. We find that the ferromagnetic and antiferromagnetic contributions develop rather differently for the same structural changes. Therefore, the key parameter $\alpha$ for the ground state is especially sensitive for small structural changes that might be induced by temperature or pressure variation.   

Quantum criticality of dipolar spin chains

We show that a one-dimensional chain of Heisenberg spins, interacting with long-range dipolar forces in a magnetic field perpendicular to the chain, exhibits a quantum critical point belonging to the two-dimensional Ising universality class. Within linear spin-wave theory (corresponding to the so-called Gaussian approximation) the long-wavelength magnon dispersion is characterized by a logarithmic singularity in the magnon velocity for vanishing momenta, due to the long range nature of dipolar interactions in one-dimension. However, in the vicinity of the critical point this logarithmic correction is renormalized to zero by the effects of quantum fluctuations, signaling the reemergence of scale invariance, in accordance with the Ising critical scenario. The quantum critical regime where linear spin-wave theory breaks down is studied using two independent non-perturbative methods, namely the density-matrix renormalization group (DMRG) and the functional renormalization group (FRG). The Ginzburg regime where non-Gaussian fluctuations are important is found to be rather narrow on the ordered side of the transition, and very broad on the disordered side.   

Spin transport in spin chains: Possible applications for spintronics

One of the main issues in modern electronics is that as devices get ever smaller the removal of waste energy generated by Joule heating becomes problematic. A possible solution to this problem is offered by spintronics in non-itinerant systems. Here, we theoretically propose the spin-equivalent in such systems of several different components that are used in traditional electronics: the resistance, the diode, and the capacitance. The system we consider here consists of an antiferromagnetic spin chain with anisotropic exchange interaction, connected to two three-dimensional spin reservoirs. We use inhomogeneous Luttinger liquid theory to describe the system, and non-equilibrium methods to calculate the relevant transport properties. \begin{thebibliography}{100} \bibitem{1} K. A. van Hoogdalem and D. Loss, Phys. Rev. B {\bf 84}, 024402 (2011). \bibitem{2} K. A. van Hoogdalem and D. Loss, in preparation. \end{thebibliography}   

Fractionalization of electron's spin and orbital degrees of freedom in 1D

We show that electron's spin and orbital degrees of freedom can fractionalize in 1D antiferromagnets: although the orbital excitations are inherently coupled to spinons in antiferromagnetic Mott insulators, in 1D they separate into a {\it pure} orbiton and a single spinon. This is similar to the spin-charge separation in 1D but corresponds to an exotic regime where spinons are faster than holons [1]. The resulting large dispersion of the pure orbiton can be detected in e.g. quasi-1D cuprates [2]. [1] K. Wohlfeld, M. Daghofer, S. Nishimoto, G. Khaliullin, and J. van den Brink, Phys. Rev. Lett. {\bf 107}, 147201 (2011). [2] J. Schlappa {\it et al.}, to be published (2011).   

Accurate determination of the Gaussian transition in spin-1 chains with single-ion anisotropy

The Gaussian transition in the spin-one Heisenberg chain with single-ion anisotropy is extremely difficult to treat, both analytically and numerically. We introduce an improved DMRG procedure with strict error control, which we use to access very large systems. By considering the bulk entropy, we determine the Gaussian transition point to 4-digit accuracy, $D_{c}/J = 0.96845(8)$, resolving a long-standing debate in quantum magnetism. With this value, we obtain high-precision data for the critical behavior of quantities including the ground-state energy, gap, and transverse string-order parameter, and for the critical exponent, $\nu = 1.472(2)$. Applying our improved technique at $J_{z} = 0.5$ highlights essential differences in critical behavior along the Gaussian transition line.   

Generalized Fidelity Susceptibilities as Applied to the $J_1-J_2$ Heisenberg Chain

In this talk slightly generalized quantum fidelity susceptibilities for the antiferromagnetic Heisenberg $J_1-J_2$ chain will be introduced. The differential change in these fidelities differ from the typical fidelity in that they are measured with respect to a term other than the one used for driving the system towards a quantum phase transition. We study three fidelity susceptibilities; $\chi_{\rho}$, $\chi_D$ and $\chi_{AF}$, which are related to the spin stiffness, the dimer order and antiferromagnetic order, respectively. I will discuss how these quantities can accurately identify the quantum critical point at $J_2$=0.241167$J_1$ in this model. This phase transition, being in the Berezinskii-Kosterlitz-Thouless universality class, is controlled by a marginal operator and is therefore particularly difficult to observe. In addition more recent work on the anisotropic Heisenberg triangular model will be discussed.   

Excitation structure of frustrated spin chains with dimerization and the description by the effective field theory

Heisenberg antiferromagnetic chain with alternating exchange interaction is an important model, which describes magnetic properties of real materials. Field theoretical approach is a powerful tool to investigate such kind of one-dimensional quantum magnets, and it is known that this lattice model is related with corresponding sine-Gordon effective field theory through the bosonization technique. We investigate the excitation spectrum and the correspondence between $S=\frac{1}{2}$ and 1 frustrated chain with dimerization and their effective field theories by both analytical and numerical methods, focusing on the mass ratio $r$ of second breather to soliton. In the result, the $S=\frac{1}{2}$ and 1 cases are understood in a unified way. $r$ becomes $\sqrt{3}$, the value predicted from sine-Gordon model by the introduction of next-nearest neighbor coupling $J_2=J_{2{\rm c}}$ where the marginal term in effective field theory vanishes. The universality class of transition is Tomonaga-Luttinger liquid and first order for $J_2 [Preview Abstract]

Statistically interacting particles with shapes

Ising spin $s=\frac{1}{2},1,\frac{3}{2}$ chains with nearest and next-nearest neighbor coupling are interpreted as systems of floating particles. The particles are classified into species according to structure and into categories according to function. Species are distinguished by motifs consisting of several consecutive spins that interlink by sharing one or two sites. The four categories include compacts, hosts, tags, and hybrids. All particles from one set are excited from a selected Ising product state serving as pseudo-vacuum. Compacts and hosts float in segments of pseudo-vacuum. Tags are located inside hosts. Hybrids are tags with hosting capability. All particles are free of binding energies but subject to a generalized Pauli principle. In the Ising context, all particle energies are functions of the Hamiltonian parameters. However, the exact statistical mechanical analysis can be performed for particles with arbitrary energies. The entropy is a function of the particle populations from each species. Applications to jamming of granular matter in narrow channels and to DNA overstretching are in the works.   

Fisher zeros and correlation functions in Ising models

Phase transitions take place at singular points of the free energy. These correspond to zeros of the partition function when one tuning parameter is extended into the complex plane (so called Lee-Yang or Fisher zeros). It has been known since the 1960s that transition temperatures and critical exponents can be calculated from the distributions of these partition function zeros. We use this technique to calculate the spin-spin correlation function for the 1d Ising model and notice that it forms a spiral with a wavevector dependent on the position of the complex temperature on the contour of zeros. To extend this we will look at results from the 2d Ising model as well as the Ising model in a transverse field.   

Integrability in anyonic quantum spin chains via a composite height model

Recently, properties of collective states of interacting non-abelian anyons have attracted a considerable attention. We study an extension of the `golden chain model', a model of interacting Fibonacci anyons, where two- and three-body interactions are competing. Upon fine-tuning the interaction, the model is integrable. This provides an additional integrable point of the model, on top of the integrable point, when the three-body interaction is absent. To solve the model, we construct a new, integrable height model, in the spirit of the restricted solid-on-solid model solved by Andrews, Baxter and Forrester. The model is solved by means of the corner transfer matrix method. We find a connection between local height probabilities and characters of a conformal field theory governing the critical properties at the integrable point. In the anitferromagnetic regime, the criticality is described by the $Z_{k}$ parafermion conformal field theory, while the $su(2)_{1} \times su(2)_{1} \times su(2)_{k-2}/su(2)_{k}$ coset conformal field theory describes the ferromagnetic regime.   

Out of equilibrium energy dynamics in low dimensional quantum magnets

We investigate the real-time dynamics of the energy density in spin-1/2 $XXZ$ chains using two types of quenches resulting in initial states which feature an inhomogeneous distribution of local energies [1]. The first involves quenching bonds in the center of the chain from antiferromagnetic to ferromagnetic exchange interactions. The second quench involves an inhomogeneous magnetic field, inducing both, an inhomogeneous magnetization profile [2] and local energy density. The simulations are carried out using the adaptive time-dependent density matrix renormalization group algorithm. We analyze the time-dependence of the spatial variance of the bond energies and the local energy currents which both yield necessary criteria for ballistic or diffusive energy dynamics. For both setups, our results are consistent with ballistic behavior, both in the massless and the massive phase. For the massless regime, we compare our numerical results to bosonization and the non-interacting limit finding very good agreement. The velocity of the energy wave-packets can be understood as the average velocity of excitations induced by the quench. \\[4pt] [1] Langer et al. Phys. Rev. B in press; arXiv:1107.4136\\[0pt] [2] Langer et al. Phys. Rev. B 79, 214409 (2009)   

Dissipative phases in the one-dimensional Kondo-Heisenberg model

Atomic-sized magnetic structures built on clean metallic surfaces are currently under intense investigation {[}1{]}. Besides their potential uses in quantum information storage and processing, these systems allow to ask fundamental questions in condensed matter physics. In particular, the interplay between the Kondo effect (i.e., the screening of the atomic magnetic moment by conduction electrons) and Heisenberg exchange interactions between magnetic impurities has been recently investigated with scanning tunneling microscopy (STM) {[}2{]}. Inspired by the above developments, we study an one-dimensional chain of S=1/2 Kondo impurities coupled by anisotropic Heisenberg-Ising exchange and embedded in a two-dimensional metallic substrate. Remarkably, in the case of easy-plane exchange, we find a novel quantum phase exhibiting long-range order at zero temperature. We discuss implications of the existence of this phase for possible experiments. References: {[}1{]} R. Wiesendanger, RMP 81, 1495 (2009). {[}2{]} P. Wahl et al, PRL 98, 056601 (2007) and references therein.   

String solutions and Mott physics in quasi-one-dimensional antiferromagnets

Mott insulators are caused by repulsive interactions between electrons, whereas band insulators are due to full filling of a single-particle band. In the one-dimension (1D) Hubbard model, the upper Hubbard band (UHB) has been identified with $k$-$\Lambda$ string solutions [1]. Similarly, in the 1D spin-1/2 antiferromagnetic Heisenberg model, the high-energy magnetic excitations in a magnetic field have been identified primarily with 2-string solutions [2]. We can intuitively understand the correspondence between the high-energy states and the UHB, by mapping the Heisenberg model to the hard-core boson model with repulsive interactions. Furthermore, noting that the high-energy states persist in anisotropic triangular antiferromagnets [3], we can interpret the high-energy magnetic excitations observed in quasi-1D antiferromagnets such as Cs$_2$CuCl$_4$ and CuCl$_2$$\cdot$2N(C$_5$D$_5$) in a magnetic field in the context of the Mott physics: the high-energy magnetic excitations, whose origin can be traced back to the string solutions, are due to repulsive interactions between hard-core bosons (down-spins) mapped from the Heisenberg model. [1] M.K., Phys. Rev. Lett. 105, 106402 (2010). [2] M.K., Phys. Rev. Lett. 102, 037203 (2009). [3] M.K., Phys. Rev. Lett. 103, 197203 (2009).   

A family of spin-S chain representations of SU(2) level k Wess-Zumino-Witten models

We investigate a family of spin-$S$ chain Hamiltonians recently introduced by one of us [M. Greiter, Mapping of Parent Hamiltonians, Springer Tracts in Modern Phyiscs, Vol 244 (Springer, Berlin, 2011)]. For $S=1/2$, it corresponds to the Haldane--Shastry model. For general spin $S$, we numerically show that the low--energy theory of these spin chains is described by the SU(2)$_{k}$ Wess--Zumino--Witten model with coupling $k=2S$. In particular, we investigate the $S=1$ model whose ground state is given by a Pfaffian for even number of sites $N$. We reconcile aspects of the spectrum of the Hamiltonian for arbitrary $N$ with trial states obtained by Schwinger projection of two Haldane--Shastry chains.   

Inhomogeneous Spin Chains and Luttinger Liquids

We consider a one-dimensional spin chain with inhomogeneous coupling, which can also be modeled as an inhomogeneous Luttinger liquid. The Luttinger liquid paradigm has proved a very successful theoretical tool for investigating one-dimensional wires. However, there remain open questions about what happens when such a system becomes inhomogeneous. The mapping between the spin chain and the Luttinger liquid allows us to use both numerics and field theory to analyze the problem. Of particular interest is the case where the Luttinger liquid is attached to external leads, as is necessary for example when measuring the conductance of the wire. In this paper we use an abrupt shift in the parameters of the Luttinger liquid to model these connections and see how this affects its behavior. In particular we analyze the relevant back-scattering perturbations at the connections, and identify a case where this relevant operator can be tuned to zero within an otherwise still inhomogeneous system. This of course has consequences not only for transport in the Luttinger liquid system but also for the magnetic susceptibility of the spin chain.   

Session B14: Focus Session: Spins in Semiconductors - Spin Relaxation in Semiconductors and Diamond

3D Electron Spin Relaxation Control by Electric Field in Quantum Wells

We have measured the electron spin relaxation time in (111)-oriented GaAs quantum wells by time-resolved photoluminescence. By embedding the QWs in a PIN or NIP structure we demonstrate the tuning of the conduction band spin splitting and hence the spin relaxation time with an applied external electric field applied along the growth z direction . The application of an external electric field of 50 kV/cm yields a two-order of magnitude increase of the spin relaxation time which can reach values larger than 30 ns; this is a consequence of the electric field tuning of the spin-orbit conduction band splitting which can almost vanish when the Rashba term compensates exactly the Dresselhaus one [1]. The spin quantum beats measurements under transverse magnetic field prove that the D'Yakonov-Perel (DP) spin relaxation time is not only increased for the Sz electron spin component but also for both Sx and Sy. These results contrast drastically with the (001) and (110) quantum wells.The role of the cubic Dresselhaus terms on the spin relaxation anisotropy will finally be discussed. The tuning or suppression of the DP electron spin relaxation demonstrated here for GaAs/AlGaAs quantum wells grown on (111) substrates is also possible in many other III-V and II-VI zinc-blende nanostructures since the principle relies only on symmetry considerations. \\[4pt] [1] A. Balocchi, Q. H. Duong, P. Renucci, B. L. Liu, C. Fontaine, T. Amand, D. Lagarde, and X. Marie, Phys. Rev. Lett 107, 136604(2011)   

Rotating Frame Spin dynamics of a Single Nitrogen Vacancy Center in Diamond Nanocrystal

We investigate the spin dynamics of a nitrogen-vacancy (NV) center contained in individual diamond nanocrystals with an average diameter of 70 $\pm $ 20 nm in the presence of continuous microwave excitation. Upon periodic reversal of the microwave phase, we observe a train of rotary (Solomon) echoes that effectively extends the system coherence lifetime to reach several tens of microseconds, depending on the microwave power and phase inversion rate [1]. Starting from a model where the NV center interacts with a bath of paramagnetic defects on the nanocrystal surface, we use average Hamiltonian theory to compute the signal envelope from its amplitude at the echo maxima. A comparison between the effective Rabi and Solomon propagators shows that the observed response can be understood as a form of higher-order decoupling from the spin bath. The observed rotary echoes can be thought of as the rotating frame analog of Hahn's spin echoes, implying that the present scheme may find use for nanodiamond-based magnetic sensing. [1] A. Laraoui, C. A. Meriles, Phys. Rev. B \textbf{84}, 161403(R) (2011).   

Persistence of single spin coherence above room temperature in diamond

The nitrogen vacancy (NV) center in diamond is unique among single spin systems because of its robust optical spin initialization, optical spin readout, and room temperature spin coherence. However, there is not yet an understanding of what processes limit the NV center's spin properties at higher temperatures. We address this question by performing pulsed electron spin resonance and spin-resolved optical lifetime measurements on single defects at elevated temperatures [1]. The measurements demonstrate that the NV center's spin coherence remains robust at high temperatures while its spin-dependent photoluminescence diminishes above 600 K due to nonradiative relaxation. These results provide an enhanced understanding of the NV center orbital structure and also suggest the possibility of using single spins in diamond for nanoscale thermometry with sensitivities on the order of 100 mK Hz$^{-1/2}$ over a broad temperature range. \\[4pt] [1] D. M. Toyli*, D. J. Christle*, A. Alkauskas, B. B. Buckley, C. G. Van De Walle, D. D. Awschalom, (\emph{submitted}).   

Optically trapped fluorescent nanodiamonds

The electronic spin state of the nitrogen-vacancy (NV) center in diamond has gained considerable interest because it can be optically initialized, coherently manipulated, and optically read out at room temperature. In addition, nanoparticle diamonds containing NV centers can be integrated with biological and microfluidic systems. We have constructed and characterized an optical tweezers apparatus to trap fluorescent nanodiamonds in a fluid and measure their fluorescence. Particles are held and moved in three dimensions using an infrared trapping laser. Fluorescent detection of these optically trapped nanodiamonds enables us to observe nanoparticle dynamics and to measure electron spin resonance of NV centers. We will discuss applications using the electron spin resonance of trapped NV centers in nanodiamonds for magnetic field imaging in fluidic environments.   

Nuclear Polarization of Nanodiamond

Nanoparticles with long nuclear spin relaxation times are candidates for use in the context of targeted therapeutic delivery [1] and magnetic resonance imaging [1,2]. We report progress towards the development of contrast agents [3] based on 13C in nanodiamond. Nuclear relaxation and electron spin resonance data is presented for particles produced using detonation and the high-pressure high temperature technique. We describe the development of a milli-Kelvin nuclear polarization setup that makes use of a dilution refrigerator and X-band microwave resonator with fast sample exchange. [1] Huang H., Pierstorff E., Osawa E., Ho D., ``Active Nanodiamond Hydrogels for Chemotherapeutic Delivery'', Nano Lett, 7, 3305-3314 (2007). [2] Aptekar J.W., Cassidy M. C., Johnson A. C., Barton R. A., Lee M. Y., Ogier A. C., Vo C., Anahtar M. N., Ren Y., Bhatia S. N., Ramanathan C., Cory D. G., Hill A. L., Mair R. W., Rosen M. S., Walsworth R. L., Marcus C. M., ``Silicon nanoparticles as hyperpolarized magnetic resonance imaging agents'', ACS Nano, 3, 4003-4008 (2009). [3] Manus L. M., Mastarone D. J., Waters E. A., Zhang X., Schultz-Sikma E. A., MacRenaris K. W., Ho D., Meade T. J., ``Gd(III)-nanodiamond conjugates for MRI contrast enhancement'', Nano Lett, 10, 484-489 (2010).   

Spin Lifetime Measurements of GaAsBi Films

Substituting a small amount of As with Bi, the largest non-radioactive group V element, leads to a large reduction in the GaAs band gap and expected large spin-orbit effects \footnote{B. Fluegel et al., \textbf{Giant Spin-Orbit Bowing in GaAs$_{1-x}$Bi$_{x}$}, Phys. Rev. Lett. \textbf{97}, 067205 (2006).}. Both properties are advantageous with potential applications ranging from infrared detectors to spin valves. Compressively strained GaAsBi films with varying bismuth compositions were grown on GaAs using molecular-beam epitaxy. Spin lifetimes were measured using the Hanle effect, a magneto-optical technique where an out-of-plane spin polarization is generated by circularly polarized light and then made to precess about an in-plane magnetic field. A Lorentzian lineshape can be fit to the field-dependent photoluminescence polarization to extract $gT_s$, where $g$ is the Lande g-factor and $T_s$ is a function of the carrier recombination time and spin dephasing time and provides a lower bound for both. Temperature and power dependent measurements were conducted and our extracted values for $gT_s$ vary from 100ps to 1ns.   

Carrier and Spin Dynamics in Narrow Gap Multi Quantum Well Structures

We studied carrier/spin dynamics in several $InSb/Al_{x}In_{1-x}Sb$ multiple-quantum well structures using several time resolved differential transmission schemes in the mid-infrared. Our results demonstrate the unique and complex dynamics in InSb heterostructures that can be important for electronic and optoelectronic devices. We present experimental observations and compare them with theoretical calculations. Calculations are based on the 8-band Pidgeon-Brown model generalized to include confinement potential as well as pseudomorphic strain. Optical properties are calculated within the golden rule approximation and compared with one and two color, time-resolved pump-probe differential transmission.   

Electric field-driven coherent spin reorientation and spin rephasing of optically generated electron spin packets in InGaAs

Full electric-field control of spin orientations is one of the key tasks in semiconductor spintronics. We demonstrate that electric field pulses can be utilized for phase-coherent 2$\pi$ spin rotation of optically generated electron spin packets in InGaAs epilayers using time-resolved Faraday rotation. Through spin-orbit interaction, the electric-field pulses act as local magnetic field pulses (LMFP). By the temporal control of the LMFP, we can turn on and off electron spin precession and thereby rotate the spin direction into arbitrary orientations in a 2-dimensional plane [1]. Moreover, using two subsequent pulses of opposite polarity allows us to perform spin echo measurements by reversing the spin precession direction. Although our spin transport experiment is in the diffusive regime, we unexpectedly observe that electric field-induced spin dephasing is reversible to a large extend.\\ $[1]$ S. Kuhlen \textit{et al}. arXiv 1107.4307   

Using spin fluctuations to reveal long hole spin lifetimes and hole-nuclear coupling in (In,Ga)As quantum dots

``Spin noise spectroscopy'' is a recently-developed technique for passively measuring the spin dynamics of electrons and holes via their intrinsic random spin fluctuations. In accord with the fluctuation-dissipation theorem, the frequency spectra of this spin noise alone reveals spin dephasing times and Land\'{e} $g$-factors. Using these methods we measure hole spins confined in self-assembled (In,Ga)As/GaAs quantum dots (QDs). Owing to their \emph{p}-type wavefunctions, holes experience much less hyperfine interaction with lattice nuclei as compared with confined electrons, leading in principle to long spin decoherence times which are favorable for potential qubit applications. We observe very long hole spin correlation times ($\sim$400 ns) in zero magnetic field, ultimately limited by dephasing from hole-nuclear hyperfine interactions. Suppressing this dephasing with small longitudinal fields ($<$ 100 G) directly reveals the hyperfine coupling strength, and unveils intrinsic hole spin relaxation times up to $\sim$5 $\mu$s. Importantly, the lineshape of the noise evolves from a Lorentzian to a power-law as the hole-nuclear dephasing is suppressed.   

Spin lifetimes in InGaAs quantum wells

We have carried out Hanle spin relaxation time measurements in InGaAs quantum well structures as well as in Fe-based spin-LEDs that incorporate InGaAs QWs. In the InGaAs QW structures the spin lifetime T$_{S}$ at T = 5 K is equal to 2 ns while in spin LEDs T$_{S}$ is only 0.17 ns. For the undoped QWs, T$_{S}$ increases from 2ns at 5K to 4.5ns at 15K and then decreases monotonically. On the other hand, T$_{S}$ in the LEDs increases monotonically with temperature. The origin for the difference in the spin lifetimes between undoped QWs and that within a LED structure can be understood by considering the corresponding differences in the band structure between the symmetric quantum well and the spin-LED. In the latter, the Rashba spin-orbit interaction due to the built-in asymmetry brings about a significant enhancement in spin relaxation. In addition, bias conditions in the spin-LED play a crucial role in determining the temperature trend of the spin relaxation. The bias voltage tunes the electron density and accordingly the spin dephasing could be either suppressed or enhanced by electron-electron scattering.   

T1 spin lifetimes in n-doped quantum wells and dots

We have used a pump probe technique to measure $T_1$ spin lifetimes in $n$-type GaAs quantum wells and InAs self-assembled quantum dots. The circularly polarized pump laser pulse aligns the spins; the linearly polarized probe laser pulse probes the spin states of the selected well (or dots) via the Kerr (or Faraday) effect at some later time. Results for the quantum well sample include a spin-filling effect that depends on the direction from which the probe laser wavelength approaches that of the well, and spin lifetimes ranging from 50 to 2000 ns (depending on temperature and field conditions). The InAs quantum dots, doped such that each dot has approximately one extra electron, display $T_1$ lifetimes longer than 5 ms at 1 T and 1.5 K.   

Frequency-domain optical probing of coherent spins in semiconductors

We have demonstrated a technique for measuring GHz-scale coherent spin dynamics in semiconductors using narrow-linewidth modulated lasers. This scheme, based on the Faraday effect, is carried out by adding a sinusoidal modulation at frequency $\Omega$ to continuous-wave pump and probe lasers. The result is to effectively shift the Fourier component of the Faraday rotation signal at $\Omega$ to zero frequency, where it can be measured by a low-bandwidth detector. By producing the Fourier transform of the time-domain spin dynamics, this method yields the same information as pulsed pump-probe measurements without the need for complex pulsed lasers, and with minimal spectral bandwidth, allowing for high-resolution spectroscopic measurements. The ability to perform Faraday rotation measurements with narrow-linewidth lasers is essential for observing spins in individual quantum dots, or for avoiding unintentional carrier excitation. We have employed this technique to observe coherent spin dynamics in CdSe nanocrystals using standard diode lasers. By fitting these results to the expected model, we can extract electron g-factors, and spin coherence and dephasing times in agreement with time-domain measurements.   

Quadratic magnetic field dependence of magnetoelectric photocurrent

We experimentally study the spin and electric photocurrents excited by a linearly polarized light via direct interband transitions in an InGaAs/InAlAs quantum well. In the absence of a magnetic field, the linearly polarized light induces a pure spin current due to the spin-orbit coupling, which may be transformed into a measurable electric current by applying an in-plane magnetic field. The induced electric photocurrent is linear with the in-plane magnetic field. Here, we report a quadratic magnetic field dependence of the photocurrent in the presence of an additional perpendicular component of the magnetic field. We attribute the observation to the Hall effect of magnetoelectric photocurrent.   

Session B15: Focus Session: Spins in Metals: Spin Torque Switching and Magnetic Anisotropy Control

Switching Energy Barrier and Current Reduction in MTJs with Composite Free Layer

We investigate the properties of a penta-layer magnetic tunnel junction (MTJ) with a composite soft layer by exhaustive micromagnetic simulations. The structure CoFe/spacer (1nm)/Py (4nm)/spacer (1nm)/ CoFe (Py is Ni$_{81}$Fe$_{19}$) with an elliptical cross-section (major axes 90nm and 35nm, correspondingly) is considered. The system with the composite soft layer is obtained by removing a central stripe of 5nm width from the monolithic free layer. The MTJ with a composite free layer switches two to three times faster than the one with a monolithic layer [1]. We have found that in the MTJ structure with the composite layer the switching energy barrier is decreased and becomes equal to the shape anisotropy energy barrier responsible for thermal stability. This results in a switching current density reduction. The physical reasons for the switching energy barrier reduction are discussed.\\[4pt] [1] A. Makarov {\it et al., Phys. Status Solidi RRL} {\bf 5}, No. 12, 420-422 (2011).   

Switching Distributions in all perpendicular spin-valve nanopillars under thermal activation and spin-transfer torques

The free layer switching field distributions of spin-valve nanopillars with perpendicular magnetization have been studied. The distributions are consistent with expectations of a model based on thermal activation over a single field dependent energy barrier. However, at zero applied current there is a strong asymmetry between the P and AP states and the reverse, with energy barriers more than 50\% higher for P to AP transitions. The inhomogeneous dipolar field from the polarizer is demonstrated to be at the origin of this symmetry breaking\footnote{D. B. Gopman et al, arXiv:1111.1342v1 [cond-mat.mes-hall]}. I will show results on the effects of varying lateral geometry on this symmetry breaking. Also, I will introduce a low-cost method for studying the switching current distributions that applies a continuous waveform to sweep the dc current and simultaneously probe the magnetoresistance. This method permits the acquisition of over $10^{6}$ switching events in six hours, which presents the possibility to obtain deep statistics on the reversal process. The effect of the magnitude and direction of applied dc currents on the thermal stability of a nanomagnet has been investigated and the results will be examined within the acquired statistics. Supported by NSF Grant DMR-1006575.   

Landau-Lifshitz Bloch Macrospin Simulations of Magnetization Switching Dynamics in Perpendicular Anistropy CoNi/Pd Magnetic Multilayer Thin Films

Heating magnetic multilayer thin films close to their Curie temperature (Tc) for brief periods of time, in the presence of a magnetic field, will potentially enable ultra-high density magnetic recording while maintaining thermal stability of perpendicular anisotropy magnetic media. This idea is to be exploited in thermally assisted magnetic recording (TAR) technology, which is currently being pursued by many academic and industrial research labs. In our study, we have performed macrospin simulations of the magnetization switching based on the numerical solution of Landau-Lifshitz-Bloch equation at such elevated temperatures (close to Tc) for a strongly exchange coupled and high perpendicular anisotropy CoNi/Pd magnetic multilayer thin film structure (Tc=448 K). We will discuss the results of a comprehensive model for this material system taking into account temperature dependencies of anisotropy, saturation magnetization and transverse and longitudinal susceptibilities and their effects on the switching process.   

Statistical and Time Resolved Studies of Switching in Magnetic Tunnel Junction based Orthogonal Spin Transfer Devices

We report statistical and single-shot time-resolved studies of spin transfer switching in OST-MRAM devices. These devices consist of a perpendicular polarizer integrated into a layer stack with an in-plane magnetized free and reference layer, which form the electrodes of a magnetic tunnel junction [1]. The perpendicular polarizer provides an initial torque -- designed to reduce the incubation delay in switching. The demagnetization field created during the switching can further accelerate the reversal process [2]. The devices switch reliably at 0.7 V and 500 ps duration for both voltage polarities. We record the change of the device resistance in real time during the pulse to obtain the time needed to initiate the switching $\tau _{start}$ and the time between the initiation and the end of the switching $\tau _{switch}$ for every single switching event. $\tau _{switch}$ is determined to be less than a few hundreds of picoseconds, on the order of the precession time due to the demagnetization field and we find evidence for precession reversal under certain conditions. We further present results on the effects of pulse amplitude and applied field on $\tau _{start}$ and $\tau _{switch}$. This work was supported by Spin Transfer Technologies. [1] H. Liu et al., APL 97, 242510 (2010). [2] A. D. Kent et al., APL 84, 3897 (2004).   

Magnon contribution to the spin torque and magnetoresistance properties of FeCoB/MgO/FeCoB magnetic tunnel junctions

We have studied the spin-torque excited ferromagnetic resonance (ST-FMR) and the tunneling magnetoresistance (TMR) properties of FeCoB/MgO/FeCoB magnetic tunnel junctions as a function of temperature from 300K to 10K. We find that while the TMR increases by $\sim $ 50{\%} upon cooling to 10 K, the in-plane spin torque and the perpendicular or field-like torque both decrease substantially. The results demonstrate that while magnon-assisted tunneling degrades TMR, it acts to significantly enhance ST in MTJs, in accord with theoretical prediction. Moreover, the bias-dependent structure in both the asymmetry of the in-plane ST and the parallel conductance of the MTJ is more pronounced at low temperature, indicative of this asymmetry being due substantially to the interfacial electronic structure of the electrodes.   

Magnetic field dependence of spin torque switching in nanoscale magnetic tunnel junctions

Magnetic random access memory based on spin transfer torque effect in nanoscale magnetic tunnel junctions (STT-RAM) is emerging as a promising candidate for embedded and stand-alone computer memory. An important performance parameter of STT-RAM is stability of its free magnetic layer against thermal fluctuations. Measurements of the free layer switching probability as a function of sub-critical voltage at zero effective magnetic field (read disturb rate or RDR measurements) have been proposed as a method for quantitative evaluation of the free layer thermal stability at zero voltage. In this presentation, we report RDR measurement as a function of external magnetic field, which provide a test of the RDR method self-consistency and reliability.   

Shot noise in double barrier epitaxial magnetic tunnel junctions

Shot noise has been shown to be an effective tool to study statistics of electron tunnelling in single barrier magnetic tunnel junctions [1-2]. Here we report on shot noise and tunnelling magnetoresistance measurements in fully epitaxial Fe/MgO/Fe/MgO/Fe double barrier magnetic tunnel junctions. We observed that the shot noise is essentially determined by the barriers' symmetry and relative magnetic configuration. Enhanced barrier asymmetry effectively suppresses electron correlations, and the noise approaches the Poissonian limit. The proposed model of sequential tunnelling (with spin relaxation) through the magnetic layer inside the tunnel barrier satisfactory explains the experimental observations. [1] R.Guerrero, et al., Phys. Rev. Lett. 97, 0266602 (2006). [2] R.Guerrero, et al, Appl. Phys. Lett. 91, 132504 (2007).   

Planar approximation for spin-transfer devices with tilted polarizer

Planar spin-transfer devices with dominating easy-plane anisotropy can be described by an effective one-dimensional equation for the in-plane angle [1-3]. Such a description provides an intuitive qualitative understanding of the magnetic dynamics. We apply the effective planar equation to describe magnetic switching and precession states in devices with a tilted polarizer [4]. The approach allows one to understand the dynamic regimes and sketch the switching diagram without pefroming the detailed calculations. \\[4pt] [1] Ya. B. Bazaliy, Appl. Phys. Lett. 91, 262510 (2007).\\[0pt] [2] Ya. B. Bazaliy, Phys. Rev. B 76, 140402(R) (2007).\\[0pt] [3] Ya. B. Bazaliy and F. Arammash, Phys. Rev. B 84, 132404 (2011).\\[0pt] [4] Ya. B. Bazaliy, arXiv:1109.1331 (2011).   

Modification of the Stoner-Wohlfarth Astroid by a Spin-Polarized Current

Conventional Stoner-Wohlfarth astroid describes the field-induced switching of a nanomagnet with a uniaxial anisotropy. Both theory and experiments show that the spin-transfer torque can change the switching behavior of nanomagnets and therefore modify the astroid. Such a modification was recently analyzed in the limit of small currents [1]. To explore the modification of the astroid by a current of arbitrary magnitude we propose an exact method capable of finding the switching boundaries for a magnet described by an LLG equation with the Slonczewski's spin-torque term. Our approach takes into account both the local destabilization of equilibria and the equilibrium collisions. The self-crossing nature of the modified astroid is explained and a novel region with three stable equilibria is predicted in our result. \\[4pt] [1] Y. Henry, S. Mangin and J. Cucchiara, J. A. Katine, and E. E. Fullerton, Phys. Rev. B 79, 214422(2009).   

Origin of perpendicular magnetic anisotropy in Co/Ni multilayers on Ti layer

Magnetic materials in which their magnetic moment direction is oriented perpendicular to the plane of the magnetic layers in thin film heterostructures have been much studied for their potential application to spintronic devices. In particular, theories of current induced excitation, via the phenomenon of spin torque transfer, show that perpendicularly magnetized layers can be more easily excited or their magnetization direction switched than in-plane magnetized layers. In particular, Co/Ni multilayers are promising due to high spin polarization and small Gilbert damping compared to Co/Pt or Fe/Pt. However, their perpendicular magnetic anisotropy (PMA) is highly sensitive to the underlayer that is critical in device performance because, for instance, the current shunting can substantially reduce the spin transfer torque in magnetic racetrack memory. We observed an excellent PMA in annealed Co/Ni on Ti underlayer whose resistance is significantly greater than those of Co/Ni, thereby minimizing the current shunting. It is found that the PMA does not simply originate from magneto-crystalline effect (spin-orbit interaction) but mainly from magnetoelastic effect caused by compressive strain along (111) direction. We will present systematic results and quantitative analyses.   

Perpendicular magnetization of CoFeGe alloy films induced by MgO interface

The perpendicular magnetization of CoFeGe alloy films was achieved in the structures of CoFeGe/MgO with the perpendicular magnetic anisotropy energy density ($K_{u})$ of $\sim $ 1 x 10$^{6 }$erg/cm$^{3}$. The CoFeGe thickness dependence of $K_{u}$ was investigated, indicating that the perpendicular anisotropy of CoFeGe is contributed by the interfacial anisotropy between CoFeGe and MgO. High-resolution transmission electron microscope images clearly show formation of bcc crystalline structure of CoFeGe well lattice matched with the (100)-oriented MgO barrier. Gilbert damping constant for the films was evaluated by using ferromagnetic resonance measurement.   

Voltage-Induced Ferromagnetic Resonance in Magnetic Tunnel Junctions

We demonstrate excitation of ferromagnetic resonance in CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) by the combined action of voltage-controlled magnetic anisotropy (VCMA) and spin transfer torque (ST). Our measurements reveal that GHz-frequency VCMA torque and ST in low-resistance MTJs have similar magnitudes, and thus that both torques are equally important for understanding high-frequency voltage-driven magnetization dynamics in MTJs. As an example, we show that VCMA can increase the sensitivity of an MTJ-based microwave signal detector to the sensitivity level of semiconductor Schottky diodes.   

Spin Torque in Asymmetric CoFeB/MgO/FeB Magnetic Tunnel Junctions

Recent studies have shown that the use of asymmetric electrodes in MTJs can significantly affect spin torque (ST) behavior. We will report on the measurement via spin torque ferromagnetic resonance (ST-FMR) and switching phase diagram (SPD) studies of the in-plane and field-like (out-of-plane) torkance of low resistance, asymmetric IrMn/FeB/MgO/FeCoB) and IrMn/FeCoB/MgO/FeB MTJ nanopillars in the as-grown state (TMR$\sim $22{\%}) and the annealed state (TMR$\sim $90{\%}), and in comparison to that of symmetric counterparts; IrMn/FeB/MgO/FeB) and Ir/Mn/FeCoB/MgO/FeCoB MTJs. For the asymmetric MTJs only,the ST-FMR data show a strong field-like torkance for low voltage bias V that reverses sign when the free and pinned layers are reversed. At the higher V regime explored by the SPD the equivalent linear term in the field-like torque can dominate over the in-plane torque, resulting in either the parallel or antiparallel alignment being favored for both bias polarities, depending on the composition of the free layer.   

Influence of growth parameters on the spin-filtering properties of epitaxial ferrite tunnel barriers

In spintronics, spin-filtering is a physical phenomenon which has the potential to produce highly spin-polarized currents by the spin-selective transport of electrons across a ferromagnetic tunnel barrier. The insulating ferrites MnFe$_{2}$O$_{4}$, CoFe$_{2}$O$_{4}$ and NiFe$_{2}$O$_{4}$, whose Curie temperatures are above 300K, are promising candidates for spin-filtering at room temperature. In this work, we report on the in-depth study of structural, chemical and physical properties of epitaxial ferrite ultra-thin films and associated spin-filtering measurements as a function of different growth parameters. We analyze the effect of oxidation on the physical properties and the resultant spin-polarization. The influence of structural defects on the spin-filter efficiency is also put on view by tunneling magnetoresistance. Finally, we show the impact of the MgAl$_{2}$O$_{4}$(001) substrates on the magnetic behavior of cobalt ferrite tunnel barriers revealing the important role played by strains in the spin-filter properties.   

Magnetoelectric Control of Magnetic Anisotropy in Ultrathin Fe Films

Magnetoelectric switching of the magnetization vector could enable new low-power logic devices and non-volatile memory cells. Magnetoelectric switching typically requires complex multiferroic oxides or strain coupled magnetostrictive/piezoelectric composites. However, recently it has been demonstrated that surface magnetic anisotropy in ultrathin ferromagnetic metal films can be directly controlled by application of a strong electric field [1]. In this work we apply an electric field across a high-k oxide stack of MgO and ZrO$_{2}$ to induce charge at the surface of an ultrathin Fe film. By using high-k dielectric materials more charge can be induced at the surface of the ferromagnetic film and the efficiency of the magnetoelectric effect can be enhanced. Under application of just a few volts across the oxide stack we observe a strong magnetoelectric effect which results in a shift of the spin reorientation thickness by 0.5 atomic layers and a change in perpendicular surface anisotropy of $\sim $120$\mu $J/m$^{2}$. Moreover, by engineering the high-k oxide stack we realize a novel charge pumping mechanism that permits optical imprinting of the magnetic state in the continuous Fe film. [1] T. Maruyama \textit{et al.} Nature Nanotechnology 4, 158 - 161 (2009)   

Session B16: Hubbard Model

Charge and magnetic order in the triangular lattice Hubbard model at one-third filling

Experimental work over the last decade on layered triangular lattice materials with itinerant electrons has increased the motivation to study these systems theoretically. As in insulating magnets, frustration in itinerant systems enriches the phase diagram of the material. In a class of organic charge transfer salts and sodium cobalt oxide spin liquid, superconducting, charged ordered, and magnetically ordered phases have been observed. With the kinetic energy of electrons, a Hubbard type model on a triangular lattice is an appropriate starting point to study much of the correlated physics of these systems. We consider a single band Hubbard model on a triangular lattice with variable electron filling and anisotropic hopping. Within mean-field theory we observe a transition at one-third filling on the isotropic lattice to a charge ordered antiferromagnet on a honeycomb sublattice at a critical interaction $U_c/t = 5.0$. We will also present preliminary results for other regions of the phase diagram.   

Electronic reconstruction of doped Mott insulator heterojunctions

Correlated electron heterostructures became a possible alternative when thin-film deposition techniques achieved structures with a sharp interface transition [1]. Soon thereafter, Okamoto \& Millis introduced the concept of ``electronic reconstruction'' [2]. We study here the electronic reconstruction of doped Mott insulator heterostructures based on a Cluster Dynamical Mean Field Theory (CDMFT) calculations of the Hubbard model in the limit where electrostatic energy dominates over the kinetic energy associated with transport across layers. The grand potential of individual layers is first computed within CDMFT and then the electrostatic potential energy is taken into account in the Hartree approximation. The charge reconstruction in an ensemble of stacked planes of different nature can lead to a distribution of electron charge and to transport properties that are unique to doped-Mott insulators.\\[4pt] [1] J. Mannhart, D. G. Schlom, Science 327, 1607 (2010).\\[0pt] [2] S. Okamoto and A. J. Millis, Nature 428, 630 (2004).   

Solving the Parquet Equations for the Hubbard Model beyond Weak Coupling

We find that imposing the crossing symmetry in the iteration process considerably extends the range of convergence for solutions of the parquet equations for the Hubbard model. When the crossing symmetry is not imposed, the convergence of both simple iteration and more complicated continuous loading (homotopy) methods are limited to high temperatures and weak interactions. We modify the algorithm to impose the crossing symmetry without increasing the computational complexity. We also imposed time reversal and a subset of the point group symmetries, but they did not further improve the convergence. We elaborate the details of the latency hiding scheme which can significantly improve the performance in the computational implementation. With these modifications, stable solutions for the parquet equations can be obtained by iteration more quickly even for values of the interaction that are a significant fraction of the bandwidth and for temperatures that are much smaller than the bandwidth. This may represent a crucial step towards the solution of two-particle field theories for correlated electron models.   

Correlation effects of one band hubbard model beyond the Gutzwiller Approximation

A novel scheme is introduced to go beyond the Gutzwiller approximation (GA). Starting from the scheme, we can see how the standard GA is recovered by relaxing physical constraints step by step. This not only adds to validity of the current scheme, but provides new insights into understanding the GA. Performance of the scheme on several testing cases is superior to the standard GA. We studied the one band Hubbard model in one, two and three dimensions to revisit relevant conclusions made under the GA as well as the slave boson formalism.   

Local electronic nematicity in the 2-dimensional Hubbard model

Recent measurements on magnetic and transport properties of some strongly correlated materials show a local electronic nematic phase which locally breaks the $C_4$ symmetry but keeps translational symmetry. We studied the 2-dimensional Hubbard model using the variational cluster approach and found a similar phase. The results showed this local nematicity is a property of the electron liquid and would appear even if there is no lattice distortion. Calculations of spin correlation were preformed and compared to results from magnetic neutron scattering of real materials, which showed a distinct asymmetry along x and y directions.   

Evolution of insulator-metal-insulator transitions under staggered lattice potentials

It is known that in the ionic Hubbard model metallic phases exist between band and Mott insulators in the presence of staggered lattice potentials. We investigate how the phase diagram depends on the strength of the staggered lattice potential, by means of the dynamical mean-field theory combined with the continuous-time quantum Monte Carlo method. Observed at finite temperatures is the crossover between metallic and band insulating phases while a first-order transition ending up with a critical point shows up between the metallic and Mott insulating phases. It is discussed how such transition behaviors evolve as the lattice potential grows at low temperatures.   

Mott transition and ferrimagnetism in the Hubbard model on the anisotropic kagome lattice

Mott transition and ferrimagnetism are studied in the Hubbard model on the anisotropic kagome lattice using the variational cluster approximation and the phase diagram at zero temperature and half-filling is analyzed. The ferrimagnetic phase rapidly grows as the geometric frustration is relaxed, and the Mott insulator phase disappears in moderately frustrated region, showing that the ferrimagnetic fluctuations stemming from the relaxation of the geometric frustration is enhanced by the electron correlations. In metallic phase, heavy fermion behavior is observed and mass enhancement factor is computed. Enhancement of effective spatial anisotropy by the electron correlations is also confirmed in moderately frustrated region, and its effect on heavy fermion behavior is examined.   

Graphene two-leg ladder. A system with conducting, insulating and superconductive properties

We use DMRG to study the ground-state phases of the Hubbard model defined on a one dimensional ladder of edge-sharing hexagons - a model which may be relevant to the electronic structure of polyacenes or graphene strips. At half filling we find a robust insulating phase with a large spin-gap, even at small U/t which, as a function of the strength of the third-neighbor hopping, exhibits a non-trivial cross-over from a band -insulator to a Mott insulator. The doped system exhibits a variety of conducting phases. Possible relevance of our results to the recently discovered high temperature superconductivity in K doped dibenzpentacene will be discussed, as well.   

Dominant superconducting fluctuations in the 1D extended Holstein-extended Hubbard model

The search for realistic 1D models that exhibit dominant superconducting (SC) fluctuations has a long history. In these 1D systems, the effects of commensurate band fillings - strongest at half-filling - and electronic repulsions typically lead to a finite charge gap and the favoring of insulating density wave ordering over superconductivity. We study a model - the extended Hubbard-extended Holstein (EHEH) model - with non-local electron-phonon interactions, in addition to electron-electron interactions. The EHEH model unambiguously possesses dominant superconducting fluctuations at half filling in a large region of parameter space. Using multi-scale functional renormalization group for the full model and a renormalization group for a bosonized form of the model, we prove the existence of dominant SC fluctuations in this model. Dominant SC fluctuations arise because the spin-charge coupling at high energy is weakened by the non-local electron-phonon interaction and the charge gap is destroyed by the suppression of the Umklapp process. The existence of the dominant SC pairing instability in this half-filled 1D system suggests that non-local boson-meditated interactions may be important in the superconductivity observed in high $T_c$ cuprate and organic superconductors.   

Ground state and finite temperature behavior of 1/4-filled zigzag ladders

We consider the simplest example of lattice frustration in the $\frac{1}{4}$-filled band, a one-dimensional chain with next-nearest neighbor interactions. For this zigzag ladder with electron-electron as well as electron-phonon interactions we present numerical results for ground state as well as thermodynamic properties. In this system the ground state bond distortion pattern is independent of electron-electron interaction strength. The spin gap from the ground state of the zigzag ladder increases with the degree of frustration. Unlike in one-dimension, where the spin-gap and charge ordering transitions can be distinct, we show that in the ladder they occur simultaneously. We discuss spin gap and charge ordering transitions in $\frac{1}{4}$-filled materials with one, two, or three dimensional crystal structures. We show empirically that regardless of dimensionality the occurrence of simultaneous or distinct charge and magnetic transitions can be correlated with the ground state bond distortion pattern.   

Phases of the two-leg Hubbard ladder in the large U limit

We study the phase diagram of the two-leg Hubbard ladder in the large U limit using the density matrix renormalization group (DMRG). Already in the limit of infinite on-site repulsion U, we find a rich phase diagram in which commensurability effects are unexpectedly prominent: A fully spin-polarized ``Nagaoka'' metallic phase occurs for electron density, n, in the range $1> n> n_1$, where $n_1 \approx 0.8$ is not obviously locked by any commensurability. There is an insulating, anti-ferromagnetic commensurate plaquette phase at n=3/4, and two-phase coexistence for $n_1 > n > 3/4$. For $3/4 > n > n_2 \approx 0.6$, there is a partially spin-polarized metallic state with a magnetization peak centered at n=2/3. For the most part, the ground state is a paramagnetic Luttinger liquid for $n_2 \geq n$, although an antiferromagnetic phase with a substantial charge gap (and which may or may not have a small spin-gap) arises at n=1/2. Interesting soliton excitations with fractional charge are found for the plaquette phase at n=3/4. We have also explored the evolution of these phases as a function of decreasing (but still large) U, both by studying the t-J model and of the underlying Hubbard model.   

Quantum Phase Transitions in an Ionic Hubbard Model in One Dimension

We study quantum phase transitions in an ionic Hubbard model in one dimension. This model accounts for electrons in alternating potentials with a lattice period of 2. For a specified alternating potential of strength $\Delta$, we change the local repulsion between spin-up and spin-down electrons, $U$, for the model to exhibit a quantum phase transition from a band insulator to a Mott insulator. Via exact diagonalization with a modified Lanczos method, we find that, as we tune $U$, the ground-state energy shows a level crossing at half-filling. We obtain a phase diagram of the model by using a finite-size scaling method. Also, we find an interesting feature of double occupancy around the phase transitions.   

Ground state properties of the ionic Hubbard model on a two-leg triangular ladder at 3/4 filling

We study numerically the ionic Hubbard model on a two-leg triangular ladder at 3/4 filling. This model is believed to be the minimal microscopic model describing the physics of Na$_{x}$CoO$_{2}$ at $x=0.5$, and shows a rich phase diagram that depends on a delicate balance between the Coulomb interaction $U$, the hopping amplitude $t$, and the ionic potential $\Delta$. Motivated by experiments analyzing the dopant distribution on Na$_{0.5}$CoO$_{2}$, we focus in the case of a stripe-type ionic potential. In the correlated limit where the Coulomb interaction is large, the ground state of the model is a charge-transfer insulator for large ionic potential and turns metallic for zero ionic potential. Electronic structure calculations point to the regime $\Delta\sim |t|, t<0$ as the one relevant for Na$_{0.5}$CoO$_{2}$, but previous calculations [1] have not fully clarify the nature of ground state of the model in such regime. The aim of this work is to study the metal-insulator transition that occurs in the region $U\gg\Delta\sim |t|$ of the phase diagram as well as the magnetic and charge structures of the associated ground states. \newline [1] J. Merino et al. Phys. Rev. B 80, 045116 (2009).   

Quantum Monte Carlo Studies on Attractive Hubbard Model with Anisotropic Spin-dependent Hopping on Two-leg Ladder Lattice

Using spin-dependent hopping, it is possible to have a fully paired state with a gap for single fermion excitation and gapless Cooper pair excitation, called `Cooper-pair Bose-metal' phase. Recently, the existence of this phase was suggested by a density matrix renormalization group studies\footnote{ Feiguin, A.E. and M.P.A. Fisher, \textit{Exotic paired phases in ladders with spin-dependent hopping.} Physical Review B, 2011. \textbf{83}(11): p. 115104.} on the attractive Hubbard Model with two-leg ladder geometry and anisotropic spin-dependent hopping. We here present a detailed Quantum Monte Carlo (QMC) studies on this model to investigate its finite temperature properties, including correlation function, s-wave and d-wave pairing function, and finally to deduce the existence and behavior of `Cooper-pair Bose-metal' phase.   

An Exotic Spin Mode in a Non-Polarized Fermi Liquid With Net Spin-Current

We find an exotic spin excitation in an ordered magnetic system with an order parameter with a net spin current but no net magnetization. Starting from a Fermi liquid theory, similar to that for a weak ferromagnet, this excitation emerges from a state that is protected by a Pomeranchuck instability. We derive the propagating mode using Landau kinetic equation, using two different approaches and find that the dispersion of the mode is the same for both approaches in leading order.   

Session B17: Focus Session: Thermoelectrics - Nanowires and Other Nanostructures

Thermoelectric Transport in Bismuth Telluride Nanoplates, Semiconductor Nanowires, and Silicide Nanocomposites: Effects of Low Dimensionality, Surface States, Interface Structures, and Crystal Complexity

This presentation will review recent measurement results of thermoelectric properties of individual bismuth telluride nanoplates, semiconductor nanowires, and silicide nanocomposites. In experiments with these realistic nanostructured materials, a number of factors influence the transport properties. For example, unintentional doping, interface roughness and impurities can often obscure the predicted effects of the low-dimensional electronic density of states and the protected surface states, the latter of which have been suggested for bismuth telluride and other thermoelectric materials, now also referred as topological insulators. Similarly, impurities and defects as well as contact thermal resistance can play an important role in phonon transport in nanostructures, making it nontrivial to quantify the actual effects of phonon-surface scattering and other intriguing low-dimensional phonon transport phenomena. Because of these experimental complications, diverse theoretical interpretations of experimental results have appeared in the literature, and will be discussed. Moreover, the effects of twin defects and crystal complexity on thermoelectric transport in nanostructures will be examined based on measurement results of III-V and silicide nanostructures.   

Universal Scaling Relations for the Thermoelectric Power Factor of Semiconducting Nanostructures

We present a model for the power factor (\textit{PF}) of wires and thin films which bridges between strongly confined and bulk behavior. Relevant scattering mechanisms are considered in the framework of the relaxation time approximation. Previous models for the transport properties of nanostructured thermoelectric materials predicted vast improvements in the \textit{PF} values over bulk due to discretization of the electron density-of-states function as the result of confinement. Using this model, we find that the \textit{PF} of nanowires and thin films in fact falls below the bulk value for most of the experimentally-accessible size range. We find a non-monotonic relationship between \textit{PF} and system size in all systems studied---regardless of the particular materials parameters and dominant scattering mechanisms. The effects of the size, dimensionality, temperature, carrier concentration and dominant scattering mechanism in single-carrier semiconductors will be discussed. In the framework of the \textit{constant} relaxation time approximation, universal scaling relations for the power factor of all single-carrier semiconductors are obtained.   

Thermal conductivity of silicon nanowires: interplay between core defects and surface roughness

Recent experiments [1] suggested that the thermal conductivity ($\kappa$) of Si nanowires may be reduced by about two orders of magnitude compared to that of bulk Si ($\kappa_{bulk}$), making them attractive materials for thermoelectric applications. Size reduction plays an important role [2] in determining such reduction but it does not fully account for recent measurements [1]. We investigated $\kappa$ in wires with 15 nm diameter (comparable to experimental sizes) using large scale molecular dynamics simulations. We show that, unlike the case of thin [3] (2-3 nm diameter) rods, the presence of an amorphous layer at the surface accounts only for a decrease by a factor of 4 in $\kappa$ with respect to that of wires with smooth surfaces. It is the combined effect of defects in the core and rippled surfaces that enables a decrease up to a factor of 90 with respect to $\kappa_{bulk}$. Work supported by DOE/SciDAC-e.\\[4pt] [1] A.I. Hochbaum et al., Nature 451, 163 (2008); A. I. Boukai et al., Nature, 451, 168 (2008).\\[0pt] [2] D.Li et al. APL 2003.\\[0pt] [3] D. Donadio and G. Galli, Phys. Rev. Lett. 102, 195901 (2009).   

Electric-Field Guided Synthesis of Standalone Nanowire Arrays for Thermoelectric Applications

Theoretical studies have suggested that figure of merits of thermoelectric materials can be improved through fabrications of nanoscaled thermoelectric materials. Thin films are expected to result in up to a seven fold improvement in efficiency over bulk materials; even greater enhancement, up to 15 times in efficiency, is expected for very thin wires. Researchers have already succeeded in increasing the efficiency by making thin-layered materials and nanowires of a non-thermoelectric material, i.e. silicone. For practical applications, however, arrays of standalone nanowires or isolated thermoelectric nanowire devices without any template will be required. Here I present an electromagnetic field guided nanostructured synthesis of an array of standalone thermoelectric nanowires. This technique utilizing electric field as a guide in building highly ordered nanostructures will be an elegant, ``bottom-up'' method for nanofabrication without the need of a template. An array of quasi-one dimensional chalcogenide nanowires has been successfully grown in between two conducting plates. Thermoelectric transport measurements including thermalconductivity, thermoelectric power and figure of merit can be easily performed in the device, without any need of complicated electron beam lithography technique.   

