Session W1: Photonic Systems and Entanglement Generation

Efficient quantum computing using coherent photon conversion

Single photons make very good quantum information carriers, but current schemes for photonic quantum information processing (QIP) are inefficient. We describe a new scheme, \emph{coherent photon conversion (CPC)}, using classically pumped nonlinearities to generate and process complex multiquanta states\footnote{Published in Nature \textbf{478}, 360 (2011)}. One example based on four-wave mixing provides a full suite of QIP tools for scalable quantum computing from a single, versatile process, including: deterministic multiqubit entanglement gates based on a novel photon-photon interaction, high-quality heralded multiphoton states without higher-order imperfections, and robust, high-efficiency detection. Using photonic crystal fibres, we present observations of quantum correlations from a four-colour nonlinear process suitable for CPC and study the feasibility of reaching the deterministic regime with current technology. The scheme could also be implemented in optomechanical, electromechanical and superconducting systems.   

Dephasing of multiparticle Rydberg excitations for fast entanglement generation

We propose an approach to fast entanglement generation based on Rydberg dephasing of collective excitations (spin waves) in large, optically thick atomic ensembles. Rather than trying to prevent multiple excitations via the Rydberg blockade mechanism, our idea is to allow multiple Rydberg level excitations to self-interact and dephase. The strong interaction required to dephase multiple excitations is induced by mixing adjacent, opposite-parity Rydberg levels with a microwave field. These levels experience resonant $1/r^3$ dipole-dipole interactions ($ns + n'p \rightarrow n'p + ns$) that extend over the whole ensemble in contrast to the weaker, short range $1/r^6$ Van der Waals coupling due to non-resonant processes ($ns + ns \rightarrow np + (n-1)p$). The interaction-induced phase shifts suppress the contribution of multiply excited states in phase matched optical retrieval. The dephasing mechanism therefore permits isolation and manipulation of individual spin wave excitations. High quality single photons can be created with $1/e$ maximum efficiency in few microseconds. The dephasing mechanism is shown to have favorable, approximately exponential, scaling. Required long coherence times are achieved via four-photon excitation and read-out of long wavelength spin waves.   

Unconventional ultra-efficient photon blockade and single-photon emitters from weakly nonlinear systems based on coupled cavities

Single photons are usually generated by non-resonant excitation of single (artificial) atoms or by resonant excitation of Kerr systems with a giant nonlinear interaction much larger than the losses of the system (standard photon blockade). Here, we present a general class of destructive quantum interference effects [1,2], which provide a robust protocol to achieve strong photon antibunching and single-photon emission in a double cavity system, where one resonantly driven cavity is coupled to an auxiliary nonlinear cavity. An original scheme [2] shows the single-photon emission can be also produced by the auxiliary cavity with orthogonal polarization with respect to the pump beam, hence providing a direct way to get spatial and polarization selection. \\[4pt] [1] M. Bamba, A. Imamo\u{g}lu, I. Carusotto, C. Ciuti, {\it Origin of strong photon antibunching in weakly nonlinear photonic molecules}, Phys. Rev. A {\bf 83}, 021802 (2011) and references therein. \\[0pt] [2] M. Bamba, C. Ciuti, {\it Counter-polarized single-photon generation from the auxiliary cavity of a weakly nonlinear photonic molecule}, Appl. Phys. Lett. {\bf 99}, 171111 (2011).   

Anomalous Stokes scattering by an atomic ensemble: detection of entanglement and vector metrology of field gradient

We investigate the collective Stokes scattering by a typical atomic cloud, i.e. with size much larger than the light wavelength and with density much lower than that required for Dicke superradiance. We show that the diffraction pattern of the Stokes photon can be used to detect entanglement in the atomic ensemble. When the atomic cloud is placed in a static magnetic field or electric field, the change of diffraction pattern by the evolution in the field can also provide sensitive vector metrology of the spatial gradient of the field.   

Unique Properties and Prospects: Quantum Theory of the Orbital Angular Momentum of Ince-Gauss Beams

The Ince-Gauss modes represent a new addition to the standard solutions to the paraxial wave equation. Parametrized by the ellipticity of the beam, they span the solution space between the Hermite-Gauss and the Laguerre-Gauss modes. These beams may be decomposed in either basis, and single photons in the Ince-Gauss modes exist naturally as superpositions of either Laguerre-Gauss or Hermite-Gauss modes. We present the fully quantum theory of the orbital angular momentum of these beams. Interesting features that arise are: stable beams with fractional orbital angular momentum, non-monotonic behavior of the OAM with respect to ellipticity, and the possibility of orthogonal modes possessing the same OAM. We believe that these modes may open up a fully new parameter space for quantum informatics and communication, and thus are worthy of thorough study.   

Entanglement of Ince-Gauss Modes of Photons

Ince-Gauss modes are solutions of the paraxial wave equation in elliptical coordinates [1]. They are natural generalizations both of Laguerre-Gauss and of Hermite-Gauss modes, which have been used extensively in quantum optics and quantum information processing over the last decade [2]. Ince-Gauss modes are described by one additional real parameter -- ellipticity. For each value of ellipticity, a discrete infinite-dimensional Hilbert space exists. This conceptually new degree of freedom could open up exciting possibilities for higher-dimensional quantum optical experiments. We present the first entanglement of non-trivial Ince-Gauss Modes. In our setup, we take advantage of a spontaneous parametric down-conversion process in a non-linear crystal to create entangled photon pairs. Spatial light modulators (SLMs) are used as analyzers. [1] Miguel A. Bandres and Julio C. Guti\'{e}rrez-Vega ``Ince Gaussian beams", Optics Letters, Vol. 29, Issue 2, 144-146 (2004) [2] Adetunmise C. Dada, Jonathan Leach, Gerald S. Buller, Miles J. Padgett, and Erika Andersson, ``Experimental high-dimensional two-photon entanglement and violations of generalized Bell inequalities", Nature Physics 7, 677-680 (2011)   

Entangling two spatially separated qubits via interaction with nonclassical radiation

We propose a scheme for entangling two spatially separated noninteracting qubits using two-mode squeezed light in a cavity. Unlike other methods that typically require dipolar coupling for creating entanglement, our proposal relies solely on the interaction of the qubits with the squeezed cavity field. The squeezed field induces exchange of correlated photons which leads to transfer of entanglement from the field to discrete entanglement between qubits. Our scheme exhibits substantial steady-state entanglement which is robust against decoherence under the strong squeezing condition. In addition, we find that the entanglement generated between two asymmetric qubits is stronger than that generated by identical ones and crucially depends on the degree of squeezing.   

Generating coherent states of entangled spins

A coherent state of many spins contains quantum entanglement, which increases with the decrease of the collective spin value. We present a scheme to engineer this class of pure state based on incoherent spin pumping with several collective raising and lowering operators. In a pumping scenario aimed for maximum entanglement, the $N$-qubit steady state realizes the ideal resource for the $1\to N/2$ quantum telecloning. We show how the scheme can be implemented in the cold atomic system in an optical lattice. Error analysis shows that high-fidelity state engineering is possible for $N\sim O(100)$ spins in the presence of decoherence. The scheme can also prepare the large-scale Affleck-Kennedy-Lieb-Tasaki state.   

Non-destructively probing matter-photon correlations described by the Dicke-Hubbard Lattice model

The Dicke-Hubbard Lattice (DHL) Hamiltonian is a prototypical system to study photon matter entanglement across a symmetry breaking quantum phase transition in the matter subsystem. The model describes a cavity containing a periodic lattice, with a single mode photon field delocalized across the cavity. Like the Bose-Hubbard model, the Hamiltonian includes on-site repulsion between atoms and nearest neighbor hopping of an atom from one site to another. In addition, matter-light coupling in the DHL model can excite an atom to a higher band by absorbing a photon and the reverse process. We focus on the DHL model in a region of parameter space in which light is ``superradiant'' and matter is either a Mott-insulator or superfluid of both bands. Through mean field, exact diagonalization, and quantum Monte Carlo calculations we examine photon statistics across the matter quantum phase transition in order to elucidate how the photon statistics reflect the matter correlations. Doing so provides a novel technique to non-destructively probe the Mott-insulator to superfluid phase transition.   

The Impact of Geometry on the TM PBGs of Photonic Crystals and Quasicrystals

Here we demonstrate a novel quantitative procedure to pursue statistical studies on the geometric properties of photonic crystals and photonic quasicrystals (PQCs) which consist of separate dielectric particles. The geometric properties are quantified and correlated to the size of the photonic band gap (PBG) for wide permittivity range using three characteristic parameters: shape anisotropy, size distribution, and feature-feature distribution. Our concept brings statistical analysis to the photonic crystal research and offers the possibility to predict the PBG from a morphological analysis.   

A low-dimensional population-competition model for analyzing transverse optical patterns

Under favorable conditions, laser beams passing through a nonlinear medium (e.g. atomic vapors) can undergo directional instabilities, generating transverse optical patterns in the far field. In particular, a low intensity all-optical switching scheme using these transverse patterns in semiconductor quantum well microcavities was numerically demonstrated. Trying to understand the switching mechanism through the simulation results has turned out to be a complicated task. In this Contribution, we present a low-dimensional ``population-competition' model that (i) exhibits nearly all the pattern selection and switching behaviors and (ii) is simple enough to allow a comprehensive analysis of its solution structure in the relevant region of parameter space. We will explain the relation between this simple model and the realistic theory. Using elementary methods in Catastrophe Theory, we analyze the ``phase diagrams'' of our model's steady state solutions in parameter space, with the help of which we construct an organized picture of the behaviors of the realistic simulation results.   

Phonon effects on analog quantum simulation with ultracold ions in a linear Paul trap

Linear Paul traps have been used to simulate the transverse field Ising model with long-range spin couplings. Here, we study the effects of phonon creation on the spin state probability and spin entanglement. The effective spin models are created by applying a spin-dependent force with a laser that couples the spin state to the phonons of the ion crystal. Adiabatically removing the phonons creates an action described by a static spin Hamiltonian plus additional quantum time-dependent phases. In appropriate limits, the system is described predominantly by the static spin Hamiltonian. Here, we solve for the evolution of the coupled spin-phonon system exactly using exact diagonalization and examine the effect of phonon creation during the simulation on the probabilities of different spin states and on their entanglement. In particular, we examine phonon effects on the possibility for seeing the kink transition when the laser is detuned between the two phonon modes that lie below the COM mode.   

Entanglement Effects in Highly Dense Systems

We study the effect of entanglement in Jaynes-Cummings model using Neumann Entropy in the highly dense systems. We study a highly dense system such as neutron star as a good application of our results.   

Session W2: Invited Session: Instrumentation and Measurement Science for Energy Research: PV and Batteries

Using Deep Level Transient Spectroscopy (DLTS) to characterize defects in semiconductor devices

Deep Level Transient Spectroscopy (DLTS) is a member of the class of instrumentation methods that utilizes the detection of trapped electronic charge to characterize defects in solids. Such methods detect this charge either directly, e.g. via capacitance measurements, or indirectly, e.g. via the current associated with the release of trapped charge. These types of instrumentation have been widely used since the dawn of solid-state physics, particularly for nonradiative defects in semiconductors and insulators. In the case of semiconductor devices, the highly sensitive capacitive detection of trapped charge in the junction depletion layer makes these methods particularly powerful. The DLTS method introduced the concept of time-domain filtering (the so-called ``rate window'') to create a defect spectrum from the transient response of the device versus temperature. This talk will give an overview of DLTS, with particular emphasis on the correlation between defects and device performance.   

Role of Defects and Their Analysis in Photovoltaics

Defects are intrinsically related to the performance of solar cells. In solar cells the generation and collection of charge carriers determines their efficiency. Effective transport of charge carriers across interfaces and minimization of their recombination in the bulk or at surfaces and interfaces is of utmost importance. In this talk we will discuss the role of surface and bulk defects. First, the role of surface passivation is very important in limiting the rate of carrier of recombination. Here we will combine spectroscopic evaluation of the surface of a Si device with electrical lifetime measurements to ascertain what factors determine the quality of a solar cell passivation. We have also utilized time-resolved photoluminescence (TRPL) to assess the quality of materials grown under varying conditions. TRPL decay is best fit by a biexponential model that includes both the minority carrier lifetime and the rate of trap filling. At low laser powers the trap state recombination dominates and decay times are very short. At higher laser powers the trap states become saturated and we can extract the minority carrier lifetime. By evaluating the relative contributions of the trap-filling and minority carrier lifetimes we can assess the density of traps (defects) as a function of the growth conditions and guide refinement of growth recipes to improve material quality.   

The Integration of Scanning Electron Microscopy, Scanning Probe Microscopy, and Luminescence Spectroscopy in one Platform: New Opportunities and Applications in Photovoltaics

We have recently integrated scanning tunneling microscopy (STM), atomic force microscopy (AFM), and near-field scanning optical microscopy (NSOM) onto the mechanical stage of a scanning electron microscope compatible with operation under high vacuum and the use of cryogenics. This instrument is unique in the sense that is not just the assembly of different microscopes but an integrated platform in which both the electron beam and the ultrasharp tip of the AFM/STM/NSOM can be controlled simultaneously and independently as excitation or sensing elements, providing innovative modes of operation and access to optoelectronic properties in the micro and nanoscale not accessible before. Furthermore, this instrument is equipped with focused laser illumination of the tip and detection of luminescence and can be used to measure cathodoluminescence, scanning tunneling luminescence, photoluminescence, and electroluminescence, all with high resolution. In this contribution, we review the application of these techniques to the development of second- and third-generation photovoltaics (PV) beyond those commercially available today. Among these applications, we present the luminescence and electron transport across single grain boundaries in chalcopyrite and kesterite compounds, the detection of single molecule species using plasmonics, the nanoscale imaging of the exciton transport in organic semiconductors, and the insitu manipulation and measurement of nanowires.   

Li ion nanowire batteries and their \textit{in situ} characterization in the TEM

The ability to measure the morphological, chemical, and transport characteristics with nanoscale resolution in electrochemical energy storage devices is critical for understanding the complex interfacial reactions and phase transformation that accompany cycling of secondary batteries. In this talk I will describe the use of an all-nanowire Li ion battery for \textit{in situ} characterization of charge and discharge reactions. The nanowire batteries (NWBs) consist of a metalized core, a LiCoO$_{2}$ cathode, LiPON solid electrolyte, and a thin film Si anode. Measuring several micrometers in length and several hundred nanometers in diameter, the NWBs can be readily imaged and analyzed in transmission electron microscopes (TEM, STEM). We use focused ion beam milling and electron beam induced deposition to separate the cathode and anode and fabricate Pt contacts to a NWB. \textit{In situ} electrical cycling of NWBs in TEM reveals that the most of the structural changes due to cycling happens in the electrolyte layer especially near the cathode/electrolyte interface. Electrical response from a single NWB was measured in the sub-pA range. For NWBs with the thinnest electrolyte, approximately 100 nm, we observe rapid self-discharge, along with void formation at the electrode/electrolyte interface, indicating electrical and chemical breakdown. The analysis of the NWB's electrical characteristics reveals space-charge limited electronic conduction, which effectively shorts the anode and cathode electrodes. When the electrolyte thickness is increased, the self-discharge rate is reduced substantially and the NWBs maintain a potential above 2 V. Our study illustrates that at reduced dimensions the increase in the electric field can lead to large electronic current in the electrolyte effectively shorting the battery even when the electrolyte layer is uniform and pinhole free. The scaling of this phenomenon provides useful guidelines for design of 3D Li ion batteries.   

In-Situ TEM Electrochemistry of Individual Nanowire and Nanoparticle Electrodes in a Li-Ion Cell

Recently, we created the first Li-ion electrochemical cell inside a transmission electron microscope (TEM) and observed, in real time with atomic scale resolution, the lithiation/delithiation processes. This experiment opened the door for a suite of experimental studies involving in-situ TEM characterization of Li-ion battery materials. In this presentation, I'll first review our latest progress of using the in-situ electrochemical cell setup inside the TEM to reveal the intrinsic electrochemistry of several high energy density anode materials such as SnO$_{2}$, ZnO, Si, Ge, Al nanowires, Si nanoparticles, carbon nanotubes, and graphene. Several electrochemical mechanisms were observed and characterized in real-time, including lithiation induced stress, volume changes, phase transformations, pulverization, cracking, embrittlement, and mechanical failure in anode materials. These results indicate the strong material, size and crystallographic orientation dependent electrochemical behavior and degradation mechanisms that occur in Li-ion battery anodes. In the future, we will need further advancements in in-situ characterization for understanding important processes in Li-ion batteries. For example, liquid cells are required in order to examine the electrochemical reactions between battery materials and the standard battery electrolytes, which are ethylene carbonate-based. Furthermore, in-situ studies need to be correlated with electrochemical studies performed on bulk electrodes. I will present a comparison between our in-situ results and electrochemical studies on conventional battery electrodes and highlight how in-situ studies can have important impact on the design of Li-ion batteries. Finally I will discuss outstanding challenging issues and opportunities in the field of Li-ion battery research. \\[4pt] References: \textbf{Science} 330, 1515 (2010); 330, 1485 (2010); \textbf{Nano Lett}. Doi: 10.1021/nl200412p, 10.1021/nl2024118, 10.1021/nl201684d, 10.1021/nl202088h, \textbf{ACS Nano}, doi: 10.1021/nn200770p, 10.1021/nn202071y; \textbf{PRL} 106, 248302 (2011); \textbf{Eng. Env. Sci.} doi: 10.1039/c1ee01918j   

Session W3: Invited Session: Fractional Topological Insulators

Fractional Topological Insulators in 2 and 3 dimensions

I will present a family of exactly solvable models whose low energy physics is that of a 3D topological band insulator of fractionally charged fermions. When time reversal is broken at the surface, these insulators display a fractional magnetoelectric effect, leading to fractional quantum Hall surface states. Further, some -- but not all -- of them can be shown to be genuine topological insulators, whose gapless surface states are protected by time reversal. This gives an explicit construction of fractional topological insulators in 3D. This work has been done in collaboration with Michael Levin (University of Maryland), Maciej Koch-Janusz (Weizmann Institute), and Ady Stern (Weizmann Institute).   

Composite fermions for fractionally filled Chern bands

We consider fractionally filled bands with a non-zero Chern index that exhibit the Fractional Quantum Hall Effect~in zero external field\footnote{R. Roy and S. Sondhi, \textit{Physics }\textbf{4}, 46 (2011) and papers reviewed therein.} a possibility supported by numerical work.\footnote{Ibid.} Analytic treatments are complicated by a non-constant Berry flux and the absence of Composite Fermions (CF), which would not only single out preferred fractions, but also allow us compute numerous response functions at nonzero frequencies, wavelengths and temperature using either Chern-Simons field theory or our Hamiltonian formalism.\footnote{G. Murthy and R. Shankar, Rev. Mod. Phys., \textbf{75}, 1101, (2003)} We describe a way to introduce CF's by embedding the Chern band in an auxiliary problem involving Landau levels. The embedded band can be designed to approximate a prescribed Chern density in k space which determines the commutation relations of the charge densities and hence preserve all dynamical and algebraic aspects of the original problem. We find some states for which the filling fraction and dimensionless Hall conductance are not equal. The approach extends to two-dimensional time-reversal invariant topological insulators and to composite bosons.   

Fractional Topological Insulators

The prediction and experimental discovery of topological band insulators and topological superconductors are recent examples of how topology can characterize phases of matter. In these examples, electronic interactions do not play a fundamental role. In this talk I shall discuss cases where interactions lead to new phases of matter of topological character. Specifically, I shall discuss fractional topological states in lattice models which occur when interacting electrons propagate on flattened Bloch bands with non-zero Chern number. Topologically ordered many-particle states can emerge when these bands are partially filled, including a possible realization of the fractional quantum Hall effect without external magnetic fields. I shall also ponder on the possible practical applications, beyond metrology, that the quantized charge Hall effect might have if it could be realized at high temperatures and without external magnetic fields in strongly correlated materials.   

Correlated topological insulators and the fractional magnetoelectric effect

I will describe the recent theoretical construction of electronic phases in 3d that combine the physics of electron fractionalization with that of topological insulators. Called fractional topological insulators, these states of matter host protected surface states and fractionally charged quasiparticle excitations. I will then discuss the emergent gauge theory description of these phases with an emphasis on the crucial role of deconfinement at low energies. I will also describe a wide variety of experimental signatures of fractional topological insulators as well as suggesting directions for experimental searches.   

Session W4: Strongly Interacting Fermi Gases

Revealing the Superfluid Lambda Transition in the Universal Thermodynamics of a Unitary Fermi Gas

We have observed the superfluid phase transition in a strongly interacting Fermi gas via high-precision measurements of the local compressibility, density and pressure down to near-zero entropy. We perform the measurements by in-situ imaging of ultracold $^6$Li at a Feshbach resonance. Our data completely determine the universal thermodynamics of strongly interacting fermions without any fit or external thermometer. The onset of superfluidity is observed in the compressibility, the chemical potential, the entropy, and the heat capacity. In particular, the heat capacity displays a characteristic lambda-like feature at the critical temperature of $T_c/T_F = 0.167(13)$. This is the first clear thermodynamic signature of the superfluid transition in a spin-balanced atomic Fermi gas. We provide a new value of the Bertsch parameter $\xi_S$. The experimental results are compared to recent Monte-Carlo calculations. Our measurements provide a benchmark for many-body theories on strongly interacting fermions, relevant for problems ranging from high-temperature superconductivity to the equation of state of neutron stars.   

Probing upper branch physics in strongly interacting Fermi gases

Motivated by a recent experiment at MIT, we consider the collision of two clouds of spin-polarized atomic Fermi gases close to a Feshbach resonance. We explain why two dilute gas clouds, with attractive interactions between its constituents, bounce off each other as if they were billiard balls. Our hydrodynamic analysis, in excellent agreement with experiment, gives strong evidence for a novel metastable many-body state, the so-called upper branch, with repulsive effective interactions. We also propose another experiment, measuring spin decoherence rates, to study the physics of the upper branch.   

Strongly interacting atomic Fermi gases in a trap with mass and population imbalances at finite temperature

A great advantage of studying atomic Fermi gases is the easy tunability of multiple physical parameters, including interaction strength, mass and population imbalances, as well as species dependent trapping potential. Indeed, the mixture of $^6$Li and $^{40}$K gases has been of great interest, with and without population imbalance. In this talk, we will address the finite temperature phase diagrams of two component atomic Fermi gases with both mass and population imbalances in a trap, using a pairing fluctuation theory. We show that in certain parameter ranges, there exist intermediate temperature superfluids as well as phase separation with exotic sandwich-like shell structure with superfluid or pseudogapped normal state in the middle. We consider pairing strength over the entire range of BCS-BEC crossover. Our result is relevant to future experiment on mixtures of $^6$Li and $^{40}$K and possibly other Fermi atoms. References: H. Guo, C.-C. Chien, Q.J. Chen, Y. He, and K. Levin, Phys. Rev. A 80, 011601(R) (2009); C.-C. Chien, Q.J. Chen, Y. He, and K. Levin, Phys. Rev. Lett. 98, 110404 (2007); C.-C. Chien, Q.J. Chen, Y. He, and K. Levin, Phys. Rev. Lett. 97, 090402 (2006); Q.J. Chen, I. Kosztin, B. Janko, and K. Levin, Phys. Rev. Lett. 81, 4708 (1998).   

Two-particle current from Superfluid Fermi Gases in the BCS-BEC Crossover

In recent years, the crossover from the BCS-type superfluid to the Bose-Einstein condensation (BEC) of tightly-bound molecules has been realized in ultracold atomic Fermi gases using a tunable pairing interaction associated with a Feshbach resonance.In the BCS-BEC crossover it will be important to reveal the nature of fermion pairs. In this paper, we propose that two-particle (double photoemission) current (DPE current) is a powerful technics to provide direct insight into the pair-correlations. The DPE from superconductors has been studied both theoretically and experimentally, where a pair of electrons is emitted from the system upon the absorption of one photon. In this study, we consider an analogous situation in ultracold atomic gases, and derive a general expression for DPE current from superfluid Fermi gases in the BCS-BEC crossover.We show DPE current as a function of energy and momentum transfers, and identify the contributions of the condensed pair components and uncorrelated pair states, and discuss the possibility of distinguishing between weakly-bound Cooper pairs and tightly-bound molecules.   

Feshbach resonances and BCS-BEC crossover in optical lattices

In this talk we study Feshbach resonances of fermionic atoms placed in a periodic potential. We investigate the criteria when such a system can be described by a Hubbard model with variable interaction strength in case of broad resonance, or by a tight binding model of atoms and molecules with can convert into each other on sites of the lattice in case of narrow resonances. Assuming the applicability of these models, we first study the BCS-BEC crossover for broad resonance. We find that while below half filling the system undergoes the conventional crossover from a BCS superconductor to a Bose condensate of molecules, above half filling the nature of the BEC phase changes to that of a condensate of molecules made of holes. Switching our attention to the case of narrow resonance, we find that the crossover takes the system from a BCS to hole-BEC regime, than back to BCS, and finally to a conventional BEC of atomic molecules. In the latter crossover, we find that the size of Cooper pairs/molecules changes non-monotonously, being larger in the BCS and smaller in the BEC regimes. Finally, at a unity filling we find a quantum phase transition from a band insulator to a BCS-BEC superfluid replacing the crossover.   

Resonant enhancement of the FFLO state in 3D by an optical potential

In a two component Fermi gas, spin-imbalance leads to a competition between Cooper-pairing with zero momentum and with nonzero momentum. The latter gives rise to the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. Hitherto this state has not been observed in a 3D Fermi gas. We propose a new way to enhance the presence of the FFLO state, by adding a 1D periodic potential. To investigate the effect of this potential, we study the ground state properties of the system, starting from the partition sum of an imbalanced Fermi gas in path-integral representation. To describe the FFLO state, a saddle point is chosen in which the pairs can have nonzero momentum. Minimizing the resulting free energy leads to the phase diagram of the system. The stability region of the FFLO state is found to be greatly enlarged due to the presence of the periodic potential, compared to the ordinary 3D case. We find that the FFLO state can exist at higher spin imbalance if the wavelength of the optical potential becomes smaller. We propose that this concept can be used experimentally to enhance the FFLO state.\\[4pt] [1] Jeroen P.A. Devreese, S.N. Klimin, and J. Tempere, Phys. Rev. A 83, 013606 (2011).\\[0pt] [2] Jeroen P.A. Devreese, M. Wouters, and J. Tempere, J. Phys. B 44, 115302 (2011); Phys. Rev. A 84, 043623 (2011).   

Single magnetic impurity in a spin-imbalanced superfluid Fermi gas

A spin-imbalanced superfluid Fermi gas harmonically trapped in a two-dimensional optical lattice with a single classical magnetic impurity is investigated by Bogoliubov-de Gennes equations. In spin-balanced and weak spin-imbalanced case, we show that a strong magnetic impurity can change sign of the pairing order parameter. The amplitude of the sign-changed order parameter caused by impurity is affected by the strength of impurity potential, temperature and particle density. Compared to spin-balanced case, we find that an additional in-gap bound state can be induced by a strong magnetic impurity in weak spin-imbalanced case. In strong spin-imbalanced case where the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state is established as the ground state, the impurity induces wide spatial oscillations of pairing order parameters and can enhance the order parameters by suppressing the local spin-imbalance. Our results can be used to create and manipulate the FFLO state with magnetic impurities in spin-imbalanced Fermi gases.   

Proposal for interferometric detection of the topological character of modulated superfluidity in ultracold Fermi gases

A system with unequal populations of up and down fermions may exhibit a Larkin-Ovchinnikov (LO) phase characterized by periodic domain walls across which the order parameter changes sign and the excess polarization is localized. Despite fifty years of theoretical and experimental work, there has so far been no unambiguous observation of an LO phase. We propose an experiment in which two fermion clouds, prepared with unequal population imbalances, are allowed to expand and interfere. We show that a pattern of staggered fringes in the interference is unequivocal evidence of LO physics. The resilience of these interference signatures against thermal and quantum fluctuations is also discussed, and our results are supported with time-of-flight simulations of the experiment. Y.-L. Loh and N. Trivedi, Phys. Rev. Lett. 104, 165302 (2010). M. Swanson et al., arXiv:1106.3908   

A single impurity atom in a two-dimensional Fermi gas

We consider a single impurity atom immersed in a Fermi gas in two dimensions and interacting via an attractive, short-range potential. Using variational wave functions for polarons, molecules, trimers, and quadrumers, we arrive at the ground state phase diagram as a function of mass ratio and interaction strength. We show that the phase diagram includes a Fulde-Ferrell-Larkin-Ovchinnikov phase for experimentally relevant mass ratios.   

Realization of a Resonant Fermi Gas with a Large Effective Range

We have measured the interaction energy and three-body recombination rate for a two-component Fermi gas near a narrow Feshbach resonance and found both to be strongly energy dependent. Even for deBroglie wavelengths greatly exceeding the van der Waals length scale, the behavior of the interaction energy as a function of temperature cannot be described by atoms interacting via a contact potential. Rather, energy-dependent corrections beyond the scattering length approximation are required, indicating a resonance with an anomalously large effective range. For fields where the molecular state is above threshold, the rate of three-body recombination is enhanced by a sharp, two-body resonance arising from the closed-channel molecular state which can be magnetically tuned through the continuum. This narrow resonance can be used to study strongly correlated Fermi gases that simultaneously have a sizeable effective range and a large scattering length.   

Anomalous Dimers in Quantum Mixtures near Broad Resonances:Pauli Blocking, Fermi Surface Dynamics and Implications

We study the energetics and dispersion of anomalous dimers that are induced by the Pauli blocking effect in a quantum Fermi gas of majority atoms near interspecies resonances. Unlike in vacuum, we find that both the sign and magnitude of the dimer masses are tunable via Feshbach resonances. We also investigate the effects of particle-hole fluctuations on the dispersion of dimers and demonstrate that the particle-hole fluctuations near a Fermi surface (with Fermi momentum $\hbar k_F$) generally reduce the effective two-body interactions and the binding energy of dimers. Furthermore, in the limit of light minority atoms the particle-hole fluctuations disfavor the formation of dimers with a total momentum $\hbar k_F$, because near $\hbar k_F$ the modes where the dominating particle-hole fluctuations appear are the softest. Our calculation suggests that near broad interspecies resonances when the minority-majority mass ratio $m_B/m_F$ is smaller than a critical value (estimated to be 0.136), dimers in a finite-momentum channel are energetically favored over dimers in the zero-momentum channel. We apply our theory to quantum gases of $^{6}$Li$^{40}$K, $^{6}$Li$^{87}$Rb, $^{40}$K$^{87}$Rb and $^{6}$Li$^{23}$Na near broad interspecies resonances, and discuss the implications.   

Unpolarized Fermi gas in squeezed anisotropic harmonic trap by Quantum Monte Carlo methods

Using diffusion Monte Carlo (DMC) method, we calculate the ground state properties of unpolarized Fermi gas at unitarity regime in both isotropic and anisotropic harmonic potentials. We study the effects of anisotropy by increasing the frequency in z direction $\omega_z$ of the harmonic potential while keeping the frequency in x and y direction unchanged. The true unitarity regime is obtained by extrapolating the interaction range to zero and the calculations are done using the fixed-node diffusion Monte Carlo method. The trial function is of the BCS form with the pairing function expanded in appropriate linear combinations of the anisotropic oscillator eigenstates. We evaluate the binding energies for varying particle numbers and we estimate its behavior in the limit of large number of atoms. We estimate dependence of projected density profile and momentum distribution on the X-Y plane with respect to $\omega_z$. Our results can be readily used as a benchmark for the cold atom experiment with similar experimental set-up. Supported by ARO and NSF.   

A Theory for Normal Fermi Gases at Unitarity

In this study, we will develop a simple, yet accurate, mean-field-like theory for the normal phase of a unitarity Fermi gas. First, we derive a self-consistent equation for the self-energy using a momentum-dependent coupling constant. Using zero temperature Monte Carlo results as a starting point, we then derive an analytical expression for the momentum-dependent self-energy within one-step iteration. Lastly, we determine the validity of our analytical self-energy by comparing it to fully numerical calculations. Our theory shows excellent agreement with pressure measurements made by Nascimbene, \textit{et al.} in a recent experiment performed by the ENS group.   

Session W5: Thin Film Growth and Processing

Etch induced losses in high Q-value superconducting resonators

We have investigated how the microwave loss in coplanar wave-guide titanium nitride resonators fabricated on Si wafers is affected by the choice of etch method used to pattern the resonators. Three different etches has been investigated, one fluorine based reactive etch, one chlorine based reactive ion etch, and one argon ion mill. At high microwave probe powers, the two different reactive etches show low internal loss whereas the milled samples show dramatically higher loss. At single photon powers we observe that the fluorine etch resonators exhibit substantially lower loss than the chlorine etched resonators. In the single photon limit we observe loss tangents of $1\cdot 10^{-6}$ for the fluorine etched, $4\cdot 10^{-6}$ to $5.5\cdot 10^{-6}$ for the chlorine etch and $1.4\cdot 10^{-4}$ for the argon ion mill. We compare these results to numerically calculated filling factors and find that the chlorine etch Si surface has a higher loss tangent than the fluorine etched surface. We also find that re-deposition of Silicon onto the titanium nitride surfaces is the probable cause of the high loss observed for argon ion milled resonators.   

Atomic Layer Deposition Films as Diffusion Barriers for Silver Artifacts

Atomic layer deposition (ALD) was investigated as a means to create transparent oxide diffusion barrier coatings to reduce the rate of tarnishing for silver objects in museum collections. Accelerated aging by heating various thicknesses (5 to 100nm) of ALD alumina (Al$_{2}$O$_{3})$ thin films on sterling and fine silver was used to determine the effectiveness of alumina as a barrier to silver oxidation. The effect of aging temperature on the thickness of the tarnish layer (Ag$_{2}$S) created at the interface of the ALD coating and the bulk silver substrate was determined by reflectance spectroscopy and X-Ray Photoelectric Spectroscopy (XPS). Reflectance spectroscopy was an effective rapid screening tool to determine tarnishing rates and the coating's visual impact. X-Ray Photoelectric Spectroscopy (XPS), and Time of Flight Secondary Ion Mass Spectroscopy (ToF-SIMS) analysis showed a phase transformation in the Ag$_{2}$S tarnish layer at 177\r{ }C and saturation in the thickness of the silver sulfide layer, indicating possible self-passivation of the tarnish layer.   

Normal versus anomalous roughening of electrodeposited Prussian Blue layers

Electrochemically deposited Prussian Blue films on gold over silicon substrates are studied by various microscopy methods. Film surface features and roughness scaling suggest faceted anomalous roughening. However, accounting for the time increase of adsorption rate, which reduces surface diffusion lengths as the film grows, a scenario of diffusion-dominated growth (Mullins-Herring class) emerges. A significant effect of the diffusion-to-deposition ratio on the roughness scaling is found, consistently with the close packed surface morphology and formation of a film with single crystalline grains. That effect also explains the striking difference of exponents obtained from the anomalous and normal scaling interpretations.   

Continuum Description of Atomistics for Nanomechanics of Grain Boundary Embrittlement in FCC Metals

A nonlinear field projection method has been developed to study nanometer scale mechanical properties of grain boundaries in nanocrystalline FCC metals [1]. The nonlinear field projection is based on the principle of virtual work, for virtual variations of atomic positions in equilibrium through nonlocal interatomic interactions such as EAM potential interaction, to get field-projected subatomic-resolution traction distributions on various grain boundaries. The analyses show that the field projected traction produces periodic concentrated compression sites on the grain boundary, which act as crack trapping or dislocation nucleation sites. The field projection was also used to assess the nanometer scale failure processes of Cu $\Sigma 5$ and $\Sigma 9$ grain boundaries doped with Pb. It was revealed that the most significant atomic rearrangement is dislocation emission which requires local GB slip, and some Pb locks the local GB slip and in turn, embrittles the GB. Reference: [1] C.-K. Wang, et al., 2011, \textit{MRS Proceedings}, Vol. 1297, DOI: 10.1557/opl.2011.678.   

Coherent Surface X-ray Scattering: Observation of Pt (001) Step-Flow Motion

We recently [1] observed oscillations of speckle intensities from the Pt (001) surface at high temperatures (T $>$ 1620K), persisting for tens of minutes. Using a model of hex-reconstructed terraces we showed that the observed oscillations come from surface dynamics due to sublimation induced step-flow motion. Our results demonstrate the possibility that coherent surface x-ray scattering (CSXS) can be applied to monitor the real-time evolution of surfaces. Hopefully CSXS can be extended further to in-situ ``real-world'' environments. This work and the use of the Advanced Photon Source were supported by the U.S. DOE, Office of Basic Energy Sciences under Contract No. DE-AC02-06CH11357. The work at Safarik University was supported by VEGA 1/0138/10 and VVCE-0058-007.\\[4pt] [1] M.S. Pierce, et al., Appl. Phys. Lett. 99, 121910 (2011).   

Study of Electrode Surface Dynamics Using Coherent Surface X-ray Scattering

We present successful efforts to develop a new surface x-ray scattering technique that allows in-situ measurements of surface dynamics in electrochemical systems. The technique, sensitive to the microstates of the system, can measure the transient dynamics of phase relaxation from one phase to another upon changing electrochemical conditions, as well as the equilibrium dynamics of microstates even if the electrode macroscopic state appears static. We will discuss the underlying physics of surface x-ray speckle correlation spectroscopy and present our recent study of Au (100) surface in vacuum, water, and electrolytes.   

Effect of tensile misfit dislocation on diffusion of Ni adatom on Ni/Cu(111): a Molecular Dynamics study

We apply molecular dynamics and molecular static methods to calculate the effect of tensile misfit dislocation on Ni adatom diffusion for Ni/Cu(111) system and compare the results with those calculated previously for Cu adatom on the Cu/Ni(111) system [1] which has compressive dislocation. Our Ni/Cu(111) substrate model system consists of 5 layers of Ni on top of a 7-layer Cu(111) slab which after energy minimization displays an isolated misfit dislocation buried at the Cu-Ni interface, causing the Ni film to be under tensile stress. In contrast to the isotropic trajectory that emerges on a defect-free surface, in this tensile stressed system we find that presence of the defect under the surface strongly affects the adatom trajectory, introducing anisotropy in atomic diffusion similar to compressive system, but with the difference: tensile misfit dislocation enhances diffusion in the direction perpendicular to the misfit dislocation line and decreases it in the direction parallel to it, whereas compressive dislocation induces the opposite behavior. We present the calculated energy barriers for the adatom and compare them with adatom diffusion on defect -- free and on the surface containing compressive dislocation. \\[4pt] [1] M. Aminpour, O. Trushin, and T. S. Rahman, Physical Review B, 84, 035455 (2011).   

Enhancement of Ag Cluster Mobility on Ag(111) Surface by clustering with Chlorine

Chlorine is observed to accelerate the fragmentation of Ag nanostructures deposited on graphite. To understand the role of chlorine in the stability of Ag nanostructures, we have studied the formation and diffusion of Ag$_{n}$ and Ag$_{n}$Cl$_{m}$ (n= 1 to 4) clusters on Ag(111) surface, using density functional theory (DFT) with generalized gradient approximations (GGA) and the projector-augmented wave method. Our calculation shows that the formation energies and diffusion barriers of Ag$_{n}$ clusters are both lowered by clustering with chlorine when n=1, 3 and 4, indicating the enhancement of mass transport on Ag(111) surface. (AgCl)$_{n}$ clusters (n=1, 3 and 4) are good candidates for surface mass transport units. We have also studied a chloridized Ag$_{55}$ cluster and an Ag$_{55}$-Ag$_{55}$ neck structure. Chlorine is found to loosen the Ag$_{55}$ structure and weaken the binding between the Ag$_{55}$ bulbs in the neck structure.   

Growth and Characterization of Metal Oxides Layers on CVD Graphene

Thin metal oxide layers deposited on graphene can be utilized as dielectric barriers between ferromagnetic metals and graphene to help overcome conductivity mismatch between the metal and graphene. Furthermore, these layers have been shown to increase the spin relaxation time measured utilizing non local detection and spin precession measurements. However, simply depositing metal oxide layers such as aluminum oxide on graphene results in non uniform film lowering the quality of the interface barrier. This presentation will show our work growing uniform aluminum oxide layers on graphene under ultra high vacuum conditions utilizing a Ti seed layer. The surface roughness of the films was measured with atomic force microscopy with and without titanium seed layers. The results show titanium seed layers reduced the surface roughness by a factor of 4 indicating a more uniform film. In addition, X-ray photoelectron spectroscopy results will be presented to confirm the stoichiometry of the films.   

Ab-initio study on stabilized ZnO (0001) polar surface with graphene substrate

First-principles calculations have been performed on interfaces between zinc oxide (0001) surfaces and graphene layer. By modifying the initial interface configurations we have found the energetically stable interface structure. Results indicate that the unstable polar ZnO (0001) surface could be stabilized by the graphene layer. Further analyses show that the structure stabilization could be well understood by the charge transfer between the carbon atoms and oxygen atoms. This suggests that polar ZnO (0001) surfaces can be obtained in experiments by growing them on graphene substrates.   

Session W6: Focus Session: Carbon Nanotube Synthesis, Structure and Defects

Chirality selective growth of carbon nanotubes from one-dimensional fusion of aromatic compounds

We have investigated the formation of carbon nanotubes (CNTs) from one-dimensional coalescence of various polyaromatic compounds with different edge structures within an outer tube template. Transformation of the filled precursors into an inner tube was confirmed upon high-temperature thermal annealing. These newly formed inner tubes were then extracted through ultrasonication, as reported in our previous study [1]. High resolution transmission electron microscope observations of the samples together with the photoluminescence analyses of the extracted dispersions reveal that the chirality of the inner tubes generated were greatly affected not only by the edge structures but also by the intermediates formed. In particular, graphene nanoribbon-like intermediates obtained from perylene-derivative can be preferentially converted into near-zigzag CNTs with (8,1) chirality. The present method may provide the controlled growth of chiral-specific CNTs, which has been difficult to achieve using ordinary synthesis approaches.\\[4pt] [1] Y. Miyata, et al., ACS Nano 2010, 4, 5807.   

Hierarchical 3D microstructures from pyrolysis of epoxy resin

Nature is replete with examples of microscale dendrites connected to tree-like backbones ranging from the overall structures of trees to vascular networks. These branched structures have emerged as a result of an optimization between the maximization of a surface area and the minimization of transport losses. Elucidating these sophisticated designs proposed by nature is of paramount importance for the creation of higher-efficiency materials. The fabrication of such structures is however particularly challenging at small scale. In this paper, we focus on amorphous carbon microstructures, which provide a wide electrochemical stability window, excellent bio-compatibility, and cost-effective fabrication. However, relatively few methods have been developed for the fabrication of hierarchical amorphous carbon microstructures.Here we show that novel anisotropic microarchitectures comprising vertically aligned amorphous carbon nanowires CNWs can be made by oxygen plasma treatment of epoxy resins, followed by pyrolysis. Interestingly, these structures can also be shaped into deterministic three-dimensional (3D) hierarchical structures where nanowires are anchored to a microsized solid carbon core. These structures could play a key role in the development of new electrodes for microsensors, bioprobes, batteries, and fuel cells.   

Semiconducting nanotube dominant chemical vapor deposition synthesis of isopropanol carbon feedstock

The development of guided chemical vapor deposition (CVD) growth of single wall carbon nanotubes provides great platform for wafer-scale integration of aligned nanotube into circuits and systems. However, the co-existence of the metallic and semiconducting nanotubes is still a major problem for the development of carbon nanotube based nanoelectronics. To address this limitation, we developed a method to get semiconducting dominant nanotube by using isopropanol carbon feedstock. We achieved a purity of 87{\%} of semiconducting nanotube growth, which was verified by measuring single nanotube transistors fabricated from aligned nanotube arrays. Besides, Raman spectrum was characterized to confirm the enhanced fraction of semiconducting nanotube as well. To further understand chemical mechanism of synthesis at atomic level, we performed the mass spectrum study and compared the measurement results from different carbon source. Furthermore, to discuss the future application of this synthesis method, we fabricated thin-film transistor from as-grown nanotube network. Transistor with on/off ratio over 104 and mobility up to 116 cm2/v\textbullet s was achieved, which shows great potential for thin-film transistor applications.   

Synthesis and Characterization of Carbon Nanotubes Produced From Thermal Decomposition of Nickellocene

We have employed a direct thermal deposition technique, which used Nickellocene both as the catalyst as well as the carbon source, to grow films of carbon nanotubes (CNT). The CNT films obtained using this procedure were characterized using Transmission Electron Microscopy which indicated the presence of thin diameter carbon nanotubes as well as single walled CNT ropes. Volumetric adsorption measurements were performed to determine the porosity and specific surface areas of these samples. Electrical transport measurements performed on long ropes of CNTs extracted from these bulk films will be presented and will be discussed in the framework of transport theories of quasi-one dimensional systems.   

Covalently Functionalized Carbon Nanotubes for Electronics

Covalent chemistry on carbon nanotubes generates useful and stable functionalities, but it also generally damages their electronic properties, which is a critical drawback for device applications. Here we present two approaches to achieve covalent functionalization of carbon nanotubes without compromising on their electronic properties. For each case, we demonstrate the fabrication of functional carbon nanotube devices. First, double-walled carbon nanotubes (DWNTs) are functionalized using a monovalent reaction with aryldiazonium salts. Absorption and Raman spectroscopy along with electrical measurements show that the functionalization occurs strictly on the outer wall and preserves the optical and transport properties of the inner wall. Functionalized-DWNT devices are operated with similar characteristics as pristine single-walled carbon nanotube (SWNT) devices [1]. Second, SWNTs are functionalized with different addends using a divalent carbene reaction. For both metallic and semiconducting species, electrical measurements of numerous functionalized and unfunctionalized SWNT devices show identical characteristics. Ref: [1] Bouilly D. et al. ACS Nano, 5 (6), 4927 (2011)   

Mechanocapillary forming of carbon nanotube microstructures

The hierarchical structure and organization of filaments within both natural and synthetic materials can determine a wide variety of collective chemical and physical functionalities. Carbon nanotubes (CNTs) are known for their record properties, and densely packed CNTs are therefore expected to enable new materials having outstanding multifunctional performance. However, it remains a significant challenge to build highly ordered assembles of CNTs, and this challenge has largely limited the design and properties of macroscale CNT yarns and sheets, and CNT-based surfaces and interfaces. We have created a versatile technique called \textit{capillary forming} to manipulate patterned vertically aligned (VA-) CNTs into diverse 3D microarchitectures, and to enable their integration in applications ranging from microsystems to macroscale functional films. Capillary forming relies on shape-directed capillary rise during solvent condensation, followed by evaporation-induced shrinkage. Three-dimensional transformations result from shrinkage of the vapor-liquid-solid interface and the resulting heterogeneous strain distribution in the microstructures. A portfolio of microscale CNT assemblies with highly ordered internal structure and freeform geometries including straight, bent, folded and helical profiles, are fabricated using capillary forming. The mechanical stiffness and electrical conductivity of capillary formed CNT micropillars are 5 GPa and 10$^4$ S/m respectively. These values are at least hundred-fold higher than as-grown CNT forests and exceed the properties of typical microfabrication polymers. Finally, the potential applications of these structures are demonstrates as vertical microsprings with geometrically tunable compliance, and hydrogel-driven microtransducers.   

{\em Ab initio} studies of defects in carbon nanofoam structures

We combine {\em ab initio} density functional and GW calculations to study the stability, electronic and elastic properties of hypothetical cellular foam-like carbon nanostructures. These systems with a mixed sp$^2$/sp$^3$ bonding character are structurally related to bundles of carbon nanotubes. The cross-sectional honeycomb structure may accommodate the same type of structural defects as the honeycomb lattice of graphene. The infinite 3D foam structure is a narrow-gap semiconductor, with the binding energy of atoms nearly 0.5 eV lower than graphene. Quasi-2D layered foam structures of finite thickness may be stabilized by terminating caps. When exposed to hydrostatic pressure, the unit cells of the foam deform significantly from their initially hexagonal shape.   

Energetical Stability of Near-Armchair Carbon Nanotubes: A Systematic First-Principles Study

We perform the systematic first-principles study of 57 kinds of carbon nanotubes (CNTs) and investigate the energetical stability of so-called ``near-armchair'' CNTs. The density functional theory computational code which utilizes the helical symmetry and has been developed in our group is extensively used in the present work. Because the computational effort can be drastically reduced by using the helical symmetry of CNTs, the systematic study of fully-optimized CNTs including the experimentally abundant CNTs was finally achieved in this work. As a result, it is found that ``near-armchair'' CNTs including (6,5) and (7,5) CNTs are energetically more stable than other CNTs. This result corresponds well with the high abundance of these near-armchair CNTs experimentally reported so far. In addition, by performing the systematic analysis of CNT bond-lengths and angles, the presence of the geometrical family pattern after their structural optimizations has been revealed for the first time. It is also confirmed that the geometrical optimization plays a very important role in predicting electronic structures of chiral CNTs as well as achiral CNTs. The fundamental gap corrections associated with the geometrical optimizations are sizable even in one nm diameter CNTs.   

Diameter and Chiral Selective Purification of SWNT and DWNT Using CO$_2$

Oxidation of carbon nanotubes in air is a well known method to eliminate carboneous impurities from raw material. Recently, it has also been used to selectively remove single-wall carbon nanotubes (SWNT) present in double-walled carbon nanotubes (DWNT) soot[1]. Here, we propose a more efficient purification process for both SWNT and DWNT based on a high temperature oxidation in a pure CO$_2$ gas flow. This treatment, combined with a standard reflux in nitric acid, provides fast oxidation of amorphous carbon and removal of other impurities without affecting the structure of the carbon nanotubes. Parameterization of the treatment allowed us to observe both diameter and chirality dependence of the nanotubes reaction rate with CO$_2$. This selective character was applied to produce thin films of clean and highly enriched DWNT and of semiconducting SWNT. Microscopy and spectroscopy analyses will be shown and reveal that those films are composed of very high quality carbon nanotubes (micrometers long, very low impurity and catalyst concentration, low Raman $I_D/I_G$ ratios). Also, no significant nanotube shortening is observed following the different diameter or chirality enrichment treatments.\\*[1em] [1] K. Iakoubovskii et al., J. Phys. Chem. C, 112, 30 (2008)   

Radial Elasticity Measurement of Single-walled Carbon Nanotubes by Atomic Force Microscopy

By applying well-calibrated tapping mode and contact mode AFM upon horizontally aligned SWCNTs grown directly on quartz substrates, we have obtained effective radial modulus (E$_{radial})$ of SWCNTs with diameters less than 2 nm. The measured E$_{radial}$ decreases from 57 to 9 GPa with the increase of the SWCNT diameter from 0.92 to 1.91 nm. Our experimental result is consistent with the computational data obtained using the modified molecular structure mechanics model. We have also compared our measurements with the reported experimental results obtained on SWCNTs with diameters from 2 to 3 nm. Our measurements of large diameter SWCNTs (diameter close to 2nm) are in agreement with the reported data and modeling. However, our measurements of SWCNTs with smaller diameters deviate from this previous study. This has been explained by the compressibility of the substrate and the AFM tip. By employing Hertzian theory, we specifically exploit the components of deformation of the AFM tip-SWCNT-substrate system. Further calculation using our measured E$_{radial}$ indicates that under the same normal force the deformation of our quartz-SWCNT-silicon tip system can be as much as 96{\%} more than the deformation of the SWCNT compressed between a rigid substrate and an AFM tip.   

Synthesis and high temperature stability of amorphous Si(B)CN-MWCNT composite nanowires

We demonstrate synthesis of a hybrid nanowire structure consisting of an amorphous polymer-derived silicon boron-carbonitride (Si-B-C-N) shell with a multiwalled carbon nanotube core. This was achieved through a novel process involving preparation of a boron-modified liquid polymeric precursor through a reaction of trimethyl borate and polyureasilazane under atmospheric conditions; followed by conversion of polymer to glass-ceramic on carbon nanotube surfaces through controlled heating. Chemical structure of the polymer was studied by liquid-NMR while evolution of various ceramic phases was studied by Raman spectroscopy, solid-NMR, Fourier transform infrared and X-ray photoelectron spectroscopy. Electron microscopy and X-ray diffraction confirms presence of amorphous Si(B)CN coating on individual nanotubes for all specimen processed below 1400 degree C. Thermogravimetric analysis, followed by TEM revealed high temperature stability of the carbon nanotube core in flowing air up to 1300 degree C.   

Session W7: Focus Session: Graphene Devices - Mechanical Effects

Nanomechanical Field-Effect Magnetometry in Graphene

This presentation will describe our studies of graphene nanomechanical resonators in the quantum Hall (QH) regime. We observe strong magneto-mechanical coupling to de Haas-van Alphen oscillations of magnetization, resulting in oscillatory frequency shifts of up to 1 MHz. This response is over two orders of magnitude larger than the expected response in a ``standard'' torque magnetometry framework. Instead, we find that the electric field-effect modulation of the magnetic energy provides a gradient force without a magnetic field gradient. Modeling of the effect produces excellent agreement with experiment, using only the disorder as a free parameter. We further use this novel mechanism to quantify the many-body exchange interaction of broken-symmetry QH states. This new mechanism may prove to be a useful tool for magnetic studies across low-dimensional materials and in sensing applications.   

High frequency graphene resonators

We study mechanical resonance properties of suspended graphene devices through radio frequency electromechanical~downmixing techniques. Taking advantage of graphene's atomically thin nature, unusually large transconductance, and extremely high Young's Modulus, we have fabricated high-frequency suspended graphene resonators.~Our global-gate devices have resonated at over 500MHz in a low temperature environment with reasonable quality factors. This research will pave the way for high-quality resonators in the GHz regime which will be used in high frequency applications such as ultra high sensitive mass, chemical and charge detectors.   

Optical Detection of Vibrations and Mass Loading of Graphene Mechanical Resonators Compatible with TEM and AFM

We produce arrays of exceptionally clean, suspended graphene mechanical resonators in high-yield using a simple, polymer-free procedure by transferring CVD-graphene to a flexible perforated substrate. The lack of a backing substrate facilitates Transmission Electron Microscopy (TEM) characterization, yet the membranes are still compatible with Atomic Force Microscopy (AFM) studies. We detect mechanical vibrations of resonators through optical interferometry and find excellent agreement with a theoretical model based on the 2D wave equation. We find quality factors of 1.25 $\mu$m diameter circular membranes to be as high as Q $\sim$ 800-1000 at room temperature, and observe lifting of mode degeneracies in square membranes. TEM and AFM studies reveal graphene folding, nanoparticle contamination, holes, tears, and other defects that can lead to the observed degeneracy splitting in square membranes. Controlled mass loading is also explored to suppress certain vibrational modes and tune vibrational frequencies for possible high density archival memory applications. The graphene membrane devices reported here open up a host of new possibilities for correlating TEM and AFM studies of an individual graphene membrane to its performance and properties as a mechanical resonator.   

Graphene xylophone: Mechanical properties of graphene based nano resonators

Mechanical properties of graphene xylophone structure were investigated. The arrays of the graphene ribbon patterns were prepared using nano lithography and suspended graphene structure was realized on the pre-patterned trench by the micro contact transfer printing method. Xylophone like structure was prepared for studying length and thickness dependence of mechanical properties of graphene resonator. The various mechanical behaviors, such as frequency tuning, non-linearity and bistability of single, bi and multi layer graphene structure will be discussed in this presentation. The potential application of our graphene xylophone structure for RF component will be suggested in the end of this presentation.   

Cavity Optomechanics with Graphene Resonators

Optical manipulation of micromechanical and nanomechanical resonators promises control of quantum states of macroscopic systems, among other applications. Because the spring constant of a resonator scales with its mass, there are advantages associated with using the lightest possible membranes as the mechanical elements. Here, we demonstrate that graphene, a one-atom-thick membrane, can be used as the mechanically active part of an optomechanical system. We show that a laser coupled to a Fabry-Perot cavity between a graphene resonator and a reflective backplane can both enhance and damp graphene motion. The enhancement of resonator motion is sufficient to induce self-oscillation, which is useful for applications in sensing and signal processing. These experiments demonstrate that graphene resonators are useful for optomechanical applications and show promise for resonator cooling toward the quantum ground state.   

Ultrathin circular membranes with high quality factors

We have fabricated large ultrathin circular drum resonators from monolayer graphene (up to 90 $\mu $m in diameter) and silicon nitride ($\sim $15 nm, up to 1mm in diameter). Resonant frequency, quality factor (Q) of different modes of these self tensioned graphene drums and high tensile stress ($\sim $ 1GPa) silicon nitride were measured using optical interferometric detection technique. We measured extremely high quality factors (up to 5,000 for graphene and up to 4,000,000 for ultrathin silicon nitride membranes) at room temperature. High quality factors observed in these resonators indicate dissipation mechanisms which differ from the conventional high surface to volume ratio resonators that show very low quality factors. The measured mechanical dissipation (Q$^{-1})$ shows a strong size and modal dependence, possibly indicating the influence of clamping losses in these tensioned membranes. These findings pave the way for identifying optimum size and modes for achieving high Q oscillators for applications in mass sensing and optomechanical coupling experiments which are underway.   

Novel Fabrication Techniques for Wafer-Scale Graphene Drum NanoElectroMechanical Resonators

Graphene NanoElectroMechanical Systems (NEMS) have shown excellent mass sensitivity as well as resonant and oscillatory behaviors that are desirable in mass sensors and active elements in Radio Frequency Integrated Circuit (RFIC) design. Out of many structures proposed for graphene NEMS, it has been recently shown that a drum resonator exhibits higher Q-factor than other structures such as a bar resonator. However, fabricating a large array of drum graphene resonator has been problematic because liquid or gas can be trapped inside the drum. Such issues led to designs with a hole in the center of a drum or with a drainage trench, either at the cost of additional lithography step or lowered Q-factor. Here, we demonstrate two novel fabrication methods that are free of the trapping without any compromise in additional lithography step or Q-factor degradation. In one method, wafer scale graphene is dry-stamped on prefabricated leads, holes and local gates. In the other method, an resist strip with a circular hole at the center holds graphene underneath. I will discuss direct electrical readout and characterization of devices using these two methods. These drum structures may provide a practical way to achieve wafer scale high Q graphene NEMS.   

Graphene NanoElectroMechanical Oscillator

Graphene based NanoElectroMechanical Systems (NEMS) working in Radio Frequency (RF) regime possess considerable advantages own to the remarkable electrical and mechanical properties of this atomic thin material. Here we demonstrated self-sustained nanoelectromechanical oscillator made from graphene. Our recent developed transduction scheme enables the direct conversion from mechanical motion to electrical domain, then subsequently being fed back to the system as excitation. The absence of extra RF actuation and the stable performance shows the possibility of practical application of graphene oscillator, for example, as signal filters, mass sensors or timing devices.   

Uniaxially Strained Graphene Resonators

Strained graphene allows one to explore the connection between the mechanical and electrical properties of graphene. Exploring this interplay between the mechanical and electrical properties in graphene may enable tuning graphene's mechanical and electrical properties as well as opening up new exotic electronic states. We have developed a technique to fabricate uniaxial strained graphene transistors and mechanical resonators with strains as high as 0.5{\%}. We demonstrate how strain perturbs both the mechanical and electrical properties of graphene, highlighted by the strain quenching of flexural phonons.   

Local Optical Probe of Motion and Stress in a NEMS

Nanoelectromechanical systems (NEMSs) are emerging nanoscale elements at the crossroads between mechanics, optics and electronics, with significant potential for actuation and sensing applications. The reduction of dimensions compared to their micronic counterparts brings new effects including sensitivity to very low mass, resonant frequencies in the radiofrequency range, mechanical non-linearities and observation of quantum mechanical effects. An important issue of NEMS is the understanding of fundamental physical properties conditioning dissipation mechanisms, known to limit mechanical quality factors and to induce aging due to material degradation. There is a need for detection methods tailored for these systems which allow probing motion and stress at the nanometer scale. Here, we show a non-invasive local optical probe for the quantitative measurement of motion and stress within a multilayer graphene NEMS provided by a combination of Fizeau interferences, Raman spectroscopy and electrostatically actuated mirror. Interferometry provides a calibrated measurement of the motion, resulting from an actuation ranging from a quasi-static load up to the mechanical resonance while Raman spectroscopy allows a purely spectral detection of mechanical resonance at the nanoscale. Such spectroscopic detection reveals the coupling between a strained nano-resonator and the energy of an inelastically scattered photon, and thus offers a new approach for optomechanics.   

Nonlinear Damping Mechanism in Mechanical Graphene Resonators.

Based on a continuum mechanical model for single-layer graphene\footnote{J. Atalaya, A. Isacsson, and J. M. Kinaret, Nano Letters 8, 4196 (2008).} we propose and analyze a microscopic mechanism for dissipation in nano-electromechanical graphene resonators. We find that coupling between flexural modes and in-plane phonons leads to linear and nonlinear damping of out-of-plane vibrations. By tuning external parameters, such as static gate voltage, one can cross over from a linear to a nonlinear-damping dominated regime. We discuss how the effective quality factor depends on parameters such as temperature, and compare our results with recent experiments.   

Graphene Nanoelectromechanical Systems are Unique

Graphene Nanoelectromechanical systems (GNEMS) with their light mass show a lot of interesting novel physics effects compared to conventional Nanoelectromechanical systems (NEMS). Superior gate tunability of the order of $>$10MHz/V at room temperature as well as high quality factors of $\sim$10$^5$ at mK temperatures for the fundamental mode have already been obtained. Studying their high frequency modes, we have observed for the first time avoided crossing between graphene modes as well as between graphene modes and their suspended contacts. In addition we have studied the atypical variation of the mode dispersion versus gating resulting from tensioning and electrostatic coupling to the gate. In particular, we measure an additional frequency shift near 0Vg which could be due to a strong electromechanical coupling near the Dirac Point.   

Current-induced forces in graphene-based nanoelectromechanical systems

Transport currents have distinct effects on the vibrational dynamics of nanoelectromechanical systems. Recently, we have developed a comprehensive scattering-matrix approach to treat out-of-equilibrium current-induced forces [c.f. Phys. Rev. Lett. 107, 036804 (2011)]. We apply our method to the vibrational dynamics of a suspended graphene membrane, paying special attention to the different coupling mechanisms between Dirac fermions and flexural modes in graphene.   

Session W8: Focus Session: Frustrated Magnetism - Pyrochlore lattice

Topological phases in R2Ir2O7

We construct and analyze a theoretical model for the pyrochlore iridates R$_2$Ir$_2$O$_7$ with R (= Pr, Nd, Sm, Gd, Tb, Dy, Ho, Yb) magnetic. The electrons on trivalent rare earth ions R$^{3+}$ form local Ising doublets due to the local crystal field. Based on a space group symmetry analysis, we write down the generic Kondo coupling between the Ising spin at R sites and the effective spin at Ir sites. Besides this interaction, we also include direct electron tunneling between Ir sites and indirect electron tunneling via intermediate oxygens for Ir-Ir coupling. This simple minimal model gives a rich phase diagram with broad regions of topological semi-metal and axion insulator phases. Based on these findings, we propose R$_2$Ir$_2$O$_7$ to be one of the most promising candidates to realize the topological semi-metal and axion insulator phases. Implications for existing and future experiments are discussed.   

Muon Spin Relaxation Studies of Pyrochlore Iridates R$_2$Ir$_2$O$_7$, (R= Y, Yb, and Nd)

We report results for zero field muon spin relaxation measurements over the range 2 K $<$ T $<$ 220 K taken at the ISIS facility for three members of the pyrochlore iridate family, R$_2$Ir$_2$O$_7$, comprised of both magnetic (R = Yb, Nd) and non-magnetic (R = Y) species. The formation of a static internal field is detected via a loss of asymmetry below the bifurcation temperature observed in magnetic susceptibility for all three compounds. We will present results describing the nature of this internal field based on short time muon depolarization data taken at PSI. Longitudinal field measurements reveal the internal field is of order 1000 G for all three materials, and the existence of dynamic fluctuations at 2 K for R = Yb, Nd; the depolarization is quasistatic near 2 K for R = Y. We discuss these results as applied to understanding the unusual magnetic and transport properties observed in the iridate family, and the implications regarding long range magnetic order and the low temperature ground state.   

Chiral RKKY interaction in Pr2Ir2O7

Pr$_2$Ir$_2$O$_7$ experimentally realizes a chiral spin liquid selected out of a degenerate quantum spin-ice manifold. We propose that such a chiral state is selected by an analogue of the magnetic RKKY effect, whereby the chiral fluctuations of conducting Ir electrons close to a Mott transition promote correlations between the chirality of the local Pr moments. Pr$_2$Ir$_2$O$_7$ provides the perfect setting for such a \emph{chiral RKKY} effect, as its spin ice manifold naturally contains chiral states, and chiral fluctuations in the Ir are enhanced by the proximity to the metal insulator transition between Pr$_2$Ir$_2$O$_7$ and Nd$_2$Ir$_2$O$_7$. We calculate the chiral susceptibility within a simplified model, showing that this chiral RKKY coupling can be ferro-chiral and estimating the magnitude.   

Emergent spin superstructures and degeneracy in models of itinerant magnets

In recent years, there has been immense research interest in frustrated magnets with metallic character, such as the pyrochlores R$_2$Mo$_2$O$_7$, where R denotes a rare-earth element (Science {\bf 291}, 2573 (2001)). The frustration in magnetic sector in such systems can have interesting consequences for the electronic properties. More interestingly, the electronic degree of freedom can aid the system in selecting unconventional magnetic states. Motivated by these possibilities we investigate the double-exchange model with competing super-exchange interactions on various lattices. On a triangular lattice we find a novel chiral spin state to be the ground state at half filling (PRL {\bf 105}, 216405 (2010)). On checkerboard and Kagome lattices, the itinerant electrons play a crucial role in lifting the degeneracy of the magnetic sector and select specific ground states with intriguing superstructures. A very interesting effect takes place on a honeycomb lattice, where a Yafet-Kittel type magnetic structure emerges from the interplay between the super-exchange and double-exchange interactions (PRL {\bf 107}, 076405 (2011)). Some of these emergent spin states have a macroscopic degeneracy related to a symmetry which is intermediate between local and global.   

