Session P2: Invited Session: Large Fluctuations Far From Equilibrium

Characterizing Order in Glassy Systems

Crystals and quasicrystals can be characterized by an order that is a purely geometric property of an instantaneous configuration, independent of particle dynamics or interactions. Glasses, on the other hand, are ostensibly amorphous arrangements of particles. A natural and long-standing question has been whether they too have, albeit in a hidden way, some form of geometric order. I will examine a recent proposal for a coherence length that applies to systems which are typically characterized as amorphous, as well as to those that are conventionally ordered. The question of whether exotic order can arise in physical systems will be addressed.   

Drift, diffusion and barrier crossing of small objects on a surface assisted by an external noise: roles of non-linear friction

We study experimentally the behaviors of several driven diffusive systems that involve the sliding and rolling of small solid objects or liquid drops on a surface with an external noise and an external field. The displacement statistics here are non-Gaussian at short observation time, but they tend towards a Gaussian behavior at long time scale. Furthermore, in each of these cases, the drift velocity increases sub-linearly, but the diffusivity increases super-linearly with the strength of the noise. These observations reflect the underlying non-linear friction control of their stochastic dynamics. Specific experiments have also been designed to study the hopping of a small object over a physical barrier assisted by an external noise. These results mimic the classical Arrhenius behavior from which an effective temperature may be deduced. However, the regimes controlled by a Coulombic like friction and a linear kinematic friction need to be treated somewhat differently. All the drifted diffusive systems studied here exhibit substantial negative fluctuations of displacement at a short observation time that diminishes at longer time scale. Using the integrated fluctuation theorem, we characterize the persistence time of negative fluctuations in terms of the diffusivity and the drift velocity that can be measured experimentally.   

Noise induced stabilization in population dynamics

We investigate a model where strong noise in a sub-population creates a metastable state in an otherwise unstable two-population system. The induced metastable state is vortex-like, and its persistence time grows exponentially with the noise strength. A variety of distinct scaling relations are observed depending on the relative strength of the sub-population noises.   

Speeding up spontaneous disease extinction

The dynamics of epidemic in a susceptible population is affected both by the random character of interactions between the individuals and by environmental variations. As a consequence, the sizes of the population groups (infected, susceptible, etc.) fluctuate in the course of evolution of the epidemic. In a small community a rare large fluctuation in the number of infected can result in extinction of the disease. We suggest a novel paradigm of controlling the epidemic, where the control field, such as vaccination, is designed to maximize the rate of spontaneous disease extinction. We show that, for a limited-scope vaccination, the optimal vaccination protocol and its impact on the epidemics have universal features: (i) the vaccine must be applied in pulses, (ii) the spontaneous disease extinction is synchronized with the vaccination. We trace this universality to general properties of the response of large fluctuations to external perturbations.   

Cooperativity-Driven Singularities in Cooperative Asymmetric Exclusion

We investigate the effect of cooperative interactions on the asymmetric exclusion process. In the simplest case a particle can hop to its vacant right neighbor only if its left neighbor is occupied. We show that an initial density downstep develops into a rarefaction wave that can have a jump discontinuity at the leading edge, while an upstep results in a shock wave. We also investigate a more general model in which the particle velocity can be an increasing function of the density. Within a hydrodynamic theory, initial density upsteps and downsteps can evolve into: (a) shock waves, (b) continuous compression or rarefaction waves, or (c) a mixture of shocks and continuous waves. These unusual phenomena arise because the current versus density relation has an inflection point, so that the group velocity can either be an increasing or a decreasing function of the density on either side of the inflection point.   

Session P3: Invited Session: Alkaline Iron Selenides vs Iron Pnictides: Properties and Their Implications

Neutron Scattering Study on the New 245 Iron Selenide Superconductors

We determine using neutron and x-ray diffraction method the sample composition, crystalline structure and magnetic order of the recently discovered $A_2$Fe$_4$Se$_5$ superconductors ($A$=K, Rb, Cs, Tl/K or Tl/Rb). Contrary to initial belief that these materials are heavily electron-doped variety of the BaFe2As2 family of Fe-based superconductors, they are almost charge balanced with the Fe valence close to 2+ as in the 11 iron selenide superconductors, and crystalize in an Fe vacancy-ordered lattice structure [1,2]. Coexisting with superconductivity is a novel block antiferromagnetic order which conforms to the tetragonal crystalline symmetry and possesses a very large ordered magnetic moment 3.3$\mu_B$ per Fe and a very high ordering temperature above 500 K [1]. Such Fe vacancy ordered crystal structure and coexisting antiferromagnetism and superconductivity occur in all 5 types of new iron selenide superconductors discovered so far. With Fe vacancy number departs from the chemical formulas $A_2$Fe$_4$Se$_5$, an imperfect version of the Fe vacancy order results at base temperature while phase separation into two vacancy-ordered phases exists at the intermediate temperature range [4]. The Fe site disorder renders the materials insulating and destroys the superconductivity as spin-glass disorder does in previous 11 iron selenide superconductors [5]. \\[4pt] [1] W. Bao et al., Chin. Phys. Lett. {\bf 28}, 086104 (2011).\\[0pt] [2] P. Zavalij etal., Phys. Rev. B {\bf 83}, 132509 (2011).\\[0pt] [3] F. Ye et al., Phys. Rev. Lett. {\bf 107} 137003 (2011).\\[0pt] [4] W. Bao et al., arXiv: 1102.3674 (2011).\\[0pt] [5] T.J. Liu et al., Nat. Materials {\bf 9}, 716 (2010).   