Thermoelectric Property Characterization of Suspended Silicon Nanowires

A key challenge in the measurement of thermal and electrical properties of suspended nanowires (NWs) is the ability to obtain clean, reliable electrical and thermal contact between the nanowire and device. We report on a technique for aligning NWs to sub-micron accuracy on a suspended device made of two SiNx membranes with the assistance of a polymethyl methacrylate (PMMA) carrier layer. Electrical and thermal contact is made by using electron beam lithography to pattern a window in the PMMA over the device electrodes, followed by oxide etching and surface passivation in wet etchant, metal deposition through a shadowmask, and lift-off. We demonstrate this technique on rough Si nanowires grown by metal-assisted chemical etching. The whole assembly is clean and without contamination. Thermoelectric properties and their correlation with crystal structure will be discussed   

Thermoelectric Properties of Low-Dimensional Si and Ge Based Nanostructures

Low-dimensional thermoelectric nanostructures based on Si and Ge are promising candidates for high performance energy conversion and generation applications. 1D nanowires (NWs) and 2D superlattices of Si, Ge and Si/SiGe have experimentally demonstrated excellent performance. In these confined systems the electrical conductivity, the thermal conductivity, and the Seebeck coefficient can be designed to some degree independently so as to achieve enhanced ZT values as compared to the related bulk material values. In this work, we calculate the thermoelectric coefficients of scaled Si and Ge NWs and thin-layers. We use the sp3d5s* tight-binding model for the electronic structure and linearized Boltzmann transport theory. Our calculations include structures of feature sizes up to 12nm containing over 5500 atoms. This study indicates that the confinement length scale can be exploited as a degree of freedom in designing the material properties. We examine n-type and p-type materials of different cross section sizes and confinement/transport orientations and provide optimization directions for power factor improvement. Finally, using measured values for the lattice thermal conductivity, the ZT is estimated.   

Gate Controlled Tuning of Seebeck Coefficient in InAs Nanowires

Here we present measurements of the Seebeck Coefficient in InAs nanowires grown by Chemical beam epitaxy. Nanowires were mechanically transferred onto a SiO2 substrate with a global metallic backgate, and Ohmic contacts to a single nanowire were made by standard electron beam lithography techniques. We were able to tune the measured thermovoltage in the nanowire by field effect gating and correlate this behavior with the conductance through the nanowire. Interestingly, large enhancements in the thermoelectric power factor were seen at low temperature for certain gate voltages. Such controllability allows for optimizing the thermoelectric response of the nanowire at different substrate temperatures.   

Phonon transmission and thermal transport in GaAs-AlAs superlattices from first-principles

Using the Green's function method and Landauer's formula, we formulate phonon transport in a 3D superlattice. The theory is harmonic and describes coherent (elastic) transport of heat through a periodic structure which may also have disorder present at its interfaces. We compute the force constants from first-principles density functional calculations and use them to compute the transmission, and the thermal conductance of a GaAs-AlAs superlattice versus length and temperature. We also investigate the effect of mass disorder at the interface and anharmonicity, to locate the transition from coherent to incoherent transport. Results are finally compared with experimental measurements.   

Phonon density of states in nanocrystalline Si1-xGex explored by inelastic neutron scattering

Recently there have been significant advances in the efficiencies of traditional thermoelectric compounds gained via the creation of thermoelectric nanocomposites possessing substantially reduced thermal conductivity relative to their bulk counterparts [1,2]. The dramatic reduction in the heat transport of these nanocomposites is often attributed to the increased interface scattering of phonons or induced surface/boundary modes; however notably little work has been put worth into exploring the detailed changes in the phonon density of states in many of these functional nanocomposite samples. Here we present inelastic neutron scattering measurements exploring the phonon density of states in a series of Si1-xGex thermoelectric nanocomposites. The evolution of the phonon spectral weight distribution and linewidths as a function of Ge-doping will be discussed and compared to the known bulk phonon density of states in this system. \\[4pt] [1] Giri Joshi et al., Nano Letters 8, 4670 (2008). \\[0pt] [2] X. Wang et al., App. Phys. Lett. 93, 193121 (2008).   

Ab-initio investigation of thermal transport in alloyed and nanostructured materials

A whole spectra of intriguing physical properties appears in conventional materials when structural features reach nanoscale. Since thermal conductivity is controlled by the heat carriers' mean free paths, it becomes of paramount importance to understand and engineer the role of alloying and nanostructuring on transport coefficients. First-principles calculations often provide accurate microscopic parameters, but at significant computational cost even for ideal, perfect systems. We present a hybrid classical-quantum method to compute thermal conductivity from both harmonic and anharmonic terms using Boltzmann transport formalism. We combine first-principles calculations of harmonic terms and force-field calculations of third-order and fourth-order force constant. Results for SiGe will be discussed to show the validity of approach. We also discuss the effects of nanostructuring by introducing boundary scattering contributions, as well as mechanisms of filler rattling in thermoelectric skutterudites.   

ZT enhancement using nanocomposite materials

The effect of interface scattering on the performance of disordered, nanocomposite thermoelectric materials is studied theoretically (within a linear response formalism), using effective medium theory, and direct numerics. The general relation between interfacial and bulk transport properties which results in an enhanced ZT is determined. Given these requirements of interfacial transport properties, a series of microscopic calculations of interface scattering are presented to assess the feasibility of using nanocomposites for ZT enhancement.   

Anderson Localization of Phonons in Random Multilayer Thin Film Thermoelectrics

Anderson localization of phonons in random multilayer thin films has been explored as a means for reducing latttice thermal conductivity in thermoelectric materials. Silicon based systems have been explored due to silicon's high crust abundance and Seebeck coefficient. Reverse non-equilibrium molecular dynamics simulations have been used to determine the thermal conductivity of silicon in which randomly selected atomic planes (20\% of lattice planes) are subject to mass increase. The simulation results indicate that the lattice thermal conductivity of silicon can be decreased by a factor of over ten thousand (to 15 $\mathrm{mW}/{m \cdot K}$). Based on models in which the charge carrier mean free path is limited by scattering from the planes with mass disorder, the mobility of silicon is expected to reach values of 10 $\mathrm{cm}^2/{V \cdot s}$. At this mobility the thermoelectric figure of merit, ZT, is found to be greater than ten when the mass ratio of the disordered planes to that of silicon approaches 10. These results indicate that the pursuit of nanostructured silicon thermoelectric materials in the form of random multilayers may provide a path to efficient and sustainable thermoelectric materials.   

Session B18: Focus Session: Nanostructures and Metamaterials, Growth, Structure, and Characterization -- Metamaterials with Gain and Active

Metamaterials with Gain

Nanoplasmonic metamaterials are the key to an extreme control of light and allow us to conceive materials with negative or vanishing refractive index. Indeed, metamaterials enable a multitude of exciting and useful applications, such as subwavelength focusing, invisibility cloaking, and ``trapped rainbow'' stopping of light. The realization of these materials has recently advanced from the microwave to the optical regime. However, at optical wavelengths, metamaterials may suffer from high dissipative losses owing to the metallic nature of their constituent nanoplasmonic meta-molecules. It is therefore not surprising that overcoming loss restrictions by gain is currently one of the most important topics in metamaterials' research. At the same time, providing gain on the nanoplasmonic (metamolecular) level opens up exciting new possibilities such as a whole new type of metamaterial nano-laser with a cavity length of about a tenth of the wavelength. The talk gives an overview of the state of the art of gain-enhanced metamaterials. Particular focus will be placed on nano-plasmonic metamaterials (such as double-fishnet metamaterials) with integrated laser dyes as gain medium. The successful compensation of loss by gain is demonstrated on the meta-molecular level. On the basis of a comprehensive, microscopic Maxwell-Bloch Langevin approach of spatio-temporal light amplification and lasing in gain-enhanced nanoplasmonic (negative-index) metamaterials a methodology based on the discrete Poynting's theorem is introduced that allows dynamic tracing of the flow of electromagnetic energy into and out of ``microscopic'' channels (light field, plasmons, gain medium). It is shown that steady-state amplification can be achieved in nanoplasmonic metamaterials. Finally, a complex spatio-temporal interplay of light-field and coherent absorption dynamics is revealed in the lasing dynamics of a nanoplasmonic gain-enhanced double-fishnet metamaterial.   

Metamaterials with gain and interpretation of transmission in pump-probe experiments

We establish a new approach for pump-probe simulations of metallic metamaterials coupled to the gain materials. It is of vital importance to understand the mechanism of the coupling of metamaterials with the gain medium. Using a four-level gain system, we have studied light amplification of arrays of metallic split-ring resonators (SRRs) with a gain layer underneath. We find that that the differential transmittance $\Delta T/T$ can be negative for SRRs on the top of the gain substrate, which is not expected, and $\Delta T/T$ is positive for the gain substrate alone. These simulations agree with pump-probe experiments and can help to design new experiments to compensate the losses of metamaterials.We numerically investigate loss compensation and transmission in pump-probe experiments in resonant metamaterials with gain using an FDTD algorithm coupled to semiclassical rate equations. We explain experimentally observed negative differential transmission by gain-dependent impedance.   

Spontaneous emission enhancement in a plasmonic nanocavity

Recently, metallic optical cavities containing coupled emitters have been fabricated that operate at visible frequencies. These cavities are capable of tightly confining light, greatly modifying the electromagnetic density of states in the location of the optical emitters. Here, we present measurements from a metal-based optical cavity that greatly modifies both the spectral and temporal characteristics of the coupled emitters. Our design is based on plasmonic coupling between a silver nanowire and a planar silver substrate, with a layer of optical emitters within the gap between the two silver components. The field confinement of the structure results in a 1000-fold enhancement of the spontaneous emission rate of the coupled emitters. These results suggest that metal-based optical cavities can allow quantum cavity electrodynamics of emitters such as colloidal quantum dots and organic dyes.   

Directionally emitting plasmon laser and circuits

Scaling down of the laser promises unprecedented ultra-dense and ultra-fast integrated photonics. The research of nanoscale lasers is rapidly advancing and a variety of approaches have been explored including whispering gallery lasers, photonic crystal lasers, metallic lasers. However, a major obstacle of integrating nanolasers with other components is the strong divergence of emission from a sub-wavelength laser cavity due to the diffraction of light. Here, we demonstrate a deep sub-wavelength plasmon laser that directs more than 70{\%} of its radiation into an embedded semiconductor waveguide. The laser naturally integrates photonic and electronic functionality allowing both efficient electrical modulation and wavelength multiplexing. A maximum modulation depth of 11 dB for a small 1 V of bias sweep is achieved. We demonstrate an ultra-compact plasmonic circuit integrating five independently modulated multi-colored laser sources multiplexed onto a single semiconductor waveguide, illustrating the potential of plasmon lasers for large scale, ultra-dense photonic integration.   

Electromagnetically induced transparency and absorption in metamaterials: self-consistent theory and experiments

There has recently been a lot of interest in slow-light metamaterials that provide transparency windows combining low absorption with high group delay. This phenomenon was explained by a two-resonator model involving a radiative resonator that couples directly to the incident field and a dark resonator that can only be excited through coupling with the radiative resonator. However, in our most recent experiments on wire/SRR metamaterials, we have observed a much richer behavior---we measure not only transparency windows with incisions in the absorption spectrum (electromagnetically induced transparency), but also narrow spectral features with absorption larger than the background absorption of the radiative element (electromagnetically induced absorption). We have developed a model in which the coupling of the electromagnetic waves to the radiative resonator is treated explicitly. An important attribute of this model, which is in excellent agreement with our experiments and full-wave simulations, is the self-consistent treatment of the spectral broadening of the bright resonator originating from the dipole radiation as opposed to the bare linewidth due to dissipation. We discuss the conditions under which electromagnetically induced transparency/absorption can be observed.   

Lighting up dark plasmon modes with electron beam

Dark plasmon modes couple weakly with free-space photons. It is, therefore, generally believed that far-field detection of dark plasmon modes is difficult. In this talk, we discuss how an electron beam can light up dark plasmon modes and show theoretically and experimentally that bright and dark modes in a single metal bowtie nanoantenna can be studied at the far field using cathodoluminescence spectroscopy.   

Direct modulation of lanthanide emission at sub-lifetime scales using electric and magnetic dipole transition

Lanthanide ions, such as trivalent Europium (Eu3+) and Erbium (Er3+) are technologically important, high quantum yield light emitters that exhibit both magnetic dipole (MD) and electric dipole (ED) transitions. It is well know that the transition rate of an emitter in an inhomogenous optical environment, e.g. an emitter near a mirror, is modified due to self-interference effects, leading to either enhancement or inhibition of spontaneous emission. However, due to the opposite symmetry of their emitted fields, ED and MD transitions exhibit differing self-interference. Here, we leverage this difference to show large spectral tuning and sub-lifetime dynamic modulation of Eu3+ emission. Specifically, we use a moving gold mirror to selectively enhance the ED and MD transitions in Eu3+ doped Y2O3. Controlling the emitter-mirror distance allows us to tune the emission spectra from 580 nm to 715 nm. Modulating the mirror position with a piezoelectric crystal allows us to dynamically tune the Eu3+ emission at speeds faster than the excited state lifetime.   

Strong Coupling of Molecular Absorption and Mid-Infrared Metamaterial Resonances

Here we present numerical, analytical, and experimental evidence for strong optical coupling between a mid-infrared perfect absorber thin-film metamaterial and a molecular absorption resonance within a dielectric medium. Clear anti-crossing behavior is exhibited numerically and experimentally when the metamaterial resonance is scanned through the dielectric molecular resonance; a coupled oscillator model is used to present a further analytical description. The anti-crossing is used to demonstrate and quantify the strength of the optical coupling within the thin film. Such a device can potentially be developed for mid-infrared sensing applications and actively tunable metamaterial optical components.   

Magnetic Dipole Emission Lines in Trivalent Lanthanide Ions

Magnetic dipole (MD) transitions offer a unique opportunity to study light-matter interactions beyond the electric dipole approximation. Modified spontaneous emission from MD transitions can serve as an ideal way to study optical magnetic fields in metamaterials and related nanostructures. While many MD absorption lines have been identified, only a few MD emission lines in solid state systems are widely known. Here we seek to expand the known MD emission lines by calculating all such transitions in the trivalent lanthanide ions. We use intermediate coupling to construct a detailed free-ion Hamiltonian including electrostatic, spin-orbit, two- and three-body, spin-spin, spin-other-orbit, and electrostatically correlated spin-orbit interactions. Determining the free-ion energy levels allows the calculation of ground-state MD absorption lines, and their respective oscillator strengths, as well as spontaneous emission rates for all MD mediated transitions. Many strong MD emission lines are predicted throughout the visible and near infrared spectrum. Corresponding electric quadrupole transitions were also calculated and found to have negligible contribution. If time permits, we will also present direct experimental measurements characterizing these new MD emission lines.   

Self-assembled quantum dots as active emitters for silicon photonic crystal nanocavities

We present a study of the properties of active (Si)Ge self-assembled quantum dot (QD) emitters coupled to Si photonic crystal (PC) nanocavities up to room temperature and evaluate the potential of such nanostructures for the realization of efficient silicon based light sources in the near-infrared regime. The Si-Ge system allows the formation of type I and type II band alignment QD, depending on the epitaxy conditions. We first discuss the emission properties of different types of QD. In particular we present type I SiGe islands designed to produce a strong emission due to a large wave function overlap of confined electrons and holes. We then investigate the coupling properties of (Si)Ge QD emitters to 2D Si PC cavities. We interpret the experimentally observed correlation between the photoluminescence intensity of the QDs and the quality factor of the PC cavity using simulations based on a dissipative master-equation approach [1]. As a further step towards efficient light sources, we currently study the coupling of electrically pumped (Si)Ge QDs to 2D PC in contacted diode structures. [1] N. Hauke, et al. Phys. Rev. B 84, 085320 (2011)   

Broadband Transparency of Graded Anisotropic Metamaterial

We have investigated the scattering of electromagnetic waves from a graded anisotropic sphere whose dielectric permittivity is radially anisotropic, with different radial and tangential components described by the graded Drude model. The Rayleigh scattering cross section (RSCS) has been calculated analytically and numerically by the Rayleigh scattering theory. The electric polarization and the electromagnetic field distribution are also examined in a quasi-static condition. Due to the scattering cancellation mechanism, the results reveal that the RSCS can be rather smaller over a wider frequency range, indicating the so called broadband transparency. Furthermore, compared with the non-graded and graded isotropic structure, the anisotropic structure leads a better broadband transparency.   

Reconfigurable Gradient Index using VO$_2$ Memory Metamaterials

We have demonstrated tuning of a metamaterial device that incorporates a form of spatial gradient control. Electrical tuning of the metamaterial was achieved through a vanadium dioxide layer which interacts with an array of split ring resonators. Through design of the device and contact geometry, we achieved a spatial gradient in the magnitude of permittivity, writeable using a single transient electrical pulse. This induced gradient in our device was observed on spatial scales on the order of one wavelength at 1 THz. Thus we have demonstrated the viability of elements for use in future devices with potential applications in beamforming and communications.\footnote{M. D. Goldflam, \textit{et. al.}, Appl. Phys. Lett. 99, 044103 (2011).} Various contact geometries are currently being investigated with the goal of implementing finer control over gradients and expanding on the possible applications of such devices.   

Waves and rays in meta-microcavities: from positive ``n'' to negative ``n'' and from order to chaos

The emergence of metamaterials and, in particular, negative index metamaterials (NIMs) triggers reconsideration of many fundamental physical phenomena. Importantly, the majority of unique properties of NIMs stand out when NIMs are combined with conventional positive index materials (PIMs). In this talk, we consider the wave and ray properties of electromagnetic wave interaction in two-dimensional microcavities that contain a combination of PIM and NIM with negative dielectric permittivity and magnetic permeability. We consider closed and open cavities and total and partial reflection and refraction at the boundaries between the media with different indices of refraction. By using a combination of analytic and numerical methods we are able to classify the different types of possible solutions in this type of mixed PIM-NIM regions. In particular, we derive the special properties of whispering gallery modes as well as the constructive and destructive interference due to wave refraction/reflection across different refractive media boundaries. Finally, we discuss possible practical applications of this type of mixed refractive indices microcavities.   

Session B19: Invited Session: One Hundred Fifty Years of Maxwell's Equations (1862-2012)

The discovery of Maxwell's equations

In January 1865, Maxwell at age 34 wrote a letter to his cousin Charles Cay describing various doings, including his work on the viscosity of gases and a visit from two of the world's leading oculists to inspect the eyes of his dog ``Spice''. He added, ``I have also a paper afloat, with an electromagnetic theory of light, which, till I am convinced to the contrary, I hold to be great guns.'' That paper ``A Dynamical Theory of the Electromagnetic Field'' was his fourth on the subject. It was followed in 1868 by another, and then in 1873 by his massive two volume \underline {Treatise on Electricity and Magnetism}. Even so, by the time of his death in 1879 as he was beginning a radically revised edition of the \underline {Treatise}, much remained to be done. We celebrate here the 150$^{th}$ anniversary of Maxwell's first astonished realization in 1862 of the link between electromagnetism and light. So revolutionary was this that 15 or more years went by before Lorentz, Poynting, FitzGerald, and others came to address it, sometimes with improvements, sometimes not. Not until 1888 did Hertz make the essential experimental discovery of radio waves. What is so remarkable about Maxwell's five papers is that each presents a complete view of the subject radically different from the one before. I shall say something about each, emphasizing in particular Maxwell's most unexpected idea, the displacement current, so vastly more interesting than the accounts of it found in textbooks today. Beyond lie other surprises. The concept of gauge invariance, and the role the vector potential would play in defining the canonical momentum of the electron, both go back to Maxwell. In 1872 came a paper ``On the Mathematical Classification of Physical Quantities'', which stands as an education in itself. Amid much else, there for the first time appears the distinction between axial and polar vectors and those new operational concepts related to quaternion theory: \textit{curl, divergence, }and\textit{ gradient}.   

Maxwellians and the Remaking of Maxwell's Equations

Although James Clerk Maxwell first formulated his theory of the electromagnetic field in the early 1860s, it went through important changes before it gained general acceptance in the 1890s. Those changes were largely the work of a group of younger physicists, the Maxwellians, led by G. F. FitzGerald in Ireland, Oliver Lodge and Oliver Heaviside in England, and Heinrich Hertz in Germany. Together, they extended, refined, tested, and confirmed Maxwell's theory, and recast it into the set of four vector equations known ever since as ``Maxwell's equations.'' By tracing how the Maxwellians remade and disseminated Maxwell's theory between the late 1870s and the mid-1890s, we can gain a clearer understanding not just of how the electromagnetic field was understood at the end of the 19th century, but of the collaborative nature of work at the frontiers of physics.   

Using Maxwell's Equations in the late 1800s

Between the publication of Maxwell's \textit{Treatise on Electricity and Magnetism }in 1873 and the early 1900s his field equations were not considered to be fundamental by many Cambridge-trained physicists Instead, they were thought to derive from Hamilton's principle given an appropriate energy expression. Such an expression usually assigned a velocity or a position function to field quantities, though this was not invariably done. Precisely because the Hamiltonian, and not the derivative field equations, was taken to be basic, new effects could be generated by adding terms to the energy expression. This was how the Faraday and Kerr magneto-optic effects were handled. The program however never did generate a method for incorporating dissipative phenomena, as Oliver Heaviside (who disliked the use of Hamilton's principle) demonstrated. The procedure was in the end decisively abandoned when J. G. Leathem, a student of Joseph Larmor a Cambridge, demonstrated that it could not handle a particularly subtle magneto-optic process.   

Taking Off From Maxwell's Equations

I will discuss how we are still discovering new continents in the conceptual world opened up by Maxwell's equations. I will very briefly sketch, in particular, how very natural extensions of those equations describe superconductivity and the deep structure of fundamental particle interactions. Then I'll show how a newer cluster of extensions connects ideas about (perhaps no longer all that) exotic quantum particles and materials.   

Session B20: Invited Session: Astronomy's Detectors and Physics Education

The basic physics of astronomical detectors, our eyes on the Universe

The universe is an amazingly huge place. While humankind has directly explored Earth's sister planets with space probes, we don't have the means to venture beyond the solar system, and so almost all information about the universe comes from sensing light that happens our way. Astronomy is constantly striving to find better ways to sense the feeble amount of energy from distant stars and galaxies. This quest has led to a new generation of large telescopes on the ground and in space. Possibly more important than the development of bigger telescopes is the rapid advancement in solid state detector technology. In the x-ray, visible and infrared wavelengths, the most advanced detectors are based on two fundamental technologies: (1) nearly perfect detector materials that efficiently convert photon energy to electrical charge, and (2) very sensitive transistors that convert a few electrons into a measurable voltage. This talk presents the basic physics of astronomical detectors and provides an introduction to the more specialized talks that follow in this session of presentations. Since detectors of light are critical to nearly every aspect of scientific research and involve a wide range of physical phenomena, this session of talks will provide the audience with physics lessons that can be readily incorporated in an undergraduate physics curriculum.   

Advanced CCD and CMOS image sensor technology at MIT Lincoln Laboratory

The Advanced Imaging Technology (AIT) program area at Lincoln Laboratory addresses a broad range of complex imaging problems by using a wide variety of silicon-based imager technologies, including charge-coupled devices (CCDs), active-pixel sensors (APSs), photodiode arrays, and Geiger-mode avalanche-photodiode (GMAPD) arrays that are single-photon sensitive. Many of these devices, including some very large imaging devices that require low defect levels, are fabricated by us from silicon wafers in our class-10 Microelectronics Laboratory. We also operate a fully equipped packaging facility that is capable of developing and performing innovative device packaging of imaging (and other) devices. In this talk, we present an overview of Lincoln Laboratory's image sensor technologies, describe how they work and how they are built. We discuss several imaging device parameters can be optimized for high sensitivity. These include quantum efficiency (including fill-factor), charge-transfer efficiency (moving the charge from the pixel to the output port without loss or added spurious charge), and the noise to read this charge out. The overall goal is to convert most or all of the photons that impinge on the device to photoelectrons and then to read out these photoelectrons without losing any and without adding read noise. Further, we will describe design elements or methods that can help with different specific applications: the orthogonal-transfer CCD (OTCCD), an electronic shutter for back illuminated imagers, the Geiger-mode avalanche photodiode (GMAPD) circuit element, and three-dimensionally integrated CMOS focal planes. Several examples of application to high-sensitivity, high-speed, and broad-wavelength range problems will presented.   

Gamma-ray Bursts, Black Holes, and Exoplanets: How CCD Detectors have Revolutionized Astronomy

I will tell the story of my research group's role in the development of astronomical charge-coupled detectors (CCDs) by relating the contributions of four MIT research students to projects which we have undertaken together over the past three decades. These projects have empowered observations extending over four decades of the electromagnetic spectrum, enabling discoveries ranging from gamma-ray burst emitting collapsars at cosmological distances, to accretion-driven black holes in the Galaxy, and to exoplanets in the solar neighborhood. This story will illustrate the key contributions which student researchers can make when a novel detector technology arrives on the scene. Finally, I will also describe some of the ways in which their early education in these possibilities has impacted my students' future careers as astronomers and experimental physicists.   

Coupling physics to understanding the performance of detector arrays

Over the last few decades developments in microelectronics have led to the development of arrays of detectors that can be used to measure unprecedentedly small levels of signal. Such arrays have been used over to detect electromagnetic radiation ranging in energy from the X-ray through sub-millimeter wavelengths and also particles. Perhaps nowhere have the improvements been more astonishing than in devices available for the visible part of the spectrum (400 -- 1000 nm). The most successful detector array in this spectral region is the Charge Coupled Detector (CCD) whose inventors were recognized with the Nobel Prize in Physics in 2009. In this talk I will review some of the detectors and technologies that are used in low light level imaging. I will also describe a full year sequence of classes (i.e. a theory class, a CCD camera building class and a CCD camera performance measurement class) that students at the Rochester Institute of Technology can take to make them knowledgeable as to the physics underlying the operation and performance of such detector arrays. Finally I will discuss the associated laboratory classes that students must take to measure the performance of the camera they have built and what aspects of fundamental physics are integrated into their understanding. These classes have been taken by both calculus and non-calculus trained students. The classes appeal to students with both types of backgrounds as it couples an understanding of Physics to something that they have built and use.   

Detecting the Cosmic Microwave Background at the Frontier of Cosmology and in the Classroom

The 3K blackbody Cosmic Microwave Background (CMB), while exceedingly faint, is the most abundant light in the Universe, permeating all of space as a relic of the hot, dense, primordial fireball. Its detection in 1965 established the Big Bang as the standard model of cosmology and earned its co-discoverers Penzias and Wilson a Nobel Prize. Over the past two decades, advances in detector technology driven by CMB research have produced telescopes with ever-increasing numbers of photon background-limited microwave detectors, capable of mapping fine structure of the CMB to micro-Kelvin precision. These have had enormous impact, determining the geometry of the universe, quantifying the dark matter and dark energy that dominate it, and detecting the faint polarization arising from the primordial seeds of structure. The current frontier is defined by new arrays of thousands of superconducting, polarized detectors producing maps approaching nano-Kelvin precision. In this decade, these measurements will answer questions about the physics driving the earliest moments of the Big Bang and will survey the large-scale structure of the universe, determining neutrino masses and constraining the nature of dark energy. The advanced detector technology fueling this frontier provides superb device-physics training for graduate students and postdocs working on current-generation CMB telescopes. At the same time, careful experimental techniques developed for CMB observations can now be combined with inexpensive high-quality satellite TV detectors to allow even undergraduates to detect the CMB, reproducing Penzias and Wilson's famous discovery. I describe one such undergraduate class at Harvard, which builds CMB telescopes from scratch in a few weeks with a modest budget, teaching students about microwave techniques and detectors and allowing them to find their own evidence for the Big Bang.   

Session B21: Focus Session: Search for New Superconductors: Methodologies and New Materials

An Enlightened Combinatorial Search for New Superconductors

I describe a methodology for the fast search for new superconducting materials. This method consists of a parallel synthesis of a highly inhomogeneous alloy covering large areas of the metallurgical phase diagram combined with a very sensitive, fast, microwave-based method, which allows large non-superconducting portions of the sample to be discarded. Once an inhomogeneous sample containing a minority phase superconductor is identified, we revert to well-known, thorough identification methods, which include standard physical and structural methods. We show how a systematic structural study helps in avoiding miss-identification of new superconducting materials when there are indications from other methods of new discoveries. The application of these ideas to the La-Si-C system, which exhibits promising normal state properties, sometimes correlated with superconductivity, will be discussed. Although this system shows indications of a new superconducting compound, the careful analysis described here shows that the superconductivity in this system can be attributed to intermediate binary and single phases. Searches in other Rare Earth-Si based systems will also be described. Work done in collaboration with J. de la Venta, Ali C. Basaran, J. G. Ramirez, T. Grant, A. J. S. Machado, M. R. Suchomel, R. T. Weber, Z Fisk, P. Guptasarma, O. Shpyrko and D. Basov.   

Search for New Superconductors in RE-Si and Al-B Systems: a High Pressure High Temperature Approach

We have searched for the presence of superconductivity in the RE-Si and Al-B systems using HP-HT synthesis. The RE-Si system has some of the common features that are present in high TC superconducting materials. We have synthesized Ce, Pr, Nd and Gd silicides undoped and doped with C and B. On the other hand, AlB$_{2}$ has some similarities with the superconducting MgB$_{2}$. We have tried to synthesize AlB$_{2}$ way off stoichiometry using HP-HT and thin films Phase Spread Alloy. The Al$_{0.67}$ B$_{2}$ would be the MgB$_{2}$ equivalent and good reason to expect superconductivity. We discuss the results for both systems after a careful analysis of several physical properties (SQUID, Modulated Microwave Absorption) and x-ray powder diffraction.   

Search for new materials: phase spread alloy thin film fabrication and characterization

We use the phase spread alloy (PSA) method of fabricating compositionally heterogeneous thin films as an efficient way to produce and screen new, interesting materials (e.g. superconductors, magnetoresistive compounds, etc.). This method uses co-sputtering to deposit material with smoothly varying element concentration across a substrate. Both local and non-local probes are used to verify the composition of the sample. Using the La-Si-C system as an example, we perform x-ray fluorescence from a synchrotron source, x-ray diffraction from a lab source, atomic force microscopy, and infrared spectroscopy on one sample to verify the presence of different phases and their properties.   

Search for new superconductors in rare-earth silicide systems

We have searched for the presence of superconductivity in the RE$_{5}$Si$_{3}$ system doped with C or B as a light element (RE: La, Ce, Pr, and Eu). High temperature superconductors and RE$_{5}$Si$_{3}$ systems have some common properties. Both systems have a layered tetragonal crystal structure. They are multi-element compounds and are also doped with a light element to introduce the superconductivity. In this study, multiphase bulk samples were made using arc-melting. Phase spread alloy thin films were also prepared in a sputtering system. We used magnetic field modulated microwave absorption spectroscopy (MFMMS), which is a very sensitive contactless technique to detect superconductivity, as the first screening for the existence of superconductivity in an inhomogeneous sample. We will present some of our results from MFMMS and SQUID measurements and compare them with structural refinement from X-Ray data.   

Superconductivity in the K-Mo-O system

The rutile-type structure belongs to space group P4$_{2}$\textit{/mnm}. Some transition metals form dioxides with variants rutile structure are known as pseudorutiles. These dioxides have interesting physical properties but they are still poorly understood. MoO$_{2}$ is one of them. Polycrystalline samples of MoO$_{2}$ can be easily prepared using stoichiometric amounts of Mo and MoO$_{3}$ through solid state reaction at temperatures near 700\r{ }C. This material is a highly conductive oxide and exhibits Mo-Mo metallic bounds along $c$-axis. On the other hand, previous results show that the physical properties of the MoO$_{2}$ are substantially changed with potassium doping [1]. This work unambiguously demonstrates that the K$_{x}$MoO$_{2}$ system exhibits superconductivity. Electrical resistivity and magnetization measurements were carried out from 2 to 300 K. The electrical and magnetic measurements show that the superconducting critical temperature ranges from 3 to 10 K. The phase composition responsible for the superconductivity is still under investigation. \\[4pt] [1] L. M. S. Alves et al., Phys. Rev. B \textbf{81, }174532 (2010).   

High spin-low spin transition in insulating CaMn$_2$Sb$_2$

Layered manganese pnictides are often interesting compounds to compare with the iron pnictide superconductors. To this end, we have synthesized high quality flux-grown single crystals of CaMn$_2$Sb$_2$, which forms in a trigonal CaAl$_2$Si$_2$-type structure characterized by corrugated triangular Mn layers. Previously reported as a bad metal, we observe instead that this compound exhibits a distinct insulating trend in temperature-dependent resistivity measurements, including an enhancement of up to two orders of magnitude between 200 K and T$_N$ = 85 K. Measurements of ac susceptibility exhibit an orientation- and highly field-dependent plateau across the same temperature range, while heat capacity measurements reveal a sharp feature at 85 K as well as a broad anomaly centered near 195 K. Curie-Weiss behavior above 300 K indicates the presence fluctuating moments with prevailing ferromagnetic interactions, corresponding to less than half the static moment reported for the antiferromagnetic ordered state. These results imply a temperature-induced high spin-low spin insulator-insulator transition.   

An infrared study of electron delocalization in Mn-based relatives of the pnictides

Current data suggest that the viability of parent compounds to become superconducting is intimately tied to electron correlations.\footnote{M. Quazilbash \textit{et. al.} Nature Physics 5, 647 (July 2009)}$^,$\footnote{P.A. Lee \textit{et. al.} Reviews of Modern Physics 78, 17 (January 2006)} Further comparisons between the ground states of the cuprate and pnictide parent compounds indicate that doping across an electron delocalization transition (EDT) may be key to obtaining a higher critical temperature.\footnote{J. Simonson \textit{et. al.} ArXiv:11105938} These insights lead us to study the Mn-based compounds that are isostructural with pnictides and are antiferromagnetic insulators like the cuprates. Specifically, we have explored the effects of doping on LaMnPO$_{1-x}$F$_x$ and Ca$_{1-x}$La$_x$Mn$_2$Sb$_2$ via optical spectroscopy, transport, and magnetic measurements in parallel to theoretical band structure calculations. Our studies show that LaMnPO is highly resistant to electron delocalization. Likewise, in CaMn$_2$Sb$_2$, full delocalization was not attained even though a shift in the band edge was observed.   

Strong Electronic Correlations in YMn$_{2}$Ge$_{2}$

Exotic phases, like superconductivity, often emerge near electron delocalization transitions in strongly interacting systems. Magnetization, heat capacity and resistivity measurements were performed on single crystals of the antiferromagnetic metal YMn$_{2}$Ge$_{2}$, which is isostructural to the ThCr$_{2}$Si$_{2}$-type iron pnictides. Above the antiferromagnetic ordering temperature T$_{N}$=425 K, the magnetic susceptibility displays Curie-Weiss like behaviour with a fluctuating moment $\mu$ = 3.3 $\mu_{B}$/Mn atom, larger than the ordered moment of 2.2 $\mu_{B}$/Mn atom. Heat capacity measurements yield a Sommerfeld coefficient $\gamma$ = $\frac{C}{T}$ = 8.5 mJ/mol Mn K$^{2}$, nearly three times larger than $\gamma_{Ru}$ = 3.3 mJ/mol Mn K$^{2}$ for its non-magnetic isostructual analog YRu$_{2}$Ge$_{2}$, indicating strong electronic correlations in YMn$_{2}$Ge$_{2}$. The quasiparticle mass enhancement $\frac{m*}{m_{Ru}}$ = $\frac{\gamma}{\gamma_{Ru}}$ = 2.6 is similar to the value observed in the 122-type iron pinctides. Fermi-liquid behaviour of the resistivity $\rho = \rho_{0} + A T^{2}$ is observed over a very broad range of temperatures between 0.5 K and 300 K, with the resistivity at low temperature $\rho$(0.5 K) = 8 $\mu\Omega$ cm indicating high sample quality   

Magnetic, Thermal and Transport Properties of LaNi$_2$(Ge$_{1- x}$P$_x$)$_2$

Polycrystalline samples of LaNi$_2$(Ge$_{1-x}$P$_x$)$_2$ ($x=$ 0, 0.25, 0.50, 0.75, 1) with the tetragonal ${\rm ThCr_2Si_2}$ structure were investigated by heat capacity $C_{\rm p}$, magnetic susceptibility $\chi$, and electrical resistivity $\rho$ measurements for temperatures $1.8~{\rm K}\leq T \leq 300$~K\@. The $\rho(T)$ data for each sample reveal metallic behavior that follows the Bloch-Gr\"uniesen theory. The low-$T$ $C_{\rm p}(T)$ data for the series yielded Sommerfeld coefficients $\gamma = 6$--12~mJ/mol\,K$^2$ and Debye temperatues $\Theta_{\rm D} = 300$--480~K\@. The $\chi(T)$ data showed nearly $T$-independent paramagnetism except for LaNi$_2$Ge$_2$, where data up to 1000~K exhibit a broad peak at $\approx 300$~K\@. A possible onset of superconductivity is seen for ${\rm LaNi_2P_2}$ at 2.1~K\@. Analytic functions accurately representing the Bloch-Gr\"uniesen and Debye functions are presented that are very useful for fitting $\rho(T)$ and lattice $C_{\rm p}(T)$ data, respectively.   

Superconductivity in WO2.6F0.4 synthesized by reaction of WO3 with Teflon

WO3-xFx (x $<$ 0.45) perovskite-like oxyfluorides were prepared by a chemically reducing fluorination route using the polymer polytetrafluoroethylene (Teflon). The symmetry of the crystal structures of WO3-xFx changes from monoclinic to tetragonal to cubic as the fluorine content increases. Fluorine doping changes insulating WO3 to a metallic conductor, and superconductivity (Tc = 0.4 K) was discovered in the samples with fluorine contents of 0.41 $<$ x $<$ 0.45. This easy fluorination method may be applicable to other systems and presents an opportunity for finding new oxyfluoride superconductors.   

Type-I Superconductivity in Ytterbium Diantimonide

The layered antimonide compound YbSb$_2$ crystallizes with a ZrSi$_2$-type orthorhombic structure, different from other rare earth diantimonides. Unusual for a binary compound, Type-I superconductivity has been observed in YbSb$_2$. In this talk, we present the results from anisotropic magnetization, resistivity, heat capacity and magneto-optical Faraday effect measurements on YbSb$_2$ single crystals, showing a clear superconducting transition at $T_c$ = 1.25 K. The estimated electron-phonon coupling $\lambda$ = 0.51, together with the jump in electronic specific heat $\Delta C_{es}/\gamma T_{c}$ = 1.36, indicate the system to be a weak-coupling BCS superconductor. Magnetization, as well as heat capacity measured under field, clearly suggest a Type-I behavior, which is confirmed by the estimated Ginzburg-Landau parameter $\kappa$ = 0.13. According to the $H$-$T$ phase diagram, the critical field $H_c$ is around 60 Oe.   

Cu$_{1-x}$BiSO: the first Fe-pnictide-structured compound without Fe or Pnictogen

The electronic structure of 1111 transition metal pnictides offers a large variety of low-energy phenomena depending on the electronic filling explored. Based on first principles calculations I study the electronic filling $d^{10-x}$ represented by Cu$_{1-x}$BiSO: a band insulator that becomes metallic upon hole doping. I argue that the electron-phonon coupling is very strong in this material, and probably drives superconductivity. The critical temperature is however strongly depressed by the proximity to ferromagnetism. The competition between these two different order parameters brings about a high tunability of the system that can go from conventional to unconventional superconductivity by varying such parameters as doping or pressure.   

Possible Evidence for Novel Superconductivity in LaRu$_{3}$Si$_{2}$

Superconductivity in LaRu$_{3}$Si$_{2}$ with the honeycomb structure of Ru has been investigated. It is found that the normal state specific heat C/T exhibits a deviation from the Debye model down to the lowest temperature. A relation C/T = $\gamma _{n}+\beta $T$^{2}$-ATlnT which concerns the electron correlations can fit the data very well. The suppression to the superconductivity by the magnetic field is not the mean-field like, which is associated well with the observation of strong superconducting fluctuations. The field dependence of the induced quasiparticle density of states measured by the low temperature specific heat shows a non-linear feature, indicating the significant contributions given by the delocalized quasiparticles. The Wilson ratio estimated here is about 3.3, indicating also a strong correlation effect. Interestingly, the Fe-doping can suppress the superconductivity very easily, while Co-doping kills the superconductivity very slowly. The possible reasons are discussed. All these results suggest that the electronic correlation effect exists in LaRu$_{3}$Si$_{2}$ and superconductivity may be novel.   

Session B22: Focus Session: Fe-based Superconductors- ARPES and Fermi Surfaces

Distinct Fermi Surface Topology in A$_x$Fe$_{2-y}$Se$_2$ Revealed by ARPES

The discovery of superconductivity with a transition temperature above 30 K in A$_x$Fe$_{2-y}$Se$_2$ (A=alkali metal or Thallium) triggered a new wave of research on the iron-based superconductors. The new A$_x$Fe$_{2-y}$Se$_2$ superconductor exhibits many unique characteristics which make it a new platform to uncover the pairing mechanism of the iron-based superconductors. In this talk, we will show the electronic structures of A$_x$Fe$_{2-y}$Se$_2$ by means of ARPES, including Fermi Surface topology, superconducting gap structure and electron dynamics. The observed Fermi surface topology with only electron-like pockets is distinct from other iron-based superconductors and challenges the pairing mechanism based on scattering between electron-like and hole-like Fermi surface sheets. Nearly isotropic superconducting gap without nodes is revealed for all Fermi surface sheets which favors s-wave pairing symmetry. Other related topics, such as Fe vacancy order and phase separation, will also be discussed from viewpoint of our ARPES results. \\[4pt] [1] D. Mou, S. Liu, J. He, et. al, Phys. Rev. Lett. 106, 107001 (2011).\\[0pt] [2] L. Zhao, D. Mou, S. Liu, et al., Phys. Rev. B 83, 140508(R) (2011).   

Role of Degeneracy, Hybridization, and Nesting in the Properties of Multi-Orbital Systems

To understand the role that degeneracy, hybridization, and nesting play in the magnetic and pairing properties of multiorbital Hubbard models we here study numerically two types of two-orbital models, both with hole-like and electron-like Fermi surfaces (FS's) that are related by nesting vectors ($\pi$, 0) and (0, $\pi$) [1]. In one case the bands that determine the FS's arise from strongly hybridized degenerate dxz and dyz orbitals, while in the other the two bands are determined by non-degenerate and non-hybridized s-like orbitals. In the weak coupling regime it is shown that only the model with hybridized bands develops metallic magnetic order, while the other model exhibits an ordered excitonic orbital-transverse spin state that is insulating and does not have a local magnetization. Thus this state would be observed by ARPES experiments, but not by neutron scattering. However, both models display similar insulating magnetic stripe ordering in the strong coupling limit when Coulomb interactions create strong hybridization of the orbitals.\\[4pt] [1] A. Nicholson, et al., Phys. Rev. B 84, 094519 (2011).   

Thermoelectric power of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$, $0\le x\le 0.025$: tracking changes in Fermi surface topology

Temperature-dependent, in-plane, thermoelectric power (TEP) data will be presented for single crystals of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$, ($0\le x \le 0.025$). Previously reported TEP data for this system showed a big jump in the TEP data from x=0.02 to x=0.024 suggesting a Lifshitz transition, a result which was later confirmed by ARPES measurement. Given that TEP and ARPES delineated a rather large region for the Lifshitz transition to occur, and the underdoped side of the phase diagram is poorly explored, newly careful measurements of TEP on tightly spaced concentrations of Co, $0\le x \le 0.025$, were carried out. The data show clear evidence of a Lifshitz transition, but instead of a discontinuous jump in TEP between $0\le x \le 0.025$, there is a more gradual evolution in the S(T) plots as x is increased.   

Angle-resolved photoemission spectroscopy study of Ba(Fe$_{1-x}$Ru$_{x})_{2}$As$_{2}$

Ru-doped BaFe$_{2}$As$_{2}$ compounds were discovered to show superconductivity in a relatively wide doping range. We have performed angle-resolved photoemission spectroscopy measurements on a series of Ru-doped BaFe$_{2}$As$_{2}$ samples. We observed that band dispersions become more three-dimensional and Fermi velocities increase significantly with Ru doping. We will report these results and discuss implications to its superconductivity.   

What rome does the Fermi surface play in tuning the properties of iron arsenic superconductors?

External control parameters such as pressure or chemical substitution are the key to extend the phase space and achieve high temperature (T$_{c})$ superconductivity in the FeAs family. These materials show interesting properties where it is important to understand the role of Fermi surfaces (FS's) in the mechanism of yielding higher T$_{c}$. Here, we use angle-resolved photoemission to study the electronic structure of the Ba(Fe$_{1-x}$Ru$_{x})_{2}$As$_{2}$ as a function of Ru concentration ($x)$. We find that the substitution of Ru for Fe is isoelectronic, i. e., it does not change the value of the chemical potential. More interestingly, there are no measured significant changes in the shape of the FS or in the Fermi velocity over a wide range [1]. We contrast this unusual behavior with the Co substitution, where even small substitutions induce large changes not only in the size of the FS pockets but also in the FS topology [2]. Given that the suppression of the antiferromagnetic and structural phase has been associated with the emergence of the superconducting state, Ru substitution must achieve this via a mechanism that does not involve changes of the Fermi surface. We speculate that this mechanism relies on magnetic dilution that leads to the reduction of the effective Stoner enhancement. \\[4pt] [1] R. S. Dhaka, \textit{et al.}, PRL, (2011). \\[0pt] [2] Chang Liu, \textit{et al.}, Nature Physics, \textbf{6}, 419 (2010).   

Fermi surface in BaFe$_2$As$_2$ via SdH measurements on detwinned crystals

We have completely determined the Fermi surface in the antiferromagnetic orthorhombic phase of BaFe$_2$As$_2$ by measuring Shubnikov-de Haas oscillations in detwinned single crystals (T. Terashima et al., PRL 107, 176402 (2011)). The determined Fermi surface consists of one hole and two electron pockets, and the carrier compensation is satisfied, the carrier number being 0.024 holes and electrons per primitive unit cell. The Fermi surface can well be accounted for by an LSDA band-structure calculation using the experimental crystal structure. The mass enhancements $m^*/m_{band}$ are found to be 2--3. The Sommerfeld coefficient estimated from the determined Fermi surface and effective masses agrees well with an experimental value. Previous ARPES reports are not very consistent with our determined Fermi surface.   

Fermi surface topology and low-lying electronic structure of a new iron-based superconductor Ca$_{10}$(Pt$_{3}$As$_{8})$(Fe$_{2}$As$_{2})_{5}$

We report a first study of low energy electronic structure and Fermi surface topology for the recently discovered iron-based superconductor Ca$_{10}$(Pt$_{3}$As$_{8})$(Fe$_{2}$As$_{2})_{5}$ (the 10-3-8 phase, with Tc $\sim $ 8 K), via angle resolved photoemission spectroscopy (ARPES). Despite its triclinic crystal structure, ARPES results reveal a fourfold symmetric band structure with the absence of Dirac-cone-like Fermi dots (related to magnetism) found around the Brillouin zone corners in other iron-based superconductors. Considering that the triclinic lattice and structural supercell arise from the Pt$_{3}$As$_{8}$ intermediary layers, these results indicate that those layers couple only weakly to the FeAs layers in this new superconductor at least near the surface, which has implications for the determination of its potentially novel pairing mechanism.   

Recent advance in ARPES data analysis of dispersive features in Fe-based superconductors

Angle-resolved photoemission spectroscopy (ARPES) is a powerful tool to image the electronic band dispersion of materials, especially in multi-band systems such as the Fe-based superconductors. Here we present a new method to visualize ARPES data based on the mathematical concept of curvature, which improves the advantages and the reliability of the second derivative method in tracking the positions of extrema from the experimental data. We apply it to the Fe-based superconductors. We reveal clear kink features and FS contours, making it easier to capture the essential physics from the data. \\[4pt] [1] P. Zhang, P. Richard, T. Qian, Y.-M. Xu, X. Dai and H. Ding, \emph{A precise method for visualizing dispersive features in image plots}, Rev. Sci. Instrum. \textbf{82}, 043712 (2011).   

Fermi Surfaces of Iron-Pnictide High-T$_{\rm c}$ Superconductors from the Limit of Local Magnetic Moments

We study a 2-orbital t-J model for an isolated square lattice of iron atoms, which stack up to form an iron-pnictide high-T$_{\rm c}$ superconductor. The two orbitals in question are the degenerate $d\pm = 3d_{(x\pm iy)z}$ ones, which maximize the Hund's Rule coupling. First-neighbor and second-neighbor hopping (t) and Heisenberg exchange (J) are included. A Schwinger-boson-slave-fermion mean-field analysis yields a hidden half metal state in which holes hop through a $\nwarrow_{d+}\searrow_{d-}$ spin background without much hopping across orbitals. This state is characterized by an inner and an outer Fermi surface pocket centered at the $\Gamma$ point. The Fermi surface pockets resemble those predicted by band structure calculations that include all five $3d$ orbitals. By sweeping the Hund's coupling, we also identify a quantum-critical point (QCP) where zero-energy spin-wave excitations exist at the momenta associated with commensurate spin-density-wave (cSDW) order. These low-energy spin-waves result in nested Fermi-surface pockets centered at cSDW momenta. Exact diagonalization of one hole in the 2-orbital t-J model over a 4$\times$4 square lattice yields low-energy spectra that are consistent with the nested Fermi surfaces that are predicted to exist at the QCP.   

dHvA measurements of the Fermi surface of LiFeP and its relation to the nodal gap structure

The iron-pnictides are highly unusual in that there appears to be considerable variation in the structure of the superconducting gap across the different materials. The 111 compounds, LiFeX (X=As, P) superconduct at ambient pressure in their undoped stoichiometric form. LiFeP (($T_c$==5K) was found to have superconducting gap nodes whereas LiFeAs ($T_c$=17K) does not. Linking these differences in gap structure to Fermi surface features could provide a key test of microscopic theories which seek to explain superconductivity in iron pnictides. Here we report de Haas-van Alphen effect data which determine, almost completely, the {\it bulk} Fermi surface of LiFeP. The topology of the Fermi surface, which consists of quasi nested electron and hole sheets, is in good agreement with DFT band structure calculations when allowance for small band energy shifts is made. We find that one hole sheet has a anomalously small mass enhancement (compared to the others) which suggest it interacts weakly. This is probably because of its mixed orbital character rather than for any geometrical reason. We suggest that this could be the driver for node formation in this material.   

Evidence of Strong Coupling in Antiferromagnetic Ordered Iron Chalcogenide Fe$_{1.02}$Te Observed by Photoemission

The role of many-body effects is one of the central questions for unconventional superconductivity. For the recently discovered iron-based superconductors, the strength of electronic correlations is still an unsettled issue. For one of them, iron chalcogenides, a strong correlation scenario has both been proposed by theory and suggested by experiments. However, the metallic behavior in the antiferromagnetic ordered state in Fe$_{1.02}$Te seems to deviate from such scenario. Our discovery of evidence of strong coupling in electronic bandstructure probed by angle resolved photoemission (ARPES) reconciles this contrast. Our finding also highlights the non-trivial enrichment of many-body effects when multiple ingredients of interactions reinforce each other.   

Three-dimensionality and orbital characters of Fermi surface in Tl$_{0.5}$Rb$_{0.3}$Fe$_{1.63}$Se$_{2}$

We report a comprehensive study of the tridimensional electronic bands in the recently discovered Iron-selenide superconductor Tl$_{0.5}$Rb$_{0.3}$Fe$_{1.63}$Se$_2$ (T$_c$$\sim$32 K) with angle-resolved photoemission spectroscopy (ARPES). We determined the orbital characters and the $k_z$ dependence of the low-energy electronic structure by tuning the polarization and the photon energy of the incident photons. We observed a small 3D electron pocket near the Brillouin Zone (BZ) center and a 2D like electron pocket near the zone boundary. The photon energy dependence, the polarization analysis and the LDA calculations suggest a significant contribution from the Se 4$p_z$, Fe 3$d_{xy}$ and the Fe 3$d_{z^2}$ orbitals. Comparing with iron-pnictide superconductors, the emergence of Se 4$p_z$ states may be the cause of the different magnetic properties between iron-chalcogenides and iron-pnictides.   

ARPES measurements of superconducting gaps in iron-chalcogenide superconductors

The size and momentum dependence of the superconducting gap are crucial to the determination of the mechanism leading to Cooper pairing. Previous ARPES results on iron-pnictides superconductors reveal nearly-isotropic superconducting gaps with size varying from one Fermi surface to another. Here we show that this scheme is also valid in the iron-chalcogenide superconductors. We demonstrate that the superconducting gaps can be fitted by a single function derived from local pairing scenarios. Our finding of an apparent universality in iron-based superconductivity is a serious challenge to weak coupling approaches and rather favors pairing from local antiferromagnetic exchange interactions.   

Session B23: Superconductivity Theory I: Cluster DMFT, t-J model, renormalization group

Pair Structure and the Pairing Interaction in a Bilayer Hubbard model

The bilayer Hubbard model with an intra-layer hopping $t$ and an inter-layer hopping $t_\perp$ provides an interesting testing ground for several aspects of what has been called unconventional superconductivity. One can study the type of pair structures which arise when there are multiple Fermi surfaces. One can also examine the pairing for a system in which the structure of the spin-fluctuation spectral weight can be changed. Using a dynamic cluster quantum Monte Carlo approximation, we find that near half-filling, if the splitting between the bonding and anti-bonding bands $t_\perp/t$ is small, the gap has $B_{1g}$ ($d_{x^2-y^2}$-wave) symmetry but when the splitting becomes larger, $A_{1g}$ ($s^\pm$-wave) pairing is favored. We also find that in the $s^\pm$ pairing region, the pairing is driven by inter-layer spin fluctuations and that $T_c$ is enhanced.   

Influence of the cluster-shape on the d-wave-transition temperature $T_c$ in the dynamical cluster approximation: A case study for the 2D repulsive Hubbard model

The dynamical cluster approximation (DCA) is a systematic extension beyond the single site approximation of dynamical mean field theory to include non-local correlations. The method maps the infinite lattice self-consistently on a cluster with $N_c$ sites, which is embedded into a mean field. Since the correlations within the cluster are treated exactly, it is ideal to investigate phase-transitions such as d-wave superconductivity. Since a Quantum Monte Carlo integration is used to obtain the DCA self-consistency, the sign-problem prevents us to solve large cluster problems ($N_c>32$). Meanwhile, the transition temperatures $T_c$ from smaller clusters fluctuate due to a large dependency on the cluster-shape. Here, we investigate whether this cluster-dependency originates from the quantum impurity problem, or from the mesh on which we solve the Bethe-Salpeter (BS) equation. By exploiting the localized nature of the self-energy $\Sigma$ and the vertex function $\Gamma$ in real space, we can reconstruct these functions on arbitrary fine meshes in momentum-space and investigate its impact on the solutions of the BS equation. We will show that the $8$-site cluster has a finite $T_c$ for small doping-levels and that the 16-site clusters have converged transition temperatures.   

Superconductivity and the Pseudogap in the 2d Hubbard model

Using a numerically exact continuous-time quantum Monte Carlo impurity solver and the DCA cluster dynamical mean field method with cluster sizes up to 16, we have been able to access the superconducting phase of the two dimensional Hubbard model for parameters believed to be relevant to high temperature copper oxide superconductivity. We present results for the phase diagram, the gap to transition temperature ratio, and the interplay of the pseudogap and the superconducting gap. The gap results are obtained by direct inference from imaginary frequency data and analytically continued spectral functions.   

Strongly correlated superconductivity and Mott transition

Whether the pseudogap temperature $T^*$ intercepts or merges with the superconducting dome is one of the key questions in the field of high-temperature superconductors. We study the normal and the d-wave superconducting phases at finite temperature in the two-dimensional Hubbard model within cellular dynamical mean-field theory and continuous-time quantum Monte Carlo. Above the critical value for the Mott transition, the superconducting $T_c$ has a dome-like shape as a function of doping. The pseudogap temperature $T^*$ intercepts the superconducting dome. Removing superconductivity, one finds that in the normal state, $T^*$ ends at a finite-doping first-order transition that occurs at temperatures below the superconducting dome. That first order transition between a pseudogap metal and a strongly correlated metal is linked to the Mott transition at half-filling. Refs: G. Sordi et al., PRL 104, 226402 (2010); G. Sordi et al., PRB 84, 075161 (2011); G. Sordi et al., arXiv:1110.1392 (2011).   

Multi-band Hubbard Model Simulation for Unconventional Superconducting System

To simulate the new experiment findings on unconventional superconductor materials such as Cuprates and iron pnictides, it is inevitable to go beyond single-band model and capture the physics from multi-band effects. We carried out a novel Dynamical Cluster Quantum Monte Carlo study on multi-band Hubbard model, implementing the Continuous Time Quantum Monte Carlo algorithm as quantum solver. We studied various of single-particle properties aiming to observe the experiment concerned issues such as the spontaneous symmetry breaking to nematic order.   

Scaling of the transition temperature of hole-doped cuprate superconductors with the charge-transfer energy

We use first-principles calculations to extract two essential microscopic parameters, the charge-transfer energy and the inter-cell oxygen-oxygen hopping, which correlate with the maximum superconducting transition temperature $Tcmax$ across the cuprates. We explore the superconducting state in the three-band model of the copper-oxygen planes using cluster Dynamical Mean-Field Theory with an exact diagonalization impurity solver. We find that variations in the charge-transfer energy largely accounts for the trend in $Tcmax$ across the cuprate families.   