Resolving magnetic frustration in a Laves lattice

CeFe2 is a ferromagnet that exhibits antiferromagnetic fluctuations in its ground state. We use x-ray diffraction and diamond-anvil-cell techniques to directly measure the transition to antiferromagnetism in pure CeFe2 at high pressure which couples to the change in the lattice symmetry. Numerical simulations are adopted to identify the magnetic structure of the ground states and to quantitatively illustrate effects of competing magnetic energy scales and geometrical frustration on the magnetic phase diagram. Comparison of phase transitions under both chemical substitution and applied pressure suggests a general solution to the physics of Laves phase magnets.   

Neutron Diffraction Investigation of Magnetic and Orbital Order in FeV$_{2}$O$_{4}$

The vanadium spinels, AV$_{2}$O$_{4}$, with divalent cations on the diamond sublattice are model magnetic systems for the study of interacting orbital, lattice and spin degrees of freedom. Studies of both systems with diamagnetic (e.g. Zn$^{2+}$, Cd$^{2+}$, Mg$^{2+})$ and spin-only (e.g. Mn$^{2+})$ cations on the A-site sublattice have revealed multiple phase transitions and ground state properties heavily influenced by V$^{3+}$ orbital degrees-of-freedom. I will report on neutron powder diffraction measurements of another spinel system, FeV$_{2}$O$_{4}$, which additionally has two-fold orbitally degenerate Fe$^{2+}$ cations on the A-site sublattice. Previous x-ray and Mossbauer studies have reported four structural phase transitions in this material and at least one magnetic transition. Our data confirm the existence of three structural transitions and reveal distortions of local polyhedra with important implications for orbital order. We confirm the existence of hypothesized collinear antiferromagnetism below a temperature T$_{N1}$=110K and further identify a second magnetic transition at T$_{N2}$=60K where V$^{3+}$ moments cant away from the Fe$^{2+}$ spin direction to form a 2-in-2-out spin structure on the pyrochlore sublatice. I will discuss these observations in the context of recent predictions for orbital order in vanadate spinels.   

Universal exchange-driven phonon splitting

We report on a linear dependence of the phonon splitting on the non-dominant exchange coupling $J_{nd}$ in the antiferromagnetic monoxides MnO, Fe$_{0.92}$O, CoO and NiO, and in the highly frustrated antiferromagnetic spinels CdCr$_{2}$O$_{4}$, MgCr$_{2}$O$_{4}$ and ZnCr$_{2}$O$_{4}$. For the monoxides our results directly confirm the theoretical prediction of a predominantly exchange induced splitting of the zone-centre optical phonon [1,2]. We find the linear relation $\hbar\Delta\omega = \beta J_{nd} S^2$ with slope $\beta$ = 3.7. This relation also holds for a very different class of systems, namely the highly frustrated chromium spinels. Our finding suggests a universal dependence of the exchange-induced phonon splitting at the antiferromagnetic transition on the non-dominant exchange coupling [3].\\[4pt] [1] S. Massidda et al., Phys. Rev. Lett. 82, 430 (1999).\\[0pt] [2] W. Luo et al., Solid State Commun. 142, 504 (2007).\\[0pt] [3] Ch. Kant et al., arxiv:1109.4809.   

First-principles study of the structural and magnetic phase transition in CdCr$_2$O$_4$

We use first-principles calculations to investigate the mechanism for the paramagnetic (PM) cubic to an antiferromagnetic (AFM) tetragonal structural phase transition at 7.8 K in the spinel oxide compound CdCr$_2$O$_4$. Because of the magnetic frustration associated with AFM interactions on the pyrochlore lattice formed by the Cr ions, we focus on the spin-lattice coupling, specifically (a) the forces on the atoms and resulting atomic displacements induced by the spin configuration and (b) the spin-phonon coupling. Using the linear response method, we determine the full phonon dispersion relations for the PM, FM and AFM orderings in cubic and tetragonal structures. We determine the strength of spin-phonon couplings using IFCs for various magnetic orderings and show that the spin-phonon couplings are large but do not lead to any unstable modes that could alone drive the structural transition at low temperatures. Instead, we find that it is the symmetry-lowering forces and stresses induced by AFM ordering that drive the cubic to tetragonal phase transition at low temperature. The results are compared with a recent experimental determination of the phonon dispersion in the in the cubic and tetragonal phases of CdCr$_2$O$_4$ and the implications for related compounds discussed.   

Frustration and Jahn-Teller ordering in magnetic spinel oxides

Geometrically frustrated magnetic systems have become extremely important for the study of novel phenomena often present in highly degenerate magnetic ground states. Extensive study of the canonically frustrated ZnCr$_2$O$_4$ and MgCr$_2$O$_4$ has increased the understanding of frustration in three-dimensional systems, however little is known about the structural effect of dilute magnetism on the $A$ site. We present the effects on magneto-structural coupling of dilute magnetic cations on the diamagnetic $A$ site of ZnCr$_2$O$_4$ and MgCr$_2$O$_4$. In ZnCr$_2$O$_4$ and MgCr$_2$O$_4$, a spin-driven structural distortion concomitant with the onset of long-range magnetic order resolves the large ground state degeneracy at temperatures far below the theoretical Curie-Weiss temperature. Employing variable temperature high-resolution synchrotron powder diffraction studies and magnetic susceptibility measurements, we show the changes in spin-lattice coupling in the solid solutions Zn$_{1-x}$Co$_x$Cr$_2$O$_4$, Zn$_{1-x}$Cu$_x$Cr$_2$O$_4$ and Mg$_{1-x}$Cu$_x$Cr$_2$O$_4$ for $x \leq 0.2$. These results highlight the effect of dilute $A$ site magnetism on magnetic frustration as well as the role of dilute Jahn-Teller active Cu$^{2+}$ on spin-lattice coupling in these frustrated systems.   

Ultrasound Velocity Measurements in the Geometrically Frustrated Spinel MgCr$_2$O$_4$

Magnesium chromite spinel MgCr$_2$O$_4$ is a geometrically frustrated magnet with the N$\acute{e}$el temperature $T_N\simeq$13 K, and the Weiss temperature $\theta_W = -390$ K. Recent inelastic neutron scattering experiments provided a compelling evidence for the spin molecular ground states in not only the paramagnetic phase but also the antiferromagnetic phase. We performed ultrasound velocity measurements of MgCr$_2$O$_4$ in all the symmetrically independent elastic moduli of $C_{11}$, $(C_{11}-C_{12})/2$, and $C_{44}$. Temperature dependence of all of these elastic moduli exhibits a remarkable softening in the paramagnetic phase. Taking into account the absence of orbital degrees of freedom in Cr$^{3+}$ (3$d^3$) in MgCr$_2$O$_4$, the spin degrees of freedom should play a significant role for the elastic softening. The most probable origin for the elastic softening in the paramagnetic phase is the strong coupling of the acoustic phonons to the molecular spin fluctuations.   

Magnetic correlations and magnetic order in the pyrochlore Er$_2$Ti$_2$O$_7$

We analyse short-range magnetic correlations in the pyrochlore magnet Er$_2$Ti$_2$O$_7$. Four unique nearest-neighbour exchange interactions are permitted by the space group symmetry of the pyrochlore lattice; the four corresponding nearest-neighbour exchange constants for Er$_2$Ti$_2$O$_7$ are extracted from diffuse neutron scattering maps. Low-temperature magnetic order in Er$_2$Ti$_2$O$_7$ is discussed in light of these results. The results are compared to recently published values for the sister compound Yb$_2$Ti$_2$O$_7$, which has similar features.   

Low Temperature Spin Structure of Gadolinium Titanate

Many rare earth pyrochlore oxides exhibit exotic spin configurations at low temperatures due to frustration. The nearest neighbor coupling between spins on the corner-sharing tetrahedral network generate geometrical magnetic frustration. Among these materials, gadolinium titanate (Gd2Ti2O7) is of particular interest. Its low temperature ordered phases are not yet understood theoretically. Bulk thermal measurements such as specific heat and magnetic susceptibility measurements find two phase transitions in zero external field, in agreement with simple mean field calculations. However, recent neutron scattering experiments suggest a so-called 4-k spin structure for intermediate phase and a so called canted 4-k structure for lower temperature phase that does not agree with either mean-field theory or Monte Carlo simulation which find the 1-k state and Palmer-Chalker state respectively as the lowest free energy configuration for those phases. In our work, we study the 4-k structure in detail and present a new phase diagram for dipolar Heisenberg spins on a pyrochlore lattice, certain portions of which describe gadolinium titanate.   

Elastic properties of the titanate pyrochlore Tb$_{2}$Ti$_{2}$O$_{7}$

The presence of geometric frustration inhibits the formation of long-range spin ordering in Tb$_{2}$Ti$_{2}$O$_{7}$ even at very low temperatures, making this compound the prototype of a ``spin liquid.'' We have initiated a study of the elastic properties of Tb$_{2}$Ti$_{2}$O$_{7}$ as a function of temperature (0.5-300 K) and magnetic field (0-15T) using Resonant Ultrasound Spectroscopy (RUS). All three elastic constants show a pronounced softening below 50 K, indicative of a possible structural transition at very low temperatures. Application of a magnetic field partially suppresses the elastic softening in this compound, suggesting that the magnetoelastic coupling plays a significant role in the spin liquid physics of Tb$_{2}$Ti$_{2}$O$_{7}$ at low temperatures.   

Imaging Magnetic Order in Magnetostructural Phases of Mn3O4

Frustration in A-site spinels due to the competition between complex structure and strong interactions has been the focus of many theoretical and experimental studies recently. Mn3O4 is one such material with a three way interplay between complex lattice geometry, strong spin-lattice coupling and magnetic interactions. Mn3O4 is known to have two distinct phases below the Neel temperature that differ in both structure and magnetic order, including a tetragonal phase with disordered spins, and at lower temperatures, an orthorhombic phase exhibiting long-range commensurate magnetic order. In this talk, we present a nanometer-scale imaging investigation of the transition between these two phases. With cryogenic magnetic force microscopy (MFM), we observe novel magnetic stripe patterns accompanying this phase transition. Complementary electron backscatter diffraction (EBSD) measurements indicate that the magnetic patterns are closely related to local crystalline orientation. The onset and spatial periodicity of the magnetic patterns show variations with temperature and external magnetic field. We will also discuss possible causes of this phenomenon and their implications.   

Spin state of Mn$_{3}$O$_{4}$ investigated by $^{55}$Mn NMR

The $^{55}$Mn nuclear magnetic resonance spectrum for spinel oxide Mn$_{3}$O$_{4}$ was measured in the temperature range of 6 K - 30 K to investigate the spin structure in the ground state. The spectrum consists of three peaks in the frequency range of 250 - 265 MHz, which corresponds to the hyperfine field range of 24 - 25 T. The temperature dependence of the spectrum and the rf enhancement factor show that Mn$^{3+}$ ions have two different magnetic moments, one of which is strongly related with the commensurate-incommensurate phase transition. This is consistent with the picture of two magnetic moments, R and S, claimed from the result of a neutron experiment. The dipolar hyperfine field was calculated to explain the splitting of two peaks coming from R and S and to estimate the magnetic moments. The spin-spin relaxation time has a frequency dependence that induces spectrum broadening and further splitting of the peak coming from S, indicating that the Suhl-Nakamura interaction is the major relaxation mechanism in Mn$_{3}$O$_{4}$.   

Session W9: Focus Session: Complex Bulk Oxides: Multiferroics

Evolution of spin wave excitations in Sr$_{2}$FeSi$_{2}$O$_{7}$ under an external magnetic field

Evolution of static and dynamic spin correlations in a new multiferroics material Sr$_{2}$FeSi$_{2}$O$_{7}$ under an external magnetic field was investigated by elastic and inelastic neutron scattering techniques. An external magnetic field up to $B$ = 14 Tesla induces four different magnetic and ferroelectric phases in Sr$_{2}$FeSi$_{2}$O$_{7}$. The static magneto-electric coupling can be understood as the spin-dependent metal-ligand hybridization proposed for a related material Ba$_{2}$CoGe$_{2}$O$_{7}$. By analyzing the inelastic neutron scattering data obtained from a single crystal of Sr$_{2}$FeSi$_{2}$O$_{7}$ without field, we have determined the effective spin Hamiltonian in this material that includes isotropic nearest neighbor exchange interaction in the two-dimensional Fe square plane and easy plane single ionic anisotropies. The spin wave excitations show interesting changes as upon ramping up the system enters the field-induced phases for $B \quad >$ 6.5 Tesla, which will also be discussed.   

THz spectroscopy of spin waves in multiferroic Ba$_2$CoGe$_2$O$_7$ in high magnetic fields

By applying external magnetic field the square-lattice antiferromagnet Ba$_2$CoGe$_2$O$_7$ can be transformed to a chiral form, evidenced by large optical activity when the light is in resonance with spin excitations at sub-terahertz frequencies. We found that the magnetochiral effect, the absorption difference for the light beams propagating parallel and anti-parallel to the applied magnetic field, has an exceptionally large amplitude close to 100\% and persists to fields up to 30\,T. All these features are ascribed to the magnetoelectric nature of spin excitations as they interact both with the electric and magnetic components of light. We observe a spin flop at 15\,T, that is consistent with our theoretical calculations.   

Ground State Properties and Magnetodielectric Coupling in Sm$_{0.5}$Nd$_{0.5}$Fe$_{3}$(BO$_{3})_{4}$

We report x-ray scattering and polarized infrared reflectivity measurements of the substituted ferroborate Sm$_{0.5}$Nd$_{0.5}$Fe$_{3}$(BO$_{3})_{4}$. Below T$_{N}$ = 33 K, a new set of commensurate peaks with resonant enhancements at the rare earth L edges indicates a doubling of the magnetic structure along the c-axis and simultaneous ordering of the rare earth and iron ions. Rare earth spin polarizations decrease rapidly with increasing temperature, in contrast to that of the iron ions. Shell-specific measurements of the rare earth spin polarizations indicate similar behaviors of the Sm and Nd 5d states, while the Sm 4f and 5d states have different temperature dependences. Along the c-axis we observe negative thermal expansion below $\sim $75 K and a strong phonon softening from room temperature down to T$_{N}$, at which it freezes in frequency. Also at T$_{N}$ we observe the appearance of an electromagnon in the ab-plane that gets its spectral weight from the lowest frequency phonon. These results indicate a lattice instability linked to magnetism with a strong coupling between magnetic and elastic properties.   

Spin-driven ferroelectricity in ferroaxial crystals

Spin-driven ferroelectricity in most non-collinear magnets, such as TbMnO3, is induced by the so-called inverse Dzyalonshinskii-Moriya mechanism and requires a cycloidal magnetic structure, an ordered magnetic state that is not truly chiral (or lacks helicity). Conversely, in a truly chiral magnetic state (proper helix), the pseudo-scalar helicity can not couple directly to the electric polarization, and therefore can't induce ferroelectric order. However, in systems of specific crystal symmetry, named here ``ferroaxials,'' the presence of collective structural rotations mediates an indirect coupling between magnetic helicity and ferroelectricity. I will review our recent experimental results for new compounds of this class, obtained by magnetic X-ray and neutron diffraction techniques, including a clear demonstration that the magnetic helicity can be controlled by an electric field.   

Multiferroic behavior in trimerized Mott insulators

We demonstrate multiferroic behavior in trimerized Mott insulators through the interplay between spins and electric dipole moments resulting from electronic charge fluctuations in frustrated units [L. N. Bulaevskii, C. D. Batista, M. V. Mostovoy, and D. I. Khomskii, Phys. Rev. B 78, 024402 (2008)]. The model consists of stacked triangular layers of trimers with small inter-trimer exchange interactions $J'$ and $J''$. We construct a phase diagram using a semi-classical approach. Ferroelectric states coexist with ferro- or antiferromagnetic orderings depending on the value of the magnetic field $H$ and the sign of the inter-layer exchange $J''$. The electric polarization undergoes abrupt changes as a function of $H$.   

Electric field control of magnetic chiralities in ferroaxial multiferroic RbFe(MoO$_{4})_{2}$

The onset of ferroelectric polarisation in high symmetry, proper-screw type multiferroic materials cannot be explained in terms of conventional microscopic mechanisms or symmetry analysis since the direction of the magnetic propagation vector is orthogonal to the plane of the spins. RbFe(MoO$_{4})_{2 }$undergoes a structural distortion at T$_{c}$ = 190 K in which the oxygen tetrahedra rotate, imposing an ``axiality'' to the crystal structure. We show that a simple symmetric-exchange driven coupling of this axiality with the magnetic chiralities below T$_{N}$ = 4 K explains the appearance of a ferroelectric polarisation parallel to both the axial vector and direction of magnetic propagation. We present spherical neutron polarimetry data that are sensitive to both helical and triangular chiral domains of the magnetic structure. We are able to distinguish the magnetic order in the two axial domains that are present in equal proportions, and to demonstrate direct control of the magnetic structure by an applied electric field. The above formalism may be readily generalised to other `ferroaxial' systems in which the magnetic ordering breaks the inversion symmetry in a point group supporting an axial vector.   

Electric field control of nonvolatile four-state magnetization at room temperature

We find the realization of large converse magnetoelectric (ME) effects at room temperature in a multiferroic hexaferrite Ba$_{0.52}$Sr$_{2.48}$Co$_{2}$Fe$_{24}$O$_{41}$ single crystal, in which rapid change of electric polarization in low magnetic fields (about 5 mT) is coined to a large ME susceptibility of 3200 ps/m. The modulation of magnetization then reaches up to 0.62 $\mu _{B}$/f.u. in an electric field of 1.14 MV/m. We find further that four ME states induced by different ME poling exhibit unique, nonvolatile magnetization versus electric field curves, which can be described by an effective free energy with a distinct set of ME coefficients. *These authors contributed equally to this work.   

Magnetoelectric Effects and Related Phenomena in Spin-spiral Hexaferrites

Among various multiferroics, extensive studies of ferroelectrics originating from magnetic orders, i.e., \textit{magnetically-induced ferroelectrics }in which the inversion simmetry breaking and resultant ferroelectricity are induced by complex magnetic orders, have been triggered almost a decade ago by the discovery of multiferroic nature in a perovskite-type rare-earh manganites TbMnO$_{3}$. The magnetically-induced ferroelectrics often show giant magnetoelectric effects, remarkable changes in electric polarization in response to a magnetic field, since the origin of their ferroelectricity is driven by magnetism which sensitively responds to an applied magnetic field. Though a large number of new magnetically-induced ferroelectrics have been reported in the past decade, so far there has been no practical application employing the magnetoelectric effect of the magnetically-induced ferroelectrics. This is partly because none of the existing magnetically-induced ferroelectrics have combined large and robust electric and magnetic polarizations at room temperature until quite recently. The situation is changed by the discoveries of magnetoelectricity in hexagonal ferrites (\textit{hexaferrites}) with spin-spiral structures.\footnote{T. Kimura, G. Lawes, and A. P. Ramirez, Phys. Rev. Lett. 94, 137201 (2005).}$^,$\footnote{Y. Kitagawa \textit{et al.}, Nature Mater. 9, 797 (2010).}$^,$\footnote{K. Okumura \textit{et al.}, Appl. Phys. Lett. 98, 212504 (2011).} In this presentation, I show our recent studies on magnetoelectric effects and related phenomena in the new series of magnetically-induced ferroelectrics which are promising candidates for multiferroics operating at room temperature and low fields. This work has been done in collaboration with Y. Hiraoka, T. Ishikura, K. Okumura, Y. Kitagawa, H. Nakamura, Y. Wakabayashi, M. Soda, T. Asaka, and Y. Tanaka.   

Pressure-induced Polarization Reversal in Z-type Hexaferrite Single Crystal

Multiferroic materials with a gigantic magnetoelectric (ME) coupling at room temperature have been searched for applications to novel devices. Recently, large direct and converse ME effects were realized at room temperature in the so-called Z-type hexaferrite (Ba,Sr)$_{3}$Co$_{2}$Fe$_{24}$O$_{41}$ single crystals [1,2]. To obtain a new control parameter for realizing a sensitive ME tuning, we studied ME properties of the crystals under uniaxial pressure. Upon applying a tiny uniaxial pressure of about 0.6 GPa, magnetic field-driven electric polarization reversal and anomaly in a $M-H$ loop start to appear at 10 K and gradually disappear at higher temperature above 130 K. By comparing those results with longitudinal magnetostriction at ambient pressure, we propose the pressure-dependent variations of transverse conical spin configuration as well as its domain structure under small magnetic field bias, and point out the possibility of having two different physical origins of the ME coupling in this system. [1] Y. Kitagawa \textit{et al}., Nat. Mater. \textbf{9}, 797 (2010) [2] S. H. Chun \textit{et al}., submitted.   

The complex multiferroic phase diagram of Mn$_{1-x}$Co$_x$WO$_4$

MnWO$_4$ is a classical multiferroic where ferroelectricity is induced by an inversion symmetry breaking helical spin order. The origin of the helical order is found in competing magnetic exchange interactions with strong uniaxial anisotropy, resulting in magnetic frustration. The microscopic parameters can be tuned by chemical substitution of Fe, Zn, or Co for Mn. The effects of Co substitution up to 30\% on the magnetic structure and the ferroelectric (FE) phase are investigated. The multiferroic phase diagram of Mn$_{1-x}$Co$_x$WO$_4$ is completely resolved. At low Co content, the FE polarization is oriented along the b-axis and decreases with increasing x. At doping levels between 7.5\% and 12\% Co, the polarization points along the a-axis and it reaches a maximum value of 90 $\mu$C/m$^2$ at x=10\%. With further increasing x, the FE polarization rotates back to the b-axis. Its magnitude decreases continuously and vanishes above 30\% Co content. This complex behavior comes along with a delicate sequence of different magnetic phases which are explored by magnetization, heat capacity, and neutron scattering experiments.   

Spin flop transition in the multiferroic Mn$_{1-x}$Co$_{x}$WO$_{4}$ studied by neutron diffraction

Elastic neutron diffraction is employed to investigate the ground state magnetic structure of the multiferroic Mn$_{1-x}$Co$_{x}$WO$_{4}$. Unlike the undoped MnWO$_{4 }$that has low-$T$ collinear spin structure, the doped MnWO$_{4}$ exhibits complex evolution of magnetic structure. Samples at lower concentration ($x<$0.10) have similar elliptical spiral structure as MnWO$_{4}$ in the multiferroic phase. With increasing $x$, the magnetic structure undergoes a sudden spin-flop transition which switches electric polarization from crystalline $b$-axis to $a$-axis. Polarized neutron scattering is also used to study the correlation between the bulk electricity and magnetic helicity.   

Session W10: Invited Session: Novel Bose-Einstein Condensates: Photons, Excitons, Magnons, Rydberg Atoms, and Polar Molecules

Bose-Einstein condensation of photons

Bose-Einstein condensation, the macroscopic ground state accumulation of particles with integer spin (bosons) at low temperature and high density, has been observed in several physical systems, including cold atomic gases and solid state physics quasiparticles. However, the most omnipresent Bose gas, blackbody radiation (radiation in thermal equilibrium with the cavity walls) does not show this phase transition. The photon number is not conserved when the temperature of the photon gas is varied (vanishing chemical potential), and at low temperatures photons disappear in the cavity walls instead of occupying the cavity ground state. Here I will describe an experiment observing a Bose-Einstein condensation of photons in a dye-filled optical microcavity [1]. The cavity mirrors provide both a confining potential and a non-vanishing effective photon mass, making the system formally equivalent to a two-dimensional gas of trapped, massive bosons. By multiple scattering of the dye molecules, the photons thermalize to the temperature of the dye solution. In my talk, I will begin with a general introduction and give an account of current work and future plans of the Bonn photon gas experiment. \\[4pt] [1] J. Klaers, J. Schmitt, F. Vewinger, and M. Weitz, Nature \textbf{468}, 545 (2010).   

Superfluid Phase Transition of Long-Lifetime Polaritons

Exciton-polaritons are quanta of electronic excitation which can have their properties tailored in semiconductor structures to have extremely light mass, about four orders of magnitude less than a free electron. One can think of them as photons dressed with an effective mass and an atom-like interaction. Because of their very light mass, exciton-polaritons show Bose quantum effects even at moderate densities and temperatures from tens of Kelvin up to room temperature. In the past five years, multiple experiments have shown effects of polaritons analogous to Bose condensation of cold atoms, such as a bimodal momentum distribution, quantized vortices, a Bogoliubov excitation spectrum, spatial condensation in a trap, and Josephson junction oscillations. In these experiments, though, the lifetime of the polaritons has been just a little longer than their thermalization time, which means that nonequilibrium effects play an important role; in particular, the transition to superfluidity has been smeared out rather than a sharp transition. In this talk I report new results with polaritons that have very long lifetime compared to their thermalization time. We see a discontinuous jump in the properties of the polariton gas indicative of a true phase transition, and we see ballistic transport over hundreds of microns. We also now have a way to use a laser to create a potential barrier for the polaritons.   

Session W11: Focus Session: Graphene Structure, Stacking, Interactions: Local Probes and Microscopy

Direct Imaging of a Two-Dimensional Silica Glass on Graphene

Large-area graphene substrates [1] are a promising lab bench for synthesizing and characterizing novel low-dimensional materials such as two-dimensional (2D) glasses. Unlike 2D crystals such as graphene, 2D glasses are almost entirely unexplored--yet they have enormous applicability for understanding amorphous structures, which are difficult to probe in 3D. We report direct observations of the structure of an amorphous 2D silica supported on graphene. To our knowledge, these results represent the first discovery of an extended 2D glass. The 2D glass enables aberration-corrected scanning transmission electron microscopy and spectroscopy, producing the first atomically-resolved experimental images of a glass. The images strikingly resemble Zachariasen's seminal 1932 cartoons of a 2D continuous random network glass [2] and allow direct structural analyses not possible in 3D glassy materials. DFT calculations indicate that van der Waals interactions with graphene energetically favor the 2D structure over bulk SiO$_{2}$, suggesting that graphene can be instrumental in stabilizing new 2D materials. [1] J. C. Meyer et al., Nature \textbf{454}, 319--322 (2008). [2] W. H. Zachariasen, J. Am. Chem. Soc. \textbf{54}, 3841--3851 (1932).   

Imaging defects on epitaxial graphene/SiC(0001) using non-contact AFM with a Q-plus sensor

STM has been commonly used to study the atomic structures of resonant scatters such as vacancies and adsorbates on graphene, which are leading factors limiting its mobility. However, since STM probes primarily the local density of states, complex patterns are often observed when imaging defects on graphene, making it challenging to determine their atomic structures using STM alone. In this work, we carried out an integrated study of defects on epitaxial graphene/SiC(0001) using non-contact AFM with Q-plus sensors in addition to STM. With atomic resolution AFM imaging, straight forward identifications of single- and di-vacancy defects can be made. These results and their implications for understanding electron scattering in epitaxial graphene on SiC(0001) will be presented at the meeting.   

Direct Imaging of Charged Impurities in Substrates used for Graphene Devices

The use of hexagonal boron nitride (h-BN) as a substrate for graphene led to approximately an order of magnitude improvement in electron mobility compared to graphene on SiO$_{2}$. One hypothesis for the improvement is a reduction in trapped charge density on the surface of h-BN compared to SiO$_{2}$. We address this directly by mapping local potential fluctuations above the bare substrates h-BN and SiO$_{2}$ using Kelvin probe microscopy in ultra-high vacuum. We compare the results to a model of randomly distributed charges in a 2D plane at the surface of an insulating substrate. For SiO$_{2}$, the results are well modeled by a 2D charge density of $\sim$ 2.5x10$^{11}$ cm$^{-2}$. Previous measurements of charged impurity scattering in graphene indicates that this density of substrate charges would limit graphene mobility to 20,000 cm$^{2}$/Vs, in good agreement with the maximum values reported for graphene on SiO$_{2}$. h-BN displays potential fluctuations that are approximately an order of magnitude lower than SiO$_{2}$, consistent with an order of magnitude improvement in mobility in graphene/h-BN devices. This work was supported by the US ONR MURI program, and the U. of MD NSF-MRSEC under Grant No. DMR 05-20471.   

Self-Organized Graphene Nanoribbons on SiC(0001) Studied with Scanning Tunneling Microscopy

Graphene nanoribbons grown directly on nanofacets of SiC$(0001)$ offer an attractive union of top-down and bottom-up fabrication techniques. Nanoribbons have been shown to form on the $<1\bar{1}0n>$ facets of templated silicon carbide substrates,\footnote{Sprinkle et al., \emph{Nat. Nanotech.} \textbf{5}, 727 (2010).} but also appear spontaneously along step-bunches on vicinal SiC(0001) miscut slightly towards $<1\bar{1}00>$. These self-organized graphene nanoribbons were characterized with low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES) in ultra-high vacuum. Our measurements indicate that the graphene forms a continuous ``buffer layer'' across the SiC(0001) terraces during nanoribbon formation, with the zigzag edge of the buffer layer aligned parallel to the step-bunched nanofacets. Scanning tunneling microscopy/spectroscopy (STM/STS) was used to characterize the topography and electrical characteristics of the graphene nanoribbons. These measurements indicate that the graphene nanoribbons are highly-crystalline with predominantly zigzag edges.   

Imaging epitaxial graphene on SiC(0001) using STM with functionalized W tips

Epitaxial graphene on SiC(0001) is studied using scanning tunneling microscopy with W tips functionalized by transition-metal (Cr, Fe) coatings, enabling the imaging of states within a few meV of the Fermi level that are not accessible with conventional W tips. First-principles modeling of these tips as pyramidal structures on W(110) indicates that an apex atom is stable for the Cr/W(110) tip but not for the Fe/W(110) or W/W(110) tips. Further calculations of the tunneling current show that the Cr- and Fe-coated tips can get significantly closer to the substrate than a bare W tip at a given current, and that the Cr (Fe) tip states contributing to the tunneling at low bias are spatially more localized than the W tip states. These characteristics lead to increased resolution, making possible the selective imaging of the complex electronic properties of the epitaxial graphene on SiC(0001)1,2.   

Scanned probe studies of dielectric screening and charge puddles in epitaxial graphene on SiC(0001)

Epitaxial growth of graphene on SiC(0001) produces wafer-scale monolayer films suitable for large scale device applications. However, the presence of the buffer layer beneath the graphene produces n-doping of 10$^{12}$-10$^{13}$ cm$^{-2}$ and limits mobility to $\sim $10$^{3}$ cm$^{2}$/Vs. Recently H intercalation has produced p-doped samples with similar carrier density and improved mobility. Transport measurements on processed devices show evidence of charge impurity scattering, but these measurements cannot show whether the transport behavior is due to top gate dielectrics or intrinsic to the as-grown graphene. Here we use scanning microwave microscopy to look at graphene samples and bare SiC substrates to extract information about the screening role of the buffer layer. This data is complemented by earlier results suggesting charge puddles due to random impurities to not exist in epitaxial samples.   

Scanning Tunneling Microscopy Study of Mechanically-Stacked Double Layer Graphene

Bilayer graphene has drawn considerable attention due to deviation from Dirac Fermion picture such as anomalous quantum hall effect and a tunable band gap in their spectrum. While a pristine Bernal (AB) stacked bilayer graphene can be synthesized by mechanical exfoliation, growth on a SiC single crystal and epitaxial growth on metal substrates, separate control of the top and the bottom layers has seldom been performed. In this study, artificially modified 2D layers were demonstrated with individually stacked double layer graphene. Large-area graphene was grown on a Cu foil by chemical vapor deposition (CVD). CVD-grown graphene layers were transferred successively onto several insulating substrates with minimum chemical process for realizing bilayer graphene. Mechanically-stacked double layer graphene was mainly investigated using scanning tunneling microscopy and spectroscopy. The artificial bilayer graphene showed Moire patterns as determined by misalignment angle. In spatially resolved spectrums of local density of states, dependence on separation distance between two graphene layers and their corrugation was measured. In addition, we confirmed less charging effects of graphene on BN thin film than on SiO2 or SiN.   

Scanning Tunneling Spectroscopy of Potassium on Graphene

We investigate the effect of charged impurities on the electronic properties of large single crystal CVD grown graphene using scanning tunneling microscopy. Mono- and multilayer crystals were prepared by transferring graphene from copper onto exfoliated boron nitride flakes on 300 nm SiO$_2$ substrates. The boron nitride provides an ultra flat surface for the graphene. Potassium atoms are controllably deposited on the graphene at low temperature by heating a nearby getter source. Scanning tunneling spectroscopy and transport measurements were performed in ultra high vacuum at 4.5 K. Transport measurements demonstrate the shifting of the Dirac point as the samples are doped, while STM measurements demonstrate the size, arrangement and local electronic influence of the potassium atoms.   

Nanoscale electronic and optical investigations of functionalized graphene

A rigorous understanding of light-matter interactions at the nanometer scale is pivotal in the development of nanoscale device applications. Graphene and its functionalized derivatives, due to their unique properties, promise unexpected capabilities as a platform for such devices, which has led to significant interest in graphene-based nano-optical, optoelectronic, and photovoltaic applications. Here, we will describe our efforts to resolve and understand the structural, electronic and optical properties of these systems. We will present a UHV STM study of the structural and electronic properties of C60 molecules deposited on graphene that was grown epitaxially on SiC(0001), which serves as a model system for the study of molecule-surface interactions. Our results indicate reduced coupling of the molecules to the graphene and underlying substrate, compared to those on metallic substrates, suggesting a path for developing molecular-scale electronic and optically active ``devices'' that are not dominated by the substrate. We will also discuss our efforts to correlate these STM studies with the optical properties of the system using a UHV STM that incorporates confocal optical microscopy and spectroscopy at the tip-sample junction with integrated high-numerical aperture optics.   

Three Different Translations Can Each Convert the Top Plane of Graphite to Graphene

The discovery of graphene, a unique two-dimensional electron system with extraordinary physical properties, has ignited tremendous research activity in both science and technology. Graphene can be obtained from graphite by moving its top layer until it becomes locally decoupled from the bulk. However, a detailed microscopic understanding of this process has yet to be completed. Here we present STM images of the top plane of graphite, which has been transformed into graphene. In addition, we also present STM images which reveal several intermediate stages in between pure graphene and pure graphite. Density functional theory was used to simulate STM images from a six-layer slab of graphite. We also moved the top layer of graphite incrementally in three different directions. Vertical movement of the top layer by about 0.1 nm created graphene. A continuous transition from pure graphite to pure graphene was observed with the simulations. Horizontal movement of the top layer can also create graphene in two ways. In one configuration, the carbon atoms perfectly align with the layer below, while in the second the carbon atoms have no vertical alignment with the layer below. Other significant details found between graphite and graphene will be discussed.   

Graphene on Ru(0001): The four hills in the 25 on 23 structure

A single layer of sp2 hybridized carbon on Ru(0001) accommodates in a 23 on 25 superstructure with 625 carbon honeycombs as found by surface x-ray diffraction (SXRD) [1]. In a significant computational effort 25x25 unit cells of graphene were relaxed on 23x23 Ru(0001) unit cells with up to 6 substrate layers. The density functional theory calculations that take van der Waals interactions into account predict 4 protrusions, quantum dots [2] or ``hills'' in the unit cell with two kinds of hills: 3 $\Omega$-type hills with a honeycomb around the summits, and one T-type hill with a single carbon atom on the summit. This prediction is confirmed by state of the art low temperature scanning tunneling microscopy and is in line with the SXRD data in Ref.[1].\\[4pt] [1] Martoccia et al. Phys. Rev. Lett. 101 (2008) 126102.\\[0pt] [2] Zhang et al. J. Phys.: Condens. Matter 22 (2010) 302001.   

Thickness Determination of Multilayer Graphene Using Transmission Electron Microscopy

With dark field transmission electron microscopy and select area electron diffraction (SAED) crystallographic grain boundaries in graphene can be easily imaged. We present a complete, quantitative theoretical model of the SAED pattern that allows determination of the number of layers. Grain boundary maps of single and multilayer graphene grown by chemical vapor deposition will be shown.   

High resolution transmission electron microscopy of lattice dynamics of graphene

Lattice dynamics of carbon atoms in graphene was investigated by aberration corrected ultra-high resolution transmission electron microscopy near the holes and the grain boundaries. We studied in-situ formation of various unusual defect structures in graphene under various conditions. In this presentation we will show the stability and dynamics of the atoms at the holes, grain boundaries and the defects and discuss their formation mechanism. This work will also elaborate on the electronic properties of the defects in light of recent experimental and theoretical progress.   

High resolution transmission electron microscope Imaging and first-principles simulations of atomic-scale features in graphene membrane

Ultra-thin membranes such as graphene[1] are of great importance for basic science and technology applications. Graphene sets the ultimate limit of thinness, demonstrating that a free-standing single atomic layer not only exists but can be extremely stable and strong [2--4]. However, both theory [5, 6] and experiments [3, 7] suggest that the existence of graphene relies on intrinsic ripples that suppress the long-wavelength thermal fluctuations which otherwise spontaneously destroy long range order in a two dimensional system. Here we show direct imaging of the atomic features in graphene including the ripples resolved using monochromatic aberration-corrected transmission electron microscopy (TEM). We compare the images observed in TEM with simulated images based on an accurate first-principles total potential. We show that these atomic scale features can be mapped through accurate first-principles simulations into high resolution TEM contrast. [1] Geim, A. K. {\&} Novoselov, K. S. \textit{Nat. Mater. }\textbf{6}, 183-191, (2007). [2] Novoselov, K. S.\textit{et al. Science }\textbf{306}, 666-669, (2004). [3] Meyer, J. C. \textit{et al. Nature }\textbf{446}, 60-63, (2007). [4] Lee, C., Wei, X. D., Kysar, J. W. {\&} Hone, J. \textit{Science }\textbf{321}, 385-388, (2008). [5] Nelson, D. R. {\&} Peliti, L. \textit{J Phys-Paris }\textbf{48}, 1085-1092, (1987). [6] Fasolino, A., Los, J. H. {\&} Katsnelson, M. I. \textit{Nat. Mater. }\textbf{6}, 858-861, (2007). [7] Meyer, J. C. \textit{et al. Solid State Commun. }\textbf{143}, 101-109, (2007).   

Interference effects in electronic transport in mesoscopic graphene

Graphene consists of a monolayer of carbon atoms arranged in a honeycomb lattice and has been intensively studied due to its fascinating physical properties. We study transport in mesoscopic graphene systems, in particular, conductance oscillations due to interference of the Dirac electrons in phase-coherent transport. Green's functions are calculated in the tight-binding model via $T$-matrix formalism, and the conductance is then obtained using the Landauer-B\"{u}ttiker formalism. We consider a measurement set-up consisting of two STM tips, as well as transport from a contact and a STM tip. Regular Bloch oscillations are obtained, as well as richer structures when different types of isolated impurities are considered.   

Session W12: Focus Session: Graphene: Growth, Mechanical Exfoliation, and Properties - Exfoliation and Doping

Graphene Carrier Control and Band Gap Formation through Stacked Graphene Sheets

Graphene's use in RF transistors and frequency doublers is attractive since its high mobility and high saturation velocity translate into operation at high frequencies while utilizing little power. However, further graphene development for device applications is hindered by high metal contact resistance, poor control of channel conductivity, and the absence of a band gap. In this talk, I will present our efforts at NRL to address these challenges using two strategies: 1) substitutional insertion of group III-V atoms into graphene's lattice to control the carriers and 2) through a synthetic means to create bilayer graphene with a band gap. Substitutional incorporation of atoms into graphene can result in doping, if their concentration does not drastically affect the $\pi$-network. Using selective oxidation to remove C atoms from the graphene lattice, we are able to backfill the C vacancies using molecular beam deposition of dopants with controllable ultra-low fluxes. We will show that boron and phosphorus dopants can provide extra holes and electrons to the graphene $\pi$-network, respectively, modifying the carrier concentration in transport measurements. Bernal-stacked graphene bilayers have a relatively small band gap ($\sim$few meV). However, if the symmetry of the system is broken by the application of a large applied electric field, the band gap can be increased ($\sim$250 meV). Alternatively, we find it is possible to obtain such large built-in electric fields when graphene sheets of opposite doping are stacked. By bonding a p-type, CVD-grown graphene monolayer transferred from Cu to an n-type, epitaxially grown graphene monolayer on SiC, we formed a p-n graphene bilayer. Transport measurements and modeling of the resulting electric field generated by opposite doping of the graphene sheets indicate the creation of a 100-300 meV band gap in the synthetic bilayer.   

Atomic characterization of monolayer doped graphene sheets synthesized by chemical vapor deposition

Large-area, high-quality monolayer nitrogen (or boron)-doped graphene sheets were synthesized on copper foils by a modified chemical vapor deposition (CVD) apparatus. As-grown graphene sheets could be easily transferred from copper foils onto different substrates (e.g. silicon/silicon dioxide wafers). Compared with pristine graphene, nitrogen (or boron)-doped graphene shows strong D-band caused by doping and structural defects formed within the lattice. Scanning tunneling microscopy (STM) and spectroscopy (STS) reveal that the defects in the doped graphene samples arrange in different geometrical configurations. The localized states in the valence and conduction bands are in accordance with the type of dopant and bonding type. These experimental results are in agreement with first principles calculations of LDOS of doped graphene and STM image simulations.   

Graphene melting by molecular dynamics simulations

Mechanisms of melting of graphene were studied by molecular dynamics (MD) using two different interatomic potentials: the Reactive Empirical Bond Order (REBO) and recently developed Screened Environment Dependent (SED) -REBO potentials. Melting was investigated in two-dimensional (2-D) and three-dimensional (3-D) coordinate space. It was shown that the loss of long-range order and melting proceeds through generation and in-plane aggregation of Stone-Wales (S-W) defects in REBO-graphene, followed by the formation of a complex 3D network of carbon chains. Although S-W defects are also formed in the SED-REBO-graphene, they do not cluster. Instead, the melting proceeds through the formation of dangling bonds and vacancies. The melting temperature of graphene using REBO was found to be 5,200 K, whereas for SED-REBO it is lower by $\sim $800K. The melting in 2-D occurs at higher temperatures compared to 3-D because of in-plane geometrical constraints.   

Unexpected Structures for Intercalation of Sodium in Epitaxial SiC-Graphene Interfaces

We show using scanning tunneling microscopy and spectroscopy and calculations from first principles that several different intercalation structures exist for Na in epitaxial graphene on SiC(0001). Intercalation takes place rapidly at room temperature and tunneling spectroscopy shows that it significantly electron dopes the graphene. Upon annealing above room temperature a quite different intercalation structure is formed which removes the carbon-rich interface layer and transforms this into a second graphene layer. In addition, we find that direct deposition of Na onto the carbon rich buffer layer graphene precursor decouples it from the SiC substrate leading to formation of a new sheet of graphene. This interface-layer decoupling is unambiguously demonstrated by transforming bare buffer layer to a graphene layer. Our observations show that intercalation in graphene is fundamentally different than in graphite and provides a very versatile approach to metal-graphene functionality and electronic-property control.   

Electron phonon renormalization in N-doped graphene

Current research efforts are aimed at controlling the electronic properties graphene sheets using electron (or hole) doping for successful device fabrication. The presence of strong coupling between electronic and vibrational properties in graphene greatly assists Raman spectroscopy in probing the dopant-induced electronic energy changes. Previously, Raman spectroscopy was employed as a tool to probe the electron and phonon renormalization in doped single-walled carbon nanotubes (SWNT). It was found that the increase in electron velocity in?uences lattice vibrations locally near a negatively charged defect. These local renormalization effects were observed to result in a new effectively downshifted (up-shifted) Raman peak below the G' peak for n-doped (p-doped) SWNTs. In case of graphene, we find that the several Raman features for CVD grown N-doped graphene vary depending upon local dopant bonding environment. For instance, non-graphitic dopants (pyridinic, pyrrolic) were observed to result in highly intense D {\&} D'-band unlike the graphitic dopants. We explain these results in terms of the zig-zag (armchair) edges formed by graphitic (non-graphitic) bonding environment of the dopant.   

Transport properties of pristine and doped graphene

Graphene has attracted a lot of attention for various applications recently. Chemically exfoliated graphene is one of the best methods to prepare good quality and large amount of few-layer graphene sheets. We prepared pristine and doped graphene using chemical exfoliation through high energy tip sonication technique. The exfoliated graphene was later sintered for studying thermoelectric properties. The thermopower of these samples exhibits valleys, tentatively assigned to phonon drag, which shift towards higher temperature upon vacuum annealing and electron doping. Such a similar behavior was previously observed in doped carbon nanotubes. The effects of vaccum annealing and doping upon the fundamental behavior of thermoelectric power and thermal conduction of graphene will be presented.   

Experimental investigation of carrier quantum confinement in graphene quantum dots and nanoribbons

Recently, Zhu \textit{et al.} (Chem. Comm., 2011. \textbf{47}(24), 6858) and Pan \textit{et al}. (Adv. Mat., 2010. \textbf{22}(6), 734) have successfully synthesized graphene quantum dots (width $<$10 nm). They probed the carrier quantum confinement in such GQDs using photoluminescence spectroscopy (PL). However, a self-consistent explanation for the observed PL spectra is lacking. Interestingly, we find that the organic reducing agents used for synthesizing GQDs have PL signature similar to the GQDs themselves. Thus, deconvoluting solvent effects is extremely important to achieve further progress in synthesis and application of GQDs. We studied the PL behavior of hydrothermally synthesized graphene nanoribbons (GNRs) and GQDs to understand the solvent effects and the underlying mechanism for the observed PL. We will discuss the effects of size, shape and morphology on the PL behavior of GQDs and GNRs.   

Scanned Probe Measurements of Graphene on Ferroelectrics

We present results on scanned probe measurements of graphene and few-layer graphite (FLG) on ferroelectric thin films. The graphene was mechanically exfoliated onto the PbZr$_{0.2}$Ti$_{0.8}$O$_{3}$ and other ferroelectric films, and its topography and polarization were characterized using atomic force microscopy, Kelvin Probe (surface potential) Force Microscopy, and Piezoresponse Force Microscopy. We discuss how graphene can be used as a top electrode for ferroelectric materials, as changing the potential of the graphene can cause the ferroelectric region beneath it to switch polarization. We demonstrate that the change in polarization is reversible. We also show how the surface potential of FLG on PZT depends on the number of layers of graphene. *The authors acknowledge Grant DMR-1124696, Sub-Award 235743-3668.   

Can Structured Mixed Solvents be Used for Graphene Exfoliation?

Using a combination of computational and experimental techniques we investigate graphene exfoliation and suspension in C6H6, C6F6 and their mixtures. Our MD simulations show that an equimolar mixture of C6H6/C6F6 has the highest affinity for graphene. This is manifested in the formation of translational and orientational order normal to the graphene surface, with no translational ordering parallel to the graphene surface. The solvent structure is driven by quadrupolar interactions and consists of stacks of alternating C6H6/C6F6 molecules rising from the surface of the graphene. These stacks give rise to density oscillations in registry with the graphene surface. The period of the density oscillations is on the order of 3.4 {\AA}, corresponding to the van der Waals diameter of carbon and this ordered structure extends 30 {\AA} from each side of the graphene sheet. To experimentally verify the results of the molecular dynamics simulations we use dynamic contact angle measurements. These measurements demonstrate an increase in solvent affinity for HOPG in the case of 1:1 mixtures in comparison with pure components. The quality of the exfoliated material and flakes after sonication is verified by AFM, SEM, and TEM techniques. The graphene sheets produced in the equimolar mixture can be freeze-dried at room temperature, (T=300K) producing sponge-like graphene structures held together entirely by graphene sheet interactions and reflecting the structure of the graphene sheets in solution.   

Fabrication and Characterization of folded Graphene on Graphite

Graphene is one of the most studied carbon compounds due to its electronic properties. Applying an electric pulse between a Graphite surface and STM tip the upper layer of a Graphite surface could be folded due to mechanical and electrostatic forces forming voids, bend, move, and decouple or transfer part of the tip to the surface. Ab-initio electrostatic calculations and MD thermal simulations support our experimental method. Due to the different new discovered and studies physical-effects in Graphene edges, which can significantly influence the overall electronic and magnetic properties of Graphene nanostructures, this results may be exploited as an method to obtain different structures. Here we study Graphene flakes obtained with this Method. Folded Graphene's structure and electronic properties are studied to determine its degree of coupling to the graphite substrate. Cross-sectional analysis of the fold shown reveals that it consists of a single sheet of graphite folded. Graphene edges and folded line reveals new electronic and magnetics properties. Among others the possibility of a multi folded Graphene sheet system gains importance due to the opportunity of getting a Semi-Carbon-Nanotube-Graphene device.   

Controlling desorption of H, O, atoms and OH group from graphene by pulse laser

Possibilities of reduction of graphene oxide and one-side dehydrogenation of graphane (H-terminated graphene) with using ultra-short pulse ($\sim$2 fs) laser are discussed. We have performed molecular dynamics (MD) simulation of O-, H-, and OH-adsorbed graphene sheets induced by electronic excitation upon irradiation with laser pulse. The time-dependent density functional theory treating real-time propagation of electron wave functions combined with the Ehrenfest approximation for the MD was employed and FPSEID [1] code was used to check the energy conservation rule under dynamical external field [2]. We found asymmetric pulse shape as a function of time causes an efficient desorption of O atoms and OH groups from graphene which can be applicable for reduction of graphene oxide alternative to chemical and thermal treatment. Meanwhile, such asymmetric pulse shape is beneficial for one-side H-desorption from graphane that will trigger further structural changes such as spontaneous shrink/rippling or heterogeneous termination on side-by-side.\\[4pt] [1]O. Sugino, Y. Miyamoto, PRB{\bf 59}, 2579, (1999);B{\bf 66}, 089901(E) (2002)\\[0pt] [2]Y. Miyamoto, H. Zhang, PRB{\bf 77}, 165123 (2008)   

Graphene on Au-coated SiO$_{x}$ substrate: Its visibility and intrinsic core-level photoemission

With the motivation of precisely and intrinsically characterizing a exfoliate graphene using photoelectron spectroscopy, a conducting substrate having high optical contrast is greatly desired. Here, we demonstrate that exfoliated graphene can be optically visible on a thin 9-nm Au-coated SiO$_{x}$ substrate, and can be easily conducted into scanning photoelectron microscopy/spectroscopy (SPEM/S) studies. Because of the elimination of charging effect, precisely core-level characterization of exfoliated graphene is presented with different numbers of layers. Consequently, the usage of Au-coated SiO$_{x}$ substrate serves a simple but effective method to study pristine graphene by photoelectron spectroscopy and other electron-detection techniques.   

Stage-1 intercalation compounds of few graphene layers by anhydrous ferric chloride

Anhydrous ferric chloride (FeCl3) was used to intercalate few graphene layers into stage-1 intercalation compounds. The intercalant, staging, stability, and doping of the resulting intercalation compounds are characterized by Raman scattering. The G peak of pure stage-1 compounds upshifts to $\sim $1626 cm-1, which is similar to that of heavily-doped monolayer graphenes by 18M sulfuric acid. A single Lorentzian line shape for the 2D band of stage-1 compounds were observed, which indicates that each layer behaves as a decoupled heavily doped monolayer. By performing Raman measurements at different excitation energies, we show that, for a given doping level, the variation of the 2D intensity relative to the G peak with excitation energy allows one to assess the Fermi energy. This allows us to estimate a Fermi level shift of up to $\sim $0.85 eV, which agrees well with that estimated from the 2D/G intensity ratio and is close to $\sim $0.9 eV measured in stage-1 GICs by electron energy loss spectroscopy. The stage-1 intercalation compound of few graphene layers is thus ideal test-beds for the physical and chemical properties of heavily doped graphenes.   

Session W13: Focus Session: Low-Dimensional and Molecular Magnetism - Metallorganics and Spincrossover Molecular Magnets

Magnetic properties of selected Prussian Blue Analogs (PBAs)

Prussian Blue Analogs consists of MC$_{6}$ and AN$_{6}$ octahedra connected by cyanide ligands (M, A= metals). They typically crystallize in cubic structures. We have studied temperature and field dependence of the magnetization and the susceptibility of selected Prussian Blue Analogs such as hexacyanocobaltates, -ferrates and -chromates. All compounds exhibit modified Curie --Weiss behavior in the paramagnetic region. The observed effective moments of those compounds were compared with the ones of the respective free-ion values. Furthermore, we find evidence that a few of the compounds exhibit a transition to long-range magnetic order at low temperatures.   

Persistent Photocontrolled Magnetism in Core-Shell Prussian Blue Analogues

Cubic heterostructured (\textbf{BA}) particles of Prussian blue analogues, composed of shells of ferromagnetic K$_j$Ni$_k$[Cr(CN)$_6$]$_l\cdot n$H$_2$O (\textbf{A}), \mbox{$T_c\sim70$~K}, surrounding bulk cores ($\sim350$~nm) of photoactive ferrimagnetic Rb$_a$Co$_b$[Fe(CN)$_6$]$_c\cdot m$H$_2$O (\textbf{B}), $T_c\sim20$~K, have been studied. \mbox{Below} $T_c\sim 70$~K, these samples exhibit a persistent photoinduced decrease in low-field magnetization, resembling results from previous core-shell particles\footnote{M.F. Dumont \emph{et al.}, Inorg. Chem. \textbf{50} (2011) 4295.} and analogous \textbf{ABA} films.\footnote{D.M. Pajerowski \emph{et al.}, J. Am. Chem. Soc. \textbf{132} (2010) 4058.} This net decrease suggests that the photoinduced lattice expansion in the \textbf{B} layer generates a strain-induced decrease in the magnetization of the \textbf{A} layer, similar to a pressure-induced decrease observed by others in a pure \textbf{A} material\footnote{M. Zentkov\'a \emph{et al.}, J. Phys.: Condens. Matter \textbf{19} (2007) 266217.} and by us in the \textbf{BA} cubes. To quantify the length scale over which the photoinduced strain dissipates into the \textbf{A} layer, a series of \textbf{B} and \textbf{BA} cubes of varying shell thickness have been characterized.   

Pressure-induced local lattice distortions in Co(dca)$_2$

We employed vibrational spectroscopy along with complementary lattice dynamics and spin density calculations to investigate local structure and magnetism through the series of pressure-induced transitions in Co(dca)$_2$. Analysis of several ligand bending modes reveals compression and distortion of molecular linkages and a major change in the crystal lattice through the 1 GPa transition, whereas a modified local structure (due to a change in molecular symmetry) but similar crystal lattice is anticipated above 3 GPa. We discuss our findings in terms of the competition between antiferromagnetic and ferromagnetic exchange interactions.   

Magnetotransport Properties of Switchable Valence Tautomer Films

We report tunneling magnetoresistance (TMR) and spectroscopy results in trilayer stacks of spincoated molecular films sandwiched between two metallic thin film electrodes. The molecules, cobalt dioxolene complexes, exhibit switchable bistable paramagnetic states as a result of intramolecular electron transfer accompanied by a spin-crossover of the Co ion, the so-called valence tautomerism (VT). Temperature dependent differential conductance measurements show changes in the density of states that correspond to the temperature dependent VT transition. The observed electronic transition is shown to be switchable by light exposure, which correlates the counterpart in the magnetic susceptibility. In trilayers with two ferromagnetic electrodes, e.g. Permalloy and Co, spin valve effects have been observed above and below the VT transition, including room temperature. The dependence of the TMR effect on bias voltage, light exposure, and molecular film thickness has been examined systematically as a function of temperature, aimed at exploring the effects of spin dependent states in the VT molecules.   

Computational studies of the Fe(II) spin crossover compound Fe[H$_2$B(pz)$_2$]$_2$(bpy)

Using calculations from first principles, we studied the electronic and transport properties of the Fe(II) spin crossover (SCO) compound Fe[H$_2$B(pz)$_2$]$_2$(bpy). The magnetic transition has been imposed by constrained magnetization calculations and the computed electronic structure agrees with available experimental data. Besides the characterization of the single molecule, we constructed a ?-stacking molecular chain of the compound and evaluated electronic transport in the direction of the chain for both the low-spin and the high-spin configurations. We found the high-spin configuration to be more conductive than the low-spin case, in agreement with experimental measurements of corresponding currents through disordered thin films. Molecule-molecule interactions are taken into account by the London dispersion forces. The spin-switchable electronic transport properties of this kind of Fe(II) SCO compound systems provide viable proofs for future switchable molecular spintronic devices and applications.   

Highly Unquenched Orbital Moment In Fe Phthalocyanine

Metal-Phthalocyanine molecules (MPc) form a family of compounds with a wide range of commercial application such as catalysts or dyes, and more recently in thin film technology. In an early work we found that in the $\alpha $-phase of FePc, where the FePc molecules are stacked in a herringbone structure, the Fe atoms are strongly magnetically coupled into ferromagnetic Ising chains with very weak antiferromagnetic interchain coupling. The chains achieve 3D long range ordering at T$_{N}$=10 K, and strong irreversibility (slow relaxation) below 5K. The Fe(II) is in a S=1 state and the hyperfine field in the ordered phase reaches a record value in Fe(II) of B$_{hf}$=66.2 T. This result is consistent with a large, unquenched orbital moment. It has been measured directly in a X-ray Magnetic Circular Dichroism (XMCD) spectroscopic study on FePc thin films deposited parallel on a Au surface predeposited on a Si substrate. The XMCD spectra at the L$_{3}$ and L$_{2}$ edges were measured as a function of incident angle $\gamma $. The orbital moment is $\left| {m_L } \right|=0.53\pm 0.04\mu _B $ and the isotropic spin component is $m_S =0.64\pm 0.05\mu _B $. The origin of this unusually high orbital moment is the incompletely filled e$_{g}$ level lying close to the Fermi energy. The ferromagnetically coupled Fe moments show strong, in-plane anisotropy [1]. Angular dependent measurements at the Fe K-edge also show strong quadrupolar excitations associated to a strong orbital moment, confirming the above result of the existence of a large, unquenched orbital moment in this molecule. Submonolayer FePc thin films deposited on Au, recently studied my XAS and XMCD have shown that there is charge transfer from the substrate to the Fe atom, modifying the electronic structure and magnetic properties [2] \\[4pt] [1] J. Bartolom\'{e} et al., Phys. Rev. B 81, 195405 (2010) \\[0pt] [2] S. Stepanow et al., Phys. Rev. B 83, 220401(R) (2011).   

Theoretical modelling of exchange interactions in metal-phthalocyanines

The theoretical understanding of exchange interactions in organics provides a key foundation for quantum molecular magnetism. Recent SQUID magnetometry of a well know organic semiconductor, copper-phthalocyanine [1,2] (CuPc) shows that it forms quasi-one-dimensional spin chains. Green's function perturbation theory calculation [3] is used to find the dominant exchange mechanism. Hybrid density functional theory simulations [4] give a quantitative insight to exchange interactions and electronic structures. Both calculations are performed for different stacking and sliding angles for lithium-Pc, cobalt-Pc, chromium-Pc, and copper-Pc. The exchange interactions depend strongly on stacking angles, but weakly on sliding angles. Our results qualitatively agree with the experiments, and remarkably $\alpha $-cobalt-Pc has a very large exchange above liquid-Nitrogen temperature. Our theoretical predictions on the exchange interactions can guide experimentalists to design novel organic semiconductors. \\[0pt] [1] S. Heutz, et. al., Adv. Mat., 19, 3618 (2007) [2] Hai Wang, et. al., ACS Nano, 4, 3921 (2010) [3] Wei Wu, et. al., Phys. Rev. B 77, 184403 (2008) [4] Wei Wu, et. al., Phys. Rev. B 84, 024427 (2011)   

Magneto-optical spectroscopic studies of solid and solution-phase tetra-phenyl porphyrin

Tetraphenylporphyrin (TPP) is a heterocyclic model system for porphyrins found in heme proteins, cytochromes and photosynthetic cofactors. TPP can accommodate a metal ion in the center; D-shell ion porphyrin complexes with a crystalline solid phase are of interest for magnetic studies because of the possibility of macroscopic long-range magnetic order of the ion spins. We have investigated the 5K magnetic properties of poly-crystalline thin films of TPP complexed with Zn, Mn and Cu and deposited through a room temperature capillary pen technique that produces grain size in the 100 micron to 1mm range. Our novel setup measures the UV/VIS, linear dichroism and MCD simultaneously and incorporates a photoelastic modulator and a microscopy superconducting magnet for high-field (5T) measurements. In addition, we present 25T data on samples from the new split magnet at NHMFL. We present solution and crystalline data on metal-complexed TPP; data are analyzed in terms of A and B-type MCD using a perimeter model. We find good agreement with previous solution data, and novel crystalline phase spectra that are correlated to the long range ordering of the solid state.   

Spin exchange interaction in quasi-1D Cu-phthalocyanine crystalline thin film measured by Magnetic Circular Dichroism (MCD) spectroscopy

Highly-oriented Cu-phthalocyanine (PC) pen-written crystalline thin films can be viewed as quasi-1D spin 1/2 magnetic chains. In order to reveal the nature of spin exchange between localized S=1/2 Cu spins, MCD spectroscopy was performed on films with millimeter-sized grains fabricated from a soluble CuPc derivative in magnetic fields up to 10 Tesla at temperatures ranging from 0.4 K to 77K. At T$<$2K and B$<$4T the MCD associated with transitions between Q-band $\pi$ electron states exhibits a non-linear temperature-dependent Brillouin-like increase with magnetic field. For B$>$4T the MCD evolves linearly with magnetic field, as expected from diamagnetic carbon-based systems. Theoretical modeling\footnote{W. Wu et. al.,PRB 84,024427(2011)} of electronic structure and exchange interactions in this system predicts an indirect exchange mechanism mediated by delocalized ligand states. Our MCD measurements identified the states responsible for this exchange.   

Introducing copper phthalocyanine as a qubit

Quantum information processing (QIP) has been shown to solve certain useful problems faster than its classical counterpart. However finding a physical system upon which to execute these algorithms is a challenging task. One promising implementation is to use an electron spin in a magnetic field as the information bearing quantum system. Numerous options have been proposed along these lines. Here I discuss a new candidate qubit, copper phthalocyanine. The copper atom at the centre of the molecule carries an unpaired electron. Pulsed electron paramagnetic resonance measurements of relaxation times reveal that it has potential for QIP. We measure the spin-lattice and spin-spin relaxation times of this electron and demonstrate single qubit manipulations. Solid-state electronic devices can be built with this low cost material, which is optically active, and offers great opportunities for chemical and physical modification, leading to significant control of magnetic and other properties.   

Structure-Controlled Coercivity of Low-Dimensional Metallo-Organic Thin Films

Metallo-organic thin films based on iron phthalocyanine were prepared to form self-assembled structures. Based on the substrate, the metal-ion chains can be grown perpendicular or parallel to the substrate. Individual iron chains are separated by 1.3 nm and interact weakly at low temperatures. The magnetic response of these structurally templated thin films is studied for a fixed film thickness and varied deposition temperatures. The magnetic hysteresis loops are wasp-shaped and mark long-range ferromagnetic interaction below a temperature of 4.5 K. The hysteresis loop coercivity can be correlated with the grain size of the iron phthalocyanine thin film. The enhanced coercivity is attributed to longer iron chains that are formed in larger grains.   

The Otto thermodynamic cycle using the magnetic molecule Ni$_2$

In order to design realistic molecular heat engines, the study of quantum thermodynamics is essential since classical thermodynamics does not apply in this extreme miniaturization limit [1,2]. Realizing a thermodynamic cycle on an existing magnetic molecule embodies a novel and unique approach to understand and exploit the thermodynamic properties of spin at the molecular level.\\ Here we propose an Otto cycle in the Ni$_2$ dimer based on a fully ab-initio calculation of the electronic states and the perturbative inclusion of spin-orbit coupling. A laser pulse, described by the time-dependent Schr\"{o}dinger equation, is used to heat the Ni$_2$ dimer. The pulse not only excites the electrons to higher, many-body electronic states, but also influences the spin of the system due to spin-orbit coupling. Using a low-temperature thermal bath the system is cooled back to the ground state. The adiabatic work exchange between the Ni$_2$ and the environment is described by the quasi-static expansion or compression of the bond length of the dimer. The calculated efficiency of the cycle is up to 34\%.\\ $[1]$ T. D. Kieu, Phys. Rev. Lett. {\bf 93 } 140403 (2004)\\ $[2]$ H. T. Quan, Phys. Rev. E {\bf 79} 041129 (2009)\\ $[3]$ T. Zhang \emph{et al.}, Phys. Rev. A {\bf 75} 062102 (2007)   

Session W14: Focus Session: Spins in Semiconductors - Ferromagnetism and Spin Dynamics in Semiconductors

Spin Dynamics in Bi2Se3 / GaAs Heterostructures

The narrow band gap semiconductor Bi$_2$Se$_3$ has been characterized as a topological insulator (TI), wherein strong spin-orbit coupling and time-reversal symmetry give rise to spin-polarized surface conduction states. Molecular beam epitaxy (MBE) of Bi$_2$Se$_3$ thin films onto conventional semiconductors such as GaAs [1] provides an attractive pathway for the creation of hybrid devices, coupling the exotic spin physics of TIs with the well-understood properties of spin coherence in semiconductors. We employ spatio-temporally resolved optoelectronic techniques to probe the carrier spin dynamics at the heterointerface between a TI and GaAs. Results are compared with interface band structure calculations.\\[4pt] This work is supported by ONR and NSF. \\[4pt] [1] A.~Richardella, D.~M.~Zhang, J.~S.~Lee, A.~Koser, D.~W.~Rench, A.~L.~Yeats, B.~B.~Buckley, D.~D.~Awschalom and N.~Samarth, {\it Appl.~Phys.~Lett.} {\bf 97}, 262104 (2010).   