Distinct Fermi Surface Topology and nearly Isotropic Superconducting Gap in A$_{x}$Fe$_{2-y}$ Se$_{2}$ (A=K, Tl, Rb) Superconductors

High resolution angle-resolved photoemission measurements have been carried out to study the electronic structure and superconducting gap of the newly discovered A$_{x}$Fe$_{2-y}$Se$_{2}$ [A=K, (Tl,K) and (Tl,Rb)] superconductors[1,2,3] 1. Distinct Fermi surface topology, consisting of two electron-like Fermi surface sheets around the $\Gamma $(0,0) point and an electron-like Fermi surface sheet near the M($\pi $,$\pi )$ point, was revealed in all these samples. This is in strong contrast to the Fermi surface topology of other Fe-based superconductors where hole-like Fermi surface sheets are present near the $\Gamma $(0,0) point. 2. Both the electron-like Fermi surface sheet near M point and the large electron-like Fermi surface sheet near $\Gamma $ point show nearly isotropic superconducting gap without nodes 3. The doping evolution of the electronic structure from insulating samples to the superconducting samples is consistent with a phase separation picture. The information on the Fermi surface topology and superconducting gap of this new A$_{x}$Fe$_{2-y}$Se$_{2}$ superconductor will provide key insights and constraints to understand the superconductivity mechanism in iron-based superconductors. \\[4pt] [1]. D. X. Mou, X. J. Zhou et. al, Phys. Rev. Lett. \textbf{106}, 107001 (2011). \\[0pt] [2]. L. Zhao, X. J. Zhou et. al, Phys. Rev. B \textbf{83}, 140508(R) (2011). \\[0pt] [3]. L. Yu, X. J. Zhou et al., unpublished.   

Pairing strength and symmetries of 122 iron selenides in comparison with iron pnictides

High temperature superconductivity with comparable transition temperatures has been observed in the vicinity of an antiferromagnetic phase, in both 122-alkaline iron selenides and 122-iron pnictides. In contrast to iron pnictides, where the parent state is an antiferromagnetic semimetal, the parent state of 122-iron selenides is a large moment, antiferromagnetic insulator. This provides a clear indication of strong electronic correlations. The 122-selenides possess only electron pockets, while the pnictides have both hole and electron pockets. In addition, the observed block spin magnetic order in 122-selenides can not be explained by Fermi surface nesting. At the same time, the comparable $T_c$ suggests a commonality in the underlying mechanism for superconductivity in the two classes of materials. Motivated by these observations and considerations, we present a comparative strong coupling analysis of the pairing strength and symmetries in these two classes of materials [1,2]. The analysis of appropriate five orbital $t-J_1-J_2$ models, reveals a similar pairing phase diagram for both materials, with $A_{1g}$ $s(x^2y^2)$ and $B_{1g}$ $d(x^2-y^2)$ as two dominant pairing channels. The pairing amplitudes in both materials are of comparable strength, making it natural for a comparable maximum $T_c$ . In contrast to the pnictides case, an $A_{1g}$ $s(x^2+y^2)$ state is not competitive, making the dominant pairing channels fully gapped. We also discuss the magnetism of the vacancy-ordered insulating 122 iron selenides [3], showing that the observed block-spin state occurs over a wide parameter range. The predicted magnetic excitation spectrum has been verified by inelastic neutron scattering experiments. Our study also reveals some commonality with the magnetism of the parent iron pnictides [4].\\[4pt] Work was done in collaboration with Rong Yu, Predrag Nikolic, Jian-Xin Zhu, Qimiao Si and Elihu Abrahams. \\[4pt] [1] Rong Yu, Pallab Goswami, Qimiao Si, Predrag Nikolic, and Jian-Xin Zhu, ``Pairing strength and symmetries of 122 iron selenides in comparison with iron pnictides,'' to be published; arXiv:1103.3259. \\[0pt] [2] Pallab Goswami et al, ``Superconductivity in Multi-orbital $t-J_1-J_2$ Model and its Implications for Iron Pnictides,'' Europhys. Lett. {\bf 91}, 37006 (2010).\\[0pt] [3] Rong Yu, Pallab Goswami, and Qimiao Si, ``The magnetic phase diagram of an extended $J_1-J_2$ model on a modulated square lattice and its implications for the antiferromagnetic phase of $\mathrm{K}_y\mathrm{Fe}_x\mathrm{Se}_2$,'' Phys. Rev. B {\bf 84}, 094451 (2011). \\[0pt] [4] Pallab Goswami et al, ``Spin Dynamics of a $J_1-J_2$ Antiferromagnet and its Implications for Iron Pnictides,'' Phys. Rev. B {\bf 84}, 155108 (2011).   