A contractor-renormalization study of Hubbard plaquette clusters

We implement the contractor-renormalization method to study the checkerboard Hubbard model on various finite-size clusters as function of the inter-plaquette hopping $t'$ and the on-site repulsion $U$ at low hole doping. We find that the pair-binding energy and the spin gap exhibit a pronounced maximum at intermediate values of $t'$ and $U$, thus indicating that moderate inhomogeneity of the type considered here substantially enhances the formation of hole pairs. The rise of the pair-binding energy for $t'< t'_{\rm max}$ is kinetic-energy driven and reflects the strong resonating valence bond correlations in the ground state that facilitate the motion of bound pairs as compared to single holes. Conversely, as $t'$ is increased beyond $t'_{\rm max}$ antiferromagnetic magnons proliferate and reduce the potential energy of unpaired holes and with it the pairing strength. For the periodic clusters that we study the estimated phase ordering temperature at $t'=t'_{\rm max}$ is a factor of 2--6 smaller than the pairing temperature.   

Effects of disorder in the Checkerboard Hubbard model

The checkerboard Hubbard model (CHM) is an unusual example of a model for strongly correlated electrons that has a region in parameter space where a controlled perturbative solution is possible. The square lattice in two dimensions is divided into four-site plaquettes for which the intra-plaquette hopping ($t$) is stronger than the inter-plaquette hopping ($t'$). We study the ground state properties of the CHM in the presence of disorder using exact diagonalization on clusters of up to twelve sites as a function of $t'/t$, disorder strength and interaction strength. We consider both site and bond disorder and calculate the pair binding energy and the spin gap. We comment on the implications of our results for superconductivity in this model.   

Superconducting pair-pair correlations in the half-filled Hubbard model on the anisotropic triangular lattice: absence of long-range order

We report calculations of superconducting pair-pair correlations for the half-filled band Hubbard model on an anisotropic triangular lattice using the path-integral renormalization group (PIRG) method. Mean-field studies have suggested that $d_{x^2-y^2}$ superconductivity occurs near the boundary between metallic and antiferromagnetic phases in this model. We calculate bond orders, spin structure factors, and superconducting pair-pair correlations at zero temperature. Our results are consistent with previous studies of the metal-insulator transition and antiferromagnetism in this model. However, we do not find any parameter region where pair-pair correlations are enhanced by the Hubbard U, except for trivial enhancement of on-site correlations. The superconducting pair-pair correlations at larger distances decrease monotonically with increasing U, with a distance dependence approaching that of noninteracting fermions, indicating the absence of frustration-driven superconductivity within the model.   

Two temperature scales in the d-wave pair correlation length of the 2D t-J model

Two temperature scales are observed in the d-wave pair correlation length for doping $\delta\sim0.25$. Both the s-wave and d-wave pair correlation lengths increase with decreasing temperature starting at very high temperatures. By $T\sim3J$ both symmetries have grown to $\sim1/3$ of a lattice spacing. For $T>3J$ the s-wave correlation length is a few percent larger than d-wave. At $T\sim3J$ both symmetries are saturating, with the growth in the correlation length flattening for both symmetries. The s-wave correlation length does not grow at lower temperatures. The d-wave correlation length does resume growing at $T\sim0.8J$, with the growth at low temperatures much stronger than it was at high temperatures. The d-wave correlation length reaches a size of one lattice spacing at $T\sim0.25J$ with no sign of saturating. Interestingly, both of these temperature scales also occur in the t-J model momentum distribution $n_{\bf k}$. The features that are found in the temperature dependence of the 2D $n_{\bf k}$ on the zone diagonal are very similar to the temperature dependence of the 1D t-J model $n_{\bf k}$. These observations can be understood by having spin-charge separated degrees of freedom.   

Two routes to disorder-induced magnetism and nematicity in the cuprates

We study disorder-induced magnetism within the Gutzwiller approximation applied to the t-J model relevant for cuprate superconductors. We identify two distinct disorder-induced magnetic phases depending on the strength of the scatterers. For weak potential scatterers, charge reorganization may push local regions in-between the impurities across the magnetic phase boundary, whereas for strong scatterers a local static magnetic moment is formed around each impurity. We calculate the density of states and find a universal low-energy behavior independent of both disorder and magnetization. However, the magnetic regions are characterized by larger (reduced) superconducting gap (coherence peaks) [1]. Recent studies have highlighted the role of a electronic nematic liquid underdoped cuprates. We calculate the spin susceptibility with a small explicitly broken rotational symmetry to show how the induced spin response asymmetry is enhanced by correlations. In the disorder-induced stripe phase, impurities become spin nematogens with a C2 symmetric impurity resonance state, and the disorder-averaged spin susceptibility remains only C2 symmetric at low energies, similar to recent data from neutron scattering on underdoped YBCO [2].\\[4pt] [1] R. B. Christensen et al., accepted Phys. Rev. B (2011).\\[0pt] [2] B. M. Andersen et al., submitted to EPL (2011).   

Band structure effects on superconductivity in the weak-coupling Hubbard model

The repulsive Hubbard model is the paradigmatic model for the study of unconventional superconductivity. In order to explore the influence of various features of the band structure on the magnitude and character of the pairing, we use a well-controlled perturbative renormalization group (RG) method to study the weak coupling limit of the model on the square lattice with various modifications: The first is the checkerboard model described by the strong hopping $t$ and the weak hopping $t^{\prime}$. The second is the bilayer model described by the intra-layer hopping $t$ and the inter-layer hopping $t_{\perp}$. We obtain the pairing symmetry and strength as functions of the relevant band parameters. Large changes in the effective pairing strength are found and their origins explained.   

Effects of longer-range interactions on unconventional superconductivity

We analyze the effect of the non-vanishing range of electron-electron repulsion on the mechanism of unconventional superconductivity. We present asymptotically exact weak-coupling results for dilute electrons in the continuum and for the 2D extended Hubbard model, as well as density-matrix renormalization group results for the two-leg extended Hubbard model at intermediate couplings, and approximate results for the case of realistically screened Coulomb interactions. We show that $T_c$ is generally suppressed in some pairing channels as longer range interactions increase in strength, but superconductivity is not destroyed. Our results confirm that electron-electron interaction can lead to unconventional superconductivity under physically realistic circumstances.   

Three-particle interactions in effective one-band models for unconventional superconductivity

We discuss different approximations for effective low-energy interactions in multiband models for weakly interacting correlated electrons. In the study of Fermi surface instabilities of the conduction band(s), the standard approximation consists in keeping just those terms in the bare interactions that couple only to the conduction band(s), while corrections due to virtual excitations into bands away from the Fermi surface are typically neglected. In order to include important aspects of these virtual interband excitations, we present an improved truncation of the functional renormalization group (fRG) that keeps track of the three-particle vertex in the conduction band. Within a simplified two-patch treatment of a two-band model, we demonstrate that these corrections can have a rather strong effect in parts of the phase diagram by changing the critical scale for $d $-wave pairing close to a phase boundary. The improved truncation scheme is applied as well to the Emery model within a multi-patch channel-decomposed fRG.   

Effective interactions in multi-band systems from constrained summations

The application of many-body techniques for the study of correlation effects and unconventional superconductivity requires the formulation of an effective low-energy model that contains only the relevant bands near the Fermi level. However the bands away from the Fermi level are known to renormalize the low-energy interactions substantially. Here we compare different schemes to derive low-energy effective theories for interacting electrons in solids. The frequently used constrained random phase approximation (cRPA) is identified as a particular resummation of higher-order interaction terms that includes important parts of of the leading virtual corrections. We then propose an adapted renormalization group scheme that includes the cPRA, but also allows one to go beyond the cRPA approximation. We study a simple two-band model in order to demonstrate the differences between the different approximations.   

Session B24: Electronic Structure: Theory and Spectra

Valence band effective Hamiltonians in nitride semiconductors

Valence band effective Hamiltonians are useful to determine the electronic states of shallow impurities, quantum wells, quantum wires and quantum dots within the effective mass approximation. Although significant experimental and theoretical work has been performed, basic parameters such as the Rashba Sheka Pikus (RSP) Hamiltonian parameters are still uncertain. In this work, the electronic band structures of AlN, GaN and InN, all in the wurtzite crystal structure, as well as the RSP Hamiltonian parameters~are determined by using the QSGW approximation in a FP-LMTO implementation. The corrections offered by this approach beyond the LDA are important to obtain the splittings and effective masses accurately. The present GW implementation, which allows for a real space representation of the self-energy, enables us to interpolate exactly to a fine k-mesh and hence to obtain accurate effective masses. We find the crystal field splitting in GaN (12 meV) in much closer agreement with experiment than previous work and obtain a negative SO coupling for InN. Moreover, we have generalized the method of invariants to crystals with orthorombic symmetry, such as ZnSiN$_{2}$ ZnGeN$_{2}$, ZnSnN$_{2}$ and CdGeN$_{2}$ and determined the corresponding Hamiltonian parameters.   

Hybrid Density Functional Study of (Hg,Cd)Te Systems

The HgCdTe alloy is used in high-performance infrared detection applications with a band gap range extending across the infrared spectrum. HgTe in particular has sparked interest for its topological insulating behavior in quantum well devices due to its band inverted nature. We test the quality of the newer hybrid screened functional, HSE, on the two contrasting materials: HgTe (semimetal) and CdTe (semiconductor) to see how well it performs under a range of computational setups [1]. A direct comparison of HSE with the standard DFT functional PBE to experiment for the HgCdTe alloy reveals HSE is able to reproduce the experimental crossover composition of 17\% Cd concentration when the alloy goes from a semimetal to semiconductor, whereas PBE overestimates this composition at 67\% Cd concentration. HSE also predicts a higher valence band offset of 0.53 eV in the HgTe/CdTe heterostructure than previous first-principle and early experimental results, but in good agreement with the more recent experimental results. Supported by DOE-Basic Energy Science DOE-BES-DMS (DEFG02-99ER45795). Computing resources are provided by NERSC and OSC. \\[4pt] [1] Jeremy W. Nicklas and John W. Wilkins, Phys. Rev. B 84, 121308(R) (2011)   

Ab-initio description of satellites in graphite

The GW method from Many-Body Perturbation Theory has been very successful in describing photoemission spectra in a variety of systems. In particular, GW is known to give good quasiparticle properties like band-gaps, but it has shown some limitations in the description of complex correlation effects like satellites. Satellite peaks in photoemission come from higher-order excitations and are still poorly studied in the valence bands. In perturbative GW the spectral function can describe additional features beside the quasiparticle peaks, but these satellites are known to be too weak and too low in energy, as it appears from calculations on the Homogeneous Electron Gas and some real materials. We have recently shown that including additional diagrams in the Green's function (similarly to what has been done with the cumulant expansion) we obtain an excellent description of satellites series in the test case of bulk silicon, where GW is unable to cope. We now focus on a more complex system, i.e. graphite, with this same approach. Using our newly measured XPS valence data, we investigate the effects of anisotropies on satellites and give a prediction on the spectral changes following the transition towards a single graphene layer.   

Isotope dependence of band gap in semiconductors

Recently, carrier confinement was observed in diamond superlattice consisting of pure $^{12}$C and $^{13}$C layers. This interesting phenomenon comes from large isotope dependence of fundamental gap width in diamond. To understand this carrier confinement and design related nanodevices, we investigate the effect of electron-phonon couplings on quasiparticle energies in semiconductors composed of carbon and other light elements using first-principles methods based on the density functional theory. The calculated isotope dependence as well as temperature dependence of band gap in diamond is found to agree well with the experiment. We also discuss the isotope dependence in binary compounds such as boron nitride and silicon carbide.   

Clarification of the relations between stacking structures of ${\it sp}^3$ network materials and their band gaps

Silicon carbide (SiC) has been discovered in various polymorphs. Each polymorph is characterized by its stacking of atomic planes. The band gap varies substantially in each polymorph from 2.40 eV to 3.33 eV in spite that the local atomic structures are identical to each other [1]. The mechanism of this intriguing property have been poorly understood. To clarify the fundamental reasons for this band-gap variation, we have performed the electronic-structure calculations in the density functional theory. We have found that the Kohn-Sham orbital at the conduction-band bottom distributes broadly around the interstitial channel, thus floating in the matter. The way of the floating depends on the stacking of the atomic planes and determines the band gap in each polymorph. We also find that the floating state appears in other ${\it sp}^3$-bonded materials, and the band-gap variation is common to the covalent materials. References [1] Properties of Silicon Carbide edited by G. L. Harris (INSPEC, London, 1995).   

Nonequilibrium ``melting'' of a charge density wave insulator via an ultrafast infrared laser pulse

In equilibrium, electrons interacting with lattice vibrations have a transition either to a charge density wave phase (a static modulation of the electronic charge) or to a superconductor (electron pairs move without resistance). If the coupling is weak, the system orders in the Bardeen-Cooper-Schrieffer scenario, where the ordering occurs at some transition temperature Tc and a gap simultaneously forms in the density of states. In strong-coupling, preformed pairs bind at a high temperature (forming a gap in the density of states) and the ordering only occurs at a lower temperature. We employ an exact solution of a model for pump-probe time-resolved photoemission spectroscopy to show how, in nonequilibrium, a third scenario arises: the gap disappears in the presence of a nonzero order parameter, and then reforms well after the pulse has passed. This nonequilibrium ``phase transition'' scenario qualitatively describes all of the available experiments on the ultrafast melting of a charge density wave.   

Engineering Electronic Band Structure for New Elpasolite Scintillators

The utilization of scintillator materials is one of the primary methods for radiation detection. Elpasolites are a large family of quaternary halides that have recently attracted considerable interest for their potential applications as $\gamma $-ray and neutron scintillators. A large number (on the order of 10$^{3})$ of different chemical compositions exist in the elpasolite family of compounds. This wide range of compositions offers numerous opportunities for fine-tuning the material chemistry to target specific scintillation properties, but they also pose significant challenges in identifying the most promising ones. Many elpasolite scintillator materials currently under development suffer from low light output and long scintillation decay time. The low light output is partially due to a large band gap while the long scintillation decay time is a result of the slow carrier transport to Ce impurities, where carriers recombine to emit photons. We suggest that these problems may be mitigated by optimizing the band gap and carrier mobility by selecting constituent elements of proper electronegativity. For example, cations with lower electronegativity may lower the conduction band and increase the conduction band dispersion simultaneously, resulting in higher light output and faster scintillation. We demonstrate by first-principles calculations that the strategy of manipulating electronegativity can lead to a number of new elpasolite compounds that are potentially brighter and faster scintillators.   

Origin of the variation of exciton binding energy in semiconductors

Electron-hole interaction plays a crucial role in optical properties, and the exciton binding energy ($E_{b})$ in technologically important semiconductors varies from merely a few meV to around 100 meV, which is not well understood. In this work, we investigate the origin of the variation of $E_{b}$ in semiconductors, employing first-principle calculations based on the density functional theory (DFT) and the many-body perturbation theory with Green's function (GW/BSE). Our results clearly show that $E_{b}$ decreases as the spread of electron distribution, which measures the magnitude of electron delocalization, increases. This is due to the increased electronic screening when electrons tend to be more delocalized. Furthermore, the spread distribution of the top valence electrons is of central importance in determining excitonic screening, which leads to weakly bound electrons and holes in semiconductors. Thus, the variation of exciton binding energy in semiconductors can be understood from the computed magnitude of electron delocalization of top valence bands of these materials using DFT.   

Modeling Shallow Core-Level Transitions in the Reflectance Spectra of Gallium-Containing Semiconductors

The electronic structure of covalent materials is typically approached by band theory. However, shallow core level transitions may be better modeled by an atomic-scale approach. We investigate shallow d-core level reflectance spectra in terms of a local atomic-multiplet theory, a novel application of a theory typically used for higher-energy transitions on more ionic type material systems. We examine specifically structure in reflectance spectra of GaP, GaAs, GaSb, GaSe, and GaAs$_{1-x}$P$_{x}$ due to transitions that originate from Ga3d core levels and occur in the 20 to 25 eV range. We model these spectra as a Ga$^{+3}$ closed-shell ion whose transitions are influenced by perturbations on 3d hole-4p electron final states. These are specifically spin-orbit effects on the hole and electron, and a crystal-field effect on the hole, attributed to surrounding bond charges and positive ligand anions. Empirical radial-strength parameters were obtained by least-squares fitting. General trends with respect to anion electronegativity are consistent with expectations. In addition to the spin-orbit interaction, crystal-field effects play a significant role in breaking the degeneracy of the d levels, and consequently are necessary to understand shallow 3d core level spectra.   

Ab-initio study of dilute nitride substitutional and split-interstitial impurities in gallium antimonide (N-GaSb)

We present an ab-initio density functinal theory study of dilute-nitride GaSb. Adding dilute quantities of nitrogen causes rapid reduction in bandgap of GaSb ($\sim$300 meV for 2\% N). Due to this rapid reduction in bandgap, dilute-nitrides provide a pathway for extending the emission of GaSb based type-I diode lasers into the mid-infrared wavelength region (3-5 micron). In this study we look at the effect of substitutional N impurity on the electronic properties of our system and compare it with the band-anticrossing model, a phenomenological model, which has been used to explain giant band bowing observed in dilute-nitride alloys. We also study the effect of Sb-N split interstitials which are known to be non-radiative recombination centers. Furthermore we also discuss the stability of the Sb-N split interstitial relative to substitutional nitrogen to determine if the split interstitials can be annihilated using post-growth annealing to improve the radiative lifetime of the material which essential for laser operation.   

Electronic properties of InP in terms of an ab-initio LDA

We present results from ab-initio local density approximation (LDA) calculations of electronic and related properties of zinc blende indium phosphide (InP). Our computations employed the Ceperley and Alder LDA potential and the linear combination of atomic orbital (LCAO) formalism. The implementation of the LCAO formalism followed the Bagayoko, Zhao, and Williams (BZW) method. Consequently, we solved self consistently both the Kohn sham equation and the one giving the ground state charge density in terms of the wave functions of the occupied states. Self-consistency, for the latter equation, requires a search for the optimal basis set. This search entails increases of the size of the basis set and the related modifications of angular symmetry and of available radial functions. Our calculated, direct band gap of 1.398 eV (1.40 eV), at the ? point, is in excellent agreement with experimental values. The calculated density of states (DOS) also agree with experimental finding. The calculated electron and hole effective masses differ by 10\% from some corresponding experimental ones. We discuss the equilibrium lattice constant and optical properties.   

Self Consistent Calculations of Electronic Properties of Systems with an Energy or a Band Gap

We re-examine the process of performing self consistent calculations of electronic and related properties of finite systems (with an energy gap) and of crystals with a band gap. This work applies to calculations utilizing density functional and X? potentials and to other approaches that entail solving a system of inherently coupled equations. In particular, the local density approximation (LDA) is defined by a system of equations that reduces to two equations upon the selection of Vxc. We show how the Bagayoko, Zhao, and Williams (BZW) method solves the relevant system of equations and leads to results in excellent agreement with experimental ones. We discuss such results for w-ZnO, rutile TiO2, w-CdS, zb-CdS, zb-InP, Ge, Ca B6, and other materials. Work funded by the National Science Foundation, through LASiGMA [NSF AwardEPS-1003897 and No. NSF (2010-15)-RII-SUBR], LONI [Award No. 2-10915], and Ebonyi State, Federal Republic of Nigeria.   

Infrared Refractive Index of Silicon: Parity and Sum-Rule Tests

We have resolved conflicting reports for the IR refractive index of silicon using general considerations of linear response theory. We find that use of unphysical series expansions in the analysis of channel spectra has been a significant source of systematic error. Recognition that the index is an even function of photon energy is crucial for analysis of these measurements and clarifies data presentation. In the region of high IR transparency of elemental semiconductors, the index may be expanded in a rapidly convergent Taylor series. Coefficients of terms in the (2n)$^{th}$ power of energy are proportional to the (2n+1)$^{th}$ inverse moment of the electronic absorption spectrum. In the favorable case of intrinsic Si, the electronic absorption is sufficiently well known that independent values of the intercept, slope and curvature of plots of index \textit{vs}. the square of photon energy may be calculated. Index data sets with parameters significantly different from these suffer from systematic errors or refer to impure samples. Using these parity and sum-rule tests we have prepared a composite index data set for intrinsic silicon that represents a best fit to reliable measurements from microwaves to the visible. Applications to germanium and diamond will be discussed.   

Electronic Structure of NiFe$_2$O$_4$ using screened Hybrid Functionals

As an insulating ferrimagnet with a high Curie temperature, NiFe$_2$O$_4$ (NFO) may be a promising candidate for future spin-based applications. Recent demonstration of spin-Seeback effect in magnetic insulators indicates that important new phenomena may be discovered in such materials. Unfortunately, density functional theory cannot give a full account of its properties; most notably, LDA calculations find it to be metallic. LDA+U yields an insulator but underestimates the band gap. The recently-implemented screened hybrid functionals method (HSE06) represents only a moderate increase in computational effort compared to traditional DFT. This method allows one to modify the PBE-approximated exchange potential with a portion of the Hartree-Fock exchange. We present LDA, LDA+U, and HSE06 calculations of the density of states and band structure of NFO. We show that hybrid methods greatly improve agreement with the experimental band gap over LDA +U. We find that NFO is an indirect band gap system with the spin-down channel having the lower, indirect gap, whereas the majority channel possess a direct gap with over a 0.5eV difference with the minority gap. Comparison of our theoretical results with recent optical measurements on NFO thin films is also presented.   

Session B25: Focus Session: Simulation of Matter at Extreme Conditions - Shock-Induced Chemistry

Extending DFT to Long Time-Scales: Using the Density Functional Tight Binding Approach for Materials Under Extreme Temperatures and Pressures

We report here on density functional tight binding (DFTB) simulations of covalently bonded materials over a pressure range of 10 -- 2,000 GPa and a temperature range of 300 -- 30,000 K using both standard and new interaction potentials we have created for these conditions. Density Functional Theory (DFT) has been shown to accurately reproduce the high pressure-temperature chemistry, phase boundaries, and EOS of many materials. DFT-MD simulations, though, scale poorly with computational effort and thus are generally limited to nanometer system sizes and picosecond time-scales. In contrast, chemical kinetic effects and phase changes can span up to nanosecond timescales and significantly longer length scales. The DFTB method holds promise as a high throughput simulation capability by providing orders of magnitude increase in computational efficiency while retaining most of the accuracy of Kohn-Sham DFT. We show that DFTB interaction potentials can be created by (a) fitting the DFTB repulsive energy to measured and computed compression data, and (b) using an extended basis set that includes $d$-orbital interactions, as needed. Our new potential for carbon yields accurate material properties for diamond, graphite, the BC8 phase, and simple cubic carbon, as well as for the shock Hugoniot of diamond compressed up to the conducting liquid. We also discuss simulations of the long-time scale reactivity of H$_{2}$O$_{2}$ under detonation conditions, and shock compression of astrophysical ice mixtures and the subsequent synthesis of pre-biotic materials. Our results provide a straightforward method by which DFTB can be made to provide equation of state and long-time scale chemical kinetic data at a similar accuracy to standard quantum codes. Our approach could be extended to any number of materials related to geology and planetary science, including silicon, SiO$_{2}$, and hydrocarbon systems.   

Pressure-induced Polymerization in Substituted Acetylenes

A fundamental understanding of shock-induced chemical reactions in organics is still lacking and there are limited studies devoted to determining reaction mechanisms, evolution of bonding, and effect of functional group substitutions. The fast timescale of reactions occurring during shock compression create significant experimental challenges (diagnostics) to fully quantify the mechanisms involved. Static compression provides a complementary route to investigate the equilibrium phase space and metastable intermediates during high pressure chemistry, although at a much slower timescale. In this study, we present our results from our ongoing high pressure in situ synchrotron x-ray diffraction and vibrational spectroscopy experiments on substituted acetylenes: tert-butyl acetylene [TBA: (CH3)3-C$\equiv $CH] and ethynyl trimethylsilane [ETMS: (CH3)3-SiC$\equiv $CH]. We observed that the onset pressure of chemical reactions (at room temperature) in these compounds is significantly higher in static compression (TBA: 11 GPa and ETMS: 26 GPa) when compared to shock input pressures (TBA: 6.1 GPa and ETMS: 6.6 GPa). The products were polymeric in nature, recovered to ambient conditions with little degradation and fully characterized using spectroscopy, calorimetry, and other techniques to identify reaction mechanisms.   

Non-equilibrium Molecular Dynamics Studies of Interfacial Chemistry in Shocked Ni/Al Nanolaminates

The response of Ni/Al composite materials to shock loading has been studied using non-equilibrium molecular dynamics and an EAM force field. The simulation cells consist of layered Ni and Al laminates with at least 3 million particles in a 1:1 mole ratio. The main thrust of our research is to gain a better understanding of the chemistry that occurs at the Ni/Al interface when the real material is shocked. Initial geometries were chosen so as to identify the factors important to reaction in the complex macro-scale material. Specifically, we vary the orientation of the interface with respect to the shock wave and the geometry of the interface ($i.e.$, deviation from planarity) to study how mixing and reactivity of Ni and Al are affected. Our results show a clear dependence of pressure and temperature on interface orientation, in agreement with continuum-scale simulations.   

Near-Equivalency of Inter and Intramolecular Hydrogen Bonding Under High-Pressure

Triamino-trinitro-benzene (TATB, C$_{6}$H$_{6}$N$_{6}$O$_{6})$ exhibits unusually strong intramolecular hydrogen bonding as evidenced by the high rotational energy barrier of the nitro and amino groups. In the condensed phase, the competing intermolecular hydrogen bonding becomes pronounced at high-pressure in its graphitic-like crystal structure. We report density functional theoretical calculations of the equation of state of TATB under hydrostatic compression of up to 250 GPa. Our results show Our results show increasing bond equivalency between the intramolecular and intermolecular hydrogen bonds of the amino and nitro groups in the region 30$<$P$<$70 GPa, beyond which the difference between the two bond distances remains constant. This approximate bond equivalency is manifested by a rapid decrease of the intermolecular -NO---HN- distance along the b lattice direction from 2.6 {\AA} at the zero pressure equilibrium geometry to 1.72 {\AA} at 67 GPa, and by a decrease of the intramolecular -NO---HN- bond from 1.65 {\AA} to 1.57 {\AA} for the same pressure region. It is expected that vibrational motions involving the NO---HN modes are sensitive to the nearly equivalent hydrogen bonding, as recent spectroscopic IR analysis of the NH$_{2}$ stretches revealed.   

Modeling of electron-ion coupling in shocked materials

This work describes and implements a quasi-statistical approach to electron-ion coupling in shocked matter. By combining this approach with the multi-scale shock technique (MSST) and a tight-binding model, the magnitude and role of electronic excitations in shocked energetic materials are studied theoretically using quantum molecular dynamics simulations. Focusing on the detonating primary explosive HN3 (hydrazoic acid), this work finds that the material transiently exhibits a high level of electronic excitation characterized by carrier densities in excess of 10$^{21}$ cm$^{-3}$, or one electronic excitation for every 8 molecules. Electronic excitations enhance the kinetics of chemical decomposition by about 30\%. The electronic heat capacity has a minor impact on the temperatures exhibited, on the order of 100 K.   

Quantum-based Molecular Dynamics Simulations of Shock-induced Reactions with Time-resolved Raman Spectra

Shock-induced reactions in liquid hydrocarbons have been studied using quantum-based, self-consistent tight-binding (SC-TB) molecular dynamics simulations with an accurate and transferable model for interatomic bonding. Our SC-TB code LATTE enables explicit simulations of shock compression using the universal liquid Hugoniot. Furthermore, the effects of adiabatic shock heating are captured precisely using Niklasson's energy conserving extended Lagrangian Born-Oppenheimer Molecular Dynamics formalism. We have been able to perform relatively large-scale SC-TB simulations by either taking advantage of the sparsity of the density matrix to achieve $O(N)$ performance or by using graphics processing units to accelerate $O(N^3)$ algorithms. We have developed the capability for the on-the-fly computation of Raman spectra from the Fourier transform of the polarizability autocorrelation function via the density matrix perturbation theory of Niklasson and Challacombe. These time-resolved Raman spectra enable us compare the results of our simulations with identical diagnostics collected experimentally. We will illustrate these capabilities with a series of simulations of shock-induced reaction paths in a number of simple molecules.   

Molecular Simulation of Shock Hugoniot for Polymers

The behavior of polymers under extreme conditions (high pressure and temperature) is of interest for a variety of applications, such as polymer-bonded explosives, coatings, adhesives, and light-weight armor. Material properties and response at extreme conditions can be determined through shock experiments, which are often difficult to measure experimentally because of difficulties in traversing a large range of pressures (up to hundreds of gigapascals) and temperatures (thousands of kelvin) with available instrumentation. In addition, interesting behavior, such as observed behind a shock front, occurs at extremely short time- and length-scales (nanoscale), which poses problems in characterizing the material using current experimental capabilities. To further understand shocked systems, simulation methods such as molecular dynamics (MD) and quantum mechanics (QM) can be used to provide insight into atomic-level phenomena. Using classical MD and QM, we have calculated the principle shock Hugoniot curves for four polymers: poly[methyl methacrylate], poly[ethylene], poly[styrene], and hydroxyl-terminated poly[butadiene]. In the MD calculations, we considered both a non-reactive (i.e. PCFF) and reactive (i.e. ReaxFF) forcefield, respectively, where calculations were performed in LAMMPS. The QM calculations were performed with density functional theory (DFT) using dispersion corrections as implemented in CP2K. We have applied both atom centered pseudopotentials (DCACPs) and Grimme van der Waals corrections in our study. Overall, results obtained by QM show much better agreement with available experimental data for the range of up to 20 GPa than classical force fields. At pressures where reactions can occur the short simulation time available in MD modeling prevents us from fully exploring possible reaction pathways.   

Amorphous Polymeric Nitrogen from Dynamic Shock Simulation

In recent years there has been significant interest in polymeric phases of nitrogen at low pressure for potential application as an energetic material. This interest was bolstered by experimental evidence of metastable amorphous polymeric nitrogen at low pressure.\footnote{Goncharov, A. F. {\it et al}., Phys. Rev. Lett. {\bf 85}, 1262 (2000)}$^,$\footnote{Eremets, M. I. {\it et al}., Nature {\bf 411}, 170 (2001)} While considerable theoretical work has been done on many crystal phases of nitrogen, simulating amorphous polymeric nitrogen has been more challenging. Starting from first principles dynamic shock simulation of cubic-gauche nitrogen\footnote{Mattson, William D. and Balu, Radhakrishnan, Phys. Rev. B {\bf 83}, 174105 (2011)} we demonstrate a form of polymeric nitrogen at low pressure that may be directly related to amorphous polymeric nitrogen.   

Ethane and Xenon mixing: density functional theory (DFT) simulations and experiments on Sandia's Z machine

The combination of ethane and xenon is one of the simplest binary mixtures in which bond breaking is expected to play a role under shock conditions. At cryogenic conditions, xenon is often understood to mix with alkanes such as Ethane as if it were also an alkane, but this model is expected to break down at higher temperatures and pressures. To investigate the breakdown, we have performed density functional theory (DFT) calculations on several xenon/ethane mixtures. Additionally, we have performed shock compression experiments on Xenon-Ethane using the Sandia Z - accelerator. The DFT and experimental results are compared to hydrodynamic simulations using different mixing models in the equation of state. Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, a wholly owned subsidiary of the Lockheed Martin company, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.   

Dynamic Response of a Carbon Fiber -- Epoxy Composite

The dynamic response of carbon fiber reinforced epoxy composite materials was investigated under planar impact loading. The samples were unidirectional (all carbon fibers oriented in a single direction) with fiber fill volumes from 62 to 68\%. Gun driven planar impact tests with impact velocities of 0.1-2.0 km/s were conducted allowing samples to be compressed up to about 15 GPa. Velocity interferometry was used to measure particle velocities from which the compressed state of the samples was determined. Wave speeds for shocks traveling along the carbon fibers are significantly higher than for those traveling transverse to the fibers or through the bulk epoxy. As a result, the dynamic material response is dependent on the relative shock - fiber orientation. Shocks traveling along the fiber direction exhibit both elastic and plastic characteristics over the stress range tested. Shocks traveling transverse to the fibers show only a single wave response similar to but slightly stiffer than the bulk response of the epoxy. Results will be presented detailing these findings which provide a basis for modeling this class of directional composite materials.   

Calculation of Transport Coefficients in Binary Yukawa Mixtures

We employ classical molecular dynamics (MD) to estimate species diffusivity and viscosity in binary Yukawa Mixtures. The Yukawa potential is used to describe the screened Coulomb interaction between the ions, providing the basis for models of dense stellar materials, inertial confined plasmas, and colloidal particles in electrolytes. We calculate transport coefficients in equilibrium simulations using the Green-Kubo relation over a range of thermodynamic conditions including the viscosity and the self-diffusivity for each component of the mixture. The inter-diffusivity (or mutual diffusivity) can then be related to the self-diffusivities by using a generalization of the Darken equation. We have also employed non-equilibrium MD to estimate inter-diffusivity during the broadening of the interface between two regions each with a high concentration of either species. The main motivation in this work is to build a model that describes the transport coefficients in binary Yukawa mixtures over a broad range of thermodynamic conditions.   

Fracture of Constructional Materials with the Covering at Shock-Wave Loadings

The behavior of constructional materials with a covering subjected to shock load is numerically modeled. The covering on a material is applied by the method of high velocity oxygen fuel. The materials created by a similar method, are widely applied in aerospace branch, both for creation of engines and for creation of details of cases. Possibility of application of multilayered coverings essentially expands the spectrum of researches for the analysis of separate layer influence on behavior of a design as a whole. Features of behavior of this sort of materials is an actual problem as well as construction of authentic models of behavior of materials with coverings as a whole. Influence of a material of a covering, quantity of layers and their geometrical parameters on fracture and shock-wave processes in a material is investigated. The range of velocities of interaction from 50 to 2000m/sec is considered. As projectiles steel cylinders and spheres were used.   

Session B26: Focus Session: Physics of Energy Storage Materials - Advanced Materials

How can one tell if a lithium-ion battery will last for 10 years in experiments that only take a few weeks?

Lithium-ion batteries are now being used in electric vehicles. There are four main factors which will determine the success of Li-ion batteries in this application: a) safety; b) cost; c) performance and d) lifetime. Each of the factors is presently the subject of much debate and much R+D. I will only speak about lifetime here. Testing the lifetime of Li-ion batteries for automotive applications under \textbf{realistic} conditions of temperature and number of cycles per day is very time consuming. In fact, such a test should take a decade or more, if the batteries are expected to last a decade in the field. Tests of such duration slow the product improvement cycle immensely. In this lecture, I will discuss how high precision measurements of the coulombic efficiency of Li-ion cells and batteries can be used to predict the relative lifetimes, on the decade-long scale, of these devices in measurements that only take a few weeks. The measurements enable the rapid comparison of technologies using new electrode materials, electrolyte additives and cell designs so that the product improvement cycle can be significantly shortened. I will describe the requirements of the instruments needed to make these measurements and point out that nothing suitable is, as yet, commercially available. There is a major need for such equipment and an associated business opportunity.   

Organic electrodes for high rate capability Lithium-ion batteries

Lithium-ion batteries (LIBs) for power-intensive applications such as in electric vehicles require high discharge rate, i.e., high Li diffusion rate (or low Li diffusion barrier) in electrode and electrolyte materials. Based on first-principles calculations, we found that organic salt, di-lithium terephthalate (Li2TPA), a promising anode material recently tested in experiment, could have high rate capability. We further predict that di-potassium terephthalate (K2TPA) could exhibit even lower Li diffusion barrier. The calculated Li diffusion barrier in fully lithiated K2TPA is only 150 meV, which yields Li diffusion rate orders of magnitude higher than that in Li-intercalated graphite at room temperature. The calculated anode voltage vs metal lithium and specific energy density are 0.62 V and 209 mAh/g, respectively. In addition, the volume change of K2TPA in charging/discharging is only 5{\%}, much smaller than that in Li-intercalated graphite. These unique advantages call for further investigation of the organic salts, both the TPA-based and beyond, for power-intensive LIB applications either as anode or cathode materials.   

First Principles Study for Lithium Intercalation and Diffusion Behavior in Orthorhombic Nb2O5 Electrochemical Supercapacitor

Unlike batteries, electrochemical supercapacitors require not only high energy density, but also very fast rates of electronic and ionic transport. Experimental results show that niobium oxide exhibits an outstanding power density and fast ionic charging rates. We investigate lithium intercalation and diffusion behavior in orthorhombic niobium oxide (T-Nb2O5) by using first-principles density-functional theory (DFT) calculations. We find that the Li ions can only intercalate in the {001} family of lattice planes with the lowest niobium occupancy due to electrostatic-repulsion between Li+ and Nb5+. Besides, since Li diffusion along the z direction is hindered by a high diffusion barrier (2.64 eV), the overall Li intercalation and diffusion can only occur within {001} planes. Furthermore, the diffusion barriers within the {001} planes are found to have a broad distribution of values form around 50 meV to 1000 meV; the diffusion barrier is determined by the neighboring oxygen-oxygen distance. The barrier remains low (around 60 meV) when the neighboring oxygen distance along the diffusion path is larger than 3.9 {\AA}, thus leading to fast Li ion diffusion. These results explain the excellent performance of Nb2O5 as a cathode material for electrochemical supercapacitors.   

Multiscale Simulations of Energy Storage in Polymers

Polypropelene is the most used capacitor dielectric for high energy density storage. However, exotic materials such as copolymerized PVDF and, more recently, polythiourea, could potentially lead to an order of magnitude increase in the stored energy density [1,2]. In our previous investigations we demonstrated that PVDF-CTFE possesses non-linear dielectric properties under applied electric field. These are characterized by transitions from non-polar to polar phases that lead enhanced energy density. Recent experiments [3] have also suggested that polythiourea may be another potential system with high energy-density storage and low loss. However, the characteristics of this emerging material are not yet understood and even its preferred crystalline phases are not known. We have developed a multiscale approach to predicting polymer self-organization using the REAX force field and molecular dynamics simulations. We find that polythiourea chains tend to coalesce in nanoribbon-type structures and prefer an anti-polar interchain ordering similar to PVDF. These results suggest a possible role of topological phase transitions in shaping energy storage in this system.\\[4pt] [1] B. Chu et al, Science 313, 334 (2006).\\[0pt] [2] V. Ranjan et al., PRL 99, 047801 (2007).\\[0pt] [3] Q. Zhang, private communication   

Addressing the challenges of solar thermal fuels via atomic-scale computational design and experiment

By reversibly storing solar energy in the conformations of photo-isomers, solar thermal fuels (STFs) provide a mechanism for emissions-free, renewable energy storage and conversion in a single system. Development of STFs as a large-scale energy technology has been hampered by technical challenges that beset the photo-isomers of interest: low energy density, storage lifetime, and quantum yield; UV absorption; and irreversible degradation upon repeated cycling. In this talk, we discuss our efforts to design new STFs that overcome these hurdles. We present computational results on various STFs based on our recently proposed photo-isomer/template STF concept [Kolpak and Grossman, Nano Letters 11, 3156 (2011)], as well as new experimental results on azobenzene-functionalized carbon nanotube STFs. Our approach yields significant improvements with respect to STFs studied in the past, with energy densities similar to Li-ion batteries, storage lifetimes $>$ 1 year, and increased quantum yield and absorption efficiency. Our strategy also suggests mechanisms for inhibiting photo-isomer degradation. With a large phase space yet to be explored, there remain numerous possibilites for property enhancement, suggesting that STFs could become a competitive renewable energy technology.   

Photoswichable Molecular Rings for Solar-Thermal Energy Storage

Solar-thermal fuels reversibly store solar energy in the chemical bonds of molecules by photoconversion, and can release this stored energy in the form of heat upon activation. Many conventional photoswichable molecules could be considered as solar thermal fuels, although they suffer from low energy density and short lifetime in the photo-excited state, rendering their practical use unfeasible. We present a new approach to design systems for solar thermal fuel applications, wherein well-known photoswitchable molecules are connected by different linker agents to form molecular rings. This approach allows for a significant increase in both the amount of stored energy per molecule and the stability of the fuels. Our results suggest a range of possibilities for tuning the energy density and thermal stability as a function of the type of the photoswitchable molecule, the ring size, and/or the type of linkers.   

Physics of Materials for Sodium-Ion Batteries

Sodium is used as a functional ionic species in a variety of electrochemical energy storage devices. This talk will examine several manifestations of sodium ion use, and will focus deeply on a novel class of aqueous electrolyte sodium ion batteries that have been developed over the past 5 years. While these new systems are typically lower in energy density, they are very robust and low in cost, making them appealing for a number of stationary energy storage applications. Specific topics to be covered include: sodium ion intercalation compounds, sodium/carbon interaction at potentials below 0V vs NHE, the behavior of porous thick electrodes in different electrolyte solutions, and the future outlook for sodium ion battery research.   

Computational studies of carbon-onions for electrochemical capacitor applications

Supercapacitors bridge the gap between conventional batteries and electrolytic capacitors. Recently, onion-like carbon structures have [1] shown to have capacitances four orders of magnitude higher and energies an order of magnitude higher than conventional capacitors, making them the fastest growing competitors for energy storage. We study the formation of carbon-onions from nanodiamonds using reactive force-fields [2]. Our study suggests that the temperature and mechanical stability as well as the final-equilibrium structure are strongly dependent on the inclusion of long-range forces. We are currently developing reactive-force fields to allow mesoscopic modeling of reactions of carbon nanostructures with aqueous electrolytes. Progress along these lines will also be presented. This material is based upon work supported as part of the Fluid Interface Reactions, Structures and Transport (FIRST) Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.\\[4pt] [1] D. Pech et. al, Nature Nanotechnology 5, 651 (2010)\\[0pt] [2] Adri C. T. van Duin et.al, J. Phys. Chem. A 105, 9396 (2001)   

Supercapacitor Electrodes with High-energy and power densities prepared from monolithic NiO/Ni Nanocomposite

Despite significant progresses in the development for high-performance supercapacitors, it lacks techniques to realize the full potential of electrode material by achieving simultaneously tailored pore structure, electrode conductivity, and crystallinity. Moreover, the problem of being difficult for industrial scale manufacture still exists. For an attempt to address all these issues, we recently developed a simple and cost-effective process, which is also scalable, for achieving supercapacitor electrodes with both high energy and power densities. We first produce nickel nanoparticles with a modified polyol method. A simple mechanical compaction of nanoparticles and a followed thermal treatment result in compact, stable, highly porous Ni/NiO electrodes that do not require a support. During the charging process, OH$^{-}$ electrolytic ions are bound to the NiO, giving off electrons. The process is reversed when the stored electrical energy is drawn off as current. The high granularity NiO provides a large inner surface area, and the conductive network of the metal particles is maintained. High energy density of about 60 Wh kg$^{-1}$ and power density of 10 kW kg$^{-1}$ were simultaneous achieved with a slow charge/fast discharge process.   

An ab-initio approach to modeling high temperature thermodynamics of non stoichiometric ceria

Ceria is a very promising material for fuel cell electrolytes because of its high oxygen ion diffusivity. Also, its ability to thermodynamically create oxygen vacancies in its structure at high temperatures and get oxidized at low temperatures has found use in thermochemical splitting of water to generate hydrogen. Although the experimental phase diagram for ceria is well established in the composition of interest, there have not been significant attempts at studying the oxygen vacancy thermodynamics at elevated temperatures from first principles. We performed GGA+$U$ calculations ceria to study the electronic structure and ground state energies of various concentrations and configurations of oxygen vacancies in ceria. The energies are then fitted to a Cluster expansion Hamiltonian to efficiently model the interactions between the different species : Ce$^{3+}$, Ce$^{4+}$ , O$^{2-}$ and oxygen vacancies . Lattice Monte Carlo simulations are then performed to obtain the free energy as a function of temperature and oxygen chemical potential through thermodynamic integration. We are also investigating the effect of lattice vibrational contribution to the phase diagram by including a temperature dependent cluster expansion.   

Defect-Induced Segregation and Lattice Stability of BSCF Perovskites

Among novel advanced materials for clean energy, \textbf{Ba}$_{x}$\textbf{Sr}$_{1-x}$\textbf{Co}$_{1-y}$\textbf{Fe}$_{y}$\textbf{O}$_{3-\delta }$(\textbf{BSCF}) are considered as promising materials for cathodes in solid oxide fuel cells (SOFC) and oxygen permeation membranes. BSCF exhibits a good oxygen exchange performance, mixed ionic and electronic conductivity, high oxygen vacancy concentration, and low diffusion activation barrier, which largely define the oxygen reduction chemistry. However, understanding the interplay between the structural disorder and crystalline stability in BSCF is extraordinarily complex and essentially unexplored. We present first principles calculations of an ideal BSCF crystal and the crystal containing point defects, Frenkel and Schottky disorders, cation and antisite exchanges, and a set of relevant solid-solid solutions. We discuss possible mechanisms of defect-induced (in)stability, solid-state decomposition reactions, and phase transitions of the BSCF lattice as a function of oxygen vacancy concentration for cubic and hexagonal BSCF in the context of available experiments. This research explains the observed SOFC performance reduction and provides insights on enhancing energy conversion.   

Session B27: Invited Session: Magneto-Electric and Magneto-Optical Properties of Topological Insulators

Magneto-Optical Properties of Electrically Gated Topological Insulators

Topological insulators (TI) are a predicted new quantum state of matter in which spin-orbit coupling gives rise to topologically protected surface states with unpaired spin-helical Dirac cones. TIs are predicted to have exotic properties including Majorana fermions induced by the proximity effect from a superconducting film, a intrinsic magnetoelectric effect and hybridized spin-plasmon modes. The magneto-electric effect leads to Faraday and Kerr rotations quantized in units of the fine structure constant. This effect corresponds to $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $ integer quantum Hall step and is predicted both in field and \textbf{\textit{in the absence of a magnetic field when magnetic order gaps the Dirac spectrum}}. A key difference between this half integer QHE in TIs and the usual integer QHE is that the former cannot be measured by a dc transport experiment. I will describe experiments designed to measure the Kerr rotation in Bi2Se3, one example of a topological insulator. Gating the surface isolates the surface response from the bulk signals due to unavoidable bulk carriers from defects and impurities. Preliminary results will be presented on the surface state Kerr rotation for Bi2Se3 doped with Mg (ungapped) and Bi2Se3 doped with Sm (magnetically gapped).   

Magneto-Electric Effect in Three-Dimensional Topological Insulators from Surface Magnetic Disorder and Ferromagnetic Thin Film

Topologically nontrivial gapped phases can be characterized by the bulk topological indices and the surface gapless modes. The topological magneto-electric (ME) effect is a novel manifestation of the bulk-surface correspondence in which the bulk magnetization is generated by a circulating quantized Hall current flowing at the surface of topological insulators. To realize the topological ME effect, there are two difficulties: (a) one needs to attach an insulating ferromagnetic layer with the magnetization normal to the surface all pointing out or in. (b) The Fermi energy must be tuned accurately within the small gap of the surface Dirac spectrum opened by the exchange interaction. In this talk we discuss the anomalous quantized Hall current on the surface of a magnetically doped topological insulator, basing on the two-dimensional surface Dirac Hamiltonian with magnetic disorder. The scaling analysis indicates that, in sharp contrast to the time-reversal-invariant cases, the all surface states tend to be localized while the Hall conductivity is quantized no matter whether the Fermi level resides within or out of the surface gap. This resolves problem(b). Furthermore it is shown that this also resolves problem (a) with the simultaneous application of magnetic and electric fields parallel or antiparallel to each other. By this method, doped local spins can be controlled by the bulk energy which can overcome the magnetic anisotropy and Zeeman splitting at the surface. We also comment on the generalization of the topological responses to the case of topological superconductors and superfluids. This work was done in collaboration with Naoto Nagaosa, Shinsei Ryu, and Akira Furusaki. K. Nomura and N. Nagaosa, Phys. Rev. Lett. 106, 166802 (2011); K. Nomura, S. Ryu, A. Furusaki, N. Nagaosa, arXiv:1108.5054.   

Scotch Tape and Spectroscopy, Probing and Manipulating the surface of a Topological Insulator

Recently there has been a great deal of interest in studying the surfaces of materials with topologically non-trivial electronic states. In addition to probing the surfaces of topological insulators it is highly desirable to put them in proximity with other materials (ferromagnets and superconductors) to induce new particles such as Majoranna Fermions. I will discuss our groups efforts to study these materials using mechanical exfoliation and a variety of spectroscopic techniques (Raman, IR and tunneling). In addition I will detail a new method we have devised that enables us to produce high temperature superconductivity in a topological insulator via the proximity effect.   

Giant terahertz Faraday rotation in graphene

The Faraday rotation of the polarization of light in a medium, where the time-reversal symmetry is broken due to external magnetic field, is an optical analogue of the Hall effect. We recently demonstrated that graphene, the thinnest existing material, can turn the polarization of terahertz radiation by several degrees in modest magnetic fields, which is a spectacular manifestation of the cyclotron resonance. In this talk I will review our Faraday rotation spectroscopy studies of single-layer and twisted multilayer epitaxial graphene with an emphasis on the physical information that one can extract from these measurements and potential applications.\\[4pt] [1] I. Crassee et al. Nature Physics \textbf{7}, 48 (2011).\\[0pt] [2] I. Crassee et al. Phys. Rev. B, \textbf{84}, 035103 (2011).   

Ultrafast Probing of Dynamical Spin-Charge Coupling in Topological Insulators

The three-dimensional topological insulator (TI) is a new quantum phase of matter that exhibits quantum-Hall-like properties, even in the absence of an external magnetic field. Charge carriers on the surface of a TI behave like a two-dimensional gas of massless helical Dirac fermions for which the spin is ideally locked perpendicular to the momentum. In this talk, I will discuss recent experiments in which we used the angular momentum of circularly polarized ultrafast laser pulses to directly visualize and manipulate the spin-charge coupling in TIs. By using laser pulses in the UV region, we performed novel time of flight based angle-resolved photoemission spectroscopy that enabled simultaneously mapping all three components of spin over the entire Dirac cone of a TI. We find that an idealized description of helical Dirac fermions only applies within a small energy window about the Dirac point, beyond which strong textural deformations occur. Utilizing the pump-probe technique, we selectively obtained time-resolved dynamics of surface and bulk excitations. By using circularly polarized laser pulses in the optical region, we achieved preferential excitation of spin species on one side of the surface Dirac cone, resulting in a charge imbalance in momentum space and thus causing a current flow with a direction dependent on photon helicity.   

Session B28: Focus Session: Dopants and Defects in Semiconductors - Nitrides

Optical signatures of defects in nitride semiconductors

Despite successful commercialization of GaN-based light emitting diodes (LEDs) and high frequency transistors, crystal defects continue to have a strong and often undesired impact on the opto-electronic properties of the III-Nitride family of materials. Fully realizing the potential of this fascinating materials system requires a better understanding of the physical origin of defects, their dependence on both substrate quality and epitaxial growth conditions, and their influence on electrical and optical properties. This talk will discuss the use of deep level optical spectroscopy (DLOS) to quantitatively study defect states in GaN-based materials. As a photocapacitance technique, DLOS is able to probe defect levels that are otherwise inaccessible to thermally-stimulated defect spectroscopies in wide band gap materials, such as the III-Nitrides. DLOS quantifies both the energy level and density of defects and probes defect states with nanoscale depth resolution. Beyond the canonical application of DLOS to thin films, this talk will describe new developments of DLOS to quantitatively study defect states in a wide variety of structures with nanoscale dimensionality, including InGaN/GaN multi-quantum wells and AlGaN/GaN core-shell nanowires. The microscopic origin of observed defect states and their influence on the electrical and optical properties of GaN-based LEDs and nanowire devices will be discussed. The reported studies establish DLOS as a critical technique for nanoscale dimensional defect metrology that is able to advance the development of conventional and emerging opto-electronic devices.   

Shallow versus deep nature of Mg acceptors in nitride semiconductors

Although Mg doping is the only known method for achieving p-type conductivity in nitride semiconductors, Mg is not a perfect acceptor. Hydrogen is known to passivate the Mg acceptor, necessitating a post-growth anneal for acceptor activation. Furthermore, the acceptor ionization energy of Mg is relatively large (200 meV) in GaN, thus only a few percent of Mg acceptors are ionized at room temperature. Surprisingly, despite the importance of this impurity, open questions remain regarding the nature of the acceptor. Optical and magnetic resonance measurements on Mg-doped GaN indicate intriguing and complex behavior that depends on the growth, doping level, and thermal treatment of the samples. Motivated by these studies, we have revisited this topic by performing first-principles calculations based on a hybrid functional. We investigate the electrical and optical properties of the isolated Mg acceptor and its complexes with hydrogen in GaN, InN, and AlN. With the help of these advanced techniques we explain the deep or shallow nature of the Mg acceptor and its relation to optical signals often seen in Mg-doped GaN. We also explore the properties of the Mg acceptor in InN and AlN, allowing predictions of the behavior of the Mg dopant in ternary nitride alloys.   

Effect of Doping Profile and Concentration on the Near-Infrared Optical Properties of AlGaN/GaN and AlInN/GaN Heterostructures

Intersubband (ISB) devices utilizing III-nitrides have recently attracted attention for near- and far- infrared optoelectronic applications. In order to achieve efficient ISB transitions, large doping densities are typically required ($>$1E18 cm$^{-3}$). The large impurity density has significant effects on the band structure and material quality, effects that are reflected in important device parameters such as transition energies and linewidths. To determine the optimal doping concentration and profile for III-N intersubband devices, we carried out a systematic study of optical and structural properties of strained AlGaN/GaN and lattice-matched AlInN/GaN heterostructures grown by MBE on quasi-bulk GaN substrates. The lattice-matched AlInN/GaN system is targeted because it allows growth of thick strain-free materials. However, it also presents some considerable growth challenges due to the vastly different optimal growth conditions for Al and In containing nitrides. The transition energy and line profile were determined by direct and photoinduced absorption measurements, while the material quality was assessed using TEM and high resolution x-ray diffraction. The FWHM of the ISB transition at 1.9 $\mu$m was found to vary up to 60\% with the position of delta doping in the quantum well.   

Field-enhanced vacancy diffusion in AlGaN

Room-temperature (RT) native defect diffusion does not generally occur in semiconductors because of high activation energies ($>$1.5 eV). However, recent observations of plastic deformation in AlGaN/GaN High Electron Mobility Transistors (HEMTs) have been attributed to diffusive processes. Here we report first-principles density-functional calculations of the formation and migration energies of vacancies, including the effect of strain and electric fields. We find that triply-negatively charged cation vacancies are the enablers of self-diffusion, as follows: though strain alone is insufficient, we find significant activation barrier lowering due to the applied electric field acting on charged vacancies, reducing cation vacancy barriers in AlGaN to $\sim $1 eV or lower where RT diffusion becomes significant. The described mechanism of electric field enhanced vacancy diffusion is relevant for other materials, including several oxides that also feature charged vacancies with low formation energy.   

Role of native defects and related complexes in absorption and luminescence of AlN

AlN is a wide-band-gap material that has being considered as a substrate for GaN-based optoelectronic devices, or in its own right for deep ultraviolet light-emitting diodes and laser diodes. The band gap of 6.2 eV in principle allows transparency in the visible to UV range, but in practice AlN crystals exhibit several sub-band-gap absorption and emission bands, likely due to the presence of native defects and impurities (the most common being oxygen). Using first-principles calculations with the screened hybrid functional of Heyd, Scuseria, and Ernzerhof (HSE), we investigate the structural, electronic, and optical, properties of N and Al vacancies, and their complexes with O impurities in AlN. Defect charge transition levels and stable charge states are determined from the calculated formation energies, and absorption and emission energies are obtained by constructing configuration coordinate diagrams. Our results indicate that Al vacancies and O impurities are responsible for several absorption/emission lines observed experimentally, in particular for the absorption band around 2.85 eV (which gives AlN crystals a yellowish color) and the emission around 3.20 eV emission (375 nm), often observed in O-doped AlN. Mechanisms for each of these processes will be discussed in detail.   

Effective mass calculations for shallow acceptors in nitrides

In the effective mass approximation for shallow acceptors in semiconductors, the defect eigenstates are written as a product of a slowly varying envelope function and the band extrema Bloch functions. The Kohn-Luttinger Hamiltonian describing the valence band manifold in zincblende, or its generalization for other crystals structures, then becomes a set of coupled differential equations for the envelope function. These can be solved by a variational approach with hydrogenic type basis functions. We have implemented this approach for the appropriate Hamiltonians for zincblende, wurtzite and an orthorhombic crystal structure occurring for II-IV-N2 semiconductors. The Hamiltonian parameters used were extracted from first-principles GW calculations. The central cell correction to the Coulomb potential was added based on pseudopotential differences as proposed by Mireles and Ulloa (Phys. Rev. B 58, 3879 (1998)). Results are presented for various acceptors in GaN, AlN, InN, ZnGeN2 and ZnSnS2. The effects of varying the crystal field splitting parameter, and the type of pseudopotentials (including or not semicore d-states) were investigated.   