Optical Aharonov-Bohm Effect in Al$_{0.08}$Ga$_{0.92}$As/Al$_{.25}$Ga$_{0.75}$As Quantum Wells

The photoluminescence (PL) from Al$_{0.08}$Ga$_{0.92}$As/Al$_{.25}$Ga$_{0.75}$As quantum wells (QW) was studied as function of magnetic field applied along the normal to the QW planes. The PL intensity exhibits two maxima at 2.3 and 4.9 tesla. The time-resolved PL from the same sample has a decay time which is one order of magnitude longer than the PL from a GaAs/AlGaAs QW, indicating that the recombination in the AlGaAs QW is spatially indirect. The PL intensity oscillations are attributed to the optical Aharonov-Bohm effect associated with spatially quasi-indirect excitons, which are located in the vicinity of islands with lower Al composition. The holes are localized by the islands, while the electrons move around them in a ring-like geometry. This model is in agreement with the interpretation of earlier results from Al$_{x}$Ga$_{1-x}$As/Al$_{y}$Ga$_{1-y}$As Fe spin-LEDs.   

Optical response of relativistic electrons in the polar BiTeI semiconductor

The transitions between the spin-split bands by spin-orbit interaction are relevant to many novel phenomena such as the resonant dynamical magneto-electric effect and spin Hall effect. Here, we present a combined experimental and theoretical study of the dynamics of relativistic electrons in the recently discovered giant bulk Rashba spin splitting system BiTeI. Several novel features are observed in the optical spectra including sharp edge singularity due to the reduced-dimensionality of joint density of states and a systematic doping dependence of the intraband transitions between the Rashba-split branches. These confirm the bulk nature of the Rashba-type splitting in BiTeI and manifest relativistic nature of the electron dynamics in a solid.   

Room temperature ferromagnetism in transparent conducting Fe-doped In$_{2}$O$_{3}$ films

Oxide semiconductors have been widely studied as a host compound for spintronic devices since they can be doped with transition metals to realize a higher Curie temperature and can produce high n-type carriers by either doping with Group IV elements or introducing oxygen vacancies. Among various oxide semiconductors, Fe-doped In$_{2}$O$_{3}$ is a promising ferromagnetic semiconductor due to the high solubility of Fe-ions into the In$_{2}$O$_{3}$ lattice. Recently, at NRL, In$_{2-x}$Fe$_{x}$O$_{3}$ thin films have been deposited on MgO, sapphire, and YSZ substrates by pulsed laser deposition. The lattice constant decreases linearly with increasing Fe-doping concentration suggesting the incorporation of Fe ions into the In$_{2}$O$_{3}$ lattice matrix. Magneto-transport characteristics including anomalous Hall effect along with structural analysis demonstrate that an intrinsic ferromagnetism is observed for some films grown under optimized conditions. In this presentation, we will discuss our work to date on the growth of In$_{2-x}$Fe$_{x}$O$_{3}$ thin films grown by pulsed laser deposition with various deposition conditions and present the structural, optical, magnetic, and transport properties along with spin-polarization measurements.   

Selective optical excitation of in-plane and out-of-plane spin polarizations with linearly polarized light in InGaAs

Excitation with circularly polarized light is a standard technique for optical spin orientation in semiconductors. This method is based on the transfer of angular momentum from the photons to the electrons and yields a polar spin polarization directed along the propagation direction of the exciting laser beam. Here we present linearly polarized all-optical pump-probe experiments to excite and detect coherent electron spins in InGaAs [1]. We find the magnitude and the orientation of the spin polarization strongly depend on the polarization axes of the exciting light. While in general the excited spin ensemble is composed of both polar and transverse spin components, the polarization axis of the exciting light can be chosen such that polar and transverse spin components can be excited separately. Thus, selective excitation of in-plane and out-of-plane spin polarizations is feasible with linearly polarized light.\\[4pt] [1] K. Schmalbuch \textit{et al.}, Phys. Rev. Lett. 105, 246603 (2010)   

Carrier Dynamics in Narrow Gap Ferromagnetic Semiconductors

Narrow gap ferromagnetic semiconductors are promising materials for spin photonic and spin transport devices because of their small effective masses, small energy gap, and high carrier mobility. We use time resolved differential transmission (TRDT) experiments to study carrier dynamics in ferromagnetic InMnAs and InMnSb. Electronic structure for InMnAs and InMnSb is calculated using an 8-band Pidgeon-Brown model generalized to include the effects of an external magnetic field. Our model includes the effects of the ferromagnetic Mn ions and their coupling to electrons and holes with or without an external magnetic field. Optical transitions are calculated from Fermi's Golden rule and interband transitions at a given pump or probe laser energy are identified. This allows us to understand a sign change seen in the TRDT. Our results show that 1) Phase-Space Filling, 2) Band Gap Renormalization and 3) Free Carrier Absorption all contribute to the TRDT and that the relative importance of these effects depends on the laser probe energy.   

Ferromagnetism in cobalt-doped SrTiO3 on Si grown by molecular beam epitaxy

We report the epitaxial growth of ferromagnetic cobalt-doped SrTiO$_{3}$ directly on silicon without the use of any buffer by molecular beam epitaxy (MBE). Magnetization as a function of magnetic field was performed for samples with varying doping concentration at room temperature and at 10 K. Room-temperature ferromagnetism is confirmed in single phase samples with composition 20-30\% cobalt. We also performed x-ray photoelectron spectroscopy of the Co and Ti 2p levels to determine stoichiometry and cobalt oxidation state. In order to elucidate the origin of ferromagnetism, we also performed first-principles calculations of cobalt-doped SrTiO$_{3}$ with different doping concentrations and dopant configurations. The calculations show that intrinsic ferromagnetism can be stabilized beyond a critical concentration in SrTiO$_{3}$ under particular conditions. The ability to directly integrate a ferromagnet on silicon in epitaxial form may potentially overcome the problems of impedance mismatch and interface losses in applications involving spin injection in silicon.   

Two-dimensional MnGaN Layer formed by Nitridation of Mn $\surd $3 $\times $ $\surd $3-R30 structure on Wurtzite GaN (000\underline {1})

There has been much interest in dilute Mn-doped GaN as a spintronic material. Recently, it has become of interest to consider the possible advantages of delta-doped magnetic layers, rather than a bulk alloy. Here we investigate experimentally the growth of single Mn-containing layers on top of wurtzite GaN as well as the overgrowth of GaN onto the Mn-containing layer, using a combination of N-plasma molecular beam epitaxy and reflection high energy electron diffraction. Sub-monolayer Mn deposition on GaN(000\underline {1}) results in a novel $\surd $3 $\times \quad \surd $3-R30 structure [1]. Upon nitrogen plasma exposure, this periodicity is removed; whereas, Mn is found remaining on the surface as measured by Auger electron spectroscopy. Unexpectedly, and in dramatic comparison to tiny lattice constant changes seen for bulk films, we find a huge -3.3 percent surface lattice contraction in both [10\underline {1}0] and [11\underline {2}0] azimuthal directions. This suggests the possible formation of a Mn0.25Ga0.75N 2-D alloy. Furthermore, subsequent overgrowth of GaN layers does not show significant lattice constant change compared to the nitridation process, up to a thickness of 20 ML or more.\\[4pt] [1] Chinchore et al., Applied Physics Letters \textbf{93(18)}, 181908 (2008).   

Structural transition and giant spintronic response of two dimensional Manganese-Gallium $\surd $3$\times \surd $3 R30\r{ } surface structure

In recent experiments, we have found that gallium nitride surface when exposed to transition metal atoms, results in novel well-ordered two dimensional spintronic structure, with tunable spintronic properties. A 2000 {\AA} N-polar $w$-gallium nitride (000\underline {1}) layer is grown on a sapphire substrate, by molecular beam epitaxy. The growth is monitored using reflection high energy electron diffraction system. Post growth, the standard 1$\times $1 gallium nitride surface, is exposed to sub monolayer doses of manganese. At low deposition temperature the diffraction patterns show manganese atoms forming a metastable 3$\times $3 structure, on supplying little heat to the manganese 3$\times $3 structure, it undergoes an irreversible transition to form a stable $\surd $3$\times \surd $3R30\r{ } structure. Both the structures are studied using a scanning tunneling microscope. The $\surd $3$\times \surd $3R30\r{ } structure shows a giant change in spectroscopic response on application of a very small out-of-plane magnetic field. The new findings suggest that the two dimensional magnetic nitride systems have excellent potential for both fundamental investigations and for use in future spintronic devices.   

Spinglass Dynamics of Amorphous Ferromagnetic Ge:Mn

Ge$_{0.84}$Mn$_{0.16}$ is an amorphous ferromagnetic semiconductor with a Curie temperature of about 160 K (measured at 1000 Oe). Magnetic field-cooled and zero-field-cooled experiments show existence of a spinglass phase well below the Curie temperature. The spinglass temperature is about 23 K at a 50 Oe field. The spinglass temperature scales monotonically with applied magnetic field. Magnetic hysteresis experiments show a non-zero coercive field below the spinglass temperature, and a very small coercive field or a superparamagnetic phase above it. Under a rapid quench, thermoremanent magnetization (TRM) decay experiments (at 14 K) exhibit a very large effective waiting time. This is associated with a slow rate of cooling, allowing communication between phase space states before the measuring temperature is reached, leading to an effective reduction in the attempt frequency. This is consistent with experiments on other more typical spin glass systems.   

Magnetism in Iron-Titanium Oxide Nanostructures

Modification of the titania nanotubes with magnetic transition metal additions is anticipated to provide the opportunity of creating a novel multifunctional nanostructured material with magnetic, semiconducting and catalytic properties. Incorporation of Fe into titania to produce ordered arrays of amorphous (Fe+Ti)O$_{2}$ nanotubes was achieved by electrochemical anodization. As-made nanotubes were subjected to a systematic thermal treatment in a variety of atmospheres for crystallization; their associated morphological, structural and magnetic character was examined. Preliminary results indicate that the Fe-modified nanotubes possess a unit cell volume that is slightly larger than that of titania (137 \textit{vs} 136 {\AA}$^3$) confirming Fe incorporation into the lattice. The temperature-dependent magnetic susceptibility data obtained from the samples may be decomposed into a Curie-Weiss component that represents the localized magnetic character of titania nanotubes and a Pauli paramagnetic component that represents the semiconducting behavior of the nanotubes. It is noted that Fe incorporation causes an increase in both components of the magnetic signal, suggesting modification of the electronic structure of the crystalline titania phase with iron incorporation.   

Magnetoluminescence Studies of Mn-doped PbS Quantum Dots

Diluted magnetic semiconductor quantum dots are interesting model systems for the investigation of carrier dopant exchange interactions. In this work, we report the carrier spin polarization studies in narrow band gap Mn-doped PbS quantum dots, a much less studied system compared to their II-VI counterparts. The PbMnS quantum dots were synthesized by hot colloidal solution technique. They are single crystalline with cubic structure. The doping concentration is 3-4{\%} as measured by energy dispersive X-ray spectroscopy. The system is paramagnetic down to 2 K, as measured by VSM. We have recorded the PL spectra in the Faraday geometry for magnetic fields of up to 7 tesla in the 0-50 K temperature range. The PL was excited at 780 nm and the emission is centered at 940 meV with a FWHM of 100 meV. In the presence of a magnetic field the emission becomes strongly $\sigma $+ polarized (P = 35{\%} at 4 tesla), suggesting carrier spin polarization. The polarization is temperature sensitive and decreasing sharply with increasing temperature. The polarization vanishes at around T = 40 K. The degree of spin polarization can be tuned by quantum confinement.   

All-Optical observation of Nuclear Magnetic Resonance in a 2D electron system

Electron-nuclear spin interaction may be utilized to manipulate nuclear states coherently in quantum computation. Here we report on an all-optical observation of nuclear magnetic resonance (NMR) in a 2D electron system embedded in a GaAs/AlGaAs heterostructure. In analogy to radio-frequency fields used in traditional NMR, circularly polarized light creates electron spins in semiconductors whose hyperfine coupling with nucleis could tip nuclear moments. At a fixed time-delay $\sim $12.5ns, time-resolved Kerr-rotation (TRKR) signals were measured as a function of the modulation frequency (from 1 KHz to 100 kHz) of the pump laser. Spin-polarized carriers generated by the pump laser pulse train acts on the nuclear spins as an effective rf magnetic field synchronized at the pulse repetition frequency, which resonates nuclear spins at suitable frequencies in an external magnetic field in Voigt geometry. Several dips were observed in a TRKR trace at, which shift linearly with increasing magnetic field. They are ascribed to be NMR signals of elements As-75, Ga-69, Al-27 and Ga-71.   

Confinement and Diffusion Effects in Dynamical Nuclear Polarization in Low Dimensional Nanostructures

We investigate the dynamic nuclear polarization as it results from the hyperfine coupling between nonequilibrium electronic spins and nuclear spins in semiconductor nanostructures. The natural confinement provided by low dimensional nanostructures is responsible for an efficient nuclear spin - electron spin hyperfine coupling [1] and for a reduced value of the nuclear spin diffusion constant [2]. In the case of optical pumping, the induced nuclear spin polarization is position dependent even in the presence of nuclear spin diffusion. This effect should be measurable via optically induced nuclear magnetic resonance or time-resolved Faraday rotation experiments. We discuss the implications of our calculations for the case of GaAs quantum well structures.\\[4pt] [1] I. Tifrea and M. E. Flatt\'{e}, Phys. Rev. B 84, 155319 (2011).\\[0pt] [2] A. Malinowski and R. T. Harley, Solid State Commun. 114, 419 (2000).   

Session W15: Focus Session: Spins in Metals - Domain Wall, Vortex, Magnonic Based Devices

High speed domain wall motion in MgO-based magnetic tunnel junctions driven by perpendicular current injection

The ability to efficiently drive fast domain wall (DW) motion will pave the way for revolutionary new electronic devices ranging from DW-MRAMs to spintronic memristors. The majority of domain wall devices use a lateral, current-in-plane configuration in which critical current densities for domain wall motion remain quite high, typically being on the order of 100 MA/cm$^{2}$ with velocities generally limited to about 100 m/s. In this contribution we show that critical current densities can be decreased by up to two orders of magnitude using the current-perpendicular-to-plane geometry. Indeed, we demonstrate that a DW can be propagated back and forth along the free layer of a MgO-based magnetic tunnel junction (MTJ) in the absence of an external magnetic field using current densities that are on the order of 5 MA/cm$^{2}$. More notably however, we obtain high domain wall velocities for these low current densities: the MTJ's large resistance variations allow us to carry out time-resolved measurements of the wall motion from which we evidence DW velocities exceeding 500m/s.   

Thermal activation energy for domain wall motion in out-of-plane magnetized submicron strips

We have experimentally studied micrometer-scale domain wall (DW) motion driven by magnetic field and electric current in 500-nm wide out-of-plane magnetized Co/Pt multilayer strips with Co layer thicknesses 0.5-0.7 nm. The scaling of thermal activation energy for DW motion with driving field and current has been extracted directly from the temperature dependence of the DW velocity. For DWs driven by field, the activation energy follows creep and depinning dynamics below a critical field and collapses to zero above the critical field. DW motion assisted by current shows velocity enhancement independent of current polarity, but causes no measurable change in the activation energy barrier. Through this analysis, the observed current-induced DW velocity enhancement in these Co/Pt multilayer strips can be entirely and unambiguously attributed to Joule heating.   

Intrinsic domain wall flexing from current-induced spin torque

Spin torque generated by coherent carrier transport in domain walls [1] is a major component in the development of spintronic devices [2]. We model spin torque in N\'eel walls [3] using a piecewise linear transfer-matrix method [4] to calculate spin torque on interior wall segments. For a $\pi$ wall with a total positive torque (current left-to-right), we find the largest positive and negative spin torques left of the central region, 4-5 orders of magnitude larger than the center. The wall's rightward push comes from the back of the wall; all other significant regions pull to the left. Adding a second wall (both walls with positive total torque) changes the first wall little, but produces spin torques in the second wall with large canceling torques on the left, and the push rightward from a smaller torque on the right. The gradient of torque across the wall generates an intrinsic domain wall flexing (distinct from extrinsic wall flexing from pinning centers [5]). Work supported by an ARO MURI.\\[4pt] [1] M. Yamanouchi et al., Nature 428, 539 (2004).\\[0pt] [2] S. Parkin et al., Science 320, 190 (2008)\\[0pt] [3] G. Vignale and M. Flatt\'e, Phys. Rev. Lett. 89, 098302 (2002)\\[0pt] [4] E. Golovatski and M. Flatt\'e, Phys. Rev. B, 84, 115210 (2011)\\[0pt] [5] A. Balk et al., Phys. Rev. Lett. 107, 077205 (2011).   

Spinmotive force due to domain wall motion in high field regime

Spinmotive force associated with a moving vortex domain wall is investigated numerically. Dynamics of magnetization textures such as a domain wall exerts a non-conservative spin-force on conduction electrons [1], offering a new concept of magnetic devices [2]. This spinmotive force in permalloy nanowires has been detected by voltage measurement [3] where magnitude of the signal is limited less than 500 nV. Theoretically it is suggested that the spinmotive force signal increases as a function of external magnetic fields. At higher magnetic fields, however, the wall propagation mode becomes rather chaotic involving transformations of the wall structure and it remains to be seen how the spinmotive force appears. Numerical simulations show that the spinmotive force scales with the field even in a field range where the wall motion is no longer associated coherent precession. This feature has been tested in a recent experiment [4]. Further enhancement of the spinmotive force is explored by designing ferromagnetic nanostructures [5] and materials. [1] S. Barnes and S. Maekawa, PRL (2007). [2] S. Barnes, J. Ieda, and S. Maekawa, APL (2006). [3] S. A. Yang et al., PRL (2009). [4] M. Hayashi, J. Ieda et al., submitted. [5] Y. Yamane, J. Ieda et al., APEX (2011).   

All-magnonic spin-transfer torque and domain wall propagation

In this talk, we will discuss the spin-transfer torque (STT) between magnons and magnetic domain wall (DW). It is found that a spin wave passes through a transverse magnetic DW in a magnetic nanowire without reflection. A magnon, the quantum of the spin wave, carries opposite spins on the two sides of the DW. As a result, there is a spin angular momentum transfer from the propagating magnons to the DW. This magnonic STT can efficiently drive a DW to propagate in the opposite direction to that of the spin wave. In comparison with the electronic STT, the energy consumption is much lower when the magnonic STT is used to drive a DW propagating at a useful velocity. Since this STT does not require any itinerant electrons, it opens a door of using magnetic insulators like YIG in spintronics devices. One extra benefit of using magnetic insulators is the low damping coefficient so that it should further lower the energy consumption and increase operation efficiency.   

Disentangling the physical contributions to the anomalous Hall effect and domain wall resistance in isoelectronic L1$_{0}$-FePd and L1$_{0}$-FePt alloys

We analyze the origin of the electrical resistance arising in domain walls of perpendicularly magnetized materials by considering a superposition of anisotropic magnetoresistance and the resistance implied by the magnetization chirality. The domain wall profiles of L1$_{0}$-FePd and L1$_{0}$-FePt are determined by micromagnetic simulations based on which we perform first principles calculations to quantify electron transport through the core and closure region of the walls. The wall resistance, being twice as high in L1$_{0}$-FePd than in L1$_{0}$-FePt, is found to be clearly dominated in both cases by a high gradient of magnetization rotation, and not by the spin-orbit interaction driven anisotropic magnetoresistance effect. Concerning the anomalous Hall effect on the other hand, we show that difference in spin-orbit interaction strength of Pt and Pd atoms leads to a pronounced cross-over from an extrinsic side jump mechanism in L1$_{0}$-FePd to an intrinsic Berry-phase anomalous Hall effect in L1$_{0}$-FePt.   

Stochastic dynamics of vortex cores in a pinning potential observed via torsional magnetometry

Measurements of a single 1 micrometer diameter permalloy disk fabricated on a silicon nitride torsional resonator are presented. Sensitivity of this device is sufficiently high to allow study of the vortex core interaction with very weak pinning sites. Low speed stochastic dynamics are revealed and attributed to the vortex core hopping between bistable states around pinning sites. The temperature and field dependence of this noise around the pinning site was investigated. Analysis employing an analytical description of the vortex in a thin disk and an Arrhenius model allows determination of the pinning potential.   

Dimensionality crossover in magnetic vortex dynamics

The ground state of a micron diameter ferromagnetic disk is often a single magnetic vortex, in which the gyrotropic mode of the vortex core is the lowest frequency excitation. In thin disks (thickness $L\ll $ diameter $D$), the vortex can be treated two-dimensionally (2D), and the gyrotropic frequency $f_G$ is determined by $L/D$. We have observed a crossover from 2D to 3D dynamics as the thickness increases. Using time-resolved Kerr microscopy, we have investigated the gyrotropic mode in 1~$\mu$m diameter Ni$_{80}$Fe$_{20}$ disks as a function of $L$, which was varied from 20 nm to 200 nm. In thin disks ($L < $ 80~nm), $f_G$ is approximately proportional to $L$. For $L > 100$~nm, $f_G$ increases less rapidly than predicted by the 2D model, and an additional gyrotropic mode appears at higher frequencies. We have explored the thickness dependences of both modes. In the ordinary gyrotropic mode, which is magnetostatic in character, the core oscillates uniformly through the thickness of the disk. The second gyrotropic mode is exchange-dominated, and the core oscillates with larger amplitude at the surfaces and a node in the equatorial plane of the disk. In the thickest disks, the exchange-dominated mode is the lowest in frequency.   

Magnonic band gaps in films with periodically modified surfaces

It is of current interest to understand the electromagnetic response of different nano-structures. In this study we focus on the role of geometry in ferromagnetic modes and response. Specifically we consider a ferromagnetic thin film with periodically perturbed surfaces, in the magnetostatic limit. We focus on the changing behavior of surface modes of the unperturbed film. Our film shows a behavior of interest since magnons propagate in it with band gaps associated to the geometry, i.e. they may be controlled by design. A reduced Brillouin zone scheme is introduced to describe the modes, which are of the Bloch type. Different bands are identified, and they are calculated numerically. For small geometric perturbations we develop a perturbation theory that agrees with our numerical results, and we obtain analytic expressions for the band gaps at the edges of the Brillouin zone. The underlying theory used to calculate the modes was previously developed, and relies on solving integral equations along the edges of the sample for the magnetostatic potential. We also calculate the response to magnetic field waves of a given wavevector that travel along the modulation direction, finding effective line width increments at the edge of the Brillouin zone, where the bands strongly couple.   

Manipulation of propagating spin waves in straight and curved magnetic microstrips

The main challenges in realizing magnonics devices are the generation, manipulation and detection of spin waves, especially in metallic magnetic materials where the length scales are of interest for applications. We have studied the propagation of spin waves in transversely magnetized Permalloy (Py) microstrips of different shapes using micro-Brillouin light scattering. The Py stripe was 30-nm thick, several micrometers wide and $>$50 $\mu$m long. Spin waves were excited in the Py strip using a 2-$\mu$m wide antenna. We compare the spin wave propagation along a straight wire to the propagation along a magnetic microstrip with a smooth bend. We will also discuss the use of a current through a gold wire under the Permalloy to provide a local magnetic field to maintain a transverse magnetization around the bend.   

Dynamics of wavepacket of magnetostatic spin wave in magnet

A semiclassical equation of motion of a wave packet of the magnetostatic spin wave is theoretically constructed. There appears the Berry curvature as an anomalous velocity term in the equation of motion, which causes characteristic orbital motions of the wave packet such as a self-rotational motion and a motion along the edge of the system. Due to the symmetry, the Berry curvature in the case of a thin film of an insulating ferromagnet appears when the magnetization is perpendicular to the film. We show a numerical calculation of the Berry curvature for this mode, i.e., the magnetostatic forward volume wave mode. Around the degeneracy point, the Berry curvature from the highest energy band enhances and that from other bands decreases. We also propose experimental settings to observe this effect.   

Nonlinear Behavior for the Uniform Mode and Horizontal Standing Spin Wave Modes in Metallic Ferromagnetic Microstrips: Experiment and Theory

Micron sized rectangular ferromagnetic bars have a variety of spin excitations, including a quasi-uniform mode, horizontal and vertical standing spin wave modes, and edge and corner modes. When driven by a strong microwave field, these modes differ from those found in the linear regime. For example, the resonance field or frequency becomes amplitude dependent. We study the nonlinear spin dynamics in such microstrips both experimentally and theoretically for a geometry where the static magnetic field is perpendicular to the plane of the sample. Experimentally it is found that, at a fixed microwave frequency, the resonance field for the uniform mode is significantly reduced as the microwave power is increased. In contrast, the resonance fields for the standing horizontal spin wave modes are only slightly reduced. This behavior is confirmed theoretically using micromagnetic calculations, and an intuitive explanation for this behavior is developed.   

Efficient computation of Magnon dispersions within Time Dependent Density Functional Theory Using Maximally Localized Wannier Functions

An efficient scheme is presented to compute the transverse magnetic susceptibility within time dependent density functional theory from which magnon dispersions can be extracted. The scheme makes use of maximally localized Wannier functions in order to interpolate the band structure onto a fine k-mesh in order to converge sums on the first Brillouin zone. An optimal real space basis set containing few basis functions is shown to be sufficient to extract the magnon dispersion, making computations very efficient. The gap error in the magnon dispersion at $\Gamma$, numerically violating Goldstone's theorem, is analysed and a correction scheme is devised which can be generalized to systems where Goldstone's theorem does not apply. The method is applied to the computation of the magnon dispersion of bulk bcc iron and fcc nickel.   

Electron Paramagnetic Resonance of Transition Metal Ions: New Relativistic Effects at High Magnetic Fields

We calculate the shift in the Electron Spin Resonance (ESR) frequency due to an inhomogeneous term in the equation of motion describing the precession of the angular momenta of relativistic electrons coupled to a static magnetic field. It is proposed that the calculated frequency shift may be observed in transition metal complexes in which the contributions from the ligand field are completely and precisely known. Furthermore, it is shown that a measurable transient oscillation of the dipole moment occurs after the external magnetic field is suddenly switched off.   

Session W16: Quantum Criticality

Pairing of critical Fermi-surface states

States of matter with a sharp Fermi-surface but no well-defined Landau quasiparticles are expected to arise in a number of physical systems. Examples include i) quantum critical points associated with the onset of order in metals, ii) the spinon Fermi-surface (U(1) spin-liquid) state of a Mott insulator and iii) the Halperin-Lee-Read composite fermion charge liquid state of a half-filled Landau level. In this work, we use renormalization group techniques to investigate possible instabilities of such non-Fermi-liquids to pairing. We show that for a large class of phase transitions in metals, the attractive interaction mediated by order parameter fluctuations always leads to a superconducting instability, which preempts the non-Fermi-liquid effects. On the other hand, the spinon Fermi-surface and the Halperin-Lee-Read states are stable against pairing for a sufficiently weak attractive short-range interaction. However, once the strength of attraction exceeds a critical value, pairing sets in. We describe the ensuing quantum phase transition between i) the U(1) and the Z$_2$ spin-liquid states, and ii) the Halperin-Lee-Read and Moore-Read states.   

Fractionalized Fermi liquids as ground states of single band tJ models

Recently it has been argued that the normal state of underdoped cuprates might realize a fractionalized Fermi liquid (FL*), where electron-like quasiparticles couple to fractionalized excitations of a fluctuating antiferromagnetic background. The corresponding Fermi surface consists of pockets, the area of which is determined by the density of doped charge carriers alone as opposed to the total density of electrons, thereby violating Luttinger's theorem. Most previous studies of the FL* phase were based either on two-band models such as the Kondo lattice model, or on phenomenological models where Fermions are coupled to a fluctuating unit vector field representing the local Neel order. In this talk I will show that the FL* phase can indeed arise as the ground-state of a single band tJ model and discuss its implications.   

Doping the Kane-Mele-Hubbard model: A Slave-Boson Approach

We study the Kane-Mele-Hubbard model both at half-filling and away from half-filling using a slave-boson mean-field approach at zero temperature. We obtain a phase diagram at half-filling and discuss its connection to recent results from quantum Monte Carlo, cellular dynamical mean field, slave-rotor, and $Z_2$ mean-field studies. In particular, we find a small window in parameter space where a spin liquid phase with gapped spin and charge excitations reside. Upon doping, we show the spin liquid state becomes a superconducting state by explicitly calculating the singlet pairing order parameters. Interestingly, we find an ``optimal" doping for such superconductivity. Our work reveals some of the phenomenology associated with doping an interacting system with strong spin-orbit coupling and intermediate strength electron-electron interactions.   

Stripe melting and quantum criticality in correlated metals

In the last several years evidence for the occurrence of stripe and related orders has accumulated in many underdoped cuprates. With increasing doping the stripe ordering tendency disappears. This has given rise to the idea that a stripe melting quantum phase transition in the ``underlying normal state'' may play a role in some of the physics of the optimally doped strange metal. However there is currently no controlled understanding of a stripe-disordering phase transition in the presence of a Fermi-surface of electrons. We obtain a controlled critical theory of a continuous melting transition of charge stripes in a metal by proliferating pairs of dislocations in the stripe-order parameter, without proliferating single dislocations. At such a (deconfined) quantum critical point (QCP) the fluctuations of the stripe order parameter are strongly coupled, yet tractable. They also decouple dynamically from the Fermi-surface. We find that the full Fermi-surface and the associated Landau quasiparticles remain sharply defined at the QCP. On the stripe ordered side the reconstruction of the Fermi surface occurs at an energy scale that is parametrically different from that associated with the onset of stripe order.   

Quantum Criticality for Extended Nodes on the Bethe Lattice

Theoretical description of anisotropic systems, such as layered superconductors and coupled spin chains, is often a challenge due to the different natures of interactions along different directions. As a model of such a system, we present an analytical study (1) of $d$-dimensional ``nodes" arranged as the vertices of a Bethe lattice, where each node has nonzero spatial dimension and is described by an $O(N)$ quantum rotor model, and there is hopping between neighboring nodes. In the limit of large connectivity on the Bethe lattice, the hopping can be treated by constructing a self-consistent effective action for a single node. This procedure is akin to dynamical mean field theory, but generalized so that spatial as well as quantum fluctuations are taken into account on each node. The quantum phase transition is studied using this effective action for both infinite and finite $N$. The importance of the Perron-Frobenius uniform mode on the Bethe lattice is discussed, and its elimination via an ``infinite range hopping" term shifts the transition, leading to nontrivial critical behavior. We calculate critical exponents and find that the internode hopping reduces the upper and lower critical dimensions each by one. \\ (1) arXiv:1111.2011   

Quantum Criticality on Graphs

One of the tenets of our understanding of strongly correlated systems is that their macroscopic critical behavior is often universal and independent of any microscopic details. Continuous phase transitions driven by either thermal or quantum fluctuations can be placed in universality classes defined by the spatial dimension and the symmetry of a Landau-like order parameter. In an attempt to further understand the role of the spatial dimension in universality, we imagine a system whose symmetries and connectivities are controlled by the low-lying spectrum of degrees of freedom on a graph. As an elucidating example, we study a quantum phase transition on large random regular graphs, with coupled vertices described by $d$-dimensional $\mathrm{O}(N)$ quantum rotor models.   

Entropic Signatures of Quantum Criticality in Itinerant Ferromagnets and Metamagnetic Systems

We investigate the thermodynamic properties of itinerant ferromagnets near quantum critical points described by a quantum Landau-Ginzburg $\phi^4$ theory. We show that the quartic coupling in this theory, which is dangerously irrelevant, has a singular contribution to the renormalized Gaussian free energy. We trace this singularity to some ultraviolet contributions, thereby demonstrating its unphysical nature. We introduce a procedure to regularize this singularity, and apply the prescription to calculate thermodynamic quantities across ferromagnetic quantum critical points in both two and three dimensions. Our calculation illustrates various thermodynamic signatures of quantum criticality, including the entropy accumulation at the quantum critical point which was first proposed on scaling grounds [1]. We systematically compare our theoretical results with the experimental data on entropy and specific heat as a function of magnetic field in Sr3Ru2O7 [2]. We demonstrate that the thermodynamic data are compatible with a quantum critical scenario, but the critical behavior does not agree well with the conventional itinerant ferromagnetic quantum criticality picture. [1] L. Zhu et al PRL 91, 066404 (2003). [2] A.W. Rost et al, Science 325, 1360 (2009).   

Quantum criticality in a dissipative (2+1)-dimensional $XY$ model of circulating currents in high-$T_{\rm c}$ cuprates

We present large-scale Monte Carlo results for the dynamical critical exponent $z$ and the spatio-temporal two-point correlation function of a (2+1)-dimensional quantum $XY$ model with bond dissipation, proposed to describe a quantum critical point in high-$T_c$ cuprates near optimal doping. The phase variables of the model, originating with a parametrization of circulating currents within the CuO$_2$ unit cells in cuprates, are compact, $\{ \theta_{vvr,\tau} \}$ $\in [-\pi,\pi \rangle$. The dynamical critical exponent is found to be $z \approx 1$, and the spatio-temporal correlation functions are explicitly demonstrated to be isotropic in space-imaginary time. The model thus has a fluctuation spectrum where momentum and frequency enter on equal footing, rather than having the essentially momentum-independent marginal Fermi liquid-like fluctuation spectrum previously reported for the same model.   

Strong Electron Correlation by Virtual Phonon Exchange in Jahn-Teller Crystals

In Jahn-Teller crystals - crystals with at least one sublattice of ions with orbitally degenerate electronic states - virtual phonon exchange is a major source of strong electron correlation. This type of electron correlation leads to different structural and magnetic transitions the interplay of which is especially interesting in case of the triple degeneracy of the electronic ground states. The interest to these systems lately has increased as it is related to some unusual situations in perovskite and spinel structure compounds that are of big practical interest (multiferroicity, piezomagnetism, and others). Mutual influence of the antiferrodistortive XY-type and ferroelastic ZZ-type orderings mediated by magnetic external or internal interactions in such a type of crystals is under discussion. It is found that even in case of stronger interactions leading to the ferroelastic ordering the presence of magnetic interaction causes new type of structural phase transition from the ZZ- type of ordering to the XY-type. These structural transformations are accompanied by specific anomalies in the temperature and external magnetic field dependences. The thermodynamics of the systems with strong electron correlation leading to such a phase transitions is analyzed.   

Features of Fermi Systems near $\ell$=0 Pomeranchuk Instabilities: A Crossing Symmetric Approach

In Fermi systems, interactions can cause symmetry-breaking deformations of the Fermi surface, called Pomeranchuk instabilities. In Fermi liquid (FL) language, this occurs when one of the Landau harmonics F$_{\ell}^{a,s}$ $\rightarrow$ -(2$\ell$ + 1); e.g. F$_{0}^{a,s}$ = -1 are related to ferromagnetic (a), and density instabilities (s) resepctively. The corresponding point in parameter space may be viewed as a quantum critical point (QCP). Using graphical and numerical methods to solve coupled non-linear integral equations of a crossing symmetric equation (TSCE) scheme, we study the behavior of spin/density excitations; effective mass; ferromagnetic, spin density wave, phase separation, and pairing transitions near $\ell$=0 Pomeranchuk instabilities in a 3D Fermi system. Considering momentum dependence of the renormalized FL interactions, we find a number of results for repulsive and attractive couplings of arbitrary strengths; viz. attraction in both singlet and triplet pairing amplitudes (though singlet pairing is primarily favored); possibility of a second ferromagnetic transition due to spin waves, and possibility of phase separation with and without ferromagnetic transition. Some of our results may apply to ferromagnetic superconductors, such as UGe$_{2}$ and UIr.   

Phase reconstruction near to the two-dimensional ferromagnetic quantum critical point

We study the formation of new phases in two dimensions near to the putative quantum critical point of the itinerant ferromagnet to paramagnet phase transition. In addition to the first order and helimagnetic behaviour found in non-analytic extensions to Hertz-Millis theory [1] and in the quantum order-by-disorder approach [2], we find a small region of spin nematic order. Our approach also admits a concurrent formation of superconducting order. We further study the effect of small deformations from quadratic electron dispersion -- as previously found in three dimensions, these enlarge the region of spin nematic order at the expense of spiral order.\\[4pt] [1] D. Belitz, T.R. Kirkpatrick and T. Vojta, Rev. Mod. Phys. \textbf{77}, 579 (2005),. V. Efremov, J.J. Betouras, A.V. Chubukov Phys. Rev. B\textbf{ 77}, 220401(R), (2008)\\[0pt] [2] G. J. Conduit Phys. Rev. A \textbf{82}, 043604 (2010)   

Divergence of the effective mass in a strongly-interacting 2D electron system

The diffusion thermopower in a low-disorder, strongly-interacting 2D electron system in silicon increases with decreasing electron density and tends to infinity at a finite density $n_t$. Comparison with earlier data for a high-disorder silicon system indicates that the critical density $n_t$ does not depend on the degree of disorder. The increase of the thermopower is associated with a diverging electron mass in the close vicinity of an interaction-induced phase transition.   

A New View of the Mott-Hubbard Transition: Renormalization of the Fermi-Surface Topology

We present the renormalization of the (underlying) Fermi-surface topology in the Hubbard model on a square lattice with frustrating hopping, that is relevant for the physics of high-temperature superconductors. With the help of novel high precision variational tools, including Jastrow factors and backflow correlations, we show that the Fermi surface renormalizes to perfect nesting at the interaction-driven Mott-Hubbard transition and in the large interaction limit. Moreover, we present new results for the density-driven Mott-Hubbard transition, investigating the Fermi-surface renormalization flow as a function of doping, where the renormalization occurs only when the half-filled case is insulating. We associate the flow to the appearance of a van Hove singularity at the Fermi level at small doping, that is interpreted as an instability to magnetic order. Finally, we show also that Fermi surface renormalization is associated to a strong crossover at finite doping for the critical $U$ corresponding to the Mott-Hubbard transition.   

Detecting Quantum Phase Transitions using Classical Noises

We theoretically propose that the classical noise spectra provide an efficient and straightforward way to detect the quantum phase transition points in low-dimensional quantum spin systems. By using Ornstein-Uhlenbeck noise, we employ both a quadratic response theory and time-dependent density matrix renormalization group method to study the quantum system. In the non-Markovian region, the time evolutions of physical observables exhibit distinct behaviors for different quantum phases. In addition, we have the freedom to choose various noises to detect peculiar quantum phases. This method can be used to measure the three body correlation function directly. We demonstrate that the method can determine faithfully the quantum transition points of the transverse Ising model as well as spin-1 bilinear-biquadratic Heisenberrg model. The possible experimental realizations of noise detection are discussed.   

Topological non-Fermi liquids and continuous transitions to Fermi liquids in fractional Chern states

We develop a slave-particle formulation of the Halperin-Lee-Read (HLR) non-Fermi liquid state and extend it to situations without an external magnetic field. We use this formulation to develop a theory of a continuous transition to a Fermi liquid, producing an example in which we can understand how a Fermi surface is continuously destroyed to obtain a fractionalized Fermi liquid. We discuss senses in which the HLR state should be viewed as a topological non-Fermi liquid, and finally we discuss experimental possibilities for inducing such transitions by tuning the bandwidth of a topologically non-trivial bandstructure.   

Session W17: Optics of Semiconductor Nanowires

Scalable Synthesis of Vertically Aligned, Catalyst-Free Gallium Arsenide Nanowire Arrays -- Towards Optimized Optical Absorption and Reflection.

Vertically aligned, catalyst-free nanowires hold great potential for photovoltaic applications, where scalable synthesis and optimized optical absorption are critical. Here, we report using nanosphere lithography, scalable synthesis of vertical gallium arsenide nanowires grown by selected area MOCVD. A comparative study was done between regular nanowires arrays using electron beam lithography and slightly more defective nanowire arrays using nanosphere lithography.~ Reflection of light by the nanowire array has been used as a measure to study the effects of defects in the patterned structures using NSL both experimentally and by simulation. Both studies show similar reflection behavior between nanowire prepared by EBL and NSL. GaAs nanowires as short as 130 nm show reflection of $<$10{\%} over the visible range of solar spectrum. Optimized nanowire configuration to maximize the absorption has also been discussed.   

Theory of photoluminescence polarization reversal in GaAs nanowires

The polarization of photoluminescence (PL) in wurtzite (WZ) GaAs nanowires (NW) of diameter ~100 nm has been observed to reverse as a function of temperature from perpendicular to parallel to the NW axis. We use the weak confinement limit for excitons and the envelope function approximation to study this phenomenon. The WZ GaAs crystal field and spin-orbit splittings were determined using GW calculations and agree well with resonant Raman spectra on WZ NWs. In contrast to zincblende (ZB) NWs, the crystal field splitting in WZ NWs leads to a perpendicularly polarized exciton ground state. The first excited state, however, has a parallel component and can be mixed in at slightly elevated temperature, leading to a polarization reversal. We find that a reversal can only take place for much smaller crystal field splittings than the one obtained in pure WZ. Strain induced reduction of the crystal field splitting would require an unrealistically large strain. On the other hand, multiple twinning, can lead to a substantially lower crystal field splitting as obtained from our GW calculations for lower hexagonality polytypes, such as 4H GaAs, and can thus explain the observed polarization reversals.   

Phonon Spectrum and Thermal Properties of free standing $<$100$>$ and $<$111$>$ InGaAs alloy nanowires

The phonon spectra in zinc blende InAs, GaAs and their ternary alloy nanowires (NWs) are computed using an enhanced valence force field (EVFF) model. The physical and thermal properties of these nanowires such as sound velocity, elastic constant, specific heat (Cv), phonon density of states, phonon modes, and the ballistic thermal conductance are explored. The calculated transverse and longitudinal sound velocities along $<$100$>$ direction in these NWs are $\sim $25{\%} and 20{\%} smaller compared to the bulk velocities, respectively. These velocities along $<$111$>$ direction are about twice smaller than bulk values. The Cv for NWs are about twice as large as the bulk values due to higher surface to volume ratio (SVR) and strong phonon confinement in the nanostructures. The temperature dependent Cv for InAs and GaAs nanowires show a cross-over at 180K and 155K along $<$100$>$ and $<$111$>$ directions respectively. It happens due to higher phonon density in InAs nanowires at lower temperatures. With the phonon spectra and Landauer's model the ballistic thermal conductance is reported for these III-V NWs. The results in this work demonstrate the potential to engineer the thermal behavior of III-V NWs.   

How and why a magnetized quantum wire can act as an optical amplifier

The fundamental issues associated with the magnetoplasmon excitations are investigated in a quantum wire characterized by a confining harmonic potential and subjected to a perpendicular magnetic field. Essentially, we embark on the device aspects of the intersubband collective (magnetoroton) excitations which observe a negative group velocity between the maxon and the roton. The computation of the gain coefficient suggests an interesting and important application: the electronic device based on such magnetoroton modes can act as an optical amplifier.\footnote{M.S. Kushwaha, J. Appl. Phys. {\bf 109}, 106102 (2011).}   

Electronic band structure calculations of bismuth-antimony nanowires

Alloys of bismuth and antimony received initial interest due to their unmatched low-temperature thermoelectric performance, and have drawn more recent attention as the first 3D topological insulators. One-dimensional bismuth-antimony (BiSb) nanowires display interesting quantum confinement effects, and are expected to exhibit even better thermoelectric properties than bulk BiSb. Due to the small, anisotropic carrier effective masses, the electronic properties of BiSb nanowires show great sensitivity to nanowire diameter, crystalline orientation, and alloy composition. We develop a theoretical model for calculating the band structure of BiSb nanowires. For a given crystalline orientation, BiSb nanowires can be in the semimetallic, direct semiconducting, or indirect semiconducting phase, depending on nanowire diameter and alloy composition. These ``phase diagrams'' turn out to be remarkably similar among the different orientations, which is surprising in light of the anisotropy of the bulk BiSb Fermi surface. We predict a novel direct semiconducting phase for nanowires with diameter less than $\sim$15 nm, over a narrow composition range. We also find that, in contrast to the bulk and thin film BiSb cases, a gapless state with Dirac dispersion cannot be realized in BiSb nanowires.   

Gas phase interactions with bare and gold nanoparticle decorated gallium nitride nanowires by ultraviolet photoelectron spectroscopy

Ultraviolet photoelectron spectroscopy (UPS) has been used to characterize the interaction of CO and H$_{2}$O with the surface of bare and gold nanoparticle (Au NP) decorated gallium nitride nanowires at 298 K, 77 K and 20 K. The average diameter of the Au NPs is 4.5 $\pm $ 0.5 nm and the average nanowire diameter is 105 $\pm $ 75 nm. CO and H$_{2}$O do not bond to the surface of the bare GaN nanowires at 298K, 77K, or 20K. Temperature dependent UPS analysis reveals that CO and H$_{2}$O weakly physisorbed to the Au NP decorated GaN nanowires with heats of adsorption of 4.37 $\pm $ 0.03 meV and 1.25 $\pm $ 0.04 meV , respectively. The adsorption at 298K of 50 Langmuir of CO followed by 50 Langmuir of H$_{2}$O showed that CO adsorption promotes H$_{2}$O adsorption, while 50 Langmuir of H$_{2}$O followed by 50 Langmuir of CO showed that H$_{2}$O inhibits CO adsorption. The findings of this study that the adsorption of H$_{2}$O inhibits CO adsorption onto the Au NP-GaN nanowires explains previous studies of the gas sensing properties of mats of Au NP- GaN nanowires.   

Genomic design of strong direct-gap optical transition in Si/Ge core/multishell nanowires

Converting the electronically superior but optically impractical indirect-gap Si and Ge semiconductors into a strongly light-absorbing system has been a long-standing challenge, given that the phonon-assisted optical transition of the indirect gap has weak intensity, requiring thick absorbers. One of main strategies has been the use of two-dimensional (2D) layer-by-layer growth of Si/Ge superlattices (SLs). However, the maximum thickness of SLs that can be grown coherently on a substrate is limited by the lattice-mismatch-induced strain. This limitation can be greatly relaxed by changing from 2D SLs to one-dimensional quantum nanowire (NW), where much higher strain can be accommodated. With developed Vapor-Liquid-Solid based technique, experimental growth of Si/Ge core-multishell NWs has recently demonstrated a significant level of synthetic control. However, the number of possible core/multishell sequences and thicknesses might easily reach an astronomic value. We will present here a genomic search for targeted core/multishell NW geometries that give both a direct gap and a significantly enhanced dipole-allowed optical transition in the Si/Ge system, by using a combination of genetic algorithm with atomistic pseudopotential electronic-structure calculations.   

Axially-Resolved Luminescence of Individual ZnSe Nanowires

Axially resolved distributions of luminescence intensity and lifetimes along individual ZnSe nanowires were studied using two-photon excited luminescence imaging and time-correlated single photon counting techniques. The nanowires were grown on GaAs substrates via the self-catalyzed VLS mode. An intense tip, to which a gallium particle is attached, is found for the deep level (DL) emissions via luminescence imaging, while the intensity for the near band edge (NBE) emissions is more uniform. The luminescence decays at all locations of the nanowires are dominated by a fast process at early times, followed by a slow one. In addition, the shape of distribution of the lifetimes along the length of nanowire resembles a flattened letter ``U'' for the NBE emissions, but it resembles a long tailed letter ``L'' for the DL emissions. Possible explanations of these results will be discussed.   

Studies of electronic excitations of rectangular ZnO nanorods by electron energy-loss spectroscopy

Electronic excitations of single ZnO rectangular nanorod have been investigated by electron energy-loss spectroscopy in conjunction with scanning transmission electron microscopy (STEM-EELS). We focus primarily on the surface excitations greatly enhanced at the grazing incidence parallel to the surfaces of ZnO nanorods. An uncommon kind of surface excitation known as surface exciton polaritons occurring near interband transitions is found to dominate in the spectral range between the band gap at 3.4 eV and the surface plasmon peak at 15.8 eV. In addition, the dielectric function of ZnO up to 25 eV has also been derived from the bulk excitation spectra using the Kramers-Kronig analysis on a single nanorod. Theoretical EELS simulations are also compared with the experimental results and good agreements are obtained.   

Precise investigation of optical/electronic fine structures of nanostructures via cathodoluminescence spectroscopy

High special/energy resolution cathodoluminescence (CL) spectroscopy enables us to make precise investigation on the optical/electronic fine structures in nanostructures. The linear distribution of strain gradient from tensile to compression in bent ZnO nano/microwires provides ideal conditions to address the modification of the electronic structures by strain in semiconductor materials. Radial line scan of the CL spectroscopy along bent ZnO wires at liquid helium temperature shows very fines excitonic emission structures, which demonstrates systematic red shift towards the increase of tensile strain, and blue shift as well as excitonic peak splitting towards the increase of compressive strain. First-principle simulations reveal an electronic band structure evolution under continuously tuned strain.   

Raman scattering study of mixed metal oxide Ir$_{x}$Ru$_{1-x}$O$_{2}$ nanowires grown by chemical vapor deposition

We present Raman scattering results of mixed metal oxide Ir$_{x}$Ru$_{1-x}$O$_{2}$ nanowires that have been studied for their stability and for activity as electrocatalysts. For our study, 1-dimensional metallic mixed oxide single crystalline Ir$_{x}$Ru$_{1-x}$O$_{2}$ nanowires were synthesized, for the first time, via a simple physical vapor transport process by controlling relative ratios of two precursors, RuO$_{2}$ and IrO$_{2}$, respectively. We measured Raman spectra of Ir$_{x}$Ru$_{1-x}$O$_{2}$ nanowires using excitation laser sources with wavelengths of 488 nm and 632.8 nm. We observed that an E$_{g}$ phonon mode of an Ir$_{x}$Ru$_{1-x}$O$_{2}$ nanowire is being blue-shifted linearly with respect to the Ir contents. We could use our observation of frequency shift of the E$_{g}$ phonon to determine stoichiometry information of nanowires which we also measured and confirmed by using EDS. From the shape of the phonon modes we measured, we could get information regarding crystalline quality that was also measured by HRTEM. We show that Raman scattering spectroscopy can provide a simple, prompt, and effective mean to measure the stoichiometry and crystalline quality of mixed metal oxide nanowires.   

Strain effects in polytypical wurtzite/zinc-blend nanowhiskers

The recent interest on III-V nanowhiskers has led to the growth of high quality samples of systems with two different crystalline structures [1]. The crystals grown in [111]-direction for the zinc-blend (ZB) phase and [0001]-direction for the wurtzite (WZ) phase are very similar and can both be described as stacked hexagonal layers. The effect of two different structural phases coexisting in the same nanostrucure is known as polytypism and creates confinement profiles similar to a heterostructure. One can notice band offsets at the interface and the formation of electronic minibands that can be explored to design systems for device applications. Although some of the III-V compounds do not present WZ structure in bulk form, recent calculations [2] presented a theoretical prediction of their band structure. However, as they considered that ZB and WZ to have the same lattice parameters no strain effects should appear on a first approach. Our theoretical approach introduces strain effects in our previous model [3] by using group theory arguments. It allows the analysis of the biaxial strain effects for both structures in a single matrix. [1] P. Caroff et al. Nature Nanotech. 4, 50, 2009. [2] A. De and C. E. Pryor, Phys. Rev. B 81, 155210, 2010 [3] http://arxiv.org/abs/1012.022   

Quantum Zigzag Phase Transition in Quantum Wires

We use Quantum Monte Carlo (QMC) techniques to study the quantum phase transition of interacting electrons in quantum wires to a quasi-one dimensional zigzag phase. The phase diagram of particles with Coulomb interaction that undergo a linear to zigzag transition is relevant to electrons in quantum wires [Meyer et al, PRL 2007] and ions in linear traps [Simshoni et al., PRL 2011]. Interacting electrons confined to a wire by a transverse harmonic potential form a Wigner crystal at low densities; as density increases, symmetry about the axis of the wire is broken and the electrons undergo a transition to a quasi-one-dimensional zigzag phase. Using QMC, we characterize this phase transition by measuring the power spectrum and addition energies.   

Raman Spectroscopy and Strain Mapping in Individual Ge- Si$_{x}$Ge$_{1-x}$ Core-Shell Nanowires

Core-shell Ge-Si$_{x}$Ge$_{1-x}$ nanowires (NWs) are expected to contain large strain fields due to the lattice-mismatch at the core/shell interface. Here we report measurement of the core strain in such NW heterostructures by Raman Spectroscopy. We measure the diameter dependence of Raman spectra in individual Ge NWs, as well as Ge-Si$_{x}$Ge$_{1-x}$ core-shell NW heterostructures. We find that the bare Ge NWs show no diameter-dependence of the Ge-Ge peak at 300.5 $cm ^{-1}$. On the other hand, the Ge-Ge peak of the Ge-Si$_{x}$Ge$_{1-x}$ core-shell NW shows a blue shift by comparison to the bare Ge NWs. This blue shift increases with reducing the NW diameter as a result of larger compressive strain in the Ge core. While the elastic strain is expected to split the triply degenerate Ge-Ge mode into separate singlet and doublet peaks, only the singlet mode was observed in experiment, a finding explained by the NW absorption and emission anisotropy. Using lattice dynamical theory and the Raman spectroscopy results we determine the strain in Ge-Si$_{x}$Ge$_{1-x}$ core-shell NWs as a function of the NW diameter. We compare the experimental results with the strain values calculated using a continuum elasticity model.   

Session W18: Focus Session: Interfaces in Complex Oxides - Devices and Nanostructures

Mesoscopic transport properties of LaAlO$_{3}$/SrTiO$_{3}$ devices

The conducting interface between the two band insulators LaAlO$_{3}$ and SrTiO$_{3}$ [1], with its unique electronic properties, stands as a prominent example of an oxide heterostructure for the realization of multifunctional devices. Using the electric field effect the ground state of this system can be tuned from insulating to superconducting [2]. Recently we demonstrated also the possibility to tune the carriers mobility by changing the deposition conditions of the LaAlO$_{3}$ film (3) and measured Shubnikov-de Haas oscillations, whose analyses demonstrate the 2D nature of the electronic states at the interface [3,4]. We are currently focusing on the realization of devices where the 2D quantum nature of the electronic states can be fully exploited. We show the feasibility of structures with lateral dimensions down to few hundreds nanometers, using an electron beam lithography based process. The lateral confinement of the electron gas in these devices is demonstrated by a phase coherent transport regime, which can be tuned by electric field.\\[4pt] [1] A.Ohtomo et al., Nature 427, 423 (2004)\\[0pt] [2] A.D.Caviglia et al., Nature 456, 624 (2008); C.Bell et al., PRL 103, 226802 (2009)\\[0pt] [3] A.D.Caviglia et al., PRL 105, 236802 (2010)\\[0pt] [4] M.Ben Shalom et al., PRL 105,206401 (2010)   

Sketched Oxide Single-Electron Transistor

Devices that confine and process single electrons represent an important scaling limit of electronics. Such devices have been realized in a variety of materials and exhibit remarkable electronic, optical and spintronic properties. Here, we use an atomic force microscope tip to reversibly ``sketch'' single-electron transistors by controlling a metal-insulator transition at the interface of two oxides.\footnote{Cheng \textit{et al.}, Nature Nanotechnology \textbf{6}, 343 (2011).} In these devices, single electrons tunnel resonantly between source and drain electrodes through a conducting oxide island with a diameter of $\sim$1.5 nm. We demonstrate control over the number of electrons on the island using bottom- and side-gate electrodes, and observe hysteresis in electron occupation that is attributed to ferroelectricity within the oxide heterostructure. These single-electron devices may find use as ultradense non-volatile memories, nanoscale hybrid piezoelectric and charge sensors, as well as building blocks in quantum information processing and simulation platforms.   

Unexpected magnetic Exchange Interaction between Epitaxial La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ film and a two Dimensional Electron Gas at an LaAlO$_{3}$/SrTiO$_{3}$ Interface

We have examined the electrical and magnetic properties of LSMO films epitaxially grown on STO, LAO and LAO/STO substrates. Compared to LAO and STO the LSMO films on LAO/STO shows a higher metal-insulator transition temperature and also an order of magnitude larger magnetization. The magnetic hysteresis loops measured in pinning fields show a large exchange coupling for the case of the LAO/STO substrate and the sign of the coupling supports a ferromagnet to a ferromagnet exchange. The magnetization measured arises from a combination of substrate magnetism and that of the film and the enhanced conductivity, metal-insulator transition temperature and magnetization can be accounted for by the exchange coupling between the magnetic phase observed in the LAO/STO interfaces and the LSMO layer. The decay length of this interaction in LSMO is 90 nm which is surprisingly much longer than has been observed in ferromagnetic metals. Furthermore, the discovery of a long range oscillatory magnetic exchange coupling with LAO thickness suggests a role for the LAO layer beyond a simple insulator in this magnetic heterostructure.   

Quantum spin hall effect in LaAlO$_3$/SrTiO$_3$ nanostructures

LaAlO$_3$/SrTiO$_3$ heterostructures are known to exhibit strong spin-orbit coupling. We investigate local and non-local transport behavior of nanoscale Hall crosses created by conductive AFM lithography. The four-terminal resistance of these structures is consistently found to be $\sim$$h/e^2$, independent of the length of the channel. We also observe large (1-10 k$\Omega$) non-local resistances and zero-field Hall resistance that are attributed to quantum spin Hall phase with a spin-orbit derived pseudo-magnetic fields $B_{eff}\sim 15$ T. The pure spin current is blocked by Cooper pairs that form below Tc~200 mK, leading to a collapse of the non-local and zero-field Hall resistances.   

Nonvolatile Ferroelectric Manipulation of Electronic Structure at LAO/STO Heterointerface

Hetero-interfaces between different oxide insulators have attracted a lot of interests. One of the most important system is the 2D electron gas at LaAlO$_{3}$(LAO) and SrTiO$_{3}$ (STO), which had been reported to possess metallic conduction and superconductivity. In this study, the top-patterned Pb(Zr$_{0.2}$Ti$_{0.8})$O$_{3}$ ferroelectric layer epitaxially grown on LAO/STO was proposed as a nonvolatile electronic modulation, and the interface band deformation was investigated using photoelectron spectroscopy (PES). Result showed different thickness and polarization state of top PZT significantly affected the band structure and its corresponding valence band offset at the LAO/STO hetero-interface. The transport data indicated that the as-grown PZT would deplete the conducting interface of LAO/STO, while switching the polarization of PZT would enhance the interface conduction.   

Non-local piezoresponse in 3 u.c. LaAlO$_3$/SrTiO$_3$

Nanoscale control of the metal-insulator transition in 3 unit cell (u.c.) LaAlO$_3$/SrTiO$_3$ heterostructures can be achieved by the conducting AFM lithography,\footnote{C. Cen, S. Thiel, G. Hammerl, C. W. Schneider, K. E. Andersen, C. S. Hellberg, J. Mannhart, and J. Levy, Nat. Mater. \textbf{7}, 2136 (2008)} however the mechanism behind this transition is still not well understood. One proposed mechanism invokes ionic transport through the LaAlO$_3$ layer.\footnote{A. Kumar, S. Jesse, A. Gruverman, C. B. Eom, S.V.Kalinin, unpublished} We have performed a variety of local and non-local piezoforce measurements on 3 u.c. LaAlO$_3$/SrTiO$_3$ heterostructures. The existence and nature of the non-local piezoelectric effect places strong constraints on the origin of the piezoelectric response. This work is supported by NSF DMR-1104191.   

Piezoresponse Force Microscopy of Gated LaAlO$_3$/SrTiO$_3$ Heterostructures

The quasi-two-dimensional (q-2DEG) electron liquid at the LaAlO$_3$/SrTiO$_3$ (LAO/STO) interface can be tuned through the metal-insulator transition using a metallic top gate. At low carrier densities, the capacitance between the top gate and q-2DEG is significantly enhanced beyond the geometric capacitance.\footnote{Lu Li, C. Richter, S. Paetel, T. Kopp, J. Mannhart, R.C. Ashoori, Science \textbf{332}, 825 (2011)} In order to understand the origins of this enhancement in capacitance, we have performed spatially resolved piezo-force microscopy (PFM) on a top-gated 5u.c. LAO/STO structure. A large enhancement in piezoresponse is observed as the interface is switched to the conducting phase. Within the transition region, spatial structures or domains are observed. We propose that such measurement can provide new insights into the metal-insulator transition of the interface, and the associated capacitance enhancement.   

Ferroelectric control of two dimensional electron gas in oxide heterointerface

Oxide heterointerfaces are emerging as one of the most exciting materials systems in condensed-matter science. One remarkable example is the LaAlO$_{3}$ /SrTiO$_{3}$ (LAO/STO) interface, a model system in which a highly mobile electron gas forms between two band insulators. Our study to manipulate the conductivity at this interface by using ferroeletricity of Pb(Zr,Ti)O$_{3}$. Our transport data strongly suggests that down polarization direction depletes the conducting interface of LAO/STO. After switching the polarization direction (up), it becomes accumulation. In addition, our experiments show there is obvious the band structure changed by cross-sectional scanning tunneling microscopy and combining with X-ray photoelectron spectroscopy (XPS) measurements. The transport properties are measured to build up the connection between macroscopic properties and local electronic structures that have been applied to study this structure. Controlling the conductivity of this oxide interface suggests that this technique may not only extend more generally to other oxide systems but also open much potential to ferroelectric field effect transistors.   

Anomalous High Mobility in LaAlO$_3$/SrTiO$_3$ Nanowires

Nanoscale control over the LaAlO$_3/$SrTiO$_3$ interface\footnote{C. Cen, S. Thiel, K. E. Andersen, C. S. Hellberg, J. Mannhart, and J. Levy, Nature Materials \textbf{7}, 2136 (2008).} provides a possible pathway for reconfigurable oxide-based nanoelectronics at densities that exceed conventional silicon electronics. One of the central challenges in replacing silicon relates to energy dissipation, which in turn depends on the carrier mobility. We have performed four-terminal transport measurements of LaAlO$_3/$SrTiO$_3$ nanowires at room temperature (300 K) and at low temperature ($\sim 500$ mK). We find that the equivalent 2D mobility of nanowires greatly exceeds that of bulk SrTiO$_3$ ($\mu_{STO}=6$ cm$^2$/Vs), and approaches that of optimally doped Si at room temperature. Low-temperature mobilities can exceed 30,000 cm$^2$/Vs. We discuss possible physical mechanisms to explain the anomalously high mobility and the implications for future device technologies.   

First-principles modeling of Pt/LaAlO$_{3}$/SrTiO$_{3}$ nanocapacitors under an external bias potential

We study the electronic, structural and electrical properties of Pt/LaAlO$_{3}$/SrTiO$_{3}$ nanocapacitors under the action of an external applied bias, using first-principles calculations performed at fixed electric displacement $D$. In particular, we deduce a complete set of \emph{ab initio} band diagrams and a simple analytical expression for the electric field within the LaAlO$_{3}$~(LAO) film as a function of thickness and applied potential. In addition, we investigate the capacitance of the metal-oxide heterostructure in a field-effect transistor setup. We find that the electric field within LAO is a non-intrinsic quantity that monotonically decreases with increasing LAO thickness. The occurrence of spontaneous Zener tunneling in this system, therefore, is ruled out. We discuss the implications of our results in the light of recent experimental observations involving biased LAO/STO junctions and metallic top electrodes.   

THz rectified optical response of LaAlO$_{3}$/SrTiO$_{3}$ nanojunctions

Conducting AFM lithography can be used to create a variety of nanoscale devices at the LaAlO$_{3}$/SrTiO$_{3}$ interface\footnote{C.Cen et al., Nature Material, \textbf{7}, 298 (2008)}\footnote{C.Cen et al., Science, \textbf{323}, 1026 (2009)}. Nanoscale junctions have been shown to exhibit strongly localized photoconductivity over a range of wavelengths spanning the visible and near-infrared regime\footnote{P.Irvin et al., Nature Photonics, \textbf{4}, 849 (2010)}. Power-dependent and interferometric measurements of these nanostructures with ultrafast laser pulses reveal a nonlinear photoconductive response attributed to second-order nonlinear susceptibility of SrTiO$_{3}$. The breaking of inversion symmetry comes from the strong local electric field that extends across the junction. The ultrafast response of these nanojunctions make them attractive candidates for generation and detection of THz radiation at molecular scales. This work is supported by NSF DMR-1104191.   

Tuning the two-dimensional electron gas at the LaAlO$_{3}$/SrTiO$_{3}$(001) interface by metallic contacts

Density functional theory calculations reveal that adding a metallic overlayer on LaAlO$_{3}$/SrTiO$_{3}$(001) reduces/eliminates the electric field within the polar LaAlO$_{3}$ film and thus suppresses the thickness-dependent insulator-to-metal transition observed in uncovered films. Independent of the LaAlO$_{3}$ thickness both the surface and the interface are metallic, with an enhanced interface carrier density relative to LaAlO$_{3}$/SrTiO$_{3}$ (001) after the metallization transition. Moreover, a monolayer thick metallic Ti-contact exhibits a finite magnetic moment and for a thin SrTiO$_{3}$-substrate induces a spin-polarized 2D electron gas at the $n$-type interface due to confinement effects. The height of the Schottky barrier formed between the metal contact and LaAlO$_{3}$ depends strongly on the choice of the overlayer and allows to tune the carrier density at the interface [1]. \\[4pt] [1] V. Ruiz L\'{o}pez, R. Arras, W. E. Pickett, and R. Pentcheva, arXiv:1106.4205v1.   