NMR investigation of iron-selenide and iron-arsenide high $T_{c}$ superconductors

We have investigated the electronic, magnetic, and superconducting properties of the iron-selenide high $T_{c}$ superconductor K$_{x}$Fe$_{2-y}$Se$_{z}$ ($T_{c}=33$\ K) with $^{77}$Se NMR [1]. We will compare the results with those observed for FeSe in ambient and applied pressures ($T_{c}>9$\ K) [2], and with iron-arsenides [3]. Similarities and dissimilarities will be pointed out, with primary focus on the anomalous normal state properties. Our latest work on K$_{x}$Fe$_{2-y}$Se$_{z}$ was carried out in collaboration with D. Torchetti, M. Fu, D. Christensen, K. Nelson (McMaster), H. Lei, and C. Petrovic (Brookhaven National Lab).\\[4pt] [1] D. Torchetti et al., PR {\bf B83}, 104508 (2011).\\[0pt] [2] T. Imai et al. PRL {\bf 102}, 177005 (2009).\\[0pt] [3] F.L. Ning et al., PRL {\bf 104}, 037001 (2010); JPSJ {\bf 78}, 103711 (2009).   

Session P4: Multi-Component BECs and Mixtures

Analytical solutions to the spin-1 Bose-Einstein condensates

We present classes of exact stationary solutions for the one-dimensional coupled nonlinear Gross-Pitaevskii equations which describe the $F=1$ spinor Bose-Einstein condensates, both with and without the Zeeman splitting. The spin magnetization configurations and the spin currents are investigated. The soliton or the soliton train complexes are naturally produced.   

Geometry of Spinor Condensates with Large Spins

Laser cooled atoms with spin can become magnetically ordered in a variety of ways, like electrons in a frustrated lattice, but the phases are more geometrical in this setting. For spin one, two and three atoms, states have been predicted with a nematic symmetry (the symmetry of a toothpick), a hexagon, and an octahedron, as well as other possibilities. As the spin becomes larger, the phase diagrams become more and more complicated. I will present a geometrical way of predicting regularities in the phases as the spin increases. For a certain form of interaction, we find a phase diagram for arbitrarily large spin. (In particular, a nematic phase does not occur beyond spin 2.) We have used a mapping to a classical problem of interacting particles arranging themselves on a sphere.   

Pomeranchuk cooling of SU(N) ultracold fermions in optical lattice

We investigate thermodynamic properties of a half-filled SU($2N$) Fermi-Hubbard model in two-dimensional square lattice using the determinantal Quantum Monte Carlo simulation. We address the question how the large number of hyperfine-spin components makes thermodynamic properties of SU($N$) ultracold fermions different from the conventional Hubbard model with $N$=2. Various thermodynamic quantities such as entropy, charge fluctuations, and spin correlations have been calculated. We devote special attention to the interaction-induced adiabatic cooling: an analogue of the Pomeranchuk effect in Helium-3.   

Determinant Quantum Monte Carlo simulations on quantum magnetism of theSU(2N) ultra-cold fermions

We investigate the quantum magnetism of the repulsive $SU(2N)$-Hubbard model on a two-dimensional square lattice at half-filling. At $2N=4$, our numerical results suggest that there exists a long-range Neel ordering at large $U$. In this regime, both of the antiferromagnetic structure factor per site and the farthest two-point spin-spin correlation functions are saturated to finite values in the thermodynamic limit. The single-particle excitations are finite and the spin gaps vanish. All of above evidences support the presence of the Neel ordering in the SU(4)-Hubbard model. For $2N > 4$, such features are not explicitly observed.   

Multi-component integrable models in cold atoms

The quantum gases are intensively studied for the inspiring advances in ultra-cold atomic physics. Various kinds of lattice and gas systems are created in different dimensions. The multi-component cold atomic systems have rich phase diagrams. Our works focus on the one-dimensional integrable quantum systems. We find a series of integrable models of $Sp(2s+1)$ fermions and $SO(2s+1)$ bosons and solve them via Bethe ansatz techniques, where $s$ the hyperfine spin of the atoms. We find the paired bosons exist in both repulsive and attractive $SO(3)$ integrable bosonic gases with hyperfine spin-1. For the $Sp(2s+1)$ repulsive fermions, there are no bound states in the ground state, while 2-string bound solutions appear in the spin sector. We also calculate the spin-wave velocities and low temperature specific heat of the repulsive fermions. These systems have spin-charge separation property of Luttinger liquid. Different from the $SU(2s+1)$ repulsive models, the spin-wave velocities of the $Sp(2s+1)$ models are no longer the same. The holes of 1-strings (real rapidities) of different branches in the spin sector have a same spin-wave velocity $v_1$ and the ones of 2-strings share a same spin-wave velocity $v_2$. The two velocities are different.   

Haldane Phase of Ultra Cold Atom Gas Loaded on Pseudo-One-Dimensional Optical Lattice

Ultracold Fermi gas loaded on optical lattice (FGOL) has attracted considerable attention since its temperature, interaction, and filling factor are flexibly controllable and various quantum phases are accessible. In this study, we examine properties of pseudo-one-dimensional (P1D) FGOL obtained by including effects of the transverse excitations. At first, we prove that the P1D system at half-filling can be theoretically mapped on spin-1 Heisenberg chain. Secondly, we reveal by using DMRG scheme that Haldane phase emerges in the P1D FGOL. Finally, we clarify effects of trap potential and spin imbalance on not only the central Haldane phase but also various different magnetic structures around the Haldane phase.   

