Session H1: Focus Session: Advances in Scanned Probe Microscopy I: Novel Approaches to Complex Systems

Hybrid STM/AFM study of half-metallic surface states in cobaltates

Na$_{x}$CoO$_{2}$ is a well-known compound that has been studied at various Na concentrations, and has drawn much attention for its unconventional superconductivity and antiferromagnetic phase. In its stoichiometric concentration, NaCoO$_2$ has has recently been proposed as a system for observing topological superconductivity when mixed with a superconductor's electronic states through the proximity effect. We examine NaCoO$_2$ using an ultrahigh-vacuum low-temperature hybrid scanning tunneling and atomic force microscope at 4 K. We use the tuning-fork AFM mode to study the topography of this bulk insulating material when no tunneling is possible, and utilize a special electrical contact scheme to access the electronic surface states for spectroscopy.   

Trend and novel challenges in spectroscopic imaging scanning tunneling microscopy for correlated electron materials research

In this presentation we would like to discuss the recent progress of the spectroscopic imaging scanning tunneling microscopy (SI-STM) and how the real- and momentum-space sensitive spectroscopic tool is evolving into a more robust quantum theoretical tool for the phase transition study of the correlated electron materials. \\[4pt] [1] Jhinhwan Lee, et al., Science 325, 1099 (2009).\\[0pt] [2] Michael J. Lawler, et al., Nature 466, 374 (2010).\\[0pt] [3] Colin V. Parker, et al., Nature 468, 677 (2010).\\[0pt] [4] Ilija Zeljkovic, et al., arXiv:1104.4342v1 (2011).   

Unique vortex states on nanosize superconducting islands observed by LT-STM

With rapid progress of fabrication methods for nano-size superconductors, many novel properties of their superconductivity have been studied. Several vortex states unique to the nano superconductors, such as, vortex clustering, giant vortex, and anti-vortex, have been reported/predicted on specific size and shape of the islands. In this study we used Pb island structures with atomically flat surface formed on the Si(111) substrate under ultrahigh vacuum conditions as the superconducting sample. We investigate vortex states formed on the Pb islands under magnetic fields using low-temperature scanning tunneling microscopy (LT-STM), which enable us to observe the surface topograph and the superconducting gap (DOS) at atomic scale spatial resolutions simultaneously in real space. We performed precise tunneling spectroscopy on the Pb islands to take two dimensional DOS mapping at the Fermi level and succeeded in observing several kinds of vortex states (ex. multi vortex state an d giant vortex state) in real space. The details will be discussed in the presentation.   

Scanning SQUID microscopy: A powerful tool for probing magnetism and superconductivity in complex oxides

Magnetic measurements are useful in investigating novel materials because they probe the behavior of electrons and their interactions. Superconducting Quantum Interference Devices (SQUIDs) are ultra-sensitive flux magnetometers and, when used in imaging mode, they become a powerful tool for mapping magnetic fields above a sample. This talk will outline the basics of scanning SQUID microscopy and highlight our recent measurements on a new material system: complex oxide interfaces. Our scanning SQUID technique uncovered the coexistence of superconductivity and magnetism in the LAO/STO oxide system. These measurements highlight many key benefits of the scanning SQUID technique including a 3 micron imaging kernel, excellent flux sensitivity, and an on-chip field coil to simultaneously measure both the intrinsic magnetism and the sample's response to an applied magnetic field.   

Spatial Dependence of Kondo Screening and Magnetic Anisotropy on Saturated Copper Nitride Islands

Co adatoms on a copper nitride surface constitute a unique system for studying the interplay between Kondo physics and anisotropy in a high spin atom (S=3/2). By using scanning tunneling spectroscopy techniques, we can determine both the energy scales of the Kondo screening and the spin excitations on a single atom. Here we study the case of Co adatoms on large nitride islands that form as the copper surface is saturated with nitrogen. These islands present a rich spatial variation of their electronic structure. We observe how changes in the electronic structure of the insulator result in dramatic changes in the spectroscopy of the cobalt adatoms: a considerable increase of the anisotropy energy occurs as the Kondo resonance disappears. These results allow us to explore in detail the interplay between these two phenomena.   

Evolution of the Kondo Resonance for Screened Atoms on Metals and Thin Insulators

We study the magnetic anisotropy and the Kondo screening of the spin of Co atoms deposited on Cu$_{2}$N using scanning tunneling microscopy and spectroscopy. We find that for Co atoms placed on Cu$_{2}$N islands the Kondo screening is weaker when the atom is very close to the edge and at the same time has significant changes in the magnetic anisotropy. Furthermore we observe that Co atoms that are placed on the Cu surface but near the Cu$_{2}$N islands still show anisotropy and an unusually small Kondo temperature. At larger distances from the Cu$_{2}$N islands the usual large Kondo temperature recovers. We examine possible causes for these dramatic changes in the Kondo screening and magnetic anisotropy, including a possible extension of the electronic properties of the Cu$_{2}$N islands compared to the topographic influence.   

Atomic force microscopy as nano-stethoscope to study living organisms, insects

Atomic force microscopy (AFM) is a known method to study various surfaces. Here we report on the use of AFM to study surface oscillations (coming from the work of internal organs) of living organisms, like insects. As an example, ladybird beetles (Hippodamia convergens) measured in different parts of the insect at picometer level. This allows us to record a much broader spectral range of possible surface vibrations (up to several kHz) than the previously studied oscillations due to breathing, heartbeat cycles, coelopulses, etc. (up to 5 -10 Hz). The used here AFM method allows collecting signal from the area as small as $\sim $100nm2 (0.0001$\mu $m2) with an example of noise level of (2$\pm $0.2)$\times $10-3 nm r.m.s. at the range of frequencies $>$50Hz (potentially, up to a MHz). Application of this method to humans is discussed. The method, being a relatively non-invasive technique providing a new type of information, may be useful in developing of what could be called ``nanophysiology.''   

Scanning tunneling microscopy study of the assembly and structure of filamentous virus M13 bound to graphite

Viruses are an important class of biomaterials used for placing nano particles on inorganic substrates. To accomplish greater control over viral assembly on a substrate it is important to determine the in situ nanoscale structure of the viral protein coat. Scanning tunneling microscopy offers the unique potential for determining the structure and arrangement of the proteins of a virus adsorbed on a conducting substrate. In this work, I develop an experimental technique for isolating and studying M13 viruses that bind to graphite. Using scanning tunneling microscopy in ambient conditions I obtain the correct lateral dimension of the virus and the periodicity of its protein structure when it is bound to graphite. I also analyze the tunneling conductance fluctuations in these measurements and introduce a simple model for tunneling through an assembly of proteins to obtain an accurate estimation of the vertical dimension of a virus bound to a conducting substrate. I discuss broader implications of this scanning tunneling microscopy study for the in situ structure determination of other biomolecules.   

Nanomechanics of Murine Articular Cartilage Reveals the Effects of Chondroadherin Knockouts

With high resolution nanotechnology tools, quantification of cartilage biomechanical properties provides important insights into the role of low abundance matrix molecules on cartilage function and pathology. In this study, the role of chondroadherin (CHAD) on cartilage mechanical properties was assessed via atomic force microscopy-based nanoindentation (0.1-10 $\mu $m/s z-piezo displacement rates) of murine knee cartilage from wild type (WT) and CHAD knockout (KO) animals ages 1 year, 4 month, and 11 weeks (n$\ge $4 joints/age-group). A significant increase in indentation modulus, E, with indentation rate in all specimens (p$<$0.05, Friedman) suggested poro-viscoelastic behavior. For all age groups, CHAD KO significantly reduced E at all indentation rates (p$<$0.05, 2-way ANOVA); e.g., at 1-year, E was 0.77+/-0.1 MPa for WT (mean+/-SEM 1$\mu $m/s rate) and 0.25+/-0.07 MPa for CHAD KO cartilage. Lack of CHAD appears to delay development of load bearing extracellular matrix. This could affect the effective cross-link density of the tissue network and, hence, decrease local osmotic swelling while increasing the hydraulic permeability of the aggrecan-filled network. Ongoing studies are investigating the biochemical properties and nanostructure of CHAD KO joints.   

Nanoindenter Stiffness Measurements on a MEMS Sound Sensor

We demonstrate a novel technique to extract the various components of the stiffness (or compliance) measured along the surface of a MEMS directional sound sensor. Because the sensor comprises a cantilever beam mounted on torsion springs, the overall stiffness consists of various compliance components added in series. Stiffness measurements made using a nanoindenter are found to agree with an analytical model and a finite element model (FEM) of the sensor. Moreover, by exploiting the differing power-law characteristics of the individual compliance components, we demonstrate extraction of the separate components from a logarithmic plot of the overall stiffness. Finally, we measure the ultimate (failure) strength of the sensor, from which we obtain the maximum acoustic intensity the sensor can tolerate.   

Primary electron beam generation in Near-Field-Emission SEM

Due to low electron energies used in Near-Field-Emission SEM (NFESEM), the understanding of the physical phenomena governing the primary electron beam generation is of fundamental relevance. The geometry and the chemical composition of the ultra-sharp field emitter have been therefore investigated by using different well-known electron microscopy techniques. The last hundreds of nanometers of such a field emitter, produced by electrochemical etching of a tungsten wire, can be macroscopically approximated by a cone with angle of aperture of about $6^\circ \pm 1^\circ$. The shape of the very apex is strongly dependent on the preparation conditions. Moreover, the only remaining contamination after the annealing procedure of the tungsten tips is a tungsten-oxide coating, uniformly distributed on the surface.   

Chemical contrast in Near-Field-Emission SEM

In Near-Field-Emission SEM the primary electron beam, of some tens of eV, is generated by cold field emission from a polycrystalline W-tip. Recently, topography images have been obtained by scanning a W(110) sample with a tip at constant height, typically of tens of nm, recording the secondary electron yield and the emission current. We report on the observation of a chemical contrast of a W(110) surface covered by submonolayer of Fe achieved with the NFESEM technique. The chemical contrast is caused by a significant lower secondary electron yield for Fe with respect to W. The Fe islands with a diameter of 2 nm to 5 nm are clearly distinguishable, giving a direct indication of the microscope lateral resolution. The adsorbate position, size and shape are confirmed by STM. Moreover, this technique shows the presence of Fe growing along the step edges of the substrate, which can not be identified with STM.   

Electrostatic characterization of Near-Field-Emission SEM

The properties of the primary electron beam in Near-Field-Emission SEM (NFESEM) are uniquely determined by the actual geometry and position of the conducting components in the experimental apparatus. The reciprocal dependence of the accessible quantities, namely the voltage applied to the emitting tip with respect to the conducting sample surface ($V$), the relative distance between the tip and the sample ($d$) and the field-emission current ($I$), has been thoroughly characterized. In particular, the voltage $V$ needed to produce a given current $I$ has been measured as a function of $d$; the values of $I$ have been chosen in the range from 0.05 nA to 1.5 nA, while $d$ has been varied from 4 to 1500 nm. For values of $d$ smaller than a certain threshold $\bar d$, dependent on the tip, $V$ turns out to be directly proportional to the distance between the tip and the sample surface. At larger distances, $d > \bar d$, we found $V \propto I^a \cdot d^b$, with $a$ and $b$ generally varying from one tip to the another. These results are supported by preliminary theoretical calculations which assume electrostatic geometries directly inspired by the NFESEM setup, such as an hyperboloid-shaped emitter with a conduction plane lying at a generic $d$.   

Session H2: Invited Session: Novel and Proven Methods of Communicating Science to the Public

Creating Catalytic Collaborations between Theater Artists, Scientists, and Research Institutions

Catalyst Collaborative@MIT (CC@MIT) is a collaboration between MIT and Underground Railway Theater (URT), a company with 30 years experience creating theater through interdisciplinary inquiry and engaging community. CC@MIT is dedicated to creating and presenting plays that deepen public understanding about science, while simultaneously providing artistic and emotional experiences not available in other forms of dialogue about science. CC@MIT engages audiences in thinking about themes in science of social and ethical concern; provides insight into the culture of science and the impact of that culture on society; and examines the human condition through the lens of science that intersects our lives and the lives of scientists. Original productions range from Einstein's Dreams to From Orchids to Octopi -- an evolutionary love story; classics re-framed include The Life of Galileo and Breaking the Code (about Alan Turing). CC@MIT commissions playwrights and scientists to create plays; engages audiences with scientists; performs at MIT and a professional venue near the campus; collaborates with the Cambridge Science Festival and MIT Museum; engages MIT students, as well as youth and children. Artistic Director Debra Wise will address how the collaboration developed, what opportunities are provided by collaborations between theaters and scientific research institutions, and lessons learned of value to the field.   

Drawing at the Speed of Talk: doodling complex discussions in real-time to create animated ``Conversation Portraits''

What does your research look like as a drawing? Imagine the guy who delivered your pizza understanding the latest paper you delivered. Flash Rosenberg will describe how she ``live-draws'' discussions between prominent authors and scientists to create lucid animations. She will offer strategies for how to translate the impenetrable into the understandable, convert equations into sensations, and enable everyone to appreciate issues in physics as relevant, intriguing fun. View sample ``Conversation Portraits'' at http://vimeo.com/flashrosenberg/videos   

Celebrating 24 years of Public Outreach of Science and Engineering in Portland Oregon

There have been several core strategies in our highly successful 24-year Science, Technology and Society outreach program. However, the strategy for each season is also dynamic, requiring innovation and novel coalitions. As Bob Dylan put it so succinctly, ``He not busy being born is busy dying.'' Public outreach programs - as the Chautauquas of the past - should be positioned in the cultural milieu along with the opera, symphony and theatre. Support for the enterprise needs to be a broad and diverse coalition, based ideally on the creative formation of win-win relationship. You want people to see your success as their success: ``Together we can enhance the intellectual environment in ways that none of us could do alone.'' Being multi-disciplinary presents challenges but has considerable advantages. For instance, enlightened managers of established organizations recognize the value of exposing their employees to a diversity of problem solving approaches. Instead of inviting speakers for one large lecture we now invite them to be Resident Scholars for two-three days and develop a range of additional smaller public engagements. Science and engineering topics must be relevant - placed in the broader Science, Technology and Society framework. We avoid ``gee-whiz'' in favor of what stimulates reflection on who we are, where we came from, and our role in the universe. I will briefly review how we have survived and thrived and, finally, what I see as future trends and opportunities.   

Session H3: Invited Session: Electronic Properties of the Pseudogap Phase in Cuprates

Extremely Correlated Fermi Liquids

A new framework is reported for calculating the properties of extremely correlated electronic systems with eliminated double occupancy. Based on Schwinger's approach to field theory, it avoids using auxiliary variables, and leads to a low (particle) density expansion with equations that approximately double the complexity of the standard theory for interacting electrons. Concrete results for the one electron spectral function of the $t$-$J$ model in 2-dimensions are presented to lowest non trivial order in density. These already show considerable promise in the context of cuprate superconductors. A distinguishing characteristic of this theory is the low energy long wavelength asymmetry between adding holes and particles. Prospects for the experimental observation of this asymmetry are discussed. \\[4pt] [1] ``Extremely Correlated Fermi Liquids,'' B. S. Shastry, arXiv:1102.2858 (2011), Phys. Rev. Letts. 107, 056403 (2011).\\[0pt] [2] ``Dynamical Particle Hole Asymmetry in Cuprate Superconductors,'' B. S. Shastry, arXiv: 1110.1032 (2011)   

The new extremely correlated electron perspective of the normal state of high temperature superconductors

In this talk, two recent angle resolved photoelectron spectroscopy (ARPES) studies on high temperature superconductors are discussed. These studies show the importance of the ``extreme electron correlation'' a la t-J model. First, we will discuss the normal state single particle spectral function, which has been considered both anomalous and crucial to understand. Here, we report [1] an unprecedented success of applying the new t-J model based ``extremely correlated Fermi liquid theory'' by Shastry, to describe both laser ARPES data and conventional synchrotron ARPES data on Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ and synchrotron ARPES data on La$_{1.85}$Sr$_{0.15}$CuO$_4$. It fits all data sets with the same physical parameter values, satisfies the particle sum rule and successfully addresses two widely discussed kink anomalies in the dispersion. Second, new ARPES investigation [2] of the Fermi surface geometry of Ca$_{2-x}$Na$_x$CuO$_2$Cl$_2$ from underdoping to overdoping shows that the ``weak correlation'' Luttinger sum rule, based on Fermi surface only, clearly breaks down in the underdoped case. We note that a t-J model based theory by Yang, Rice and Zhang provides an alternative ``extreme correlation'' Luttinger sum rule, based on both Fermi surface and ``Luttinger surface.'' This extreme correlation Luttinger sum rule offers much more natural explanation for the observed ARPES data. These two studies imply that the extreme correlation as embodied in the t-J model is essential for understanding high temperature superconductors over a wide doping range. \\[4pt] [1] Gweon, Shastry, and Gu, Phys. Rev. Lett. 107, 056404 (2011).\\[0pt] [2] Meng, Gweon et al., Phys. Rev. B 84, 060513(R) (2011).   

A Phenomenological Theory of the Anomalous Pseudogap Phase in Underdoped Cuprates

A consistent theoretical description of the many anomalous properties that characterize the pseudogap phase in the underdoped region of the cuprate phase diagram has proved challenging. The continuous progress in spectroscopic and other experiments suggests a phenomenological approach. An ansatz based on analogies to the transition to Mott localization at weak coupling in lower dimensional systems, has been proposed by Yang, Rice and Zhang some years back [1]. This ansatz has had success in describing a wide range of experiments [2]. The motivation underlying this ansatz will be described and some of the comparisons to experiment reviewed. The implications for a more microscopic theory will be discussed together with the relation to microscopic theories that start directly from strongly coupled Hamiltonians. \\[4pt] [1] K-Y. Yang, T. M. Rice {\&} F. C. Zhang, Phys. Rev. B\textbf{73},174501 (2006)\\[0pt] [2] T. M. Rice, K.-Y. Yang {\&} F. C. Zhang, arXiv 1109.0632   

Fermi pockets and the pseudogap in underdoped $Bi_2Sr_2CaCu_2O_8$

The Fermi surface topologies of underdoped samples of the high $T_c$ superconductor $Bi_2Sr_2CaCu_2O_8$ have been measured with angle resolved photoemission. By examining thermally excited states above the Fermi level, we show that the observed Fermi surfaces in the pseudogap phase are actually components of enclosed hole-pockets. The spectral weight of these pockets is vanishingly small at the magnetic zone boundary, creating the illusion of Fermi ``arc''. The area of the pockets as measured in this study is consistent with the doping level, and hence carrier density, of the samples measured. Furthermore, the shape and area of the pockets is well reproduced by phenomenological models of the pseudogap phase as a spin liquid. The demonstration that the pseudogap in the anti-nodal region is a gap symmetric about the chemical potential is a clear indication that singlet pairing takes place in the normal state.   

Nodal-antinodal dichotomy in underdoped cuprates: a cluster-dynamical mean-field theory perspective.

A distinctive feature of the normal state of underdoped cuprates is the strong dichotomy between nodal and antinodal regions. The nodal regions display well-defined quasiparticle excitations. In contrast, single-particle lineshapes are very broad near the antinodes, where a pseudogap opens. Cluster extensions of dynamical mean-field theory provide an understanding of this phenomenon as a momentum-selective Mott transition. This will be reviewed in this talk and confronted to experiments and to other theoretical approaches.   

Session H4: Spin-orbit Coupling in Ultracold Gases

Topological phase transitions in ultra-cold Fermi superfluids: the evolution from BCS to BEC under artificial spin-orbit fields

We discuss topological phase transitions in ultra-cold Fermi superfluids induced by interactions and artificial spin orbit fields. We construct the phase diagram for population imbalanced systems at zero and finite temperatures, and analyze spectroscopic and thermodynamic properties to characterize various phase transitions. For balanced systems, the evolution from BCS to BEC superfluids in the presence of spin-orbit effects is only a crossover as the system remains fully gapped, even though a triplet component of the order parameter emerges. However, for imbalanced populations, spin-orbit fields induce a triplet component in the order parameter that produces nodes in the quasi-particle excitation spectrum leading to bulk topological phase transitions of the Lifshitz type. Additionally a fully gapped phase exists, where a crossover from indirect to direct gap occurs, but a topological transition to a gapped phase possessing Majorana fermions edge states does not occur.   

BCS-BEC Crossover and Topological Phase Transition in 3D Spin-Orbit Coupled Degenerate Fermi Gases

We investigate the BCS-BEC crossover in three-dimensional degenerate Fermi gases in the presence of spin-orbit coupling (SOC) and Zeeman field. We show that the superfluid order parameter destroyed by a large Zeeman field can be restored by the SOC. With increasing strengths of the Zeeman field, there is a series of topological quantum phase transitions from a nontopological superfluid state with fully gapped fermionic spectrum to a topological superfluid state with four topologically protected Fermi points (i.e., nodes in the quasiparticle excitation gap) and then to a second topological superfluid state with only two Fermi points. The quasiparticle excitations near the Fermi points realize the long-sought low-temperature analog of Weyl fermions of particle physics. We show that the topological phase transitions can be probed using the experimentally realized momentum-resolved photoemission spectroscopy. Phys. Rev. Lett. 107, 195303 (2011).   

BCS-BEC crossover in 2D spin-orbit coupled degenerate Fermi gases

The recent experimental realization of spin-orbit coupling for ultra-cold atoms has generated much interest in the physics of spin-orbit coupled degenerate Fermi gases. Although recently the BCS-BEC crossover in 3D spin-orbit coupled Fermi gases has been intensively studied, the corresponding 2D crossover physics has remained unexplored. In this talk, we discuss the BCS-BEC crossover physics in 2D degenerate Fermi gases in the presence of spin-orbit coupling. We derive the zero temperature mean field gap and atom number equations suitable for the 2D spin-orbit coupled Fermi gases, from which the dependence of the ground state properties (pairing gap, chemical potential, etc.) on the system parameters (e.g., binding energy, spin-orbit coupling strength) is obtained, both numerically and analytically. We characterize the dependence of the BKT transition temperature as well as the vortex-antivortex lattice melting temperature on the spin-orbit coupling strength and the external Zeeman field.   

One-dimensional Ultracold Fermi Gases with Spin-orbit Coupling

We study one-dimensional ultracold Fermi gases with spin-orbit coupling. We use the Bogoliubov-de Gennes equations to determine pairing order parameter, and find out new phases in addition to fully polarized normal phase, fully paired BCS phase and the FFLO phase. We complete the phase diagram in terms of polarization, interaction parameter and strength of spin-orbit coupling.   

Who is the Lord of the Rings in the Zeeman-spin-orbit Saga: Majorana, Dirac or Lifshitz?

Zeeman, spin-orbit fields and interactions can be tuned in the context of ultra-cold atoms and allow for the visitation of several different phases. For systems with zero Zeeman field, the evolution from BCS to BEC superfluidity in the presence of spin-orbit effects is only a crossover [1]. In contrast, for finite Zeeman fields, spin-orbit coupling induces a triplet component in the order parameter that produces nodes in the quasiparticle excitation spectrum leading to bulk topological phase transitions of the Lifshitz type [2]. A fully gapped phase also exists, where a crossover from indirect to direct gap occurs. For spin-orbit couplings with equal Rashba and Dresselhaus strengths the nodal quasi-particles are Dirac fermions that live at and in the vicinity of rings of nodes. Transitions from and to nodal phases can occur via the emergence of zero-mode Majorana fermions at phase boundaries, where rings of nodes of Dirac fermions annihilate [3]. Lastly, we characterize different phases via spectroscopic and thermodynamic properties and conclude that Lifshitz is the ``Lord of the Rings.'' \\[4pt] [1] Li Han, C. A. R. Sa de Melo, arXiv:1106.3613v1.\\[0pt] [2] Kangjun Seo, Li Han and C. A. R. Sa de Melo, arXiv:1108.4068v2.\\[0pt] [3] Kangjun Seo, Li Han and C. A. R. Sa de Melo, arXiv:1110.6364v1.   

Quantum phases of atomic Fermi gases with spin-orbit coupling

We consider a general anisotropic spin-orbit coupling and analyze the phase diagrams of both balanced and imbalanced Fermi gases for the entire BCS-BEC evolution. First we use the self-consistent mean-field theory at zero temperature, and show that the topological structure of the ground-state phase diagrams is quite robust against the effects of anisotropy. Then we go beyond the mean-field description, and investigate the effects of Gaussian fluctuations near the critical temperature. This allows us to derive the time-dependent Ginzburg-Landau theory, from which we extract the effective mass of the Cooper pairs and their critical condensation temperature in the molecular BEC limit.   

Low-density molecular gas of tightly-bound Rashba-Dresselhaus fermions

We study interacting Rashba-Dresselhaus fermions in two spatial dimensions. First, we present a new exact solution to the two-particle pairing problem of spin-orbit-coupled fermions for arbitrary Rashba and Dresselhaus spin-orbit interactions. An exact molecular wave function and the Green function are explicitly derived along with the binding energy and the spectrum of the molecular state. In the second part, we consider a thermal Boltzmann gas of fermionic molecules and compute the time-of-flight velocity and spin distributions for a single fermion in the gas. We show that the pairing signatures can be observed already in the first-moment expectation values, such as time-of-flight density and spin profiles.   

Order by Disorder in Spin-Orbit Coupled Bose-Einstein Condensates

Motivated by recent experiments, we investigate the system of isotropically-interacting bosons with Rashba spin-orbit coupling. At the non-interacting level, there is a macroscopic ground-state degeneracy due to the many ways bosons can occupy the Rashba spectrum. Interactions treated at the mean-field level restrict the possible ground-state configurations, but there remains an accidental degeneracy not corresponding to any symmetry of the Hamiltonian, indicating the importance of fluctuations. By finding analytical expressions for the collective excitations in the long-wavelength limit and through numerical solution of the full Bogoliubov- de Gennes equations, we show that the system condenses into a single momentum state of the Rashba spectrum via the mechanism of order by disorder. We show that in 3D the quantum depletion for this system is small, while the thermal depletion has an infrared logarithmic divergence, which is removed for finite-size systems. In 2D, on the other hand, thermal fluctuations destabilize the system. This work is supported in part by JQI-PFC.   

Exotic 3D Spin-Orbit Couplings

We describe a scheme for creating an isotropic three-dimensional spin-orbit coupling, dubbed Weyl spin-orbit coupling, in systems of ultracold atoms. This coupling is induced by Raman transitions that link four internal atomic states with a tetrahedral geometry. This spin-orbit coupling gives rise to a Dirac point that is robust against environmental perturbations. We then propose a general procedure for generating exotic three-dimensional spin-orbit couplings with degenerate ground states on more complex manifolds. The procedure is applied to produce a spin-orbit coupling with a toroidal ground state manifold. Finally, we discuss the many-body implications of the exotic spin-orbit couplings.   

Cold-atom systems with synthetic SU(3) spin-orbit coupling

Recently, the ability to create and control artificial gauge fields in cold gases has been experimentally demonstrated. Here, we propose a scheme to realize synthetic SU(3) spin-orbit interactions and derive an effective single-particle Hamiltonian, parameterized by the Gell-Mann matrices. We then investigate a many-body system of SU(3)-spin-orbit-coupled bosons and derive and analyze numerically the Gross-Pitaevskii equation to describe the effect of interaction on the possible ground states. The time-of-flight density profiles to probe various many-body states in the rich phase diagram of the system are calculated.   

Superfluidity of Bosons in Optical lattices with Spin-Orbit coupling

Recent experimental and theoretical work has explored artificial spin-orbit coupling induced among two species of boson. Here we examine superfluidity of a cold gas of bosons with spin-orbit coupling in a periodic optical lattice, in the presence of additional short-range interactions. We compute the density distribution after free expansion from the lattice as a probe of superfluidity, and phase transitions, of the trapped gas.   

Spinor Bose-Einstein Condensates Under Synthetic Gauge Field

Due to the recent popularity of synthetic gauge field in ultracold atoms, I will talk about the combined effects of Rashba spin-orbit coupling (SOC) and rotation in spin -1/2 condensates [X.-Q. Xu et al, Phys. Rev. Lett. 107, 200401 (2011)]. Novel features appear in the ground state wave function, such as the existence of a half-quantum vortex or giant vortex, domains of stripe-like phase, suppressed Skyrmion order. Additionally, I will talk about the interesting mapping between pure Rashba BECs and chiral magnets with Dzyaloshinskii-Moriya (DM) interaction.   

BCS-BEC crossover induced by a synthetic non-Abelian gauge field

We investigate the ground state of interacting spin-$half$ fermions(3D) at a finite density ($\rho \sim k_F^3$) in the presence of a uniform non-Abelian gauge field (with magnitude $\lambda$) that generates a generalized Rashba spin-orbit interaction. For a weak attractive interaction in the singlet channel described by a small negative scattering length $(k_F |a_s| \la 1)$, the ground state in the absence of the gauge field ($\lambda=0$) is a BCS superfluid with large overlapping pairs. With increasing $\lambda$, a non-Abelian gauge field engenders a crossover of this BCS ground state to a BEC ground state of bosons even with a weak attractive interaction. For large gauge couplings $(\lambda/k_F \gg 1)$, the BEC attained is a condensate of bosons whose properties are solely determined by the gauge field (and not by the scattering length); we call these bosons ``rashbons.'' In the absence of interactions ($a_s = 0^-$), the shape of the Fermi surface of the system undergoes a topological transition at a critical gauge coupling $\lambda_T$. For high symmetry gauge field configurations we show that the crossover from the BCS superfluid to the rashbon BEC occurs in the regime of $\lambda$ near $\lambda_T$.   

Trapped fermions in a synthetic non-Abelian gauge field

On increasing the coupling strength ($\lambda$) of a non-Abelian gauge field that induces a generalized Rashba spin-orbit coupling, the topology of the Fermi surface of a homogeneous gas of non-interacting fermions of density $\rho \sim k_F^3$ undergoes a change at a critical value, $\lambda_T \approx k_F$ [PRB {\bf 84}, 014512 (2011)]. We analyze how this affects the size/shape of a cloud of fermions trapped in a harmonic potential. We develop an adiabatic formulation, with Pancharatnam-Berry phase terms, for the one particle states in a trap with the gauge field. Local density approximation reveals that the cloud shrinks in a {\em characteristic fashion with increasing $\lambda$} and predicts a spherical cloud for all gauge field configurations. We show, via a calculation of the cloud shape using exact eigenstates, that for certain gauge fields there is systematic anisotropy in the cloud shape that increases with increasing gauge coupling $\lambda$. An important spin-off of our adiabatic formulation is that it reveals exciting possibilities for the cold-atom realization of interesting Hamiltonians (eg. quantum hall spherical geometry) by using a non-Abelian gauge field in conjunction with another potential.   

Effects of the interplay between spin-orbit coupling and interaction on bosons

We show that spin-orbit coupling drastically changes the properties of bosons. The interplay between the spin-orbit coupling and interaction determines the fate of Bose-Einstein condensate, which may even not exist in the presence of isotropic spin-orbit coupling. For anisotropic spin-orbit coupling, condensates survive and are characterized by anisotropic energy spectrum, with a slower sound velocity along the direction of weaker spin-orbit coupling. The spectrum can also be used to distinguish the plane wave phase and the tripe phase.   

Session H5: Focus Session: Interfaces in Complex Oxides - Polar Interfaces and Ferroelectrics

First-principles study of intermixing and polarization at the DyScO$_3$ /SrTiO$_3$ interface

Exploring oxide interfaces is an attractive challenge, due to the emerging novel behaviors which don't exist in the corresponding parent bulk compounds. E. g. joining two simple band insulators LaAlO$_3$ and SrTiO$_3$ with different polarity can induce conductivity at the interface. We carried out density functional theory (DFT) calculations based on the full potential linearized augmented planewave (FLAPW) method as implemented in the {\tt FLEUR} code (www.flapw.de) for studying the polar to non-polar interface of DyScO$_3$ and SrTiO$_3$. Due to the polar discontinuity, arising from nominally charged DyO or ScO$_2$ layers, sharp interfaces induce a strong ferroelectric-like polarization in the SrTiO$_3$ , while in off-stoichiometric interfaces this discontinuity is avoided and no such polarization can be found. In both scenarios the interface remains insulating with only a small reduction of the bandgap. Our calculations show that chemically mixed interfaces are energetically more favorable than sharp ones. Our DFT calculations explore also different configurations of the Dy and Sr atoms within the mixed interface plane. The calculated ground state configuration is confirmed by experimental observations.   

Growth and transport properties of fractionally \textit{$\delta $}-doped oxide superlattices

LaTiO$_{3}$/SrTiO$_{3 }$(LTO/STO) heterostructures are interesting as they show an intriguing 2D conduction, and their bulk counterpart, La$_{x}$Sr$_{1-x}$TiO$_{3}$ (LSTO), exhibits a filling-controlled insulator-metal transition (IMT). In this study, we investigated the filling controlled IMT in 2D geometry by fabricating monolayer-thick fractionally \textit{$\delta $}--doped LSTO/STO superlattices (SLs), in order to find ways to enhancing the carrier mobility of two dimensional electron gas (2DEG). Fractional layers of LSTO have been grown in between STO using advanced PLD. It is found that the SLs' transport properties are governed by a multichannel conduction with at least two distinctly different carriers: (1) High-density-low-mobility carriers presenting at the LSTO interface layer and (2) low-density-high-mobility carriers residing in the STO layers away from the \textit{$\delta $}--doped layer. By optimizing $x$, we could tune the effective mass and carrier density to enhance the carrier mobility by about an order of magnitude, selectively for the high-density-low-mobility carriers. This proves that the fractional \textit{$\delta $}--doping is an effective way to controlling the filling controlled IMT, resulting in highly improved transport properties   

Characterization of extreme-concentration 2DEGs at the SrTiO$_{3}$/GdTiO$_{3 }$interface

Heterostructures of Mott and band insulators exhibit unique interface properties, including two-dimensional electron gases (2DEGs) with extremely high sheet carrier densities due to the polarization discontinuity at the interface. Of paramount importance for the properties is the location, spatial extent and confinement of the 2DEG. Here, we study the 2DEG with carrier densities of 3x10$^{14}$ cm$^{-2}$ formed at GdTiO$_{3}$/SrTiO$_{3}$ interfaces grown by MBE. Using a self-consistent Schr\"{o}dinger-Poisson solver, we estimate the majority of the carriers are confined in a narrow region ($<$ 3 nm) at the SrTiO$_{3}$-side of the interface. Given the large and rapid spatial variation in charge density, experiments are needed to verify the assumptions underlying such models. We measure the admittance as a function of frequency at different fixed DC bias. To extract the carrier distribution in the depletion approximation, a distributed model is used to account for loss and series resistance effects. The resulting CV profile corresponds to a p-type layer. This is explained with a highly conductive space charge layer, resulting in significant depletion only in the p-type GdTiO$_{3}$. We report on the carrier distribution near the interface.   

Dynamical conductivity of the extreme concentration 2DEGs in GdTiO$_3$/SrTiO$_3$ heterostructures

Metallic conductivity with extremely high 2D carrier concentration is observed at the interface between the Mott insulator GdTiO$_3$ and the band insulator SrTiO$_3$. Irrespective of layer thickness or repeats, MBE-grown GdTiO$_3$/SrTiO$_3$ heterostructures have carrier density of approximately 1/2 electron per interface unit cell, or $3 \times 10^{14}$ cm$^{-2}$ per interface, in excellent agreement with the polar discontinuity model. To probe the orbital character, confinement, and correlations of carriers in this system we have measured the static (dc) and dynamical conductivity of a variety of heterostructures, using a combination of THz time-domain and FTIR spectroscopies. Samples with SrTiO$_3$ layers exceeding $\sim$10 nm thickness exhibit a Drude conductivity that may arise from $d_{xz}$ and $d_{yz}$ electric subbands at the interface. A discrepancy between the measured dynamical and dc conductivity measurements indicates the presence of a few additional carriers with very low scattering rate. By contrast, the Drude response of samples with thinner SrTiO3 layers shows an increased scattering rate with excellent agreement between the dc and THz frequency dynamical conductivity.   

Electrostatic carrier doping of GdTiO$_{3}$/SrTiO$_{3}$ heterostructures

Two-dimensional electron gases (2DEGs) at interfaces between Mott insulators and band insulators have attracted significant attention because they can exhibit unique properties, such as strong electron correlations, superconductivity and magnetism. At interfaces between SrTiO$_{3}$ and the rare earth titanates (Mott insulators) an interfacial fixed polar charge arises due to a polarization discontinuity, which can be compensated by a 2DEG, residing in the bands of the Mott/band insulator. In this presentation, we report on intrinsic electronic reconstructions at a Mott/band insulator interface between stoichiometric GdTiO$_{3}$ and SrTiO$_{3}$ that were grown using molecular beam epitaxy. The sheet carrier densities of all GdTiO$_{3}$/SrTiO$_{3}$ heterostructures containing more than one unit cell of SrTiO$_{3}$ are approximately 1/2 electron per unit cell, independent of layer thickness and growth sequence. These carrier densities closely meet the electrostatic requirements for compensating the fixed charge at these polar interfaces. Based on the experimental measurements, insights into the location and confinement of the charge and the influence of different electrostatic boundary conditions are obtained.   

High mobility electron gas in the surface vicinity of the cubic-perovskite KTaO$_3$ via Ar$^+$-irradiation

KTaO$_{3}$ (KTO), like SrTiO$_{3}$ (STO), can be doped to create a high mobility perovskite oxide semiconductor. However, while the low dimensional electronic states of STO have attracted intense interest via spectroscopic [1,2] and transport [3] studies, analogous investigations in KTO have been limited. Of particular interest is the fact that the cubic crystal symmetry in KTO is preserved at low temperatures, in contrast to STO. Here we electron dope KTO via oxygen-vacancy formation by Ar$^+$-irradiation [4]. Below $T$ = 10 K, the Hall mobility ($>$ 10$^4$ cm$^2$/Vs) of the electrons is significantly higher than in previous studies of STO [4]. The angular dependence of the Shubnikov-de Haas oscillations indicates a Fermi surface without cubic symmetry, in contrast to that expected for bulk KTO. We discuss the possible origins of these data, including the formation of quantum well structures, the coexistence of surface and bulk electrons, and the suppression of cyclotron motion by finite size effects.\\[4pt] [1] A. F. Santander-Syro \textit{et al}., Nature \textbf{469}, 189 (2011).\\[0pt] [2] W. Meevasana \textit{et al}., Nat. Mater. \textbf{10}, 114 (2011).\\[0pt] [3] Y. Kozuka \textit{et al}., Nature \textbf{462}, 487 (2009).\\[0pt] [4] J. H. Ngai \textit{et al.}, Phys. Rev. B \textbf{81}, 241307 (2010).   

Memristive behavior of ferroelectric tunnel junctions

Employment of polarization reversal in ultrathin ferroelectric layers opens new possibilities for development of electronic devices with novel functional properties not available in conventional systems. A particularly promising aspect is realization of resistive switching in the ferroelectric tunnel junctions (FTJ), which can be used as non-charge based logical switches in nonvolatile memory devices. Functionality of FTJs is intrinsically linked to a relationship between polarization orientation and tunneling resistance, which brings about a problem of ferroelectric switching and polarization retention in ultrathin ferroelectric barriers. Here, we demonstrate a giant resistive switching effect of more than 105\% at room temperature in the FTJ device structure composed of an epitaxial BaTiO$_3$ ferroelectric barrier sandwiched between bottom and top electrodes. We provide experimental evidence of the memristive behavior of these FTJs where both the low- and high-resistance states can be tuned by the external voltage by several orders of magnitude. The mechanism of the memristive behavior in our junctions is discussed in terms of the modifications of the tunneling potential profile driven by the charge accumulation in the BaTiO$_3$ layer.   

Exploring and alleviating detrimental interface dipole effects in ultra-thin all-oxide metal-ferroelectric-metal heterostructures

Ultrathin-film metal-ferroelectric-metal heterostructures present an exciting prospect for switchable nanoelectronic memories and devices such as ferroelectric tunnel junctions. The main challenge is to realize ferroelectricity in ultrathin-films where detrimental interface effects become increasingly more pronounced as ferroelectric film thicknesses approach the nanoscale. We studied the ferroelectric polarization of BaTiO$_{3}$ in epitaxial SrRuO$_{3}$/BaTiO$_{3}$/SrRuO$_{3}$ junctions by first-principles density functional theory and phenomenological modeling. The calculations show that the presence of a RuO$_{2}$/BaO termination sequence at the SrRuO$_{3}$/BaTiO$_{3}$ interface leads to a pinned interface dipole and is therefore detrimental to the stability of ferroelectricity, leading to the disappearance of switchable polarization under a certain thickness. Here, we propose to alleviate this behavior by depositing a thin layer of SrTiO$_{3}$ at this interface to suppress the RuO$_{2}$/BaO interface termination sequence, thereby eliminating the associated unfavorable pinned interface dipole. By doing this we find, and experiments confirm, that a switchable ferroelectric state can be stabilized in much thinner heterostructures.   

Domains and Electrostatic Coupling in Ferroelectric Superlattices

Superlattices composed of ferroelectric and paraelectric oxides have been the subject of numerous studies, delving into fundamental questions about ferroelectric size effects, revealing novel interfacial phenomena, and opening new possibilities for tailoring the functional properties of these artificially layered ferroelectrics. Here we examine the role of periodic 180 degree ferroelectric nanodomains on the structural and electrical properties of PbTiO$_3$/SrTiO$_3$ superlattices. Using a combination of X-ray diffraction and electrical measurements, nanoscale motion of domain walls under applied field has been detected and linked to the large enhancement of the dielectric response. Electrostatic interactions between the ferroelectric layers have been studied in detail, revealing an unexpected decoupling of the ferroelectric layers once the paraelectric layer thickness exceeds just a few perovskite unit cells. Recent advances in transmission electron microscopy allowed us to map out the local structural distortions across the superlattice using electron energy loss spectroscopy with unit-cell resolution, revealing highly inhomogeneous polarization profiles near the interfaces and giving new microscopic insight into the behavior of these fascinating materials.   

In situ x-ray study of oxide superlattice growth in reactive molecular-beam epitaxy

Improper ferroelectrity found in ultrashort period PbTiO3/SrTiO3 superlattices has attracted interests as one of new `interfacially engineered' materials.[1] At the interface of PbTiO3/SrTiO3 superlattices, the coupling between the ferroelectric mode and antiferrodistortive rotations of oxygen octahedra creates improper ferroelectricity with a large dielectric constant. PbTiO3/SrTiO3 superlattices were grown using reactive molecular-beam epitaxy in a chamber with in situ x-ray diffraction capability at the Advanced Photon Source. The use of in situ surface x-ray diffraction allows one to study the evolution of oxide heterostructures. Here we present initial studies of PbTiO3 and SrTiO3 single layers as well as superlattices. [1] E. Bousquet, M. Dawber, N. Stucki, C. Lichtensteiger, P. Hermet, S. Gariglio, J.-M. Triscone, and P. Ghosez, Nature 452, 732 (2008).   

The memristive magnetic tunnel junction as a nanoscopic synapse-neuron system

Memristors cover a gap in the capabilities of basic electronic components by remembering the history of the applied electric potentials, and are considered to bring neuromorphic computers closer by imitating the performance of synapses. We used memristive magnetic tunnel junctions based on MgO to demonstrate that the synaptic functionality is complemented by neuron-like behavior in these nanoscopic devices. The synaptic functionality originates in a resistance change caused by a voltage-driven oxygen vacancy motion within the MgO layer. The additional functionality provided by magnetic electrodes enabled a current-driven resistance modulation due to spin-transfer torque. We report on memristive magnetic tunnel junctions characterized by the simultaneous occurrence of resistive switching and tunnel magnetoresistance. Since resistivity provides a natural measure of the synaptic strength, and because of the bipolar nature of the resistance change, long term potentiation and long term depression were emulated. Furthermore, we show that the flux is a good variable for describing voltage-induced resistance variation, which provides the scope for the emulation of spike timing dependend plasticity as well.   

Session H6: Focus Session: van der Waals Bonding in Advanced Material - Methods and Application to Water and Ice

van der Waals interactions in water and ice from density functional theory simulations: improvements and challenges

Accounting for long range van der Waals (vdW) type correlations in the description of water and ice has proven to be one of the most important improvements in density functional theory (DFT)-based simulations of water. I will show how our understanding of the network structure in liquid water has changed with the newly available vdW density functionals, derived from the original version of Dion et al. [Phys. Rev. Lett. 92, 246401 (2004)]. In particular I will focus on the links between the density of water and the interplay between the hydrogen bonds and the so called vdW-bonds, easily identifiable in the liquid. These new vdW-DFs have also proven to be very important in the description of ice. In particular I will show how they are capable of accurately describing the anomalous nuclear quantum effects in ice, bringing DFT simulations and experimental results very close together. Our results show that current vdW-DFs are capable of correctly describing the subtle interplay between inter-molecular libration modes and intra-molecular stretching modes in ice, thus reproducing the experimental results once the zero point of these modes is accounted for in the simulations.   

The Effect of van der Waals Interactions on the Structure of Liquid Water.

In this work, we demonstrate the importance of including van der Waals (vdW) interactions in the theoretical prediction of the structure of liquid water. These effects are investigated by computing and analyzing the oxygen-oxygen, oxygen-hydrogen, and hydrogen-hydrogen radial distribution functions (RDFs) obtained from highly accurate \textit{ab initio}molecular dynamic simulations that explicitly account for vdW interactions. In particular, we utilize an efficient order(N) algorithmic implementation of the self-consistent energy and analytical forces of the density functional based vdW correction proposed by Tkatchenko and Scheffler (PRL 102, 073005 (2009)) to demonstrate the importance of vdW interactions in obtaining RDFs that are in close agreement with experiment. In addition, we also provide an analysis of finite size effects in vdW-based liquid water simulations as well as a comparison to several other competitive theoretical methods for treating vdW interactions.   

Van der Waals interactions and vibrational effects in ice from first principles

We present a comparative study of the equation of state and of the electronic and vibrational properties of ice XI and VIII, as obtained with ab-initio calculations using semi-local (PBE) and nonlocal, van der Waals functionals. The two functionals yield similar electronic properties for both phases, however they perform very differently in describing their vibrational properties, and the transition pressure from the low to the high pressure phase. The latter is overestimated by a factor of about 6 when using PBE and in agreement with experiment when dispersion forces are taken into account. The inclusion of zero point energy contributions does not affect the computed transition pressure, while it substantially affects structural properties, including equilibrium volumes and bulk moduli, especially for the high pressure phase.   

Hydrogen Bonds and van der Waals Forces in Ice at Ambient and High Pressures

The balance between van der Waals (vdW) forces and hydrogen bonding in ambient and high pressure phases of ice has been examined with the first principles approaches, density-functional theory (DFT) and quantum Monte Carlo. At higher pressure, the contribution to the lattice energy from vdW increases and that from hydrogen bonding decreases, leading vdW to have a substantial effect on the transition pressures between the crystalline ice phases. An important consequence, likely to be of relevance to molecular crystals in general, is that the transition pressures obtained from DFT functionals which neglect vdW forces are greatly overestimated [Phys. Rev. Lett. \textbf{107}, 185701 (2011)].   

Higher-order van der Waals coefficients from static multipole polarizability

van der Waals interaction is a long-range nonlocal correlation arising from instantaneous charge fluctuations on each fragment. Though very weak, it considerably affects the properties of molecules and solids. Evaluation of van der Waals coefficients is of strong current interest. In this work, we have derived a general expression for these coefficients in terms of static multipole polarizability only. Applications of the present theory to atom as well as molecular pair interactions have been made.   

Quantum Monte Carlo Simulation of condensed van der Waals Systems

Van der Waals forces are as ubiquitous as infamous. While post-Hartree-Fock methods enable accurate estimates of these forces in molecules and clusters, they remain elusive for dealing with many-electron condensed phase systems. We present Quantum Monte Carlo [1,2] results for condensed van der Waals systems. Interatomic many-body contributions to cohesive energies and bulk modulus will be discussed. Numerical evidence is presented for crystals of rare gas atoms, and compared to experiments and methods [3]. Sandia National Laboratories is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. DoE's National Nuclear Security Administration under Contract No. DE-AC04-94AL85000.\\[4pt] [1] J. Kim, K. Esler, J. McMinis and D. Ceperley, SciDAC 2010, J. of Physics: Conference series, Chattanooga, Tennessee, July 11 2011 \\[0pt] [2] QMCPACK simulation suite, http://qmcpack.cmscc.org (unpublished)\\[0pt] [3] O. A. von Lillienfeld and A. Tkatchenko, J. Chem. Phys. 132 234109 (2010)   

Van der Waals interactions based on maximally localized Wannier functions in ABINIT

We review the recent implementation\footnote{C. Espejo et al. Computer Phys. Comm. In press (2011), doi:10.1016/j.cpc.2011.11.003} of the method to evaluate van der Waals (vdW) interactions based on maximally localized Wannier functions\footnote{P. L. Silvestrelli. Phys. Rev. B., \textbf{100}, 053002 (2008)}$^,$\footnote{P. L. Silvestrelli. J. Phys. Chem. A., \textbf{113}, 5224 (2009)} in the DFT software ABINIT\footnote{X. Gonze et al. Computer Phys. Comm. \textbf{180}, 2582 (2009)}. The implementation allows for the evaluation of vdW interaction energies for molecular and periodic systems on the same grounds and at a low additional computational cost as compared with a normal DFT calculation. Some results on test systems such as Ar$_2$, benzene dimer and graphene bilayer show both its reliabilty and performance. Discussion of new defined variables controlling the calculation and guide lines for the user will be presented along with an application to MoS$_2$ structure.   

Density-Functional Theory with Screened van der Waals Interactions for the Modeling of Hybrid Inorganic/Organic Systems

The electronic properties and the function of hybrid inorganic/organic systems (HIOS) are intimately linked to their geometry, with van der Waals (vdW) interactions playing an essential role for the latter. Here we show that the inclusion of the many--body collective response of the substrate electrons inside the inorganic bulk enables us to reliably predict the HIOS geometries and energies. Specifically, dispersion-corrected density-functional theory (the DFT+vdW approach) [\textit{PRL} {\bf 102}, 073005 (2009)], is combined with the Lifshitz-Zaremba-Kohn theory [\textit{PRB} {\bf 13}, 2270 (1976)] for the non--local Coulomb screening within the bulk. Our method (DFT+vdW$^{\rm surf} $) includes both image-plane and interface polarization effects. We show that DFT+vdW$^{\rm surf}$ yields geometries in remarkable agreement ($ \approx $~0.1 \AA) with normal incidence x--ray standing wave measurements for the 3,4,9,10--perylene--tetracarboxylic acid dianhydride (C$_{24}$H$_{8}$O$_{6}$, PTCDA) molecule on Cu(111), Ag(111), and Au(111). Similarly accurate results are obtained for xenon and benzene adsorbed on metal surfaces.   

Stress and Vibrational Properties of non-local van der Waals exchange-correlation functionals

Van der Waals interaction is an essential component in the description of soft matter and plays an important role in many other systems, from adsorbates to water interaction. Within the framework of Density Functional Theory in the last years a great effort has been made to overcome the limitations of LDA or GGA functionals, and a new class of non-local functionals is now filling the gap giving interesting results. We present here several new improvements in this field, and some selected applications. In particular, i) we worked out the theoretical formalism needed to define both the stress tensor and the phonon vibrations associated to the non-local functional form proposed by Dion[1], and ii) we developed and implemented in the Quantum ESPRESSO simulation package a new functional inspired by the work of Vydrov and Van Voorhis[2], simple to compute efficiently in plane wave approach and with great performances on the S22 set. Finally we present some results obtained on aminoacid crystal and other simple molecules where these new tools are benchmarked and compared with experimental results and other theoretical approaches. \\[4pt] [1] M. Dion, B. I. Lundqvist et al., Phys. Rev. Lett. 92, 246401 (2004); \\[0pt] [2] Oleg A. Vydrov and Troy Van Voorhis , Phys. Rev. Lett. 103, 06300   

Dispersion interactions in Density Functional Theory

Semilocal functionals in Density Functional Theory (DFT) achieve high accuracy simulating a wide range of systems, but miss the effect of dispersion (vdW) interactions, important in weakly bound systems. We study two different methods to include vdW in DFT: First, we investigate a recent approach [1] to evaluate the vdW contribution to the total energy using maximally-localized Wannier functions. Using a set of simple dimers, we show that it has a number of shortcomings that hamper its predictive power; we then develop and implement a series of improvements [2] and obtain binding energies and equilibrium geometries in closer agreement to quantum-chemical coupled-cluster calculations. Second, we implement the vdW-DF functional [3], using Soler's method [4], within ONETEP [5], a linear-scaling DFT code, and apply it to a range of systems. This method within a linear-scaling DFT code allows the simulation of weakly bound systems of larger scale, such as organic/inorganic interfaces, biological systems and implicit solvation models. [1] P. Silvestrelli, JPC A 113, 5224 (2009). [2] L. Andrinopoulos et al, JCP 135, 154105 (2011). [3] M. Dion et al, PRL 92, 246401 (2004). [4] G. Rom{\'a}n-P{\'e}rez, J.M. Soler, PRL 103, 096102 (2009). [5] C. Skylaris et al, JCP 122, 084119 (2005).   

A Qbox Implementation of van der Waals Density Functionals with Applications

We present an implementation of the non-local van der Waals correlation functional proposed by Dion et al. [1] in the Qbox code [2]. We develop a simple approach to remove the logarithmic singularity in the kernel function, and derive the non-local potential needed for self-consistent calculations of energies, ionic forces and stress for simulations in arbitrary-shaped unit cells. We compare the performance of five different van der Waals functionals in applications to the benzene-water dimer, the benzene crystal and other organic molecular crystals. \\[4pt] [1] M. Dion et al. Phys. Rev. Lett. 92, 246401 (2004)\\[0pt] [2] http://eslab.ucdavis.edu/software/qbox   

Session H7: Focus Session: Carbon Nanotube Sensing

Selective, Reversible Near-Infrared Fluorescence Quenching of Boronic Acid--Single-Walled Carbon Nanotube Complexes in Response to Glucose

We present the high throughput screening of a library of 30 boronic acid derivatives to form complexes with sodium cholate suspended single-walled carbon nanotubes (SWNTs) to screen for their ability to reversibly report glucose binding via a change in SWNT fluorescence. The screening identifies 4-cyanophenylboronic acid which uniquely causes a reversible wavelength red-shift in SWNT emission. The results also identify 4-chlorophenylboronic acid which demonstrates a turn-on fluorescence response when complexed with SWNTs upon glucose binding in the physiological range of glucose concentration (0 to 30 mM). The mechanism of fluorescence modulation in both of these cases is revealed to be a photo-induced excited-state electron transfer that can be disrupted by boronate ion formation upon glucose binding. This ``turn-on'' sensing scheme that uses the reversible fluorescence quenching and wavelength shift of the BA--SWNT complex offers a new approach for nIR optical sensing of glucose.   

The transduction mechanism of carbon nanotube transistors monitoring single molecule protein dynamics

Recently we have demonstrated high resolution, real-time monitoring of single molecule chemistry using molecules attached to a single-walled carbon nanotube (SWCNT) transistor. The transduction mechanism of SWCNT sensing is often claimed to be due to charge transfer, but here we clearly show the entire effect to be electrostatic. In this study, the chemical system of interest is lysozyme and its enzymatic processing of its binding substrate, peptidoglycan. We investigate the interaction of a lysozyme molecule with the SWCNT conductivity by building devices out of eight different lysozyme variants synthesized by mutagenesis. Each lysozyme variant has a different sequence of surface charges near the SWCNT attachment site, providing a calibrated method of looking at the electrostatic interactions. We observe that positively- and negatively-charged amino acids induce signals of opposite magnitude, while quasi-neutral amino acids like alanine induce little signal at all. The results indicate that careful consideration and manipulation of the charge environment near the attachment site may enhance the signal-to-noise of SWCNT sensors for studying single molecule interactions.   

Electrical Detection of Resonance in a Helically Coiled Carbon Nanowire

Helically coiled Carbon Nanowires (HCNWs) are promising elements, both for their promise as components for NEMS devices as well as for fundamental research. This is due primarily to their exotic geometry. We present here the electrostatic excitation of a HCNW cantilever to resonance and an entirely electrical measurement of the same using harmonic detection of resonance (HDR). The correlation between the directly observed resonance and the electrical signal is shown and, in addition to calculating a lateral spring constant from the observed resonance frequency, we examine the nonlinear behavior of the HCNW when driven to large amplitudes of vibration. Specifically, elliptical oscillation is visually evident and we have measured the electrical response of the corresponding combination mode.   

Arrays of Versatile DNA-Carbon Nanotube Hybrid Chemical Sensor for Vapor Analyte Detection

Large arrays of nanoscale sensors were fabricated for the detection of small molecules based on single-stranded DNA (ssDNA) for chemical recognition and single-walled carbon nanotube field effect transistors (SWNT FETs) for electronic read-out. These versatile sensors are capable of detecting very low concentrations of molecules ranging from volatile organic compounds, whose detection could provide a method for detection or identification of individuals, to noxious compounds designed to harm them. In this work, we deposited enriched semiconducting SWNTs on Si/SiO2 with an APTES adhesion layer. Photolithographically defined contacts resulted in high yield, high performance arrays of SWNT FETs, which were then individually coated in different ssDNA. The arrays of devices were then simultaneously exposed to analytes down to ppb concentrations. The sensing response of a single device is both analyte and ssDNA sequence dependant. The response and recovery to baseline are both fast (seconds) and repeatable without need for refreshing. By using large arrays of differently functionalized devices, we distinguished similar analytes and established electronic signatures indicative of their presence, paving the way for incorporation of ssDNA/SWNT FET arrays in ``electronic nose''-type systems.   

Environmental Charge Noise in Suspended Carbon Nanotube Biosensors

Carbon nanotube field effect transistors (CNT FETs) are a promising platform for probing biological systems at nanometer length scales. The sensitivity of a CNT FET sensor is ultimately limited by intrinsic fluctuations in the conductance of the sensor. Our work aims to understand the mechanisms responsible for these conductance fluctuations, and therefore understand the fundamental detection limits of charge-sensitive biosensors. We have measured conductance fluctuations in both surface-bound CNTs and suspended CNTs. Experiments are performed in physiological buffers -- the typical environment for real-time biosensing. We compare our measurements to a charge noise model and the Hooge model. We find good agreement with the charge noise model, and find that charge noise is reduced by 10 --fold when CNTs are suspended rather than surface-bound. By measuring conductance fluctuations in a variety of liquid buffers we uncover new clues about the origins of charge noise in electrolyte environments.   

Generalized Protein Attachment Chemistry for Highly Sensitive Carbon Nanotube-Based Biosensors

We developed a label free covalent functionalization procedure for attaching proteins to carbon nanotube field effect transistors (CNTFETs). Biomarker proteins are becoming increasingly useful for early diagnosis of disease, ranging from cancer to arthritis to stress. Current clinical immunoassays for measuring patient protein levels are costly and require significant processing time. Using diazonium salts followed by stabilization of carboxylic acid groups, we can attach a variety of proteins to carbon nanotubes as confirmed by atomic force microscopy. Proteins maintain the integrity of their epitope and bind to their corresponding complementary proteins. Carbon nanotube transistors are superior readout elements for such protein binding events due to their speed and comparable scale. Resulting changes in the electronic transport properties of CNTFETs demonstrate a concentration-dependent response. Binding of osteopontin (OPN), a biomarker for prostate cancer, to its complementary single chain variable fragment (scFv) can be detected down to 1 pg/mL with these methods. Moreover, these devices exhibit selectivity for OPN. Such high sensitivity biosensors could be used in parallel to test a single small volume patient sample for any number of potentially ominous biomarker proteins.   

Detection of carbon nanotubes in plant roots through microwave-induced heating

We demonstrate a novel technique for quantitative detection of CNTs in biological samples by utilizing the thermal response of CNTs under microwave irradiation. In particular, rapid heating of CNTs due to microwave absorption was employed to quantify the amount of CNTs present in alfalfa plant roots. Alfalfa roots were prepared by injecting a known amount of CNTs (single walled and multi walled) and exposed to 30-50 W microwave power to generate calibration curves (temperature rise vs. CNT mass). These calibration curves serve as a characterization tool to determine the unknown amount of CNTs absorbed by alfalfa plant roots grown in CNT-laden soil with superior accuracy and sensitivity. Moreover, the threshold for detectable CNT concentration is much lower than common analytical methods of detecting nanomaterials, such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Raman spectroscopy. Considering the lack of effective detection methods for CNT uptake in plants, this method is not only unique but also practical, as it addresses a major problem in the field of nanotoxicology risk assessment.   

Adsorption of noble gases on individual suspended single-walled carbon nanotubes

Suspended single-walled carbon nanotubes can act as nanomechanical resonators, offering the ability to detect an adsorbed substance with very high sensitivity via the frequency shift due to the adsorbed mass. By measuring the resonance frequency electrically in the presence of vapors at controlled temperature and pressure we have obtained isotherms for 4He, Ar, Kr and Xe, on multiple nanotubes. The behavior resembles that on graphite but with notable differences, including weaker binding energies. The lower binding allows access to behavior at lower 2D chemical potential than on conventional substrates. For 4He the binding energy is reduced by as much as a factor of two. For Ar the derived two-dimensional phase diagram is similar to that on conventional substrates. For Kr there is variation between nanotubes which may be related to commensurability and area per site on the surface of a cylinder. Work supported by NSF DMR 0907690.   

Conductance isotherms for adsorption of noble gases on individual single-walled carbon nanotubes

Using transistors made from suspended carbon nanotubes allows one to probe the interaction of adsorbed atoms and molecules with the carbon substrate electrons. We have studied the effects of adsorbing He, Ne, Ar, Kr, Xe, and other gases on the electrical properties of individual suspended single-walled nanotubes, as a function of pressure and temperature. The conductance changes measurably, and sometimes dramatically, as a monolayer forms and undergoes phase transitions. It yields complementary information to the coverage, which is obtained from the mass shift in the natural vibrational frequency of the nanotube. For example, measurements below the 2D critical point show nonmonotonic features and fluctuations heralding the first-order phase transition. Conductance changes can be measured on a timescale of milliseconds, permitting studies of the dynamics of the monolayer. In the nonlinear regime we observe features in the I-V characteristics as phase transitions are induced by the current and nonequilibrium stationary states occur.   

Dependence of adsorption kinetics on the geometry of substrate

We report on the results of an adsorption kinetics study of linear alkanes on planar graphite and on aligned multiwalled carbon nanotubes. The kinetics study was performed by monitoring the equilibration times for methane, butane and pentane adsorbed on these two substrates as a function of fractional coverage.~ For methane, the time required to reach equilibration was found to decrease as the surface coverage increased. However, for butane and pentane, a systematic increase in the equilibration time with increasing fractional coverage was observed. Although similar results were found for both substrates, the waiting times for longer alkanes adsorbed on multiwalled carbon nanotubes, were found to be longer than the ones obtained for planar graphite. We speculate that the observed increase in the equilibration time with coverage for the longer alkanes is due to the rearrangement of the adsorbed molecules. Results of the current study will also be compared to our previous study of adsorption kinetics of linear alkanes on single-walled carbon nanotubes.   

New concepts in molecular and energy transport within carbon nanotubes: thermopower waves and stochastically resonant ion channels

Our laboratory has been interested in how carbon nanotubes can be utilized to illustrate new concepts in molecular and energy transfer. In the first example, we predict and demonstrate the concept of thermopower waves for energy generation [1]. Coupling an exothermic chemical reaction with a thermally conductive CNT creates a self-propagating reactive wave driven along its length. We realize such waves in MWNT and show that they produce concomitant electrical pulses of high specific power $>$7 kW/kg. Such waves of high power density may find uses as unique energy sources. In the second system, we fabricate and study SWNT ion channels for the first time [2] and show that the longest, highest aspect ratio, and smallest diameter synthetic nanopore examined to date, a 500 $\mu $m SWNT, demonstrates oscillations in electro-osmotic current at specific ranges of electric field, that are the signatures of coherence resonance, yielding self-generated rhythmic and frequency locked transport. The observed oscillations in the current occur due to a coupling between stochastic pore blocking and a diffusion limitation that develops at the pore mouth during proton transport. \\[4pt] [1] Choi W, Hong S, Abrahamson JT, Han JH, Song C, Nair N, Baik S, \textbf{Strano MS}: Chemically driven carbon-nanotube-guided thermopower waves. NATURE MATERIALS, 9 (2010) 423-429.\\[0pt] [2] Lee, CY, Choi W, Han, JH, \textbf{Strano MS}: Coherence Resonance in a Single-Walled Carbon Nanotube Ion Channel. SCIENCE, 239   

Neon adsorption on oxidized single-walled carbon nanohorns

We will present the results of a study of neon adsorption on oxidized single-walled carbon nanohorns. Our adsorption isotherm measurements were conducted at temperatures below 24.5 K, the triple point for Ne. Results for the effective specific surface area and for the effective pore volume of the nanohorn aggregates will be presented. We will also report on the sorbent-loading dependence of the isosteric heat of neon on the nanohorns, and on the binding energy. Our results for this system will be compared with those obtained for Ne on a sample of dahlia-like nanohorns annealed at 520 K.   

Carbon dioxide adsorption on open single walled carbon nanohorn aggregates

The adsorption of carbon dioxide was measured on aggregates of single-walled carbon nanohorns treated with H2O2. Volumetric adsorption isotherms were conducted at several temperatures between 162 and 212 K (below the triple point for CO2). Results on adsorption equilibration times, on the substrate-loading dependence of the isosteric heat, and on the binding energy will be presented. The effective specific surface area values obtained in this study will be compared with the ones obtained for other carbon nanohorn samples.   

Session H8: Focus Session: Frustrated Magnetism - Kagome I

Confinement transitions of Z2 spin liquids on the kagome lattice

Motivated by numerical evidence of a Z2 spin liquid ground state of the Heisenberg model on the kagome lattice and indications of a proximate valence bond solid (VBS) phase (see S. Yan et al, Science 332, 1173 (2011)), we study quantum phase transitions between Z2 spin liquids and VBS states, in which the space group of the kagome lattice is broken. These confinement transitions are driven by the condensation of elementary vortex excitations of the Z2 spin liquid, so called visons. In this talk I will show how a projective symmetry group (PSG) analysis of effective models for the visons can be used to construct quantum field theories for such confinement transitions, which in turn allow for a classification of the spatial symmetries of possible VBS states. Interestingly, the kagome lattice is unique in the sense that the critical properties at the confinement transition are seemingly not described by the Wilson-Fisher fixed point, as is the case for other lattice geometries.   

NMR study of the spin-1/2 near-kagome system Vesignieite

The spin-1/2 kagome lattice antiferromagnet is understood to be an ideal setting in which to find novel quantum spin liquid physics. Here, $^{51}$V NMR results are presented on the quantum spin system Vesignieite, which closely approximates such an antiferromagnetic kagome model, possessing a minute 0.7\% length difference between inequivalent Cu-Cu bonds. We obtain a measure of the intrinsic magnetic susceptibility of the near-kagome lattice, which shows commonalities with other kagome systems, in particular Herbertsmithite. Meanwhile, the system is found to undergo partial spin freezing at a surprisingly high temperature of $T_C = 9 K \simeq J/6$. Through a loss of NMR intensity and detailed analysis of the spectral linewidth, we infer a heterogeneous ground state in which 50\% of the spins are very weakly frozen, with a moment of $\sim 0.2$ $\mu_B$ and the remaining 50\% remain dynamic down to very low temperatures. These results are found to be highly consistent with $\mu$SR studies, which find a similar frozen fraction and small size of magnetic moment. We propose that the elevated transition temperature and weakly frozen ground state are explained by the Dzyaloshinskii-Moriya interaction and a proximity to the resulting quantum phase transition.   