Spatial Analogue of Quantum Spin Dynamics via Spin-Orbit Interaction

We theoretically demonstrate the mapping of electron spin dynamics from time to space in quantum wires with built-in spatially uniform and oscillating Rashba spin-orbit coupling in orthogonal directions. The presence of the spin-orbit interaction introduces pseudo-Zeeman couplings of the electron spins to effective magnetic fields. By periodically modulating the spin-orbit coupling along the quantum wire axis, it is possible to create the spatial analogue of spin resonance, without the need for real magnetic fields. The mapping of time-dependent operations onto a spatial axis suggests a new mode for quantum information processing in which gate operations are encoded into the band structure of the material. We describe a potential realization of such a material within nanowires at the interface of LaAlO$_{3}$/SrTiO$_{3}$ heterostructures.   

Session W19: Invited Session: Spin Coupling and Kondo Screening in Individual Magnetic Spins

The Impact of the Local Environment on the Kondo Screening of a High-Spin Atom

Spin 1/2 Kondo systems have been investigated extensively in theory and in a variety of experimental geometries. However the magnetic atoms that give rise to the Kondo effect in metals often have a larger spin, which makes the properties of the system more complex. Using low-temperature scanning tunneling microscopy and spectroscopy, we explore the Kondo effect of individual high-spin magnetic atoms on small islands of the thin insulator copper nitride (Cu$_{2}$N) in Cu(100) surfaces. Using a combination of elastic spectroscopy to probe the local density of states features arising from the Kondo screening and inelastic tunneling spectroscopy to study the higher energy spin excitations, we determined the spin of the atom and explore its impact on the Kondo resonance [1]. We find that the local magnetic anisotropy plays a decisive role in the physics of Kondo screening. In addition, we find that the splitting of the Kondo peak matches the splitting of the underlying unscreened spin levels, and surprisingly does not show any evidence of a renormalization of energy scales even though large renormalizations have been predicted for lower spin system. In addition, we find remarkably large variations in the strength of both the Kondo screening and magnetic anisotropy for Co atoms on both small and large Cu$_{2}$N islands. In both cases, the anisotropy and Kondo screening are inversely related: the Kondo resonance weakens as the anisotropy increases. For small islands, the Kondo screening is strongest near the center of the island, while for large island this trend is reversed. We examine the possible origins of this phenomenon, including variations in the physical and electronic structure of the Cu$_{2}$N surface. \\[4pt] [1] AF Otte et al., Nature Physics 4, 847 (2008).   

Mechanical Control of Spin States in Spin-1 Molecules and the Underscreened Kondo Effect

The ability to make electrical contact to single molecules creates opportunities to examine fundamental processes governing electron flow on the smallest possible length scales. We report experiments in which we controllably stretched individual molecules with spin S = 1/2 and S = 1 while simultaneously measuring Kondo-assisted current flow through the molecule as a probe of its spin states. For molecules with S = 1/2, the temperature dependence of the Kondo signal is in excellent agreement with the predicted universal scaling curves for the S = 1/2 Kondo effect and the molecular spin states exhibit no energy splitting with stretching, consistent with Kramers' theorem. However, for cobalt complexes we observe temperature scaling curves that are very different from the S = 1/2 Kondo predictions, and instead are in quantitative agreement with the predictions of the underscreened Kondo model for S = 1, in which conduction electrons only partially compensate the molecular spin. This allows us to identify the spin state as S = 1. As a function of stretching, the S = 1 molecules exhibit energy splittings of the spin states in the absence of magnetic field due to magnetic anisotropy arising from modification of the molecular symmetry. These findings demonstrate a mechanism of spin control in single-molecule devices and establish that they can serve as model systems for making precision tests of correlated-electron theories.   

STEM in Kondo Lattices: a new window on correlated electron materials

The tremendous developments in scanning tunneling electron spectroscopy over the past decade, applied with tremendous success to the cuprate superconductors, are now beginning to be applied to other strongly correlated electron systems. One area where they offer tremendous potential, is in the context of heavy fermion materials. In the last few years, it has become possible to start probing the physics of the Kondo lattice using STEM methods. In this talk I will review this field, discussing the physics of tunneling into the Kondo lattice, showing how tunneling involves a co-operative process of electron transfer and spin-flip, called ``cotunnelling'' [1,2]. I will provide an overview of latest results in this field, especially URu2Si2 [3,4], YbRh2Si2 [5] and CeCoIn5 [6], discussing how STEM can be used to probe various new theoretical proposals [7,8] for the exotic order and critical behavior. \\[4pt] [1] M. Maltseva, M. Dzero, and P. Coleman, Phys. Rev. Lett. 103, 206402 (2009).\\[0pt] [2] J. Figgins and D. Morr, Phys. Rev. Lett. 104, 187202 (2010).\\[0pt] [3] A. R. Schmidt et al, Nature 465, 570-576 (2010).\\[0pt] [4] P. Aynajian et al., Proc. Natl. Acad. Sci. U.S.A. 107, 10383 (2010).\\[0pt] [5] S. Ernst et al, Nature (2011).\\[0pt] [6] S. Ernst et al, Physica Status Solidi 247, 624 (2010).\\[0pt] [7] Y. Dubi and A.V. Balatsky, Phys. Rev. Lett. 106, 196407 (2011).\\[0pt] [8] P. Chandra, P. Coleman and R. Flint, to be published (2012).   

Pseudo-spin Resolved Transport Spectroscopy of the Double Dot Kondo Effect

The Kondo effect is a paradigmatic example of a highly correlated many-body state, describing how conduction electrons screen a localized spin via spin-flip scattering. Experimentally probing the spin physics of the Kondo effect is challenging, as spin-resolved measurements require either large magnetic fields that break the spin degeneracy of the localized site or ferromagnetic contacts that give the spin states different tunnel rates to the localized site. We demonstrate how the desired spin resolution can be achieved for arbitrary tunnel rates and fields in a double quantum dot system. A quantum dot consists of a confined droplet of electrons connected by tunnel barriers to conducting leads. We study two dots that are capacitively coupled with negligible inter-dot tunneling. In this system, one can have a degeneracy associated with an electron being in either dot 1 or dot 2, and this degeneracy acts as a pseudo-spin degree of freedom that gives rise to Kondo screening. We present transport spectroscopy measurements that show the zero-bias peak that is the hallmark of the Kondo effect. We use gate voltages to break the degeneracy between the pseudo-spin states, creating a pseudo-magnetic field. We show that spectroscopy measurements in this regime are analogous to measurements of the spin Kondo effect in a magnetic field. Finally, we demonstrate how measuring transport through each dot individually as a function of the bias voltage on the measured dot gives us the ability to perform pseudo-spin resolved measurements that are difficult to achieve in spin systems.   

Evolution of Kondo Resonance from a Single Impurity Molecule to the Two-Dimensional Kondo Lattice

We investigated the Kondo resonance formed by the adsorption of magnetic molecule, iron(II) phthalocyanine (denoted as FePc), on Au(111) from a single impurity regime to the two-dimensional Kondo lattice by using scanning tunneling microscopy, the density functional theory (DFT) and numerical renormalization group (NRG). In the single molecule regime, FePc takes ontop and bridge configurations. These species show characteristic Kondo signatures in their one-particle energy spectra depending on the adsorption site. The ontop species shows a broad peak accompanied by a sharp dip, while the bridge species only a broad peak. The origin of these features comes from the difference in the local symmetry around the Fe ion. The two-stage Kondo screening occurs for two localized electrons in the different 3d orbitals characterized by low and high Kondo temperatures, reflecting the different coupling strength of these orbitals with Au(111). For the ontop species, highly-symmetric SU(4) Kondo effect is realized, leading to the sharp dip. The dip observed for the ontop species is also evolved from the single impurity regime to the two-dimensional lattice. The spectral evolution and the quantum phase of the lattice are discussed by the competition of the Kondo effect with antiferromagnetic RKKY coupling between the molecular spins.   

Session W20: Invited Session: Nuclear Power, One Year After Fukushima

The Accident at TEPCO's Fukushima-Daiichi Nuclear Power Plant: Technical Description of What Happened and Lessons Learned for the Future

Tsunami that followed M9.0 earthquake on March 11$^{th}$ left the Fukushima-Daiichi Nuclear Power Plants without power and heat sink. While water makeup continued by AC-independent systems to keep the fuel core covered by coolant, operating team tried to depressurize and enable low pressure injection to the reactor to avoid overheating but was not successful enough primarily due to limited available resources. This resulted in core melt, hydrogen explosion and release of radioactivity to the environment. Key lessons learned are; 1) safety regulation and safety culture, 2) workable/executable severe accident management procedure, 3) crisis management and 4) design. Implications on security include revealed vulnerability and the nexus of safety and security. Given the scale of damage to the environmental, attention must be paid to defense against it and to societal safety goal of nuclear power by considering offsite remedial costs, compensation to damage, energy replacement cost etc. A sort of root cause analysis first by asking ``Why nuclear community failed to prevent this accident?'' was initiated by the University of Tokyo.   

The U.S. nuclear industry following the Fukushima event

The nuclear industry is executing a coordinated, comprehensive and safety focused strategic plan in response to the accident in Japan. This beyond design basis external hazard driven event resulted in loss of core cooling due to a loss of offsite power as a result of the significant earthquake which was followed by a loss of on-site power due to the tsunami. Site response actions were impacted due to difficulty accessing the site. The current US fleet of operating reactors has over the last decades implemented numerous reviews assessing external hazards with post 911 actions being the most current. The industry took immediate actions following the March accident to verify existing capabilities to address events driven from external hazards and is now implementing a ``Way Forward'' plan that has as a priority the continued safe operation of the existing US fleet while examining both the technical and organizational root causes of the Fukushima Dai-ichi accident. Industry focus is to provide additional defense in depth through a ``Flex'' approach that adds the most significant safety benefit in the most efficient time frame. From a topical perspective, the industry is in agreement with the tier 1 recommendations from the NRC near term task force report. Southern Nuclear Operating Company operates 6 reactors and is building Vogtle 3\&4 which is expected to be granted a COL in the near term. This generation III+ reactor design has passive safety features that would have mitigated an event similar to the Fukushima accident.   

Lessons from Fukushima for Improving the Safety of Nuclear Reactors

The March 2011 accident at the Fukushima Daiichi nuclear power plant has revealed serious vulnerabilities in the design, operation and regulation of nuclear power plants. While some aspects of the accident were plant- and site-specific, others have implications that are broadly applicable to the current generation of nuclear plants in operation around the world. Although many of the details of the accident progression and public health consequences are still unclear, there are a number of lessons that can already be drawn. The accident demonstrated the need at nuclear plants for robust, highly reliable backup power sources capable of functioning for many days in the event of a complete loss of primary off-site and on-site electrical power. It highlighted the importance of detailed planning for severe accident management that realistically evaluates the capabilities of personnel to carry out mitigation operations under extremely hazardous conditions. It showed how emergency plans rooted in the assumption that only one reactor at a multi-unit site would be likely to experience a crisis fail miserably in the event of an accident affecting multiple reactor units simultaneously. It revealed that alternate water injection following a severe accident could be needed for weeks or months, generating large volumes of contaminated water that must be contained. And it reinforced the grim lesson of Chernobyl: that a nuclear reactor accident could lead to widespread radioactive contamination with profound implications for public health, the economy and the environment. While many nations have re-examined their policies regarding nuclear power safety in the months following the accident, it remains to be seen to what extent the world will take the lessons of Fukushima seriously and make meaningful changes in time to avert another, and potentially even worse, nuclear catastrophe.   

Nuclear Power in China

In response to the Fukushima accident, China is strengthening its nuclear safety at reactors in operation, under construction and in preparation, including efforts to improve nuclear safety regulations and guidelines based on lessons learned from the accident. Although China is one of the major contributors in the global nuclear expansion, China's nuclear power industry is relatively young. Its nuclear safety regulators are less experienced compared to those in other major nuclear power countries. To realize China's resolute commitment to rapid growth of safe nuclear energy, detailed analyses of its nuclear safety regulatory system are required. This talk explains China's nuclear energy program and policy at first. It also explores China's governmental activities and future nuclear development after Fukushima accidents. At last, an overview of China's nuclear safety regulations and practices are provided. Issues and challenges are also identified for police makers, regulators, and industry professionals.   

Nuclear Power in India

India has ambitious plans for expanding nuclear power over the next few decades. A major accident in a densely populated country like India can be catastrophic and thus of concern. There are both technical and organizational requirements for safety of nuclear facilities. This talk will describe some of the organizational factors that safety theorists have identified and examine, from the publicly available information about incidents and failures at India's nuclear facilities to see if these requirements are met. It will also describe some of the reactions to the Fukushima accident from officials associated with nuclear energy in India.   

Session W21: Focus Session: Search for New Superconductors- Electron-Phonon Coupling

Correlation-enhanced electron-phonon coupling produces high-temperature superconductors

The microscopic origin of superconductivity has been established in numerous classes of materials. In elemental metals it results from the exchange of phonons whereas in copper oxides and iron pnictides it is intimately connected to magnetism. On the other hand, the cause of superconductivity in a large third class of isotropic high temperature superconductors is still mysterious and subject of debate. In this talk, we will demonstrate that dynamical correlations among electrons enhance their coupling to phonons in a large number of superconductors, raising significantly the previously underestimated theoretical superconducting critical temperature T$_c$ of up to a few Kelvin to their experimental values of tens of Kelvin. The mechanism we propose is quite general, explains the magnitude and the doping dependence of superconductivity in many materials such as the celebrated Ba$_{1-x}$K$_x$BiO$_3$ ($T_C$=32 K) compounds and the electron-doped $\beta$-HfNCl compounds ($T_c$ = 25.5 K) and can be used to design other high temperature superconductors which have not yet been synthesized. We will also propose a few novel materials which are good superconductors in our theory and can be easily synthesized and tested by experiments.   

Spectral properties of correlated systems with electron-phonon coupling

Results from a variety of experiments, including single particle probes such as ARPES and STM, and multi-particle spectroscopies such as optical and Raman responses, have revealed the importance of the electron-phonon interaction in strongly correlated electron materials. We present a determinant quantum Monte Carlo study of the single-band Hubbard-Holstein model, which treats electron-electron and electron-phonon interactions on an equal footing. We focus on the behavior of the single- and multi-particle dynamical properties of the model, such as the spectral function and optical conductivity, as a function of electron-phonon coupling and Hubbard U.   

Vibrational spectrum and electron-phonon coupling of doped solid picene from first principles

The search for new intercalated hydrocarbon superconductors was initiated by the report of a superconducting critical temperature ($T_c$) of 18 K in K- and Rb- doped picene (C$_{22}$H$_{14}$), followed by phenanthrene, coronene and di-benz-picene (27 K). These compounds, formed by justappoxed benzene rings, bear a strong resemblance both to fullerenes and intercalated graphites. Using first-principles linear response calculations have performed calculations of the phonon spectrum and electron-phonon (ep) interaction, we have shown that the coupling of the high-energy C bond-stretching phonons to the $\pi$ molecular orbitals for a doping of ~3 electrons per picene molecule is sufficiently strong to reproduce the experimental Tc of 18 K within Migdal-Eliashberg theory. For hole doping, we predicted a similar coupling leading to a maximum Tc of 6 K. However, we argue that, due to its molecular nature, picene may belong to the same class of strongly correlated $ep$ superconductors as fullerides [1]. Our results are in agreement with estimates based on molecular orbital models;we also discuss possible reasons and implications of the discrepancy with linear response calculations that include explicitely the dopant.   

Strong Correlation Physics in Aromatic Hydrocarbon Superconductors

We show, by means of ab-initio calculations, that electron-electron correlations play an important role in doped aromatic hydrocarbon superconductors, including potassium doped picene with $T_c = 18K$ [1], coronene and phenanthrene [2]. For the case of picene the inclusion of exchange interactions by means of hybrid functionals reproduces the correct gap for the undoped compound and predicts an antiferromagnetic state for $x=3$, where superconductivity has been observed [3]. The latter finding is compatible with a sizable value of the correlation strength. The differences between the different compounds are analyzed and results of Dynamical Mean-Field Theory including both correlation effects and electron-phonon interactions are presented. Finally we discuss the consequences of strong correlations in an organic superconductor in relation to the properties of Cs$_3$C$_{60}$, in which electron correlations drive an antiferromagnetic state [4] but also lead to an enhancement of superconductivity [5]. \\ 1. R. Mitsuhashi et al. Nature 464, 76 (2010)\\ 2. X.F. Wang et al, Nat. Comm. 2, 507 (2011)\\ 3. G. Giovannetti and M. Capone, Phys. Rev. B 83, 134508 (2011)\\ 4. Y. Takabayashi et al., Science 323, 1585 (2009)\\ 5. M. Capone et al. Rev. Mod. Phys. 81, 943 (2009   

Transport Anomalies and Possible High Tc Superconductivity in interconnected multiwall carbon nanotube sheets doped by ion implantation

Ion implantation offers an alternative doping method. In searching for superconductivity,we describe here the ion-implantation doping of MWCNT interconnected networks by boron and other dopants (phosphorous, sulfur, arsenic) and report transport anomalies in oriented networks of ion implanted MWCNT sheets as compared to cross coated (non-oriented multilayer MWCNT sheets). The strong drop of resistance R(T) with temperature decrease starting at T$_{c1}$= 50-60 K and even at higher T is reminiscent of inhomogeneous superconducting islands appearing in the non-SC matrix. An unusual anomaly of the 4-terminal resistance is observed in many samples, R(T) becoming negative at lower T$<$ T$_{c2} \quad \sim $ 10-20 K, This negative resistance is found to be associated with unusual I-V curves with s-shape at low T $<$ Tc2 and R(T) shows nonlinear dependence on excitation current and other features that are studied carefully in MWCNTs with different lengths and densities. This negative-resistance behavior gives a hint for the possible incorporation of superconducting areas and can be explained in terms of an imbalanced resistance bridge.   

Large Magnetoresistance and low temperature Transport anomalies in Ion implanted HOPG

Strong positive magnetoresistance (MR) was found in highly oriented pyrolytic graphite (HOPG) upon ion implantation by boron and phosphorous. Similar effects, but with smaller amplitude, are induced by carbon ion implantation, but due to structural disorder and defect formation without carrier concentration increase. The magnetic field dependence of the MR is linear at high fields with no sign of saturation, but different contact geometries result in a wide range of parameters. Two possible explanations of strong MR are suggested and analyzed. While the MR remains large at all temperatures, plots of R(T) in constant field show a drop at lower T. Future experiments will clarify the origin of the R(T) drop, in particular, whether this might be interpreted as the onset of inhomogeneous superconductivity, as proposed in some previous work, or could be explained in the context of strong linear MR effects seen in high-mobility, disordered materials.   

Quantiative reliability of the Migdal-Eliashberg theory for strong coupling superconductors

The Migdal-Eliashberg (ME) theory for strong electron-phonon coupling and retardation effects of the Morel-Anderson type form the basis for the quantitative understanding of conventional superconductors. The validity of the ME theory for values of the electron-phonon coupling strength $\lambda>1$ has been questioned by model studies. By distinguishing bare and effective parameters, and by comparing the ME theory with the dynamical mean field theory (DMFT), we clarify the range of applicability of the ME theory. Specifically, we show that ME theory is very accurate as long as the product of effective parameters, $\lambda \omega_{\rm ph}/D$, where $\omega_{\rm ph}$ is an appropriate phonon scale and $D$ an electronic scale, is small enough [1]. The effectiveness of retardation effects is usually considered based on the lowest order diagram in the perturbation theory. We analyze these effects to higher order and find modifications to the usual result for the Coulomb pseudo-potential $\mu^*$. Retardation effects are weakened due to a reduced effective bandwidth. Comparison with the non-perturbative DMFT corroborates our findings~[2]. \\[4pt] [1] J Bauer, J E Han, and O Gunnarsson, Phys. Rev. B. 84, 184531 (2011).\\[0pt] [2] J Bauer, J E Han, and O Gunnarsson, in preparation (2011).   

Influence of Surface Termination of Boron-Doped Diamond on Superconducting Property

In 2004, a heavily boron-doped diamond was found to be a superconductor Since then, a superconducting diamond has attracted considerable attention, mainly explored for fundamental properties and a theoretical basis. Meanwhile, it is known that the surface of diamond is easily modified by a chemical treatment, and the physical properties, such as surface conductivity, could be modulated through the surface modification. Here, we report modulation of superconducting properties of a heavily boron-doped diamond by tuning the surface electronic state. A heavily boron-doped diamond was prepared onto a silicon wafer substrate by a microwave plasma-assisted chemical vapor deposition method. The surface of a boron-doped diamond was changed between hydrogen- and oxygen-termination by thermal and electrochemical reactions, respectively. As a result, the critical current and the diamagnetic magnetization value could be modulated in a reversible manner between the hydrogen- and oxygen-terminated diamonds with maintenance of the superconducting transition temperature. It is assumed that the carrier density at grain boundaries would change due to the induced dipole moment via surface modification, resulting in modulation of the magnetic flux pinning effect at grain boundaries.   

Energy landscape of fullerene materials: A comparion of boron to boron nitride and carbon

After the discovery of the C60 fullerene some 25 years ago, many more hollow and endohedrally doped structures made out of various elements have been proposed theoretically. However, since no other fullerenes have been synthesized up to date, the question arises whether experimentalists have just not yet found a way to synthesize these theoretically predicted fullerenes, or whether they do not exist at all in nature. Following the theoretical discovery of the $B_{80}$ fullerene by Szwacki et al, various other fullereneand stuffed fullerene structures were proposed but none of them could be synthesized in the laboratory yet. Using the minima hopping global geometry optimization method on the density functional potential energy surface we show that the energy landscape of boron clusters is glass like. Medium size boron clusters exhibit many structures which are lower in energy than the cages. This is in contrast to carbon and boron nitride systems which can be clearly identified as structure seekers. The differences in the potential energy landscape explain why carbon and boron nitride systems are found in nature whereas pure boron fullerenes have not been found. We thus present a methodology which can make predictions on the feasibility of the synthesis of new nano structures.   

Light element ternary compounds -- searching for new superconductors in the ``upper left corner''

We propose here a new class of ternary compounds, composed entirely of light elements drawn from the upper left corner of the Periodic Table, as a new family of superconductors with the promise of high transition temperatures ($T_c$). In this explorative computational study, we have investigated stoichiometric 1:1:1 compounds of lithium, beryllium, and boron. We find layered metallic phases that are thermodynamically stable at P=1 atm, with still others stabilized at relatively low pressures and hence in principle accessible to synthesis and experimental characterization. At high pressures, close packed structures are again stabilized and a metal-to-insulator transition is predicted. Superconducting transition temperatures for the most structurally attractive metallic phases are estimated using BCS theory. An outlook on other stoichiometries, as well as the incorporation of different constituents, Mg instead of Be in particular, is given.   

Progress on Determining the alpha-beta Phase Boundary of Elemental Boron

Recently, it was reported that the phase boundary between alpha-boron and beta-boron has been directly determined using high-pressure and temperature experiments down to P$\sim $4GPa and T$\sim $1400K [Scientific Reports 1, \textbf{96} (2011)]. Based on linear extrapolation of their results to lower pressure and temperature, these authors proposed that at P=0GPa alpha-boron is the stable form below about T$\sim $933(20)K, in conflict with the recent theoretical works based on DFT total energy calculations [JACS \textbf{129}, 2458 (2007); PRB \textbf{77}, 064113 (2008); JACS \textbf{131}, 1903 (2009) ], where it was concluded that beta-boron is the most stable at all temperature below melting temperature and down to zero Kelvin. At the talk, we show that the theoretical alpha-beta boundary obtained with a few approximations agrees well with the aforementioned experimental results within the error bars except for the lowest $P, T$ point, and in this case, the ground state is still beta-boron [submitted]. We will also discuss on the recent experimental efforts in measuring the specific heat of boron allotropes that lead to a tentative conclusion supporting the aforementioned DFT results.   

Calculation of electron-phonon coupling in Arsenic under pressure

Elemental As undergoes a structural transformation from a rhombohedral A7 phase to a simple cubic (sc) phase at around 25 GPa as pressure is increased. At pressures near this phase transformation, As is superconducting, with a maximum superconducting transition temperature $T_c$ of about 2.5 $K$. Experiments indicate that this maximum $T_c$ occurs at the transition pressure for structural transformation, and the increase in $T_c$ as the transition pressure is approached has been attributed to phonon softening. In this work, we calculate from first principles the electronic structure, phonon dispersions, and electron-phonon coupling constant $\lambda$ for As in the A7 and sc phases at various pressures near the A7 to sc transition. Using these detailed quantitative calculations, we explain the trends in $T_c$ as function of pressure in terms of phonon softening, electronic density of states, and electron-phonon matrix elements. We discuss the implications of these results for finding new superconducting materials.   

Physical properties of heavily boron doped silicon

The discovery of superconductivity (SC) in heavily boron doped silicon in 2006 by [1] occurred shortly after diamond in 2004 by [2]. However, the SC in these 2 materials occurs differently. For diamond, the SC is obtained for a boron concentration close to the metal-insulator transition (MIT), while for silicon, the onset of superconductivity is obtained well above the MIT threshold. The aim of this study is to determine the influence of different parameters that impact the SC, such as the doping concentration nB, or the thickness of the layer. Interpolation between resistivity measurements of Tc(nB) and ab initio calculations of the electron phonon coupling $\lambda $(nB) showed a complete mismatch of the dependency of $\lambda $(Tc) with the BSC MacMillan exponential law. The results obtained suggest rather a power law dependence such as $\lambda $ $\alpha $ Tc$^{2}$. This dependency suggests a fractal dimension of the superconducting wave function as reported by Feigel'man et al. [3]. \\[4pt] [1] E. Bustarret \textit{et al}., Nature (London) 444, 465 (2006).\\[0pt] [2] E. Ekimov \textit{et al.} (2004).\textit{ Nature} \textbf{428}: 542\\[0pt] [3] Feigel'man et al., arXiv:1002.0859   

Eliashberg-McMillan Parameters of Polysulfur Nitride, (SN)$_{x}$

Thirty-seven years following the discovery of superconductivity in polysulfur nitride, (SN)$_{x}$ remains the lone conducting polymer exhibiting this phenomenon. The transition temperature is only 0.3 -- 0.4 K, and details of its origin remain largely unknown, although it very likely arises from conventional phonon-mediated BCS pairing of normal state carriers. In pursuit of such a possible mechanism, we have performed density functional theory (DFT) investigations of the phonon and electron-phonon dispersion relationships in (SN)$_{x}$, and will present values for the coupling strength of the latter along with an estimate of T$_{C}$.   

Probing electronic order via coupling to phonons in Bi2Sr2CuO6 (Bi2201)

Recent work on several members of the cuprate family of high-temperature superconductors has revealed the occurrence of broken lattice translational, rotational, and time reversal symmetries in various materials at low temperatures. We report on measurements of acoustic phonons on Bi$_2$(La,Sr)$_2$CuO$_{6+\delta}$, which reveal a coupling to an underlying electronic density-wave state. Longitudinal acoustic phonons are found to be anomalously broadened at a wavevector of q $\approx$ 1/4 rlu along the Cu-O direction. At low temperatures a disparity between the scattered intensity at positive and negative energy transfer is seen. These measurements indicates a breaking of time reversal and inversion symmetry in the bulk.   

Session W22: Focus Session: Fe-based Superconductivity - Magnetism in AxFe(1-y)Se2

NMR Study of Superconductivity and Spin Fluctuations in Intercalated Iron Selenides A$_y$Fe$_{2-x}$Se$_2$

The role of spin fluctuations in superconductivity is an essential topic in both cuprate and Fe-based superconductors. NMR works in several Fe-based superconductors proposed that the low-energy antiferromagnetic spin fluctuations (AFSF) is a possible pairing glue for superconductivity. However, studies on other systems such as KFe$_2$As$_2$ and Li$_{1-x}$FeAs does not support a strong correlation between low-energy spin fluctuations and superconductivity. In the newly discovered A$_y$Fe$_{2-x}$Se$_2$ superconductors with $T_c\sim$ 32 K, our NMR study identifies unambiguously a paramagnetic superconducting phase, which is phase separated from the block antiferromagnetic state. The low-energy AFSF is not seen at all, even though the T$_c$ is high. The A$_y$Fe$_{2-x}$Se$_2$ are singlet superconductors evidenced from the NMR Knight shift $K$; However, the absence of the coherence peak in the spin-lattice relaxation rate $1/T_1$ suggests an unconventional behavior of superconductivity. In fact, we found that both the $K$ and the $1/T_1T$ increase dramatically with temperature and follow a $a+bT^2$ form from Tc up to 300 K. Such behavior is strong evidence for spin fluctuations with a high-energy, local nature in 3D systems, and inconsistent with a band-gap effect. Furthermore, $K$ and $1/T_1T$ saturate above 400 K, indicating an energy scale of 35 meV, which is distinct from the low-energy spin fluctuations. The above temperature enhanced spin fluctuations seem to be universal in Fe-based superconductors. \\[4pt] References: W. Yu et al., Phys. Rev. Lett. 106, 197001 (2011); Long Ma et al., Phys. Rev. B 83, 174510 (2011); L. Ma et al., arXiv:1103.4960.   

Spin Waves and magnetic exchange interactions in insulating Rb0.89Fe1.58Se2

Superconductivity in alkaline iron selenide AFe$_{1.6+x}$Se$_{2}$ (A = K, Rb, Cs) may have a different origin from the sign reversed s-wave electron pairing mechanism, because they are insulators near x = 0 and form a blocked AF structure that is completely different from the iron pnictides. We use neutron scattering to map out spin waves in the AF insulating Rb$_{0.89}$Fe$_{1.58}$Se$_{2}$. A comparison of the fitted effective exchange couplings using a local moment Heisenberg Hamiltonian in Rb$_{0.89}$Fe$_{1.58}$Se$_{2}$, (Ba,Ca,Sr)Fe$_{2}$As$_{2}$, and iron chalcogenide Fe1.05Te reveals that their next nearest neighbor (NNN) exchange couplings are similar. Therefore, superconductivity in all Fe-based materials may have a common magnetic origin that is intimately associated with the NNN magnetic exchange interactions, even though they have metallic or insulating ground states, different AF orders and electronic band structures.   

77Se NMR Study of K{x}Fe{2-y}Se{2-z}S{z}

We will present a $^{77}$Se NMR study of the superconducting properties of the recently discovered K$_{x}$Fe$_{2-y}$Se$_{2}$ (T$_{c} \sim$ 33 K), in a temperature range of 4 to 250 K [1]. Our Knight shift data reflect the progressive decrease in uniform spin susceptibility with temperature, in analogy with FeSe and iron-arsenide systems. Nuclear spin-lattice relaxation rate data shows no Hebel-Slichter coherence peak, nor any enhancement of antiferromagnetic spin fluctuations (AFSF) toward T$_{c}$. We have also conducted $^{77}$Se NMR measurements on K$_{x}$Fe$_{2-y}$Se$_{0.4}$S$_{1.6}$ (non-superconducting) and K$_{x}$Fe$_{2-y}$Se$_{1.2}$S$_{0.8}$ (T$_{c} \sim$ 21 K) to study the effect of sulphur substitution in this superconductor [2]. Sulphur applies a chemical pressure on the lattice, because it has the same valence as Selenium but less than half the ionic radius. We again measure NMR Knight shift and nuclear spin-lattice relaxation rate 1/T$_{1}$, and find that both are suppressed with S substitution. We will discuss these results in comparison with K$_{x}$Fe$_{2-y}$Se$_{2}$. \\[4pt] [1] D. Torchetti et. al., PRB 83, 104508 (2011)\\[0pt] [2] D. Torchetti et. al., arXiv:1111.2552 (2011)   

Magnetic state of K$_{0.8}$Fe$_{1.6}$Se$_2$ from a five-orbital Hubbard model in the Hartree-Fock approximation

The five-orbital Hubbard model (without lattice distortions) is investigated to study theoretically the recently discovered Fe-based superconductors K$_{0.8}$Fe$_{1.6}$Se$_2$, by using the real-space Hartree-Fock approximation and employing a 10$\times$10 Fe cluster with Fe vacancies in a $\sqrt{5}\times\sqrt{5}$ pattern[1]. The phase diagram contains an insulating state with the same spin pattern as observed experimentally, involving 2$\times$2 ferromagnetic plaquettes coupled with one another antiferromagnetically. The magnetic moment $\sim$3$\mu_B$/Fe is in good agreement with experiments. Several other competing phases are also stabilized in the phase diagram, in agreement with recent calculations using phenomenological models. [1] Qinlong Luo {\it et al.}, Phys. Rev. B {\bf 84}, 140506(R) (2011), and references therein.   

Electronic structure and magnetic excitations in iron selenides

We calculate the electronic structure and magnetic excitations in the K$_{4}$Fe$_{4+x}$Se$_{5}$ alloy, with x between 0 and 1, within the local-density approximation. Analysis of the electronic structure with varying x leads to a prediction of the coexistence of two phases: one, strongly magnetic and another, weakly or nonmagnetic. Using linear response techniques we calculate spin wave spectra in K$_{4}$Fe$_{4+x}$Se$_{5}$, and find it is in excellent agreement with a recent experiment. The spectrum can be described rather well by an anisotropic J$_{1}$-J$_{2}$ model. We confirm that exchange coupling between NN Fe magnetic moments is strongly anisotropic, and show directly that in the ideal system this anisotropy can be associated with higher order terms in spin Hamiltonian (biquadratic coupling). Structural relaxation provides an additional source of the exchange anisotropy of approximately the same strength. The dependence of spin wave spectra on filling of Fe vacancy sites is discussed.   

Novel Magnetism in K$_{0.8}$Fe$_{1.6}$Se$_2$ Explained in the Unified Picture

The novel block checkerboard antiferromagnetism in Fe-vacancy-ordered insulating K$_{0.8}$Fe$_{1.6}$Se$_2$ is investigated theoretically [1]. Neither of the Fermi surface nesting and the Mott insulator scenarios, which were widely employed to model previous Fe-vacancy-free iron-based superconductors, is supported by our first-principles analysis of its electronic structure including unfolded Fermi surface, band gap, and orbital polarization. Instead, the orbital-degenerate double-exchange model, previously proposed to unify the metallic collinear and bicollinear antiferromagnetism of iron-based superconductors, is found sufficient to explain this insulating spin order as well as associated quantum magnetic transition induced by tetramer lattice distortion. Our findings demonstrate that the iron-based superconductors be universally described in the framework of coexisting itinerant and localized electronic states, which are coupled by Hund's rule coupling on the Fe atoms. Work supported by DOE DE-AC02-98CH10886.\\[4pt] [1] W.-G. Yin, C.-H. Lin, and W. Ku, arXiv:1106.0881v1.   

Vacancy-driven orbital and magnetic order in (K,Tl,Cs)$_y$Fe$_{2-x}$Se$_2$

We investigate the effects of the $\sqrt{5}\times\sqrt{5}$ Fe vacancy ordering on the orbital and magnetic order in (K,Tl,Cs)$_y$Fe$_{2-x}$Se$_2$ using a three-orbital ($t_{2g}$) tight-binding Hamiltonian with generalized Hubbard interactions. We find that vacancy order enhances electron correlations, resulting in the onset of a block antiferromagnetic phase with large moments at smaller interaction strengths. In addition, vacancy ordering modulates the kinetic energy differently for the three $t_{2g}$ orbitals. This results in a breaking of the degeneracy between the $d_{xz}$ and $d_{yz}$ orbitals on each Fe site, and the onset of orbital order. Consequently, we obtain a novel inverse relation between orbital polarization and the magnetic moment. We predict that a transition from high-spin to low-spin states accompanied by a crossover from orbitally-disordered to orbitally-ordered states will be driven by doping the parent compound with electrons, which can be verified by neutron scattering and soft X-ray measurements.   

Phase separation and magnetic order in K-doped iron selenide superconductor

The newly discovered alkali-doped iron selenide superconductors exhibit unique characters that are absent in other iron-based superconductors, such as anti-ferromagnetically ordered insulating phases, extremely high Neel transition temperatures, and the presence of Fe vacancies and ordering. We have grown high-quality K$_{x}$Fe$_{2-y}$Se$_{2}$ thin films on graphene by molecular beam epitaxy and measured their atomic and electronic structures by low-temperature scanning tunneling microscopy. We demonstrate that a typical K$_{x}$Fe$_{2-y}$Se$_{2}$ sample contains two distinct phases: an insulating phase with well-defined $\surd 5 \times \surd $5 order of Fe vacancies, and a superconducting KFe$_{2}$Se$_{2}$ phase containing no Fe vacancies. An individual Fe vacancy can locally destroy superconductivity in a similar way as a magnetic impurity in conventional superconductors. The measurement of magnetic field dependence of the Fe-vacancy-induced bound states reveals a magnetically-related bipartite order in the tetragonal iron lattice. These findings elucidate the existing controversies on this new superconductor and provide atomistic information on the interplay between magnetism and superconductivity in iron-based superconductors.   

Magnetism and Superconductivity in Rb$_{1-x}$Fe$_{2-y}$Se$_2$

We report on the structural, magnetic, and superconducting properties of Rb$_{1-x}$Fe$_{2-y}$Se$_2$ single crystals. The system exhibits a strongly anisotropic antiferromagnetic behavior below 400~K. For 1.53 $<2-y<$ 1.6 superconductivity is found, whereas for Fe concentrations $2-y<$ 1.5 and $2-y>$ 1.6 insulating and semiconducting behavior is observed, respectively. The sharpest transition to the superconducting state and the highest transition temperature $T_c$ of 32.4~K is found for compositions close to Rb$_2$Fe$_4$Se$_5$. Comparison of the magnetic behavior of non-superconducting and superconducting samples provides evidence for the coexistence of superconductivity and static antiferromagnetic order [1]. THz time-domain transmission spectroscopy in superconducting samples evidences a metallic response and the superconducting transition of the system [2].\\[4pt] [1] V. Tsurkan et al., Phys. Rev. B 84, 144520 (2011).\\[0pt] [2] A. Charnukha et al., arXiv:1108.5698.   

Defect formation, magnetic interaction and electron phonon coupling in iron selenides M$_{1-x}$Fe$_{2-y}$Se$_{2}$

We perform a systematic study to explore the electronic and magnetic structures in iron selenide superconductors M$_{1-x}$Fe$_{2-y}$Se$_{2}$ using first principles calculations. We show that there is an intimate relationship between Se-height and the underneath Fe-spin square in M$_{1-x}$Fe$_{2-y}$Se$_{2}$. A displacement of the Se atom by as small as 0.2{\AA} is enough to change the amount of charge in the Fe-plane as much as 0.7 e per Fe. The Se-height increases as the number of ferromagnetic Fe-Fe bonds increases, yielding an expansion of 2{\AA} expansion in the c-axis for fully ferromagnetic spin configuration, which indicates a giant magneto-elastic coupling in these systems. Our calculations also explain why the formation of Fe vacancies is favorable in iron selenides, but not in iron pnictides. Finally, we calculate the spin-resolved electron-phonon coupling in MFe$_{2}$Se$_{2}$ and M$_{1-x}$Fe$_{2-y}$Se$_{2}$ to shed light on the mechanism of superconductivity in these materials.   

Phononic, magnetic, and inter-band Raman scattering in K$_{0.75}$Fe$_{1.75}$Se$_{2}$ superconductor

We have analyzed collective excitations in K$_{0.75}$Fe$_{1.75}$Se$_{2}$ single crystal ($T_{c}\sim $ 32 K) by polarized Raman scattering in the energy shift range of 20-8000 cm$^{-1}$, the temperature range of 10-300 K, and laser excitation energies from 1.8 to 3.0 eV. Seven $B_{g}$ and nine $A_{g}$ phonon modes are observed at 300K. Below $\sim $150 K an extra $A_{g}$ mode appears at 165 cm$^{-1}$. The amplitudes of the $A_{g}$ modes at $\sim $67, 112, and 124 cm$^{-1}$ are reduced, while the amplitude of 183 cm$^{-1} \quad A_{g}$ mode is enhanced by factor of five as temperature decreases from 300 to 40 K. Magnetic scattering bands at 1000-2000 cm$^{-1 }$consist of at least three distinct peaks each, implying different Fe-Fe AFM exchange coupling constants for underlying structure. Inter-band transitions are observed at $\sim $3700 and 4600 cm$^{-1}$ at 300 K in the $A_{g}$ and $B_{g}$ channels, respectively. Below 140 K these excitations are hardened to $\sim $4040 and 4820 cm$^{-1}$.   

Electronic structure and magnetism of doped $A_{x}Fe_{2-y}Se_{2}$

We develop a new multiorbital t-J Hamiltonian with realistic tight-binding and Heisenberg parameters to study the electronic and magnetic structure of $A_xFe_{2-y}Se_2$ superconductors for 0$<$y$<$0.4. The ARPES experiments are fitted by a tight-binding lattice model with random vacancy order. We find that the vacancy order greatly affects the electronic band structure. For intermediate doping levels 0 $<$ y $<$ 0.4, the stable electronic structure is a compromise between the solution for y=0 and y=0.4. Based on this model, we study the paramagnetic and antiferromagnetic (AFM) phases of $A_{0.8}Fe_{1.6}Se_2$. In the AFM phase the calculated spin susceptibility for the bare band structure agrees with a block-spin structure. This theoretical result is in good agreement with neutron scattering experiments of the spin structure. Furthermore, we show the results on the evolution of low-energy quasiparticle states with electron filling factor in the vacancy-ordered magnetic state.   

Nodeless d-wave Superconductivity and spin resonance in iron-selenide superconductors

Iron-selenide based layered compounds have been realized to be high-transition temperature superconductor in December, 2010. The superconductivity is tuned by varying number of iron vacancies in the crystal. This unique tunability of the high-temperature superconducting properties has reinforces the debate of universal properties in Fe based superconductors. Experiments and band structure calculations have shown that the electronic and magnetic structures of these compounds are significantly different from other iron-based superconductors. This fact leads us to propose that the superconducting state is nodeless d-wave pairing which is still driven by magnetic interactions. Nodeless gap leads to the fully gapped quasiparticle spectrum. Sign-changing gap lends itself naturally to the sharp feature in neutron scattering spectrum, the so called spin resonance. We predict the upward ``horseshoe'' dispersion of the spin resonance, in a sharp contrast with the ``hourglass'' dispersion in high\_Tc oxides where similar spin resonance is ubiquitously seen. In conclusion, despite iron-selenide systems exhibit very different observables, we show that the underlying pairing mechanism is driven by similar spin-fluctuation instabilities as in other high-temperature superconductors.   

Session W23: Focus Session: Fe-based Superconductors - AFeAs and RFeAsO Families

Quantum oscillations in LiFeAs

Quantum oscillations are a powerful technique to establish with accuracy the three-dimensional topology of the Fermi surface and it has been successfully used in the study of iron-based superconductors. Here we report quantum oscillations in the 111 pnictide superconductor, LiFeAs, with T$\sim $18K, by using highly sensitive torque magnetometry in high magnetic fields (up to 58T) and at very low temperatures (0.3-4.2K). We observe clearly three different orbits around 1.5kT, 2.4kT and 2.9kT. By comparing the angular dependence of the measured frequencies with the predictions given by the first principle band calculations we conclude that the observed orbits belong to the electronic bands. The values of the quasiparticle masses for these orbits are significantly enhanced as compared with the band masses (a factor 4-5) suggesting that either that electron-electron and/or electron-phonon correlations are significant. We will compare our data with available APRES data on the same material and discuss the effect of the spin-orbit coupling. The details of the Fermi surface of LiFeAs will be compared with other iron-based superconductors. This work was supported by EPSRC (UK), EuroMagNET II, KAKENHI from JSPS and National Science Foundation, State of Florida and the U.S. Department of Energy.   

Effects of correlations in LiFeAs and LiFeP

We will discuss the role of electronic correlations in the iron-based superconductors LiFeAs and LiFeP by considering the effects on band structure, mass enhancements, and Fermi surface in the framework of density functional theory combined with dynamical mean field theory calculations. We show that LiFeAs shows characteristics of a moderately correlated metal and that the strength of correlations is mainly controlled by the value of the Hund's rule coupling $J$. The hole pockets of the Fermi surface show a distinctive change in form and size with implications for the nesting properties. We discuss our results in view of recent angle-resolved photoemission spectroscopy and de Haas-van Alphen experiments.   

Anisotropic Energy-Gaps of Iron-based Superconductivity from Intra-band Quasiparticle Interference in LiFeAs, Part I

Cooper pairing in the iron-based high-T$_c$ superconductors is thought to occur due to the projection of the antiferromagnetic interactions between neighboring iron atoms onto the complex momentum-space electronic structure. It is thus pivotal to have an exact measurement of the electronic structure in these materials. In this talk, I will introduce intra-band Bogoliubov quasiparticle scattering interference (QPI) to iron-based superconductor studies. We report a precise determination of the low energy band structure of LiFeAs using QPI. We observe three hole-like bands, in qualitative agreement with dHvA and ARPES studies (``$\gamma$, $\alpha_2$ \& $\alpha_1$''). The quantitative determination of the bandstructure is the foundation that we later use to measure the superconducting gap structure with QPI.   

Non-magnetic impurity effects in LiFeAs studied by STM/STS

Detecting the possible sign reversal of the superconducting gap in iron-based superconductors is highly non-trivial. Here we use non-magnetic impurity as a sign indicator. If the sign of the superconducting gap is positive everywhere in momentum space, in-gap bound state should not be observed near the impurity site unless it is magnetic. On the other hand, if there is a sign-reversal in the gap, even non-magnetic impurity may create in-gap bound state~[1]. We performed STM/STS experiments on self-flux and Sn-flux grown LiFeAs crystals and examined the effects of Sn impurity. In STM images of Sn-flux grown samples, we found a ring-like object which may represent Sn. Tunneling spectrum taken at this defect site exhibits in-gap bound state. Together with flat-bottom superconducting gap observed far from the defects, sign-reversing $s$-wave gap is the most plausible gap structure in LiFeAs. [1] T. Kariyado and M. Ogata, JPSJ {\bf 79}, 083704 (2010).   

Bogoliubov Electronic Structure at Individual Impurity Atoms in LiFeAs

Individual impurity atoms in a superconductor can strongly perturb the surrounding electronic environment and can therefore be used to probe high-temperature superconductivity at the atomic scale. Spectroscopic imaging scanning tunneling microscopy (SI-STM) is an ideal technique for the study of such effects. This has provided the motivation for several theoretical studies predicting the spatial and energetic structure of Bogoliubov electronic states at single impurity atoms in Fe-based superconductors. Here we report on a type of impurity atom that forms a strong in-gap quasi-particles interference (QPI) pattern nearby in the Fe-based superconductor LiFeAs. Our result puts a number of restrictions on theoretical studies of Fe-based superconducting mechanism.   

Theory of quasiparticle vortex bound states in Fe-based superconductors: application to LiFeAs

Spectroscopy of vortex bound states can provide valuable information on the structure of the superconducting order parameter. Quasiparticle wavefunctions are expected to leak out in the directions of gap minima or nodes, if they exist, and scanning tunneling spectroscopy (STS) on these low-energy states should probe the momentum dependence of the gap. Anisotropy can also arise from band structure effects, however. We perform a quasiclassical calculation of the density of states of a single vortex in an anisotropic superconductor, and show that if the gap itself is not highly anisotropic, the Fermi surface anisotropy dominates, preventing direct observation of superconducting gap features. This serves as a cautionary message for the analysis of STS data on the vortex state on Fe-based superconductors, in particular LiFeAs, which we treat explicitly. YW and PJH were supported by the DOE under DE-FG02-05ER46236, and I. V. under DE-FG02-08ER46492.   

Local electronic structure near a vortex in LiFeAs within self-consistent BdG

A major question in Fe-based superconductors is whether the pairing is of an unconventional nature with a sign change. The electronic structure in the presence of vortices can serve as a platform for phase sensitive measurements to answer this question. However as Fe-based superconductors are in the intermediate regime of correlation strength, a delicate balance between band structure effects and interaction effects may challenge a simple guess. We perform a microscopic self-consistent BdG calculation for LiFeAs in the presence of a perpendicular magnetic field and calculate the energy-dependent local electronic structure near a vortex. We use a band structure in agreement with recent experiments and compare different gap-symmetry possibilities. We find the low-energy local density of states to be dominated by the geometry of the Fermi surface, with tails along the directions perpendicular to the flat portions of the Fermi surface. These are the directions of the gap {\it maxima} on the square-like hole pocket around the $\Gamma$ point according to recent observations. We discuss how the gap symmetry affects high-energy local density of states.   

Elastic neutron scattering on Co-doped NaFeAs superconductors

NaFeAs and LiFeAs are the only two members in the Pnictide superconductors 111 family. Without doping, NaFeAs shows filamentary superconductivity coexisting with static antiferromagnetic (AF) order .In addition to the superconducting transition at 9 K, structural and AF transitions appear at $\sim $50 K and $\sim $ 40 K, respectively. By gradually doping Co on Fe sites, the AF order is suppressed and bulk superconductivity occurs. Although the electronic phase diagram of Co-doped NaFeAs is similar to the phase diagram of Co-doped 122 family, the NaFeAs system is much simpler because it does not have strong c-axis magnetic coupling and therefore a detailed investigation of this system will shed new light to our understanding on the common features amongst different classes of Fe-based superconductors. We will report our elastic neutron scattering results about this system.   

Temperature-dependent resistivity in single crystals Na$_{1-\delta}$Fe$_{1-x}$Co$_x$

Stoichiometric NaFeAs superconductor is representative of the slightly underdoped part of the doping phase diagram, with a sequence of tetragonal-to-orthorhombic, $T_{s} \approx$ 60 K, magnetic, $T_{m}$=45 K, and superconducting, $T_c$=12~K transitions. Doping level in the compound can be tuned with Co substitution of Fe, acting as electron donor. This doping suppresses structural and magnetic instabilities and induces superconductivity with $T_c$ up to 25~K. Doping with Co allows for studying complete doping phase diagram. We performed systematic measurements of the temperature-dependent in-plane, $\rho _a(T)$, and inter-plane, $\rho _c(T)$, electrical resistivities in the compounds. At optimal doping, both $\rho _a(T)$ and $\rho _c(T)$ show close to $T$-linear temperature dependence above the superconducting $T_c$. With doping this dependence gradually evolves towards $T^2$. At much higher temperatures a slope-change is observed in $\rho _a(T)$, which we relate with onset of carrier activation over a pseudogap.   

Oxidative deintercalation of single crystal Na$_{1-\delta}$FeAs upon Interaction with the environment

Due to high mobility of Na ions, NaFeAs superconductor exhibits pronounced reaction with the environment, leading to a bulk change change in stoichiometry. We study the doping evolution of the same single crystals as a function of time of the environmental exposure. In NaFeAs, a controlled reaction with air increases the superconducting transition temperature, $T_{c}$, from the initial value of 12 K to 27 K as probed by transport and magnetic measurements. Temperature dependent resistivity, $\rho_{a}$(T), shows a dramatic change with the exposure time. In freshly prepared samples, $\rho_{a}$(T) reveals clear features at the structural, $T_{s} \approx$ 60 K, and magnetic, $T_{m}$=45 K, transitions and superconductivity with onset $T_{c;ons} \approx$16 K and offset $T_{c;off} \approx$12 K. The exposed samples show $T-$linear variation of $\rho_{a}$(T) above $T_{c;ons} \approx$30 K ($T_{c;off} \approx$26 K). This suggests bulk doping and implies the existence of a quantum critical point at the optimal doping. The resistivity for different doping levels is affected below $\sim$ 200 K suggesting the existence of a characteristic energy scale that caps the $T-$linear regime, which could be identified with a pseudogap.   

Some magnetic and electronic properties of Vanadium (hole) doped NaFeAs

We examine the magnetic properties of the hole doped system V$_{x}$Na$_{1-x}$FeAs. It's known that the Na111 system exhibits structural, magnetic, and superconducting phase transitions. As opposed to the better studied electron doped systems, we successfully grew homogeneous single crystals with holes doped into the iron sites. Preliminary results show a total suppression of the superconducting phase and partial suppression of the AF order with doping as low as 1{\%}. I will present a range of susceptibility measurements to examine the effect of Vanadium doping on T$_{c}$ and the Neel Temperature.   

Cerium-Iron Magnetic Coupling in Single Crystal CeFeAsO at Low Temperatures

Neutron and synchrotron resonant X-ray scattering techniques have been used to determine the intricate magnetic structure of single crystal CeFeAsO at low temperatures, way below the spin-density wave (SDW) transition around 130 K associated with Fe moments ordering. Our synchrotron X-ray scattering results at the Ce $L_{II-}$edge clearly show a magnetic transition that is specific to the Ce ordering at $T_{Ce}$ = 4 K, whereas neutron diffraction data indicate a transition at $T*$ = 12 K with unusual order parameter. Detailed order parameter measurements on the (100) {\&} (101) magnetic reflections by neutrons show an anomaly at $T$ = 4 K which we associate with the Ce ordering. The successive transitions at $T_{Ce}$ and $T$* can also be clearly identified by two anomalies in heat capacity measurements. We argue that the higher transition temperature observed in neutron measurements reflects Fe-Ce combined rearrangement prior to the complete ordering of the Ce. The effect of the weak Ce-Fe coupling on the rearrangement of Fe ordering is yet another example of the vulnerability of the Fe-SDW as influenced by minute doping or by relatively low applied pressures.   

FT-IR evaluation of SmFeAsO$_{1-x}$F$_{x}$ (x = 0, 0.069)

Optical properties of superconducting SmFeAsO$_{1-x}$F$_{x}$ (x=0, 0.069) were demonstrated by reflection measurement with FT- IR method. Polycrystalline SmFeAsO$_{1-x}$F$_{x}$ samples were synthesized using two-step solid state reaction described elsewhere [New J. Phys.\textbf{12}, 033005 (2010)]. Purity of samples was checked by X-ray diffraction patterns using Cu K- alpha radiation. The reflection measurement was performed at the range from 9000 cm$^{-1}$ to 18000 cm$^{-1}$ that was corresponded to an energy region from 1.12 eV to 2.25 eV. A photoconductivity of SmFeAsO$_{1-x}$F$_{x}$ was determined by Kramer-Kroning (KK) relation. Reflectivity and photoconductivity measurements, as well as by FT-IR, at various areas were performed to define an energy level of materials [EPL, \textbf{84} 67013 (2008), and J. Phys. Soc. Jpn. \textbf{80} 013707 (2011)]. Obtained photoconductivity and reflection spectra were similar to those of LaFeAsO$_{1-x}$F$_{x}$ that was a basic compound of LnFeAsO$_{1-x}$F$_{x}$ (Ln=La, Ce, Sm), reported by Z. G. Chen et al [Phys. Rev. B \textbf{81}, 100502 (2010)]. Our result suggests that the energy band structure of SmFeAsO was affected by F-doping even in visible area. Details and temperature dependence of the reflection and photoconductivity spectra will be presented at the conference.   

Superconducting and normal state properties of single crystalline RFeAsO-based materials

We report electrical resistivity, Hall effect, magnetoresistance, magnetization, and specific heat measurements on single crystals of several RFeAsO-based materials (R = rare-earth). We also discuss the effects of lattice compression on the magnetic and superconducting transition temperatures. These single crystal studies benefit from significantly sharper signatures of the transitions, compared to earlier studies on polycrystalline samples.   

Growth conditions of oxypnictide compounds LaFePnO Pn=\{P,As,Sb\}

The discovery of superconductivity in layered ferro-oxynictides LaFePO $(T_c\sim4\ K)$ and LaFeAsO$_{1-x}$F$_x$ $(T_c\sim26\ K)$ lead to an intense search for other iron based superconducting materials. For the hypothetical compound LaFeSbO, ab initio density functional theory (DFT) calculations predict an enhanced density of states at the Fermi level together with increased nesting between the electron and hole sheets in the tetragonal structure (isostructural to LaFeAsO) and an enhanced spin-density wave ground state in a closely related orthorhombic structure; indicating the potential for superconductivity with a higher transition temperature [1]. We report ab initio DFT calculations of the phase stability of the oxypnictides LaFe$Pn$O $Pn$=\{P,As,Sb\} and find growth conditions where LaFePO and LaFeAsO are thermodynamically stable, but LaFeSbO is unstable with respect to the formation of La$_2$SbO$_2$. Indeed, our attempt to synthesize LaFeSbO led to the synthesis and characterization of La$_2$SbO$_2$. The phonon spectrum of hypothetical LaFeSbO shows no soft modes, indicating that LaFeSbO is potentially metastable and leaving open the possibilty of a nonequilibrium synthesis route. \\[4pt] [1] C-Y. Moon, S.~Y. Park, and H. J. Choi, Phys. Rev. B, {\bf 78}, 212507 (2008)   

Session W24: Integer Quantum Hall Effect

A quantitative examination of the collapse of spin splitting in the quantum Hall regime

There is a great deal of current interest in understanding electron spin physics in semiconductors for potential quantum computation applications. The quantum Hall effect in the two-dimensional electron system (2DES) has proved to be a unique system in this avenue due to a tunability in the difference of spin population and thus the strength of exchange interaction provided by the formation of Landau levels. In this talk, we want to present our experimental results to \textit{quantitatively} examine the theoretical model of spin splitting collapse in the quantum Hall regime [by Fogler and Shklovskii, Phys. Rev. B 52, 17366 (1995)] at fixed magnetic fields as a function of electron density in a high quality heterojunction insulated-gate field effect transistor. In the density range between $n$ = 2$\times $10$^{10}$ and 2$\times $10$^{11}$ cm$^{-2}$, the Landau level number $N$ follows a power-law dependence on the critical electron density $n_{c}$, where the spin splitting collapses, and $N$=11.47$\times n_{c}^{0.64\pm 0.01}$. This power law dependence is in good agreement with the theoretical prediction in the low density regime.   

Single electron wave packets probed by Hanbury-Brown and Twiss interferometry

The ballistic propagation of electronic waves in the quantum Hall edge channels of a 2DEG bears strong analogies with photon optics which inspired a whole set of experiments, including the realization of electronic Mach-Zehnder [1] and Hanbury-Brown and Twiss [2] interferometers. So far, these experiments have been performed with continuous sources, but the recent realization of on-demand single electron emitters [3] has risen the hope to reach, in these experiments, the single charge control. We report here on the first realization of a Hanbury-Brown and Twiss experiment on a single electron beam generated by the single electron emitter recently developed by our group [3]. Using the chiral edge channels of the quantum Hall effect, single electron emitted by the source are directed towards an electronic beam-splitter. From the low frequency current correlations at the output of the beam splitter, we are able to count and characterize the elementary excitations produced by the source. By analyzing their antibunching with thermal excitations, we show that we are able to shape single particle states in a tuneable way. [1] Ji et al., Nature 422, 415 (2003) [2] Henny et al., Science 284 296 (1999) [3] F\`{e}ve et al., Science 316, 1169 (2007)   

Hall Viscosity I: Linear Response Theory for Viscosity

In two dimensional systems with broken time-reversal symmetry, there can exist a non-dissipative viscosity coefficient [1,2,3]. This Hall viscosity is similar in nature to the non-dissipative Hall conductivity. In order to investigate this phenomenon further, we develop a linear response formalism for viscosity. We derive a Kubo formula for the frequency dependent viscosity tensor in the long wavelength limit. We compute the viscosity tensor for the free electron gas, integer quantum Hall systems, and two-dimensional paired superfluids. In the zero frequency limit, we show how the known results [3,4] for the Hall viscosity are recovered.\\[4pt] [1] J. Avron, R. Seiler, and P. Zograf, Phys. Rev. Lett. {\bf 75}, 697 (1995).\\[0pt] [2] P. Levay, J. Math. Phys. {\bf 36}, 2792 (1995).\\[0pt] [3] N. Read, Phys. Rev. B {\bf 79}, 045308 (2009).\\[0pt] [4] N. Read and E. Rezayi, Phys. Rev. B {\bf 84}, 085316 (2011).   

Hall Viscosity II: Extracting Viscosity from Conductivity

When time reversal symmetry is broken, the viscosity tensor of a fluid can have non-dissipative components, similarly to the non-dissipative off-diagonal Hall conductivity. This ``Hall viscosity'' was recently shown to be half the particle density times the orbital angular momentum per particle. Its observation can thus help elucidate the nature of the more exotic quantum Hall states and related systems (e.g., p+ip superconductors). However, no concrete measurement scheme has hitherto been proposed. Motivated by this question we use linear response theory to derive a general relation between the viscosity tensor and the wave-vector dependent conductivity tensor for a Galilean-invariant quantum fluid. This relation enables one to extract the Hall viscosity, as well as other viscosity coefficients (shear and bulk) when relevant, from electromagnetic response measurements. We also discuss the connection between this result and a similar one recently derived by C. Hoyos and D. T. Son [arXiv:1109.2651].   

Four-point characterization using capacitive and ohmic contacts

A four-point characterization method is developed for semiconductor samples that have either capacitive or ohmic contacts. When capacitive contacts are used, capacitive current- and voltage-dividers result in a capacitive scaling factor which is not present in four-point measurements with only ohmic contacts. Both lock-in amplifier and pre-amplifier are used to measure low-noise response over a wide frequency range from 1 Hz -- 100 kHz. From a circuit equivalent of the complete measurement system after carefully being modeled, both the measurement frequency band and capacitive scaling factor can be determined for various four-point characterization configurations. This technique is first demonstrated with a discrete element four-point test device and then with a capacitively and ohmically contacted Hall bar sample using lock-in measurement techniques. In all cases, data fit well to a circuit simulation of the entire measurement system over the whole frequency range of interest, and best results are achieved with large area capacitive contacts and a high input-impedance preamplifier stage. Results of samples (substrates grown by Max Bichler Dieter Schuh, and Frank Fischer of the WSI) measured in the QHE regime in magnetic fields up to 15 T at temperatures down to 1.5 K will also be shown.   

Growth and magnetotransport measurements of triple-valley high mobility, miscut (111) AlAs quantum wells

We optimize growth of AlAs on (111)B GaAs substrates and perform magnetotransport measurements on vicinal (111)AlAs quantum wells (QWs). Previous literature reports that MBE growth on exactly oriented GaAs (111)B substrates is difficult, and the grown epi-layers are obscured by pyramid-like surface faceting and twin defect formation; slight substrate miscut results in stable step-flow growth. We perform a combined structural analysis with AFM, TEM and XRD to correlate MBE growth conditions with defect density scaling. We find that a high growth temperature of 690$^{\circ}$ C and low As beam fluxes reduce micro-twin formation for exactly oriented substrates and eliminate them for miscut substrates. A slight miscut of 2$^{\circ}$, at which slip-step growth is known to occur, lead to AlAs QWs with record electron mobility $\mu$ = $13000 ~$cm$^2$/Vs at a sheet density $n_{\mathrm{2D}}$ = $2.17\times 10^{11}$ cm$^{-2}$. Numerical calculations reveal that valley splitting is about 1 meV per degree of miscut, which compares to typical Fermi energies of 2DEGs in AlAs QWs. Signatures in the transport data indicate that not only miscut but also exchange splitting between valleys can play an important role. Magnetotransport data at 15 mK in magnetic fields up to 15 T will also be presented.   

Chiral heat transport in driven quantum Hall and spin Hall edge states

We consider a model for an edge state of electronic systems in the quantum Hall regime with filling $\nu=1$ as well as in the quantum spin Hall regime. In both cases the system is in contact with two reservoirs by tunneling at point contacts. Both systems are locally driven by applying an ac voltage in one of the contacts. By weakly coupling them to a third reservoir, the transport of the generated heat is studied in two different ways: i) when the third reservoir acts as a thermometer the local temperature is sensed, and ii) when the third reservoir acts as a voltage probe the time-dependent local voltage is sensed. Our results indicate a chiral propagation of the heat along the edge in the quantum Hall case and in the quantum spin Hall case (if the injected electrons are spin polarized). The temperature profile shows that electrons along the edge thermalize with the closest upstream reservoir.   

Wigner path integral solution for the integer quantum Hall effect

The real time propagator of the Wigner distribution function can be constructed from the Wigner-Liouville equation as a phase space path integral. By analogy with the Feynman path integral one can define a new effective Lagrangian of the system in the Wigner-Weyl representation. The effects of gauge transformations and geometric constraints on the action are discussed. In particular we discuss the dynamics of a non-interacting 2DEG on a Hall strip.   

Microwave reflection study of GaAs/AlGaAs devices in the regime of the radiation-induced magnetoresistance oscillations

The microwave-induced magnetoresistance oscillations are revealed in the GaAs/AlGaAs two dimensional electron system (2DES) under microwave and terahertz photo-excitation at liquid helium temperatures. Such oscillations are understood in terms of the displacement and inelastic models for photo-excited transport in this system. In order to identify the relative physical contributions, we have concurrently examined magnetotransport and microwave reflection from the 2DES. For the reflection measurements, a sensitive microwave detector was assimilated into the standard experimental setup. Here, we report on the observed magnetic field induced changes in the microwave reflection, and correlate the observations with concurrent transport response of the photo-excited 2DES.   

Observation of linear-polarization-sensitivity in the microwave-radiation-induced magneto-resistance oscillations

In the quasi two-dimensional GaAs/AlGaAs system, we investigate the effect of rotating \textit{in-situ} the electric field of linearly polarized microwaves relative to the current, on the microwave-radiation-induced magneto-resistance oscillations. We find that the frequency and the phase of the photo-excited magneto-resistance oscillations are insensitive to the polarization. On the other hand, the amplitude of the resistance oscillations are remarkably responsive to the relative orientation between the microwave antenna and the current-axis in the specimen. The results suggest a striking linear polarization sensitivity in the radiation-induced magnetoresistance oscillations.   