Evidence for mobile electrons in p-type GaN:Mg

Although Mg-doping is the only successful means of achieving p-type conductivity in GaN, little is known about the local environment of the impurity. Our work focuses on a unique phenomena revealed in the electron paramagnetic resonance (EPR) spectrum attributed to Mg: an angular dependent line-shape suggesting the presence of free carriers. 10 GHz EPR measurements were made at 4 K with the magnetic field in the plane of the c-axis. Samples included 0.5 -- 1.5 um thick Mg-doped GaN films grown on sapphire by molecular beam epitaxy or chemical vapor deposition. As expected, the angular dependence of the g value reflects axial symmetry. Unexpected is the line shape change from pure Lorentzian with the magnetic field 30$^{o}$ from the c-axis to increasingly Dysonian upon rotation. The latter reflects the presence of mobile carriers due to, for instance, an interfacial conducting layer, polarization charge, or loosely bound electrons from Ga near neighbors. The new EPR analysis suggests local fields surround the Mg impurity which influence the acceptor's response.   

A Combined Excitation Experiment and the Emission Nature of Eu in GaN

We have developed a fiber based confocal optical microscope that operates inside of a commercial SEM instrument (JEOL 6400) enabling the excitation of a sample either by a laser or by electron beam, and hence combining the complimentary techniques of photoluminescence (PL) and cathodoluminescence (CL). The capabilities of the instrument are demonstrated by experiments involving the excitation of europium ions in-situ doped in Mg doped GaN thin films. The combination of the Eu and Mg defect create new optically active centers (Eu/Mg centers) that absorb energy from electron-hole pairs (EHP) more efficiently than the normal Eu centers in non co-doped GaN (Eu centers). However, the luminescence from these centers decrease as a function of EHP exposure time. The instrument used enables us to perform PL studies before and after electron beam exposure to investigate the nature of this effect. We use a below band gap PL excitation source tuned to resonantly excite the Eu centers using their $^7$F$_0$ to $^5$D$_0$ transitions. This allows us to determine their relative numbers and monitor changes of each of the relevant centers. Uitilizing these data, the nature of the decrease in emission is investigated with our new and unique experimental apparatus.   

Magneto-Optical Studies of Rare Earth Doped III-V Nitrides

We investigated the site selective optical and magneto-optical properties of Neodymium doped Gallium and Aluminum Nitride and Erbium doped Gallium Nitride. For our current study, we applied magnetic fields parallel and antiparallel to the C-axis of the crystals and observed the resulting Zeeman splitting both in excitation and emission transitions. On the basis of these measurements, we determined the effective g-factors of all the states involved in the Nd$^{3+}$ transitions. For erbium doping, we observed the Zeeman splitting of the $^{4}$I$_{13/2 }$and $^{4}$I$_{15/2 }$levels. Due to small crystal field splitting and large Zeeman splitting, the assignment of levels and corresponding g-factors is very complex. In addition, we observed unexpected asymmetries in the emission intensities when we compared the spectra obtained for fields parallel and antiparallel to the growth direction. The degree of this asymmetry depends on the substrate material and is unambiguously related to the strain and resulting internal fields that are induced by lattice mismatch. ~The asymmetry behavior parallels the ferromagnetic behavior that is induced by the rare earth ions in GaN and hence our observation suggests that magnetization can be controlled by strain.   

Origins of Persistent Photoconductivity in Highly Mismatched Semiconductor Alloys

Persistent photoconductivity (PPC) is a nonequilibrium phenomenon in which an illumination-induced increase in conductivity of a semiconductor persists following the termination of illumination. The PPC effect has been explained in terms of the large-lattice-relaxation (LLR) model, in which photoexcited carriers are unable to relax to equilibrium due to an energy barrier between shallow donor and deep donor complex (DX) center configurations. To date, an experimental identification of atomistic configurations in support of a model for the PPC effect has yet to be reported. Here, we examine the origins of the PPC effect in GaAsN alloy films, providing the first direct correlation between the concentration of interstitials and the strength of the PPC effect. Thus, the PPC effect in GaAsN is attributed to a change in the bond orientation or a shift in the center of mass of either N-N or N-As pairs. Similar investigations of GaAsBi alloy films will be discussed.   

Influence of excitation frequency on A$_{1}$(LO) and E$_{2}$ Raman modes in In rich In$_{1-x}$Ga$_{x}$N thin films

MBE grown In$_{1-x}$Ga$_{x}$N ($x$ = 0, 0.1, 0.3 and 0.54) thin films with bandgap energies varying from 0.77 to 1.85 eV have been investigated using Raman spectroscopy with 1.58 and 2.41eV excitation energies. The carrier mobility for InN film is 900 cm$^{2}$/V$\cdot $s and decreases with increasing $x$ with its value being 20 cm$^{2}$/V$\cdot $s for In$_{0.46}$Ga$_{0.54}$N . We observe a one-mode behavior of the A$_{1}$(LO) and E$_{2}$ modes. An enhancement in intensity of A$_{1}$(LO) and 2A$_{1}$(LO) replica modes in In$_{1-x}$Ga$_{x}$N films with bandgap energies close to the excitation energy is observed. For samples with $x \quad >$ 0, the A$_{1}$(LO) mode shows a higher intensity relative to E$_{2 }$mode which indicates a resonant enhancement of the A$_{1}$(LO) mode due to Fr\"{o}lich interaction. We find that the energies of longitudinal optical modes (A$_{1}$(LO) and 2A$_{1}$(LO)) vary nonlinearly, unlike the E$_{2}$ mode, with increasing Ga fraction. The width and asymmetry of the A$_{1}$(LO) band is higher for the lower excitation energy (1.58 eV). This is perhaps due to the structural disorder in the deeper regions of the films or due to the distribution of regions with different indium fractions. This may explain the lower carrier mobilities observed in In$_{1-x}$Ga$_{x}$N films with higher values of $x$.   

Defect and Impurity Properties of Hexagonal Boron-nitride

By using both GGA and hybrid functional calculations, we have systematically calculated the properties of defects and impurity in hexagonal boron-nitride ($h$-BN). Our calculations show that the defect configurations and the local bond lengths around defects are sensitive to their charge states. The possible highest negative charge states of defects are largely determined by the nearly- free-electron state at the conduction band minimum. The in-gap defect levels got from hybrid functional calculations are much deeper than those got from GGA calculations. The formation energies of neutral defects calculated by hybrid functional and GGA are close to each other, but the defect transition energy level between charge states and neutral state respect to valence band maximum are quite different in GGA and hybrid functional calculations. Finally, we show that the charged defect configurations as well as the transition energy levels exhibit interesting layer effects.   

Non-linear piezoelectric polarization in III-V and nitride semiconductors

Piezoelectricity can have a large impact on the electronic and optical properties of quantum well and quantum dots based devices such as lasers, light emitting diodes, infrared photodetectors. In particular it has been shown to be important in III-V and nitride semiconductors. The piezoelectric effect in quantum Well or in Quantum Dots is usually taken into account by neglecting the non linear term in the piezoelectric tensor. We have calculated the second order piezoelectric tensor for all the III-V (including the nitrides) semiconductors in the Wurtzite and Zincblende structure. And we have derived a relation between the proper and the improper second order piezoelectric coefficients. This relation is used to calculate the proper coefficients which are the experimentally measurable ones. We have calculated the piezoelectric field in several Quantum wells and compare our values to experiment. We show that the second order can be so large for Zinc-Blende materials that it cancels the first order term, we demonstrate also that for Nitrides this effect is much lower. However we show that for severely strained structure such as quantum dots or thin films, the second order piezoelectric effect can even exceed the spontaneous polarization in the nitrides.   

Session B29: Focus Session: Quantum Optics with Superconducting Circuits: Many-Body Physics, Phase Transitions, & Bistability

Cavity-Free Photon Blockade Induced by Many-Body Bound States

We show theoretically that a variety of strong quantum nonlinear phenomena occur in a completely open one-dimensional waveguide coupled to an N-type four-level system. This system could be realized, for example, in experiments using superconducting circuits. We focus on photon blockade, photon-induced tunneling, bunching or anti-bunching, and the creation of single-photon states, all in the absence of a cavity. Many-body bound states appear due to the strong photon-photon correlation mediated by the four-level system. These bound states cause photon blockade, generating a sub-Poissonian single-photon source [1]. Such a source is crucial for quantum cryptography and distributed quantum networking; our work thus supports the notion that open quantum systems can play a critical role in the manipulation of individual, mobile quanta, a key goal of quantum communication. [1] H. Zheng, D. J. Gauthier, and H. U. Baranger, Phys. Rev. Lett. in press (2011), arXiv:1107.0309.   

Experimental investigation of a nonequilibrium delocalization-localization transition of photons in circuit quantum electrodynamics

Strong photon-qubit coupling in the circuit quantum electrodynamics architecture may lead to quantum phase transitions of light. Recent theoretical and experimental efforts have been made toward examining such quantum phase transitions in large systems; however, interesting crossovers may also exist in significantly smaller and more controllable systems. A sharp nonequilibrium self-trapping transition of light has been predicted in a system comprising two coupled resonators each containing a single qubit. A delocalized regime, where photons coherently oscillate between the two cavities, transitions via dissipation into a localized regime, where photons cannot tunnel. We realized this system experimentally using two capacitively coupled superconducting microwave coplanar waveguides each containing a single transmon qubit. We present our experimental investigation of the system using time and frequency domain measurements to probe its dynamics.   

Disorder in a Kagome Lattice of Superconducting Coplanar Waveguide Resonators

It has been proposed that arrays of electromagnetic cavities, coupled to two level quantum systems can be used to realize quantum phase transitions of polaritons. One possible experimental realization is a circuit quantum electrodynamics architecture, in which transmon qubits are coupled to superconducting coplanar waveguide resonators (CPWRs); however, for this to be successful, arrays of resonators must be fabricated with low disorder. Results will be reported on characterization of an array of 12 niobium resonators on a sapphire substrate in a honeycomb pattern with the photonic lattice sites forming a Kagome star. These arrays were characterized by measuring many devices of the same design, and using statistical methods for analysis. Furthermore we investigate the origins of disorder, and its dependence on fluctuations in the CPWR geometry.   

Double symmetry breaking and 2D quantum phase diagram in spin-boson systems

When the collective coupling between a chain of spins (two-levels systems) and a bosonic mode becomes comparable with the two-level transition frequency, superradiant quantum phase transitions for the cavity vacuum can occur, for instance within the Dicke model [1]. Here, the quantum ground state properties of two independent chains of pseudo-spins interacting with the same bosonic field are investigated [2] . Each chain is coupled to a different quadrature of the field, leading to two independent symmetry breakings for increasing values of the two spin-boson interaction constants. A 2D phase diagram is provided with 4 different phases that can be characterized by the complex bosonic coherence of the ground states and can be manipulated via non-abelian Berry effects. Possible realizations of such model in circuit QED are discussed, generalizing the previous proposals to implement the standard Dicke model [3,4]. \\[4pt] [1] C. Emary, and T. Brandes, Phys. Rev. Lett. 90, 044101 (2003).\\[0pt] [2] P. Nataf, A. Baksic and C. Ciuti, arXiv:1111.1617 (2011).\\[0pt] [3] P. Nataf and C. Ciuti, Nat. Commun. 1 :72 (2010).\\[0pt] [4] P. Nataf and C. Ciuti, Phys. Rev. Lett. 104, 023601 (2010).   

Quantum hysteresis in coupled qubit-radiation systems

We study theoretically the dynamical response of a set of solid-state qubits arbitrarily coupled to a radiation field which is confined in a cavity. Driving the coupling strength in round trips, between weak and strong values, we quantify the hysteresis or irreversible quantum dynamics. The matter-radiation system is modeled as a finite-size Dicke model which has previously been used to describe equilibrium (including quantum phase transition) properties of systems such as quantum dots in a microcavity, and superconducting circuit QED. Here we extend this model to address {\it non-equilibrium} situations. Analyzing the system's quantum fidelity, we find that the near-adiabatic regime exhibits the richest phenomena, with a strong asymmetry in the internal collective dynamics depending on which phase is chosen as the starting point. We identify significant deviations from the conventional Landau-Zener-Stuckelberg formulae, in particular from cycles starting in the superradiant phase. In the diabatic or impulsive regime, the system remains quenched and there is little hysteresis. By contrast, depending on the specifications of the cycle, the radiation subsystem can exhibit the emergence of non-classicality, complexity and sub-Planckian structures as evidenced by its Wigner function.   

Coupled Qubit-Cavity Arrays: Evolution of resonance with hopping

Recent experiments on the light-matter interaction in superconducting qubits have sparked interest in the prospect of studying collective behaviour of coupled qubit-cavity arrays. Any such behaviour will necessarily be non-equilibrium, as the photon loss present in real cavities must be compensated by pumping. We study an array of coupled qubit-cavity systems, with the simplest pumping scheme, using a coherent field. Such pumping might be thought to destroy any interesting physics by imposing coherence on the system. Yet we show that the emerging phenomena are remarkably rich, focussing on the evolution from the antiresonance feature known in the Jaynes-Cummings model [1] as one increases hopping strength between the coupled cavities. We study both the coherent field (as can be measured by homodyne detection) and the fluorescence spectrum, comparing numerical simulations to analytic approximations valid in the limits of large and small hopping. The steady state coherent field depends non-monotonically on the hopping strength, as a crossover occurs from polariton blockade physics at small hopping to semiclassical behaviour at large hopping. \\[4pt] [1] Bishop et al., Nat. Phys. 5, 105 - 109 (2009)   

Superradiant Phase Transitions and the Standard Description of Circuit QED

We investigate the equilibrium behavior of a superconducting circuit QED system containing a large number of artificial atoms. It is shown that the currently accepted standard description of circuit QED via an effective model fails in an important aspect: it predicts the possibility of a superradiant phase transition, even though a full microscopic treatment reveals that a no-go theorem for such phase transitions known from cavity QED applies to circuit QED systems as well. We generalize the no-go theorem to the case of (artificial) atoms with many energy levels and thus make it more applicable for realistic cavity or circuit QED systems.   

Photon thermalization and condensation in circuit QED by engineered dissipation

The ability to engineer the coupling between a quantum system and its environment opens the possibility to dissipatively prepare entangled and quantum many- body states. Of particular interest is the case in which the system undergoes a phase transition driven by the coupling to a reservoir. Here we show how to engineer dissipation in the context of coupled cavity arrays, and more specifically in circuit QED. We propose an implementation based on coupled LC resonators and superconducting qubits, which under asymmetric coherent driving, leads to thermalization of photons to the symmetric state between neighboring sites. Above a critical threshold of the driving intensity, a macroscopic occupation of this symmetric mode is found.   

Stabilizing manifolds of quantum states by reservoir engineering

We consider the problem of stabilizing a manifold of states by reservoir engineering. Qubits are coupled to resonators in the strong dispersive limit for which the dispersive shift is much larger than the cavity decay rate. The resonators are driven by microwave fields. By adequately choosing the frequencies of these fields, we can transfer the entropy of the quantum system into its environment. This stabilization is autonomous and continuous in time, and does not rely on a precise control of the drive field amplitudes. The scheme does not require any knowledge of measurement outcomes thus simplifying its experimental realization. Experimental data on dynamical cooling of a transmon qubit coupled to a compact resonator will be shown. Finally, applications to quantum error correction will be discussed.   

Adiabatic state preparation of interacting two-level systems

We consider performing adiabatic rapid passage (ARP) using frequency-swept optical pulses to excite a collection of interacting two-level systems. Such a model arises in a wide range of many body quantum systems, such as circuit QED or interacting quantum dots, where a nonlinear component couples to light. We analyse the one-dimension case using the Jordan-Wigner transformation. In the mean field limit, the system can be described by a Lipkin-Meshkov-Glick Hamiltonian. Both approaches provide complementary insights into the behaviour of an interacting model under ARP, suggesting our results are generically applicable. We find ARP can still be used for state preparation in the presence of interactions but the parameters required to achieve full occupation depend on the strength of the interaction. In particular, for a fixed pulse time, stronger interactions require a larger pulse bandwidth, introducing new restrictions on the pulse form required.   

Collective quantum coherence in large superconducting circuits with approximate $S_N$ symmetry

The vast majority of superconducting circuits consist of a minimum number of circuit elements, following an implicit conjecture that any increase in circuit complexity thwarts quantum coherence. Recent experiments have evidenced that this conjecture is not compelling and quantum coherence can persist for much larger circuits~[1]. A tool for the design of future circuits, we present theory for the fluxonium qubit, a device which includes a large number, $N$, of array junctions. Taking into account the degrees of freedom of all junctions and the approximate $S_N$ permutation symmetry, we identify the relevant collective mode and pinpoint an approximate decoupling of additional modes. This allows us to derive the effective models previously used. We also discuss corrections going beyond these models which include subspaces of states that transform as non-trivial representations of the permutation group. \par\noindent [1]~V.\ E.\ Manucharyan, J.\ Koch, L.\ I.\ Glazman, and M.\ H.\ Devoret, Science {\bf 326\/}, 113 (2009)   

Quantum crossover of the switching rate of a modulated oscillator

Experiments with Josephson bifurcation amplifiers have reached the regime where switching between coexisting stable vibrational states is due to quantum fluctuations. In switching the oscillator goes over the effective dynamical barrier that separates the states. It was found earlier that, for small damping, the barrier height calculated for $T\to 0$ is smaller than for $T=0$. Respectively, the switching rates calculated in these two limits are exponentially different, the effect of fragility. If other parameters are fixed, both barrier heights are proportional to the number of bound quantum states localized mostly in the basin of attraction of the corresponding stable state. Here we show that for large but finite values of the number of states the $T=0$ solution is stabilized. For some temperature $T_c$ there occurs a sharp crossover to the finite-temperature regime. Our analytical results are corroborated by numerical results.\\[4pt] [1] Vijay et al., Rev. Sci. Instr. (2009)\\[0pt] [2] M. Dykman et al., JETP (1988)\\[0pt] [3] M. Marthaler et al, PRA (2005)   

Observation of single artificial atom optical bi-stability and its application to single-shot readout in circuit quantum electrodynamics

The high power transient behavior of superconducting qubit-cavity systems has recently been used to perform high fidelity readout of transmon qubits [1]. We show that in the steady state, the system exhibits a bi-stable behavior that can be observed on the single-shot level, with the cavity state switching stochastically between dim and bright states. The switching times are shown to be long compared to the cavity and qubit lifetimes. Some features of the bi-stability can be explained by mean field theory, while its switching dynamics is studied with large scale simulations. Understanding these dynamics will be crucial for studying the transient response, an essential aspect of the qubit readout. We will discuss progress on optimizing readout by shaping the measurement pulse. \\[4pt] [1] M. D. Reed, L. DiCarlo, B. R. Johnson, L. Sun, D. I. Schuster, L. Frunzio, and R. J. Schoelkopf, Phys. Rev. Lett. 105, 173601 (2010)   

Session B30: Quantum Error Correction and Quantum Control

Fault-tolerant quantum computation with asymmetric Bacon-Shor codes

Bacon-Shor codes are quantum subsystem codes which are constructed by combining together two quantum repetition codes, one protecting against $Z$ (phase) errors and the other protecting against $X$ (bit flip) errors. In many situations, for example flux qubits, the noise is biased such that faults that produce $Z$ errors are much more common than faults that produce $X$ errors; in these cases it is natural to consider an asymmetric Bacon-Shor code where the code protecting against $Z$ errors is longer than the code protecting against $X$ errors. This work describes fault-tolerant constructions for gadgets that achieve universal fault-tolerant quantum computation using asymmetric Bacon-Shor codes. Gadgets take advantage of the Bacon-Shor structure by breaking up into parallel smaller gadgets that act on a single row or column, with majority voting of the separate results. For a bias of $ \epsilon/\epsilon' = 10^{4}$, we prove a threshold around $2.5 \times 10^{-3}$. The effective error strength is shown to decrease rapidly (faster than polynomial) with decreasing $\epsilon$. Therefore it may be practical to use Bacon-Shor codes directly with no additional concatenation. This could greatly reduce the resource overhead required for fault-tolerant computation with biased noise.   

Quantum Error Correction: Optimal, Robust, or Adaptive? Or, Where is The Quantum Flyball Governor?

In \emph{The Human Use of Human Beings: Cybernetics and Society} (1950), Norbert Wiener introduces feedback control in this way: \begin{quote} \begin{small} ``This control of a machine on the basis of its actual performance rather than its expected performance is known as \emph{feedback} ... It is the function of control ... to produce a temporary and local reversal of the normal direction of entropy.'' \end{small} \end{quote} \emph{The} classic classroom example of feedback control is the all-mechanical flyball governor used by James Watt in the 18th century to regulate the speed of rotating steam engines. What is it that is so compelling about this apparatus? First, it is easy to understand how it regulates the speed of a rotating steam engine. Secondly, and perhaps more importantly, \emph{it is a part of the device itself}. A naive observer would not distinguish this mechanical piece from all the rest. So it is natural to ask, where is the \emph{all-quantum} device which is self regulating, ie, the Quantum Flyball Governor? Is the goal of quantum error correction (QEC) to design such a device? Devloping the computational and mathematical tools to design this device is the topic of this talk.   

Unified approach to approximate quantum error correction via the transpose channel

Much of the existing work on error correction focuses on the standard paradigm of perfect quantum error correction(QEC), where the recovery operation perfectly reverses the effects of a noise channel. Recent studies on approximate QEC(AQEC) have demonstrated possible advantages that arise from relaxing the requirement for perfect correction. However, while the recovery operation for perfectly correctable codes is well-known, finding the recovery for approximately correctable codes often requires difficult numerical procedures. We demonstrate an analytical, universal and near-optimal recovery map-the transpose channel- for AQEC codes, with optimality defined in terms of the worst-case fidelity. Using the transpose channel, we provide an alternative interpretation of the QEC conditions and generalize them to a set of conditions for AQEC codes. This forms the basis of a simple algorithm for finding AQEC codes. Our analytical approach is a departure from earlier work relying on exhaustive numerical search for the optimal recovery map, with optimality defined based on entanglement fidelity. Our results can also be extended to the general case of approximate operator quantum error correction, thus bringing us closer to a unified, analytical framework for AQEC.(Ref:PRA,81,062342(2010))   

Design of additive quantum codes via the codeword-stabilized framework

Codeword stabilized (CWS) construction defines a quantum code by combining a classical binary code with some underlying graph state. In general, CWS codes are non-additive but become additive stabilizer codes if derived from a linear binary code. Generic CWS codes typically require complex error correction; however, we show that the CWS framework is an efficient tool for constructing good stabilizer codes with simple decoding. We start by proving the lower Gilbert-Varshamov bound on the parameters of an additive CWS code which can be obtained from a given graph. We also show that cyclic additive CWS codes belong to a previously overlooked family of single-generator cyclic stabilizer codes; these codes are derived from a circulant graph and a cyclic binary code. Finally, we present several families of simple stabilizer codes with relatively good parameters, including a family of the smallest toric-like cyclic CWS codes which have length, dimension, and distance as follows: $[[t^2+(t+1)^2,1,2t+1]]$, $t=1,2, \ldots$ \\[4pt] [1] A. A. Kovalev, I. Dumer, and L. P. Pryadko, preprint arXiv:1108.5490 (2011).   

Error-threshold for topological subsystem quantum error-correcting codes

In general, stability against noise in a quantum computer can be achieved by storing quantum information redundantly. For instance, topological quantum error correction averts decoherence effects by encoding qubits in non-local degrees of freedom, while actively correcting for local errors. The key merit of these topological stabilizer codes lies in the intrinsic locality of the operations for syndrome measurement and error correction. Topological subsystem codes further facilitate practical applications by requiring only measurements of adjacent qubit pairs. We numerically determine the error threshold of topological subsystem codes for the depolarizing channel by mapping the problem onto a classical statistical spin model with bond disorder, which is analyzed via large-scale Monte Carlo simulations. In this picture, faulty qubits correspond to antiferromagnetic interactions between classical spins and the point in the disorder--temperature phase diagram where ferromagnetic order is lost corresponds to the error threshold of the underlying quantum bit system.   

Correlated Errors in the Surface Code

A milestone step into the development of quantum information technology would be the ability to design and operate a reliable quantum memory. The greatest obstacle to create such a device has been decoherence due to the unavoidable interaction between the quantum system and its environment. Quantum Error Correction is therefore an essential ingredient to any quantum computing information device. A great deal of attention has been given to surface codes, since it has very good scaling properties. In this seminar, we discuss the time evolution of a qubit encoded in the logical basis of a surface code. The system is interacting with a bosonic environment at zero temperature. Our results show how much spatial and time correlations can be detrimental to the efficiency of the code.   

Quantum Circuits for Measuring Levin-Wen Operators

We give explicit quantum circuits (expressed in terms of Toffoli gates, CNOTs and single qubit rotations) which can be used to perform quantum non-demolition measurements of the commuting set of vertex and plaquette operators that appear in the Levin-Wen model [1] for the case of doubled Fibonacci anyons. Such measurements can be viewed as syndrome measurements for the quantum error correcting code defined by the ground states of the Levin-Wen model --- a scenario envisioned in [2]. A key component in our construction is a quantum circuit ${\cal F}$ that acts on 5 qubits at a time and carries out a so-called $F$-move, a unitary operation whose form is essentially fixed by a self-consistency condition known as the pentagon equation. In addition to our measurement circuits we also give an explicit 7 qubit circuit which can be used to verify that ${\cal F}$ satisfies the full pentagon equation as well as a simpler 2 qubit circuit which verifies the essential nontrivial content of this equation. \newline [1] M.A. Levin and X.-G. Wen, Phys. Rev. B {\bf 71} 045110 (2005). \newline [2] R. Koenig, G. Kuperberg, and B.W. Reichardt, Ann. Phys {\bf 325}, 2707 (2010).   

Exchange-Only Dynamical Decoupling in the 3-Qubit Decoherence Free Subsystem

The Uhrig dynamical decoupling sequence achieves high-order decoupling of a single system qubit from its dephasing bath through the use of bang-bang Pauli pulses at appropriately timed intervals [1]. This high-order decoupling property of the Uhrig sequence has been extended to decouple general noise from single [2] and multiple [3] qubit systems, using single-qubit Pauli pulses. For the 3-qubit decoherence free subsystem (DFS) and related subsystem encodings, Pauli pulses are not naturally available operations; instead, exchange interactions provide all required encoded operations [4]. Here we demonstrate that exchange interactions alone can achieve high-order decoupling against general noise in the 3-qubit DFS. We present decoupling sequences for a 3-qubit DFS coupled to classical and quantum baths and evaluate the performance of the sequences through numerical simulations. References: [1] G. S. Uhrig, Phys. Rev. Lett. 98, 100504 (2007). [2] J. R. West, B. H. Fong, and D. A. Lidar, Phys. Rev. Lett. 104, 130501 (2010). [3] Z.-Y. Wang and R.-B. Liu, Phys. Rev. A 83, 022306 (2011). [4] J. Kempe et al., Phys. Rev. A 63, 042307 (2001).   

Upper bounds on coherence preservation in dynamical decoupling

We explore the fundamental limits on coherence preservation by dynamical decoupling in terms of control time scales and the spectral bandwidth of the environment. Our main focus is a decohering qubit controlled by arbitrary sequences of $\pi$ pulses. Using methods from mathematical analysis, we establish a non-perturbative lower bound for the coherence loss in terms of the minimum pulse separation and the cutoff frequency of the environment. We argue that similar bounds are applicable to a variety of open-loop unitary control methods while we find no explicit dependence of such lower bounds on the total control time. We use these findings to generate bandwidth adapted dynamical decoupling sequences that can preserve a qubit up to arbitrary long times within the best fidelities theoretically possible given the available control resources. Our analysis reinforces the impossibility of fault-tolerance accuracy thresholds under purely reversible error control.   

Generalized Uhrig Dynamical Decoupling for Multi-Level Quantum Systems

Dynamical decoupling can efficiently suppress decoherence induced by the system-environment interaction. Recently, Uhrig proposed an efficient dynamical decoupling scheme, which uses only $N$ pulses to suppress dephasing noise to $O(T^{N+1})$ for a qubit system with total time evolution $T$. We generalize Uhrig's dynamical decoupling scheme from 2-level to $L$-level quantum systems. We find that $M=(L-1)N$ pulses are sufficient to suppress dephasing noise to $O(T^{N+1})$. We observe interesting patterns in the timing of these pulses, which depend on both $L$ and $N$ with various asymptotic forms for large $L$ or large $N$.   

Universal Dynamical Decoupling and Quantum Walks in Functional Spaces

We investigate the universal dynamical decoupling (DD) schemes, which can restore the coherence of quantum system independent of the details of system-environment interaction. We introduce a general mapping between DD sequences and quantum walks in functional spaces, and use it to prove the universality of various DD schemes such as quadratic DD, nested Uhrig DD, and Uhrig concatenated DD, as well as previously known universal schemes of concatenated DD, Uhrig DD and concatenated Uhrig DD.   

Protecting adiabatic quantum computation by dynamical decoupling

Adiabatic quantum computation (AQC) relies heavily on a systems ability to remain in its ground state with high probability throughout the entirety of the adiabatic evolution. System-environment interactions present during the evolution manifest decoherence, thereby increases the probability of excitation. In this work, it is shown that the existence of such noise-producing terms can be dramatically reduced by Dynamical Decoupling (DD). In particular, we consider a multi-qubit system subjected to a classical bath modeled by random Gaussian-correlated noise. The performance of deterministic schemes such as Concatenated Dynamical Decoupling (CDD) and Nested Uhrig Dynamical Decoupling (NUDD) are analyzed for Grover's search algorithm and the two-qubit Satisfiability (2-SAT) problem. The CDD evolution substantially increases noise suppression with increasing concatenation level. In contrast, improvements in performance are only observed for specific sequence orders in the NUDD scheme. These results are verified for both adiabatic evolutions in terms of the total adiabatic run time and minimum pulse interval.   

Polar codes for achieving the classical capacity of a quantum channel

We construct the first near-explicit, linear, polar codes that achieve the capacity for classical communication over quantum channels. The codes exploit the channel polarization phenomenon observed by Arikan for classical channels. Channel polarization is an effect in which one can synthesize a set of channels, by ``channel combining'' and ``channel splitting,'' in which a fraction of the synthesized channels is perfect for data transmission while the other fraction is completely useless for data transmission, with the good fraction equal to the capacity of the channel. Our main technical contributions are threefold. First, we demonstrate that the channel polarization effect occurs for channels with classical inputs and quantum outputs. We then construct linear polar codes based on this effect, and the encoding complexity is O(N log N), where N is the blocklength of the code. We also demonstrate that a quantum successive cancellation decoder works well, i.e., the word error rate decays exponentially with the blocklength of the code. For a quantum channel with binary pure-state outputs, such as a binary-phase-shift-keyed coherent-state optical communication alphabet, the symmetric Holevo information rate is in fact the ultimate channel capacity, which is achieved by our polar code.   

Improved coded optical communication error rates using joint detection receivers

It is now known that coherent state (laser light) modulation is sufficient to reach the ultimate quantum limit (the Holevo bound) for classical communication capacity. However, all current optical communication systems are fundamentally limited in capacity because they perform measurements on single symbols at a time. To reach the Holevo bound, joint quantum measurements over long symbol blocks will be required. We recently proposed and demonstrated the ``conditional pulse nulling'' (CPN) receiver -- which acts jointly on the time slots of a pulse-position-modulation (PPM) codeword by employing pulse nulling and quantum feedforward -- and demonstrated a 2.3 dB improvement in error rate over direct detection (DD). In a communication system coded error rates are made arbitrary small by employing an outer code (such as Reed-Solomon (RS)). Here we analyze RS coding of PPM errors with both DD and CPN receivers and calculate the outer code length requirements. We find the improved PPM error rates with the CPN translates into $>$10 times improvement in the required outer code length at high rates. This advantage also translates increase the range for a given coding complexity. In addition, we present results for outer coded error rates of our recently proposed ``Green Machine'' which realizes a joint detection advantage for binary phase shift keyed (BPSK) modulation.   

Session B31: Topological Insulators: Semimetals and Interactions

Adler-Bell-Jackiw anomaly in Weyl semi-metals: Application to Pyrochlore Iridates

Weyl semimetals are three dimensional analogs of graphene where the energy of the excitations are a linear function of their momentum. Pyrochlore Iridates are conjectured to be examples of such a system, with the low energy physics described by twenty four Weyl nodes. An intriguing possibility is that these materials provide a physical realization of the Adler-Bell-Jackiw anomaly. In this talk we report on the properties of pyrochlore iridates in an applied magnetic field. We find that the dispersion of the lowest landau level depends on the direction of the applied magnetic field. As a consequence the magneto-conductivity in an electric field, applied parallel to the magnetic field is highly anisotropic, providing a detectable signature of the semi-metallic state.   

Dirac Semimetal in Three Dimensions

In a Dirac semimetal the conduction and valence bands contact at discrete points in the Brillouin zone, dispersing linearly away from these critical points with the low energy physics described by a four band Dirac Hamiltonian. In 2D this situation is realized in graphene in the absence of spin-orbit coupling. 3D Dirac semimetals are predicted to exist at the phase transition between a topological insulator and an ordinary insulator when inversion symmetry is preserved. Here we show that 3D Dirac points can also be protected by crystallographic symmetries in particular space groups and enumerate the criteria necessary to identify these groups. As an example of a Dirac semimetal, we present calculations for $\beta$-Cristobalite ${\rm BiO_2}$ which exhibits Dirac points at the three symmetry related $X$ points of the FCC Brillouin zone. We find that $\beta$-Cristobalite ${\rm BiO_2}$ is metastable, so it can be physically realized as a 3D analog to graphene. We provide a systematic approach that includes crystallographic symmetry arguments and physical and chemical considerations to identify other such materials and rule out possible alternatives such as HgTe. This would greatly expand the range of applications that take advantage of properties arising from Dirac points.   

Semi-metal and Topological Insulator in Perovskite Iridates

The two-dimensional (2D) layered perovskite Sr$_2$IrO$_4$ was proposed to be a spin-orbit (SO) Mott insulator, where the effect of Hubbard interaction is amplified on a narrow J$_{eff}$=$\frac{1}{2}$ band due to strong spin-orbit coupling. On the other hand, the three-dimensional (3D) orthorhombic perovskite SrIrO$_3$ remains metallic. We construct a tight-binding model for SrIrO$_3$ to understand the physical origin of the metallic behaviour and study possible metal-insulator transitions. In particular, we identify possible perturbations that turn the material into a topological insulator.   

Topological Insulators and Semimetals with Point Group Symmetries

In this work, we study the theory of topological phases in systems with point group symmetries (PGSs) in one-, two- and three-dimension. The systems we study in general do not require time-reversal symmetry, and hence may be realized in both non-magnetic and magnetic materials. We show that a point group symmetry introduces new quantum numbers which reveal themselves in the entanglement spectrum as mid-gap states. PGSs also define a series of topological semimetals, in which the band touching points are protected by certain symmetries. We apply our theory to 3D ferromagnetic semimetal HgCr$_2$Se$_4$ which possesses a double-vortex band crossing protected by $C_4$ rotation symmetry.   

Diamagnetism of Weyl semimetals

We present a study of the diamagnetic orbital response in a Weyl semimetal, the recently discovered gapless topological phase of matter. Weyl semimetal is a three-dimensional (3D) material, characterized by the presence of isolated Dirac (Weyl) point nodes in its band structure. It can be thought of as the closest 3D analog of graphene. It is known from graphene studies that two-dimensional (2D) Dirac fermions have a highly nontrivial singular diamagnetic response to an applied perpendicular magnetic field, reflecting the quantum critical nature of the ground state of undoped graphene. Here we investigate the analogous orbital response of 3D Dirac fermions in a Weyl semimetal to an applied magnetic field. As in 2D graphene, we find strong signatures of quantum criticality in the diamagnetic response of 3D Weyl semimetal. In particular, we find that the orbital susceptibility has a characteristic logarithmic dependence on the applied field, deviation of the chemical potential from the charge-neutral position and temperature. This unusual diamagnetic response can be used for experimental characterization of Weyl semimetals.   

Weyl Semimetal in a Topological Insulator Multilayer

We propose a simple realization of the three-dimensional (3D) Weyl semimetal phase, utilizing a multilayer structure, composed of identical thin films of a magnetically-doped 3D topological insulator (TI), separated by ordinary-insulator spacer layers. We show that the phase diagram of this system contains a Weyl semimetal phase of the simplest possible kind, with only two Dirac nodes of opposite chirality, separated in momentum space, in its bandstructure. This Weyl semimetal has a finite anomalous Hall conductivity, chiral edge states, and occurs as an intermediate phase between an ordinary insulator and a 3D quantum anomalous Hall insulator. We discuss unusual transport properties of the Weyl semimetal, and in particular point out quantum critical-like scaling of the DC and optical conductivity.   

3D Weyl Semimetal in a Honeycomb Array of Topological Nano-wires

The Weyl semimetal phase has been recently introduced and suggested to exist in strongly correlated pyrochlore iridates as well as in the non-interacting layered Normal/Topological band insulator systems. This unusual phase has a number of interesting properties in the bulk and at the surface arising from the appearance of isolated point-like hedge-hog topological defects (known as Weyl-Dirac points) in the Bloch-state manifold. Here we discuss the possible emergence of this phase in a honeycomb arrangement of the parallel topological insulator nano-rods each exposed to a half-integer multiple of magnetic flux quantum. We consider direct hoping between rods as well as the electron-electron interaction between them. We discuss how the initially degenerate Weyl points can be separated in the Brillouin zone by various perturbations breaking either inversion or time-reversal symmetry.   

Charge transport in Weyl semimetals

Weyl semimetals are three-dimensional phases with band touchings, whose low-energy excitations are governed by the Weyl equation. They can be thought of as higher dimensional cousins of graphene. Recent theoretical work predicted certain pyrochlore iridates such as Y2Ir2O7 to be in this phase. We study charge transport in Weyl semimetals in the presence of Coulomb interactions or disorder at temperature T and compare our results to existing data on Y2Ir2O7 and Eu2Ir2O7. In the interacting clean limit, we determine the conductivity by solving a quantum Boltzmann equation within a ``leading log'' approximation and find it to be proportional to T, upto logarithmic factors arising from the flow of couplings. In the noninteracting disordered case, we compute the finite-frequency Kubo conductivity and show that it exhibits distinct behaviors for low and high frequencies compared to T. The behavior of Weyl semimetals in a magnetic field will also be briefly discussed.   

Magnetic Instability on the Surface of Topological Insulators

Gapless surface states that are protected by time reversal symmetry and charge conservation are among the manifestations of 3D topological insulators. In this work we study how electron-electron interaction may lead to spontaneous breaking of time reversal symmetry on surfaces of such insulators. We find that a critical interaction strength exists, above which the surface is unstable to spontaneous formation of magnetization, and study the dependence of this critical interaction strength on temperature and chemical potential.   

Plasmons in Topological Insulators

A theory is presented for calculating the plasmon mode dispersion relation in three-dimensional topological insulators (TI). There are two-dimensional (2D) conducting surface states. The conducting states localized close to the surface of the semi-infinite slab have a well defined Dirac cone. The bulk energy gap is large and comparable with room temperature. We investigate plasmon excitations of those surface bound electrons in the long wavelength limit employing the random-phase approximation. Results from our calculations show that for a quasi-1DTI, the plasmon dispersion relation is given by $\omega_p \approx q \left({ 1- \omega_{0} \ln(q)}\right)$ where $\omega_0 = \frac{2 e^2}{\pi \epsilon_0} \frac{3}{10}$. On the other hand, for the conventional 1DEG, the plasmon dispersion satisfies $\omega_p \approx q \sqrt{-\omega_{0} \ln(q)}$, with $\omega_0 = 2n_{1D} e^2/\epsilon_0 m$ and $n_{1D}$ denoting the linear electron density. The plasmons in 1DTI are density-independent as they are in metallic armchair graphene nanoribbons but obey different dispersion relation. The material parameters we chose correspond to $\texttt{Bi}_2 \texttt{Te}_3$ crystals.   

Topological Insulators with electron-electron interactions

We consider the effect of interaction for the $3$ dimensional Topological insulators. We show that effectively the system is equivalent to non-interacting Topological Insulators in 4+1 dimensions.   

Actinide Topological Insulator Materials with Strong Interaction

Topological band insulators have recently been discovered in spin-orbit coupled two and three dimensional systems. In this work, we theoretically predict a class of topological Mott insulators where interaction effects play a dominant role. In actinide elements, simple rocksalt compounds formed by Pu and Am lie on the boundary of metal to insulator transition. We show that interaction drives a quantum phase transition to a topological Mott insulator phase with a single Dirac cone on the surface.   

Fractional quantum Hall effect and plasmons in topological insulators

I will discuss theoretical studies of the effect of Coulomb interactions at the topological insulator surface in the presence of a magnetic field. Coulomb interaction can cause composite fermion formation and the fractional quantum Hall effect. We predict the stability of the fractional quantum Hall effect by considering the form of the effective interparticle interaction: if it is sufficiently short ranged, then there will be composite fermion formation. We will also study plasmons and magnetoplsmons of the surface states both for an ideal topological insulator and for a topological insulator with bulk conduction.   

Half quantum spin Hall effect on the surface of weak topological insulators

We investigate interaction effects in three dimensional weak topological insulators (TI) with an even number of Dirac cones on the surface. We find that the surface states can be gapped by a surface charge density wave (CDW) order without breaking the time-reversal symmetry. In this sense, timereversal symmetry alone can not robustly protect the weak TI state in the presence of interactions. If the translational symmetry is additionally imposed in the bulk, a topologically non-trivial weak TI state can be obtained with helical edge states on the CDW domain walls. In other words, a CDW domain wall on the surface is topologically equivalent to the edge of a two-dimensional quantum spin Hall insulator. Therefore, the surface state of a weak topological insulator with translation symmetry breaking on the surface has a ``half quantum spin Hall effect,'' in the same way that the surface state of a strong topological insulator with time-reversal symmetry breaking on the surface has a ``half quantum Hall effect.'' The on-site and nearest neighbor interactions are investigated in the mean field level and the phase diagram for the surface states of weak topological insulators is obtained.   

2D symmetry protected topological orders and their protected gapless edge excitations

Topological insulators/superconductors with time reversal or particle-hole symmetry protected gapless edge excitations have been well characterized and classified in free fermion systems. However, it is not clear in general interacting boson or fermion systems, when such symmetry protected topological(SPT) orders exist with gapless edge excitations that are protected even against strong interactions. Here, we present a systematic construction of 2D interacting bosonic models with non-trivial SPT orders for any on-site symmetry of group $G$. We demonstrate the non-trivialness of the models by rigorously proving that the gapless edge excitations of the system is stable against any interaction as long as symmetry is not broken. We prove this result by developing the tool of matrix product unitary operator to study the nonlocal symmetry transformation on the edge degrees of freedom and revealing its relation to the non-trivial 3-cocycles of the symmetry group $G$. This relation between SPT orders and group cocycles has actually been established in 1D interacting systems and led to a complete classification of 1D SPT orders. We show here that this relation also extends to $>2$ spatial dimensions and possibly provides a (partial) classification of SPT orders in all interacting systems.   

Session B32: Focus Session: Dielectric, Ferroelectric, and Piezoelectric Oxides - Relaxors, Nanostructures, and Morphotropic Phase Boundary

Symmetry determination of piezoelectric (1-x)Pb(Mg$_{1/3}$Nb$_{2/3})$O$_{3}$-xPbTiO$_{3}$ single crystal near morphotropic phase boundary

The symmetry of (1-x)Pb(Mg$_{1/3}$Nb$_{2/3})$O$_{3}$-xPbTiO$_{3}$ single crystal (PMN-xPT) is investigated at the morphotropic phase boundary with x=31{\%}. The XRD result indicates that the average symmetry of annealed PMN-31{\%}PT single crystal is in close agreement with the monoclinic M$_{C}$ (Pm) phase, but with strain. The local symmetry of PMN-31PT are then investigated at three different directions of [001]$_{C}$, [010]$_{C}$, [011]$_{C}$ and [111]$_{C}$ by using convergent electron diffraction (CBED) technique with a probe size of 2 nm. The dynamical theory is used to simulate the CBED patterns for the reported phases of PMN-xPT. The CBED results show that significant deviations of the experimental CBED patterns from the simulations. The deviations in the CBED results are considered as a result from a local distortion induced by the different B cations of Mg$^{2+}$, Nb$^{5+}$ and Ti4$^{+}$ The local symmetry of PMN-31{\%}PT is not defined by any reported phases and the symmetry of PMN-31{\%}PT seen in X-ray diffraction is the average of local structures   

Phase diagram and skin effect of the relaxor ferroelectric $(1-x)$Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_3$+$x$PbTiO$_3$

We revisit the phase diagram of the relaxor ferroelectric PMN-$x$PT using neutron powder diffraction to test suggestions that defects in the oxygen stoichiometry and/or strain affect the ground state crystal structure. Two identical sets of PMN-$x$PT powders were prepared with $x=0.10$, 0.20, 0.30, and 0.40. One set was annealed in air at 873K for 2h. For a given composition and temperature the same structural phase is observed in each set, thus indicating that the effects of strain and oxygen vacancies are minimal. But the distortions measured in the annealed samples are consistently smaller than those in the as-grown samples. In addition, the average grain size of the annealed samples is approximately twice that of the as-grown samples ($1.2\pm0.6$ microns vs $0.6\pm0.3$ microns). This result is consistent with a skin effect in which Ti-poor bulk crystals show significantly smaller distortions than do powders of the same composition. The diffraction patterns for both the as-grown and annealed samples with compositions $x=0.10$ and $x=0.20$ are best refined using the monoclinic $Cm$ space group, which agrees with recent speculation by Singh et al., Phys. Rev. B {\bf74}, 024101 (2006).   

Model for Relaxor Ferroelectrics

We consider a model of a lattice made of polarizable unit cells with local and dipole interactions and disorder. Without disorder, it is well known that dipole interactions alone do not lead to ferroelectricity. We show by a cluster expansion in disorder and by a high temperature series expansion that in the presence of sufficient disorder, the model with both local and dipole interactions has locally correlated regions at low temperatures but has no long-range order. We compare our results with measurements of the static and dynamic structure factor by neutron scattering~[1]. References: 1. S.N.~Gvasaliya, B.~Roessli, R.A.~Cowley, P.~Huber, S.~Lushnikov, J. Phys.: Condens. Matter 17 4343 (2005).   

Molecular Dynamics Studies of Structure, Dynamics and Dielectric Response in a Relaxor Ferroelectric

Since the first synthesis of the classic PbMg$_{1/3}$Nb$_{2/3}$O$_3$ (PMN) material in 1961, relaxor ferroelectrics have been the subject of ongoing experimental and theoretical investigation due to their fundamental scientific interest and their importance in technological applications. We use atomistic molecular dynamics simulations to study relaxor behavior in the 0.75PMN-0.25PT material. Even for a fairly small simulation size of 1000 atoms, the system exhibits frequency dispersion and deviation from the Curie-Weiss law typical of relaxor materials. Analysis of the time autocorrelation functions for individual atoms allows us to identify the Nb atoms with a high concentration of neighboring Ti atoms as the nucleation sites for the relaxor behavior. This is due to the higher coupling between the cation displacements induced by the presence of overbonded oxygen atoms. We also analyze local structure and dynamics in PMN-PT using instantaneous, time-averaged and frequency resolved pair distribution functions (PDF). We find that dynamic Pb and Ti off-centering is present even in the paraelectric phase, below $T_b$ the rate of growth of local Pb off-centering increases, followed by the freezing in of the local displacement direction at an intermediate temperature $T_c$ and a transition to a ferroelectric-like phase at $T_f$. Thus there is a sequence of four phases, PE, dynamic relaxor, mixed dynamic and frozen phase, and the non-ergodic frozen relaxor phase. We identify the average instantaneous local cation off-centering as the order parameter for the dynamic relaxor phase, and the time-averaged local cation off-centering as the order parameter for the two lower-temperatures relaxor phases. Examination of the dynamic PDF data reveals the shape and the range of correlation between the cation displacements. We also show that the relaxor phase is characterized by the appearance of strong nearest-neighbor correlation between the off-center displacements along the Cartesian directions.   

First-principles calculations of finite temperature Sc and O NMR parameters in Pb(Sc$_{2/3}$W$_{1/3}$)O$_3$

Understanding the dynamics of complex relaxor ferroelectrics is important to characterizing their large electromechanical coupling. Preliminary NMR measurements of Sc electric-field-gradients (EFG) in Pb(Sc$_{2/3}$W$_{1/3}$)O$_3$ (PSW) show a strong temperature dependence in the range $T = 250 - 330$ K. To understand this behavior, we use the first-principles GIPAW\footnote{C.~\ J.~\ Pickard and F.~\ Mauri, Phys. Rev. B {\bf 63}, 245101 (2001);} method within the Quantum Espresso (QE) package\footnote{P. Giannozzi et al., Journal of Physics: Condensed Matter {\bf 21}, 395502 (2009)} to calculate $^{45}$Sc and $^{17}$O chemical-shifts and EFG tensors. To study finite temperature effects, we incorporate the thermal expansion of the lattice and sample thermal disorder, using the phonon degrees of freedom. As in our previous studies of perovksites,\footnote{D.~\ L.~\ Pechkis, E.~\ J.~\ Walter, and H.~\ Krakauer. J.~\ Chem. Phys. {\bf 135}, 114507 (2011); {\em ibid.} {\bf 131}, 184511 (2009)} we show that the $^{17}$O chemical shifts in PSW also exhibit a linear correlation with the nearest-neighbor B-O bond length.   

Critical Slowing Down in the Relaxor Ferroelectric K$_{1-x}$Li$_{x}$TaO$_{3}$(KLT)

In this report, we illustrate an essential characteristic of mixed crystals such as KLT: the strong dependence of their macroscopic properties on the spatial distribution of the mixed ions in the crystal. As a prototypical relaxor ferroelectric, KLT exhibits a large dielectric constant, low frequency dispersion and a broad relaxation peak. Lithium randomly substitutes for potassium and, because of its smaller size, moves off-center in one of six possible $<$100$>$ directions thus forming a local dipole. Correlations between these dipoles lead to the appearance of Polar Nanodomains (PNDs), the size and polarization of which depend on local density fluctuations or type of distribution of the Li ions (random homogeneous or locally clustered). The dielectric constant of two KLT crystals with almost identical average Li concentrations displays two radically different behaviors, which can be traced to two very different distributions of the lithium ions in the two crystals. This is particularly striking of the critical behaviors in the two separate crystals. A first order structural transition is observed in one crystal but critical slowing down is observed in the other. The type of spatial distribution present in each crystal can be inferred from the dielectric results.   

EIT-like effect due to hetero-phase oscillations near the phase transition of relaxor ferroelectrics

We report the observation of a remarkable ``transparency window'' in the dielectric resonant absorption spectrum of the relaxor ferroelectric K$_{1-x}$Li$_{x}$TaO$_{3 }$(KLT) in the vicinity of its weakly first order transition. This phenomenon is shown to be conceptually similar to the electro-magnetically induced transparency (EIT) phenomenon observed in certain atomic vapors. In KLT however, it reveals the presence of hetero-phase (cubic-tetragonal) fluctuations and provides unique information on the nature and mechanism of the phase transition in relaxors.   

Temperature evolution of the linear birefringence in striated single crystals of KTa$_{1-x}$Nb$_{x}$O$_{3 }$(KTN)

We report the temperature evolution of a special linear birefringence in 3 crystals of KTa$_{1-x}$Nb$_{x}$O$_{3 }$(KTN), with x=0.155, 0.27 and 0.36 respectively, upon approaching the cubic-tetragonal phase transition. This birefringence, which is in violation of crystalline symmetry conditions, is caused by growth striations in the crystal that give rise to local strain and result in an average uniaxial behavior due to the photoelastic effect. Simultaneously, the set of parallel striations acts as a volume phase grating which can produce diffracted beams. Upon approaching the phase transition, the measured birefringence displays a rapid temperature dependence which is due to the formation of polar nano-domains (PND). These are incipient tetragonal uniaxial domains preferentially oriented with their c-axis perpendicular to the plane of the striations. As the birefringence increases, the diffraction efficiency unexpectedly decreases, indicating that the phase grating amplitude is occluded by the PND formation. The striation pattern is well defined in the 15.5{\%} crystal, more diffuse in the 36{\%} crystal, and there are no obvious striations in the 27{\%} crystal. Experimental results are presented and a simple phenomenological model for the birefringence behavior is proposed and discussed.   

Strain effects on Coherent Epitaxial Ferroelectric Pb(Zr0.2Ti0.8)O3

A comprehensive study of strain coupling to ferroelectricity in coherent epitaxial Pb(Zr0.2Ti0.8)TiO3 thin films is presented. The epitaxial strain variants are obtained by growing coherent PZT thin films on three different substrates, SrTiO3, DyScO3 and GdScO3 by pulsed laser deposition technique. The strain sensitivity of remnant polarization is found to be less in the epitaxial strain variants with larger tetragonality. Despite the fact that the tetragonality of PZT is more sensitive to the epitaxial strain than that of BaTiO3, the polarization-strain coupling is weaker in PZT than in BTO. These results underpins that the strong sensitivity of ferroelectricity to epitaxial strain is not a universal characteristic of complex oxide ferroelectrics and may depend on the intricate details of individual material systems.   

Structural and Ferroelectric Properties of Epitaxial ultrathin PbZr$_{0.52}$Ti$_{0.48}$O$_{3 }$Films Prepared on La$_{0.67}$Sr$_{0.33}$MnO$_{3}$/(LaAlO$_{3})_{0.3}$(Sr$_{2}$AlTaO$_{6})_{0.7}$ Substrates

The existence of ferroelectricity in ultrathin films open the possibility to further miniaturize devices based on FE materials, i.e. ferroelectric tunnel junctions take advantage of tunnel electroresistance effect. We have fabricated epitaxial PbZr$_{0.52}$Ti$_{0.48}$O$_{3 }$thin and ultrathin films using pulsed laser deposition on (001) on La$_{2/3}$Sr$_{1/3}$MnO$_{3}$/(LaAlO$_{3})_{0.3}$(Sr$_{2}$AlTaO$_{6})_{0.7}$ (LSMO/LSAT) substrates. The film thickness ranged between 3 to 100 nm. X-ray diffraction analysis revealed PZT and LSMO films are (00l) oriented perovskite structure. Atomic force microscopy of the PZT/LSMO(40nm)/LSAT structures show the surface is smoothness, densely packed, and free of cracks. The surface roughness on a 3 x 3 $\mu $m$^{2}$ area of the 100 nm and 3 nm thick films is $\sim $2 nm and $\sim $0.3 nm respectively. Well defined ferroelectric loop was observed in Pt/PZT(100nm)/LSMO(40nm)/LSAT structure with a remanent polarization $\sim $38 $\mu $C/cm$^{2}$ and a coercive field $\sim $80 kV/cm. The ferroelectric nature of the PZT ultrathin films (7--3 nm) was characterized using piezo force microscopy, a clear contrast between up and down ferroelectric domains was observed after writing positive and negative polarized in 2x2 $\mu $m$^{2}$ and 1x1 $\mu $m$^{2}$ areas respectively.   

Ab Initio Investigation of Free Energy Landscape near Morphotropic Phase Boundary

For pressure induced morphotropic phase boundary (MPB) in PbTiO3, although ground states have been investigated intensely, the overall free energy landscapes and so the transition paths are never systemically considered by ab initio method. Also there is little information about the oxygen octahedral tilts in monoclinic (M), orthorhombic (O) and triclinic (Tri) structures. In this work, in order to obtain the free energy landscapes, necessary oxygen octahedral tilts are considered not only in tetragonal (T) and rhombohedral (R) but also in M, O and Tri. According to the landscapes in the vicinity of MPB, firstly, the T-R transition path is not unique, since T-R and T-O-R paths have similar barriers; secondly, T-R barrier is ultra-low. Those explain the easy polarization rotation and so the ultra-high piezoelectric constant. Also, ground states are obtained by considering the oxygen octahedral tilts in T, R, M and O, and our results consist with the conclusion by J. Frantti, et.al in 2007. The fully relaxed M is actually T or R, which is indicated by the free energy landscapes. The ground state goes directly from T to R through MPB.   

Measuring the curves of dispersion for dielectrics using a low-energy laser and a thermal source of radiation

We propose a simple and accurate method for finding curves of dispersion for solid and liquid dielectrics using polarized light reflected by their surface near the Brewster angle. A precision of 0.0001 for indices of refraction can be achieved from running a parabolic fit through the experimental data for the parallel component of the reflectance normalized to the total reflectance in a region only 15 degrees wide around the Brewster angle [1]. This precision allows accurate measurements of small changes in the indices of refraction that cannot be measured with other existing methods. For example, such changes can be produced by the temperature variation on the dielectric surface. In this paper we show that using a low-energy laser beam (such as the inexpensive red diode laser of 650 nm) and a thermal (blackbody) source of radiation one can easily generate precise curves of dispersion in the visible and ultraviolet spectrum for any dielectric transparent to this radiation. Our interpretation is based on the Lorentz model for the interaction between dipoles on the dielectric surface and the incident optical field. The average thermal energy of the blackbody source can be associated to an effective frequency. This thermal energy contributes to a higher frequency of oscillation of these dipoles and can be measured as a slight increase in the value of the refractive index. The project was partially supported by the McNair Scholars Program and STAIRSTEP-NSF-DUE grant{\#} 0757057. [1] Bahrim C and Hsu W-T, 2009 \textit{Am. J. Phys.} \textbf{77} (4) 337-343.   