Half-quantum vortex state and its excitations in a spin-orbit coupled spinor Bose-Einstein condensate

We investigate theoretically the condensate state and collective excitations of a spin-orbit coupled spinor Bose gas in two-dimensional harmonic traps. In the weakly interacting regime, when the inter-species interaction is larger than the intra-species interaction ($g_{\uparrow \downarrow} > g$), we find that the condensate state has a half-quantum-angular-momentum vortex configuration (half-vortex state) with spatial rotational symmetry and skyrmion-type spin texture. We investigate the stability of half-vortex state in the regime when $g$ is greater than a threshold $g_c$, and in the regime when $g_{\uparrow \downarrow} < g$, by solving the Bogoliubov equations for collective density oscillations. In addition, we also investigate the dynamical properties of the half-vortex state. We present the phase diagram as a function of interatomic interaction and spin-orbit coupling.   

Searching for Non-abelian Phases in Bose-Einstein Condensate of Dy

Recently Bose-Einstein condensate of a spin 8 element, Dysprosium, has been realized. We compute the mean-field ground state and Bogoliubov excitation of Dy condensate for different scattering lengths and in presence of a magnetic field, and find various non-abelian phases in the parameter space. We suggest an experimental scheme for detecting the remaining discrete point group symmetry of these phases simply by looking at population of each spin components and the degeneracy of Goldstone modes. A BEC whose remaining symmetry is a non-abelian group can support exotic non-abelian vortices.   

Quasiparticles in a Bose-Fermi mixture in optical lattice

We investigate a mixture of interacting bosons and fermions using a generalized dynamical mean-field theory combined with the numerical renormalization group. We focus on many-body effects in the presence of the superfluidity. It is elucidated that fermionic particles are strongly renormalized by low-energy bosonic excitations via the boson-fermion interaction, giving rise to an anomalous peak structure in the density of states for fermions. We also address how the renormalization effects appear in the phases with long-range order such as a supersolid phase.   

Ultracold bosons in the presence of a second species in the Tonks-Girardeau regime: dynamics and quantum features

We develop a framework to study the interaction between two ultracold bosonic species in different regimes. One species is in the low correlation limit forming a Bose-Einstein condensate (BEC), while the other is in the strongly correlated Tonks-Girardeau regime. We use a Bose-Hubbard-like model where, due to the momentum distribution of the Tonks gas, many single particle states are considered and study the dynamics of the system by numerical simulation of the equations obtained. The atoms in the Tonks gas act as impurities submerged in the BEC, and the excitations created by their interactions with the BEC gas can be understood in terms of polarons. We focus on the fundamental quantum properties of the system and investigate the effects the condensed environment has on the Tonks-Girardeau gas as a function of the interspecies scattering strength.   

The Many-Body Correlation of Bose-Fermi Mixture in the Ring Trap

Since the realization of Bose-Einstein Condensation in alkali atoms in 1995, studies on cold atomic gases have greatly advanced. The cold atoms are precisely controlled by electromagnetism and optics, and flexible to design quantum systems. In 2001, A. G\"{o}rlitz's group has realized the Bose-Einstein condensates in the quasi-one-dimensional system [1]. One have now obtained the ideal system to study the one-dimensional many-body physics experimentally. The purpose of our study is to clarify the effect of quantum many-body correlation beyond the mean-field approximation. To accomplish this purpose, we first prepare the bosons and fermions in the ring trap [2]. We prepare the initial state in the trap with the small distortion and obtained that both kinds of particles tend to be localized. After taking off this distortion, we solved the time-dependent Schr\"{o}dinger equation. We derived the energy spectrum, density profile, and studied pair-correlation effect. Our results predict that the many-body correlation emerges, which has never been observed experimentally up to now. \\[4pt] [1] A. G\"{o}rlitz et al., Phys. Rev. Lett. 87, 130402 (2001)\\[0pt] [2] O. Morizot et al., Phys. Rev. A. 74, 023617 (2006)   

Cooling by corralling: a route to antiferromagnetism in optical lattices

Cold atoms in optical lattices have emerged as a promising tool for emulating condensed matter Hamiltonians. Current experiments have observed ``Mott insulating'' behavior in the Fermi-Hubbard model at an average entropy $S/N \approx 1 k_B$/atom. Our quantum Monte Carlo simulations [1], in agreement with other methods, show that $S/N \approx 0.65 k_B$/atom is low enough to produce antiferromagnetism (AF) at the center of a harmonic trap. However, further progress in the field requires even lower entropies that are beyond the reach of traditional cooling techniques. I have proposed a way to attain very low temperatures and entropies ($S/N < 0.03 k_B$/atom) by trapping fermions in a corral formed from another species of atoms [2]. This Fermi system can then be evolved into an antiferromagnet by morphing the lattice into a set of double wells, quasi-adiabatically. Quantum dynamics simulations have, so far, given promising results.\\[4pt] [1] Thereza Paiva, Yen Lee Loh, Mohit Randeria, Richard T. Scalettar, and Nandini Trivedi, ``Fermions in 3D optical lattices: Cooling protocol to obtain antiferromagnetism,'' PRL 107, 086401 (2011).\\[0pt] [2] Yen Lee Loh, ``Proposal for achieving very low entropies in optical lattice systems,'' arxiv:1108.0628.   