Magnetization Process of Spatially Anisotropic Kagome Heisenberg Model

Motivated by recent experiments on volborthite, a typical spin-1/2 antiferromagnet with a kagome lattice structure, we study magnetization process of a Heisenberg model on a kagome lattice with a spatial anisotropy in applied magnetic fields. First, for the classical Heisenberg model, by using the Monte Carlo method, we find a magnetization step due to the anisotropy at low temperatures and low magnetic fields. The magnetization step signals a first-order transition, between two phases distinguished by distinct and well-developed short-range spin correlations, one characterized by a local $120^{\circ}$ structure and the other by a partially spin-flopped structure. These states are also evident in magnon dispersions based on a classical spin configuration for each phase. Then, to clarify how quantum fluctuations affect the magnetization process, we calculate the sublattice magnetization by using the exact diagonalization method. We find that the sublattice magnetization process of the quantum model looks qualitatively similar to that of the classical model, which indicates that the spin structure observed in the classical model also appears in the quantum model. Finally, we point out the relevance of our results to the magnetization steps observed in volborthite.   

Investigation of fermi-liquid-like specific heat and spin-density-wave signatures in the distorted kagome compound Volborthite, Cu$_3$V$_2$O$_7$(OH)$_2$$\cdot2$H$_2$O

The distorted kagome compound Volborthite shows signatures of spin-density-wave magnetic order and Fermi-liquid specific heat at low temperatures and magnetic fields. A recent density functional study [O. Janson {\it et al}., Phys. Rev. B {\bf{82}}, 104434 (2010)] suggests that Volborthite can be viewed as coupled frustrated Heisenberg spin chains, a model we approach using a slave-fermion representation of the spins. For a certain range of couplings, our mean-field theory finds a Fermi surface of spinons, a portion of which contains nesting. We investigate whether the coexistence of a U(1) spin liquid with a spinon Fermi surface, along with a spinon spin-density-wave, may describe the aforementioned features of the low-field phase. Furthermore, at higher fields, conventional magnetically ordered states are found. We examine if higher magnetic fields can lead to the destruction of the fermi surface, prompting a spinon confinement transition into such a conventionally magnetically ordered state.   

Experimental signatures of spin liquid physics on the S=1/2 kagom\'{e} lattice

I will describe our recent experimental progress on the quest to study novel ground states in frustrated magnets. New states of matter may be produced if quantum effects and frustration conspire to prevent the ground state from achieving classical order. Materials based on the kagom\'{e} lattice appear to be ideal hosts for the possibility of a quantum spin liquid ground state in two-dimensions. I will discuss our work which includes single crystal growth, bulk characterization, and neutron scattering measurements of the S=1/2 kagom\'{e} lattice material ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$ (also known as herbertsmithite). Recent susceptibility measurements on single crystals yield valuable information on the additional terms in the spin Hamiltonian beyond nearest neighbor Heisenberg exchange, and anomalous x-ray diffraction yields detailed information on the presence of a small amount of atomic impurities. Most interestingly, inelastic neutron scattering measurements of the spin correlations in a single crystal sample reveal a continuum of spinon excitations in this two-dimensional insulating magnet. We will discuss our results in relation to recent theories for spin liquid physics on the S=1/2 kagom\'{e} lattice.   

Spin cluster operator theory for the kagome lattice antiferromagnet

The spin-1/2 quantum antiferromagnet on the kagome lattice provides a quintessential example in the strongly correlated electron physics where both effects of geometric frustration and quantum fluctuation are pushed to their limit. Among possible non-magnetic ground states, the valence bond solid (VBS) with a 36-site unit cell is one of the most promising candidates. Improving the bond operator theory, we propose a new approach dubbed as the spin cluster operator theory in which extended clusters of spin are treated as fundamental building blocks of the system. As a result, it is shown that the lowest spin excitation has a gap much lower than the previous value obtained by the bond operator theory, narrowing the difference against exact diagonalization results.   

Low Energy Spectrum and Correlation Functions of the $S=1/2$ Kagome Antiferromagnet

We report a large scale Exact Diagonalization study of the $S=1/2$ Heisenberg antiferromagnet on samples of up to 48 sites. The singlet spectrum at low energies involves only energy levels at the Gamma point, indicating the probable absence of spontaneous translation symmetry breaking in the thermodynamic limit. The spin-spin correlations are short ranged as expected, and the dimer-dimer correlation functions reveal traces of diamond-like resonances. We discuss the compatibility of our results with different theoretical proposals for the ground state of the Kagome antiferromagnet   

Valence bond crystals in the kagome spin-1/2 Heisenberg antiferromagnet: Symmetry classification and projected wave function study

We present a hierarchical group theoretical classification and representation of Valence bond crystal (VBC) phases on the kagome lattice. Starting from the most symmetric parent VBC, we enumerate and give the ansatz for all 6, 12, and 36-site unit cell VBC's in order of increasing number of broken point group symmetry elements. We treat the VBC's within the class of Gutzwiller projected fermonic variational wave functions, which are optimized using a sophisticated implementation of the stochastic reconfiguration method. In particular, for the spin-1/2 quantum Heisenberg antiferromagnetic model, we show that the U(1) Dirac spin liquid is remarkably stable (locally and globally) with respect to all possible VBC patterns enumerated. However, upon addition of a small ferromagnetic next-nearest-neighbor coupling we find that the lowest energy state is a non-trivial generalized 36-site VBC, which can be regarded as being continuously connected to a uniform RVB spin liquid. We also communicate the ground state energy on the kagome 48 site cluster for the nearest-neighbor spin-1/2 quantum Heisenberg antiferromagnetic model, using the technique of application of a few Lanczos steps (within a variational Monte carlo scheme) on the U(1) Dirac spin liquid and the uniform RVB wave function.   

Neutron Scattering and Thermodynamic Studies of Low Spin Kagome Magnets

Materials containing low quantum number spins arranged on a kagome lattice are some of the most promising candidates to display spin liquid ground states due to the high degree of geometric frustration. Cu(1,3-bdc) is a hybrid organometallic compound featuring antiferromagnetically coupled S=$\frac{1}{2}$ Cu$^{2+}$ ions on a kagome lattice. Below T=1.8 K the magnetic moments enter a quasi-static phase with no long range magnetic order but extremely slow spin fluctuations. Application of a magnetic field quickly leads to a competing magnetic phase, with a 1 Tesla field able to completely polarize the magnetization below T=2 K. We present inelastic neutron scattering measurements of Cu(1,3-bdc) and note the emergence of low-energy modes in the quasi-static phase. We also present new thermodynamic data on the compound BaNi$_{3}$(OH)$_{2}$(VO$_{4}$)$_{2}$, recently synthesized by our group, which features S=1 Ni$^{2+}$ ions on a kagome lattice.   

Magnetic studies of S=1/2 kagom\'e lattice single crystals

Herbertsmithite ZnCu3(OH)6Cl2--one of the most promising quantum spin liquid candidates--presents a promising system for studies of frustrated magnetism on an S=1/2 kagom\'e lattice. Following our recent success in crystal growth, we have measured anisotropies in the magnetic susceptibility and specific heat. The implication on the Hamiltonian will be discussed. Specific heat has been measured at dilution fridge temperatures up to 18 T on a single crystal sample which gives further information on the low temperature phases. In addition, inelastic neutron scattering has been performed and the broad continuum observed is consistent with deconfined 2D spinons which lends further support of herbertsmithite's quantum spin liquid candidacy.   

Single Crystal NMR Study of Frustrated Spin-Liquid in S = 1/2 Kagome Lattice $ZnCu_{3}(OD)_{6}Cl_{2}$

Herbertsmithite $ZnCu_{3}(OD)_{6}Cl_{2}$ is one of the most promising examples for a quantum spin liquid state. Despite the remarkable absence of long range magnetic order down to at least 50mK, understanding the magnetic properties of $ZnCu_{3}(OD)_{6}Cl_{2}$ remains a challenge. This is mainly due to the difficulty in locating the defects, and in understanding the possible role of defects in the physical properties of this material. We have investigated the local magnetic and lattice environment of $ZnCu_{3}(OD)_{6}Cl_{2}$ single crystals\footnote{T. H. Han \textit{et al}., Phys. Rev. B {\bf 83}, 100402(R) (2011)} using NMR techniques\footnote{T. Imai \textit{et al}., Phys. Rev. B {\bf 84}, 020411(R) (2011); see also Phys. Rev. Lett. {\bf 100}, 077203 (2008)}. With successful identification of $^{2}D$ NMR signals arising from the nearest neighbors of $Cu^{2+}$ defects substituting Zn, we find that $14(2)\%$ of Zn sites are occupied by these weakly interacting $Cu^{2+}$ defect spins, which contribute to the large Curie-Weiss enhancement of bulk susceptibility at low temperatures. We then discuss the key aspects of nuclear spin-lattice relaxation rate $1/T_{1}$ measured near the defect and intrinsic sites.   

Session H9: Focus Session: Complex Bulk Oxides: Ruthenates

Nonlinear conduction phenomena in the Mott insulator Ca$_{2}$RuO$_{4}$

The 4$d$-electron Mott insulator Ca$_2$RuO$_4$ has attracted considerable attention because of its rich electronic states dramatically varied by temperature change, pressure, and isovalent Sr substitution. Recently, Nakamura {\it et al}. reported an intriguing result of current-voltage characteristics in Ca$_2$RuO$_4$ and found an $E$-induced insulator-to-metal transition caused by relatively low electric field $E$ $\sim$ 50 V/cm [1]. In this study we investigate the nonlinear conduction phenomena in the Mott insulating phase of Ca$_2$RuO$_4$ with a proper evaluation for self-heating effects. By utilizing a non-contact infrared thermometer, sample temperature was accurately determined even in the presence of large Joule heating. The resistivity shows a typical insulating behavior featured by thermal activation with an energy gap, but clearly depends on the applied currents. The result is highlighted by a strong suppression of the energy gap by the electrical currents. A striking similarity to the current dependence of charge-order gap in organic insulators is discussed in terms of the nonequilibrium phase transition. \\[4pt] [1] F. Nakamura {\it et al.}, (submitted).   

Emergent electronic and magnetic state in Ca$_{3}$Ru$_{2}$O$_{7}$ induced by Ti doping

We report an emergent electronic and magnetic state in the bilayer ruthenate Ca$_{3}$Ru$_{2}$O$_{7}$ upon doping with a small concentration of Ti on the Ru sites. In contrast to a quasi-two-dimensional metallic state in Ca$_{3}$Ru$_{2}$O$_{7}$, which has an antiferromagnetic state formed by ferromagnetic bilayers stacked antiferromagnetically along the c-axis [1,2], we find an insulating state with a ``G''-type nearest neighbor antiferromagnetic order in Ca$_{3}$(Ru$_{1-x}$Ti$_{x})_{2}$O$_{7}$ for $x$ $>$= 0.03 [3]. The close proximity of these two distinct electronic and magnetic states demonstrates unique competing magnetic interactions in Ca$_{3}$Ru$_{2}$O$_{7}$, which provides a rare opportunity to investigate the interplay between correlated metal physics and Mott physics. Work supported by U.S. DOE. \\[4pt] [1] W. Bao \textit{et al}., Phys. Rev. Lett. \textbf{100}, 247203 (2008).\\[0pt] [2] X. Ke \textit{et al}., Phys. Rev. B \textbf{84}, 014422 (2011).\\[0pt] [3] X. Ke \textit{et al}., Phys. Rev. B \textbf{84}, 201102 (R) (2011).   

From quasi-2D metal with ferromagnetic bilayers to Mott insulator with G-type antiferromagnetic order in Ca$_{3}$(Ru$_{1-x}$Ti$_{x}$)$_{2}$O$_{7}$

Ca$_{3}$Ru$_{2}$O$_{7}$ exhibits unique electronic and magnetic properties, such as giant magnetoresistance, a quasi-2D metallic ground state, and antiferromagnetic (AFM) order comprising of ferromagnetic bilayers coupled antiferromagnetically along the c-axis [1-3]. In this talk, we will show that only a few percent of Ti-doping at Ru sites can tune the ground state to a Mott-insulating state with ``G''-type, nearest neighbor AFM order [4]. We have established the electronic and magnetic phase diagram of this doped system to address the underlying physics of such a Mott-transition. We find that a strong scattering effect due to the Ti ions'' empty 3d orbital significantly reduces electrons'' itinerancy, playing a pivotal role in the suppression of the bilayer ferromagnetism and inducing the Mott transition. These findings imply that Ca$_{3}$Ru$_{2}$O$_{7}$ involves competition between the antiferromagnetism due to the Mott transition and the itinerant ferromagnetism due to a Stoner instability. [1] X.N. Lin et al., Phys. Rev. Lett. 95, 017203(2005). [2] W. Bao et al., Phys. Rev. Lett. 100, 247203 (2008). [3] Y. Yoshida et al., Phys. Rev. B 69, 220411 (R) (2004). [4] X. Ke et al., Phys. Rev. B 84, 201102 (R) (2011).   

Orbital selective phase transition

Orbital selective phase transition (OSPT), proposed to be responsible for the coexistence of localized and itinerant electrons, has attracted extensive interest from both experimentalists and theoreticians, since the observation of an anomalous behavior with a Curie-Weiss-like local spin in the metallic phase of Ca$_{2-x}$Sr$_x$RuO$_4$ at $0.2 \leq x \leq 0.5$. Recently, even more attentions have been paid to OSPT since the coexistence of localized and itinerant electrons may reconcile the strong debates on how to understand the origin of magnetism in various iron-based superconductors. Here, various mechanisms for OSPT are reviewed and a new mechanism will be proposed. The distinct band dispersion of different orbitals, which should be generally satisfied in various materials, is identified to be the crucial point for OSPT with magnetic order. Such an OSPT are not sensitive to the strength of Hund's rule coupling. Heavy doping favors collinear antiferromagnetic state over the OSPT. Discussions are made related to the pnictides.   

Magnetic order arising from chemical chaos in A2MnRuO6 (A = Sr, Ca) double perovskites

Experimentally Sr$_{2}$MnRuO$_{6}$ is observed to be a c-type antiferromagnetic insulator with a tetragonal structure, while Ca$_{2}$MnRuO$_{6}$ is found to be a metallic ferrimagnet with an orthorhombic structure. Both compounds display magnetic ordering even in the absence of any recognizable chemical ordering of Mn and Ru ions. In this work, we present first principles calculations to show that the change in properties of the two compounds is only a consequence of pressure and hence can be tuned either by epitaxial growth or by alloying at A-site with Sr$^{2+}$ or Ca$^{2+}$ ions. We find that the Mn $e_{g}$-states are highly sensitive to pressure. Compression raises their energy in Sr$_{2}$MnRuO$_{6}$, accompanied by a transfer of electrons from the eg states to the Ru-Mn hybridized $t_{2g}$-states. This results in a transition from c-type antiferromagnetism to ferrimagnetism. Ferrimagnetic ordering in-turn allows for greater delocalization of the up-spin Ru $t_{2g}$-electrons, hence increasing the conductivity. We also show that in the presence of disorder, the Mn$_{Ru}$ antisites couple ferromagnetically with their neighboring Mn$_{Mn}$ sites. This allows for the interesting possibility of preserving highly spin-polarized conduction even in the absence of chemical ordering.   

Surface dynamics and electronic properties of parent and Mn doped Sr$_{3}$Ru$_{2}$O$_{7}$

High resolution Electron Energy Loss Spectroscopy has been utilized to measure the temperature dependence of the low energy excitations at the surface of cleaved Sr$_{3}$(Ru$_{1-x}$Mn$_{x})_{2}$O$_{7}$ single crystals (x = 0, 0.16). Four loss peaks are observed and assigned as the A$_{1g}$(2), A$_{2u}$(3), A$_{2u}$(2), and A$_{1g}$(1) vibration modes. The continuous electronic excitation spectra, Drude tail, shows that the parent compound is metallic at all temperatures investigated (80K to RT). But the A$_{1g}$(1) mode splits into two peaks between 145K and 210K indicating a structural transition at the surface. For the 16{\%} Mn doped samples the bulk is insulating and antiferromagnetic below 160K. In contrast, the surface is always metallic. Upon cooling from RT the A$_{1g}$(1) mode hardens with its width broadening from RT to 160K, and then softens and narrows quickly until 80K. The Drude tail exhibits similar behavior. Evidently the presence of the surface suppresses AF ordering and kills the insulating phase.   

The Physics of Hunds metals and its relevance for ruthenades, iron pnictides and chalchogenides

I will discuss the physics of Hund's metals. In these systems the Coulomb interaction among the electrons is not strong enough to fully localize them, but it significantly slows them down, such that low-energy emerging quasiparticles have a substantially enhanced mass. This enhanced mass emerges not because of the Hubbard interaction U, but because of the Hund's rule interactions J that tend to align electrons with the same spin but different orbital quantum numbers when they find themselves on the same atom. I will show a few examples of such Hund's metals, including Sr2RuO4, iron pnictides and iron chalchogenides materials. The electronic structure, computed by the Dynamical Mean Field Theory in combination with Density Functional Theory, successfully reproduces several experimental results and explains the key properties of these material: such as the mass renormalizations and anisotropy of quasiparticles, the crossover into an incoherent regime above a low temperature scale, the magnetic moments in iron compunds, etc. While at very low temperature our simulations predict these materials to be Fermi liquids, at finite temperature they strongly deviate from Fermi liquid prediction and can be characterized by self-energy which follows a powerlaw, with non-integer exponents. The origin of this non-Fermi liquid fixed point will be discussed.   

Anomalous $E-$type Antiferromagnetism in the ground state of Mn-substituted Sr$_{3}$Ru$_{2}$O$_{7}$

The bi-layer perovskite, Sr$_{3}$Ru$_{2}$O$_{7}$, has sparked a lot of interest because of the quantum critical behavior---related to a metamagnetic (magnetic field-tuned) phase transition. One of the key issues is related to the magnetism in the system. Here we report an investigation of the effects on magnetism resulting from chemical substitution in this compound. Our neutron scattering investigation reveals an unusual $E-$type antiferromagnetic (AFM) structure induced by Mn-substitution in the ground state of Sr$_{3}$(Ru$_{1-x}$Mn$_{x})_{2}$O$_{7}$ ($x$ = 0.16). The AFM structure exhibits a long-range order in \textit{ab}-plane but almost only a single bilayer-thickness correlation along the $c$-direction, thus characterizing the system as a quasi-two-dimensional antiferromagnet while the AFM order parameter shows almost three-dimensional-like scaling character as $T$ approaches $T_{N}$ ($\sim $ 82 K). The magnetic moments are aligned along the c-axis with an upper limit of $\sim $ 0.70 \textit{$\mu $}$_{B}$/Ru site. The induced AFM order most likely results from the enhancement of super-exchange interactions rather than from structural distortions or from freezing of electronic instabilities due to the nesting character of Fermi surface in the parent compound.   

Anisotropy of magnetoresistivities in Sr$_{3}$Ru$_{2}$O$_{7}$: Evidence for orbital-dependent metamagnetism

Sr$_{3}$Ru$_{2}$O$_{7}$ has been studied extensively due to its rich electronic and magnetic ground state properties, such as its quantum criticality and electronic nematic phase [1,2]. In this talk we will present the results of in-plane angle-resolved directional magnetotransport anisotropy measurements, a technique we used previously to elucidate the orbital-selective nature of the itinerant metamagnetism in Sr$_{4}$Ru$_{3}$O$_{10}$ [3]. We find that the \textit{c}-axis magnetoresistivity anisotropy undergoes a drastic change in symmetry from fourfold to twofold through the metamagnetic transition, consistent with the behavior expected for the strong spin polarization. In contrast, the in-plane magnetoresistivity anisotropy remains fourfold through the transition accompanied by only a gradual shift in phase, and only trends towards twofold symmetry at fields well above the transition. These findings suggest the $d_{xz,yz}$ bands, should play a pivotal role in the metamagnetic transition.\\[4pt] [1] S. Grigera \textit{et al}., Science \textbf{294}, 329 (2001)\\[0pt] [2] R. Borzi \textit{et al}., Science \textbf{315}, 214 (2007)\\[0pt] [3] D. Fobes \textit{et al}., Phys. Rev. B \textbf{81}, 172402 (2010)   

Study of an electronic nematic with precise control of the applied magnetic field vector

The layered perovskite metal Sr$_{3}$Ru$_{2}$O$_{7}$ has gained considerable interest since the discovery of its field tuned quantum criticality [1] and the subsequent discovery of a new electronic phase with a high magnetoresistive anisotropy, consistent with the existence of an electronic nematic fluid [2]. This anisotropy may be oriented by applying a moderate field ($H_{ab}$) in the plane of the RuO layers. The study of the behaviour of the electronic nematic state requires precise control over both the magnitude and direction of $H_{ab}$. For this purpose, we operate a 3-axis 9/1/1 tesla vector magnet, which offers full control of the magnetic field vector with a high degree of precision. Here, we present recent magnetotransport data for Sr$_{3}$Ru$_{2}$O$_{7}$ measured in the vector magnet. We confirm the two-fold to four-fold rotational symmetry breaking and show that it occurs even in the limit of small values of $H_{ab}$. Additionally, we address pinning of the anisotropy underlying crystal lattice. Finally, we show the dependence of the anisotropy on magnetic field and temperature, which may help explain its origin at the microscopic scale. \\[4pt] [1] S. A. Grigera \textit{et al}., \textit{Science }\textbf{294}, 329 (2001) [2] R. A. Borzi \textit{et al}., \textit{Science} \textbf{315}, 214 (2007)   

Optical Polarization Microscopy of the Electron Nematic Phase in Sr$_3$Ru$_2$O$_7$

We report the implementation of a fiber-based optical microscope, capable of operating at temperatures below 100 mK and in magnetic fields in excess of 9 Tesla, with sub-micron spatial resolution. This microscope is integrated into the bore of a dilution refrigerator with an optical fiber coupling light to an external optical table. Bench-top optical elements allow for polarization analysis of the reflected light from a surface and thus the detection of magnetic or other polarization-sensitive properties of mater at low temperature and high fields. As a first application of the instrument, we are studying the proposed electron nematic phase of the n=2 Ruddlesden-Popper material Sr$_3$Ru$_2$O$_7$, which exhibits a low-temperature phase transition in the form of an in-plane conduction anisotropy. We plan to detect this phase optically by analyzing the polarization rotation of the reflected light through the phase boundary, with the aim of imaging domain structure in the nematic phase.   

Suppression of An Antiferromagnetic Insulating Phase in Sr$_{3}$(Ru$_{1-x}$Mn$_{x})_{2}$O$_{7}$ by Magnetic Field

Double-layered Sr$_{3}$Ru$_{2}$O$_{7 }$is a paramagnetic metal. The partial substitution of Mn for Ru results in metal-insulator transition at T$_{MIT}$ and antiferromagnetic ordering at T$_{M}$ in Sr$_{3}$(Ru$_{1-x}$Mn$_{x})_{2}$O$_{7. }$Interestingly, both T$_{MIT}$ and T$_{M}$ can be easily suppressed by the application of magnetic field, especially for low-doping compounds (x $<$ 0.1). This behavior can be explained as Mn-doping-induced antiferromagnetic-insulating domains below T$_{MIT}$. The application of magnetic field suppresses the antiferromagnetic coupling, thus converting the insulating domains back to metallic.   

The Orbital-Selected Mott Phase of the Nondegenerate Two-Orbital Hubbard Model

We use the dynamical mean-field theory to study the optical conductivity and orbital susceptibility of the nondegenerate two-orbital Hubbard model in the orbital-selective Mott phase. The optical conductivity of the wide band presents expectedly a nonzero Drude peak, while the localization character is observed for the optical conductivity of the narrow band. Particularly, a rapidly reverse shape in the orbital susceptibility emerges right at Fermi surface, implying the coexistence of the orbital ordering with the orbital-selected Mott phase. We also find that the orbital-selected Mott transition can be suppressed by the negative crystal field splitting. Applying the present findings to compound Ca$_{2-x}$Sr$_{x}$RuO$_{4}$, we demonstrate that the orbital-selected Mott phase can not survive in wide doping region from $x$=0.2 to 2.0.   

Session H10: Invited Session: Strongly Interacting Photons

Strong photon-photon interaction in a coupled quantum dot- photonic crystal nanocavity

Quantum dots (QDs) in photonic crystal nanocavities are interesting both as a testbed for fundamental cavity quantum electrodynamics (QED) experiments, as well as a platform for classical and quantum information processing. In addition to providing a scalable, on-chip, semiconductor platform, this system also enables very large dipole-field interaction strengths, as a result of the field localization inside of sub-cubic wavelength volumes (vacuum Rabi frequency is in the range of 10's of GHz). We have demonstrated controlled amplitude and phase modulation between two continuous wave (CW) optical beams at the single photon level (power less than a photon per cavity photon lifetime) interacting via a strongly coupled quantum dot - photonic crystal cavity system, and have subsequently extended this experiment to weak time-varying control field and a CW signal field. Recently, we have performed all-optical switching at the single photon level between two pulsed, resonant optical beams (with 40ps pulses and 80MHz repetition rate). In this experiment, we have measured transmission through the strongly coupled QD-cavity system as a function of delay between the two pulses, and have demonstrated a 22{\%} increase in the transmission at zero delay. The increase in the transmission is a result of the saturation of the strongly coupled QD-cavity system. We have also studied the effects of the photon blockade and photon induced tunneling which result from the anharmonicity of the ladder of dressed states in a strongly coupled QD-nanocavity system. These effects lead to dramatic changes in the transmitted photon statistics, which can be varied from sub-Poissonian to super-Poissonian, and can be employed to generate nonclassical states of light (such as Fock or NOON states) with high efficiency.   

Single-photon nonlinearity

Optical nonlinearity at the level of single photons will enable a variety of novel effects and applications, including the possibility of quantum gates between individual photons. We generate such a nonlinearity in an atomic ensemble by replacing the strong (classical) coupling field of electromagnetically induced transparency by the mode of an optical resonator. Then the resonant transmission of light through the atomic ensemble can be substantially altered even by the cavity vacuum. The vacuum induces a group delay of the optical pulse that corresponds to a group velocity of 1600 m/s. We also discuss possibilities for implementing a single-photon switch.   

Slow-light polaritons in Rydberg gases

Slow-light polaritons are quasi-particles generated in the interaction of photons with laser-driven atoms with a $\Lambda$- or ladder-type coupling scheme under conditions of electromagnetically induced transparency (EIT). They are a superposition of electromagnetic and collective spin excitations. If one of the states making up the atomic spin is a high lying Rydberg level, the polaritons are subject to a strong and non-local interaction mediated by a dipole-dipole or van-der Waals coupling between excited Rydberg atoms. I will present and discuss an effective many-body model for these Rydberg polaritons. Depending on the detuning of the control laser the interaction potential between the polaritons can be repulsive or attractive and can have a large imaginary component for distances less than the so-called blockade radius. The non-local effective interaction gives rize to interesting many-body phenomena such as the generation of photons with an avoided volume, visible in stronlgy suppressed two-particle correlations inside the blockade volume. Moreover the long-range, power-law scaling of the interaction can in the repulsive case give rize to the formation of quasi-crystalline structures of photons. In a one dimensional system the low-energy dynamics of the polaritons can be described in terms of a Luttinger liquid. Using DMRG simulations the Luttinger K parameter is calculated and conditions for the formation of a quasi-crystal are derived. When confined to a two-dimensional geometry, e.g. using a resonator with quasi-degenerate transversal mode spectrum, Rydberg polaritons are an interesting candidate to study the bosonic fractional quantum Hall effect. I will argue that the formation of photons with an avoided volume is essential for explaining recent experiments on stationary EIT in Rydberg gases [1,2].\\[4pt] [1] J.D. Pritchard et al., Phys. Rev. Lett. 105, 193603 (2010). \\[0pt] [2] D. Petrosyan, J. Otterbach, and M. Fleischhauer, arXiv:1106.1360   

Fast optical control of atom-light interactions using quantum dots coupled to photonic crystal cavities

Quantum dots (QDs) are stable, bright, semiconductor based light emitters that exhibit a quantized energy spectrum. For these reasons they are excellent candidates for development of lasers, optoelectronic components, and could serve as basic building blocks for future quantum information technology. By coupling these nanostructures to optical cavities the interaction strength between QDs and light can be significantly increased. Photonic crystals (materials with a periodic index of refraction) are particularly promising for enhancing these interactions due to their ability to guide and confine light on the size scale of an optical wavelength. Photonic crystal based optical cavities have already been shown to enable the strong coupling regime of cavity quantum electrodynamics (cQED). In this regime a significant modification of both the QD emission spectrum and cavity reflectivity can be observed due to quantum mechanical mixing of atom-photon states. Control of QD-photon interactions on fast timescales is an important capability that enables strong nonlinear optical effects, opening up the door for a new class of opto-electronic devices at ultra-low light levels. It could also provide a promising route towards quantum information processing using photons and QDs to store and transmit quantum coherence. Here we describe a method to achieve fast all-optical control of atom-light interactions using indium arsenide (InAs) QDs coupled to photonic crystal cavities. We show that a QD strongly coupled to a photonic crystal cavity can exhibit very large optical Stark shifts due to resonant cavity enhancement of the electromagnetic field. Stark shifts as large as 20 GHz are demonstrated with as few as 10 photons in the cavity. These shifts can be used to control the QD resonant frequency on fast time scales, and therefore modify its interactions with the optical cavity through resonant detuning. Using this approach we demonstrate the ability to perform all optical switching with control pulse energies as small as 400 photons and switching times as fast as 140 ps. The approach can be improved through better cavity coupling methods to approach nonlinear optics near the single photon level.   

Session H11: Focus Session: Graphene Structure, Stacking, Interactions: Interfaces and Adsorbates

Confinement of organic solvents by wet transfer of graphene

Transfer of graphene grown by chemical vapor deposition (CVD) on Cu requires a polymer support for the graphene and wet-etching of the Cu growth substrate. After etching, the polymer/graphene film must go through several solvent baths to remove contaminants. Water is commonly used for this cleaning due to its capability as a solvent and its ability to support the polymer/graphene film through high surface tension. By contrast, common organic solvents have lower surface tension, causing the film to tear and fold within the liquid. To bypass this challenge, we have implemented a polymer-bound truss to reinforce the polymer/graphene film, which avoids the need of proper solvent surface tension. We have transferred graphene grown on Cu using common organic solvents like methanol, isopropanol, ethylene glycol, and dimethyl sulfoxide for the final transfer liquids. This process traps the solvents between the graphene and the final substrate. Confinement effects are determined via optical microscopy, atomic force microscopy, and Raman spectroscopy for both the trapped solvent molecules and the graphene. Our procedure opens up the possibility of confining biological materials suspended in organic solvents under graphene.   

Photo-assisted polymerization of one-dimensional molecular arrays on epitaxial graphene

Self-assembled monolayers provide opportunities to tailor the materials properties of graphene and template subsequent chemical functionalization at molecular length scales. In particular, 10,12 pentacosadiynoic acid (PCA) is found to assemble into one-dimensional arrays on epitaxial graphene on SiC(0001) as revealed by ultra-high vacuum scanning tunneling microscopy (UHV-STM). Furthermore, UV-triggered polymerization of PCA in UHV is achieved, leading to distinct conformational changes at the molecular scale. The sub-5 nm widths of these one-dimensional polymers make them promising candidates for templating the formation of graphene nanoribbons with significant band gaps. Molecular resolution UHV STM characterization of the structure and electronic properties of this nanostructured chemical functionalization strategy will be presented.   

Chemical reactivity imprint lithography on graphene: Controlling the substrate influence on electron transfer reactions

The chemical functionalization of graphene enables control over electronic properties and interactions with other materials. Graphene's chemical reactivity is strongly influenced by the underlying substrate. In this paper, we show a stark difference in the rate of electron transfer chemistry with aryl diazonium salts on monolayer graphene supported on a broad range of substrates. Reactions proceed rapidly when graphene is on SiO$_{2}$ and Al$_{2}$O$_{3}$ (sapphire), but negligibly on alkyl-terminated and hexagonal boron nitride (hBN) surfaces. The effect cannot be explained by the overall graphene doping levels alone, and can instead be described using a reactivity model accounting for substrate-induced electron-hole puddles in graphene. Raman spectroscopic mapping is used to characterize the effect of the substrates on graphene. Reactivity imprint lithography (RIL) is demonstrated as a technique for spatially patterning chemical groups on graphene by patterning the underlying substrate, and is applied to the covalent tethering of proteins on graphene.   

Graphene on Metals: Interface Structure and Defects

The epitaxial growth of graphene on metal substrates is one of the major methods of graphene production for electronic applications. Therefore, the metal/graphene interface interactions as well as the graphene defects appeared during the growth affect in a substantial way the electronic properties of both graphene and graphene/metal contacts, which are both important for device applications. Structural and electronic properties of simple and complex graphene/metal as well as graphene/metal-alloy interfaces were investigated using first principles density functional theory. The point defect structures in graphene on metal substrate were studied and compared with those in free standing graphene.   

Topologically Frustrated Bonding in Dual-sided Adsorption to an Atomically Thin Membrane

A seamless sp2 atomically thin layer cuts space in half, and prevents penetration of atoms through the sheet, while still allowing cross-sheet charge transfer. This geometrical frustration separates charge donating (e.g. alkali) and charge accepting (e.g. halogen) ions in opposite subspaces and generates a collective planar dipole. In this unusual geometry we observe new physics and chemistry. For graphene layer the uncompensated Coulomb interactions generate a system with multiple nearly degenerate structures which are either metallic or small-gap semiconductors, in contrast to the insulating behavior of unfrustrated salt crystals. When the layer is changed to h-BN, the collective dipole imposes a large Stark effect on the halogen and alkali-derived valence and conduction bands, resulting in a large band gap tuning with areal adsorbate density.   

Correlation between adatom adsorption properties and growth morphology of metal on graphene

We present a systematic study of various metal adatom adsorption on graphene by \textit{ab initio} calculations. The correlation between the adatom adsorption properties and the growth morphology of the metals on graphene is investigated. We show that the growth morphology is related to the ratio of the adsorption energy to the bulk cohesive energy (E$_{a}$/E$_{c})$ of the metals and the diffusion barrier ($\Delta $E) of the metal adatom on graphene. The growth morphology is also affected by the strain induced by metal adsorption on graphene. We also show that most of the metal nanostructures on graphene are thermally stable again coarsening. The first-principles calculations are consistent well with the observations from recent experiments.   

Lithium Intercalation Induced Decoupling of Epitaxial Graphene on SiC(0001): Electronic Property and Dynamic Process

This work presents first-principles investigations of the dynamic process of lithium (Li) penetration through the buffer layer on 6H-SiC(0001) surface, as well as the Li-insertion induced change of electronic structure. It is found that the penetration is kinetically forbidden for perfect buffer layer, because of the size confinement of its honeycomb structure. From the analysis of rate coefficient under the experimental conditions, topological defects no smaller than 8-membered ring are predicted to be essential for Li intercalation. Along with the Li insertion, the electronic property of the buffer layer is changed from n-type doping (Li-adsorption) to that of quasi-free-standing graphene (Li-intercalation). It is the electron injection by Li that results in the dissociation of the Si-C bonds and the decoupling of Li-intercalated buffer layer from the substrate. Moreover, we demonstrate the influence of such topological defects on the electronic property of epitaxial graphene, which provides some useful hints for understanding the observed gap and midgap state behavior.   

Single atom doping of graphene -- A theoretical study

We present first principles results and analysis for the electronic structure of chemically modified graphene. We show the relationship between different physical parameters and the electronic band structure of the modified material and its doping level. Finally, we discuss the possible effects of a substrate and of charge transfer patterns for device applications.   

Hydrogen adsorption induced structural and electronic changes in graphene grown on metal substrate

Graphene hydrogenation proposed to open a band gap has also been shown to be the case for graphene on metal substrates. Our carbon specific soft x-ray (photoelectron, absorption, emission) spectroscopy studies on single and few layer graphene on Pt(111) do not indicate band opening due to hydrogenation. The graphene layer is weakly interacting with the Pt(111) substrate but hydrogenation induces structural changes which lead to observation of density of states at the Fermi level (contrary to band opening hypothesis) due to strong hybridization with substrate. Hydrogenation observed to occur only on the surface atoms of few layers of graphene induces interlayer carbon-carbon bonding due to structural distortions initiated at the surface, i.e. propagation of sp$^{3}$ hybridization to underneath carbon layers. This structure is stabilized due to hybridization of the carbon atoms in the bottom layer with the Pt(111) substrate.   

Vibrational Spectrum of ``Crystalline Graphane''

Since the discovery of graphene, a flat monolayer of carbon atoms, a great interest was attracted to synthesis of chemically modified carbon sheets. In particular, it was proposed that graphane, representing a graphene sheet saturated by hydrogen adsorbed from both sides, would be stable [1], and this prediction was confirmed by TEM [2]. Recently, hydrogenated graphite with a composition close to CH has been synthesized by exposing the graphite to gaseous hydrogen at P=2 to 7 GPa and T=350 to 450$^{o}$C [3]. The formation of hydrographite is accompanied by a 40{\%} increase in the $c$-parameter of the unit cell. The IR spectrum shows a strong band near 2850 cm$^{-1}$ due to stretching vibrations of the C-H covalent bonds. In the present work, we studied the vibrational spectrum of hydrographite by inelastic neutron scattering. The obtained spectrum is very similar to that calculated for a single graphane plane [4]. This suggests a weak interaction between the graphane layers in hydrographite, so it could be considered as a ``crystalline graphane'' material. 1. J.O. Sofo et al., PRB \textbf{75}, 153401 (2007). 2. D.C. Elias et al., Sci. \textbf{323}, 610 (2009). 3. I.O. Bashkin et al., Int. Symp. Metal-Hydr. Syst., Reykjavik, Iceland, 2008. 4. G. Savini et al., PRL \textbf{105}, 037002 (2010).   

Direct determination of the dominant scatterer in graphene on silicon oxide

Previously the density of native scatterers in graphene on silicon oxide was shown to be proportional to the number of adsorption sites for atomic hydrogen [1]. However, this study provided limited information about the sites in graphene with affinity to atomic hydrogen. We employed a detailed temperature programmed desorption study on hydrogen-dosed graphene sheets. The determined desorption energy is used to reveal the nature of the dominant scatterer in graphene on silicon oxide. \\[4pt] [1] J. Katoch, J.H. Chen, R.Tsuchikawa, C.W. Smith, E.R. Mucciolo, and M. Ishigami, \textit{Uncovering the dominant scatterer in graphene sheets on SiO$_{2}$}, Physical Review B Rapid Communications, 82, 081417 (2010).   

Electron Heat Capacity of Nitrogen Doped Graphene in Low Temperature

We calculate the electron heat capacity of nitrogen doped graphene with a simple method. There are four kinds of bonds to be considered, the pi and anti-pi bonds of C-C and C-N. And the extra electrons are treated as free constituents. We found a small amount of difference in the electron heat capacity between a pure graphene and a thin film CNx. Nevertheless, with a precise measurement on the electron heat capacity of the thin film or layer structure of CNx, we can determine the doping concentration of nitrogen.   

Chemical reactions and thermal stability of oxygen impurities on graphene

Oxygen as an impurity is known to degrade conductivity in graphene, but annealing at moderate temperature reverses the effect. Here we report first-principles calculations of oxygen binding and reactions on graphene that elucidate the underlying physics. We find that two O atoms can form an O dimer that can desorb from graphene with an overall activation barrier of 1.3 eV. Oxygen can also be removed in a more complicated reaction in which C atoms in graphene are consumed. We find that structural defects such as Stone-Wales defect and grain boundaries show enhanced binding to O atoms due to the local strain, facilitating the O reaction. If H atoms coexist, an O atom can bind to an H atom forming an OH group, which can also be removed by thermal annealing due to the weak binding, resulting in defect-free graphene.   

Efficient adsorbate transport on graphene by electromigration

Chemical functionalization of the surface of graphene holds promise for various applications ranging from nanoelectronics to surface catalysis and nano-assembling. In many practical situations it would be beneficial to be able to propel adsorbates along the graphene sheet in a controlled manner. We propose to use electromigration as an efficient means to transport adsorbates along the graphene surface. Within the tight-binding approximation for graphene, parametrized by density functional theory calculations, we estimate the contributions of the direct force and the electron wind force to the drift velocity of electromigration and demonstrate that the electromigration can be rather efficient. In particular, we show that the drift velocity of atomic oxygen covalently bound to graphene can reach up to 4 cm/s for realistic graphene samples. Further, we discuss ways to dynamically, i.e., during experiment, control the efficiency of electromigration by charging and/or local heating of graphene.   

Session H12: Chemical Doping of Graphene and Applications: Solar Cells, Sensors

I-V characteristics of orgarnic molecules coated graphene

Thin layers of organic molecules, OTS and DMF, are coated on top of graphene which was extracted by the exfoliation. I-V characteristics of the organic molecules coated graphene were investigated at low temperature by varying the gate voltage. P-doping with slight enhancement of mobility was observed with the OTS coating. On the other hand, n-doping with relatively higher enhancement of mobility was obtained with the DMF coating. CNT is nested on top of the organic molecules coated graphene by dielectrophoresis and Coulomb blockade was observed in the sandwich device.   

Investigation of Electrochemical Gate Controlled Charge Transport in Large Area Boron-Nitrogen Doped Graphene

We report on the investigation of charge transport measurements of B and N doped graphene C (B,N) under the influence of an electrochemical gate. These C (B,N) systems are expected to have unique electronic properties due to the combination of impurities including both atomistically separated B and N species, as well as hexagonal boron nitride ($h-$BN) units within the graphitic C lattice. Investigations were performed on large area BN doped graphene devices fabricated with different BN doping levels. The electrochemically gate controlled interfacial capacitance and quantum capacitance of BN doped graphene devices were measured. The effect of doping on the quantum capacitance and electron mobility will be discussed.   

Stable Chemical Doping of Graphene for Low Electrical Resistance and High Optical Transparency

Since becoming experimentally available, graphene has been used in various devices such as field effect transistors (FETs), solar cells and sensing applications. Although graphene based devices with modest characteristics have been reported, in some device geometries a lower graphene resistance with different Fermi level values is still desired. Here, we describe our use of a hydrophobic organic dopant with strong electronegativity, environmental stability and high optical transmittance which is spin cast onto CVD-grown graphene films. We observe a typical 70{\%} reduction in resistance upon chemical modification of the graphene. Magnetoresistance measurements imply that the modified graphene is hole doped, and time-dependent resistance measurements show excellent stability. Using Raman spectroscopy we confirm the doping of graphene sheets from the shifts in G and 2D peak positions and intensity ratios. We show transmittance and SEM characteristics of the graphene sheets before and after doping. These results may serve as a guide for modification of graphene's properties as desired for various applications.   

Reversible and Robust Carrier Doping in Graphene \textit{via} Mechanical Actuation of Tethered Azobenzene

machines -- molecules capable of responding to external stimuli - have gained great interest due to their applications in molecular actuation nanodevices. In this talk, we demonstrate that ultrathin graphene exhibits high-sensitivity to orientation, surface-vicinity, electronegativity, and density-of-states of interfaced molecules. This enables the realization of reversible doping of graphene \textit{via} molecular mechanics on its surface. Here, few-layer-graphene (FLG) sheets were functionalized with electronegative and isomerizable azobenzene-molecules. The optical transformation of these azo-molecules from their \textit{trans} conformation to \textit{cis} conformation dopes 7.5 X 10$^{3}$ holes/$\mu $m$^{2}$ in the underlying graphene. This corresponds to $\sim $20 azobenzene molecules producing 1 hole (hole-mobility of 301 $\mu $m$^{2}$/V/s) in the azobenzene-FLG (AFLG) device. Further, we demonstrate the facile fabrication of the AFLG device and the mechanism of electrical modulation due to molecular mechanics. We also compare the response of the AFLG device with an FLG device directly doped with electronegative perylene tetracarboxylic acid, which led to $\sim $3 fold increase in the hole density. -abstract-   

Tuning the Electrical Properties of Large Area Graphene through Boron-Nitrogen Co-Doping

We report on investigation of low temperature electrical transport measurements of B and N co-doped graphene layers C (B, N). We find that the temperature dependence of resistance (5K $<$ T $<$ 400 K) of pure graphene shows a metallic behavior, whereas the C (B, N) samples show an increasingly semiconducting behavior with increasing doping levels. Within the studied temperature range, at higher temperatures, the doped samples showed a band-gap dominated Arrhenius-like temperature dependence. At the lowest temperatures, the temperature dependence deviates from an activated behavior, and presents evidence for a conduction mechanism that is consistent with Mott's 2D-Variable Range Hopping (2D-VRH).   

Raman studies of ultra-clean graphene on hexagonal boron nitride with controlled doping

Graphene prepared by exfoliation on hexagonal boron nitride (h-BN) provides an ideal platform for studies of the intrinsic properties of Dirac electrons because of its unprecedented charge homogeneity. With this system, many of the fascinating phenomena hidden by charge inhomogeneity in conventional graphene samples have recently been revealed. Here we describe progress in examining both electron-phonon and electron-electron interactions by means of Raman scattering by the G- and the 2D-modes. In our studies, we were able to observe a symmetric energy shift in the Raman 2D peak at low doping levels when the Fermi level was tuned from the electron side to the hole side. This shift is understood as a change in the double-resonance condition induced by the renormalization of the electron and phonon band structures. At the same time, the 2D peak is broadened under electron or hole doping. Additionally, we observed very weak doping dependence of the G peak (both position and width) at Fermi energies less than half of the phonon energy and the subsequent usual removal of non-adiabatic Kohn anomaly with increased doping, which reflects again the extremely homogeneous charge distribution in our samples.   

Scanning Tunneling Microscope (STM) Study of B-doped Monolayer Graphene

Chemical doping is a promising technique to tailor the electronic properties of graphene. Here we focus on an atomic scale characterization of Boron-doped monolayer graphene sheets using primarily STM, assisted by Raman Spectroscopy and X-ray Absorption Spectroscopy (XAS). We will show in topography that there are two major structures that result from B-doping, and in spectroscopy that each of these structures plays different roles in modifying the electronic properties of graphene. Raman Spectroscopy and XAS provide complementary information about the nature of the B-C bonds in the sample.   

Substituent Effects for the Control of Covalent Electronic Doping of CVD Graphene

Controlling the electrical properties of graphene is of particular interest for future electronic, optoelectronics and sensing applications. We investigate the doping effect of different chemical functional groups covalently attached to CVD-grown graphene devices with a polymer electrolyte top-gate. The covalent reaction is based on a diazonium chemistry, specifically the type and degree of doping for a diazonium salt with a nitro group, a bromo group or an alkyne group attached are investigated. We use three different approaches to inspect various aspect of the doping in graphene: Gaussian calculations, Raman measurements and transfer-characteristics. The transfer curves show that nitro groups induce p-doping while the bromo- and alkyne groups induce n-doping. A new model that takes into account both coulombic and resonant scattering as well as a asymmetric electron and hole transport is developed to fit the transfer-curves. The graphene transistors are very robust and reproducible, suggesting this is a simple and facile way to control the electronic properties of graphene.   

Charge transfer and density of states modifications of graphene upon molecular adsorption - Implications for gas and molecular sensors

The adsorption of molecules on single layer graphene can result in significant modifications to the band structure and density of states near the Dirac point and can result in the introduction of scattering centres which can modify the carrier mobility. Understanding how the competing interactions of increased carrier density and density of scattering centres is therefore an important consideration in the description of the properties of graphene. We have used ab initio methods to explore the degree of charge transfer, modification to the band structure and density of states associated with the adsorption of a range of open and closed shell molecules, organometallic molecules and planar organic molecules. We show how the charge transfer can be related to the position of the molecule related energy levels on adsorption relative to the Dirac point. We find low levels ($<$0.05e) of charge transfer for NH$_{3}$, NO and NO$_{2}$ molecules but larger values for cobaltocene (n-type, 0.31 e/molecule) and about 0.3 e/molecule for the organic molecules TDAE (n-type) and DDQ (p-type) respectively. These molecules open up ways to dope graphene to high levels and are important considerations in sensing. We also discuss the factors that control the charge transfer.   

Biochemical Sensing in Solution Gated Graphene Field Effect Transistors

Epitaxial graphene is a promising material for the construction of label-free chemical and biochemical sensors. In this work, graphene of few-layers is used as a sensor for pH and ionic strength in a solution gated field effect transistor (SGFET). In order to improve the sensitivity of the SGFET to pH and ionic changes the transistor is connected in a four-point (van der Pauw) configuration. Results for the shift in the Dirac point when the pH or ionic strength is changed are shown.   

Combined transport-Scanning Probe Microscopy study of reduced graphene oxide sensors

We present an in-depth study of the sensing properties of chemically reduced graphene oxide (rGO) based devices. Graphene oxide is an electronically hybrid material that can be controllably tuned from an insulator to a semiconductor material via reduction chemistry. Due to their chemical structure and large surface to volume ratio rGO sensors can detect gas adsorption at very low concentrations. rGO devices are created by dielectrophoretic assembly of rGO platelets onto interdigitated electrode arrays, which are lithographically pre-patterned on top of SiO2/Si wafers. The gas sensing properties of these devices are characterized using novel combined transport-Scanning Kelvin Probe Microscopy and transport-Electrostatic Force Microscopy measurements in the presence of different gas analytes. These measurements show unique, very sensitive and repeatable responses to various volatile organic compounds and other gases. Maps of the electrostatic potential and charge distribution across these circuits are used to model the dynamics of electronic transport through the rGO system.   

Biomimetic graphene sensors: functionalizing graphene with peptides

Non-covalent biomimetic functionalization of graphene using peptides is one of more promising methods for producing novel sensors with high sensitivity and selectivity. Here we combine atomic force microscopy, Raman spectroscopy, and attenuated total reflection Fourier transform infrared spectroscopy to investigate peptide binding to graphene and graphite. We choose to study a dodecamer peptide identified with phage display to possess affinities for graphite and we find that the peptide forms a complex mesh-like structure upon adsorption on graphene. Moreover, optical spectroscopy reveals that the peptide binds non-covalently to graphene and possesses an optical signature of an ?-helical conformation on graphene.   

Solution-gated graphene field-effect transistors as local pH sensors in microfluidic systems

We report a study of solution-gated graphene field-effect transistors (SGGFETs) as high-performance local pH sensors in microfluidic devices. Previous experiments have shown that SGGFETs can function as pH sensors for bulk volumes of solutions, and a response of ~20 mV/(unit pH) shift in the Dirac point was typically observed. In our study, we investigated SGGFETs micro-fabricated out of CVD graphene grown on copper foil and found a robust pH sensitivity of ~50 mV/(unit pH). This value is close to the thermodynamically allowed maximum value, i.e., the Nernst value of 59 mV/(unit pH) at room temperature. We further integrated the SGGFETs into microfluidic systems for lab-on-chip applications. We found the SGGFETs are capable of real-time detection of local pH changes in microfluidic channels, thus providing reliable measurement of the local pH for small volumes of liquids (a few nL). Possible applications of this microfluidic detection system, for example, in monitoring chemical diffusion and reactions in microfluidic channels, will be discussed.   

A low cost construction method for Graphene based resistive chemical sensors

Graphene is a 2D material with distinctive properties and a large surface area that can be exposed to surface adsorbates from a target gas, making it attractive as a sensing material. This enables studies on the interaction of gas molecules with the graphene surface and subsequent changes in its properties. Due to its high electron mobility at room temperature, graphene exhibits high sensitivity, making it a good candidate for environmental and industrial sensing applications. Several models of graphene based sensors have been put forth previously based on high-resolution lithographic techniques and for individual electrode attachment to the sensing film with e-beam lithography. These techniques can produce small numbers of devices that explore the limits of molecular scale sensing, but the methods are currently impractical for large scale production of low cost sensors. We present our graphene based sensor with the focus on designing small, cost effective and reliable sensors with high sensitivity towards the target gas, detailing the assembly of graphene/acrylic devices, their characterization and investigation of their performance as resistive chemical sensors using differential voltage measurements.   

Finite temperature quantum transport in nanosensors based on graphene nanoribbons

We study finite temperature quantum conductance of nanosensors based on graphene nanoribbons exposed to carbon and nitrogen oxides. Using ab-initio-based Green's function formalism, the quantum conductance of the nanoribbon with and without adsorbed oxide molecules is calculated. We investigate the effects of molecular vibrations and electron-vibron coupling on conductance modulation. The implication of the results concerning nanosensor functionality under desired environmental temperatures, and the differences with the low-temperature cases, are discussed   

Session H13: Focus Session: Low-Dimensional and Molecular Magnetism - Magnetism and Transport of Isolated Molecular Magnets - Molecular spintronics

Manipulating Molecular Kondo Effect by Chemical Reactions

Motivated by spintronic applications, the control of Kondo effect arising from spin exchange interaction between isolated spins and conduction electrons of non-magnetic metals, has been explored. A method to control the molecular Kondo effect is demonstrated via chemical reactions. A spontaneous binding between molecules was exploited to control the molecular Kondo effect on Au(111). The Kondo effect was switched back on using local scanning tunneling microscope manipulation. This method relies on the hybridized pairing of unpaired spins two molecules, as supported by our density functional theory calculation results.   

Electronic readout of a single nuclear spin using a molecular spin transistor

Quantum control of individual spins in condensed matter devices is an emerging field with a wide range of applications ranging from nanospintronics to quantum computing [1,2]. The electron, with its spin and orbital degrees of freedom, is conventionally used as carrier of the quantum information in the devices proposed so far. However, electrons exhibit a strong coupling to the environment leading to reduced relaxation and coherence times. Indeed quantum coherence and stable entanglement of electron spins are extremely difficult to achieve. We propose a new approach using the nuclear spin of an individual metal atom embedded in a single-molecule magnet (SMM). In order to perform the readout of the nuclear spin, the quantum tunneling of the magnetization (QTM) of the magnetic moment of the SMM in a transitor-like set-up is electronically detected. Long spin lifetimes of an individual nuclear spin were observed and the relaxation characteristics were studied. The manipulation of the nuclear spin state of individual atoms embedded in magnetic molecules opens a completely new world, where quantum logic may be integrated.\\[4pt] [1] L. Bogani, W. Wernsdorfer, Nature Mat. 7, 179 (2008).\\[0pt] [2] M. Urdampilleta, S. Klyatskaya, J.P. Cleuziou, M. Ruben, W. Wernsdorfer, Nature Mat. 10, 502 (2011).   

DFT calculations of the charged states of N@C60 and {\{}Fe4{\}} single molecule magnets investigated in tunneling spectroscopy

For device applications of single molecule magnets (SMMs) in high-density information storage and quantum-state control it is essential that the magnetic properties of the molecules remain stable under the influence of metallic contacts or surface environment. Recent tunneling experiments [1, 2] on N@C60 and {\{}Fe4{\}} SMM have shown that these molecules preserve their magnetic characteristics when they are used as the central island of single-electron transistors. Although quantum spin models have been used extensively to study theoretically tunneling spectroscopy of SMMs, it has been shown recently that the orbital degrees of freedom, which is absent in spin models, can significantly affect the tunneling conductance [3]. In this work we present first-principles calculations of the neutral and charged states of N@C60 and {\{}Fe4{\}} SMMs, and discuss a strategy to include their properties into a theory of quantum transport. We also present results of the magnetic anisotropy for the different charge states of Fe4 and discuss their relevance for experiments [2] in the sequential tunneling and cotunnelling regimes. \\[4pt] [1]. N. Roch et al., Phys. Rev. B 83, 081407 (2011). \\[0pt] [2]. A.S. Zyazin et al., Nano Lett. 10, 3307 (2010). \\[0pt] [3]. L. Michalak et al., Phys. Rev. Lett. 104, 017202 (2010).   

Beller Lectureship: Single Molecular Magnets on Conductive Surfaces

For more than a decade molecules showing magnetic bistability, generally known as Single Molecule Magnets, have represented a playground to study quantum effects that appear in magnetism when the nanometer scale is attained. The field is now mature to investigate more complex nanostructures where the interplay between transport and magnetism at the molecular scale can be exploited. The first step along this direction requires to organize SMMs on surfaces rather than as bulk phases. Pursuing this apparently straight forward task has already encountered many difficulties because of the complex nature and fragility of SMMs and of the peculiar origin of their magnetic bistability. Also robust SMMs based on lanthanide ions and phthalocyanine loose most of their SMM properties when the crystalline phase is abandoned. Thanks to a collaboration with Prof. Cornia in Modena, Italy, and Prof. Sainctavit in Paris, France, we have carried out a very low temperature synchrotron investigation showing that a polynuclear SMM cluster, based on a propeller-like tetranuclear iron(III) core, Fe4, retains the typical hysteresis also when the molecules are chemically grafted to a gold surface. The tailoring of the anchoring ligand has allowed the control of the orientation of the molecules on the substrate and has given the possibility to observe the resonant quantum tunneling of the magnetization. Preliminary investigations on Fe4 SMMs thermally evaporated in UHV conditions on a conducting ferromagnetic oxide like Lanthanium Strontium Manganite, have shown an unprecedented phenomenon. While the reported studies of paramagnetic molecules on magnetic substrates have in general shown a sizeable magnetic interaction with the substrate but no evidences of SMM behavior, in our investigation the magnetic hysteresis of Fe4 exceeds in coercive field that of the substrate, recorded at the Mn L3 edge. More interestingly, the zero field step of the hysteresis, typical of quantum tunneling of magnetization that characterizes Fe4 SMMs, disappears when deposited on LSMO, opening the perspective of a novel hybrid magnetism at the nanoscale.   

Theoretical study of hysteresis in electron transport through spin-crossover molecules

Recent advances in nanoscale molecular systems stimulate experimental studies of electron transport across molecular junctions formed by single molecules or nanoparticles bridged between electrodes, or molecular monolayers adsorbed onto surfaces, using three-terminal set-ups or scanning tunneling microscope. Among them, spin-crossover molecular systems draw attention due to their unusual coupling between spin degrees of freedom and external stimuli. Spin magnetic moments of these molecular systems increase with increasing temperature or pressure, or shining light, and their magnetization shows hysteresis behavior with temperature, pressure, or light. Recent transport measurements across nanoparticles made of such spin-crossover molecules reveal hysteresis behavior in current-voltage characteristics, driven by voltage at a given temperature. In this talk, we present our work on understanding of hysteresis in electron transport through a nanoparticle consisting of Fe-based spin-crossover molecules, using a model-Hamiltonian approach and insight obtained from density functional theory.   

Individual Magnetic Molecules on Ultrathin Insulating Surfaces

Single molecule magnets have attracted ample interest because of their exciting magnetic and quantum properties. Recent studies have demonstrated that some of these molecules can be evaporated on surfaces without losing their magnetic properties [M. Mannini \textit{et al}., \textit{Nature} 468, 417, (2010)]. This remarkable progress enhances the chances of real world applications for these molecules. We present STM imaging and spectroscopy data on iron phthalocyanine molecules deposited on Cu(100) and on a Cu$_{2}$N ultrathin insulating surface. These molecules have been shown to display a large magnetic anisotropy on another thin insulating surface, oxidized Cu(110) [N. Tsukahara \textit{et al.}, \textit{Phys. Rev. Lett.} 102, 167203 (2009)]. By using a combination of elastic and inelastic electron tunnelling spectroscopy, we investigate the binding of the molecules to the surface and the impact that the surface has on their electronic and magnetic properties.   

Interfacial Electronic and Magnetic Coupling in Organic-metal System Studied by Scanning Tunneling Microscopy

Organic materials have drawn much attention in spintronics studies because of their tunable properties by functional groups and potential to achieve molecular magnets. An important factor influencing these properties is the interfacial effect. In organic-metal systems, different interfaces lead to strong modulation of electronic structures and even magnetic behaviors like spin coupling. In our study, Mn-phthalocyanine (MnPc) deposited on Cu(111) surface have been measured by scanning tunneling microscopy (STM) and spectroscopy (STS) at 4.5 K. With different deposition amount, MnPc are adsorbed as isolated molecules or in an ordered assembled structure. From STS curves, assembled MnPc possess a broadened state near the onset of Cu(111) surface state comparing to islolated ones. According to ab initio calculation, distance between the central Mn atom and the substrate in assembled molecules is reduced due to intermolecular interaction and affects the electronic structures. Magnetic behaviors of MnPc on ferromagnetic metal substrate are further investigated by spin-polarized STM (SP-STM). Spin contrast of isolated molecules on Co nanoislands on Cu(111) is found near the Fermi level in STS maps, which is considered to be ferromagnetic coupling between MnPc and Co islands.   

The Kondo effect in molecular magnets from first principles

When a magnetic molecule is deposited on a metallic substrate or attached to metallic contacts its magnetic moment may actually be screened by the conduction electrons due to the Kondo effect. In view of possible applications of molecular magnets such as nanoscale spintronics and magnetic storage devices, it is important to being able to predict whether the Kondo effect will take place or not in a given system. Also one would like to understand in detail how the Kondo effect emerges in a given situation and how it is controlled by the various parameters such as the molecular conformation and the type of substrate. Using a recently developed ab initio approach for molecular devices [1,2] that explicitly takes into account the strong electronic correlations that give rise to the Kondo effect, we have calculated the electronic structure and transport properties of different magnetic molecules coupled to nanocontacts [3] and surfaces [4]. Our calculations shed light on the complex nature of the Kondo effect in molecular-scale devices. [1] D. Jacob {\it et al.}, PRL {\bf 103}, 016803 (2009); [2] D. Jacob {\it et al.}, PRB {\bf 82}, 195115 (2010); [3] M. Karolak {\it et al.}, PRL {\bf 107}, 146604 (2011); [4] K. J. Franke {\it et al.}, Science {\bf 20} 940 (2011)   

Electronic transport in Co-based valence tautomeric conjugated polymers

Using first principle density functional theory (DFT) methods combined with maximally localized Wannier function (MLWF), real-space basis sets and a Green's function transport scheme within the Landauer ballistic transport regime, we investigated the electronic structures and electronic transport properties of a Co-based valence tautomeric (VT) conjugated backbone polymeric system. We found that GGA+U induced high-spin structure not in satisfactorily agreement with realistic circumstances from the computed Co projected density of states (PDOS). So we instead employed constrained magnetization calculations to induce the low-spin to high-spin magnetic transition computationally. Transport calculations showed that the high-spin structure is two orders of magnitude more conductive than the low-spin structure, thus supporting the vision that this kind of Co-based VT polymer can function as basis for switchable molecular spintronic devices. Finally, we will briefly discuss the chemisorption of this VT system on metallic substrates the spin transport properties of metal-molecule-metal configurations.   

Dynamical magnetic anisotropy in spin--1 molecular systems

We study electronic transport through a deformable spin-1 molecular system in a break junction setup, under the influence of a local vibrational mode. Our study shows that the magnetic anisotropy, which arises due to stretching along the transport axis[Science 328 1370 (2010)], is renormalized by the interactions with vibrations. The coupling induces additional spin--asymmetric hybridizations that contribute to the net molecular anisotropy. We show that the low temperature physics of such device can be described by an anisotropic Kondo model ($J_{\perp} > J_{\parallel}$), with a magnetic anisotropy term, $A_{Net}S_z^2$, negative at zero stretching. A quantum phase transition (QPT) is explored by stretching the molecule, driving $A_{Net}$ into positive values, and changing the character of the device from a non--Fermi--liquid (NFL) to a Fermi liquid (FL) ground state. This transition can be directly observed through the zero--bias conductance, which we find to be finite for negative anisotropy, zero for positive anisotropy, and to reach the unitary limit at $A_{Net} \approx 0$. At that point, an underscreened spin-1 Kondo ground state appears due to the restitution of the spin-1 triplet degeneracy.   

Hot electron spin transport in C$_{60}$ fullerene

Carbon-based molecular materials are interesting for spin transport application mainly due to their small sources of spin relaxation [1]. However, spin coherence lengths reported in many molecular films do not exceed a few tens of nanometers [2]. In this work we will present results showing how hot spin-polarized electrons injected well above the Fermi level in C$_{60}$ fullerene films travel coherently for hundreds of nanometers. We fabricated hot-electron vertical transistors, in which the current created across an Al/Al$_{2}$O$_{3}$ junction is polarized by a metallic Co/Cu/Py spin valve trilayer and subsequently injected in the molecular thin film. This geometry allows us to determine the energy level alignment at each interface between different materials. Moreover, the collector magnetocurrent excess 85{\%}, even for C$_{60}$ films thicknesses of 300 nm. We believe these results show the importance of hot spin-polarized electron injection and propagation in molecular materials. [1] V. Dediu, L.E. Hueso, I. Bergenti, C. Taliani, Nature Mater. 8, 707 (2009) [2] M. Gobbi, F. Golmar, R. Llopis, F. Casanova, L.E. Hueso, Adv. Mater. 23, 1609 (2011)   

A Copper Nitride Nanotemplate for Individual Magnetic Molecules

Molecular magnets hint at a promising future in information technology applications because of their interesting quantum and magnetic properties in the bulk. If these molecules are to be useful for device applications, it may be necessary to place them on surfaces, and work has begun to concentrate in this area. However, the practical issues of isolating and accessing these molecules have yet to be resolved. We present scanning tunnelling microscopy (STM) data on FePc and (DyPc$_{2}$) deposited on a (Cu$_{2}$N-Cu(100)) surface. (Cu$_{2}$N-Cu(100)) forms a quasi-periodic lattice, and has been shown to force porphyrin molecules to sit at specific sites (D. Ecija et al., Appl. Phys. Lett. 92, 223117 (2008)). As with the porphyrins, we find that these molecules sit at the intersection of the copper lines. This spatial separation restrains any potential dipolar or exchange interaction between the molecules, and it allows for individual, independent spins to be addressed. Using data taken with an STM we investigate the properties of these molecules, and furthermore show that the molecules on this surface are immobile up to room temperature.   

Session H14: Focus Session: Spins in Semiconductors - Electrical Spin Injection

Electrical Spin Injection and Detection in Silicon: Effect of Interface States

We have observed (using the local Hanle method) electrical spin injection into n and p type Si through a Fe/MgO/Si tunnel, with an effective spin lifetime of $\sim $130ps and an extremely large spin RA product as high as $\sim $0.1 M$\Omega $*$\mu $m$^{2}$ at low bias and temperature. Both the spin-RA and the differential resistance decrease exponentially with bias at temperatures below 150K. The effective spin lifetime weakly depends on temperature, decreasing by $\sim $30{\%} from 10K to 300K. We observe the inverse Hanle effect when an external magnetic field is applied parallel to the magnetization, possibly indicating the presence of stray fields near the Si surface. These observations roughly agree with other local Hanle spin injection studies in Silicon and GaAs, but differ strongly from the results expected for injection into bulk Silicon. The two stage tunneling model through localized states (LS) developed by Tran \textit{et al }(\textbf{PRL} 102; p. 036601) can explain the large magnitude of the observed spin RA, and we have developed an extended LS model which can explain the voltage dependence, which will be discussed in another talk.   

Non-local spin transport and accumulation measurements in Si:AlGaAs with tunable carrier density

The spin lifetime in GaAs varies strongly with carrier density near the insulator to metal transition (IMT), possibly peaking at the transition [1]. However, determining the optimal spin lifetime in this material is challenging because many replica samples need to be fabricated and measured. This difficulty can be circumvented by employing Si:Al$_{0.3}$Ga$_{0.7}$As, a persistent photoconductor, as the spin transport medium. This material has been characterized and has an effective carrier density which can be tuned \textit{in situ} via photo-excitation from 10$^{14}$ to 10$^{18 }$cm$^{-3}$ and a critical carrier density for the IMT of 9.0 x 10$^{16 }$cm$^{-3}$ at 5K [2]. Heterostructures have been grown by MBE, consisting of 2 $\mu $m Si:AlGaAs, a thin epitaxial Fe layer, and an AlGaAs graded junction to create Schottky tunnel barrier contacts. Non-local spin devices have been fabricated and measured. Based on non-local 4 terminal (NL 4T) and local and NL 3T Hanle effect measurements, the initial electrical spin transport and accumulation measurements in this material are reported. The spin lifetimes range from 600 ps to 2.8 ns for multiple carrier densities, ranging from 3.5 x 10$^{16}$ to 2.4 x 10$^{17 }$cm$^{-3}$. [1] J. M. Kikkawa et al., Phys. Rev. Lett. 80, 4313 (1998). [2] J. Misuraca et al., Phys. Rev. B. 82, 125202 (2010).   