Session B34: Focus Session: Impact of Ultrafast Lasers II: Multidimensional Methods

Control over coherent light fields enables multidimensional coherent spectroscopy and multispectral coherent control

Using a combination of spatial and temporal shaping of optical laser fields, fully coherent spectroscopy and coherent control can be carried out to high order from optical to THz spectral ranges. A single beam with a single femtosecond pulse can be transformed into multiple beams and multiple pulses, reconfigurably under computer control with no human alignment needed, retaining full phase coherence among all the noncollinear fields. This enables multiple-quantum 2D and 3D Fourier transform optical spectroscopy of excitons and exciton-polaritons in inorganic quantum wells and microcavities, in organic J-aggregate films, and in inorganic/organic hybrid structures, the results of which will be discussed. Spatiotemporal shaping also enables coherent control over THz phonon-polariton waves in ferroelectric crystals. The THz waves can be coherently superposed to reach extremely large field amplitudes both in the host crystals and in free space, and the fields can be further enhanced in dipolar antenna and metamaterial structures, enabling highly nonlinear coherent spectroscopy and coherent control in the THz regime. Results from solid, liquid, and gas phases, including multiple-quantum rotational coherences in molecular gases and THz-induced phase transitions in crystalline solids, will be presented. Prospects for further generalization of the approach all the way to the hard x-ray regime will be discussed.   

Solvent Influenced Fluxionality Studied by Ultrafast Chemical Exchange Spectroscopy

Two-dimensional infrared spectroscopy (2DIR) allows unprecedentedly detailed understanding of the dynamics of chemical systems in the condensed phase. Carbonyl vibrations of small transition metal complexes report intramolecular dynamics and solvent-solute interactions due to their strong oscillator strengths and moderate environmental sensitivity. We studed the fluxional dynamics of iron pentacarbonyl (Fe(CO)$_{5}$), which is unique in that it contains nearly perfectly uncoupled vibrational modes. We seek to probe the ``molecularity'' of condensed phase activated barrier crossings beyond the continuum Kramers theory picture. Using 2DIR chemical exchange spectroscopy, we show how the dynamics of Berry pseudorotation, the only significant mechanism for vibrational mode mixing on our experimental timescale, is sensitive to interactions with the environment. In a wide range of solvents, we have investigated the effects of hydrogen bonding with alcohols and friction from high viscosity alkanes. In addition, we have monitored vibrational energy redistribution as a solvation shell probe. Moreover, recently implemented mid-infrared pulse shaper based methods allow increased flexibility in experimental design, enabling experimental techniques that are not possible using passive optics.   

Constant-Speed Vibrational Signaling along Polyethyleneglycol Chain up to 60{\AA} Distance

A series of azido-PEG-succinimide ester oligomers with a number of repeating PEG units of 0, 4, 8, and 12 (azPEG0, 4, 8, and 12) was investigated using a relaxation-assisted two-dimensional infrared (RA 2DIR) spectroscopy method. The RA 2DIR method relies on the energy transport in molecules and is capable of correlating the frequencies of vibrational modes separated by large through-bond distances. Excitation of the azido group in the compounds at ca. 2100 cm-1 generates an excess energy which propagates in the molecule as well as dissipates into the solvent. We discovered that a part of the excess energy propagates ballistically via the covalent backbone of the molecules with a constant speed of ca. 550 m/s. The transport is described as a propagation of a vibrational wavepacket having a mean-free-path length of ca. 11{\AA}. The discovery has the potential for developing new efficient signal transduction strategies for molecular electronics and biochemistry. It also permits extending the distances accessible in RA 2DIR structural measurements up to ca. 60{\AA}.   

Effects of Symmetry on Intense-field Ionization of Heterocyclic Organic Molecules

We report on the ultrafast photoionization of pyridine, pyridazine, pyrimidine and pyrazine. These four molecules represent a systematic series of perturbations into the structure of a benzene ring which explores the substitution of a C-H entity with a nitrogen atom, creating a heterocyclic structure which remains isoelectronic with benzene. Other than pyridine, each molecule has the same molecular formula, with the only difference being the placement of the perturbing nitrogen atoms (ortho-, meta- or para-substitutions). Differences in the intense-field behavior of these molecules are caused by the symmetry of the perturbation to the benzene system, primarily influenced through the interactions of lone-pair states surrounding the nitrogen atoms. Data is recorded under intense-field, single-molecule conditions. 50 fs, 800 nm pulses are focused into the molecular vapor, and ion mass spectra are recorded for intensities of $\sim $10$^{13}$ W/cm$^{2}$ to $\sim $10$^{15}$ W/cm$^{2}$. We measure ion yields in the absence of the focal volume effect without the need for deconvolution of the data. For all targets, stable singly- and doubly-charged parent ions (C$_{6-n}$H$_{6-n}$N$_{n}^{+(+)})$ are observed with features suggesting resonance enhancement (REMPI).   

Experimental determination of Hamiltonian via 3D Fourier-transform spectroscopy

Prediction and control of quantum mechanical processes requires knowledge about the system Hamiltonian. For coherent control, information about interfering quantum pathways or the underlying Hamiltonian is essential for achieving deterministic control. Even in cases of closed-loop control, a priori knowledge about the system Hamiltonian provides guidance for designing an efficient learning algorithm and a good initial guess for faster convergence. The complete Hamiltonian of a complicated system, especially the effects of inter-particle interactions and coupling to the environments, can only be determined experimentally. Here we demonstrate an experimental determination of the Hamiltonian of an atomic vapor, achieved by using 3-dimensional Fourier transform (3DFT) spectroscopy. The 3DFT spectra provide complete information about the third-order coherent response of the vapor. The contributions from different quantum pathways are unambiguously isolated such that the components of the Hamiltonian, including energy levels, dipole moments and relaxation rates, can be determined. The 3DFT spectroscopy opens an avenue towards identifying the Hamiltonian of complex molecular systems, which can be useful for designing coherent control strategies and for studying the molecular dynamics.   

Nonradiative-decay mechanisms in CdSe nanoparticles: MUPPETS (multiple population-period transient spectroscopy) in excitonic systems

Nonradiative decay in semiconductor nanoparticles on the picosecond to nanosecond time scale is complex and poorly understood. Here, two-dimensional (2D) incoherent spectroscopy (MUPPETS) is applied to these processes in CdSe nanoparticles. For the first time, MUPPETS is extended to multilevel, excitonic systems to yield an analog of 2D coherent correlation spectroscopies. In core-only CdSe particles, the transfer of an excited electron from the core to the surface follows a highly dispersed, power-law decay in 1D measurements. 2D-MUPPETS measurements show that the rate dispersion is not due to relaxation nor due to multi-step kinetics, but results solely from particle-to-particle heterogeneity in the barrier to the surface. A model in which surface defects are distributed within the dipolar electric field of the particle accounts for the power-law decay. A second study of CdSe:ZnS core-shell particles uses correlation MUPPETS to distinguish biexcitons from photoproducts with a fast relaxing single exciton. Even when both species have similar lifetimes, they are distinguishable by having opposite signs and different symmetries in the two time intervals of a 2D experiment. Potential correlations between biexciton and exciton rates are sought, but are not found.   

Session B35: General Theory

The evolution of dielectric properties of sodium, silicon and argon clusters

We used a computational scheme based on site-specific polarizabilities to study the evolution of the dielectric properties of sodium, silicon and argon clusters. In this approach, the total cluster polarizability is decomposed into local dipole (LD) and charge-transfer (CT) parts. The local dipole part measures the redistribution of charge within an atomic volume, while the CT part describes the movement of charge between volumes. We find distinct differences in the relative contributions of the LD and CT components to the total polarizability as a function of cluster size for the different cluster types and relate this to the development of metallic behavior. The method also directly probes the extent of electrostatic screening of the cluster interior to an applied electric field.   

Topological Solitons between Two Magic Number In-clusters on a Si(100) Surface Break the Even/Odd Symmetry in the Self-Selection of Their Distance

Depositing particles randomly on a 1D lattice is expected to result in an equal number of particle pairs separated by even or odd lattice units. Unexpectedly, the even-odd symmetry is broken in the self-selection of distances between indium magic-number clusters on a Si(100)-2x1 reconstructed surface. Cluster pairs separated by even units are less abundant because they are linked by silicon atomic chains carrying topological solitons, which induce local strain and create localized electronic states with higher energy. Our findings reveal a unique particle-particle interaction mediated by the presence or absence of topological solitons on alternate lattices.   

Determination of ground state structures of selected medium-sized clusters via first-principles global search

Due to the existence of numerous local isomers on the potential energy surface, determining the most stable structures of medium-sized clusters is one of the most challenging tasks in cluster physics. Recent years, we have explored the potential energy surface of medium-sized clusters using first-principles calculations combined with global search methods like genetic algorithm and simulate annealing. The examples on a few selected examples such as B$_{80}$/B$_{101}$, Na$_{1-3}$Si$_{1-11}$, and (WO$_{3})_{2-12}$ clusters will be briefly illustrated. The size-dependent electronic properties of these clusters will also be discussed and compared with available experimental data.   

Novel building blocks for materials by design: Janus particles and other patchy colloids

The emergent assembly of nonisotropically structured colloidal particles can lead to novel materials with requisite optical or mechanical properties. We have developed two models---one that includes detailed interactions between particles and another that coarse-grains the interactions---so as to explore the equilibrium and dynamics effected by varying interaction heterogeneities. In particular, we have performed a series of simulations of systems consisting of Janus particles---in which each of two hemispheres can be characterized by a single interaction type such as charge or degree of hydrophobicity. The equilibrium structure of Janus clusters has been the subject of experimental and theoretical studies by Grannick and coworkers. We find that the bulk Janus systems give rise to surprising equilibrium structure and dynamics which can be tuned through both the volume fraction and the interactions. The coarse-grained model provides surprisingly good agreement with the more detailed particle-model for the equilibrium structure while overestimating the relaxation rates.   

Structure and Diffusion of Nanoparticles at the Liquid-Vapor Interface

Large-scale molecular dynamics has been used to simulate a layer of nanoparticles diffusing on the surface of a liquid. Both a low viscosity liquid, represented by Lennard-Jones monomers, and a high viscosity liquid, represented by linear homopolymers, are studied. The organization and diffusion of the nanoparticles are studied as the coverage and the contact angle between the nanoparticles and liquid are varied. Results are compared to simulations of identical nanoparticles in two-dimensions. We show that when the interaction between the nanoparticles and liquid is reduced the contact angle increases and the nanoparticles ride higher on the liquid surface, which enables them to diffuse faster. In this case the short range order is also reduced as seen in the pair correlation function. For low contact angles, nanoparticles diffuse into the liquid for high coverages.   

Supersymmetric Quantum Mechanics For Atomic Electronic Systems

We employ our new approach to non-relativistic supersymmetric quantum mechanics (SUSY-QM), (J. Phys. Chem. A 114, 8202(2010)) for any number of dimensions and distinguishable particles, to treat the hydrogen atom in full three-dimensional detail. In contrast to the standard one-dimensional radial equation SUSY-QM treatment of the hydrogen atom, where the superpotential is a scalar, in a full three-dimensional treatment, it is a vector which is independent of the angular momentum quantum number. The original scalar Schr\"odinger Hamiltonian operator is factored into vector ``charge'' operators: $\vec Q$ and $\vec Q^{\dagger}$. Using these operators, the first sector Hamiltonian is written as $H_1 = \vec Q^{\dagger}\cdot \vec Q + E_{0}^1$. The second sector Hamiltonian is a tensor given by $H_{2} = \vec{Q}\vec{ Q}^{\dagger} + E_{0}^11$ and is isospectral with $H_1$. The second sector ground state, $\vec\psi_{0}^{(2)}$, can be used to obtain the excited state wave functions of the first sector by application of the adjoint charge operator. We then adapt the aufbau principle to show this approach can be applied to treat the helium atom.   

Ion correlations in the electrical double layer near liquid/liquid interfaces

Equilibrium properties of the electrical double layer of monovalent salts near the interface between two immiscible electrolyte solutions are studied via Monte Carlo simulations, for several electrolyte concentrations. The corresponding results are collated with experimental data and the classical Verwey-Niessen theory of point ions. The observed differences between the simulation data and the mean field approach of ``back-to-back" electrical double layers stresses the importance of including properly ionic correlations near dielectric discontinuities.   

Molecular Dynamics Simulation of Adsorption of Methane and Chloromethane on Molybdenum via Hybrid EAM/OPLS Interactions

The question of liquid adsorption on the surface of a solid substrate has been of major interest to computational science even from its inception. Accurate depictions of adsorption phenomena require accurate modeling of both phases of the simulated system. In cases in which accuracy of the models leads to the adoption of potentially inconvenient or incompatible force fields, questions arise as to whether joint systems can perform as well as needed to gather optimal data. The current investigation uses molecular dynamics (MD) tools to focus, in part, on how well-developed models for liquid simulation (OPLS, built on a Lennard-Jones foundation) can combine with tested models for metals (based on functionals of electron density) to describe the adsorption of methane and chloromethane on the surface of molybdenum. Comparisons have been made between the differences in effect of a hybrid EAM/OPLS system and systems focusing solely on Lennard-Jones-based interactions, as well as the effect of large asymmetry and polarity differences in adsorbate species, on the structure and dynamics of layers adsorbed directly at the surface of the metal substrate. Preliminary results suggest that the surface landscape and corrugation play a significant role in choice of surface binding sites.   

First Principles Study of Adsorption of $O_{2}$ on Al Surface with Hybrid Functionals

Adsorption of $O_{2}$ molecule on Al surface has been a long standing puzzle for the first principles calculation. We have studied the adsorption of $O_{2}$ molecule on the Al(111) surface using hybrid functionals.In contrast to the previous LDA/GGA, the present calculations with hybrid functionals successfully predict that $O_{2}$ molecule can be absorbed on the Al(111) surface with a barrier around 0.2$\sim$0.4 eV , which is in good agreement with experiments. Our calculations predict that the LUMO of $O_{2}$ molecule is higher than the Fermi level of the Al(111) surface, which is responsible for the barrier of the $O_{2}$ adsorption.   

Octaselenododecane $(C_4 H_8 Se_{8} )$: a novel polyselenoether crown macrocycle

In this work we have used density-functional theory (DFT/GGA-PBE) to calculate the structural, electronic, and vibrational properties of octaselenododecane $(C_4 H_8 Se_{8} )$, a novel twelve-membered crown-shaped heterocycle which contains four diselenide groups.\footnote {G. Hua, J. M. Griffin, S. E. Ashbrook, A. M. Z. Slawin, and J. D. Wollins, {\it Angew Che. Int. Ed.} {\bf 2011,} 50, 4123-4126.} Our all-electron DFT calculations have yielded results that are in excellent agreement with the observed experimental x-ray diffraction data and infrared and Raman vibrational spectra for the solid state phase of octaselenododecane. In addition to obtaining good general agreement with the selected IR and Raman frequencies reported to lie within the range of 282-2925 cm$^{-1}$, we have obtained other vibrational modes which have not been reported in the literature. In particular, we have computed a Raman active mode at 267 cm$^{-1}$ which is in good agreement with the experimental band at 282 cm$^{-1}$ and have determined that it represents significant asynchronous stretches of diselenide groups within the heterocycle. Our gas phase calculations also show the presence of strong low frequency distortions that are supressed in the crystal due to close Se-Se intramolecular interactions.   

Bias Dependent Switching Behavior of STM-Induced Melamine/Cu(001) Switch

The manipulation or stimulation of molecules using Scanning Tunneling Microscopy (STM) is a technique that recently has deserved deep attention for its potential applications in molecular electronics. The melamine/Cu(001) system was found to show switching behavior in very wide range of applied bias. Although its mechanism was analyzed by a statistic model, the relationship between the switching rate and bias is still far to be fully clarified. In this context, we performed a campaign of exhaustive first-principles calculations to obtain most of the parameters for resonance model; such model is able to predict the switching rate as functions of bias and current. The energy barrier was calculated using the nudged elastic band method, with the aid of recent implementation of current-induced forces into SMEAGOL code, which is based on the nonequilibrium Green's function method with Density Functional Theory. The electron-phonon coupling and then the Inelastic Tunneling Spectroscopy signal are calculated to validate the one-phonon approximation. The spatial distribution of molecular orbitals and their coupling with vibrational modes are very useful to understand the switching behavior.   

Kirkwood-Buff analysis of liquid mixtures in AdResS: Towards an open boundary simulation scheme

Many biophysical processes in water are determined by interactions of cosolvents with the hydration shells of dissolved (bio)molecules. Computational approaches to study these systems are mostly limited to the closed boundary simulations. While closed boundaries are perfectly suitable in many cases, problems arise when concentration fluctuations are large, or intimately linked to the physical phenomenon. For example, in non-ideal mixtures of water/cosolvent and a biomolecule, the excess of water/cosolvent, close to a protein surface, leads to water/cosolvent depletion elsewhere. This complicates a comparison with experiments that are conducted under osmotic conditions. Therefore, we use Adaptive Resolution Simulation (AdResS) scheme, which describes a small sub-volume of a much larger system in atomistic detail, maintaining thermodynamic equilibrium with a surrounding coarse grained reservoir. We show that the Kirkwood-Buff integrals (KBI), which directly connect thermodynamic properties to the molecular distributions, can be efficiently calculated within the small open boundary all atom region and the coarse-grained reservoir maintains the correct particle fluctuations. Results will be presented for the methanol/water mixture and solvation of amino acids in urea/water mixture.   

Session B36: Focus Session: New Energy II

Theories of plasmon enhanced optical processes important in solar energy

This talk will focus on the development of electronic structure methods that can be combined with electrodynamics calculations to describe enhancement in chemical reaction rates that arise from plasmon excitation of noble metal nanoparticles. Two types of enhancement are described: passive and active. In the passive case, the nanoparticle produces an enhanced electromagnetic field that acts externally to the nanoparticle to enhance a photochemical process, while in the active case, plasmon excitation leads to electron transfer to or from the noble metal nanoparticle. Examples and applications of each type will be described.   

Porphyrin Molecular Multilayer Thin-Films on Gold (111) Electrodes for Electro-optical Applications

We have developed a Layer-by-Layer (LbL) method for the fabrication of thin-film molecular multilayers on gold (111) electrodes. Copper(I) catalyzed azide-alkyne cycloaddition (CuAAC) coupling reactions were used for surface attachment and subsequent LbL deposition of porphyrin building blocks. The electrochemical and photophysical properties of the thin-films can be tuned through synthetic modification of the individual components, resulting in new porphyrin multilayers for applications in light harvesting and molecular electronics. Herein, we demonstrate the reproducible growth trends and optical properties of these films. Multilayer growth was followed by UV-Vis absorption and reflectance spectroscopy. Film thickness (FT) and optical constants were obtained from spectroscopic ellipsometry. Topology and surface roughness was examined by TM-AFM, while the copper content was quantified by XPS. The redox characteristics were studied by electrochemical methods, whereas the conductance of individual porphyrin constructs was examined by STM using the molecular break junction method. The multilayers show consistent linear growth in absorbance and FT over tens of layers and continuity in their molecular structure.   

Nonadiabatic Excited-State Molecular Dynamics (NA-ESMD): Numerical tests of convergence and parameters

Nonadiabatic molecular dynamics simulations, involving multiple coupled potential energy surfaces, often requires a large number of independent trajectories in order to achieve the desired convergence of the results, and simulation relies on different parameters that should be tested and compared. In addition to influencing the speed of the simulation, the chosen parameters combined with frequently implemented approximations can lead to unanticipated changes in the accuracy of the simulated dynamics. We have previously developed a nonadiabatic excited state molecular dynamics (NA-ESMD) methodology employing Tully's fewest switches surface hopping (FSSH) algorithm. In this study, we seek to investigate the impact of the number of trajectories and the various parameters on the simulation of the photoinduced dynamics of distyrylbenzene (a small oligomer of polyphenylene vinylene) within our developed framework. Various user-defined parameters are analyzed: classical and quantum integration time steps, and the number of trajectories used for statistical averaging. Common approximations such as reduced number of nonadiabatic coupling terms and the classical path approximation (CPA) are also investigated.   

Probing Nanointerfaces of Nanoparticle-Based Solar Energy Conversion: Molecular Dynamics on the Angstrom Scale

Charge-induced surface chemistry following solar-illumination of nanometer-scale semiconductor particles is one possible route to low-cost solar energy conversion. Because of their high surface to volume ration interfaces play a major role in the performance of this technology and thus the physics of charges interacting with adsorbed molecules is of central interest. In this talk, after a brief review of related work, we will focus on our advances in understanding molecular-dissociation-dynamics at the interface of nanometer-scale TiO$_{2}$ crystals and large organic molecules. Clearly the standard probes of beam-based molecule dynamics are not easily adaptable to small nanointerfaces. Instead our work uses STM imaging to examine dynamics of fragments following tip-induced electron injection into organic molecules on TiO$_{2}$ (110) and on $\sim $10nm nanocrystals (110). Our experiments have used halogenated anthracene to probe the efficiency and fragment trajectory following dissociative electron capture. Our work has examined nanoparticle synthesis, adsorbate-molecule orientation, thermal and injected-electron chemistry, and adsorbate charge-mediated fragmentation trajectories.   

Band gap engineering iron pyrite for sustainable solar energy conversion

In the quest to develop sustainable materials for solar energy conversion, iron pyrite (FeS2) holds great promise as a solar absorber. The electronic band gap of FeS2, however, is not well matched to the solar spectrum. Here we explore chemical doping as a strategy to engineer the band gap of FeS2, as has been successfully demonstrated with other semiconducting materials. Using first-principles calculations, we first establish the relationship between pressure, lattice distortions, and the electronic structure of FeS2, and rationalize the results in terms of distortions in the crystal-field splitting of Fe. We then investigate the effects of doping FeS2 with transition metal elements, wherein our strategy is guided by the knowledge of band gap dependence on local distortions.   

Photochemistry of chemisorbed and physisorbed O$_{2}$ on reduced rutile TiO$_{2}$(110)

The ultraviolet (UV) photon-stimulated reactions of oxygen on TiO$_{2}$(110) are studied. For chemisorbed O$_{2}$, the photochemistry depends on the O$_{2}$ coverage. For small coverages, only $\sim $14{\%} desorbs while the rest either dissociates during UV irradiation, or remains molecularly adsorbed on the surface. For the maximum coverage of chemisorbed oxygen, the fraction of O$_{2}$ that photodesorbs is $\sim $40{\%}. However when physisorbed O$_{2}$ is also present, $\sim $70{\%} of the initially chemisorbed O$_{2}$ photodesorbs. Experiments using O$_{2}$ isotopologues show that UV irradiation results in exchange of atoms between the chemisorbed and physisorbed oxygen. Annealing chemisorbed oxygen to 350 K maximizes these exchange reactions. The exchange products photodesorb in the plane perpendicular to the bridge-bonded oxygen rows at an angle of 45\r{ }. Remarkably, the chemisorbed species is stable under multiple cycles of UV irradiation with physisorbed O$_{2}$, and the atoms in the chemisorbed species can be changed from $^{18}$O to $^{16}$O and then back to $^{18}$O via the exchange reactions. The results show that annealing oxygen adsorbed on TiO$_{2}$(110) to $\sim $350 K produces a stable chemical species with interesting photochemical properties. Possible forms for the photoactive species include O$_{2}$ adsorbed in a bridging oxygen vacancy or tetraoxygen.   

Energy Materials in Extreme Environments

The critical shortage of abundant, affordable, and clean energy calls upon novel materials with extreme properties for energy production, storage, conversion, and transfer that are superior to materials that now exist or are in use today. Twenty-first century energy technology also demands enhanced performance from existing materials under extreme environments of pressure, temperature, chemistry, radiation, and electromagnetic fields. Investigating the behavior of materials in extreme pressures and temperatures, in particular, provides the fundamental knowledge needed to address these problems. These studies are leading to the discovery of both new materials with enhanced performance as well as new physical and chemical phenomena, and take advantage of advances at national x-ray, infrared, neutron, and laser facilities. An important example is the continued study of hydrogen-rich materials, from investigations of transformations in pure hydrogen, which has now been pressurized well above 300 GPa, to the discovery of new hydrogen storage materials and hydriding reactions induced by extreme conditions. Other examples include studies of carbon-based materials, which are also deepening our understanding of carbon sources and cycling within the planet.   

Session B37: SPS Undergraduate Research I

Committee Work or How I Learned to Stop Complaining and Love the Process

Over the course of their career, most American scientists, be it at an independent university, government lab, or in the private sector, will at some point receive public funding to perform their research.~ Yet most of these individuals have only a vague sense of how their money was originally allocated. Most may be familiar with the intricacies of NSF, NIST, or any one of the agencies that compose the alphabet soup of the Federal Government, but what about before that, when the money and subsequent directives are being allocated to these agencies? In other words, what about congress? In an effort to better understand this process and to contribute to the reciprocal understanding by congress I embarked on a 10 week internship with the House Committee on Science, Space, and Technology, funded by the John and Jane Mather Foundation as part of the SPS Summer Internship Program. In my time with the committee I was able to attend multiple hearings and seminars on the Hill and see how policies with regard to science are created. In this talk I will discuss my experience and observations, for example that no one wished to be viewed as anti-science and that even those who are looked upon as very critical are often more critical of the opposing side of the aisle than of the actual science itself. In fact, I saw that the biggest hindrance to science in the political setting is the lack of understanding of just what it is. This the same problem faced by the general public and the steps toward fixing this issue may very well be the same.   

Wetting in Color: Designing a colorometric indicator for wettability

Colorimetric litmus tests such as pH paper have enjoyed wide commercial success due to their inexpensive production and exceptional ease of use. While such indicators commonly rely on a specific photochemical response to an analyte, we exploit structural color, derived from coherent scattering from wavelength-scale porosity rather than molecular absorption or luminescence, to create a Wetting-in-Color-Kit (WICK). This inexpensive and highly selective colorimetric indicator for organic liquids employs chemically encoded inverse-opal photonic crystals to translate minute differences in liquids' wettability to macroscopically distinct, easy-to-visualize color patterns. The highly symmetric re-entrant inter-pore geometry imparts a highly specific wetting threshold for liquids. We developed surface modification techniques to generate built-in chemistry gradients within the porous network. These let us tailor the wettability threshold to specific liquids across a continuous range. As wetting is a generic fluidic phenomenon, we envision that WICK could be suitable for applications in authentication or identification of unknown liquids across a broad range of industries.   

EPR spectral study and modeling of lithium borovanadate RLi$_{2}$OB$_{2}$O$_{3}$KV$_{2}$O$_{5}$ glasses

Utilizing electron paramagnetic resonance EPR spectroscopy, lithium borovanadate RLi$_{2}$OB$_{2}$O$_{3}$KV$_{2}$O$_{5}$ glasses with R = 0.4 and K ranging from 0.1 to 0.5 were analyzed in order to elucidate the environment of unpaired 3d$^{1}$ electrons. Transitions associated with coupling of such electrons to vanadium nuclear spins were identified and modeled to reveal both g factor and A factor values. For a system with K = 0.3, representative data include: g$_{ll}$ = 1.9242, g$_{\bot }$ = 1.9693, A$_{ll}$ = 184.3768 cm$^{-1}$, A$_{\bot }$ = 64.6568 cm$^{-1}$, $\Delta $g$_{ll}$ = 0.0781, $\Delta $g$_{\bot }$ = 0.0330, and $\Delta $g$_{ll}$/$\Delta $ g$_{\bot }$ = 2.3670. A comparison revealing g$_{ll}<$ g$_{\bot } \quad <$g$_{e}$ is indicative of localized electrons residing in tetragonally-distorted octahedral sites. A slight increase observed in $\Delta $g$_{ll}$/$\Delta $ g$_{\bot }$ values when K = 0.1 to K = 0.3 is further evidence of a possible elongation of the octahedral site associated with increasing K values. This pattern, however, is not present in systems with K values greater than 0.3, suggesting that perhaps no further elongation of the site is possible due to bond constraints. A comprehensive model will be presented to summarize data for the entire family of lithium borovanadates studied here.   

Metal-Metal Oxide Nanoomposite for Solar Cell Applications

Currently, there is a large need for alternative energies and one good option is solar cells. A High efficiency solar cell generally consists of a number of thin layers: active layer consisting of a material having high absorption in the solar spectrum, transparent conducting layer, $p$- and $n$-type materials used to fabricate the junction, and electrodes for good Ohmic contacts. The presence of metal nanoparticles in metal oxide films improves significantly the solar absorbance of metal oxide films. The absorption depends on the bandgap of metal oxides which can be tuned by incorporation of metal nanoparticles. Tuning of the bandgap and absorption are the very important parameters to fabricate the solar cell devices. Thin films of M-MO (M = transition metals Co and Ni) nanocomposite have been grown on quartz substrate using pulse laser deposition technique. Structural properties have been characterized using X-ray diffraction and scanning electron microscopy. Electrical properties with and without light and absorption spectra have been measured using I-V characterization and UV-VIS spectroscopy techniques. Detailed results will be discussed in the presentation.   

Electrical Characterization of Flexible Titanium Dioxide Memristors

The memristor is a new fundamental circuit element with a resistance that depends on the magnitude and polarity of the voltage applied to it and the length of time that voltage is applied. Memristors are also nonvolatile, which means that when the bias is removed, a memristor retains its last resistive state. While memristors have potential applications ranging from alternative computer architectures to memory in inexpensive lightweight wearable sensors, the mechanism behind its switching is not well understood. One of the major questions to be resolved is whether memristive switching is electric field dependent or charge dependent. In the former case, a minimum bias is needed for switching to occur. In the latter case, a minimum amount of charge needs to pass through the device to cause switching. I will present the results of electrically characterizing flexible memristors that consist of a nm-thick layer of titanium dioxide sandwiched between two metal contacts in an effort to help establish whether their switching is charge or field-driven.   

Two-Dimensional Ordering of DNA Origami Using Stacking Bonds

Utilizing the DNA Origami method we have designed nano-scale self-assembled structures. These structures are made using a 7000 base pair long single stranded DNA as a scaffold that is held in place by shorter single stranded DNA molecules using Watson-Crick DNA base pairings. The staples were chosen to attach at certain specific sites of the scaffold DNA so that a well-defined planar structure of double stranded DNA can be created at room temperature. In designing these origami structures we made use of the computer application caDNAno. Two geometrical structures with differing symmetries were created using the same scaffold. Edges of these structures were modified in such a way that the double stranded DNA of one structure's edge can stack onto the edge of the second structure. Similar modifications were recently shown by Woo and Rothemund (Nat Chem., 1755-4330, 2011) to enable the formation of extended DNA origami structures. We intend to extend this method to create two-dimensional square and triangular lattice structures. We discuss our experimental results and implications of this method for nano-scale self-assembly.   

Synthesis of Magnetic Nanoparticles for Biosensing Studies Using Magneto-Impedance Technology

Polymer nanocomposites (PNCs) have been shown to be a compact and durable solution for applications such as electromagnetic interference shielding and magnetically tunable microwave devices. We report studies aimed at exploring applications of PNCs to aid in bio-sensing, using the Giant Magneto-Impedance (GMI) effect. GMI is a change in the ac impedance of a ferromagnetic conductor in a varying dc magnetic field, and has been shown to be about 500 times more sensitive than its counterpart, Giant Magneto-Resistance (GMR). In our study, magnetite (Fe$_{3}$O$_{4})$ nanoparticles (mean size, 6$\pm $2 nm) were synthesized by thermal decomposition and dispersed in a polymer provided by the Rogers Corporation to create PNCs with 20, 50, and 80 wt{\%} compositions. The GMI of an amorphous magnetic ribbon was measured with and without the PNCs layered on the ribbon. The effects of nanoparticle concentration on the GMI sensitivity were studied, with a view toward applications in highly sensitive bio-molecular detection.   

Development of a Protocol to Study the Conformational Stability of Protein Using the Model Protein Cytochrome $c$

The function of a protein is inherently dependent upon the proper folding and the resulting tertiary structure of the molecule. The development of an unfolding procedure is desirable so that the structural stability of a protein molecule can be determined through the change in thermodynamic properties of the unfolding reaction. The protein cytochrome$ c$ has long been used in protein structural studies and monitored by circular dichroism (CD), absorption, and fluorescence spectroscopic techniques. Single amino acid mutations of wild type cytochrome $c$ were unfolded both chemically and thermally using the developed protocol and the unfolding was monitored by CD spectroscopy. Thermodynamic properties such as Gibbs free energy, enthalpy and melting temperature were used to interpret the results. The mutant proteins were calculated to have different thermodynamic properties than that of the native cytochrome $c$ during the unfolding process. When denatured at a lower pH, the proteins thermally unfolded more readily. The objective of this session is to present recent work addressing the denaturation of wild type and mutant proteins.   

Genotyping and phenotyping of an epigenetic modifier \textit{Unstable factor for orange1 (Ufo1)} in maize

Pericarp color 1 is a model system for the study of epigenetic gene regulation. It has more than 100 alleles that contribute to the color of the pericarp and cob glume of maize. Unstable factor for orange 1 (Ufo1) is a spontaneous dominant mutation that leads to a gain in pigmentation due to a decrease in methylation in p1 genes. This decrease in methylation of cytosine in the DNA leads to changes in chromatin structure. Finding the mechanism for this spontaneous mutation can lead to way of preventing the mutation increasing production colorless maize for food production. Through genotyping and phenotyping fine gene mapping, gene expression and whole genome profiling can be accomplished for plants with the Ufo1 mutation present.   

Computational and Electronic Analog Implementation of the Hodgkin-Huxley Model of Action Potentials in Neurons

Alan Loyd Hodgkin and Andrew Huxley's mathematical model of action potential initiation and propagation in neurons is one of the greatest hallmarks of biophysics. Two techniques for implementing the Hodgkin-Huxley model were explored: computational and electronic analog. Computational modeling was done using NEURON 7.1. NEURON is a free, robust, and relatively user friendly simulation environment that enables quantitatively accurate computational modeling of neurons and neural networks. An analog electronic circuit was built using field-effect transistors (FETs) to simulate the non-linear, voltage-dependent (sodium and potassium) conductances that are responsible for membrane excitability. While the electronic analog qualitatively reproduces many of the key features of the action potential including overall shape, inactivation period, and propagation, it was difficult to quantitatively reproduce the Hodgkin-Huxley model. In addition, while the relative cost to build circuits equivalent to small membrane patches is minimal ($\sim ${\$}50), implementation of larger cells or networks would prove uneconomical. Still, both techniques are viable avenues toward introducing interdisciplinary research into either a computational or electronics lab setting at the undergraduate level.   

First-principles calculation of structural and electronic properties of memantine (Alzheimer's disease) and adamantane (anti-flu) drugs

Memantine is currently used as a treatment for mild to severe Alzheimer's disease, although its functionality is complicated. Using various density functional theory calculations and basis sets, we first examine memantine alone and then add ions which are present in the human body. This provides clues as to how the compound may react in the calcium ion channel, where it is believed to treat the disease. In order to understand the difference between calcium and magnesium ions interacting with memantine, we compute the electron affinity of each complex. We find that memantine is more strongly attracted to magnesium ions than calcium ions within the channel. By observing the HOMO-LUMO gap within memantine in comparison to adamantane, we find that memantine is more excitable than the anti-flu drug. We believe these factors to affect the efficiency of memantine as a treatment of Alzheimer's disease.   

Construction and Operation of a Differential Hall Element Magnetometer

A Differential Hall Element Magnetometer (DHEM) was constructed to measure the magnetic saturation and coercive fields of small samples consisting of magnetic nanoparticles that may have biomedical applications. The device consists of two matched Hall elements that can be moved through the room temperature bore of a 9 Tesla superconducting magnet. The Hall elements are wired in opposition such that a null response, to within a small offset, is measured in the absence of a sample that may be located on top of one unit. A LabVIEW program controls the current through the Hall elements and measures the net Hall voltage while simultaneously moving the probe through the magnetic field by regulating a linear stepper motor. Ultimately, the system will be tested to obtain a figure of merit using successively smaller samples. Details of the apparatus will be provided along with preliminary data.   

Magnetostriction of engineered magnetorheological elastomers

We have completed a study of the magnetostriction and poison ratio of several types of magnetorheological elastomers (MREs), including both hard and soft magnetic materials in silicone rubber matrices. While both random and aligned soft magnetic particles gave large ($\sim$1\%) magnetostriction, hard magnetic powders provided minimal actuation, regardless of whether they were aligned or not. In addition, we have created engineered lattices of magnetic wires and find the actuation highly dependent on the sample shape, and the angle of the magnetic field with respect to the alginment axis. We also propose some new structures based on hard magnetic wires which should provide piezomagnetic response.   

Physical Patterns Associated with 27 April 2011 Tornado Outbreak

The National Weather Service office in Memphis, Tennessee has aimed their efforts to improve severe tornado forecasting. Everything is not known about tornadogenesis, but one thing is: tornadoes tend to form within supercell thunderstorms. Hence, 27 April 2011 and 25 May 2011 were days when a Tornado Outbreak was expected to arise. Although 22 tornadoes struck the region on 27 April 2011, only 1 impacted the area on 25 May 2011. In order to understand both events, comparisons of their physical features were made. These parameters were studied using the Weather Event Simulator system and the NOAA/NWS Storm Prediction database. This research concentrated on the Surface Frontal Analysis, NAM40 700mb Dew-Points, NAM80 250mb Wind Speed and NAM20 500mb Vorticity images as well as 0-6 km Shear, MUCAPE and VGP mesoscale patterns. As result of this research a Dry-Line ahead of a Cold Front, Dew-points \r{ }5C and higher, and high Vorticity values$^{ }$were synoptic patterns that influenced to the formation of supercell tornadoes. Finally, MUCAPE and VGP favored the possibility of tornadoes occurrence on 25 May 2011, but shear was the factor that made 27 April 2011 a day for a Tornado Outbreak weather event.   

Session B39: Electronic Structure: Calculations I

Acceleration of Hartree-Fock Exchange Computations using Recursive Subspace Bisection

We use the recursive subspace bisection algorithm [1] to accelerate the computation of the Hartree-Fock exchange operator in electronic structure computations involving hybrid density functionals. This approach leads to a reduction of the computational cost of the exchange operator from $O(N^3 \log N)$ to $O(N^2 \log N)$ and allows for controlled accuracy through a threshold parameter. The subspace bisection method is extended to invariant subspaces including excited states. Applications to molecular dynamics simulations and computations of energy band gaps in large systems using the PBE0 hybrid functional will be presented. \\[4pt] [1] F. Gygi, Phys. Rev. Lett. 102, 166406 (2009).   

A Discontinuous Galerkin Framework for Electronic Structure Calculations

It is generally accepted that a good basis set for any calculation should possess a number of salient features, including systematic improvability, adaptive resolution of multiscale features, and fidelity in capturing the pertinent physics. Considering the progenitors of most modern electronic structure basis sets to be Gaussian-type orbitals or planewaves, descendants of these methods have inherited features that address either systematic improvability (planewaves) or adaptive resolution (Gaussians) separately, and use a variety of tricks to differentiate the core and valence physics. Discontinuous Galerkin methods provide a framework for defining adaptive local basis sets, that may be based on these canonical basis sets, that can be mixed and matched to simultaneously achieve all of these goals. Our group is presently developing a new electronic structure code to enable Density Functional and Hartree-Fock calculations within this framework, particularly in the context of all-electron formulations wherein the accurate resolution of both core and valence states is necessary. Numerous implementation details will be addressed, including the incorporation of hardware- and software-based acceleration, such as GPU-based parallelism, and fast electrostatics solvers.   

New method of optimizing the Jastrow factor for solids with the transcorrelated method

Transcorrelated (TC) method[1-5] is one of the promising theories for \textit{ab initio} electronic structure calculation of solids. It is one of the wave-function-based approaches which are considered to be advantageous for high-accuracy calculation. In the TC method, the total wave function is approximated as the Jastrow-Slater-type wave function, and the many-body Hamiltonian is similarity-transformed by the Jastrow factor. Then we solve an SCF equation and optimize one-body orbitals in the Slater determinant with relatively low computational cost. On the other hand, optimization of the Jastrow factor has been computationally much more demanding although it is indispensable to high-accuracy calculation. In this study, a new method of optimizing the Jastrow factor is developed by use of variance minimization of the total energy. It is demonstrated that, by truncating the basis-set expansion of the variance, the optimization is realized with low computational cost. [1] S. F. Boys and N. C. Handy, Proc. R. Soc. London Ser. A 309, 209 (1969). [2] S. Ten-no, Chem. Phys. Lett. 330, 169 (2000). [3] N. Umezawa and S. Tsuneyuki, J. Chem. Phys. 119, 10015 (2003). [4] R. Sakuma and S. Tsuneyuki, J. Phys. Soc. Jpn. 75, 103705 (2006). [5] H. Luo, J. Chem. Phys. 133, 154109 (2010).   

Adaptive local basis set for Kohn-Sham density functional theory in a discontinuous Galerkin framework

Uniform discretization of the Kohn-Sham Hamiltonian generally results in a large number of basis functions per atom in order to resolve the rapid oscillations of the Kohn-Sham orbitals around the nuclei even in the pseudopotential framework. Atomic orbitals and similar objects significantly reduces the number of basis functions, but these basis sets generally require fine tuning of the parameters in order to reach high accuracy. We present a novel discretization scheme that adaptively and systematically builds the rapid oscillations of the Kohn-Sham orbitals around the nuclei as well as environmental effects into the basis functions. The resulting basis functions are localized in the real space, and are discontinuous in the global domain. The continuous Kohn-Sham orbitals and the electron density are evaluated from the discontinuous basis functions using the discontinuous Galerkin (DG) framework. Our method is implemented in parallel and the current implementation is able to handle systems with at least thousands of atoms. Numerical examples indicate that our method can reach very high accuracy (less than $1$meV) with a very small number ($4\sim 40$) of basis functions per atom.   

Retrofit of the HSE density functional

The original parameterization of the Heyd-Scuseria-Ernzerhof (HSE) density functional was dependent on the choice of a hybrid fraction and a range-separation length for separating out a portion of the exchange energy to compute exactly. For backward compatibility with the PBE0 functional, the hybrid fraction was fixed to 0.25. Here, we examine the full hybrid fraction / separation length phase space. With respect to multiple error metrics, the phase space does not have a well-developed point of minimum error. Instead, there is a ``valley'' of functionals with increasing hybrid fraction and decreasing separation length of similarly good quality. This enables a reduction of the separation length without degrading the accuracy of the HSE06 parameterization, which in turn reduces the computational cost of evaluating the exchange energy.   

Projector Augmented Wave formulation of orbital-dependent exchange-correlation functionals

The use of orbital-dependent exchange-correlation functionals within electronic structure calculations has recently received renewed attention for improving the accuracy of the calculations, especially correcting self-interaction errors. Since the Projector Augmented Wave (PAW) method\footnote{ P. Bl\"{o}chl, {\em{Phys. Rev. B}} {\bf{50}}, 17953 (1994).} is an efficient pseudopotential-like scheme which ensures accurate evaluation of all multipole moments of direct and exchange Coulomb integrals, it is a natural choice for implementing orbital-dependent formalisms. Using Fock exchange as an example of an orbital-dependent functional, we developed the formulation and numerical implementation of the approximate optimized effective potential formalism of Kreiger, Li, and Iafrate (KLI)\footnote{ J. B. Krieger, Y. Li, and G. J. Iafrate {\em{Phys. Rev. A}} {\bf{45}}, 101 (1992).} within the PAW method, comparing results with the analogous Hartree-Fock treatment.\footnote{ Xiao Xu and N. A. W. Holzwarth, {\em{Phys. Rev. B}} {\bf{81}}, 245105 (2010); {\bf{84}}, 155113 (2011).} Test results are presented for ground state properties of two well-known materials -- diamond and LiF. This formalism can be extended to treat orbital-dependent functionals more generally.   

Solution of the Bethe-Salpeter equation without empty electronic states: Applications to solids, nanostructures and molecules

A method to solve the Bethe-Salpeter equation that avoids the explicit calculation of empty electronic states and the storage and inversion of dielectric matrices has been recently introduced [1-3]. This approach is suitable to compute the absorption spectra of large systems in a wide energy range and without relying on the Tamm-Dancoff approximation. We show the accuracy and scalability of this method by presenting calculations of absorption spectra of solids, molecules and nanostructures, including Si quantum dots and nanowires. In the case of nanowires, we discuss the influence of size and surface reconstruction on the optical properties.\\[4pt] [1] D. Rocca, D. Lu, and G. Galli, J. Chem. Phys. 133, 164109 (2010)\\[0pt] [2] D. Rocca, Y. Ping, R. Gebauer, and G. Galli, submitted to PRB \\[0pt] [3] Y. Ping, D. Rocca, D. Lu, and G. Galli, submitted to PRB   

Iterative diagonalization of non-Hermitian eigenproblems in time-dependent density-functional and many-body perturbation theory

We present a technique for the iterative diagonalization of random-phase approximation (RPA) matrices, which are encountered in the framework of time-dependent density-functional theory (TDDFT) and in the solution of the Bethe-Salpeter equation (BSE) [1]. The non-Hermitian character of these matrices does not permit a straightforward application of standard iterative techniques used, i.e., for the diagonalization of ground state Hamiltonians. We first introduce a new block variational principle for RPA matrices. We then develop an algorithm for the simultaneous calculation of multiple eigenvalues and eigenvectors, with convergence and stability properties similar to techniques used to iteratively diagonalize Hermitian matrices. The algorithm is validated by computing multiple low-lying excitation energies of molecules at both the TDDFT and BSE level.\\[4pt] [1] D. Rocca, Z. Bai, R.-C. Li, and G. Galli, submitted to J. Chem. Phys.   

New \emph{ab initio} approaches for calculating the microscopic electron-density response matrix

The electron-density response matrix is a key quantity for excitation calculations such as GW-BSE. Typically, the response matrix is obtained at the level of the random phase approximation (RPA), with the wavefunction from local density approximation (LDA) density functional theory. The expense of this approach grows quickly with the number of atoms, and its accuracy depends on both the accuracy of the LDA and the RPA. We investigate a series of approaches to overcome these difficulties, based on an eigenvalue decomposition of the molecular response matrix.   

Cluster expansion of the electron-density response function: GW+BSE with molecular environments

Molecular excitations in dielectric environments have drawn great interest because environmental manipulation provides the possibility to engineer photo-excitation processes. In exciton calculation the environments often are replaced by dielectric continuum media. These approaches have been successful for solvated molecules, but they lack molecular detail, and hence miss microscopic features. We present a new method to represent environments that allows a more accurate treatment of a wide range of systems by employing cluster expansions of environmental response functions. This initial work, at the GW+BSE level, presents results for shifts in excitation energies due to environments consisting of either individual molecules or conjugated polymers.   

Unified description of ground and excited states of finite systems: the self-consistent \textit{GW} approach

Fully self-consistent $GW$ calculations -- based on the iterative solution of the Dyson equation -- provide an approach for consistently describing ground and excited states on the same quantum mechanical level. Based on our implementation in the all-electron localized basis code FHI-aims [1], we show that for finite systems self-consistent $GW$ reaches the same final Green function regardless of the starting point. The results show that self-consistency systematically improves ionization energies and total energies of closed shell systems compared to perturbative $GW$ calculations ($G_0W_0$) based on Hartree-Fock or (semi)local density-functional theory. These improvements also translate to the electron density as demonstrated by a better description of the dipole moments of hetero-atomic dimers and the similarity with the coupled cluster singles doubles (CCSD) density. The starting-point independence of the self-consistent Green function facilitates a systematic and unbiased assessment of the performance of the $GW$ approximation for finite systems. It therefore constitutes an unambiguous reference for the future development of vertex corrections and beyond $GW$ schemes. [1] V. Blum \textit{et al.}, Comp. Phys. Comm. {\bf 180}, 2175 (2009).   

Stress formulation in the all-electron full-potential linearized augmented plane wave method

Stress formulation in the linearlized augmented plane wave (LAPW) method has been proposed in 2002 [1] as an extension of the force formulation in the LAPW method [2]. However, pressure calculations only for Al and Si were reported in Ref.[1] and even now stress calculations have not yet been fully established in the LAPW method. In order to make it possible to efficiently relax lattice shape and atomic positions simultaneously and to precisely evaluate the elastic constants in the LAPW method, we reformulate stress formula in the LAPW method with the Soler-Williams representation [3]. Validity of the formulation is tested by comparing the pressure obtained as the trace of stress tensor with that estimated from total energies for a wide variety of material systems. Results show that pressure is estimated within the accuracy of less than 0.1 GPa. Calculations of the shear elastic constant show that the shear components of the stress tensor are also precisely computed with the present formulation [4].\\[4pt] [1] T. Thonhauser {\it et al.}, Solid State Commun. {\bf 124}, 275 (2002).\\[0pt] [2] R. Yu {\it et al.}, Phys. Rev. B {\bf 43}, 6411 (1991).\\[0pt] [3] J. M. Soler and A. R. Williams, Phys. Rev. B {\bf 40}, 1560 (1989).\\[0pt] [4] N. Nagasako and T. Oguchi, J. Phys. Soc. Jpn. {\bf 80}, 024701 (2011).   

Real Space DFT by Locally Optimal Block Preconditioned Conjugate Gradient Method

Real space approaches solve the Kohn-Sham (KS) DFT problem as a system of partial differential equations (PDE) in real space numerical grids. In such techniques, the Hamiltonian matrix is typically much larger but sparser than the matrix arising in state-of-the-art DFT codes which are often based on directly minimizing the total energy functional. Evidence of good performance of real space methods - by Chebyshev filtered subspace iteration (CFSI) - was reported by Zhou, Saad, Tiago and Chelikowsky [1]. We found that the performance of the locally optimal block preconditioned conjugate gradient method (LOGPCG) introduced by Knyazev [2], when used in conjunction with CFSI, generally exceeds that of CFSI for solving the KS equations. We will present our implementation of the LOGPCG based real space electronic structure calculator. \\[4pt] [1] Y. Zhou, Y. Saad, M. L. Tiago, and J. R. Chelikowsky, ``Self-consistent-field calculations using Chebyshev-filtered subspace iteration,'' J. Comput. Phys., vol. 219,pp. 172-184, November 2006. \\[0pt] [2] A. V. Knyazev, ``Toward the optimal preconditioned eigensolver: Locally optimal block preconditioned conjugate gradient method,'' SIAM J. Sci. Comput, vol. 23, pp. 517-541, 2001.   

Bridging density-functional and many-body perturbation theory: orbital-density dependence in electronic-structure functionals

Energy functionals which depend explicitly on the orbital densities (ODD), instead of the total charge density, appear when applying self-interaction corrections to density-functional theory. In these cases (e.g. the Perdew-Zunger [1] and the non-Koopmans [2] approaches) the total energy loses invariance under unitary rotations of the orbitals, and the minimization of the functionals leads to orbital-dependent Hamiltonians. We show that it is possible to identify the orbital-dependency of densities and potentials with an effective and discretized frequency-dependency, in close analogy to the quasi-particle approximation of frequency-dependent self-energies and naturally oriented to interpret electronic spectroscopies [3]. Some of the existing ODD functionals are analyzed from this new perspective. Numerical results for the electronic structure of gas-phase molecules (within the Koopmans-corrected class of functionals) are computed and found in excellent agreement with photoemission (UPS) data. [1] J.-P. Perdew and A. Zunger, Phys. Rev. B 23, 5048 (1981). [2] I. Dabo, A. Ferretti, N. Poilvert, Y. Li, N. Marzari, M. Cococcioni, Phys. Rev. B 82, 115121 (2010). [3] M. Gatti, V. Olevano, L. Reining, I.-V. Tokatly, Phys. Rev. Lett. 99, 057401 (2007).   

A new type of pseudopotentials: effective atomic pseudopotentials

We derive a new type of pseudopotentials from conventional norm-conserving pseudopotentials for the treatment of a large number of atoms. The pseudopotentials are not aimed at the calculation of the total enegy, but of band edge states relevant for optical processes. We describe the pseudopotential construction and benchmark its quality and transferability by comparison to standard DFT calculations.   

Session B40: Focus Session: Cytoskeleton and Biomechanics - Entanglement and Crosslinking

Seeing is believing: New insight into structure-mechanics relationships in entangled and crosslinked microtubule networks

The microtubule cytoskeleton is essential in maintaining the shape, strength and organization of cells and its misregulation has been implicated in neurological disorders and cancers. To better understand the structure-mechanics relationships in microtubule networks, we measure the force-dependent viscoelastic responses of entangled and sparsely crosslinked microtubule networks to precise microscale manipulation. We use magnetic tweezers devices to apply calibrated step stresses and measure the resultant strain as a function of time. At short times the material behaves as an elastic solid. The linear regime is large, with gentle stiffening observed in entangled networks above $\sim $70{\%} strains. Crosslinked networks are stiffer, and show an extended linear regime. At longer times, we find a creeping regime, suggesting that structural rearrangements of the network dominate the mechanical response. To understand the molecular origins of this behavior, we use a newly-developed portable magnetic tweezers device to observe the network morphology using a confocal microscope while simultaneously applying point-like stresses to embedded magnetic particles. We observe substantial network compression in front of the bead with no evidence of long-length scale filament flow, and find that the spatial extent of the deformation field depends sensitively on network architecture and connectivity. Our results are important to understanding the role of the cytoskeleton in regulating cargo transport in vivo, as well as the basic physics of non-affine deformations in rigid rod polymer networks.   

Actin filament curvature biases branching direction

Actin filaments are key components of the cellular machinery, vital for a wide range of processes ranging from cell motility to endocytosis. Actin filaments can branch, and essential in this process is a protein complex known as the Arp2/3 complex, which nucleate new ``daughter'' filaments from pre-existing ``mother'' filaments by attaching itself to the mother filament. Though much progress has been made in understanding the Arp2/3-actin junction, some very interesting questions remain. In particular, F-actin is a dynamic polymer that undergoes a wide range of fluctuations. Prior studies of the Arp2/3-actin junction provides a very static notion of Arp2/3 binding. The question we ask is how differently does the Arp2/3 complex interact with a straight filament compared to a bent filament? In this study, we used Monte Carlo simulations of a surface-tethered worm-like chain to explore possible mechanisms underlying the experimental observation that there exists preferential branch formation by the Arp2/3 complex on the convex face of a curved filament. We show that a fluctuation gating model in which Arp2/3 binding to the actin filament is dependent upon a rare high-local-curvature shape fluctuation of the filament is consistent with the experimental data.   

Condensation of F-Actin by Dimensional Reduction

We present a Brownian Dynamics simulation of the equilibrium condensation of F-actin in the presence of linker molecules. The filaments are modeled as worm-like chains, using finite element analysis. At low linker concentrations, the systems forms a gel whose physical properties do not depend on the linker molecules. If the linker concentration is increased then for isotropic linkers only a single mode of condensation is encountered: bundle formation. If the linker molecules impose a preferential angle between F-actin filaments, then condensation takes place either into a either a hexatic or squaratic two-dimensional liquid crystal phase or into a heterogeneous cluster. Condensation is driven by competition between linker and filament entropy, which imposes dimensional reduction on the F-actin aggregate.   

Stress Enhanced Gelation in $\alpha$-Actinin-4 Cross-linked Actin Networks

A hallmark of biopolymer networks is their exquisite sensitivity to stress, demonstrated for example, by pronounced nonlinear elastic stiffening. Typically, they also yield under increased static load, providing a mechanism to achieve fluid-like behavior. In this talk, I will demonstrate an unexpected dynamical behavior in biopolymer networks consisting of F-actin cross-linked by a physiological actin binding protein, $\alpha$-Actinin-4. Applied stress actually enhances gelation of these networks by delaying the onset of structural relaxation and network flow, thereby extending the regime of solid-like behavior to much lower frequencies. By using human kidney disease-associated mutant cross-linkers with varying binding affinities, we propose a molecular origin for this stress-enhanced gelation: It arises from the increased binding affinity of the cross-linker under load, characteristic of catch-bond-like behavior. This property may have important biological implications for intracellular mechanics, representing as it does a qualitatively new class of material behavior.   

Simulations and theory of model microtubule self-assembly

We used molecular dynamics simulations to study the self-assembly of artificial microtubules. The model monomer has a wedge-shape to promote formation of rings that stack to form tubules. Attractive interaction sites are on the sides for ring formation and top/bottom for filament growth. We have studied the assembly kinetics and dynamics as a function of these lateral and vertical interaction strengths. A full structure diagram was calculated. The range of interaction strengths that best form tubules has been determined. We found that tubules form better when the lateral strength is stronger than the filamental stength, which contrast the picture for microtubules. The interaction strengths must be weak enough to allow for reformation of the clusters that initially form. Besides tubules, a variety of structures form depending on the interaction parameters. Interestingly, helical tubes and other helical structures are frequently observed despite the fact that the minimum energy substructure is a nonhelical ring. We have used a simple Flory-Huggins type theory to characterize the structure diagram.   