Magnetic order in the doped Hubbard model and the crossover from two- to three-dimensions: a Hartree-Fock study

We report unrestricted Hartree-Fock (UHF) results for the ground state of the single-band Hubbard model in three-dimensions (3D), with repulsive onsite interactions and nearest-neighbor hopping. Magnetic and charge properties are determined by full numerical solutions of the self-consistent UHF equation in large supercells, and quantified as a function of hole doping $h$. We focus on weak to intermediate interaction strengths, where UHF has been shown to capture the main characteristics of the magnetic correlations in two-dimensions (2D).~\footnote{J. Phys.: Condens. Matter, in press (arXiv:1107.0976v2).} We find linear spin-density wave (SDW) states with AFM order and a long wavelength modulation whose wavelength is inversely proportional to $h$. We also study the dimensional crossover from 2D to 3D as the inter-plane distance is increased.   

Continuous measurement quantum state tomography of atomic spins

Quantum state tomography is a fundamental tool in quantum information science and technology. It requires estimates of the expectation values of an ``informationally complete'' set of observables. This is, in general, a very time-consuming process that requires a large number of measurements to gather sufficient statistics. There are, however, systems in which the data acquisition can be done more efficiently. An ensemble of quantum systems can be prepared and driven by external fields while being continuously and collectively probed, producing enough information in the time evolving measurement record to estimate the initial state. Such protocol has the advantage of being fast and robust. In this talk, we present a study of a continuous-measurement quantum-state tomography protocol and its application to controlling large spin ensembles. We perform reconstruction of quantum states prepared in the 16 dimensional ground-electronic hyperfine manifolds of an ensemble of 133Cs atoms, controlled by microwaves and radio-frequency magnetic fields and probed via polarization spectroscopy. We present theoretical and experimental results of its implementation and discuss two estimation methods: constrained maximum likelihood and compressed sensing.   

Measuring spin correlations in optical lattices

The study of ultracold atoms in optical lattices has produced several groundbreaking results. A major, but presently unrealized goal, is to study quantum magnetism using atoms in optical lattices. We suggest three different experimental methods for probing both short- and long-range spin correlations of atoms in optical lattices. The first method involves an adiabatic doubling of the periodicity of the underlying lattice to probe neighboring singlet (triplet) correlations for fermions (bosons) by the occupation of the resulting vibrational ground state. The second method utilizes a time-dependent superlattice potential to generate spin-dependent transport by any number of prescribed lattice sites, and probes correlations by the resulting number of doubly occupied sites. The third method relies on the difference in tunneling times for the vibrational ground state and the first excited state. Correct timing then allows for the spin correlations to be fingerprinted. For experimentally relevant parameters, we demonstrate how all three methods yield large signatures of antiferromagnetic correlations of strongly repulsive fermionic atoms in a single shot of the experiment.   

Session P5: Surfaces, Interfaces, and Thin Films: Kinetics, Dynamics, and Reactions

Direct versus hydrogen assisted CO dissociation on metal surfaces

We present investigations of the formation of precursor hydrocarbon species relevant to production of liquid hydrocarbons on low index surfaces of various important noble and transition metals. The formation could occur via the so-called carbide mechanism where direct CO dissociation takes place, followed by stepwise hydrogenation of C yielding CH$_{x }$ species. Formation of precursor CH$_{x }$ species could also potentially take place through hydrogenated CO intermediates. First-principles calculations of energetics and barriers of CO conversion to hydrocarbons species were performed using plane-wave periodic density functional theory. Our calculations indicate that the two pathways are generally competitive on transition metals. A microkinetic model, with input thermodynamics and kinetic parameters estimated from electronic structure calculations, has been developed. The two pathways will be further examined using microkinetic approach to determine whether the aforementioned finding holds at realistic conditions.   

Multi-lattice approach to first-principles kinetic Monte Carlo simulations: Application to catalytic CO oxidation at Pd(100)

First-principles kinetic Monte Carlo (1p-kMC) simulations enable a quantitative microkinetic modeling of heterogeneous chemical reactions while accounting for the full spatial distributions at the surface. Application to reaction-induced surface morphological transitions is hitherto prevented by the inability to describe the system within prevalent fixed-lattice 1p-kMC and the excessive cost of off-lattice 1p-kMC variants. To this end we develop a novel multi-lattice 1p-kMC approach and apply it as a case in point to the CO oxidation at Pd(100). In the catalytically active state this system is suspected to undergo transitions from the pristine metal surface to a PdO surface-oxide film. As a first step towards a comprehensive simulation we focus on the initial oxide destruction step induced by clustering of oxygen vacancies. First simulations confirm the stability of the oxide film at stoichiometric feed as predicted by preceding fixed-lattice 1p-kMC simulations [1].\\[4pt] [1] J. Rogal, K. Reuter, and M. Scheffler, Phys. Rev. Lett. 98, 046101 (2007).   