Electrical spin injection and detection in Fe/MgO/Si: influence of interface states

We report electrical spin injection and detection in Fe/MgO/Si tunnel diodes using a 3-terminal (3T) geometry. Analysis of our Hanle curves yields an effective spin life-time of $\sim $0.1 ns and a spin-RA product $\sim $1 M$\Omega *\mu $m$^{2}$, both of which are in rough agreement with previous 3T studies. However, according to our analysis the spin-RA is $\sim $ 6 orders of magnitude larger than expectations for bulk Si, and the 0.1 ns effective spin life-time is much smaller than reported value in Si by ESR or non-local methods. Here we provide a detailed analysis of electrical injection and detection in the 3T geometry. We present an alternative expression for the 3T spin signal than is usually used, and we propose that spin is accumulating in localized states (LS) at the MgO/Si interface rather than just in bulk Si. Incorporating a theory of spin accumulation in LS developed by M. Tran \textit{et al} (\textbf{PRL} 102, 036601), we propose an energy distribution for the density of localized states, and introduce a model that agrees well with our anomalously large spin-RA and can explain the strong bias dependence of both spin and charge transport.   

Spin accumulation in Fe/MgO/Ge heterostructures

We have investigated the injection of spins into n-type Ge(001) from Fe through an MgO tunnel barrier using 3-terminal Hanle measurements. While significant progress has been made in Si, spin research in Ge is still at a nascent stage, due in part to the fact that significant Fermi level pinning at the Ge interface makes it difficult to efficiently inject carriers. We observe here precessional dephasing of the spin accumulation in an applied magnetic field (the Hanle effect) in Fe/MgO/Ge structures for both forward and reverse bias. We determine spin lifetimes and corresponding spin diffusion lengths for injection into Ge substrates of varying carrier concentration and see a trend of increasing spin lifetime with decreasing doping density. At room temperature, spin lifetimes range from $\tau_{s}$ = 50 ps to 123 ps as the carrier concentration is reduced from n=8x10$^{17}$cm$^{-3}$ to 2x10$^{16}$cm$^{-3}$. We will discuss the spin-RA product as a function of carrier concentration and the role of interface states. The observed room temperature injection of spins shows that despite persistent Fermi level pinning, spin accumulation is possible in the surface of Ge. This work was supported by core programs at NRL.   

Spin Transfer from a Ferromagnet into a Semiconductor through an Oxide barrier

We present results on the magnetoresistance of the system Ni/Al203/n-doped Si/Al2O3/Ni in fabricated nanostructures. The results at temperature of 14K reveal a 75{\%} magnetoresistance that decreases in value up to approximately 30K where the effect disappears. We observe minimum resistance in the antiparallel configurations of the source and drain of Ni. As a possibility, it seems to indicate the existence of a magnetic state at the Si/oxide interface. The average spin diffusion length obtained is of 650 nm approximately. Results are compared to the window of resistances that seems to exist between the tunnel barrier resistance and two threshold resistances but the spin transfer seems to work in the range and outside the two thresholds.   

Comparing nonlocal and three terminal Hanle experiments in Silicon

We have recently shown electrical spin injection in the technologically important material Si up to 500K and demonstrated a dependence of the spin lifetime with carrier concentration. In previous work on GaAs, we have seen excellent agreement between the spin lifetime derived from Hanle data measured directly at the injector contact using a three terminal measurement and the simultaneously measured nonlocal signal outside of the charge path. Unfortunately, simultaneous measurement of three terminal and nonlocal measurements is impractical on the Silicon devices due to the high resistance of the 6x100 $\mu $m$^{2}$Fe/AlO$_{x}$/2e18 n-type Si injector contact. Instead we used a separate 150x100 $\mu $m$^{2}$Fe/AlO$_{x}$/Si contact on the same substrate to do three terminal measurements and nonlocal measurements independently. Lorentzian fits to the data shows a spin lifetime of 280 ps measured directly underneath the spin injecting contact. This data is in excellent agreement with the spin lifetime vs. carrier concentration for NiFe/SiO$_{2}$/Si contacts, however it is a factor of 3 lower than the spin lifetime of $\sim $ 1ns measured at the nonlocal contact. We discuss this observed difference and other properties such as temperature dependence and bias dependence of the three terminal vs. nonlocal experiments.   

Electrical injection and detection of spin accumulation in Si at 500 K with magnetic metal/SiO$_{2}$ contacts

Electrical spin injection into Si (001) from a ferromagnetic metal through an Al$_{2}$O$_{3}$ tunnel barrier has been demonstrated.\footnote{B. T. Jonker et al., Nature Phys. 3, 542 (2007); S. P. Dash et al., Nature 462, 491 (2009).} However, the utilization of SiO$_{2}$ as the tunnel barrier can have significant impact on the development of Si based spintronics. Here we demonstrate the electrical injection, detection and precession of spin accumulation in Si, via injection from ferromagnetic contacts such as Ni$_{0.8}$Fe$_{0.2}$ and Co$_{0.9}$Fe$_{0.1}$ through a SiO$_{2}$ tunnel barrier.\footnote{C. H. Li et al., Appl. Phys. Lett. 95, 172102 (2009).} The injection of spin-polarized carriers produce a net spin polarization and an imbalance in the spin-dependent electrochemical potential under the contact, which is detected as a voltage at the same contact. The decrease of this voltage with increasing out-of-plane magnetic field due to spin dephasing, i.e., Hanle precession of the electron spin, is observed up to 500 K. We observe Hanle precession of electron spin accumulation in Si for a wide range of bias, and demonstrate that the spin lifetime (extracted from the Lorentzian fit to the Hanle data) varies with Si carrier density. Details of the bias and temperature dependence of the spin lifetime and spin diffusion length will also be presented at the meeting. These results confirm spin accumulation in the Si transport channel up to 500 K rather than trapping in localized interface states, and demonstrate the practical aspect of spin-based semiconductor device technology. Supported by ONR.   

Spin-filter effect of quantum dot with spin-orbit interaction in magnetic field

We theoretically investigate a spin-polarized current generation in a semiconductor quantum dot (QD) with spin-orbit interaction in a magnetic field. In the absence of magnetic field, a spin-polarized current is generated only when the QD is connected to more than two leads.\footnote{M.\ Eto and T.\ Yokoyama, J.\ Phys.\ Soc.\ Jpn.\ {\bf 79}, 123711 (2010).} In the presence of magnetic field, on the other hand, we show that the two-terminal QD works as a spin filter due to the spin-orbit interaction even if the Zeeman effect is negligibly small. First, we focus on the vicinity of current peaks of Coulomb oscillation due to the resonant tunneling, considering the two energy levels around the Fermi level in the leads, and obtain an analytical form of spin-dependent current. The spin-polarization of the current is largely enhanced when the spacing between the two levels is smaller than the level broadening due to the tunnel coupling to the leads. Second, we perform a numerical simulation using a realistic model for the confining potential of the QD. We find more than 40\% spin-filtering efficiency around some current peaks.   

Tailoring Chirp in Spin-Lasers

The interplay of spin injection in lasers and their nonlinear response leads to novel spintronic devices [1]. Such spin-lasers can enable desirable properties including threshold reduction, bandwidth enhancement, and low chirp [1-3]. These lasers can also be viewed as spin-amplifiers, since high circular polarization in the output can be achieved even with nearly spin-unpolarized injection [2,3]. In the present work, we study chirp in spin-lasers and suggest new modulation schemes to improve their performance. Supported by NSF-ECCS, U.S. ONR, AFOSR-DCT, and NSF-NEB 2020. \\[4pt] [1] M. Holub et al., Phys. Rev. Lett. 98, 146603 (2007); J. Rudolph et al., Appl. Phys. Lett. 87, 241117 (2005). \\[0pt] [2] J. Lee, W. Falls, R. Oszwadowski, and I. \v{Z}uti\'c, Appl. Phys. Lett. 97, 041116 (2010).\\[0pt] [3] C. G{\o}thgen, R. Oszwadowski, A. Petrou, and I. \v{Z}uti\'c, Appl. Phys. Lett. 93, 042513 (2008).\\[0pt] [4] G. Boeris, J. Lee, K. V\'yborn\'y, and I. \v{Z}uti\'c, preprint (2011).   

Session H15: Focus Session: Spin and Dynamics in Metal, Spin logic and Spin-Based Devices

A novel three-terminal spintronics device utilizing the spin Hall effect

Previous work\footnote{Luqiao Liu \textit{et al.,}arXiv:1110.6846 and Luqiao Liu invited presentation this conference (focus topic 10.1.4)} has established that the spin Hall effect (SHE) in certain thin film metallic layers can generate a transverse spin current large enough to effect, through spin transfer torque (STT), the reversible magnetic switching of an adjacent ferromagnetic layer having perpendicular magnetic anisotropy. Here we discuss a new three-terminal spintronics device that utilizes the SHE induced STT to efficiently and reversibly switch the magnetic orientation of a thin free layer electrode of an MgO magnetic tunnel junction having in-plane magnetization. The low write currents ($\le $ 1mA), large output impedance and good thermal stability (45k$_{B}$T) that has been achieved with this SHE three-terminal device approach, which separates the write and read operations in a manner that is relatively straightforward to fabricate, demonstrate an attractive candidate for application in next generation STT MRAM and non-volatile spin logic circuits.   

Electric field assisted switching in magnetic tunnel junctions

It is of great interest to acquire large effects of electric field on magnetic properties, partly driven by the premise that voltage-controlled magnetization reversal would be far more energy efficient and be compatible with the ubiquitous voltage-controlled semiconductor devices. Normally the effect of electric field in metallic systems is negligible because the electric field can only penetrate into the materials by a few monolayers due to screening by the free electrons. Here we report the pronounced effects of electric field in magnetic tunnel junctions (MTJs) with very thin CoFeB electrodes, where the magnetic anisotropy originates solely from the CoFeB/MgO interfaces. The MTJs have the key structure of Co40Fe40B20(1.2-1.3nm)/MgO(1.2-2nm)/Co40Fe40B20(1.6nm) and the tunneling magnetoresistance in all junctions is in excess of 100{\%}. Due to the redistribution of electrons among the different 3d orbitals of Fe and Co, the perpendicular magnetic anisotropy of the CoFeB electrodes can be significantly modified by an applied electric field. As a result, the coercivity, the magnetic configuration, and the tunneling magnetoresistance of the MTJs can be manipulated by voltage pulses, such that the high and low resistance states of the MTJ can be reversibly controlled by voltages less than 1.5 V in magnitude and with much smaller current densities.   

Nanomagnetic triangles for a non-volatile logic applications

Single domain triangular nanomagnet is a base element in the recently proposed non-volatile logic application [1]. Dependent on the triangle shape and dimensions ``Y'' or ``buckle'' magnetization alignment ground states are defined by configurational anisotropy. In the ``Y'' ground state the local magnetization is aligned to point either towards or away from the triangle vertexes. The three triangle vertexes are used for information storage while switching between different ground states performs logic operations. In this work we study nanomagnetic triangle ground states and switching between them using micromagnetic simulations. We show that triangle shape engineering within the fabrication method tolerance allows maintaining the ``Y'' ground state despite the shape distortions typical for fabrication process. We correlate the height and profile of the energy barrier between the triangle ground states with the triangle shape and dimensions. We discuss ground state switching mechanisms and asses the triangle use for non-volatile logic and memory applications. \\[4pt] [1] A. Kozhanov, C.J. Palmstrom, S.J Allen ``Spin Torque Triad for a non-volatile logic gates'' US Patent pending.   

Ultra low-energy hybrid spintronics and straintronics: multiferroic nanomagnets for memory, logic and ultrafast image processing

We have theoretically shown that multiferroic nanomagnets (consisting of a piezoelectric and a magnetostrictive layer) could be used to perform computing while dissipating $\sim $ few 100 kT/bit (Appl. Phys. Lett. 97,173105, 2010) at clock rates of $\sim $1GHz. They can act as memory elements (Appl. Phys. Lett. \textbf{99}, 063108, 2011), logic gates (Nanotechnology, 22, 155201, 2011, http://arxiv.org/abs/1108.5758v1) and associative memory for higher order computing such as ultrafast image reconstruction and pattern recognition (J. Phys. D: Appl. Phys. 44, 265001 (2011), http://arxiv.org/abs/1109.6932v1). This talk will provide an overview of our research in: \begin{enumerate} \item Theoretical study of stress induced magnetization dynamics in isolated multiferroic nanomagnets (memory) and dipole coupled nanomagnetic arrays laid out in specific geometric patterns to implement a universal logic gate. \item Monte Carlo simulations of the magnetization trajectories in such systems described by the stochastic Landau-Lifshitz-Gilbert (LLG) equation, that show error-free ($>$99.99{\%}) \textit{fast} ($\sim $1 GHz) switching with very low dissipation (few 100kT/bit/magnet). \item Demonstrating that multiferroic nanomagnets possessing biaxial anisotropy could be used for four-state logic and perform image processing applications such as image reconstruction and pattern recognition. \item Experimental fabrication of such devices using e-beam lithography and deposition to create $\sim $ 100 nm diameter elliptical nanostructures and study them with magnetic force microscopy. \end{enumerate}   

Reliable switching in MRAM and multiferroic logic

Low reliable writing in spintronic devices limits their applicability in the automotive and defense industries. Coupling stochastic macromagnetic simulator with quantum transport, we show how greater reliable switching can be achieved in MRAM and multiferroic logic. Using a combination of spin-transfer torque and small applied perpendicular field in MRAM, the error rate can be considerably reduced for a given voltage pulse. In multiferroic logic, strain plays the role of the magnetic field. Information is passed along an array of nanomagnets (NM) (magnetostrictive + piezoelectric layers) through dipole coupling with neighboring NMs. A low voltage applied to the piezoelectric element causes the NM's magnetization to switch to its hard axis. Upon releasing the stress, the magnetization of the NM relaxes to the easy axis, with its final orientation determined by the dipolar coupling with the left NM, thus achieving a low power Bennett clocked computation. In the face of stagnation points along the potential energy landscape, the success rate of the straintronic switching can be controlled with by how fast the stress is removed from the NM. (Funding: DARPA, GRANDIS, NSF-NEB).   

Low-energy information transfer between dipolar-coupled magnetic disks observed by time resolved magnetic soft X-ray microscopy

The coupling between oscillators allows to mutually transfer energy and also to propagate information signals. Utilizing the concept of coupled oscillators, we experimentally demonstrated a new mechanism for energy transfer between spatially separated dipolar-coupled magnetic disks by stimulated vortex gyration. Direct experimental evidence was obtained by state-of-the-art experimental time-resolved soft X-ray microscopy probe. The rate of energy transfer from one disk to the other was derived from the two normal modes' frequency splitting caused by dipolar interaction. This mechanism provides tunable energy transfer rates, low-power input signals and negligible energy loss in the case of negligible intrinsic damping. Coupled vortex-state disks might find applications in future information-signal processing. H. Jung, et al., NPG - Scientific Reports 1 59 (2011); M.-W. Yoo, et al., Phys. Rev. B 82, 174437 (2010); H. Jung, et al., Appl. Phys. Lett. 97, 222502 (2010); Y.-S. Choi, et al., Phys. Rev. B 80, 012402 (2009); P. Fischer, et al., Phys Rev B 83 212402 (2011).   

Four-state straintronics: Ultra low-power collective nanomagnetic computing using multiferroics with biaxial anisotropy

Two-phase multiferroic nanomagnets, consisting of elastically coupled magnetostrictive/piezoelectric layers, can be endowed with four stable magnetization states by introducing biaxial magnetocrystalline anisotropy in the magnetostrictive layer. These states can encode four logic bits. We show through extensive modeling that dipole coupling between such 4-state magnets, combined with stress sequences that appropriately modulate the energy barriers between the stable states through magnetoelastic coupling, can be used to realize 4-state NOR logic (J. Phys. D: Appl. Phys. 44, 265001 (2011)) as well as unidirectional propagation of logic bits along a ``wire'' of nanomagnets (arXiv:1105.1818). As very little energy is consumed to ``compute'' in such a system, this could emerge as an ultra-efficient computing paradigm with high logic density. We show, by solving the Landau-Lifshitz-Gilbert (LLG) equation, that such nanomagnet arrays can be used for ultrafast image reconstruction and pattern recognition that go beyond simple Boolean logic. The image processing attribute is derived from the thermodynamic evolution in time, without involving any software. This work is supported by the NSF under grant ECCS-1124714 and VCU under PRIP.   

GMAG Dissertation Award Talk: All Spin Logic -- Multimagnet Networks interacting via Spin currents

Digital logic circuits have traditionally been based on storing information as charge on capacitors, and the stored information is transferred by controlling the flow of charge. However, electrons carry both charge and spin, the latter being responsible for magnetic phenomena. In the last few decades, there has been a significant improvement in our ability to control spins and their interaction with magnets. All Spin Logic (ASL) represents a new approach to information processing where spins and magnets now mirror the roles of charges and capacitors in conventional logic circuits. In this talk I first present a model [1] that couples non-collinear spin transport with magnet-dynamics to predict the switching behavior of the basic ASL device. This model is based on established physics and is benchmarked against available experimental data that demonstrate spin-torque switching in lateral structures. Next, the model is extended to simulate multi-magnet networks coupled with spin transport channels. The simulations suggest ASL devices have the essential characteristics for building logic circuits. In particular, (1) the example of an ASL ring oscillator [2, 3] is used to provide a clear signature of directed information transfer in cascaded ASL devices without the need for external control circuitry and (2) a simulated NAND [4] gate with fan-out of 2 suggests that ASL can implement universal logic and drive subsequent stages. Finally I will discuss how ASL based circuits could also have potential use in the design of neuromorphic circuits suitable for hybrid analog/digital information processing because of the natural mapping of ASL devices to neurons [4]. \\[4pt] [1] B. Behin-Aein, A. Sarkar, S. Srinivasan, and S. Datta, ``Switching Energy-Delay of All-Spin Logic devices,'' \textit{Appl. Phys. Lett.}, 98, 123510 (2011).\\[0pt] [2] S. Srinivasan, A. Sarkar, B. Behin-Aein, and S. Datta, ``All Spin Logic Device with Inbuilt Non-reciprocity,'' \textit{IEEE Trans. Magn}., 47, 10 (2011).\\[0pt] [3] S. Srinivasan, A. Sarkar, B. Behin-Aein and S. Datta, ``Unidirectional Information transfer with cascaded All Spin Logic devices: A Ring Oscillator,'' \textit{IEEE Device Research Conference} (2011).\\[0pt] [4] A. Sarkar, S. Srinivasan, B. Behin-Aein and S. Datta, ``Multimagnet networks interacting via spin currents'' \textit{IEEE} \textit{International Electron Devices Meeting} 2011. (to appear).   

``Spin inverter'' as building block of All Spin Logic devices

All-spin logic (ASL) represents a new approach to information processing where the roles of charges and capacitors in charge based transistors are played by spins and magnets, without the need for repeated spin-charge conversion. In our past work, we have presented numerical simulations based on a coupled spin transport and Landau Lifshitz Gilbert model showing that ring oscillators and logic circuits with intrinsic directionality [IEEE Trans. Magn. 47,10, 4026, 2011; Proc. IEDM, 2011)] can be implemented by manipulation of spins in magnetic nanostructures. The aim of this talk is (1) to identify a basic ASL unit that can be interconnected to build up spin circuits analogous to the way transistors are interconnected to build conventional circuits and (2) to present a compact model for this basic unit that can be used to design and analyze large scale spin circuits. We will show that this basic ASL unit is a one-magnet ``spin inverter'' with gain that can be cascaded to accomplish a spin circuit implementation of almost any logic functionality~   

Ultra low-power straintronics with multiferroic nanomagnets: magnetization dynamics, universal logic gates and associated energy dissipation

Stress induced magnetization dynamics of dipole coupled multiferroic nanomagnet arrays is modeled by solving the Landau-Lifshitz-Gilbert (LLG) equation. We show that in such multiferroic nanomagnets, consisting of magnetostrictive layers elastically coupled to piezoelectric layers, the single domain magnetization can be rotated by a large angle ($\sim $ 90$^{\circ})$ in $\sim $ 1 ns if a tiny voltage of a few tens of millivolts is applied across the piezoelectric layer [Nanotechnology, 22, 155201, 2011, Appl. Phys. Lett. 99, 063108, 2011]. Arrays of such multiferroic nanomagnets can be laid out in specific geometric patterns to implement combinational and sequential logic circuits by exploiting inter-magnet dipole coupling and Bennett clocked with specific stress cycles to propagate logic bits and implement dynamic logic. In this work, we theoretically demonstrate logic propagation in and fan-out characteristics of a universal NAND gate and discuss energy dissipation in the magnet and in the external clock. We show that this energy dissipation can be 3 orders of magnitude more energy-efficient than current CMOS technology for a reasonable clock speed of 1 GHz. This work is supported by the NSF under grant ECCS-1124714.   

Hybrid spintronics and straintronics: A paradigm for ultra-low-energy computing

We have shown in the past that the magnetization of a two-phase multiferroic single-domain nanomagnet can be electrically switched (flipped) with very little energy dissipation at low temperatures. This heralds a new energy-efficient magnetic logic and memory technology [Appl. Phys. Lett., \underline {99}, 063108, 2011, Phys. Rev. B, \underline {83}, 224412, 2011, Nanotechnology, \underline {22}, 155201, 2011]. Here, we extend our low-temperature result to room temperature where thermal noise can cause switching failures and increase average energy dissipation and switching delay. Using Monte Carlo simulations of switching trajectories described by the stochastic Landau-Lifshitz-Gilbert (LLG) equation, we show that even at room temperature, nearly error-free \textit{fast} switching is possible with very low dissipation. The energy dissipated to switch an appropriately designed nanomagnet with $>$ 99.99{\%} probability at room temperature is only $\sim $400 kT for a switching delay of sub-nanosecond. This is enabled by the complex interplay between the in-plane and out-of-plane excursions of the magnetization vector which \textit{aids} switching. This work is supported by the NSF under grant ECCS-1124714.   

Session H16: Quantum Fluids: Mainly Helium, Vortices, and Solitons

Weak to strong limits in confined superfluid helium

\newcommand{\boxes}[1]{$\left( #1~\mu \rm{m} \right)^3$} Experiments with $^4$He confined in \boxes{2} boxes connected via a thin film 33~nm thick have shown that the boxes act as isolated entities when spaced 4~$\mu$m edge-to-edge [1], whereas when spaced 2~$\mu$m edge-to-edge, they are strongly coupled to each other [2]. To investigate the spatial dependence of this coupling we are currently measuring \boxes{2} boxes spaced 1~$\mu$m edge-to-edge. We report measurements of the specific heat and superfluid density of helium confined in this geometry. These new data will help us to map the transition between fully isolated to fully coupled boxes which, in this limit, should behave like a 2 $\mu$m thick film. Questions involving our understanding of the correlation length $\xi$ arise, since it is observed that coupling is manifest over much larger distances than $\xi$.\\[4pt] [1] Perron J~K, Kimball M~O, Mooney K~P and Gasparini F~M 2010 {\em Nat. Phys.\/} {\bf 6} 499--502\\[0pt] [2] Perron J~K, and Gasparini F~M 2011 {\em submitted to PNAS\/}   

Critical point coupling in liquid helium and the significance of the correlation length

\newcommand{\boxes}[1]{$\left( #1~\mu \rm{m} \right)^3$} Recent measurements of liquid helium confined to \boxes{2} boxes connected through a 33~nm film have shown coupling effects between boxes spaced distances much larger than the correlation length $\xi(t,L)$[1,2] and proximity effects on the connecting film[3]. An analysis of data suggests that $\xi(t,L)$ is the relevant parameter in these effects. This dependence on $\xi(t,L)$ is used to argue that the enhancement in the specific heat due to coupling is a reflection of the finite-size correlation length in the boxes and hence its scaling function. All this raises some profound questions about our physical understanding of $\xi(t,L)$. \\[4pt] [1] Perron J~K, Kimball M~O, Mooney K~P and Gasparini F~M 2010 {\em Nat. Phys.\/} {\bf 6} 499--502\\[0pt] [2] Perron J~K, and Gasparini F~M 2011 {\em submitted to PNAS\/}\\[0pt] [3] Perron J~K, and Gasparini F~M 2011 {to be published in \em JPCS\/}   

Creating Only Isotropic Homogeneous Turbulence in Liquid Helium near Absolute Zero

Flow through a grid is a standard method to produce isotropic, homogeneous turbulence for laboratory study. This technique has been used to generate quantum turbulence (QT) above 1 K in superfluid helium\footnote{S. R. Stalp, L. Skrbek, and R. J. Donnelly, \textit{Phys. Rev. Lett}. \textbf{\textit{82}}, 4831 (1999).} where QT seems to mimic classical turbulence. Efforts have been made recently\footnote{G.~G.~Ihas, G.~Labbe, S-c.~Liu, and K.~J.~Thompson\textit{, J. Low Temp. Phys}. \textbf{150}, 384 (2008).} to make similar measurements near absolute zero, where there is an almost total absence of normal fluid and hence classical viscosity. This presents the difficulty that most motive force devices produce heat which overwhelms the phenomena being investigated. The process of designing and implimenting a ``dissipation-free'' motor for pulling a grid through superfluid helium at millikelvin temperatures has resulted in the development of new techniques which have broad application in low temperature research. Some of these, such as Meissner-affect magnetic drives, capacitive and inductive position sensors, and magnetic centering devices will be described. Heating results for devices which can move in a controlled fashion from very low speed up to 10 cm/s will be presented. Acknowledgement: We thank W.F. Vinen for many useful discussions.   

Superfluid Onset and 2D phase transitions of Helium-4 on Lithium and Sodium

We have fabricated lithium and sodium films on quartz crystal microbalances (QCM) using in situ low temperature pulsed laser deposition. The frequency shift and dissipation of the QCM was measured as a function of helium pressure and chemical potential and used to construct the phase diagram of helium films on these substrates. Pressure measurement techniques based on an RGA mass spectrometer, which provides accurate measurement below 10$^{-8}$ Torr will be described. Lithium and sodium are predicted to be intermediate strength substrates which are strong enough to be wetted by He-4 but weak enough that solid-like layers do not form, so they are candidates for observing sub-monolayer superfluidity in direct contact with a metallic surface. Helium adsorption isotherms and quenches between 0.5K and 1.6K on both lithium and sodium indicated continuous, sub-monolayer helium film growth and superfluid onsets in sub-monolayer films. Features below 1K indicate a collision between a classical 2D liquid/vapor phase transition and the Kosterlitz-Thouless superfluid phase transition. We see no evidence for the pre-wetting step instability predicted for helium on sodium.   

Discrepancies Between Theory and Experiment for Field-Dependence and $f$-wave Interactions in Superfluid $^3$He-$B$

We have performed transverse acoustics experiments in superfluid $^3$He-B, exploring the magnetic field splitting of the imaginary squashing mode (ISQ), a collective mode of the order parameter labelled by its total angular momentum $J=2$. We have compared theoretical calculations\footnote{J.A. Sauls and J.W. Serene, Phys. Rev. Lett. {\bf 49}, 1183 (1982).} of the Zeeman splitting, $g_{2^-}$, and its dependence on the strength of $f$-wave pairing interactions, $x_3^{-1}$, with our recent experimental data, showing unexpected discrepancies. We suggest that the origin of these discrepancies can be traced to limits on the applicability of the theoretical calculations at high magnetic field and at frequencies some distance from the order parameter collective mode. We discuss the analysis done by Davis \textit{et al.} in light of those limitations.\footnote{J.P. Davis {\it et al.}, Phys. Rev. Lett. {\bf 100}, 015301 (2008).} This work was supported by the National Science Foundation DMR-1103625.   

Role of the order parameter manifold on surface Majorana fermions and spin susceptibility of superfluid $^{3}$He-B

Here, we theoretically investigate surface Andreev bound states (SABS) in superfluid $^{3}$He-B confined to a slab geometry. It is known that the Majorana property gives rise to the Ising anisotropy of spin susceptibility on the surface, which reflects the assumption that the order parameter manifold of the B-phase is restricted to the subspace. In this talk, we first demonstrate that the SO(3) manifold, which describes the relative rotation of spin and orbital, plays a critical role on the various properties associated with the SABS, such as the Majorana nature, gapless dispersion, topological invariant, and spin susceptibility. Then, based on the quasiclassical Eilenberger theory which takes account of the dipole interaction, we quantitatively discuss thermodynamics and the Majorana property of the SABS in superfluid $^{3}$He-B under a magnetic field, where the Majorana property and spin susceptibility is determined by the interplay of the magnetic field and dipole interaction.   

Macroscopic quantum tunneling of a single vortex in a rotating Bose-Einstein condensate

A vortex can be created in a metastable state near the center of a harmonically trapped Bose condensate when it is rotated with suitable velocities. This state has a finite lifetime before which the vortex tunnels outwards to the edge of the trap. We estimate this tunneling rate semiclassically with the vortex treated as a point particle. This is followed by a more exact treatment incorporating the dynamics and mass of the vortex. The calculation is based on a Thomas-Fermi approximation, relevant to the typically large atomic densities used in experiments, but we also analyze the low density limit in a ``weakly nonlinear'' perturbative framework. We discuss the feasibility of detecting this effect with currently achievable experiments.   

Expansion Dynamics of a Ring Bose--Einstein Condensate

We studied the dynamics of BECs when released from a ring trap under conditions similar to those that obtained in a recent experiment done at NIST. In that experiment a ring--shaped BEC was formed in an all--optical trap created by intersecting a horizontal light sheet and a vertical Laguerre-Gaussian beam. Condensates were created in these traps and then ``stirred'' by applying Raman pulses having orbital angular momentum (OAM). We modeled the dynamics of condensates formed under these conditions by first solving the 2D time--dependent Gross--Pitaevskii equation (GPE) in imaginary time to obtain the initial condensate shape. We accounted for the OAM by applying a phase imprint to this wave function and then propagated it using the GPE in real time with the trap off. We found that, after release, the condensate expands both inward and outward. When no OAM was applied, this inward expansion causes the hole in the ring to close up entirely in turn causing a buildup of atom density there. Inflow and outflow of atoms from the center caused expanding interference rings to form. With non-zero applied initial OAM similar behavior was observed except that the central hole never closes with hole size increasing with increasing initial OAM. We compare our results with the NIST experiment.   

Quantum Dynamics of a Bose Superfluid Vortex

Quantum vortex dynamics remain poorly understood despite decades of theoretical investigation. The vortex is a topological soliton, arising from the same medium as the quasiparticles with which it interacts. Hence the coupling between the vortex ``zero mode'' and the quasiparticles has no term linear in the quasiparticle variables -- the lowest order coupling is quadratic. We present a fully quantum-mechanical derivation of the vortex equation of motion valid at low temperatures where the normal fluid density is small. The resulting equation of motion is naturally expressed as a function of the dimensionless frequency $\tilde \Omega = \hbar \Omega/k_BT$. The usual Hall-Vinen/Iordanskii equations are valid when $\tilde \Omega \ll 1$ (the ``classical regime''), but elsewhere, the equations are strongly memory dependent. We will discuss the experimental implications of this frequency dependence in Bose superfluids and cold atomic gases.   

Fragmented Many-Body states of definite angular momentum and stability of attractive 3D Condensates

We consider a 3D Bose-Einstein Condensate (BEC), with attractive interparticle interactions, embedded in a harmonic, spherically symmetric trap. This system is metastable only if the total number of bosons $N$ and the interaction strength $lambda_0$ do not exceed some critical values. Otherwise the system collapses. Gross-Pitaevskii (GP) theory predicts the maximum (critical) number of bosons $N_{cr}^{GP}$ that, for a given $\lambda_0$, can be loaded to the system, without its collapse. But, what happens to the excited states? To investigate the structure and stability of these states we must go beyond GP theory; these states have definite values of angular momentum (AM) L, are highly fragmented and can support number of bosons much greater than $N_{cr}^{GP}$. Secondly, we investigate the impact of external rotation of the trap to the AM and stability of the gas. We find that, for all allowed values of rotation frequency no significant stabilization occurs. The symmetry of the ground state does not change and no AM is transferred to the gas. This behaviour is attributed to the attractive nature of the interparticle interaction.   

Origins of bright soliton transparency to Bogoliubov quasi-particles

Bogoliubov quasi-particles can pass through a one-dimensional bright soliton without reflection at all energies.\footnote{D. J. Kaup, Phys. Rev. A {\bf 42}, 5689 (1990).} Reflectionless properties of this kind usually originate from a supersymmetric structure of the corresponding Hamiltonian.\footnote{E. Witten, Nucl. Phys. B {\bf 188}, 513 (1981).}$^,$\footnote{C. V. Sukumar, J. Phys. A {\bf 18}, 2917 (1985).} However, we give a strong indication that in this case\footnotemark[1], the mathematical mechanism enabling full spectrum transparency of a scattering object does not fall into any of the conventional paradigms.   

Supersymmetric Structure of two Families of Solitons

Solitons have generated considerable interest in the cold atoms and condensed matter communities. We demonstrate that two families of $n$-soliton solutions (with $n$ an integer) -- one for the attractive nonlinear Schr\"{o}dinger (NLS) equation, and one for the sine-Gordon (sG) equation -- originate from a quantum-mechanical supersymmetric (QM-SUSY) chain connecting a set of reflectionless operators $\hat{H}_n$. The families consist of breather-type solitons for NLS\footnote{D. Schrader, IEEE J. Quantum Electron. {\bf 31}, 2221 (1995).} and multi-(anti)kink solitons with specific velocities for sG. The operators $\hat{H}_n$, which we refer to as Akulin`s Hamiltonians\footnote{V. M. Akulin, \underline{Coherent Dynamics of Complex Quantum Systems} (Springer, Heidelberg, 2006).}, form reflectionless direct-scattering initial conditions for the inverse scattering method. Such a QM-SUSY chain is analogous to the known connection between QM-SUSY chains of P\"{o}schl-Teller potentials and solitons of the Korteweg-de Vries (KdV) equation\footnote{Sukumar, J. Phys. A {\bf 19}, 2297 (1986)}. The existence of QM-SUSY chains connecting soliton solutions, now for three different integrable nonlinear equations, sheds light on the underlying mechanisms responsible for soliton generation.   

From Dark Solitons to Vortex Clusters in Bose-Einstein Condensates

In this talk, we 'll start by considering the experimental, theoretical and numerical dynamical properties of dark solitons in quasi-one-dimensional trapped Bose-Einstein condensates. We will identify the oscillations and interactions of such coherent structures and we will then aim towards generalizing the corresponding notions to quasi-two-dimensional vortex states. In the latter setting, we will use two approaches: one from the linear limit of the underlying system that will permit us to identify the bifurcations of multi-vortex cluster states, while the second in the large density limit will enable our consideration of the vortices as precessing and interacting particles within the condensate. We will corroborate our analytical predictions within these limits and numerical results bridging the limits with experimental observations for the dynamics of few-vortex clusters.   

Multifractals in Soliton sea on Fibonacci Lattice

Systems exhibiting Bose-Einstein condensation are suitable for fabricating artificially designed structures, e.g., the bichromatic potentials have attracted much attention. More exotic structures also appear to be experimentally feasible in the optical imaging system with high resolution. We consider one of exotic structures, Fibonacci potential, which is quasiperiodic. On the Fibonacci potential without nonlinear interaction, it is known that the spectrum is singular continuous and all the eigenvectors are called a critical state, in which multifractal states are included. On the other hand, the physical properties of the Bose-Einstein condensates confined in a optical lattice can be described in terms of the Schr\"odinger equation with nonlinear term via particle-particle interactions. We numerically and mathematically investigated nonlinear Schr\"odinger equation on Fibonacci potential focusing on the competition between nonlinear fluctuation and criticality~[M.~Takahashi {\it et al.}, arXiv:1110.6328]. The conclusion is that the critical states with the spectrum in the Cantor set retains their profile irrespective of the strength of the nonlinearity. The spectrum for the critical states is in a sea of ``stationary solitons" which appear as a result of nonlinear effects.   

Session H17: Focus Session: Thermoelectrics - Characterization/Molecular junctions/Magnetic and Low Temp

Strategies for developing optimal thermoelectric metrology protocols

The Seebeck coefficient is an essential physical property routinely measured to evaluate the potential performance of new thermoelectric materials. These materials facilitate the inter-conversion of thermal and electrical energy and are useful in power generation or solid-state refrigeration applications. However, the diversity in Seebeck coefficient measurement techniques, conditions, and probe arrangements has resulted in conflicting materials data, further complicating the inter-laboratory confirmation of reported higher efficiency thermoelectric materials. In an effort to identify optimal thermoelectric measurement protocols, we have developed a complimentary strategy to both evaluate and compare these different probe arrangements and measurement methodologies: first, through the design of an innovative experimental apparatus, and second, through error modeling of Seebeck coefficient measurements using finite element analysis. This talk will include a discussion of key measurement challenges, example diagnostics, and recommended practices to effectively manage uncertainty in Seebeck coefficient measurements.   

Experimental Determination of the Lorenz Number

In an effort to improve the dimensionless thermoelectric figure of merit (ZT), thermal conductivity reduction is imperative. Most efforts are made to reduce the lattice portion of the thermal conductivity through nanostructuring. However there is no direct way to measure the lattice contribution and typically the lattice thermal conductivity is approximated by various methods. By experimentally determining the Lorenz number, the lattice thermal conductivity can be directly calculated. A method for determining the Lorenz number of thermoelectric materials Bi$_{2}$Te$_{2.7}$Se$_{0.3}$ and Bi$_{0.88}$Sb$_{0.12}$ will be presented.   

Anisotropic Two-band Transverse Thermoelectrics in Zero Magnetic Field

Narrow gap materials with anisotropic electron and hole band conductance are shown to function as anisotropic two-band transverse (A2T) thermoelectrics, whereby longitudinal electrical currents generate transverse Peltier heat flow. Unlike the Ettingshausen effect which requires external magnetic field, a large transverse Seebeck coefficient in A2T thermoelectric results from the anisotropic electron and hole mass tensors without magnetic field. Compared to synthetic transverse thermoelectrics, A2T thermoelectric coolers can be scaled to nanoscale, and the intrinsic nature of this phenomenon is promising for cryogenic applications. With exponentially tapered coolers, arbitrary $\Delta$T can be reached with sufficiently thick layers and a small electric field. Equations for A2T thermoelectric transport from an electron-hole band model yield the optimal orientation to achieve maximum transverse figure of merit $Z_\perp T$. The InAs/GaSb type II superlattice is shown to have the appropriate anisotropic band structure, and bandgaps of order $kT$ are calculated to give a competitive $\Delta$T = 14~K at room temperature. Thermal conductivity of the superlattice is 4~W/m$\cdot$K at 300~K using 3$\omega$ method. Preliminary data on in-plane Seebeck coefficient will also be presented.   

Magnetotransport in thermoelectric materials.

The mechanisms behind the magnetic field and temperature dependent thermoelectric effects that have recently been found experimentally in the electrical resistivity, thermopower and thermal conductivity of nanocomposite samples of Bi2Te3 and related compounds is studied. Large deviations in the transport coefficients in the presence of an applied magnetic field, both when the magnetic field is parallel and perpendicular to the transport direction has been observed in Bi2Te3 compounds, despite the small electronic mean free paths in these materials. An accurate analysis of the experimentally measured data leads to the estimation of electron mean free path and extraction of lattice thermal conductivity contribution from the total thermal conductivity. We will apply a Boltzmann transport equation based formalism to analyze the experimental data and improve the accuracy of the estimation of electronic and lattice part of the thermal conductivity. A particular attention will be paid to the analysis of the anisotropic effects in these samples.   

Separation of Joule Heating and Peltier Cooling via Time-Resolved X-Ray Di?raction in Si/SiGe Superlattice

We present detailed measurements of the thermal pro?le in a pulsed current SiGe-based thermoelectric micro-cooler. The evolution of heat ?ow in thermoelectric materials has been previously studied using time-domain thermore?ectance imaging; however, such methods are typically only sensitive to the surface temperature of the device, and the heat ?ow into the material remains hidden. Using time-resolved x-ray di?raction, we probe the transient temperature change in both the surface gold electrode and the underlying Si/SiGe superlattice using the shift in diffraction pattern caused by thermal expansion. We are also able to resolve Joule heating vs. Peltier cooling taking place in the gold through separation of timescales made possible by the relatively short duration (100ps) of the Advanced Photon Source.   

Reassessment of the carrier concentration in GeTe-based thermoelectric materials by $^{125}$Te NMR

Ge$_{1-x}$Ag$_{x/2}$Sb$_{x/2}$Te $p$-type thermoelectric materials (``TAGS-$n$'') were studied extensively in the 1970s and then again recently. They exhibit an unusual combination of large thermopower, $S$, and high hole concentration, $p$, reported based on the Hall effect data, which has not been explained. To solve this puzzle, we have synthesized GeTe, GeTe:Bi, and TAGS-$n$ with $n $= 97, 94, 90, and 85 and studied XRD, thermopower, electrical resistivity, thermal conductivity, and $^{125}$Te NMR. Most importantly, we have determined the carrier concentrations using $^{125}$Te NMR spin-lattice relaxation and Knight shift. In GeTe and GeTe:Bi, we found that carrier concentrations generally agree with the values reported from Hall effect. In TAGS-$n$, they are much lower but agree better with the values expected from $S$ vs. $p$ for GeTe-based materials, solving the puzzle partially. The NMR vs. Hall effect discrepancy in TAGS-$n$ can be due to the presence not only of holes but also electrons generated by Sb atoms, which results in artificially high hole concentration from Hall effect. Even though the true hole concentration is lower than reported, the thermopower of TAGS-$n$ is still significantly larger than that of GeTe and GeTe:Bi at similar carrier concentration. This can be explained by energy filtering enhanced by potential barriers formed due to Ag-Sb pairs in the TAGS-$n$ lattice.   

Charge and Energy Transport in Molecular Junctions

Charge and energy transport in molecular junctions, created by trapping short organic molecules ($\sim $1nm) between inorganic electrodes, is expected to be fundamentally different from transport in bulk materials due to their discrete electronic structure. In fact, numerous computational studies have suggested that it may be possible to utilize the novel thermoelectric and thermal transport phenomenon in nanoscale molecular junctions to create efficient energy conversion devices (e.g. thermoelectric devices). However, a large number of these effects remain to be experimentally verified. We will describe our experimental studies where thermoelectric properties of junctions were studied at the single/few molecule level enabling novel insights into the relationship between molecular structure and the thermoelectric properties of junctions. We will also present our recent experimental efforts to probe thermal transport in nanoscale molecular junctions and point contacts. In order to accomplish this goal, it is necessary to accurately measure heat currents as small as 10 picowatts. Towards this goal, we will present a novel device developed by us that is capable of resolving heat currents as small as 4 picowatts.   

The impact of electron-vibration interactions on the thermoelectric efficiency of molecular junctions

From first--principles approaches, we investigate the thermoelectric efficiency of a molecular junction where a benzene molecule is connected directly to the platinum electrodes. We calculate thermoelectric figure of merits ZT in the presence of electron--vibration interactions with and without local heating under two scenarios: linear response (zero bias) and finite bias (non--zero bias) regimes. In the linear response regime, ZT saturates around the electrode temperature $T_{e}=25$~K in the elastic case, while in the inelastic case we observe a non--saturated and a much larger ZT beyond $T_{e}=25$~K attributed to the tail of the Fermi--Dirac distribution. In the finite bias regime, the inelastic effects reveal the signatures of the molecular vibrations in the low temperature regime. The normal modes exhibiting structures in the inelastic profile are characterized by large components of atomic vibrations along the current density direction on top of each individual atom. In all cases, the inclusion of local heating leads to a higher wire temperature $T_{w}$ and thus magnifies further the influence of the electron--vibration interactions due to the increased number of local phonons.   

Interfacial Interactions in Polymer-Nanocrystal Thermoelectric Composites Provide a Novel Route for Power Factor Enhancement

The highest performing thermoelectric materials currently available are fabricated via expensive high-temperature vacuum processing techniques. Recently, there has been an increasing interest in the thermoelectric properties of solution-processable materials, which have the potential to dramatically reduce module fabrication costs. These solution-processed materials however often exhibit poor transport properties, which undermines their competitive advantage over the more traditional expensive thermoelectric materials. Here, we present the thermoelectric transport properties of a new class of solution-processable conducting-polymer/inorganic composite materials as a function of nanocrystal loading. In the Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and Tellurium nanowire composite devices fabricated for this study, the thermoelectric performance of the composite exceeds that of either pure organic or inorganic component alone. This result suggests an interface-driven mechanism for this enhanced performance and provides an exciting route for improving the power factors of organic-inorganic hybrid thermoelectrics.   

Nernst-Ettingshausen Effect in Elemental Rare-Earth Single Crystals

The transverse Nernst-Ettingshausen (N-E) coefficient $N $measurements of the elemental rare-earth (R-E) single-crystal are for the first time presented from 80 to 420 K. Since they have mainly hexagonal symmetry at room temperature, measurements are given with the heat flux along the [100] and the [001] axes. Due to their complex band structure and Fermi surface, their small thermopower (S) and their multicarrier systems involving electron (e) and hole (h) pockets, their $N$ are expected to be large. Indeed, for such systems, both $S $and $N$ can be expressed as$^{1} \quad S=(S_{e}$\textit{$\sigma $}$_{e}+ S_{h}$\textit{$\sigma $}$_{h}$\textit{)/( $\sigma $}$_{e}$\textit{+$\sigma $}$_{h})$ while $N=[(N_{e}$\textit{$\sigma $}$_{e}+ N_{h}$\textit{$\sigma $}$_{h}$\textit{)( $\sigma $}$_{e}$\textit{+$\sigma $}$_{h})+(S_{h}-S_{e})(R_{Hh}$\textit{$\sigma $}$_{h}- R_{He}$\textit{$\sigma $}$_{e}$\textit{)$\sigma $}$_{e}$\textit{$\sigma $}$_{h}$\textit{]/( $\sigma $}$_{e}$\textit{+$\sigma $}$_{h})^{a}$, where \textit{$\sigma $} is the electrical conductivity and $R_{H}$ the Hall coefficient and the subscript correspond to either carriers. Since $S_{h}>$0 and$ S_{e}<$0, the resulting $S$ should be low thus leading to a large $N$ . These solids are useful in single-material thermoelectric N-E coolers. They create a large temperature differences using thermomagnetic effects, without having to be cascaded. This would resolve th problem of contact resistances of actual multi-stage Peltier coolers, especially in the cryogenic temperature range. The dimensionless figure of merit of N-E coolers is \textit{zT}$_{N}=B^{2}N^{2}$\textit{$\sigma $(B)T/$\kappa $(B),} with $B$ is the magnetic field, $T$ the absolute temperature and \textit{$\kappa $} the thermal conductivity. a.E.H. Putley, \textit{The Hall Effect and Semiconductor Physics} , New York: Dover publication, 1968.   

Giant Bipolar Nernst Effect in the Quasi-One-Dimensional Metal, Li$_{0.9}$Mo$_6$O$_{17}$

The Nernst coefficient for the quasi-one-dimensional metal, Li$_{0.9}$Mo$_6$O$_{17}$, is found to be among the largest known for metals ($\nu\simeq 500\ \mu$V/KT at $T\sim 20$~K), and is enhanced in a broad range of temperature by orders of magnitude over the value expected from Boltzmann theory for carrier diffusion. A comparatively small Seebeck coefficient implies that Li$_{0.9}$Mo$_6$O$_{17}$ is bipolar with large, partial Seebeck coefficients of opposite sign. A very large thermomagnetic figure of merit, $ZT\sim 0.5$, is found at high field in the range $T\approx 35-50$~K.   

Cryogenic Thermoelectric Properties of the Bismuth-Magnesium and Bismuth-Antimony-Magnesium Systems

There is a need to increase the Figure of Merit of thermoelectric materials used in low temperature cooling applications. Band structure calculations show that substitutional magnesium in bismuth can form sharp density of states peaks, suggesting the presence of a resonant level. Single crystal samples of (Bi$_{1-x}$Sb$_{x})_{1-y}$Mg$_{y}$ (0 $\le $ x $\le $ 12{\%} and 0 $\le $ y $\le $ 0.7{\%} nominally) were synthesized in evacuated ampoules. The composition of each ingot was analyzed using x-ray diffraction, and transport properties were measured using a Thermal Transport Option (TTO) in a Physical Properties Measurement System (PPMS) from 300K to 2K. It is apparent that the addition of magnesium strongly influences thermopower; the data for Bi$_{90}$Sb$_{10}$Mg$_{0.7}$ shows a second minimum in thermopower at 20K, in addition to the expected minimum at approximately 50-60K. This could be due to the resonant scattering at the cryogenic temperatures which arises from the excess density of states. The addition of magnesium also appears to decrease thermal conductivity below 30K. We present systematic experimental approaches and the results to elucidate the role of magnesium in bismuth and bismuth-antimony systems.   

Low Temperature Electronic and Magnetic Properties of CePd$_{3}M_{x}$

The intermediate valence compound CePd$_{3}$ is a strong candidate for low temperature thermoelectric applications due to its unusually large Seebeck coefficient which peaks at approximately 115 $\mu $V/K near 125K. This phenomenon results from a sharp peak in the density of states near the Fermi level due to the hybridization of conduction electrons with those in the partially occupied cerium f-shell, thus making the system highly sensitive to changes in the average cerium valence state. We have systematically studied the structural and thermoelectric properties of various CePd$_{3}M_{x}$ compounds, where $M$ is an s- or p-block element and 0 $<$ x $<$ 0.1, in order to explore the effects of such partial filling on the cerium valence. The results of X-ray diffraction, Seebeck coefficient, and magnetic susceptibility measurements are reported. We have found that incorporating $M$ elements of various valence configurations has similar effects on the electronic and magnetic properties as changing the $M$ concentration, thus establishing an effective new mechanism for tailoring the thermoelectric properties of the system. \newline   

Session H18: Focus Session: Electron, Ion, and Exciton Transport in Nanostructures - Nanowires and Superlattices

Energy Transfer of Excitons and Electron-Hole Plasmas between Quasi-two-dimensional Semiconductor Layers

We study the energy transfer mechanisms of excitons and electron-hole plasmas between two quasi-two dimensional (2D) semiconductor quantum wells. It is shown that dipolar (i.e., Foerster) energy transition (ET) mechanism dominates at a short distance while photon-mediated ET mechanism plays a more important role for transfer over a long distance. The magnitude and the dependence of the transfer rates of plasmas and excitons on the temperature, the well-to-well distance, and the density are compared for both ET mechanisms. The spatial dependence of the 2D-2D transfer rates will be compared with that of the ET rate between two quantum dots.   

Charge transport physics of individual PbSe Nanowire field effect transistors

We report the charge transport properties of individual PbSe nanowire (NW) field-effect transistors (FETs) fabricated from single crystalline, PbSe NWs 10 nm in diameter synthesized by wet-chemical methods. PbSe is a particularly interesting semiconductor to study in one-dimension as the diameter of the NWs is smaller than the electron, hole and exciton Bohr radii, allowing study of strongly quantum confined NWs. We investigate the temperature dependent charge transport properties of ambipolar and reversibly, surface, p-doped PbSe NW FETs. We demonstrate that PbSe NW FETs behave as Schottky Barrier (SB) FETs, in which the off current is limited by the SB height and decreases as temperature decreases, while the on current is achieved by gate thinning and increases as temperature decreases. We calculate the SB heights for electron and hole injection in ambipolar and hole injection in p-type PbSe NW FETs. The hole mobility in surface-doped, p-type FETs is temperature dependent, rising monotonically from 200 cm2/Vs at room temperature to 2000 cm2/Vs at 4.5K, without signatures of impurity scattering which commonly limits carrier mobilities at low temperatures in substitutionally doped NWs.   

Electronic transport in individual, vertical, catalyst free GaN/AlN nanowires

In the recent years, the advances in the THz technology, for example the development of the quantum cascade laser, promotes the possibility to use resonant tunneling diodes (RTD) to enhance such technologies. Coaxial m-plane AlN/GaN nanowire based resonant tunneling diode structures were formed by plasma assisted molecular beam epitaxy (MBE) using a two-step growth method that allows for control of vertical and lateral growth [1]. They are spontaneously formed MBE nanowires and they are integrated in Si (111). We discuss ongoing work on the electronic transport in these individual, vertical nanowires using two nanoprobes to contact the top of the nanowire and the substrate. The IV characteristics reveal a clear negative differential resistance (NDR) at room temperature (RT). The NDR was observed $\sim $ 4V with a peak-to-valley ratio as high as 10. [1] S.D. Carnevale \textit{et al}., \textit{Nano Letters, }11, (2), 2011.   

Correlated study of individual nanowires with electronic transport and scanning tunneling microscopy

The electronic conductance in quantum wires is often dictated by quantum instabilities and strong localization at the atomic scale. We present a novel nano-transport technique which combines local nano-contacts and four-probe STM. The approach allows for correlated study of electron transport and scanning tunneling spectroscopy in individual nanowires. We first apply it to the GdSi2 quantum wires, which show that isolated nanowires exhibit a metal-insulator transition upon cooling, driven by the defect-induced localizations, while wire bundles maintain a robust metallic state, stabilized by interwire electronic coupling. We then demonstrate applications of this transport technique with cabon nanotubes and copper wires in situ. The method bridges the gap between the transport and the local electronic and structural properties down to the atomic scale.   

The influence of excitonic effects on charge separation at hybrid nanoscale interfaces

Efficient charge separation is critical in nanostructured photovoltaic systems, but the accurate prediction of charge separation rates remains an extremely difficult task. Although considerable progress has been made, reliable theoretical schemes to describe the charge-separation mechanisms and to compute the interfacial charge transfer dynamics have not been fully developed. In this work, we embrace the excitonic effects that are of critical importance in nanoscale geometries to derive a criterion for charge separation at hybrid nanoscale interfaces beyond the traditional quasiparticle energy-level alignment. The approach utilizes calculations from many-body perturbation theory with Green functions to accurately account for self-energy and electron-hole interactions. Four representative interfaces between Si quantum dots and small molecules are considered using this approach, in order to demonstrate that both excitonic and Coulomb stabilization effects are essential for correctly predicting charge separation at nanostructured interfaces. Interestingly, our calculations suggest that preemptory exciton transfer across interfaces only suppresses the subsequent charge separation. We also compute charge separation rates using the Marcus theory and other methods, and their accuracy an   

QM/EM simulation of Junctionless FinFET

We present here the simulations of a junctionless transistor. Its source, channel and drain are embeded in a piece of uniformly doped silicon nanowire. In the earlier stage, it has been designed to be a normally ``ON'' device. Quite reversely, the first experimentally presented junctionless transistor is in the ``OFF" state when the applied gate voltage is absent. Simulations show that the depletion occurs between the nanowire and hetero-doped gate. A ``P-N junction'' is formed in the junctionless transistor, whose direction is perpendicular to the direction of the current flowing. Our simulation considers the depletion effect in the Qauntum mechanical calculation. I-V curves of transistors with the gate doped by the same and different type of dopants have been obtained. The results match the experiments.   

Tuning the hole mobility in InP semiconductor nanowires

Transport properties of holes in InP nanowires were calculated considering the effect of temperature and the presence of realistic strain fields. The mobility of holes is obtained analytically by considering electron-phonon interaction via deformation potential through longitudinal optical (LO) phonons. Using molecular dynamics with realistic force potentials, we simulate nanowire structures and the associated phonon density of states; the structures show effects of LO phonon energy renormalization due to the reduced dimensionality and variation of the phonon lifetimes important for carrier mobility. Our mobility calculations include heavy and light hole subbands in a Luttinger Hamiltonian formalism and consider how the valence band ground state changes between light- and heavy-hole character, as both the strain field configuration and the nanowire size are changed. Depending on the dimensions and characteristics of the nanowire, we find interesting sudden changes in the mobility, which arise with the onset of a resonance between the LO phonon frequencies and the subband separation between the ground and first hole state. We will present the effect of strains and temperature on this resonant behavior and discuss the consequences for carrier mobility in these systems.   

First-principles study on metal-TaO$_x$-metal heterostructures: response to applied bias voltages

Metal-TaO$_x$-metal heterostuructures are promising as a novel nonvolatile memory device [1]. The formation of conduction paths in the TaO$_x$ layer is responsible for the low resistance state and the switching mechanism is understood as the electrochemical redox reaction involved with the bias-voltage application. However, microscopic details of the conduction path and switching mechanism have not been clarified yet. We examine electronic states, and electron and ion transport in Cu-TaO$_x$ (x$\sim$2.5)-Pt(or Cu) heterostructures from first principles, focusing on the response to applied bias voltages. We show that Cu interstitials in crystalline Ta$_2$O$_5$ enhances electronic conduction considerably [2], while does not in amorphous one. We also show that the potential change due to the bias application is sensitive to the structure of TaO$_x$ layer and/or metal-TaO$_x$ interface: in some case, the potential change may be very small near the Cu/TaO$_x$ interface in the TaO$_x$ layer so that the bias application hardly change the mobility of Cu ions in this region. These results will be discussed in terms of electronic states. [1] T. Sakamoto et al., APL 91 (2007) 092110; [2] T. K. Gu et al., ACS Nano 4 (2010) 6477.   

Theory of spatially inhomogneous Bloch oscillations in semiconductor superlattices

In a semiconductor superlattice with long scattering times, damping of Bloch oscillations due to scattering is so small that nonlinearities may compensate it and Bloch oscillations persist even in the hydrodynamic regime. To demonstrate this, we propose a Boltzmann-Poisson transport model of miniband superlattices with inelastic collisions and derive hydrodynamic modulation equations for the electron density, the electric field and the complex amplitude of the Bloch oscillations. For appropriate parameter ranges, we solve numerically these equations and show that there are solutions having the form of stable Bloch oscillations with spatially inhomogeneous field, charge, current density and energy density profiles. These Bloch oscillations disappear as scattering times become sufficiently short. For sufficiently low lattice temperatures, Bloch and Gunn type oscillations mediated by electric field, current and energy domains coexist for a range of voltages. For larger lattice temperatures (300 K), there are only Bloch oscillations with stationary amplitude and electric field profiles.   

Stochastic current switching in semiconductor superlattices: observation of non-exponential kinetics

We report the experimental measurement of first-passage-time distributions associated with noise-induced current switching in doped, weakly-coupled GaAs/AlAs superlattices, in a regime of nonlinear electronic transport where the static current-voltage ($I - V$) curves exhibit multiple branches and bistability. For applied voltages near the end of each branch, internal shot noise induces switching of measured current to the next branch with a stochastically varying switching time. Switching time distributions are constructed by carrying out up to $10^5$ measurements under identical initial conditions. We have implemented a novel, high bandwidth technique that permits measurement of switching times over very large dynamic range of approximately $10^9$, with measured times ranging from $4$ ns to $10$ s. For relatively small times ($<$ 10$\mu$s), the switching time distributions show exponential tails, as expected for activated escape from an initial metastable state. However, at larger times ($>$ 10 $\mu$s), the distributions exhibit approximate power law tails that extend over several decades of time, with additional fine structure. A rate equation model indicates the possible role of multiple, nearly degenerate metastable states in producing the long tail behavior.   

Pseudospin Transfer Torques in Semiconductor Electron Bilayers

We use self-consistent quantum transport theory to investigate the influence of interactions on interlayer transport in semiconductor electron bilayers in the absence of an external magnetic field. We conclude that even though spontaneous pseudospin ferromagnetism does not occur at zero field, interaction-enhanced quasiparticle tunneling does alter the resultant interlayer I-V curves. We find that the system exhibits a critical bias voltage that is similar to that of the pseudospin ferromagnetic system, but whose properties depend heavily on the charge imbalance between the two layers and their relative spatial separation. When the bias voltage exceeds the critical value, interlayer current is gradually droped due to the charge imbalance between the layers until the transport current no longer reaches steady state values.   

Effects of Fermion Flavor on Excitonic Condensation in Double Layer Systems

We perform fermionic path integral quantum Monte Carlo (PIMC) simulations to study the physical properties of dipolar exciton condensates in symmetric double layer systems. Recently, the role of screening of additional fermion flavors has been a source of contention in exciton condensates. A room temperature superfluid state has been predicted in bilayer graphene assuming that the condensate screens out additional fermion flavors.\footnote{H. K. Min, R. Bistritzer, J. J. Su and A. H. MacDonald.\emph{Phys. Rev.} B \textbf{78}, 121401 (2008)} On the other hand, large-N calculations have resulted in much weaker screening of fermion flavors and have placed the transition temperature far lower, around one millikelvin.\footnote{M. Y. Kharitonov and K. B. Efetov, \emph{Semicond. Sci. Technol.} \textbf{25}, 034004 (2010)} We demonstrate the effect of added fermion flavor on the Kosterlitz-Thouless transition temperature ($T_{KT}$) in symmetric electron-hole bilayers by collecting static and dynamic response functions.\footnote{J. Shumway and M. J. Gilbert, \emph{arXiv:1108.6107} (2011)} We find that the addition of fermion flavors decreases $T_{KT}$, however, due to strong exciton binding, the decrease we observe is not as drastic as is predicted in the earlier large-N calculations.   

Magnetic-Field Modulated Josephson Oscillations in a Planar Semiconductor Microcavity

An exciton-polariton Josephson junction in a planar semiconductor microcavity is studied by considering an external magnetic field applied normal to its plane [1]. Theoretical results show that there is a competition between the Zeeman energy and the interactions of exciton polaritons, and a critical magnetic field can be determined. Below the critical magnetic field, there are time-independent extrinsic and intrinsic Josephson currents which manifest the dc Josephson effect; while above the critical magnetic field, the ac Josephson effect occurs with the oscillating extrinsic and intrinsic Josephson currents. The oscillation frequency and oscillation amplitude of the Josephson currents are modulated by the magnetic field, and also the spontaneous polarization separation and the macroscopic quantum self-trapping of condensate can be realized under an appropriate magnetic field. The physical origin behind is exposed and the analogy with the Josephson effect in a conventional superconducting Josephson junction is discussed. It is suggested that magnetic fields can be used to facilitate the experimental investigations of the exciton polaritons Josephson effect in semiconductor microcavities. \\[4pt] [1] Chuanyi Zhang and Guojun Jin, Phys. Rev. B 84, 115324 (2011).   

Lagrange formalism of memory circuit elements: classical and quantum formulations

The general Lagrange-Euler formalism for the three memory circuit elements, namely the memristor, memcapacitor, and meminductor [1,2] is introduced for circuits with voltage or current sources. In addition, \textit{mutual meminductance}, i.e., mutual inductance with a state depending on the past evolution of the system, is introduced. The Lagrange-Euler formalism for a general circuit network and the corresponding work-energy theorem are also obtained. We provide examples of this formalism applied to specific circuits both in the classical and quantum regimes showing under which conditions the quantum excitations of the memory degrees of freedom can be observed experimentally. Work was supported in part by NSF.\\[4pt] [1] M. Di Ventra, Y. V. Pershin, and L. O. Chua, \textit{Proc. IEEE} \textbf{97}, 1717 (2009).\\[0pt] [2] Y.V. Pershin and M. Di Ventra, \textit{Advances in Physics} \textbf{60}, 145-227 (2011).   

Electrolyte Gated Transistors based on Solution Processed Mesoporous Tungsten Trioxide Thin Films

Tungsten trioxide (WO3) is an important material for electrochromic displays, gas sensors, and photoelectrochemical cells. Despite intensive research efforts, the charge transport properties of nanostructured WO3 films, as well as of other metal oxide films, are still largely undiscovered. Electrolyte gating provides a powerful platform to study the charge transport properties of nanostructured WO3 films permitting to achieve high charge density regimes. In turn, this opens the possibility to improve the film transport properties for a wide range of applications. Here we report on electrolyte gated transistors making use of WO3 films as the semiconductor and H2SO4(aq) 1M as the gate dielectric. WO3 films, prepared by sol-gel method, were deposited on source and drain patterned ITO substrates. The liquid electrolyte was confined using a PDMS well. Atomic force microscopy and scanning electron microscopy images show a mesoporous film structure where the electrolyte can easily penetrate. The mesoporous structure permits an efficient electrolyte gating compared to bulk WO3 films because of the higher surface available for electrical double layers, which are the underpinning of the electrolyte gating. Upon application of gate bias in the 0-1 V range, with an applied drain voltage ranging between 0-1 V, we were able to tune the conductivity in the WO3 transistor channel: electrolyte gating of the films led to clear transistor behaviour. Electrolyte gating of WO3 electrochromism is presently under investigation.   

Session H19: Invited Session: Current-Driven Spin Textures

Doppler velocimetry of a current driven spin helix

We present direct observation of the translational motion of spin helices in GaAs quantum wells under the influence of applied electric fields. Previously, the lifetime of such helices was observed by time-resolving the amplitude of light diffracted from the periodic spin polarization [1]. This technique cannot be applied to tracking the motion of current-driven spin helices because diffraction amplitude is insensitive to translation of the center of mass of a periodic structure. In this talk, we describe a new experimental technique, Doppler spin velocimetry, capable of resolving displacements of spin polarization at the level of 1 nm on a picosecond time scale [2]. This is accomplished through the use of heterodyne detection to measure the optical phase of the diffracted light. We discuss experiments in which this technique is used to measure the motion of spin helices as a function of temperature, in-plane electric field, and photoinduced spin polarization amplitude. Several striking observations will be reported -- for example, the spin helix velocity changes sign as a function of wavevector and is zero at the wavevector that yields the largest spin lifetime. Another important observation is that the velocity of spin polarization packets becomes equal to the drift velocity of the high-mobility electron gas in the limit of small spin helix amplitude. Finally, we show that spin helices continue propagate at the same speed as the Fermi sea even when the electron drift velocity exceeds the Fermi velocity of 10$^{7}$ cm-s$^{-1}$. In collaboration with J. D. Koralek and J. Orenstein, UC Berkeley and LBNL, D. R. Tibbetts, J. L. Reno, and M. P. Lilly, SNL. Supported by DOE under Contract No. DE-AC02-05CH11231 and DE-AC04-94AL85000. \\[4pt] [1] J. D. Koralek et al., ``Emergency of the persistent spin helix in semiconductor quantum wells,'' Nature 458, 610-613 (2009). \\[0pt] [2] L. Yang et al, ``Doppler velocimetry of spin propagation in a two-dimensional electron gas,'' to appear in Nature Physics.   

Experimental studies of skyrmion textures and spin torque effects in chiral magnets

Small angle neutron scattering and measurements of a topological Hall signal identify the formation of skyrmion lattices in the non-centrosymmetric B20 compounds MnSi [1], Mn$_{1-x}$Fe$_{x}$Si, Mn$_{1-x}$Co$_{x}$Si and the strongly doped semiconductor Fe$_{1-x}$Co$_{x}$Si [2]. This observation has been confirmed by Lorentz force microscopy in thin samples of Fe$_{1-x}$Co$_{x}$Si, FeGe and, most recently, MnSi, where even individual skyrmions have been spotted [3]. Because the skyrmion lattices are exceptionally weakly pinned to the crystal lattice, extreme care has to be exercised when studying the precise intrinsic morphology of related spin textures in bulk samples. As a particularly striking property each skyrmion supports precisely one quantum of emergent magnetic flux. This permits a highly efficient coupling between skyrmions and conduction electrons which results in spin torque effects at ultra-low current densities as seen in small angle neutron scattering [4] and the emergent electric field when the skyrmions move [5].\\[4pt] Work in collaboration with: T. Adams, A. Bauer, B. Binz, P. B\"oni, G. Brandl, R. A. Duine, K. Everschor, C. Franz, M. Garst, R. Georgii, S. Gottlieb-Sch\"onmeyer, W. Heusler, M. Janoschek, F. Jonietz, T. Keller, K. Mitterm\"uller, S. M\"uhlbauer, W. M\"unzer, A. Neubauer, P.G. Niklowitz, C. Pfleiderer, A. Rosch, T. Schulz, A. Tischendorf, M. Wagner.\\[4pt] [1] S. M\"uhlbauer et al., Science {\bf 323}, 915 (2009); A. Neubauer et al., Phys. Rev. Lett. {\bf 102}, 186602 (2010); C. Pfleiderer et al., J. Phys. Cond. Matter {\bf 22}, 164207 (2010); T. Adams et al., Phys. Rev. Lett., in press, arXiv/1107.0993. \\[0pt] [2] W. M\"unzer et al., Phys. Rev. B {\bf 81}, 041203(R) (2010). \\[0pt] [3] X. Z. Yu et al., Nature {\bf 465}, 901 (2010); X. Z. Yu et al., Nature Materials {\bf 10}, 106 (2010). \\[0pt] [4] F. Jonietz et al., Science, {\bf 330}, 1648 (2010). \\[0pt] [5] {\it Emergent electrodynamics of skyrmions in a chiral magnet}, T. Schulz, R. Ritz, A. Bauer, M. Halder, M. Wagner, C. Franz, and C. Pfleiderer, K. Everschor, M. Garst, and A. Rosch, preprint 2011.   