Dynamical Length-Regulation of Microtubules

Microtubules (MTs) are vital constituents of the cytoskeleton. These stiff filaments are not only needed for mechanical support. They also fulfill highly dynamic tasks. For instance MTs build the mitotic spindle, which pulls the doubled set of chromosomes apart during mitosis. Hence, a well-regulated and adjustable MT length is essential for cell division. Extending a recently introduced model [1], we here study length-regulation of MTs. Thereby we account for both spontaneous polymerization and depolymerization triggered by motor proteins. In contrast to the polymerization rate, the effective depolymerization rate depends on the presence of molecular motors at the tip and thereby on crowding effects which in turn depend on the MT length. We show that these antagonistic effects result in a well-defined MT length. Stochastic simulations and analytic calculations reveal the exact regimes where regulation is feasible. Furthermore, the adjusted MT length and the ensuing strength of fluctuations are analyzed. Taken together, we make quantitative predictions which can be tested experimentally. These results should help to obtain deeper insights in the microscopic mechanisms underlying length-regulation. \\[4pt] [1] L.Reese, A.Melbinger, E.Frey, Biophys. J., {\bf 101}, 9, 2190 (2011)   

On the origins and extent of mechanical variation among cells

Why would any one biological cell be mechanically different from another from the same population? Prompted by findings of broad distributions of cell stiffness within populations, we investigate possible origins of intrinsic mechanical heterogeneity among single cells. Through optical stretching, a non-contact technique for deforming cells in the suspended state, we obtain the creep compliance and complex modulus of single cells. Measurements of hundreds of human mesenchymal stem cells and murine fibroblasts in the time and frequency domains reveal that mechanical heterogeneity is not detectably dependent on cell lineage, cell cycle, cytoskeletal crosslinking, or repeated loading. However, adenosine triphosphate (ATP) depletion reduces heterogeneity of both stiffness and fluidity values. We explore the connection between these two parameters by positing that mechanical variation predominantly arises from Gaussian fluctuations in cell fluidity, which can be interpreted as emergent agitation in the energy landscape of soft glassy materials. Our findings ultimately link relatively small structural variations within cytoskeletal networks to large mechanical differences among cells and cell populations.   

Non-Equilibrium Cell Mechanics Studied with a Dual Optical Trap

Cells communicate with their surroundings biochemically, but at the same time also sense the active and passive mechanical properties of their micro-environment. Cells can ``feel'' mechanical stress and they generate contractile forces through their acto-myosin network to actively probe the mechanical response of the material they adhere to or are embedded in. These mechanosensory interactions result in cellular responses. We have used a dual optical trap to perform force measurements on cells suspended between two fibronectin-coated beads. We analyzed the correlated fluctuations of the beads with high spatial and temporal resolution. Using a combination of active and passive microrheology, we can simultaneously determine the (non-thermal) forces generated by the cells and actively probe their visco-elastic response properties. Here, we present data on contractile forces and elastic response of 3T3 fibroblasts, demonstrating that the transmitted force depends on the trap stiffness (i.e. rigidity of the environment). Using biochemical perturbations, we have studied the contributions of different cytoskeletal elements to the active and passive mechanical properties of the cell.   

M {\&} M's: Mechanosensitivity and Mechanotransduction in Myoblasts

The effect of external mechanical stimulation of muscle precursor cells (myoblasts) during exercise is a crucial step in myogenesis. This effect takes place many hours later while muscles are in a resting state; however it remains unclear to what extent the role of force application has on the promotion of myogenesis. Here, we combine Traction Force Microscopy (TFM) and Atomic Force Microscopy (AFM) to directly measure the magnitude of generated cellular traction forces (CTFs) in myoblasts, as a result of controlled mechanical loading. Precise nanonewton forces (1 {\&} 10 nN) were applied to live cells with the AFM tip while simultaneous TFM measurements were performed. The experiment was performed on substrates ranging in elastic moduli ($E)$, (16-89 kPa) mimicking resting and active muscle tissue, respectively. The results of this analysis demonstrated that the magnitude of CTFs was dependent on substrate $E,$ as expected. However, CTFs only increased in response to applied force (compared to controls) on substrates with $E$ greater than 62 kPa. Our results suggest that muscle precursor cells are most sensitive to mechanical force when the surrounding muscle tissue is stiff and contracted, whereas myogenesis itself proceeds optimally on softer, resting tissue.   

Polymer Coated Surface Acoustic Wave Biosensor for Living Cells

A shear horizontal surface acoustic wave (SH-SAW) biosensor is fabricated on quartz wafer for measurement of mechanical properties of living cells. The SAW device was fabricated with a top film of polymer (PMMA, SU-8) to avoid immense attenuation in aqueous media. Several models were designed to operate under different frequencies such as 20MHz, 40MHz and 80MHz and higher in order to identify how frequency affect the sensitivity. A network analyzer was used to capture the resonant frequency of inter-digitated transducers (IDT) of SAW, and it is found that resonant frequency shift is closely correlated to the cell deposition on the sensing area of SAW.   

Modeling spinal cord biomechanics

Regeneration after spinal cord injury is a serious health issue and there is no treatment for ailing patients. To understand regeneration of the spinal cord we used a system where regeneration occurs naturally, such as the lamprey. In this work, we analyzed the stress response of the spinal cord to tensile loading and obtained the mechanical properties of the cord both in vitro and in vivo. Physiological measurements showed that the spinal cord is pre-stressed to a strain of 10$\%$, and during sinusoidal swimming, there is a local strain of 5$\%$ concentrated evenly at the mid-body and caudal sections. We found that the mechanical properties are homogeneous along the body and independent of the meninges. The mechanical behavior of the spinal cord can be characterized by a non-linear viscoelastic model, described by a modulus of 20 KPa for strains up to 15$\%$ and a modulus of 0.5 MPa for strains above 15$\%$, in agreement with experimental data. However, this model does not offer a full understanding of the behavior of the spinal cord fibers. Using polymer physics we developed a model that relates the stress response as a function of the number of fibers.   

Matrix elasticity perturbation and Lamin-A/C expression in stem cells modulate their mechanics and lineage specification

Commitment of stem cells to different lineages is regulated by many cues in their local microenvironment. They are particularly sensitive to the mechanical properties of their extracellular matrix. Nuclear lamins are fibrous proteins providing structural function and transcriptional regulation in the cell nucleus. In particular Lamin A/C levels could influence cellular mechanical sensitivity. Here we show that perturbation of the extracellular matrix and nucleus mechanics can direct stem cells lineage specification. We studied the behavior of human mensechymal stem cells (hMSC) cultured on thin highly ordered collagen nanofilms. To tune the mechanical properties of the nanofilms we used the enzyme transglutaminase as a crosslinking agent. AFM imaging and manipulation is used to examine the nano topography and mechanical properties of the films and cells. Film stiffening affects cells morphology, cytoskeleton organization and their elastic response. hMSCs cultured for two weeks on collagen nanofilms initially tune their stiffness with matrix elasticity but later continuously change it with time. We observed upregulation of osteogenic markers on cross-linked films and increased lamin A/C expression. We show that manipulating Lamin-A/C expression in stem cells also directs cell lineage with knockdown favoring adipogenesis and over expression favoring osteogenesis. We found positive correlation between matrix and nucleus mechanics and that they have a synergistic effect on hMSCs differentiation potential.   

Simulation of Second Harmonic Generation from Heterogeneous Microtubule Structures

Second harmonic generation imaging is a coherent nonlinear microscopy with contrast arising from certain asymmetric endogenous structures in cells, including spindle microtubules. As a second-order nonlinear optical process, SHG requires a noncentrosymmetric macromolecular organization to generate signal, so it can be used as a measure of microtubule polarity within spindles or other microtubule structures. We developed a simulation of SHG microscopy accounting for 3-dimensional orientation and circularly polarized excitation in order to quantify the dependence of SHG signal on microtubule density, spacing, polarity, and rotational order. SHG can be used to assess spindle polarity in living cells using simultaneous ratio imaging with two-photon excited fluorescence from labeled tubulin. The results from simulation are used to quantify microtubule polarity from SHG and TPEF images of spindles in the one-cell C. elegans embryo and Xenopus oocyte extract.   

Session B41: Lipid Bilayers and Biological Membranes

Computer Simulation of Cytoskeleton-Induced Blebbing of Lipid Membranes

Blebs are balloon-shaped membrane protrusions that form during many physiological processes such as cytokinesis, cell motility and apoptosis. Using computer simulation of a particle-based model for self-assembled lipid bilayers coupled to an elastic meshwork, we investigated the phase behavior and kinetics of blebbing. We found that for small values of the mismatch parameter, defined as the ratio between the area of the lipid bilayer divided by the rest area of the cytoskeleton, the equilibrium state is that of a homogeneous vesicle with the cytoskeleton conforming to the bilayer. However, for large values of a mismatch parameter, the equilibrium state is that of a blebbed vesicle. We also found that blebbing can be induced when the cytoskeleton is subject to a localized ablation or a uniform compression. The obtained results are qualitatively in agreement with the experimental evidence and the model opens up the possibility to study the kinetics of bleb formation in detail. For more information see Spangler {\em et al.}, Phys. Rev. E {\bf 84}, 051906 (2011).   

Coarse-grained simulation of lipid vesicles with ``n-atic'' orientational order

We perform coarse-grained simulation studies of fluid lipid vesicles with in-plane ``n-atic'' orientational order associated with the shape of lipid head group, to test the theoretical predictions of Park, Lubensky and MacKintosh [1] for resulting vesicle shape and defect structures. Our simulation model uses a single layer coarse-grained implicit-solvent approach proposed by Yuan et al [2], with addition of an extra vector degree of freedom representing in-plane orientational order. We carry out simulation studies for n=1 to 6, examining in each case the spatial distribution of defects and resulting deformation of the vesicle. An initially spherical vesicle (genus zero) with n-atic order has a ground state with 2n vortices of strength 1/n, as expected, but the observed equilibrium shapes are sometimes quite different from those predicted theoretically. For the n=1 case, we find that the vesicle may become trapped in a disordered, long-lived metastable state with extra +/- defects whose pair-annihilation is inhibited by local changes in membrane curvature, and thus may never reach its predicted ground state. \\[4pt] [1] J. Park, T. C. Lubensky, and F. C. MacKintosh, Europhys. Lett. 20, 279 (1992)\\[0pt] [2] H. Yuan, C. Huang, Ju Li, G. Lykotrafitis, and S. Zhang, Phys. Rev. E 82, 011905 (2010)   

Morphologies of Elastic Membranes with Fluctuating Connectivity

We numerically investigate the effects of topological defects in single-component two-dimensional elastic membranes with spherical topology allowing changes in shape. The membrane is simulated as a closed, triangulated elastic surface in three dimensions, where the vertices are permitted to move in space and the connectivity of the triangulation is able to fluctuate. Fluctuations in connectivity allow the creation of topological defects. A Monte Carlo simulated annealing method is utilized to explore optimal shapes and connectivities. The familiar defect-driven buckling transition [Seung \& Nelson, PRA 1988, 38:1005] from a sphere to an icosahedron is shifted as a result of the fluctuating connectivity.   

Cooperative Motion in Lipid Bilayer Membranes

Lipid bilayer membranes, like the cell membrane, are complex biological systems. Transport of specific molecules in and out of cells are controlled by these membranes. Therefore, it is vital to understand the detailed dynamics that ultimately control membrane transport. We use molecular dynamics simulations of a coarse-grained and solvent-free lipid model that has been previously shown to spontaneously assemble a bilayer structure. Approaching the crossover to the gel-like state of the bilayer, the lipid dynamics become extremely slow. We analyze the cooperativity of the lipid motion and compare it with the cooperativity that has been well-characterized in liquids nearing a glass transition. Future simulations will examine the generality of this behavior using more realistic models, and comparing with experiments.   

How the antimicrobial peptides destroy bacteria cell membrane: Translocations vs. membrane buckling

In this study, coarse grained Dissipative Particle Dynamics simulation with implementation of electrostatic interactions is developed in constant pressure and surface tension ensemble to elucidate how the antimicrobial peptide molecules affect bilayer cell membrane structure and kill bacteria. We find that peptides with different chemical-physical properties exhibit different membrane obstructing mechanisms. Peptide molecules can destroy vital functions of the affected bacteria by translocating across their membranes via worm-holes, or by associating with membrane lipids to form hydrophilic cores trapped inside the hydrophobic domain of the membranes. In the latter scenario, the affected membranes are strongly corrugated (buckled) in accord with very recent experimental observations [G. E. Fantner \textit{et al.}, Nat. Nanotech., 5 (2010), pp. 280-285].   

Interaction of PLGA and trimethyl chitosan modified PLGA nanoparticles with mixed anionic/zwitterionic phospholipid bilayers studied using molecular dynamics simulations

Poly(lactic-co-glycolic acid) (PLGA) is a biodegradable polymer. Nanoparticles of PLGA are commonly used for drug delivery applications. The interaction of the nanoparticles with the cell membrane may influence the rate of their uptake by cells. Both PLGA and cell membranes are negatively charged, so adding positively charged polymers such as trimethyl chitosan (TMC) which adheres to the PLGA particles improves their cellular uptake. The interaction of 3 nm PLGA and TMC-modified-PLGA nanoparticles with lipid bilayers composed of mixtures of phosphatidylcholine and phosphatidylserine lipids was studied using molecular dynamics simulations. The free energy profiles as function of nanoparticles position along the normal direction to the bilayers were calculated, the distribution of phosphatidylserine lipids as a function of distance of the particle from the bilayer was calculated, and the time scale for particle motion in the directions parallel to the bilayer surface was estimated.   

Atomic Force Microscopy Study of Changes to the Mechanical Properties of \textit{Pseudomonas aeruginosa }Cells Induced By Antimicrobial Peptides

The cell envelope of Gram-negative bacteria plays a key role in the maintenance of cell shape and the selective transfer of small molecules in and out of the cell. Both the inner and outer membranes of the cell envelope can be major targets for antimicrobial peptides, which can ultimately compromise the mechanical integrity of the cell. We have applied a new, AFM-based creep deformation technique (1) to study changes to the mechanical properties of individual \textit{Pseudomonas aeruginosa }cells as a function of time of exposure to polymyxin B (PMB), a well-known cyclic antimicrobial peptide. The results can be understood in terms of simple viscoelastic models of arrangements of springs and dashpots. These measurements provide a direct measure of the mechanical integrity of the bacterial cell, and time-resolved creep deformation experiments reveal that the time of action for PMB is very fast (of the order of a minute). This measurement provides new insight into the mechanism of action of antimicrobial peptides. (1) V. Vadillo-Rodriguez, T. J. Beveridge, and J. R. Dutcher, \textit{J. Bacteriol}. \textbf{190}, 4225-4232 (2008).   

Interactions between HIV-1 Neutralizing Antibodies and Model Lipid Membranes imaged with AFM

Lipid membrane interactions with rare, broadly neutralizing antibodies (NAbs), 2F5 and 4E10, play a critical role in HIV-1 neutralization. Our research is motivated by recent immunization studies that have shown that induction of antibodies that avidly bind the gp41-MPER antigen is not sufficient for neutralization. Rather, it is required that antigen designs induce polyreactive antibodies that recognize MPER antigens as well as the viral lipid membrane. However, the mechanistic details of how membrane properties influence NAb-lipid and NAb-antigen interactions remain unknown. Furthermore, it is well established that the native viral membrane is heterogeneous, representing a mosaic of lipid rafts and protein clustering. However, the size, physical properties, and dynamics of these regions are poorly characterized and their potential roles in HIV-1 neutralization are also unknown. To understand how membrane properties contribute to 2F5/4E10 membrane interactions, we have engineered biomimetic supported lipid bilayers (SLBs) and use atomic force microscopy to visualize membrane domains, antigen clustering, and antibody-membrane interactions at sub-nanometer z-resolution. Our results show that localized binding of HIV-1 antigens and NAbs occur preferentially with the most fluid membrane domain. This supports the theory that NAbs may interact with regions of low lateral lipid forces that allow antibody insertion into the bilayer.   

Modeling curvature-dependent subcellular localization of a small sporulation protein in Bacillus subtilis

Recent experiments suggest that in the bacterium, B. subtilis, the cue for the localization of small sporulation protein, SpoVM, that plays a central role in spore coat formation, is curvature of the bacterial plasma membrane. This curvature-dependent localization is puzzling given the orders of magnitude difference in lengthscale of an individual protein and radius of curvature of the membrane. Here we develop a minimal model to study the relationship between curvature-dependent membrane absorption of SpoVM and clustering of membrane-associated SpoVM and compare our results with experiments.   

Relationship between peptide amino acid sequence and membrane curvature generation

Amphipathic peptides and amphipathic domains in proteins can perturb and restructure biological membranes. For example, it is believed that the cationic, amphipathic motif found in membrane active antimicrobial peptides (AMPs) is responsible for their membrane disruption mechanisms of action. And ApoA-I, the main apolipoprotein in high density lipoprotein contains a series of amphipathic $\alpha $-helical repeats which are responsible for its lipid associating properties. We use small angle x-ray scattering (SAXS) to investigate the interaction of model cell membranes with prototypical AMPs and consensus peptides derived from the helical structural motif of ApoA-I. The relationship between peptide sequence and the peptide-induced changes in membrane curvature and topology is examined. By comparing the membrane rearrangement and corresponding phase behavior induced by these two distinct classes of membrane restructuring peptides we will discuss the role of amino acid sequence on membrane curvature generation.   

Family of pH-Low-Insertion-Peptides (pHLIPs)

pHLIP (pH (Low) Insertion Peptide) is a novel delivery system for targeting of acidic diseased tissue such as solid tumors, sites of inflammation, arthritis and other pathological states. The molecular mechanism of pHLIP action is based on pH-dependent insertion and folding of pHLIP in membrane. We performed sequence variation and investigated 16 pHLIP variants with main goals of understanding the main principles of peptide-lipid interactions and tune delivery capability of pHLIP. The biophysical studies including thermodynamics and kinetics of the peptides interaction with a lipid bilayer of liposomes and cellular membranes were carried out. We found that peptides association to membrane at neutral and low pH could be modulated by 3-4 times. The apparent pK of transition from surface bound to membrane-inserted state could be tuned from 6.5 to 4.5. The rate of peptide's insertion across a bilayer could be enhanced 100 times compared to parent pHLIP. As a result, blood clearance and tumor targeting were modulated in a significant degree. The work is supported by NIH grants CA133890 to OAA, DME, YRK.   

Membrane-associated peptide folding: pH triggered insertion and helical structure formation

We are interested in the molecular events that occur when a peptide inserts across a membrane or exits from it. pHLIP (pH (Low) Insertion Peptide) provides an opportunity to study membrane insertion/exit and folding/unfolding, since its insertion is modulated by pH and since it forms helical structure as it inserts. We found that pHLIP inserts across a POPC phospholipid bilayer in several steps: first is the rapid formation (100 ms) of an interfacial helix, which is then followed by a slow insertion pathway that contains several intermediates. We show that while the number of protonatable residues at the inserting end does not affect the formation of helical structure in the membrane, it correlates with the time for transmembrane insertion, the number of intermediate states on the folding pathway, and the rate of unfolding and exit. We conclude that particular intermediate states on the folding and unfolding pathways are not mandatory and, in the simple case of a polypeptide with a non-charged and non-polar inserting end, the folding and unfolding is well described as an all-or-none transition. A model for membrane-associated insertion/folding and exit/unfolding is proposed.   

Modulation of the transmembrane helix insertion pathway by polar cargo

In an earlier study, we found a series of kinetic steps in the pH-triggered insertion of the pHLIP{\textregistered} (pH (Low) Insertion Peptide). In the present work we observe that the polarity of the inserting end, including its cargo, modulates the number of intermediates, and that insertion can be described as a two state process for a simple case. Each investigated pHLIP variant preserve the pH-dependent properties of surface binding to membrane at neutral pH and insertion at low pH to form a transmembrane helix. However, there are thermodynamic and kinetic properties that are determined by the degree of cargo polarity. The presence of a polar cargo at the peptide's inserting end leads to the appearance of two additional intermediate states on the insertion pathway of the pHLIP-2E peptide, which itself (when no cargo is attached) shows an all-or-none transition from the partially unstructured membrane-surface to the transmembrane state described well by the two-state model at 800 ms timescale. We discuss the utility of our observations for the design of new delivery agents for the direct translocation of polar therapeutic and diagnostic cargo molecules across cellular membranes. The work is supported by NIH grants CA133890 to OAA, DME, YRK.   

Exploration of phase separation in heterogeneous lipid monolayers

A Langmuir monolayer is a well established model of a single leaflet of a lipid membrane. In this work, we investigate the phase separation behavior of a model Langmuir monolayer as a function of both Langmuir surface pressure and ratio of saturated lipid : unsaturated lipid : cholesterol. The specifics of domain separation behavior, or ``rafting,'' in membranes are generally thought to be responsible for much of the behavior of living membranes, specifically in protein integration and transport. Off-specular x-ray scattering is used to probe in-plane structure of the membrane at the sub-micron scale. Additionally, atomic force microscopy imaging is taken on samples transferred to a rigid support. In-plane order is found to grow as a function of surface pressure. Also, the in-plane order is found to depend on cholesterol concentration in the monolayer. The phase space of the in-plane order as a function of lipid and cholesterol concentration is presented.   

Domain formation in multicomponent lipid bilayers coupled to elastic substrate

We will discuss the physics that governs the lipid localization and domain formation in multicomponent lipid bilayers coupled to an elastic substrate. Lipid localization and domain formation has been studied extensively in biological cell membranes. In this talk we will extend a previous model for membrane energetics to account for the coupling between the bending and the local lipid composition of the two leaflets. Our aim is to determine the relationship between the localization and domain formation in the presence of lipid flip-flops between the two leaflets and the effect of intrinsic curvature of the lipids. Using a lattice model for the membrane, we simulate the system and study the effect of lipid flip-flop on lipid organization in the membrane.   

Session B42: Focus Session: BioChip Physics-Detection and Transport

On-chip Metamaterials for Ultra-sensitive Spectroscopy and Identification of Biomolecules

Infrared absorption spectroscopy is a unique tool for identifying and characterizing molecular bonds. For most organic and inorganic molecules (such as proteins, chemical toxins and gases), vibrational and rotational modes are spectroscopically accessible within the mid-infrared (mid-IR; 3-20 $\mu $m) regime of the electromagnetic spectrum. Characteristic vibrational modes are associated with unique IR absorption spectral bands that are bond-specific. Because of that, the IR wavelength range is also known as ``finger print'' region. However, because of the Beer-Lambert law, its sensitivity has been limited to perform analytical/functional studies on small samples often available from biological specimens. In this talk we will describe how we use plasmonic metametrials to overcome these challenges. We will introduce tailoring of the resonances to selectively address fingerprint signatures of proteins. We will also describe novel designs and fabrication methods to exploit extreme near-field enhancements in small gaps for vibrational signal enhancements.\\[4pt] In collaboration with In collaboration with Ronen Adato, Serap Aksu, Alp Artar, Arif Cetin, and Boston University.   

Ultrasensitive Plasmonic Biosensors for Direct Detection of Biomarker Proteins with The Naked Eye

We introduce an ultrasensitive label free biodetection technique based on asymmetric plasmonic Fano resonances. Our sensors bring a number of advantages: (i) ultrasensitive detection limits surpassing gold standard Kretschmann configuration plasmon sensors, (ii) detection of biomarker molecules with ``the naked eye'', (iii) massive multiplexing capabilities. By exploiting extraordinary light transmission phenomena through high quality factor sub-radiant dark modes, we experimentally demonstrate record high figures of merits for intrinsic detection limits surpassing the gold standard BiaCore devices. Our experiments show an order of magnitude improved device performances over the state of art metamaterial and other plasmonic biosensors. Steep dispersion of the plasmonic Fano resonance profiles in engineered plasmonic sensors exhibit dramatic light intensity changes to the slightest perturbations within their local environment. As a spectacular demonstration, we show direct detection of a single monolayer of biomolecules with naked eye using these Fano resonances and the associated Wood's anomalies. The demonstrated sensing platform offers point-of-care diagnostics in resource poor settings by eliminating the need for fluorescent labeling and optical detection instrumentation (such camera, spectrometer, etc.).   

High-Throughput On-Chip Diagnostic System for Circulating Tumor Cells

We have developed a novel, low-cost and high-throughput microfluidic device for detection and molecular analysis of circulating tumor cells (CTCs). The operation is based upon a size-selective cell separation, which was enabled by a weir-style physical barrier with a gap in the fluidic channel. The new system is a versatile CTC analysis platform with many advantages. First, it supports extremely high throughput operation, since the use of weir structure reduces fluidic resistance and enables flow-through separation ($>$ 20,000-fold CTC enrichment from whole blood at the flow rate of 10 mL/h). Second, the CTC-chip facilitates visual verification and enumeration of CTCs during/after operation. By implementing microwell-shaped structures on the physical barrier, CTCs can be individually captured at sites for single-cell resolution analyses. Furthermore, the captured cells could be profiled in situ by introducing antibodies or small molecular probes. The chip thus assumes not only high detection sensitivity but also molecular specificity for CTC identification. Finally, the CTC-chip can retrieve captured CTCs. By reversing the flow direction, the cells can be dislodged from their capture sites and collected for downstream investigation.   

Mass transport to suspended waveguide biosensors

The response of a biosensor is controlled both by the kinetics of analyte adsorption as well as the mass transport to the device. Improving the affinity between a target molecule and the functionalized sensor can pose significant challenges in terms of biochemistry and surface chemistry. The careful design of sample flow systems presents a more convenient route for decreasing the time required for a measurement Using finite element methods, we model mass transport to a novel integrated photonic biosensor suspended within a microfluidic channel in an effort to understand how boundary layer flow patterns may be engineered to improve the transient response of the device. By monitoring the surface concentration of bound analyte over a range of inlet concentrations and vertical positions within the channel, we compare the behavior of suspended devices to that of planar sensors located on the floor of the channel. Thinner boundary layers and increased effective sensing area lead to consistently faster transient responses for the suspended sensor, with optimal performance resulting from the symmetric placement of the sensor with respect to the channel height.   

Weighing single cells in two fluids: measuring mass, volume and density

The Suspended Microchannel Resonator (SMR) is a highly sensitive cantilever-based mass sensor shown to be capable of weighing the buoyant mass of living single cells. We have engineered SMR-based microfluidic systems to achieve consecutive weighing of single cells in two different fluids, with controlled exposure times. By choosing fluids of two different densities, the paired buoyant mass measurements are used to characterize single-cell volume, mass and density. With density precision of 0.001 g.cm$^{-3}$, we explore the application of our techniques to samples ranging from bacterial to mammalian cells and show that cellular density is a tightly regulated biological property within populations, up to 100-fold more so than the other size parameters.   

Label-free screening of niche-to-niche variation in satellite stem cells using functionalized pores

Combinations of surface markers are currently used to identify muscle satellite cells. Using pores functionalized with specific antibodies and measuring the transit time of cells passing through these pores, we discovered remarkable heterogeneity in the expression of these markers in muscle (satellite) stem cells that reside in different single myofibers. Microniche-specific variation in stem cells of the same organ has not been previously described, as bulk analysis does not discriminate between separate myofibers or even separate hind-leg muscle groups. We found a significant population of Sca-1+ satellite cells that form myotubes, thereby demonstrating the myogenic potential of Sca-1+ cells, which are currently excluded in bulk sorting. Finally, using our label-free pore screening technique, we have been able to quantify directly surface expression of Notch1 without activation of the Notch pathway. We show for the first time Notch1-expression heterogeneity in unactivated satellite cells. The discovery of fiber-to-fiber variations prompts new research into the reasons for such diversity in muscle stem cells.   

Demonstration and analysis of the harmonic dithering technique for a high-sensitivity silicon waveguide biosensor

A label-free biosensor readout technique is demonstrated based on a silicon-on-insulator ring resonators and a harmonic dithering technique using a distributed feedback (DFB) laser and a lock-in amplifier. The 400 $\mu$m ring resonator is integrated with a microfluidic sample delivery channel formed with Polydimethylsiloxane (PDMS). Dithering the frequency of the DFB laser across the Lorentzian lineshape of the drop port at high frequency eliminates 1/f noise, and broadband noise is reduced by narrow-band detection with the lock-in amplifier. Biosensor system noise is analyzed and compared with more conventional readout methods, and in our case is dominated by thermal noise of the receiver, shot noise, and relative intensity noise (RIN) of the DFB laser. Because the readout automatically latches onto the drop port and does not require a complex scanning process, this methodology may provide a pathway for high-sensitivity, real-time, and low-cost biosensing.   

Label-free detection of DNA on silicon surfaces using Brewster angle straddle interferometry (BASI)

Label-free sensing of biomolecular interactions is of great importance for drug screening and a variety of clinical assays. Ultrasensitive detection of dsDNA on silicon substrates can be achieved using our new label-free sensing method - Brewster angle straddle interferometry (BASI) which exploits the removal of destructive interference to detect binding of target molecules on a silicon surface functionalized by probe molecules. By exploiting the fact that reflections of p-polarization undergo 180 degree phase shifts above the Brewster angle and none below it, we are able to use unprocessed silicon substrates with native oxide serving as the interference layer. Destructive interference in the geometry we use results in reflectivities $\sim$ 0.01\%. Reflectivity from the chip is a quantitative measure of the amount of bound target molecules and can be imaged in real time in microarray format. We demonstrate detection of DNA intercalation on pyrene modified surfaces. The substrates are shown to exhibit excellent binding toward dsDNAs. This work provides an avenue for understanding the binding specificity of small molecule-DNA interactions that can be potentially helpful in developing anticancer agents.   

Real-time molecular detection using a nanoscale porous silicon waveguide biosensor

A grating-coupled porous silicon waveguide with an integrated PDMS flow cell is demonstrated as a platform for real-time detection of chemical and biological molecules. This sensor platform not only allows for quantification of molecular binding events, but also provides a means to improve understanding of diffusion and binding mechanisms in constricted nanoscale geometries. The large internal surface area of porous silicon enables the capture of molecules inside the waveguide, which causes a large perturbation of the guided mode field and improves detection sensitivity by more than one order of magnitude as compared to evanescent wave-based detection methods. Molecular binding events in the waveguide are monitored by real-time angle-resolved reflectance measurements. Diffusion, adsorption and desorption coefficients of different sized chemical linker and nucleic acid molecules are determined based on the rate of change of the measured resonance angle. Both the magnitude of the waveguide resonance angle shift and kinetic parameters are observed to depend on molecule size. Experimental results are shown to be in good agreement with calculations based on rigorous coupled wave analysis and finite element simulation.   

On-chip Magnetic Separation and Cell Encapsulation in Droplets

The demand for high-throughput single cell assays is gaining importance because of the heterogeneity of many cell suspensions, even after significant initial sorting. These suspensions may display cell-to-cell variability at the gene expression level that could impact single cell functional genomics, cancer, stem-cell research and drug screening. The on-chip monitoring of individual cells in an isolated environment could prevent cross-contamination, provide high recovery yield and ability to study biological traits at a single cell level These advantages of on-chip biological experiments contrast to conventional methods, which require bulk samples that provide only averaged information on cell metabolism. We report on a device that integrates microfluidic technology with a magnetic tweezers array to combine the functionality of separation and encapsulation of objects such as immunomagnetically labeled cells or magnetic beads into pico-liter droplets on the same chip. The ability to control the separation throughput that is independent of the hydrodynamic droplet generation rate allows the encapsulation efficiency to be optimized. The device can potentially be integrated with on-chip labeling and/or bio-detection to become a powerful single-cell analysis device.   

Chip-based magnetic cytometer for high-throughput cellular profiling in unprocessed biological samples

Quantitative, high-throughput measurement of biomarkers in individual cells is a cornerstone of biomedical research, but prohibitive size, cost, and requisite sample processing have kept this technology from being more widely adapted in the clinic. We have developed a miniaturized magnetic cytometer ($\mu $MCM), a hybrid semiconductor / microfluidic chip, to rapidly measure the magnetic moments of individual immunomagnetically tagged cells. The use of magnetic detection enables measurements to be done on native specimens, thus decreasing the loss of rare cells and removing the need for expensive sample processing equipment. Benefiting from the high speed and sensitivity of semiconductor technology, the $\mu $MCM offers high-throughput operation (upwards of 10$^{7}$~cells/sec) with a detection resolution of $\sim $2000 magnetic nanoparticles/cell. The clinical utility of the $\mu $MCM was demonstrated by detecting scant tumor cells (20 cells) in whole blood and by molecularly profiling cells from solid tumor to monitor longitudinal drug efficacy.   

Miniature magnetic resonance system for robust and portable diagnostics

We have recently developed a new diagnostic platform, microNMR($\mu$NMR), specifically designed for clinical applications This new $\mu$NMR system performs rapid, accurate, and robust measurements of cells, proteins and small molecules in point-of-care settings. The system utilizes magnetic nanoparticles (MNPs) to amplify the analytical signals in NMR detection. When molecularly-specific MNPs identify their targets, the particles induce large, amplified changes in the transverse relaxation of water protons by producing local magnetic fields. A major challenge in achieving reliable NMR detection is the fluctuation of NMR frequency ($f$0) with temperature, which originates from the the temperature-dependent drift of the magnetic field. To overcome the challenge, we have implemented a new, automated feedback controller that keeps track of $f$0 and reconfigures measurement settings. The mechanism enables robust $\mu$NMR measurements in realistic clinical environments (4-50 $^{\circ}$C). Moreover, the $\mu$NMR interfaces with mobile devices for its operation, maximizing the portability of $\mu$NMR. The clinical utility of the new $\mu$NMR system is demonstrated by detecting and molecularly profiling cancer cells from patient samples.   

Paper Inside? - New Thinking for Biochip and Other Applications

The drive to improve the performance and reduce the cost of electronic, photonic and fluidic devices is starting to focus on the use of materials that are exotic for these applications but actually readily available in other fields. In this talk the use of paper in biochip and other applications will be reviewed. Paper is a very attractive material for many device applications: very low cost, available in almost any size, versatile surface finishes, portable and flexible. From an environmental point of view, paper is a renewable resource and is readily disposable (incineration, biodegradable). Applications of paper-based electronics currently being considered or investigated include biochips, sensors, communication circuits, batteries, smart packaging, displays. The potential advantages of paper-based devices are in many cases very compelling. For example, biochips fabricated on paper can use the capillary properties of paper to operate without the need of external power sources, greatly simplifying the design and reducing the cost. For e-reader devices, in addition to flexibility, the ideal solution for providing the look-and-feel of ink on paper is to have \textit{e-paper on paper}.   

Session B43: Invited Session: Physical Mechanisms of Cell Growth

Processive motions of MreB micro-filaments coordinate cell wall growth

Rod-shaped bacteria elongate by the action of cell-wall synthesis complexes linked to underlying dynamic MreB filaments, but how these proteins function to allow continued elongation as a rod remains unknown. To understand how the movement of these filaments relates to cell wall synthesis, we characterized the dynamics of MreB and the cell wall elongation machinery using high-resolution particle tracking in \textit{Bacillus subtilis}. We found that both MreB and the elongation machinery move in linear paths across the cell, moving at similar rates ($\sim $20nm / second) and angles to the cell body, suggesting they function as single complexes. These proteins move circumferentially around the cell, principally perpendicular to its length. We find that the motions of these complexes are independent, as they can pause and reverse,and also as nearby complexes move independently in both directions across one surface of the cell. Inhibition of cell wall synthesis with antibiotics or depletions in the cell wall synthesis machinery blocked MreB movement, suggesting that the cell wall synthetic machinery is the motor in this system. We propose that bacteria elongate by the uncoordinated, circumferential movements of synthetic complexes that span the plasma membrane and insert radial hoops of new peptidoglycan during their transit.   

Spatial Patterning of Newly-Inserted Material during Bacterial Cell Growth

In the life cycle of a bacterium, rudimentary microscopy demonstrates that cell growth and elongation are essential characteristics of cellular reproduction. The peptidoglycan cell wall is the main load-bearing structure that determines both cell shape and overall size. However, simple imaging of cellular growth gives no indication of the spatial patterning nor mechanism by which material is being incorporated into the pre-existing cell wall. We employ a combination of high-resolution pulse-chase fluorescence microscopy, 3D computational microscopy, and detailed mechanistic simulations to explore how spatial patterning results in uniform growth and maintenance of cell shape. We show that growth is happening in discrete bursts randomly distributed over the cell surface, with a well-defined mean size and average rate. We further use these techniques to explore the effects of division and cell wall disrupting antibiotics, like cephalexin and A22, respectively, on the patterning of cell wall growth in \textit{E. coli.} Finally, we explore the spatial correlation between presence of the bacterial actin-like cytoskeletal protein, MreB, and local cell wall growth. Together these techniques form a powerful method for exploring the detailed dynamics and involvement of antibiotics and cell wall-associated proteins in bacterial cell growth.\\[4pt] In collaboration with Kerwyn Huang, Stanford University.   

Mechanics of cell division in fission yeast

Cytokinesis is the stage of cell division in which a cell divides into two. A paradigm of cytokinesis in animal cells is that the actomyosin contractile ring provides the primary force to squeeze the cell into two. In the fission yeast \textit{Schizosaccharomyces pombe}, cytokinesis also requires a actomyosin ring, which has been generally assumed to provide the force for cleavage. However, in contrast to animal cells, yeast cells assemble a cell wall septum concomitant with ring contraction and possess large (MPa) internal turgor pressure. Here, we show that the inward force generated by the division apparatus opposes turgor pressure; a decrease in effective turgor pressure leads to an increase in cleavage rate. We show that the ring cannot be the primary force generator. Scaling arguments indicate that the contractile ring can only provide a tiny fraction of the mechanical stress required to overcome turgor. Further, we show that cleavage can occur even in the absence of the contractile ring. Instead of the contractile ring, scaling arguments and modeling suggest that the large forces for cytokinesis are produced by the assembly of cell wall polymers in the growing septum.   

Decoding the topology of vascular organization

Distribution and structural networks permeate virtually all life, from the cellular to the organismic level. They have allowed organisms to grow in size and complexity by ensuring efficient distribution of nutrients and structural support. Given their importance, these vascular and structural webs have been under strong evolutionary selection and their form frequently reflects important aspects of their function. We discuss the design principles behind the evolution of the architecture and topology of vascular and structural networks and present some examples (leaf venation, arterial vasculature of the neocortex and others) that elucidate them.   

Mechanics and Dynamics of Plant Cell Division

The division of eukaryotic cells involves the assembly of complex cytoskeletal structures to exert the forces required for chromosome segregation and cytokinesis. In plants, tensional forces within the cytoskeleton constrain cells to divide according to a small number of area minimizing configurations. We have shown that the probability of observing a particular division configuration increases inversely with its relative area according to an exponential probability distribution known as the Gibbs measure. The distribution is universal up to experimental accuracy with a unique constant that applies for all plants studied irrespective of the shape and size of their cells. Using a maximum entropy formulation, we were able to demonstrate that the empirically observed division rule is predicted by the dynamics of the tense cytoskeletal elements controlling the positioning of the division plane. Finally, by framing this division rule as a dynamical system, we identified a broad class of attractors that are predictive of cell patterns observed in plants. Plant cell division thus offers a remarkable example of how interactions at the molecular level can lead to strikingly complex behaviors at the cellular and multicellular levels.   

Session B44: Focus Session: Translocation through Nanopores - Measurements and Theoretical Models

Sequence-dependent ion current modulations in biological and synthetic nanopores

The possibility of DNA sequence detection by measuring the blockade ionic current in nanopores has been the driving force for the spectacular development of the nanopore research field. Nevertheless, fifteen years after the first measurements, the molecular mechanism(s) of ion current modulation by the sequence of DNA nucleotides remains elusive. Here, we report the results of extensive all-atom molecular dynamics and Brownian dynamics simulations of three nanopore systems: a biological nanopore MspA, a solid-state nanopore and a graphene nanopore, aimed at elucidating the microscopic mechanism of the ion current modulation. In the case of solid-state and graphene nanopores, we determined the effect of sequence convolution on the ionic current value by simulating the ionic current blockades produced by all 64 permutations of the DNA nucleotide triplets. In the case of MspA, we determined the effect of the sequence, the global orientation, and the conformation of a DNA strand on the distribution of the ion current blockades. Based on the results of our simulations, we suggest possible routes for increasing the resolution of DNA sequence detection by measuring the nanopore ionic current and describe the inherit limitations of the method.   

Unexpected stop-and-go when DNA is pulled through a network

We perform single-molecule imaging of lambda-DNA chains when DC electric fields drive them through agarose networks in which they are heavily entangled. Velocity is decidedly unsteady. Exhaustive statistics reveal how motion switches between ``mobile'' and ``pause'' states, the latter differing from well-known ``hooking.'' As these observations appear to be inconsistent with the prevailing theories of DNA electrophoresis, we are also engaged in measurements that discriminate between motion of the chain ends and the chain centers, by direct two-color fluorescence imaging.   

Non-Equilibrium DNA Dynamics Probed by Delayed Capture and Recapture by a Solid-State Nanopore

We studied the relaxation of $\lambda $-DNA following its translocation through a voltage-biased solid-state nanopore. The translocation process drives DNA into a non-equilibrium state because the $\sim $2 ms translocation time is roughly fifty times shorter that the polymer's characteristic (Zimm) relaxation time. By reversing the applied voltage at controlled delay times after a translocation event, the nanopore probed the configurations of recaptured molecules at various stages of relaxation. We monitored the disruptions of the ionic current through the nanopore and computed the integrated charge deficits (ECDs) resulting from DNA translocations. As the delay time between voltage reversals was decreased from 50 ms to 5 ms, the distribution of ECDs shifted to lower values. Furthermore, an increasing fraction of recapture events occurred in a shorter interval from the voltage reversal than the delay time. These observations are explained by the expansion of the DNA coil as it approaches equilibrium. Finally, we show that recapturing a molecule multiple times and averaging the ECDs reduces the measurement error, which is useful for molecular diagnostic applications. The variance decreases approximately as the inverse number of passes through the pore.   

Forced Translocation of Polymer through Nanopore: Deterministic Model and Simulations

We propose a new theoretical model of forced translocation of a polymer chain through a nanopore. We assume that DNA translocation at high fields proceeds too fast for the chain to relax, and thus the chain unravels loop by loop in an almost deterministic way. So the distribution of translocation times of a given monomer is controlled by the initial conformation of the chain (the distribution of its loops). Our model predicts the translocation time of each monomer as an explicit function of initial polymer conformation. We refer to this concept as ``fingerprinting''. The width of the translocation time distribution is determined by the loop distribution in initial conformation as well as by the thermal fluctuations of the polymer chain during the translocation process. We show that the conformational broadening \textit{$\Delta $t} of translocation times of $m$-th monomer \textit{$\Delta $t}\textit{$\propto $m}$^{1.5}$ is stronger than the thermal broadening\textit{ $\delta $t}\textit{$\propto $m}$^{1.25}$ The predictions of our deterministic model were verified by extensive molecular dynamics simulations   

Force-Driven Translocation of a Polymer through a Nanopore

We study the far from equilibrium translocation of a DNA molecule through a nanopore. The pore is much narrower than the DNA, so the electrically driven DNA undergoes dramatic deformations during its passage. Using an idealized model in which the DNA is assumed to be a very long and flexible homopolymer driven by a force exerted only in the pore, we modify a previously developed method by introducing the concept of ``iso-flux trumpet''. We show that although the speed of the process is determined by the friction of the trailing part with the solvent, friction dissipates a small portion of the work performed by the electric field on the polymer, and the work is mostly dissipated by the irreversible stretching and destretching of the polymer squeezed into the small pore. Moreover, due to such stretches essentially caused by the membrane, a net heat transfer occurs during translocation from the post-translocation to the pre-translocation side of the membrane. The current theory can be improved by accounting for the nonzero field outside the pore and by considering the coupling between electric and hydrodynamic fields. The forces exerted by such fields on the DNA bulk not only alter the passage dynamics, but also introduce deformations on the initial conformation of the polymer.   

Non-equilibrium tension propagation as a unifying description of driven polymer translocation

We present results from a new Brownian dynamics model of driven polymer translocation$^1$, in which non-equilibrium memory effects due to tension propagation (TP) along the cis side subchain are included as a time-dependent friction. To solve the effective friction, we develop a finite chain length TP formalism, expanding on the work of Sakaue$^{1,2}$. The model yields results in excellent quantitative agreement with molecular dynamics simulations in a wide range of parameters. Our results show that non-equilibrium TP along the cis side subchain dominates the dynamics of driven translocation. In addition, the model explains the different scaling of translocation time with chain length observed both in experiments and simulations as a combined effect of finite chain length and pore-polymer interactions. \\ $^1$T. Ikonen, A. Bhattacharya, T. Ala-Nissila and W. Sung, submitted.\\ $^2$T. Sakaue, Phys. Rev. E {\bf 76}, 021803 (2007)\\ $^3$T. Sakaue, Phys. Rev. E {\bf 81}, 041808 (2010).\\   

Graphene Nano-Electrodes for DNA Sequencing: an Ab initio Perspective

The proposal was made that a graphene nanogap could be used to probe the transverse conductance of individual nucleotides in DNA to rapidly identify the associated base sequence. Experimentally, the characteristic drop in ionic current associated with translocation events of DNA passing through a graphene nanopore was measured. Using first-principles methods, we evaluated the performance of two graphene nano-electrodes configurations for nucleobase identification. In the first study, Nano Lett. 11, 1941 (2011), we investigated the electronic transport properties of the four nucleotides when located in a graphene nanogap by employing density functional theory and the non-equilibrium Green's function method. In particular, we determined the electrical current variation at finite bias due to changes in the nucleotides orientation. Our second study, Adv. Funct. Mater. 21, 2674 (2011), utilized molecular dynamics simulations in conjunction with electronic transport calculations to explore specifically the effect of the hydrogenated graphene edges on the translocating DNA. It is found that edge-hydrogenated graphene electrodes facilitate the temporary formation of weak H-bonds with suitable atomic sites in the nucleotides.   

Preprotein translocation across the endoplasmic reticulum membrane in milieus crowded by proteins

Translocation of preproteins chains between the cytoplasm and the endoplasmic reticulum lumen takes place in a milieu crowded primarily by proteins. We compute translocation and retrotranslocation times for chains of different length in a milieu crowded by spherical agents at volume fractions equivalent to that found in cells. These numerical times obtained from a diffusion-equation model subject to a potential given by the free energy of one chain, indicate that crowding increases the translocation time by up to five times compared to those in dilute conditions for average-size chains and by up to a thousand times for long chains. Retrotranslocation times become smaller than translocation ones, in approximately 75\%. Translocation rates obtained in this work are similar to those found in a theoretical model for Brownian-ratchet translocation and coincide with in vitro experimental results (1-8 aminoacid/s) only in the limit of very long chains; for shorter chains, translocation rates are much faster. Our prediction that for long chains translocation rates would be significantly slowed by crowding can be tested experimentally using vesicles. Discrepancy of time-scales with experiments for short chains indicates that other factors beside crowding must be included in our model.   

Nanopore Mass Spectrometry

We describe a concept for single-DNA analysis called nanopore mass spectrometry, which seeks to combine the benefits of nanopores with the speed, sensitivity, and robustness of single base detection by mass spectrometry. The basic idea is to cleave the individual nucleotides from a DNA polymer as they transit a nanopore in sequence, and to identify each one by determining its charge-to-mass ratio in a mass spectrometer. We describe how nanopore mass spectrometry can addresse the challenges faced by other nanopore-based DNA analysis approaches. We also describe the design, construction, and testing of a prototype instrument that interfaces a nanopore ion source with a quadrupole mass filter and a single ion detector. We are using this new instrument to test the key scientific questions bearing on our analysis strategy: 1) Can DNA nucleotides be reliably transferred from their native liquid phase into the vacuum environment of a mass spectrometer? 2) Can nucleotides be detected with near 100$\%$ efficiency? 3) Can DNA polymers be controllably cleaved to isolate ionized bases or nucleotides in the mass spectrometer?   

Direct observation of DNA translocation influenced by electrically gated nanopores

One of remarkable recent developments in the solid state nanopore based DNA analysis is adding the ability to control electric potential near nanopore as a gate electrode by patterning metal in or on nanopore. In this approach, better control of DNA translocations for example, slowing down the translocation speed might be expected. We have fabricated insulator-metal-insulator nanopores of rather large 100 nm pore in diameter. The 100 nm diameter pores allow us to observe the translocation of lambda-DNA molecules directly by means of fluorescence microscopy without heavy clogging of the DNA molecules into the pores. By controlling ?gate voltage? on metal relative to the cis and trans voltages, the translocation rates of DNA are able to change. Interestingly, applying pulse voltage to the gate metal near 100 ms to reverse the direction of the electric field near the cis side of nanopore reverses the direction of the DNA translocation instantaneously. This in fact provides us a new way to repeat translocation of the same DNA molecule. Furthermore, repeating the pulse tends to clear off the clogged DNA molecules in nanopore. We will present more details of these phenomena caused by the gate voltages.   

Thermophoretic stretch of a polymer confined to a nanofluidic channel

The precise manipulation of macromolecules by thermophoresis is quite promising. Indeed, Thamdrup \textit{et al.} (Nano Lett., 10, 2010) successfully moved double-stranded DNA (dsDNA) filaments in microfluidic geometries and subsequently inserted them into nano-channels using thermophoretic forces originating from highly localized thermal gradients and also showed that once in the channel, DNA can be symmetrically stretched under thermophoresis. This last procedure could be used to better expose the backbone of a nano-confined polymer to study its properties or the binding activity of some enzyme. We present a novel approach to model this symmetric stretch using blobs and the Flory free-energy of a polymer chain. Our model describes the monomer concentration profile reported in the aforementioned study. A value close to what is found in the literature is obtained for the Soret coefficient of the segments of dsDNA, characterizing thermophoresis. We further corroborate the validity of our model using molecular dynamics simulations. In these calculations, excluded volume interactions are shown to play a key role especially when temperatures are close to the solvent's $\theta$ value.   

Small Interfering RNA Transfection Across a Phospholipid Membrane

Small interfering RNA (siRNA) molecules play a pivotal role in silencing gene expression via the RNA interference mechanism. We have performed steered MD simulations to study the transfection of a bare siRNA and siRNA/Oleic Acid (OA) complex across the dipalmitoylphosphatidycholine (DPPC) bilayer at T = 323 K. Bare siRNA induces the formation of frustrated lipid gel domains, whereas in the presence of siRNA/OA complex the membrane is found to be in the liquid-ordered phase. In both cases the stress profiles across the membrane indicate that the membrane is under tension near the head groups and highly compressed at the water-hydrophobic interface. During transfection, the membrane is deformed and the lateral stress is significantly lowered for the bare siRNA and siRNA/OA complex. The bare siRNA transfects through a lipid-nanopore of hydrophilic head-groups and hydrophobic carbon chains, whereas the siRNA/OA complex transfects through a lipid-nanopore of hydrophilic head groups.   

DNA base-specific modulation of $\mu$A transverse edge currents through a metallic graphene nanoribbon with a nanopore

We study two-terminal devices for DNA sequencing which consist of a zigzag graphene nanoribbon (ZGNR) and a nanopore in its interior through which the DNA molecule is translocated. Using the nonequilibrium Green functions combined with density functional theory, we demonstrate that each of the four DNA nucleobases inserted into the nanopore, whose edge carbon atoms are passivated by either hydrogen or nitrogen, will lead to a unique change in the device conductance. Unlike other recent biosensors based on transverse electronic transport through translocated DNA, which utilize small (of the order of pA) tunneling current across a nanogap or a nanopore yielding a poor signal-to-noise ratio, our device concept relies on the fact that in ZGNRs local current density is peaked around the edges so that drilling a nanopore away from the edges will not diminish the conductance. Inserting a nucleobase into the nanopore affects the charge density in the surrounding area, thereby modulating edge conduction currents whose magnitude is of the order of $\mu$A at bias voltage $\simeq 0.1$ V. The proposed biosensors could also be realized with other nanowires supporting transverse edge currents, such as chiral GNRs or wires made of two-dimensional topological insulators.   

Session B45: Focus Session: Thin Film Block Copolymers - Swelling and Ordering

Effect of Solvent Removal Rate on the Morphology of Solvent Vapor Annealed ABA Triblock Copolymer Thin Films

Solvent vapor annealing (SVA) treatments can be used to kinetically trap unique self-assembled nanostructures in block copolymer thin films that are not achievable by traditional thermal annealing methods. In this work, we kinetically trapped the thin film morphologies of a cylinder-forming ABA triblock copolymer at key points during the solvent removal process following SVA in order to gain insight into the re-ordering mechanisms associated with solvent removal. Specifically, we identified morphology transformations as a function of solvent removal rate, showed that the mechanism for cylinder reorientation involved the propagation of changes at the free surface through the film as a front, and validated a film etching scheme to image the through-film morphology using successive ultra-violet ozone (UVO) etching steps followed by atomic force microscopy (AFM). This facile real-space analysis of the thin film internal structure is more easily implemented in comparison to cross-sectional imaging. The results and methodology of our work are significant not only for improving our understanding of block copolymer thin film self-assembly, but also for tailoring solution processing methods to fabricate nanostructured materials (e.g., for nanotemplate and membrane applications) in general.   

Obtaining Perpendicular Block Copolymer Morphologies with Solvent Annealing

Being able to control block copolymer (BCP) thin film morphology and orientation is of interest for lithographic applications where creation of feature sizes ranging from 10-100nm is desirable. Perpendicular oriented cylinders and lamellae are especially valuable due to their high aspect ratios but are difficult to achieve in BCP systems with a large Flory-Huggins interaction parameter ($\chi )$. We explore the morphological phase behavior that films (30-200nm) of poly(styrene-b-dimethylsiloxane) (PS-PDMS, 45kg/mol, $\chi $=0.26) exhibit under different solvent conditions with focus on conditions that produce perpendicular microdomains. The microdomains are revealed by selectively etching the PS with an oxygen plasma (50W CF4). Variation in the solvent vapor conditions results in selective swelling of the different blocks of the copolymer depending on the relative Hildebrand solubility parameters (e.g. PS- 18.5, toluene-18.3 MPa$^{1/2}$), affecting the microdomain morphologies, and the solvent evaporation and deswelling process influences the orientation of the microdomains. Two different strategies are presented involving solvent vapor annealing that result in perpendicular morphologies in films of PS-PDMS and the results are compared with self-consistent field theory modeling of solvent-polymer systems.   

Mesoscale Dynamics of Solvent Evaporation in Block Copolymer Thin Films

Block copolymer thin films are being investigated for a wide variety of applications ranging from separation membranes, organic photovoltaics, and lithographic masks. In order to accelerate defect annihilation in the periodic structures that develop within these films, solvent annealing techniques are often employed that exploit control over solvent atmosphere to modify the free surface thermodynamics and evaporation rate in an attempt to influence the alignment of ordered domains. The inherently non-equilibrium nature of this problem complicates standard theoretical treatments based on relative free energy calculations, so we have employed a dynamical extension of Self-Consistent Field Theory coupled with a solvent evaporation mechanism to gain insights into the interplay between component-surface interactions, evaporation rate, and observed film morphology. The effects of these factors on the micro-phase separation trajectory of the film will be discussed.   

Optimizing the Combination of Solvent Annealing and Directed Self-Assembly in Thin Film Block Copolymer Systems Using Self-Consistent Field Theory

We show how self-consistent field theory simulations can be used to optimize the combination of solvent annealing and directed self-assembly using topographical and chemical templating in order to achieve an ultimate goal of arbitrary pattern generation in thin film block copolymer systems. The simulations use a combination of field boundary conditions to model topographical features such as posts and chemically distinct surfaces along with a variety of compositions to model a range of solvent, homopolymer, and block copolymer multi-component blends [Macromolecules 2010, 43, 8290--8295]. Computational results are compared with experimental systems that use polydimethylsiloxane, polystyrene, and polyferrocenylsilane polymers, heptane, toluene, and chloroform solvents, and electron beam lithography created hydrogen silsesquioxane posts. By varying the different polymers used and thus their $\chi $ interaction parameters, the relative fractions of solvent and different polymer blocks, and surface feature affinity and shape, the optimal requirements to create arbitrary complex features for nanolithography applications will be demonstrated.   

Sub-10 nm block copolymer patterns with mixed morphology and period using electron irradiation and solvent annealing

High resolution patterns with controllable period and feature geometry are of intense interest for nanolithography applications, but to date this has been challenging to accomplish from a single block copolymer, which produces patterns of a fixed period and morphology. Here we show how patterns consisting of coexisting sub-10 nm spheres and cylinders and sphere patterns with a range of periods can be created using a combination of serial solvent anneal processes and electron-beam irradiation of selected areas of a film of poly(styrene-block-dimethylsiloxane). We also addtionally combined with topographical templates consisting of either removable polymeric regions to make patterns with highly aligned lines, dots and featureless regions, or post arrays which can simultaneously align and register the line and dot arrays. These techniques offer the possibility of forming a wide range of aperiodic pattern geometries including single lines or ordered line segments, and significantly extend the ability of block copolymer lithography to produce patterns essential for nanoscale device fabrication.   