The role of non-conventional supports for single-atom Platinum-based catalysts in fuel cell technology: A theoretical surface science approach

Platinum-based heterogeneous catalysts are known to play a key role in the fuel cell technology, such as their use in the low-temperature proton exchange membrane (PEM) fuel cells. However, the high cost and low lifecycle of these PEM fuel cells are the major hindrances to its large-scale commercial production. To elevate the high-cost and to optimize its catalytic activity, it was recently proposed that catalysts with single-Pt atom dispersions and a more durable, corrosion-resistant TiN support could play an important role in the next generation of Pt-based PEM fuel cells [1]. As a first step towards a microscopic understanding of single-Pt atom-dispersed catalysts on these new supports, we present density-functional theory (DFT) calculations to investigate the adsorption properties of Pt atoms on pristine TiN(001). Optimized atomic geometries, energetics, and analysis of the electronic structure of the Pt/TiN system are reported for various surface coverages of Pt. We find that atomic Pt does not bind preferably to the clean TiN surface, but under operational conditions, TiN surface vacancies play a crucial role in anchoring the Pt atom for its catalytic function. $[1]$ B. Qiao, $et al.$ Nat. Chem. 3 (2011) 634; B. Avasarala and P. Haldar, Int. J. Hydrog. Energy 36 (2011) 3965   

Water-Pd Interface in Catalytic Biomass Conversion: Atomic-Scale Structure and Properties

Biomass pyrolysis and other relevant catalytic reactions often occur at the liquid-solid interface. It is therefore of great importance to investigate the interfacial structure and other properties in order to achieve a deep understanding about the catalytic reactions for biomass conversion. We used \textit{ab initio }molecular dynamics simulations to study the interfaces formed by liquid water and the palladium surfaces. Such interfaces are involved in many catalytic reactions for biomass conversion. We report results about the structural properties of the water/Pd(100) and water/Pd(111) interfaces, the interaction between liquid water and the metal surfaces, and how the interaction affects the structure. We found that while the interaction between water and the metal surface is weak, it could still cause considerable effects. In particular, the interaction promotes the formation of close-packed local clusters of liquid water.   

Multilayer epitaxial graphene oxide: structural and chemical properties from a combined theoretical and experimental XPS study

Multilayer graphene oxide (GO) is a material holding great promise in future energy storage and nano-electronic technologies. This material remains qualitatively known to date. In this work, we present a combined density functional theory (DFT) and experimental X-ray photo-emission spectroscopy (XPS) study of the structural, chemical, and thermal stability of multilayer GO grown epitaxially on silicon carbide. This investigation shows that at room temperature multilayer GO is a metastable material. GO films undergo spontaneous modifications and chemical reduction with a relaxation time of about one month. These processes lead multilayer GO toward a longer-living quasi-equilibrium state, consisting a structure deprived of epoxide groups, rich of hydroxyl groups, and with a O/C ratio of 0.38. Our study suggests that the presence of excess H chemisorbed on the graphitic sheets is the origin of the metastable character of multilayer GO. These H species favor the reduction of epoxide groups and the consequent transformation of hydroxyls into water molecules intercalated between the graphitic layers. Our DFT calculations show that these molecular transformations are controlled by diffusion processes.   

Switching of molecules at metal surfaces

Understanding the switching ability of molecules on surfaces upon excitation with external stimuli is a prerequisite for the development of functional molecular devices with possible applications to information processing, storage, or switching. While the switching mechanisms in many classes of molecular switches are thoroughly studied and understood in solution, their counterparts on surfaces still remain largely unresolved. In particular, many switching processes are suppressed or irreversible when the molecules are anchored to a metallic substrate. The adsorption configuration and steric hindrance are only one factor influencing the switching capability. More important is the electronic coupling strength between adsorbate and substrate and accordingly the lifetime of molecular excited states which is significantly reduced at metal surfaces. I will discuss several examples of optically and thermally induced conformational changes in molecular switches at noble metal surfaces.   

Competing Atomic and Molecular Mechanisms of Thermal Oxidation of Si vesus SiC

Thermal oxidation is a universal process in solids and is of practical importance in semiconductor technology. The oxidation of Si and SiC provide a unique opportunity for studying the oxidation mechanism because the products are the same oxide SiO$_{2}$. The oxidation of Si follows a linear-parabolic law with molecular oxygen identified as the oxidant. The oxidation of SiC obeys the same linear-parabolic law as Si but with different rates and temperature dependences. Using results from first-principle calculations, we first show that an atomic oxygen mechanism can account for the oxidation of Si-face SiC. We then discuss implications of the results and identify the determining factors in the competition between atomic and molecular mechanism. This work is supported by NSF GOALI grant DMR-0907385.   