Current-driven spin dynamics in spin-orbit coupled superconductors

The study of the interplay between spin-orbit coupling (SOC) and superconductivity in two-dimensional electron gases (2DEG) has recently gained impetus following the discovery of i) 2DEGs in InAs or GaAs semiconductor heterostructures that are proximized by ordinary s-wave superconducting leads -- a class of systems which plays a key role in the quest for Majorana fermions -- and ii) 2DEGs that form at interfaces between complex oxides such as ${\rm LaAlO}_3$ and ${\rm SrTiO}_3$, which display tunable SOC and, under appropriate conditions, superconductivity. Motivated by this body of experimental and theoretical literature, we investigate the collective spin dynamics of an archetypical 2DEG model Hamiltonian with Rashba SOC in the presence of {\it repulsive} electron-electron (e-e) interactions. In the absence of superconductivity a Rashba 2DEG exhibits spin oscillations, which, at long wavelength and for weak repulsive interactions, have a frequency $\approx 2 \alpha k_{\rm F}$, $\alpha$ being the strength of SOC and $k_{\rm F}$ the usual 2D Fermi wavenumber in the absence of SOC. These oscillations, however, are damped and quickly decay due to the emission of (double) electron-hole pairs, which, in the normal phase, are present at arbitrary low energies. In the presence of superconductivity, collective spin oscillations continue to exist in a wide range of parameters, because the Cooper pairs are mixtures of singlet and triplet components. Further, these excitations are undamped because they lie inside the superconducting gap where no other excitation exists. These spin oscillations can be excited by the application of a magnetic field or a supercurrent and can be used to realize persistent spin oscillators operating in the frequency range of $10~{\rm GHz} - 1~{\rm THz}$.\\[4pt] Work supported by EU FP7 Programme Grant No. 215368-SEMISPINNET, No. 234970- NANOCTM and No. 248629-SOLID, and by NSF DMR-0705460.   

Emergent electrodynamics from moving magnetic whirls in chiral magnets

In chiral magnets a lattice of magnetic whirls -- so-called skyrmions -- is stabilized in a small temperature and field range by thermal fluctuations [1]. We discuss how electric and spin currents couple to these skyrmions. As the spin of the electrons locally adjusts to the magnetic texture, the electron picks up a Berry phase. The effects of these time-dependent Berry phases are best described by ``artificial'' electric and magnetic fields of an emergent electrodynamics which couple to the spin and the spin currents. The efficient Berry phase coupling together with a partial cancellation of pinning forces due to the stiffness of the skyrmion lattice allows to explain theoretically experiments [2], which show that skyrmion lattices can be controlled by ultrasmall current densities. Using tiny gradients of temperature or magnetic field it is also possible to induce rotations of the skyrmion lattice. The topologically quantized winding number of the skyrmions induces exactly one quantum of emergent magnetic flux per skyrmion. Therefore one can also determine quantitatively the emergent electric field induced by a moving skyrmion following Faraday's law of induction as has been measured in recent experiments [3].\\[4pt] [1] {\it Skyrmion Lattice in a Chiral Magnet}, S. M\"uhlbauer, B. Binz, F. Jonietz, C. Pfleiderer, A. Rosch, A. Neubauer, R. Georgii, P. B\"oni, Science {\bf 323}, 915 (2009). \\[0pt] [2] {\it Spin Transfer Torques in MnSi at Ultralow Current Densities}, F. Jonietz, S. M\"hlbauer, C. Pfleiderer, A. Neubauer, W. M\"unzer, A. Bauer, T. Adams, R. Georgii, P. B?ni, R. A. Duine, K. Everschor, M. Garst, and A. Rosch, Science {\bf 330}, 1648 (2010).\\[0pt] [3] {\it Emergent electrodynamics of skyrmions in a chiral magnet}, T. Schulz, R. Ritz, A. Bauer, M. Halder, M. Wagner, C. Franz, and C. Pfleiderer, K. Everschor, M. Garst, and A. Rosch, preprint 2011.   

Session H20: Invited Session: Hydrodynamics and Microstructure: From Single Self-Propelled Particles to Active Soft Matter

Swimming {\&} Propulsion in Viscoelastic Media

Many microorganisms have evolved within complex fluids, which include soil, intestinal fluid, and mucus. The material properties or rheology of such fluids can strongly affect an organism's swimming behavior. A major challenge is to understand the mechanism of propulsion in media that exhibit both solid- and fluid-like behavior, such as viscoelastic fluids. In this talk, we present experiments that explore the swimming behavior of biological organisms and artificial particles in viscoelastic media. The organism is the nematode \textit{Caenorhabditis elegans}, a roundworm widely used for biological research that swims by generating traveling waves along its body. Overall, we find that fluid elasticity hinders self-propulsion compared to Newtonian fluids due to the enhanced resistance to flow near hyperbolic points for viscoelastic fluids. As fluid elasticity increases, the nematode's propulsion speed decreases. These results are consistent with recent theoretical models for undulating sheets and cylinders. In order to gain further understanding on propulsion in viscoelastic media, we perform experiments with simple reciprocal artificial `swimmers' (magnetic dumbbell particles) in polymeric and micellar solutions. We find that self-propulsion is possible in viscoelastic media even if the motion is reciprocal.   

Assembly and dynamics of synthetic cilia

From motility of simple protists to determining the handedness of complex vertebrates, highly conserved eukaryotic cilia and flagella are essential for the reproduction and survival of many biological organisms. Despite extensive studies, the exact mechanism by which individual components coordinate to produce ciliary beating patterns remains unknown. We describe a novel approach towards studying ciliary beating. Instead of deconstructing a fully functional organelle from the top-down, we describe a process by which synthetic cilia-like structures are assembled from the bottom-up. We find that simple mixtures of microtubules, kinesin clusters, and a bundling agent produce spontaneous oscillations in MT bundles, suggesting that self-organized beating may be a generic feature of internally driven bundles. Furthermore, bundles in close proximity spontaneously coordinate their beating to generate metachronal traveling waves, reminiscent of the waves seen in ciliary fields. These findings and future refinements of the system can potentially provide insights into general design principles required for engineering synthetic cilia as well as understanding the biological analogues.~   

Polar patterns in active fluids

Active fluids are a new class of soft materials composed of interacting units that consume energy and collectively generate motion and mechanical stress. Examples include bacterial suspensions, mixtures of cytoskeletal filaments and motor proteins, and migrating epithelial cell layers. Due to their elongated shape, active particles can exhibit orientational order at high concentration and have been likened to ``living liquid crystals'', with either nematic or polar symmetry. In this talk I will discuss the spatio-temporal dynamics of continuum models of active fluids in two dimensions, focusing on the case of a system with polar symmetry as relevant to bacterial suspensions. Upon increasing activity, the active fluid displays increasingly complex patterns, including traveling bands, traveling vortices and chaotic behavior. The nonlinear hydrodynamic equations can be mapped onto a diffusion-reaction-convection model, highlighting the connection between the complex dynamics of active system and that of excitable systems.   

Swimming of Paramecium in confined channels

Many living organisms in nature have developed a few different swimming modes, presumably derived from hydrodynamic advantage. Paramecium is a ciliated protozoan covered by thousands of cilia with a few nanometers in diameter and tens of micro-meters in length and is able to exhibit both ballistic and meandering motions. First, we characterize ballistic swimming behaviors of ciliated microorganisms in glass capillaries of different diameters and explain the trajectories they trace out. We develop a theoretical model of an undulating sheet with a pressure gradient and discuss how it affects the swimming speed. Secondly, investigation into meandering swimmings within rectangular PDMS channels of dimension smaller than Paramecium length. We find that Paramecium executes a body-bend (an elastic buckling) using the cilia while it meanders. By considering an elastic beam model, we estimate and show the universal profile of forces it exerts on the walls. Finally, we discuss a few other locomotion of Paramecium in other extreme environments like gel.   

Helical swimming in viscoelastic and porous media

Many bacteria swim by rotating helical flagella. These cells often live in polymer suspensions, which are viscoelastic. Recently there have been several theoretical and experimental studies showing that viscoelasticity can either enhance or suppress propulsion, depending on the details of the microswimmer. To help clarify this situation, we study experimentally the motility of the flagellum using a scaled-up model system - a motorized helical coil that rotates along its axial direction. A free-swimming speed is obtained when the net force on the helix is zero. When the helix is immersed in a viscoelastic (Boger) fluid, we find an increase in the force-free swimming speed as compared with the Newtonian case. The enhancement is maximized at a Deborah number of approximately one, and the magnitude depends not only on the elasticity of the fluid but also on the geometry of the helix. In the second part of my talk, I will discuss how spatial confinements, such as a porous medium, affect the flagellated swimming. For clarity, the porous media are modeled as cylindrical cavities with solid walls. A modified boundary element method allows us to investigate a situation that the helical flagella are very close to the wall, with high spatial resolution and relatively low computational cost. To our surprise, at fixed power consumption, a highly coiled flagellum swims faster in narrower confinements, while an elongated flagellum swims faster in a cavity with a wider opening. We try understanding these effects with simple physical pictures.   

Session H21: Superconductivity: Electronic Structure of BSCCO; ARPES and theory

Extraction of Normal Electron Self-Energy and Pairing Self-Energy in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$ via Laser ARPES

Super-high resolution laser-based angle-resolved photoemission measurements have been performed on a high temperature superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$. The band back-bending characteristic of the Bogoliubov-like quasiparticle dispersion is clearly revealed at low temperature in the superconducting state which gives rise to two peaks in the momentum distribution curves. This makes it possible for the first time to experimentally extract the normal electron self-energy and pairing self-energy in the superconducting state. These information can be used to further determine the Bosonic spectral function that will provide key insight and constraints on the origin of electron pairing in high temperature superconductors.   

Momentum Dependence of the Self-energy and Fluctuation Spectrum of Bi2212 from High Resolution Laser ARPES

We report deduction of the Eliashberg function $\alpha^2 F(\theta,\omega)$ at energy $\omega$ and along momentum cuts at angles $\theta$ normal to the Fermi surface from the high resolution laser angle resolved photoemission spectroscopy on slightly underdoped Bi2212 in the normal and superconducting states. Our principle result is that despite the angle dependence of the extracted single-particle self-energy, the Eliashberg function in the normal state collapses onto a single function of $\omega$ independent of the angle. It has a peak around 0.05 eV, flattens out above 0.1 eV with the angle dependent cut-off. The cut-off energy is given by the intrinsic value of about 0.4 eV or the energy of the band bottom in direction $\theta$, whichever is lower. These results are consistent only with fluctuation spectra which have the correlation length of the lattice constant or shorter. In the superconducting state, the deduced $\alpha^2 F(\theta,\omega)$ exhibits a new peak around 0.015 eV in addition to the 0.05 eV peak and flat spectrum as in the normal state. Both peaks become enhanced as $T$ is lowered or the angle moves away from the nodal direction. The implication of these findings is discussed.   

Mechanisms for Superconductivity in Cuprates compared with results from the Generalized MacMillan-Rowell Analysis of High Resolution Laser- ARPES

The spectra of fluctuations and their coupling to fermions has been deduced from extensive high resolution laser ARPES in several BISCCO samples and quantitatively analyzed. We ask the question whether some of the theories for superconductivity in Cuprates are consistent or inconsistent with the frequency and the momentum dependence of the deductions. We find that any fluctuation spectra, for example that of Antiferromagnetic Fluctuations, whose frequency dependence depends significantly on momentum dependence are excluded. We consider the quantum-critical spectra of the loop-current order observed in under-doped cuprates and its coupling to fermions and find it consistent with the data.   

Comparative study of the MDC and two-dimensional analysis of ARPES intensity for Bi2212 bilayer superconductor

The momentum distribution curve (MDC) analysis is commonly used to analyze the ARPES data. A problem of the MDC analysis, however, is that the matrix elements turn out to be energy dependent. To remedy this we take the two-dimensional (2D) analysis of the ARPES intensity as reported by Meevasana et. al. [1]. We analyze the overdoped Bi2212 superconductor ARPES data by performing both 2D and MDC analysis taking the bilayer splitting into consideration. The deduced self-energy and Eliashberg function from both analysis will be compared and presented. \\[4pt] [1] Meevasana et. al. Phys. Rev. B 77, 104506 (2008).   

Ultra-High Resolution Time- and Angle-Resolved Photoemission Experiments on High Temperature Superconductor Bi$_2$Sr$_2$CaCu$_2$O$_8$

Ultra-high resolution laser-based time- and angle-resolved photoemission measurements have been carried out on various dopings of Bi$_2$Sr$_2$CaCu$_2$O$_8$ high temperature superconductor. In this talk, we will report on the study of the dynamical quasiparticle excitation and recombination of the nodal electronic states in cuprate.   

Momentum-Dependent Ultrafast Quasiparticle Dynamics in Optimally-Doped Bi-2212 Monitored with Time-Resolved ARPES

We have employed the novel technique of time- and angle-resolved photoelectron spectroscopy (t-ARPES) to investigate the quasiparticle dynamics in photoexcited, optimally-doped Bi2Sr2CaCu2O8 superconductor across the superconductor-metal transition. In this talk, we will present and analyze the substantially different ultrafast dynamics, tracked in momentum-dependent fashion, by probing the nodal and antinodal regions of the Brillouin zone, on a 35 femtosecond timescale. The consequences of these findings in terms of reentrant superconductivity will be discussed.   

Tracking Cooper Pair Formation in a Cuprate Superconductor: An Ultrafast ARPES Study

A basic mystery in high temperature superconductivity is the process that drives quasiparticles to form Cooper pairs, the fundamental charge carriers in any superconductor. We use time- and angle-resolved photoemission spectroscopy (TR-ARPES) to measure the relaxation dynamics of low energy excitations in the optimally doped cuprate superconductor Bi-2212. Results are discussed within the context of the Rothwarf-Taylor model of quasiparticle recombination.   

Nodal Gap of Heavily Under-doped Bi2201 Revealed by VUV Laser ARPES

We have carried out VUV laser-based angle-resolved photoemission (ARPES) on heavily under-doped $Bi_2(Sr_{2-x}La_x)CuO_{6+\delta}$ (abbreviated as La-Bi2201) samples with different dopings from antiferromagnetic insulators to superconductors. We find that, along the $(0,0)-(\pi,\pi)$ nodal direction, there is a gap opening that is strongly dependent on the hole doping level. The momentum and temperature dependence of the gap is investigated and the implication of the observations will be discussed.   

Doping Dependence of Pair-breaking and Pair-forming Processes in BSCCO

Angle resolved photo-emission spectroscopy (ARPES) provides a direct measure of electronic scattering processes in materials. However traditional methods (e.g. MDC widths) of determining the scattering processes from ARPES fail to discriminate between pairing and non-pairing interactions. Our new analysis technique, the ARPES tunneling spectrum (ATS), isolates the pairing channel from all other processes. We will report doping, angle and temperature dependence of both the pair-forming strength and the pair-breaking rate in BSCCO.   

Momentum dependence of Fermi surface maps in angle-resolved photoemission spectrum of ${\rm Bi_2Sr_2CuO_6}$ (Bi2201)

We have investigated the effect of ARPES matrix element on photointensity for emission from the Fermi energy as a function of photon energy in Bi2201 using first-principles as well as tight-binding model calculations. Our results show that as the photon energy increases and photoemitted electrons are spread over a larger area of momentum space, the highest spectral intensities generally remain pinned to the largest momenta probed at any given photon energy. The tight-binding calculations, which involve a three band Hubbard Hamiltonian based on Cu $d_{x^2-y^2}$ and O $p_x$, $p_y$ orbitals, give insight into the role of the ARPES matrix element in shaping the photointensities. A relatively simple formula is derived for the matrix element, showing how much of the zone-to-zone variation of the photointensity is controlled by the structure factor associated with the Bloch wavefunction.   

Detecting the minimum gap locus in ARPES spectra of Bi2201

Recent angle-resolved photoemission (ARPES) studies have reported a direct evidence for the competing nature of the pseudogap by showing that the pseudogap dispersion is not tied to Fermi momentum ($k_{F}$)[1,2]. In this study, to get more detailed information on how the competing pseudogap evolves across the pseudogap temperature (T*), we introduce a new analysis method for spectral weight. We found a clear indication that the pseudogap opens at T* with the minimum gap locus deviating from $k_{F}$, which is completely different manner from the gap opening by simple superconductivity, and strongly supports that the pseudogap is another distinct order. [1] M. Hashimoto and R.-H. He {\it et~al.}, Nature Phys. {\bf 6}, 14-418 (2010). [2] R.-H. He and M. Hashimoto {\it et~al.}, Science {\bf 331}, 1579-1583 (2011).   

Evidence for Charge-Density-Wave in Underdoped Bi2201 from ARPES and LEED

While there is mounting evidence for a broken symmetry in the pseudogap state of the high-$T_c$ cuprates, the identification of a specific phase remains elusive. Through the combination of electronic (ARPES) and structural (LEED) probes, we uncover a temperature dependent evolution of the CuO$_2$ plane band dispersion in highly-ordered Bi2201, which is directly associated with a hitherto-undetected evolution of the incommensurate superstructure. The quasilinear, continuous variation of the modulation wavelength $2\pi/Q_2$ from $\sim\!66$ to $43$\AA, below a characteristic $T_{Q_2}\!\simeq\!130$\,K, provides evidence for an electronically-driven charge-density-wave ordering. This points to a remarkable electron-lattice coupling, in which the footprint of the BiO-layer-induced superstructure is found in the modulated electronic structure of the CuO$_2$ plane.   

Electronic Scattering Rates in the High-T$_{c}$ Superconductor Bi$_{2.1}$Sr$_{1.9}$Ca(Cu$_{1-y}$Fe$_{y})_{2}$O$_{x}$

We investigate the effects of Fe impurities in bi-layer BSCCO. It is known that substituting Fe for Cu in this material reduces T$_{c}$, but the mechanism for this decrease is not well understood. We have developed a technique that utilizes ARPES to quantitatively measure the effects of impurities on electronic scattering. Using this technique we investigate the details of how Fe impurities cause an increase in the pair-breaking scattering rate in bi-layer BSCCO.   

Universal 115meV Feature and High Energy Spectral Weight Transfer in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\Delta}$ Superconductor Revealed by Laser ARPES

Quasiparticle dispersion and spectral weight transfer have been investigated by laser-based angle-resolved photoemission spectroscopy in Bi$_2$Sr$_2$CaCu$_2$O$_{8+{\Delta}}$. A universal energy scale near $\sim$115meV can be clearly identified in superconducting state which is insensitive to momentum and doping levels. A concomitant observation with this energy scale is the spectral weight transfer over a large energy range when the sample goes from the normal state to the superconducting state. The origin and implications of these observations will be discussed.   

Towards the Standard Model for Fermi Arcs from a Wilsonian Reduction of the Hubbard Model

Two remarkable features emerge from the exact Wilsonian procedure for integrating out the high-energy scale in the Hubbard model. At low energies, the number of excitations that couple minimally to the electromagnetic gauge is less than the conserved charge, thereby implying a breakdown of Fermi liquid theory. In addition, two charge $e$ excitations emerge in the lower band, the standard projected electron and a composite entity (comprised of a hole and a charge $2e$ bosonic field) which give rise to poles and zeros of the single-particle Green function, respectively. The poles generate spectral weight along an arc centered at $(\pi/2,\pi/2)$ while the zeros kill the spectral intensity on the back-side of the arc. The result is the Fermi arc structure intrinsic to cuprate phenomenology. The presence of composite excitations also produces a broad incoherent pseudogap feature at the $(\pi,0)$ region of the Brillouin zone, thereby providing a mechanism for the nodal/anti-nodal dichotomy seen in the cuprates.   

Session H22: Focus Session: Fe-based Superconductors - Anisotropic Transport and Anisotropy

Temperature-dependent anisotropic resistivity in electron, hole and isoelectron - doped BaFe$_2$As$_2$ superconductors

Anisotropic electrical resistivity, $\rho(T)$, was studied in iron-arsenide superconductors, obtained by doping the parent BaFe$_2$As$_2$ compound on three different sites: (1) electron donor transition metal (Co,Ni,Rh,Pd) substitution of Fe [1,2]; (2) hole donor K substitution of Ba [3]; (3) isoelectron P substitution of As. For all three types of dopants a range of $T$-linear behavior is found at the optimal doping in both the in-plane and the inter-plane $\rho(T)$ above $T_c$. At some higher temperature this range of $T$-linear resistivity is capped by a slope-changing anomaly, which, by comparison with NMR, magnetic susceptibility and Hall effect measurements, can be identified with the onset of carrier activation over the pseudogap [1]. The doping-evolution of anisotropic temperature dependent $\rho(T)$ and of the pseudogap are quite different for three types of doping. A three-dimensional $T-H$ phase diagram summarizing our results will be presented. Furthermore, potential correlation of the anisotropic normal state transport and anisotropic superconducting state heat transport will be discussed. \\[4pt] In collaboration with N. Ni, A. Thaler, S.L.Bud'ko, P.C. Canfield, R. Prozorov, Bing Shen, Hai-Hu Wen, K. Hashimoto, S. Kasahara, T. Terashima, T. Shibauchi and Y. Matsuda. \\[4pt] [1] M.A.Tanatar et al. PRB \textbf{82}, 134528 (2010)\\[0pt] [2] M.A.Tanatar et al. PRB \textbf{84}, 014519 (2011)\\[0pt] [3] M.A.Tanatar et al. arXiv:1106.0533   

Enhanced conductance in the normal state of Fe pnictides {\&} chalcogenides measured by quasiparticle scattering spectroscopy (QPS): evidence of orbital fluctuations

QPS reveals a conductance enhancement at a temperature, T$_{onset}$,~for RFe$_{2}$As$_{2}$ (R=Ca, Sr, Ba) and Fe$_{1.13}$Te. For Ba/Sr Fe$_{2}$As$_{2}$ and Fe$_{1.13}$Te the enhancement survives well above the magnetic and structural transition temperatures (Ba: T$_{N}\sim $132 K, T$_{onset}\sim $175 K; Sr: T$_{N}\sim $192 K, T$_{onset}\sim $240 K; Fe$_{1.13}$Te: T$_{N}\sim $60 K, T$_{onset}\sim $75 K) while for most CaFe$_{2}$As$_{2 }$junctions it disappears below T$_{N}$ and T$_{S }$(11 junctions tested, only 2 showed weak enhancement above T$_{N})$. For Co underdoped BaFe$_{2}$As$_{2}$ the enhancement coexists with the superconducting Andreev peaks while it is not observed for Co overdoped Ba122. We construct a modified phase diagram for Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ to reflect the presence of this feature for the underdoped regime.\footnote{Arham et. al, arXiv:1108.2749.} We discuss this conductance enhancement in the context of non-Fermi liquid behavior of these compounds due to orbital fluctuations.\footnote{Lee et. al, arXiv:1110.5917.} This work is supported by the Center for Emergent Superconductivity, an Energy Frontier Research Center funded by the US DOE, Office of Science, Award No. DE-AC0298CH1088. Work at Cambridge supported by EPSRC. Work at BNL under DOE Award No.DE-AC0298CH10886. Ames Lab. operated by ISU for the U.S. DOE under Award No. DE-AC02-07CH11358.   

Nematic order in the vicinity of a vortex in superconducting FeSe

We present a phenomenological theory of the interplay between nematic order and superconductivity in the vicinity of a vortex induced by an applied magnetic field [1]. Nematic order can be strongly enhanced in the vortex core. As a result, the vortex cores become elliptical in shape. For the case where there is weak bulk nematic order at zero magnetic field, the field-induced eccentricity of the vortex core has a slow power-law decay away from the core. Conversely, if the nematic order is field induced, then the eccentricity is confined to the vortex core. We discuss the relevance of our results to recent scanning tunneling microscopy experiments on FeSe [2]. \\[4pt] [1] D. Chowdhury, E. Berg and S. Sachdev, to appear in Phys. Rev. B, arXiv: 1109.2600 (2011).\\[0pt] [2] Can- Li Song et al., Science 332, 1410 (2011).   

T-matrix Impurity Effects on Nematicity in Iron-Based Supeconductors

In iron-based superconductors, nematicity has been reported in transport measurements and a broad range of spectroscopies, including angle-resolved photoemission, neutron scattering, and scanning tunneling microscopy. These observed anisotropies of broken tetragonal symmetry have been attributed to pure spin physics or unequal occupation of the iron d-electron orbitals, referred to as orbital ordering. To address this issue, we use realistic multi-orbital tight-binding Hamiltonians and T-matrix formalism to explore the effects of non-magnetic impurities. In particular, we present a detailed examination of the local density of states around impurities, and highlight the interplay of magnetic and orbital degrees of freedom.   

Surface Structure of Stripe ordered 1x2 phase on (Ba, Ca)(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$

Low energy electron diffraction (LEED) and scanning tunneling microscopy/spectroscopy (STM/S) have been utilized to investigate the geometric structure of the stripe 1$\times $2 surface phase of (Ba, Ca)(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ iron pnictides. STM images show that the surface consists of competing ordered and disordered regions. The 1$\times $2 phase appears on the surface of all compounds but coexist with ($\surd $2 $\times \surd $2)R45$^{\circ}$ phase on the surface of Ba122. Quantitative structural analysis of LEED-I(V) using the fractional spots of the 1$\times $2 phase on both parent compounds as well as Ca(Fe$_{0.925}$Co$_{0.075})_{2}$As$_{2}$ gives a similar surface structure with a termination layer of 50{\%} Ca/Ba atoms. The surface Ca/Ba layer has a large inward relaxation $\sim $ 0.5 {\AA} and the underneath As-Fe-As layer displays a buckling distortion. The Pendry Rp factor ($\sim $ 0.24) obtained in the structural refinement is excellent for all three systems.   

Scanning tunneling spectroscopy in Co-doped BaFe$_{2}$As$_{2}$: what density functional theory can tell us

We use density functional theory to simulate the scanning tunneling spectra and topographic images of Co-doped BaFe$_{2}$As$_{2}$. The matrix element effects are evaluated and the specific contributions of the different surface atoms to the spectra are considered. The results give a better understanding of the measured spectra and assess the resolution of STS measurements in these systems.   

Scanning tunneling spectroscopic studies of the iron-arsenic superconducting Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ single crystals

Scanning tunneling spectra of Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ (x=0.06, 0.08, 0.12) single crystals are studied as a function of temperature ($T)$ and magnetic field ($H)$. For $H$ = 0, direct evidence for two-gap superconductivity at energies $\omega$ = $\Delta _{\beta}$ and $\Delta _{\alpha ,\gamma /\delta}$ ($\approx $ 2$\Delta _{\beta })$ and for magnetic resonance modes at $\Omega \quad \approx \quad \Delta _{\beta }+\Delta _{\alpha ,\gamma /\delta}$ are found for all samples at $T \quad < \quad T_{c}$. Fourier transformation of the tunneling spectra reveals $x$- and $\omega$-dependent quasiparticle interference (QPI) wave-vectors \textbf{q}$_{2}$ near ($\pm \pi $,0)/(0,$\pm \pi )$ and \textbf{q}$_{1}$ near ($\pm $2$\pi $,0)/(0,$\pm $2$\pi )$. The spectral intensity of \textbf{q}$_{2}$ exhibits strong $\omega$-dependence, peaking sharply at $\omega$ = $\Delta _{\beta }$, $\Delta _{\alpha ,\gamma /\delta}$ and $\Omega$. This is in stark contrast to the Bragg diffraction peaks that are independent of $\omega$, $T$ and $x$. For $H \quad >$ 0, additional QPI wave-vector \textbf{q}$_{3}$ appears near ($\pm \pi $,$\pm \pi )$. These findings are consistent with the sign-changing $s$-wave pairing symmetry. Additionally, for the optimally doped sample, a pseudogap at $\omega$ $\sim \quad \Delta _{\gamma /\delta }$ is found inside the vortex core, possibly due to coexisting superconductivity and spin density waves. This result is in contrast to the zero-bias conductance peaks observed inside the vortex core of (Ba$_{1-x}$K$_{x})$Fe$_{2}$As$_{2}$, implying asymmetry in the hole and electron-doping of the iron arsenides. This work was supported by NSF DMR-0907251.   

Examining multiple spectroscopic techniques with multiband Eliashberg theory in BaCo$_{x}$Fe$_{2-x}$As$_2$

A wealth of experimental data from multiple probes are available for the newly-discovered pnictide superconductors. Here, we exploit this fact and examine a multi-band Eliashberg model for optimal doped BaCo$_x$Fe$_{2-x}$As$_2$ assuming a broad spin-fluctuation boson spectrum and retaining the full 3D bandstructure. Our focus is on comparing the model directly with data collected by our group from ARPES, STM and optics experiments performed on samples from the same growth batch. We find that the model captures all of the important aspects of the three probes including the behavior of the gap structure in the dI/dV characteristics and contributions from low-energy interband transitions in the optical conductivity. Our results also indicate the role of matrix elements in establishing the strong particle-hole asymmetry observed in the dI/dV spectra.   

Point-Contact Andreev Reflection Spectroscopy in Pt-Substituted BaFe2As2

We have investigated the superconducting order parameter of BaFe2-xPtxAs2 using point-contact Andreev reflection spectroscopy (PCAR). The samples used were large single crystals with measured Pt concentrations consistent with optimal doping (x = 0.15). Junctions were made between gold or lead tips and the c-axis of the superconducting samples. Conductivity spectra were measured over a range of temperatures and fit to curves generated using the Blonder-Tinkham-Klapwijk (BTK) model for two gaps, one isotropic and one angle-dependent gap with nodes.   

Scanning tunneling microscopy study of K-doped iron selenide superconductor film by MBE

The alkali-doped iron selenide superconductors have generated considerable excitements as well as confusions, regarding the delicate interplay between Fe vacancies, magnetism and superconductivity. We have grown high-quality K$_{x}$Fe$_{2-y}$Se$_{2}$ thin film with (001) surface orientation on STO substrate by molecular beam epitaxy. The scanning tunneling microscopy (STM) measurement demonstrates that there are two superconducting phases: striped KFe$_{2}$Se$_{2}$ in adjacent to the phase with $\surd 5 \times \surd $5 Fe vacancy order and doped KFe$_{2}$Se$_{2}$ with Fe and K vacancies. Both phases have a superconducting gap of 9 meV. These findings elucidate the existing controversies on the role of $\surd 5 \times \surd $5 Fe vacancy order in superconducting K$_{x}$Fe$_{2-y}$Se$_{2}$. Based on the atomic level information by STM, we will discuss the mechanism of the two different superconducting phases.   

The Surface Structure of FeTe$_{1-x}$Se$_{x}$: A LEED/STM Study

We have utilized low energy electron diffraction (LEED $I-V)$ and scanning tunneling microscopy (STM) to investigate the structural properties of FeTe$_{1-x}$Se$_{x}$ ($x$ = 0 and 0.45) surface. The LEED pattern indicates there is no surface reconstruction on both parent and doped compounds. However, the detailed surface structure calculations from LEED $I-V$ show the cleaved surface of FeTe is Te termination with a 0.06 {\AA} compression on the top layer. The STM topography shows the extreme flatness of the FeTe surface with a corrugation of less than 8 pm. The high resolution STM topography indicates that nanoscale chemical phase separation between Te and Se atoms is unambiguously revealed on the surface of FeTe$_{0.55}$Se$_{0.45}$. Chemical phase separation on the nanoscale makes the LEED $I-V$ surface structural analysis of FeTe$_{1-x}$Se$_{x}$ (001) very challenging. We will discuss the solution of the LEED analysis and the STM observations as a function of $x.$   

Cryomagnetic Point-Contact Andreev Reflection Spectroscopy on Single Crystal Iron-Chalcogenide Superconductors

We report on cryomagnetic point-contact Andreev reflection spectroscopy performed on single crystals of superconducting FeTe$_{1-x}$S$_{x}$ and FeTe$_{1-x}$Se$_{x}$. The samples are cleaved in-situ and the measurements are carried out at temperatures down to 4.2K and in a field up to 9T. At base temperature and zero field, we observe a cone-shaped hump at lower voltages in the conductance spectra with no dips at zero bias and a linear background at higher voltages. The spectral evolution of gap size, zero-bias conductance, and excess spectral area are analyzed as a function of temperature and field. Further spectral analysis is carried out using theoretical models of conductance spectra in multiband superconductors [1,2] and of gap symmetry in Fe-based superconductors [3]. The role of interstitial iron is also considered, by comparison with atomically-resolved scanning tunneling spectroscopy data.\\[4pt] [1] V. Lukic and E.J. Nicol, PRB 76, 144508 (2007) [2] A. Golubov \emph{et al.}, PRL 103, 077003 (2009) [3] P.J. Hirschfeld \emph{et al.}, RPP 74, 124508 (2011)   

Transport properties of disordered iron-pnictides in case of coexistence between superconducting and spin-density wave order

We present a theoretical description of the transport properties of a dirty multi-band superconductor in the case when both superconducting and spin-density wave orders coexist. We focus on differential conductance spectra of normal metal-superconductor junctions. In pure SC phase, we demonstrate that the interband impurity scattering broadens the coherent peak near the superconducting gap and significantly reduces its height even at relatively low scattering rates. This broadening is consistent with a number of recent experiments performed for both tunnel junctions and larger diffusive contacts. We further analyze the effect of the SDW order parameter on the differential conductance and other transport properties in the coexistence phase.   

Session H23: Metals Structural

Cr based Alloy Cr-Y-Mo-W Oxidation Study from First Principals Molecular Dynamics Simulation

First principles molecular dynamics simulations have been performed to study the stability and oxidation progress of Cr based alloys at high temperatures. The bulk phase of cubic Cr-based alloys is investigated with density functional theory (DFT) calculations and \textit{ab initio} molecule dynamic (MD) method. Diffusion of oxygen atoms within different densities of Y, Mo, and W doping and temperatures in Cr-Y systems is studied in this research. The effects of Y, Mo, and W doping are also studied. The properties of the optimized Y, Mo, and W co-doped Cr-based alloys also studied by using \textit{ab initio} MD method. Further improvement of the oxidation resistance and surface corrosion of Cr-based alloys will be discussed.   

Enhancing mechanical toughness of aluminum surfaces by nanoboron implantation: An {\em ab initio} study

We use {\em ab initio} density functional theory to study the formation dynamics and equilibrium atomic arrangement in aluminum surfaces exposed to energetic boron aggregates. Results of our molecular dynamics simulations indicate that after using up their excess kinetic energy to locally melt the aluminum surface, boron atoms prefer to remain in subsurface sites. We perform extensive structure optimization studies to identify the optimum structural arrangement and changes in the electronic structure associated with the formation of strong Al-B bonds, which are responsible for the stability enhancement of the compound. Nano-indentation simulations based on constrained optimization suggest that presence of boron atoms enhances the mechanical hardness and wear-resistance of aluminum surfaces.   

Phase Stability and Deformation Behavior of Mo-Si-B System and effect of alloying

Molybdenum silicides are promising materials for ultra-high temperature applications above 1300 \r{ }C. One of the main drawbacks is their brittleness at low temperatures, which may be improved by additions. We employ first principles calculations with the highly precise FLAPW method to investigate the effect of alloying with 3$d$, 4$d$ and 5$d$ transition metals on phase stability, cleavage and shear characteristics of the 3-component system Mo -- Mo$_{3}$Si -- Mo$_{5}$SiB$_{2}$. We determined site preference, phase partitioning of alloying elements, and their effect on shear behavior and preferred deformation modes. We show that in Mo$_{3}$Si alloying with 3$d$ transition metals results in a significant reduction of energy barriers to shear deformation (softening effect), while 4$d$ and 5$d$ additions increase shear barriers (hardening effect). In Mo$_{5}$SiB$_{2}$, 3$d$ transition metals (except for Ti) act as weak softeners, while 4$d$ and 5$d$ show mixed behavior -- hardening for early elements and softening for late ones. The softening potency of additions increases with atomic number, but exhibits non-monotonic behavior as a result of a competition between size and electronic effects. The results are discussed in conjunction with possible pathways to ductility enhancement through alloying.   

Structural, elastic and thermal properties of cementite from Modified Embedded Atom Method (MEAM) potential

Structural, elastic and thermal properties of cementite were studied using a newly developed MEAM potential for the Fe-C alloy system. The single element potential of C correctly predicts graphite and diamond as the two stable structures. Parameters for the Fe-C alloy potential were constructed based on the structural and elastic properties of elements in the L$_{\rm{12}}$ reference structure, calculated from ab-initio simulations. Parameters were further adjusted to reproduce structural properties of cementite and the interstitial energies for Fe correctly. Pair potential parameters were optimized using a method combining Latin hypercube sampling of the N-dimensional parameter space and multi-objective optimization. Elastic constants, surface formation energies, melting temperature and specific heat of cementite were calculated using the potential. The values computed from the Fe-C alloy MEAM potential are in good agreement with DFT calculations and experiments.   

High Strain Rate Deformation of BCC Materials: Molecular Dynamics Simulations

To improve machining processes, a good understanding of plasticity of bulk metals and at high strain rates is required. During machining processes, it is thought that strain rates of $\sim$10$^{6}$s$^{-1}$ are reached. These strain rates are currently not achievable in experimental high strain rate testing techniques. Here we investigate high strain rate deformation of bcc Fe using Molecular Dynamics simulations. Simulations include those of nano-scale single crystal pillars and of a nano-crystalline sample. We show that bcc materials may exhibit different deformation behavior at high strain rates. Under compression at strain rates of $\sim$10$^{8}$s$^{-1}$, nano-scale single crystal pillars may exhibit slip on atypical planes if the direction of maximum resolved shear stress points along that plane. The stress-strain behavior of these pillars is characterized by sudden strain bursts due to slip events. The yield stress required for these slip events can be as high as 2 GPa, much higher than the Peierls stress which is around 25 MPa for an edge dislocation in Fe. The nano-crystalline sample showed stress-strain behavior closer to that of a bulk metal. At high temperatures, pillars exhibit surface premelting. A large portion of the pillar turns amorphous when strained.   

Effect of lattice vibrations on magnetic phase transition in BCC Iron

From the first principle calculation of BCC iron, we build a classical Heisenberg model with the interaction coupling as a function of both the distance and local environments(e.g. volume). Using the Johnson potential and Finnis-Sinclair potential, we perform Monte Carlo simulations of BCC iron including the effect of lattice vibrations. The validity of classical Heisenberg model in describing the magnetic phase transition of BCC iron is explained.   

Phase-Stabilizers in Titanium Alloys

Titanium alloys exhibit three distinct crystal structures: alpha, beta and omega. For various applications alloying elements can be used to stabilize the desired phase. While alloy designers have well established rules of thumb, rigorous theory for non-equilibrium single-phase crystal stability is less well established. Here we tackle this problem using electronic structure calculations. We use two different methods based on density functional theory with pseudopotentials and plane waves, with either explicit atoms or the virtual crystal approximation (VCA). The former is highly reliable, while the latter makes a number of drastic assumptions that typically lead to poor results. Surprisingly, the agreement between the methods is good, showing that the approximations in the VCA are not important in determining the phase stability and elastic properties. This allows us to generalize, showing that the single-phase stability can be related linearly to the number of d-electrons, independent of the actual alloying elements or details of their atomistic-level arrangement. This leads to a quantitative measure of beta-stabilization for each alloying transition metal.   

Structural properties and phase stability of Ti alloys containing V and Cr by first-principles calculation

We apply the first-principles plane-wave pseudopotential method and virtual crystal approximation (VCA), to investigate structural properties and phase stability of ternary titanium alloy Ti-V-Cr. The lattice parameters of the alloy vary almost linearly with the number of d electrons in spite of the different ratio of V to Cr, which agrees with the available experimental results. At 0K, we find that an extra 0.4 electrons per atom can stabilise the beta Ti-V-Cr. Debye approximation is used to consider the temperature effect, and it is found that at 973K, with an extra 0.1 d electron per atom, the beta Ti-V-Cr can be stabilised, compared with 0.15-0.20 d electron required experimentally at 973K. It is shown that V and Cr move the Fermi energy to lower values of the density of the states of the beta phase, which accounts for the stabilisation effect.   

Dynamics of twinning dislocations in Tantalum

Twinning is one of the major deformation modes of plastic deformation in crystals, being particularly important in systems under extreme conditions of low temperature or high strain rates. Despite decades of work, the nucleation and growth mechanisms of twining are still poorly understood, especially in bcc metals. Nucleation of twinning dislocations loops on the coherent twin boundary has been considered a principal mechanism of growth of deformation twins. We have used molecular dynamics simulation to study the behavior of twinning dislocations in Tantalum, in particular the dependence of dislocation velocities on shear stress and temperature. The dynamics of edge and screw twinning dislocations is isolated and analyzed. Finally we show how kinetic parameters extracted from these simulations help inform a multiscale strength model for Tantalum that includes both twinning and slip as deformation mechanisms in the regime of high strain rates.   

Vacancy Assisted Climb in Continuum Dislocation Dynamics

Using a mesoscale continuum theory of dislocation dynamics, we study the physical effects of vacancy assisted climb. New physics emerges at high temperatures where dislocations are also able to move in the climb direction due to the absorption and emission of vacancies. We investigate this high temperature behavior using our minimal continuum dislocation dynamics model, which produces fractal cell structures in 2 and 3 dimensions. By coupling the dislocation density to a vacancy field we are able to study the effects of vacancies on diffusion-limited dislocation motion. We calibrate our model using measurements of climb velocities for straight, parallel dislocations and check the limit of no climb by freezing out vacancy motion. We use our model to explore applications of vacancy assisted climb, including dislocation creep and absorption of dislocations at grain boundaries.   

Stacking-fault energy and anti-Invar effect in FeMn alloys at high temperature

High-Mn steels (20-30at\%Mn, 2-4wt\%Si and Al) are of interest for the automotive industry due to their outstanding mechanical properties. Their deformation behavior has been empirically correlated to the stacking-fault energy (SFE), an important quantity in steel design that can be calculated \emph{ab-initio}. Using state-of-the-art methods within density-functional theory together with Monte Carlo simulations, we calculate the free energy of the Fe-22.5at\%Mn binary alloy between 300-800~K. Experimentally, the alloy is completely random and in the paramagnetic state, which we model via the coherent potential approximation and the disordered local moment approach, respectively. We treat magnetic excitations by including longitudinal spin-fluctuations and find that the FeMn alloy is an itinerant paramagnet. Our calculations confirm the experimentally observed strong magneto-volume coupling, realized in the anti-Invar behavior. We then obtain the structural stability and the SFE from free energy differences and find very good agreement with measurements. Our results demonstrate that the interplay between magnetic excitations and the thermal lattice expansion is the main factor determining the anti-Invar effect, the \emph{fcc-hcp} martensitic transformation temperature and the SFE.   

Microstructural and mechanical characterization of 0.2mass\% Carbon content steel

The The microstructures of low carbon content steel are comprised of bainite, martensite, tempered martensite and retained autenite. These structures are obtained by different heat treatments. The effect of heat treatment on microstructure and mechanical properties were investigated using X-ray diffraction, focused ion beam - scanning electron microscope (FIB-SEM), electron backscatter diffraction (EBSD), and nanoindentation. The experimental misorientation distribution revealed most grain boundaries had misorientation range between 50$^{\circ}$ and 60$^{\circ}$. The lattice relation between bainite and parent austenite is Kurdjomov-Sachs ($<111> || <110>$). FIB-SEM images and nanoindentation were revealed the grain size can influence the hardness.   

Ab initio-based interatomic potentials for body-centered cubic refractory metals

A fundamental understanding of transformation and deformation processes in the bcc refractory metals (V, Nb, Ta, Mo, and W) is vital for designing new bcc-based commercial alloys with desired properties. Such an understanding is aided by computational methods capable of reaching length and time scales needed for meaningful simulations of phase transformations and extended defects responsible for plastic deformation. Classical interatomic potentials are indispensable for simulating such phenomena inaccessible to first-principles methods. We develop accurate and robust embedded-atom method (EAM) [1] and modified-EAM (MEAM) [2] potentials for the bcc metals by fitting the model parameters to accurate first-principles data. The potentials are applicable for studying mechanical and thermodynamic properties, yielding excellent agreement with both experiments and first-principles calculations. {\newline} {\newline} [1] M. S. Daw and M. I. Baskes, Phys. Rev. Lett. 50, 1285 (1983). {\newline} [2] M. I. Baskes, Phys. Rev. Lett. 59, 2666 (1987).   

Influence of Nano-Alumina and Micro-Size Copper on Microstructure and Mechanical Properties of Magnesium Alloys AZ31

In this paper, magnesium composites are synthesized through the addition of nano-alumina and micron size copper particulates in AZ31 magnesium alloy using the technique of disintegrated melt deposition. The simultaneous addition of Cu and nano-alumina particulates led to an overall improvement in physical, microstructural characteristics and mechanical response of AZ31. Small size and reasonably distributed second phases were formed. The 0.2{\%} yield strength increased from 180 to 300 MPa (67{\%}), while the ductility increased by almost 24{\%}. The overall tensile properties assessed in terms of work of fracture improved by 66{\%}. An attempt is made to correlate the tensile response of composites with their microstructural characteristics. The results suggest that these alloy composites have significant potential in diverse and wider engineering applications.   

Session H24: Electrical and Thermal Properties of Semiconductors

Electrical characterization of SiGeSn grown on Ge substrate using ultra high vacuum chemical vapor deposition

There has been recently considerable interest in growing Si$_{y}$Ge$_{1-x-y}$Sn$_{x}$ alloys for the fabrication of photonic devices that could be integrated with Si technologies. We report temperature dependent Hall (TDH) measurements of the hole concentration and mobility from high quality p-type doped Si$_{0.08}$Ge$_{0.90}$Sn$_{0.02}$ layers grown on p-type doped Ge substrates using ultra high vacuum chemical vapor deposition. The TDH measurements show the hole sheet density remains constant at low temperatures before slightly decreasing and dipping at $\sim $ 125 K. It then exponentially increases with temperature due to the activation of shallow acceptors. At temperatures above $\sim $ 450 K, the hole sheet density increases sharply indicating the onset of intrinsic conduction in the SiGeSn and/or Ge layers. To extract the electrical properties of the SiGeSn layer alone, a parametric fit using a multi layer conducting model is applied to the measured hole concentration and mobility data. The analysis yields boron and gallium doping concentrations of 3x10$^{17}$ cm$^{-3}$ and 1x10$^{18}$ cm$^{-3}$ with activation energies of 10 meV and 11 meV for the SiGeSn layer and Ge substrate, respectively. Furthermore, a temperature independent hole sheet concentration of $\sim $5x10$^{15}$ cm$^{-2}$ with a mobility of $\sim $250 cm$^{2}$/Vs, which is believed to be due to an interfacial layer between the SiGeSn layer and the Ge substrate, is also determined.   

Informatic Strategies for Screening Electron Mobility of Candidate Semiconducting Materials

While carrier transport properties are critical to semiconductor efficiency, screens for potentially new materials based upon mobility measurements can be problematic. In the early stages of materials development, measured electron mobilities are often unreliable indicators of their eventual performance and serve as a poor basis to assess the longer term potential of candidate materials. In this paper, we describe an information-based approach for estimating an effective upper limit, using the specific case of the II-VI semiconductors. The optical (polaron) electron mobility has been developed as a screening property, supported by informatic estimates of dielectric properties. This mobility represents a practical screen, providing an estimate of the potential bounding value at room temperature. Using this basis, partial screening criteria based on compositional factors can also be constructed.   

Measurement of Electron Effective Mass in GaAs$_{1-x}$Bi$_{x}$

Magnetic field and temperature dependent resistivity measurements on n-type GaAs$_{1-x}$Bi$_{x}$ epitaxially grown films show clear Shubnikov de Haas oscillations in the range 0 $\le x \le $ 0.0088. An overall decrease in the electron effective mass is observed for this range of compositions. Accounting for the known giant bandgap bowing of GaAs$_{1-x}$Bi$_{x}$, the measured changes in the electron effective mass are in qualitative agreement with perturbation theory applied to the known bandgap reduction for this alloy, confirming that bismuth mainly perturbs the valence band. The stronger compositional dependence of the measured masses is attributed to effects from the bismuth isolated state.   

Piezo-resistance in $n$-type Si$_{1-x}$Ge$_x$ alloys as a function of alloy composition and strain

We use first-principles electronic structure methods[1,2] to predict the piezoresistance of $n$-type Si$_{1-x}$Ge$_x$ at various alloy compositions and strain configurations. The gauge factor, $G = d\rho/d\epsilon/\rho$, where $\rho$ is resistivity and $\epsilon$ is strain, varies strongly with both composition and strain. Nonlinear changes in resistivity with strain may arise due to changing occupancy of the higher-conductance $L$ valley relative to the lower-conductance $\Delta$ valley, coupled to a change in inter-valley alloy and phonon scattering. [1] F. Murphy-Armando and S. Fahy, J. Appl. Phys., accepted for publication (2011). [2] F. Murphy-Armando and S. Fahy, Phys. Rev. Lett., 97, 096606 (2006)   

Non-diffusive thermal conductivity in semiconductors at room temperature

The ``textbook'' value of phonon mean free path (MFP) in silicon at room temperature is $\sim $40 nm. However, a large contribution to thermal conductivity comes from low-frequency phonons with much longer MFPs. We find that heat transport in semiconductors such as Si and GaAs significantly deviates from the Fourier law at distances much longer than previously thought, $\ge $1 $\mu $m at room temperature and above. We use the laser-induced transient thermal grating technique in which absorption of crossed laser pulses in a sample sets up a sinusoidal temperature profile monitored via diffraction of a probe laser beam. By changing the period of the thermal grating we vary the thermal transport distance within the range $\sim $1-10 $\mu $m. In measurements performed on thin free-standing Si membranes and on bulk GaAs the thermal grating decay time deviates from the expected quadratic dependence on the grating period, thus providing model-independent evidence of non-diffusive transport. The simplicity of the experimental configuration permits analytical treatment of non-equilibrium phonon transport with the Boltzmann transport equation. Our analysis shows that at small grating periods the effective thermal conductivity is reduced due to diminishing contributions of ``ballistic'' low-frequency phonons with long MFPs.   

Thermal conductivity and isotope effect in wide bandgap semiconductors

We have calculated the lattice thermal conductivity, $k$, of wurtzite InN, GaN, and AlN using an exact numerical solution of the phonon Boltzmann transport equation and \textit{ab initio} calculations of interatomic force constants. We find good agreement with experiment for the thermal conductivities around and above room temperature. The large frequency gap between the acoustic and the optic phonon branches in these materials limits anharmonic phonon scattering leading to large enhancements to $k$ with isotopic enrichment. We comment on the roles of various phonon scattering mechanisms on $k$ in these wide bandgap semiconductors.   

Conductance beyond the Landauer limit and charge pumping in quantum wires driven by linearly polarized radiation

Periodically driven systems, which can be described by Floquet theory, have been proposed to show characteristic behavior that is distinct from static Hamiltonians. Floquet theory proposes to describe such periodically driven systems in terms of states that are indexed by a photon number in addition to the usual Hilbert space of the system. In this work, we propose a way to measure directly this additional state by the measurement of the conductance of a single channel quantum point contact. Specifically, we show that a single channel wire augemented with a grating structure when irradiated with microwave radiation can show a DC conductance above the limit of one conductance quantum set by the Landauer formula. Another interesting feature of the proposed system is that being non-adiabatic in character, it can be used to pump a photo-current even with linearly polarized light. This circumvents the topological restrictions of adiabatic pumping, which necessisate the use of circularly polarized light. J.S. thanks the Harvard Quantum Optics center for support.   

Positive and Negative Coulomb Drag in a 1D Quantum Circuit

We report Coulomb drag measurements between tunable vertically-coupled quantum wires. The wires are fabricated in a GaAs/AlGaAs double quantum well heterostructure with a 15 nm barrier separating the quantum wells. The Coulomb drag signal is mapped out versus the number of subbands occupied in each wire, and regions of both positive and negative drag are observed (D. Laroche \textit{et. al.} Nature Nanotechnology, doi:10.1038/nnano.2011.182). The observation of negative Coulomb drag at a high one-dimensional electronic density is not predicted by the usual momentum-transfer model for Coulomb drag and shows that the existing picture of the drag effect in one-dimension is incomplete. In order to clarify the origin of this negative signal, temperature dependencies of the Coulomb drag are presented both in the positive and in the negative drag regimes. 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.   

Phonon Mediated Optical Stark Effect for Organic-Semiconductor Heterostructures

The system of macroscopic equations of hybrid excitons, photons and phonon is investigated in order to model the optical Stark effect for the hybrid systems with combination of semiconductor and organic materials. The unique properties of the hybrid system will allow tuning the system to get the most preferable outcomes. One of the key points is to study the interaction of different components of the system with each other to get the condition to facilitate the observation of phonoriton and the phonon-mediated Stark effect.   

Electron phonon coupling and carrier lifetimes in III-nitride ternary semiconductors

III-nitride semiconductors have a large bandgap and find applications in high power devices, in which the thermal management of energy generated becomes a key issue. The transfer of energy from the energetic carriers to the lattice is determined by the electron-phonon coupling for the crystal. In this work, we present our results on the electron-phonon coupling in ternary and quaternary semiconductors. The full phonon dispersion and electron-phonon coupling is determined using ab-initio methods. We then evaluate the carrier lifetimes for the emission of LO phonons by including the full zone and not only the zone center phonons. We expect that the treatment of the electron-phonon coupling effects over the full Brillouin zone could be critical for III-nitride thermal management, and will be directly compared to the results where only the zone center phonons are considered.   

Vibrational structure of defect luminescence bands in GaN from electronic structure calculations

Optical methods are among the most powerful to characterize defects in materials. The study of optical signatures based on state-of-the-art electronic structure methods is therefore very important. In this work we investigate the vibrational structure of luminescence bands pertaining to deep defect levels in GaN. Since luminescence lineshapes depend sensitively on defect geometries and vibrational frequencies, these should be described accurately. The latter is achieved through the use of hybrid density functionals. Both quasi-localized and bulk phonons are included in our description. In the case of transitions accompanied by very large lattice relaxations, anharmonic effects become sizeable, and these are also accounted for. For the defects studied a very good agreement with available experimental data is achieved. For instance, in the case of wide luminescence bands the resulting line widths are within 0.05 eV of the experimental values. This work was supported by the Swiss NSF and by NSF.   

Ultrafast Carrier Dynamics in GaAs(110) Studied by Time- and Angle-Resolved Photoelectron Spectroscopy

Ultrafast carrier dynamics in GaAs is of particular importance to optoelectronic devices and solar cell technologies. We employ Time- and Angle-Resolved Photoelectron Spectroscopy to elucidate the dynamics of both the occupied and unoccupied states of GaAs(110) upon femtosecond infrared laser excitation. We observe in the conduction band an optically excited population and energy shift, which both decay in $\sim$10 ps. The valence band also exhibits energy shifting dynamics which encompasses multiple temporal regimes. More intriguingly, valence band dynamics are also observed for negative pump-probe delays. We explain these observations by a carrier-transport-induced electrostatic potential change within a Drude-like picture.   

Intraexcitonic Autler-Townes effect in terahertz-driven semiconductor quantum wells

When a two-level system is resonantly driven by intense light non-perturbative phenomena such as Rabi oscillations and their frequency equivalent, the AC Stark or Autler-Townes effect, can be observed. The latter one manifests itself in an absorption line splitting where the magnitude is linear in the light field strength and where the symmetry of the splitting is determined by the detuning from resonance. Known from molecular spectroscopy [1], the effect has also been observed in solid state structures with its much broader line widths, e.g. for intersubband transitions [2]. Here, we present the first unambigous evidence of this effect in undoped GaAs/AlGaAs quantum wells using the hydrogen atom like intraexcitonic 1s and 2p states of the heavy-hole exciton. These states with a transition energy of 9 meV are resonantly coupled by strong terahertz light from a free-electron laser. For low fields our findings are qualitatively explained by a simple two-level model whereas deviations occur in the 10 kV/cm field range where the rotating-wave approximation of the simplified model breaks down and exciton ionization occurs. Due to the small Rydberg energy we can easily reach a highly non-trivial regime where the Rabi energy and the transition energy become comparable to the photon energy.\\[4pt] [1] S. H. Autler and C. H. Townes, Phys. Rev. 100, 703 (1955). \\[0pt] [2] S. G. Carter et al., Science 310, 651 (2005). \\[0pt] [3] M. Wagner et al., Phys. Rev. Lett. 105, 167401 (2010).   

Raman Spectroscopy and Scanning Electron Microscopy of the Bismuth Sillenites Bi$_{25}$InO$_{39}$ and Bi$_{25}$FeO$_{39}$

The Raman spectrum of Bi$_{25}$InO$_{39}$, a new type of bismuth sillenite, is reported along with the spectra of Bi$_{25}$FeO$_{39}$. Their spectra are remarkably similar to each other and to other bismuth sillenites reported in the literature. The similarities show that the new samples were successfully grown in the sillenite structure. The parameters of each Raman mode were obtained by fitting the spectra to a Lorentzian oscillator model, and the modes were assigned to symmetry-allowed modes of the I23 space group. The assignments were made by comparison to other materials with the sillenite structure. Scanning Electron Microscope images of the samples are also presented.   

Two-dimensional Fourier-Transform spectra of PbS semiconductor quantum dots

Investigating the correlations of multiple excitons in semiconductor quantum dots is a challenging many-body problem that has drawn considerable experimental and theoretical attention over the last two decades. Nonlinear four-wave mixing experiments have long been known to provide direct probes for the many-body effects in the ultrafast dynamics of excitons in semiconductor nanostructures. With the advent of two-dimentional Fourier-transform (2DFT) spectroscopy many-body contributions can be isolated and identified. 2DFT spectra of colloidal quantum dots will be presented, providing new understanding of the role of many-body interactions in the excitonic decoherence of these nanomaterials.   

Session H25: Focus Session: Simulation of Matter at Extreme Conditions - Shock Compression of Metals

Invariance of the Dissipative Action at Ultrahigh Strain Rates above the Strong Shock Threshold

We have directly resolved shock structures in pure aluminum in the first few hundred picoseconds subsequent to a dynamic load, at peak stresses up to 43 GPa and strain rates of in excess of 10$^{10}$ s$^{-1}$. For strong shocks we obtain peak stresses, strain rates, and rise times. From these data, we directly validate$^{1}$ the invariance$^{2}$ of the dissipative action in the strong shock regime, and by comparing with data obtained at much lower strain rates show that this invariance is observed over at least 5 orders of magnitude in the strain rate. Over the same range, we similarly validate the fourth-power scaling of strain rate with peak stress (the Swegle-Grady relation). 1. J. C. Crowhurst, M. R. Armstrong, K. B. Knight, J. M. Zaug, E. M. Behymer, Phys. Rev. Lett, 107, 144302 (2011). 2. D. E. Grady, J. Appl. Phys. 107, 013506 (2010). This work was also supported by the EFree, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Grant No. DESC0001057.   

Ultrashort elastic and plastic shock waves in nickel generated by femtosecond laser pulses

The structure and evolution of ultrashort shock waves generated by femtosecond laser pulses in thin nickel films were investigated by molecular dynamics and two-temperature hydrodynamics simulations. Ultrafast laser energy deposition results in the formation of a highly-pressurized 100-nm-thick layer below the surface of the film. Relaxation of the stress-confined state leads to the creation of a strong compression wave that later transforms into an ultrashort shock wave. Analysis of experimental data shows that such a shock wave, generated by low absorbed laser fluence, can exhibit a pure elastic structure despite an amplitude exceeding the conventional Hugoniot elastic limit. For absorbed fluences above $\sim 0.6 \rm{\:J/cm^{2}},$ two independent processes of elastic and plastic wave breaking are observed with the elastic precursor appearing before formation of the plastic wave. It is found that the amplitude of the elastic precursor is almost independent of the absorbed fluence, but closely related to the pressure at the melting front. The decay rate of the plastic wave amplitude is much higher than that of the elastic wave, which may result in the complete disappearance of the plastic wave within the metal film.   

Molecular dynamics simulations of steady shock waves in nickel

Shock waves in single-crystal nickel samples were investigated by molecular dynamics (MD) simulations. Standard piston simulations were used to investigate the elastic-plastic split-shock-wave regime, whereas regimes having a single steady shock-wave structure were studied by a novel moving window (MW-MD) technique. Two distinct regimes were investigated, including the regimes of split elastic and plastic shock waves and the steady two-zone elastic-plastic single wave. Split shock waves were shown to form at moderate piston velocities out of a metastable high-pressure elastic state that decays into a two-wave structure consisting of a slow plastic wave and fast elastic precursor. At higher piston velocities, the plastic wave overtakes the elastic precursor but does not overrun it. Instead, both waves were found to move in tandem with the same average speed while maintaining a finite, and in some cases strongly fluctuating, separation width that may extend to several microns. The dependence of shock wave structure on crystallographic orientation and concentration of defects was investigated.   

A Molecular dynamics study of a Richtmyer-Meshkov instability

We simulated a single-mode Richtmyer-Meshkov instability (RMI) using the SPaSM (Scalable Parallel Short-range Molecular-dynamics) code on the RoadRunner supercomputer and its Cerrillos counterpart. The simulations consisted of approximately 60 million atoms shocked along the $<111>$ direction. The single crystal simulations had a sinusoidal groove with a wavelength of 257 nm. We conclude from the simulations that the RMI interfaces shows an inversion for most of the conditions we studied. For certain amplitudes and stresses, we observe the spike saturating. Simulations have been carried out at shock strengths both above and below the melt transition. A discussion of the spike and bubble characteristics will be discussed.   

Orientation dependence of shock-induced melting in crystalline aluminum

The complete evolution of metastable states during shock-induced solid-liquid phase transitions in crystalline aluminum was observed in moving window molecular dynamics simulations. The orientation-dependent transition pathways towards orientation-independent final equilibrium states include both ``cold melting'' followed by resolidification in [110]- and [111]-oriented shock waves, and crystal overheating followed by melting in [100] shock waves. Such orientation-dependent dynamics of solid-liquid phase transitions takes place within an extended zone up to hundreds of nanometers behind the shock front, which makes it accessible for experimental observation.   

Strain rate influence on the Hall-Petch effect in Cu

The increased strength of materials with a decreasing grain size has been known for several decades as the Hall-Petch effect. This trend is true down to a specific grain size below which the strength starts decreasing again, due to a change in the underlying plasticity mechanisms caused by the increased grain boundary network density. We are interested in the strain rate influence on the Hall-Petch and ``reverse Hall-Petch" effects. We use molecular dynamics simulations to study polycrystalline samples of copper of different grain sizes between 5 nm and 30 nm, and under uniaxial compression at a wide range of strain rates (10$^{8}$ to 10$^{11}$ s$^{-1}$). We verify that the yield stress of the material increases with the strain rate, due to the time scale required to generate plasticity. Moreover, we observe that the grain size for which the yield stress is highest depends on the strain rate studied. These results are of particular importance for high strain rate loading conditions, such as shock compression.   

High Strain Rate and Shock-Induced Deformation in Metals

Large-scale non-equilibrium molecular Dynamics (MD) simulations are now commonly used to study material deformation at high strain rates (10$^9$-10$^{12}$ s$^{-1}$). They can provide detailed information-- such as defect morphology, dislocation densities, and temperature and stress profiles, unavailable or hard to measure experimentally. Computational studies of shock-induced plasticity and melting in fcc and bcc single, mono-crystal metals, exhibit generic characteristics: high elastic limits, large directional anisotropies in the yield stress and pre-melting much below the equilibrium melt temperature for shock wave propagation along specific crystallographic directions. These generic features in the response of single crystals subjected to high strain rates of deformation can be explained from the changes in the energy landscape of the uniaxially compressed crystal lattice. For time scales relevant to dynamic shock loading, the directional-dependence of the yield strength in single crystals is shown to be due to the onset of instabilities in elastic-wave propagation velocities. The elastic-plastic transition threshold can accurately be predicted by a wave-propagation stability analysis. These strain-induced instabilities create incipient defect structures, which can be quite different from the ones, which characterize the long-time, asymptotic state of the compressed solid. With increase compression and strain rate, plastic deformation via extended defects gives way to amorphization associated with the loss in shear rigidity along specific deformation paths. The hot amorphous or (super-cooled liquid) metal re-crystallizes at rates, which depend on the temperature difference between the amorphous solid and the equilibrium melt line. This plastic-amorphous transition threshold can be computed from shear-waves stability analyses. Examples from selected fcc and bcc metals will be presented employing semi-empirical potentials of the embedded atom method (EAM) type as well as results from density functional theory calculations.   

Metallurgical Effects Upon the Shock Response of Tantalum: Cold Work and Dilute Alloying

The response of the body centred cubic metal tantalum to shock loading has been studied for several decades, due to its use by the military in explosively formed projectiles. It can also be considered as an ideal body centred cubic metal, thus rendering it ideal for studies of fundamental mechanical and microstructural behaviour. Previous studies on well controlled, annealed specimens has shown that deformation is controlled by the motion of rather than the generation of a/2$<$111$>${\{}110{\}} screw dislocations in straight segments, which result in little if any post shock hardening. In situ-shear strength measurements have also shown a significant strength reduction behind the shock front, suggesting that the motion of these dislocations acts as a stress relief mechanism. Similar effects have also been noted in tungsten and its alloys, but very recently, measurements in niobium and molybdenum show shear strength to be near constant behind the shock front. Other factors, such as variation of Peierls stress effecting ease of dislocation generation and the propensity to twin also have a strong effect upon the shock response. In this presentation, we return to tantalum, investigating the differences in shock response between a low dislocation density (annealed) and high dislocation density (cold rolled) material. We also examine the effects of dilute alloying through the addition of 2.5wt{\%} tungsten to tantalum. Results are discussed in terms of the shear strength and its variation with time behind the shock front.   

High Strain Rate Behavior of Nanoporous Tantalum

Nano-scale failure under extreme conditions is not well understood. In addition to porosity arising from mechanical failure at high strain rates, porous structures also develop due to radiation damage. Therefore, understanding the role of porosity on mechanical behavior is important for the assessment and development of materials like metallic foams, and materials for new fission and fusion reactors, with improved mechanical properties. We carry out molecular dynamics (MD) simulations of a Tantalum (a model body-centered cubic metal) crystal with a collection of nanovoids under compression. The effects of high strain rate, ranging from $10^{7}$$s^{-1}$ to $10^{10}$$s^{-1}$, on the stress strain curve and on dislocation activity are examined. We find massive total dislocation densities, and estimate a much lower density of mobile dislocations, due to the formation of junctions. Despite the large stress and strain rate, we do not observe twin formation, since nanopores are effective dislocation production sources. A significant fraction of dislocations survive unloading, unlike what happens in fcc metals, and future experiments might be able to study similar recovered samples and find clues to their plastic behavior during loading.   