Non-equilibrium bosonization and its applications to quantum Hall systems

Bosonization is a powerful theoretical method allowing one to treat interactions in 1D systems non-perturbatively. This technique in its original formulation applies to equilibrium states. In order to describe recent experiments on 1D systems far from equilibrium, we introduce a new bosonization method: A non-equilibrium state is described by imposing non-trivial boundary conditions for collective boson fields. This method allows to reduce the problem of finding correlation functions in an interacting 1D system to the calculation of full counting statistics of a process, which creates a non-equilibrium state. The full counting statistics has been extensively studied, and it is well known in several important situations. We apply the non-equilibrium bosonization technique to explain recent experiments on noise-induced dephasing in quantum Hall interferometers and to the energy equilibration at quantum Hall edge states.   

Single-electron quantum tomography in quantum Hall edge channels

The recent demonstration of an on demand single electron source [1,2] has opened the way to a new generation of``electron quantum optics'' experiments aimed at preparing, manipulating and measuring coherent single electron excitations propagating in ballistic conductors such as the edge channels of a 2DEG in the integer quantum Hall regime. In this talk, I will describe a proposal [3] for measuring single electron coherence using an Hanbury Brown and Twiss interferometer. This quantum tomography protocol could be used to characterize single electron sources and to perform quantitative studies of decoherence.\\[4pt] [1] Science 316, 1169 (2007)\\[0pt] [2] Phys. Rev. B 82, 201309 (2010)\\[0pt] [3] New Journal of Physics 13, 093007 (2011)   

Linear polarization rotation study of the radiation-induced magnetoresistance oscillations

The polarization sensitivity of microwave-radiation-induced magneto-resistance oscillations is investigated by rotating, by an angle \textit{$\theta $}, the polarization of linearly polarized microwaves with respect to the long-axis of GaAs/AlGaAs Hall-bar electron devices. At low microwave power, $P$, experiments show a strong sinusoidal variation in the diagonal resistance $R_{xx}$ vs. \textit{$\theta $} at the oscillatory extrema, indicating linear polarization sensitivity in the microwave radiation-induced magneto-resistance oscillations. Surprisingly, the phase shift \textit{$\theta $}$_{0}$ for maximal oscillatory $R_{xx}$ response under photo-excitation appears dependent upon the radiation-frequency $f$, the extremum in question, and the magnetic field orientation or \textit{sgn}(B).   

Wave function multifractality and dephasing at quantum Hall transitions: A numerical investigation

To understand the effect of the Coulomb interaction is one of the most challenging problems in the context of Anderson localization and the quantum Hall effect. In our study we address this question by following a perturbation theory in the interaction near the non-interacting fixed point. In each order diagrams appear which contain correlation functions characterizing the fluctuation properties of wavefunctions at the (non-interacting) critical fixed point. It turns out that the correlators relevant for dephasing combine in a way such that the {\em leading} multifractal powerlaws cancel; the subleading terms govern the interaction corrections. We present a numerical study based on the Chalker-Coddington network, in which we determine quantitatively the subleading multifractal exponents of the salient wavefunction correlators.   

Corrections to scaling near the quantum Hall transition

Corrections to scaling near critical points are important to understand, because they superimpose and often obscure the true asymptotics of critical scaling laws. This is true, in particular, for studies near the quantum Hall transition where recent numerical work by Slevin and Ohtsuki (Phys. Rev. B {\bf 80}, 041304 (2009)) reports a very small value for the leading irrelevant scaling index $|y|\approx 0.17$. We here report a numerical study of two-point conductances and two-terminal conductances at the integer quantum Hall transition within the Chalker-Coddington network. The scaling of these observables will be analyzed in the two-dimensional and the quasi-onedimensional geometries. We confirm the relation between the conductance exponents $X_q$ and the anomalous dimensions $\Delta_q$ known from the multifractal wavefunction analysis: $X_q=2\Delta_q$. For a consistent picure it is essential to carefully account for corrections to scaling due to subleading power laws and irrelevant scaling operators.   

Session W25: Focus Session: Modeling of Rare Events: Methods and Applications I

Pathways to forming glass

Upon supercooling at a finite rate, many liquids transform to glass. It is a non-equilibrium transition that depends upon both the material and the cooling protocols. We have used methods of transition path sampling to study this phenomenon in models of glass forming liquids. For the models considered, we establish the order of the transition and the nature of excitations that distinguish a melt from a glass.   

The kinetic activation-relaxation technique: an off-lattice, self-learning kinetic Monte Carlo algorithm with on-the-fly event search

While kinetic Monte Carlo algorithm has been proposed almost 40 years ago, its application in materials science has been mostly limited to lattice-based motion due to the difficulties associated with identifying new events and building usable catalogs when atoms moved into off-lattice position. Here, I present the kinetic activation-relaxation technique (kinetic ART) is an off-lattice, self-learning kinetic Monte Carlo algorithm with on-the-fly event search [1]. It combines ART nouveau [2], a very efficient unbiased open-ended activated method for finding transition states, with a topological classification [3] that allows a discrete cataloguing of local environments in complex systems, including disordered materials. In kinetic ART, local topologies are first identified for all atoms in a system. ART nouveau event searches are then launched for new topologies, building an extensive catalog of barriers and events. Next, all low energy events are fully reconstructed and relaxed, allowing to take complete account of elastic effects in the system's kinetics. Using standard kinetic Monte Carlo, the clock is brought forward and an event is then selected and applied before a new search for topologies is launched. In addition to presenting the various elements of the algorithm, I will discuss three recent applications to ion-bombarded silicon, defect diffusion in Fe and structural relaxation in amorphous silicon.\\[4pt] This work was done in collaboration with Laurent Karim B\'{e}land, Peter Brommer, Fedwa El-Mellouhi, Jean-Fran\c{c}ois Joly and Laurent Lewis.\\[4pt] [1] F. El-Mellouhi, N. Mousseau and L.J. Lewis, Phys. Rev. B. \textbf{78}, 153202 (2008); L.K. B\'{e}land \emph{et al.}, Phys. Rev. E \textbf{84}, 046704 (2011).\newline [2] G.T. Barkema and N. Mousseau, Phys. Rev. Lett. \textbf{77}, 4358 (1996); E. Machado-Charry \emph{et al.}, J. Chem Phys. 135, 034102, (2011).\newline [3] B.D. McKay, Congressus Numerantium \textbf{30}, 45 (1981).   

An improved Activation-Relaxation Technique method for finding transition pathways

The Activation-Relaxation Technique nouveau (ARTn) is an eigenvector following method for systematic search of saddle points and transition pathways on a given potential energy surface. We propose a variation of this method aiming at improving the efficiency of the local convergence close to the saddle point. The efficiency of the method is demonstrated in the case of point defects in body centered cubic iron. We also prove the convergence and robustness of a simplified version of this new algorithm. Joint work with E. Cances, M.-C. Marinica, K. Minoukadeh and F. Willaime.   

Entropic stabilization of nanoscale voids in materials under tension

While preexisting defects are known to act as nucleation sites for plastic deformation in strained materials, the kinetics of the early stages of plastic yield are still poorly understood. We use a wide range of atomistic simulation techniques (molecular dynamics, accelerated molecular dynamics, umbrella sampling, etc) to investigate the kinetics of plastic yield around small nanoscale voids in copper under uniaxial tensile strain. We demonstrate that, at finite temperatures, these voids are stabilized by strong entropic effects and show that their lifetime is significant even when the static mechanical instability limit is exceeded. This stabilization phenomenon dramatically affects the yield kinetics: the lifetime of the voids is seen to increase with increasing temperature, in contrast with the usual thermally-activated behavior. Even accounting for thermal activation, very small voids prove to be extremely inefficient nucleation sites for plasticity.   

Estimate Exponential Large Mean Exit Time for Diffusion Process

We propose an efficient numerical method to estimate the mean exit time of a high dimensional diffusion process associated with an Ito SDE with a gradient drift and small $\epsilon$ diffusion coefficient, starting from a stable equilibrium of the drift. It is well-known that the mean exit time is exponential large in $\epsilon$ and thus the direct simulation of the SDE requires long time integration. Our method only requires the simulation time $O(1 / \epsilon)$ and is based on the Ornstein-Uhlenbeck diffusion in each dimension.   

K-ART study of defect evolution on experimental time scales in bcc iron and Cu--Zr interfaces

Defects in metals are challenging to study in linear simulation schemes like molecular dynamics (MD), as diffusive activation barriers are typically high compared to $k_BT$, and most ressources are devoted to integrating out thermal vibration. Simultaneously, low-energy non-diffusive rearrangements (so-called basins) are serious obstacles for methods that use state-to-state trajectories like kinetic Monte Carlo (KMC). We combined the kinetic Activation-Relaxation technique (k-ART, [1]), an off-lattice, self-learning KMC method which correctly reproduces long-range interactions, with an autonomous basin identification scheme that averages over all in-basin transitions. This allows us to study defect evolution on much longer time scales than MD. In this talk, we present results on the vacancy cluster formation in bcc iron and interface diffusion in the Cu--Zr system.\\[4pt] [1] B\'{e}land, Brommer, \emph{et al.}, \emph{Phys.\ Rev.\ E}, {\bf 84}, 046704 (2011).   

Accelerated Molecular Dynamics of GaAs(001) Homoepitaxy: Effects of Long-Range Disorder

GaAs homoepitaxy and heteroepitaxy are both fundamentally and technologically significant. From a fundamental perspective, recent experimental work has shown that the GaAs(001) $\beta $2(2x4) substrate can transform and play an active role in diffusion and the morphologies that form during thin-film growth. Structural transformations lead to a substrate with local (2x4) domains that exhibit long-range disorder characteristic of c(2x8). At experimentally relevant temperatures, these transformations are mediated by rare events that occur over time scales ranging from ns to ms, which makes it difficult to probe the kinetics with conventional rare-event techniques. We develop an accelerated molecular dynamics (MD) protocol to deal with this challenge and we apply it to describe the temperature-dependent long-range structure of the surface. Our results are in agreement with experiment. Adatom diffusion occurs over longer time scales than those associated with transformations of the surface. Our accelerated MD studies indicate that adatom diffusion on this surface occurs via different mechanisms than those suggested by previous theoretical studies based on first-principles density-functional theory.   

Ultimate self-learning metabasin escape algorithm for supercooled liquids and glasses

A generic history-penalized metabasin escape algorithm is presented in this work without any predetermined parameters. The configuration space location and volume of imposed penalty functions are determined in self-learning processes as the complete 3N-dimensional potential energy surface is sampled. The computational efficiency is demonstrated using the binary Lennard-Jones liquid supercooled to the glass transition temperature, which shows an exponential enhancement over previous algorithm implementations.   

Modeling Correlation and Defect Effects on Mixed Polaronic and Ionic Transport in Rare-Earth Phosphates

Rare-earth phosphates are promising candidates for intermediate-temperature proton-conducting fuel cell membranes, with Sr-doped CePO$_{4}$ showing particularly high conductivity in oxidizing conditions. Since this high conductivity is likely due to hole-polarons rather than protons, the defect chemistry of incorporating charge carriers (protons versus holes versus oxygen vacancies) is currently being investigated with experiments and calculations. In this work, the dominant charge carrier and its transport in CePO$_{4}$ is calculated with density functional theory and compared to conductivity and photon beam experiments. In particular, we present ab-intio calculations on the thermodynamics of the dopants and defects and proton and hole-polaron mobilities. For the case of hole-polaron mobilities, the strength of the coupling with the lattice and the electronic coupling between sites will be investigated.   

Session W26: Focus Session: What is Computational Physics? New Technologies and Their Application

The Future of Scientific Computing

Computing technologies are undergoing a dramatic transition. Multicore chips with up to eight cores are now available from many vendors. This trend will continue, with the number of cores on a chip continuing to increase. In fact, many-core chips, e.g., NVIDIA GPUs, are now being seriously explored in many areas of scientific computing. This technology shift presents a challenge for computational science and engineering--the only significant performance increases in the future will be through the increased exploitation of parallelism. At the same time, petascale computers based on these technologies are being deployed at sites across the world. The opportunities arising from petascale computing are enormous--predicting the behavior of complex biological systems, understanding the production of heavy elements in supernovae, designing catalysts at the atomic level, predicting changes in the earth's climate and ecosystems, and designing complex engineered systems. But, petascale computers are very complex systems, built from multi-core and many-core chips with 100,000s to millions of cores, 100s of terabytes to petabytes of memory, and 10,000s of disk drives. The architecture of petascale computers has significant implications for the design of the next generation of science and engineering applications. In this presentation, we will provide an overview of the directions in computing technologies as well as describe the petascale computing systems being deployed in the U.S. and elsewhere.   

GPU Acceleration of the Qbox First-Principles Molecular Dynamics Code

The availability of double precision graphics cards provides an opportunity to speed up electronic structure computations. We modify the Qbox [1] code to utilize Fermi GPUs on the Keeneland [2] platform. We use the CUFFT library to speed up Fourier transforms and perform asynchronous communication to cut down the cost of data transfers. The modified code is used in simulations of a 64-molecule water system with an 85 Ry plane wave energy cut off. Preliminary results show a 2-3 times speedup in the calculation of the charge density and in the application of the Hamiltonian operator to the wave function. We present these findings as well as further speedups measured in other parts of the code. \\[4pt] [1] http://eslab.ucdavis.edu/software/qbox\\[0pt] [2] http://keeneland.gatech.edu   

Quantum Monte Carlo in the era of petascale computers

Continuum quantum Monte Carlo (QMC) methods are a leading contender for high accuracy calculations for the electronic structure of realistic systems, especially on massively parallel high-performance computers (HPC). The performance gain on recent HPC systems is largely driven by increasing parallelism: the number of compute cores of a SMP and the number of SMPs have been going up, as the Top500 list attests. However, the available memory as well as the communication and memory bandwidth per element has not kept pace with the increasing parallelism. This severely limits the applicability of QMC and the problem size it can handle. (OpenMP,CUDA)/MPI hybrid programming provides applications with simple but effective solutions to overcome efficiency and scalability bottlenecks on large-scale clusters based on multi/many-core SMPs. We discuss the design and implementation of hybrid methods in QMCPACK and analyze its performance on multi-petaflop platforms characterized by various memory and communication hierarchies. Also presented are QMC calculations of bulk systems, including defects in semiconductors.   

GPU accelerated replica-exchange simulations of polymers

Precise estimation of physical quantities using Monte Carlo computer simulations strongly depends on the amount of statistical data gathered during the simulation. Being able to increase the performance of the sampling process will allow more accurate results in a shorter time period. To employ the parallel tempering replica-exchange algorithm on parallel hardware such as multicore CPUs and GPUs turns out to be very suitable for the task. We achieve rapid speedups in our investigation of an exemplified bead-spring polymer model. Identification and classification of phase-like transitions were done by analyzing the microcanonical entropy.   

Graphics Processing Unit Accelerated Hirsch-Fye Quantum Monte Carlo

In Dynamical Mean Field Theory and its cluster extensions, such as the Dynamic Cluster Algorithm, the bottleneck of the algorithm is solving the self-consistency equations with an impurity solver. Hirsch-Fye Quantum Monte Carlo is one of the most commonly used impurity and cluster solvers. This work implements optimizations of the algorithm, such as enabling large data re-use, suitable for the Graphics Processing Unit (GPU) architecture. The GPU's sheer number of concurrent parallel computations and large bandwidth to many shared memories takes advantage of the inherent parallelism in the Green function update and measurement routines, and can substantially improve the efficiency of the Hirsch-Fye impurity solver.   

A simple yet powerful open-source tool for scientific computing

I will introduce new open-source software for easily carrying out common mathematical task (plotting, animating, differentiating, integrating, and solving systems of equations) and fitting experimental data. This software requires no special syntax or programming, and it is designed to allow you to communicate mathematical results to your colleagues or students in a manner that is interactive, productive, and efficient. The current version can be downloaded from http://www.compadre.org/osp/items/detail.cfm?ID=11250.   

Multicanonical Modeling of Commercial WDM Optical Communication Systems

The multicanonical method has been extensively applied to optical and more recently wireless communication systems. Here we outline our recent work on the simulation of electronically compensated polarization multiplexed wavelength-division-multiplexed quadrature phase shift keyed optical communication systems influenced by polarization mode dispersion and fiber nonlinearites. This constitutes to our knowledge the first complete multicanonical analysis of a realistic commercial system.   

Mantid, A high performance framework for reduction and analysis of neutron scattering data

The use of large scale facilities by researchers in the field of condensed matter, soft matter and the life sciences is becoming ever more prevalent in the modern research landscape. Facilities such as SNS and HiFNR at ORNL and ISIS at RAL have ever increasing user demand and produce ever increasing volumes of data. One of the single most important barriers between experiment and publication is the complex and time consuming effort that individual researchers apply to data reduction and analysis. The objective of the Manipulation and Analysis Toolkit for Instrument Data or MANTID [1] framework is to bridge this gap with a common interface for data reduction and analysis that is seamless between the user experience at the time of the experiment and at their home institute when performing the final analysis and fitting of the data.\\[4pt] [1] http://www.mantidproject.org/   

SU(N) Clebsch-Gordan coefficients and non-Abelian symmetries

The numerical treatment of models with SU($N$) benefits greatly from the Wigner-Eckart theorem. Its application requires the explicit knowledge of the Clebsch-Gordan coefficients (CGCs) of the group SU($N$). We present an algorithm for the explicit numerical calculation of SU($N$) CGCs based on the \emph{Gelfand-Tsetlin pattern} calculus. Further exploitation of the Weyl symmetry of SU($N$) irreducible representations (irreps) leads to a significant speed-up compared to our previous algorithm (J.~Math.~Phys.\ 52, 023507, 2011). Our algorithm works for arbitrary $N$ and tensor products of two arbitrary SU($N$) irreps. It is well-suited for numerical implementation; we provide a well-tested computer code for download and online use. Possible applications of our code include numerical treatments of quantum many-body systems using the numerical renormalization group (NRG), the density matrix renormalization group (DMRG), and general tensor network methods.   

Efficient interacting many body similations using GPUs

Graphics Processing Units (GPUs) provide an ideal tool to study interacting systems using classical machanics with huge speedups for example in molecular dynamics. The quantum-mechanical calculations of many-body systems require additional work, but are feasible using additional degrees of freedom to incorporate quantum-mechanical effects [1]. As an example of the method I show the self-consistent solution to the current transport in a magnetic field can be obtained from a microscopic model with thousands of Coulomb interacting electrons. This yields a microscopic model of the Hall effect [2]. For few electron systems I compare the electronic density evolution based on the GPU classical-quantum model to TD-DFT calculations and discuss prospects of GPUs for solving the Schrodinger equation for many-particles. \\[4pt] [1] Time dependent approach to transport and scattering in atomic and mesoscopic systems, T. Kramer AIP Conf. Proc., 1334, 142 (2011) \\[0pt] [2] Self-consistent calculation of electric potentials in Hall devices, T. Kramer, V. Krueckl, E. Heller, and R. Parrott Phys. Rev. B, 81, 205306 (2010)   

Session W27: Invited Session: Electrons, Spins, and Collective Modes in Nanofilms

Plasmons under extreme dimensional confinement

In our studies, we explore how surface and bulk plasmons emerge under extreme dimensional confinement, i.e., dimensions that are orders of magnitude smaller than those employed in `nanoplasmonics'. Atomically-smooth ultrathin Mg films were epitaxially grown on Si(111), allowing for atomically-precise tuning of the plasmon response.\footnote{M.M. \"{O}zer, E.J. Moon, A.G. Eguiluz, and H.H. Weitering, Phys. Rev. Lett. \textbf{106}, 197601 (2011).} While the single-particle states in these 3-12 monolayer (ML) thick films consist of a series of two-dimensional subbands, the bulk-plasmon response is like that of a thin slice carved from bulk Mg subject to quantum-mechanical boundary conditions. Remarkably, this bulk-like behavior persists all the way down to 3 ML. In the 3-12 ML thickness range, bulk loss spectra are dominated by the n=1 and n=2 normal modes, consistent with the excitation of plasmons involving quantized electronic subbands. The collective response of the thinnest films is furthermore characterized by a thickness-dependent spectral weight transfer from the high-energy collective modes to the low-energy single-particle excitations, until the bulk plasmon ceases to exist below 3 ML. Surface- and multipole plasmon modes even persist down to 2 ML. These results are striking manifestations of the role of quantum confinement on plasmon resonances in precisely controlled nanostructures. They furthermore suggest the intriguing possibility of tuning resonant plasmon frequencies via precise dimensional control.   

Radial band structure of a ultrathin liquid metal film

Understanding the properties of strongly disordered materials has been one of the long standing and most challenging problems in condensed matter physics. One major difficulty lies in the failure of the band structure concept to describe electronic properties due to the lack of the periodicity, as notorious for electrons in liquid metals without any well-established and well-tested alternative until now. In this work, we experimentally, using angle-resolved photoelectron spectroscopy, establish the formation of an intriguing ``double radial band structure'' in a strongly disordered electronic system of a liquid metal Pb. A monolayer Pb film was formed on Si(111) with an unusually low melting temperature and its detailed band dispersions and Fermi contours were mapped throughout the melting process. Furthermore, we introduce the way to understand this characteristic band structure based on an old theoretical idea proposed in 1962 invoking the coherent radial scattering of electrons, which can be widely encountered in wave scatterings within disordered media. In conclusion, liquid metals, or possibly other strongly disordered electronic systems, have well defined radial bandstructures through the coherent radial scattering of electrons and the radial correlation of atoms.   

Ultrafast mass transport during decay of gigantic Pb mesas on Ni(111)

We have studied the initial growth of Pb films on Ni(111) at elevated temperatures of 424 K and 474 K. Quantum Well States (QWS's) have been found to be responsible for the morphology of these Pb films on Ni(111). The delicate balance between surface energies, elastic energies and QWS's is initially tilted towards QWS's, as discrete layer heights are observed. First, a strong preference for 5 and 7 layers high, flat topped Pb islands is observed, showing several striking similarities with Pb films on Si(111) and on Cu(111). Key examples of these will be discussed. When the character of the rough film gradually changes from 2D to 3D, the balance between these forces becomes more and more dominated by interfacial energies. A tipping point is reached by very slowly heating the surface to about 520 K. As the energetic balance is tipped for good in favor of the interface free energy, the electronically stabilized, extended, about 40 layers high mesas suddenly decay towards compact hemispheric structures. The spectacular speed at which the transition takes place (billions of atoms move over several microns during a few milliseconds!) is many orders of magnitude larger than what is expected, based on arguments involving thermally activated behavior on atomic scales. I will discuss peculiarities of the wetting layer and its changes, which appear to coincide with the ultrafast transition of the film morphology. With a widespread interest in nanostructures in general, our results illustrate the generic need to characterize all aspects of nanostructures, both structural and electronic, since small excursions away from equilibrium can have dramatic consequences. T.R.J. Bollmann, R. van Gastel, H.J.W. Zandvliet and B. Poelsema; Phys. Rev. Lett. 107, 116101 (2011); T.R.J. Bollmann, R. van Gastel, H.J.W. Zandvliet and B. Poelsema; New J. Phys. 13, 103025 (2011).   

Regulating spin and Fermi surface topology of a quantum metal film by the surface (interface) monatomic layer

Spin and current controls in solids have been one of the central issues in researches of electron and spin transport. Nowadays, electronics/spintronics deals with nanometer- or atomic-scale structures and miniaturization of these systems implies emergence of various quantum phenomena, intimately linked to the formation of electronic states different from those of the corresponding bulk materials. For example, valence electrons of films with the thickness comparable to the electron wavelength form discrete quantum-well states (QWSs) under opportune conditions of confinement (quantum size effect). Furthermore, the size reduction also increases the surface/volume ratio and a film possibly changes its electronic (spin) properties by the surface effect. Concerning metal films, the quantum size effect requires the thickness in a range of nanometers and the length corresponds to several tens of atoms, indicating the very large ratio of a surface (interface) monatomic layer to film atomic layers. Thus, we have been interested in combining the quantum size effects and the surface effect on the metal films to induce new physical phenomena. In the present talk, two research cases are shown. 1) Instead of isotropic two-dimensional in-plane states expected for an isolated metal film, quasi-one-dimensional quantized states were measured by photoemission spectroscopy in an epitaxial Ag(111) ultra thin film, prepared on an array of atomic chains [1]. 2) High-resolution spin-resolved photoemission and magneto-transport experiments of ultrathin Ag(111) films, covered with a /3$\times$/3-Bi/Ag surface ordered alloy, were performed. The surface state (SS) bands, spin-split by the Rashba interaction, selectively couple to the originally spin-degenerate QWS bands in the metal film, making the spin-dependent hybridization [2,3]. Magnetoconductance of the films, measured in situ by the micro-four-point probe method as a function of the applied magnetic field [4], has shown that the formation of the Rashba-type surface alloy reduces the spin-relaxation time in the ultrathin film significantly [5]. These results demonstrate that spin and Fermi surface topology of a quantum metal film can be regulated by the surface (interface) monatomic layer.\\[0pt] [1] T. Okuda, Y. Takeichi, K. He, A. Harasawa, A. Kakizaki, and I. Matsuda, Phys. Rev. B 80, 113409 (2009).\\[0pt] [2] K. He, T. Hirahara, T. Okuda, S. Hasegawa, A. Kakizaki, and I. Matsuda, Phys. Rev. Lett. 101, 107604 (2008).\\[0pt] [3] K. He, Y. Takeichi, M. Ogawa, T. Okuda, P. Moras, D. Topwal, A. Harasawa, T. Hirahara, C. Carbone, A. Kakizaki, and I. Matsuda, Phys. Rev. Lett. 104, 156805 (2010).\\[0pt] [4] N. Miyata, R. Hobara, H. Narita, T. Hirahara, S. Hasegawa, and I. Matsuda, Japanese Journal of Applied Physics 50, 036602 (2011).\\[0pt] [5] N. Miyata, H. Narita, M. Ogawa, A. Harasawa, R. Hobara, T. Hirahara, P. Moras, D.Topwal, C.Carbone, S.Hasegawa, and I. Matsuda, Phys. Rev. B, 83, 195305 (2011).   

Quantum Oscillations of Surface Electronic Structure: Inter-relation Between Quantum Well States, Work Functions and Surface Energies

Quantum size effects (QSE) in ultra-thin metallic film has been a topic of intense investigations. Of particular interests are the inter-relationship between the quantum well states (QWS), work function (W) and surface energy (E{\_}s). In ultrathin Pb films on semiconductors, quantum oscillations of E{\_}s as a function of layer thickness (L) have been investigated by various experimental methods which have all yielded identical results. Experimental studies of work function, however, took a longer journey. Photoemission can probe the work function for an uniform film but in this case uniform film only exists for certain thicknesses. Scanning tunneling microscopy, can probe ``local'' properties for all thicknesses, but the very existence of QWS in these films profoundly affects the measured tunneling decay constant $\kappa $. Consequently, L-dependence of $\kappa $ also depends on the bias voltage. It was then discovered that at a very low sample bias ($\vert $Vs$\vert <$ 0.03 V) the measured $\kappa $ vs. L accurately reflects the quantum size effect on the work function [1]. With this last obstacle removed, we are able to simultaneously measure the W vs. L, E{\_}s vs. L and to correlate these quantities with the measured QWS locations, yielding the quantitative phase relationship between the quantum oscillations of work function and surface energy. To our surprise, instead of a predicted $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 4$} $ wavelength phase shift, we find that the quantum oscillations of these two quantities are exactly in-phase. A new model is proposed.   

Session W29: Focus Session: Superconducting Qubits: Resonators and Loss Mechanisms

Microwave Loss in Josephson Junction Embedded Superconducting Resonators

We report on progress to identify and mitigate sources of microwave frequency loss in Josephson junction resonant circuits at low temperature and power--the operating regime of superconducting qubits. Large critical current junctions ($>$500nA) were embedded in lumped element $4-8$ GHz superconducting resonators based on single crystal silicon dielectric overlap capacitors. Small critical current junctions (25-100nA) were characterized by measuring the $T_{1}$ relaxation time of transmon qubits. For both sets of measurements, the critical current density was varied from 20-800 $A/cm^{2}$. We present the dependence of junction loss on junction area and environmental factors such as shielding and filtering.   

Superconducting Microwave Resonators

High quality factor superconducting microwave resonators play a key role in applications to quantum computation and single photon detection schemes. We have optimized our aluminum quarter wavelength coplanar waveguide resonators in an effort to improve energy decay times. As the characteristic decay times in our samples begin to approach the requirements set out by fault tolerant error correction algorithms, reproducibility becomes a growing focus. Consistent reproduction of high quality factor resonators requires reliable determination of device parameters independent of experimental imperfections and environmental influences. These measurements permit an improved understanding of the variations between nominally identical resonators as well as variations in an individual sample over time. Recent experimental results will be discussed.   

Improving the Quality Factor of Superconducting Resonators

Superconducting resonators hold great promise for quantum information storage in quantum computing. Improving the coherence lifetimes is therefore of central interest. We have focused on improving the interface between the resonator's underlying dielectric and superconducting metallization, as simulations have shown this interface to be a major source of loss, possibly associated with two-level states. After mitigating for stray light and magnetic fields, we have achieved low power intrinsic quality factors in excess of one million at single photon energies, with high power Q's in excess of ten million. Attaining such high quality factors is dependent on substrate preparation before depositing the superconductor, as well as the deposition method. We will describe the fabrication method and characterization of resonators that consistently achieve quality factors above one million.   

Investigation of superconducting resonator designs for measuring the microwave response of vortices

The microwave response of superconductors can be influenced by the presence of vortices and the dynamics they exhibit at high frequencies. We present measurements of vortices trapped in superconducting resonators fabricated from thin aluminum films, a common material for superconducting qubit circuits. In particular, we are studying the dependence of the threshold magnetic field for trapping vortices on the resonator geometry. We perform field-cooled measurements of various configurations of coplanar waveguide resonators to study the magnetic field, frequency, and temperature dependence of the microwave vortex response. The addition of vortices results in a downward shift in resonant frequency and a reduction in the resonator quality factor. We discuss the optimization of the resonator layout for detecting the response from only a few vortices trapped in the superconducting film.   

Anomalous magnetic field effects in high Q superconducting resonators

Superconducting coplanar wave guide resonators are an important tool in quantum computing for use as memory elements. Recent process improvements have allowed for quality factors in excess of 1.5 million at single photon excitations. While allowing for more sensitive experiments, the most recent group of resonators exhibit very high sensitivity to magnetic fields. Ordinarily Abrikosov vortex physics is expected to govern the magnetic response of the resonators. During field cooling, vortices begin to form at a threshold field, $B_{th}$, that depends quadratically on the width of the resonator. However these resonators show an observed $B_{th}$ two orders of magnitude lower than predicted by theory and without any scaling with resonator width. We explore increased sensitivity to frequency fluctuations at nonzero field as a possible explanation for reduced quality factor long before vortices are expected to form.   

Quality Factor Measurements with Improved Superconducting Stripline Resonators

Superconducting microwave resonators can be coupled to a superconducting qubit to manipulate, read out and protect the qubit from the environment. Improvements in resonator quality factors, Q, offer a variety of benefits. One benefit of higher Qs is the possibility for longer coherence times. Alternatively, higher Q resonators could be implemented as a quantum memory. We fabricated and tested superconducting stripline resonators which are designed to minimize sensitivity to surface dielectric losses. These devices allow the possibility of sensitive testing of superconducting film quality and/or losses in the bulk substrate dielectric. By measuring these losses we can better understand possible mechanisms of decoherence in superconducting qubits. Preliminary quality factor results, taken at milliKelvin temperatures, will be given.   

Dielectric loss analysis using linear resonators with different impedances

It is known that amorphous dielectrics are a major source of decoherence in superconducting qubits due to energy absorption by two-level systems coupled to the electric fields. Linear resonators have been applied extensively to study loss in different dielectrics used in qubit circuits due to their versatility and relative simplicity in design, fabrication and measurement. We have designed linear resonators with multi-turn inductors and parallel-plate capacitors with resonance frequencies of 4.8-6.4 GHz. We achieve substantially different L/C values and capacitor volumes by varying the number of inductance turns and the area of the capacitors. We will present results of continuous wave measurements with SiNx capacitors and show how loss tangent and phase noise are related to impedance and capacitor volume.   

Reduction of Microwave Loss in Titanium Nitride CPW Resonators

Titanium Nitride (TiN) thin films, when optimally grown and processed, exhibit very low microwave loss at high and low power. We investigate reducing the loss by systematically removing Si substrate material from the gap region in TiN coplanar waveguides (CPWs) fabricated on intrinsic Si substrates. By exploiting the radial dependence of the etch rate in a parallel plate reactive ion etcher, otherwise identical CPWs with only the gaps etched to varying depth, i.e. trenched, were created in a single TiN film within a single processing step. The high power loss is similar for all resonators, $<$ $2\times10^{-7}$. However, when comparing the loss from all trench depths in the single photon regime at 50 mK we find that loss was reduced for the deeper trenches with the deepest reduced by a factor of 2. Predictions from finite-element analysis, with a reduced participation of lossy surface oxides in the deeper trenched CPW gaps, fit well to the measured reduction.   

Cooper Pair Transistor Embedded in a dc-Biased High-Q Microwave Cavity

A high-Q microwave cavity design based on the circuit quantum electrodynamics architecture has been developed to introduce a dc bias to the center conductor of the cavity without significantly degrading the Q at high frequencies [1]. Here we directly couple Cooper pair transistors (CPTs) to such a cavity. In the subgap region of the CPT, the dc bias generates a tunable oscillating current through the CPT via the ac Josephson effect. Evidence of such self-oscillations has been observed as current peaks in our dc measurements, which are in good agreement with calculated cavity modes, and indicate the strong coupling between the CPT and the cavity. Recent experimental results will be discussed.\\[4pt] [1] F. Chen, A. J. Sirois, R. W. Simmonds and A. J. Rimberg, Appl. Phys. Lett., 98, 132509 (2011).   

Photon Emission from a Self-oscillating Cavity-Embedded-Cooper Pair Transistor

A strongly non-linear superconducting device consisting of a Cooper pair transistor embedded in a dc voltage biased microwave cavity is investigated. The cavity-embedded-Cooper pair transistor (CECPT) is driven via the ac Josephson effect by an applied dc bias and exhibits self-oscillation without an external ac drive. Tunneling Cooper pairs can both emit photons into and absorb photons from microwave cavity modes. Photons emitted into the cavity are directly probed and are in good agreement with dc measurements. Photon emission arising from both sequential tunneling and cotunnelling processes has been observed. The CECPT offers an interesting system for studying nonlinear quantum dynamics and the quantum-to-classical transition. Recent experimental results will be discussed.   

Superconducting microstrip resonators for circuit QED experiments

Superconducting microstrip resonators have several advantages when designing scalable circuit QED systems. Their simple geometry facilitates the implementation of additional circuit elements and control lines, and, most importantly, their spectrum tends to exhibit nearly no parasitic modes up to 20 GHz even for more complicated geometries. However, due to their specific field configuration they are not expected to yield high Q-factors at very low temperatures. We analyzed such resonators at Millikelvin temperatures and find experimentally useful quality factors of approximately 1500 even in the low temperature low power limit. Our analysis indicates that even ten times higher quality factors can be achieved straightforwardly by choosing substrates with better dielectric properties. Supported by the DFG via SFB 631 and by the German Excellence Initiative via NIM   

Superconducting Resonators with Parasitic Electromagnetic Environments

Microwave losses in niobium superconducting resonators are investigated at milli-Kelvin temperatures and with low drive power. In addition to the well-known suppression of Q-factor that arises from coupling between the resonator and two-level defects in the dielectric substrate [1-4], we report strong dependence of the loaded Q-factor and resonance line-shape on the electromagnetic environment. Methods to suppress parasitic coupling between the resonator and its environment are demonstrated.\\[4pt] [1] Day, P.K. et al., Nature 425, 817-821 (2003).\\[0pt] [2] Wallraff, A. et. al., Nature 451, 162-167 (2004).\\[0pt] [3] Macha, P. et. al., Appl. Phys. Lett., 96, 062503 (2010).\\[0pt] [4] O'Connell, A.D. et. al., Appl. Phys. Lett., 92, 112903 (2008).   

Session W30: Quantum Entanglement

Entanglement, Fluctuations, and Quantum Critical Points

We show that bipartite fluctuations F can be considered an entanglement measure. We further demonstrate that the concept of bipartite fluctuations F provides a very efficient tool to detect quantum phase transitions in strongly correlated systems. We investigate paradigmatic examples for both quantum spins and bosons in one and two dimensions. As compared to the von Neumann entanglement entropy, we observe that F allows to find quantum critical points with a much better accuracy in one dimension. We further demonstrate that F can be successfully applied to the detection of quantum criticality in higher dimensions with no prior knowledge of the universality class of the transition. Promising approaches to experimentally access fluctuations are discussed for quantum antiferromagnets and cold gases.   

A statistical portrait of the entanglement decay of two-qubit memories

We present a novel approach to the study of entanglement decay, which focuses on collective properties. As an example, we investigate the entanglement decay of a two-qubit memory, produced by local identical reservoirs acting on the qubits, for three experimentally and theoretically relevant cases: depolarizing, dephasing and amplitude-damping channels. We study the probability distributions of disentanglement times, a quantity independent of the measure used to quantify entanglement, and the time-dependent probability distribution of concurrence. Uniformly distributed pure states are assumed for the two-qubit system. The calculation of these probability distributions gives a clearer insight on how different decoherence channels affect the entanglement initially contained in the set of two-qubit pure states. The entanglement evolution of mixed states, under the Hilbert-Schmidt metric, is also considered.   

Quantum Mutual Information Capacity for High Dimensional Entangled States

High dimensional Hilbert spaces used for quantum communication channels offer the possibility of large data transmission capabilities and improved security. We propose a method of characterizing the channel capacity of an entangled photonic state in high dimensional position and momentum bases. We use this method to measure the channel capacity of a parametric downconversion state, achieving a channel capacity over 7 bits/photon in either the position or momentum basis, by measuring in up to 576 dimensions per detector. The channel strongly violated an entropic separability bound, indicating the performance cannot be replicated classically.   

Entangling Qubits in a One-Dimensional Harmonic Oscillator

We present a method for generating entanglement between qubits associated with a pair of particles interacting in a one-dimensional harmonic potential. By considering the effect of the interaction on the energy spectrum of the system, we show that, under certain approximations, a ``power-of-SWAP" operation is performed on the initial two-qubit quantum state without requiring any time-dependent control. Initialization errors and deviations from our approximation are shown to have a negligible effect on the final state. Using a GPU-accelerated iteration scheme to find numerical solutions to the two-particle time-dependent Schr\"{o}dinger equation, we demonstrate that it is possible to generate maximally entangled Bell states between the two qubits with high fidelity for a range of possible interaction potentials.   

Entanglement entropy for arbitrary quantum lattice models from quantum Monte Carlo

We present a general scheme to numerically calculate the Renyi entropy for the reduced density matrix of a subsystem in a quantum lattice model at finite and (physically) zero temperature. This scheme is based on an extended-ensemble formulation of quantum Monte Carlo, which can be applied in principle to any quantum Monte Carlo algorithm. It improves on the existing approach of R. G. Melko et al., Phys. Rev. B 82, 100409(R) (2010) and of M. B. Hastings et al., Phys. Rev. Lett. 104, 157201 (2010) in that it allows to probe the ground-state properties of lattice models regardless of their symmetry - as long as they admit an efficient quantum Monte Carlo algorithm. We test the entanglement entropy scaling of fundamental quantum spin models, showing e.g. that the two-dimensional XX model, describing lattice hardcore bosons, exhibits an area law despite lacking an intrinsic length scale for the decay of correlations.   

Different Measures of Entanglement in Spin Chains

Different measures of entanglement in spin chains are considered. Main example is VBS state, it is important because of measurement based quantum computation. Entanglement spectrum and negativity are considered in the lecture. These measures are calculated analytically in one dimension. In 2D we have only estimates. Lecture follows the papers: http://arxiv.org/abs/1109.4971 and http://arxiv.org/abs/1110.3300 \\[4pt] [1] Heng Fan, Vladimir Korepin, Vwani Roychowdhury, PRL, {\bf vol 93}, (2004), 227203   

R\'enyi entropy of $d$-wave Bose metal phases on multi-leg ladders

In recent years, much progress has been made toward understanding 2D Bose metal-type phases by accessing them through a series of controlled quasi-1D ladder studies [1]. Crucially, such quasi-1D descendants of these exotic phases are expected to have a number of gapless Luttinger modes, $c$, that grows with the width of the ladder. Therefore, characterizing scaling of the entanglement entropy has become an essential tool for establishing the existence of these phases, as it provides a direct measure of $c$. With density-matrix renormalization group (DMRG) methods, it is easy to calculate the entanglement entropy but the results converge prohibitively slowly as the ladder becomes wider. Here, we present results where we have calculated, using Variational Monte Carlo (VMC), the R\'enyi entropy $S_2$ for Gutzwiller-projected $d$-wave Bose metal (DBM) [2] trial wave functions on ladders, following the method employed in [3]. We compare with DMRG results (where they are available), and comment on what our findings mean for the ability of our trial wave functions to faithfully represent the DBM phase. \\[4pt] [1] D. N. Sheng et. al., PRB {\bf 78}, 054520 (2008).\\[0pt] [2] O. I. Motrunich and M. P. A. Fisher, PRB {\bf 75}, 235116 (2007).\\[0pt] [3] Y. Zhang et. al., PRL {\bf 107}, 067202 (2011).   

R\'{e}nyi Entanglement Entropies and the Entanglement Spectrum

We describe a simple method for computing the full entanglement spectrum of any finite density matrix from the R\'{e}nyi entropies of integer order. This has important implications for non-interacting fermionic systems where the R\'{e}nyi entropies are directly related to the cumulants of charge number fluctuations, and for quantum Monte Carlo simulations where it is now becoming possible to compute the R\'{e}nyi entropies but not the von Neumann entropy or the full entanglement spectrum.   

Entanglement Entropy of Fermi Liquids via Multi-dimensional Bosonization

The logarithmic violations of the area law, i.e. an ``area law'' with logarithmic correction of the form $S \sim L^{d-1} \log L$, for entanglement entropy are found in both 1D gapless system and for high dimensional free fermions. The purpose of this work is to show that both violations are of the same origin, and in the presence of Fermi liquid interactions such behavior persists for 2D fermion systems. In this paper we first consider the entanglement entropy of a toy model, namely a set of decoupled 1D chains of free spinless fermions, to relate both violations in an intuitive way. We then use multi-dimensional bosonization to re-derive the formula by Gioev and Klich [Phys. Rev. Lett. 96, 100503 (2006)] for free fermions through a low-energy effective Hamiltonian, and explicitly show the logarithmic corrections to the area law in both cases share the same origin: the discontinuity at the Fermi surface (points). In the presence of Fermi liquid (forward scattering) interactions, the bosonized theory remains quadratic in terms of the original local degrees of freedom, and after regularizing the theory with a mass term we are able to calculate the entanglement entropy perturbatively up to second order in powers of the coupling parameter for a special geometry via the replica trick.   

Multipartite Entanglement classes via Negativity Fonts

The number and types of K-way negativity fonts in canonical form of an N-qubit state depends on the nature and amount of quantum coherences in the state. Non zero determinants of negativity fonts, characterizing a given state, are easily written down and reflect the entanglement microstructure of the superposition. A classification criterion for multipartite entangled states, based on negativity fonts in canonical state and decomposition of global partial transpose in terms of K-way partially transposed operators, is proposed. Inequivalent sub-classes are labelled by N-qubit local unitary invariants. A complete classification of four qubit states is obtained. The number of major families for N$>$3 is found to be $2^N-2N$. Classification of four qubit states indicates that a small number of relevant polynomial invariants is enough to classify N-qubit states.   

Entanglement Spectrum Classification of Disordered Class AII Symplectic Systems

Of the available classes of random matrices which have been shown to contain topologically non-trivial properties\footnote{A.~P.~Schnyder, S.~Ryu, A.~Furusaki, and A~.W.~W.~Ludwig, \emph{Phys. Rev. B} \textbf{55}, 195125 (2008).}, one of the most intriguing is class AII, which is characterizes a system that possesses time-reversal symmetry. This class of random matrices has been the subject of significant attention as it encompasses Z$_2$ topological systems of which the quantum spin Hall (QSH) state is a member~\footnote{C.~L.~Kane and E.~J.~Mele, \emph{Phys. Rev. Lett.} \textbf{95}, 146802 (2005).}. We calculate the entanglement spectrum for disordered class AII symplectic systems in two-dimensions as a function of disorder strength, chemical potential, and bulk inversion asymmetry. We show that there is a one to one correspondence between the full system Hamiltonian and that of the entanglement spectrum not only in terms of level statistics but also in terms of the scaling of the inverse participation ratios. We also use the properties of the entanglement spectrum to illustrate the nature of the symplectic metal phase which appears when inversion symmetry is broken.   

Experimental entanglement estimation for a general unknown input state of a multiqubit system

Measurement of entanglement remains an important problem for quantum information. We present the design and simulation of an experimental method for entanglement estimation for a general, unknown, state of a multiqubit system. The state can be in pure or mixed, and it need not be ``close'' to any particular state. Our method, based on dynamic learning, does not require prior state reconstruction or lengthy optimization. Results for three-qubit systems compare favorably with known entanglement measures. The method is then extended to four- and five-qubit systems, with relative ease. As the size of the system grows the amount of training necessary diminishes, raising hopes for applicability to large computational systems.   

Entanglement Spectrum of the Kugel-Khomskii Model

We study the entanglement spectrum of the Kugel-Khomskii model in one dimension. The Kugel-Khomskii Hamiltonian describes transition metal oxides with orbital degeneracy, and is rich with both gapless and gapped phases with interesting symmetries. The entanglement spectrum reveals much more information than the commonly studied entanglement entropy. In this work, we investigate the entanglement spectrum for different phases and different partitions. We also make comparisons with previous field theoretic and numerical studies.   

Entanglement Spectrum In Topological Phasess

I will review the information that entanglement spectra give for a wide range of systems in condensed matter physics, such as fractional quantum hall effect, quantum spin chains, topological insulators, and disordered systems. I will also show how the entanglement spectrum is a unique tool to examine previously unknown many-body wavefunctions such as the ground-states of Fractional Chern Insulators (the results are based on a series of works performed in collaboration with N. Regnault, M. Hermanns, B. Estienne, Yangle Wu, Aris Alexandadinata, R. Thomale, A Sterdyniak, Z. Papic, T.L. Hughes, E. Prodan, D.P. Arovas, P. Bonderson)   

Topological Order in Three Dimensions and Entanglement Entropy of Gapped Phases

In this talk, I will present two very general, yet easy to understand results in the entanglement entropy of the ground states corresponding to the gapped phases of matter. In particular, I will focus on the following two results: 1) In contrast to the familiar result in two dimensions, a size independent constant contribution to the entanglement entropy can appear for non-topological phases in any odd spatial dimension. 2) The ``topological entanglement entropy'' corresponding to discrete gauge theories in any given spatial dimension $D$ (and in particular, $D = 3$) has an interesting dependence on the Betti numbers of the boundary manifold defined by the entanglement cut.   

Session W31: Focus Session: Topological Insulators: Synthesis & Characterization - Quantum Transport & Nanostructures

Evidence for a Dirac Spectrum in the Topological Insulator Bi$_2$Te$_2$Se from High-Field Shubnikov-de Haas Oscillations

The transport properties of surface states in the 3D topological insulators based on bismuth have been observed in a number of experiments. However, there is still no direct evidence for the Dirac dispersion predicted for these states. We have measured the Shubnikov-de Haas (SdH) oscillations in Bi$_2$Te$_2$Se in intense dc fields. At B$>$40 T, we can reach the n=1 surface Landau Level. In the index plot (of 1/B versus n), the relatively large oscillation amplitudes in our crystals (as large as $17\%$ of the total conductance) enables us to resolve, in the high-field limit, the $\frac{1}{2}$-shift predicted from the Berry phase in the Dirac spectrum. In addition, the linearity of the index plot shows that the surface Lande g-factor is quite small (less than 5) in Bi$_2$Te$_2$Se, in contrast with recent inferences based on low-B SdH experiments. \\ Supported by NSF DMR 0819860   

Thin films of topological insulators in a parallel or tilted magnetic field

In thin films of topological insulators, the surface states on the opposite edges are coupled by a tunneling coupling $t$. We discuss the energy spectrum and the transport properties of the system in a magnetic field. When an electron tunnels between the edges, the Lorentz force due to the in-plane magnetic field $\mathbf{B =(0,B_y,0)}$ changes the in-plane electron momentum by $\Delta p_x\propto B_y$. As a result, the Fermi circles on the opposite edges shift by $\Delta p_x$ in the momentum space. We propose that this effect can be detected by measuring the tunneling conductance $\sigma_{zz}(B_y)$ between the edges of the system. We show that $\sigma_{zz}(B_y)$ has special signatures due to the helical spin configuration of the surface Dirac cones. In case of a tilted field $\mathbf{B=(0,B_y,B_z)}$, the perpendicular component $B_z$ quantizes the in-plane motion to the Landau levels, while the in-plane component $B_y$ spatially shifts the wave functions on the different edges. As the overlap between the wave functions changes, the tunneling amplitude $t$ is renormalized and acquires dependence on both $B_y$ and $B_z$. This effect can be observed as the dependence of the interlayer conductance and in-plane conductivity on the tilt angle $\theta$ of the magnetic field tan$\theta=B_y/B_z$.   

Clear Revelation of Topological Surface States in Bi$_{2}$Se$_{3}$ Thin Films by \textit{in situ} Al Passivation

We report that \textit{in situ} aluminum (Al) passivation of Bi$_{2}$Se$_{3}$ can inhibit the degradation process and clearly reveal the massless, Dirac-like topological surface states. In this work, an 8 quintuple layers Bi$_{2}$Se$_{3}$ film was passivated with 2 nm Al, immediately after the MBE growth, which prevents native oxide (BiO$_{x})$ formation, isolates the film from ambient $n$-type doping or contaminations in the subsequent fabrication process. Dual evidences from both Shubnikov-de Hass (SdH) oscillations and weak antilocalization (WAL) effect, originated from the $\pi $ Berry phase of the nontrivial surface states, were clearly revealed in the sample with \textit{in situ} Al passivation. In contrast, we show that the two dimensional carrier density was increased 39.2{\%} for the un-passivated control samples. Also, the SdH oscillations were completely absent and a large deviation from the WAL was observed   

High Field Magnetoresistance in ultra pure Bi2Se3

The longitudinal and transverse components of magnetoresistance were measured in bulk crystals of undoped, high purity Bismuth Selenide in pulsed magnetic fields of up to 60 Tesla. Data is presented from samples with a range of carrier concentrations extending into the quantum limit. Measurements were also performed at multiple angles along the plane containing the current direction to investigate the angular dependence of the linear behavior of the magnetoresistance in this material.   

Surface-dominated conduction in a 6nm-thick Bi2Se3 thin

We report a direct observation of surface dominated conduction in an intrinsic Bi$_{2}$Se$_{3}$ thin film with a thickness of 6 quintuple layers (QLs) grown on lattice-matched CdS (0001) substrates by molecular beam epitaxy (MBE). Shubnikov-de Haas (SdH) oscillations from the topological surface states suggest that the Fermi level falls inside the bulk band gap and is 53 +/-5 meV above the Dirac point, in agreement with 70 +/- 20 meV obtained from scanning tunneling spectroscopies (STS). Our results demonstrate a great potential of producing genuine topological insulator devices using Dirac Fermions of the surface states.   

Magnetoconductance in high-mobility topological insulator Bi2Se3 devices

We report the fabrication and measurement of gate-tunable high mobility exfoliated ($<$100nm thick) Bi$_{2}$Se$_{3}$ devices. We measure electronic transport of these devices in magnetic fields up to 35T, and find a complex pattern of quantum oscillations consistent with both the surface and the bulk channels. We study the dependence of the oscillations on the magnetic field angle and gate voltage and discuss models for coexistence of surface and bulk oscillations.   

Magnetic Field Signatures of Topological States in 3D Time-Reversal Invariant Insulators

While the topological behavior of Bi$_2$Se$_3$ has been identified experimentally\footnote{Y.L. Chen et al., \emph{Science} {\bf 325}, 178 (2009).} \footnote{P. Roushan et al., \emph{Nature} {\bf 460}, 7259 (2010).}, characterization by electron transport has been difficult due to high bulk transport caused by inadvertent doping of the crystal.\footnote{N. P. Butch et al., \emph{Phys. Rev. B.} {\bf 81}, 24 (2010).} We perform self-consistent quantum transport calculations to show that patterned surfaces offer a unique environment in which the system may be characterized by resultant magnetic field distributions. We compare doped and undoped Bi$_2$Se$_3$ samples with normal metals to show a qualitative difference in current flow around the patterned surface. We find that the surface to bulk conductance ratio can be inferred from the magnetic field in patterned systems due to the spatial separation of bulk and surface currents created by the corrugation, which applies even in heavily doped systems. The magnetic field is sufficiently large so as to be observed using ultracold atom microscopy.   

Quantum Hall effect from Dirac fermions of the 3D topological insulator HgTe

Three dimensional (3D) topological insulators (TI) exhibit two dimensional (2D) topologically protected conducting surface states created by strong spin-orbit coupling. Time reversal invariance (TRI) of those states manifest itself in spin-momentum locking and a dispersion relation of massless Dirac fermions. We present our results of magneto-transport of the 3D TI HgTe [1]. Our samples are 70 nm HgTe films strained on slightly Zn doped CdTe substrates. Tensile strain due to the lattice mismatch between the HgTe film and the substrate lifts the heavy hole - light hole degeneracy, which results in TI states at the Brillouin zone center. We observe evidence for a quantized Hall (QH) resistance developing at temperatures below T=50K. The observed effect is confirmed to derive from Dirac fermions of the two TI surfaces as shown through a non-zero Berry's phase by an extrapolation of the filling factors of the QH plateaus to the large magnetic field limit. We have also confirmed the 2D character of the probed states through tilted magnetic field measurements. If time allows, we will discuss our results for very high magnetic fields and dilution refrigeration temperatures experiments. Work supported by the Gordon and Betty Moore Foundation. [1] C. Br\"une et.al., PRL 106, 126803 (2011)   

Topological insulator Bi$_{2}$Te$_{3}$ nanowire field effect devices

Bismuth telluride (Bi$_{2}$Te$_{3})$ has been studied extensively as one of the best thermoelectric materials and recently shown to be a prototype topological insulator with nontrivial conducting surface states. We have grown Bi$_{2}$Te$_{3}$ nanowires by a two-step solution phase reaction and characterized their material and structural properties by XRD, TEM, XPS and EDS. We fabricate both backgated (on SiO$_{2}$/Si) and top-gated (with ALD high-k gate dielectric such as Al$_{2}$O$_{3}$ or HfO$_{2})$ field effect devices on such nanowires with diameters $\sim $50nm. Ambipolar field effect and a resistance modulation of up to 600{\%} at low temperatures have been observed. The 4-terminal resistance shows insulating behavior (increasing with decreasing temperature) from 300~K to 50K, then saturates in a plateau for temperatures below 50K, consistent with the presence of metallic surface state. Aharonov--Bohm (AB) oscillations are observed in the magneto-resistance with a magnetic field parallel to the nanowire, providing further evidence of the presence of surface state conduction Finally, a prominent weak anti-localization (WAL) feature that weakens with increasing magnetic field and/or temperature is observed in the magneto-resistance with a magnetic field perpendicular to the nanowire.   

Analysis of quantum interference in mesoscopic channels of epitaxial Bi$_2$Se$_3$

Predictions of topologically protected surface states lead to expectations of longer scattering lengths from the surface channel in candidate topological insulators such as Bi$_2$Se$_3$. In this context, we probe coherent transport in mesoscopic channels of MBE-grown Bi$_2$Se$_3$ at temperatures down to 0.5K and magnetic fields up to 6T. The magnetoresistance reveals two types of quantum corrections superimposed upon a classical background: low-field weak antilocalization and an aperiodic, reproducible fingerprint. Analysis and comparison of the quantum corrections data are used to extract important length scales and provide insights into the origin of these corrections. The channel length and temperature dependence of the magnetofingerprint is consistent with the theory of universal conductance fluctuations for diffusive systems that are two dimensional in phase coherent phenomena. Periodic oscillations in the autocorrelation of the fingerprint, persistent to high fields and high temperatures, point towards the presence of dominant scattering centers. Work supported by NSF-MRSEC and ONR.   

Synthesis and Transport Measurements of Catalyst-Free Topological Insulator Bi$_{2}$Se$_{3}$ Nanostructures

The semiconductor bismuth selenide (Bi$_{2}$Se$_{3}$) was predicted to be a topological insulator (TI) with a single Dirac cone, which was observed using Angle-Resolved-Photo-Emission-Spectroscopy in 2008. TI's are materials which exhibit electrically insulating properties in the bulk, but they have metallic surface states. The surface states are topologically protected from perturbations, defects, and impurities. Nano-sized structures might be well suited for the study of surface states because the surface effects are likely to dominate over bulk properties due to the high surface-to-volume ratio. So far, nanowires and nanoribbons of TI s have been synthesized using metal catalysts, such as gold, iron, or nickel. However, it has been found that these catalysts dope the nanostructures, which has the potential to modify their properties. We show catalyst-free growth of nanowires and nanoribbons of Bi$_{2}$Se$_{3}$ using the Vapor-Liquid-Solid method. Analysis by EDAX and TEM imaging suggest high purity samples. We have fabricated devices from these nanostructures and present electron transport measurements.   

Topological Insulator Nanoribbon Synthesis by Metal-Organic Chemical Vapor Deposition

We report a method for metal-organic chemical vapor deposition (MOCVD) synthesis of Bi$_2$Se$_3$ topological insulator nanoribbons. We use gold nanoparticles to catalyze nanoribbon growth on silicon substrates. Trimethyl Bismuth and Diethyl Selenium are used as the metal-organic precursors. The growth parameters can be varied to control the morphology from narrow nanoribbons to wide platelets. Resulting nanostructures are characterized by electron diffraction, energy dispersive X-ray spectroscopy, and low-temperature transport measurements. We also investigate the synthesis of ternary compounds using this growth method.   

Electrical, Thermal, and Thermoelectric Characterizations of Vapor Solid Bi$_{2}$Te$_{3}$ Nanoplates

Single-crystal nanoplates of Bi$_{2}$Te$_{3}$ synthesized by the vapor solid method are characterized by electrical, thermal, and thermoelectrical measurements. The Bi$_{2}$Te$_{3}$ domains investigated are less than 12 nm thick and are suspended to remove substrate doping effects. A room temperature thermal conductivity of 1.5 Wm$^{-1}$K$^{-1}$ was measured, lower than the 1.8--3.3 Wm$^{-1}$K$^{-1}$ range reported for bulk crystals with different carrier types and concentrations. The room temperature electrical conductivity was measured at 1.5$\times $10$^{5}$ Sm$^{-1}$. Applying the Wiedemann-Franz Law, the electron contribution to the total thermal conductivity is nearly 40 {\%} at room temperature. The electrical conductivity is similar to that reported for bulk single crystals at an electron concentration of 3.5$\times $10$^{19}$ cm$^{-3}$.~ However, the room-temperature Seebeck coefficient of -66 $\mu $VK$^{-1}$ indicates $n$-type doping and is lower than that reported for $n$-type Bi$_{2}$Te$_{3}$ single crystals at an electron concentration as high as 14.6$\times $10$^{19}$ cm$^{-3}$. Consequently, the figure of merit is only 0.11 at room temperature, a factor of 7.9 lower than the highest ZT reported for $n$-type single crystals at the optimized doping level.   

Hybrid Bismuth Selenide Nanostructures Synthesized by Chemical Vapor Deposition

The recent demonstration of theoretically predicted topologically ordered states in real bismuth based chemical compounds such as bismuth antimony, bismuth selenide, bismuth telluride etc opened the field of topological insulators (materials exhibiting insulator properties in bulk, but metallic behavior on the surface) for a plethora of possible applications. Topological insulator nanostructures in particular are of great interest due to their large surface to volume ratio. We will report on the CVD synthesis of various hybrid 1D and 2D nanostructures of the bismuth-selenium complex, and their morphological and structural properties (investigated by SEM and TEM imaging coupled with EDAX and XRD spectroscopy). Optical and transport properties will be also presented and related to possible spintronics and no dissipation electronics applications.   

Electronic transport of Sb-doped Bi$_{2}$Se$_{3}$ topological insulator nanoribbons

Vapor-liquid-solid (VLS) grown nanoribbons, having large surface / volume ratio and high crystal quality, provide a unique opportunity to study topological insulator materials by electronic transport. However, clear observation of the surface states is often hindered due to materials' imperfections. Bulk impurities and environmental doping effects are known to contribute to dominant background transport signal, so that appropriate doping and surface protection are necessary to reduce the excess carriers. We report that Antimony (Sb), known to be an effective compensational dopant for bulk crystals, can be incorporated into Bi$_{2}$Se$_{3}$ nanoribbons and it reduces the bulk electron contribution significantly. With a Zinc Oxide protective layer, the carrier density of thin ribbons reaches below 10$^{12}$cm$^{-2}$. This talk will include magnetotransport studies and temperature dependant transport of nanoribbons as well.   

Session W32: Focus Session: Dielectric, Ferroelectric, and Piezoelectric Oxides - Bandgaps, Surfaces, Oxides on Semiconductors

Band Gap and Edge Engineering of SrTiO$_{3}$ and Related Compounds via Ferroic Distortion and Anisotropic Strain

Due to its electronic band edge energies and its stability in water, the perovskite strontium titanate (SrTiO$_{3}$) is a promising water splitting photocatalyst. However, its ability to use solar photons to split water into hydrogen and oxygen would be more efficient if its wide optical band gap (3.2 eV) better matched the solar spectrum. Therefore, there is interest in modifying the crystal structure of SrTiO$_{3}$ (e.g., via chemical doping or epitaxial strain) to tune its electronic and optical properties. We use density functional theory (DFT) and many-body perturbation theory within the GW approximation to calculate the effects of structural and chemical modifications of SrTiO$_{3}$ on its band gap and edges. Much of our work (Berger, Fennie, and Neaton, Phys. Rev. Lett. 107, 146804 [2011]) focuses on the effects of epitaxial strain and the associated ferroic distortions. Anisotropic strains are shown to reduce the SrTiO$_{3}$ gap by breaking degeneracies at the band edges. Ferroic distortions are shown to widen the gap by allowing new band edge orbital mixings. To reduce the SrTiO$_{3}$ gap, one must lower the symmetry from cubic while suppressing ferroic distortions. Our calculations indicate that for engineered orientation of the growth direction along [111], the SrTiO$_{3}$ gap can be controllably and considerably reduced at room temperature. Chemical doping and substitution can, in combination with strain and distortion, further tune the band gap and edges. Our results and their favorable qualitative agreement with experiment suggest achievable paths toward engineering efficient solar water splitting catalysts, and more generally provide fundamental insight into the relationships between crystal and electronic structure in the property-rich perovskite family.   

Epitaxial strain tuning of polarization and band gap in perovksite SnTiO$_3$

Lead toxicity has motivated theoretical studies of a tin-based perovskite ferroelectric material. Density-functional calculations predict a polar perovksite ground state for SnTiO$_3$. Simulated epitaxial strain up to $\pm$2\% tunes both the magnitude of the polar distortion, its direction, and the electronic band gap --- compressive bi-axial strain creates the largest polar distortions, which occur entirely in the growth direction, while tensile strain reorients the polar displacements, enlarging the band gap. Projected densities of states indicate that the broken four-fold symmetry of the non-growth-oriented distortion allows Ti $d_{xy}$ bands to mix with O $p_x$ bands, further separating the valence band maximum and conduction band minimum. Comparing Sn and Pb in the perovskite titanate phases shows similar trends and suggests that SnTiO$_3$ ferroelectrics may be viable thin-film alternatives to Pb-based oxides.   

Ferroelectric Properties of TiO$_2$ Rutile Bulk and Surfaces Investigated by Ab Initio Calculations

TiO$_2$ rutile is an incipient ferroelectric material [Lee et al., Phys. Rev. B \textbf{50}, 13379 (1994)] and theoretical studies have shown that a ferroelectric transition can be enforced [Montanari et al., J. Phys. Condens. Matter \textbf{16}, 273 (2004)]. The experimental realization of ferroelectric TiO$_2$ (110) surfaces would have great technical impact and already the tuning of the electric permittivity would be of interest, e.g., for optical coating. In order to get an insight into the ferroelectric trends, we have studied the atomic and electronic structure, as well as phonon spectra, dipolar interactions and the polarization, using density functional theory (VASP and PWscf). Accordingly, ferroelectric states polarized along (110) and (001) can be stabilized within bulk. We demonstrate the different strain dependencies of the corresponding polar modes, which opens up the possibility of strain engineering the polarization direction, and the resulting dielectric response [Gr\"unebohm et al., Phys. Rev. B \textbf{84}, 132105 (2011) and Gr\"unebohm et al. arxiv:1111.2575]. Although the dipolar interaction and the short range repulsion are both modified at the (110) surface, stable local dipoles are obtained within the surface planes, which increase with increasing strain.   

XPS measurements of interface dipole switching at the a-Al2O3/Si interface

In this work, we describe work on polarization switching at a high-k oxide-silicon interface. The procedure involves inserting a monolayer (ML) of ZrO2 in-between an amorphous-Al2O3 and Si. Theoretical calculations using density functional theory (DFT) predict that the ZrO2 should display two stable configurations of the polarization. Deposition of the ZrO2 in UHV is used to avoid SiO2 formation. The device is transferred in vacuum and the interface chemistry analyzed using x-ray photoelectron spectroscopy (XPS) to determine the oxidation state of the Si. When the ZrO2 is in direct contact with the Si, chemical shifts as large as 0.58 [eV] are observed, implying a polar interface. In addition, XPS measurements on devices under applied voltage, along with electron transport measurements, show a switching of the interface dipole of 0.25 [eV].~ These voltage dependent XPS results are consistent with the magnitude and direction of hysteresis loops observed in Capacitance-Voltage measurements. Finally, the microscopic structure has been investigated using extended x-ray absorption fine structure (EXAFS) at the Zr K-edge. The results are compared to DFT-calculated atomic positions.   