Scanning tunneling microscopy uncovers the mechanism of silicon oxidation in aqueous solutions

Because of their immense technological importance, silicon oxidation reactions have been studied intensely for decades under a variety of conditions. However, the disordered nature of the reaction product, silicon oxide, makes these reactions notoriously difficult to understand. In this work, silicon oxidation is coupled with a subsequent etching reaction, allowing the oxidation reactions to literally write an atomic-scale record of their reactivity into the etched surface -- a record that can be decoded into site-specific reaction rates, and thus chemical understanding, with the aid of simulations and infrared spectroscopy. This record overturns the long-standing and much-applied mechanism for the (low-temperature) oxidation of the technologically important face of silicon, Si(100), and shows that the unusually high reactivity of a previously unrecognized surface species leads to a self-propagating etching reaction that produces near-atomically-flat surfaces terminated by a single monolayer of hydrogen atoms. This finding shows that, contrary to expectation, the low-temperature oxidation of Si(100) is a highly site-specific reaction and suggests strategies for the uniform functionalization of the technologically relevant face of silicon by low-temperature reactions.   

Metal pulled-off effect: A unique explanation of different oxidation process on Cu and Al surfaces

One interesting oxidation phenomenon is the difference of the oxidation of Cu and Al. Cu forms disordered domains, large surface reconstructions and oxide islands on the surface with some O atoms diffuse into inner layers to further oxidize inner Cu atoms. Al forms a dense oxide layer which protects the inner Al atoms from oxidation. In this talk, we demonstrate a possible electronic origin of this oxidation difference by using the first-principles method to calculate the initial oxidation of different metal surfaces and nanoclusters. On Cu 55 Icosahedron surface, we found that 2 O atoms at neighboring sites form a structure with a Cu atom in the middle pulled off from the surface. We also found the similar pull-offs on Cu, Pd, Zn surfaces, but not on Al surface, which is not a transition metal. This pulled off effect is explained by the strong metal d and O p coupling. We also checked different O concentration on Cu (111) surface and on Cu cluster surface and found that O atoms form chain or ring like structures. Our first principle molecular dynamic calculation confirms that these structures are stable. With this pull-off effect, additional O atoms can further oxidize inner Cu atoms and make Cu relative easy to oxidize. This finding enhances the scientific understanding of the initial oxidation of metallic nano-particles and surfaces, which may have important applications in catalysis, thermal storage and other surface science fields.   

Electrical and Structural Properties of Thin Films Fabricated by E-Beam Lithography from Gold Nanoparticle Resists

Drop- and spin-coated solutions of ligand-coated nanoparticles act as novel ``direct write'' e-beam resist, which can be prepared with metallic, magnetic and semiconducting nanoparticles. We prepared thin films from gold nanoparticles, in which we varied the film thickness. Small angle X-Ray scattering experiments as well as SEM imaging of the samples were performed to determine structural properties of the nanoparticles films at various stages of the fabrication process, after drop coating, ebeam exposure and annealing. We further performed DC charge transport measurements in the 2-350K temperature range and report different conductivity mechanisms based on the film thickness, ranging from insulating to Mott hopping conduction to metallic.   

Self-Diffusion of small Ag and Ni islands on Ag(111) and Ni(111) using the self-learning kinetic Monte Carlo method

We have applied a modified Self-Learning Kinetic Monte Carlo (SLKMC) method [1] to examine the self-diffusion of small Ag and Ni islands, containing up to 10 atom, on the (111) surface of the respective metal. The pattern recognition scheme in this new SLKMC method allows occupancy of the fcc, hcp and top sites on the fcc(111) surface and employs them to identify the local neighborhood around a central atom. Molecular static calculations with semi empirical interatomic potential and reliable techniques for saddle point search revealed several new diffusion mechanisms that contribute to the diffusion of small islands. For comparison we have also evaluated the diffusion characteristics of Cu clusters on Cu(111) and compared results with previous findings [2]. Our results show a linear increase in effective energy barriers scaling almost as 0.043, 0.051 and 0.064 eV/atom for the Cu/Cu(111), Ag/Ag(111), and Ni/Ni(111) systems, respectively. For all three systems, diffusion of small islands proceeds mainly through concerted motion, although several multiple and single atom processes also contribute. [1] Oleg Trushin et al. Phys. Rev. B 72, 115401 (2005) [2] Altaf Karim et al. Phys. Rev. B 73, 165411 (2006)   

Collective Excitations in Ultrathin Magnesium Films on Silicon

We present a systematic study of plasmon excitation in ultrathin Mg overlayer on Si(111) substrate. Our numerical results qualitatively reproduce the experimentally observed plasmon spectra of the Mg/Si systems [1]. The underlying physics of the formation of various absorption peaks can be understood using the simple hybridization concept. Based on this concept, the coexistence of surface and bulk plasmons in the experimental observation turns out to be a clear evidence for the existence of multiple surface plasmons due to the quantum confinement in Mg thin films [2]. In addition, we clearly see the plasmon enhanced substrate absorption, which comes from the screening of the substrate to the oscillatory charges.\\[4pt] [1] Ao Teng et al.(to be published).\\[0pt] [2] Xiaoguang Li et al.(to be published).   

Scaling Properties of Exciton Binding Energies in Two-Dimensional Insulating Films

Using the GW and Bethe-Salpter Equation (BSE) methods with inclusion of many-electron effects, we carry out a systematic study of the quasiparticle energy and optical absorption spectra of various two-dimensional (2D) thin-film insulators, including boron nitride, graphane, fluorographene, fluorosilicene, etc. All the calculated band gaps of these systems are increased by the GW corrections, and their optical responses are dominated by the strongly bound excitons, which can be attributed to the enhanced Coulomb attraction in 2D. Most strikingly, we find a well-defined linear scaling dependence between the exciton binding energy and band gap, and this scaling relationship is in stark contrast with the established ones in 3D and 1D insulating systems. The likely underlying physical mechanism for the linear scaling relationship will also be discussed.   