High-Pressure Strength Determination via Quasi-Elastic Optimization Analysis

The analysis of unloading profiles from ramp wave experiments on Sandia's Z machine for the purposes of extracting strength information can be greatly influenced by the presence of a window. An impedance mismatch between the sample and the window generates a reflected ramp wave which perturbs the incoming wave, particularly at later times when, during unloading, the material strength becomes evident. In an effort to analyze the waveforms for an accurate estimate of the strength, the experimental data is coupled with optimized numerical simulations. Simulations were performed with LASLO, a one-dimensional magneto-hydrodynamics code. The deviatoric response was calculated using a modified rate-independent Steinberg - Guinan model in which a quasi-elastic response was implemented during unloading by linearly varying the shear modulus. A best fit of relevant parameters in this strength model along with the magnetic field at the drive surface were estimated over the course of thousands of simulations using the Dakota optimization package. These results may then be used to estimate the in situ wave profiles from which the strength can be extracted. Initial results will be presented for ramp wave compression of tantalum with a lithium fluoride window to peak stresses of $\sim $120 GPa. Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin company, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.   

Calculation of diffusivity and viscosity of Al-Cu molten mixtures using molecular dynamics

We use equilibrium classical molecular dynamics and Green-Kubo techniques to calculate the diffusivity and viscosity of Al-Cu molten mixtures. We calculate both the self-diffusivities and the Maxwell-Stefan diffusivities, and evaluate the validity of the Darken relation for this system. We compare the results with those from experiments reported in the literature. We have constructed an analytic model that is fit to the MD results. This transport model has been implemented in a continuum hydrodynamics code. Both the continuum code and extremely large-scale molecular dynamics have been used to simulate the development of vortices due to the Kelvin-Helmholtz instability in a shear layer, and we discuss the results of that comparison.   

Mechanical Behaviour of Light Metal Alloys at High Strain Rates. Computer Simulation on Mesoscale Levels

Researches of the last years have allowed to establish that the laws of deformation and fracture of bulk ultrafine-grained and coarse-grained materials are various both in static and in dynamic loading conditions. Development of adequate constitutive equations for the description of mechanical behavior of bulk ultrafine-grained materials at intensive dynamic influences is complicated in consequence of insufficient knowledge about general rules of inelastic deformation and nucleation and growth of cracks. Multi-scale computational model was used for the investigation of deformation and fracture of bulk structured aluminum and magnesium alloys under stress pulse loadings on mesoscale level. The increment of plastic deformation is defined by the sum of the increments caused by a nucleation and gliding of dislocations, the twinning, meso-blocks movement, and grain boundary sliding. The model takes into account the influence on mechanical properties of alloys an average grains size, grain sizes distribution of and concentration of precipitates. It was obtained the nucleation and gliding of dislocations caused the high attenuation rate of the elastic precursor of ultrafine-grained alloys than in coarse grained counterparts.   

Estimation of spectral characteristics of particles ejected from free surfaces of metals and liquids under shock wave effect

The authors present approximated relations for estimations of the basic characteristics of flow of particles ejected from free surface of substance after shock wave arrival (shock-wave ejecta). The problem is considered as a particular case of the Richtmayer-Meshkov instability. Periodic perturbations on free surface, which are sinusoidal and having triangular shape, are considered as the initial perturbations causing formation of jets and particles. The medium is assumed to be liquid with surface tension. The role of viscosity is estimated. In the work, the authors obtained equations for estimations of the following characteristics of the particle flow: -~dependence of integral mass of ejected substance on time; -~space-time distribution of density of ejected substance; -~space-time distribution of velocity of ejected substance; -~distribution of particles in sizes; -~correlation of sizes and velocities of particles. Estimations are presented concerning influence of shear strength and plasticity on substance ejecta. Analytical relations are compared with results of numerical calculations and experiments.''   

Session H26: Focus Session: Friction, Fracture and Deformation Across Length Scales - Friction Across Length Scales

Rock Friction from the Nanoscale to the San Andreas Fault

Nucleation of earthquakes (EQs) and the resistance of faults to shearing during EQs are determined by nano-to-micro- scale frictional processes that occur on tectonic-scale faults. A first-order observation from rock-friction studies is that of ageing, i.e., the linear increase in friction with the log of the time of stationary contact, manifest as a positive or negative dependence of friction on sliding rate. A necessary condition for EQ nucleation is a negative rate dependence of friction. In spite of the success of friction `laws' which encapsulate the rate and time dependences of friction in fitting experimental data and reproducing natural phenomena in EQ models, these laws lack a physical basis. Atomic force microscope (AFM) experiments on silica-silica contacts explore the physics of ageing, more specifically increases in adhesion of nanometers-sized contacts with time (Li et al., \textit{Nature}, 2011). The experiments reveal prominent ageing which increases with humidity, as in rock friction tests, without increases in contact area due to creep (the canonical explanation for ageing in rock-friction tests). Ageing in the AFM tests is in fact much larger than in rock-friction tests, a discrepancy explained with a simple multi-asperity contact model. At EQ slip rates ($\ge $1 m/s) a variety of dynamic fault-weakening mechanisms may decrease the shear resistance of faults, which would have important consequences for the magnitudes of EQ stress drops, strong ground motions and accelerations, for the EQ energy budget, and for the state of stress on faults. Experiments on rocks found in the Earth's crust for slip rates up to $\sim $0.4 m/s over $\sim $40 mm of slip, reveal a dramatic 1/$V$ decrease in frictional strength above a characteristic weakening velocity $V_{w}$ of $\sim $0.1 m/s (Goldsby and Tullis, \textit{Science}, 2011). Friction is also revealed to be a nearly pure function of slip rate, i.e., it adjusts to the ambient slip rate over only microns of slip. The observations are explained by `flash heating', whereby microscopic asperity contacts become intensely frictionally heated and weakened above $V_{w}$. Dramatically lower friction due to flash heating may explain why heat flow along active faults like the San Andreas Fault is much lower than expected. Strong velocity-weakening friction and the rapid strength recovery with decreasing slip rate from flash heating may explain why EQ ruptures propagate as slip pulses rather than as cracks.   

Avalanche Distributions and the Effect of Inertia in Strained Amorphous Solids

We present results from two and three-dimensional simulations of a disordered, binary Lennard-Jones solid under quasi-static, steady-state shear. The solid responds to the applied shear strain with bursts of particle movement and plasticity. The energy E of these avalanches spans a wide range and follows a power-law distribution $N(E) \propto E^{-\tau}$ with three distinct exponents, depending on the importance of inertia. In the limit of overdamped dynamics, or no inertia, we find $\tau \approx 0.8$, consistent with previous energy minimization simulations. As inertia becomes more important, the system approaches an unstable critical point where $\tau = 1$. In the underdamped limit, where inertia plays a large role, the distribution of avalanches follows a power-law with exponent $\tau = 1.4$ with an excess of system-spanning events. The three regimes have distinct finite-size-scaling exponents. The fact that consistent exponents are found in two and three dimensions indicates that both may be in the mean-field limit. Spatial correlations in avalanches under different damping regimes will be contrasted.   

Dynamic Phase Diagram and Jamming for Driven Dislocation Assemblies

By using large scale numerical simulations for driven dislocations in 2D, we show that the resulting dynamic phase diagram has the same features found for driven vortex matter and charge density waves in the presence of random disorder. For low loads the system is pinned or jammed. Just above the onset of motion we observe strong velocity fluctuations with $1/f$ noise properties and bimodal velocity distributions associated with rapidly fluctuating dislocation structures. At higher loads there is a dynamic reordering into a state of partially ordered domain walls with a pronounced drop in the velocity fluctuations as well as a reduction in the noise power. These features have all the hallmarks observed for dynamic phases such as the pinned, fluctuating, and dynamical reordering transitions found in driven vortex matter. We discuss the implications of work in terms of dynamic phase transitions at the onset of motion and the onset of the dynamical ordering.   

Scale invariant avalanches: a critical confusion

In the last decades considerable efforts have been devoted to understanding single events related to friction, fracture and unjamming transition, commonly denominated avalanches. However, in many different natural scenarios -from subcritical fracture to earthquake dynamics- these events are of all scales; a situation that has often been interpreted within the formalism of critical phenomena, and having as a relevant consequence the inherently unpredictability of scale-invariant avalanches. A revision of this interpretation which departs from standard ideas is presented here, resulting in [1]: (i) critical systems are not necessarily unpredictable; (ii) slowly driven systems evolving through power-law distributed avalanches are not necessarily critical; and (iii) scale-invariant avalanches are not necessarily unpredictable. Simple simulations and granular experiments [2] confirm the findings. \\[4pt] [1] O. Ramos, Scale invariant avalanches: a critical confusion; in B. Veress and J. Szigethy (eds.) Horizons in Earth Science Research. Vol. 3 (Nova Science Publishers) pp 157-188 (2011) arXiv:1104.4991v1. \\[0pt] [2] O. Ramos, E. Altshuler, and K. J. M{\aa}l{\o}y, Avalanche prediction in a self-organized pile of beads, Phys. Rev. Lett. 102, 078701 (2009).   

Unjamming of amorphous films probed with a transverse shear ultrasonic oscillator

Friction between solids depends essentially on the response of the interfacial amorphous layer to shear and compressive stresses. Hence, the transition from static to dynamic friction corresponds to the unjamming transition of confined amorphous materials [1]. With a shear ultrasonic oscillator, we study the boundary lubrication due to molecular films confined between a plane and a sphere [2]. We observe a linear viscoelastic behaviour at low oscillation amplitude and a nonlinear frictional microslip regime at high amplitude. In a new set of experiments, the system is brought near the unjamming transition by applying a static force. The interfacial layer softens before unjamming, as indicated by the linear response of the oscillator. We suggest an interpretation based on a stress-induced decrease of the free volume, and propose a corresponding heuristic model. Last, we show how ultrasonic in-plane oscillations of $\sim 10$ nm amplitude can trigger unjamming, and we discuss the possible related mechanisms. \\[4pt] [1] P. Thomson and G. Grest and M.O. Robbins, Phys. Rev. Lett \textbf{68}, 3448 (1992)\\[0pt] [2] J. L\'eopold\`es and X. Jia, Phys. Rev. Lett \textbf{105}, 266101 (2010)   

Universality of Deformation Down to the Nanoscale

Deformation on macroscopic scales is often modeled as a continuous process, which in reality occurs via a sequence of nanometer-sized discrete slips. We report statistical analyses of slip size distributions obtained by uniaxial compression experiments on nano-crystals of different crystal structures and sizes down to 75 nm. We show that a simple mean field theory (MFT) correctly predicts the statistical behavior by collapsing data using the MFT exponents and scaling function. This study demonstrates that a simple model captures the statistics and universality class of discrete deformation events in a variety of metallic nano-crystals down to the smallest experimentally accessed length scales.   

Boundary lubrication under pressure: could the friction jump down, instead of up?

The sliding friction during pressure squeezout of a boundary lubricated contact has been shown [1,2] to undergo upward jumps every time a lubricant atomic layer is expelled. Here we ask the question whether the jump could not be downward. Whereas most studies focus on the layered structure which the confined lubricant takes in the normal direction, the element we wish to consider is a possible change of parallel periodicity occurring at the squeezout transition. Such changes have been reported in simulations [3], but their effect has not been discussed so far. One possible effect could be a transition of the slider-lubricant interface commensurability, producing a switch of the frictional mechanism, from lubricant melting-freezing in a commensurate state, to superlubric in an incommensurate one -- in this case with a drop of friction for increasing load. We exemplify this effect by MD simulations, where we replace for convenience the open squeezout system with a closed system, where the lubricant is sealed between the sliders. As the number of layers drops under pressure, the planar lubricant structural lattice parameter also drops. This change reflects in a sliding friction jump, which is easily observed to be downwards. The potential observability of load-induced friction drops will be discussed. \\[4pt] [1] J.N. Israelachvili et al., Science 240, 189 (1988). \\[0pt] [2] J. Gao et al., J. Phys. Chem. B 102, 5033 (1998). \\[0pt] [3] U. Tartaglino et al., J. Chem. Phys. 125, 014704 (2006).   

Plasticity as a Depinning Phase Transition

Crystalline materials deform in an intermittent way via slip-avalanches, which exhibit a variety of scale-invariant behaviors that have been interpreted as a pinning-depinning transition. We use discrete dislocation dynamics at zero temperature to resolve the temporal profiles of slip-avalanches and extract the finite-size scaling properties of the dislocation system, thus going beyond gross aggregate statistics. We provide a comprehensive set of scaling exponents, which establishes that the dynamics of plasticity, in the absence of hardening, is consistent with the mean field interface depinning universality class, even though there is no quenched disorder. Finally, we show how Phase Field Crystal simulations shed light on the effect of temperature on intermittent dislocation dynamics and its critical scaling behavior.   

Tribo-induced melting and temperature gradients at sliding asperity contacts

Tribo-induced nanoscale surface melting mechanisms have been investigated by means of a combined QCM-STM technique [1] for a range of Au and Au-Ni alloys with varying compositional percentages and phases. The QCM-STM setup allows studies to be performed at sliding speeds of up to m/s, and also reveals valuable information concerning tip-substrate temperature gradients.[3] A transition from solid-solid to solid-``liquid like'' contact was observed for each sample at sufficiently high asperity sliding speeds. Pure gold, solid-solution and two-phase Au-Ni (20 at.{\%} Ni) alloys were compared, which are materials of great relevance to MEMS RF switch technology.[2] The transition points agree favorably with theoretical predictions for their surface melting characteristics. We acknowledge NSF and AFOSR support for this research. \\[4pt] [1] B. D. Dawson, S. M. Lee, and J. Krim, Phys. Rev. Lett. 103, 205502 (2009) \\[0pt] [2] Zhenyin Yang; Lichtenwalner, D.J.; Morris, A.S.; Krim, J.; Kingon, A.I, Journal of Microelectromechanical Systems, April 2009, Volume: 18 Issue:2, 287-295 \\[0pt] [3] C.G. Dunkle, I.B. Altfeder, A.A. Voevodin, J. Jones, J. Krim and P.Taborek, J. Appl. Phys., 107, art{\#}114903, (2010)   

Formation of Stable Metallic Nanocontacts by mechanical annealing

Metallic nanocontacts (NC) can be fabricated using STM or related techniques. In these experiments the size of the NC can be followed, down to the atomic contact, by measuring its electrical conductance. Such evolution will normally differ for each experimental realization and therefore conductance histograms are used to identify preferential configurations. It can be shown that occasionally there are some atomic configurations that can be repeated during consecutive cycles of mechanical deformation of the contacts. Here we report experiments for gold NC where the same trace of conductance can be obtained for hundreds of cycles of formation and rupture. We have studied the process leading to such repetitiveness of the traces and found that this is obtained when limiting the indentation depth between the two surfaces to a conductance value of approximately 5-6 G$_{0}$. Using molecular dynamics simulations we have obtained the same behaviour and observed how, after repeated indentations, the two metallic contacts are shaped into a stable configuration by mechanical annealing. This confirms and explains the fact that repeated indentation of a tip into a metallic substrate can be used as a method to sharpen or clean STM tips, but only when such indentation does not exceed a limit.   

Scaling in earthquake models with inhomogeneous damage

We study the scaling of earthquake models that are variations of Olami-Feder-Christensen and Burridge-Knopoff models, in order to explore the effect of spatial inhomogeneities on earthquake-like systems when interaction ranges are long, but not necessarily longer than the distances associated with the inhomogeneities of the system. For long ranges and without inhomogeneities, such models have been found to produce scaling similar to GR scaling found in real earthquake systems. In the earthquake models discussed here, damage is distributed inhomogeneously throughout and the interaction ranges, while long, are not longer than all of the damage length scales. We find that the scaling depends not only on the amount of damage, but also on the spatial distribution of that damage.   

Stress Corrosion Fracture of Silicate Glasses: How Far Can Water Penetrate?

Although glass can be considered as homogeneous at scales as small as a few tens of nanometers, since it exhibits no density fluctuations beyond, its amorphous structure makes it a disordered material with respect to fracture properties. Fluctuations of the orientations of Si-O bonds with respect to the external stress make it unlikely that bonds closest to the crack tip break first, despite the high stress concentration. As a consequence, glass behaves in a quasi-brittle manner rather than in a purely elastic way. \textit{In situ} Atomic Force Microscopy experiments tracking the slow progression of a stress corrosion crack seemed to show indeed the opening and growth of nano size flaws ahead of the tip. However, these results are controversial, because of artifacts which may seriously affect the observations. Furthermore, the low diffusion coefficient of water in silica should forbid hydrolysis at a distance from the crack tip, except at the free surface. Nevertheless, our recent neutron reflectivity experiments show that water actually penetrates into the material during stress corrosion fracture. Comparing two experiments performed for different crack velocities shows that the diffusion coefficient is hugely increased under stress, allowing for bond breakings at $\sim $ten nanometers ahead of the main crack tip.   

Magnetic friction: From Stokes to Coulomb behavior

We demonstrate that in a ferromagnetic substrate, which is continuously driven out of equilibrium by a field moving with constant velocity $v$, at least two types of friction may occur when $v$ goes to zero: The substrate may feel a friction force proportional to $v$ (Stokes friction), if the field changes on a time scale which is longer than the intrinsic relaxation time. On the other hand, the friction force may become independent of $v$ in the opposite case (Coulomb friction). These observations are analogous to e.g.\ solid friction. The effect is demonstrated in both, the Ising (one spin dimension) and the Heisenberg model (three spin dimensions), irrespective which kind of dynamics (Metropolis spin-flip dynamics or Landau-Lifshitz-Gilbert precessional dynamics) is used. For both models the limiting case of Coulomb friction can be treated analytically. Furthermore we present an empiric expression reflecting the correct Stokes behavior and therefore yielding the correct cross-over velocity and dissipation. arXiv:1111.2494   

Session H27: Invited Session: McGroddy Prize, Adler Lectureship, and Pake Prize: Superconductivity and Spin Transport

Assembling, understanding, auguring phase diagrams for Fe-based superconductivity

The quest for improved examples of novel, potentially useful, superconductors reached another milestone in 2008 with the discovery of Fe-based superconductivity in wide range of structurally related arsenide and selenide compounds. In particular, the AFe$_{2}$As$_{2}$ (A = Ba, Sr, Ca) compounds proved to have the highly desirable combination of intriguing properties that imply intimate coupling between electronic, magnetic and structural degrees of freedom, exceptionally high and relatively isotropic upper critical field curves and readily grown, homogeneous single crystals. Over the past three years the CMP community has been able to develop a broad and deep empirical understanding of substitutional and pressure based phase diagrams of these materials that is leading to theoretical as well as synthetic insights. In this talk I will broadly review some of our key findings and speculate about future directions for research in this field.   

James C. McGroddy Prize for New Materials Lecture: New Superconductors and other Research in New Materials

Superconductors and other electronic materials can often display subtle relationships between their structural characteristics and their electronic properties. Though the primary interest in these relationships is within the condensed matter physics community, often at their foundation are the concepts of bonding and structure familiar to inorganic and solid state chemists. Thus a hybridized view, combining physics and chemistry, is one way of approaching the discovery and characterization of new materials. In this talk I will describe some of our research in this context and comment on some broader aspects of interdisciplinary research in new materials.   

David Adler Lectureship Award in the Field of Materials Physics: Racetrack Memory - a high-performance, storage class memory using magnetic domain-walls manipulated by current

Racetrack Memory is a novel high-performance, non-volatile storage-class memory in which magnetic domains are used to store information in a ``magnetic racetrack'' [1]. The magnetic racetrack promises a solid state memory with storage capacities and cost rivaling that of magnetic disk drives but with much improved performance and reliability: a ``hard disk on a chip''. The magnetic racetrack is comprised of a magnetic nanowire in which a series of magnetic domain walls are shifted to and fro along the wire using nanosecond-long pulses of spin polarized current [2]. We have demonstrated the underlying physics that makes Racetrack Memory possible [3,4] and all the basic functions - creation, and manipulation of a train of domain walls and their detection. The physics underlying the current induced dynamics of domain walls will also be discussed. In particular, we show that the domain walls respond as if they have mass, leading to significant inertial driven motion of the domain walls over long times after the current pulses are switched off [3]. We also demonstrate that in perpendicularly magnetized nanowires there are two independent current driving mechanisms: one derived from bulk spin-dependent scattering that drives the domain walls in the direction of electron flow, and a second interfacial mechanism that can drive the domain walls either along or against the electron flow, depending on subtle changes in the nanowire structure. Finally, we demonstrate thermally induced spin currents are large enough that they can be used to manipulate domain walls. \\[4pt] [1] S.S.P. Parkin, US Patent 6,834,005 (2004); S.S.P. Parkin et al., Science 320, 190 (2008); S.S.P. Parkin, Scientific American (June 2009). \\[0pt] [2] M. Hayashi, L. Thomas, R. Moriya, C. Rettner and S.S.P. Parkin, Science 320, 209 (2008). \\[0pt] [3] L. Thomas, R. Moriya, C. Rettner and S.S.P. Parkin, Science 330, 1810 (2010). \\[0pt] [4] X. Jiang et al. Nat. Comm. 1:25 (2010) and Nano Lett. 11, 96 (2011).   

George E. Pake Prize Lecture: Pulsed Laser Deposition and the Oxide Electronics Revolution

The discovery of the Pulsed Laser Deposition (PLD) Process at Bellcore was followed by a stream of advances in the epitaxial growth of oxides and a variety of heterostructures and interfaces. Today Oxide Electronics is a fascinating field with a great deal of new Science and potential for applications. Following a discussion of these events, my talk will focus on the adventure involved in creating a new company, Neocera, and, at the same time, pushing ahead in the evolving field of oxide electronics. There, electron spin, pairing, and alignment to create superconductivity and magnetism have opened up new frontiers for research and materials development.   

A New Avenue towards Colossal Magnetoresistance in Organic Tunneling Junctions

A major challenge for the field of organic spintronics is how to achieve large magnetoresistance (MR) in a reliable manner. We have developed a new approach that dramatically improves MR of organic spin valves. Our approach involves using buffer layer assisted growth to prepare magnetic nanodot layers on top of the organic spacer layer. Interdiffusion between magnetic electrode and organic spacer layer has been largely suppressed in devices prepared by this method. Consequently, devices become highly reliable and large magnetoresistance up to a few hundred percent has been obtained. Moreover, we have attempted to insert a single magnetic nanodot layer inside the organic spacer layer. In such a tunneling junction device, even when the electrodes are nonmagnetic, a colossal MR up to 100000\% has been achieved at relatively high temperatures. The underlying mechanism has been discussed based on temperature-dependent I-V curves and resistivity measurements.   

Session H28: Focus Session: Dopants and Defects in Semiconductors - Si

Reliability of III-V electronic devices -- the defects that cause the trouble

Degradation of electronic devices by hot electrons is universally attributed to the generation of defects, but the mechanisms for defect generation and the specific nature of the pertinent defects are not known for most systems. Here we describe three recent case studies [1] in III-V high-electron-mobility transistors that illustrate the power of combining density functional calculations and experimental data to identify the pertinent defects and associated degradation mechanisms. In all cases, benign pre-existing defects are either depassivated (irreversible degradation) or transformed to a metastable state (reversible degradation). This work was done in collaboration with R.D. Schrimpf, D.M. Fleetwood, Y. Puzyrev, X. Shen, T. Roy, S. DasGupta, and B.R. Tuttle. Devices were provided by D.F. Brown, J. Speck and U. Mishra, and by J. Bergman and B. Brar. \\[4pt] [1] Y. S. Puzyrev et al., Appl. Phys. Lett. \textbf{96}, 053505 (2010); T. Roy et al., Appl. Phys. Lett. \textbf{96}, 133503 (2010); X. Shen et al., J. Appl. Phys. \textbf{108}, 114505 (2010).   

Near-Interface Defects in SiO$_{2}$/SiC MOS Devices

The implementation of SiO$_{2}$/SiC MOSFETS for high power applications has been hindered by the high density of near-interface states. We have developed a method to distinguish both the energy and spatial distribution of defect states near insulator-semiconductor interfaces through a comparison of the thermal emission energy extracted from constant capacitance transient spectroscopy (CCDLTS) measurements and the interface Fermi energy (F$_{P})$. The dependence of F$_{P}$ on trap filling voltage at the CCDLTS peak temperature is determined from temperature-dependent 1MHz C-V curves. Capture by tunneling into oxide traps is detected in 4H- and 6H-SiC capacitors fabricated by oxidation followed by NO-annealing, with the difference in thermal emission energies consistent with the conduction band offsets of the two polytypes at the SiO$_{2}$/SiC interface. Comparison with results from first principles calculations suggests that the observed oxide traps are C$_{O}$=C$_{O}$ and interstitial Si [1]. SiC defects having energies close to the SiC conduction band are suggested to be carbon di-interstitial defects, (C$_{2})_{i}$, introduced during standard oxidation [1]. Well-known traps introduced in SiC by ion-implantation are observed in 4H-SiC MOS capacitors fabricated by N-implantation followed by standard oxidation, thus validating this new method [2]. \begin{enumerate} \item A.F. Basile, \textit{et al}., J. Appl. Phys. \textbf{109}, 064514 (2011) \item A.F. Basile, \textit{et al}., J. Appl. Phys. \textbf{109}, 114505 (2011). \end{enumerate}   

Doping in Si/SiO$_{2}$ Structures: A first-principles metadynamics study

Dopant diffusion in semiconductor devices is a field of study with tremendous technological importance. We have performed first-principles metadynamics simulations of the diffusion of n-type dopants at the Si/SiO$_{2}$ interface using the ab-initio MD method. After the generation of a vacancy in the Si region, arsenic, phosphorus and silicon atoms show varying mechanisms of diffusion, including both substitutional and interstitial. Although at the near-interface silicon region arsenic is the first to diffuse interstitially, its interstitial position is more stable and thus less likely to diffuse across the interface relative to phosphorus and silicon. As arsenic crosses the interface, however, its relative stability decreases with respect to phosphorus and silicon and diffusion into the oxide becomes unfavorable. This is in agreement with experimentally observed arsenic pile-up at the Si/SiO$_{2}$ interface. We quantify the diffusion mechanisms by comparing free energy barriers in the silicon region as well as at the interface. We find the largest barriers exist for silicon, while the smallest barriers exist for arsenic.   

The lifetime recovery puzzle in intermediate band materials: A new experimental approach

We have recently observed that deep-level impurities in silicon -- such as the chalcogens S and Se -- can drive an insulator-to-metal transition. The existence of this transition has potential ramifications for the development of intermediate band (IB) solar cells, and significant progress has recently been made toward explaining the origin of this transition. Recently, however, theoretical disagreement has arisen regarding whether impurity concentrations beyond the insulator-to-metal transition should result in increased non-radiative recombination rates or instead yield ``lifetime recovery.'' Very few measurements of carrier lifetime in IB-candidates have been reported that could help clarify this issue, which has important impacts on the selection of IB-materials and the design of IB solar cells. We have developed a technique based on transient optical absorption to measure the trapping rate of the intermediate states and will discuss the results of these measurements in the hyperdoped silicon system (Si doped with chalcogens to concentration $>$10$^{20}_{ }$cm$^{-3})$. We will also discuss the impact of these measurements on the current theoretical disagreement.   

Single-Electron Capacitance Spectroscopy of Individual Dopants in Silicon

Motivated by recent transport experiments and proposed atomic-scale semiconductor devices, we present measurements that extend the reach of scanned-probe methods to discern the properties of individual dopants tens of nanometers below the surface of a silicon sample. Using a capacitance-based approach, we have both spatially resolved individual subsurface boron acceptors and spectroscopically detected single holes entering and leaving these minute systems of atoms. A resonance identified as the B $^{+}$ state is shown to shift in energy from acceptor to acceptor. The resonance is absent in a control sample that does not contain the boron acceptors. By directly measuring the quantum levels and testing the effect of dopant-dopant interactions, this method represents a valuable tool for the development of future atomic-scale semiconductor devices.   

Charge States of Individual Group V Donors on n-doped Si(111)-(2x1) Surface

Functionality of semiconductor devices now relies upon only a few atoms and study of individual dopants in silicon has thus been rapidly growing in importance. Group V donors are especially interesting due to their potential applications in quantum computing and spintronics. The charge state of the dopants is of fundamental importance for conventional semiconductor devices as well as in concept QIP and spintronic devices. We combine density functional theory simulations and ion implantation and cross-sectional scanning tunneling microscopy (XSTM) to study individual Group V donors in cleaved n-doped Si(111)-(2x1) surface. We present a detailed analysis of the dopant-induced charging effects and discuss the surface charge dependence on the local reconstruction induced by the individual dopants.   

First-principles calculations for Er impurities in Si

Erbium-doped Si is a promising material for the development of silicon-based light sources that can interface with CMOS technology, optical fiber, and spin centers for quantum computing. Using density functional theory with a screened hybrid functional we examine the structural and electronic properties of Er(III) impurities in Si, focusing on the site preference and the Er effects on the electrical properties of the Si host. We find that Er is stable either at the tetrahedral intersitital site or at the substitutional site, depending on the Fermi-level position; Er sitting at the hexagonal interstitial site is higher in energy at all Fermi levels, in agreement with experimental observations. In p-type Si, i.e., for Fermi levels near the valence band, Er prefers the tetrahedral interstitial site and acts as a donor. In n-type Si, i.e., for Fermi levels near the conduction band, Er prefers the substitutional site and acts as an acceptor. We will also discuss the impurity-to-band optical transitions determined from calculated configuration coordinate diagrams.   

Deactivation of deep level impurities in hyperdoped silicon

Extremely high concentrations of deep level impurities in silicon have exhibited unique properties of interest for optoelectronic and photovoltaic applications. For example, silicon hyperdoped with chalcogens demonstrates significant infrared absorption at wavelengths longer than the band edge of silicon. Hyperdoped silicon is fabricated by high-dose ion implantation followed by pulsed laser melting and rapid re-solidification. The result is a metastable supersaturated solid solution with doping concentrations orders of magnitude above the room temperature solubility limit. Thermal annealing results in a deactivation of the sub-gap absorption in this material, suggesting that the precise chemical state of the deep level impurities is a critical component of the enhanced absorption. To gain further insight to the absorption mechanism and the stability this material, we present a detailed investigation of the deactivation induced by rapid thermal annealing of silicon hyperdoped with sulfur.   

Single crystal silicon hyperdoped with transition metals

Silicon hyperdoped with sulfur and selenium by ion implantation and pulsed laser melting has recently been shown to undergo an insulator to metal transition. While experimental and theoretical investigations have begun to unravel the nature of this transition, little has been done to generalize this work to other dopants. This talk will discuss recent progress in hyperdoping silicon with transition metal dopants focusing on challenges not present in the silicon-chalcogen system. In particular, experimental results (e.g., SIMS, RBS, TEM) on the role of dopant selection and resolidification velocity in preventing segregeation and cellular breakdown of the solidification front will be addressed in detail. In addition, measurements of the optoelectronic properties of silicon hyperdoped with transition metal dopants will be reported.   

Theoretical and Experimental Framework of an Insulator-to-Metal Transition in Selenium-Hyperdoped Silicon

Following the discovery of black silicon in 1998, hyperdoping - doping to concentrations orders of magnitude larger than the solubility limit - has emerged as a promising method for designing semiconductors with unique optical and electronic properties. Black silicon (silicon hyperdoped with chalcogens), synthesized by pulsed laser techniques, has recently received substantial interest owing to its broad, sub-band gap absorption down to photon energies as low as 0.5 eV, suggesting applications towards infrared detection and intermediate band photovoltaics. Until now, there has not been a clear explanation of these properties. In this presentation, we combine computational and experimental evidence to probe the origin of sub-band gap optical absorption and metallicity in black silicon. Temperature-dependent conductivity measurements show that black silicon undergoes an insulator-to-metal transition at a critical dopant concentration. Our computational analysis based on density functional theory and quantum Monte Carlo methods suggest that the enhanced optical properties result from this insulator-to-metal transition that appears to be a classic impurity-driven Mott transition, driven largely by many-body effects.   

Optical Absorption Mechanisms in Sulfur Hyper-doped Silicon

Silicon that is doped with sulfur, a deep-level donor, to concentrations approaching 1{\%} at. demonstrates sub-band gap optical absorption, and has potential applications as an intermediate band solar cell material and a short-wavelength infrared (SWIR) photodetector. Understanding the nature of the absorption mechanism will aid in creating future devices with this exciting material. To elucidate the absorption mechanism, the reflectivity and absorption coefficient have been measured to photon energies down to 75 meV. We report on these new measurements as well as data fitting that give insight into absorption mechanism within these materials.   

Atomistic study of heavy doping in Si nanowires

Dopant atoms are becoming increasingly important in the nanoscaled field-effect transistors (FET) because of their tendency to influence device parameters such as sub-threshold current-voltage characteristics and gate-to-channel electrostatic coupling. Achieving high doping concentrations is essential for the realization of Si nanowire FET where low resistance contacts or tunnel junctions and narrow depletion widths are needed. In an effort to understand the dopants effect on these devices as a function of scaling parameters, we use self-consistent field (SCF) tight-binding (TB) method as implemented in NEMO3D to obtain an accurate quantitative description of the band structure, confinement geometries and valley-orbit interaction from a full band-structure technique as a function of dopant location, concentration and applied electrical field. Our method solves the Poisson equation iteratively coupled with the atomistic TB Hamiltonian for charge self-consistency to provide an accurate description of the electrostatics. Our simulations show how the band structure of the nanowire is affected by the presence of few impurities.   

Femtosecond-laser hyperdoping: controlling sulfur concentrations in silicon for band gap engineering

Doping silicon to concentrations above the metal-insulator transition threshold yields a novel material that has potential for photovoltaic applications. By focusing femtosecond laser pulses on the surface of a silicon wafer in a sulfur hexafluoride (SF6) environment, silicon is doped with 1\% atomic sulfur. This material exhibits near-unity, broadband absorption from the visible to the near infrared ($<$ 0.5 eV, deep below the silicon band gap), and metallic-like conduction. These unusual optical and electronic properties suggest the formation of an intermediate band. We report on the femtosecond laser doping techniques we employ and the resulting material properties. By changing the laser parameters and ambient environment we can control the dopant profiles, crystallinity, and surface morphology. We perform optical absorption and temperature-dependent Hall measurements to investigate electron transport and to identify the energy states of the sulfur donors.   

Session H29: Focus Session: Superconducting Qubits: Amplifiers and Read-out

Frequency tunable non-degenerate Josephson amplifier for qubit readout

We have developed a new ultra low noise microwave amplifier based on the Josephson parametric converter (JPC), which overcomes a practical weakness of devices of previous generations: having sufficient frequency tunability to easily match the qubit readout frequency. The JPC consists of two superconducting microwave resonators that are coupled to each other through a ring of four Josephson junctions, threaded by a magnetic flux and providing the non-linearity for the amplification process. The non-linearity is of the trilinear form involving the minimal number of modes, and allows ideal non-degenerate parametric amplification at the quantum limit of noise. In our new tunable version, the junctions responsible for amplification are shunted by a cross of four larger junctions, which for our purpose can be regarded as linear inductors, as in the work of Roch et al.[1]. The JPC has now a unique bias point at any applied flux and is tunable over more than half a gigahertz. We are currently using this amplifier in conjunction with a quantum non-demolition measurement of a transmon qubit and have observed quantum jumps with fidelity larger than 90{\%}. [1] N. Roch, E. Flurin, F. Nguyen, P. Morfin, P. Campagne-Ibarcq, M. H. Devoret, and B. Huard, in preparation.   

Microstrip SQUID amplifiers for quantum information science

Recent progress in SQUID amplifiers suggests that these devices might approach quantum-limited sensitivity in the microwave range, thus making them a viable option for measurement of superconducting quantum systems. In the microstrip SQUID amplifier configuration, gains of around 20dB are possible at frequencies of several hundred MHz, and the gain is limited by the maximum voltage modulation available from the SQUID. One route for increasing the voltage modulation involves using larger resistive shunts, however maintaining non-hysteretic device operation requires smaller junction capacitances than is possible with conventional photolithographically patterned junctions. Operating at higher frequencies requires a shorter input coil which reduces mutual inductance between the coil and washer and therefore gain. We have fabricated microstrip SQUID amplifiers using submicron Al-AlOx-Al junctions and large shunts. The input coil and SQUID washer are optimized for producing high gain at frequencies in the gigahertz range. Recent measurements of gain and noise temperature will be discussed as well as demonstrations of these devices as a first stage of amplification for a superconducting system   

Measurement of the phase non-reciprocity of the Josephson parametric converter

Non-reciprocal devices such as circulators and isolators play a pivotal role in many microwave experiments on quantum superconducting circuits. However, non-reciprocity in these devices is achieved using ferrites and permanent magnets which are not suitable for on chip integration. We have built a symmetric two port device by pairing two Josephson parametric converters (JPCs) working in pure conversion mode, each of which is driven with an independent pump tone. We observed a non-reciprocal phase shift between the two ports, which depends on the phase difference between the two pumps. The present noiseless system constitutes an important step towards the implementation of a noiseless gyrator, the main building block of an on-chip circulator for back-action free quantum measurement.   

Performance of the doubly pumped four-wave Josephson parametric amplifier

The degenerate Josephson Parametric Amplifier (JPA) is a promising quantum-limited amplifier for the measurement of mesoscopic systems. In the single pump scheme, the amplification process utilizes two photons at the drive frequency to produce a signal and an idler photon. However the large reflected pump tone at the drive frequency strongly acts back on the measured system. Several circulators prevent this back action, but come at a non-negligible cost in system noise temperature. Two drive tones symmetrically detuned from the original drive frequency provide the necessary pump energy but with the reflected tones now far removed from the signal frequency. We have investigated the performance of the doubly pumped JPA with pump detunings from 50 to 1000 MHz. The expected gain profile has been measured with at least 20 dB of gain and no loss of bandwidth. A gigahertz range of tunability, no degradation in noise temperature, and improvements in dynamic range are also expected.   

Analytical Calculation of Gain and Noise of DC Squid Microwave Amplifier

The dc SQUID microwave amplifier, based on Josephson junctions, is employed in a wide spectrum of applications ranging from dark matter detection to the readout of superconducting qubits. A crucial advantage offered by this device is the separation of input and output channels, unlike conventional Josephson parametric amplifiers, so that it does not require a nonreciprocal device such as a circulator for its operation. The mechanism underlying the directional gain in the SQUID microwave amplifier, however, has so far remained elusive. We present a first principles, analytical calculation, based on scattering theory, of a practical SQUID amplifier which elucidates the underlying nonlinear mode mixing responsible for the directional operation of the device. The gain and quantum noise characteristics of a SQUID operated as a microwave voltage amplifier are discussed. Work supported by IARPA and ARO (AK, MHD and JC) and NSF (AK and MHD).   

Observing quantum jumps of a transmon qubit with a Josephson parametric converter

A high fidelity linear quantum non-demolition (QND) readout of a superconducting qubit opens up the possibility of observing quantum jumps and is a prerequisite for quantum feedback and error correction. This readout is challenging since the qubit, the readout resonator and the following amplifier chain have to be simultaneously optimized to achieve the desired performance. We fabricated a superconducting transmon qubit at 5.7 GHz coupled to a compact resonator at 7.5 GHz, designed to produce a dispersive shift ($\chi )$ of 6 MHz of the resonator frequency when the qubit is excited. The resonator linewidth matches $\chi $ to produce maximum readout contrast in a transmission measurement, while maintaining a Purcell limited T$_{1}$ of about 3 $\mu $s. Using a Josephson parametric converter that is tuned to match the resonator frequency, we achieved a system noise temperature of the following amplifier chain of about 0.5 K, roughly thrice the standard quantum limit. Using these optimized parameters, we measured the qubit state with about 5 photons in the readout resonator and observed quantum jumps with fidelity above 90 {\%}. Further, by looking at the statistics of the jumps and the evolution of the qubit population in single shot traces, we find that the average qubit T$_{1}$ during the readout matches the Purcell limited T$_{1}$, as expected for a QND measurement.   

Quantum measurement in action with the transmon qubit

High fidelity, rapid quantum non-demolition readout of superconducting qubits greatly facilitates tests of single qubit measurement theory. We have realized such readout in an experiment comprised of a transmon coupled to a compact resonator, which is in turn connected via an isolator and circulator to a tunable Josephson parametric converter (JPC) operated as a phase-preserving parametric amplifier. When the qubit state is measured with an rf tone corresponding to an average cavity circulating power of 5 photons, fidelity exceeds 90{\%} for a measurement duration of 240 ns ($\sim $0.1 T1). This performance allows the observation of quantum trajectories of the qubit, showing discrete jumps and a bimodal distribution of measurement results, despite the linear character of the amplifier. This provides further support for the quantum nature of superconducting artificial atoms. We have conducted Stern-Gerlach type experiments, in which the qubit is repeatedly measured along different axes. Results are in good agreement with theoretical predictions of the effect of partial measurement on qubit state evolution.   

Monitoring the state of a superconducting quantum bit using weak measurements

We demonstrate continuous weak measurement of the state of a superconducting transmon qubit in the circuit QED architecture using a Josephson parametric amplifier. The near quantum-limited noise performance of the amplifier enables us to obtain a high-fidelity measurement record which can be used to reconstruct the qubit state evolution during measurement by employing the quantum Bayesian formalism. While simultaneously driving the qubit at its Larmor frequency and measuring it weakly, we are able to resolve the spectral signature of Rabi oscillations present in the measurement record with a high signal-to-noise ratio (SNR). We discuss current limitations in this measurement scheme and suggest ways to further optimize the SNR. Our results suggest a route to implement quantum feedback to steer the state of a superconducting qubit.   

Stabilizing Rabi oscillations in a superconducting qubit using quantum feedback

Recent progress in the development of quantum-noise-limited superconducting parametric amplifiers has enabled high-fidelity, continuous measurements of superconducting quantum bits (qubits). We exploit this functionality while leveraging improved coherence times in transmon qubits to show that it is now possible to obtain a faithful record of real-time qubit dynamics during measurement. We weakly measure the qubit while it is continuously driven at the Larmor frequency. The phase of the resulting Rabi oscillations diffuses slowly, primarily due to the measurement. We monitor this phase diffusion and correct for it by feeding back on the qubit drive amplitude. This locks the Rabi oscillations to a classical reference signal and the oscillations persist indefinitely with a reduced visibility set by the feedback efficiency. We perform tomography on the feedback stabilized state and suggest routes to further optimize the feedback efficiency. Such capabilities suggest a measurement based route for implementing continuous quantum error correction.   

High fidelity readout of a superconducting qubit using heralded state preparation

We report measurements of a superconducting flux qubit, coupled via a shared inductance, to a quasi-lumped element 5.78-GHz readout resonator formed by the parallel combination of an interdigitated capacitor and a meander line inductor. A Josephson parametric amplifier with near-quantum-limited noise performance is used to increase the measurement sensitivity. We demonstrate a continuous, high-fidelity readout with sufficient bandwidth and signal-to-noise ratio to resolve quantum jumps in the flux qubit. We achieve a readout fidelity of 91\%, limited primarily by $T_1$ decay between state preparation and measurement. The fast, high-visibility, QND character of the readout allows for many successive readouts within a time $T_1$. We exploit this capability to herald pure ground and excited state ensemble populations by post-selecting only for certain states after an initial readout. This method enables us to eliminate errors due to imperfect state preparation, increasing the fidelity to 94\%. We also present a precise budget of fidelity loss and an analysis of the readout backaction.   

Experimental approaches to improve the single shot measurement fidelity of a superconducting charge qubit

We discuss various experimental approaches to improve the single shot measurement fidelity of a superconducting charge qubit. Dispersive readout is optimized on a transmon coupled to a superconducting coplanar waveguide resonator. Measurement parameters, such as microwave power and frequency are varied. Also control theory is adapted to construct a genetic algorithm which optimizes the shape of the drive pulse. Additionally, we attempt to reduce noise and increase SNR by employing a SLUG amplifier. Using these techniques, we discuss the feasibility of reaching the measurement fidelity needed for scalable quantum computation with superconducting circuits.   

Multiplexed dispersive readout for the superconducting phase qubit

Scaling to multiple qubit circuits requires state readout with maximum reliability and the minimum number of readout lines. Here, we introduce a multiplexed readout scheme for superconducting phase qubits. We replace our standard readout SQUIDs with inductively coupled resonators so that the measured state of the qubit (left or right side of the potential well) is read out as a shift of the resonator frequency. We connected several readout resonators to a single feedline and use a multi-tone microwave reflection measurement to simultaneously read out the states of multiple qubits using a single cable. Together with the compact lumped LC resonator design, the efficiency of chip space usage is greatly improved.   

Ground State Misidentification in Superconducting Qubits

To achieve fault tolerant quantum computation, it is necessary to maximize measurement fidelity and minimize readout-induced measurement errors. A new protocol was developed to measure $P_1(|g>)$, the probability of measuring the excited state without exciting the qubit, while not including stray tunneling present in superconducting phase qubits. We have confirmed the expected trend in $P_1(|g>)$ with device temperature. We then compared $P_1(|g>)$ for phase qubits with different readout mechanisms and found that it is $\sim$3\% for our dispersive readout scheme and $\sim$1.5\% for our prior SQUID-based readout scheme. We have further applied microwave power to the flux bias and microwave drive lines to understand the source of this difference.   

Autoresonant readout of a high Q resonator coupled to a superconducting quantum bit

The frequency of a nonlinear oscillator changes with oscillation amplitude. When a high-Q, nonlinear oscillator is excited with a frequency chirped drive, the system can respond at either low or high oscillation amplitude depending on whether the drive excitation is below or above a critical value, respectively -- a phenomenon known as autoresonance. We exploit this nonlinear phenomenon to read out the state of a superconducting transmon qubit coupled to a high-Q nonlinear resonator. Because the excitation is non-equilibium, the resonator can be read out faster than its energy decay time. The fidelity for mapping the qubit state onto the oscillator can be as high as $80\%$ and is limited by the $T_1$ lifetime of the qubit, and readily achievable pulse parameters.   

Qubit Measurement with a Nonlinear Cavity Detector Beyond Linear Response

We consider theoretically the use of a driven, nonlinear superconducting microwave cavity to measure a coupled superconducting qubit. This is similar to setups studied in recent experiments.\footnote{M. Hatridge {\it et al.} Phys.Rev.B, 83,134501 (2011)}$^,$\footnote{F.R. Ong {\it et al.} PRL 106,167002 (2011)} In a previous work, we demonstrated that for weak coupling (where linear response theory holds) one misses the quantum limit on QND detection in this system by a large factor proportional to the parametric gain.\footnote{C. Laflamme and A.A. Clerk, Phys. Rev. A 83, 033803 (2011)} Here we calculate measurement backaction beyond linear response by using an approximate mapping to a detuned degenerate parametric amplifier having both linear and dispersive couplings to the qubit. We find surprisingly that the backaction dephasing rate is far more sensitive to corrections beyond linear response than the detector response. Thus, increasing the coupling strength can significantly increase the efficiency of the measurement. We interpret this behavior in terms of the non-Gaussian photon number fluctuations of the nonlinear cavity. Our results have applications to quantum information processing and quantum amplification with superconducting microwave circuits.   

Session H30: Semiconductor Qubits - Device Development

Screening of charged impurities with multi-electron singlet-triplet spin qubits in quantum dots

Charged impurities in semiconductor quantum dots comprise one of the main obstacles to achieving scalable fabrication and manipulation of singlet-triplet spin qubits. We theoretically show that using dots that contain several electrons each can help to overcome this problem through the screening of the rough and noisy impurity potential by the excess electrons [1]. We demonstrate how the desired screening properties turn on as the number of electrons is increased, and we characterize the properties of a double quantum dot singlet-triplet qubit for small odd numbers of electrons per dot. We show that the sensitivity of the multi-electron qubit to charge noise may be an order of magnitude smaller than that of the two-electron qubit. \\[4pt] [1] E. Barnes, J.P. Kestner, N.T.T. Nguyen, and S. Das Sarma, arXiv:1108.1399.   

Measurements of undoped accumulation-mode SiGe quantum dot devices

We report transport measurements of undoped single-well accumulation-mode SiGe quantum dot devices with an integrated dot charge sensor. The device is designed so that individual forward-biased circular gates have dominant control of dot charge occupancy, and separate intervening gates have dominant control of tunnel rates and exchange coupling. We have demonstrated controlled loading of the first electron in single and double quantum dots. We used magneto-spectroscopy to measure singlet-triplet splittings in our quantum dots: values are typically $\sim $0.1 meV. Tunnel rates of single electrons to the baths can be controlled from less than 1 Hz to greater than 10 MHz. We are able to control the (0,2) to (1,1) coupling in a double quantum dot from under-coupled (t$_{c} \quad <$ kT$\sim $ 5$\mu $eV) to over-coupled (t$_{c} \quad \sim $ 0.1 meV) with a bias control of one exchange gate. Sponsored by the United States Department of Defense. Approved for Public Release, Distribution Unlimited. The views expressed are those of the author and do not reflect the official policy or position of the Department of Defense or the U.S. Government.   

Fabrication of dual-gated devices on undoped Si/SiGe heterostructures

Undoped accumulation mode Si/SiGe heterostructures have recently emerged as a promising platform for the fabrication of few-electron silicon spin qubits. Spin blockade has been observed in an accumulation mode double dot [1] and a record mobility of 1.6 million cm$^2$/(Vs) has been achieved in undoped wafers grown at National Taiwan University [2]. We develop a fabrication process for dual-gated accumulation mode structures and form a stable two-dimensional electron gas by applying positive bias to a global top gate. The resulting 2DEG has charge densities of $2-5\times10^{11}$/cm$^{2}$ and mobilities up to 200,000 cm$^2$/(Vs). We present preliminary data from quantum point contacts fabricated in this geometry. \\ References:\\ \noindent [1] M. G. Borselli \textit{et al.}, Appl. Phys. Lett. {\bf99}, 063109 (2011).\\ \noindent [2] T. M. Lu \textit{et al.}, Appl. Phys. Lett. {\bf94}, 182102 (2009).\\   

Optimization of realistic silicon double quantum dots through simulation

We present results obtained using a newly developed semiclassical and Poisson-Schrodinger simulation tool which is able to simultaneously optimize many solution parameters. We discuss the benefit this capability has on realistic device design, and report general trends seen when targeting few-electron quantum dots in silicon and silicon-germanium structures. This work was supported by the Laboratory Directed Research and Development program at Sandia National Laboratories. 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.   

Anatomy of the exchange gate action in undoped accumulation-mode SiGe quantum dot devices

We discuss simulations of an undoped accumulation-mode SiGe device containing an electrostatically formed double quantum dot in its active area. We validate our virtual model by extensive device characterization (in terms of gate actions, dot addition energies, etc.) and quantitative comparisons to concurrent experimental data. Next, we trace and map in detail the turn-on of the inter-dot exchange interaction by the exchange gate located between the dot gates. Of primary interest is the ability to control (i.e., both to completely shut off and to gradually modulate in the neV to $\mu$eV range) the exchange energy between the two separated electrons. We identify a potential obstacle to proper device operation, the formation of additional dot states under the progressively more forward-biased exchange gate. This effect is limited, however, to the case of large dot gate diameter and/or large dot-dot separation. Lastly we quantify and analyze the consequences of cross-capacitance between adjacent exchange and dot gates. Sponsored by the United States Department of Defense. Approved for public release, distribution unlimited.   

Effect of fixed charge gate oxide defects on the exchange energy of a multi-valley silicon double quantum dot

Exchange interaction at the MOS interface has been proposed as a qubit coupling approach for both MOS quantum dots and donor qubits. An intrinsic source of disorder in the MOS system is the charge defects in silicon dioxide, nanometers away from the interface and qubit electrons. The presence of a charge defect so near the qubit can significantly perturb the confinement potential and alter the intended coupling. Using a large-scale atomistic tight-binding method coupled to a full configuration interaction technique, we investigate the role of these defects on the two-electron coupling of a double quantum dot (DQD) as a function of detuning bias. We show how the multi-valley character of silicon is manifested in the two-electron spectrum, and hence in the exchange energies of excited triplet states corresponding to different valley configurations. Our results show that defects near the tunnel barrier can adversely affect the tunability of the DQD, while defects distributed asymmetrically relative to the two dots can act as an additional detuning source.   

Modeling split gate tunnel barriers in lateral double top gated Si-MOS nanostructures

Reliable interpretation of quantum dot and donor transport experiments depends critically on understanding the tunnel barriers separating the localized electron state from the 2DEG regions which serve as source and drain. We analyze transport measurements through split gate point contacts, defined in a double gate enhancement mode Si-MOS device structure. We use a square barrier WKB model which accounts for barrier height dependence on applied voltage. This constant interaction model is found to produce a self-consistent characterization of barrier height and width over a wide range of applied source-drain and gate bias. The model produces similar results for many different split gate structures. We discuss this models potential for mapping between experiment and barrier simulations. This work was performed, in part, at the Center for Integrated Nanotechnologies, a U.S. DOE, Office of Basic Energy Sciences user facility. The work was supported by the Sandia National Laboratories Directed Research and Development Program. 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. DOE's National Nuclear Security Administration under contract DE-AC04-94AL85000.   

Enhancement-mode buried strained-silicon channel double quantum dot

We demonstrate a relaxed-SiGe/strained-Si enhancement-mode gate stack for quantum dots. The devices were fabricated within a 150 mm Si foundry setting that uses implanted ohmics and chemical-vapor-deposited dielectrics. Thermal budget was minimized to prevent Ge/Si interdiffusion and strain relaxation. A mobility of 1.6x10$^{5}$ cm$^{2}$/Vs at 5.8x10$^{11}$/cm$^{2}$ is measured in Hall bars that witness the same device process flow as the quantum dot. Periodic Coulomb blockade measured in a double-top-gated lateral quantum dot nanostructure terminates with open diamonds up to +/- 10 mV of dc voltage across the device. Charge sensing indicates a lithographically defined double quantum dot with tunable coupling. This work was performed, in part, at the Center for Integrated Nanotechnologies, a U.S. DOE, Office of Basic Energy Sciences user facility. The work was supported by the Sandia National Laboratories Directed Research and Development Program. 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.   

Hybrid Donor-Dot Devices made using Top-down Ion Implantation for Quantum Computing

We present progress towards fabricating hybrid donor -- quantum dots (QD) for quantum computing. These devices will exploit the long coherence time of the donor system and the surface state manipulation associated with a QD. Fabrication requires detection of single ions implanted with 10's of nanometer precision. We show in this talk, 100{\%} detection efficiency for single ions using a single ion Geiger mode avalanche (SIGMA) detector integrated into a Si MOS QD process flow. The NanoImplanter (nI) a focused ion beam system is used for precision top-down placement of the implanted ion. This machine has a 10 nm resolution combined with a mass velocity filter, allowing for the use of multi-species liquid metal ion sources (LMIS) to implant P and Sb ions, and a fast blanking and chopping system for single ion implants. The combination of the nI and integration of the SIGMA with the MOS QD process flow establishes a path to fabricate hybrid single donor-dot devices. 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.~   

Computer assisted design of poly-silicon gated enhancement-mode, lateral double quantum dot devices for quantum computing

We discuss trade-offs of different double quantum dot and charge sensor lay-outs using computer assisted design (CAD). We use primarily a semi-classical model, augmented with a self-consistent configuration interaction method. Although CAD for quantum dots is difficult due to uncontrolled factors (e.g., disorder), different ideal designs can still be compared. Comparisons of simulation and measured dot characteristics, such as capacitance, show that CAD can agree well with experiment for relevant cases. CAD results comparing several different designs will be discussed including a comparison to measurement results from the same designs. Trade-offs between poly-silicon and metal gate lay-outs will also be discussed. This work was performed, in part, at the Center for Integrated Nanotechnologies, a U.S. DOE, Office of Basic Energy Sciences user facility. The work was supported by the Sandia National Laboratories Directed Research and Development Program. 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.   

Correlation functions of the electric and magnetic fields in the vicinity of a metal surface

The Johnson noise-induced relaxation rate of a charge or spin qubit for a transition at a particular frequency in the vicinity of a metal boundary is proportional to the temporal Fourier component at that frequency of the electric or magnetic correlation function evaluated at the position of the qubit. These correlation functions are shown to be greatly enhanced compared to the blackbody result in the near vicinity of the metal due to the contribution of evanescent waves. As such, we expect a measurable enhancement of qubit decoherence due to the contribution of evanescent waves. We use a Green's dyadic approach to calculate the correlation functions of the fluctuating electric and magnetic fields in the vicinity of a conducting surface. In a local treatment of the dielectric properties of the metal this enhancement diverges as the inverse cube of the distance from the boundary, and for distances less than the order of the Fermi wavelength of the metal a nonlocal treatment is necessary to obtain an accurate result. We present a calculation of the correlation function for the full range of distances.   

Fabrication and measurement of quantum dots in double gated, dopantless Si/SiGe heterostructures

Significant progress has been made towards quantum dot spin qubits in Si/SiGe single and double quantum dots. In the past, these structures have been created by depleting a modulation-doped 2DEG that forms at the Si/SiGe interface. The modulation doping in such devices is believed to be a source of charge noise. Recently, undoped structures have been explored for the formation of both 2DEGs and quantum dots in Si/SiGe. Here we discuss measurements on double gated, dopantless quantum dots in Si/SiGe heterostructures. The devices are based on a new ``island mesa'' design incorporating micro-ohmic contacts. We present transport measurements on a double quantum dot showing a smooth transition from single dot to double dot behavior.   

Unintentional Quantum Dots in Silicon: Deducing the Location and Cause

When attempting to use local gates to electrostatically define quantum dots in silicon, additional unintentional quantum dots (U-QDs) that are not defined by the gates are often observed. U-QDs are typically blamed on random charged defects such as dopants or interface traps. We use measured gate capacitances and a capacitance simulator to determine the location of the U-QDs with a precision of a few nanometers. Since we have observed U-QDs in similar locations in multiple devices, we suggest that some U-QDs are not caused by random charged defects instead are a systematic but unanticipated consequence of the fabrication. We will discuss strain as a potential cause of the U-QDs. This allows us to suggest methods to reduce the frequency of U-QDs in future devices. Given the variety of groups suffering from U-QDs and the simplicity of this technique, we think that many groups might benefit from our methods.   

The divacancy in SiC: A new solid-state qubit

The nitrogen-vacancy center in diamond has attracted interest due to promising applications as a room-temperature solid-state qubit (the basic unit of a quantum computer). It is, however, desirable to identify defects that possess similar properties, but in alternative semiconductors that are either cheaper or more technologically mature. One notable defect system is the divacancy in 4H-SiC, which has recently been the subject of extensive experimental investigation. In this work, we employ advanced computational methods, particularly density functional theory using a hybrid functional, to investigate the stability and excitation energies of multiple forms of the divacancy in the various polytypes of silicon carbide. The hybrid functional gives band gaps and lattice parameters that are in excellent agreement with experiments. This allows for quantitative predictions of defect levels and zero-phonon line energies for excitation and emission processes, aiding in experimental identification of these defects.   

Large Stark effect for Li donor spins in Si

We study the effect of a static electric field on lithium donor spins in silicon. The anisotropy of the effective mass leads to the anisotropy of the quadratic Stark susceptibility, which we determined using the Dalgarno-Lewis exact summation method [1]. The theory is asymptotically exact in the field domain below Li-donor ionization threshold, relevant to the Stark-tuning Electron Spin Resonance (ESR) experiments [2]. With the calculated Stark susceptibilities at hand, we were able to predict and analyze several important physical effects. In particular, we demonstrate that the Stark effect anisotropy, combined with unique valley-orbit splitting of a Li donor in Si, spin-orbit interaction and specially tuned external stress, may lead to a very strong modulation of the donor spin g-factor by the electric field. Also we investigate the influence of random strains on the g-factor shifts and quantify the random strain limits and requirements to Si material purity necessary to observe the ESR-Stark shifts experimentally. Finally, we discuss possible applications of our results to quantum information processing with Li spin qubits in Si. \\[4pt] [1] A. Dalgarno and J. T. Lewis, Proc. Roy. Soc. 233, 70 (1955).\\[0pt] [2] F. R. Bradbury et al. Phys. Rev. Lett. 97, 176404 (2006).   

Session H31: Focus Session: Topological Insulators: Synthesis and Characterization: Bulk Crystals

Low carrier concentration crystals of the topological insulator Bi$_2$Te$_2$Se

We report the characterization of Bi$_2$Te$_2$Se crystals obtained by the modified Bridgeman and Bridgeman-Stockbarger crystal growth techniques. X-ray diffraction study confirms an ordered Se-Te distribution in the inner and outer chalcogen layers, respectively, with a small amount of mixing. The crystals displaying high resistivity ($> 1~ \mathrm{\Omega cm}$) and low carrier concentration ($\sim 5\times 10^{16}$/cm$^3$) at 4 K were found in the central region of the long Bridgeman-Stockbarger crystal, which we attribute to very small differences in defect density along the length of the crystal rod. Analysis of the temperature dependent resistivities and Hall coefficients reveals the possible underlying origins of the donors and acceptors in this phase.   

Structural study of topological insulator Bi$_{2}$(Se$_{3-x}$Te$_{x})$

With neutron diffraction, we systematically investigated the local and average structures of topological insulator Bi$_{2}$(Se$_{3-x}$Te$_{x})$ (x=0, 1, 1.5, 2, and 3) from 5 to 500 K. The average crystal structure of Bi$_{2}$Se$_{3}$ is rhombohedral R-3m symmetry with 2 unique chalcogen sites. This gives rise to 2 types of Bi-(Se/Te) bonds in quintuple layer composed of Se1-Bi-Se2 --Bi-Se1 layers, covalent Se1-Bi bond with the bonding length of 2.85 {\AA} and ionic Se2-Bi bond with the bonding length of 3.07 {\AA} at 5 K. The Se1-Se1 bonds between quintuple layers are governed by Van der Walls of the length of 3.47 {\AA} at 5 K. With increasing temperature, it is observed that the quintuple layer unit contracts from the anti-parallel motion of the Bi-(Se1)$_{3}$ tetrahedra toward the Se2-center layer. Also such a contraction implies the Van der Waals bonding between quintuple layers weakens with temperature. A similar temperature dependence of atomic structure is observed on Bi$_{2}$Se$_{3}$.   

Hole doping of p-type thin topological insulators

Recent studies of intrinsically n-type topological insulator (TI) Bi$_{2}$Se$_{3}$ demonstrated that doping with Cu introduces electrons into this system, and that in the narrow range of doping Cu$_{x}$Bi$_{2}$Se$_{3}$ becomes a superconductor below Tc 5 K. It is presumed that Cu intercalates into Van der Waals gaps in the tetradymite structure of layered Bi$_{2}$Se$_{3}$, although this remains to be confirmed. We report on hole doping experiments on the naturally p-type Te-based TIs: thin films of Bi$_{2}$Te$_{3}$ and nanocrystalline plates of Sb$_{2}$Te$_{3}$. The samples were iodine doped either during the growth or by the exposure to iodine vapor. Transport measurements on films indicate a very unusual T$^{3}$ temperature dependence of longitudinal resistivity. It drops significantly below 30 K with decreasing temperature, although the drop appears arrested by a metal-insulator transition in the 300 mK range. Magnetization measurements on nanoplates indicate the development of large diamagnetic signal. These results will be discussed in comparison with the superconductivity obtained by Cu doping in Bi$_{2}$Se$_{3}$. * Supported in part by NSF-DMR-1122594.   

Synthesis and Characterization of New Topological Insulators

In this talk, I will show detailed information on synthesizing process and characterization results of new topological insulator (TI) materials with interesting properties. Among the synthesized materials, TlBiSe$_2$ was the first ternary TI and has the largest bulk band gap [1], TlBi(S$_{1-x}$,Se$_x$)$_2$ presents a topological phase transition with unexpected Dirac mass [2], BiTe$_2$Se presents a large bulk resistivity [3], and Bi$_{1.5}$Sb$_{0.5}$Te$_{1.7}$Se$_{1.3}$ has finally achieved the surface-dominated transport in bulk single crystals [4]. It is essentially easy to grow single crystals of all the chalcogenides above, because those compounds melt congruently at relatively low temperatures. Therefore, the melt-growth method is applicable if the raw materials are in a sealed condition, e.g., in a quartz tube. However, crucial techniques for obtaining high-quality samples vary between the systems. Besides the growth method, characterizations of the transport properties, ARPES, the X-ray diffraction, and quantitative chemical analysis will also be presented. \\[4pt] This work is in collaboration with A. A. Taskin, S. Sasaki, Zhi Ren, K. Eto, T. Minami, and Y. Ando (Osaka Univ.), and T. Sato, S. Souma, H. Guo, K. Sugawara, K. Kosaka and K. Nakayama, and T. Takahashi (Tohoku Univ.). \\[4pt] \noindent [1] T. Sato, Kouji Segawa, T. Takahashi, Y. Ando {\it et al.}, Phys. Rev. Lett. {\bf 105}, 136802 (2010). \\ \noindent [2] T. Sato, Kouji Segawa, Y. Ando, T. Takahashi {\it et al.}, Nature Physics, {\bf 7}, 840 (2011). \\ \noindent [3] Zhi Ren, Kouji Segawa, Y. Ando {\it et al.}, Phys. Rev. B (Rapid Comm.) {\bf 82}, 241306(R) (2010). \\ \noindent [4] A. A. Taskin, Kouji Segawa, and Y. Ando {\it et al.}, Phys. Rev. Lett. {\bf 107}, 016801 (2011).   

Transport properties of new Pb-based Topological Insulators

A topological insulator (TI) has a gapped insulating bulk and a gapless metallic surface. So far, such materials as Bi$_{1-x}$Sb$_{x}$, Bi$_{2}$Se$_{3}$, and TlBiSe$_{2}$ are known to be TIs. Recently, several theoretical predictions have been made for new TI materials. In this work, we focus on the Pb-based ternary chalcogenides as new candidate TIs. We have grown a number of single crystals in the systems of Pb-Bi-Se, Pb-Bi-Te, Pb-Sb-Te and Pb-(Sb,Bi)-Te. After selecting single-phase samples, we measured the transport properties to check for their bulk-insulating nature. It was found that Pb(Sb$_{x}$Bi$_{1-x}$)$_{2}$Te$_{4}$ shows a change in the carrier type at around x= 0.55 as inferred both by the themopower and by the Hall effect, but the temperature dependences of the resistivity remained metallic in all the samples studied. We discuss the prospect of making a bulk-insulating material in the Pb-based TIs.   

Millikelvin transport of high quality Bi2Se3 crystals at high pressure

In this study we present electronic transport in Bi2Se3 single crystals in a diamond anvil pressure cell. Reaching temperatures down to 20 mK and pressures exceeding 30 GPa we measure changes in electrical resistivity, magnetoresistance, and the Hall effect and discuss the results as they pertain to topologically interesting behavior.   

Entropy transport in Bi$_{2}$Se$_{3}$

Bi$_2$Se$_3$ and Bi$_2$Te$_3$ are well known compounds in the thermoelectricity community as they present a high figure of merit [1]. Although the thermoelectric power of Bi$_2$Se$_3$ has been extensively studied at high temperature, little is known about its behaviour at the low temperature limit. In this presentation, we will report the results of our entropy measurement of Bi$_{2}$Se$_{3}$ at low temperature and high magnetic field for a bulk carrier concentration from 10$^{17}$cm$^3$ to 10$^{19}$cm$^3$. In all compounds we show significant quantum oscillations in the Seebeck and Nernst responses. Based on the bulk Fermi surface, we propose a simple description of the entropy transport measurement in Bi$_2$Se$_3$ (in the range of concentrations studied). Indeed, Bi$_2$Se$_3$ (non compensated system) appears as a complementary system of bismuth [2] and graphite [3] (compensated systems) to understand the entropy transport in the low carrier concentration limit. \\[4pt] [1] G.S. Nolas et al, Thermoelectrics Basic Principles and New Materials Developments, Springer.\\[0pt] [2] K.Behnia et al, PRL, 98, 166602 (2007)\\[0pt] [3] Z.Zhu et al, Nature Physics, 6, 26 (2009)   

Search and Design of Topological Insulators by High-throughput Method

Topological insulators (TIs) have attracted enormous interest because of their novel surface conduction effects which are protected by time-reversal symmetry. A high-purity TI with a highly insulating bulk is necessary to realize the potential practical applications of this class of materials. Therefore, numerous attempts are being made to search for TIs with desired properties (e.g., a large band gap and amenability to high-quality crystal growth). In this presentation, we will introduce an effective high-throughput approach to search and design TIs from a vast electronic structure database such as Inorganic Crystal Structure Database (ICSD) and Heusler alloys.   