Macroscopic electrostatics at the nanoscale: From ferroelectric capacitors to confined electron gases

Complex oxides are characterized by a multitude of coupled electronic and lattice degrees of freedom, and therefore constitute an unusually rich playground for experimentalists and theoreticians alike. These microscopic variables manifest themselves in macroscopically measurable quantities such as polarization, magnetization and strain, whose mutual coupling is sought after for applications in multifunctional electronic devices. To understand the interplay of these many factors, density functional theory (DFT) has proven an invaluable tool. However, the treatment of the macroscopic electrical variables (electric fields and polarization), which are a crucial ingredient in describing the experimentally observed response properties, has traditionally been difficult within first-principles calculations. In this talk I will first review a number of recent methodological developments that removed this limitation, thus extending the scopes of first-principles theory to the simulation of realistic devices within arbitrary electrical boundary conditions. Next, I will discuss the evolution of the band offset at a metal/ferroelectric interface as a function of polarization, and its implications for the electrical properties of nanocapacitors. Finally, I will show that, depending on the polarization of the film, a problematic regime might occur where the metallic carriers populate the energy bands of the insulator, making it metallic. As the most common approximations of density functional theory are affected by a systematic underestimation of the fundamental band gap of insulators, this scenario is likely to be an artifact of the simulation. I will discuss a number of criteria to systematically identify this situation in simulations, and effective modeling strategies to describe this peculiar charge compensation mechanism.   

Band Alignment of Plasma-Enhanced ALD High-k Dielectrics on Gallium Nitride

GaN-based transistors have shown immense promise because of their high saturation velocity and breakdown field, but their performance is limited by the high gate leakage. This limitation is mitigated with the use of metal/high-$k$ oxide/III-N structures. This experiment investigates three promising high-$k$ dielectrics deposited by plasma enhanced ALD: Al$_{2}$O$_{3}$, HfO$_{2}$, and La$_{2}$O$_{3}$. The band gaps of these materials are 6.5eV, 5.8eV, and 4.3eV, while the dielectric constants are 9, 20, and 27, respectively. The large band gap associated with Al$_{2}$O$_{3}$ reduces the leakage current; however, the lower dielectric constant increases the equivalent oxide thickness. The band alignment of the high-$k$ oxide/GaN interface plays a critical role in determining the confinement properties of semiconductor carriers and ultimately device performance. In situ photoemission gave valence band offsets for Al$_{2}$O$_{3}$, HfO$_{2}$, and La$_{2}$O$_{3}$ with GaN as 1.8eV, 1.3eV, and 0.9eV. The results are described by the charge neutrality level and interface dipole models. We also investigated the use of Al$_{2}$O$_{3}$ as an interfacial passivation layer between HfO$_{2}$ and GaN. This research is supported by the Office of Naval Research.   

High-k dielectrics on n-Al$_{0.25}$Ga$_{0.75}$N via atomic layer deposition

AlGaN/GaN and AlInN/GaN high-electron-mobility transistors (HEMTs) are promising devices for high-temperature and high-power electronics applications. A key issue with these devices is the high gate leakage current, particularly for enhancement-mode HEMTs. There has been an increased interest in developing high quality gate insulators to reduce gate leakage current. Al$_{2}$O$_{3 }$and HfO$_{2}$ layers (21nm thick)$_{ }$were deposited via atomic layer deposition on n-Al$_{0.25}$Ga$_{0.75}$N pretreated with one of two different surface preparations, H$_{2}$O$_{2}$:H$_{2}$SO$_{4}$ (1:5) (piranha) or HF:H$_{2}$O (1:3). Dielectrics were characterized using spectroscopic ellipsometry, X-ray photoelectron spectroscopy, atomic force microscopy (AFM), and capacitance-voltage (C-V) measurements. AFM shows that Al$_{2}$O$_{3 }$and HfO$_{2}$ layers are continuous and uniform in thickness on both HF and piranha pretreated surfaces. However, C-V measurement shows smaller (15{\%}) hysteresis for HF pretreated samples. The estimated dielectric constants ($\varepsilon )$ are 9 and 18 for Al$_{2}$O$_{3}$ and HfO$_{2}$ on HF pretreated surfaces, respectively, in general agreement with theoretical values of 9 and 25. Al$_{2}$O$_{3}$ layers on Al$_{0.25}$Ga$_{0.75}$N exhibited a lower leakage (7x10$^{-8}$ A/cm$^{2}$ at 5 V) current and higher forward breakdown voltage of 7.5 MV/cm compared to that of HfO$_{2}$ layer. The higher breakdown voltage and lower leakage current for Al$_{2}$O$_{3}$ is due to larger conduction band offset with Al$_{0.25}$Ga$_{0.75}$N.   

The growth of c-axis oriented BaTiO$_{3}$ films on Si and Ge for a non-volatile field-effect transistor

The reorientable polarization of a ferroelectric material can be utilized in a variety of applications, including the development of novel memory devices. Of particular interest is the use of a ferroelectric's spontaneous polarization to maintain ``on'' and ``off'' conductivity states in a field effect transistor. BaTiO$_{3}$ has been proposed as a Pb-free ferroelectric for such an application. Direct coupling of the ferroelectric polarization with the channel of a field effect transistor requires c-axis oriented BaTiO$_{3}$ films to be grown on Si or Ge. However, due to the small thermal expansion coefficients of Si and Ge, BaTiO$_{3}$ films tend to be a-axis oriented, having the polarization lying in the plane of the film. In order to achieve c-axis oriented BaTiO$_{3}$ films, we have developed a graded buffer layer that imparts in-plane compressive strain to overcome the incompatibility in thermal expansion. Ferroelectric, c-axis oriented, BaTiO$_{3}$ films with thicknesses exceeding 120 nm have been achieved. We will discuss the growth and characterization of these films in the development of a non-volatile, ferroelectric transistor.   

First principles analysis of the capacitance density of high-k nanocapacitors utilizing the orbital separation approach

In order to realize further scaling of electronic devices, capacitors and transistors with higher capacitance density are necessary. To this end, higher-k films of nanometer thickness are being considered for next-generation devices. However, high-k/metal interfaces often suffer from degraded dielectric properties due to, e.g., contamination, defects, and the intrinsic dead layer effect [1]. In this work, we developed a new first principles approach based on the Kohn-Sham formalism that we call orbital separation approach [2] to calculate the capacitance of nano-sized capacitors and examine the limit in capacitance density. In this method, the Kohn-Sham orbitals around the Fermi level of a metal/insulator/metal slab are separated into left and right electrode. The separated orbitals are occupied according to different Fermi levels to simulate the effect of bias voltage. We applied this approach to Au/MgO/Au and SrRuO$_3$/SrTiO$_3$/SrRuO$_3$ systems to examine the effect of the interface on the dielectric response. We confirmed that this method gives reasonable results. The impact of the intrinsic dead layer and that of defects on the capacitance are also examined. \\[4pt] [1] M. Stengel and N. Spaldin, Nature 443, 679 (2006).\\[0pt] [2] S. Kasamatsu et al., Phys. Rev. B 84, 085120 (2011).   

Properties of Ultrathin Al$_{2}$O$_{3}$-TiO$_{2}$ Nanolaminate Films for Gate Dielectric Applications Deposited by Plasma-Assisted Atomic Layer Deposition

High permittivity dielectrics such as Al$_{2}$O$_{3}$, HfO$_{2}$, Ta$_{2}$O$_{5}$, TiO$_{2}$, etc., are an essential component of aggressively-scaled III-V and graphene field effect transistors (FETs) where insulators are necessary to reduce gate leakage current while maintaining high gate capacitance and charge control of the channel. Atomic layer deposition (ALD) has the capability to deposit hybrid films, or nanolaminates, of two or more dielectrics that have unique properties. Thin [Al$_{2}$O$_{3}$+TiO$_{2}$] nanolaminates with varying TiO$_{2}$ and Al$_{2}$O$_{3}$ content were deposited on $n$-Si substrates at $\sim $225-300\r{ }C using ALD. A nanolaminate is composed of bilayers, defined as the sum of (x)Al$_{2}$O$_{3}$ and (y)TiO$_{2}$, where x, and y indicate the number of times a component monolayer is repeated. While the overall thickness of the dielectric was held at $\sim $ 17-20 nm, the relative ratio of Al$_{2}$O$_{3}$ to TiO$_{2}$ in the bilayer stack was varied to evaluate changes in the material properties and electrical performance of the oxides. C-V and I-V measurements on various [(x)TiO$_{2}$+(y)Al$_{2}$O$_{3}$] MOS capacitors were taken. The high-TiO$_{2}$-content films show limited evidence of oxide charge trapping and relatively large dielectric constants ($\kappa \sim $15), whereas the high-Al$_{2}$O$_{3}$-content films offer a larger optical bandgap and improved suppression of leakage current. We will discuss the properties of very thin nanolaminates and their possible use as gate oxides. Morphological, electrical, and XPS composition assessments will be presented.   

Ferroelectric Surface Chemistry: FIrst-principle study of NOx Decomposition

NOx molecules are critical and regulated air pollutants produced during automotive combustion. As part of a long-term effort to design viable catalysts for NOx decomposition that operate at higher temperatures and thus would allow for greater fuel efficiency, we are studying NOx chemistry on ferroelectric perovskite surfaces. Changing the direction of the ferroelectric polarization can modify surface properties and thus can lead to switchable surface chemistry. We will discuss our results for NO and NO2 on the polar (001) surfaces of PbTiO3 as function of ferroelectric polarization, surface stoichiometry, and various molecular or dissociated binding modes.   

Session W33: Physics of Hydrogen Production, Storage, Delivery

Boron Doping Carbon Structures Using Decaborane? A Theoretical Study

Boron-doped carbon materials have been shown to improve hydrogen storage. Boron-doped activated carbons have been produced using a novel process involving the pyrolysis of a boron containing compound and subsequent high-temperature annealing. A model for the boron doping process based on a Langmuir isotherm is presented. A theoretical study of the interaction of the boron containing compound with the undoped carbon precursor will be presented. Ab-initio calculations of the potential energy surface and the Langmuir isotherm parameters derived from them are also presented. The theoretical study outlines the unique capabilities and limits of this doping procedure.   

Mass Transport in the Dehydrogenation Reaction of B$_{20}$H$_{16}$

The compound B$_{20}$H$_{16}$ has been predicted to decompose directly into 20B and 8H$_{2}$ with favorable hydrogen release (6.9 wt. \%) and equilibrium temperature ($T$ = 20 $^{\circ}$C at a pressure of 1 bar H$_2$) [W.Q. Sun, et. al. Phys. Rev. B. \textbf{83}, 064112, 2011]. The segregation of B and H during this reaction is investigated using density functional theory assuming that this process is mediated by the diffusion of native point defects in solid B$_{20}$H$_{16}$. Using the calculated formation energies under relevant chemical conditions, those defects that form in the largest concentrations, and thus those that facilitate mass segregation, are identified. These results are used to gain insight into the possible kinetic limitations of this hydrogen storage reaction.   

Theoretical prediction of intermediates in the decomposition of Mg(BH$_4$)$_2$

We have studied the decomposition pathway of Mg-borohydride using density-functional theory (DFT) calculations of the free energy (including vibrational contributions) in conjunction with a Monte Carlo-based crystal structure prediction method, the prototype electrostatic ground state (PEGS) method. We find that a recently proposed Mg(B$_3$H$_8$)$_2$ intermediate [Chong, etc, {\it Chem. Commun.} {\bf 47}, 1330, (2011)] is energetically highly unfavorable with respect to decomposition into MgB$_{12}$H$_{12}$. We systematically search for low-energy structures of Mg-triboranes [Mg(B$_3$H$_8$)$_2$, MgB$_3$H$_7$, and Mg$_3$(B$_3$H$_6$)$_2$], closo-borane MgB$_{\rm n}$H$_{\rm n}$ (n=6,7,8,9,10,11), and Mg(B$_{11}$H$_{14}$)$_2$ compounds using PEGS+DFT simulations. We find that only the reaction enthalpy to Mg$_3$(B$_3$H$_6$)$_2$ is close to the stable MgB$_{12}$H$_{12}$ pathway, and falls within the thermodynamic conditions for reversibility [e.g., $\Delta$ H = 20$\sim$50 kJ/(mol H$_2$)]. Careful control over experimental conditions might allow for Mg$_3$(B$_3$H$_6$)$_2$ as a possible intermediate in the decomposition of Mg(BH$_4$)$_2$, and might allow Mg$_3$(B$_3$H$_6$)$_2$ to be rehydrided back to Mg(BH$_4$)$_2$ under modest H$_2$($T,p$) conditions.   

Industrial Scale Measurements of Hydrogen Uptake and Delivery in KOH Activated Carbons

The Alliance for Collaborative Research in Alternative Fuel Technologies (ALL-CRAFT) has been producing high surface area activated carbons. Here we will investigate the hydrogen adsorption characteristics of these activated carbons using a custom built 10 liter hydrogen adsorption apparatus filled with 4 kg of activated carbon. We will discuss problems and solutions specific to filling and delivering hydrogen from industrial scale systems. Results show that activated carbons can produce a significant but surmountable, amount of impedance to hydrogen flow. The 10 liter hydrogen storage system measures adsorption at temperatures between -78 Celsius and 100 Celsius and at pressures between zero and 100 bar. The 10 liter hydrogen adsorption uptakes are compared against results obtained with the Hiden Isochema HTP1 volumetric gas analyzer.   

Reversible Storage of Hydrogen and Natural Gas in Nanospace-Engineered Activated Carbons

An overview is given of the development of advanced nanoporous carbons as storage materials for natural gas (methane) and molecular hydrogen in on-board fuel tanks for next-generation clean automobiles. High specific surface areas, porosities, and sub-nm/supra-nm pore volumes are quantitatively selected by controlling the degree of carbon consumption and metallic potassium intercalation into the carbon lattice during the activation process. Tunable bimodal pore-size distributions of sub-nm and supra-nm pores are established by subcritical nitrogen adsorption. Optimal pore structures for gravimetric and volumetric gas storage, respectively, are presented. Methane and hydrogen adsorption isotherms up to 250 bar on monolithic and powdered activated carbons are reported and validated, using several gravimetric and volumetric instruments. Current best gravimetric and volumetric storage capacities are: 256 g CH4/kg carbon and 132 g CH4/liter carbon at 293 K and 35 bar; 26, 44, and 107 g H2/kg carbon at 303, 194, and 77 K respectively and 100 bar. Adsorbed film density, specific surface area, and binding energy are analyzed separately using the Clausius-Clapeyron equation, Langmuir model, and lattice gas models.   

The Stationary States of Adsorbed Hydrogen

In order to investigate the impact of quantum effects on hydrogen adsorption, it is important to understand the stationary states occupied by adsorbed hydrogen molecules. We present experimental inelastic neutron scattering spectra which provide evidence for significant mixing of degrees of freedom which are normally decoupled in free space. Results suggest that simultaneous treatment of translational and rotational degrees of freedom and consideration of the potential corrugation are necessary for improved theoretical understanding of the problem. Numerical calculations of the full five dimensional single body potential are used to understand the origins of the experimentally observed stationary states. In addition, we briefly discuss how our results may be used to understand the shape of hydrogen adsorption isotherms.   

Measured Enthalpies of Adsorption of Boron-Doped Activated Carbons

There is significant interest in the properties of boron-doped activated carbons for their potential to improve hydrogen storage.\footnote{Multiply Surface-Functionalized Nanoporous Carbon for Vehicular Hydrogen Storage, P. Pfeifer et al. DOE Hydrogen Program 2011 Annual Progress Report, IV.C.3, 444-449 (2011).} Boron-doped activated carbons have been produced using a process involving the pyrolysis of decaborane (B$_{10}$H$_{14})$ and subsequent high-temperature annealing. In this talk, we will present a systematic study of the effect of different boron doping processes on the samples' structure, hydrogen sorption, and surface chemistry. Initial room temperature experiments show a 20{\%} increase in the hydrogen excess adsorption per surface area compared to the undoped material. Experimental enthalpies of adsorption will be presented for comparison to theoretical predictions for boron-doped carbon materials. Additionally, results from a modified version of the doping process will be presented.   

Performance of Carbon Hydrogen Storage Materials as a Function of Post-Production Thermal Treatment

High-surface-area activated carbons for hydrogen storage were investigated as a function of post-synthesis surface treatment. Thermal treatment of the initial carbon in high vacuum at temperatures 200-1000 \r{ }C leads to materials with significantly different surface chemistries and hydrogen storage capacities. Results from nitrogen pore-structure analyses, FT-IR spectroscopy before and after the treatment, and thermogravimetric analysis and mass spectroscopy of volatile reaction products during treatment, are reported. For treatment at 600 \r{ }C, excess hydrogen adsorption at 80 K and 303 K is found to be 20-30{\%} higher than for the untreated sample. At temperatures below 450 \r{ }C, volatiles are mostly water and air; volatiles above 450 \r{ }C are mostly carbon dioxide and carbon monoxide. The results are interpreted as that high-temperature treatment produces materials with a large fraction of high-binding-energy sites.   

Size-scaling behavior of hydriding phase transformations in nanocrystals

By partnering data obtained from a novel in-situ luminescence-based probe with a statistical mechanical model we derive size-scaling laws for hydriding phase transformations relevant for hydrogen storage. We conclude that the observed experimental trends are consistent with thermally-driven nucleation, and derive scaling relations that reveal the fundamental size-dependence of nucleation barriers in nanocrystals for first-order phase transformations: near the critical point, the barrier to nucleation is controlled directly by the size of the nanocrystal. Consequently, phase transformation can occur in a nanocrystal even at the critical point, in stark contrast to the classical bulk scenario. Our results provide a detailed framework for understanding how nanoscale interfaces impact broad classes of thermally-driven solid-state phase transformations of relevance for hydrogen storage, catalysis, batteries, and fuel cells.   

Hydrogen affinity and structural stability of Mg-films on substrates

Using first-principles density functional theory we investigated the binding mechanism of hydrogen to thin Mg films and alloyed films. In ultrathin Mg films the stability of hydrides is much lower than in the corresponding bulk systems and it can be modified by metal alloying. We calculated the chemical potential of hydrogen in Mg films for different dopant species and film thicknesses while including all vibrational degrees of freedom. By comparing the chemical potential with that of free hydrogen gas at finite temperature and pressure, we construct a hydrogenation phase diagram and identify the conditions for hydrogen absorption/desorption. Experimentally those films were synthesized on the Si substrate using MBE technique, where we observed the formation of both Mg and Mg silcides. We studied the competing mechanism of Mg/Mg$_{2}$Si formation and their structural stabilities on the substrate.   

Hydrogen storage through spillover at sun-2nm Pt nanoparticle - support interfaces

Hydrogen generation and storage are essential components in the increasingly important field of energy storage. Electrochemical generation of Hydrogen atoms at the surface of Pt like metals at select potentials is a widely studied phenomenon. However, moving these adsorbed Hydrogen atoms to high surface area support systems (primarily Carbon) for storage is an issue. We show spillover of these adsorbed Hydrogen atoms to the conducting transition-metal oxide support for sub-2 nm Pt nanoparticles sputtered on fluorine doped tin oxide (FTO) using cyclic voltammetry. The sub-2 nm Pt nanoparticles are deposited on oxide and carbon support systems using a unique tilted target sputtering (TTS) system developed by our lab. The resultant Pt nanoparticles are highly homogeneous, have high number density and are crystalline in nature. We propose integrating this sub-2 nm Pt nanoparticle-FTO system with different carbon structures to see if the spilled over hydrogen can be stored reversibly on adjacent carbon support systems and study the involved hydrogen spillover and storage mechanisms.   

Activated carbon monoliths for methane storage

The use of adsorbent storage media for natural gas (methane) vehicles allows for the use of non-cylindrical tanks due to the decreased pressure at which the natural gas is stored. The use of carbon powder as a storage material allows for a high mass of methane stored for mass of sample, but at the cost of the tank volume. Densified carbon monoliths, however, allow for the mass of methane for volume of tank to be optimized. In this work, different activated carbon monoliths have been produced using a polymeric binder, with various synthesis parameters. The methane storage was studied using a home-built, dosing-type instrument. A monolith with optimal parameters has been fabricated. The gravimetric excess adsorption for the optimized monolith was found to be 161 g methane for kg carbon.   

A DFT Study of Atomic Hydrogen and Oxygen Chemisorption on the $\gamma $-U Surface

Generalized gradient approximation to density functional theory has been used to compute O and H atomic adsorption properties on the (100) surface of $\gamma $-U. The computational method used is the all-electron full-potential linearized augmented plane wave plus local orbitals basis method as implemented in the WIEN2k code. The adatom was allowed to approach the five-layer slab surface at the top, center and bridge sites. The bridge site was found to be most stable with chemisorption energies of 8.43 and 3.76 eV for O and H, respectively, spin-orbit coupling (SOC) included. The optimized distances to the surface for the O atom was 1.98 at the top, 0.75 at center and 1.32 {\AA} and at the bridge sites. For H, these distances were found to be 2.07, 0.57, and 1.40 {\AA} at the corresponding sites, respectively. Inclusion of SOC has significant effects on the energies, changing the chemisorption energy by as much as 0.63 eV for O and 0.43 eV for H. Possible changes in work function, magnetic moment, and charge density distributions have been investigated and will be presented. We will also present results on interstitial sites, by incorporation of O and H atoms inside the slab. Different properties were observed for the O adatom as compared to the H atom.   

Effect of Nitrogen Doping on the Electronic and Optical Properties of TaON

TaON is considered as a potential candidate as a visible-light responsive photocatalyst. We report the results of ab initio studies of electronic structure of TaON in monoclinic and hypothetical cubic phases using VASP code. Specifically, we show that the position of conduction and valence band can be modified by varying the nitrogen (N) concentration in TaO1+xN1-x. The bandgap decreases monotonically with the increase of N concentration from near 2.7eV to just over 1.1eV (i.e. by 230{\%}) when N concentration is reduced from x=0.5 to 1.5. The bandgap reduction is mostly associated with the change in the position of the valence band, while the conduction band is not sensitive to nitrogen content. We calculated the optical absorption spectra and discuss the effect of nitrogen doping on the photocatalytic activity of oxinitrides.   

Determination of crystal structure and the study of electronic properties of AgBiW$_{2}$O$_{8}$ by density functional theory

AgBiW$_{2}$O$_{8}$ shows promising features in solar energy to hydrogen conversion through photoelectrochemical (PEC) approach because of moderate band gap, stability in the liquid solution, suitable band edge positions and low production cost. However, there is not much study available to determine its crystal structure. Using mineral database of relevant oxides and density functional theory (DFT) total energy calculation, we have determined the crystal structures of AgBiW$_{2}$O$_{8}$ to be a wolframite structure, which contradicts a previous report. Our theoretical electronic structure and optical properties calculation agrees well with the recent experimental result.   

Session W34: Focus Session: Nano V: Nanoscale Materials and Properties II

In Quest of a Systematic Framework for Unifying and Defining Nanoscience

A \textit{central paradigm driven, Mendeleev-like nano-periodic system} has been cited as a critical missing link in the transformation of nanotechnology from an empirical to a highly predictive science. A systematic framework is proposed based on the same first principles underpinning ``central paradigms'' for chemistry/physics.\footnote{D.A. Tomalia, \textit{J. Nanopart. Res.} (2009), 11, 1251.} As such, a \textbf{Nanomaterials Classification Roadmap} considers \textit{structure controlled} nanoparticles defined by \textbf{Critical Nanoscale Design Parameters (CNDPs); }namely, \textbf{size, shape, surface chemistry, flexibility, architecture and elemental composition}. Classified as either \textbf{hard (H) (}inorganic) or \textbf{soft (S) (}organic)\textbf{ nano-element categories}$, $these nanoparticles (e.g., nano-clusters) generally manifest pervasive \textbf{atom mimicry }features.\footnote{S.N. Khanna, A.W. Castleman, et al., \textit{PNAS }(2006), 103 (49), 18405.} Many literature examples demonstrate chemical bonding/assembly of these nano-element categories to produce extensive libraries of \textbf{hard-hard [H}$_{n}$\textbf{:H}$_{n}$\textbf{], soft-soft [S}$_{n}$\textbf{-S}$_{n}$\textbf{]or hard-soft [H}$_{n}$\textbf{-S}$_{n}$\textbf{]} nano-element combinations, referred to as \textbf{nano-compounds}. Due to their quantized CNDP features, these nano-element/compounds exhibit many well-defined\textbf{ nano-periodic property patterns}. These property patterns are observed in their intrinsic physico-chemical properties (i.e., melting points, reactivity/self-assembly, sterics), as well as important functional/ performance properties (i.e., magnetic, photonic, and electronic behavior). The importance of these CNDP directed property patterns was recently demonstrated by publication of \textbf{first Mendeleev-like nano-periodic tables} by Percec, et al.\footnote{V. Percec, et al., \textit{J. Am. Chem. Soc}. (2009), 131, 17500.} Similarly, Mirkin, et al.\footnote{C.A. Mirkin, et al., \textit{Science }(2011), 334, 204.} recently reported six CNDP dependent nano-periodic rules for predicting hard-soft nano-element assemblies. These two independent reports appear to fulfill/validate this proposed nano-periodic concept. This lecture will overview this unifying \textbf{nano-periodic system }suitable for tuning optimal nanostructure/application properties, as well as predicting important risk/benefit/performance boundaries in the nanoscience field.   

DFT-based Modeling of Field-Dependent Control and Response of Nanomagnetic Molecules

Regardless of whether one is interested in characterizing, utilizing or controlling molecular-scale systems [1], one requisite to their understanding, design, and improvement is the ability to realistically model their response to electromagnetic fields. Since such responses are often collective their description requires an understanding of the interplay between bonding, spin, spin-orbit, vibrations, and electromagnetic fields. Inclusion of spin and magnetism influences the behaviors significantly. I provide an overview of a density-functional-based method (NRLMOL) for determining resonant tunneling of magnetization and Berry's phase oscillations in molecular magnets (primarily Mn$_{12}$-Acetate and derivatives) [2] and spin-electric effects in frustrated spin systems [Na$_{12}$Cu$_3$(AsW$_9$O$_{33}$)$_2\cdot$3H$_2$0] [3]. The complexities related to spin- and magnetically dependent transport are compared to those of a nonmagnetic case [4]. Direct comparisons to experiments will be made. Challenges and recent progress associated with incorporating these effects into a realistic description of the frequency and amplitude dependent field driven response of many-electron/spin nanosystems will be discussed.\\[4pt] [1] MRP and SN Khanna, PRB {\bf 60} 9566 (1999).\\[0pt] [2] AV Postnikov, J. Kortus \& MRP, PSSB {\bf 243} 2533 (2006).\\[0pt] [3] MF Islam, JF Nossa, CM Canali, \& MRP, PRB {\bf 82} 15546 (2010).\\[0pt] [4] N.A. Zimbovskaya, MRP, AS Blum, BR Ratna and R. Allen, JCP {\bf 130} 094702 (2009).   

On the mechanism of enhanced photocatalytic activity of composite TiO2/carbon nanofilms

We fabricated and analyzed well-defined model samples consisting of anatase and graphitic carbon films with and without modifying the interface between them by a thin SiO2 space layer. The study was performed in the search for the origin of the enhanced photocatalytic activity of composite TiO2--carbon systems observed previously by us, but also reported in number of publications. We found that the films with a TiO2/C interface show noticeably lower photoluminescence intensity and shorter carrier life times compared to single TiO2 films with the same thickness and composition. The stronger non-radiative recombination was mainly assigned to charge carrier leakage (transfer) at the interface between TiO2 nanocrystallites and the carbon film.   

Light-sensitive gold nanoparticles designed for solar energy use

Light-sensitive organic ligands are incorporated with gold nanoparticles (AuNPs) to utilize solar energy. The physical properties of the ligand-AuNP systems are mainly modulated by the ligand/AuNP ratio. Hydrodynamic size measurement, laser Doppler electrophoresis and transmission electron microscopy are used for physical characterization. The interconnectivity of the AuNPs by dual-functional ligands also has a great impact on the physical properties. In addition, fs-THz spectroscopy is applied to evaluate electron activation of the designed AuNPs. Electron beams of different energy levels are applied to change the surface energy of AuNPs, which strongly affects the absorption energy band. This study contributes to the basic understanding on the nanoparticle technology for solar energy use.   

Factors controlling thermodynamic properties at the nanoscale: Ab initio study of Pt nanoparticles

We analyze via density-functional-theory calculations how factors such as size, shape, and hydrogen passivation influence the bond lengths, vibrational density of states (VDOS), and thermodynamic quantities of 0.8-1.7 nm diameter Pt nanoparticles (NPs), whose shape was previously characterized via extended X-ray absorption fine structure spectroscopy (EXAFS) [1]. For a given shape, unsupported NPs display increasingly broader bond-length distributions with decreasing size. Since the VDOS is remarkably non-Debye-like (even for the largest NPs), the VDOS and the thermal properties are not correlated as they are in the bulk. Generally, the fundamental vibrational frequency of a NP is associated with the shape and decreases with increasing size, as in macroscopic systems. Not surprisingly, we find that the frequency of this fundamental mode largely characterizes the thermal properties. We demonstrate that the qualitative difference between the atomic mean-square-displacement and the corresponding mean bond-projected bond-length fluctuations should be taken into account when interpreting the Debye-Waller factor of NPs measured by X-ray (or neutron) scattering or EXAFS. We find that in H-passivated Pt NPs, H desorption with increasing temperature explains the appearance of negative thermal expansion.\\[4pt] [1] B. Roldan Cuenya, et al. (2011), preprint available   

Nucleation in the Anatase-to-Rutile Transformation of Nanocrystalline Anatase

We use molecular dynamics (MD) simulations to investigate the anatase-to-rutile transformation of titania nanocrystals in vacuum. By comparing energies of various Wulff-shaped nanocrystals, we find that rutile becomes favored over anatase past a critical size that is significantly smaller than that predicted by thermodynamic models based on surface energies, indicating that edges play a profound role in the energetics of nanocrystals. We develop a local order parameter to distinguish anatase from rutile and intermediate anatase (112) twins at the resolution of a single TiO$_{2}$ unit and we apply it in direct MD simulations of spherical and Wulff-shaped anatase nanocrystals, as well as nanocrystal aggregates. To further characterize the transformation, we simulate X-ray diffraction of the nanoparticles. The anatase-to-rutile transformation originates at surfaces and interfaces, where alternating anatase (112) twin planes form. Rutile nuclei form via transformation of anatase (112) twins and they grow rapidly when they reach a critical size that can be as small as 10 TiO$_{2}$ units. Rutile nucleii tend to have planar structures bounded by (101) surfaces and the ease with which they form is dependent on the structure of the nanocrystal.   

Local Ionic Environment around Polyvalent Nucleic-Acid Functionalized Gold Nanoparticles

Polyvalent oligonucleotide-functionalized gold nanoparticles (DNA-AuNPs) are remarkably stable in a cellular environment against degradation by nucleases, a property that was recently attributed to the local high concentration of mono- and divalent ions (Ref 1). In order to evaluate this hypothesis, we investigated the composition of the ion cloud around spherical nanoparticles that are functionalized by stiff, highly charged polyelectrolyte chains by means of classical density functional theory and molecular dynamics simulations. We developed a cell model that includes ligands explicitly and both applies over the entire relevant parameter space and is in excellent quantitative agreement with simulations (Ref 2). The ion distribution around the DNA-AuNPs as a function of DNA grafting densities and bulk ionic concentrations, as well as different sizes of nanoparticles and chains, is studied. For small particles with high DNA surface densities, we find strongly enhanced local salt concentrations, a pronounced localization of divalent ions near the surface of the nanoparticle, and a large radial component of the electric field between the ligands. Therefore, we conclude that enzyme activity in general may be heavily influenced by the local environment around DNA-AuNPs.   

Mechanical properties of Ge nanowire

Nanowires possess unique properties owing to their low dimensionality and high surface-to-volume ratio. Although numerous calculations exist for the electronic properties of nanowires, the mechanical properties have not been addressed to the same extent. Here, we present real-space pseudopotential calculations for the mechanical properties of Ge nanowires. In particular, we examine three different orientations of Ge nanowires, with the axis along the [111], [110], and [100] directions. We present calculations for the elastic properties as a function of wire diameter. We find that Young's modulus is decreased as the surface-to-volume ratio increases, except for the [110] orientation, which shows the opposite trend. In addition, we will discuss the band structure under strain for each nanowire system.   

Chemisorption on Palladium and Silver Clusters

We continue our interest on the chemisorption of different atomic and molecular species on small clusters of metallic elements, by examining the interactions of H, O and F atoms with Pd$_{n}$ and Ag$_{n}$ clusters (n = 6 thru 12). The hybrid ab initio methods of quantum chemistry (particularly the DFT-B3LYP model) are used to derive optimal geometries for the clusters of interest. We compare calculated binding energies, bond-lengths, ionization potentials, electron affinities and HOMO-LUMO gaps for the clusters of the two different metals. Of particular interest are the comparisons of binding strengths at the three important types of sites: edge (E) sites, hollow sites (H) site and on-top (T) sites. Effects of crystal symmetries corresponding to the bulk structures for the two metals will also be investigated. Our theoretical results will be compared with the experimental studies where they are available. Implications for the molecular dissociation of the H$_{2}$ and O$_{2}$ species will be considered.   

Blinking in nanoscale systems: a universal theoretical framework

Fluctuations of fluorescence intensity (blinking) is observed in many different kinds of optically active nanoscale objects. These fluctuations with extremely long-term correlations manifest on timescales longer than seconds and were observed in the emission of colloidal and self-assembled quantum dots, nanorods, nanowires, and some organic dyes. We suggest the idea of a universal physical mechanism underlying the blinking phenomenon. Here we show that the features of this universal mechanism can be captured phenomenologically by the multiple recombination center model (MRC) we proposed in a recent work to explaining single colloidal QD intermittency. Within the framework of the MRC model we qualitatively explain all the important features of fluorescence intensity fluctuations for a broad spectrum of nanoscale emitters.   

Session W35: Focus Session: DFT VIII: Time-Dependent Processes II: Excitations

Watching excitons move: the time-dependent transition density matrix

Time-dependent density-functional theory allows one to calculate excitation energies and the associated transition densities in principle exactly. The transition density matrix (TDM) provides additional information on electron-hole localization and coherence of specific excitations of the many-body system. We have extended the TDM concept into the real-time domain in order to visualize the excited-state dynamics in conjugated molecules. The time-dependent TDM is defined as an implicit density functional, and can be approximately obtained from the time-dependent Kohn-Sham orbitals. The quality of this approximation is assessed in simple model systems. A computational scheme for real molecular systems is presented: the time-dependent Kohn-Sham equations are solved with the OCTOPUS code and the time-dependent Kohn-Sham TDM is calculated using a spatial partitioning scheme. The method is applied to show in real time how locally created electron-hole pairs spread out over neighboring conjugated molecular chains. The coupling mechanism, electron-hole coherence, and the possibility of charge separation are discussed.   

A minimal TDDFT model for excitons

Optical processes in insulators and semiconductors, including excitonic effects, can be accurately described with linear-response TDDFT, provided one uses suitable exchange-correlation kernels. We have developed a conceptually and computationally simple formalism for calculating exciton binding energies with TDDFT, based on a two-band approximation. This formalism is implemented in a one-dimensional Kronig-Penney model, and we discuss the requirements for excitonic binding in this model. The performance of different types of exchange-correlation kernels (long- versus short-ranged, adiabatic versus nonadiabatic) is analyzed, with a particular emphasis on the excitonic Rydberg series.   

Nonlinear ultrafast optical response in organic molecular crystals

We analyze possible nonlinear excitonic effects in the organic molecule crystals by using a combined time-dependent DFT and many-body approach. In particular, we analyze possible effects of the time-dependent (retarded)interaction between different types of excitations, Frenkel excitons, charge transfer excitons and excimers, on the electric and the optical response of the system. We pay special attention to the case of constant electric field and ultrafast pulses, including that of four-wave mixing experiments. As a specific application we examine the optical excitations of pentacene nanocrystals and compare the results with available experimental data.[1] Our results demostrate that the nonlinear effects can play an important role in the optical response of these systems. [1] A. Kabakchiev, ``Scanning Tunneling Luminescence of Pentacene Nanocrystals'', PhD Thesis (EPFL, Lausanne, 2010).   

Electronic and dielectric properties of organic photovoltaic compounds from first principles

The initial step towards the design of organic photovoltaic (OPV) devices from first principles is to predict the electronic spectra and dielectric responses of molecular and polymer OPV compounds to quantitative accuracy [{\it Phys. Rev. B} {\bf 71}, 041306 (2005)]. To date, determining the frontier levels and dielectric properties of materials within conventional density-functional theory approximations has been elusive. To address current limitations, orbital-dependent density-functional methods, namely, hybrid density-functional theory (hybrid-DFT) and self-interaction-corrected density-functional theory (SIC-DFT) approximations, represent promising alternatives. In this presentation, we provide a critical comparison of hybrid-DFT and SIC-DFT functionals in determining electronic energies for families of OPV materials. We demonstrate that SIC-DFT based upon Koopmans' condition [{\it Phys. Rev. B} {\bf 82}, 115121 (2010)] is apt at describing donor and acceptor levels within 0.1-0.4 eV and 0.2-0.6 eV relative to experiment. Furthermore, SIC-DFT dielectric responses for semiconducting polymers are predicted in close agreement with more expensive wave-function methods, thereby allowing the accurate and computationally tractable description of donor-acceptor molecular complexes.   

Discontinuities of the exchange-correlation kernel and charge-transfer excitations in time-dependent density functional theory

We identify the key property that the exchange-correlation (XC) kernel of time-dependent density functional theory (TDDFT) must have in order to describe long-range charge-transfer excitations. We show that the discontinuity of the XC potential as a function of particle number induces a frequency-dependent discontinuity of the XC kernel which diverges in the dissociation limit. This divergency compensates for the exponentially small overlap between the acceptor and donor orbitals, thereby yielding a finite correction to the Kohn-Sham eigenvalue differences. This mechanism is illustrated to first order in the Coulomb interaction.   

Density Functional Resonance Theory: The Complex Density Function, Orbital Energies and Lifetimes, and Results for Simple Systems

Density Functional Resonance Theory (DFRT) is a recently developed complex-scaled version of ground-state Density Functional Theory (DFT) for metastable systems. This work is a detailed study of the formalism itself, its consequences and its application. The meaning of the complex ``density'' function, which is used as the primary variable, is discussed along with its possible application to study the reactivity of metastable systems. In addition, orbital energies and lifetimes are defined and related to physical quantities. Finally, results for energies and lifetimes are presented for simple systems.   

Meta-GGA-based adiabatic time-dependent density-functional theory

The local-density approximation (LDA) to the ground-state density functional theory (DFT) is well known to allow for a generalization to the time-dependent case [1]. The assumption of the adiabaticity of the process greatly simplifies the theory. The further extension of the time-dependent DFT (TDDFT) to the generalized gradient approximation (GGA) is trivial. Here we address lifting the adiabatic TDDFT to the third rung of the ``Jacobs ladder'' [2] : We work out the kinetic energy density dependent (meta-GGA) TDDFT formalism. The new theory possesses remarkable properties not present in LDA and GGA: (i) It is non-local with respect to the particle density; (ii) In the case of bulk semiconductors, it supports the 1/q$^2$ singularity of the exchange-correlation kernel, where q is the wave-vector, the latter being important to reproduce the excitonic effect. We also present illustrative calculations of the optical absorption in semiconductors [3]. \\[4pt] [1] A. Zangwill and P. Soven, Phys. Rev. A, 21, 1561 (1980).\\[0pt] [2] J. Tao, J. P. Perdew, V. N. Staroverov, and G. E. Scuseria, Phys. Rev. Lett. 91, 146401 (2003).\\[0pt] [3] V. U. Nazarov and G. Vignale, Phys. Rev. Lett. 107, 216402(2011).   

Describing electronic excitations using electron-hole density functional theory (eh-DFT)

The electron-hole interaction play a crucial role in calculating optical properties of atoms, molecules, clusters and solids. In this talk, the density functional treatment of electron-hole quasiparticle interaction will be presented in the framework of electron-hole density functional theory(eh-DFT). The electron-hole correlation functional plays a central role in accuracy of any eh-DFT calculations. In the present work, the development of eh-correlation functional using the eh-reduced density matrix will be presented. Benchmark calculation using the developed functional will be compared with HF, full CI, R12-full CI, and explicitly correlated Hartree-Fock calculations. Exciton binding energies in CdSe quantum dots have been calculated and the eh-DFT results will be compared with experimental and pseudopotential+CI results. Discussion on construction of electron-hole adiabatic connection curve (ACC) using density-constrained minimization will be presented and the results from the ACC will be compared with eh-DFT calculations. Finally, similarity and difference between the GW+Bethe-Salpeter method and eh-DFT approach for treating electron-hole interaction will be presented.   

Applying state-of-the-art signal processing to time-dependent density functional theory

Real-time time-dependent density functional theory (TDDFT) is a computationally efficient method that can be used to calculate optical absorption spectra, circular dichroism spectra, and other properties including non-linear ones. These properties are often obtained via time-propagation methods, and a discrete Fourier transform is used to convert from the time domain to the frequency domain. However, a Fourier transform requires long time propagations to resolve spectra, especially if they contain closely-spaced frequencies. Instead, we apply a state-of-the-art signal processing technique known as compressed sensing to the spectral analysis of electron dynamics. Compared to a Fourier transform, compressed sensing provides higher-resolution absorption spectra with shorter propagation times. In the systems we study, the method requires about 80\% less time series data to obtain comparable frequency resolution, thus reducing total computational cost by approximately a factor of five. By combining the computational efficiency and parallelizability of real-time TDDFT for large systems with the dramatic reduction in simulation time enabled by compressed sensing, we increase the feasibility of studying electron dynamics in large biological molecules and organic photovoltaics.   

Density Functional Theory Investigation of Tunable Surface Composition of CdS Quantum Dots

It has recently been observed that surface composition of CdS QDs greatly affects photoluminescence. Band edge emission is quenched in sulfur terminated CdS QDs and recovered when QDs were cadmium terminated. However, in all cases the absorption spectra remained relatively unchanged. To understand the origin of this phenomenon, the density of states in a stoichiometric, Cd rich, and S rich dots was investigated using density functional theory (DFT). It was found the in the S rich system states within the band gap were introduced, providing a channel for non-radiative electronic relaxation. Gap states were also introduced in the Cd rich system compared to the stoichiometric system, but a significant band gap remained in even the most Cd rich systems. Finally, time dependent (TD)DFT was used to calculate the spectra of the various systems. It was found that created gap states were largely optically inactive. Therefore the new states would not participate in optical absorption or emission, but could participate in electron phonon relaxation. This study clarifies the role of surface defects in QD optical properties and provides a route for tuning those optical properties by controlling surface composition.   

First-Principles Approach to Chemical Raman Enhancement of Organic Adsorbates on Metal Surfaces

A first-principles density functional theory (DFT)-based approach is developed [1] to determine chemical contributions to surface-enhanced Raman spectroscopy (SERS) for molecular adsorbates on metal surfaces. While SERS applications are often directed at sensing trace amounts of chemical species, quantitative calculations of how Raman spectra of molecules are altered on chemisorption [1], coupled to experiments, can contribute significantly to our understanding of the electronic structure of the metal-adsorbate interface. For two adsorbates on Au -- benzene thiol and trans-1,2-two(4-pyridyl) ethylene (BPE) -- DFT calculations of the static Raman tensor demonstrate a strong mode-dependent modification of Raman spectra by Au substrates. Raman active modes with the largest enhancements result from stronger contributions from Au to their electron-vibron coupling, as quantified through a deformation potential. Based on our calculations, we introduce a straightforward analysis to extract ``chemical enhancement'' from measurements, and demonstrate how SERS spectra of BPE change as a function of the relative fraction of BPE molecules chemisorbed to the substrate.\\[4pt] [1] Zayak et al, Phys. Rev. Lett. 106, 083003 (2011)   

Electron-Hole Pairs during Adsorption Dynamics of O$_2$ on Pd(100) -- Exciting or not?

{\it Ab initio} modeling can provide important atomistic insights into the dynamics of elementary reaction steps in heterogeneous catalysis. But already an apparently simple example like the adsorption of O$_2$ on a frozen Pd(100) surface brings up several fundamental challenges: The corresponding six-dimensional potential energy surface (PES) is required, and excitations of electron-hole (eh) pairs as well as the transition from the $^3\Sigma_{\rm g}^-$ spin-triplet in the gas phase to a singlet-like state of adsorbed oxygen might require to go beyond the Born-Oppenheimer approximation. We tackle the PES by neural network interpolation, based on a coordinate transformation that correctly includes and exploits the underlying symmetry. Classical molecular dynamics thereon yields the initial sticking coefficient in good agreement with experimental data. Scrutinizing this result, we calculate spin-resolved eh pair spectra for several trajectories of different statistical relevance using a computationally appealing perturbative approach building on time-dependent DFT.\footnote{J. Meyer and K. Reuter, New J. Phys. {\bf 13}, 085010 (2011)} Although non-adiabatic energy losses do not exceed 5\% of the chemisorption energy, their importance for the spin transition is finally discussed.   

Self-consistent GW calculations with basis of dominant products

Hedin's $GW$ approximation (GWA) is a well known method to study charged excitations in electronic systems with a moderate computational cost [1]. Already one-shot GWA delivers a considerable improvement if compared with Green's functions from density-functional theory (DFT). However, the one-shot results are dependent on the used starting point. This unphysical dependence can be eliminated by iterating a $GW$ calculation to self-consistency. We implemented self-consistent GWA for molecules [2], within our original framework of dominant products basis. We use the DFT calculation by SIESTA code as starting point. The framework allowed to calculate Green's functions on a fine frequency mesh for such small molecules as benzene. We demonstrate the level of independence on starting point achievable within pseudo-potential framework, validating the implementation. Effects of the self-consistency on the interacting Green's function will be discussed along with different levels of self-consistency and mixing schemes. Finally, we compare the self-consistency with so-called quasi-particle self-consistent $GW$ [3]. \\[0pt] [1] F.Aryasetiawan, O.Gunnarsson, Rep. Prog. Phys. 61, 237 (1998).\\[0pt] [2] D.Foerster, P.Koval, D.Sanchez Portal, J. Chem. Phys. 135, 074105 (2011).\\[0pt] [3] T.Kotani, M.van Schilfgaarde, S.V.Falleev, Phys. Rev. B 76, 165106 (2007).   

Session W37: Topological Insulators: Fractionalization

Zoology of Fractional Chern Insulators

We study four different models of Chern insulators in the presence of strong electronic repulsion at partial fillings. We observe that all cases exhibit a Laughlin-like phase at filling fraction $1/3$. We provide evidence of such a strongly correlated topological phase by studying both the energy and the entanglement spectra. In order to identify the key ingredients of the emergence of Laughlin physics in these systems, we show how they are affected when tuning the band structure. We also address the question of the relevance of the Berry curvature flatness in this problem. Using three-body interactions, we show that some models can also host a topological phase reminiscent of the $\nu=1/2$ Pfaffian Moore-Read state. Additionally, we identify the structures indicating cluster correlations in the entanglement spectra.   

Time-reversal symmetric hierarchy of fractional incompressible liquids

In this talk I shall establish a hierarchical construction for a class of BF theories and present it as a candidate topological field theory to describe fractional topological insulators in 2D. The structure of the K matrix and the universal properties that it describes will be discussed, and the properties of the time-reversal symmetric edge theory will be derived via the bulk-edge correspondence.   

Fractional Chern Insulator

Chern insulators are band insulators exhibiting a nonzero Hall conductance but preserving the lattice translation symmetry. We conclusively show that a partially filled Chern insulator at $1/3$ filling exhibits a fractional quantum Hall effect and rule out charge-density wave states that have not been ruled out by previous studies. By diagonalizing the Hubbard interaction in the flat-band limit of these insulators, we show the following: the system is incompressible and has a $3$-fold degenerate groundstate whose momenta can be computed by postulating an generalized Pauli principle with no more than $1$ particle in $3$ consecutive orbitals. The groundstate density is constant, and equal to $1/3$ in momentum space. Excitations of the system are fractional statistics particles whose total counting matches that of quasiholes in the Laughlin state based on the same generalized Pauli principle. The entanglement spectrum of the state has a clear entanglement gap which seems to remain finite in the thermodynamic limit. The levels below the gap exhibit the identical counting of Laughlin $1/3$ quasiholes. The $3$ groundstates and excited states exhibit spectral flow upon flux insertion. All the properties above disappear in the trivial state of the insulator.   

Fractional Chern Insulators from the nth Root of Bandstructure

I will describe some recent theoretical results pertaining to fractional Chern insulators. These are interacting lattice models of partially filled Chern bands which have been numerically shown to realize some universal aspects of fractional quantum Hall physics. We use parton/slave-particle techniques to provide model wavefunctions for these phases. We also provide a strong coupling expansion that gives new insights into the foundations of the parton approach. I will conclude by describing some of the practical uses of our results, including suggesting candidate models to realize non-Abelian fractional Chern insulators.   

Wannier states and pseudopotential Hamiltonians for fractional Chern insulators

Fractional Chern insulators are fractional quantum Hall states realized in lattice models with full lattice translational symmetry in the absence of an external magnetic field. In fractional quantum Hall systems, pseudopotential Hamiltonians have been constructed for which the ideal ground states such as Laughlin states are exact ground states. In this work, we constructed pseudopotential Hamiltonians for the fractional Chern insulators by making use of the Wannier function representation. The physical interaction Hamiltonians can be expanded in pseudopotentials, from which one can analyze the preferred fractional quantum Hall states in the corresponding systems. The results of this analysis are compared with exact diagonalization. Our approach may be generalized to studying nonabelian fractional Chern insulator states and time-reversal invariant fractional topological insulators.   

Topological insulators and fractional quantum Hall effect on the ruby lattice

We study a tight-binding model on the two-dimensional ruby lattice. This lattice supports several types of first and second neighbor spin-dependent hopping parameters in an $s$-band model that preserves time-reversal symmetry. We discuss the phase diagram of this model for various values of the hopping parameters and filling fractions, and note an interesting competition between spin-orbit terms that individually would drive the system to a $Z_2$ topological insulating phase. We also discuss a closely related spin-polarized model with only first and second neighbor hoppings and show that extremely flat bands with finite Chern numbers result, with a ratio of the band gap to the band width approximately 70. Such flat bands are an ideal platform to realize a fractional quantum Hall effect at appropriate filling fractions. The ruby lattice can be possibly engineered in optical lattices, and may open the door to studies of transitions between quantum spin liquids, topological insulators, and integer and fractional quantum Hall states.   

Charge and spin fractionalization in strongly correlated topological insulators

The recently discovered two-dimensional topological insulators (TI) with time-reversal symmetry are closely related to integer quantum Hall states in which electron spin plays the role of charge. The appearance of protected edge states in these systems can be understood by describing the spin-orbit coupling as the source of an SU(2) (spin-dependent) magnetic flux. However, the absence of spin conservation cripples the quantum spin-Hall effect. In this talk we will explore the possibility of obtaining strongly correlated TIs with fractional quasiparticles. Such states are the SU(2) analogues of fractional quantum Hall states, but with modified topological orders due to the spin non-conservation. We will discuss two heterostructure designs featuring a ``conventional'' TI quantum well that could host a fractional TI state of Cooper pairs or excitons. These devices exploit a quantum critical point for electron localization to provide a fragile spectrum that can be dramatically reshaped by the strong spin-orbit coupling. Then, we will present a topological spinor-field theory of fractional TIs and explain how the spin-orbit coupling can produce a combined charge and spin fractionalization despite the spin non-conservation.   

Symmetry protected fractional Chern insulators and fractional topological insulators

We construct fully symmetric wavefunctions for the spin-polarized fractional Chern insulators (FCI) and time-reversal-invariant fractional topological insulators (FTI) using the parton approach. We show that the lattice symmetry gives rise to many different FCI and FTI phases even with the same filling fraction $\nu$ (and the same quantized Hall conductance $\sigma_{xy}$ in FCI case). They have different symmetry-protected topological orders, which are characterized by different projective symmetry groups. We mainly focus on FCI phases with which are realized in a partially filled band with Chern number one and filling fraction $\nu=1/m$. Examples of FCI/FTI wavefunctions on honeycomb lattice and checkerboard lattice are explicitly given. Possible non-Abelian FCI phases which may be realized in a partially filled band with Chern number two are discussed. Generic FTI wavefunctions preserving all lattice symmetries in the absence of spin conservation are also presented for filling fraction $\nu=2/m$.   

High-temperature fractional quantum Hall states

Using a suitable combination of geometric frustration, ferromagnetism, and spin-orbit coupling in a hopping model on the kagome lattice, we obtain a flat band with nonzero Chern number. Partial filling of this band could give rise to the fractional quantum Hall effect in this system which, when considering realistic parameters, would persist at room temperature. Possible material realizations that indicate new directions for exploration and synthesis are discussed.   

Fractional Quantum Hall states in strongly correlated multi-orbital systems

For topologically nontrivial and very narrow bands, Coulomb repulsion between electrons has been predicted to give rise to a spontaneous fractional quantum-Hall (FQH) state in absence of magnetic fields. We will discuss how orbital degrees of freedom in frustrated lattice systems lead to a narrowing of topologically nontrivial bands [1]. This robust effect does not rely on fine-tuned long-range hopping parameters and is directly relevant to a wide class of transition metal compounds. In addition, we will show that strongly correlated electrons in a $t_{2g}$-orbital system on a triangular lattice self-organize into a spin-chiral magnetic ordering pattern that induces precisely the required topologically nontrivial and flat bands [2]. On top of a self-consistent mean-field approach, we use exact diagonalization to study an effective one-band model for the emerging flat band in the presence of longer-range interactions and establish the signatures of a spontaneous $\nu = \frac{1}{3}$ FQH state. \\[4pt] [1] J. W.F. Venderbos, M. Daghofer, J. van den Brink, PRL {\bf 107}, 076405 (2011) \\[0pt] [2] J. W.F. Venderbos, S. Kourtis, J. van den Brink, M. Daghofer, arXiv:1109.5955   

Hall Crystal States in Fractionally Filled Chern Bands

Two-dimensional time-reversal-invariant topological insulators can be thought of as a time-reversed pair of Chern bands. Numerical evidence shows the existence of states at fractional filling which are analogous to FQH states[1]. In [2] it was noted that at small momenta, the algebra of the density operators projected to the Chern band resembles the magnetic translation algebra. The authors have constructed a mapping[3] between Chern bands and a Landau level in a periodic potential which works at all momenta. This mapping is dynamically faithful, and reproduces the commutators of the projected density operator. There turn out to be Hall Crystal states, characterized by a Hall conductance, and another integer which described the charged dragged when the potential is adiabatically moved by a lattice unit. Using the Hamiltonian formalism developed by the authors some time ago for the FQHE[4], we calculate gaps and collective mode dispersions for such states. 1. D. N. Sheng et al, arxiv:1102.2568, N. Regnault and B. A. Bernevig, arxiv:1105.4867. 2. S. Parameswaran, R. Roy, and S. L. Sondhi, arxiv:1106.4025. 3. G. Murthy and R. Shankar, arxiv:1108.5501 4. G. Murthy and R. Shankar, Rev. Mod. Phys. 75, 1101 (2003)   

Chiral Topological Phases and Fractional Domain Wall Excitations in One-Dimensional Chains and Wires

According to the general classification of topological insulators, there exist one-dimensional chirally (sublattice) symmetric systems that can support any number of topological phases. We introduce a zigzag fermion chain with spin-orbit coupling in magnetic field and identify three distinct topological phases. Zero-mode excitations, localized at the phase boundaries, are fractionalized: two of the phase boundaries support $\pm $e/2 charge states while one of the boundaries support $\pm $e and neutral excitations. In addition, a finite chain exhibits $\pm $e/2 edge states for two of the three phases. We explain how the studied system generalizes the Peierls-distorted polyacetylene model and discuss possible realizations in atomic chains and quantum spin Hall wires.   

Quantum Hall Effect and Bound Fractional Charge in Topological Insulator Magnetic Tunnel Junctions

Proximity coupling 2D and 3D time-reversal invariant topological insulators to ferromagnetic domain walls is known to lead to bound fractional charge and an integer quantum Hall effect respectively. Here we show that by correctly engineering the sample geometry these effects can appear in the presence of only a single magnet with no domain walls, thus reducing the experimental complexity. We will prove that a magnetic layer sandwiched between 3D topological insulator films will exhibit the quantum Hall effect, possibly leading to a room-temperature realization of the quantum Hall effect.   

Phase Diagram of a Simple Model for Fractional Topological Insulator

We study a simple model of two species of (or spin-1/2) fermions with short-range intra-species repulsion in the presence of opposite (effetive) magnetic field, each at filling factor 1/3. In the absence of inter-species interaction, the ground state is simply two copies of the 1/3 Laughlin state, with opposite chirality. Due to the overall time-reversal symmetry, this is a fractional topological insulator. We show this phase is stable against moderate inter-species interactions. However strong enough inter-species repulsion leads to phase separation, while strong enough inter-species attraction drives the system into a superfluid phase. We obtain the phase diagram through exact diagonalization caluclations. Nature of the fractional topological insluator-superfluid phase transition is discussed using an appropriate Chern-Simons-Ginsburg-Landau effective field theory.   

Fractional Chern Insulators and the $W_\infty$ Algebra

A set of recent results indicates that fractionally filled bands of Chern insulators in two dimensions support fractional quantum Hall states analogous to those found in fractionally filled Landau levels. We provide an understanding of these results by examining the algebra of Chern band projected density operators. We find that this algebra closes at long wavelengths and for constant Berry curvature, whereupon it is isomorphic to the $W_\infty$ algebra of lowest Landau level projected densities first identified by Girvin, MacDonald and Platzman [Phys. Rev. B 33, 2481 (1986).] For Hamiltonians projected to the Chern band this provides a route to replicating lowest Landau level physics on the lattice.   

Session W39: Focus Session: Materials and Functional Structures for Biological Interfaces: Cells

Controlling Cell Function with Geometry

This presentation will describe the use of patterned substrates to control cell shape with examples that illustrate the ways in which cell shape can regulate cell function. Most cells are adherent and must attach to and spread on a surface in order to survive, proliferate and function. In tissue, this surface is the extracellular matrix (ECM), an insoluble scaffold formed by the assembly of several large proteins---including fibronectin, the laminins and collagens and others---but in the laboratory, the surface is prepared by adsorbing protein to glass slides. To pattern cells, gold-coated slides are patterned with microcontact printing to create geometric features that promote cell attachment and that are surrounded by inert regions. Cells attach to these substrates and spread to adopt the shape defined by the underlying pattern and remain stable in culture for several days. Examples will be described that used a series of shapes to reveal the relationship between the shape of the cell and the structure of its cytoskeleton. These geometric cues were used to control cell polarity and the tension, or contractility, present in the cytoskeleton. These rules were further used to control the shapes of mesenchymal stem cells and in turn to control the differentiation of these cells into specialized cell types. For example, stem cells that were patterned into a ``star'' shape preferentially differentiated into bone cells whereas those that were patterned into a ``flower'' shape preferred a fat cell fate. These influences of shape on differentiation depend on the mechanical properties of the cytoskeleton. These examples, and others, reveal that shape is an important cue that informs cell function and that can be combined with the more common soluble cues to direct and study cell function.   

Role of fiber functionality and angle on cell migration.

In order to determine the role of surface interactions on cell migration we compared the cell velocity on electrospun PMMA fibers which were either etched with UV/ozone plasma, had pre-adsorbed Fibronectin or both. It shows that dermal fibroblasts (CF-29, ATCC) did not adhere to the fibers without treatment, and the migration of cells was fastest on with both etching and pre-coat FN fibers. Vinculin was used to stain for the focal adhesion points and the largest number per cell were found on the FN pre-incubated samples, and nearly none on the plasma etched surface, despite good proliferation and migration. The results indicate that the migration velocity need not directly correlate to the cell adhesion. Using FN coated fibers we also studied the effect of angle on crossed fibers. We found that there was a clear preference by the cells for crossing a matrix where the fibers were oriented at 30 degrees. At this angle the migration velocity was slowest. Movies of the migrating cells indicate that the residence time of the cells at junctions with this angle is the longest cut to the interactions of the motion between fibers. The largest speed was observed for fibers placed at 90 degree.   

Anomalous cell migration properties on electrospun fibers

We have studied the influence of substrate morpholiogy on the en-mass cell migration from an agarose droplet. On flat surfaces, the cell velocity decreases asymptotically towards the single cell value as the radial distance increases, and remains constant thereafter. On fibers, the velocity remains constant at the single cell limit for the first 24 hours and then begins to increase continuously for the next four days. On flat surfaces we have shown that migration was triggered by nuclear deformation [Pan Z. et al, 2009], whereas on fibers the nucleus is constantly deformed as the cell assumers the shape of the fiber and hence does not seem to play as major a role. Vinculin and paxillin immunofluorescent staining were performed to determine the role of traction forces. We found that whereas polarization remains constant on flat surfaces with time, it increases on the fiber surfaces after the first 24 hours, and may explain the increased migration speed.   

Control cell adhesion with dynamic bilayer films

Interfacially-directed assembly of amphiphilic block copolymers was employed to create ultrathin films having the potential to correlate the dynamics of ECM cues with cell adhesion and cytoskeletally-generated forces. The mobility of the polymeric bilayer films were tuned by the incorporation of hydrophobic homopolymer chains, which are thought to reduce interlayer friction. Labeling of the block copolymer chains with an adhesive peptide ligand (RGD) provided a specific means to study integrin-mediated cellular processes and the corresponding mechanotransduction. By seeding anchorage-dependent cells on ``dynamic'' (laterally mobile) and ``static'' films that display the same amount of RGD, we have found that cells recognize the difference in RGD diffusivity and develop distinct responses over time. We intend to examine changes in cell response by controlling the extent of cytoskeletally-generated forces and the assembly dynamics of focal adhesion complexes. Such films provide a unique platform to unveil the biomechanical signals related with ECM dynamics, and may ultimately facilitate a deeper understanding of cellular processes.   

Diblock Copolymer Foams with Adhesive Nano-domains Promote Stem Cell Differentiation

Adhesions play an important role in cell behavior, including differentiation. Substrates are typically modified with homogeneous protein coatings; extracellular matrices \textit{in vivo} provide heterogeneous adhesive sites. To mimic adhesive heterogeneity, internal phase emulsion foams were polymerized with polystyrene-polyacrylic acid (PAA) and polystyrene-polyethylene oxide (PEO) to determine if interface de-mixing would form~patch-like surfaces. PEO/PAA mole ratios were~confirmed by XPS and water contact angle while spatial distribution was measured~by chemical force spectroscopy. This method confirmed the presence of patch-like PAA domains. Protein differentially adsorbs on PEO and PAA, so adsorption on foam mixtures was copolymer ratio dependent. Bone marrow-derived~mesenchymal stem cell (BMSC) adhesion was ratio dependent, but the highest density and vinculin expression was observed for 75PEO/25PAA. BMSCs appeared to change lineage expression the most on this composition, suggesting that this foam, which exhibits small adhesive PAA domains, may be more biomemetic than uniformally adhesive scaffolds, e.g. 0PEO/100PAA.   

Dental Pulp Stem Cell Differentiation on Poly-4-vinyl-pyridine surfaces

In the regeneration of a natural tissue, the mechanics and the chemical properties of the artificial substrate play a critical role. In this study, the influence of poly-4-vinyl-pyridine scaffold morphology on dental pulp stem cell differentiation was analyzed. Cells were plated on spun cast films and electrospun fibers with diameters ranging from nano to micrometers. Confocal microscopy showed the presence of various cell morphologies: on microfibers cells conform precisely to the main axis of elongation, while on nanometric scaffolds they result spread and in contact with several fibers. Even if the surface chemistry was identical, a great variation in the curvature was present. From day 9 of incubation, spontaneous biomineralization in the absence of induction agents occurred only on the fibrous structures. The SEM revealed template deposits directly on the microfibers, while on the nanofibers large spherical islands were also present. EDAX determined hydroxyl apatite nature of the deposits. RT-PCR indicated upregulation of osteogenic markers, confirming differentiation. SEM also revealed the presence of ECM fibers covering the polymer structure, which may enhance the expression of focal adhesion sites on the cell membrane.   

Spontaneous Differentiation of Dental Pulp stem cells on Dental polymers

Dental pulp stem cells were plated on two dentally relevant materials i.e. PMMA commonly used for denture and Titanium used for implants. In both cases, we probed for the role of surface interaction and substrate morphology. Different films of PMMA were spun cast directly onto Si wafers; PMMA fibers of different diameters were electro spun onto some of these substrates. Titanium metal was evaporated onto Si surfaces using an electron beam evaporator. In addition, on some surfaces, P4VP nanofibers were spun cast. DPSC were grown in alpha-MEM supplemented with 10\% fetal bovine serum, 0.2mM L-ascorbic acid 2-phosphate, 2mm glutamine and 10mM beta-glycerol phosphate either with or without 10nM dexamethasone. After 21 days samples were examined using confocal microscopy of cells and by scanning electron microscopy (SEM) and Energy dispersive X-ray Analysis (EDAX). In the case of Titanium biomineralization was observed independent of dexamethasone, where the deposits were templated along the fibers. Minimal biomineralization was observed on flat Titanium and PMMA samples. Markers of osteogenesis and specific signaling pathways are being evaluated by RT-PCR, which are up regulated on each surface, to understand the fundamental manner in which surfaces interact with cell differentiation.   