Raster solvent vapor annealing of block copolymer thin films

Nanoscale phase separation in block copolymer (BCP) thin films makes them attractive for a variety of applications such as membranes, organic electronics, and nanoscale templating. Annealing BCP thin films (thermal or solvent) promotes ordering of their microphase-separated structures into useful patterns. Solvent vapor annealing (SVA) is an attractive approach as it avoids thermal degradation and provides greater flexibly in morphology control compared to thermal annealing through overall film swelling and preferential swelling of one or more blocks. SVA is typically conducted in an annealing chamber that permits exposure of the film to a controlled vapor environment. However, control over the vapor environment locally may be more desirable for nano-patterning applications. In this work we introduce raster solvent vapor annealing, a method that gives us precise control over the annealed region and excellent point control of the resulting morphology allowing for specific mixed morphologies on a single sample. The simplest setup, needle directed solvent vapor with controlled solvent flow rate and rastering speed, allows ``drawn'' ordered parallel and perpendicular cylinders in a cylinder forming BCP thin film. The efficacy of this method will be discussed in the oral presentation.   

Confined drying of copolymer solutions

We developed a simple tool for the rapid screening of phase diagrams of polymer and surfactant solutions. Our technique is based on the controlled drying of a droplet solution in a confined geometry. A $\mu$L-sized droplet of an aqueous solution is confined between two wafers (diameter 3 cm), separated by a controlled thickness ($\approx$ 150 $\mu$m). The confinement casts a well-defined timescale to the drying kinetics, mainly governed by the wafer area. Indeed, water removal only occurs through a diffusive process from the edge of droplet to the edge of the wafer. Confinement also permits a simple 2D description, and allows simple observations of the drying. We studied the drying of an aqueous solution of a tribloc copolymer (Pluronics, P104) thanks to three different techniques: polarized microscopy, fluorescent microscopy, and Raman imaging. With our tool and techniques, we not only build an accurate phase diagram of the solution (with one microliter only) but also measure both the mutual diffusion coefficient and the activity of the solution as a function of its concentration, including the Flory-Huggins parameter.   

Rapid Self- Assembly and Perpendicular Alignment in lamellar PS-b-PEO System for Fabrication of Sub 20 nm Nanolithography Templates

Creating perpendicular alignment in lamellar block copolymer systems has considerable industrial and commercial significance. In general, these lamellar systems require careful interface engineering to obtain vertical orientation of the blocks. To avoid the strong preferential adsorption of one block to either the substrate or the polymer/air interface, the surface must be `neutralised' by chemical brushes or external forces e.g. solvent fields. Reported here is a stepwise thermo/solvent annealing process called ``combinatorial annealing'' allowing the formation of perpendicular domains of polystyrene-b-polyethylene oxide (PS-b-PEO) lamellar structures while avoiding brush or other surface modification. The Thermo/solvent annealing is done in a commercial microwave reactor and perpendicular alignment is observed within a few minutes. This BCP has a very This BCP has a small minimum feature size, relevant to the fabrication of nano-features in electronic devices and results are presented here.   

Controlled Deposition of Ordered Block Copolymer Thin Films by Electrospray

Electrospray offers a potentially useful platform for the controlled delivery of a variety of materials, but surprisingly, its application to block copolymer thin film deposition remains unexplored. Here we show that under appropriate conditions, well ordered films of PEO cylinder-forming poly(styrene-b-ethylene oxide) may be continuously deposited by electrospray. Ordered film formation is predicated on fast thermal equilibration relative to rate of deposition. We conduct time-resolved observations and investigate the effects of process parameters that underpin film morphology including solvent selectivity, substrate temperature and flow rate of the electrospray feed solution. For the particular system studied, we uncover a wide temperature window from 90$^{o}$C to 160$^{o}$C and an ideal flow rate (2$\mu$L/min) for ordered film growth, but no strong influence of solvent selectivity was observed. PEO cylinders were observed to align with their long axes perpendicular to the substrate at optimal spray conditions.   

Two distinct timescales in the ordering of symmetric diblock copolymer films

At equilibrium, an ordered symmetric diblock copolymer film forms lamellae parallel to the substrate interface. Furthermore, unless a film's thickness is exactly commensurate with the intrinsic height of the layered structure, the free surface will break up into holes or islands with a thickness corresponding to one lamellae. This ensures that the preferred lamellar spacing of the film is achieved while volume is conserved. We study the internal dynamics of ordering as a lamellar forming thin film transitions from the disordered state to its equilibrium morphology. In particular, before the free surface nucleates to form islands or holes, a film must begin by ordering internally. Using ellipsometry, we measure small changes in the internal film structure as the diblock orders prior to any change of the free surface topology. We probe two distinct timescales along the pathway to an equilibrium state: 1) An initial ordering time where molecules begin to align and form lamellae; and 2) an incubation time where the structure remains constant before nucleation of holes or islands. We will show the effect that film height and commensurability has on these two timescales, allowing us to better understand the effect of confinement on the ordering dynamics of lamellar forming thin films.   

The hydration and ordering of lamellar block copolymer films prior to the formation of polymer vesicles

Polymersomes -- vesicles based on self-assembled bilayers in turn composed of amphiphilic copolymers -- are good candidates for molecular delivery systems; hydrophilic molecules can be enclosed within the aqueous core, to be released by a trigger, which disrupts the vesicle's wall. The key to the use of these polymer vesicles as effective molecular delivery system is in the ability to efficiently encapsulate a molecular payload within the vesicle. To understand the formation mechanism of polymer vesicles via the thin film rehydration method, we have evaluated the hydration and ordering of PEO-PBO diblock copolymer thin films in a controlled water vapor atmosphere. We have performed Neutron Reflectivity, Ellipsometry and Atomic Force Microscopy measurements during the hydration process. These results show that the film swells slowly in the initial stage. It then swells rapidly at a certain critical point and makes ordered structure at the same time. The lamellae are gradually oriented parallel to the substrate with increasing water absorption.   

Effect of thickness on microdomain orientation in shear-aligned, cylinder-forming PS-PHMA thin films

Previous work has shown that application of a shear stress can impart well-defined orientational order to the microdomains of cylinder-forming poly(styrene)-b-poly(n-hexylmethacrylate) (PS-PHMA) thin films. Unlike many block copolymer thin films, PS-PHMAs tend not to form terraces (islands/holes). As a result, film thickness plays an important role in determining the orientation of the cylindrical microdomains. Here we study the effect of film thickness on the morphology of aligned and unaligned PS-PHMA films. Films with a gradient in thickness were generated, via flow coating, and then imaged using atomic force microscopy to examine the microdomain morphology before and after shear alignment. As a function of thickness the unsheared film morphology oscillated between dots (either spheres or cylinders oriented perpendicularly to the substrate) and lines (cylinders oriented parallel to the substrate), with the highest fraction of lines occurring at film thicknesses corresponding to an integral number of cylinder domain spacings. For thicknesses larger than three domain spacings, the oscillation ceased and complete coverage by line patterns was observed. Once shear-aligned, the thicknesses that exhibited the largest fraction of lines pre-shear, showed the highest quality of alignment post-shear.   

GISAXS simulation and analysis on GPU clusters

We have implemented a flexible Grazing Incidence Small-Angle Scattering (GISAXS) simulation code based on the Distorted Wave Born Approximation (DWBA) theory that effectively utilizes the parallel processing power provided by the GPUs. This constitutes a handy tool for experimentalists facing a massive flux of data, allowing them to accurately simulate the GISAXS process and analyze the produced data. The software computes the diffraction image for any given superposition of custom shapes or morphologies (e.g. obtained graphically via a discretization scheme) in a user-defined region of k-space (or region of the area detector) for all possible grazing incidence angles and in-plane sample rotations. This flexibility then allows to easily tackle a wide range of possible sample geometries such as nanostructures on top of or embedded in a substrate or a multilayered structure. In cases where the sample displays regions of significant refractive index contrast, an algorithm has been implemented to perform an optimal slicing of the sample along the vertical direction and compute the averaged refractive index profile to be used as the reference geometry of the unperturbed system. Preliminary tests on a single GPU show a speedup of over 200 times compared to the sequential code.   

Modeling Line Edge Roughness in Lamellar Block Copolymer Systems

Block copolymers offer an appealing alternative to current lithographic techniques with regard to fabrication of the next generation microprocessors. However, if copolymers are to be useful on an industrial manufacturing scale, they must meet or exceed lithography specifications for placement and line edge roughness (LER) of resist features. Here we discuss a field theoretic approach to modeling the LER in the lamellar phase of a strongly segregated block copolymer system. Our model is based on the Leibler-Ohta-Kawasaki free energy functional, which takes the Flory-Huggins parameter and index of polymerization as inputs. We consider a domain with a finite number of phase separated microdomains; at the system boundary, we apply conditions akin to a chemical pinning field. Using a path integral formalism, we determine how fluctuations of the microdomain boundaries depend on distance from the system boundary, number of microdomains, the Flory-Huggins parameter, and index of polymerization.   

Session B46: Invited Session: Industrial Applications of Advanced Polymer-Based Nanomaterials

Polymer and Material Design for Lithography From 50 nm Node to the sub-16 nm Node

Microlithography is one of the technologies which enabled the Information Age. Developing at the intersection of optical physics, polymer science and photochemistry, the need for ever smaller high fidelity patterns to build integrated circuits is currently pushing the technology evolution from 193 nm immersion lithography to extreme ultraviolet lithography (13.5 nm) to alternate patterning technologies such as directed self assembly (DSA) of block copolymers. Essential to the success of this progression is a rapid application of new concepts and materials in polymer science. We will discuss the requirements for 193 immersion lithography and how advanced acrylic random polymers are being designed with chemical amplification functionality to meet these needs. The special requirements of a water immersion lithography led to the invention and rapid commercial application of surface assembled embedded barrier layer polymers. Design of polymers for EUV lithography is having to respond to much different challenges, prominent being the dearth of photons in the exposure step, and the other being how to maximize the efficiency of photoacid production. In parallel, alternative lithographic approaches are being developed using directed self assembly of block copolymers which realize pattern frequency multiplication. We will update with our progress in the applications of polymers designed for DSA.   

Development of nanostructured surfaces for ice protection applications

Ice accretion on surfaces of aircrafts, wind turbine blades, oil and gas rigs and heat exchangers, to name a few examples, presents long recognized problems with respect to efficiency and cost of operation. For instance, significant ice accretion on critical surfaces of an aircraft will cause problems during lift off (and will change the aerodynamics of the wings during flight. On the other hand, ice built up on wind turbine blades in cold climates (T $<$ -20\r{ }C) drastically reduces the efficiency of power generation. Despite considerable number of studies and significant progress toward development of icephobic coatings, development of \textit{robust ice-resistance or anti-icing coatings} is still elusive. Several approaches towards development of anti-icing surfaces have recently postulated that the superhydrophobic properties of hierarchically textured coatings, with contact angles $>$ 150\r{ }, may lead to a significant reduction and perhaps elimination of snow and ice accretion. However, the exact mechanism of delayed icing on these surfaces is still under debate. Here we present a systematic study of early stages of ice formation upon water droplet impact on a range of hydrophobic, hydrophilic, textured and chemically patterned surfaces. We show that, in addition to a significant reduction in ice-adhesion strength on superhydrophobic surfaces, decreasing the water-substrate contact area plays a \textit{dual} role in delaying ice nucleation: first by reducing heat-transfer and second by reducing the probability of heterogeneous nucleation at the water-substrate interface. The study presented here also offers a comprehensive perspective on the efficacy of textured surfaces for practical non-icing applications.   

Tough Block Copolymer Organogels and Elastomers as Short Fiber Composites

The origins of the exceptional toughness and elastomeric properties of gels and elastomers from block copolymers with semicrystalline syndiotactic polypropylene blocks will be discussed. Using synchrotron X-radiation small angle (SAXS) and wide angle X-ray scattering (WAXS) experiments were simultaneously performed during step cycle tensile deformation of these elastomers and gels. From these results the toughness can be attributed to the formation, orientation and elongation of the crystalline fibrils along the tensile direction. The true stress and true strain $\varepsilon _{H}$ during each cycle were recorded, including the true strain at zero load $\varepsilon _{H,p}$ after each cycle that resulted from the plastic deformation of the sPP crystals in the gel or elastomer. The initial Young's modulus E$_{init}$ and maximum tangent modulus E$_{max}$ in each cycle undergo dramatic changes as a function of $\varepsilon _{H,p}$, with E$_{init}$ decreasing for $\varepsilon _{H,p} \quad \le $ 0.1 and then increasing slowly as $\varepsilon _{H,p}$ increases to 1 while E$_{max }$increases rapidly over the entire range of $\varepsilon _{H,p}$ resulting in a ratio of E$_{max}$/E$_{init} \quad >$ 100 to 1000 at the highest maximum (nominal) strain. Based on SAXS patterns from the deformed and relaxed gels, as well as on previous results on deformation of semicrystalline random copolymers by Strobl and coworkers, we propose that the initial decrease in E$_{init}$ and increase in E$_{max}$ with $\varepsilon _{H,p}$ are due to a breakup of the network of the original sPP crystal lamellae and the conversion of the sPP lamellae into fibrils whose aspect ratio increases with further plastic deformation, respectively. The gel elastic properties can be understood quantitatively as those of a short fiber composite with a highly deformable matrix. At zero stress the random copolymer midblock chains that connect the fibrils cause these to make all angles to the tensile axis (low E$_{init})$, while at the maximum strain the stiff, crystalline sPP fibrils align with the tensile axis producing a strong, relatively stiff gel$. $The evolution of the crystalline structure during deformation is confirmed by WAXS and FTIR measurements.   

Interfacial, Thin Film, and Structural Measurements to Facilitate Polymer Nanomanufacturing

There is a growing interest in coupling the relatively mature roll-to-roll manufacturing processes with advanced lithographic pattering methods to enable high volume manufacturing of technologies with the added functionality that can only be realized by nanoscale pattering. Nanoimprint lithography is particularly attractive for roll-to-roll processes because the patterning is achieved though a simple embossing technique that relies upon the mechanical deformation of a liquid, melt, or solid material via a squeeze-flow process. Embossing techniques on a roll-to-roll substrate are routinely encountered in the graphic arts community. The difference with nanoimprint lithography is that the printed features can be on the order of 10 nm or smaller. In this presentation we will look at some of the difficulties encountered when these roll-to-roll embossing processes are scaled to the nanoscale. In particular we will look at issues related to the viscous flow of high molecular mass polymer melts into the nanoscale cavities with high throughput. We will show how fast nanoscale squeeze-flow pattering can lead to significant plastic deformation of the polymer melt. This can have implications on the ability of the material to fill the mold and the residual stresses that are generated in the pattern through the imprint process. Techniques to quantitatively evaluate these processes will be discussed and related to fundamental concepts of polymer physics.   

Self-Healing Polymer Networks

Supramolecular chemistry teaches us to control non-covalent interactions between organic molecules, particularly through the use of optimized building blocks able to establish several hydrogen bonds in parallel. This discipline has emerged as a powerful tool in the design of new materials through the concept of supramolecular polymers. One of the fascinating aspects of such materials is the possibility of controlling the structure, adding functionalities, adjusting the macroscopic properties of and taking profit of the non-trivial dynamics associated to the reversibility of H-bond links. Applications of these compounds may include adhesives, coatings, rheology additives, high performance materials, etc. However, the synthesis of such polymers at the industrial scale still remains a challenge. Our first ambition is to design supramolecular polymers with original properties, the second ambition is to devise simple and environmentally friendly methods for their industrial production. In our endeavours to create novel supramolecular networks with rubbery elasticity, self-healing ability and as little as possible creep, the strategy to prolongate the relaxation time and in the same time, keep the system flexible was to synthesize rather than a single molecule, an assembly of randomly branched H-bonding oligomers. We propose a strategy to obtain through a facile one-pot synthesis a large variety of supramolecular materials that can behave as differently as associating low-viscosity liquids, semi-crystalline or amorphous thermoplastics, viscoelastic melts or self-healing rubbers.   

Session B47: Focus Session: Jamming, Glass Transition, and Gelation in Colloids and Soft Matter Systems

Geometrical analysis of suspension flows near jamming

The viscosity of suspensions was computed early on by Einstein and Batchelor in the dilute regime. At high density however, their rheology remains mystifying. As the packing fraction increases, steric hindrance becomes dominant and particles move under stress in a more and more coordinated way. Eventually, the viscosity diverges as the suspension jams into an amorphous solid. Such a jamming transition is reminiscent of critical points: the rheology displays scaling and a diverging length scale. Jamming bear similarities with the glass transition where steric hindrance is enhanced under cooling, and where the dynamics is also observed to become more and more collective as it slows down. In all these examples, understanding the nature of the collective dynamics and the associated rheology remains a challenge. Recent progress has been made however on a related problem, the unjamming transition where a solid made of repulsive soft particles is isotropically decompressed toward vanishing pressure. In this situation various properties of the amorphous solid, such as elasticity, transport or force propagation, display scaling with the distance to threshold. Theoretically these observations can be shown to stem from the presence of soft modes in the vibrational spectrum, a result that can be extended to thermal colloidal glasses as well. Here we focus on particles driven by shear at zero temperature. We show that if hydrodynamical interactions are neglected an analogy can be made between the rheology of such a suspension and the elasticity of simple networks, building a link between the jamming and the unjamming transition. This analogy enables us to unify in a common framework key aspects of the elasticity of amorphous solids with the rheology of dense suspensions, and to relate features of the latter to the geometry of configurations visited under flow.   

Finite Size Effects at the Jamming Transition

Packings of spheres at zero temperature and shear stress exhibit a jamming/unjamming transition as a function of density. For spheres that repel when they overlap and do not otherwise interact, packings are jammed with a nonzero static shear modulus when the density, $\phi$, exceeds a critical density, $\phi_c$. This jamming transition displays characteristics of both first and second order phase transitions with, for example, a discontinuous jump in the coordination number (average number of interacting neighbors per particle) and a power-law increase in the shear modulus. In addition, multiple length scales have been identified that diverge as $\phi$ decreases towards $\phi_c$, emphasizing the second order nature of the transition. The existence of diverging length scales suggests that quantities such as the coordination number and shear modulus should exhibit finite size scaling as $\phi_c$ is approached, but until now this has not been observed. We report the first measurements of finite size scaling at the jamming transition of soft frictionless repulsive spheres and explore the implications of these results on our current understanding of the jamming transition.   

Stability of jammed systems to generalized boundary deformations

At zero temperature and applied stress, amorphous packings of repulsive spheres exhibit a jamming transition to rigidity. As pointed out by Torquato and Stillinger, some of these ``collectively jammed'' configurations may not be stable with respect to boundary deformations, while others, ``strictly jammed,'' may be stable with respect to any change of the boundaries. We explore this by considering systems with periodic boundary conditions at packing fractions above the jamming transition as comprising the basis of an infinite square (or cubic) lattice. The displacement fields of particles corresponding to normal modes of vibration are generally not consistent with the periodic boundary conditions used to construct the basis packing. Therefore, by studying the vibrational modes we determine the linear response to boundary deformations that do not respect the periodicity of the initial system. Hence, we are able to explore the stability of packings with respect to a large class of boundary deformations. We discover that some configurations have modes with negative energies, indicating instability. We report the effects of system size and packing fraction upon the probability that a collectively jammed packing is also stable with respect to boundary deformations.   

Vestige of $T = 0$ jamming transition at finite temperature in 3D

When a random packing of spheres at T = 0 is compressed to the jamming transition, the system becomes rigid and the first peak of the pair-correlation function, $g(r)$, diverges [1]. We study the manifestation of this signature and the associated particle dynamics when the temperature, $T$, is no longer negligible. To this end, we employ a three-dimensional packing of monodisperse, micron-size, colloids made from n-isopropyl acrylimide (NIPAM). NIPAM particles change size and hence the packing fraction of the system in response to environmental temperature. Thus by changing sample temperature we can probe all packing fractions of interest using a single sample. These particles are compressible so the system can reach packing fractions and configurations inaccessible to hard colloids. We observe a vestige of the T = 0 divergence as a maximum in the first peak of $g(r)$ versus packing fraction coincident with dynamical arrest of the particles. The general features in 3D are in agreement with a previous study in a two-dimensional bi-disperse NIPAM system [2]. We report the dependence of $g(r)$ and particle motion on packing fraction.\\\ [1] C. S. O'Hern, et al., Phys. Rev. E 68, 011306 (2003). [2] Z. Zhang, N. Xu, et al., Nature 459, 230 (2009).   

Localization model for relaxation in glass-forming materials

As a material approaches its glass transition, its structural relaxation time $\tau $ rapidly increases as its particles become localized. A model relating this relaxation time increase to an experimentally accessible measure of localization (e.g. the Debye-Waller factor $\langle u^2\rangle )$ would have a fundamental impact on our understanding of glass formation and would be practically useful in the design of new materials. After examining such a relationship proposed by Leporini and coworkers and finding it to be inadequate, we develop an activated transport model that accurately describes relaxation data from a variety of simulated and experimental glass-forming materials. This model naturally extends the Hall-Wolynes and free volume models relating $\tau $ to $\langle u^2\rangle $ (or free volume). The model parameters are physically meaningful and reflect the anharmonicity and anisotropy of particle localization. The Vogel-Fulcher-Tamman relation also describes relaxation in the simulated and experimental glass-forming materials we considered, and by exploring consistency between these two relations we gain new insight into the characteristic temperatures of glass formation in terms of the extent of particle localization.   

Lattice Model of Dynamic Heterogeneity in Glassy Systems

Free volume in a fluid provides space for molecular motion; this creates local mobility and avoids ``kinetic jamming'' as the sample is rapidly cooled towards its glass transition. This physical picture has recently garnered significant interest in the polymer materials realm, where free volume is suspected to be the key factor in suppressing the glass transition of a polymer thin film near its exposed surfaces. We have developed a simple kinetic model that describes how free volume is transported in a near-glassy liquid. Model simulation results reveal that spatial fluctuations of free volume grow large near the glass transition, and this gives rise to hallmark glassy characteristics such as dynamic heterogeneity, intermittency and, ultimately, kinetic arrest. We will also discuss the response of glassy and molten states to perturbations, as a probe for characterising fluctuations near the glass transition.   

Building a Colloidal Proxy for Binary Metallic Glasses

Current experimental techniques for determining the atomic structure of metallic glasses and testing structural theories such as the efficient cluster packing model are limited to diffraction and scattering. These techniques give only average structural information that could result from many different unique structures. Simulations of metallic glasses, on the other hand, give the exact structure of every atom in the system, but are limited by computing power to a few thousand atoms which are equilibrated over a few nanoseconds. This leads to uncertainties in the reliability of reproducing real metallic glass structures. To overcome these deficiencies, we have created a proxy experimental system that can be treated much like a simulation. We have synthesized suspensions of larger, colloidal scale particles (2-3um in diameter) to build pseudo binary metallic glasses. Using confocal microscopy imaging techniques, we determine the three dimensional position of hundreds of thousands of individual particles (atoms), and calculate structural information such as the radial distribution function, Voronoi volume, and partial coordination numbers. We compare these results to both theoretical calculations and experimental results of real metallic glasses. The focus here will be on a building a proxy for the CuZr binary system.   

Contact processes in crowded environments

Periodically sheared colloids at low densities demonstrate a dynamical phase transition from an inactive to active phase as the strain amplitude is increased. To begin to answer the question of what happens to this system at higher densities, we investigate a conserved-particle-number contact process with a three-body interaction as opposed to the usual two-body interaction. In particular, one active (diffusing) particle collides with two inactive (non-diffusing) particles such that they can become active. In mean-field, this system exhibits a continuous absorbing phase transition belonging to conserved directed percolation universality class. Simulations on 2D lattices support our result. In contrast, the three-body interaction with two active particles colliding and activating one inactive particle exhibits a first-order transition. Inspired by kinetically-constrained models of the glass transition, we investigate the ``caging effect'' at even higher particle densities to look for a second dynamical phase transition back to an inactive phase. While mean-field calculations demonstrate a continuous transition, 2D lattice simulations show a hint of a first-order transition, suggesting a possible fluctuation-driven transition due to the highly localized geometric constraint.   

Experiments on Rearrangements and Forces in 2D Emulsion Hopper Flow

We did experiments with a quasi-two-dimensional binary emulsion flowing through a hopper. Our samples are oil-in-water emulsion confined between two close-spaced parallel plates, so that the droplets are deformed into pancake shapes. In this system, there is only viscous friction and no static friction between droplets. The hopper flow induces a high rate of rearrangement events allowing us to understand how stresses and forces change during the process. By imaging the droplets during flow, we observed T1 events, which are topological changes when droplets exchange neighbors. Simultaneously, we measured forces between the droplets using a technique we have developed and studied the evolution of forces between droplets during rearrangements.   

The Role of Solvent-Solute Interactions on The Behavior of Low Molecular Mass Organo-Gelators

Low molecular mass organo-gelators (LMOGs) are a class of small molecules that can self-assemble in organic solvents to form three-dimensional fibrillar networks. This has a profound effect on the viscoelastic properties of the solution causing physical gelation. These gels have uses in a range of industries including cosmetics, foodstuffs, plastics, petroleum and pharmaceuticals. A fundamental question in this field is: What makes a good LMOG? This talk will discuss the relationships between the viscoelastic properties and thermodynamic phase behavior of LMOG/solvent solutions. The regular solution model was used to fit the liquidus line and sol/gel transition temperature vs. concentration in different solvents to determine LMOG-solvent interaction parameters ($\chi $ = A/T). This parameter A was found to scale with the solubility parameter of the solvent, especially for non-polar solvents. This demonstrates that gelation is strongly linked to LMOG solubility and indicates that the bulk thermodynamic parameters of the LMOG (solubility parameter and melting temperature) are useful to predict the solution behavior of LMOGs.   

Effects of penalty function type on the history-penalized metabasin escape algorithm for supercooled liquids

The history-penalized metabasin escape algorithm provides an autonomous transition state pathway for a trapped system to escape from deep metastable energy minima and larger basins of attraction. The effects of penalty function type on the efficiency of the algorithm are demonstrated by sampling portions of the potential energy surface of a binary Lennard-Jones liquid close to the glass transition temperature. Our results indicate that optimal penalty functions prefer both large 3N+1 dimensional volumes and the ability to force the system over a large range. The Gaussian, which until now has served as the standard penalty function used for activation, serves as the benchmark against other representative penalty function types. Analysis shows that the triangle function results in four fold improvements in efficiency over the Gaussian, thereby furthering the reach of simulation into timescale regimes largely inaccessible to molecular dynamics.   

Nonlinear effective medium theory of disordered spring networks

Disordered soft materials, such as fibrous networks in biological contexts exhibit a nonlinear elastic response. We study such nonlinear behavior with a minimal model for networks of simple Hookian elements with disordered spring constant. We develop a mean-field approach to calculate the differential elastic bulk modulus for the macroscopic network response of such networks under large isotropic deformations. We find that the nonlinear mechanics depends only weakly on the lattice geometry and is governed by the average network connectivity. In particular, the nonlinear response is controlled by the isostatic connectivity, which depends strongly on the applied strain. Our predictions for the strain dependence of the isostatic point as well as the strain-dependent differential bulk modulus agree well with numerical results in both two and three dimensions. In addition, by using a mapping between the disordered network and a regular network with random forces, we calculate the non-affine fluctuations of the deformation field and compare it to the numerical results. Finally, we discuss the limitations and implications of the developed theory.   

Session B48: Electrically and Optically Active Polymers

Micellar Electrolytes in Organic Electrochemical Transistors

Organic electrochemical transistors (OECTs) are promising for applications in sensing and bioelectronics. OECTs consist of a conducting polymer film (transistor channel) in contact with an electrolyte. A gate electrode immersed in the electrolyte controls the doping/dedoping level of the conducting polymer. OECTs can be operated in aqueous electrolytes, making possible the implementation of organic electronic materials at the interface with biology. The inherent signal amplification of OECTs has the potential to yield sensors with low detection limits and high sensitivity. In this talk we will present recent studies on OECTs using ionic surfactants (such as hexadecyl-trimethyl-ammonium bromide) as electrolytes. As the conducting polymer we used PEDOT:PSS, i.e. (Poly,3-4 ethylenedioxythiopene) doped with Poly(styrene sulphonate). Interestingly, ionic surfactant electrolytes result in large transistor current modulation, especially beyond the critical micellar concentration (CMC). Since micelles play a primary role in biological processes and drug-delivery systems, the use for micellar electrolytes opens new exciting opportunities for the use of OECTs in bioelectronics.   

Quadratic Electro-Optic Effect in the Nonconjugated Conductive Co-polymer Iodine-doped Styrene-Butadiene-Rubber Measured at 633 nm and 1550 nm

The quadratic electro-optic effect in the nonconjugated conductive \textit{co-polymer} film of styrene-butadiene-rubber (SBR) has been measured using field-induced birefringence method. Thin films of styrene-butadiene-rubber have been prepared on various substrates from a chloroform solution and characterized using optical absorption spectroscopy, FTIR and DSC before and after doping with iodine. The optical absorption spectrum at low doping shows two peaks: one at 4.27 eV and the other at 3.2 eV corresponding to the radical cation and charge-transfer transition. FTIR data indicate =C-H vibration bands (964 cm$^{-1}$ and 910 cm$^{-1})$ of polybutadiene decrease upon doping due to transformation of the double bonds into radical cations. The Kerr coefficients as measured at 633 nm and at 1550 nm are 3.1x10$^{-10}$ m/V$^{2}$ and 1.3x10$^{-10}$ m/V$^{2}$ respectively. These exceptionally large values have been attributed to the subnanometer metallic domains formed upon doping and charge-transfer involving isolated double-bonds.   

Photoluminescence of P3HT nanoparticles

Polythiophenes are semiconducting polymers that have been designed to crystallize. The photophysics of semicrystalline polythiophene and polythiophene-blends are the focus of intense research efforts across different disciplines. In these systems there is a competition between charge separation and recombination. Exciton diffusion length in organic-semiconductors is a major road-block for efficient solar energy harvesting devices since, for direct bandgap organic materials, this distance is about 10 nanometers. Thus, efficient extraction of photogenerated electrons and holes requires engineering polymer domain dimensions in this size range. In our initial investigations of the photophysics of isolated P3HT nanoparticles (15 - 130 nm), we have observed several intriguing size-dependent features in the single-particle photoluminescence (PL) connected with exciton diffusion and dissociation dynamics. In addition to the short-time behavior, we also observe size-dependent differences in PL decay at long times. In the 10 - 100 ns time regime, the PL originates not from radiative transitions of bound excitons, but rather from charge-separation followed by bi-polaron recombination--and thus provides an interesting measure of exciton fission probability within the nanoparticle.   

Crystallizing Conjugated Polymer Chains of P3HT Stretched for Dramatic Enhancements in Optoelectronic Efficiencies

Previously the amorphous conjugated polymer MEH-PPV was shown to illustrate huge enhancement in photoluminescence (PL) efficiencies upon stretching to large molecular strains. In this work the crystallizing polymer of poly(3-hexylthiophene) (P3HT), dispersed in the polystyrene (PS) matrix, was stretched in the PS local deformation zones to study the effect of molecular deformation. A huge enhancement of the PL intensity was observed that when normalized to the fraction of the strained polymer corresponded to an increase of 15 folds, significantly larger than that of the stretched MEH-PPV. Moreover, when examined under a con-focal micro-PL (spot size $\sim$5$\mu$m), the emission from the local deformation zones subject to a high stress ($\sim$40MPa) manifested marked increase of the intrachain emission relative to the effect on the interchain. These emission peak positions, however, were unaffected by the stretching. The PL enhancements were attributed to the depression of electron-phonon interactions of the stretched P3HT chains. Constraining conjugated polymers to yield high efficiencies thus may provide a feasible way for improving the performance of polymer-based devices.   

Manipulating meso-structure and electrical conductivity in polymer-acid doped polyaniline by exploiting redox chemistry

Template synthesis of polyaniline on poly(2-acrylamido-2-methyl-1-propane sulfonic acid) yields electrostatically stabilized particles that can be aqueously dispersed and cast into thin films; electrical conductivity in these films scales with inter-particle connectivity. Solvent annealing with dichloroacetic acid induces structural relaxation of the polymer acid, thereby eliminating the particulate nature of thin films and consequently increasing their conductivity by up to two orders of magnitude (from 0.4 to 40 S/cm). Alternatively, the electrostatic interactions between polyaniline and its template can be neutralized through chemical reduction with hydrazine monohydrate, after which the polymer acid can be plasticized by water vapor to encourage structural relaxation. Exposure to nitric oxide leads to oxidation of polyaniline and concurrent reassociation with its polymer acid dopant. Enhanced conductivity is observed following this redox process, and is attributed to extensive polymer chain relaxation and the simultaneous elimination of the particulate nature of template-synthesized polyaniline.   

An Integrated Electrochromic Nanoplasmonic Optical Switch

We describe an electrochemically-driven optical switch based on absorption modulation of surface plasmon polaritons (SPPs) propagating in a metallic nanoslit array waveguides containing the electrochemical polymer polyaniline (PANi). Optical transmission modulation of near 100{\%} is achieved by electrochemically switching PANi between oxidized and reduced states using voltages below 1 V. High spatial overlap and long interaction length between the SPP and the active material are achieved by preferential growth of PANi on the nanoslit sidewalls. The resulting orthogonalization between the directions of light propagation, and that of charge transport from the electrolyte to ultra-thin active material inside the nanoslit waveguide offers significant promise for the realization of electrochromic devices with record switching speeds.   

Resistive Switching in Ag Nanowire/Polymer Composite Materials

Bulk composites of electrically conductive nanoparticles within an insulating polymer matrix are insulating when the conductive particle concentration is below the electrical percolation threshold and conductive above it. However, we have observed reversible resistive switching with increasing voltage at room temperature in Ag nanowire/polystyrene composites with nanowire concentrations close to the percolation threshold. We have found the reversibility of the observed switching behavior to be temperature dependent which implies a diffusive process is involved. We propose the basis for resistive switching in these materials is the formation of field-induced filaments between adjacent nanowires that extend the percolated electrical network and increase the overall conductivity of the system. Here, we will compare our observations of resistive switching in Ag nanowire/polystyrene and Ag nanowire/poly(methylmethacrylate) bulk nanocomposites, explore the breadth of metal nanowire and polymer systems that exhibit resistive switching, and explore the underlying mechanism for filament formation.   

Far-infrared through visible optical characterization of polymer-based electrochromic devices on single-walled carbon nanotube electrodes

Electrochromic polymers (ECPs) exhibit reversible optical modulation in a wide spectral range as a function of an externally applied voltage. In this work, ECPs have been used in absorptive/transmissive electrochromic devices as candidates for smart window applications. The electrochromic devices were fabricated on flexible polyethylene substrates and used ECPs sandwiched between thin films of single-walled carbon nanotubes serving as conductive and flexible electrodes. Unlike ITO, the nanotube films are highly transmissive in the visible and infrared region of the spectrum. The transmission and reflection of the individual device components as well as assembled devices were measured over a wide spectral range (FIR to UV). The devices were switched in situ in the spectrometers. The optical constants of the constituent layers were calculated using the Drude-Lorentz model. The devices demonstrated high transmission contrasts between their colored and bleached states in the VIS, NIR, and MIR spectra, enabling electrically tunable control over the transmission or reflection of both light and heat. This control could lead to reduced heating or cooling costs in real world applications and the flexible nature of the device components allows many applications.   

Consequence of the miscibility and mesostructure of the photoactive layer on organic solar cell performance

Recent work has found that mixed phases exist in polythiophene/fullerene solar cells. Nevertheless, the consequence of miscibility between the electron donor and acceptor is not fully understood. Through model polythiophene/fullerene mixtures, we have characterized charge transport in amorphous mixed phases. These results suggest that partial miscibility may be important for device performance, due to the interplay between minimizing large scale phase separation and maximizing charge transport in the photoactive layer. However, in some systems crystallization of either the electron donor or acceptor complicates the role of miscibility on device performance by modifying the composition of amorphous phases. As a result, we utilize grazing-incidence small angle X-ray scattering results to quantitatively describe solar cell device performance from the structure of the photoactive layer.   

Magnetic field alignment of supramolecular perylene/block copolymer complexes for electro-optic thin films

The realization of nanostructured electro-optic materials by self-assembly is complicated by the persistence of structural defects which render the system properties isotropic on macroscopic length scales. Here we demonstrate the use of magnetic fields to facilitate large area alignment of a supramolecular system consisting of a poly(styrene-b-acrylic acid) (PS-b-PAA) diblock copolymer host and a semiconducting perylene ligand. Hydrogen bonding between the carboxylic acid groups of PAA and imidazole head group of the perylene species results in hierarchically ordered materials with smectic perylene layers in a matrix of hexagonally packed PS cylinders at appropriate stoichiometries. The smectic layers and the PS domains are strongly aligned by the application of large ($>$ 2T) magnetic fields in a manner reflective of the positive diamagnetic anisotropy and the planar anchoring of perylene units at the PS interface. We use a combination of SAXS studies in-situ with applied magnetic fields, GISAXS and polarized optical transmission measurements to characterize the system. Magnetic fields thus offer a viable route for directing the self-assembly of functional materials based on rigid chromophores and further, that supramolecular approaches can be complementary to such efforts.   

Structure/property relationships in high hole mobility regioregular PT based copolymers

The synthesis of novel solution processable conjugated polymers with high hole mobilites is an active field of study due to the potential to fabricate low cost, high though-put, lightweight organic field effect transistors (OFET). Two regioregular copolymers, based on cyclopenta[2,1-$b$:3,4-$b'$]dithiophene (CDT) and pyridal[2,1,3]thiadiazole (PT) structural units, have been prepared by using polymerization reactions involving reactants specifically designed to avoid random orientation of the PT heterocycle. Compared to their regiorandom counterpart, the regioregular polymers exhibit a two orders of magnitude increase in hole mobility from 0.005 to 0.6 cm$^{2 }$V$^{-1}$ s$^{-1}$. Grazing incidence wide angle X-ray scattering (GIWAXS), near edge X-ray absorption fine structure (NEXAFS) spectroscopy, and transmission electron microscopy were carried out to obtain further insight into possible differences of structural order within the bulk and interfaces of the thin films. It was found that the backbone regioregularity leads to significant differences in the structural arrangement of the chains and indicates the importance of regioregularity for achieving optimal electronic properties.   

Electric Field Effect on Adhesion of Poly(ethylene oxide) Physical Hydrogels

The objective of this study was to characterize the effect of electric field on adhesion of poly(ethylene oxide) physical hydrogels to an aluminum substrate. The normal load necessary to disbond two plain aluminum surfaces joined by a thin layer of a poly(ethylene oxide) physical hydrogel can be reduced by about at least one order of magnitude if a reduced normal load is applied to aluminum-hydrogel interfaces simultaneously with an electric potential difference. Two aluminum surfaces joined by the hydrogel serve as a cathode and an anode. The current densities of about ten amperes per square meter determine the tenths-of-watt power dissipated in a sample. The effect of electric field on the adhesion strength of poly(ethylene oxide) hydrogels to aluminum depends on polymer concentration.   

Optical behavior of the conjugated polymer MEH-PPV thin films stretched in bi-layer dwetting by an unstable layer

Molecular packing and chain conformation play important roles in the optoelectronic performance of conjugated polymer thin films. It has been shown that by virtue of stretching via dewetting, the photoluminescence (PL) efficiencies of rarefied MEH-PPV thin films may be dramatically enhanced. To result similar effects in the stable non-diluted pristine MEH-PPV thin films, bi-layer dewetting was attempted in samples of MEH-PPV thin films ($\sim$7nm) covered by one layer of polystyrene (PS) ($\sim$40nm) that dewetted in toluene vapor to form droplets (height $\sim$300 nm) and ultrathin residual layer ($\sim$3nm) on the substrate. The instability was initiated from the PS layer in which small pinholes first emerged upon the intake of the solvent vapor. The pinholes then expanded and deepened into the underlying MEH-PPV, forcing the conjugated film to dewet. As a result of the stretching induced by the dewetting, the PL peak blue-shifted 20 nm to 540 nm and the intensity was enhanced around 10 times. Revealed by the position-sensitive confocal PL data, the huge enhancement came from both the droplet and residual layer, caused by molecular separation and stretching. Electroluminescence devices are being made based on these stretched MEH-PPV films.   

High pressure optical studies of donor-acceptor polymer heterojunctions

Bulk heterojunction polymer solar cells are based on a composite blend of two materials with electron donating and electron accepting properties. We present optical studies of a ladder-type poly(para-phenylene) and a regioregular poly(3-hexylthiophene) polymer blended with a fullerene derivative under hydrostatic pressure. The photoluminescence and absorption spectra reveal different pressure coefficients for the pristine polymer compared with the blended system. Using a phenomenological model to determine the volume change of the system under pressure, we attribute the difference in the pressure coefficient to a change in the band-edge offset at the heterojunction upon enhanced interaction. The band-edge offset is found to increase with increasing pressures for both the ladder-type and thiophene systems.   

Electrical transport properties of FeCl$_{3}$ doped poly(phenylenevinylene-co-3,4-ethylenedioxythiophene)

Poly(arylenevinylene) copolymers, in which 3,4-ethylenedioxythiophene (EDOT) and dialkoxy phenylenes are alternatively linked by vinylene unit, were synthesized by the Horner-Emmos reaction. The samples were doped with FeCl$_{3 }$and the temperature dependence of conductivity, magnetoresistance (MR), and thermoelectric power (TEP) were measured. The temperature dependence of conductivity follows exp[-(T$_{0}$/$T)^{1/2}$] and positive MR is observed up to $H $= 14 tesla. The TEP can be described by $S (T)$ = A + B/$T$ + C$T$. These behaviors are understood in the frame of charging energy limited tunneling conduction between metallic islands separated by insulating barriers.   

Session B49: Focus Session: Long-time, Entangled Dynamics in Polymers - Rods, Rings, Fibers, Particle Tracking

An improved dissipative particle dynamics model for simulation of entangled polymers

We develop an improved polymer model to capture entanglements within the DPD framework by using simplified bond-bond repulsive interactions to prevent bond crossings. We show that structural and thermodynamic properties can be improved by applying a segmental repulsive potential (SRP) that is a function of the distance between the midpoints of the segments, rather than the minimum distance between segments. The alternative approach, termed the modified segmental repulsive potential (mSRP), is shown to produce chain structures and thermodynamic properties that are similar to the softly-repulsive, flexible chains of standard DPD. Parameters for the mSRP are determined from topological, structural and thermodynamic considerations. The effectiveness of the mSRP in capturing entanglements is demonstrated by calculating the diffusion and mechanical properties of an entangled polymer melt. This improved DPD method was used in simulations for entangled polymer networks to explore impact of branched architectures on the mechanical response to the tensile and compressive deformation.   

Theory of the nonlinear rheology of topologically entangled rod fluids

Our first-principles microscopic theory of the tube confinement field and dynamics of topologically entangled rod fluids is extended to describe nonlinear rheology. Stress generically weakens tube constraints, resulting in a competition between reptation and transverse activated entropic barrier hopping. For a step-strain deformation, four distinct nonlinear relaxation regimes are predicted with increasing strain amplitude: quiescent-like reptation, strongly accelerated reptation due to stress-induced tube dilation, relaxation dominated by lateral barrier hopping, and, beyond a critical strain of order unity, an initially complete destruction of the tube constraint (microscopic yielding) followed by a re-entanglement process with complex kinetics and multi-step stress relaxation. A theory for continuous start up shear has also been formulated. In the nonlinear regime, deformation-rate-dependent stress overshoots are predicted. In the nonequilibrium steady state, strong shear thinning occurs determined largely by the rate-dependent dilated tube diameter. At very high Weissenberg numbers, a stress plateau in the flow curve is predicted and relaxation is controlled by a convective-constraint-release-like process that self-consistently emerges within the theory.   

Theoretical and computational studies of entangled rod-coil block copolymer diffusion

Despite continued interest in the thermodynamics of rod-coil block copolymers for functional nanostructured materials in organic electronics and biomaterials, relatively few studies have investigated the dynamics of these systems which are important for understanding diffusion, mechanics, and self-assembly kinetics. Here, the diffusion of coil-rod-coil block copolymers through entangled melts is simulated using the Kremer-Grest molecular dynamics model, demonstrating that the mismatch between the curvature of the rod and coil blocks results in dramatically slower reptation through the entanglement tube. For rod lengths near the tube diameter, this hindered diffusion is explained by a local curvature-dependent free energy penalty produced by the curvature mismatch, resulting in a rough energy surface as the rod moves along the tube contour. Compared to coil homopolymers which reptate freely along the tube, rod-coil block copolymers undergo an activated diffusion process which is considerably slower as the rod length increases. For large rods, diffusion of the rod through the tube only occurs when the coil blocks occupy straight entanglement tubes, which requires ``arm retraction'' as the dominant relaxation mechanism.   

Self-Consistent Field Theory of Gaussian Ring Polymers

Ring polymers, being free from chain ends, have fundamental importance in understanding the polymer statics and dynamics which are strongly influenced by the chain end effects. At a glance, their theoretical treatment may not seem particularly difficult, but the absence of chain ends and the topological constraints make the problem non-trivial, which results in limited success in the analytical or semi-analytical formulation of ring polymer theory. Here, I present a self-consistent field theory (SCFT) formalism of Gaussian (topologically unconstrained) ring polymers for the first time. The resulting static property of homogeneous and inhomogeneous ring polymers are compared with the random phase approximation (RPA) results. The critical point for ring homopolymer system is exactly the same as the linear polymer case, $\chi N$ = 2, since a critical point does not depend on local structures of polymers. The critical point for ring diblock copolymer melts is $\chi N \approx$ 17.795, which is approximately 1.7 times of that of linear diblock copolymer melts, $\chi N \approx$ 10.495. The difference is due to the ring structure constraint.   

Self-consistent field theory for polymer dynamics

We develop a self-consistent field theory (SCFT) for polymer dynamics. We reformulate a Rouse model for interacting monomers as a dynamical functional integral over field variables, using standard techniques. Novel aspects include our use of the functional Fokker-Planck equation to describe single-chain dynamics, and our extremization of the functional integral, resulting in a set of self-consistent equations for the time-dependent monomer density and the mean force field on a monomer. Our theory is distinct from published dynamical SCFTs that combine elements of equilibrium SCFT with phenomenological dynamical evolution schemes. The time scale in our theory is known exactly; no phenomenological kinetic coefficient needs to be introduced. Dynamical quantities in our theory have analogs in equilibrium SCFT, allowing sophisticated numerical techniques developed for equilibrium SCFT to be applied directly to study the dynamics. Our approach is flexible and can be used, for example, to study polymer melt dynamics. To test the self-consistent nature of the theory in space-time, we examine the simple case of the dynamics of trapped, interacting particles, and binary mixtures, in one dimension.   

Structure Formation in Semi-Dilute Polymer Solution during Electrospinning

In our recent work it was shown that longitudinal stretching of electrospun highly entangled semi-dilute polymer solution caused by jet hydrodynamic forces, transforms the topological network to an almost fully-stretched state within less than 1 mm from the jet start (PRE, 2011). Further evolution of the polymer network is related to a disentanglement of polymer chains and transformation of the topological network structure. As was sown by Malkin et al., (Rheol. Acta, 2011) high deformation rate of a topological polymer network, results in reptations of macromolecules caused by uncompensated local forces, whereas Brownian motion effect is negligible. Based on this conclusion, we examine the disentanglement process, using a mechanical pulley-block system assembled from multiple pulleys suspended by elastic springs, and taut string connecting two blocks. Each pulley corresponds to a topological knot; the taut string corresponds to a reptated chain; the springs correspond to surrounded polymer chains; and the blocks correspond to local deformation force. It turned out that the system is sensitive to system parameters. The pulleys can approach each other and the string stops to move. Such a behavior corresponds to formation of bundle of knots of entangled chains. In other conditions, the string continuously moves while the pulleys did not approach each other which corresponds to disentanglement of polymer chains. These experiments clarify the disentanglement kinetics in rapid-deformed polymer system.   

Edge electrospinning from a fluid-filled bowl for high throughput production of quality nanofibers

We present a stationary, edge-cylinder geometry for high throughput electrospinning that utilizes a reservoir filled with polymer solution and a concentric cylindrical collector [\textit{Nanotechnology} \textbf{22}, 345303 (2011)]. In this ``bowl'' electrospinning configuration, under high voltage initiation, multiple jets spontaneously form on the fluid surface, rearrange until they are approximately equidistant along the reservoir-edge and spin towards the collector, producing high quality fibers after the voltage is reduced to a working value. The technique produced poly(ethylene oxide) nanofibers with average diameter of 225 nm and a demonstrated throughput $\sim$40 times higher than traditional single-needle electrospinning. The electric field patterns generated by traditional, bowl, and our previously reported edge-plate [\textit{Polymer} \textbf{51}, 4928 (2010)] geometries show significant similarity in field magnitude and gradient along a path towards the collector, which may underlie the ability to form similar quality fibers. We discuss how the interaction between fluid properties and the applied electric field determines the effective flow rate, jet stability versus time, throughput, and fiber quality.   

Size-dependent behavior of as-spun nanofiber vs. polymer molecular weight

The size-dependent behavior of nano-objects is a well-known and widely-accepted phenomenon, but up to now it has no satisfactory explanation. From the physical point of view, such a behavior is to be related to an internal scale parameter which is comparable with the scale of the system. Recently Ji et al. showed that the elastic moduli of polystyrene nano-fibers of different molecular weights can be described by one universal curve as a function of fiber radius, scaled by radius of gyration $R_g $ (EPL, 84, 56002, 2008). However, the crossover to the size-dependent behavior in the above dependence occurs at $R \mathord{\left/ {\vphantom {R {R_g }}} \right. \kern-\nulldelimiterspace} {R_g }\sim 25-30$, therefore the radius of gyration, $R_g $, is too small in order to play a role of the required scale parameter. This discrepancy requires an explanation. Utilizing a number of well-known scaling dependences, on the base of the model of confinement mechanism of polymer nanofiber reinforcement, proposed by us earlier, it is demonstrated that elastic modulus of polymer nanofibers is described by the function $F\left[ {\left( {R \mathord{\left/ {\vphantom {R {R_g }}} \right. \kern-\nulldelimiterspace} {R_g }} \right)^\alpha } \right]$ ($\alpha \sim 1.5)$. This function, conforming to experimental observation, increases at small argument values, whereas for large argument values tends to value of the bulk elastic modulus, in doing so the crossover scale to the size-dependent behavior also agrees with experimental data.   

Mechanical properties and morphology of polymer gels

Understanding morphology and mechanical response of polymeric gels is of particular importance to design materials with required energy dissipation characteristics. We will present our latest results for polymer gels based on 1) self-assembled block copolymers and 2) chemically cross-linked polymers. The dissipative particle dynamics (DPD) was used to predict morphology in good agreement with atomic force microscopy. We have performed DPD non-equilibrium oscillatory shear calculations predicting elastic modulus of unentangled gels that correlates well with experimental rheology data. However, this methodology fails to predict mechanics of entangled polymer networks due to unphysical chain crossing brought by the soft potentials used in DPD simulations. Recently, we have introduced an improved segmental repulsion potential that removes the bond crossing allowing for reptation dynamics. The improved DPD method was used in simulations for entangled gels to explore impact of branched architecture of solvent on the mechanical response to the tensile deformation. Novel architectures of solvent resulting in a dramatic increase of the elastic modulus were identified. The topological analysis was applied to understand contributions of chemical cross-links and entanglements to the stress.   

Rapid, accurate single particle tracking based on radial symmetry center determination

Accurately tracking particles in images is a crucial task for applications as diverse as super-resolution microscopy, membrane biophysics, and microrheology. Tracking errors can easily propagate into flawed conclusions about mechanisms underlying particle dynamics. The commonly used method of locating the center of a particle by direct fitting a Gaussian function to a measured intensity profile is very accurate, but is computationally intensive and not generalizable to non-point-like particles. Its slowness is a necessary consequence of numerically searching a large parameter space. I introduce a new approach to sub-pixel particle tracking based on exploiting radial symmetry, valid for any radially symmetric particle intensity profile. I provide an algorithm that analytically, non-iteratively calculates the best-fit symmetry center to determine the particle location. Over a wide range of signal-to-noise ratios, this approach yields accuracies nearly identical to those of Gaussian fitting with execution times over two orders of magnitude faster and with greater robustness in the presence of nearby particles. This algorithm is tested on simulated data as well as real images from several experimental systems, including colloidal assemblies and single fluorescent proteins.   

Molecular Simulations of Passive Particle Rheology

In this work, we demonstrate that nanoscale viscoelastic properties of polymer melts can be obtained from molecular dynamics (MD) simulations by using an approach analogous to the experimental passive microrheology. We carry out MD simulations of a system consisting of a probe particle that is embedded in a polymer melt represented using the bead-spring model. The mean squared displacement of the probe particle determined from these MD simulations is analyzed to calculate the storage and the loss moduli of the medium. Our results indicate that calculation of the viscoelastic properties from the straightforward Generalized Stokes Einstein Relationship leads to unphysical values for these quantities in the high frequency regime. We show that this problem can be alleviated by accounting for the inertial effects in the system. Our particle rheology simulation results are quantitatively compared with the literature results that were obtained from other simulation approaches. Results will also be presented for the effects of the particle size, rigidity and the chain length on the viscoelastic properties.   

NMR-based Molecular Rheology of Entangled Polymers in Bulk and in Nanoscopic Confinement

We demonstrate the use of simple proton low-field NMR to probe the validity of the tube model of polymer dynamics. The method yields a time-domain measure of the segmental orientation autocorrelation function $C(t)$, which in turn is directly related to the stress relaxation modulus $G(t)$, thus providing a true molecular measure of rheologically relevant quantities. The fixed-tube model does not describe actual data well, and current work focuses on deuteron labeling schemes to investigate the relevance of contour-length fluctuation (CLF) or constraint release (CR) effects. As first results, we found that unexpectedly, CR processes are responsible for modified chain modes faster than actual reptation [1], and also that the dynamics is inhomogeneous along a given chain, stressing also the significance of CLF. We also present recent results for melt dynamics in nanoscopic confinement of long cylindrical channels of 20-400 nm diameter [2]. We consistently observe a fraction of chains whose dynamics is less isotropic on long time scales, i.e., in the Doi-Edwards regimes III (reptation) and IV (disentangled dynamics)\\[0pt] [1] F. Vaca Ch{\'a}vez, K. Saalw{\"a}chter, {\em Phys. Rev. Lett.} {\bf 104}, 198305 (2010), [2] S. Ok et al., {\em Macromolecules} {\bf 43}, 4429 (2010)   

Session B50: Focus Session: Nano to Mesoscale Structures in Ordered Systems: Liquid Crystalline Structure and Interactions

Molecular Assembly and Liquid Crystal Properties of a Near-IR Absorbing Dye

The molecules of the near-IR absorbing dye IR-806 spontaneously assemble in water at very low concentrations, forming a liquid crystal phase at room temperature when the concentration is above 0.6 wt\%. Unlike most chromonic liquid crystal systems, macroscopic phase separation between the isotropic and liquid crystal phases is not observed. Also unlike most chromonic liquid crystal systems, the absorption spectrum of IR-806 changes dramatically as the concentration increases and molecular assembly proceeds. Analysis of the absorption spectra provides evidence of an isodesmic assembly process at an extremely low concentration, followed by a second non-isodesmic assembly process at a higher concentration just before the liquid crystal phase appears.   

Studies on Sunset Yellow Chromonic liquid crystals by Polarized Raman and X-ray Scattering

Sunset Yellow FCF (SSY) molecules aggregate into columns in water and form chromonic liquid crystalline phases. Nematic SSY is aligned both in a flat capillary and in the magnetic field with columns pointing perpendicular to the long axis of capillary and perpendicular to the magnetic field direction, respectively. Temperature and concentration dependence of order parameters, both $<$P$_{200}>$ and $<$P$_{400}>$, are calculated based on polarized Raman measurement. The scission energy, E, determined from the Arrhenius thermal evolution of the longitudinal correlation length, is found to be around 4.3k$_{B}$T in the nematic N phase based on x-ray measurement. Flow behavior of 1.1 M nematic SSY chromonic solution under steady shear is predicted using the order parameters measured and the aspect ratios of columns.   

Photoisometric liquid crystals integrated with plasmonic nano-structures

Optically switching photoisometric liquid crystals can be used for a variety of applications. However, the dependence of the efficiency of photoisometric processes on optical fluence requires power levels too high for many practical applications. Recently it has been shown that symmetry breaking in deterministic~aperiodic plasmonic nano-structures boosts the efficiency of nonlinear processes by producing significant spatial field localization. This technology has the potential to dramatically enhance the performance of photoisometric liquid crystal devices. In this work, we combine photoisometric liquid crystals with lithographically fabricated periodic and aperiodic plasmonic nano-particle arrays in microfluidic channels to enhance their all optical switching properties. Using rigorous analytical multiple scattering methods we engineer particle arrays for near field enhancement at a control wavelength and the diffraction properties of a probe beam at another. Optical pump probe measurements of retardance and absorption are used to characterize the liquid crystal's structure factor as function pump power. The effects of plasmonic particle arrays on the switching dynamics on the photoisometric liquid crystals are also experimentally explored.   