Effects of Intrinsic Defects on Topological Insulator Behavior: Theory and Experiments on Ternary Tetradymite Compounds

Ternary tetradymites Bi$_{2}$Te$_{x}$Se$_{(3-x)}$ are predicted to be topological insulators via density functional theory (DFT) surface band structure calculations. Experimentally, we find that Bi$_{2}$Se$_{3}$ and Bi$_{2}$Te$_{3}$ form a continuous solid solution at the two non-equivalent group VI sites with different site preferences for Se and Te. The DFT formation energies for ordered and partially ordered compounds agree well with experimental data. Importantly, we calculated the intrinsic defect formation energies of binary and ternary tetradymites, and find they correlate well with the bulk conductivity measurement. Angle resolved photoemission spectroscopy confirms the existence of the Dirac cone in the surface band of Bi$_{2}$Te$_{2}$Se.   

Transport properties of chalcogenide and related topological insulator bulk crystals

Three-dimensional (3D) topological insulators (TIs) have attracted strong theoretical and experimental interest in the condensed matter community. We have used the Bridgman method to synthesize various chalcogenide and related 3D TI crystals. We present an experimental survey of a group of binary, tertiary, and quaternary bulk crystals of various compositions of Bi, Sb, Ge, Se, Te, and S. Our survey includes systems without intentional doping, as well as systems doped with magnetic and non-magnetic impurities. We present magnetotransport data over a range of temperatures. We also measure thin films exfoliated from these bulk crystals to examine efficacy of carrier modulation through an applied gate voltage. We discuss the results of these measurements in the context of TI properties, with the ultimate goal of identifying systems that display electronic transport properties consistent with an insulating bulk and spin-helical Dirac fermion surface states.   

Bulk Excitations in single crystal Bi$_{2}$Se$_{3}$: Electron Energy Loss Spectroscopy Study

Bi$_{2}$Se$_{3}$ with larger figure of merit has been used in room-temperature thermoelectric applications. Furthermore, it is also one of a handful known topological insulators. Most of recent studies were focused on the topological surface states of Bi$_{2}$Se$_{3}$ while few of them studied the electronic excitations of bulk Bi$_{2}$Se$_{3}$. Here, we report studies of electronic excitations of single-crystal Bi$_{2}$Se$_{3}$ by electron energy-loss spectroscopy (EELS). EELS spectrum in bulk Bi$_{2}$Se$_{3}$ reveals several spectral features at $\sim $7, $\sim $16.8, $\sim $26.4 and $\sim $28.4 eV. The peaks at $\sim $26.4 and $\sim $28.4 eV are due to excitations from Bi 5$d$ electrons. The $\sim $7 and $\sim $16.8 eV peaks are easily identified as bulk-plasmon excitations according to the frequency-dependent complex dielectric function derived from experimental spectrum with Kramers-Kr\"onig analysis. Furthermore, momentum ($q)$-dependent EELS spectra along [110], [300] and [001] directions were also performed in this study. When momentum transfer $q$ is parallel [110] and [300] directions, the 16.8 eV-peak (bulk plasmon) significantly shift to higher energy (up to 23 eV) with increasing $q$ values, while this peak shifts less than 1 eV when momentum transfer $q$ is parallel to [001] direction, revealing the distinct anisotropy of bulk plasmon dispersions. Detailed characteristics of this anisotropic behavior will also be discussed.   

Insights on the electronic and vibrational properties of Bi(111) from first principles

Bi(111) is known to have surface electron carriers close to $\Gamma $ as well as hole carriers at $\Gamma $ and along the $\Gamma $M directions. The lattice dynamics of Bi(111) is however largely unknown. We investigate both the electronic structure and lattice dynamics of Bi(111) films via density-functional-theory and density-functional-perturbation-theory calculations taking into account the spin-orbit coupling (SOC). While the splitting of the branches is dominated by the SOC almost everywhere along the $\Gamma $M direction, around the zone boundary (M), the delocalized character of this state plays an important role. Reducing the thickness of a film decreases the band gap progressively. At $\sim $3-nm thickness, the highest valence band re-crosses the Fermi level and creates extra electron pockets. We find, however, that the lattice dynamics of Bi(111) is robust with respect to film thickness. Bi(111) has a number of ``high-lying'' surface modes in the optical band almost everywhere along the $\Gamma $KM and $\Gamma $M directions, most notably, a vertical mode slightly above the bulk band. Surface acoustic modes are also present as well as some ``low frequency'' optical modes in small regions of the zone. A comparison with recent measurements will be presented, as well as the possible implications on the electron-phonon coupling.   

Electron beam irradiation effect on Bi$_{2}$Se$_{3}$ topological insulator nanodevices

Nanofabrication is found to introduce excess charge carriers and results in a strong metallic state in Bi$_{2}$Se$_{3}$. The uncontrolled carrier density causes the Fermi level to rise to the conduction band. To verify the effect of the electron beam lithography (EBL), we have measured the carrier density of Bi$_{2}$Se$_{3}$ nanodevices before and after EBL with a range of electron beam energies and doses and find that the Fermi level rises in both n- and p-type devices. To effectively control the position of the Fermi level, we have developed a nanofabrication-free technique for Bi$_{2}$Se$_{3}$ nanodevices, with which the initial state of the bulk materials can be well preserved. We deliberately choose p-type Ca-doped Bi$_{2}$Se$_{3}$ devices and systematically introduce more electrons using successive EBI. Resistiviy temperature dependence shows that the Fermi level position is gradually tuned from the valence band into the band gap. Further fine tuning of the Fermi level is accomplished by applying a gate voltage to the devices. An increase of a factor of 10 in mobility has been observed in the device as the Fermi level is brought into the band gap, which is consistent with the suppressed backscattering of the surface states in topological insulators.   

Session H32: Focus Session: Dielectric, Ferroelectric, and Piezoelectric Oxides - Conducting Domain Walls and Conduction Mechanisms

Conduction of topologically-protected charged ferroelectric domain walls

We report on the observation of nanoscale conduction at ferroelectric domain walls in hexagonal HoMnO$_3$ protected by the topology of multiferroic vortices using \textit{in situ} conductive atomic force microscopy, piezo-response force microscopy, and kelvin-probe force microscopy at low temperatures. Conductance spectra reveal that only negatively charged tail-to-tail walls, in contrast to positively charged head-to-head walls, exhibit ohmic-like conduction in addition to Schottky-like rectification. Our results pave the way for understanding the semiconducting properties of the domains and domain walls in small-gap ferroelectrics.   

Electrical dressing of domain walls in hexagonal ErMnO$_3$

Domain walls are natural mobile nanoscale objects that can exhibit structural, physical, and chemical properties which drastically differ from the surrounding bulk material. This applies to a large variety of phenomena including chemical/electrical transport, multiferroicity, or superconductivity. Unfortunately, in contrast to bulk materials, very little is known about involved length scales and control parameters when it comes to domain walls and experimental evidence is highly desirable. Here, we report on electrical dressing of trimerization-polarization walls in ErMnO$_3$. Using piezoforce-response microscopy and conductive atomic force microscopy we reveal that two characteristic length scales are to be distinguished: A first one corresponding to the structural / ferroelectric changes occurring at the wall and a second one referring to the associated electric properties. Furthermore, we demonstrate the response of the electrically dressed walls to external electric fields and develop a model that explains this response. Our results are expected to generally apply to domain walls in ferroelectric semiconductors and provide new insight into the interplay of charge and lattice degrees of freedom at domain walls.   

Anisotropic conductance at improper ferroelectric domain walls

Domain walls in ferroelectric oxides hold great potential for the development of new device paradigms in oxide nanoelectronics due to their field-tunable functionality. They are also of fundamental interest for studies of ferroic and low-dimensional systems physics. We investigate the electronic conductance of ferroelectric domain walls in the improper ferroelectric ErMnO3. We show that the conductance is a continuously tunable function of the domain wall orientation, with a range of an order of magnitude. We explain the observed behavior using first-principles density functional and phenomenological theories, and relate it to the extraordinary stability of the inherent head-to-head and tail-to-tail domain walls in hexagonal manganites, which is a direct consequence of the improper ferroelectric character of these materials.   

Conduction at domain walls in insulating Pb(Zr$_{0.2}$Ti$_{0.8}$)O$_3$ thin films

Ferroic domain walls are intrinsically nanoscale and often present functional properties beyond those of their parent material. One of the most striking examples is the recent discovery of electrical conduction\footnote{Seidel et al., Nat. Mat. {\bf8}, 229 (2009)} at domain walls in multiferroic BiFeO$_3$. Different scenarios have been proposed to explain the observed conduction, generally relating it to the complex nature of domain walls specific to BiFeO$_3$.\footnote{Lubk et al., PRB {\bf80}, 104110 (2009); Chiu et al., Adv. Mat. {\bf23}, 1530 (2011); Farokhipoor et al., PRL {\bf107}, 127601 (2011)} Here, we report on scanning probe microscopy studies of domain-wall-specific conduction in thin films of tetragonal ferroelectric (PZT). Our measurements show nonlinear asymmetric current-voltage characteristics with strong thermal activation at $T>150$ K. Moreover, the average current signals remain stable over the duration of measurement (up to four days). In light of recent transmission electron microscopy measurements at 180$^{\circ}$ domain walls in PZT,\footnote{Jia et al., Sci. {\bf331}, 1420 (2011)} we discuss the possible conduction mechanisms, highlighting the role of electrode asymmetry and microscopic domain wall structure promoting local defect segregation.   

Interplay between polarization and conductivity in BiFeO$_{3}$ thin films

BiFeO$_{3}$ (BFO) is a rhombohedrally distorted, ferroelectric, antiferromagnetic perovskite and one of the few room temperature multiferroics. We've previously reported on conduction at 71$^{o}$ domain walls in BFO thin films grown on SrRuO$_{3}$-buffered SrTiO$_{3}$ substrates. For clarifying the origin of conductivity in domain/domain walls, the conduction mechanisms have been extensively studied. The large current regime is determined by Schottky emission from the tip. The migration of oxygen vacancies to the domain walls lowers the Schottky barrier heights at the interface with the metallic tip compared to that in the domains, which results in the observed difference of conductivity in domains and domain walls. In this work we investigate the tunability of the conductivity upon changes in the electrode's work function, as well as the interplay between polarization and conductivity in BFO thin films.   

Tunable Metallic Conductivity in Ferroelectric Nanodomains

Domain wall conductivity in ferroelectric and multiferroic oxides is an essential example of new electronic properties created by topological defects. So far electron transport through domain walls in canonical BiFeO$_{3}$ and PbZr$_{0.2}$Ti$_{0.8}$O$_{3}$ (PZT) ferroelectrics has been dominated by thermally activated hopping, concealing the enabling physics and limiting potential applications. We will present a pioneering observation of metallic conductivity in nanoscale ferroelectric domains in PZT, that unambiguously identifies a new conduction channel created through the bulk of the oxide film [1]. From a corollary theoretical analysis, we conclude that metallic conductance is enabled by the interplay of charging and flexoelectric effects at tilted and curved walls of the nanodomain. Furthermore, both type and density of carriers can be tuned by manipulation of the order parameter. Thus, a new family of electronic properties may be found in multiferroic and topologically nanostructured complex oxides. [1] Maksymovych et al, \textit{Nano Lett}. in review (2011). Research conducted at the Center for Nanophase Materials Sciences, sponsored by BES, U. S. DOE.   

Ultrafast p-d charge-transfer carrier dynamics of multiferroic BiFeO3

We report first comprehensive understanding of ultrafast carrier dynamics in bulk single crystal BiFeO$_{3}$. From a wavelength dependent optical pump-probe measurement, we find that the photoexcited carriers relax to the bottom of band through electron-phonon coupling with a $\sim$1 ps time constant that does not significantly change across the antiferromagnetic transition. Following relaxation, carriers leave the conduction band or original excited electronic configuration and decay via radiative recombination, which is supported by our photoluminescence spectroscopy, reported for the first time.   

Photocurrent effect of epitaxial tetragonal-like BiFeO$_{3}$ thin film

Photovoltaic effect in ferroelectrics has recently received many attentions due to potential applications related to optoelectronic devices and solar cells. Here we report photocurrent effect of highly elongated ``tetragonal-like'' BiFeO$_{3}$ thin films grown on LaAlO$_{3}$ (001) substrates using pulsed laser deposition technique. Spatially resolved photocurrent measurements are performed with varying photon wavelength and polarization. Being combined with local ferroelectric domain structure by piezoresponse force microscopy, the spatially resolved techniques make a pathway to explore inter-relation between electric polarization and photon polarization. This study might deepen our understating of light induced conduction phenomena in ferroelectrics.   

Intrinsic defects in BiFeO3: Energetics and implication for magetism

We investigate energetics of the intrinsic defects in bulk multiferroic BiFeO3 and explore their implication for magnetization in this compound using a first-principles approach based on density functional theory. We find that dominant defects in oxidizing conditions are Bi and Fe vacancies and in reducing conditions are O and Bi vacancies. When enforcing charge neutrality, the calculated carrier concentration shows that the BiFeO3 grown in oxidizing conditions has p-type conductivity. The conductivity decreases with oxygen partial pressure and the material becomes insulating with tendency for n-type conductivity. We find that the Bi and Fe vacancies produce a magnetic moment of $\sim $1 $\mu $B and $\sim $5 $\mu $B per vacancy, respectively, for p-type BiFeO3 and none for insulating BiFeO3. O vacancies do not introduce any moment for both p-type and insulating BiFeO3. Calculated magnetic moments due to intrinsic defects are consistent with those reported experimentally for the bulk BiFeO3, however do not explain the large magnetization observed in some experiments on thin-film BiFeO3.   

Dielectric screening enhanced Hall mobility in doped ferroelectrics

A low electron mobility is the key limitation that prevents widespread device applications of complex oxide materials. However, in some perovskites, for example SrTiO$_3$ and KTaO$_3$, high mobilities in excess of 10,000 cm$^2$ s$^{-1}$ V$^{-1}$ are measured. Together with this dramatic increase in mobility as temperature is lowered, their dielectric constants also increase from a few hundred at room temperature to near 20,000 at low temperatures, suggesting a correlation between the dielectric constant and the mobility. By using electron-doped ferroelectric crystals of composition KTa$_{1-x}$Nb$_x$O$_3$, where the ferroelectric transition temperature can be tuned by changing the Ta:Nb ratio, we demonstrate an enhancement of the Hall mobility by a factor of 2-3 at the Curie temperature up to room temperature. We conclude that the mobility in these doped ferroelectrics peaks at the Curie temperature due to the increased dielectric constant, which reduces charge carrier scattering by impurities. Enhanced mobility could result in faster oxide transistors, boost the performance of thermoelectric devices, and enable more efficient photovoltaic materials. Supported by ORNL's LDRD program (W.S., H.M.C., V.C., G.E.J.), U.S. DOE, BES, MSED (M.A.M., B.C.S.) and SUFD (M.D.B., I.N.I.).   

The persistence of ferroelectric distortions in electron-doped BaTiO3: microscopic origins and critical behavior

To explore possible novel applications of the prototypical ferroelectric oxides we perform theoretical studies of electron-doping in BaTiO3. The presence of conduction electrons in a ferroelectric opens the possibility of bi-stable behavior directly in a conducting material which may lead to new functionalities. It is known, however, that conduction electrons screen the long range Coulomb interactions responsible for polar instabilities. Interestingly though, our first-principle density functional calculations reveal that ferroelectric distortions can persist in electron-doped BaTiO3 up to 0.01 e/unit cell, consistent with experimental results [1], suggesting that ferroelectricity and conductivity can coexist. To elucidate the competition between the long range Coulomb interactions and the short range bonding effects we have developed an adequate electrostatic model. Using this model, we reproduce the polarization vs. doping behavior obtained from first-principles and derive an analytical expression for the critical doping above which ferroelectric distortions disappear. [1] T. Kolodiazhnyi et al, Phys. Rev. Lett. 104, 147602 (2010).   

Multiphysics model of semiconducting ferroelectrics and its application to memory devices

Ferroelectrics are used in many electronic devices, in particular as transistors for ferroelectric memory devices. The behavior of these materials are often described via the classic time-dependent Ginzburg Landau model, where they are treated as insulators. However, it is well known that ferroelectrics are in fact wide band-gap semiconductors. It then follows that capturing the key aspects of semiconductor physics--band bending at the interface, Fermi levels, depletion layers, require ferroelectrics to be treated as semiconductors. In this work, we introduce a model that addresses these difficulties, yet at the same time is consistent with both the time-dependent Ginzburg Landau model and the classic drift-diffusion model in semiconductors. Unlike other models, our model makes no a priori assumptions on the space charge and polarization distributions and is not restricted to equilibrium profiles. We first demonstrate that charge carriers migrate to neutralize electric fields across 90\r{ } domain walls. Finally, we attempt a full simulation of a ferroelectric transistor and model current flow, electric field and polarization distributions.   

Conduction mechanism in BiFeO$_{3}$-CoFe$_{2}$O$_{4}$ columnar nanostructure

Multiferroic materials, which possess interaction between more than one ferroic ordering parameters, had attracted great scientific and technological interests. Among the bi-phase magneto-electric nanostructures, BiFeO$_{3}$-CoFe$_{2}$O$_{4}$ (BFO-CFO) is a model system with ferroelectricity and ferrimagnetism coupling to each other through stress mediation. In this study, we investigated the electron transport behavior and the leakage-current mechanism in high quality nano-composite BFO-CFO thin films. The CFO nanopillars were heteroepitaxially embedded in a BFO matrix grown on SrTiO$_{3}$ substrates. Macroscopic vertical transport result showed the interface limit model was the dominant mechanism of the large leakage. Local conduction in epitaxial BFO-CFO nanostructures was studied by conducting atomic force microscope (C-AFM) while the nature of band structure variation was demonstrated by scanning tunneling microscope (STM). This study provides a basic explanation of leakage mechanism in self-assembled composite material system.   

Current-Controlled Negative Differential Resistance Due to Joule Heating In Tio$_2$

We show that Joule heating causes current-controlled negative differential resistance (CC-NDR) in TiO2 memristive systems by constructing an analytical model of the current-voltage characteristics based on polaronic transport for Ohm's law and Newton's law of cooling and fitting this model to experimental data. This threshold switching is he ``soft breakdown'' observed during electroforming in TiO2 and other transition-metal oxide based memristors, as well as a precursor to ``ON'' or ``SET'' switching of unipolar memristors from their high to their low resistance states. The shape of the V-I curves is a sensitive indicator of the nature of the polaronic conduction, which apparently follows an adiabatic regime [1]. \\[4pt] [1] A.S. Alexandrov, A.M.Bratkovsky, B.Bridle, S.E.Savel'ev, D. Strukov, and R.S.Williams, Appl. Phys. Lett. 99, xxx (2011).   

Physical and Electrical Characterization of HfO$_{2}$, HfSiO$_{4}$, and ZrSiO$_{4}$ Memristors Based on Sol-Gel Synthesis

To date, most memristive devices have been fabricated by using TiO$_{2}$ or TaO$_{x}$ dielectric films. In order to explore the possible advantages of other high-$\kappa$ dielectrics in memristive devices, memristors were fabricated with HfO$_{2}$, HfSiO$_{4}$, and ZrSiO$_{4}$ layers synthesized from sol-gels. X-ray photoelectron spectroscopy measurements are consistent with reported spectra of HfO$_{2}$, HfSiO$_{4}$, and ZrSiO$_{4}$ films, but contain significant amounts of carbon. The films also have low densities and are flat, as measured by vacuum ultraviolet spectroscopic ellipsometry and optical profilometry measurements, respectively. This flat morphology is different from previous solution-processed dielectric films that exhibited rough surfaces [1]. Transmission electron microscopy measurements were also used to characterize these dielectric films. Current-voltage measurements indicate that, despite the contamination, the memristors exhibit nonvolatile bipolar resistive switching. The retention times measured for these memristors are $\sim$10$^{6}$ s. Capacitance and conductance measurements of these memristors indicate differences between the ON and OFF states, which will be discussed further. \\ \newline [1] J.L. Tedesco, et al., ECS Trans. $\textbf{35}$, 111 (2011).   

Session H33: Focus Session: Scaleable Technologies for Photovoltaics

Solar Glitter -- Microsystems Enabled Photovoltaics

Many products have significantly benefitted from, or been enabled by, the ability to manufacture structures at an ever decreasing length scale. Obvious examples of this include integrated circuits, flat panel displays, micro-scale sensors, and LED lighting. These industries have benefited from length scale effects in terms of improved performance, reduced cost, or new functionality (or a combination of these). In a similar manner, we are working to take advantage of length scale effects that exist within solar photovoltaic (PV) systems. While this is a significant step away from traditional approaches to solar power systems, the benefits in terms of new functionality, improved performance, and reduced cost for solar power are compelling. We are exploring scale effects that result from the size of the solar cells within the system. We have developed unique cells of both crystalline silicon and III-V materials that are very thin (5-20 microns thick) and have very small lateral dimensions (on the order of hundreds of microns across). These cells minimize the amount of expensive semiconductor material required for the system, allow improved cell performance, and provide an expanded design space for both module and system concepts allowing optimized power output and reduced module and balance of system costs. Furthermore, the small size of the cells allows for unique high-efficiency, high-flexibility PV panels and new building-integrated PV options that are currently unavailable. These benefits provide a pathway for PV power to become cost competitive with grid power and allow unique power solutions independent of grid power.   

Perspective on the Practical Limits and Roadmap for Nanostructured Photovoltaics

The practical efficiency limits for nanostructured photovoltaics including organic small molecule, dye-sensitized, polymer, and colloidal-quantum-dot architectures are assessed \textit{a posterori}. Five decades since Shockley and Queisser derived the theoretical power conversion efficiency limit of single-junction photovoltaic cells, researchers have still not demonstrated such high performance for any photovoltaic device system. Hence, in evaluating the achievable performance of a comparatively new photovoltaic technologies, such as nanostructured PVs, it is prudent to estimate the upper limit of achievable efficiencies based on trends of the best technical demonstrations across the nanostructured platforms. This analysis is utilized to give a clear perspective on the potential market viability of these technologies in the near future and outline the challenges necessary to overcome this threshold. These technologies are compared and contrasted to provide an overview for the potential of each for reducing thermal losses with ``Third Generation'' concepts accessible to nanostructured PVs that can subsequently impact cost structures.   

Role of radiative recombination in 1 eV GaInNAs solar cells

High quality GaInNAs p-i-n solar cells with depletion widths in excess of 1$\mu $m for material absorbing in the practically important 1eV band gap regime are presented [1]. This is achieved through optimization of post-growth rapid thermal annealing at a temperature of $\sim $ 910\r{ } C. Despite the improvements in material quality evidenced by a low background impurity concentration and improved minority carrier collection, the external quantum efficiency remains limited to $\sim $ 50{\%}. This is attributed to losses due to efficient radiative recombination in the bulk GaInNAs intrinsic region enhanced via localization of carriers in alloy fluctuations. \\[4pt] [1] Sellers \textit{et al}. Applied Physics Letters \textbf{99}, 151111 (2011)   

Device architectures for efficient photovoltaics from hard-to-dope semiconductors

In the search for cheap and efficient next-generation solar cells, much attention and effort has focused on semiconductor absorber materials which are difficult to dope both p and n, because of self-compensation, trap creation, or other effects. Such materials include oxides, sulfides, nanoparticles, organics, and so on. Even when the material itself has desirable electrical and optical properties for photovoltaic performance, the lack of a p-n homojunction architecture hampers device efficiency. Heterojunctions are a frequent solution, but compatible semiconductors are often unavailable or suboptimal. Therefore, we have explored new, cost-effective device architectures that promise performance comparable to a p-n homojunction, but which require neither bipolar doping nor compatible materials for heterojunctions. These architectures have the potential to bring emerging hard-to-dope semiconductors into technological and commercial relevance.   

Efficiency Analysis and Demonstration of Split-Junction Photovoltaic Solar Cells

Recently, it has been proposed that separating solar radiation with a split-junction solar cell can result in higher overall conversion efficiencies, than are possible for monolithic designs. This hypothesis is investigated by simulating and analyzing 2+1 split junction cells for efficiency comparisons with theoretical and actual multi-junction cells. Ideal band-gaps for simultaneously operating photovoltaic and thermophotovoltaic cells have been determined. With the new configuration, it is shown that the efficiency achievements previously set by Ge/InGaAs/InGaP cells can be surpassed. A total increase in power output is observed during field tests using a Cassegrain split-junction concentrator with a dichroic lens (1.1 micron cutoff wavelength). Proposed benefits such as reduced heat load on the solar cell and ease of lattice constant matching in cell design are also validated. Additionally, with the flexibility of the concentrator assembly, it is shown that similar split-junction configurations with matching dichroic lenses allow for significant improvements in high efficiency solar cell technology.   

Electronic structure and optical spectra of Bulk and Nanocrystalline CuInS$_2$

Chalcopyrite CuInS$_2$ is a promising candidate for semiconductor photovoltaic devices. Here we study the behavior of the electronic structure and optical spectra of nanocrystalline CuInS$_2$. We determine the bulk band structure using the HSE06 Hybrid Density Functional as implemented in the Vienna Ab-initio Simulation Package (VASP). We find an equilibrium structure in good agreement with experiment, and a direct band gap of 1.2~eV, as compared to the experimental value of 1.5~eV. The band gap is extremely sensitive to the position of the sulfur atoms, which suggests that it can be controlled in part by the doping of the In site with Ga. CuInS$_2$ is nearly cubic, so we fit the first-principles band structure to a k.p Hamiltonian with invariants consistent with the departure for cubic symmetry. This Hamiltonian is then used to describe the hole spectra in a CuInS$_2$ nanocrystal. We show the symmetry breaking inherent in the chalcopyrite structure can activate the optically passive ground hole state in the nanocrystal, and discuss the resulting optical behavior of the nanocrystal.   

Impact of a high resistivity transparent (HRT) layer on ultra-thin CdTe devices

Pushing the limits of the absorber layer thickness to submicron levels without compromising the efficiency in CdTe devices is important for several reasons. Reducing the thickness of the CdTe layer can increase the manufacturing speed and lower the material usage, production time, cost and energy needed for production. Thickness fluctuations in these submicron devices can lead to shunting when these fluctuations are comparable to the absorber layer thickness. Introducing a high resistivity transparent (HRT) layer in the device structure can reduce such shunting. We find that the impact of the HRT layer is higher for ultra-thin absorber layers. In this study, the effect of using sputtered ZnO as the HRT layer was investigated for devices with a 0.2 $\mu $m CdTe layer. Our results show a 40{\%} increase in efficiency in the CdTe devices with the HRT layer compared to the devices without the HRT layer. The increase in efficiency is primarily from improved fill factor due to an increase in shunt resistance from 0.15 kohm-cm$^{2}$ to 0.32 kohm-cm$^{2}$. As a result of increased shunt resistance there is an increase in the efficiency as well as a substantial increase in the yield for small dot cells.   

Solution-based synthesis of crystalline silicon thin films from liquid silane inks

Silicon (Si) dominates the photovoltaics industry and there is a need for new approaches that can significantly reduce fabrication cost. In this context, we report a non-vacuum, solution-based process for the synthesis of crystalline silicon (c-Si) thin films from liquid cyclohexasilane (CHS) in a platform that is readily applicable to large-area flexible devices. UV-polymerization during spin coating leads to the formation of thin films, which were crystallized via thermal and laser annealing. Structural changes in the films were examined using SEM, AFM and Raman spectroscopy. Subsequent chemical annealing through atmospheric-pressure hydrogen plasma treatment led to a four-decade enhancement in film conductivity, which we attribute to a disorder-order transition in a bonded Si network.   

Device modeling for organic solar cells

Organic solar cells (OSCs) are expected to play an important role in addressing our future energy needs due to their low cost and low processing requirements compared to inorganic solar cells (ISCs). However the efficiency of OSCs is still too low in comparison with ISCs for widespread applications. The biggest loss of quantum efficiency (QE) in OSCs is due to the limited free carrier generation occurring at the donor-acceptor (D-A) interface. Excitons (bound electron-hole pairs) are generated in the bulk by photo-absorption, but only a portion of them reach the D-A interface where they can dissociate into free charge carriers. Therefore, better understanding and control of exciton diffusion, free carrier generation and recombination are critical in order to improve QE for OSCs. To elucidate the physics of OSCs and aid in experimental studies, we developed a drift-diffusion model to describe the dynamics of excitons and free charge carriers. Our model predicts the performance of OSC devices by calculating their QE and current-voltage curves (I-V), as well as many other important physical quantities, such as the internal electric field, and the concentration and flux of excitons and free carriers. The effect of exciton and free carrier mobility, device temperature, and layer thickness, will be discussed. Furthermore, the exciton dissociation mechanism widely described by Onsager's model, will be investigated in detail.   

Combined Photo and Thermionic Energy Conversion with Doped Diamond Electron Emitters

Conversion of heat into electrical energy has been demonstrated using low effective work function diamond films achieved with n-type doping and surface hydrogen termination. Recently, visible light photo-electron emission has been demonstrated from the same diamond, and this work suggests that this effect could be utilized for a new approach to solar energy conversion namely combined photo and thermionic energy conversion. This work presents a spectroscopic study of photo- and thermionic electron emission from nitrogen doped diamond films on silicon substrates. In this experiment the diamond samples are heated from 100\r{ }C to 500\r{ }C, while being illuminated with light from 240 to 600 nm. The emission spectra show a significant increase of photo-emission intensity with elevated temperature and a lowering of the effective work function. The results are discussed in terms of the photo and thermal excitation, the carrier transport and the electron statistics. The results indicate the potential of diamond films in a combined photo and thermionic energy conversion solar cell. This research is supported through the Office of Naval Research.   

Metal nanoparticle-graphene superstructures as electrodes for solar cells

We present metal nanoparticle and graphene superstructures as a potential electrode for solar cells. We show the effect of various metallic nanoparticles on the work function, sheet resistance, and optical properties of graphene layers. The geometries studied include nanoparticles on single layer graphene, and embedded in a graphene sandwich. We discuss the application of these electrodes to organic and silicon solar cells.   

Study of the crystalline phases in paste coating deposition of CIGS

One or two microns of CIGS can absorbs most of the incident solar radiation because CuIn$_{1-x }$Ga$_{x}$Se$_{2}$ have a direct band gap with a high absorption coefficient. The theoretical predictions explain that the optimum photovoltaic performance should be provided by a high gallium concentration, but experimentally is observed that above 30{\%} of Ga the efficiency is reduced. This contradictory behaviour is not completely understood. Using paste coating technique for CIGS deposition, we have recently shown a strong correlation among Ga concentration, structural properties and compound stoichiometry. The desired stoichiometric compound will be obtained varying the concentration of the basic elements in the paste. Our diffraction data show that the maximum of the crystalline phase is reached when CIGS have a Ga concentration higher. Furthermore the SEM EDX quantitative analysis performed on the same samples have shown the presence of different phases. Such a phases separation find a theoretical explanation as ODC (Ordered Defect Compound) in Cu-poor system. X-ray absorpition spectroscopy reveals the structure around a specific atomic species and allow us to understand the type of phases in chalcopyrite strucuture.   

A large-area, LED-based spectral response measurement system for solar PV device characterization

Accurate and reliable measurement of the spectral responsivity (SR) of a solar cell is an important step in evaluating the electrical performance of competing photovoltaic (PV) technologies. We have investigated ways to measure the spectral responsivity, and hence the external quantum efficiency, of solar cells using measurement techniques that employ light emitting diodes (LEDs). Our setup includes one or more plates of compactly-installed, high-powered LEDs each containing up to 32 different LEDs that span the wavelength range of 375 nm to 1200 nm. Each LED plate is placed at the entrance of a tapered, highly reflective light guide for light mixing and large-area projection. Two unique measurement techniques have been investigated at NIST. The first technique consists of an LED sweep algorithm where a pulsed signal is applied to a given LED and the photogenerated current from the device under test is recorded using a lock-in technique. In the second SR technique, 32 variable-frequency, pulsed signals are applied to all LEDs at the same time, while recording the photogenerated current by a spectrum analyzer in the frequency domain. We will describe the uniqueness and advantages offered by each technique in detail and compare the accuracy of the two methods. A scheme for providing light bias and its impact on the SR measurements will be reported.   

Session H34: Focus Session: Nano II: Nanoscale Materials and Properties I

Nanoelectronics Meets Biology

Nanoscale materials enable unique opportunities at the interface between the physical and life sciences, and the interface between nanoelectronic devices and biological systems makes possible communication between these two diverse systems at the length scale relevant to biological function. In this presentation, the development of nanowire nanoelectronic devices and their application as powerful tools for the life sciences will be discussed. First, a brief introduction to nanowire nanoelectronic devices as well as comparisons to other electrophysiological tools will be presented to illuminate the unique strengths and opportunities enabled at the nanoscale. Second, illustration of detection capabilities including signal-to-noise and applications for real-time label-free detection of biochemical markers down to the level of single molecules will be described. Third, the use of nanowire nanoelectronics for building interfaces to cells and tissue will be reviewed. Multiplexed measurements made from nanowire devices fabricated on flexible and transparent substrates recording signal propagation across cultured cells, acute tissue slices and intact organs will be illustrated, including quantitative analysis of the high simultaneous spatial and temporal resolution achieved with these nanodevices. Specific examples of subcellular and near point detection of extracellular potential will be used to illustrate the unique capabilities, such as recording localized potential changes due to neuronal activities simultaneously across many length scales, which provide key information for functional neural circuit studies. Last, emerging opportunities for the creation of powerful new probes based on controlled synthesis and/or bottom-up assembly of nanomaterials will be described with an emphasis on nanowire probes demonstrating the first intracellular transistor recordings, and the development of ``cyborg'' tissue. The prospects for blurring the distinction between nanoelectronic and living systems in the future will be highlighted.   

Charge Retention by Monodisperse Gold Clusters on Surfaces Prepared Using Soft Landing of Mass Selected Ions

Monodisperse gold clusters have been prepared on surfaces in different charge states through soft landing of mass-selected ions. Gold clusters were synthesized in methanol solution by reduction of a gold precursor with a weak reducing agent in the presence of a diphosphine capping ligand. Electrospray ionization was used to introduce the clusters into the gas-phase and mass-selection was employed to isolate a single ionic cluster species which was delivered to surfaces at well controlled kinetic energies. Using in-situ time of flight secondary ion mass spectrometry (SIMS) it is demonstrated that the cluster retains its 3+ charge state when soft landed onto the surface of a fluorinated self assembled monolayer on gold. In contrast, when deposited onto carboxylic acid terminated and conventional alkyl thiol surfaces on gold the clusters exhibit larger relative abundances of the 2+ and 1+ charge states, respectively. The kinetics of charge reduction on the surface have been investigated using in-situ Fourier Transform Ion Cyclotron Resonance SIMS. It is shown that an extremely slow interfacial charge reduction occurs on the fluorinated monolayer surface while an almost instantaneous neutralization takes place on the surface of the alkyl thiol monolayer. Our results demonstrate that the size and charge state of small gold clusters on surfaces, both of which exert a dramatic influence on their chemical and physical properties, may be tuned through soft landing of mass-selected ions onto selected substrates.   

Transitioning the Superfluid Helium Droplet Assembly into a Technology: Synthesis of Nanometer Scale Energetic Films using SHeDA

Since the pioneering work of the Toennies, Scoles, and Northby groups in the early 1990's, dozen of instruments around the world have been constructed to produce and study beams of superfluid helium nanodroplets. The technique has been exploited to shed light on a wide range of topics in chemical physics such as atomic scale manifestations of superfluidity, chemistry at ultra-low temperatures, and the assembly of exotic Van der Waals complexes to name a few. The helium droplet method has been considered for more applied use as a tool for isotope enrichment, low-fragmentation ionization mass spectrometry, and synthesizing/depositing core-shell spintronic nanoparticles. Indeed, the helium droplet methodology is in the midst of transitioning from a novel cryogenic nano-scale matrix in which to perform fundamental research into a technology for synthesizing, characterizing, and manipulating material. This talk describes our efforts to engineer a robust, user-friendly, broadly-tunable helium droplet apparatus capable of synthesizing composite nanoparticles and depositing them into films. This device is now being used to assemble and deposit metallic nanoparticles, and the efficiency of the process is being investigated. The physical details of the design, performance of the instrument, and our progress at understanding the deposition process will be presented. Distro 96ABW-2011-0266.   

Screened Charge Model in the Treatment of Electrostatic Interactions

Partial atomic charges play an important role in molecular simulations of complex systems, and they are widely used to compute the electrostatic interactions in various methods. We propose a screened charge model to include charge penetration and screening effects in electrostatic modeling. In the screened charge model, the atomic charge density of an atom in a molecule is represented by a spherical smeared charge plus a point charge at the nucleus. The new model is illustrated for the electronically embedded combined quantum mechanical and molecular mechanical (QM/MM) calculations and for the electrostatically embedded many-body (EE-MB) method. For a test set of 40 complexes, the mean unsigned error of QM/MM electrostatic interactions between QM and MM regions is reduced from 8.1 to 2.8 kcal/mol and that for QM/MM induction interactions from 1.9 to 1.4 kcal/mol. In a test of five water hexamers, the mean unsigned error of the EE-MB binding energies of the clusters is decreased by a factor of 2 at both the pairwise additive (PA) and three-body (3B) levels. Moreover, we have found that the charges derived by fitting electrostatic potentials with the screened charge method are less sensitive to the positions of the fitting points, and the quality of the fit to the electrostatics is improved.   

Magnetic Superatom Assemblies and their Transport Properties

We had recently shown that magnetic superatoms can be formed by embedding 3d transition metal atoms in metallic clusters of otherwise non-magnetic elements. The hybridization between the localized exchange split atomic orbitals in 3d elements with superatomic orbitals can help stabilize the magnetic state. Through first principles studies on the electronic structure and magnetic moment of Mg$_{n}$TM (TM = Sc, Ti, V, Cr, Mn, Fe, Co, and Ni) clusters, we had identified Mg$_{8}$Fe to be a stable magnetic superatom. In this work, we will present our investigations on the magnetic properties of the assemblies of such superatoms and the nature of electronic transport through such assemblies with various electrodes. The effects of the contact geometry and gate voltage on the conductance are also studied.   

Fabrication and Characterization of Electrodeposited Nanoporous Alloys

Nanoporous Ni and NiFe thin films were created by electrodeposition of NiCu and NiFeCu followed by electrochemical dealloying to remove the Cu component. The structure and composition of the resulting materials, before and after the dealloying step, was characterized using scanning electron microscopy and energy dispersive spectroscopy. The electrochemical double-layer capacitance was measured to estimate the active surface area. The catalytic behavior of these complex nanoporous materials was investigated using hydrogen evolution as a model reaction.   

Strongly Size-Dependent High-Temperature Behavior of Bismuth Oxide Nanoparticles

Oxide nanostructures show very strong size-dependent changes in their thermal and chemical stability and reactivity. The degree of these changes depends on the type and strength of bonds at the surface: The higher the surface energy the stronger the size-dependence. Inorganic compounds are governed by strong and long ranging bonds which result e.g. in generally high melting points and surface energies. So the properties of such nanostructures could shed more light on the role that the material's surface plays. This is demonstrated here by experiments with size-selected bismuth oxide nanoparticles between 5 and 60 nm ($\pm$5\%). Characterization of the particles revealed a metastable $\beta-Bi_2O_3$ structure. That testifies a size-driven crossover in phase stability below a critical particle size. Heating experiments up to the evaporation point were performed inside the synthesis-chamber as well as with in-situ TEM, in-situ XRD and a high-temperature nanocalorimeter. Different atmospheres were used. For the first time a melting point reduction in oxide nanoparticles was directly shown: For example 10 nm particles melted max. 40\% and evaporated 12\% below the bulk values which is a considerably stronger size-effect than for metals ($\leq$5\%).   

Borane derivatives: A New Class of Superhalogens

Halogens have the largest electron affinities of all elements in the periodic table, that of Cl being the highest, namely 3.6 eV. Superhalogens have electron affinities that far exceed that of halogens. Based on density functional theory calculations, we show that the Wade-Mingo's rule, well known for describing the stability of \textit{closo}-boranes (B$_{n}$H$_{n}^{2-})$, can be used to design a new class of superhalogens by tailoring the size and composition of borane derivatives. These superhalogens do not have to have either a metal or a halogen atom unlike conventional superhalogens. We show this by taking B$_{12}$H$_{13}$ and CB$_{11}$H$_{12}$ as examples. Also, these superhalogens can be used as building blocks of hyperhalogens of the form M(B$_{12}$H$_{13})_{2}$ and M(CB$_{11}$H$_{12})_{2}$ (M=Li, Na, K, Rb, Cs). This finding opens the door to an untapped source of superhalogens and weakly coordinating anions with potential applications.   

Configurational thermodynamics of alloyed nanoparticles: a first-principles cluster expansion study

Transition-metal, alloyed core-shell nanoparticles (NPs) continue to be studied as heterogeneous catalysts because they are found to improve catalytic activity and selectivity for many energy-conversion processes. However, thermodynamic investigations have been limited mostly to NP core-shell preference, rather than both geometric structure and its chemical decoration. Here, by extending cluster expansion methods to treat alloyed nanoparticles, we study the configurational thermodynamics of bimetallic NPs, using databases from density functional theory calculations. We find that the interplay between the ordering tendency and the core-shell segregation tendency can induce site-specific preferences around the NP, as we exemplify, e.g., in AgAu. Such simulations will provide information needed for further understanding of the structure-function relationships in NP catalysis.   

Sintering of multi-metallic nanoparticles

During the thermal treatment employed to activate the Pt-based nano catalysts used in fuel cell applications, the particles undergo structural transformations that affects their chemical performance. The mechanisms of coalescence and grain growth in bimetallic/trimetallic nanoparticles supported on planar silica on silicon are investigated using in-situ synchrotron based X-ray diffraction in the temperature regime 400-900C. The sintering process was found to be accompanied by lattice contraction and L10 chemical ordering. The mass transport involved in sintering is attributed to grain boundary diffusion and its corresponding activation energy is estimated from data analysis.   

Temperature dependence of Raman spectroscopy of molecular iodine trapped in zeolite crystals

Molecular iodine has been pursued for many practical applications, such as molecular clock, molecule-based quantum information processing, due to its narrow-linewidth hyperfine optical transitions. But because of its low vapor pressure, the experimental setup employing a free-space-based iodine vapor cell is very space-consuming. Recently, it is reported that the iodine molecule can be loaded into the channels of zeolite crystals, the density there could be orders improved and its space orientation can be precisely controlled. It may drastically reduce the size of molecular iodine experiment setup, and have many potential applications in microchip technology. We have studied the Raman spectroscopy of iodine molecule confined in zeolite crystals, AlPO4-5 (AFI) and AlPO4-11 (AEL), under different temperatures. The results show that in AEL, where the molecules are intensely confined, the ground vibrational states are close to that of an ideal 1D harmonic oscillator, while in AFI, where the molecules have a bit more freedom, they vibrate like in the free space, but with a loosened spring. And we come up a reasonable theoretical model to explain the Raman width dependence on temperature for these systems   

Single molecule thermodynamics and nanopore-based thermometry

The nanopore-based resistive pulse method measures the reduction in ionic current caused by the interaction of single molecules with the pore. It has great promise in addressing problems across a range of fields that include biomedicine and genomics. The technique requires the residence time of the molecules in the pore to exceed the inverse bandwidth of the detection system ($\sim $ 10 $\mu $s). Efforts are underway to improve this by molecular modification of the pore wall, but little effort has focused on modifying the solution conditions in and around the pore. We address this issue by precisely controlling the solution temperature around a protein ion channel (alpha hemolysin) via laser-induced heating of gold nanoparticles. In this technique, the nanopore serves dual roles as both a highly local thermometer and single molecule sensor. Preliminary data suggests that the solution temperature can be controlled over a wide range, the nanopore conductance can be used to directly measure rapid changes in temperature, and the temperature change can dramatically alter the interaction kinetics of single molecules with the nanopore. The method will improve the development of biochip sensors and lead to a new platform for single molecule thermodynamic studies.   

On the Stability and Dynamics of Atmospheric Pre-Nucleation Clusters

Atmospheric new-particle formation is a complex physical phenomenon with far-reaching consequences: currently, the role of aerosols is one of the main uncertainties in predicting the climate change. However, the molecular-level particle formation mechanisms are poorly understood. It is believed that sulfuric acid is the key player with possible contributions from various base molecules, ions or organics. Here we present results from first-principles molecular dynamics simulations of molecular clusters of sulfuric acid and two atmospherically relevant bases, ammonia and dimethylamine. The dynamics and stability of the studied clusters (sulfuric acid)$_n \bullet$(ammonia)$_{n-1}$ and (sulfuric acid)$_n \bullet$ (dimethylamine)$_n$ where $n=2,3,4$ were probed for 45 ps at T=300K at BPE/TZV2P level of theory. The stability of the clusters is largely dependent on the H-bonding patterns and in most cases the equilibrium patterns emerged within the first 10 ps. Curiously, even after the equilibrium was reached the clusters showed pronounced bond rearrangement: the number of bonds remained the same, but the individual atoms forming the bonds changed. Regardless of this behavior, the clusters remained bound together.   

Session H35: Focus Session: DFT III: Weak Bonding and Liquid Phase Systems

Weakly Interacting Subsystems in DFT

In chemistry, one is frequently interested in systems that are composed of weakly interacting fragments: a solute dissolved in a solvent phase, the base pairs in DNA, molecules in a crystal. In all these cases, our physical picture is that electronic states of the assembly emerge from small perturbations of states localized on the fragments. In frustrating fashion, standard functionals completely fail to reproduce this qualitative picture: the excited states of molecular assemblies are dominated not by local, valence excitations, but by spurious charge transfer states; weak van der Waals forces that hold molecules together are typically absent; excess spin and charge tend to be strongly delocalized even when the physical coupling between centers is extremely weak. In this talk we will discuss how truly nonlocal density functionals can mollify these trends. In particular, we will highlight the recent derivation and implementation of nonlocal van der Waals density functionals that account for long-range correlation in terms of two-point density interactions.   

Towards Efficient and General Method for Many-Body van-der-Waals Interactions

Van der Waals interactions are intrinsically many-body phenomena, arising from collective electron fluctuations in a given material. Adiabatic connection fluctuation-dissipation theorem (ACFDT) allows to compute the many-body vdW interactions accurately. However, the ACFDT computational cost is prohibitive for real materials, even when the random-phase approximation is employed for the response function. We show how the problem of computing the long-range many-body vdW energy for real systems can be solved efficiently by mapping the system (molecule or condensed matter) onto a collection of quantum harmonic oscillators. Currently, our method, which couples density-functional theory with the many-body dispersion energy (DFT+MBD), is developed for non-metallic system [A. Tkatchenko, R. A. DiStasio Jr., R. Car, M. Scheffler, submitted]. The DFT+MBD method includes the hybridization effects by using the Tkatchenko-Scheffler approach [PRL 102, 073005 (2009)], the long-range Coulomb screening through classical electrodynamics [B. U. Felderhof, Physica 29, 1569 (1974)], and the many-body vdW energy from the coupled-fluctuating dipole model [M. W. Cole et al., Mol. Simul. 35, 849 (2009)]. The successes of the DFT+MBD approach and the many challenges that lie ahead will be discussed.   

Dispersion Interactions with Density-Functional Theory: Benchmarking Semiempirical and Interatomic Pairwise Corrected Density Functionals

We present a comparative assessment of the accuracy of two approaches for evaluating dispersion interactions: inter-atomic pair-wise corrections and semi-empirical meta-generalized-gradient-approximation (meta-GGA) based functionals. This is achieved by employing conventional (semi-)local and (screened-)hybrid functionals, as well as semi-empirical hybrid and non-hybrid meta-GGA functionals of the M06 family, with and without pair-wise Tkatchenko-Scheffler corrections. All those are tested against the benchmark S22 set of weakly bound systems, a representative larger molecular complex, and a representative dispersively bound solid. We also compare our results with those obtained from Grimme's pair-wise correction (DFT-D3) and Langreth-Lundqvist functionals (vdW-DF1/2). We find that the semi-empirical kinetic-energy-density dependence of the M06 functionals mimics some of the non-local correlation needed to describe dispersion. However, long-range contributions are still missing. Pair-wise corrections provide for a satisfactory level of accuracy irrespectively of the underlying functional.   

{\it Ab-inito} liquid water with hybrid functionals and dispersion interactions

We report {\it ab-initio} molecular dynamics simulations of liquid water using the hybrid PBE0 functional plus self-consistent dispersion forces based on the scheme of Ref.\footnote{A. Tkatchenko and M. Scheffler, Phys. Rev. Lett. {\bf 102}, 073005 (2009).} Simulations were performed at T=300K and at T=330K to approximately account for nuclear quantum effect on the oxygen-oxygen(O-O) RDF, as suggested by previous path integral simulations. Focusing on O-O RDF, we find that the combined effect of the hybrid functional and of the dispersion interactions significantly improves the agreement of the simulated structure with experiment.   

Understanding the role of London dispersion forces in molecular surface processes

The interactions and dynamics of molecules at surfaces and within pores are essential to many chemical processes, ranging from molecular storage to catalysis and self-assembly. A molecular level understanding of molecule-surface interactions is crucial for tuning surface/pore selectivity and reactivity. While it is clear that strong chemisorption bonds facilitate these interactions, the role of weaker van der Waals (vdW) forces, which include London dispersion and $\pi$-$\pi$ stacking interactions, are often unknown or overlooked. Recent advances in density functional theory (DFT) have now made it possible to reliably account for London dispersion interactions. In this paper, I will discuss the use of one such technique, the Rutgers-Chalmers vdW non-local correlation functional,\footnote{M. Dion, H. Rydberg, E. Schr\"{o}der, B. I. Lundqvist and D. C. Langreth, Phys. Rev. Lett., {\bf 92}, 246401 (2004)}$^,$\footnote{T. Thonhauser, V. R. Cooper, S. Li, A. Puzder, P. Hyldgaard, and David C. Langreth, Phys. Rev. B, {\bf 76}, 125112 (2007)} to demonstrate how the inclusion of London dispersion forces is critical for a truly first principles understanding of processes sensitive to molecule-surface interactions, such as the loading of H$_2$ within porous materials and the chemisorption of organic molecules at surfaces. These works highlight the fundamental importance of London dispersion interactions in the broader context of chemical physics. This work was supported by the Department of Energy, BES, Materials Sciences and Engineering Division.\footnote{Collaborators: Guo Li, Isaac Tamblyn, Yungok Ihm, Jun-Hyung Cho, Shixuan Du, Jeffrey B. Neaton, Hong-Jun Gao, Zhenyu Zhang, James R. Morris}   

First principles Monte Carlo simulations of vapor--liquid equilibria: Density functionals, basis sets, and dispersion corrections

Gibbs ensemble Monte Carlo simulations are used to compute the vapor--liquid equilibria for water, methanol, and methane using Kohn-Sham density functional theory. Results for BLYP and PBE functionals, BLYP with Grimme D2 and D3 dispersion corrections, and various basis sets are compared. Although none of the combinations of functional, dispersive correction, and basis set is found to yield highly accurate predictions for liquid densities, vapor pressures, and heats of vaporization for all three compounds, the results for dispersion corrected BLYP with large basis set are promising.   

Incorporation of an Improved Radial Distribution Function into Classical DFT

An expression for the radial distribution function $g(\vec{r})$ for a fluid can be found using classical density functional theory. We use this expression in place of the mean-field value ({\it e.g.}, $g=1$ for Lennard Jones fluids) in the excess Helmholtz free energy functional for the pair interactions to achieve self-consistent density profiles. We will discuss the differences found with this improved choice for $g(\vec{r})$ compared to other, simpler approximations. We will show results for liquid-vapor systems containing hard-sphere fluids with Lennard-Jones and dipolar interactions.   

Solvation of the chloride anion in water: ab initio simulations

We studied the structural, vibrational and electronic properties of the chloride anion in water using ab initio molecular dynamics. Our investigation has three main objectives: understand the range of perturbation exerted by the anion on the water hydrogen bonded network; identify signatures of the anion perturbation in infrared spectra of the solution and study the extent of charge localization on the anion, as predicted by semi-local (PBE) and hybrid functionals (PBE0). In agreement with recent experiments, we find that the presence of the anion substantially affects only the hydrogen bonding in the first solvation shell, due to a decrease of the dipole moment of the first shell water molecules and thus a weakening of the hydrogen bonds. Such a weakening leads to a slightly blue shifted band in the computed IR spectra. While structural and vibrational properties of the solution are similar within PBE and PBE0, the electronic properties exhibit marked differences.   

Entropy of Liquid Water from Ab Initio Molecular Dynamics

The debate on the structural properties of water has been mostly based on the calculation of pair correlation functions. However, the simulation of thermodynamic and spectroscopic quantities may be of great relevance for the characterization of liquid water properties. We have computed the entropy of liquid water using a two-phase thermodynamic model and trajectories generated by ab initio molecular dynamics simulations [1]. In an attempt to better understand the performance of several density functionals in simulating liquid water, we have performed ab initio molecular dynamics using semilocal, hybrid [2] and van der Waals density functionals [3]. We show that in all cases, at the experimental equilibrium density and at temperatures in the vicinity of 300 K, the computed entropies are underestimated, with respect to experiment, and the liquid exhibits a degree of tetrahedral order higher than in experiments. We also discuss computational strategies to simulate spectroscopic properties of water, including infrared and Raman spectra.\\[4pt] [1] C.Zhang, L.Spanu and G.Galli, {\it J.Phys.Chem. B} 2011 (in press)\\[0pt] [2] C.Zhang, D.Donadio, F.Gygi and G.Galli, {\it J. Chem. Theory Comput.} 7, 1443 (2011)\\[0pt] [3] C.Zhang, J.Wu, G.Galli and F.Gygi, {\it J. Chem. Theory Comput.} 7, 3061 (2011)   

Mean-field Density Functional Theory of Triple Junction

A triple junction in a three-phase fluid system is modeled by a mean-field density functional theory. We use a variational approach to find the Euler-Lagrange equations. Analytic solutions are obtained in the two-phase regions at large distances from the triple junction. We employ a triangular grid and use a successive over-relaxation method to find numerical solutions in the entire domain for the special case of equal interfacial tensions for the two-phase interfaces. We use the Kerins-Boiteux formula to obtain a line tension associated with the triple junction. This line tension turns out to be negative. We associate line adsorption with the change of line tension as the governing potentials change.   

Structural and vibrational properties of sulphate-water clusters from ab-initio calculations

Hydrated clusters of sulfuric acid derivatives play an important role in many processes of interest, e.g. environmental sciences and electrochemistry. Determining their structural and electronic properties is a challenging task: they exhibit a complex free energy landscape and there is no direct experimental measurement of their geometry, which is usually inferred from spectral signatures obtained, e.g. by infrared (IR) measurements [1]. We have used \textit{ab initio} molecular dynamics simulations to investigate the stability, electronic properties and IR of sulphate-water clusters containing $12$ and $13$ water molecules. We discuss how the entire hydrogen bonded network of the cluster may be affected by the presence of a single, additional water molecule, leading to geometrical arrangements not yet identified in experiments. We also show that clusters with different structures may have similar IR spectra, thus making it difficult to use spectral signatures to unequivocally determine the cluster geometry. Finally we discuss the electronic properties of the clusters and in particular differences obtained in the computed ionization potentials when using semi-local and hybrid functionals.\\[4pt] [1] Zhou \textit{et al.} J. Chem. Phys. \textbf{11,} 111102 (2006)).   

Session H36: Focus Session: Environment II: Green Processes

Using Molecular Simulation to Develop New Materials for Energy and Environmental Applications

We face two enormous challenges in the coming decades. First, we must find ways to provide clean, affordable energy to an ever-growing population. Second, as underdeveloped countries advance, we must find ways to provide access to advanced technologies that enhance human wellbeing. What makes these two challenges even more daunting is that they both must be met using sustainable technologies; simply relying on old dirty technologies is not an option. In this talk, I will show how approaches rooted in chemical physics are being used to develop new materials that can be used to meet these challenges. In particular, I will give examples where molecular-based simulations are being used in three areas: 1) the discovery of new ionic liquid solvents for capturing CO2 produced from fossil fuel combustion; 2) the development of new refrigerants that have better performance and significantly lower global warming potentials than existing refrigerants; and 3) fundamental investigations of actinide ions in solution, with the objective of developing new separation processes for nuclear waste remediation and fuel reprocessing technologies.   

Highly Selective CO$_{2}$/CH$_{4 }$Gas Uptake by a Halogen-Decorated Borazine-Linked Polymer

We report herein a synergistic approach that combines synthesis and characterization of a new borazine-linked polymer, BLP-10(Cl), and theoretical calculations based on density functional theory to investigate its performance in small gas storage and separation. We focus on the binding of H$_{2}$, CO$_{2}$, and CH$_{4}$. The choice of these gases is motivated by their impact on energy and the environment. Given the relatively small and similar kinetic diameter of the CH$_{4}$ and CO$_{2}$ molecules, their efficient separation remains a nontrivial task. In the case of a dihydrogen molecule interacting with the chlorinated borazine, we find the H$_{2}$ to be bound molecularly with a bond length of 0.75 {\AA} and at a distance of 2.76 {\AA} from the boron site. CO$_{2}$ and CH$_{4}$, on the other hand, interact with the central ring system of borazine at a distance of 3.12 {\AA} and 3.33 {\AA} respectively. The bond length between the carbon and oxygen atoms of CO$_{2}$ is 1.16 {\AA} while the distance between the carbon and hydrogen of CH$_{4}$ is 1.10 {\AA}. The binding affinities of all gases with the chlorinated borazine rings obtained from using the M06 exchange-correlation potential agree very well with experimental data collected form pure gas component isotherms. Theoretical investigations also indicate that all of the gas molecules preferentially interact with the borazine ring rather than the phenyl substituent of the nitrogen atoms which highlight the significance of including polarizable building blocks in adsorbent materials.   

Carbon dioxide intercalation in Na-fluorohectorite clay at near-ambient conditions

A molecular dynamics study by Cygan et al.[1] shows the possibility of intercalation and retention of CO$_{2}$ in smectite clays at 37 $^{o}$C and 200 bar, which suggests that clay minerals may prove suitable for carbon capture and carbon dioxide sequestration. In this work we show from x-ray diffraction measurements that gaseous CO$_{2}$ intercalates into the interlayer space of the synthetic smectite clay Na-fluorohectorite. The mean interlayer distance of the clay when CO$_{2}$ is intercalated is 12.5 {\AA} at {\-}20 \r{ }C and 15 bar. The magnitude of the expansion of the interlayer upon intercalation is indistinguishable from that of the dehydrated-monohydrated intercalation of H$_{2}$O, but this possibility is ruled out by careful repeating the measurements exposing the clay to nitrogen gas. The dynamics of the CO$_{2}$ intercalation process displays a higher intercalation rate at increased pressure, and the rate is several orders of magnitude slower than that of water or vapor at ambient pressure and temperature.\\[4pt] [1] Cygan, R. T.; Romanov, V. N.; Myshakin, E. M. \textit{Natural materials for carbon capture}; Techincal report SAND2010-7217; Sandia National Laboratories: Albuquerque, New Mexico, November, 2010.   

Monoethanolamine adsorption on TiO2 (110) for solid supported CO2 capture

Solid supported CO2 capture materials are drawing substantial attention as a promising, cost-effective and environmentally friendly alternative to aqueous amine based CO2 capture. Recently CO2 capture was observed from monoethanolamine (MEA) adsorbed TiO2 powders. In order to facilitate the rational design of future solid CO2 capture materials, it is very important to understand the interaction between MEA and the TiO2 surface at the atomic level and how it affects the CO2 capture capabilities. In this work, we report a combined experimental and theoretical study of MEA adsorption on rutile TiO2 (110). We found that 1 ML of MEAs can form a stable and ordered patten on TiO2 (110). However, the amine group in MEA (the CO2 capture site) binds preferably to the TiO2 surface in the gauche mode. The binding energy of the gauche mode is about 0.8 eV larger than the trans mode, where the amine group is free, causing the present MEA/TiO2 system unable to capture CO2. We found that this large binding energy difference is originated from a combination of surface donor-acceptor bonds, H-bonds, and dipole-induced dipole interaction. Our study suggests that these effects are key factors to design future amine-based solid supported CO2 capture materials.   

Investigation of nanoparticle transformations to guide the design of greener products and processes

Nanoscale particles and products containing nanoparticles hold promise as higher performance materials; however, there are concerns that the production and use of nanoparticles might negatively impact human health or the environment. Within the context of greener nanoscience we aim to maximize the benefits, while minimizing hazards, of nanoscale products. A significant gap in the knowledge needed to develop greener products and processes is our understanding of the formation and transformation of nanoparticles. Such studies of nanoparticle dynamics are technically challenging and few studies have been reported. In this presentation, I will describe convenient methods to monitor nanoparticle dynamics and show how knowledge of nanoparticle transformations can guide the design of greener products and processes. In one example, chemically-modified transmission electron microscopy (TEM) grids are used to directly visualize silver nanoparticle transformations on surfaces. By indexing the TEM grids, it was possible to examine the same nanoparticles repeatedly throughout exposure to different environments. These studies show that larger particles can act as a source of smaller nanoparticles and that much larger particles also produce nanoparticles. With this knowledge, an improved design of nanoparticle coatings for antimicrobial fabrics was developed. A second example involves the use of small angle x-ray scattering (SAXS) to monitor nanoparticle formation reactions in solution in real-time. A combination of beam-line and lab-scale SAXS measurements, combined with simultaneous optical studies, showed that particle growth and coalescence compete under typical synthesis conditions, leading to loss of structural definition of the product. This mechanistic insight, in turn, guided the design of efficient and greener syntheses of well-defined nanoparticles.   

Ceria Nanoparticles: Environmental Impacts on Particle Structure and Chemistry

Ceria nanoparticles are widely studied for catalytic, energy, environmental and bio-medical applications. The performance of ceria often depends on the ability of Cr to switch between +3 and +4 oxidation states. This paper summarizes observations of the impact that synthesis route, processing conditions, storage and environmental conditions have on the chemical and physical properties of ceria nanoparticles. Particles less than 10 nm in diameter are highly dynamic and change their oxidation state not just as a function of size, but also as a function of aging (time) and environmental conditions. During particle nucleation and growth, both particle size and oxidation state change with time. These observations suggest that interpretations of experimental results based primarily on particle size may be misleading. Raman and microXRD studies indicate that these changes can be more complex than anticipated. Because synthesis, analysis and relevant operational conditions often place particles in different environments, understanding how particles change with time in operational conditions is essential to predicting their properties. Time and environmentally induced changes may also play a significant role in the discrepancies reported in various studies.   

Solubility and transport of cationic and anionic patterned nanoparticles

Diffusion and transport of nanoparticles (NPs) though nanochannels is important for desalination, drug delivery, and biomedicine. Their surface composition dictate their efficiency separating them by reverse osmosis, delivering into into cells, as well as their toxicity. We analyze bulk diffusion and transport through nanochannels of NPs with different hydrophobic-hydrophilic patterns achieved by coating a fraction of the NP sites with positive or negative charges via explicit solvent molecular dynamics simulations. The cationic NPs are more affected by the patterns, less water soluble, and have higher diffusion constants and fluxes than their anionic NPs counterparts. The NP-water interaction dependence on surface pattern and field strength explains these observations. For equivalent patterns, anionic NPs solubilize more than cationic NPs since the Coulomb interaction of free anionic NPs, which are much stronger than hydrophobic NP-water interactions, are about twice that of cationic NPs.   

Hydrogen bond density and strength analysis on hydrated Rutile (110) and Cassiterite (110) surfaces

We study the dynamics of water on the surface of cassiterite (110) and rutile (110) using ab-initio molecular dynamics simulation. Water adsorbs and dissociates on these surfaces. This dynamic equilibrium is dominated by the hydrogen bond (h-bond) network at the surface. The h-bond density analysis shows that adsorbed water molecules form higher average number of h-bonds on rutile ($\sim $2.3) as compared to the cassiterite surface ($\sim $2.1). On the other hand, bridging oxygen atoms form higher average number of h-bonds on cassiterite ($\sim $1.4) than rutile surface ($\sim $1.2). Dissociated species are found to have same average number of hydrogen bonds on both surfaces. As a consequence, the rutile surface has higher density of h-bonds at the surface than cassiterite, however, their strength is lower [N. Kumar et al., J. Chem. Phys. 134, 044706 (2011)]. This delicate balance is responsible for the different dynamical properties of both surfaces.   

Microfluidics and Stimulus-Responsive Materials -- The Key to Next Generation Chemical Sensors for Widely Distributed Environmental Monitoring

The fields of chemical sensing and microfluidics have promised much, but in terms of functional devices, have delivered relatively little. Issues like biofouling and surface degradation mean that sensor characteristics change rapidly in real samples. Consequently, chemical sensors must be regularly recalibrated to ensure the information they send is reliable. This results in complex and very costly devices that must integrate fluidics, standards, and waste storage, as well as sampling and analytical procedures. The fundamental challenge for realizing sensors for widely distributed environmental monitoring is this - how can we produce low cost sensing platforms that can function reliably in an autonomous manner for periods up to years? The key to progress lies in new, and more sophisticated materials that can respond to external stimuli, and communicate with the external world. For example, materials that can be activated from a passive state, reversibly bind and release targeted guest molecules, and return to a passive form. Activation and deactivation happen as part of an external control system, which can be local (chemical in nature) or external (e.g. photonic), and the material reports its status (passive, activated-free, activated-occupied) optically materials can be incorporated into more sophisticated platforms, such as micelles, beads, or complete fluidic systems that are much more biomimetic in nature than current platforms. They include polymer actuators that expand and contract dramatically under an external stimulus (e.g. light), enabling valve and pumping functions to be fully integrated into the microfluidic device. This lecture, I will present some of the exciting possibilities for chemical sensing that are now beginning to emerge through breakthroughs in fundamental materials science.   

Session H37: SPS Undergraduate Research III

Fabrication of Micromirror Templates Using a Focused CO2 Laser

Micromirrors are an important component in quantum optics, for example in Fabry-Perot microcavities, where light can be recirculated within small volumes. Recently, a CO2 laser method has been demonstrated as a way to fabricate micromirror templates with an exceptionally high surface quality. However, these templates typically vary in size significantly, which is undesirable in many applications, for example when arrays are needed. Here we address this problem by implementing a feedback method, which uses the light emitted by the sample during the ablation process. By measuring the intensity of the light emitted and correcting, in real time, the laser intensity, we can control the feature size to be less than 5 percent.   

High-power, narrow-bandwidth laser based on a tapered amplifier

We have constructed a high-power, tunable, narrow-bandwidth, cw laser based on a semiconductor tapered amplifier at a wavelength of 780 nm. Bandwidth narrowing to less than 1 MHz is accomplished with an intra-cavity interference filter, while 10 GHz of continuous tuning is possible with a cavity mirror mounted on a PZT stack. The laser output has a slope efficiency of 0.5 W/A, optical output power of 0.7 W, and will be used for laser cooling of atomic rubidium.   