Interaction of Substrate Mechanics with Dental Pulp Stem Cells (DPSCs) differentiation to generate a scaffold for Bone regeneration

This work investigates the interaction of the substrate mechanics with the differentiation in the absence of chemical induction and only resulting from the stimuli of the substrate mechanics and chemistry. We chose enzymatically cross-linked gelatin hydrogels substrates of different stiffness varying from 8KPa to 100Pa. DPSCs were cultured and differentiated on the substrates for 7, 14 and 21 days with and without dexamethasone induction media. SEM and EDX analysis after 21 days indicate that cells produced a sheet of biomineralized deposits, several tenths of mm thick on the hard substrate irrespective of chemical induction. Modulli of the cells was independent of the induction and stiffness of the hydrogels. RT-PCR assays indicated that cells expressed more osteocalcin when cultured in non-induction media and harder substrate. The shape of the deposits was more uniform and in close packing on the harder substrate with a higher Ca:P ratio. On soft substrate the deposits were more flat with less Ca:P ratio. Further experiments indicated that conformational change due to the crosslinking of gelatin could be the reason for biomineralization.   

Neuron Growth on Carbon Nanotube Thread Bio-Scaffolds for Repair of Central Nervous System Damage

Approximately 11,000 new spinal cord injuries occur each year. Repairing such central nervous system (CNS) damage has proven to be very difficult. We report on\textit{ in vitro} experiments using carbon nanotube (CNT) threads as a bio-scaffold for promoting CNS repair via directed neuron regrowth along the CNT material. Mouse brain neurospheres, containing neuronal stem cells, neurons and support glia, were observed to attach to and grow along laminin-coated CNT threads \textit{in vitro}. However, due to their limited mobility, only neurospheres close to the threads attach. To increase cellular attachment to the threads, we exploit the fact that these cells can exhibit enhanced, directed migration along an externally applied electric field. Recent\textit{ in vitro} cell growth was carried out in chambers containing several parallel CNT threads with electrical connections extending out of the incubator so that a voltage applied across adjacent threads established an appropriate electric field. Electrochemical Impedance Spectroscopy, Cyclic Voltammetry and dc and ac IV measurements were used to monitor cell growth and attachment as a function of applied electric field and time. Cell migration and attachment were also investigated using time lapse photography in a separate growth chamber mounted on the stage of an optical microscope.   

Stiffness nanotomography of human epithelial cancer cells

The mechanical stiffness of individual cells is important in both cancer initiation and metastasis. We present atomic force microscopy (AFM) based nanoindentation experiments on various human mammary and esophagus cell lines covering the spectrum from normal immortalized cells to highly metastatic ones. The combination of an AFM with a confocal fluorescence lifetime imaging microscope (FLIM) in conjunction with the ability to move the sample and objective independently allow for precise alignment of AFM probe and laser focus with an accuracy down to a few nanometers. This enables us to correlate the mechanical properties with the point of indentation in the FLIM image. We are using force-volume measurements as well as force indentation curves on distinct points on the cells to compare the elastic moduli of the nuclei, nucleoli, and the cytoplasm, and how they vary within and between individual cells and cell lines. Further, a detailed analysis of the force-indentation curves allows study of the cells' mechanical properties at different indentation depths and to generate 3D elasticity maps.   

Snakeskin tribology: How snakes generate large frictional anisotropy

The limbless locomotion of snakes relies fundamentally upon friction. Snakes can adjust their spatial and temporal frictional properties in order to get friction anisotropies of around 2. Ventral scales play a major role in friction adjustment. In this combined experimental and theoretical study we measure the mechanical and frictional properties of snakeskin. We report the effect of substrate roughness and compliance on snake frictional anisotropy. We numerically model a snake scale interacting with an elastic rough substrate using contact mechanics. The scale is modeled as an isotropic rigid material attached to the body using a torsional spring. We find that the combined effect of the scales geometry and angle of attack leads to the scale high frictional anisotropy. Our results suggest that fabricating engineering surfaces such as artificial snakeskin with optimized geometry and orientation of scales will improve the efficiency of snake-like robots.   

Session W40: Focus Session: Single Molecule Biological Physics - Nucleic acids and Proteins

Sequence-dependent sliding kinetics of p53

Theoretical work has long proposed that one-dimensional sliding along DNA while simultaneously reading its sequence can accelerate transcription factors' (TFs) search for their target sites. More recently, functional sliding has been shown to require TFs to possess at least two DNA-binding modes. The tumor suppressor p53 has been directly observed to slide on DNA, and structural and single-molecule studies have provided evidence for a two-mode model for the protein. If the model is in fact applicable to p53, then the requirement that TFs read while they slide implies that p53's mobility on DNA should be affected by non-cognate sites and thus that its diffusivity should be generally sequence-dependent. Here we confirm this prediction with single-molecule microscopy measurements of p53's local diffusivity on non-cognate DNA. We show how a two-mode model accurately predicts the variation in local diffusivity while a single-mode model does not. Our work provides evidence that p53's sliding is indeed functional and suggests that the timing and efficiency of its activating and repressing transcription can depend on its non-cognate binding properties and its ability to change between multiple modes of binding, in addition to the much better-studied effects of cognate-site binding.   

Single-molecule study of protein-DNA target search mechanisms for dimer-active protein complexes

Protein-DNA interactions are essential to cellular processes, many of which require proteins to recognize a specific DNA target-site. This search process is well-documented for monomeric proteins, but not as well understood for systems that require dimerization at the target site for activity. We present a single-molecule study of the target-search mechanism of Protelomerase TelK, a recombinase-like protein that is only active as a dimer. We observe that TelK undergoes 1D diffusion on non-target DNA as a monomer, as expected, but becomes immobile on DNA as a dimer or oligomer despite the absence of its target site. We further show that TelK condenses non-target DNA upon dimerization, forming a tightly bound nucleo-protein complex. Together with simulations, our results suggest a search model whereby monomers diffuse along DNA, and subsequently dimerize to form an active complex on target DNA. These results show that target-finding occurs faster than nonspecific dimerization at biologically relevant protein concentrations. This model may provide insights into the search mechanisms of proteins that are active as multimeric complexes for a more accurate and comprehensive model for the target-search process by sequence specific proteins.   

Single-Molecule Encoders for Tracking Motor Proteins on DNA

Devices such as inkjet printers and disk drives track position and velocity using optical encoders, which produce periodic signals precisely synchronized with linear or rotational motion. We have implemented this technique at the nanometer scale by labeling DNA with regularly spaced fluorescent dyes. The resulting molecular encoders can be used in several ways for high-resolution continuous tracking of individual motor proteins. These measurements do not require mechanical coupling to macroscopic instrumentation, are automatically calibrated by the underlying structure of DNA, and depend on signal periodicity rather than absolute level. I will describe the synthesis of single-molecule encoders, data from and modeling of experiments on a helicase and a DNA polymerase, and some ideas for future work.   

Tuning Cargo Travel via Single Motor Velocity

Active intracellular transport is crucial for cell function, and defects are linked to diseases including neurodegeneration. Single molecule biophysical studies have revealed a great deal about the function of individual motors in vitro. However, it remains challenging to use single molecule properties of molecular motors to explain the complex range of cargo motions observed in vivo, in particular how motors work together in small ensembles. Recent reports have highlighted the sensitivity of ensemble transport to the single motor properties of processivity and force production. Here we investigate the previously unexplored role of motor velocity, and report a combined experimental and theoretical demonstration that single motor velocity crucially controls the ensemble function of two-motor transport.   

Study of Large Multimeric Biomolecules by Single-Molecule Manipulation and Imaging

Single-molecule manipulation enables us to study the properties of long chain, multimeric biomolecules. Perlecan, a giant secreted heparin sulfate proteoglycan, is a major component of basement membrane, bone stroma and blood vessels. It is involved in processes such as cell adhesion, migration and modulation of apoptosis. The changes in its synthesis and function are closely associated with many diseases, including cancer. Von Willebrand factor is a large multimeric protein circulating in blood, and is crucial for initiation of blood coagulation. We use atomic force microscope to obtain force curves and images of these proteins. We characterized the mechanical property of perlecan as well as the domain conformational changes of von Willebrand factor. The results demonstrate that single-molecule manipulation can probe directly the dynamics of large biomolecules that are usually not accessible with other methods.   

Uncovering the microscopic mechanism of strand exchange during RecA mediated homologous recombination using all-atom molecular dynamics simulations

Homologous recombination (HR) is a key step during the repair process of double-stranded DNA (dsDNA) breakage. RecA is a protein that mediates HR in bacteria. RecA monomers polymerize on a single-stranded DNA (ssDNA) separated from the broken dsDNA to form a helical filament, thus allowing strand exchange to occur. Recent crystal structures depict each RecA monomer in contact with three contiguous nucleotides called DNA triplets. Surprisingly, the conformation of each triplet is similar to that of a triplet in B-form DNA. However, in the filament the neighboring triplets are separated by loops of the RecA proteins. Single molecule experiments demonstrated that strand exchange propagation occurs in 3 base-pair increments. However, the temporal resolution of the experiments was insufficient to determine the exact molecular mechanism of the triplet propagation. Using all-atom molecular dynamics simulations, we investigated the effect of both the RecA protein and the conformation of the bound ssDNA fragment on the stability of the duplex DNA intermediate formed during the strand-exchange process. Specifically, we report simulations of force-induced unzipping of duplex DNA in the presence and absence of the RecA filament that explored the effect of the triplet ladder conformation.   

Transduction of Glycan-Lectin Binding using Near Infrared Fluorescent Single Walled Carbon Nanotubes for Glycan Profiling

In this work, we demonstrate a sensor array employing recombinant lectins as glycan recognition sites tethered via Histidine tags to Ni2+ complexes that act as fluorescent quenchers for semi-conducting single walled carbon nanotubes embedded in a chitosan to measure binding kinetics of model glycans. Two higher-affined glycan-lectin pairs are explored: fucose (Fuc) to PA-IIL and N-acetylglucosamine (GlcNAc) to GafD. The dissociation constants (KD) for these pairs as free glycans (106 and 19 $\mu $M respectively) and streptavidin-tethered (142 and 50 $\mu $M respectively) were found. The absolute detection limit for the current platform was found to be 2 $\mu $g of glycosylated protein or 100 ng of free glycan to 20 $\mu $g of lectin. Glycan detection is demonstrated at the single nanotube level (GlcNAc to GafD). Over a population of 1000 nanotubes, 289 of the SWNT sensors had signals strong enough to yield kinetic information (KD of 250 $\pm $ 10 $\mu $M). We are also able to identify the locations of ``strong-transducers'' on the basis of dissociation constant (4 sensors with KD $<$ 10 $\mu $M) or overall signal modulation (8 sensors with $>$ 5{\%} quench response). The ability to pinpoint strong-binding, single sensors is promising to build a nanoarray of glycan-lectin transducers as a method to profile glycans without protein labeling or glycan liberation pretreatment steps.   

Comprehensive single molecule dynamics and functions of lysozyme upon linear and cross-linked substrate using a carbon nanotube circuit

The dynamic processivity of individual lysozyme molecules was monitored in the presence of either linear or cross-linked peptidoglycan substrates using a single-walled carbon nanotube transistor. The substrate-driven, hinge bending motions of lysozyme induce dynamic electronic signals in the underlying transistor to allow long-term monitoring of the same molecule, all without the limitations of fluorophore quenching or bleaching. For both types of substrates, lysozyme exhibits slow, processive turnover at 20 Hz and also rapid, nonproductive motions at 300 Hz. However, the latter type of motion nearly vanishes with the linear substrate, which lacks cross-links. Specifically, the nonproductive binding fills 43{\%} of the enzyme's total activity when the substrate has cross-links, but only 7{\%} with the cross-links are absent. The continuous, uninterrupted processing indicates that lysozyme can catalytically hydrolyze glycosidic bonds all the way to the end of a linear substrate, and that the motion attributed to nonproductive binding may be the lysozyme sidestepping the peptide cross-links.   

A hierarchical coarse-grained (all-atom to all residue) approach to peptides (P1, P2) binding with a graphene sheet

Recently, Kim et al. [1] have found that peptides P1: HSSYWYAFNNKT and P2: EPLQLKM bind selectively to graphene surfaces and edges respectively which are critical in modulating both the mechanical as well as electronic transport properties of graphene. Such distinctions in binding sites (edge versus surface) observed in electron micrographs were verified by computer simulation by an all-atomic model that captures the pi-pi bonding. We propose a hierarchical approach that involves input from the all-atom Molecular Dynamics (MD) study (with atomistic detail) into a coarse-grained Monte Carlo simulation to extend this study further to a larger scale. The binding energy of a free amino acid with the graphene sheet from all-atom simulation is used in the interaction parameter for the coarse-grained approach. Peptide chain executes its stochastic motion with the Metropolis algorithm. We investigate a number of local and global physical quantities and find that peptide P1 is likely to bind more strongly to graphene sheet than P2 and that it is anchored by three residues $^{4}$Y$^{5}$W$^{6}$Y. [1] S.N. Kim et al J. Am. Chem. Soc. 133, 14480 (2011).   

Session W41: Focus Session: Quantum Coherence in Biological Systems

Quantum Coherence in Biology

Discussion of quantum mechanical effects in biology is generally restricted to molecular energetics, stability, and kinetics as determined by potential energy barriers. In recent years however, an increasing number of experiments have shown evidence for the existence of dynamical phenomena in biological systems that involve coherent quantum motion. One prominent instance is electronic quantum coherence in photosynthesis. I shall present theoretical studies that analyze the nature of this coherence, its relation to the non-local quantum correlations characteristic of entanglement, implications for relevance of quantum information processing in natural systems and address the question of whether and how such quantum coherence might result in a biological advantage.   

Absence of quantum oscillations in electronic excitation transfer in the Fenna-Matthews-Olson complex

Energy transfer in the photosynthetic Fenna-Matthews-Olson (FMO) complex of the Green Sulfur Bacteria is studied theoretically taking all three subunits (monomers) of the FMO trimer and the recently found eighth bacteriochlorophyll (BChl) molecule into account. For the calculations we use the efficient Non-Markovian Quantum State diffusion approach. Since it is believed that the eighth BChl is located near the main light harvesting antenna we look at the differences in transfer between the situation when BChl 8 is initially excited and the usually considered case when BChl 1 or 6 is initially excited. We find strong differences in the transfer dynamics, both qualitatively and quantitatively. When the excited state dynamics is initialized at site eight of the FMO complex, we see a slow exponential-like decay of the excitation. This is in contrast to the oscillations and a relatively fast transfer that occurs when only seven sites or initialization at sites 1 and 6 is considered. Additionally we show that differences in the values of the electronic transition energies found in the literature lead to a large difference in the transfer dynamics.   

Progress Towards Room-Temperature Electron Spin Detection in Biological Systems

We report on recent progress of room-temperature electron spin sensing for biological applications using nitrogen-vacancy (NV) centers in diamond. Our approach involves room-temperature detection of a small number of electron spins, situated outside the measurement substrate. Potential applications will be discussed, including detection of magnetic resonance signals from individual electron or nuclear spins of complex biological molecules, measurement of concentrations of radicals in living cells, and monitoring the ion channel function across cell membranes (important for exploring drug delivery mechanisms).   

Coherent transfer in biological systems: a study of the role of environmental noise

We study the effects of environmental noise on quantum mechanical systems, we focus mainly on biological systems in which noise is believed to assist coherent transport of energy. We use error correction methods aimed at avoiding decoherence and dissipation due to coupling to a reservoir, with the purpose of investigating if modifying the coupling to a noisy bath can provide further insight on its effects on transport mechanisms, and propose that this work is carried out experimentally.   

Noise induced quantum effects in photosynthetic complexes

Recent progress in coherent multidimensional optical spectroscopy revealed effects of quantum coherence coupled to population leading to population oscillations as evidence of quantum transport. Their description requires reevaluation of the currently used methods and approximations. We identify couplings between coherences and populations as the noise-induced cross-terms in the master equation generated via Agarwal-Fano interference that have been shown earlier to enhance the quantum yield in a photocell. We investigated a broad range of typical parameter regimes, which may be applied to a variety of photosynthetic complexes. We demonstrate that quantum coherence may be induced in photosynthetic complexes under natural conditions of incoherent light from the sun. This demonstrates that a photosynthetic reaction center may be viewed as a biological quantum heat engine that transforms high-energy thermal photon radiation into low entropy electron flux.   

FMO complex: exciton transfer and interaction with vibronic modes

We examine transport of excitons trough Fenna-Matthews-Olson (FMO) complex from a receiving antenna to a reaction center, using methods of condensed matter and statistical physics. Writing equation of motion for creation/annihilation operators, we are able to describe the exciton dynamics in the regime when the reorganization energy is of the order of the intra-system couplings. Well-known quantum oscillations of the site populations are obtained, in particular. We determine the exciton transfer efficiency and its dependencies of the system parameters. While the majority of vibronic modes are treated as a heat bath, we address the situation when specific modes are strongly coupled to excitons and examine the effects of these modes on the quantum oscillations and the energy transfer efficiency.   

Non-Markovian harmonic bath model for molecular systems: Influence of the bath spectral density

In quantum mechanical simulations of the electronic excitation dynamics in molecular complexes, like natural or artificial light-harvesting complexes, often open quantum system descriptions are applied, to treat a large number of degrees of freedom involved. A popular approach is then, to include only the electronic degrees of freedom into the system part and to couple them to a non-Markovian bath of harmonic vibrational modes. The coupling to the bath, representing intra-molecular as well as external vibrations, is usually described via the bath spectral density, which therefore is an important ingredient in this approach. Here, we discuss different aspects of the influence of the bath spectral density on dynamics and optical spectra. In particular, we consider structured spectral densities, consisting of multiple broadened peaks. It is often assumed that the strong coupling to an intra-molecular vibrational mode, which is damped by coupling to other vibrational modes, is described reasonably by such a broadened peak in the spectral density. Here we demonstrate that this interpretation should be used with caution, because the damping of the mode differs from the model for an intra-molecular mode that one would usually apply when including the mode directly in the system part.   

Curvature, torsion and temperature in energy transfer

Curvature and torsion can play a role in energy transfer in alpha-helical proteins [New J. Phys. 12, 055003 (2010)]. The phenomenon is specific to alpha-helices and not to beta-sheets in proteins due to the three strands of hydrogen bonds constituting the alpha-helical backbone. In this work we analyse and discuss how temperature can affect the role of geometry in the energy transfer.   

Quantum Process Tomography for Energy Transfer Systems via Ultrafast Spectroscopy

The description of excited state dynamics in energy transfer systems constitutes a theoretical and experimental challenge in modern chemical physics. A spectroscopic protocol that systematically characterizes both coherent and dissipative processes of the probed chromophores is desired [1,2]. In this talk, I show that a set of two-color photon-echo experiments performs quantum state tomography (QST) of the one-exciton manifold of a dimer by reconstructing its density matrix in real time. This possibility in turn allows for a complete description of excited state dynamics via quantum process tomography (QPT). Simulations of a noisy QPT experiment for an inhomogeneously broadened ensemble of model excitonic dimers show that the protocol distills rich information about dissipative excitonic dynamics, which appears nontrivially hidden in the signal monitored in single realizations of four-wave mixing experiments Progress on the experimental side will be discussed, as well as new insights that QPT has offered on the understanding of 2D electronic and vibrational spectroscopy. [1] J. Yuen-Zhou, J. J. Krich, A. Aspuru-Guzik, Quantum state and process tomography of energy transfer systems via ultrafast spectroscopy~Joel Yuen-Zhou, Jacob J. Krich, Masoud Mohseni and Al\'{a}n Aspuru-Guzik Proc. Nat. Acad. Sci. USA, Early Edition (2011).~ [2] J. Yuen-Zhou, A. Aspuru-Guzik, Quantum process tomography of molecular dimers from two-dimensional electronic spectroscopy I: General theory and application to homodimers~Joel Yuen-Zhou and Al\'{a}n Aspuru-Guzik . Chem. Phys. 134, 134505 (2011).   

Coherent vs. dissipative nonequilibrium dynamics in spectroscopy of molecular aggregates

Molecular aggregates embedded in a protein environment are the core elements in photosynthetic antennae units. Photoexcitations in these systems experience multistep relaxation, which could be traced using various time-resolved spectroscopy techniques. Initiated coherent processes turn into dissipative. Understanding of these processes is still a major theoretical task. We study theoretically spectroscopic properties of simple molecular aggregates coupled to a bath, which contains main ingredients of protein environemnt: high-energy vibrations, long-range correlations, and smooth spectrum of frequencies. At short times after the optical excitation high-energy coherent vibrational resonanses can be observed in two-dimensional rephasing spectroscopy. Their beats overlap with electronic quantum coherences, responsible for the quantum transport. We show the way to discriminate between them. At the long times we find that the conventional excitonic picture of eigenstates is valid only in the Markovian regime. In the non-Markovian regime the exciton concept breaks down and renormalized system parameters must be introduced: effective intermolecular coupling, widely used in polaron theories, can be used to account for the effects of the bath.   

New quantum state of protons and electrons in nano-confined water

Neutron Compton Scattering provides a means of directly and accurately measuring the momentum distribution of protons in water, which is determined primarily by the protons ground state wavefunction. We find that in water confined on scales of $\sim$20{\AA}, this wave function responds to the details of the confinement, corresponds to a strongly anharmonic local potential, shows evidence in some cases of coherent delocalization in double wells, and involves differences in zero point kinetic energy of the protons from that of bulk water at room temperature of -40 to +120 meV. This behavior is a generic feature of nanoscale confinement, and in particular, this state should be that which is present in water confined in biological cells. It is exhibited here in 16 {\AA} inner diameter carbon nanotubes, two different hydrated proton exchange membranes (PEMs), Nafion 1120 and Dow 858, and has been seen earlier in xerogel and 14 {\AA} diameter carbon nanotubes. The existence of this state is confirmed by xray Compton scattering measurements of the electron momentum distribution.   

Spectroscopic Characterization of the Water Oxidation Intermediates in the Blue Dimer Ru-Based Catalyst for Artificial Photosynthesis

Utilization of sunlight requires solar capture, light-to-energy conversion and storage. One effective way to store energy is to convert it into chemical energy by fuel-forming reactions, such as water splitting into hydrogen and oxygen. Ruthenium complexes are among few molecular-defined catalysts capable of water splitting. Mechanistic insights about such catalysts can be acquired by spectroscopic analysis of short-lived intermediates of catalytic water oxidation. Use of techniques such as EPR and X-ray absorption spectroscopy (XAS) are used to determine electronic requirements of catalytic water oxidation. About 30 years ago Meyer and coworkers reported first ruthenium-based catalyst for water oxidation, the ``blue dimer''. We performed EPR studies and characterized structures and electronic configurations of intermediates of water oxidation by the ``blue dimer''. Intermediates were prepared chemically by oxidation of Ru-complexes with defined number of Ce (IV) equivalents and freeze-quenched at controlled times. Changes in oxidation state of Ru atom were detected by XANES at Ru K-edges. K-edges are sensitive to changes in Ru oxidation state for Blue Dimer [3,3]$^{4+}$, [3,4]$^{4+}$, [3,4]'$^{4+}$ and [4,5]$^{3+}$ allowing a clear assignment of Ru oxidation state in intermediates. EXAFS demonstrated structural changes.   

Session W42: Focus Session: The Physics of Genome Folding I: Fractal Globules and Condensed Polymer States

How the genome folds

I describe Hi-C, a novel technology for probing the three-dimensional architecture of whole genomes by coupling proximity-based ligation with massively parallel sequencing. Working with collaborators at the Broad Institute and UMass Medical School, we used Hi-C to construct spatial proximity maps of the human genome at a resolution of 1Mb. These maps confirm the presence of chromosome territories and the spatial proximity of small, gene-rich chromosomes. We identified an additional level of genome organization that is characterized by the spatial segregation of open and closed chromatin to form two genome-wide compartments. At the megabase scale, the chromatin conformation is consistent with a fractal globule, a knot-free conformation that enables maximally dense packing while preserving the ability to easily fold and unfold any genomic locus. The fractal globule is distinct from the more commonly used globular equilibrium model. Our results demonstrate the power of Hi-C to map the dynamic conformations of whole genomes.   

Contact probability scaling of the Hilbert curve

Using Hi-C experiments, it has become possible to measure contact probability scalings for genomic polymers. However, the theoretical analysis of such scalings remains in its infancy. Here, we prove that contact probability scales with linear distance for lattice approximations of the Hilbert curve. These results point to the potential for new theoretical approaches to the study of contact probability, and shed light on the analysis behind the fractal globule, a recent model for the three-dimensional structure of the human genome.   

Territorial Polymers and Large Scale Genome Organization

Chromatin fiber in interphase nucleus represents effectively a very long polymer packed in a restricted volume. Although polymer models of chromatin organization were considered, most of them disregard the fact that DNA has to stay not too entangled in order to function properly. One polymer model with no entanglements is the melt of unknotted unconcatenated rings. Extensive simulations indicate that rings in the melt at large length (monomer numbers) $N$ approach the compact state, with gyration radius scaling as $N^{1/3}$, suggesting every ring being compact and segregated from the surrounding rings. The segregation is consistent with the known phenomenon of chromosome territories. Surface exponent $\beta$ (describing the number of contacts between neighboring rings scaling as $N^{\beta}$) appears only slightly below unity, $\beta \approx 0.95$. This suggests that the loop factor (probability to meet for two monomers linear distance $s$ apart) should decay as $s^{-\gamma}$, where $\gamma = 2 - \beta$ is slightly above one. The later result is consistent with HiC data on real human interphase chromosomes, and does not contradict to the older FISH data. The dynamics of rings in the melt indicates that the motion of one ring remains subdiffusive on the time scale well above the stress relaxation time.   

Diffusion of Particles in the Melt of Polymeric Rings and Diffusion of Proteins in the Cell Nucleus

Ring polymers in the melt are partially collapsed and partially segregated as revealed by both simulation and experiment. This behavior is qualitatively consistent with the arrangement of chromosomes in the cell nucleus which are found in distinct territories. Working under the hypothesis that a melt of nonconcatenated rings serves as a simple model for the packing of chromatin fibers in the nucleus of higher eukaryotes, we have investigated the dynamic behavior of a non-sticky spherical particle in polymer melts composed of rings using molecular dynamics simulation. Linear chains are also studied for comparison. In the case of rings such systems are thought to represent protein diffusion in the cell nucleus. The sub-diffusive motion of the particle is found to be independent of the polymer architecture and chain length but depends strongly on particle size. The long-time behavior suggests that these particles diffuse faster in the rings. We compare our results to existing models of protein diffusion.   

Spatial autocorrelation in fractal and knotted globules

The unknotted fractal globule is a model for the state of nuclear chromatin, and has properties that are distinct from those of knotted globules. One crucial property is a stronger correspondence between one-dimensional position along the polymer contour and three-dimensional position in the bulk. Here we introduce measures of spatial autocorrelation, such as Moran's I and Geary's C, and investigate spatial autocorrelation in both fractal and knotted globules. We show that fractal globules exhibit higher levels of spatial autocorrelation at both small and intermediate length scales. This autocorrelation may be relevant to the study of how proteins find their DNA targets inside the nucleus.   

In silico simulations of polymer condensation: the fractal globule as a metastable state

The fractal globule is a model for the metastable conformation of a long polymer after initial immersion in a poor solvent. Recent experimental findings describing the conformation of the human genome at the megabase scale are consistent with a fractal globule conformation. Here, we simulate the collapse of a polymer in a poor solvent using both molecular dynamics and Monte-Carlo simulations. We show that the statistics of the resulting configuration, such as radius of gyration, end-to-end distance, and contact probability, are independent of the approach used to simulate the condensation process. (* contributed equally).   

Fractal Globule as a model of DNA folding in eukaryotes

A recent study (Lieberman-Aiden et al., Science, 2009) observed that the structure of the genome, on the scale of a few megabases, is consistent with a fractal globule. The fractal globule is a quasi-equilibrium state of a polymer after a rapid collapse. First proposed theoretically in 1988, this structure had never been simulated. Fractal globule was seen as a state, in which each subchain is compact, and doesn't mix with other subchains due to their mutual unentanglement (topological constraints). We use GPU-assisted dynamics to create fractal globules of different sizes and observe their dynamics. Our simulations confirm that a polymer after rapid collapse has compact subchains. We measure the scaling of looping probability of a subchain with it's length, and observe the remarkably robust inverse proportionality. Dynamic simulation of the equilibration of this state show that it exhibits Rose type subdiffusion. Due to diffusion, fractal globule quickly degrades to a quasi-equilibrium state, in which subchains of a polymer are mixed, but topologically unentangled. We propose that separation of spatial and topological equilibration of a polymer chain might have implications in different fields of physics.   

High-order chromatin architecture shapes the landscape of chromosomal alterations in cancer

The rapid growth of cancer genome structural information provides an opportunity for a better understanding of the mutational mechanisms of genomic alterations in cancer and the forces of selection that act upon them. Here we test the evidence for two major forces, spatial chromosome structure and purifying (or negative) selection, that shape the landscape of somatic copy-number alterations (SCNAs) in cancer (Beroukhim et al, 2010). Using a maximum likelihood framework we compare SCNA maps and three-dimensional genome architecture as determined by genome-wide chromosome conformation capture (HiC) and described by the proposed fractal-globule (FG) model (Lieberman-Aiden and Van Berkum et al, 2009). This analysis provides evidence that the distribution of chromosomal alterations in cancer is spatially related to three-dimensional genomic architecture and additionally suggests that purifying selection as well as positive selection shapes the landscape of SCNAs during somatic evolution of cancer cells.   

Session W43: Invited Session: Physical Mechanisms of Collective Microbial Dynamics

Chiral patterning in Paenibacillus colonies under stress

One of the most striking examples of bacterial colony patterning occurs in the C-morphotype of Paenibacillus strains. Here, macroscopic chirality results from the interaction of local liquid-crystal ordering of the long bacterial cells with the self-propelled motility driven by the non-reflection-symmetric flagella. This talk will review some of the original experimental data from the Ben-Jacob lab as well as recent insight obtained via genomics. I will then discuss attempts to model and simulate the chiral patterns via solving reaction-diffusion equations on random lattices. At the end, I will introduce the challenges still to be faced in understanding transitions between these patterns and more common branching structures   

The effects of self-induced noise on the onset of collective behavior in suspensions of swimming bacteria

Collective dynamics of self-locomoting micro-organisms, such as bacteria and algae have attracted enormous attention, with a large number of experimental and theoretical works published in the last few years. A plethora of nontrivial properties have been predicted and consequently studied, including dynamic instabilities, anomalous density fluctuations, nontrivial stress-strain relations, rectification of chaotic motion, and viscosity reduction. Here we investigate the viscosity of a suspension of swimming bacteria analytically and numerically. We propose a simple model that takes into account excluded volume constraints and allows for efficient computation for a large number of bacteria. Our calculations show that long-range hydrodynamic interactions, intrinsic to self-locomoting objects in a viscous fluid, result in a dramatic reduction of the effective viscosity. In agreement with experiments on suspensions of \textit{Bacillus subtilis}, we show that the viscosity reduction is related to the onset of large-scale collective motion due to interactions between the swimmers. The simulations reveal that the viscosity reduction occurs only for relatively low concentrations of swimmers: further increases of the concentration yield an increase of the viscosity. We derive an explicit asymptotic formula for the effective viscosity in terms of known physical parameters and show that hydrodynamic interactions are manifested as self-induced noise in the absence of any explicit stochasticity in the system. We also explain the increase in the viscosity for pullers by analysis of the deviations from the mean field approximation   

Synchronization of flagella

Motivated by the observed coordination of nearby beating flagella, we use highly controlled simple model experiments with rotating paddles to study how hydrodynamic interactions can lead to phase-locking. The agreement between our numerical models and experimental results confirms that hydrodynamic interactions can lead to synchronization or phase-locking if the system has sufficient flexibility. We also present a simple theory, valid for weakly interacting paddles, for both viscous and viscoelastic fluids.   

Formation and propagation of high density waves during swarming of \textit{P. aeruginosa}

We will describe in this talk how the bacterium \textit{P. aeruginosa }alters its local physical environment by propagating high density waves of cells into branched tendrils during surface motility described as swarming. Biologically justified model simulations will be used to suggest a mechanism of wave propagation and branched tendril formation that depend upon competition between the changing viscosity of the bacterial liquid suspension and the liquid film boundary expansion caused by Marangoni forces. Thus, \textit{P. aeruginosa} controls physical forces responsible for liquid film expansion to efficiently colonize surfaces.   

Alignment vs noise in self-propelled particles: minimal models for collective motion and their continuous descriptions

Two important 1995 papers have marked the birth of collective motion studies in physics: Vicsek et al introduced what could now be described as the ``Ising model'' of this new subfield. This prompted Toner and Tu to propose a continuum theory of flocks which they showed to give rise to long-range orientational order even in two space dimensions. In this setting, the complexity of most natural instances of collective motion is reduced to the competition between local alignment and noise in interacting self-propelled particles. As I will show, this nevertheless gives rise to important and new physics. In this talk, I will give an update of our current knowledge about the Vicsek model, the Toner-Tu theory, and their relationship. I will also present the emerging picture of universality classes brought about by recent progress in the study of Vicsek-like models together with their continuous descriptions.   

Session W44: Focus Session: Interparticle Interactions in Polymer Nanocomposites - Mechanical, Electrical, and Optical Properties

Bio-inspired Fillers for Mechanical Enhancement

An examination of natural materials has offered a new perspective on the development of multi-functional materials with enhanced mechanical properties. One important lesson from nature is the utilization of composite structures to impart improved mechanical behavior and enhanced functionality using nanofillers. A relatively unexplored expansion of this bio-inspired, nanoscale filler approach to high performance materials is the incorporation of responsive, multi-functional reinforcing elements in polymeric composites with the goal of combining superior mechanical behavior that can be tuned with additional functionality, such as sensing and bioactivity. One approach is the use of self-assembling small molecules that form uniform, one-dimensional nanostructures as an emerging class of filler components. Another pathway toward mechanical enhancement is the incorporation of stimuli-responsive and high-modulus electrospun nanofibers. We have probed the utilization of high-aspect ratio, self-assembled small molecules and responsive electrospun nanofibers as all-organic nanofillers to achieve significant modulus changes within elastomeric matrices. The influence of matrix-filler interactions and the role of hierarchical organization in these nature-inspired composites will be discussed. Potential applications in barrier technology and drug delivery have also been explored.   

Temperature sensitive mechanical properties of Graphene-epoxy nanocomposites

Carbon-based polymeric nanocomposites have potential applications including structural parts in aerospace vehicles and civil infrastructure. In this work various aspects of mechanical properties of Graphene-epoxy nanocomposites are studied at different scales. The quasi-static tensile yield stress and stiffness of the nanocomposite are larger than those of neat epoxy. While the creep response of the nanocomposite is similar to that of neat epoxy at lower stress and room temperature, a significant discrepancy is observed at high temperature and/or large stress, the nanocomposite creeping less. The fracture toughness for the nanocomposite with the optimum filler fraction is larger than the toughness of unfilled epoxy at room temperature. This difference decreases at higher temperatures. Local mechanical properties were investigated using nanoindentation and similar trends are observed.   

Mechanical Properties of Polymer Nanocomposites under Large Amplitude Deformation

Bare silica nanoparticle dispersion in polystyrene and poly(methyl methacrylate) homopolymers are found to be controlled upon changing the evaporation condition. In this study, we deformed the polymer nanocomposites at different states of particle dispersion under large amplitude oscillatory shearing (LAOS). As the structure evolved during LAOS, the TEM images and small-angle X-ray scattering results obtained for different states of deformation together with the measured moduli allowed us to relate the mechanical reinforcement and nonlinearities such as strain stiffening/softening or shear thinning/thickening to the particle-polymer and particle-particle interactions effective at nanometer dimensions. Our results show that well-dispersed system with attractive interactions between particle and polymer becomes more elastic under large shear and behaves as attractive gels.   

Polymer Nanocomposite Mechanical Properties as a Function of Nanoparticle Dispersion

Nanoparticle (NP) dispersion critically affects the properties of polymer nanocomposites (PNCs), especially mechanical properties.~ Previous work by our group and others has shown optimal material properties occur when particles form a percolated network. Particle dispersion is a function of the interaction between the polymer and nanoparticle surface, and as such is difficult to control as an independent variable. In our previous work, dispersion was controlled via polymer grafts. Thus in order to vary the particle dispersion states the length and density of the grafted chains were necessarily simultaneously varied. Here we consider bare silica nanoparticles, 14nm in diameter, in 97kg/mol poly(2-vinyl pyridine). Although the particles are bare, the dispersion can still be controlled by proper solvent casting of the material; we use varying amounts of pyridine to charge stabilize the particles in solution and thus vary the dispersion state.~ We use TEM to probe PNP structure and NP dispersion state, and rheology is used to quantify mechanical properties. We look at the recovery of the storage modulus after large amplitude oscillatory shear and use this to probe the mechanism of particle reinforcement. Because the dispersion is an independently controlled variable, we are able to more accurately quantify its effect on nanocomposites.   

Elastic Moduli of Polymeric Thin Films of Nanocomposites and Blends via Buckling on Elastomeric Substrates

Mechanical properties are important for the long term durability of polymeric thin films. Unfortunately, there are very few methods for mechanical characterization of sub-micron thin films with high accuracy and repeatability. The technique of Strain-Induced Elastic Buckling Instability for Mechanical Measurements (SIEBIMM) was employed to determine the elastic moduli of nanocomposite and blend films, which were calculated from the buckling patterns generated by applying compressive stresses. In this study, polylactic acid (PLA) / Cloisite 30B nanocomposite thin films and polycaprolactone (PCL) / PLA blend thin films were prepared via spin-coating and then transferred to crosslinked polydimethylsiloxane (PDMS) flexible substrates. Results showed the strengthening effect of Cloisite 30B on PLA systems. The effect of nanoparticle concentrations and the influences of crystallinity and phase separation of blends will be presented.   

Electrical Conductivity in Transparent Silver Nanowire Networks: Simulations and Experiments

We model and experimentally measure the electrical conductivity of two-dimensional networks containing finite, conductive cylinders with aspect ratio ranging from 33 to 333. We have previously used our simulations to explore the effects of cylinder orientation and aspect ratio in three-dimensional composites, and now extend the simulation to consider two-dimensional silver nanowire networks. Preliminary results suggest that increasing the aspect ratio and area fraction of these rods significantly decreases the sheet resistance of the film. For all simulated aspect ratios, this sheet resistance approaches a constant value for high area fractions of rods. This implies that regardless of aspect ratio, there is a limiting minimum sheet resistance that is characteristic of the properties of the nanowires. Experimental data from silver nanowire networks will be incorporated into the simulations to define the contact resistance and corroborate experimentally measured sheet resistances of transparent thin films.   

Effect of Nanowire Size Dispersity and Orientation on Electrical Conductivity in Polymer Nanocomposites

We model the percolation threshold ($\phi _{c})$ and electrical conductivity of isotropic and oriented three-dimensional networks containing finite, conductive cylinders with experimentally typical (Gaussian) and engineered (bidisperse) distributions in their length and/or diameter. Our results show that narrow Gaussian distributions do not affect the threshold concentration or electrical conductivity significantly in either isotropic or oriented networks. In contrast, the addition of a small fraction of longer rods in a bidisperse system can improve the electrical properties considerably. We have also successfully extended the excluded volume percolation theory to predict $\phi _{c}$ of polydisperse networks of soft-core rods with finite-L/D by generalizing the monodisperse case and applying an empirical calibration factor from our simulations. Our analytical expression finds the critical concentration in nanocomposites with arbitrary distributions in L and/or D.   

Finite-size effects in nanocomposites: experimental and computational studies

Many proposed applications for electrically-conducting composite materials (smart textiles, e-m shielding coatings, tissue scaffolds) are nanostructured - that is, characteristic sample length scales may be similar to at least one dimension of the embedded particle. This is particularly true for long aspect-ratio particles such as nanotubes where the length of the particle can approach or exceed the thickness of a thin nanocomposite film or a nanofiber diameter. In these cases, the formation of a particle network and thus the electrical conductivity enhancement is affected by finite size effects, that is, percolation thresholds and the width of the transition to percolation differ with sample size [Stevens et al., \textit{Phys. Rev. E} \textbf{84}, 021126 (2011)]. We present experimental electrical conductivity and 3-D continuum Monte-Carlo simulation results on such finite-sized percolation effects for particles with aspect ratios of 1 to 1000 and discuss proposed scaling laws and techniques to improve conductance in the finite-size regime.   

Spectral and polarization modulation of quantum dot emission in a one-dimensional liquid crystal photonic cavity

We demonstrate spectral and polarization modulation of chemically synthesized core shell CdSe/ZnS quantum dots (QDs) embedded in a one-dimensional photonic cavity formed by a cholesteric liquid crystal (CLC) matrix. A Cano-wedge cell varies the pitch of the CLC leading to the formation of Grandjean steps. This spatially tunes the photonic stop band, changing the resonance condition and continuously altering both the emission wavelength and polarization state of the QD ensemble. Using high resolution spatially- and spectrally-resolved photoluminescence measurements we find that the emission is elliptically polarized and that the tilt of the ellipse, while dependent on the emission wavelength, additionally varies with distance across the Grandjean steps. This work opens up the possibility of designing new QD based optical devices, such as tunable single photon sources, where spatial control of wavelength and polarization of the embedded QDs would allow great flexibility and added functionalities.   

Self-assembly of silicon quantum dot clusters in polymer nanocomposites

We measure the influence of polymer-driven silicon nanocrystal (SiNC) self-assembly on the photoluminescent stability of SiNC clusters. Coexisting phases of varying nanoparticle density are identified in annealed SiNC-polymer nanocomposites, and the local photobleaching kinetics are measured under varied exposure to atmospheric oxygen. Increased particle packing and decreased oxygen exposure both contribute to improvements in cluster photostability, with Monte Carlo simulations of ensemble photobleaching clarifying the critical role of nanoparticle packing. The simulations further demonstrate the potential importance of nanoparticle interactions in dictating the photo-response of the self-assembled SiNC clusters.   

Utilizing photothermal heating by metal nanoparticles within polymer composites

Photothermal heating by metal nanoparticles have been extensively researched in solution environments for applications such as cancer treatment and drug delivery, but very few have explored photothermal heating in solids, such as metal nanoparticle-polymer composites. When metal nanoparticles are excited by light resonant with the particle's surface plasmon, non-radiative relaxation efficiently generates heat. Thus, this photothermal effect facilitates~\textit{in situ}~thermal processing of polymeric materials via externally-controllable light excitation.~By embedding fluorophores in the composite, a sensitive relative fluorescence approach can be utilized to dynamically monitor the average temperature within the sample as it is thermally processed. With modest light intensities and dilute nanoparticle concentrations, controllable temperature changes of several hundred degrees Celsius have been achieved.~ We discuss various cooling mechanisms and their respective effect on the heating process. The spatial specificity and temperatures achieved can potentially be used for triggering phase transitions, cross-linking, or enabling region-specific chemical reactions within a polymeric material. ~   

What determines photoluminescence and quenching when fluorophores in a polymer matrix?

A model system composed of fluorophore (pyrene), dispersed in a polymer matrix (polystyrene, PS) is investigated in order to find the relation between the structure of pyrene assembly in the PS matrix and its fluorescence/quenching. It has been shown that the pyrene disperses differently in the polymer matrix as it is prepared by different processes (namely, electrospin, solution cast and spincoat) with same composition. The difference can result in drastically variation of fluorescence response and quenching efficiency in presence of DNT. Our preliminary data indicate that the salt (tetrabutylammonium hexafluorophosphate, TBAP) applied to electrospinning process plays a crucial role in excimer fluorescence in the emission spectra, while various polymer matrices (e.g., PS and poly-methyl methacrylate, PMMA) yield similar fluorescence without TBAP. X-ray diffraction data suggests that strong dimer fluorescence of the sample may relate to the alignment of pyrene crystal structure, with a lattice length of 8.4 {\AA}. Moreover, a C$^{13}$ solid-state NMR result seems to indicate that the mobility of pyrene in the PS matrix of electrospin system is lower for samples with higher quenching efficiency.   

Microtextured Omniphobic Surfaces by Solution Spraying of Fluorodecyl POSS/PMMA Blends

We present a simple technique to prepare various micro-structured surfaces by spray coating a polymer blend of poly(methyl methacrylate) (PMMA) and the low surface energy molecule 1H,1H,2H,2H-heptadecafluorodecyl polyhedral oligomeric silsesquioxane (fluorodecyl POSS) using an air brush with a pressurized nitrogen stream. The sprayed surface morphology can be systematically tuned from a spherical or corpuscular microstructure to beads-on-string and a fiborous non-woven mesh, nearly identical to structures obtained by electrospinning similar PMMA/fluorodecyl POSS solutions. A semi-empirical framework is used to develop an operating diagram to predict the surface morphology produced during the simple spraying technique based on the polymer solution concentration and molecular weight. The presence of the low-surface-energy POSS molecules at the surface combined with the re-entrant microtextured features confers super-liquid-repellent properties to the spray-coated substrate, which are characterized by advancing and receding contact angle measurements with liquids of a range of surface tensions.   

Session W45: Focus Session: Soft Matter Physics of Heterogeneous Membranes - Theory and Simulation

Hybrid Lipids as Line Active Molecules

The lipid raft hypothesis suggests that stable nanoscopic domains in cellular membranes play an important role in several biological processes. Model membranes composed of saturated lipids, unsaturated lipids, and cholesterol (SUC membranes) exhibit coexisting chain-ordered and -disordered domains. However, these domains are unstable and the positive line tension at the interfaces between these domains drives coarsening until their size reaches of the order of the system size. This motivates the search for physical mechanisms that may reduce the line tension to zero and thus stabilize nanoscopic domains in biological membranes. There is a theoretical suggestion that the positive line tension at the interfaces between domains in SUC membranes results from the chain packing incompatibility between the ordered chains of saturated lipids and the disordered chains of unsaturated lipids. Hybrid lipids that have one saturated and one unsaturated chains may reconcile this chain packing incompatibility. We have used a phenomenological model to predict that a small concentration of hybrid lipids added to SUC membranes can reduce the line tension between coexisting domains to zero. However, this tends to occur only at low temperatures that may not be experimentally accessible, because localizing the hybrid lipids to the interfaces costs mixing entropy and this strongly suppresses the reduction of the line tension. Indeed, hybrid lipids are major components of biological membranes; unsaturated lipids are rather minor and uncommon. We have used a liquid crystal model to analyze the phase separation and line tension between domains in model membranes composed of saturated lipids, hybrid lipids, and cholesterol (SHC membranes). This model predicts that the line tension is reduced to zero at relatively higher in SHC membranes because the hybrid lipids are already at the interface and the mixing entropy of localization is no longer relevant.   

Turing Patterns on Curved Surfaces

Surface curvature modifies the emergence of Turing patterns in reaction-diffusion systems on two-dimensional interfaces. We adapt operator perturbation theory, familiar in quantum mechanics, to determine how curvature affects diffusion. When these modifications are taken into account in Turing's stability analysis, we observe new phenomena that may be relevant to patterning on cell membranes, neuron synapses, and fluid interfaces with reactive surfactants. A cylinder with longitudinal ripples illustrates how Turing patterns can lock into phase with the ripples when the most unstable mode is nearly commensurate with the ripples. The framework we introduce also sheds light on diffusion constrained to a rippled sphere, a Gaussian bump, and other shapes. More generally, it is relevant to any model that involves the Laplacian on a curved manifold.   

2D Ising Model correlation function: Precise functional forms for comparison to membrane experiments

We find a precise form for the 2D Ising correlation function in the entire scaling regime as a function of external field $H$ and temperature $T$. It is surprising that there is no functional form available, perhaps explained by the surprising complexity of the universal scaling function compared to other statistical mechanics models (e.g. 2D free energy or 3D Ising correlation function). We can use these results to test the hypothesis that heterogeneities found in a wide variety of membrane systems are manifestations of an underlying Ising critical point. We fit Monte Carlo lattice simulations in the $(H,T)$-plane with a functional form in parametric coordinates, while matching analytical results at $H=0$ and $T=T_c$ to high accuracy. This functional form allows us to interpret FRET, NMR, or ESR data from membranes, where we can map experimentally controllable variables of composition and temperature onto the Ising axes of reduced temperature and magnetization.   

Fluctuating Lipid Bilayer Membranes With Diffusing Protein Inclusions: Hybrid Continuum-Particle Model

Many proteins through their geometry and specific interactions with lipids induce changes in local membrane material properties. This can manifest in local stiffness variations and locally induced curvatures that track protein location. To study such phenomena we introduce a new hybrid continuum-particle description for the membrane-protein system that incorporates protein interactions, hydrodynamic coupling, and thermal fluctuations. We investigate how protein curvature and membrane stiffness influences protein diffusion. We discuss how collective protein effects influence membrane mechanical properties, such as the spectrum of membrane bending fluctuations and the effective elastic bending modulus of the heterogeneous protein-lipid membrane. Finally, we discuss possible roles of the membrane fluctuations influencing the distribution of proteins within the membrane.   

Influence of chain rigidity on the conformation of model lipid membranes in the presence of cylindrical nanoparticle inclusions

We employ real-space self-consistent field theory to study the conformation of model lipid membranes in the presence of solvent and cylindrical nanoparticle inclusions (''peptides''). Whereas it is common to employ a polymeric Gaussian chain model for the lipids, here we model the lipids as persistent, worm-like chains. Our motivation is to develop a more realistic field theory to describe the action of pore-forming anti-microbial peptides that disrupt the bacterial cell membrane. We employ operator-splitting and a pseudo-spectral algorithm, using SpharmonicKit for the chain tangent degrees of freedom, to solve for the worm-like chain propagator. The peptides, modelled using a mask function, have a surface patterned with hydrophobic and hydrophillic patches, but no charge. We examine the role chain rigidity plays in the hydrophobic mismatch, the membrane-mediated interaction between two peptides, the size and structure of pores formed by peptide aggregates, and the free-energy barrier for peptide insertion into the membrane. Our results suggest that chain rigidity influences both the pore structure and the mechanism of pore formation.   

Mechanics, morphology, and mobility in stratum corneum membranes

The stratum corneum is the outermost layer of skin, and serves as a protective barrier against external agents, and to control moisture. It comprises keratin bodies (corneocytes) embedded in a matrix of lipid bilayers. Unlike the more widely studied phospholipid bilayers, the SC bilayers are typically in a gel-like state. Moreover, the SC membrane composition is radically different from more fluid counterparts: it comprises single tailed fatty acids, ceramides, and cholesterol; with many distinct ceramides possessing different lengths of tails, and always with two tails of different lengths. I will present insight from computer simulations into the morphology, mechanical properties, and diffusion (barrier) properties of these highly heterogeneous membranes. Our results provide some clue as to the design principles for the SC membrane, and is an excellent example of the use of wide polydispersity by natural systems.   

Curvature-driven domain formation in ternary lipid membranes

We model vesicles formed by three-component fluid membranes whose components have differing spontaneous curvatures. We use Monte Carlo simulated annealing to find low energy configurations for a range of component characteristics. A wide variety of ordered structures are found, including highly symmetric structures with elongated domains resembling faceted edges. We also observe an effective attraction between components of highest and lowest spontaneous curvature. We relate these effects to the interplay of spontaneous curvature and underlying geometric constraints. Our results suggest that the composition-shape coupling can be an important mechanism in the formation of highly ordered structures in many-component membranes.   

Entropy driven aggregation of adhesion sites of supported membranes

Supported lipid membranes are useful and important model systems for studying cell membrane properties and membrane mediated processes. One attractive application of supported membranes is the design of phantom cells exhibiting well defined adhesive properties and receptor densities. Adhesion of membranes may be achieved by specific and non-specific interactions, and typically requires the clustering of many adhesion bonds into ``adhesion zones''. One potential mediator of the early stages of the aggregation process is the Casimir-type forces between adhesion sites induced by the membrane thermal fluctuations. In the talk, I will present a theoretical analysis of fluctuation induced aggregation of adhesion sites in supported membranes. I will first discuss the influence of a single attachment point on the spectrum of membrane thermal fluctuations, from which the free energy cost of the attachment point will be deduced. I will then analyze the problem of a supported membrane with two adhesion points. Using scaling arguments and Monte Carlo simulations I will demonstrate that two adhesion points attract each other via an infinitely long range effective potential that grows logarithmically with the pair distance. Finally, I will discuss the many-body nature of the fluctuation induced interactions. I will show that while these interactions alone are not sufficient to allow the formation of aggregation clusters, they greatly reduce the strength of the residual interactions required to facilitate cluster formation. Specifically, for adhesion molecules interacting via a short range attractive potential, the strength of the direct interactions required for aggregation is reduced by about a factor of two to below the thermal energy kT.   

Membrane lateral structure: How immobilized particles can stabilize small domains

Membranes are two-dimensional fluid environments, consisting of lipids and proteins. In model membranes, macroscopic phase separation is routinely observed, but not so in biological membranes. Instead, the lateral structure of a biological membrane is characterized by small domains. This poses an interesting puzzle because a structure of small domains inevitably implies a large amount of interface, which is unfavorable because of line tension. In this contribution, it is shown that immobilized protein obstacles provide a mechanism to compensate the cost of line tension. The presence of such obstacles in biological membranes is known to occur (arising for instance from interactions with the underlying cytoskeleton). We present results from computer simulation, which indeed show that a structure of small domains becomes stable, already at a low concentrations of quenched obstacles. In addition, these results confirm a fundamental conjecture of de Gennes, stating that a fluid with quenched obstacles belongs to the universality class of the random-field Ising model.   

In-Plane Dynamics of Membranes with Immobile Inclusions

Cell membranes are anchored to the cytoskeleton via immobile inclusions. We investigate the effect of such anchors on the in-plane dynamics of a fluid membrane and mobile inclusions embedded in it. The immobile particles lead to a decreased diffusion coefficient of mobile ones and suppress the correlated diffusion of particle pairs. Due to the long-range, quasi-two-dimensional nature of membrane flows, these effects become significant at a low area fraction (below one percent) of immobile inclusions.   

Charge correlations in multicomponent ionic crystalline membranes

We investigate the dissociation state of a polyelectrolyte membrane with charged head groups in solution. This state depends on the salt concentration and pH of the solution, but spatial correlations also highly influence it. Spatial correlations are typically neglected in these systems, as they are difficult to treat analytically, but they can qualitatively alter the results. We numerically incorporate charge correlations on both flat and curved membranes by simulating a multicomponent system on a fluctuating network with electrostatic interactions, using the replica exchange Monte Carlo approach. The salt-induced screening effects are modeled within the Debye-Huckel theory. For weak enough screening, we find a strong suppression of dissociation regardless of pH, and the membrane may exhibit a reentrant structural phase transition as pH is varied.   

Session W46: Invited Session: Silicon Spin Qubits: Relaxation and Decoherence

Single-shot readout of spin qubits in Si/SiGe quantum dots

Si/SiGe quantum dots are an attractive option for spin qubit development, because of the long coherence times for electron spins in silicon, arising from weak hyperfine interaction and low spin orbit coupling. I will present measurements of gate-defined single and double quantum dots formed in Si/SiGe semiconductor heterostuctures. Control of the gate voltages on these dots enables tuning of the tunnel coupling to the leads and to other dots. Careful tuning of these tunnel rates, in combination with fast, pulsed-gate manipulation and spin-to-charge conversion, allow spin state measurement using an integrated quantum point contact as a local charge detector. Single spin qubit readout relies on the Zeeman energy splitting from an external magnetic field for spin-to-charge conversion. Two-electron singlet-triplet qubits, on the other hand, can be measured by using Pauli spin blockade of tunneling between the dots to readout the qubit even at zero magnetic field. I will present real-time, single-shot readout measurements of both individual spin [1] and singlet-triplet qubits [2] in gated Si/SiGe quantum dots. Work performed in collaboration with J. R. Prance, Zhan Shi, B. J. Van Bael, Teck Seng Koh, D. E. Savage, M. G. Lagally, R. Joynt, L. R. Schreiber, L. M. K. Vandersypen, M. Friesen, S. N. Coppersmith, and M. A. Eriksson. \\[4pt] [1] C. B. Simmons et al. Physical Review Letters 106, 156804 (2011). \\[0pt] [2] J. R. Prance, et al., e-print: http://lanl.arxiv.org/abs/1110.6431   

Coherent manipulation of a Si/SiGe-based singlet-triplet qubit

Electrically defined silicon-based qubits are expected to show improved quantum memory characteristics in comparison to GaAs-based devices due to reduced hyperfine interactions with nuclear spins. Silicon-based qubit devices have proved more challenging to build than their GaAs-based counterparts, but recently several groups have reported substantial progress in single-qubit initialization, measurement, and coherent operation. We report [1] coherent control of electron spins in two coupled quantum dots in an undoped Si/SiGe heterostructure, forming two levels of a singlet-triplet qubit. We measure a nuclei-induced $T_{2}^{*}$ of 360 ns, an increase over similar measurements in GaAs-based quantum dots by nearly two orders of magnitude. We also describe the results from detailed modeling of our materials and devices that show this value for $T_{2}^{*}$ is consistent with theoretical expectations for our estimated dot sizes and a natural abundance of $^{29}$Si. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressly or implied, of the United States Department of Defense or the U.S. Government. Approved for public release, distribution unlimited.\\[4pt] [1] B.~M. Maune et al., ``Coherent Singlet-Triplet Oscillations in a Silicon-based Double Quantum Dot,'' accepted by Nature.   

Simulating a solid state spin qubit in a spin bath

Powerful computational methods have been developed in recent years for understanding decoherence induced by environmental spins. Specifically, the cluster correlation expansion [Phys. Rev. B 78, 085315 (2008)] and adaptations [Phys. Rev. Lett. 105, 187602 (2010)] provide successive approximations that approach the solution to the full quantum mechanical problem for small and large spin baths with good efficiency. With these methods, we are able to study the nature of spin-bath decoherence in various regimes, for different types of qubits (e.g., donors or quantum dots) and for different types of spin baths (e.g., nuclei or electrons). Our quantitative analyses have implications for solid state spin qubit prospects and materials choices. Sandia National Laboratories is a multiprogram laboratory 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.   

Two-Spin Decoherence and Disentanglement in Semiconductor Nanostructures

A crucial issue in spin-based quantum information processing is spin coherence. Decoherence of a single electron spin confined in a quantum dot or to a donor ion has been studied extensively, with hyperfine interaction to the environmental nuclear spins being identified as the most important channel of spin decoherence [1]. Decoherence of two-spin-qubit states is inevitably affected by singe-spin decoherence. Moreover, for exchange-coupled spin qubits, there are new decoherence channels beyond those for single spins because of the Coulombic nature of the exchange interaction. Here we discuss a series of studies of two-spin decoherence mechanisms [2-6], including both known single-spin decoherence and relaxation channels due to nuclear spins and new channels based on electrostatic coupling. More specifically, we examine two-spin relaxation due to hyperfine interaction and phonon emission [2], spin-orbit interaction and phonon emission [3], nuclear spin dynamics mediated by electrons, charge noise [4,5], and electron-phonon interaction [6]. We also analyze the associated disentanglement of the two spins as the decoherence processes go on. Our results show that while nuclear spins affect two-spin states in a qualitatively similar manner as for single spin states, there are interesting new twists because of the weaker hyperfine interaction due to the electron orbital symmetry. On the other hand, the charge noise and phonon induced dephasing depends strongly on the electrical features of the nanostructure, and could pose a significant constraint on two-qubit gates and quantum computing schemes based on two-spin encoding. \\[4pt] [1] L. Cywinski, W. Witzel, and S. Das Sarma, Phys. Rev. B {\bf 79}, 245314 (2009). \newline [2] M. Borhani and X. Hu, Phys. Rev. B {\bf 82}, 241302R (2010). \newline [3] M. Borhani and X. Hu, arXiv:1110.2193. \newline [4] X. Hu and S. Das Sarma, Phys. Rev. Lett. {\bf 96}, 100501 (2006). \newline [5] D. Culcer, X. Hu, and S. Das Sarma, Appl. Phys. Lett. {\bf 95}, 073102 (2009). \newline [6] X. Hu, Phys. Rev. B {\bf 83}, 165322 (2011).   

Session W47: Focus Session: Polymers for Energy Storage and Conversion - Fundamentals of Ion Transport

Decoupling of Ionic Transport from Segmental Relaxation in Polymer Electrolytes

Polymer electrolytes provide elegant solutions to many difficulties in battery technology. However, their relatively low ionic conductivity has become the bottleneck for developing batteries with higher power density, shorter charging time, and better operations at low temperatures. In this work, we present detailed studies of the relationship between ionic conductivity and segmental relaxation in a set of specially-designed polymer electrolytes with systematic variation in chain rigidity. Our analysis shows that the ionic conductivity indeed can be decoupled from segmental dynamics in rigid polymers and the strength of the decoupling correlates with the fragility, but not with the glass transition temperature. These results call for a revision of the current picture of ionic transport in polymer electrolytes. We relate the observed decoupling phenomenon to frustration in packing of rigid polymers, which also affects their fragility. The principles demonstrated in this study may provide an alternative approach to design of highly conductive materials: by incorporating relatively rigid chain structures, it is possible to develop a new class of solid polymer electrolytes with strongly decoupled ionic conductivity.   

Ion Dynamics in Solid-State Polymer Electrolyte Electrochemical Cells using \textit{in situ} Time-Resolved Infrared Spectroscopy

Understanding ion transport in solid-state polymer electrochemical cells is of great interest for the advancement of cell efficacy. However, currently there is limited experimental knowledge of ion transport on a molecular level. In this study, we report a new spectroelectrochemical experimental technique that provides \textit{in situ} molecular level detail about cation and anion transport of an ionic liquid in solid-state polymer electrolyte electrochemical cells. In situ time-resolved Fourier transform infrared attenuated total reflectance (FTIR-ATR) spectroscopy was utilized to measure the time dependent accumulation of ions at the cathode and anode interface under an applied potential. The results show that the cation and anion of the ionic liquid both transport and accumulate at the cathode under dry conditions, but only the cation accumulates at the cathode under humid conditions. This experiment was coupled with electrochemical impedance spectroscopy to simultaneously measure overall charge transport and cyclic voltammograms.   

Ion Transport in Polymerized Ionic Liquid Block and Random Copolymers

Polymerized ionic liquid (PIL) block copolymers, a new type of solid-state polymer electrolyte, are of interest for energy conversion and storage devices, such as fuel cells, batteries, supercapacitors, and solar cells. In this study, a series of PIL diblock and random copolymers with various PIL compositions were synthesized. These consisted of an IL monomer and a non-ionic monomer, 1-[(2-methacryloyloxy)ethyl]-3-butylimidazolium bis(trifluoromethanesulfonyl)imide (MEBIm-TFSI) and methyl methacrylate (MMA), and 1-[(2-acryloyloxy)ethyl]-3-butylimidazolium bis(trifluoromethanesulfonyl)imide (AEBIm-TFSI) and styrene (S), respectively, were synthesized. The anion conductivity (ion transport) and morphology were measured in all of the polymers with EIS, SAXS/WAXS, and TEM. Ion transport in block copolymers are significantly higher than random copolymers at the same PIL composition and are highly dependent on the block copolymer nanostructure. The relationship between ion transport mechanisms and the phase behavior of these materials will be discussed.   

Ion conduction in phosphonium-polysiloxane ionomers

Low Tg ionomers with phosphonium cations covalently attached as side chains have potential application in energy conversion and storage devices. For example, alkaline fuel cells rely on membranes that transport hydroxide anions and some advanced batteries rely on membranes transporting fluoride anions. To better understand ion conduction in phosphonium-polysiloxane ionomers, allyl tributyl phosphonium bromide monomer was synthesized and, along with a vinyl ethylene oxide monomer, attached to polymethylhydrosiloxane by hydrosilylation. These ionomers maintain low Tg $\approx $ -74 $^{\circ}$C with up to 10 mol{\%} phosphonium and are fully water soluble, allowing easy anion exchange and purification. We report dielectric spectroscopy results for these ionomers with a variety of counter-anions. Electrode polarization at low frequencies is analyzed to determine the number density of simultaneously conducting counter ions and their mobility. This analysis reveals higher mobility and lower activation energy for conducting anions that are larger and more diffuse, such as bis(trifluoromethane sulfonyl)imide, contributing to better performance as anion-conducting membranes.   

Ion Conduction in Polymerized Ionic Liquid Thin Films

The ion conductivity in thin films is typically assumed to be isotropic. We have developed methods to measure in-plane and through-plane ionic conductivity in thin homopolymer and block copolymers. Specifically, we are studying the conductivity in imidazolium-containing polymerized ionic liquids as a function of film thickness. These data will be compared to conductivity measurements in a triblock copolymer with a polymerized ionic liquid midblock.   

Cross-linking of Ordered Pluronic/Ionic Liquid Blends for Solid Polymer Electrolytes

Ion gels were fabricated by cross-linking PPO-PEO-PPO triblock copolymers swollen in a room temperature ionic liquid (IL). The copolymers are modified by esterification to replace the terminal hydroxyl endgroups with methacrylate endgroups. This allows the copolymer/IL blends to be cross-linked by a UV cure, forming a gel. The strong interaction of the IL with the PEO block suppresses PEO crystallization which is necessary for good ion conduction. In addition, the interaction between the IL and PEO is strongly selective for PEO, strengthening microphase separation. Despite this, the low molecular weight copolymers remain disordered in the melt even when blended with the IL. However, high molecular weight copolymers are capable of microphase separating into highly ordered block copolymer morphologies. This difference allows the effect of microphase separation on ion transport to be studied. The effect of block copolymer composition is also studied, by varying the PEO fraction of the copolymer. The resultant gels show high ionic conductivity and solid-like behavior, indicating that these materials may be effective as solid polymer electrolytes.   