Elasticity of lyotropic chromonic liquid crystal Sunset Yellow probed by magnetic Frederiks transition

By using director reorientation in the magnetic field, we determine the concentration and temperature dependencies of the splay $K_1$, twist $K_2$, and bend $K_3$ elastic constants (normalized by the anisotropy of the diamagnetic susceptibility) for a nematic lyotropic chromonic liquid crystal (LCLC) Sunset Yellow. In a sharp contrast to thermotropic liquid crystals, the Frederiks effects in LCLC show a hysteresis, which is more pronounced at high concentration and low temperatures. We attribute the hysteresis to the changes in self-assembled structure of LCLC aggregates under the influence of field-imposed deformations.   

Electric Field Induced Stable Micro Rotor in Nematic Liquid Crystal Drops Constrained on Thin Cellulosic Fibers

We directly visualize the response of nematic liquid crystal drops of toroidal topology constrained on thin fibers, suspended in air, to an AC applied electric field \textbf{E}. This new localized liquid crystal system can exhibit non-trivial point defects, which may become energetically unstable against expanding into ring disclinations depending on the fiber constraining geometries. The director anchoring tangential near the fiber surface and homeotropic at the air interface, making a hybrid shell distribution that in turn causes a ring of disclination line around the main axis of the fiber at the center of the droplet. Upon application of \textbf{E}, the disclination ring first expands and slightly moves along the fiber main axis, followed by the appearance of a stable ``spherical particle'' orbiting around the fiber at the center of the liquid crystal drop. The rotation speed of this particle was found to vary linearly with the applied voltage. This constrained liquid crystal geometry seems to meet the essential requirements in which soliton like particles can develop and exhibit stable orbiting in three dimensions upon application of an external electric field. This is another example of a soft energy transducer system which allows, at the micro scale, the transfer in a continuous way of electrical to mechanical energy.   

Synthesis and investigations of mesogen encapsulated gold nanoparticles

Nematic mesogen encapsulated gold nanoparticles with defined size and shapes are currently of great interest for a wide range of applications for electro-optical device or metamaterials. However the synthesis of most of the materials reported so far is quite cumbersome. Thus there is the need for new routes to synthesize more advanced compounds. A suitable strategy could be based on functionalizing the organic shell. In this contribution we report a new method to prepare gold nanoparticles with a bifunctional capping agent enabling control over their size and also act as a linking group for the connection with the mesogenic groups. The result showed monolayer coated gold nanoparticles without any co-ligands and lower the isotropic point. Physical and optical properties of the nematic gold nanoparticles have been characterized by HRTEM, EDS, NMR, DSC, TGA, XRD, and OPM. The results show stable nematic mesophase formation at room temperature. The investigate system displays a typical nematic schlieren texture, and a decreased viscosity when compared to other LC Au NPs. Structure properties relationships will be discussed and the materials will be compared to earlier research.   

Nonlinear Diffusion on a Sphere

We are interested in describing orientation of non-spherical particles such as liquid crystal molecules, nanoparticles and colloidal particles due to interactions with each other and with external fields. Since the orientation of a rod-like particle corresponds to a point on a unit sphere, the time evolution of the orientational distribution function corresponds to nonlinear diffusion on a sphere. We use a direct cell-based method to solve the Smoluchowski equation describing the behavior. We construct the Voronoi tessellation on the sphere, and regularize it. We then use a finite volume method to compute the particle density in the cells. Numerical results show the time evolution of the orientational probability density function. These results can describe the behavior of nanorod suspensions in electric fields.   

Elastic Torque on a Ferromagnetic Disk within a Nematic Liquid Crystal

An aspherical particle suspended in a nematic liquid crystal will impose an orientationally dependent energy due to coupling to the nematic elasticity. This energy depends strongly on the anchoring conditions on the surface of the inclusion, its shape, as well as the proximity of other boundary conditions on the fluid such as those set by the container. To study these properties, ferromagnetic nickel disks with homeotropic surface anchoring were suspended in the liquid crystal 4-cyano-4'-pentylbiphenyl (5CB) in a planar cell. The disks, 300 nm in thickness and 10 $\mu$m in diameter, possess a permanent magnetic moment confined to the disk's plane. In the absence of any external torque the disks align with the normal to their faces parallel to the director. Rotating of the disks from this preferred orientation creates an elastic deformation that is manifested by an opposing torque. Balancing this torque with the torque from an external magnetic field for various angles of rotation, we have mapped out the orientationally dependent energy. Over a large range of angles the torque shows a linear dependence as predicted by an electrostatic analogy.   

Anomalous diffusion of colloidal particles in a nematic liquid crystal

We explore the Brownian motion of colloidal microspheres in a nematic liquid crystal within time scales below 100ms that was not accessible in previous experiments. Our experimental results point towards an apparent sub-diffusion of the colloids with a mean square displacement MSD $\propto t^{1/2}$. For longer time scales, the particles exhibit normal diffusion with two anisotropic diffusion constants parallel and perpendicular to the nematic director $\bf n$ [1]. The nonlinear effect vanishes when the host is heated up to the isotropic phase; therefore the subdiffusive behavior can be attributed to the coupling of slow director fluctuations of the nematic with the colloidal particle dynamics. We also discuss the role of finite accuracy of measurements. \\[4pt] [1] J. C. Loudet, P. Hanusse and P. Poulin, Science 306, (2004).   

Colloids at a Chiral Liquid Crystal-Isotropic Liquid Interface

Whilst the behavior of particles trapped at liquid-liquid interfaces is relatively well understood the behavior as one of the phases begins to break translational symmetry is almost completely unexplored. Here the particles seed defects in the partially ordered liquid and new, effective, particle-particle interactions are induced. We use a chiral (cholesteric) liquid crystal which has a characteristic length scale, the pitch length, similar to the particle size. Our system consists of particles with planar anchoring which are trapped at an interface between the liquid crystal and an isotropic liquid (silicone oil which induces homeotropic anchoring). The creation of the cholesteric ``fingerprint'' texture allows the deformation of the cholesteric around a particle to be easily visualized. This allows us to determine the nature of the defects created and their symmetries. We have clear trends for the distribution of particles with respect to the interface as a function of particle size. Inspired by computer simulations we study the position of small particles (diameter $<$ pitch length) within the fingerprint texture. The behavior of the unadorned interface between the chiral liquid crystal and the oil is also explored.   

Deformable Colloids in the Presence of a Liquid Crystal

Spherical colloidal particles immersed in a liquid crystal experience a non-uniform pressure, as well as directional interactions among one another due to the defects they induce in the surrounding liquid crystal. Here, we use a lattice-Boltzmann algorithm to investigate the behavior of initially circular, 2D deformable colloids placed in a nematic liquid crystal. The colloidal particles, which are represented using a bead-spring model, are of a sufficient physical size to ensure that the anchoring of the liquid crystal molecules on their surface has a significant impact on the background liquid crystal. We present the resulting equilibrium particle shapes for a range of surface elasticities, and investigate the interaction between pairs of particles.   

Multistable alignment of nematic liquid crystals on patterned surfaces

A nematic in contact with a substrate patterned to promote a spatially-varying easy axis experiences a large elastic distortion adjacent to the surface and relaxes to a uniform state in the bulk. The bulk ordering may be thought of as an effective easy axis which, unlike conventional surface treatments, can be easily controlled by adjusting the geometry of the pattern. In this work, the behavior of a nematic film confined between substrates periodically patterned with rectangles is examined analytically. It is shown that multiple stable configurations exist and the effective azimuthal anchoring energy may be arbitrarily controlled by changing the aspect ratio of the rectangles. The various effects of flexoelectricity and saddle-splay elasticity, both important because of large spatial gradients in molecular orientation near the surface, are also considered. Prospects for applications of these surfaces in electrooptic devices such as displays are discussed.   

Direct Nanomechanical Measurement of an Anchoring Transition in a Nematic Liquid Crystal Subject to Hybrid Anchoring Conditions

A Surface Forces Apparatus was used to measure the normal force between two solid curved surfaces confining a film of nematic liquid crystal (5CB, 4'-$n$-pentyl-4-cyanobiphenyl) under hybrid planar-homeotropic anchoring conditions. Upon reduction of the surface separation $D$, we measured an increasingly repulsive force in the range $D$ = 35-80 nm, reaching a plateau in the range $D$ = 10-35 nm, followed by a short-range oscillatory force at $D$ $<$ 5 nm. The oscillation period was comparable to the cross-sectional diameter of the liquid crystal molecule and characteristic of a configuration with the molecules parallel to the surfaces. These results show that the director field underwent a confinement-induced transition from a splay-bend distorted configuration at large $D$, which produces elastic repulsive forces, to a uniform planar configuration with broken homeotropic anchoring, which does not produce additional elastic forces as $D$ is decreased. These findings, supported by measurements of the birefringence of the confined film at different film thicknesses, provide the first direct visualization of an anchoring transition at the nanometer scale.   

Unified theory of chiral smectic A monolayers and $\pi$-wall defects

Monodisperse suspensions of the rodlike chiral \textit{fd} viruses are condensed into one rod length thick colloidal monolayers of aligned rods by depletion forces. Twist deformations of the molecules are expelled to the monolayer edge as in a chiral smectic A (Sm-A*) liquid crystal, and a cholesteric (Ch) region forms at the edge. Coalescence of two such isolated monolayers results in a cholesteric wall sandwiched between two regions of aligned \textit{fd} viruses, dubbed $\pi$-wall defects. Based on the analogy of Sm-A* with superconductors, we develop a unified theory of the $\pi$-wall defects and the monolayer edge structure. Our model yields the molecular tilt profiles, the local thickness change, and the crossover from Sm-A*-to-Ch behavior across the monolayer and the $\pi$-wall. These allow us to determine the line tension as a function of the depletant polymer concentration and the chirality of the viruses, in agreement with experiment.   

Molecular Tilt on Monolayer-Protected Nanoparticles

We present a simple Ginzburg-Landau model to describe the order of ligands coating small metal nanoparticles (NPs). Two dimensionless parameters are introduced: a preferential tilt angle and a ratio epsilon between the energy cost due to spatial variations in the tilt of the coating molecules and that of the van der Waals interactions which favors uniform tilt. Even for the ground state, topological defects are present due to the topology of the NPs. The ground state for spherical particles is an ordered bipolar defective texture (B) for small epsilon and an untilted phase (U) for large epsilon. Octahedral particles have an additional phase (6V) at small epsilon characterized by the presence of six topological defects.   

Session B51: Focus Session: Evolutionary Systems Biology I - Evolutionary Dynamics and Rugged Fitness Landscapes

Optimal lineage principle for age-structured populations

Populations whose individuals exhibit age-dependent growth have often been studied using temporal dynamics of age distributions. In this talk, I examine the dynamics of age along lineages. We will see that the lineage point-of-view provides fundamental insights into evolutionary pressures on individuals' aging profiles. I will describe a variational principle that enables exact results for lineage statistics, in a variety of models. I will also discuss measurements on continuously dividing bacterial populations growing in microfluidics devices.   

Why Do Complex Systems Age?

Aging can be defined as the increase in probability of death with time. The observation that organisms, colonies, ecosystems, as well as larger social structures age and die in very similar ways suggest that the reasons underlying aging does not depend sensitively on molecular or cellular details. In this work we argue that aging is an inevitable outcome of the neutral co-evolution of non-aging components which with time become increasingly interdependent. Starting from this hypothesis, we construct generic dependency networks and obtain mortality rate as a function of time, as well as mean life expectancy as a function of organism size, complexity and metabolic rate.   

Learning about evolution from sequence data

Recent advances in sequencing and in laboratory evolution experiments have made possible to obtain quantitative data on genetic diversity of populations and on the dynamics of evolution. This dynamics is shaped by the interplay between selection acting on beneficial and deleterious mutations and recombination which reshuffles genotypes. Mounting evidence suggests that natural populations harbor extensive fitness diversity, yet most of the currently available tools for analyzing polymorphism data are based on the neutral theory. Our aim is to develop methods to analyze genomic data for populations in the presence of the above-mentioned factors. We consider different evolutionary regimes - Muller's ratchet, mutation-recombination-selection balance and positive adaption rate - and revisit a number of observables considered in the nearly-neutral theory of evolution. In particular, we examine the coalescent structure in the presence of recombination and calculate quantities such as the distribution of the coalescent times along the genome, the distribution of haplotype block sizes and the correlation between ancestors of different loci along the genome. In addition, we characterize the probability and time of fixation of mutations as a function of their fitness effect.   

Population genetics inside a cell: Mutations and mitochondrial genome maintenance

In realistic ecological and evolutionary systems natural selection acts on multiple levels, i.e. it acts on individuals as well as on collection of individuals. An understanding of evolutionary dynamics of such systems is limited in large part due to the lack of experimental systems that can challenge theoretical models. Mitochondrial genomes (mtDNA) are subjected to selection acting on cellular as well as organelle levels. It is well accepted that mtDNA in yeast Saccharomyces cerevisiae is unstable and can degrade over time scales comparable to yeast cell division time. We utilize a recent technology designed in Gottschling lab to extract DNA from populations of aged yeast cells and deep sequencing to characterize mtDNA variation in a population of young and old cells. In tandem, we developed a stochastic model that includes the essential features of mitochondrial biology that provides a null model for expected mtDNA variation. Overall, we find approximately 2\% of the polymorphic loci that show significant increase in frequency as cells age providing direct evidence for organelle level selection. Such quantitative study of mtDNA dynamics is absolutely essential to understand the propagation of mtDNA mutations linked to a spectrum of age-related diseases in humans.   

Rare beneficial mutations can halt Muller's ratchet

In viral, bacterial, and other asexual populations, the vast majority of non-neutral mutations are deleterious. This motivates the application of models without beneficial mutations. Here we show that the presence of surprisingly few compensatory mutations halts fitness decay in these models. Production of deleterious mutations is balanced by purifying selection, stabilizing the fitness distribution. However, stochastic vanishing of fitness classes can lead to slow fitness decay (i.e. Muller's ratchet). For weakly deleterious mutations, production overwhelms purification, rapidly decreasing population fitness. We show that when beneficial mutations are introduced, a stable steady state emerges in the form of a dynamic mutation-selection balance. We argue this state is generic for all mutation rates and population sizes, and is reached as an end state as genomes become saturated by either beneficial or deleterious mutations. Assuming all mutations have the same magnitude selective effect, we calculate the fraction of beneficial mutations necessary to maintain the dynamic balance. This may explain the unexpected maintenance of asexual genomes, as in mitochondria, in the presence of selection. This will affect in the statistics of genetic diversity in these populations.   

Clustering and Phase Transitions on a Neutral Landscape

The problem of speciation and species aggregation on a neutral landscape, subject to random mutational fluctuations rather than selective drive, has been a focus of research since the seminal work of Kimura on genetic drift. These ideas have received increased attention due to the more recent development of a neutral ecological theory by Hubbell. De Aguiar et al. recently demonstrated, in a computational model, that speciation can occur under neutral conditions; this study bears some comparison with more mathematical studies of clustering on neutral landscapes in the context of branching and annihilating random walks. Here, we show that clustering can occur on a neutral landscape where the dimensions specify the simulated organisms' phenotypes. Unlike the De Aguiar et al. model, we simulate \textit{sympatric} speciation: the organisms cluster phenotypically, but are not spatially separated. Moreover, we find that clustering occurs not only in the case of assortative mating, but also in the case of asexual fission. Clustering is not observed in a control case where organisms can mate randomly. We find that the population size and the number of clusters undergo phase-transition-like behavior as the maximum mutation size is varied.   

Mutational pathways to drug resistance through a maximally-rugged fitness landscape

Recent laboratory evolution experiments have identified surprising properties in the evolution of trimethoprim resistance in \textit{E.coli} through mutation of the drug's target, DHFR: (1) mutations are acquired in a reproducibly ordered manner; (2) multiple resistant endpoints exist; and (3) some pathways include mutation reversion or conversion. Here we investigate how these properties emerge from the fitness landscape of DHFR by characterizing all combinations of observed DHFR mutations. We see that the effects of mutations are so profoundly dependent on other mutations that sign-epistasis is nearly maximised, and the distributions of most mutations' effects are indistinguishable from randomly increasing or decreasing resistance. This almost `maximally-rugged' fitness landscape contains multiple separated peaks in drug resistance, and 20{\%} of favourable mutational steps are the loss or conversion of a previously acquired mutation. Select pathways through the rugged landscape avoid a common tradeoff between growth and resistance. Empirical characterization of this fitness landscape has identified that ordered but sometimes indirect mutational pathways to multiple endpoints arises from near-maximal levels of sign epistasis.   

Hidden Randomness between Fitness Landscapes Limits Reverse Evolution

Natural populations must constantly adapt to the ever-changing environment. A fundamental question in evolutionary biology is whether adaptations can be reversed by returning the population to its ancestral environment. Traditionally, reverse evolution is defined as restoring an ancestral phenotype (physical characteristics such as body size), and the classic Dollo's Law has hypothesized the impossibility of reversing complex adaptations. However, this ``law'' remains ambiguous unless reverse evolution can be studied at the level of genotypes (the underlying genome sequence). We measured the fitness landscapes of a bacterial antibiotic-resistance gene and analyzed the reversibility of evolution as a global, statistical feature of the landscapes. In both experiments and simulations, we find that an adaptation's reversibility declines as the number of mutations it involves increases, suggesting a probabilistic form of Dollo's Law at the molecular level. We also show computationally that slowly switching between environments facilitates reverse evolution in small populations, where clonal interference is negligible or moderate. This is an analogy to thermodynamics, where the reversibility of a physical process is maximized when conditions are modified infinitely slowly.   

Exploring the fitness landscape of poliovirus

RNA viruses are known to display extraordinary adaptation capabilities to different environments, due to high mutation rates. Their very dynamical evolution is captured by the quasispecies concept, according to which the viral population forms a swarm of genetic variants linked through mutation, which cooperatively interact at a functional level and collectively contribute to the characteristics of the population. The description of the viral fitness landscape becomes paramount towards a more thorough understanding of the virus evolution and spread. The high mutation rate, together with the cooperative nature of the quasispecies, makes it particularly challenging to explore its fitness landscape. I will present an investigation of the dynamical properties of poliovirus fitness landscape, through both the adoption of new experimental techniques and theoretical models.   

Scaling laws and universality for the strength of genetic interactions in yeast

Genetic interactions provide a window to the organization of the thousands of biochemical reactions in living cells. If two mutations affect unrelated cellular functions, the fitness effects of their combination can be easily predicted from the two separate fitness effects. However, because of interactions, for some pairs of mutations their combined fitness effect deviates from the naive prediction. We study genetic interactions in yeast cells by analyzing a publicly available database containing experimental growth rates of ~5 million double mutants. We show that the characteristic strength of genetic interactions has a simple power law dependence on the fitness effects of the two interacting mutations and that the probability distribution of genetic interactions is a universal function. We further argue that the strength of genetic interactions depends only on the fitness effects of the interacting mutations and not on their biological origin in terms of single point mutations, entire gene knockouts or even more complicated physiological perturbations. Finally, we discuss the implications of the power law scaling of genetic interactions on the ruggedness of fitness landscapes and the consequent evolutionary dynamics.   

Speeding up Evolutionary Search by Small Fitness Fluctuations

We consider a fixed size population that undergoes an evolutionary adaptation in the weak mutation rate limit, which we model as a biased Langevin process in the genotype space. We show analytically and numerically that, if the fitness landscape has a small highly epistatic (rough) and time-varying component, then the population genotype exhibits a high effective diffusion in the genotype space and is able to escape local fitness minima with a large probability. We argue that our principal finding that even very small time-dependent fluctuations of fitness can substantially speed up evolution is valid for a wide class of models.   

Experimental observation of critical slowing down as an early warning of population collapse

Near tipping points marking population collapse or other critical transitions in complex systems small changes in conditions can result in drastic shifts in the system state. In theoretical models it is known that early warning signals can be used to predict the approach of these tipping points (bifurcations), but little is known about how these signals can be detected in practice. Here we use the budding yeast Saccharomyces cerevisiae to study these early warning signals in controlled experimental populations. We grow yeast in the sugar sucrose, where cooperative feeding dynamics causes a fold bifurcation; falling below a critical population size results in sudden collapse. We demonstrate the experimental observation of an increase in both the size and timescale of the fluctuations of population density near this fold bifurcation. Furthermore, we test the utility of theoretically predicted warning signals by observing them in two different slowly deteriorating environments. These findings suggest that these generic indicators of critical slowing down can be useful in predicting catastrophic changes in population biology.   

Session B52: Granular Media: Flow and Structure

Simulations of Granular Particles Under Cyclic Shear

We perform molecular dynamics (MD) simulations of spherical grains subjected to cyclic, quasi-static shear in a 3D parallelepiped shear cell. This virtual shear cell is constructed out of rough, bumpy walls in order to minimize wall-induced ordering and has an open top surface to allow the packing to readily dilate or compact. Using a standard routine for MD simulations of frictional grains, we simulate over 1000 shear cycles, measuring grain displacements, the local packing density and changes in the contact network. Varying the shear amplitude and the friction coefficient between grains, we map out a phase diagram for the different types of behavior exhibited by these sheared grains. With low friction and high enough shear, the grains can spontaneously order into densely packed crystals. With low shear and increasing friction the packing remains disordered, yet the grains arrange themselves into configurations which exhibit limit cycles where all grains return to the same position after each full shear cycle. At higher shear and friction there is a transition to a diffusive state, where grains continue rearrange and move throughout the shear cell.   

A Continuous Time Random Walk Description of Monodisperse, Hard-Sphere Colloids below the Ordering Transition

Diffusive transport is a ubiquitous process that is typically understood in terms of a classical random walk of non-interacting particles. Here we present the results for a model of hard-sphere colloids in a Newtonian incompressible solvent at various volume fractions below the ordering transition ($\sim $50{\%}). We numerically simulate the colloidal systems via Fast Lubrication Dynamics -- a Brownian Dynamics approach with corrected mean-field hydrodynamic interactions. Colloid-colloid interactions are also included so that we effectively solve a system of interacting Langevin equations. The results of the simulations are analyzed in terms of the diffusion coefficient as a function of time with the early and late time diffusion coefficients comparing well with experimental results. An interpretation of the full time dependent behavior of the diffusion coefficient and mean-squared displacement is given in terms of a continuous time random walk. Therefore, the deterministic, continuum diffusion equation which arises from the discrete, interacting random walkers is presented. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.   

Shear failure of granular materials

Connecting the macroscopic behavior of granular materials with the microstructure remains a great challenge. Recent work connects these scales with a discrete calculus [1]. In this work we generalize this formalism from monodisperse packings of disks to 2D assemblies of arbitrarily shaped grains. In particular, we derive Airy's expression for a symmetric, divergence-free stress tensor. Using these tools, we derive, from first-principles and in a mean-field approximation, the entropy of frictional force configurations in the Force Network Ensemble. As a macroscopic consequence of the Coulomb friction condition at contacts, we predict shear failure at a critical shear stress, in accordance with the Mohr-Coulomb failure condition well known in engineering. Results are compared with numerical simulations, and the dependence on the microscopic geometric configuration is discussed. \\[4pt] [1] E. DeGiuli \& J. McElwaine, PRE 2011. doi: 10.1103/PhysRevE.84.041310   

A simple analytic theory for the statistics of avalanches in sheared granular materials

Slowly sheared granular materials at high packing fractions deform via slip avalanches with a broad range of sizes. Conventional continuum descriptions are not expected to apply to such highly inhomogeneous, intermittent deformations. Here, we show that it is possible to analytically compute the dynamics using a simple model that is inherently discrete. This model predicts quantities such as the avalanche size distribution, power spectra and temporal avalanche profiles as functions of the grain number fraction and the frictional weakening. A dynamical phase diagram emerges with quasi-static avalanches at high number fractions, and more regular, fluid-like flow at lower number fractions. The predictions agree with experiments and simulations for different granular materials, motivate future experiments and provide a fresh approach to data analysis. The simplicity of the model reveals quantitative connections to plasticity and earthquake statistics. (Reference: K.A. Dahmen, Y. Ben-Zion, J.T. Uhl, Nature Physics 7, 554-557 (2011).)   

Simulation of 2D Granular Hopper Flow

Jamming and intermittent granular flow are big problems in industry, and the vertical hopper is a canonical example of these difficulties. We simulate gravity driven flow and jamming of 2D disks in a vertical hopper and compare with identical companion experiments presented in this session. We measure and compare the flow rate and probability for jamming as a function of particle properties and geometry. We evaluate the ability of standard Hertz-Mindlin contact mode to quantitatively predict the experimental flow.   

Forces on intruders in granular media

We measure the forces acting on intruders moving in different directions in a granular medium consisting of mono-disperse spherical glass beads. We present the dependence of the drag force on the intruder's geometry and surface roughness, bead size, dragging speed and immersion depth. We also determine the distribution of the forces on the intruder's surfaces. We compare our results with lithostatic pressure (p = $\rho$ gz).   

2D Granular Impact Dynamics with Photoelasic Particles

What is the response of a granular material to a high speed impact from a foreign object? To answer this question, we use a large 2D granular system which is impacted from above by an intruder. Using photoelastic discs and a high-speed camera (frame rates at 7,000-775,00 fps at varied resolution, typically 40,000 fps at 584x256 pixels), we are able to observe the dynamics in this process in a way which has not been done previously. Data consists of the trajectory of the intruder, as well as either particle positions or interparticle force information. High frame rates allow observation of complex acoustic waves during the impact process. We examine the effects of varying the initial velocity, density, shape, and size of the intruder, with the goal of extracting the grain-scale mechanisms responsible for the dissipation of the intruder's kinetic energy. In comparing our data to macroscopic frictional models used in past work, we observe good agreement with the low-frequency behavior in our experiments, but we also observe large high-frequency fluctuations in the acceleration which are inherently granular, and not captured by these models. The large fluctuations are well correlated to the emission of localized intermittent stress pulses, seen in the photoelastic response.   

Cooperative rotations of 2d frictional disks under oscillatory shear

We explore the dynamics of the contact network under cyclic shear, with a particular focus on cooperative rolling and sliding contacts, using a molecular dynamics (MD) simulation approach and external fixed pressure. We systematically study the formation and persistance of clusters of cooperatively rolling grains for a range of reversal amplitudes. The propensity for cooperative rotation dictates structural properties of the contact network: loop configurations of even numbers of grains are able to rotate without sliding, while odd numbers of grains must have at least one sliding contract. We report on the statistics of loop structures in the contact network, as well as their relationships to cooperatively rotating grains. Finally, we demonstrate a characteristic scale over which grains can cooperatively rotate as well as the dependence on friction parameters.   

High-speed measurement of axial grain transport in a rotating drum

Over short timescales granular mixtures separate by size when tumbled in a partially filled horizontal drum. The smaller grains move toward the axis of rotation to form a central core; undulations in this core gradually increase in amplitude until they grow into axial bands. Using non-invasive high-speed synchrotron x-ray particle tracking, we investigate the axial transport properties of tracer particles traveling amongst glass spheres. This new technique allows us to gather data on time scales not previously possible. When the tracers are present in larger proportions the mixtures we used should have different tendencies to segregate axially according to size ratio; one of our findings, however, is that when the tracer concentration is low the single-particle dynamics of these mixtures do not depend on the relative particle sizes in any appreciable way. This implies that the potential for a mixture to axially segregate cannot be inferred from the microscopic dynamics of individual small particles. A second finding is that while the slope of the mean-squared displacement is close to that expected from diffusive transport, as determined from the single-particle dynamics, more detailed analyses indicate anomalous transport.   

Jamming of Cylindrical Grains in Vertical Channels

We study jamming of low aspect-ratio cylindrical Delrin grains in a vertical channel. These cylindrical grains resemble antacid tablets, poker chips, or coins since their height is less than their diameter. Grains are allowed to fall through a vertical channel with a square cross section where the channel width is greater than the diameter of a grain and constant throughout the length of the channel with no obstructions or constrictions. Within this channel, grains are sometimes observed to form jams, stable structures supported by the channel walls with no support beneath them. The probability of jam occurrence and the strength or robustness of a jam is effected by the grain dimensions and channel size. We will present experimental measurements of the jamming probability and jam strength in this system and discuss the relationship of these results to other experiments and theories.   

Ordered and disordered granular sphere packings obtained by epitaxial growth

We study granular packings obtained by depositing spheres on a substrate under the influence of gravity. By exploiting the direct particle tracking enabled by X-ray tomography, the nature of the order and disorder is investigated using statistical measures including density pair correlation function, and the orientational order parameter. We find that by using a low deposition rate, impinging particles with sufficient energy can overcome friction and come to rest in a potential minimum of a periodic substrate, giving rise to ordered face-centered cubic structures. However, impinging particles with large kinetic energy can dislodge particles in the substrate leading to disorder as mobile particles cooperatively form arches while they come to rest. Thus, a wide range of volume fractions and packing structures is accessed by simply controlling the nature of the substrate and deposition rate and energy, along with the shape of the impinging particles. We contrast the ordered and disordered phases observed as a function of packing fraction with our previous study with cyclically sheared packings. In that study, compaction, nucleation and growth of face centered cubic and hexagonal close packed crystalline order was observed after hundreds of thousands of shear cycles.   

Tunable acoustic switching and rectification in one-dimensional granular crystals

We study a new mechanism for tunable acoustic switching and rectification, which we experimentally demonstrate in a one-dimensional granular crystal. The granular crystal is composed of an array of statically compressed elastic spherical particles that interact nonlinearly via Hertzian contact. The granular crystal is uniform except for a single light-mass defect placed near one boundary of the crystal. Because of the interplay of the periodicity, nonlinearity, dissipation, and asymmetry of the granular crystal, vibrations applied near the defect position cause the system response to bifurcate from periodic non-transmitting states to quasiperiodic and chaotic transmitting states with broadband frequency content. We illustrate the nature of this bifurcation using numerical simulations and compare these results to experimental observations. Because the bifurcation causes a sharp transition between states, this mechanism can lead to phononic switching and sensing. Furthermore, as switches and rectification devices are fundamental components used for controlling the flow of energy in numerous applications, we envision that this mechanism could more generally enable the design of advanced photonic, thermal, and acoustic materials and devices.   

Granular compaction under confinement

A granular pack that is vertically vibrated undergoes rearrangements and often progresses to more dense configurations. The experiments presented here study the role of dilation in this granular compaction process. By applying a confining force to the granular pack during vibration, the dilation is inhibited and the compaction is greatly reduced. In general, systems with different accelerations during vibration will compact differently. However, these systems will compact in the same manner if the confining force is tuned to result in the same amount of dilation. Under large confining forces, there is very little dilation. In this regime, the compaction is significantly slowed and may approach a steady state packing fraction of approximately 0.60, consistent with ideas of a critical packing fraction for the onset of dilation.   

Densest columnar structures of hard spheres from sequential deposition

The rich variety of densest columnar structures of identical hard spheres inside a cylinder can surprisingly be constructed from a simple and computationally fast sequential deposition of cylinder-touching spheres, if the cylinder-to-sphere diameter ratio D is within [1,2.7013]. This provides a direction for theoretically deriving all these densest structures and for constructing such densest packings with nano-, micro-, colloidal or charged particles, which all self-assemble like hard spheres [\textit{Rapid Communication}, Physical Review E (in press)].   

Discrete Calculus as a Bridge between Scales

Understanding how continuum descriptions of disordered media emerge from the microscopic scale is a fundamental challenge in condensed matter physics. In many systems, it is necessary to coarse-grain balance equations at the microscopic scale to obtain macroscopic equations. We report development of an exact, discrete calculus, which allows identification of discrete microscopic equations with their continuum equivalent [1]. This allows the application of powerful techniques of calculus, such as the Helmholtz decomposition, the Divergence Theorem, and Stokes' Theorem. We illustrate our results with granular materials. In particular, we show how Newton's laws for a single grain reproduce their continuum equivalent in the calculus. This allows introduction of a discrete Airy stress function, exactly as in the continuum. As an application of the formalism, we show how these results give the natural mean-field variation of discrete quantities, in agreement with numerical simulations. The discrete calculus thus acts as a bridge between discrete microscale quantities and continuous macroscale quantities. \\[4pt] [1] E. DeGiuli \& J. McElwaine, PRE 2011. doi: 10.1103/PhysRevE.84.041310   

Session B53: Disordered and Glassy Systems II

Nanoscale dynamics of binary metallic glass Cu$_x$Hf$_{1-x}$ films

Scanning probe microscopy provides a valuable tool for investigating nanoscale structure of thin films. Less commonly it can be applied to study the low speed dynamical behavior of these systems as well. Presented here are scanning tunneling microscope investigations of sputtered glass Cu$_x$Hf$_{1-x}$ films which reveal the nanocrystalline structure of the films as well as hopping dynamics of crystal clusters on the surface. A correction for limited bandwidth and a range of activation energies is developed in the context of an Arrhenius process to allow extraction of the average energy barrier for cluster hopping. Concentration of the component metals in the films was varied allowing observation of the change in cluster size as well as the transition to the amorphous state. A second form of dynamics, more diffusive in character, was found for amorphous samples.   

Isothermal Pressurizations and Glass Transition Dynamics in the Intermediate Glass-Forming Liquid Glycerol

Brillouin scattering data along both a 75 \r{ }C and 100 \r{ }C isotherm to pressures as high 6 GPa are reported for glycerol an intermediate strength glass forming liquid. This represents the highest pressure data of any type reported for glycerol, and enables us to probe directly the alpha relaxation process at these high pressures. Acoustic mode frequencies and linewidths are obtained from fits to the spectra. These frequency shifts and linewidths are fit for each isotherm with an iterative technique in which parameters are adjusted until self consistency is obtained. The Tait equation of state along with a complex expression for the dynamical longitudinal modulus, M($\omega )$, and quantitative models for other physical quantities such as the adiabatic index are used in our analysis. A Cole-Davidson function is used to model the dynamical modulus, and self-consistent fits indicate that the stretching parameter, $\beta $, is pressure independent with a value of 0.37 consistent with other low pressure acoustic results in the literature. Final values for the pressure dependent dynamical longitudinal modulus and relaxation time are obtained. In contrast to the results of recent pressure-dependent dielectric studies, there does not appear to be a second process that obscures the alpha process.   

Fast Scanning Calorimetry study of non-equilibrium relaxation in 2-Ethyl-1-Hexanol

Fast scanning calorimetry (FSC), capable of heating rates in excess of 1000000 K/s, was combined with vapor deposition technique to investigate non-equilibrium relaxation in micrometer thick ultraviscous of 2-Ethyl-1-Hexanol (2E1H) films under high vacuum conditions. Rapid heating of 2E1H samples prepared at temperatures above approximately 145 K (standard glass transition temperature of 2E1H, Tgs), resulted in well manifested dynamic glass transitions at temperatures tens of degrees higher than Tgs. Furthermore, strong and complex dependence of dynamic glass transition temperature on the sample's initial state, i.e., the starting temperature of FSC scan was also observed. We discuss implications of these results for contemporary models of non-equilibrium relaxation in glasses and supercooled liquids.   

Fluctuating Relaxation Times in Glass-forming Liquids

The presence of fluctuating local relaxation times, $\tau(\vec{r},t)$ has been used for some time as a conceptual tool to describe dynamical heterogeneities~\cite{Ediger-arpc-2000}. Here we report on a new method for determining the local phase field, $\phi(\vec{r},t)\equiv\int^{t}\frac{dt'}{\tau(\vec{r},t')}$ from snapshots $\{\vec{r}(t_i)\}_{i=1...M}$ of the positions of the particles in a system, and we apply it to extract $\phi(\vec{r},t)$ from simulations of glass forming models. By studying how the phase field depends on the number of snapshots, we find that it is a well defined quantity. By studying fluctuations of the phase field, we find that they describe heterogeneities well at long distance scales. We also determine how the stretching exponent $\beta$ depends on the coarse graining volume, in order to test the hypothesis that relaxation in small regions is exponential and it only becomes non-exponential when considering large regions of the system. \\[4pt] [1] M.~D. Ediger, 2000 Annu. Rev. Phys. Chem. \textbf{51} 99   

Mapping dynamical heterogeneity in structural glasses to correlated fluctuations of the time variables

Dynamical heterogeneity is believed to play an important role in the dynamical behavior of slowly relaxing disordered materials. In this work, we test one hypothesis for its origin, namely that it emerges from soft (Goldstone) modes associated with a broken continuous symmetry under time reparametrizations. We do this by constructing coarse grained observables and decomposing the fluctuations of these observables into transverse components, which are associated with the postulated time-fluctuation soft modes, and a longitudinal component, which is unrelated to them. We perform our test on data obtained in simulations of four models of structural glasses. We find that as temperature is lowered and timescales are increased, the time reparametrization fluctuations become increasingly dominant. In particular, the ratio between the strengths of the transverse fluctuations and the longitudinal fluctuations grows as a function of the dynamical susceptibility $\chi_4$, which represents the strength of the dynamical heterogeneity; and the correlation volumes for the transverse fluctuations are approximately proportional to those for the dynamical heterogeneity, while the correlation volumes for the longitudinal fluctuations remain small and approximately constant.   

High Pressure Brillouin Scattering in the Fragile Glass Former Cumene

In recent years full-spectrum analysis in light-scattering has been utilized to explore the liquid-glass transition at variable temperature and ambient pressure. In this study we present temperature- and pressure-dependent Brillouin scattering results for the fragile glass-former cumene. Both equal-angle forward scattering and depolarized backscattering geometries are used, and high pressures are attained by the use of a diamond anvil cell mounted in a custom temperature-controlled housing. Opening up the variable pressure regime to full-spectrum analysis will allow more stringent tests of mode-coupling theory as well as greater insight into the behavior of glass-forming systems.   

Molecular modeling of ultra-stable vapor deposited glasses

Recent studies have shown that physical vapor deposition can be used to prepare glasses of small organic molecules with remarkably high kinetic stability and low enthalpy, particularly when compared to ordinary glasses prepared by cooling the supercooled liquid. The thermophysical properties of these new ultra-stable glasses are equivalent to those of common glasses after thousands of years of aging. However, experimental studies have so far been limited to relatively few types of molecules. We propose a molecular modeling scheme to prepare stable glasses that mimics the experimental procedure of vapor deposition. For simple disaccharides, such as trehalose, the thermophysical properties of our simulated glasses are consistent with those measured experimentally. We also prepare stable glasses of trehalose and glycerol mixtures, which are of interest for their use in stabilization of biomolecules in the glass state. Results for model binary Lennard-Jones glasses, which have been studied extensively in the literature, are also discussed. We find that the most stable glasses formed by vapor deposition are equivalent to ordinary glasses formed by cooling at a rate approximately 10 orders of magnitude slower than those accessible by ordinary cooling methods.   

Dynamic heterogeneity above and below the mode-coupling temperature

We study the temperature dependence of the spatial extend of the dynamic heterogeneity in a soft sphere system near the so-called mode-coupling temperature $T_c$. We utilize a recently introduced procedure\footnote{E. Flenner and G. Szamel, Phys. Rev. Lett. \textbf{105}, 217801 (2010)} to calculate the ensemble independent dynamic susceptibility $\chi_4(\tau_\alpha)$ and the dynamic correlation length $\xi(\tau_\alpha)$ at the alpha relaxation time $\tau_\alpha$. Above $T_c$, we find that $\chi_4(\tau_\alpha) \sim \xi(\tau_\alpha)^3$ and $\xi(\tau_\alpha) \sim \ln(\tau_\alpha)$, which is the same behavior found in a binary hard-sphere system. We track these relationships below $T_c$ to examine the recently reported non-monotonic temperature dependence of dynamic correlations found in the same system\footnote{W. Kob, S. Roland-Vargas and L. Berthier, Nat. Phys. DOI:10.1038/NPHYS2133}. Finally, we examine the relationship between dynamic susceptibilities that can be determined from experiments and the dynamic correlation length $\xi(\tau_\alpha)$.   

Fast Scanning Calorimetry studies of glassy and supercooled water

Despite intense efforts, development of a comprehensive system of relationships between various condensed phases of water remains an illusive goal. The lack of consensus on the nature of supercooled and glassy water is due primarily to the lack of kinetic and thermodynamic data at temperatures from 150 to 235 K. Because supercooled water undergoes rapid crystallization near 235 K, application of standard experimental methods is virtually impossible. With the objective of gaining insights into properties of water, we have developed an experimental approach which relies on rapid (1000000 K/s) heating of micro- and mesoscopic aqueous samples prepared by vapor deposition in vacuum at cryogenic temperatures. Due to high heating rates, this Fast Scanning Calorimetry approach, makes it possible to bypass crystallization and to obtain new data on molecular kinetics and thermodynamics in glassy water in previously inaccessible temperature interval. We will report the results of our FSC studies and discuss their impact on fundamental and applied research areas where glassy and supercooled water plays significant role.   

Volume and structural analysis of super-cooled water under high pressure

Motivated by recent experimental study of super-cooled water at high pressure [1], we performed atomistic molecular dynamic simulations study on bulk water molecules at isothermal-isobaric ensemble. These simulations are performed at temperatures that range from 40 K to 380 K using two different cooling rates, 10K/\textit{ns} and 10K/5\textit{ns}, and pressure that ranges from 1atm to 10000 atm. Our analysis for the variation of the volume of the bulk sample against temperature indicates a downward concave shape for pressures above certain values, as reported in [1]. The same downward concave behavior is observed at high pressure on the mean-squared-displacements (MSD) of the water molecules when the MSD is plotted against time. To get further insight on the effect of the pressure on the sample we have also performed a structural analysis of the sample.\\[4pt] [1] O. Mishima, J. Chem. Phys. 133, 144503 (2010);   

A field theory approach to the dynamics of classical particles

For nearly 30 years, mode-coupling theory (MCT) has been regarded as the \textit{de facto} theoretic description of dense fluids and the transition from the fluid to glassy state. But MCT is limited by its ad hoc construction and lacks a mechanism to institute corrections. We present a new fundamental theory for the kinetics of systems of classical particles which represents a unification of kinetic theory, Brownian motion and field theory. It is developed from first principles via a self-consistent perturbation in terms of an effective two-body potential, and we use this theory to investigate the existence of ergodic-nonergodic (ENE) transitions near the liquid-glass transition. After a brief introduction of the theory, we will address the development of a kinetic equation of the memory function form. The memory function kernel (or self-energy) determined by the theory shares properties with the MCT form, however our theory provides the crucial advantage of well-defined, perturbative corrections.   

N.Q.R measurements of low energy Chiral structures in powdered glassy As$_{2}$Se$_{3}$

Experimental and theoretical work on the As-chalcogen glasses have shown that in the glassy state the local cylindrical symmetry associated with the elemental pyramidal unit is preserved. Here we introduce a local paracrystalline model of glassy As$_{2}$Se$_{3}$. This model is based on a tight binding calculation of the electric field gradient (EFG) at the core of an As atom located at the apex of the pyramidal structure. This EFG is shown to be hyper sensitive to the bond angles and bond lengths the As atom forms with the chalcogen nearest neighbors, as well as the hybrid angle formed with second neighbor As atoms. A continuous variation of the bonding parameters produces a unique set of these pyramidal units which are shown to fit the NQR data for powdered glassy samples. The best fit to the NQR data indicates that the pyramidal units organize themselves into Chiral structures in the glass. A plot of the electronic energy per molecular site shows that the chiral structures have on average a lower electronic energy than a random configuration.   

Slow relaxations in glasses: full aging and beyond

Experiments performed in the last years demonstrated slow relaxations and aging in the conductance of a large variety of materials. Here, we present experimental and theoretical results for conductance relaxation and aging for the case-study example of porous silicon. The relaxations are experimentally observed even at room temperature over time scales of hours, and when a strong electric field is applied for a time $t_w$, the ensuing relaxation depends on $t_w$. We derive a theoretical curve and show that all experimental data collapse onto it with a single time scale as a fitting parameter. This time scale is found to be of the order of thousands of seconds at room temperature. The generic theory suggested is not fine-tuned to porous silicon, and thus we believe the results should be universal, and the presented method should be applicable for many other systems manifesting memory and other glassy effects. Reference: Phys. Rev. Lett. 107, 186407 (2011)   

Voids and molecuar hydrogen in hydrogenated amorphous silicon

Nuclear magnetic resonance (NMR) and Infrared (IR) spectroscopy experiments show that hydrogen microstructure consists of clustered and diluted hydrogen atoms as well as voids and hydrogen molecules in hydrogenated amorphous silicon. Several theoretical studies have also attempted that whether the microstructure incorporates voids and hydrogen molecules or not, by introducing hydrogen atoms within artificially created cavities, after relaxing the models of hydrogenated amorphous silicon. However, no theoretical study, up until now, has conclusively demonstrated that the voids and molecular hydrogen are built-in features of the microstructure. We generate several realistic models of hydrogenated amorphous silicon at different hydrogen concentrations by developing an information-based inverse method. The models not only satisfy structural and electronic properties but also provide correct NMR line spectra as compare to NMR experiments. The microstructure at high ($>$15\%) hydrogen concentration shows the presence of voids and some hydrogen molecules within the voids. The voids with molecular hydrogen are built-in configurations of the microstructure because they evolve themselves while relaxing the models via the first-principles density functional method.   

Structural and microscopic relaxations in glycerol: an IXS study

We present an Inelastic X Ray Scattering study of the THz dynamics of room temperature glycerol at pressures spanning the 0.66-3 Kbar range. We propose a comparison with ultrasound absorption results available in literature, which leads to infer the presence of two distinct relaxation phenomena, a slow and a fast one. Although the former relaxation has been thoroughly studied in glycerol by lower frequency spectroscopic techniques, no experimental evidences of the latter were so far reported in literature. A line-shape modeling based upon the memory function formalism allows us to observe that the characteristic timescale of the fast relaxation ranges in the sub-picosecond, tends to decrease with increasing the wave-vector and is rather insensitive to pressure changes. More in general, the observed phenomenology definitely reveals the microscopic, single particle, nature of this additional relaxation process.   

Session B54: Focus Session: Complex and co-evolving networks - Modeling Social and Biological Networks

Human travel and time spent at destination: impact on the epidemic invasion dynamics

Human mobility has a strong impact on the spatial spread of infectious diseases. Analyses of metapopulation models, that consider the epidemic spreading on a network of populations, show that topological and traffic fluctuations favor the global epidemic invasion. These studies consider markovian mobility (i.e. the memory of the origin of traveling individuals is lost) or non-markovian mobility with homogeneous timescales (i.e. individuals travel to a destination and come back with a homogenous rate). However, the time spent at destination is found to exhibit wide fluctuations. Such varying length of stay crucially affects the mixing among individuals and hence the disease transmission dynamics. In order to explore this aspect, we present a modeling framework that, by using a time-scale separation technique, allows analyzing the behavior of spreading processes on a complex metapopulation network with non-markovian mobility characterized by heterogeneously distributed timescales. Analytical and numerical results show how the degree of heterogeneity of the length of stay is able, alone, to drive a phase transition between local outbreak and global invasion. This highlights the importance of the interplay between mobility and disease timescales in the propagation of an epidemic.   

Effect of Spatial-Dependent Utility on Social Group Domination

The mathematical modeling of social group competition has garnered much attention. We consider a model originated by Abrams and Strogatz [Nature 424, 900 (2003)] that predicts the extinction of one of two social groups. This model assigns a utility to each social group, which is constant over the entire society. We find by allowing this utility to vary over a society, through the introduction of a network or spatial dependence, this model may result in the coexistence of the two social groups.   

Evolution of opinions on social networks in the presence of competing committed groups

Using a model of pairwise social influence, the {\it binary agreement} model (Xie et. al, Phys. Rev. E 84, 011130 (2011)), we study how the presence of two groups of individuals committed to competing opinions, affect the steady-state opinion of influencable individuals on a social network. We assume that two groups committed to distinct opinions $A$ and $B$, and constituting fractions $p_A$, $p_B$ of the total population respectively, are present in the network. We show using mean-field theory that the phase diagram of this system in parameter space $(p_A,p_B)$ consists of two regions, one where two stable steady-states coexist, and the remaining where only a single stable steady-state exists. For finite networks (complete graphs, Erd\H{o}s-R\'enyi networks and Barab\'asi-Albert networks), these two regions are separated by two first order transition lines which terminate and meet tangentially at $p_A = p_B \approx 0.1623$, which constitutes a second-order transition point. Finally, we quantify how the exponentially large switching times between steady states in the co-existence region depend on the distance from the second-order transition point for equal committed fractions.   

Strategy of Competition between Two Groups based on an Inflexible Contrarian Opinion Model

We introduce an inflexible contrarian opinion (ICO) model in which a fraction p of inflexible contrarians within a group holds a strong opinion opposite to the opinion held by the rest of the group. At the initial stage, stable clusters of two opinions, A and B exist. Then we introduce inflexible contrarians which hold a strong B opinion into the opinion A group. Through their interactions, the inflexible contrarians are able to decrease the size of the largest A opinion cluster, and even destroy it. We see this kind of method in operation, when companies send free new products to potential customers in order to convince them to adopt their product and influence others to buy it. We study the ICO model, using two different strategies, on both ER and SF networks. In strategy I, the inflexible contrarians are positioned at random. In strategy II, the inflexible contrarians are chosen to be the highest degrees nodes. We find that for both strategies the size of the largest A cluster decreases to zero as $p$ increases as in a phase transition. At a critical threshold value p$_c$ the system undergoes a second-order phase transition that belongs to the same universality class of mean field percolation. We find that even for an ER type model, strategy II is significantly more effective.   

Consensus in evolving networks of mobile agents

Populations of mobile and communicating agents describe a vast array of technological and natural systems, ranging from sensor networks to animal groups. Here, we investigate how a group-level agreement may emerge in the continuously evolving networks defined by the local interactions of the moving individuals. We adopt a general scheme of motion in two dimensions and we let the individuals interact through the minimal naming game, a prototypical scheme to investigate social consensus. We distinguish different regimes of convergence determined by the emission range of the agents and by their mobility, and we identify the corresponding scaling behaviors of the consensus time. In the same way, we rationalize also the behavior of the maximum memory used during the convergence process, which determines the minimum cognitive/storage capacity needed by the individuals. Overall, we believe that the simple and general model presented in this talk can represent a helpful reference for a better understanding of the behavior of populations of mobile agents.   

Epidemic and information co-spreading in adaptive social networks

We model simultaneous evolution of an epidemic and information about the epidemic on an adaptive social network. The classical Susceptible-Infectious-Susceptible (SIS) model is extended. Susceptible and infectious nodes are each divided into informed and uninformed types. Informed nodes affect the network structure by rewiring their network connections adaptively to avoid disease exposure. The impacts of mass media information and communication on the disease spreading and network structure are explored, and stochastic simulations are compared with a moment closure approximation. When the rewiring rate is high, the infection and information levels of the population show periodic oscillations for certain ranges of contact rate, and the moment closure approximation predicts similar dynamics. The epidemic threshold in the presence of rewiring and information is considered. Our results indicate that information can play a significant role in minimizing disease spread.   

Asymptotically inspired moment-closure approximation for adaptive networks

Adaptive social networks, in which nodes and network structure co-evolve, are often described using a mean-field system of equations for the density of node and link types. These equations constitute an open system due to dependence on higher order topological structures. We propose a moment-closure approximation based on the analytical description of the system in an asymptotic regime. We apply the proposed approach to two examples of adaptive networks: recruitment to a cause model and epidemic spread model. We show a good agreement between the improved mean-field prediction and simulations of the full network system.   

Epidemics on Interacting Networks

Epidemic spreading is of great importance in public health, as well as in related fields such as infrastructure. While complex network models have been used with great success to analyze epidemic behavior on single networks, the reality is that our world is made up of a system of interacting networks that do not necessarily share common characteristics. I introduce a model for constructing interacting networks and show that the phase transtion depends on the parameters $\kappa_T, kappa_A$ and $\kappa_B$, where $\kappa_T = \langle k^2 \rangle / \langle k \rangle$ over the nodes in both networks, including internetwork links, and $\kappa_A$ and $\kappa_B$ are over the networks considered individually, with no internetwork links. For strongly interacting networks ($\kappa_T > \kappa_A and \kappa_B$), there exists only one phase transition, between a disease-free phase and an epidemic phase across both networks. For weakly interacting networks ($\kappa_T < \kappa_A$ or $\kappa_B$), a third, ``mixed,'' phase exists, where the disease enters an epidemic on one network alone. The analytic predictions are confirmed by Monte-Carlo simulations.   

Epidemic spreading on interacting networks with preferred degrees

We discuss the SIS contact process on a network of two interacting communities, each with its own preferred degree of connections. Postulating various rules for an individuals to form intra-community and inter-community links, we find novel stationary (active) states, in addition to the expected absorbing states. The dynamics of infected individuals in the two communities can be quite different. Using Monte Carlo techniques, we explore the effects on both the network structure and the contact process due to different types of interactions between the communities. We will also present a mean field analysis of contact processes on a generic $M$ interacting communities and compare these results with the simulation data.   

Scaling theory of human dynamics and network science

The increasing availability of large-scale real data has fueled simultaneous advances in network theory, aiming to characterize the scaling of complex networks, and human dynamics, capturing the temporal characteristics of human activity patterns. Yet, these two areas remain disjoint, with their separate scaling laws and modeling framework. Here we show that the exponents characterizing the degree and link weight distribution of the underlying social network can be expressed in terms of the dynamical exponents characterizing human activity patterns, establishing the first formal link between the two areas.   

Network model explains why cancer cells use inefficient pathway to produce energy

The Warburg effect---the use of the (energetically inefficient) fermentative pathway as opposed to (energetically efficient) respiration even in the presence of oxygen---is a common property of cancer metabolism. Here, we propose that the Warburg effect is in fact a consequence of a trade-off between the benefit of rapid growth and the cost for protein synthesis. Using genome-scale metabolic networks, we have modeled the cellular resources for protein synthesis as a growth defect that increases with enzyme concentration. Based on our model, we demonstrate that the cost of protein production during rapid growth drives the cell to rely on fermentation to produce ATP. We also identify an intimate link between extensive fermentation and rapid biosynthesis. Our findings emphasize the importance of protein synthesis as a limiting factor on cell proliferation and provide a novel mathematical framework to analyze cancer metabolism.   

Physiological Networks: towards systems physiology

The human organism is an integrated network where complex physiologic systems, each with its own regulatory mechanisms, continuously interact, and where failure of one system can trigger a breakdown of the entire network. Identifying and quantifying dynamical networks of diverse systems with different types of interactions is a challenge. Here, we develop a framework to probe interactions among diverse systems, and we identify a physiologic network. We find that each physiologic state is characterized by a specific network structure, demonstrating a robust interplay between network topology and function. Across physiologic states the network undergoes topological transitions associated with fast reorganization of physiologic interactions on time scales of a few minutes, indicating high network flexibility in response to perturbations. The proposed system-wide integrative approach may facilitate new dimensions to the field of systems physiology.   

Detecting and evaluating communities in complex human and biological networks

We develop a simple method for detecting the community structure in a network can by utilizing a measure of closeness between nodes. This approach readily leads to a method of coarse graining the network, which allows the detection of the natural hierarchy (or hierarchies) of community structure without appealing to an unknown resolution parameter. The closeness measure can also be used to evaluate the robustness of an individual node's assignment to its community (rather than evaluating only the quality of the global structure). Each of these methods in community detection and evaluation are illustrated using a variety of real world networks of either biological or sociological importance and illustrate the power and flexibility of the approach.   

Small-world organization of self-similar modules in functional brain networks

The modular organization of the brain implies the parallel nature of brain computations. These modules have to remain functionally independent, but at the same time they need to be sufficiently connected to guarantee the unitary nature of brain perception. Small-world architectures have been suggested as probable structures explaining this behavior. However, there is intrinsic tension between shortcuts generating small-worlds and the persistence of modularity. In this talk, we study correlations between the activity in different brain areas. We suggest that the functional brain network formed by the percolation of strong links is highly modular. Contrary to the common view, modules are self-similar and therefore are very far from being small-world. Incorporating the weak ties to the network converts it into a small-world preserving an underlying backbone of well-defined modules. Weak ties are shown to follow a pattern that maximizes information transfer with minimal wiring costs. This architecture is reminiscent of the concept of weak-ties strength in social networks and provides a natural solution to the puzzle of efficient infomration flow in the highly modular structure of the brain.   

Unraveling the rules of evolution in the yeast protein-protein interaction network

A question of fundamental importance is to understand the dynamical principles according to which biological networks have acquired their topological structures and functional modules. Here, we perform an empirical study of the yeast protein-protein interactions (PPI), combined with theoretical modeling of the genomic duplication-divergence processes. Our duplication-divergence model agrees with experimental data, and provides a novel approach to reconstruct ancestral PPI networks. Following the phylogenetic tree, our analysis unravels that the ancient networks evolve into the present day yeast network by a multiplicative growth. The rule of multiplicative growth demonstrates the relationship between the topological exponents and the evolution growth rates of interactions. An important consequence of this evolutional principle is the emergence of self-similar modular structure, which is confirmed by the analysis of functional modules of proteins.