Depth of focus in digital holography using spatial partially coherent light

Digital holography is a powerful, but young, imaging technology that has a vast array of applications; its strength lies in the ability to numerically focus on any plane within a sample from a single hologram and to use both amplitude and phase information from the intensity field to reconstruct the sample's 3D characteristics on a computer. The quality of many holograms, however, is compromised by speckle and other interference noise associated with the high-coherence lasers often used to illuminate the sample. Speckle noise may be diminished by lowering the coherence length of the source. In our experiments, partially coherent light was created by directing a laser beam through a rotating ground glass. We aimed to discern whether the coherence of the source could be systematically altered by changing the position of the ground glass within the focused laser beam. We anticipated that altering the coherence length would also systematically change the depth of focus. Initial results support our hypotheses.   

Photocatalytic properties of nanostructured TiO2 surfaces

Photocatalytic chemical reactions are actively explored for direct production of chemical fuels from sun light through electrolysis or for the clean-up of organic pollutants through photocatalysis. Titanium dioxide is a prototypical photocatalyst which has been studied extensively. However, there are still unanswered questions regarding the relationship between surface morphology and photocatalytic properties. In this study, we used ion beam assisted surface nanopatterning and UV-catalysis to investigate the dependence of photoreactivity on surface nanostructures. Energetic argon gas ions were used to induce self-formation of nanopatterns on TiO2 surfaces and the structure formation was characterized by atomic force microscopy. The influence of the surface structure on the photochemical properties was assessed through photocatalytic degradation of methyl orange in aqueous solution with a flat sample and a nanopatterned sample of TiO2, respectively. The resulting absorbance spectrums were then compared.   

Pushing the Limits of Nanoscale Imaging in Atomic Force Microscopy

The invention of scanning probe microscopy has revolutionized the field of nanotechnology. Atomic force microscopy is a branch of scanning probe microscopy in which an extremely sharp tip ($\sim $50 nm diameter) is held in contact with a sample surface. By maintaining constant force between the tip and the sample, the topography of a sample surface can be measured with high precision. The lateral resolution in this technique is however limited by the size of the tip. In this presentation, we present a method to deconvolve the effect of tip size and obtain higher resolution than presented by the experimentally obtained topographic image.   

Scanning Tunneling Microscopy of Fe Doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$

We will present a low temperature scanning tunneling microscopy (STM) study of the high-temperature superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ (Bi-2212) which has been intentionally doped with magnetic (Fe) impurities in order to locally disrupt superconductivity around the impurities. We examine spatial variations in the density of states in the vicinity of Fe impurities, and compare our results with previous STM studies of Ni doped Bi-2212. Notable differences between Fe and Ni impurities include differences in the number and energy locations of the impurity peaks. Our analysis shows that Fe is a weaker magnetic impurity than Ni and that the particle-hole symmetry present in the spectra of Ni impurities is not as obvious in Fe impurities. By studying how these impurities interact with superconductivity in Bi-2212 we hope to understand more about the superconducting mechanism in high-temperature superconductors.   

Theoretical \& Experimental Design \& Optimization of Multilayer Mirrors for Soft X-Ray Reflection

The reflection of soft X-rays is relevant for the development of ultra fast attosecond cameras, X-ray lithography, and the study of solar storms. Soft X-rays are typically absorbed, as opposed to reflected, due to all materials' absorptive nature. Co/C-am multilayers composed of 40 layers were deposited on Si \& SiO2 substrates by Magneton Sputtering technique and were characterized by grazing-incidence diffraction. Theoretical interfacial roughness and layer thicknesses were simulated using the commercial software IMD, while experimental values were estimated by fitting the reflectivity data. Low reflectivity values were observed at the locations of Bragg peaks. However, through deposition optimization, reflectivity values could potentially reach values above 60\%.   

Domain Coarsening Within the Ising Model on the Hyperbolic Plane

In spite of its simplicity, both the dynamics and equilibrium statistics of the Ising model are nontrivial. Previous studies in this REU have shown that the dynamics of metastable decay is very different in the hyperbolic plane than in the Euclidean plane. Here we perform Monte Carlo simulations of domain coarsening in the hyperbolic plane. We find that domain coarsening occurs more slowly than in the Euclidean plane.   

Quantum chaos: an introduction via chains of interacting spins-1/2

We discuss aspects of quantum chaos by focusing on spectral statistical properties and structures of eigenstates of quantum many-body systems. Quantum systems whose classical counterparts are chaotic have properties that differ from those of quantum systems whose classical counterparts are regular. One of the main signatures of what became known as quantum chaos is a spectrum showing repulsion of the energy levels. We show how level repulsion may develop in one-dimensional systems of interacting spins-1/2 which are devoid of random elements and involve only two-body interactions. We present a simple recipe to unfold the spectrum and emphasize the importance of taking into account the symmetries of the system. In addition to the statistics of eigenvalues, we analyze also how the structure of the eigenstates may indicate chaos. This is done by computing quantities that measure the level of delocalization of the eigenstates.   

Solving the Feynman--Gell-Mann Equation for the Electron

There are very few cases for which the Dirac equation can be solved exactly. Moreover, the techniques familiar from undergraduate quantum mechanics provide little help in solving its linear differential equations. Working instead with a two-component formalism, the transformed Dirac equation can be solved for cases of constant electric and magnetic fields for an electron. This approach was recommend in the famous 1958 paper by Feynman and Gell-Mann and has the form: $ (\imath\hbar\frac{\partial}{\partial t}-V)^2=-\hbar^2 c^2|\nabla-\imath\frac{e}{\hbar}\vec{A}|^2-e\hbar c \vec{\sigma}\cdot (\vec{B}c+\imath \vec{E})+(mc^2)^2 $, when acting on a two component spinor $\varphi$. We will show the solution of this equation for the cases of parallel $E$ \& $B$ fields, perpendicular fields, perhaps oblique fields and finish with a discussion of the Hydrogen atom. Through taking the nonrelativistic limits, these solutions can be verified for well-known conditions.   

Second harmonic generation and non-linear corrections to the high frequency susceptibility of a multiferroic material

We consider the effects of non-linear (second order) corrections to the high-frequency susceptibilities of a material that is simultaneously ferroelectric and a canted antiferromagnet (multiferroic). The non-linear corrections introduce a second harmonic term in the magnetic, electric, and multiferroic susceptibilities. Using the Landau-Lifshitz equation of motion for the magnetic components and the Landau-Khalatnikov relaxation equation for the electric polarization we theoretically compute the non-linear corrections to the susceptibilities for the optic antiferromagnetic mode, the acoustic mode, and the electric susceptibilities up to second order. Using realistic material parameters we find that the corrections have either a noticeable or negligible effect on the first order susceptibility values.   

Aluminum Nitride Nanofibers fabricated using Electrospinning and Nitridation

Aluminum Nitride (AlN) and other nitride semiconductors are important materials in the fields of optoelectronics and electronics. AlN nanofibers were synthesized using electrospinning and subsequent heating under N$_{2}$ and NH$_{3}$ atmospheres. The precursor solution for electrospining contains aluminium nitrate and cellulose acetate. The electrospun nanofibers were heated in N$_{2}$ to eliminate the polymer and produce Al$_{2}$O$_{3}$, and then nitridized at a temperature of 1200\r{ }C under NH$_{3}$ flow. Scanning Electron Microscopy (SEM) observations demonstrate the production of fibers with diameters ranging from a few nanometers to several micrometers. X-Ray Diffraction and UV-VIs analyses show the production of AlN nanofibers with hexagonal wurzite structure and a band gap of approximately approximately 6 eV. Current-Voltage measurements on a single AlN fiber with gold electrodes suggest the formation of a Schottky contact The fabrication method and results from the fibers characterization will be presented.   

Formation of Nanostructures at Gold Surfaces Exposed to Femtosecond Laser Pulses

The evolution of free-standing gold film irradiated by ultrashort laser pulses was simulated using molecular dynamics. The spatially non-uniform deposition of laser energy was modeled by a two-dimensional temperature profile applied during time of electron-ion energy exchange. Our simulations show that the ultrafast two-dimensional heating results in the melting and pressurization of a thin surface layer. Due to a non-uniform stress distribution, this molten layer expands to form a jet-like protrusion at the laser pulse's focal point. Above some critical stress, many voids start to nucleate forming a foam-like material covered by a thin liquid shell/cupola. The still expanding cupola may rupture forming a rim around the newly-developed crater. All these processes lead to complicated surface morphology, which becomes frozen at the nanosecond time scale. Geometrical characteristics of simulated surface profiles, including crater depth and size of frozen bubbles, agree well with experiment. Our simulations help to provide better insight into the atomistic mechanisms of nanostructure formation.   

Gold coated nanoparticles on silicon substrate

The study of Gold nanoparticles is very captivating because of their significance and applications as catalysts in restructuring technologies that are used for manufacturing medicine, energy production, transportation, computation, communication, and environmental changes. In this experiment, I have analyzed the morphology of gold nanoparticles using different techniques. A sputter coating technique was used to deposit gold on silicon substrate. During the process, depositions were performed using varying plasma coating times and voltages. The Gold nanoparticles were then analyzed using the Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). The AFM and SEM data revealed that the coating surface morphology was dependent upon deposition conditions.   

Simulations of surface plasmons launched on gold nanogratings

A gold nanograting with a dielectric coating containing fluorescent molecules can enhance the intensity of fluorescence at certain frequencies due to the excitation of surface plasmons. Fluorescence is enhanced by two mechanisms: (1) an enhanced electromagnetic field at the excitation frequency of the fluorophores and (2) surface plasmon modes providing extra decay channels for fluorophores. Previous studies on corrugated film gratings show that coupling to higher diffraction order plasmons occur with lower efficiency. We find that for wire gratings, fluorophores couple to higher order plasmon modes on both sides of the gold wires with uniform efficiency. We also measure directional enhanced fluorescence on both the active (reflection) and substrate (transmission) side of the gratings. In this work, gold nanogratings with a dielectric coating were modeled using COMSOL Multiphysics software, which solves Maxwell's equations in the region of the grating. As the thickness of the dielectric layer containing the fluorophores is increased, the plasmon modes shift. The behavior of the gratings was simulated as a function of height of the gold wires and thickness of the dielectric coating. These simulations will inform future experiments.   

Session H39: Electronic Structure: Calculations II

Variational investigations of the electronic structure and energy of finite hydrogen systems with the Gutzwiller wave function within local correlation matrix renormalization approximation

We introduce the correlation matrix renormalization Hartree-Fock (CMR-HF) method in which the many-body Hamiltonian of a multi-electronic system is solved using a variational Gutzwiller-type wave-function. The Gutzwiller approximation is generalized to renormalize the one-electron density matrix and two-electron correlation matrix of the system. To achieve a clear presentation of the concept and methodology, we describe the detailed formalisms for a finite hydrogen system with minimal basis set. The resulting expectation value of the Hamiltonian have clear parallels to terms in the standard uncorrelated Hartree-Fock method, allowing an iterative self-consistent field solution of the many-electron problem analogous to the Hartree-Fock solution. We have applied the method to a series of hydrogen clusters to compare with the results of several other quantum chemical calculation methods.   

Real-time time-correlation approach for x-ray absorption and emission spectra

We present a real-time approach for calculations of X-ray absorption (XAS) and emission spectra (XES) from deep core-levels, based on time-correlation functions. XAS and XES have traditionally been calculated using Fermi's golden rule in frequency space, and alternatively, using real-space Green's function (RSGF) methods. Recently, however, with the advent of very high brightness pulsed x-ray sources, calculations of time-dependent response have become a focus of attention. Here we obtain the time-correlation functions by propagating the initial, dipole-excited wavefunction with a Crank-Nicolson time-evolution operator.\footnote{Y. Takimoto \textit{et al.}, J. Chem. Phys. {\bf127}, 154114 (2007).} The initial state is obtained using projector augmented wave (PAW) transition matrix elements and, for XAS, the propagation is carried out in the presence of a core hole. The approach is implemented using an extension of SIESTA and can be applied both to molecular and extended systems. Illustrative examples are presented for several systems, and yield results in good agreement with RSGF and Fermi golden rule approaches using FEFF and StoBe respectively. Finally, we discuss improvements in order to include dynamic many-body effects.   

Calculation of Phonon Satellites in Electron Spectral Functions

We describe a first principles approach for calculations of phonon satellites in the electron self-energy and spectral function. The method is based on cumulant expansion techniques [1] applied to the self-energy model of Eiguren and Ambrosch-Draxl [2] with the dynamical matrix and electron-phonon couplings obtained from ABINIT [3]. In particular, the electron-phonon couplings are calculated from the Eliashberg functions as in [4], and the phonon DOS is obtained from a many-pole/Lanczos representation of the phonon Green's function [5]. The method is illustrated with results for a number of systems.\\[4pt] [1] F. Aryasetiawan et al., Phys. Rev. Lett. 77, 2268 (1996).\\[0pt] [2] A. Eiguren and C. Ambrosch-Draxl, Phys. Rev. Lett. 101, 036402 (2008).\\[0pt] [3] X. Gonze et al., Comput. Materials Science 25, 478 (2002).\\[0pt] [4] S. Y. Savrasov and D. Y. Savrasov Phys. Rev. B 54, 16487 (1996).\\[0pt] [5] F. Vila et al., Phys. Rev. B 76, 014301 (2007).   

Atomic Multiplets in X-ray spectroscopies revisited

Atomic multiplets are known since a long time to produce specific, crystal field dependent, fingerprints of open d- and f- shells in X-ray spectroscopies. Older computer programs originating from gas-phase optical spectroscopy of atoms tend to be difficult to apply to crystal field environments with lower symmetries. In order to study changes of X-ray absorption near edge spectra and resonant inelastic X-ray scattering (RIXS) across symmetry lowering phase transitions, a new multiplet program was developed. Starting from a Dirac-Slater spherical atom calculation we evaluate electron-electron interaction and crystal field in the Hilbert space spanned by the open shells by diagonalization. The crystal field can be simply defined by nominal charges and cartesian atomic positions relative to the core-hole atom. To overcome limitations of the model and for fitting known spectra, spin-orbit splitting, el-el interaction and crystal-field can be scaled independently. The nominal charges may be taken as further crystal field parameters. Various XAS and RIXS examples will be discussed, in particular in view of the polarisation dependence and symmetry of the crystal.   

Simulation of electron-energy-loss spectra including both diffraction and solid-state effects

Aberration-corrected scanning transmission electron microscopes yield probe-position-dependent electron-energy-loss spectra (EELS) that can potentially provide spatial mapping of the underlying electronic states. EELS calculations, however, typically describe excitations by a plane wave travelling in vacuum, neglecting diffraction and interference effects. Here we report the development and initial application of a methodology that combines a full electronic-structure calculation with beam propagation in a thin film. The simulations are based on PAW plane-wave calculations of the excitation spectrum of the material and Bloch wave simulations of the probe propagation.   

Precise all-electron response functions from a combined spectral sum and Sternheimer approach: application to EXX-OEP

The optimized-effective-potential (OEP) method is used to construct local potentials from non-local, orbital-dependent functionals, e.g., exact exchange (EXX). The method involves two response functions, which have to be converged to very high precision to obtain smooth and stable local potentials. Usually, this requires an exceptionally large orbital basis, leading to very costly calculations, especially for all-electron methods such as FLAPW [1]. In this work, we propose a scheme that combines the usual spectral sum from standard perturbation theory with a radial Sternheimer approach. It also comprises a, albeit small, Pulay-type correction, which refines the results especially for small basis sets. We demonstrate that with this new approach already the conventional minimal LAPW basis set is sufficient to yield precise response functions. Furthermore, very few unoccupied states are required, which reduces the computational cost considerably. The numerically important Sternheimer contribution arises from the potential dependence of the LAPW basis functions and is constructed by solving inexpensive radial differential equations. We show results for complex transition-metal oxides.\newline [1] M. Betzinger \emph{et al.}, Phys. Rev. B~{\bf 83}, 045105 (2011)   

Electronic band structure of lanthanum bromide and strontium iodide from many-body perturbation theory calculations

Rare-earth based scintillators represent a challenging class of scintillator materials due to pronounced spin-orbit coupling and subtle interactions between d and f states that cannot be reproduced by standard electronic structure methods such as density functional theory. In this contribution we present a detailed investigation of the electronic band structure of LaBr$_3$ using the quasi-particle self-consistent GW (QPscGW) method. This parameter-free approach is shown to yield an excellent description of the electronic structure of LaBr$_3$. Specifically we reproduce the correct level ordering and spacing of the 4f and 5d states, which are inverted with respect to the free La atom, the band gap as well as the spin-orbit splitting of La-derived states. We furthermore present electronic structure calculations using G$_0$W$_0$ for the important scintillator material SrI$_2$. We explicitly take into account spin-orbit coupling at all levels of the theory. Our results demonstrate the applicability and reliability of the GW approach for rare-earth halides and complex halides. They furthermore provide an excellent starting point for investigating the electronic structure of rare-earth dopants such as Ce and Er.   

First-principles studies of Ce and Eu doped inorganic materials as candidates for scintillator gamma ray detectors

We have performed high-throughput DFT based (GGA+U) band structure calculations for new Ce and Eu doped wide band gap inorganic materials to determine their potential as candidates for gamma ray scintillator detectors. These calculations are based on determining the 4f ground state level of the Ce and Eu relative to the valence band of the host as well as the position of the Ce and Eu 5d excited state relative to the conduction band of the host. We find many classes of candidate materials where the 5d is in the conduction band preventing scintillation. Even when the Eu and Ce 4f and 5d levels are placed well in the gap of the host, traps on the host can also prevent the energy of the gamma ray transferring to the Eu or Ce. We therefore also performed calculations for host hole traps and electron traps to compare their energies to the Ce and Eu 4f and 5d levels.   

Test of Variational Methods for Molecular and Solid State Properties by Application to Hyperfine Interaction in Phosphorous Atom

As part of our program for testing the accuracy of variational methods for studying energy and wave-function dependent molecular and solid state properties, namely the Variational Hartree-Fock Many Body Perturbation Theory (VHFMBPT) and Variational Density Functional Theory (VDFT), we have studied the magnetic hyperfine interaction for $^{31}$P nucleus in the ground state. Our investigations provide hyperfine constants of +35.2MHz by the VHFMBPT and -11.2 MHz by VDFT procedures as compared to +55.055 MHz from experiment [1]. The VHFMBPT procedure provides the same signs for one-electron and many-body contributions as obtained earlier [2] by the HFMBPT procedure which uses the needed one-electron atomic wave-functions for the occupied and unoccupied states obtained by solving the Hartree-Fock equations through numerical integration, and not variationally as in the VHFMBPT procedure. Possible avenues for improved agreement by both variational procedures will be suggested. [1] N.C. Dutta, C. Matsubara, R.T. Pu and T.P. Das, Phys. Rev. Lett. 21, 1139 (1963) and references therein, [2] T.P. Das Hyperfine Interactions 34, 149 (1987) and reference therein to the experimental result for phosphorous atom. [3] Alfred Owusu, R.W. Dougherty, G. Gowri, J. Andriessen, T.P. Das, Phys. Rev. A 56, 305 (1997) and references therein.   

Sharp transition for single polarons in one-dimensional models with non-diagonal Su-Schrieffer-Heeger coupling

Ever since Landau pointed out the possibility of an electron becoming self-trapped in its own lattice distortion, people have looked for sharp transitions in polaronic states. Gerlach and L\"owen [1] proved the absence of such a transition in the case of a gapped (i.e. optical) phonon branch and an electron-phonon coupling $g(q)$ depending only on the phonon momentum $q$. Whether a sharp transition could be found in other models remained an open question for the last twenty years. By presenting both unbiased Diagrammatic Monte Carlo results for the single polaron with Su-Schrieffer-Heeger (SSH) coupling to optical phonons, and supporting results from no less than three other numerical and analytical approximations, we believe our previous work and that of our collaborators [2] to be the first unequivocal demonstration of a sharp transition in a polaronic system. Here we expand this work to more general models that include either an additional diagonal Holstein coupling to optical phonons, or a SSH coupling to acoustic phonons. The survival of the sharp transition in these models is investigated using the Bold Diagrammatic Monte Carlo method and the Momentum Average approximation. References: [1] Rev. Mod. Phys. 63, 63 (1991) [2] Phys. Rev. Lett. 105, 266605 (2010)   

Many-Pole Model Calculations of Inelastic Losses in XPS

Inelastic losses such as satellites in x-ray photo-emission spectra (XPS) are difficult to treat theoretically due to the importance of many-body effects beyond the quasi-particle approximation. Here we present an approach based on an exponential (i.e., cumulant) representation\footnote{M. Guzzo et al., Phys. Rev. Lett. {\bf 107}, 166401 (2011)} of the one-particle Green's function, together with\footnote{J. Kas et al., Phys. Rev. B {\bf 76}, 195116 (2007)} a many-pole model of the dielectric function, which incorporates dynamic effects beyond the GW approximation. In this method the photo-electron is treated as in inverted LEED state which couples to the system via neutral, quasi-boson excitations. The approach yields an approximation to the XPS in terms of a convolution of a quasi-particle calculation and a spectral function that includes contributions from both intrinsic and extrinsic excitations,\footnote{L. Hedin et al., Phys. Rev. B {\bf 58}, 15565 (1998)} as well as interference between them. The method is illustrated with recent experimental results.\footnote{Guzzo, op. cit.}   

Low Energy Positron Interactions with Biological Molecules

There is some experimental evidence that positrons can produce distinctive molecular fragmentation patterns. It is known that tuning the incident positron energy to near resonance with molecule vibrations can strongly enhance the positron annihilation probability for a molecule.\footnote{Gribakin, Young, and Surko, Rev. Mod. Phys. 82 (2010) 2577} This suggests that fragmentation induced by slow positrons may provide valuable complementary information to existing techniques for identification and study of proteins. In order to study this concept, we are developing a general quantum method for reliably calculating the density distribution for positrons bound to large biological molecules using NEO/GAMESS. We developed transferrable atom-centered positron basis sets for first-principles calculations for molecules containing O, N, C, and H. The positron density in the bound state is concentrated near the most electronegative atomic sites so that e$^{-}$e$^{+}$ annihilation will be most likely to occur in these regions for low incident positron energies leading to positron trapping in the bound state. Using the basis sets and approximations we have tested to predict where annihilation occurs can ultimately help us understand the resulting fragmentation patterns of larger biological molecules.   

Atomic Probing Structures of Electrolytes at Graphene Surface:~Insights from X-ray Scattering and Molecular Dynamics

The interactions of electrolyte fluids with carbon-based electrodes control many complex interfacial processes encountered in electrochemical energy storage systems. However, our knowledge of the atomic/nanoscale reactivity at interfaces of electrolytes with electrodes remain scares due to the incomplete understanding of interfacial structures and processes in-situ and real-time encountered in real operation conditions. In this talk, we will present our efforts to obtain a molecular-scale perspective of the interactions of electrolytes with carbon surfaces near ``real world'' conditions. Structures of various electrolytes including slat aqueous and ionic liquids on atomically flat graphene (epitaxially grown on a SiC substrate), an ideal model fluid-solid interface system, were investigated by coupling high-resolution interface X-ray scattering techniques with molecular modeling-simulation approaches. These results provide a base-line for understanding relevant electrolyte/carbon interactions and will lead to fundamentally new insights and provide unique tests of atomistic fluid-solid interface models for energy storage systems.   

Charge Transfer Resistance of a Pristine Graphitic Carbon Interface

Electron transfer to and from graphitic carbons is highly sensitive to the chemistry of the graphitic carbon surface. It is known that pristine graphitic carbon has a much higher interfacial resistance than defective carbon; however, a quantitative measure of the difference is limited by the difficulty of preparing truly pristine, defect-free surfaces. Here, we use an individual single-walled carbon nanotube (SWNT) and its single defect sensitivity to ensure that the graphitic carbon surface is pristine. The interfacial charge transfer resistance is measured in the context of a MnO2-coated SWNT, a psuedocapacitor device whose charge cycling performance is found to be limited by the chemistry of the carbon interface. In our devices, the SWNT is uniformly coated with 250-350 nm of MnO2 and cyclic voltammetry is analyzed using an equivalent circuit model to determine the charge transfer resistance. We prove that the defect-free carbon interface is less active than disordered interfaces of carbon or of metallic electrodes. This research is supported by the NEES Energy Frontier Research Center of the U.S. DOE Office of Basic Energy Sciences ({\#}DESC0001160).   

Applications of Light Element X-ray Raman Spectroscopy and Hard X-ray Emission Spectroscopy to the Electronic Structure of Energy Storage Materials: Prospects and Initial Results from the Spectroscopy Program at SSRL

We present the applicability of x-ray raman spectroscopy of light elements and hard x-ray emission to probe the electronic structure of energy storage materials under in-situ conditions. In particular, recent advances in resolution and throughput of x-ray raman spectroscopy (XRS) offer the capability to measure 1s x-ray absorption profiles of light elements such as lithium, boron, carbon, nitrogen and oxygen with less than 0.3eV resolution in the order of 10s of minutes. Initial results from the Spectroscopy program at SSRL and collaborations with groups in various energy storage fields will be presented, with an emphasis on Lithium Batteries and Hydrogen Storage applications.   

Session H40: Focus Session: Protein Association II: Aggregation and Fibril Formation

The Physics of Amyloid Aggregation and Templating in Prions

The problem of self-assembled amyloid aggregation of proteins in structures with beta-strands perpendicular to a one dimensional grown axis is interesting at a fundamental level (is this the most generic end state of proteins?), from a biological level (if the self-assembly can be regulated it is of use in contexts like spider silk and bacterial colony formation), for human public health (aggregation unregulated induces diseases like mad cow and Alzheimer's), and for possible materials applications (e.g., in tissue scaffolding). In this presentation, I will review the work of my group in examining the possibility that the left-handed beta helix (LHBH) structure can be the building block of the aggregates of mammalian prion and yeast prion proteins. I will also discuss our efforts to assess the possibility of a novel pH driven structural switch between LHBH and alpha-helical forms in the ordered half of the mammalian prion protein, and now the possibly pH stabilized LHBH structure can template aggregate growth of the disordered half of the protein, identified in numerous experimental studies as most relevant to disease.   

Early-Aggregation Studies of Polyglutamine in Solution

Several neurodegenerative diseases, notably Huntington's disease, are associated with certain proteins containing extended polyglutamine tracts. In all polyglutamine diseases, the age of onset is inversely correlated with the length of the polyglutamine domain beyond some pathological threshold. Diseased cells are characterized by intranuclear inclusions rich in aggregated polyglutamine. Experimental evidence suggests that oligomeric aggregate species, not mature amyloid fibrils, are the species most toxic to the cell. Little is known about the structures and aggregation dynamics of polyglutamine oligomers due to their short lifetimes. A better understanding of the pathway through which polyglutamine peptides form oligomeric aggregates will aid the design of therapies to inhibit their toxic activity. In this work, we report structural characterization of polyglutamine monomers and dimers from atomistic molecular dynamics simulations in explicit water. Umbrella sampling simulations reveal that the stability of the dimer species with respect to the disassociated monomers is an increasing function of the chain length.   

Search for Length Dependent Stable Structures of Polyglutamaine Proteins with Replica Exchange Molecular Dynamic

Motivated by the need to find beta-structure aggregation nuclei for the polyQ diseases such as Huntington's, we have undertaken a search for length dependent structure in model polyglutamine proteins. We use the Onufriev-Bashford-Case (OBC) generalized Born implicit solvent GPU based AMBER11 molecular dynamics with the parm96 force field coupled with a replica exchange method to characterize monomeric strands of polyglutamine as a function of chain length and temperature. This force field and solvation method has been shown among other methods to accurately reproduce folded metastability in certain small peptides, and to yield accurately de novo folded structures in a millisecond time-scale protein. Using GPU molecular dynamics we can sample out into the microsecond range. Additionally, explicit solvent runs will be used to verify results from the implicit solvent runs. We will assess order using measures of secondary structure and hydrogen bond content.   

Lysozyme Aggregation and Fibrillation Monitored by Dynamic Light Scattering

The aggregation of amyloidogenic proteins provides a rich phase space with significant biomedical implications, including a link with several age-related diseases. We employed dynamic light scattering to monitor the aggregation of lysozyme, a model protein, from a monomeric state until the formation of micron-sized fibrils. For an aqueous lysozyme solution buffered at pH 2, the auto-correlation function of the scattered light intensity was found to be well-fit by a single exponential function with decay time $\tau $ = 1/(2Dq$^{2})$ = 0.25 ms, which corresponds to a mean hydrodynamic radius (R$_{H})$ of 2.2 nm, very likely generated by monomers. Ethanol (4{\%} v/v final concentration) induced a partial unfolding, to R$_{H}$ = 4.6 nm. The subsequent addition of 70 mM KCl was found to shrink the size back to R$_{H}$ = 2.5 nm, as expected when a denatured protein refolds due to partial screening of the intramolecular repulsion. However, further aggregation was not observed. At pH 4, using a low-salt acetate buffer, more ethanol (10{\%} v/v) was required to initiate unfolding, but once it occurred, larger aggregates formed. These results are consistent with the model that partial unfolding, which exposes beta-motif secondary structure, is a prerequisite for aggregation and fibrillation, but the aggregation fate depends on the protein charge state (pH) and screening (salt concentration).   

Primary nucleation and linear aggregation: novel methods take us further

Understanding linear aggregation has long been of interest in the study of proteins, as it is relevant in several human diseases such Alzheimer's Disease. While much work has been carried out experimentally and numerically, relatively little has been done to develop an analytical model for the time evolution and scaling behaviour of such systems. Here, we present a novel mathematical framework, guided by physical insight, for approaching these problems. We then apply this to derive an exact expression for the evolution of the polymer length distribution of an amyloid system, driven by primary nucleation, taking into account both elongation and depolymerisation. This is presented in terms of a perturbative expansion, and is valid for negligible monomer depletion. We then apply a similar framework to solve a similar system that also includes an arbitrary number of conformational intermediates before reaching the final conformation - we uncover the time evolution of the total mass of polymers in the final conformational state, and demonstrate various properties about its time-scaling behaviour. We then apply self-consistent iterative schemes, starting from these solutions, to derive a series of approximate analytical models for these systems, taking monomer depletion into account.   

The Mechanisms of Aberrant Protein Aggregation

We discuss the development of a kinetic theory for understanding the aberrant loss of solubility of proteins. The failure to maintain protein solubility results often in the assembly of organized linear structures, commonly known as amyloid fibrils, the formation of which is associated with over 50 clinical disorders including Alzheimer's and Parkinson's diseases. A true microscopic understanding of the mechanisms that drive these aggregation processes has proved difficult to achieve. To address this challenge, we apply the methodologies of chemical kinetics to the biomolecular self-assembly pathways related to protein aggregation. We discuss the relevant master equation and analytical approaches to studying it. In particular, we derive the underlying rate laws in closed-form using a self-consistent solution scheme; the solutions that we obtain reveal scaling behaviors that are very generally present in systems of growing linear aggregates, and, moreover, provide a general route through which to relate experimental measurements to mechanistic information. We conclude by outlining a study of the aggregation of the Alzheimer's amyloid-beta peptide. The study identifies the dominant microscopic mechanism of aggregation and reveals previously unidentified therapeutic strategies.   

Amyloid growth: combining experiment and kinetic theory

The conversion of proteins from their soluble forms into fibrillar amyloid nanostructures is a general type of behaviour encountered for many different proteins in the context of disease as well as for the generation of a select class of functional materials in nature. This talk focuses on the problem of defining the rates of the individual molecular level processes involved in the overall conversion reaction. A master equation approach is discussed\footnote{Cohen et al, J Chem Phys 2011, 135, 065106} \footnote{Knowles et al, Science, 2009, 326, 1533-1537} and used in combination with kinetic measurements to yield mechanistic insights into the amyloid growth phenomenon.   

Scaling exponents report on changes in the mechanism of filamentous growth

We analyse the scaling behaviour in the proliferation and growth of protein nanostructures. We show that changes in scaling exponents that govern the lag time of the reaction within a given system can be identified with mechanistic changes that affect a molecular step in the assembly pathway. In this study, we focused on fibril growth from a representative protein, insulin, and an unstructured peptide, A$\beta$42. Our results reveal that the scaling exponent contains contributions not only from the dominant secondary nucleation mechanism in the systems studied, but also from the primary nucleation step even in cases where the generation of nuclei from the primary pathway is significantly smaller than from the secondary pathway. These results shed light on the origin of the scaling behaviour of filamentous growth.   

Control the kinetics and pathway of insulin fibril formation

Protein fibrils have been proposed as possible toxic agents for many amyloid related diseases, such as Alzheimer's disease, however the reaction pathway toward the amyloid fibrillation remain inadequately understood. In this work, we examine the conformational transition of human insulin as the model amyloid protein by single-molecule fluorescence spectroscopy and imaging. By controlling the pH cycling, insulin monomer and oligomers are indentified at given pH variation condition. Furthermore, low frequency ac-electric fields are employed to control the insulin aggregation from its monomers in a microchannel. It is observed that lag time to induce insulin fibrillation can be significantly shortened, in compassion to the commonly used cooling and seeding methods, and exhibits a strong dependence on applied ac-field strength. Additionally, the structure of insulin aggregates under ac-electric fields is observed to be drastically different from that under the temperature control.   

Time-Dependent and Low Temperature Studies of Amyloidogenic and Non-amyloidogenic Proteins by Dielectric Relaxation Spectroscopy

We present dielectric relaxation spectroscopy measurements of amyloidogenic Abeta (1-42) and non-amyloidogenic Abeta (42-1) proteins over a frequency range of 10 mHz to 10 MHz. Measurements were performed as a function of time from 0 to 24 h and temperature range of 193K-283K. Two relaxation peaks, alpha and beta, were observed at temperatures above 193 K. These peaks are attributed to the bulk and bound water. As a function of time, the dielectric signal of the Abeta (1-42) shifts towards the dielectric signal of the solvent while for the Abeta (42-1) the dielectric signal does not change. The activation energies of Abeta (1-42) and Abeta (42-1) were calculated and significant differences were found. We attribute these variations to structural changes that affect the hydration map of Abeta (1-42) aggregates. Our results are in agreement with theoretical predictions.   

Applying microfluidic techniques in quantitative studies of protein aggregation

Protein aggregation and fibrillation is involved in a number of devastating diseases, of which we have a limited understanding at present. Microfluidic techniques can be used in developing quantitative assays to study individual aspects of protein aggregation. Under certain conditions bovine insulin aggregates to give spherulites; spherical structures with fibrils growing and branching out from a central core. Drawing a parallel to actin polymerisation of the cell's cytoskeleton, fibril growth generates force. The force generated by polymerisation at fibril ends during spherulite growth can be measured in a microfluidic environment (TPJ Knowles et al, PNAS, 2009). By measuring the bending of four polydimethylsiloxane walls by a growing spherulite positioned in the centre, the force generated by polymerisation at fibril termini can be calculated. By growing the spherulites with a constant flow of monomer, the maximum force able to be generated by fibril growth, the stall force, can be calculated. This gives insight into the energy landscape of protein aggregation.   

Anomalous Formation of Multilayer Protein Aggregates on the Surface of Nanotubular $TiO_{2}$

Significant evidence links protein aggregation to the pathology and progression of most protein misfolding diseases. Protein aggregation also poses a significant problem for the safe and cost-effective production of therapeutic proteins. A comprehensive understanding of these problems requires both a detailed understanding native protein-protein interactions as well as an understanding of how protein-material interactions may alter protein aggregation phenomenon. Here we report on the anomalous formation of multilayered protein aggregates of globular proteins on the surface of $TiO_{2}$ nanotubes. Our findings suggest that minor alterations of the surface hydration properties of the nanotubes may drastically alter protein aggregation phenomenon. We further highlight the role of electrostatic and Van der Waals forces in this aggregation process.   

Effect of Lipid Bilayer on Human Islet Amyloid Polypeptide Self Assembly

Aggregates of human islet amyloid polypeptides (hIAPP, also known as human amylin) are commonly found in the pancreatic $\beta$-cells of type II diabetes patients. Experimental studies have shown that small aggregates of hIAPP, that arise during the assembly process, lead to membrane leakage and are highly cytotoxic. Due to the fast assembly kinetics, it is difficult to study the early aggregation of hIAPP experimentally. In this work, we use molecular simulation with a coarse grained (CG) model to investigate the oligomerization of hIAPP with and without the presence of lipid bilayers. We develop a CG protein model that reproduces the three thremodynamically stable structures of hIAPP, namely $\alpha$-helix, $\beta$-hairpin, and unstructured coil, and the corresponding free energy differences calculated by atomistic molecular simulations. The aggregated structure of hIAPP also agrees with that proposed by NMR experiments. We further investigate the assembly of hIAPP in the presence of a lipid bilayer and its effect on the membrane leakage. Comparing our results with the mechanism proposed based on experimental data provides a better understanding of the origins of hIAPP self assembly and its toxicity.   

Session H43: Invited Session: Physics of Systems Biology

Robustness of Biological Circuits

In responding to inputs biological systems do many types of calculations, some of which serve to maximize the signal and suppress noise. Physiology faces similar requirements and to fulfill these many sensory systems such as vision and hearing are designed to respond logarithmically to the fold change over initial conditions, a property known as ``Weber's Law.'' Mathematical modeling of the ancient Wnt signaling circuit suggested that a key design feature of that pathway was to provide a robust logarithmic output and as a consequence it does not respond robustly in a linear or hyperbolic way. We confirmed this behavior in Xenopus embryos and in human colorectal cells in culture. However, to read a robust logarithmic output from the Wnt pathway in the form of fold change in the levels of beta catenin, the responding transcriptional circuit must be able to compare initial and final levels of beta catenin accurately. This is exactly what we found in Xenopus embryos. What kind of transcriptional pathway has this property? Examination of the detailed promoter of a classic wnt responsive gene showed that two domains contribute to the calculation: the well known TCF/LEF sites that bind beta catenin near the coding sequence and a functional DNA feature upstream, which is unresponsive to beta catenin. The TCF/LEF sites on their own respond in a linear mode to the beta catenin concentration. The upstream functional feature confers the logarithmic response. Knowledge of the pathway structure should allow us to define a ``Weber's Law'' circuit that allows transcriptional systems to suppress noise by responding to fold change rather than to simple saturation.   

Epigenetic switches and network transitions

We investigate dynamics of gene networks which are regulated by both the fast binding/unbinding of transcription factors to/from DNA and the slow processes of chromatin structural change or histone modification. This heterogeneous dynamics consisting of different time scales is analyzed by the mean-field approximation and the stochastic simulation to show that the network exhibits multiple metastable states and is characterized by transitions among them. We discuss distribution and fluctuation of states of the core gene network of embryonic stem cells as an example of such heterogeneous dynamics and the simulated transitions are compared with the experimental data on the distribution of stem cell states.   

Landscape and Flux Framework for Non-Equilibrium Networks: Kinetic Paths and Rate Dynamics

We developed a general framework to quantify three key ingredients for dynamics of nonequilibrium systems through path integrals in length space. First, we identify dominant kinetic paths as the ones with optimal weights, leading to effective reduction of dimensionality or degrees of freedom from exponential to polynomial so large systems can be treated. Second, we uncover the underlying nonequilibrium potential landscapes from the explorations of the state space through kinetic paths. We apply our framework to a specific example of nonequilibrium network system: lambda phage genetic switch. Two distinct basins of attractions emerge. The dominant kinetic paths from one basin to another are irreversible and do not follow the usual steepest descent or gradient path along the landscape. It reflects the fact that the dynamics of nonequilibrium systems is not just determined by potential gradient but also the residual curl flux force, suggesting experiments to test theoretical predictions. Third, we have calculated dynamic transition time scales from one basin to another critical for stability of the system through instantons. Theoretical predictions are in good agreements with wild type and mutant experiments.We further uncover the correlations between the kinetic transition time scales and the underlying landscape topography: the barrier heights along the dominant paths. We found that both the dominant paths and the landscape are relatively robust against the influences of external environmental perturbations and the system tends to dissipate less with less fluctuations. Our theoretical framework is general and can be applied to other nonequilibrium systems.   

Experimental evolution in budding yeast

I will discuss our progress in analyzing evolution in the budding yeast, Saccharomyces cerevisiae. We take two basic approaches. The first is to try and examine quantitative aspects of evolution, for example by determining how the rate of evolution depends on the mutation rate and the population size or asking whether the rate of mutation is uniform throughout the genome. The second is to try to evolve qualitatively novel, cell biologically interesting phenotypes and track the mutations that are responsible for the phenotype. Our efforts include trying to alter cell morphology, evolve multicellularity, and produce a biological oscillator.   

Session H44: Focus Session: Nano to Mesoscale Structure in Ordered Systems: Liquid Crystal Topology and Defects

Chiral Self-Assembly

Chirality, the lack of reflection symmetry, at the molecular level has a profound influence on the ordering of molecular assemblages at the macroscopic scale. The example discussed here is the self-assembly of monolayers of rod-like fd virus particles, with the virus particles oriented on the average perpendicular to the plane of the layer, like a single layer of a smectic-A liquid crystal. Because these virus particles are chiral, they would prefer a twisted packing, which is incompatible with the layer structure. The twisted packing can only appear locally, at layer edges or in isolated defects in the interior of the layer. As chirality increases, the twisted regions achieve lower energy, until they can drive remarkable transformations to structures with longer edges and/or a greater number of defects.   

Pillar-Assisted Epitaxial Assembly of Focal Conic Domain Arrays in Smectic-A Liquid Crystals

We demonstrate a versatile approach to tailor the spacing and symmetry of periodic arrays of toric focal conic domains (TFCDs) over a large area via confinement of smectic-A liquid crystals (SmA LCs) in patterned substrates. Arrays of pillars with variable dimensions are employed to direct the assembly of TFCDs, determining both the domain positions and the size of the defects. Highly ordered square and hexagonal arrays of TFCDs result from topographical confinement of the LC in square and hexagonal arrays of pillars. Focal conic domains are shown to form only when the confined geometry provides sufficient area so that substrate-induced planar alignment of LC molecules is energetically favorable. Since the spacing and symmetry of the TFCD array can be readily pre-determined by the arrangement of the directing pillars, this pillar-assisted assembly technique serves as a model study for directed assembly of liquid crystals in three dimensions and offers improvements to the capability of LC-based templates for device fabrication and lithography.   

Shape and chirality transitions in twisted nematic elastomer ribbons: Finite element simulation studies

We use finite element simulation studies to explore transitions of shape and chirality in nematic elastomer ribbons with a twisted director configuration. Recent experimental and theoretical studies demonstrated that these fascinating materials show reversal of macroscopic chiral sense under a change of temperature, and explored shape selection as a function of the sample's aspect ratio. We explore these phenomena via three dimensional finite element simulation studies. For ribbons with width/thickness ratio above a threshold value, we find that on heating the sample undergoes a sequence of shape transitions from right handed helix -- right handed twisted ribbon -- flat ribbon -- left handed twisted ribbon -- left handed helix. Ribbons with width/thickness ratio below the threshold show fewer shape transitions, from right handed twisted ribbon -- flat ribbon -- left handed twisted ribbon. These results are in qualitative agreement with theoretical predictions, provide insight into experimental observations, and demonstrate the value of finite element methods for future engineering design of nematic elastomer devices.   

Frustrated orientational order on extrinsic geometries

The ground state of an anisotropic liquid on a curved surface has topological defects due to the surface's Gaussian curvature. The extrinsic geometry of the surface, however, frustrates the ground state arising from Gaussian curvature alone, changing the defect configurations in the ground state or expelling them from the surface altogether. We study nematic order on unduloids - a family of undulated cylinders with constant mean curvature spanning from the cylinder to a chain of spherical droplets - arising in systems ranging from liquid bridges to fluid membranes. We identify structural transitions in which pairs of disclinations of opposite signs are nucleated as the unduloid progresses from cylinder to spheres, explicitly separating the role of intrinsic geometry, which nucleates disclinations to screen Gaussian curvature, and extrinsic geometry, which expels defects from the neck. We describe some implications for the pinching off of a cylinder.   

Topology of knots and links in chiral nematic colloids

Nematic braids formed by disclinations entangling colloidal particles in chiral and achiral nematic liquid crystals are geometrically stabilized and restricted by topology. We report how self-linking number enables a classification of entangled defect lines [1, 2] and how a simple rewiring scheme for the orthogonal crossing of two half integer disclinations, based on a tetrahedral rotation of two relevant disclination segments allows us to predict nematic braids and their self-linking numbers. We further describe how using of laser micromanipulation enable the knotting of defect lines in chiral nematic colloids into knots and links of arbitrary complexity [3]. Colloids stabilized by nematic braids based on all knots and links with up to six crossings, including Hopf link, Star of David, Borromean rings are realized. We demonstrate how topology leads to the engineering of complex soft materials.\\[4pt] [1] S. Copar and S.Zumer, Nematic Braids: Topological Invariants and Rewiring of Disclinations, Phys. Rev. Lett. 106, 177801 (2011).\\[0pt] [2] S. Copar, T. Porenta and S. Zumer, Nematic Disclinations as Twisted Ribbons, Phys. Rev. E 84, 051702 (2011).\\[0pt] [3] U. Tkalec, M. Ravnik, S. Copar, S. Zumer and I. Musevic, Reconfigurable Knots and Links in Chiral Nematic Colloids, Science 333, 62 (2011).   

Modeling texture transitions in cholesteric liquid crystal droplets

Cholesteric liquid crystals can be switched reversibly between planar and focal-conic textures, a property enabling their application in bistable displays, liquid crystal writing tablets, e-books, and color switching ``e-skins.'' To explore voltage-pulse induced switching in cholesteric droplets, we perform simulation studies of director dynamics in three dimensions. Electrostatics calculations are solved at each time step using an iterative relaxation method. We demonstrate that as expected, a low amplitude pulse drives the transition from planar to focal conic, while a high amplitude pulse drives the transition from focal conic back to the planar state. We use the model to explore the effects of droplet shape, aspect ratio, and anchoring conditions, with the goal of minimizing both response time and energy consumption.   

Necklaces of Liquid Crystal Beads: Nematic Drops Constrained on Thin Cellulosic Fibers

Liquid crystal droplets dispersed in a continuous matrix have important applications in electro-optical devices. They also produce intriguing topological defect structures due to the confinement of the liquid crystal by closed boundaries that impose alignment at the interface. In this work we use a simple method to generate stable liquid crystal droplets topologically equivalent to a toroid by depositing tiny volumes of a nematic liquid on cellulosic micro-fibers (1 $\mu $m diameter) suspended in air. This system can exhibit non-trivial point topological defects which can be energetically unstable against expanding into ring defects, depending on the fibers constraining geometries. By changing temperature, the system remains stable and allows the study of the defects evolution near the nematic-isotropic transition showing qualitatively different behavior on cooling and heating processes. The necklaces of such liquid crystal drops constitute excellent systems for fundamental studies and open new perspectives for applications. This work was sponsored by Air Force Office of Scientific Research, Air Force Material Command, USAF, under grant number FA8655-10-1-3020. The US Government is authorized to reproduce and distribute reprints for Governmental purpose notwithstanding any copyright notation thereon. Other support includes the Portuguese Science and Technology Foundation grant SFRH/BD/63574/2009 and projects PEst-C/CTM/LA0025/2011 (Strategic Project - LA 25 - 2011-2012, PTDC/CTM/099595/2008, PTDC/FIS/110132/2009 and Windsor Treaty grant 2009-10 UR55.   

Controlling the location of defects in nematic shells

We study nematic shells with four s=+1/2 defects and vary the elastic constant anisotropy of the liquid crystal by approaching the nematic-to-smectic phase transition temperature. We find the defects ultimately arrange themselves along a great circle, consistent with recent expectations. Changing the elastic constant anisotropy provides an alternative route to changing the shell thickness inhomogeneity for controlling the defects location.   

Structures in liquid crystalline shells at a nematic-smectic transition

Liquid crystalline shells display phenomena that are fascinating from a fundamental physics point of view and they also hold promise for innovative applications e.g. for advanced colloids. The key feature of these shells is the unavoidable presence of topological defects, the types and numbers of which depend on the phase as well as the geometrical features of the shell. Here we present our investigation of the complex internal reordering phenomena occurring in a liquid crystalline shell undergoing a transition between the nematic and smectic phases. In the smectic phase, the topological and geometrical constraints of a spherical shell with symmetric boundary conditions imposed on the developing 1D quasi-long-range order create a conflict that triggers a series of buckling instabilities, resulting in two different characteristic defect patterns. We will also show very recent results on shells with asymmetric boundary conditions, giving rise to beautiful complex patterns, some transitory, some stable. The phase transition between nematic and smectic order yields varying textures depending on the shell size and thickness, and on the specific alignment types at the shell in- and outside, respectively.   

Visualizing, manipulating, and imprinting $\pi$-wall defects in self-assembled colloidal membranes

Geometric frustration and the resulting topological defects play an important role in determining the structural, mechanical and optical properties of materials. Here we describe the behavior of a new type of defect, called a $\pi$-wall, in a model system of colloidal membranes composed of chiral rod-like \textit{fd} viruses. We use complimentary optical microscopy techniques to study the structure and energetics of $\pi$-walls, and develop a model based on the analogy between liquid-crystals and superconductors to determine the structure and energetics of $\pi$-walls. We then focus on $\pi$-wall formation, showing that $\pi$-walls naturally assemble through a unique coalescence process in which chiral frustration plays an essential role. $\pi$-walls can also be artificially created and engineered using externally applied optical forces.   

Session H45: Focus Session: Thin Film Block Copolymers - Phase Behavior

Controlling the Orientation of Block Copolymer Thin Films with Selective and Neutral Nanoparticles

The bottom up approach using self-assembly of block copolymers (BCP) have been considered as a powerful technique which can resolve the limitations of conventional nanolithograph. For practical applications, the perpendicular orientation of microdomains with respect to the substrate is a prerequisite. However in most cases, one of the domains has a preferential interaction with the substrate and this interaction induces parallel orientation of the microdomains. To overcome the preferential interaction and to obtain vertically orientated BCP microdomains, diverse approaches have been developed. Previously, we synthesized thermally stable core-shell gold nanoparticles using UV cross-linkable BCP and also precisely tuned the surface property of nanoparticles which are selective and neutral to PS and PMMA by adjusting the composition of polymeric ligand. Moreover, we demonstrated the effect of selective and neutral gold nanoparticles on the orientation of PS-PMMA thin films. Herein, we further investigated the effect of selective and neutral gold nanoparticles on the orientation of BCP thin films of lamellar and cylindrical BCP according to the film thickness. The thin film morphologies were characterized with AFM and SEM.   

Assembly and Photo-Induced Disorder in Block Copolymer-Additive Systems

Additives that hydrogen bond selectively to one block of a weakly ordered or disordered block copolymer can drive phase segregation to yield well ordered materials. Here we show that the addition of D- or L-tartaric acid to low molecular weight, weakly segregated poly(ethylene oxide-block-tert-butyl acrylate), PEO-b-PtBA, induces strong segregation and well ordered morphologies as evidenced by Small Angle X-ray Scattering. This strong interaction between enantiopure tartaric acid and the PEO block also suppresses PEO crystallinity at room temperature. While the addition of racemic tartaric acid does not strengthen segregation nor does it suppress PEO crystallization. The UV-exposure of well ordered films of PEO-b-PtBA/tartaric acid blends containing a photo acid generator followed by a post-exposure bake results in the deprotection of the tert-butyl acrylate block to yield poly(acrylic acid) (PAA). Since PAA and PEO are miscible and tartaric acid can interact strongly with either block, the system becomes disorder, resulting in a photo-induced disordering transition which can be exploited to pattern the surfaces. The kinetic behavior of the disordering transition upon deprotection of PtBA to PAA was studied using Grazing-Incidence Small-Angle X-ray Scattering.   

Diblock copolymer morphologies in ultra thin films under shear

We demonstrate that the shear alignment and the shear-induced transitions in sphere-forming diblock copolymer single layer and bilayer films observed experimentally can be explained by cell dynamics simulation, a simple model with a Ginzburg-Landau Hamiltonian. In two layer films the spheres align in various arrangements, like (100) or (110) bcc plains, or transform to cylinders depending on the shear rate and the temperature. We present a nontrivial alignment mechanism of a single layer of spherical domains in shear via slug-like movement of transient cylindrical micelles. In addition, we clarify the formation of the perpendicular cylinders, found in the recent particle based simulation. We also present results on lamellae shearing in ultra-thin films.   

Structure and dynamics of random block copolymers in the bulk and thin films

Using a soft, coarse-grained model and a Lennard-Jones bead-spring model, we study the morphology of random block copolymers in the bulk and in contact with a hard wall that preferentially attracts one component. We show that both coarse-grained models yield similar equilibrium morphologies at intermediate and long length scales, and identify a mapping between the parameters of the two models. For most parameters we observe a disordered, microemulsion-like morphology. We study the single-chain dynamics in the bulk and in contact with a preferential surface. The relaxation times of the soft, coarse-grained model is about two orders of magnitude faster than the Lennard-Jones bead-spring model. In both models the relaxation time increases with segregation but the Lennard-Jones bead-spring model is additionally slowed down by the densification of the local packing at low temperatures. We employ the soft, coarse-grained model to generate starting configurations for the bead-spring model. Then, the bead-spring model is quenched below its glass transition temperature, and we investigate the local mechanical properties of the disordered, yet structured morphology.   

Macrophase Separation of Block Copolymer Blends in Thin Films

The behavior of macrophase-separating blends of symmetric polystyrene-b-poly(methyl methacrylate) block copolymers in thin films on non-preferential surfaces was characterized with respect to the blends' molecular weights and composition. When the molecular weight ratio of block copolymers is at least five, the blends segregate into a domain of nearly pure small block copolymer with a small lamellar period and a domain of about equal volume fraction of each block copolymer with a larger lamellar period. The composition and structure of these blends was determined by analyzing the period and area fraction of each phase from plan-view scanning electron micrographs. The lamellae propagate through the thickness of the film. The use of block copolymer blends has potential in block copolymer lithography as a strategy to pattern different period structures in the same film.   

Phase segregation at the sub-5-nm scale using high $\chi $ Poly(ethylene oxide-b-dimethylsiloxane) copolymers

Silicon containing block copolymers, such as Poly(styrene-b-dimethylsiloxane) (PS-PDMS), have recently received significant attention for nanolithographic applications [Jung et al., Nano Letters 2010, 10, 1000]. PDMS provides indeed a robust and highly selective mask to oxygen reactive ion etching. In addition, the high Flory-Huggins $\chi _{PS-PDMS}$ parameter (about 0.3 at room temperature) favors the segregation of low molecular weight (16 kg/mol) block copolymers (BCPs) into well-organized structures with pitch as small as 17 nm. In an effort to downscale further the size of structures formed by BCP, we decided to turn to copolymers with even higher $\chi $ parameters. Copolymers of Poly(ethylene oxide) (PEO) and PDMS are known to have extremely high $\chi $ parameters (0.4 -1.1) [Galin et al., Macromolecules, 1981, 14, 677], but their bulk and thin film properties have not been investigated in detail. PEO-PDMS BCPs were synthesized by chain coupling \textit{via} a versatile copper-activated azide-alkyne click reaction. The unusually high $\chi $ parameter between EO and DMS allowed strong phase segregation to occur in copolymers with molecular weight as low as 5kg/mol. The full pitch were found to be less than 10 nm and we report on their bulk and thin film characteristics.   

Decoupling Bulk Thermodynamics and Wetting Characteristics of Block Copolymer Thin Films

The incorporation of a random copolymer molecular architecture has been known to induce notable changes in the physical properties, often with commercial implications. In this presentation, the consequences of controlled degrees of epoxidation on both bulk and thin-film properties of poly(isoprene) blocks in poly(styrene-$b$-isoprene) (PS-PI) diblock copolymers and poly(isoprene) (hPI) homopolymers have been studied, where the products after epoxidation are denoted as hPIxn and PS-PIxn. Small angle X-ray scattering and dynamic mechanical spectroscopy were conducted on PS-PIxn to calculate the effective interaction parameters $\chi _{eff}$ between the PS and PIxn blocks in bulk (3-D) while the surface energy of thin-film PIxn (2-D) was estimated based on contact angle measurements on hPIxn and lamellar orientations of thin-film PS-PIxn. A non-linear change with a minimum at the intermediate degrees of modification is observed for $\chi _{eff}$ in bulk whereas thin-film experiments suggest that the surface energy of PIxn increases linearly with epoxidation. This decoupling of bulk and thin-film thermodynamic behaviors is attributed to the different roles that a random copolymer architecture plays in establishing 3-D order versus wetting at a 2-D surface.   

Continuity and connectivity of lamellar-forming block copolymers in thin films

Thin films of block copolymers are emerging as a low-cost lithographic material. In order to effectively utilize this class of materials, the structure of the self-assembled morphologies in thin films must be well understood. In this work, the network structure formed by a lamellar diblock copolymer of polystyrene (PS) and poly(methyl methacrylate) (PMMA) was explored as the compositional symmetry was varied through homopolymer addition. The volume fraction of PMMA was varied from 0.45 to 0.55. The long-range connectivity of the PS and PMMA domains, as well as the branch and endpoint density, was characterized. Increasing the compositional asymmetry of the copolymer system leads to interconnected networks that span arbitrarily large areas, increased branch density, and decreased endpoint density. The network structure for each copolymer system also depends on annealing time, annealing temperature, and surface chemistry of the substrate. Improved understanding of the variability in lamellar morphologies will enable the selection of copolymers, annealing conditions, and surface chemistries to fabricate lithographic masks by self-assembly.   

Theoretical and Experimental Investigations of Contact Hole Shrink using PS-PMMA Block Copolymers

One possible application of block copolymer directed self-assembly (DSA) involves rectification or ``shrink'' of contact holes in a polarity switched photoresist (see, e.g., J. Cheng et al., ACS Nano 8, 4815 [2010]). The block copolymer (e.g., PS-PMMA) undergoes ordering inside the cylindrical hole; the central (PMMA) domain is then etched out so that the hole diameter is effectively reduced. We utilize strong segregation theory (SST) and numerical self-consistent field theory (SCFT) to calculate the phase behavior of the block copolymers as function of their molecular weight and composition, as well as the contact hole diameter and the surface chemistry of the walls. The model predictions were compared with experimental data, and a good agreement was found. The results illustrate how modeling can serve to guide block copolymer selection for DSA contact hole rectification application.   

Tailoring block copolymer morphology via control of topographical surface: A self consistent field theoretic study

It is well known that chemically patterned or topologically complex substrates can direct self-assembly of adsorbed layers or thin films of block copolymers. In this study we have examined the self-assembly of a lamella-forming diblock copolymer guided by topological complexity, namely, substrates composed of trenches with different heights and widths. In general, when the substrate is neutral to both blocks of the copolymer, the perpendicular lamella morphology is obtained. However, when the substrate has a preferred affinity to one of the blocks, a host of novel morphologies including different bi-continuous network structures can be created by judiciously manipulating the trench height and width. Overall, this study clearly demonstrates the impact of this class of simulations in rational design of morphologies in thin multi-component polymeric films with application to technologies such as filtration, and high-surface area membranes.   

Silicon patterning using self-assembled PS-b-PAA diblock copolymer masks for anti-reflective black silicon fabrication via plasma etching

The diblock copolymer of poly(styrene-b-acrylic acid) is a novel self-assembling mask material for pattern transfer applications. This material system has high dry etch selectivity and can produce a variety of feature types and size scales. Different vertical profiles were produced by altering the etch recipes and diblock copolymer or SiO2 mask processing. This patterning technique is used to fabricate antireflective silicon metamaterials that show broadband anti-reflection properties in the visible and infrared wavelength range ($<$5{\%} total reflection). These materials are potentially useful for solar cell and light sensing applications. Similar surface roughening by chemical etch, porous silicon, nanowires and other methods have been used previously to reduce reflectance from material interfaces for photovoltaics and antireflection applications. Unlike these methods, the BCP self-assembled pattern transfer via RIE produces robust patterns that are tunable in both the horizontal and vertical directions without harsh chemicals or expensive catalysts. This simple and rapid process can also be applied to semiconductors other than silicon.   

Assembly of block copolymer films between chemically patterned and chemically homogeneous surface

Many technologically useful block copolymer systems other that poly(styrene-block-methylmethacrylate) are currently not amenable for directed assembly because one of the blocks has a lower surface energy, segregates to the free surface of the film, and disrupts directed assembly of the film (at least with respect to realizing perpendicularly oriented through-film domains) on the underlying chemical pattern. Cross-linkable random copolymer mats were developed as well as methods to deposit them on the surfaces of block copolymer films. The chemistry of these ``top coats'' can be tuned to impart preferential and non-preferential wetting properties towards the blocks of the block copolymer films. The three-dimensional morphology of block copolymers assembled between lithographically-defined chemically patterned surfaces and top coats of varying wetting properties were characterized using specialized sample preparation techniques and cross-sectional scanning electron microscopy. The resulting structures compare favorably with molecular simulations. A primary technological objective of the top coat strategy is to direct the assembly of block copolymer systems that allow for sub-10 nm patterning and perpendicularly oriented domains.   

Session H46: Invited Session: Polymer Physics Prize

Polymer Physics Prize Lecture: Polyelectrolyte complexes: New routes to useful soft materials

Mixtures of oppositely charged polyelectrolytes may form precipitates (phase-separated solids) or coacervates (phase-separated fluids). Coacervates have been known for a long time to have interesting properties such as very low interfacial tension with water and a resultant ability to coat surfaces, engulf particles and invade porous media. Most prior work on coacervate complexes has been done with structurally complex (e.g., gum Arabic), biologically derived macromolecules (e.g., gelation). Our work is focusing on phase behavior and self-assembly in classes of structurally simpler polymers. Polypeptides are one such class, where we can produce anionic, cationic and neutral, water-soluble polymers all with the some backbone and varying in small side-group structures. We are able to demonstrate very general patterns in phase behavior over different members of this class of polymers. Coacervate formation is the rule rather than the exception in these materials, with such formation quite strongly peaked at balanced stoichiometry of the polyelectrolyte components. One molar salt is usually sufficient to dissolve the coacervate phases that form. Block copolymer mixtures containing oppositely charged blocks can form self-assembled structures: micelles with diblocks and hydrogels with triblocks. The structure and properties of these assemblies can be tuned based on knowledge of the bulk phase behavior response to molecular weight, stoichiometry and salt concentration. Examples of phase behavior and structure-property relationship will be discussed.   

Physical and Biological Properties of Engineered Protein Hydrogels

Injectable hydrogels show substantial promise for use in minimally invasive tissue engineering and drug delivery procedures. A new injectable hydrogel material, developed from recombinant telechelic proteins expressed in \textit{E. coli}, demonstrates shear thinning by three orders of magnitude at large strains. Large amplitude oscillatory shear illustrates that shear thinning is due to yielding within the bulk of the gel, and the rheological response and flow profiles are consistent with a shear-banding mechanism for yielding. The sharp yielding transition and large magnitude of the apparent shear thinning allow gels to be injected through narrow gauge needles with only gentle hand pressure. After injection the gels reset to full elastic strength in seconds due to rapid reformation of the physical network junctions, allowing self-supporting structures to be formed. The shear thinning behavior is largely independent of the midblock length, enabling genetic engineering to be used to control the equilibrium modulus of the gel without loss of the characteristic yielding behavior. The shear-banding mechanism localizes shear stresses during flow into narrow regions of the gel, allowing more than 95{\%} of seeded cells to survive the injection process.   

Enhancing Biopolymer Dynamics through Destruction

Microtubules are cytoskeletal filaments that organize intracellular space structurally and through active transport along their lengths. They need to be organized and remodeled quickly during development of differentiated cells or in mitosis. Much work has focused on remodeling from the ends because these long polymers can stochastically disassemble through dynamic instability or be actively disassembled. Microtubule-severing enzymes are a novel class of microtubule regulators that create new ends by cutting the filament. Thus, these proteins add a new dimension to microtubule regulation by their ability to create new microtubule ends. Interestingly, despite their destructive capabilities, severing has the ability to create new microtubule networks in cells. We are interested in the inherent biophysical activities of these proteins and their ability to remodel cellular microtubule networks. Interestingly, despite their destructive capabilities, severing has the ability to create new microtubule networks in cells. We use two-color single molecule total internal reflection fluorescence imaging to visualize purified severing enzymes and microtubules \textit{in vitro}. We have examined two families of severing enzymes to find that their biophysical activities are distinct giving them different network-regulating abilities.   

Session H48: Polymer Gels and Solutions

Phase Segregation and Dynamics in Strongly Interacting Small Molecule Additive - Block Copolymer Surfactant Complexes

Rheology and Small Angle X-Ray Scattering (SAXS) were used to investigate order to disorder transitions (ODTs) and disorder to order transitions (DOTs) of poly(ethyleneoxide-b-propyleneoxide-b-ethyleneoxide) block copolymer surfactants mixed with hydrogen-bond-donating small molecule additives. A series of additives having a core benzene ring and systematic variation in the number of carboxylic or hydroxyl groups attached to the ring were of particular interest. Ordered cylindrical morphologies, confirmed using SAXS, were obtained only in a certain additive concentration region. ODTs were characterized by sudden changes in the linear viscoelastic properties in low frequency region upon increasing temperature. The locations of ODTs varied widely with hydrogen-bond-donating ability of the functional group and were found to be strongly dependent on the number of functional groups attached to the ring. For a given additive, the temperature at which ODT occur was strong function of the additive loading, whereas the linear viscoelastic properties of the ordered state were little changed upon varying additive concentration in ordered region. The location and dynamics of DOTs upon cooling were comparable to the ODTs upon heating. Studies using these model systems provide insight into the design of well-ordered hybrid materials.   

Using Mesoscopic Models to Design Strong and Tough Biomimetic Polymer

Using computational modeling, we investigate the mechanical properties of polymeric materials composed of coiled chains, or globules, which encompass a folded secondary structure and are crosslinked by labile bonds to form a network. In the presence of an applied force, the globules can unfold into linear. Our goal is to determine how to tailor the labile intra- and inter-molecular bonds within the network to produce material exhibiting both toughness and strength. We use the lattice spring model (LSM) to simulate the globules and the crosslinked network. We utilize our modified Hierarchical Bell model (MHBM) to simulate the rupture and reforming of $N$ parallel bonds. We demonstrate that the mechanical properties of the system are sensitive to the values of $N_{in}$ and $N_{out}$, the respective values of $N$ for the intra- and inter-molecular bonds. We find that the strength of the material is mainly controlled by the value of $N_{out}$, with the higher value of $N_{out}$ providing a stronger material. We also find that if $N_{in}$ is smaller than $N_{out}$, the globules can unfold under the tensile load before the sample fractures and thus, can increase the ductility of the sample.   

Synthesis and Characterization of Poly(hydroxyethyl methacrylate) Hydrogels Bearing Reversibly Associating Side Groups

Poly(hydroxyethyl methacrylate) (poly(HEMA)) is a technologically important hydrogel that can be processed into different shapes and is best known for its role in contact lenses. However, applications of water swollen polyHEMA are limited by its poor mechanical properties. We are studying the influence of reversibly associating side groups on the behavior of poly(HEMA) hydrogels. In non-polar media, it is well known that ureidopyrimidinone (UPy) groups self-associate to form hydrogen bonded dimers (DDAA); however their behavior in water-swollen hydrogels is unclear. A series of poly(HEMA) linear polymers of controlled molecular weight with varying UPy content have been prepared using a reversible addition-fragmentation chain transfer (RAFT) polymerization technique. UPy content significantly reduces water swelling and improves mechanical properties. The degree of hydrogen bonding within water swollen hydrogels is studied, and properties of functional hydrogel polymers and networks are compared to an unswollen hydrophobic analog.   