Ionomer Design Principles for Single Ion-Conducting Energy Materials

Single-ion conducting ionomers with low glass transition temperature, high dielectric constant and containing bulky ions with diffuse charge, are needed for polymer membranes that transport small counterions. Overarching design principles emerging from quantum chemistry calculations suggest that diffuse charge can be attained from simple considerations of atomic electronegativity. For lithium or sodium batteries, perfluorinated tetraphenyl borate ionomers with solvating polar comonomers are proposed. For fluoride or hydroxide batteries and for iodide transporting solar cells, tetra-alkyl phosphonium ionomers with anion receptors are proposed. First attempts to construct such ionomers to test these ideas will be discussed, with results from dielectric spectroscopy to measure conductivity, dielectric constant and number density of simultaneously conducting ions.   

Ion Dynamics in Model Ionomer Melts as a Function of Polymer Architecture

Ionomers, polymers with a small fraction of covalently bound ionic groups, have potential advantages as solid, single ion conducting electrolytes in future batteries. However, the strong electrostatic interactions in these materials can make counterion diffusion unacceptably slow. Understanding how controllable molecular properties affect ionomer dynamics could spur design of improved materials. With this goal, we perform molecular dynamics simulations of ionomers of various architectures and evaluate their dynamic behavior. Our model of coarse-grained polymers with explicit counterions captures the fundamental physics while allowing access to the long time and length scales relevant to ionomer melts. The simulated structure factors reproduce the trends found in experimental scattering of recently synthesized ionomers with controlled precise or pseudorandom spacing of charged groups along the chain. We calculate counterion diffusion constants and other dynamic properties, which can change significantly depending on the location and spacing of charges along the chain. Randomly spaced materials can have slower or faster dynamics than periodically spaced materials, depending on whether the sequence is completely random or pseudorandom, which will be discussed.   

Ion Conduction and Dielectric Response of Imidazolium-based Single-ion Conductors

We synthesized ionomers with imidazolium cations covalently attached as side groups with various ionic liquid counter-anions. Since these ionic polymers are single-ion conductors that are potentially useful for ionic actuators, it is of great interest to understand structure-property relations, such as the effect of different counterions and different imidazolium pendant structures, including tail and side chain lengths. Conductivities and dielectric properties of a range of monomers and polymers containing ionic liquid moieties are compared. The effects of counterions and side chain length are clearly observed in the T$_{g}$ and ionic conductivity: larger anions and/or longer side chains lead to lower T$_{g}$ and higher conductivity than smaller anions and/or shorter side chains. However, if the tail becomes too long (12 carbons) it facilitates - ion aggregation with a significantly lower dielectric constant and lower mobility for the conducting ions. Our study of counter-anions and polymer structural variations leads to insight regarding optimal design of imidazolium single-ion conductors for facile ion transport.   

Effect of matrix crystallinity on the ionic conductivity in microstructured block copolymer solid electrolytes

Polyethylene oxide (PEO)-based block copolymers have been studied extensively for use as solid electrolytes for rechargeable lithium metal batteries. Previous work has concentrated on block copolymers containing an amorphous second block, such as polystyrene, for which the modulus is sufficiently high to resist growth of dendrites that would lead to short circuiting. In this work, we instead focus on using semicrystalline polyethylene as the mechanically robust component. Polyethylene-polyethylene oxide (EEO) block copolymers doped with lithium bis(trifluoromethanesulfone) imide (LiTFSI) were characterized using AC impedance spectroscopy over a range of temperature, molecular weight, and composition values in order to determine the effect of crystallinity in the structural microphase on the conductivity of this material.   

Solvation Effects on Counterion Transport in Single-Ion Conducting Ionomers

Ionomers with short ethylene oxide side chains are synthesized by free radical polymerization, to systematically test effects of solvating anion and cation, including directly substituting the ion attached to the polymer with its counterion. Dielectric relaxation spectroscopy is used to measure the conductivity, dielectric constant and segmental relaxations in these ionomers and the electrode polarization at very low frequencies is used to assess the number density of simultaneously conducting ions and their mobility. Conductivity and conducting ion content are larger for polyanions than their corresponding polycation because the counterion can be more effectively solvated by the ether oxygens. Changing ester linkages to amide linkages in polycations boosts conductivity and conducting ion content by solvating the anionic counterion. Such findings point a clear path toward design of superior single-ion conductors.   

Correlation between superionic behavior and ion aggregation in PEO-based single-ion conductors

PEO-based ionomers as solid polymer electrolytes offer the advantage of preventing reverse polarization in batteries by covalently bonding the anion to the PEO backbone. These ionomers form ion aggregates, which reduces the polymer mobility. Since ion transport is coupled to polymer dynamics, these systems have low conductivity. We therefore need a mechanism that decouples ion conduction and PEO dynamics to improve conductivity. We investigate these conduction mechanisms using different models in MD simulations of PEO based benzene sulfonate ionomers. The study shows that these ionomers are capable of showing superionic behavior. The geometry of benzene rings helps aligning the anions, assisting in the formation of chain-like aggregates. The simulations show that the chain-like aggregates result in ionomer conductivity greater than its self-diffusion limits. The superionic behavior is attributed to a charge transfer between two chain ends (conduction sites): a cation hopping to one chain end and the cation at the other end hopping to a nearby site. This allows long range positive charge transfer while the cations only move locally. The results suggest that the superionic behavior depends on the length and lifetime of the chain aggregates. While a long chain reduces the overall number of conduction sites, a short chain prevents long range charge transfer. If the lifetime of an aggregate is shorter than the hopping time for cations, the hopping will not occur and the self-diffusion dominates conductivity.   

Simulation of Ionic Aggregation and Ion Dynamics in Model Ionomers

Ionomers, polymers containing a small fraction of covalently bound ionic groups, are of interest as possible electrolytes in batteries. A single-ion conducting polymer electrolyte would be safer and have higher efficiency than the currently-used liquid electrolytes. However, to date ionomeric materials do not have sufficiently high conductivities for practical application. This is most likely because the ions tend to form aggregates, leading to slow ion transport. A key question is therefore how molecular structure affects the ionic aggregation and ion dynamics. To probe these structure-property relationships, we have performed molecular simulations of a set of recently synthesized poly(ethylene-co-acrylic acid) copolymers and ionomers, with a focus on the morphology of the ionic aggregates. The ionomers have a precise, constant spacing of charged groups, making them ideal for direct comparisons with simulations. {\em Ab initio} calculations give insight into the expected coordination of cations with fragments of the ionomers. All-atom molecular dynamics (MD) simulations of the ionomer melt show aggregation of the ionic groups into extended string-like clusters. An extensive set of coarse-grained molecular dynamics simulations extend the results to longer times and larger length scales. The structure factors calculated from the MD simulations compare favorably with x-ray scattering data. Furthermore, the simulations give a detailed picture of the sizes, shapes, and composition of the ionic aggregates, and how they depend on polymer architecture. Implications for ion transport will be discussed. [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.]   

Session W48: New Experimental, Theoretical, and Computational Methods in Polymer and Soft Matter Physics

Coherent States Formulation of Polymer Field Theory

We developed a coherent-states formulation of equilibrium polymer field theory. Compared with the traditional Edwards' auxiliary field framework that underpins both field-theoretic simulation (FTS) and self-consistent field theory (SCFT) methods, this formulation has a number of attractive features, including much more local operators and a finite order polynomial action. The formalism is developed in the grand canonical ensemble for the Edwards model of polymers in an implicit solvent, and we show how to derive a numerically tractable scheme. We explore the efficiency and stability of the method in mean-field and~fully fluctuating simulations for a polymer solution confined to a~slit in one dimension.   

How Reliable Are Soft Potentials? Ensuring Thermodynamic Consistency Between Hierarchical Models of Polymer Melts

The use of soft effective potentials to represent macromolecular systems has become widespread in the areas of biophysics and materials science. A survey of the field reveals a vast array of various phenomenological potentials whose ability to provide quantitative information about several different properties of the same system is not evident. This talk will present a formally sound approach to obtain soft potentials for realistic models of simple linear polymer melts which reproduce the correct center of mass distribution of particles as well as the correct equation of state of the underlying system of interest. Furthermore, an analytical potential allows us to rigorously address the implications of coarse-graining on the entropy and free energies of the system and to account for the reduced degrees of freedom and smoothed energy landscape implicit to coarse-grained models. Finally, the transferability of the method to other systems and potential applications will be discussed.   

Simulation of Stress Induced Polymer-Polymer Interfacial Slip

The phenomena of stress-induced tangential slip at polymer-polymer interfaces is studied via a slip-link simulation technique. Simulations combine a slip-link model of entanglement with a self-consistent field description of the chemical potential landscape near an interface. We consider how the slip velocity depends upon shear stress, interfacial entanglement density, and polymer chain length. Our model is based on the idea that the strongly non-linear shear thinning of the interface observed in experiment is a result of stress-induced convective release (pulling-out) of entanglements across the interface.   

Tumbling dynamics of isolated polymer chains in strong shear flows and the effects of chain resolution

Using Brownian dynamics simulations, without hydrodynamic and excluded volume interactions, on polymer chain models encompassing a wide range of resolutions, we present a detailed investigation on the behavior of isolated chains in shear flow. We find a highly non-monotonic behavior for all models, with chain compression occurring at ultra-high shear rates that is consistent with the recent simulation studies. However, results obtained using highly refined models, with resolutions lower than a Kuhn step, reveal that this transition is an artifact of the level of chain discretization. Also, our results clearly indicate that, at high shear rates, the chain thickness in the shear-gradient direction is independent of the chain length, which differ from previously reported scaling law. We show that the chain thickness is fixed by the distance a sub-section of the chain can diffuse in the shear-gradient direction before convection stretches it out and suppresses further diffusion. Simple physical arguments are then used to derive the correct scaling laws for the coil width and the tumbling time at high shear rates. We believe that our findings presented here will provide the foundation for a better understanding of this basic problem in polymer dynamics.   

A Mesoscale Simulation Method for High Salt Concentrations

In mesoscale simulations of electrolyte solutions the interplay between electrostatics and hydrodynamics plays the critical role for computational efficiency and accuracy. In the past it became apparent that high salt concentrations are too costly, if every salt ion is treated explicitly as a separate particle. On the other hand, charges are highly screened at high salt concentrations and ion-ion correlations are less important than in the low-salt limit. Therefore, we have developed a dynamic mean-field treatment of charges which is expected to be perfectly sufficient in many cases leading to applications in controlled manipulation of polyelectrolytes and charged colloids by external electric fields.   

Molten Salt Eutectics from Atomistic Alchemical Simulations

Molten salt mixtures are gaining importance in solar thermal power applications. Unfortunately, their phase diagrams cannot be easily computed from first principles calculations. The eutectic mixture composition and temperature are located using the liquid mixture free energy and the pure component solid-liquid free energy differences. The liquid mixture free energy is obtained using thermodynamic integration of alchemical transformations of one atom to another. Numerical results for binary and ternary mixtures of alkali nitrates agree well with experimental measurements [1]. Some results involving mixtures of monovalent and divalent cations will also be presented.\\[4pt] [1] Jayaraman, Thompson, and von Lilienfeld, PRE, 84, 030201 (2011). \\[4pt] 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.   

An [N]PT Ensemble for Studying the Intricate Thermodynamics of Cluster Crystals

Exotic soft matter systems such as certain dendrimers can overlap with only a finite energy penalty. Systems governed by steep, bounded repulsive interactions, such as the penetrable sphere model (PSM), indeed form cluster crystals with multiple particles per lattice site under compression. Standard simulation approaches that keep $NVT$, $NPT$, or $\mu V T$ constant cannot directly equilibrate cluster crystals, because either $N$ or lattice constant is not free to relax. It is tempting to allow all fields to fluctuate by keeping ``$\mu P T$'' constant, but basic thermodynamics indicates that infinite fluctuations then result. We avoid this caveat by using a $[N]PT$ ensemble, in which $P$ and $T$ are fixed but $N$ is allowed to fluctuate within bounds $[N_{\mathrm{min}},N_{\mathrm{max}}]$ under a conjugate field analogous to $\mu$. The approach provides the equilibrium properties of neighboring state points through histogram reweighing. We solve the phase diagram of the PSM, and confirm that the FCC crystal lattice occupancy linearly increases with $\rho$ at high $T$. At low $T$ the occupancy plateaus at integer values, but the transitions remain continuous and the crystal does not phase separate. We also examine the critical behavior of another related model.   

From nucleus to phase: Growing dynamics of critical nucleus in polymer blends

In metastable system, nonclassical critical nucleus is not a drop of stable bulk in core wrapped with a sharp interface, but a diffuse structure depending on the metastability. Thus, forming a critical nucleus does not mean the birth of a new phase. Also, the dynamics of nucleus' growing before the phase is born is still unknown. In present work, the nonclassical growing dynamics of the critical nucleus is addressed in the metastable polymer blends by incorporating self-consistent field theory and external potential dynamics theory, and leads to an intuitionistic description for the results of scattering experiments. Our results suggest that the growth of nonclassical critical nucleus is not self-similarly, but forms shell structure, which gives the scattering peak at nonzero wavenumber in the experiments. This phenomenon comes from the spinodal-decomposition-like behaviors constrained within the critical nucleus. The growing dynamics of the critical nucleus can be considered as a spinodal-assistant nucleation process.   

An Interactive 3D Interface to Model Complex Surfaces and Simulate Grazing Incidence X-ray Scatter Patterns

Grazing Incidence Scattering is becoming critical in characterization of the ensemble statistical properties of complex layered and nano structured thin films systems over length scales of centimeters. A major bottleneck in the widespread implementation of these techniques is the quantitative interpretation of the complicated grazing incidence scatter. To fill this gap, we present the development of a new interactive program to model complex nano-structured and layered systems for efficient grazing incidence scattering calculation.   

Numerical Computation of Diffusion Properties in Molecular Systems on a Topology-Conforming Grid

Multiscale problems involving diffusion in molecular systems are a mainstay of computational biophysics. Given a molecular system, the local diffusion coefficient $D(\bf{r})$ as well as the equilibrium distribution function $P(\bf{r})$ that characterizes the local free energy are computed to describe the kinetics of diffusing particles at each point in space through the Smoluchowski equation (SE). An irregular grid of space-varying fineness conforming to $P(\bf{r})$ is generated via the method of topology-representing networks and a subsequent Voronoi tessellation. The discretized SE produces a rate matrix which describes the probabilities of particles hopping from point to point on the grid. We demonstrate the calculation of the rate matrix for ions diffusing through the balloon-like structure of the mechanosensitive channel of small conductance (MscS) and thence the determination of mean first-passage times that characterize conduction of ions through balloon and channel.   

Fluorescent excitation transfer as a tool for the phase transition studies

The use of the fluorescent resonant excitation transfer technique (FRET) to study the phase transition kinetics is demonstrated. The laser temperature jump is applied to the water/2,6-lutidine mixture and causes the demixing. Coumarin 480 and hydroxypyrene laser dyes form excitation transfer that interrogates the spatial structure of the system. Due to the differential solubility of these dyes in the components of the mixture, the excitation transfer ceases once the phase separation occurs. The spatial resolution of the method is determined by the Forster distance of the excitation transfer pair, and in this case is equal to 3 nm. The phase separation is completed within 1 microsecond. The rising edge of the fluorescence is consistent with polynomial growth of the phase separated domains, and not with Cahn-Hilliard fixed length instability. The theoretical model for the excitation transfer in a variety of systems such as separation of binary mixture, phase reorganization of membranes, formation of lamellar structure is developed.   

The Physics of Phase-Separation Fronts

When phase separation does occur in a sequential manner, e.g. due to the diffusion of a control parameter into the system, the resulting phase-separation structures have a markedly different appearance than homogeneous phase-separation patterns. The reason lies in the influence the already phase-separated material and the recent phase-separation dynamics exert on the newly phase-separating material. We call the region where phase-separation first occurs the phase-separation front. In this talk we will consider the simplest possible phase-separation front, i.e. a sharp transition in the control parameter moving through the system with a prescribed velocity inducing phase-separation. We show numerical and analytical solutions for the structures that are formed by such a front as a function of the front speed and the composition of the overtaken material.   

Protected TERS Probes for the Study of Polymer Surfaces

The chemistry of polymer surfaces can potentially be imaged with high ($\sim $20nm) lateral resolution using Tip Enhanced Raman Spectroscopy (TERS). The method's applicability can be tremendously broadened if the metallized tips central to the technique can be made more robust. Protecting the TERS probes with alumina coatings reduces chemical, mechanical, and thermal degradation, prolonging the use lifetime. Most recently we have focused on the detailed effect of the protective coating for cases in which the enhancement is particularly strong. ``Blinking'', which is characteristic of extreme enhancement, has been observed with TERS on polymer films for the first time. An alumina coating prolongs the duration of blinking from 20 min to 30 h, greatly extending the experimental window of extreme enhancement. The protective coating is also helpful for illuminating the mechanism behind blinking. Our results are consistent with thermal diffusion of molecules as the major mechanism facilitating blinking.   

Session W49: Focus Session: Organic Electronics and Photonics - Thin Film Transistors

Pressure effect on organic field-effect transistors

Macroscopic transport in organic semiconductors is governed by intermolecular charge transfer, necessarily resulting in its sensitivity to molecular arrangement. The effect of external pressure in such soft materials is fundamentally important because of vulnerability in molecular displacement against relatively small force. Here, we introduce a method of measuring the effect of hydrostatic pressure on the conductivity in organic semiconductor crystals inducing high-mobility charge with the application of electric field. We performed four-terminal conductivity measurement to exclude extrinsic influence of the metal/semiconductor contact resistance. In addition, Hall coefficients are simultaneously measured to deduce the pressure coefficient properly. Using rubrene single-crystal transistors, variation of mobility under pressure turned out to be about 7 times larger than in the typical experiments reported for silicon and other inorganic semiconductors. Interestingly, the mobility starts to decrease with further increasing pressure above 600 MPa. The anomalous negative pressure effect indicate that the application of pressure not only diminishes distance between centers of adjacent molecules but relative positions of equivalent atoms in the two molecules.   

Determining the elastic constants of rubrene single-crystals

Organic single crystals have opened the doors to a new generation of high-performance organic electronic devices. Exceptional charge-transport properties combined with the advent of large-area patterning techniques make organic single crystals excellent candidates for flexible electronics applications. However, in order to effectively employ organic single crystals on mechanically flexible architectures, their mechanical properties need to be understood and characterized. In this presentation, the mechanical properties of rubrene single-crystals are investigated. Given the limited dimensions of as-grown crystals and associated handling difficulty, the elastic buckling instability is chosen as a metrology tool for determining the in-plane elastic constants. Our results show that ultrathin (200nm - 1000nm) rubrene crystals exhibit anisotropic wrinkling wavelengths as a function of crystallographic direction, which can be correlated to the anisotropic nature of its molecular packing. An adaptive intermolecular reactive bond order potential (AIREBO) is employed to calculate the nine elastic constants corresponding to orthorhombic rubrene.   

Tuning contorted hexabenzocoronene crystal structure and texture for organic field-effect transistors

Crystallography conducted on single crystals reveals contorted hexabenzocoronene (HBC) can adopt either herringbone (Pbcn) or slip-stack (P21/c) packing motifs. By adjusting the molecule-solvent interactions during solvent-vapor annealing (SVA), we can controllably crystallize thin films of HBC and access both packing motifs. In HBC films annealed with dichloromethane (DCM) vapor, molecule-solvent interactions are strong and yield highly oriented Pbcn crystals. However, in films annealed with hexanes vapor, molecule-solvent interactions are weaker and randomly oriented P21/c crystals form. In addition to tuning the molecule-solvent interactions via solvent choice, the interactions may also be modulated by selectively fluorinating the peripheral aromatic rings of HBC. With increased fluorination, we decrease molecule-solvent interactions during SVA. As such, we can coax these HBC derivatives to adopt the P21/c crystal structure even with DCM SVA. Further, more fluorinated HBCs form more oriented crystals when exposed to DCM vapors. Transistors fabricated with crystalline HBC active layers suggest that the mobilities of these devices are, to first order, determined by the extent of crystal orientation and less so by the crystal structure. The ability to independently access both crystal structures with varying degrees of orientation has allowed us to decouple their relative contributions to device performance.   

Quantitative assessments of the effect of microstructure on transport in organic semiconductors

From the fundamental standpoint, organic semiconductors are fascinating as they are neither crystalline nor amorphous and their microstructure plays a central role in governing charge transport. I will show that understanding disorder is the key to determining charge transport mechanism. We are particularly interested in cumulative disorder (paracrystallinity), where the lattice parameter takes on a Gaussian distribution about its mean value. The disorder parameter g allows us to rank materials quantitatively on a continuous scale, from a perfectly crystalline material (g $<$ 1\%) to an amorphous one (g $>$ 10\%). Surprisingly, even the polymers that are considered to have the highest crystallinity (PBTTT) have a g in the pi-stacking direction close to that of an amorphous material ($\sim$7\%). Furthermore, comparison of X-ray diffraction data and optical absorption data provides insight into the nature of the disordered phase as well. Using first principle calculations and tight binding methods, I will show that paracrystallinity in the pi-stacking direction provides a fundamental mechanism for the existence of an exponential distribution of localized tail states in the gap. The larger the degree of disorder the higher the trap density and the deeper their energy. Using disorder as a ranking parameter, I will discuss the differences in transport between small molecule and polymer films as well as their respective inherent limitations and bottlenecks.   

Air-Stable Solution-Processed Thin-Thin Film Transistors with Hole Mobilities of 3.5 cm$^{2}$/Vs

We report on organic thin-film transistors fabricated on a novel soluble small molecule organic semiconductor difluoro bis(triethylgermyl) anthradithiophene. Fabrication techniques are all applicable at room temperature and ambient pressure, and include drop-casting, spin-coating, and spray deposition. Devices exhibit remarkable electronic properties, including charge carrier mobilities as high as 3.5 cm$^{2}$/Vs, on/off current ratios of 10$^{5}$, and good environmental and operational stability. Chemical treatment of the contact surface with self-assembled monolayers allows us great control of the crystalline order within the organic semiconductor layer. Because thin-film microstructure defects such as grain boundaries reduce the charge transport capabilities of the active layer, high quality single crystals are grown by physical vapor transport for comparison. By correlating the electrical properties with the structural data obtained from X-ray diffraction, we find that a good $\pi -\pi $ overlap is responsible for this superior electronic behavior.   

Variable-range hopping transport and Hall effect measurements in electrolyte-gated P3HT

Extensive charge transport measurements (1.5 -- 250 K) at gate-tuned hole concentrations between 1*1$^{20}$ and 9*10$^{20}$ cm$^{-3}$ have been made on a single ion-gel gated poly-(3-hexylthiophene) (P3HT) thin film transistor. We report observation of a robust Hall effect, having rational trends with magnetic field, gate voltage, and temperature, and yielding hole concentrations close to those measured via the charging current. At high doping we observe transitions from apparent band transport, to 3D Mott variable range hopping (VRH), to Efros-Shklovskii (ES) VRH on cooling. At lower doping ES VRH is observed at all temperatures. A detailed analysis of the temperature and field-dependence of the VRH resistivity provides information on the localization length and dielectric constant as a function of doping, providing significant insights into the approach to the insulator-metal transition in this system and the nature of the Coulomb-gapped density of states. Work at UMN supported by NSF MRSEC   

Charge Density Dependent Hole Mobility and Density of States Throughout the Entire Finite Potential Window of Conductivity in Ionic Liquid Gated Poly(3-hexylthiophene)

Ionic liquids, used in place of traditional gate dielectric materials, allow for the accumulation of very high 2D and 3D charge densities ($>$10$^{14}$ {\#}/cm$^{2}$ and $>$10$^{21}$ {\#}/cm$^{3}$ respectively) at low voltage ($<$5 V). Here we study the electrochemical gating of the benchmark semiconducting polymer poly(3-hexylthiophene) (P3HT) with the ionic liquid 1-ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate ([EMI][FAP]). The electrochemical stability of [EMI][FAP] allowed the reproducible accumulation of 2 x 10$^{21}$ hole/cm$^{3}$, or one hole (and stabilizing anion dopant) per every two thiophene rings. A finite potential/charge density window of high electrical conductivity was observed with hole mobility reaching a maximum of 0.86 cm$^{2}$/V s at 0.12 holes per thiophene ring. Displacement current measurements, collected versus a calibrated reference electrode, allowed the mapping of the highly structured and extremely broad density of states of the P3HT/[EMI][FAP] doped composite. Variable temperature and charge density hole transport measurements revealed hole transport to be thermally activated and non-monotonic, displaying a activation energy minimum of $\sim $20 meV in the region of maximum conductivity and hole mobility. To show the generality of this result, the study was extended to an additional four ionic liquids and three semiconducting polymers.   

Conductivity Maxima on the Surface of Organic Semiconductor Crystals at High Charge Densities

We have previously achieved effective carrier mobility up to 3.2 cm$^{2}$V$^{-1}$s$^{-1}$ at charge densities larger than 10$^{13}$ cm$^{-2}$ in rubrene electrical double layer transistors (EDLTs) gated with ionic liquids (ILs). At lower temperatures when a larger gate bias can be applied, the maximum attainable charge density reaches 6.5*10$^{13}$ cm$^{-2}$ (0.34 holes per rubrene molecule), and more remarkably, two pronounced maxima in channel conductivity have been reproducibly and stably observed. This feature, which has not been reported for any EDLTs gated with electrolytes, is independent of ionic liquid composition, current-voltage sweep rate, and crystallographic directions of rubrene crystals. We have identified that the first and second conductivity peaks occur at charge densities of 2.0*10$^{13}$ cm$^{-2}$ and 5.2*10$^{13}$ cm$^{-2}$, respectively. Capacitance-voltage (C-V) measurements at different frequencies have also revealed two maxima at the same gate voltages as in current-voltage measurements. Collectively, these observations imply that the conductivity maxima at high charge densities are very likely related to the electronic band structure on the surface of rubrene crystals.   

Solvent-Resistant Organic Transistors and Thermally-Stable Organic Photovoltaics Based on Crosslinkable Conjugated Polymers

Conjugated polymers in general are unstable when exposed to air, solvent, or thermal treatment, and these challenges limit their practical applications. Herein we have developed a simple, but powerful approach to achieve solvent-resistant and thermally stable organic electronic devices with improved air-stability, by introducing a crosslinkable group into a conjugated polymer. To demonstrate this concept, we have synthesized polythiophene with crosslinkable groups attached to the end of alkyl chain. Photo-crosslinking of crosslinkable P3HT dramatically improves the solvent resistance of the active layer without disrupting the molecular ordering and charge transport. This is the first demonstration of solvent-resistant organic transistors. Furthermore, the bulk-heterojunction organic photovoltaics (BHJ OPVs) containing crosslinkable P3HT show an average efficiency higher than 3.3{\%} after 40 h annealing at an elevated temperature of 150$^{\circ}$C, which represents one of the most thermally-stable OPV devices reported to date. This enhanced stability is due to an in-situ compatibilizer that forms at the P3HT/PCBM interface and suppresses macrophase separation.   

Surface enhanced Raman spectroscopic studies of the metal-semiconductor interface in organic field effect transistors

The performance of organic field-effect transistors (FETs) largely depends on the nature of interfaces of dissimilar materials. Metal-semiconductor interfaces, in particular, play a critical role in the charge injection process. Here, Raman spectroscopy is used to investigate the nature of the Au-semiconductor interface in pentacene based FETs. A large enhancement in the Raman intensity (SERS) is observed from the pentacene film under the Au layer. The enhancement is evidence of a nano-scale roughness in the morphology of the interface, which is further confirmed by electron microscopy images. The morphology of the interface is investigated by SERS as a function of the pentacene layer thickness and the Au layer thickness. The Raman spectra are found to be extremely sensitive in detecting small changes in the morphology of the interface in the sub-nanometer range. Changes in the Raman spectra are further tracked after biasing and ageing the devices. Evolution of these Raman spectra is correlated with degradation in device performance. Finally, FETs based on other donor-acceptor semiconductors are probed by Raman scattering and contrasted with those of the pentacene-based devices.   

Ultra-thin body poly(3-hexylthiophene) transistors with improved short-channel performance

The microstructure and charge transport properties of binary blend of regioregiolar (rr) and regiorandom(RRa) poly(3-hexylthiophene) (P3HT) are investigated. X-ray diffraction study shows vertical phase separation in the blend films, with rr-P3HT crystallized at the semiconductor/dielectric interface. These thin film transistors with layered structure preserve high field effect mobility when rr-P3HTcontents are reduced to as low as 5.6{\%} where the channel thickness is only a few nanometers. As a result of this ultra-thin active layer at interface, short channel effects due to bulk currents are eliminated, suggesting a new route to fabricate high performance, small size and reliable electronic devices.   

Physical Characterization of Functionalized Silk Material for Electronic Application and Devices

Naturally harvested spider silk fibers are investigated for their physical properties under ambient, humidified, iodine-doped, pyrolized, sputtered gold and carbon nanotube coated conditions. The functional properties include: humidity activated conductivity; enhanced flexibility and carbon yield of pyrolized iodized silk fibers; full metallic conductivity and flexibility of micron-sized gold-sputtered silk fibers; and high strain sensitivity of carbon nanotube coated silk fibers. Magic angle spinning nuclear magnetic resonance (MAS-NMR) and Fourier transform infrared spectroscopy (FTIR) are used to explore the nature of ambient and functionalized spider silk fiber, and significant changes in amino acid-protein backbone signature are correlated with gold sputtering, and iodine-doped conditions. The application of gold-sputtered neat spider silk fibers for making four terminal flexible, clean, ohmic contacts to organic superconductor samples and carbon nanotube coated silk fibers for heart pulse monitoring sensor are demonstrated. The role of silk thin film in organic thin film transistor will be briefly discussed.   

Spectroscopic signatures of ambipolar injection in narrow gap donor-acceptor polymer transistors

Donor-Acceptor (D-A) copolymers have recently emerged as versatile materials for use in a large variety of device applications. Specifically, these systems possess extremely narrow bandgaps, enabling ambipolar charge transport when integrated in solution-processed field-effect transistors (OFETs). However, the fundamentals of electronic transport in this class of materials remain unexplored. We present a systematic investigation of ambipolar charge injection in D-A conjugated polymers polybenzobisthiadiazole-dithienopyrrole (PBBTPD) and polybenzobisthiadiazole-dithienocyclopentane (PBBTCD) using infrared spectroscopy. We observed a significant modification of the absorption edge in both PBBTPD- and PBBTCD-based OFETs under the applied electric field. The absorption edge reveals hardening under electron injection and softening under hole injection. The most straightforward interpretation of the observed band edge modification is in terms of the linear Stark effect, implying the existence of a built-in electrical dipole moment in these polymers. Additionally, we carried out microscopic IR measurements to characterize the ambipolar injection profile between electrodes.   

Session W50: Focus Session: Micro and Nano Fluidics I: Devices and Applications

Self-Sorting of Deformable Particles in a Microfluidic Circuit

Sorting of cells, droplets, and particles based on physical characteristics including size and deformability is important for bioseparation, diagnostics, and two-phase microfluidics. While several methods have been developed to sort particles based on size, few techniques exist for sorting of particles based on deformability. Here, we present a microfluidic circuit that enables self-sorting of deformable particles based on the hydraulic resistance that the particle induces in a microchannel, which directly relates to the particle deformability. The present method employs a feed-forward circuit that biases a microfluidic Y-junction based on the hydraulic resistance induced by the particle as it enters a sensing channel. Since particles encountering a symmetric junction follow the branch with the higher flow rate, the resulting modulation of fluid flow at the junction switches the particle into one of two output channels depending on the resistance induced by the particle. Since hydraulic resistance can be influenced by particle-wall interactions, it also opens possibilities for functionalizing the sensing channel for sorting based on specific interactions. This technique may find use in cell sorting and analysis and in two-phase microfluidics.   

Accelerating Yeast Prion Biology using Droplet Microfluidics

Prions are infectious proteins in a misfolded form, that can induce normal proteins to take the misfolded state. Yeast prions are relevant, as a model of human prion diseases, and interesting from an evolutionary standpoint. Prions may also be a form of epigenetic inheritance, which allow yeast to adapt to stressful conditions at rates exceeding those of random mutations and propagate that adaptation to their offspring. Encapsulation of yeast in droplet microfluidic devices enables high-throughput measurements with single cell resolution, which would not be feasible using bulk methods. Millions of populations of yeast can be screened to obtain reliable measurements of prion induction and loss rates. The population dynamics of clonal yeast, when a fraction of the cells are prion expressing, can be elucidated. Furthermore, the mechanism by which certain strains of bacteria induce yeast to express prions in the wild can be deduced. Integrating the disparate fields of prion biology and droplet microfluidics reveals a more complete picture of how prions may be more than just diseases and play a functional role in yeast.   

Microfluidic devices for droplet injection

As picoliter-scale reaction vessels, microfluidic water-in-oil emulsions have found application for high-throughput, large-sample number analyses. Often, the biological or chemical system under investigation needs to be encapsulated into droplets to prevent cross contamination prior to the introduction of reaction reagents. Previous techniques of picoinjection or droplet synchronization and merging enable the addition of reagents to individual droplets, but present limitations on what can be added to each droplet. We present microfluidic devices that couple the strengths of picoinjection and droplet merging, allowing us to selectively add precise volume to our droplet reactions.   

Microfluidic devices for label-free separation of cells through transient interaction with asymmetric receptor patterns

Cell sorting serves an important role in clinical diagnosis and biological research. Most of the existing microscale sorting techniques are either non-specific to antigen type or rely on capturing cells making sample recovery difficult. We demonstrate a simple; yet effective technique for isolating cells in an antigen specific manner by using transient interactions of the cell surface antigens with asymmetric receptor patterned surface. Using microfluidic devices incorporating P-selectin patterns we demonstrate separation of HL60 cells from K562 cells. We achieved a sorting purity above 90{\%} and efficiency greater than 85{\%} with this system. We also present a mathematical model incorporating flow mediated and adhesion mediated transport of cells in the microchannel that can be used to predict the performance of these devices. Lastly, we demonstrate the clinical significance of the method by demonstrating single step separation of neutrophils from whole blood. When whole blood is introduced in the device, the granulocyte population gets separated exclusively yielding neutrophils of high purity ($<$10{\%} RBC contamination). To our knowledge, this is the first ever demonstration of continuous label free sorting of neutrophils from whole blood. We believe this technology will be useful in developing point-of-care diagnostic devices and also for a host of cell sorting applications.   

Role of Structural Asymmetry in Controlling Drop Spacing in Microfluidic Ladder Networks

Manipulation of drop spacing is crucial to many processes in microfluidic devices including drop coalescence, detection and storage. Microfluidic ladder networks ---where two droplet-carrying parallel channels are connected by narrow bypass channels through which the motion of drops is forbidden---have been proposed as a means to control relative separation between pairs of drops. Prior studies in microfluidic ladder networks with vertical bypasses, which possess fore-aft structural symmetry, have revealed that pairs of drops can only undergo reduction in drop spacing at the ladder exit. We investigate the dynamics of drops in microfluidic ladder networks with both vertical and slanted bypasses. Our analytical results indicate that unlike symmetric ladder networks, structural asymmetry introduced by a single slanted bypass can be used to modulate the relative spacing between drops, enabling them to contract, synchronize, expand or even flip at the ladder exit. Our experiments confirm all the behaviors predicted by theory. Numerical analysis further shows that ladders containing several identical bypasses can only linearly transform the input drop spacing. Finally, we find that ladders with specific combinations of vertical and slanted bypasses can generate non-linear transformation of input drop spacing, despite the absence of drop decision-making events at the bypass junctions.   

Synchronized Reinjection and Coalescence of Droplets in Microfluidics

In droplet-based microfluidics, one of the essential techniques is controlled addition of desired materials into the droplets. This is best achieved through the coalescence of pairs of droplets, and therefore various methods of coalescence have been developed over the last decade. However, the coalescence of two different droplets made independently in different devices still remains a challenging problem, primarily because it is difficult to synchronize the reinjection of the different droplets before their coalescence. In addition, typical coalescers require some specific conditions such as uniform droplet-droplet distances and constant flow rate, which hinders the flexible use of coalescers in practical applications. Here we present a straightforward method for synchronizing reinjection of two kinds of droplets and coalescing them. We employ a home-made emulsion collector operated by hydrostatic pressure to reinject droplets into a device, where two kinds of droplets are driven into two opposing T-junction alternatively and then pairs of droplets are merged at the new coalescer proposed here. We use the technique to create droplets with a controlled number of colloidal particles inside, so that we can observe their self-assembly into a cluster.   

Probing cell mechanical properties with microfluidic devices

Exploiting flow on the micron-scale is emerging as a method to probe cell mechanical properties with 10-1000x advances in throughput over existing technologies. The mechanical properties of cells and the cell nucleus are implicated in a wide range of biological contexts: for example, the ability of white blood cells to deform is central to immune response; and malignant cells show decreased stiffness compared to benign cells. We recently developed a microfluidic device to probe cell and nucleus mechanical properties: cells are forced to deform through a narrow constrictions in response to an applied pressure; flowing cells through a series of constrictions enables us to probe the ability of hundreds of cells to deform and relax during flow. By tuning the constriction width so it is narrower than the width of the cell nucleus, we can specifically probe the effects of nuclear physical properties on whole cell deformability. We show that the nucleus is the rate-limiting step in cell passage: inducing a change in its shape to a multilobed structure results in cells that transit more quickly; increased levels of lamin A, a nuclear protein that is key for nuclear shape and mechanical stability, impairs the passage of cells through constrictions. We are currently developing a new class of microfluidic devices to simultaneously probe the deformability of hundreds of cell samples in parallel. Using the same soft lithography techniques, membranes are fabricated to have well-defined pore distribution, width, length, and tortuosity. We design the membranes to interface with a multiwell plate, enabling simultaneous measurement of hundreds of different samples. Given the wide spectrum of diseases where altered cell and nucleus mechanical properties are implicated, such a platform has great potential, for example, to screen cells based on their mechanical phenotype against a library of drugs.   

Droplet Microfluidics for Virus Discovery

The ability to detect, isolate, and characterize an infectious agent is important for diagnosing and curing infectious diseases. Detecting new viral diseases is a challenge because the number of virus particles is often low and/or localized to a small subset of cells. Even if a new virus is detected, it is difficult to isolate it from clinical or environmental samples where multiple viruses are present each with very different properties. Isolation is crucial for whole genome sequencing because reconstructing a genome from fragments of many different genomes is practically impossible. We present a Droplet Microfluidics platform that can detect, isolate and sequence single viral genomes from complex samples containing mixtures of many viruses. We use metagenomic information about the sample of mixed viruses to select a short genomic sequence whose genome we are interested in characterizing. We then encapsulate single virions from the same sample in picoliter volume droplets and screen for successful PCR amplification of the sequence of interest. The selected drops are pooled and their contents sequenced to reconstruct the genome of interest. This method provides a general tool for detecting, isolating and sequencing genetic elements in clinical and environmental samples.   

Creating 3D chemical gradients with self-folding microfluidic networks

We describe the reversible self-folding of polymeric films into intricate three-dimensional (3D) microfluidic networks and investigate their utility as bio-inspired synthetic vasculature for in vitro tissue culture models. Our fabrication methodology relies on patterning of channels inside the films at the planar microfabrication stage followed by programmable self-folding of the two-dimensional patterned structures. Here self-folding action is enabled by stress gradients which develop in the films due to differential ultraviolet cross-linking and subsequent solvent conditioning. We achieved wafer-scale assembly of micropatterned geometries including helices, polyhedra and corrugated sheets. To demonstrate utility of such self-folded microfluidic devices we present localized chemical delivery of biochemicals in 3D to discrete regions of cells cultured on the curved self-assembled surfaces and in a thick, surrounding hydrogel. We believe that the devices can be used to mimic such natural self-assembled systems as leaves and tissues. Reference: M. Jamal et al., Nature Communications (2011; in press).   

Control of Mass Transport and Chemical Reaction Kinetics in Ultrasmall Volumes

This talk will describe means for triggering chemical reactions for studying reaction kinetics under extreme confinement with sub-millisecond temporal resolution, including on-demand generation and fusion of femtoliter (10$^{-15}$ L) volume water-in-oil droplets, and triggering reactions in femtoliter chambers microfabricated in poly(dimethylsiloxane) (PDMS). We demonstrated a reversible chemical toggle switch, which lays the groundwork for exploring more complex chemical and biochemical reaction sequences triggered and monitored in real time in discrete ultrasmall reactors, such as sequential and coupled enzymatic reactions. We are also developing methods to vary confinement and macromolecular crowding in ultrasmall, water-in-oil droplets and chambers micromolded in PDMS as biomimetic reaction vessels containing minimal synthetic gene circuits, in order to better understand how confinement, reduced dimensionality and macromolecular crowding affect molecular mechanisms involved in the operation and regulation of genetic circuits in living cells.   

Thin Polymer Films as Microvalves in Microfluidic Devices

We report on a novel technology allowing the integration of microvalves and micropumps in lab-on-a-chips made of either soft or hard materials. The approach is based on the grafting of responsive hydrogels onto the microchannel walls. These gels undergo large volume variations by absorbing or expelling water when subjected to external stimuli (here, temperature is used as the stimulus). The hydrogel thin films we study here are chemical polymer networks that are covalently bound to the surface. The first step of the elaboration of that valves is the development of the surface-attached hydrogel thin films. The objective is to obtain hydrogel films with a wide range of thicknesses. The second step is the completion of the microfluidic system by bonding a channel on the active surface. The polymer used is thermoresponsive, at room temperature the swollen gel forms a thick layer, measuring typically several micrometers. When the system is heated above the LCST (Low Critical Solution Temperature), the gel collapses, forming a submicrometric film. In this work we introduce two different applications. In the first situation, the gel layer constitutes a variable resistance. In the second situation the polymer entirely closes the channel after swelling, thus forming a valve.   

Nanofluidic Transistor Circuits

Non-equilibrium ion/fluid transport physics across on-chip membranes/nanopores is used to construct rectifying, hysteretic, oscillatory, excitatory and inhibitory nanofluidic elements. Analogs to linear resistors, capacitors, inductors and constant-phase elements were reported earlier (Chang and Yossifon, BMF 2009). Nonlinear rectifier is designed by introducing intra-membrane conductivity gradient and by asymmetric external depletion with a reverse rectification (Yossifon and Chang, PRL, PRE, Europhys Lett 2009-2011). Gating phenomenon is introduced by functionalizing polyelectrolytes whose conformation is field/pH sensitive (Wang, Chang and Zhu, Macromolecules 2010). Surface ion depletion can drive Rubinstein's microvortex instability (Chang, Yossifon and Demekhin, Annual Rev of Fluid Mech, 2012) or Onsager-Wien's water dissociation phenomenon, leading to two distinct overlimiting I-V features. Bipolar membranes exhibit an S-hysteresis due to water dissociation (Cheng and Chang, BMF 2011). Coupling the hysteretic diode with some linear elements result in autonomous ion current oscillations, which undergo classical transitions to chaos. Our integrated nanofluidic circuits are used for molecular sensing, protein separation/concentration, electrospray etc.   

The Lab-on-a-Disc: Miniature Counterpart to the Lab-on-a-CD for Driving Chip-Based Microcentrifugation

The Lab-on-a-CD concept has opened up the powerful possibility of carrying out a range of microfluidic operations simply by using a compact disc (CD) player to spin a disc on which microchannels are fabricated. Nevertheless, the bulk rotation of the entire CD structure is cumbersome, expensive and unreliable - the antithesis of microfluidic philosophy. Fluid transfer on and off the chip can also be difficult. We have instead developed a miniaturized centrifugal microfluidic platform for lab-on-a-chip applications that employs surface acoustic waves to drive the rotation of a 10 mm SU-8 disc on which microfluidic structures are patterned. Unlike its macroscopic Lab-on-a-CD counterpart, the Lab-on-a-Disc does not require moving parts, and is inexpensive, disposable, and significantly smaller both in terms of the disc itself and the portable palmtop battery-operated circuit used to power the chip-sized device. In the first proof of concept, we show the capability of the Lab-on-a-Disc platform to drive capillary-based valving, mixing and size-based concentration/separation of aqueous particle suspensions in microchannels on the disc. To the best of our knowledge, the miniature Lab-on-a-Disc concept is the first microcentrifugation platform small enough to comprise a handheld device.   

Session W52: Focus Session: Extreme Mechanics - Fluid-Structure Interactions and Swelling

Capillary rise between exible walls

We report experimental work on capillary rise of a liquid in a cell formed by parallel plates, one of which is flexible. We show that above a critical width, the cell collapses under the negative capillary pressure in the liquid. This collapse allows the liquid to rise virtually without limit between the plates. The height of the rising front is found to increase with time as $t^{1/3}$, a characteristic of capillary imbibition in a wedge.   

Motion of a rigid sphere through an elastic tube

The transport of soft objects through small rigid channels is a common problem in the biological world : red blood cells are deformed when passing through small capillaries and polymer coils can make their way through minute pores. We study the opposite model problem of a rigid sphere moving in a narrower elastic tube. Geometry, mechanical properties of the tube and friction or lubrication conditions determine the dynamics of the entrapped sphere. We present experimental results on this problem, together with scaling law analysis.   

Equilibrium and stability of an elastic meniscus

A liquid-air interface touching a solid wall gives rise to a liquid meniscus, whose shape has been well known for two centuries and results from the balance between capillarity and gravity. We investigate the case in which a portion of the liquid interface has been replaced by a soft strip, adding the elastic ingredient to this physical problem. We experimentally study the equilibrium configurations, from small to high non-linear deformations, and we compare to a 2D theoretical model. Stability of the system involving 3D corrections is also addressed.   

Capillary-Induced Self-Organization of Soft Pillar Arrays into Moir\'{e} Patterns by Dynamic Feedback Process

We report a self-organized pattern formation of polymer nanopillar arrays by dynamic feedback: two nanopillar arrays collectively structure a sandwiched liquid and pattern the menisci, which bend the pillars into Moir\'{e} patterns as it evaporates. Like the conventional Moir\'{e} phenomenon, the patterns are deterministic and tunable by mismatch angle, yet additional behaviors---chirality from achiral starting motifs and preservation of the patterns after the surfaces are separated---appear from the feedback process. Patterning menisci based on this mechanism provides a simple, scalable approach for making a series of complex, long-range-ordered structures. Reference: Sung H. Kang, Ning Wu, Alison Grinthal, and Joanna Aizenberg, Phys. Rev. Lett., \textbf{107}, 177802 (2011).~   

Study of waving of grass using a soap film model

Wind blowing over a grass field incites synchronized response from the grass blades, which appear as waves. This effect is called Mo-nami in a terrestrial setting, while in an aquatic setting it is termed as Ho-nami. We use a combination of experimental observations, numerical simulations and theoretical analysis to understand this effect. The experiment is conducted in two-dimensional realization of these phenomena in a gravity driven soap film tunnel. Nylon filaments attached to the boundaries of the soap film play the role of the grass. We provide a preliminary characterization of this analog model for the synchronized oscillations of grass.   

Direct measurements of flow and deformation of a free reed

The free reed, responsible for producing sound in a family of air-driven musical instruments, is an example of a coupled fluid-structure system engineered to vibrate efficiently at a controllable frequency. In Western free reed instruments, a flexible metal plate is clamped at one end above a slot cut into a rigid support plate. This geometry allows a constant driving pressure to produce and sustain large-amplitude vibrations. The mechanism behind this has been discussed by several investigators. However, it has yet to be verified experimentally with direct measurements of the flow speed. We present simultaneous measurements of the reed motion and the flow speed in the downstream jet, which enable characterization of the relationship between the finite-amplitude deformation of the reed and the flow.   

Control and Manipulation of Fluid Flow Using Elastic Deformations

In this work, we utilize elastic deformations within a flexible microfluidic device via mechanical actuation to control and direct fluid flow. The device consists of a microchannel with a flexible arch prepared by buckling a thin elastic film. The deflection of the arch can be predicted and controlled using the classical theory of Euler buckling. We controlled the fluid flow rate by coupling the elastic deformation of the arch to the gap within the microchannel, and matched these experimental results analytically with a perturbation of lubrication theory and with computational simulations. These results illustrate an experimental design paradigm for the preparation of portable microchannels for chemical mixing, self-healing, and in situ diagnostics.   

Curling paper

As many soft materials, paper is mechanically sensitive to humidity. Owing to its hygroscopic cellulose-based structure, it is known to wrinkle when subject to humidity fluctuations. Here, we present experimental results on the more extreme deformations observed when a sheet of tracing paper is put on a bath of water. After contact with the liquid surface, water diffuses into the hygroscopic material from below and induces differential swelling, resulting in the curling of the paper. Within seconds, a spectacular roll-up motion follows. We explain the observed shapes and curling dynamics.   

Walking and jumping spores

The Equisetum plants, more commonly called ``horsetail,'' emit 50-microns spores that are spherical in shape and present four hygroscopic arms. Under high humidity, the arms are retracted. But under lower humidity, less than 70\%, the four arms deploy beautifully. With time-lapse image recordings, we show that under repeated cycles of dry and high humidity, the spores behave as random walkers, since they move by about their size in a different direction at every cycle. The process is apparently stochastic because of the complex shape of the arms and hysteretic friction of the arms on the ground. For some spores, a decrease in humidity level results in very fast jumps, the spores taking off at a typical velocity of a meter per second, as recorded on high-speed camera. With these jumps, they reach centimetric elevations, much larger than their size. The physical mechanism at the root of these ``Levy-flight'' jumps is still under investigation. The walking and jumping phenomena thus provide motility, which we believe is helpful for the understanding of the biological dispersion of the spores. It could also bring biomimetic inspiration to engineer new motile elastic structures.   

Mechanic instabilities of swelling gels

While the study of gels takes undoubtedly its roots within the field of physico-chemistry, the interest for gels has flourished and they progressively became an important object in the study of the mechanics of polymeric materials and volumetric growth, rising some fascinating problems, some of them remaining unsolved. Because gels are multiphase objects, their study represents an important step in the understanding of the mechanics of complex soft matter as well as for the process of shape generation in biological bodies. I will present here experiments and models of swelling gels mainly in the cylindrical geometry which mimic various growth instabilities from tumors up to the morphogenesis of tubular organs.   

Patterns formed by swelling-induced folding of films

The solvent swelling of a thin polymer film attached to a rigid substrate is known to induce a creasing pattern on the free surface of the film. Here we show that if the film is weakly attached to the substrate, the swelling-induced compressive stress nucleates buckle delamination of the film from the substrate. Surprisingly, the buckles do not have a sinusoidal profile, instead, the film near the delamination buckles slides towards the buckles causing growth of sharp folds of high aspect ratio. The folds persist even after the solvent evaporates. Such fold formation depends on the size of the region of the film exposed to solvent. A very small region of exposure (realized by placing a small drop of solvent on the film) does not induce delamination. Remarkably, with moderate sized drops, the delamination and folding occurs around the perimeter of the drop, thus culminating in a corral with tall walls. We quantify the parameters (drop volume, film thickness) which demarcate the transitions between no fold formation, corral formation, and multiple fold formation.   

Swelling-Driven Shaping of Thermally Responsive Photo-Patterned Gel Sheets

Swelling-mediated shaping of patterned non-Euclidean plates offers a powerful route to design and engineer complex 3-D structures, with possible applications in biomedicine, robotics, and tunable micro-optics. We have studied the behavior of poly(N-isopropyl acrylamide) (PNIPAm) copolymers containing pendent benzophenone units that allow the degree of crosslinking to be tuned by varying the dose of ultraviolet light. A halftone (gray) gel lithography approach, wherein two photomasks enable patterning of highly-crosslinked domains within a lightly-crosslinked matrix, is shown to provide effectively continuous variations in swelling in truly two-dimensional patterns. We show how this technique can be harnessed to form complex, reversibly actuating, 3-D structures through patterned growth.   

Dynamical Actuation and Pattern Formation with Local Swelling in Microgels

In this invited talk, we present a set of study on swelling-induced actuation and pattern formation in hydrogels of three dimensional microstructures. For example, rapid actuation of a micro hydrogel device is observed by exploiting swelling-induced snap-buckling. Utilizing its fast actuation speed, the device can even jump by itself upon wetting. It is demonstrated that elastic energy is effectively stored and quickly released from the device by incorporating elastic instability. In our experiment, the micro device could generate a snapping motion within 12 milliseconds, releasing power at a rate of 34 mW/g. We also captured the evolution circumferential buckling of tubular shaped microgels. Inhomogeneous stress develops as gel swells under mechanical constraints, which gives rise to buckling instability. A simple analytical model is developed using elastic energy to predict stability and post-buckling patterns upon swelling. Our experiment demonstrates that circumferential buckling of desired mode can be created in a prescribed manner. Our study on the mechanics of three-dimensionally microstructured gels might provide new insights for in morphogenesis in tissue engineering, and provide new gateways in many emerging fields such as soft robotics and tunable matamaterials.   

Separating Viscoelasticity and Poroelasticity of Gels with Different Length and Time Scales

Viscoelasticity and poroelasticity commonly coexist in polymer gels. We propose a method capable of separating the viscoelasticity and poroelasticity of gels in various mechanical tests. The viscoelastic characteristic times and the poroelastic diffusivities of a gel can define intrinsic material length scales of the gel. The experimental setup can give sample length scales, over which the solvent migrates in the gel. By setting the sample lengths to be much larger or smaller than the material lengths, the viscoelasticity and poroelasticity of the gel will manifest at different time scales in a test. Therefore, the viscoelastic and poroelastic properties of the gel can be probed separately at different time scales of the test.   

A constitutive theory for visco-hyperelastic gels

Many gels operate in chemically saturated environments in a variety of applications. Most constitutive theories for gels are formulated using large deformation hyperelasticity coupled with fluid transport. However, in most cases the mechanical response of such gels show hysteresis and other dissipative effects which are not accounted for in present constitutive theories. We have recently developed a three dimensional continuum level theory to describe the coupled fluid permeation and large deformation response of visco-hyperelastic materials. In this work, we apply our theory and numerical simulation capability to study the indentation response among others of visco-hyperelastic gels.   

Session W53: Focus Session: Common Features of Soft Materials: Polymers, Colloids and Granular Media I

Athermal Jamming Vs Thermalized Glassiness in a Simple Model of Soft-Core Interacting Particles

Numerical simulations of soft-core frictionless disks in two dimensions are carried out to study shear viscosity $\eta$ and pressure $p$ of a simple model liquid, as a function of thermal temperature $T$, packing fraction $\phi$, and uniform applied shear strain rate $\dot\gamma$. We find that viscosity in the athermal hard-core limit, $\lim_{\dot\gamma\to 0}[\lim_{T\to 0} \eta]$, is singularly disconnected from viscosity in the hard-core thermal limit, $\lim_{T\to 0}[\lim_{\dot\gamma\to 0} \eta]$, demonstrating that thermal glassy behavior is not governed by the athermal jamming critical point, ``point J".   

Spontaneous formation of permanent shear bands in a mesoscopic model of flowing disordered matter

In this presentation we propose a coherent scenario of the formation of permanent shear bands in the flow of yield stress materials. Within a minimalistic mesoscopic model we investigate the spatial organisation of plasticity. The most important parameter is the typical time needed to regain the original structure after a local rearrangement. In agreement with a recent mean field study [Coussot \textit{et al., Eur. Phys. J. E}, 2010, \textbf{33}, 183] we observe a spontaneous formation of permanent shear bands, when this restructuring time is large compared to the typical stress release time in a rearrangement. This heterogeneous flow behaviour is different in nature from the transient dynamical heterogeneities that one observes in the small shear rate limit in flow without shear-banding [Martens \textit{et al., Phys. Rev. Lett.}, 2011, \textbf{106}, 156001]. We analyse the dependence of the shear bands on system size, shear rate and restructuring time. Further we rationalise the scenario within a mean field version of the model, that explains the instability of the homogeneous flow below a critical shear rate. Our study therefore strongly supports the idea that the characteristic time scales involved in the local dynamics are at the physical origin of permanent shear bands.   

Creep and critical scaling in random spring networks

Random networks of springs are a minimal model for physical, biological, and engineered materials ranging from foams and emulsions to biopolymer and bar-joint networks. Near the central force isostatic point, the creep response of damped networks is intimately tied to the presence or absence of floppy motions in the long time limit. We show that nearly isostatic networks display dynamical critical scaling, and that this scaling connects viscous flow and elastic deformation via a critical creep regime. We give scaling arguments for the critical exponents and confirm our predictions with numerics.   

Rheology of particle assemblies close to a jamming transition

The jamming paradigm aims at providing a unified view for the elastic and rheological properties of materials as different as foams, emulsions, suspensions or granular media. Structurally, these systems can all be viewed as dense assemblies of particles, and the particle volume fraction $\phi$ plays the role of the coupling constant that tunes the distance to the jamming transition. Enhanced experimental techniques allow to visualize the dynamics of these systems on the level of the individual particles, generating huge amount of information. Several interesting, and partially conflicting, results have emerged. It therefore seems necessary to ask in how far the experimental results reveal genuine aspects of a universal jamming transition, or justsystem-specific properties that depend on microscopic details, the driving mechanism or the preparation protocol. By comparing different computational models we will discuss the question of universality on the macroscopic level of rheological observables as well as on the microscopic level of single particle trajectories and collective particle motion.   

Inhomogeneous structure and position-dependent dynamics in complex fluids

We present computer simulation results of model complex fluids, quantifying how inhomogeneous structuring and dynamics of particles can be tuned through interactions. We first illustrate how a tracer particle's interactions can be tuned to significantly modify its long-time diffusivity. We use a recently proposed propagator-based formalism to address how this enhancement can be understood in terms of the local dynamics of neighboring particles. We then discuss how these results compare to related behaviors of model confined colloidal and molecular fluids.   

Demonstration of Secondary Currents in the Pressure-Driven Flow of a Concentrated Suspension Through a Square Conduit

It has been known for several decades now that the pressure-driven flow of polymer melts in non-axisymmetric conduits is not unidirectional; the main flow through the channel is accompanied by secondary currents, whose origin can be attributed to second normal stress differences. However, only recently was it realized [Ramachandran and Leighton, \textit{J. Fluid Mech.} (2008)] that the same may be true for concentrated suspensions, which, upon shearing, exhibit strong second normal stress differences. This work confirms the existence of these secondary flows by carrying out pressure-driven suspension flow experiments through a square (non-axisymmetric) duct. By tracking the motion of a thin stream of a contrastingly-dyed suspension introduced into the bulk flow of another, it is demonstrated that the suspension flows out of the sidewalls of the geometry towards the corners of the square cross-section, and then flows towards the center. This is found to be qualitatively consistent with calculations based on the suspension balance model of Nott and Brady [\textit{J. Fluid Mech.} (1994)]. Secondary currents have been predicted to be the dominant mechanism determining particle distribution in suspension flows, and this work lends support to that idea.   

Understanding entangled polymers: What we can learn from athermal chain packings

The study of random and ordered packings (from atoms and colloidal particles to sand grains) has been the focus of extensive research. This is not surprising since an understanding of the mechanisms that control morphology and packing is the key to the design and synthesis of novel ``smart'' materials and functionalities. In particular, the study of packings of chain molecules presents challenges but also insights which are absent in monatomic systems and further allows for a direction comparison with them. In this contribution we give an overview of our work on very dense and nearly jammed packings of athermal polymers. We show that chain molecules can be as efficiently and as densely packed as monatomic analogs up to the same maximally random jammed state. We also show that an exact correspondence can be established between the statistical-mechanical ensembles of packings of monatomic, and chain systems, which yields insights on the universality of jamming. By studying the effect of concentration on polymer size and on the underlying network of topological hindrances we precisely identify the distinct universal scaling regimes and the corresponding exponents. An unsuspected connection, valid from dilute up to very dense assemblies, is established between knots (of intermolecular origin) and entanglements (intermolecular constraints). We finally show that, against expectations, entropy-driven crystallization can occur in dense systems of athermal polymers once a critical volume fraction is reached. Such phase transition is driven by the increase in translational entropy: ordered sites exhibit enhanced mobility as their local free volume becomes more spherical and symmetric. Incipient nuclei develop well defined, stack-faulted layered crystal morphologies with a single stacking direction. The ordering transition and the resulting complex morphologies are analyzed, highlighting similarities and differences with respect to monatomic crystallization.   

Sudden Chain Energy Transfer Events in Vibrated Granular Media

In a mixture of two species of grains of equal size but different mass, placed in a vertically vibrated shallow box, there is spontaneous segregation. Once the system is at least partly segregated and clusters of the heavy particles have formed, there are sudden peaks of the horizontal kinetic energy of the heavy particles, that is otherwise small. Together with the energy peaks the clusters rapidly expand and the segregation is partially lost. The process repeats once segregation has taken place again, either randomly or with some regularity in time depending on the experimental or numerical parameters. An explanation for these events is provided based on the existence of a fixed point for an isolated particle bouncing with only vertical motion. The horizontal energy peaks occur when the energy stored in the vertical motion is partly transferred into horizontal energy through a chain reaction of collisions between heavy particles.   

Equation of state and jamming density for equivalent bi-, tri- and polydisperse, smooth, elastic sphere systems

We study binary, ternary and polydisperse mixtures of hard particle fuids as models for granular matter, colloids and other soft matter. Size ratios between 1 and 100 are studied for different size distributions. Simulation results are compared with previously found analytical equations of state by looking at the compressibility factor, Z, and agreement is found with much better than 1{\%} deviation in the fluid regime. A slightly improved empirical correction to Z is proposed. When the density is further increased, the behavior of Z changes and there is a close relationship between many-component mixtures and their two- and three-component equivalents (where our contribution is to define the term ``equivalent''). We determine the size ratios for which the liquid-solid transition exhibits crystalline, amorphous or mixed system structure. Near the jamming density, Z is independent of the size distribution and follows a -1 power law as function of the difference from the jamming density. In this limit, Z depends only on one free parameter, the jamming density itself, as reported for several different size distributions with a wide range of widths.   

Recovery of polymer glasses from mechanical perturbation

There is a longstanding debate about the nature and extent of mechanical rejuvenation in aging glasses and most related studies have concentrated on the impact of aging during plastic deformation. Here we study the time period after nonlinear creep where the glass recovers, using molecular dynamics simulations of a bead-spring model for a wide range of stress amplitudes and glass ages. We compute $\alpha$-relaxation times as well as several quantities that characterize structural changes. From an analysis of the recovery paths we find a transition from memory effects to mechanical rejuvenation that is controlled by the total strain and not the stress amplitude. Although strong mechanical perturbation can make the dynamics appear very similar to that of a freshly quenched glass, various measures of short range order as well as inherent structure energies reveal systematic differences in the underlying thermodynamic state.   

Spatiotemporal stress/strain correlations in a quasi-2D jammed emulsion

We flow quasi-2D emulsions in a flow geometry analogous to pure shear to better understand the microscopic events within jammed materials during the straining process. Our quasi-2D system serves as an experimental model system of jamming and consists of oil-in-water emulsion droplets confined between two parallel plates. Using a technique we have developed, we can determine the forces between pairs of droplets in contact based on each droplet's deformation. By imaging the motion and deformation of the droplets during the flowing process, we quantify the microscopic events using spatiotemporal correlations in strain and stress. We study these spatiotemporal correlations at various droplet concentrations to understand how the microscopic events change as we approach the jamming point.   

Low-$k$ behavior in Structure Factor and Compressibility Factor for Monodisperse and Bidisperse Packings of Frictionless Spheres

One particular structural signature of jamming transition has emerged in studies of large systems: \emph{hyperuniformity}, which is the supression of the long wavelength density fluctuations. Also, it has been observed an unusual linear dependence ($S(k)\sim k$) of the structure factor in the low $k$ limit, in monodisperse systems. The small wavenumber region of the static structure factor $S(k)$ for monodisperse systems and the compressibility factor $\theta(k)$ for bidisperse mixtures, are investigated for jammed packings of frictionless spheres with Hooke and Hertz force model, using a high precision data analysis. We have found that the zero-wavenumber intercept $S(k=0)$ and $\theta(k=0)$, as a function of the pressure, are non-zero constant, revealing a finite compressibility. This behavior is relatively insensitive to the force model but shows a dependence on the bidispersity. We have studied also zero-temperature Lennard-Jones glasses which exhibit a finite compressibility that depend weakly on the density of the glass.   

Jamming and Unjamming of the Rigid Amorphous Fraction

Semicrystalline polymers obey a three-phase model comprising crystalline, mobile amorphous (MAF), and rigid amorphous fractions (RAF) as an interphase. Using quasi-isothermal temperature modulated differential scanning calorimetry (QI-TMDSC), we investigate the formation behavior of these fractions in poly(trimethylene terephthalate), PTT. PTT was quasi-isothermally cooled step-wise from the melt which causes its crystalline fraction to be fixed below 451K, and RAF is determined as a function of temperature. For PTT, most of the RAF vitrifies between 451K and T$_{g }$step-by-step during QI cooling. With lamellar crystals acting as topological constrains, a model is proposed in which the vitrification and devitrification of RAF are interpreted using the concepts of ``jamming'' and ``unjamming.'' Constraints of the crystal surfaces reduce the mobility of the highly entangled polymer chains attached to the lamellae, and the layers which constitute RAF are formed one after another in the manner of successive jamming. In this way, several features of the RAF temperature dependence are explained for the first time, with implications in other research areas, such as topological constraints exerted on the polymer melt through effects of inclusions in polymer-based nanocomposites.\\[4pt] For support of this research, the authors thank the NSF, Polymers Program of the Division of Materials Research through DMR-0602473, and the MRI Program under DMR-0520655 for thermal analysis instrumentation. A portion of this work was conducted at the BNL, National Synchrotron Light Source, supported by the DoE. G. Georgiev thanks Assumption College for continuous research support.