Thermoreversible Supramolecular Ion Gels via Hydrogen Bonding

Ion gels are a novel class of functional materials of broad interest for advanced applications. We have developed a thermoreversible supramolecular ion gel system consisting of a poly(2-vinylpyridine-$b$-ethylene oxide-$b$-2-vinylpyridine) (P2VP-PEO-P2VP) triblock copolymer, a poly(4-vinylphenol) (PVPh) linear homopolymer, and an ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([EMI][TFSA]), where a highly inter-connected transient polymer network is formed by hydrogen bonding between the P2VP endblocks and PVPh cross-linkers. This system exhibits novel physical properties, such as interesting dynamics and homopolymer clustering in the cross-links. The applicability of time-temperature superposition to this system is striking, resulting in a master curve that extends over 20 orders of magnitude in reduced frequency. The hydrogen-bonded phase can arrange into a hexagonally packed cylindrical morphology with long-range ordering, which reveals very slow kinetics and is thermodynamically stable only within a narrow temperature window. The highly tunable relaxation dynamics as well as shear modulus might enable materials design for specific applications.   

Hydrogen Bonding Based Layer-by-Layer Assembly of Poly(vinyl alcohol) with Weak Polyacids

Multilayer thin films that consist of poly(vinyl alcohol) (PVA) and weak polyacids such as poly(acrylic acid) (PAA) and poly(methacrylic acid) (PMAA) were prepared by hydrogen bonding interactions. Both the degree of hydrolysis and molecular weight of PVA were investigated in terms of their influence on growth behavior and pH stability. Multilayer films containing PVA and PAA could be assembled successfully only by using partially hydrolyzed PVA and low pH solutions. By comparing films containing PAA with those containing a more strongly interacting partner, PMAA, it was shown that the extent of PVA hydrolysis becomes significant only when weak hydrogen bonding pairs such as PVA and PAA were used. pH-triggered dissolution experiments demonstrated that the degree of hydrolysis can be used as an additional parameter by which to tune the pH stability of the film. Also, the presence of an abundance of free hydroxyl and carboxylic acid groups in the multilayer allowed enhanced pH stability to be obtained by thermal and chemical methods as well as numerous opportunities for post-assembly functionalization.   

Gelation in a model 1-component system with adhesive hard-sphere interactions

Colloidal dispersions can undergo a dynamical arrest of the disperse phase leading to a system with solid-like properties when either the volume fraction or the interparticle potential is varied. Systems that contain low to moderate particulate concentrations form gels whereas higher concentrations lead to glassy states in which caging by nearest neighbors can be a significant contributor to the arrested long-time dynamics. Colloid polymer mixtures have been the prevalent model system for studying the effect of attraction, where attractions are entropically driven by depletion effects, in which gelation has been shown to be a result of phase separation [1]. Using the model 1-component octadecyl coated silica nanoparticle system, Eberle et al. [2] found the gel-line to intersect the spinodal to the left of the critical point, and at higher concentrations extended toward the mode coupling theory attractive driven glass line. . We continue this study by varying the particle diameter and find quantitative differences which we explain by gravity. \\ \\ 1. Lu, P.J., et al., Nature, 2008. 453(7194): p. 499-504.\\ 2. Eberle, A.P.R., N.J. Wagner, and R. Castaneda-Priego, Physical Review Letters, 2011. 106(10).   

Concentration Fluctuations in Polymer Solutions under Extensional Flow

Polymer solutions under flow are known to exhibit stress-concentration coupling that can anisotropically amplify concentration fluctuations. This phenomenon has been extensively studied for shear flow, but is less well understood for extensional flows. Using a two-fluid model, we study concentration fluctuation amplification in polymer solutions under a variety of extensional flows, including mixed shear and extension.   

Crossover length scale for viscosity experienced by Gold nanoparticles in semi dilute polymer solution

Gold nanoparticles (Au NPs) were used as a probe to characterize the crossover length scale for viscosity in semi dilute poly (ethylene glycol) (PEG)-water solutions. Fluctuation correlation spectroscopy (FCS) was used to measure the diffusion of these NPs as a function of their size (5-20 nm), PEG concentration (0 to 40\%w/w) and PEG molecular weight (5 kDa to 35 kDa). Our results indicate that for particles with radius R $>$ R$_{g}$, usual hydrodynamic conditions can be applied, but for particles with radius R $\leq$ R$_{g}$, the diffusion is approximately an order of magnitude faster than that predicted by Stokes Einstein (SE) relation. The results imply that radius of gyration R$_{g}$ of the polymer gives the crossover length scale from nanoviscosity to macroviscosity. The relative viscosity experienced by the particles was scaled as $\eta$/ $\eta$$_{0}$ = exp (b (R$_{g}$/$\xi$) $^{a}$), where $\eta$$_{0}$ is the water viscosity, $\xi$ is the correlation length, a = 0.70 $\pm$ 0.03 and b = 1.59 $\pm$ 0.07.   

Kinetics of Narrowly dispersed Latex Formation in a Surfactant-free Emulsion Polymerization of Styrene in Acetone-Water Mixture

The kinetics of narrowly dispersed latex formation in a surfactant-free emulsion polymerization of styrene in acetone-water was studied by a combination of transmission electron microscopy and light scattering. The critical nuclei were experimentally observed and the formation of narrowly dispersed PS latex is proved to be originated from competitive growth kinetics. Spherical nuclei were regenerated via a microphase inversion of PS oligomer in 50{\%} volume fraction acetone-water mixture at 70$^{\circ}$C. They follow a polydispersed log-normal distribution and the smallest nucleus with Rs 1.1nm is similar to critical nuclei. Note the spherical nuclei are not necessarily narrowly dispersed. Competitive growth kinetics makes smaller nuclei grow much faster than large nuclei in the subsequent polymerization process, resulting in narrowly dispersed PS latex. Two kinds of PS seed particles were added, separately, into two parallel surfactant-free emulsion polymerization batches of styrene in acetone-water mixture at 70$^{\circ}$C. It was found that the size of seed particles almost does not change, but the small size PS latex grows rapidly. Our fitting results proves competitive growth kinetics proposed by Vanderhoff and coworkers.   

Cluster Growth in Aqueous Sugars Observed by Dynamic Light Scattering

Dynamic light scattering of aqueous sugar solutions as a function of sugar concentration and temperature reveal the development of sugar clusters occurring in two stages. At low volume fractions of sugar, a so-called cluster phase consisting of nearly monodisperse clusters forms with a mean cluster mass that increases in proportion to the volume fraction. At a critical volume fraction, near where the clusters begin to overlap, a second stage ensues wherein cluster-cluster aggregation forces a more rapid, power law growth in advance of a percolation threshold observed near 83 wt{\%} sugar.   

Boson peak in L-cysteine: a Raman scattering study

The Boson peak is a distinctive feature of many glassy and disordered crystalline solids. Recently it has been suggested that similar feature may be correlated to anharmonic transitions observed in macromolecules like DNA and proteins. In the present work we studied the low frequency ($15-600$ cm$^{-1}$) Raman scattering response of L-cysteine and L-Cysteine hydrochloride with different hydration levels in the $15-270$ K temperature range. Our analyzes will be concerned to understand the water rule in the Boson peak inelastic light signal, its correlation to the dynamic transitions at T$^{*}\sim 80$ K and T$_{D}\sim 280$ K, and its microscopic origin as well.   

Low temperature dynamic transitions of L-cysteine and L-proline amino acids: a specific heat study

Studies have shown that several macromolecules present two dynamic transitions at T* $\sim $ 80 K and TD $\sim $ 280 K which have intrinsic correlation to their biological activity. The present work concerns the detailed analysis of the low temperature transition at T* by specific heat. This transition is usually described as originated on the CH2SH dynamics. We compared the experimental results with simulations based on rigid rotor specific heat model by Caride and Tsallis [J. Stat. Phys. \textbf{35}, 187 (1984)] and we found an excellent agreement.   

Session H49: Focus Session: Fluctuation-Induced Forces in Soft Matter & Polymeric Systems - Casimir and Interfacial Forces

Casimir Interaction at Soft Interfaces

We study Casimir interaction due to the thermal fluctuations between colloidal particles on the membranes and fluid interfaces. To calculate the Casimir energy we employ the scattering formalism. In this technique the shape and material properties of the colloids are encoded in their scattering matrices. The energy is calculated by combining the scattering matrices with the universal translation matrices, which convert between the bases used to compute scattering for each colloid, but otherwise are independent of the physical and chemical properties of the colloids and the interface. We show that in the scattering formalism one can easily implement various geometries and material properties and more importantly calculate the energy for all separation regimes.   

Critical Casimir Interactions: New fluctuation forces in colloidal science

Casimir forces arise from the confinement of fluctuations between two walls. Critical Casimir forces provide thermodynamic analogues of quantum-mechanical Casimir forces and arise from the confinement of concentration fluctuations of a critical solvent. These forces act also between colloidal particles that are suspended in this solvent, giving rise to temperature-dependent attractive interactions between the particles. We use these temperature-dependent forces to control colloidal phase transitions. In this talk, I will present a new index and density-matched model system that allows direct observation of these phase transitions with confocal microscopy. In three dimensions and real time, we follow how a colloidal gas freezes into a colloidal liquid, and the colloidal liquid freezes into a solid, all driven by critical Casimir forces. We measure the critical Casimir particle pair potential directly from the pair correlation function, and use Monte Carlo simulations to map the complete gas-liquid-solid phase diagram. Excellent agreement with the experimental observations is obtained. Our measurements include microgravity experiments on board the International Space Station (ISS) to elucidate non-equilibrium assembly of the particles achieved by controlled temperature quench.   

Thermodynamic Casimir effect in the large-$n$ limit: Exact results for slabs with free surfaces

The $O(n)$ $\phi^4$-model for a three-dimensional slab of thickness $L$ and infinite lateral extension is investigated in the large-$n$ limit. The effective (Casimir-like) forces that are induced between the two confining boundary planes by thermodynamic fluctuations in such systems at and near bulk criticality are studied. While systems with periodic or antiperiodic boundary conditions perpendicular to the planes are translationally invariant and thus can be treated analytically, the physical relevant case of free boundary conditions leads to a breaking of translational invariance perpendicular to the boundary planes. In the large-$n$ limit one arrives at a spherical model with separate constraints for each layer parallel to the confining surfaces. The resulting self-consistent Schr\"odinger-type equation is solved numerically at and near the bulk critical temperature to obtain the large-$n$ limit of the scaling functions of the Casimir force and the excess free energy. The Casimir amplitude is calculated with high accuracy to take the value $\Delta_{\mathrm{C}}=-0.01077340685025(10)$. The Casimir force scaling function shows a minimum below the bulk critical temperature similar to the $n=2$ result.   

Fluctuation-induced forces in a fluid membrane under tension

We develop an exact method to calculate thermal Casimir forces between inclusions of arbitrary shapes and separation, embedded in a fluid membrane whose fluctuations are governed by the combined action of surface tension, bending modulus, and Gaussian rigidity. Each object's shape and mechanical properties enter only through a characteristic matrix, a static analog of the scattering matrix. We calculate the Casimir interaction between two elastic disks embedded in a membrane. In particular, we find that at short separations the interaction is strong and independent of surface tension.   

The Casimir forces between inclusions in a fluid membrane

We discuss the fluctuation-induced force, a finite-temperature analogue of the Casimir force, between two foreign inclusions embedded in a stretchable fluid membrane. Specifically, we suggest a Green's-function-based method to calculate the Casimir interaction in cases where the fluctuations of a planar membrane are governed by Helfrich Hamiltonian, including the surface tension $\sigma$ and both bending $\kappa$ and Gaussian $\bar\kappa$ rigidities. For two circular inclusions in a fluid membrane, the Casimir energy scales as the inverse power law of the separation and is greatly reduced beyond the characteristic length $\ell_0=\sqrt{\kappa_0/\sigma_0}$. The impact of line tension is also discussed.   

Aspect-ratio dependence of thermodynamic Casimir forces

We consider the three-dimensional Ising model in a $L_{\perp}\times L_{\parallel}\times L_{\parallel}$ cuboid geometry with finite aspect ratio $\rho=L_{\perp}/L_{\parallel}$ and periodic boundary conditions along all directions. For this model the finite-size scaling functions of the excess free energy and thermodynamic Casimir force are evaluated numerically by means of Monte Carlo simulations [1]. The Monte Carlo results compare well with recent field theoretical results for the Ising universality class at temperatures above and slightly below the bulk critical temperature $T_{\mathrm{c}}$. Furthermore, the excess free energy and Casimir force scaling functions of the two-dimensional Ising model are calculated exactly for arbitrary $\rho$ and compared to the three-dimensional case. We give a general argument that the Casimir force vanishes at the critical point for $\rho=1$ and becomes repulsive in periodic systems for $\rho>1$. \\[4pt] [1] Alfred Hucht, Daniel Gr\"uneberg, and Felix M. Schmidt, Phys. Rev. E 83, 051101 (2011)   

Field-Theoretic Simulations of Bicontinuous Microemulsions in Polymer Blends

Long diblock copolymers introduced into a blend of thermodynamically incompatible homopolymers can act as a surfactant to supress macroscopic phase separation of the blend. As the fraction of diblock copolymer is varied, an isotropic Lifshitz tricritical point is observed in the mean-field equations, demarking the crossover from macrophase to microphase separation. Close to the Lifshitz point, fluctuations are strong enough to supress the low-temperature formation of a well-ordered microphase leading to the appearance of a long-lived bicontinuous microemulsion characterized by micron-scale continuous domains. In this work, we discuss computational strategies for simulating the equilibrium formation of the microemulsion using field-theoretic methods. We address the challenges involved with accurately localizing the order-disorder transition in a fluctuating theory, and the handling of strong fluctuations close to the Lifshitz point.   

Looking Down at Adsorption Dynamics: Filament Height Measurements with TIRFM

Polymer adsorption is important in several contexts such as chromatography, colloid stabilization, and bio-fouling. Despite a cross-disciplinary interest in the subject, there are not many techniques to observe single-molecule absorption events in real time. We use TIRF microscopy to accomplish this using the biological polymer f-actin adsorbed to a microscope slide via the well-known depletion interaction. We find TIRFM is able to quantitatively measure filament height, and we compare our results to theoretical predictions and previous results obtained from other systems.   

Partial osmotic compressibility of binary mixtures of colloidal nanoparticles and PEG

Proposed originally by Oosawa and Asakura, polymer crowding-induced attractive force between colloidal particles is being used in a variety of applications ranging from protein crystallization to nanoparticle sorting. While the force has been well studied for a pair of micro particles in the presence of polymers, direct measurement of such force between nanoparticles is very difficult. To investigate effects of crowding polymers, we propose an approach to measure the colloidal osmotic compressibility and viral coefficients in the presence of polymers and compare experimental results with theoretical models. The materials we investigated are binary mixtures of fluorescent polystyrene nanospheres (100 -- 210 nm in diameter) and polyethylene glycol (PEG). Using fluorescence microscopy to examine the change of the particle concentration in an optical trap, which exerts no force upon PEG, allows us to measure the partial osmotic compressibility of the particles. The measured partial compressibility and its virial expansion are compared with theoretical calculations to elucidate the competing effects of polymer crowding and adsorption.   

Session H50: Polymer Melts

Coil-Globule Transition of Polymer -- Tricriticality of Theta

We demonstrate the tricriticality of the theta point in the coil-to-globule transition of a single flexible polymer chain by dynamic Monte Carlo (DMC) simulation. For homopolymer, at the tricritical point, the second order theta transition line approaches the first order collapse transition line as the chain length approaches the thermodynamic limit. Theta point of homopolymer has been estimated by following the ideal behavior of chain at the theta point. Temperature at which the constant volume specific heat (Cv) shows a peak is considered as the collapse temperature and the transition is of first order. In T-N plane, collapse temperature approaches towards the theta temperature as the chain length approaches to the thermodynamic limit. For copolymer (with periodic distribution of comonomer along the chain), the collapse temperature increases with increasing the stickiness parameter (viz., higher solvophobicity of the comonomers relative to monomers) and approaching towards the theta as the value of the stickiness parameter increases. Theta points of copolymers have been estimated from the theta point of homopolymer under the condition of the ideality of the theta point. The collapse temperatures for copolymer have been in a manner similar to homopolymers. Theta temperature vs. stickiness parameter represents a second order line whereas collapse temperature vs. stickiness parameter represents a first order line. We demonstrate that as the stickiness parameter increases, these two lines meet each other, close to the ``tricritical'' theta temperature.   

Microscopic Theory of Nanoparticle Diffusivity in Entangled and Unentangled Polymer Melts

We present a statistical dynamical theory at the level of forces for the violation of the Stokes-Einstein (SE) diffusion law of a spherical nanoparticle in entangled and unentangled polymer melts. Based on a combination of mode-coupling and polymer physics ideas, the non-hydrodynamic friction coefficient is related to microscopic structure and the length-scale-dependent polymer melt collective density fluctuation dynamics. When local packing correlations are neglected, analytic expressions are derived for the non-hydrodynamic diffusivity as a function of particle size, polymer radius-of-gyration, tube diameter, degree of entanglement and temperature; local packing effects are numerically investigated under athermal and attractive conditions. The conditions for the recovery of the SE law are qualitatively distinct for unentangled and entangled melts, and entanglement effects are the origin of large SE violations consistent with recent experiments. The influences of melt packing fraction and interfacial attraction strength are also qualitatively different depending on whether the polymers are entangled or not. The anomalous time-dependence of the nanoparticle mean square displacement is studied using a self-consistent Generalized Langevin Equation approach.   

Effect of Secondary Structure on the Persistence Length of a Poly N-substituted Glycine

A polymer containing helical secondary structure is shown to be nearly as flexible as a chemically analogous polymer containing no structure. Polypeptoids or poly N-substituted glycines are a class of sequence specific polymers in which chain shape can be controlled via monomer choice involving both sterics and chirality. In this study, a polypeptoid containing aromatic chiral sidechains was synthesized. Classical measurements such as circular dichroism and NMR have shown previously that the bulky chiral side chains cause the polypeptoid to adopt a helical conformation. However, small angle neutron scattering demonstrated that in acetonitrile, the persistence length of the helical polypeptoid was approximately 1 nm, only about 15{\%} of the fully extended helical length. This small persistence length indicates that the chain likely adopts several conformations in solution and is not rigidly locked into its helical shape throughout the entire length of the polymer   

Gibbs Ensemble Calculations of Phase Coexistence in Supramolecular Assembly of Block Copolymers

We propose a new self-consistent field theory method for calculating phase behavior in reversibly bonded supramolecular polymer melts. Previous studies formulated models for supramolecular assembly in the grand canonical ensemble to make use of the constraints imposed on the chemical potentials of the products from chemical equilibrium. Instead, we formulate the model in the canonical ensemble by including a term in the Hamiltonian that accounts for the reaction favorability/penalty. The chemical equilibrium statement is obtained by optimizing the Hamiltonian with the amount of reacted polymer. The canonical partition function can be easily adapted to the Gibbs ensemble whereby phase boundaries between coexisting phases can be conveniently simulated. As an illustration of our method, we examine a blend of AB diblock and B homopolymer with the ability to reversibly bond to form ABB diblock. In the limits of infinite reaction favorability and penalty, the system approaches cases of an ABB diblock-B homopolymer blend when the AB diblock is the limiting reactant and an AB diblock-B homopolymer blend, respectively. The interplay between reactant ratios (stoichiometry) and reaction favorability/penalty is explored for intermediate values of reactivity.   

Pressure-induced structural transition in an one-component polymer poly(4-methyl-1-pentene) in its melted state

Liquid-liquid transitions or amorphous-amorphous transitions are well-known in systems with small molecular or structural units, such as those seen in water, phosphorus, and silica glass. We studied pressure-induced structural change for isotactic poly(4-methyl-1-pentene) in its melted state, and found a structural transition for this one-component polymer melt. High-pressure in-situ x-ray diffraction and specific-volume measurements on the polymer melt have uncovered discontinuities in the pressure dependences of microscopic structure as well as those of macroscopic density. The results suggest the occurrence of a liquid-liquid phase transition.   

Anomalous diffusion of a polymer chain in an unentangled melt

Contrary to common belief, the hydrodynamic interactions (HI) in polymer melts are not screened beyond the monomer length and are important in transient regimes. We show that the viscoelastic HI effects (VHI) lead to anomalous dynamics of a tagged chain in an unentangled melt at $t < t_N$ ($t_N$, the Rouse time). The chain centre-of-mass (CM) mean-square displacement is enhanced (as compared to the Rouse diffusion) by a large factor increasing with chain length. We develop an analytical theory of VHI-controlled chain dynamics yielding negative CM velocity autocorrelation function which quantitatively agrees with our MD simulations without any fitting parameter. It is also shown that the Langevin friction force, when added in the model, strongly affects the short-$t$ CM dynamics which, however, can remain strongly enhanced. The transient VHI effects thus provide the dominant contribution to the subdiffusive CM motion universally observed in simulations and experiments on polymer melts.   

Representation of Polymer melts as soft liquids with effective pair interactions

Descriptions at various levels of coarse-graining are of great interest in understanding the complex structure and dynamics of polymer liquids, as relevant processes take place at a wide variety of length and time scales. In this talk we present and analytically characterize effective interaction potentials to map a polymer melt onto a liquid of soft spheres or soft-colloid chains, with each soft colloid representing the center of mass of a chain or large subsection of a chain. The thermodynamics of the coarse-grained system using the effective potentials can be shown to agree with the thermodynamics of monomer-level descriptions across a range of thermodynamic states. The scaling of the effective pair potentials beyond the physical extent of the polymer with increasing chain interpenetration is shown to be essential to capturing the contribution to the thermodynamics of the melt due to many-polymer interactions in the soft coarse-grained description, despite its vanishingly small effect on structural correlations. Further development of the theory will consider its extension from soft-colloid chains to lower level bead and spring level descriptions with the aim of gaining insight into the thermodynamic properties of these widely used models.   

Entropy of Mixing: Rigid vs. Flexible Molecules: Effect of Varying Solvent on Dissolution Temperature

We report a study of the dissolution temperature of n-hexacontane, as a function of concentration, with 52 different solvents, aimed at understanding the effect of molecular flexibility on the entropy-of-mixing. The entropy-of-mixing of rigid molecules is commonly expected to go as ln(mole fraction), while for flexible polymers it is expected to follow Flory-Huggins ln(volume fraction). By isolating the entropy-of-mixing, we have experimentally found that rigidity, through ring structures, causes deviations from the Flory-Huggins behavior; and we have proposed and derived a cross-over form for the entropy-of-mixing which varies between ln(volume fraction) and ln(mole fraction) according to molecular rigidity   

Polymeric Fluid Flow Over Superhydrophobic Surfaces

Superhydrophobic (SHP) surfaces are characterized by their exceptionally low surface energies and distinct surface roughnesses that create a vapor layer between the fluid and the surface. The reduced contact area at the interface can create a dewetted state resulting in slip, drag reduction, and improved flow of fluids. Most previous superhydrophobic studies have utilized simple liquids (e.g. water) in focusing on characterization of the quiescent interface and on drag reduction or slip modifications of fluid flow. As polymeric fluid flows have exhibited similar slip and drag reduction phenomena, this study attempts to utilize SHP surfaces to improve the flow behavior of more complex multi-component fluids, such as polymer solutions. By merging the research fields of SHP surfaces and polymer fluids, we investigate the potential to enhance slip and drag reduction effects as a result of surface interactions. Microfluidic channels, interfacial rheometry and goniometry are used to evaluate slip length and fluid flow.   

Reconstructing Polymer Melt Dynamics Sped up due to Large-Scale Coarse-Graining

A theoretical approach to rescale the artificially fast dynamics of highly coarse-grained polymer melts is extended to chains represented as soft particles. The effective pair potential derived from first-principles to represent polymer chains with soft interactions is used to perform mesoscale molecular dynamics simulations of coarse-grained melts. This potential ensures the reproduction of the correct global structure and thermodynamics, but entropic and frictional corrections are necessary to reconstruct realistic dynamics. The behavior of the system is described by generalized Langevin equations derived for different levels of coarse-graining. The explicit analytical dependence on the thermodynamic and molecular parameters enhances the predictive power of the reconstruction method. The dynamics, reconstructed from mesoscale simulation, is in quantitative agreement with experiments and atomistic simulations.   

Steric Constraints in Fractal-Regime Star Polymers

Star polymers at high functionality, f, and high arm length, z$_{arm}$, display a collapsed core structure described by Daoud and Cotton in a colloidal regime (CR). At lower functionality (f$<\sim $8) and relatively low arm length, stars display a polymeric structure in a fractal regime (FR). For FR stars in good solvents the arms display steric interactions analogous to polymer chains tethered to a surface. We have used small-angle neutron scattering to quantify, for the first time, this steric interaction so as to understand the approach to the CR as a function of z$_{arm}$ and f as well as temperature and solvent type. Experimental data from model star polymers and literature data from polyurethane stars are considered as examples.   

Capillary break-up, gelation and extensional rheology of hydrophobically modified cellulose ethers

Cellulose derivatives containing associating hydrophobic groups along their hydrophilic polysaccharide backbone are used extensively in the formulations for inks, water-borne paints, food, nasal sprays, cosmetics, insecticides, fertilizers and bio-assays to control the rheology and processing behavior of multi-component dispersions. These complex dispersions are processed and used over a broad range of shear and extensional rates. The presence of hydrophobic stickers influences the linear and nonlinear rheology of cellulose ether solutions. In this talk, we systematically contrast the difference in the shear and extensional rheology of a cellulose ether: ethy-hydroxyethyl-cellulose (EHEC) and its hydrophobically-modified analog (HMEHEC) using microfluidic shear rheometry at deformation rates up to 10$^6$ inverse seconds, cross-slot flow extensional rheometry and capillary break-up during jetting as a rheometric technique. Additionally, we provide a constitutive model based on fractional calculus to describe the physical gelation in HMEHEC solutions.   

Session H51: Colloids II: Crystals and Phase Transitions

Effects of vacancies and interstitials on the phonon modes in a 2D colloidal crystal

We report a study of the effects of vacancies and interstitials on the phonon modes in a 2D colloidal crystal. By applying the equi-partition theorem, we extract the dispersion relation of the lattice vibrations in a two-dimensional colloidal crystal using real-time video microscopy. We find that both longitudinal and transverse modes in the spectrum are softened by the existence of vacancies and interstitials.   

Normal modes of various colloidal crystals

We measured the vibrational normal modes from particle displacements in various microgel colloidal crystals including monolayers, multi-layer thin films, three-dimensional normal and superheated crystals by video microscopy. Their density of states all agree with the Debye's theory in the low-frequency regime, but the fluctuation of the frequency is similar to that of the eigenvalues of random matrices: the distributions of the frequency spacings between successive normal modes are the Wigner surmise, the spectral rigidities are logarithmic, and the distributions of vibrational amplitudes in the majority of modes are Gaussian. In addition, the first a few low-frequency modes are plane waves and dominate the thermal vibration, and the majority of modes are delocalized.   

Microscopic observation of dynamics and structure in microgel suspensions

We use 3D confocal microscopy to understand the packing dynamics and structure of fluorescently labeled p(NIPAm-co-AAc) microgel colloidal particles. Such systems respond to changes in temperature, pH, and polymer content by changing size, morphology, and interaction behavior. We conduct experiments to understand this behavior in detail: our results show that the dynamics are dominated by attraction driven crystallization and concentration at low pH and concentration only at high pH. Crystal nucleation occurs homogeneously in the suspensions and does not appear to be restricted to geometric boundaries. The growth of crystals is nucleation-limited and can complete on the order of hours. Structural analysis of the crystals formed indicates that the stacking style is insensitive to charge, concentration, size, and stiffness of the particles and remains FCC.   

Homogeneous Melting of 3D Superheated Colloidal Crystals

We locally superheated the interior of thermal-sensitive microgel colloidal crystals and measured the homogenous melting by video microscopy. The nucleation was typically started from a local strong-vibrating region instead of precursor defects. We found that the nucleation time $t \sim (\phi-\phi_{m})^{-2}$ and critical nucleus size $r^* \sim (\phi-\phi_{m})^{-1}$ as predicted by the classical nucleation theory, while the observed non-spherical critical nuclei and the merging of subcritical nuclei are beyond the classical nucleation theory. At the superheated limit where the incubation time vanishes, the Lindemann parameter approaches 0.18 which just equals to that at the liquid-solid interface. Beyond the superheated limit, the melting becomes like a spinodal decomposition rather than a nucleation process.   

Polyhedral assembled colloids (PACs); a new family of colloids with facets

We introduce a new class of colloids with polyhedral morphology that self-assemble into well-defined clusters and crystals by means of directional attraction between facets. These micron-sized particles are prepared by controlled crystallization of metal ions and organic bridging ligands in solution. They are characterized by distinct polyhedral morphology, rhombic dodecahedra in this work. Unlike spheres that isotropically interact along a curved surface, rhombic dodecahedra particles in suspension associate in a directional facet-to-facet fashion, forming clusters whose elemental units are orderly not only in interparticle distance but also mutual orientation. Furthermore, by changing the particle concentration during the self-assembly, we observe two types of hexagonal arrangement of these rhombic dodecahedra.   

Measuring every particle's size in a confocal microscopy experiment

We have developed a technique to estimate the radius of every particle observed in a confocal microscopy experiment. From simulations, we verify that the particle radii are estimated to high accuracy in a variety of samples: dense colloidal suspensions, colloidal gels, and binary samples. This method allows us to determine {\it in situ} the particle size distribution. Furthermore, this method lets us find relationships between individual particle size and dynamical behaviors. First, crystal nucleation occurs in regions that are locally more monodisperse. Second, in dense samples, particle mobility is well correlated with the local volume fraction, defined as the true particle volume divided by the particle's Voronoi volume.   

Vibrational Phonon Modes of Two-Dimensional Soft-Particle Colloidal Crystals with Hard-Particle Dopants

We study the phonon modes of two-dimensional colloidal crystals consisting of random distributions of ``soft'' NIPA microgel particles and ``hard'' polystyrene particle dopants. Thus, the effective springs connecting nearest-neighbors are very stiff, very soft, or of intermediate stiffness, corresponding to three possible inter-particle potentials present in the crystals. We employ video microscopy to derive the phonon modes of corresponding ``shadow'' crystals with the same geometric configuration and interactions as the experimental colloidal system, but absent damping [1,2,3]. Long wavelength, Debye-like behavior is found at low frequencies, regardless of the number of hard polystyrene particles present in the crystal. Hard particles are primary participants at high frequencies, while soft spheres are primary participants at intermediate frequencies. [1] Chen \textit{et al}., PRL \textbf{105}, 025501 (2010). [2] Kaya \textit{et al}., Science \textbf{329}, 656 (2010). [3] Ghosh \textit{et al}. PRL \textbf{104}, 248305 (2010).   

Direct observation of the nucleation in colloidal solid-solid transitions

We studied the solid-solid transitions between square and triangular lattices in thermal sensitive microgel colloidal thin films by video microscopy. A novel two-step nucleation process was observed in a locally heated single crystalline domain: typically a $\sim$60-particle liquid nucleus was first from the square lattice and then rapidly transformed to a solid nucleus with triangular lattice. Such a post-critical triangular-lattice nucleus grew linearly and induced grain boundaries around it. Nuclei were triggered by the merging of stronger vibrating areas instead of precursor defects. The critical nucleus size was measured from the mean first passage time of the nucleus size.   

Colloidal gas-liquid transition: tuning nucleation and growth by Critical Casimir forces

The nucleation and growth of the liquid phase has been well studied in simulations, but direct experimental observations remain challenging. Here we present a detailed study of the colloidal gas-liquid transition induced by Critical Casimir forces that allow direct control over particle interactions via temperature-dependent solvent fluctuations. We show that with the direct control over particle interactions we can ``freeze'' a dilute colloidal gas into a dense colloidal liquid. By using dynamic light scattering to follow the evolution of liquid aggregates we observe three clearly distinct regimes: nucleation, interface limited- and diffusion limited growth. We elucidate these regimes directly in real space by using confocal microscopy. In the nucleation regime, we determine the Gibbs free energy, interfacial tension and chemical potential of the liquid aggregates directly from their size distribution. In the growth regime, we can directly follow the attachment of particles, and the collapse of liquid aggregates to large drops. Our critical Casimir colloidal system allows us to control all stages of nucleation and growth with temperature, thereby providing unprecedented insight into this gas-liquid transition.   

Temperature control of colloidal phases by Critical Casimir forces -- a simulation study

Critical Casimir forces arising from the confinement of critical solvent fluctuations between the surfaces of colloidal particles have recently been shown a promising route to control colloidal assembly. Such forces are strongly temperature dependent, and thus allow for direct temperature control of colloidal interactions. However, colloidal phase transitions controlled by this highly temperature-dependent potential are still poorly understood. Here, we report Monte Carlo simulations of critical Casimir-driven colloidal phase behavior using input potentials directly measured in experiments. We map the gas-liquid coexistence region using Gibbs ensemble simulations and the solid-fluid coexistence boundaries using Gibbs-Duhem integration, and determine the gas-liquid critical point by applying scaling theory. The constructed gas-liquid-solid phase diagram agrees quantitatively with that observed in experiments. Remarkably, the simulated gas-liquid coexistence curve exhibits 3D Ising scaling despite the strong temperature dependence of the pair potentials.   

Colloidal aggregation in microgravity by critical Casimir forces

We study aggregation and crystal growth of spherical Teflon colloids in binary liquid mixtures in microgravity by the critical Casimir effect. The critical Casimir effect induces interactions between colloids due to the confinement of bulk fluctuations (density or concentration) near the critical point of liquids. The strength and range of the interaction depends on the length scale of these fluctuations which increase as one approaches the critical point. The interaction potential can thus be tuned with temperature. We follow the growth of structures in real time with Near Field Scattering. Measurements are performed in microgravity in order to study pure diffusion limited aggregation, without disturbance by sedimentation or flow.   

Light transport through soft colloidal glasses

We have developed a novel colloidal system for the fundamental study of light propagation through disordered media. Our colloids contain core-shell particles with scattering cores and transparent shells which are self-assembled into amorphous, glassy configurations. The core-shell structure of the particles allows us to independently control two key parameters for light propagation: their scattering cross-section, which is determined by the cores, and their spacing, which is determined by the shells. Thus, our system is ideally suited for the study and manipulation of the optical properties of disordered materials. In particular, we aim to investigate how photonic stop bands arise in disordered media and how near-field coupling between scatterers affects light transport. We intend to use this knowledge to make amorphous colloids with various angularly-independent structural colors.   

3D Dynamic Light Scattering of Microgel Suspensions

We use 3D cross-correlated dynamic light scattering to investigate suspensions of microgels. We will discuss some of the theory behind this technique and present results from size- and pH-tunable pNIPAM microgels cross-linked with PEG and copolymerized with acrylic acid.   

Using colloidal packings as templates for structuring drugs

Many pharmaceutical compounds are poorly soluble in water; this is problematic because most pharmaceuticals are delivered orally and must dissolve in the gastrointestinal fluid in order to be taken up by the body. We introduce a simple method for increasing the dissolution rates of poorly water-soluble organic actives. We demonstrate that by structuring the compounds within the interconnected, nanoscale pore space of a colloidal packing we create composites which rapidly disintegrate in water, exposing the nanostructured organic active and leading to improved dissolution rates.   

Spin coating of superparamagnetic colloids with applied magnetic fields

We report experimental results on the behavior of dilute superparamagnetic colloids under shear stresses (using a commercial spin-coater and varying its rate of rotation) with applied magnetic fields. For the case of zero field, we compare the results obtained for different kind of particles (non-magnetic [1] vs PS based [2] vs silica based [3]) and solvents by analyzing the dried deposits obtained from the spin coating. All the data collapse in a single curve, when the appropriate scaling for the film thickness is performed. This agreement allows us to define a reference to measure the relative change in viscosity, when a magnetic field is applied during the spin coating. Thus, we show the magnetorheological properties of colloidal dispersions. These results shed light into the aggregation and clustering dynamics for colloids under external fields with shear and provide a new method to study the rheological properties of colloids.\\[4pt] [1] M. Giuliani et al. ``Dynamics of crystal structure formation in spin-coated colloidal films'' J. Phys. Chem. Lett. 1(9), 1481 (2010)\\[0pt] [2] M. Pichumani et al. ``Spin-coating of dilute magnetic colloids in a magnetic field'' Magnetohydrodynamics, 47(2), 191 (2011)\\[0pt] [3] M. Pichumani et al. In preparation   

Session H52: Focus Session: Extreme Mechanics - Rods

Dynamic curling of a naturally curved Elastica

We consider the motion of a naturally curved Elastica that has been flattened onto a hard surface. When it is released from one end, the Elastica lifts off the surface and curls dynamically into a moving spiral. The motion is governed by inertia, bending and geometric nonlinearity. At long times, the dynamics follows a self-similar regime: the size of the spiral grows like the cubic root of time, while the velocity of the front reaches a constant value. The asymptotic velocity is derived analytically, and compared to numerical simulations and to experiments.   

The Mechanics of Curly Hair

We explore the oft-neglected role of intrinsic natural curvature on the mechanics of elastic rods. Our testbed, a hanging hair, is a deceivingly simple system that exhibits complex mechanics and geometrically nonlinear behavior. Through a combination of precision desktop-scale experiments, numerical simulations, and theoretical analysis, we seek physical insight into the nontrivial configurations adopted by a naturally curved elastic rod that is suspended under its own weight. In particular, we aim to gain predictive understanding of the transition from planar to non-planar solutions as well as the localization of torsion in the non-planar configurations. The experimentally observed behavior of our custom-fabricated naturally curved rods is captured well by simulations and is rationalized through scaling arguments.   

The Shape of a Ponytail and the Statistical Physics of Hair Fiber Bundles

From Leonardo to the Brothers Grimm our fascination with hair has endured in art and science. Yet, a quantitative understanding of the shapes of a hair bundles has been lacking. Here we combine experiment and theory to propose an answer to the most basic question: What is the shape of a ponytail? A model for the shape of hair bundles is developed from the perspective of statistical physics, treating individual fibers as elastic filaments with random intrinsic curvatures. The combined effects of bending elasticity, gravity, and bundle compressibility are recast as a differential equation for the envelope of a bundle, in which the compressibility enters through an ``equation of state.'' From this, we identify the balance of forces in various regions of the ponytail, extract the equation of state from analysis of ponytail shapes, and relate the observed pressure to the measured random curvatures of individual hairs.   

Following the equilibria of slender elastic rods

We present a novel continuation method to characterize and quantify the equilibria of elastic rods under large geometrically nonlinear displacements and rotations. To describe the kinematics we exploit the synthetic power and computational efficiency of quaternions. The energetics of bending, stretching and torsion are all taken into account to derive the equilibrium equations which we solve using an asymptotic numerical continuation method. This provides access to the full set of analytical equilibrium branches (stable and unstable), a.k.a bifurcation diagrams. This is in contrast with the individual solution points attained by classic energy minimization or predictor-corrector techniques. We challenge our numerics for the specific problem of an extremely twisted naturally curved rod and perform a detailed comparison against a precision desktop-scale experiments. The quantification of the underlying 3D buckling instabilities and the characterization of the resulting complex configurations are in excellent agreement between numerics and experiments.   

The elasticity of magnetic chains: From self-buckling to self-assembly

Spherical neodymium magnets have become a popular toy in recent years. In this talk, we present the results of some experimental and theoretical investigations into the peculiar elastic-like behaviour exhibited by chains of these magnetic spheres. We show how the dipole-dipole interactions between spheres penalise deformation, and we find that the form of this penalty is different for a long chain compared to a short chain. Finally, we investigate the dynamic self-assembly of these chains into cylindrical structures.   

Spontaneous and Deterministic Three-dimensional Curling of Pre-strained Elastomeric Strips: From Hemi-helix to Helix

A variety of three dimensional curls are produced by a simple generic process consisting of pre-straining one elastomeric strip, joining it to another and then releasing the bi-strip. The hemi-helix, one kind of three dimensional curls, consists of multiple, alternating helical sections of half wavelength in opposite chiralities and separated by perversions. The hemi-helix wavelength and the number of perversions are determined by the strip cross-section, the constitutive behavior of the elastomer and the value of the pre-strain. Topologically, the perversions also separate regions of the helix deforming principally by bending from those where twisting dominates. Changing the prestrain and the ratio between the thickness and the width induce a phase separation of hemi-helical structure, helical structure and hybrid structure which have similarities to coiled polymer molecules and plant tendrils.   

Snakes Out of the Plane

We develop a new computational model of elastic rods, taking into account shear and full rotational dynamics, as well as friction, adhesion, and collision. This model is used to study the movement of snakes in different environments. By applying different muscular activation patterns to the snake, we observe many different patterns of motion, from planar undulation to sudden strikes. Many of the most interesting behaviors involve the snake rising out of the horizontal plane in the vertical direction. Such behaviors include a sand snake sidewinding over the hot desert sand and a cobra rearing up into a defensive striking position. Experimental videos of live snakes are analyzed and compared with computational results. We identify and explain a new form of movement previously unobserved: ``collateral locomotion.''   

Analysis of the fluid mechanical sewing machine

A thin thread of viscous fluid falling onto a moving belt generates a surprising variety of patterns, similar to the stitch patterns produced by a traditional sewing machine. By simulating the dynamics of the viscous thread numerically, we can reproduce these patterns and their bifurcations. The results lead us to propose a new classification of the stitch patterns within a unified framework, based on the Fourier spectra of the motion of the point of contact of the thread with the belt. The frequencies of the longitudinal and transverse components of the contact point motion are locked in most cases to simple ratios of the frequency $\Omega_c$ of steady coiling on a surface at rest (i.e., the limit of zero belt speed). In particular, the ``alternating loops'' pattern involves the first five multiples of $\Omega_c/3$. The dynamics of the patterns can be described by matching the upper (linear) and the lower (non-linear) portions of the thread. Following this path we propose a toy model that successfully reproduces the observed transitions from the steady dragged configuration to sinusoidal meanders, alternating loops, and the translated coiling pattern as the belt speed is varied.   

Microfabrication of a spider-silk analogue through the liquid rope coiling instability

Spider capture silk outperforms most synthetic materials in terms of specific toughness. We developed a technique to fabricate tough microstructured fibers inspired by the molecular structure of the spider silk protein. To fabricate microfibers (with diameter $\sim 30\mu m$) with various mechanical properties, we yield the control of their exact geometry to the liquid rope coiling instability. This instability causes a thread of honey to wiggle as it buckles when hitting a surface. Similarly, we flow a filament of viscous polymer solution towards a substrate moving perpendicularly at a slower velocity than the filament flows. The filament buckles repetitively giving rise to periodic meanders and stitch patterns. As the solvent evaporates, the filament solidifies into a fiber with a geometry bestowed by the instability. Microtraction tests performed on fibers show interesting links between the mechanical properties and the instability patterns. Some coiling patterns give rise to high toughness due to the sacrificial bonds created when the viscous filament loops over itself and fuse. The sacrificial bonds in the microstructured fiber play an analogous role to that of the hydrogen bonds present in the molecular structure of the silk protein which give its toughness to spider silk.   

Buckling Instability and Stress Propagation in Rods with Elastic Support

The cytoskeleton of living cells is a composite material consisting of a network of biopolymers including f-actin and microtubules (MTs). MTs are able to bear significant compressive loads in cells as a result of reinforced short wavelength buckling, due to the surrounding actin network. However, the length scale of compressive force propagation, even for macroscopic rods, remains poorly understood. Here we propose a minimal theory that incorporates elastic restoring forces from the surrounding network, elucidating the compressive force-dependence of the buckled rod shape. We identify a threshold length as the effective distance stresses can propagate in such network, and show that the decay length is tunable by modifying the longitudinal mechanical coupling coefficients. We test these predictions with experiments in macroscopic rods, and show that the degree of mechanical coupling directly controls the penetration depth of buckling, in agreement with theoretical and numerical predictions. Our results suggest that the length scale over which mechanical signals are transduced in cells may be actively controlled, and could provide design principles for novel types of fiber composite materials based on biomimetic control of the longitudinal coupling coefficients.   

Undulatory buckling of a rod constrained by an elastic matrix

Elastic instabilities of rods constrained by an elastic matrix and subjected to axial compression have long been recognized as essential for structural applications in the context of failure mitigation and, more recently, towards exploitation of functionality. Relevant fields for this class of problems include drilling, biomedical instrumentation and root growth in plants. We explore the two possible scenarios observed when, above a threshold load, compression is applied to a rod constrained by a matrix: i) the rod can develop a planar oscillatory solution (sinusoidal buckling) or ii) it can take the configuration of a helix (helical buckling). We identify the principal parameters of this system, perform a systematic parametric study and rationalize the phase diagram through a hybrid of theoretical and numerical analyses. Particular attention is devoted to the effect of the mechanical properties of the constraining matrix which is found to have a critical influence on this buckling scenario.   

Slack, stress, and noisy structures in inertial strings

Strings and chains are inextensible filaments with negligible bending and twist resistance. Local arc length conservation is enforced by the stress, a Lagrange multiplier field screened by curvature. Uniform stress fields are generated by a wide class of inertial motions that includes travelling waves of curvature and torsion, while gradients in stress result in more complicated dynamics. We will discuss a theoretical example inspired by experimental and numerical observations of the growth of an arch in a straightening chain, involving the amplification, rectification, and advection of slack.   

Sinusoidal to helical buckling of an elastic rod under a cylindrical constraint

We investigate the buckling and post-buckling behavior of an elastic rod loaded under cylindrical constraint. Our precision desktop-scale experiments comprise of axially compressing a hyper-elastic rod inside a transparent acrylic pipe. These experiments are also modeled using a discrete elastic rod simulation that includes frictional effects. Under imposed displacement, the initially straight rod first buckles into a sinusoidal mode and eventually undergoes a secondary instability into a helical buckling regime. The buckling and post-buckling behavior is found to be highly dependent on the systems' geometry, in particularly the aspect ratio of the rod to pipe diameter. We quantify the wavelength and pitch of the periodic patterns through direct digital imaging and record the reaction forces at both ends of the pipe. The observed behavior is rationalized through scaling arguments and captured by numerical simulations.   

Session H53: Focus Session: Continuum Descriptions of Discrete Materials

Failure of a loose packing of grains

One-sided repulsive interactions and history-dependent friction forces can cause the mechanics of real granular systems to deviate significantly from that of cohesive solids. Marked deviations from elastic behavior can be seen in the mechanical response and structure of sedimented loose packings of frictional spheres even for very delicate perturbations. In our experiments, particles' displacements are observed with 3D fluorescent imaging as a shear plane is displaced through the packing. As anticipated, we find the shear force is approximately linear with the displacement between discrete yielding events. However, even in this apparently linear region, structural aging continues to occur for the smallest displacements that we can apply. This suggests the inaccessibility of a reversible shear deformation regime in loose packings of granular material. We present a spatial characterization of the particle motion that distinguishes between these continuous instances of irreversibility and the larger discrete failure events.   

Jamming in Hopper Flows: Analysis of Survival Times

Many granular systems experience a transition from a fluid-like state to a solid-like state characterized by a sudden arrest in dynamics, or ``jamming.'' Recent experiments by the Behringer Group at Duke University suggest a probabilistic model of jamming in hopper flows. We will show the results of numerical simulations of dense, gravity-driven, granular flows in a two-dimensional hopper with a tapered outlet [\emph{PRE} \textbf{79}, 011303 (2009)]. We will present results for the statistics of mass flow at the outlet, and the probability of survival without a jam. We will correlate the survival times with velocity and density distributions near the hopper opening.   

Experimental analysis on how grain properties affect the performance of jammed granular systems for variable stiffness robotic applications

Jamming of granular media has become increasingly utilized as a variable stiffness mechanism for industrial and robotic applications. The goal of our work is to better understand how grain properties affect jamming so that granular systems can be designed to fulfill the requirements of a given application. We have primarily focused on experimental studies to analyze how certain grain properties---such as shape, surface roughness, size distribution, and shape distribution---affect the performance of granular systems. Potential applications that utilize jamming would typically require that a contained granular system transition between effective solid states (e.g., when particular shape or strength needs to be maintained) and effective liquid states (e.g., when the system needs to be compliant such that it can be shaped or actuated by its environment). Therefore, we are interested in quantifying 1) the strength of compacted granular systems in their effective solid states and 2) the ``ease of flow'' and compliance of granular systems in their effective liquid states.   

Shear-Transformation-Zone Theory of Glassy Diffusion, Stretched Exponentials, and the Stokes-Einstein Relation

The success of the shear-transformation-zone (STZ) theory in accounting for broadly peaked, frequency-dependent, glassy viscoelastic response functions is based on the theory's first-principles prediction of a wide range of internal STZ transition rates. Here, I propose that the STZ's are the dynamic heterogeneities frequently invoked to explain Stokes-Einstein violations and stretched-exponential relaxation in glass-forming materials. I find that, to be consistent with observations of Fickian diffusion near $T_g$, an STZ-based diffusion theory must include cascades of correlated events, but that the temperature dependence of the Stokes-Einstein ratio is determined by an STZ-induced enhancement of the viscosity. Stretched-exponential relaxation of density fluctuations emerges from the same distribution of STZ transition rates that predicts the viscoelastic behavior.   

A granular flow law based on nonlocal fluidity

A general, three-dimensional law to predict granular flow in an arbitrary geometry has been an elusive goal for decades. Recently, an elasto-plastic continuum model has shown the ability to approximate steady flow and stress profiles in multiple inhomogeneous flow environments. However, the model does not capture some phenomena observed in the slow, creeping flow regime. As normalized flow-rate decreases, granular stresses are observed to become largely rate-independent and a dominating length-scale emerges in the mechanics. This talk attempts to account for these effects using the notion of nonlocal fluidity, which has proven successful in treating nonlocal effects in emulsions. The idea is to augment the usual granular fluidity law with a diffusive second-order term scaled by the particle size that spreads flowing zones accordingly. Below the yield stress, the local contribution vanishes and the fluidity becomes rate-independent, as we require. We implement the modified law in multiple geometries and validate its predictions for velocity, shear-rate, and stress against discrete particle simulations.   

Rheology of discontinuously shear thickening suspensions beyond simple shear

The behavior of discontinuously shear thickening suspensions in flows other than simple shear is not well understood, in part due to unresolved experimental challenges. For example, such suspensions thicken most easily close to rigid boundaries due to the no-slip condition. This makes experiments highly dependent on the shape and size of the container used. We show that by placing a lubricating layer of oil between the suspension and the container we can generate flows where thickening is nearly independent of rigid boundaries. This method is particularly useful in creating quasi one- and two-dimensional flows, which can be easily visualized. We use this method to conduct capillary breakup experiments with thickening suspensions of silica and cornstarch particles, in which we observe the formation of beads-on-a-string morphologies with multiple satellite and sub-satellite bead generations, similar to the morphologies observed in breakup of viscoelastic fluids. Using a one-dimensional continuum model, we show how nonlinear rheology of thickening suspensions results in the creation of these complex morphologies.   

Activated processes in a stress landscape: a rheological model for dense driven granular materials

We model (Phil. Trans. R. Soc. A 2009 {\textbf {367}}) the rheological behavior of granular materials based on a stress-based statistical ensemble and the Soft Glassy Rheology framework (SGR). It takes into account the disordered nature of granular packings and the metastability of jammed states, as well as spatial heterogeneity and intermittency. In this model, mesoscopic subregions of a driven granular material undergo activated processes in a {\it stress} landscape with a broad distribution of barrier heights. Due to the athermal nature of granular materials, the activated processes are induced not by the thermodynamic temperature, but by a temperature-like quantity which is a measure of the fluctuation of stresses. Results and predictions of the model have been successfully applied to analyze experiments in a Couette geometry. We will discuss applications of the stress-activated framework in, for example, recent experiments that study non-local rheology in dense flows (PRL \textbf{106} 108301(2011)).   

Shear-transformation-zone theory of plasticity in hard-sphere materials

The dynamics of sheared, dense granular materials exhibits features, such as a dynamic yield stress and a glass transition, similar to those of other amorphous solids. However, strictly granular, hard-sphere systems fundamentally differ from traditional glassy and colloidal systems because at microscopic scales their dynamics and interaction energies are insensitive to thermal temperature. In this talk we present a theory of plasticity for sheared, granular materials that combines Shear Transformation Zone (STZ) theory with Edwards' statistical theory of granular materials. We find that the dynamics of a strictly granular system, like other amorphous solids, can be captured statistically in terms of entropic mechanisms. In our analysis of sheared hard spheres, the volume $V$ replaces the energy $E$ as a function of entropy $S$ in conventional statistical mechanics. In place of an effective disorder temperature, a central feature of the STZ theory for traditional glassy systems, the compactivity $X = \partial V / \partial S$ characterizes the internal state. We derive the STZ equations of motion for a granular material accordingly, and predict its macroscopic properties such as shear viscosity and macroscopic frictional coefficient under different shearing conditions.   

Width of Shear Zones in Gravity-Driven Granular Flows

Gravity-driven granular flow in a vertical hopper exhibits a flow profile that consists of a plug near the center and a shear zone near the boundary walls. It has been observed that the width of the shear zone is a few particle diameters and is independent of the channel width, however, the mechanism by which the width is selected remains unclear. Using event-driven simulations of granular flow in a two-dimensional hopper, we investigate the width of the shear zone as a function of the channel width and the boundary conditions at the wall. We focus on the role played by fluctuations in the stress as the source of activated slips near the wall as a candidate mechanism for the shear zone.   

From streamline jumping to strange eigenmodes: Learning from simple continuum models of granular mixing

Simple continuum models of granular flow can provide fundamental insight into how and why granular materials mix. Though a similar kinematic framework can be used to study both fluid and granular mixing, there are striking differences that we explore through a computational--experimental study of granular flow in a slowly rotating quasi-two-dimensional polygonal container. In the Lagrangian frame, for small numbers of revolutions, we show that the mixing pattern is captured by a model termed ``streamline jumping.'' This minimal model, arising at the limit of a vanishingly-thin surface flowing layer, possesses no intrinsic stretching or streamline crossing in the usual sense, yet it can lead to complex particle trajectories that resemble chaos. In the Eulerian frame, meanwhile, we show the presence of naturally-persistent granular mixing patterns (``strange'' eigenmodes) for intermediate numbers of revolutions. Unlike fluid mixing, however, strong diffusive effects (due to particle collisions in granular flows) result in fast decay of these transient patterns in monodisperse mixtures. Meanwhile, segregation leads to permanent excitation of eigenmodes in bidisperse mixtures.   

From liquid to stone: a polydispersive colloidal model for cement setting

The main binding phase of cement is the nano-porous C-S-H gel, (calcium-silicate-hydrate). Here we investigate the temporal evolution of the mechanical properties of cement across the rigidity transition from liquid paste to solid stone, due to the precipitation of C-S-H. This transition is named ``setting'' and occurs several hours after mixing cement powder with water. We present a numerical model of random insertion of colloidal C-S-H nano-particles in the Monte Carlo framework. The particles interact with each other according to a generalized Lennard-Jones potential. Depending on the particle size polidispersity, the packing fraction of the final assemblies ranges between 0.64 and 0.73. The mechanical properties of the assemblies indicate that the packing fraction is the key parameter for continuum mechanics models at larger scales. In fact, both the stiffness and the strength of the assemblies increase with the packing fraction, while the ductility decreases. On the other hand, the evaluation of the specific surface requires an additional parameter fixing the length scale, for example, the characteristic size of the nano-pores. We finally show the relevance of our results for cement setting with a simple semi-analytical model of micro-pore filling.   

Effect of randomness on wave propagation in granular systems

Granular systems have been shown to possess energy absorbing and potential wave mitigation characteristics due to the flexibility in tuning the properties of the particles. The present study focuses on the impact of randomness on wave propagation in 1D and 2D lattices of spherical particles where the randomness is associated with either the mass, Young's modulus or radius of the spheres. The 1D study (motivated by M. Manciu et. al. (2001)) reveals the presence of two distinct regimes of decay in peak compressive force with distance for any level of randomness. The transition between the regimes of exponential and power-law decay is shown to occur when the amplitude of the leading pulse reduces below that of the scatter. Investigation into the ensemble kinetic and potential energies of the system as a function of time shows the gradual transfer of energy from potential to kinetic with increase in the level of randomness. In 2D square packed systems simulated with a modified version of the molecular dynamics package LAMMPS, we note that the decay in peak compressive force is present due to dimensionality as well as randomness. Normalization is then used to quantify the decay due to randomness alone and we investigate the anisotropy of the randomness induced decay.   

Continuum Representation of the Mechanics of Random Fiber Networks

Solving boundary value problems over large domains of random fiber networks is important in the design of fiber-based engineering materials and in the understanding of the biophysics of biological materials. In most of these applications, systems contain a very large number of fibers, which renders nearly impossible solving boundary value problems while resolving every fiber in the problem domain. Therefore, developing a continuum model for the discrete system is desirable. This presentation focuses on conditions under which this mapping can be performed and the minimum size of the problem beyond which the continuum representation is valid. Random fiber networks are highly heterogeneous and exhibit non-affine deformation with correlated fields at different observation length scales. The scale of transition from the discrete to the continuum model must be large enough to capture all the statistically independent subdomains of the network. This scale cannot be determined exclusively based on geometric considerations (e.g. based on fiber density). These considerations, along with a constitutive model for the small deformation of continuum models of fiber networks are discussed in this talk.   

Session H54: Focus Session: Complex and co-evolving networks - Dynamics on Networks

Robustness and Assortativity for Diffusion-like Processes in Scale- free Networks

By analyzing the diffusive dynamics of epidemics and of distress in complex networks, we study the effect of the assortativity on the robustness of the networks. We first determine by spectral analysis the thresholds above which epidemics/failures can spread; we then calculate the slowest diffusional times. Our results shows that disassortative networks exhibit a higher epidemiological threshold and are therefore easier to immunize, while in assortative networks there is a longer time for intervention before epidemic/failure spreads. Moreover, we study by computer simulations a diffusive model of distress propagation (financial contagion). We show that, while assortative networks are more prone to the propagation of epidemic/failures, degree-targeted immunization policies increases their resilience to systemic risk.   

Time scales and dynamical processes in activity driven networks

Network science has undergone explosive growth in the last ten years. This growth has been driven by the recent availability of huge digital databases, which has facilitated the analysis and construction of large-scale networks from real data and the identification of statistical regularities and structural principles common to many systems. Network modeling has played an essential role in this endeavor; however models are chiefly constructed by considering as relevant ingredients only the connectivity and statistical properties of the networks, while disregarding the actual agents' behavior. Here we address this challenge by measuring the agents' interaction activity in real-world networks and defining a minimal model capable of reproducing the intrinsically additive nature of connectivity patterns obtained from time-aggregated network representations. Additionally, we demonstrate that processes such as epidemic and information spreading in highly dynamical networks can be better characterized in terms of agent social activity than by connectivity based approaches   

Network growth dynamics of fire ant ({\em Solenopsis invicta}) nests

We study the construction dynamics and topology of fire ant ({\em Solenopsis invicta}) nests. Fire ants in colonies of hundreds to hundreds of thousands create subterranean tunnel networks through the excavation of soil. We observed the construction of nests in a laboratory experiment. Workers were isolated from focal colony and placed in a quasi 2D, vertically oriented arena with wetted soil. We monitored nest growth using time-lapse photography. We found that nests grew linearly in time through tunnel lengthening and branching. Tunnel path length followed an extended power law distribution, $P ~ (l - l_0)^\beta$. Average degree of tunnel nodes was $k = 2.17 \pm 0.40$ and networks were cyclical. In simulation we model the nest growth as a branching and annihilating levy-flight process. We study this as a function of dimensionality (2D and 3D space considered) and step length distribution function $P(l_s)$. We find that in two-dimensions path length distribution is exponential, independent of the functional form of $P(l_s)$ consistent with a poisson spatial process while in three-dimensions $P(l) = P(l_s)$. Comparing simulation and experiment we attribute the slower than exponential tail of $P(l)$ in experiment as a result of a behavioral component to the ant digging program.   

The Impact of Time Delays in Network Synchronization in a Noisy Environment

Coordinating, distributing, and balancing resources in networks is a complex task and it is very sensitive to time delays. To understand and manage the collective response in these coupled interacting systems, one must understand the interplay of stochastic effects, network connections, and time delays. In synchronization and coordination problems in coupled interacting systems individual units attempt to adjust their local state variables (e.g., pace, orientation, load) in a decentralized fashion. They interact or communicate only with their local neighbors in the network, often with explicit or implicit intention to improve global performance. Applications of the corresponding models range from physics, biology, computer science to control theory. I will discuss the effects of nonzero time delays in stochastic synchronization problems with linear couplings in an arbitrary network. Further, by constructing the scaling theory of the underlying fluctuations, we establish the absolute limit of synchronization efficiency in a noisy environment with uniform time delays, i.e., the minimum attainable value of the width of the synchronization landscape.\footnote{D. Hunt, G. Korniss, and B.K. Szymanski, Phys. Rev. Lett. 105, 068701 (2010).} These results have also strong implications for optimization and trade-offs in network synchronization with delays.   

Predicting the origin of contagion processes on complex, multi-scale networks

Contagion phenomena in space often exhibit complex, multiscale spatio-temporal patterns driven by the interaction of non-local dispersal and nonlinear dynamics. A key challenge is the prediction of dynamic patterns based on information on human interactions, mobility and initial conditions. The development of computational models has thus received considerable attention. However, in many realistic situations, a process has already evolved for some period before detection and identifying the spatial origin is difficult. Surprisingly, this ``inverse problem'' has received little attention in the past. We show in a paradigmatic model for human disease dynamics that despite the spatial complexity of dynamic patterns, the origin of outbreak can be predicted with high fidelity. Based on the technique of shortest path trees in strongly heterogeneous, multi-scale human mobility networks we show that at any point in time the spatial origin can be reconstructed reliably. This novel perspective on complex spatio-temporal dynamics can be applied to systems beyond human disease dynamics for instance the reconstruction of neolithic diffusion of agriculture into Europe and related migration driven historic phenomena.   

Influence and structural balance in social networks

Models on structural balance have been studied in the past with links being categorized as friendly or antagonistic [Ref- T. Antal et al., Phys. Rev. E 72, 036121 (2005)]. However no connection between the nature of the links and states of the nodes they connect has been made. We introduce a model which combines the dynamics of the structural balance with spread of social influence. In this model, every node is in one of the three possible states (e.g. leftist, centrist and rightist) [Ref- F. Vazquez, S. Redner, J. Phys A, 37 (2004) 8479-8494] where links between leftists and rightists are antagonistic while all other links are friendly. The evolution of the system is governed by the rules of structural balance and opinion spread takes place as a result of structural balance process. The dynamics can lead the system to a number of fixed points (absorbing states). We study how the initial density of centrists $n_{c}$ affects the dynamics and probabilities of ending up in different absorbing states. We also study the scaling behavior of the expected time to converge to one of the absorbing states as a function of the initial density of centrists and under some variations of our basic model.   

Majority-vote model on a dynamic small-world square lattice

Majority-vote models are often used to study consensus building, coarsening dynamics, and phase transitions, among other phenomena. In addition to the microscope rules governing a particular model, it is well known that the relevant properties of each system depend crucially on the underlying lattice structure. Here we investigate a majority-vote with noise model on a square lattice with dynamic small-world rewiring via Monte Carlo simulation and finite size scaling analyses. We construct the order-disorder phase diagram and find the critical exponents associated with the continuous phase transition. We compare our results to those obtained from two-dimensional static small-world networks, as well as the isotropic lattice and mean-field limiting cases.   

Underlying mechanisms for commuting and migration processes

Both frequent commuting and long-term migration are complex human processes that strongly depend on socio-demographic, spatial, political, and even economic factors. We can describe both processes using weighted networks, in which nodes represent geographic locations and link weights denote the flux of individuals who commute (or migrate) between locations. Although both processes concern the movements of individuals, they are very different: commuting takes place on a daily (or weekly) basis and always between the same two locations, while migration is a rare, one-way displacement. Despite these differences, a recently proposed stochastic model, the Radiation model, provides evidence that both processes may be successfully described by the same underlying mechanism. For example, quantities of interest for either process, such as the distributions of trip length and destination populations, appear remarkably similar to the model's predictions. We explore the similarities and differences between commuting and migration both empirically, using census data for the United States, and theoretically, by comparing these commuting and migration networks to the predictions given by the Radiation model.   

A quantitative measure for organization of complex and co-evolving networks

To define evolution and self-organization in complex networks a quantitative measure for organization is necessary. Two systems should be numerically distinguishable by their degree of organization and their rate of self-organization. Here we apply as a measure for quantity of organization the inverse of the average sum of physical actions of all elements in a system per unit motion multiplied by the Planck's constant. The meaning of quantity of organization here is the number of quanta of action per one unit motion of an element. For example, a unit motion for electrons on a computer chip is the one necessary for one computation. This definition can be applied to the organization in any complex system. Systems self-organize to decrease the average action per element per unit motion in them. This is the attractor for a dynamical, nonlinear system evolving in time. Constraints increase this average action, so constraint minimization is a basic mechanism for action minimization. Increase of quantity of elements in the network, leads to faster constraint minimization through grouping, decrease of average action per element and motion and therefore faster self-organization and evolution.   

Identification of Interventions to Control Network Crises

Large-scale crises in financial, social, infrastructure, genetic and ecological networks often result from the spread of disturbances that in isolation would only cause limited damage. Here we present a method to identify and schedule interventions that can mitigate cascading failures in general complex networks. When applied to competition networks, our method shows that the system can often be rescued from global failures through actions that satisfy restrictive constraints typical of real-world conditions. However, under such constraints, interventions that can rescue the system from a propagating cascade exist over specific periods of time that do not always include the early postperturbation period, suggesting that scheduling is critical in the control of network cascades.   

Effect of degree correlations on controlability of complex networks

While significant effort was made during the past decade to understanding the structure, evolution and function of complex networks, little is known about our ability to control them. A system is considered controllable if by imposing appropriate external signals on a set of its nodes, called driver nodes, the system can be driven from any initial state to any desired final state in finite time. The controllability of a network can be quantified by calculating the minimal number of driver nodes needed to obtain complete control. We study the effect of degree correlations on network controllability via both numerical simulations and analytical calculations. Numerical simulations are preformed by systematically adding correlations to model networks using appropriate rewiring schemes inspired by simulated annealing. Analytical results are derived using the cavity method originally developed in spin glass theory. The numerical and analytical results enable us to give qualitative predictions of controllability for any networks based on their degree correlation profiles. We test our predictions on several real networks and find consistent results.   

Control Capacity in Complex Networks

By combining tools from control theory and network science, an efficient methodology was proposed to identify the minimum sets of driver nodes, whose time-dependent control can guide the whole network to any desired final state. Yet, this minimum driver set (MDS) is usually not unique, but one can often achieve multiple potential control configurations with the same number of driver nodes. Given that some nodes may appear in some MDSs but not in other, a crucial question remain unanswered: what is the role of individual node in controlling a complex system? We first classify a node as critical, redundant, or ordinary if it appears in all, no, or some MDSs. Then we introduce the concept of control capacity as a measure of the frequency that a node is in the MDSs, which quantifies the importance of a given node in maintaining Controllability. To avoid impractical enumeration of all MDSs, we propose an algorithm that uniformly samples the MDS. We use it to explore the control capacity of nodes in complex networks and study how it is related to other characteristics of the network topology.   

Controlling Complex Networks with Compensatory Perturbations

The response of complex networks to perturbations is of critical importance in areas as diverse as ecosystem management, power system design, and cell reprogramming. These systems have the property that localized perturbations can propagate through the network, causing the system as a whole to change behavior and possibly collapse. We will show how this same mechanism can actually be exploited to prevent such failures and, more generally, control a network's behavior. This strategy is based on counteracting a deleterious perturbation through the judicious application of additional, compensatory perturbations---a prospect recently demonstrated heuristically in metabolic and food-web networks. Here, we introduce a method to identify such compensatory perturbations in general complex networks, under arbitrary constraints that restrict the interventions one can actually implement in real systems. Our method accounts for the full nonlinear time evolution of real complex networks, and in fact capitalizes on this behavior to bring the system to a desired target state even when this state is not directly accessible. Altogether, these results provide a new framework for the rescue, control, and reprogramming of complex networks in various domains.