Spatially and Temperature Resolved Photoluminescence (PL) Of Excitons in Highly Oriented Phthalocyanine Films

Phthalocyanines and their derivatives are interesting alternative to polymer materials for the development of electronic devices such as organic thin field effect transistors, organic Light Emitting Diodes and photovoltaic cells. The present study focuses on spatially resolved, temperature-dependent PL of highly-oriented metal free and Zn -Octa-butoxy phthalocyanine (OBPc) polycrystalline thin films. Samples were fabricated using an in-house solution processing method\footnote{R. L. Headrick et al, APL, 92, 063302 (2008)} that results in mm-sized grains which can be individually probed using a focused laser beam. The experiments indicate the lowest optically active excitonic state which dominates the PL spectrum at 5K is optically-forbidden at room temperature. Linear Dichroism microscopy experiments indicate a reorientation of molecular planes below T$\sim$200K which may favor a mixing of Frenkel and intermolecular excitons, changing the nature of excitonic ground state.   

Grain Boundary Exploration of Excitonic States in Organic Crystalline Thin Films

The electronic states of Metal-Phthalocyanine (MPc) crystalline thin films are investigated. These samples are fabricated by solution processed pen-writing deposition technique.\footnote{R. Headrick et al, APL, 92, 063302 (2008)} Specifically, a linear dichroism mapping is performed, and excitonic emission is studied both close to and far from the large grain boundaries found in these phthalocyanine thin films. Multiple M-Pc samples are examined, including nickel, zinc and cobalt. In phthalocyanine crystalline films, it is believed that a monomer-like emission feature exclusively associated with a grain boundary is observed. The presence of this feature and its intensity are correlated with the relative orientation of neighboring grains at the boundary. The experiments are performed using a combined Linear Dichroism/Photoluminescence Microscopy experiment developed at the University of Vermont.   

Session P6: Graphene Optical Phenomena

Graphene-based photonic crystals

A novel type of photonic crystals formed by embedding a periodic array graphene and dielectric material into a background dielectric medium is discussed. One-dimensional (1D) photonic crystal formed by an array of periodically located stacks of alternating graphene and dielectric stripes, while the two-dimensional (2D) one is formed by constituent stacks of alternating graphene and dielectric discs. The electromagnetic wave propagation in 1D crystal analyzed in the framework of the Kronig-Penney model. The frequency band structure of 1D graphene-based photonic crystal is obtained analytically as a function of the filling factor and the thickness of the dielectric between graphene stripes. The photonic frequency corresponding to the electromagnetic wave localized by a defect that breaks the symmetry of the system is obtained. For 2D crystal the photonic band structure and transmittance are calculated. The graphene-based photonic crystals can be used effectively as the frequency filters and waveguides for the far infrared region of electromagnetic spectrum. Due to substantial suppression of absorption of low-frequency radiation in doped graphene the damping and skin effect in the photonic crystal are also suppressed. The advantages of the graphene-based photonic crystal are discussed.   

Graphene Plasmonic Terahertz Filters and Polarizers

Graphene has remarkably strong interaction with light, especially in the terahertz frequency range. Free carriers in graphene exhibit Drude behavior and the Drude weight can be tuned by electrostatic or chemical doping. Graphene can support surface plasmons. In this paper, we'll show that with multiple stacked CVD graphene layers, terahertz filters and polarizers can be realized by patterning them into micro-disks and ribbons. The influence of dipole-dipole interaction on the extinction spectra will be discussed.   

Surface-plasmon polaritons on graphene-metal surface

In this presentation we present a new application of graphene in the field of plasmonics. We studied excitation of surface-plasmon polaritons on graphene-metal surface. The metal surface is functionalized by transfer printing of a graphene layer grown by chemical vapor deposition on copper foils. Surface plasmon resonance (SPR) characteristics of monolayer and multilayer graphene on the metal surface are presented. The results reveal the essential features predicted by the calculations based on transfer matrix method. As an application, we fabricated a surface plasmon resonance sensor integrated with a microfluidic device to study nonspecific physical interaction between graphene layer and proteins. We obtained association and dissociation coefficient of BSA adsorbed on graphene layer. We believe that graphene functionalized SPR sensors could provide a new platform to study interactions between graphene and molecules.   

New Aspects of Photocurrent Generation at Graphene pn Junctions Revealed by Ultrafast Optical Measurements

The remarkable electrical and optical properties of graphene make it a promising material for new optoelectronic applications. However, one important, but so far unexplored, property is the role of hot carriers in charge and energy transport at graphene interfaces. Here we investigate the photocurrent (PC) dynamics at a tunable graphene pn junction using ultrafast scanning PC microscopy. Pump-probe measurements show a temperature dependent relaxation time of photogenerated carriers that increases from 1.5ps at 290K to 4ps at 20K; while the amplitude of the PC is independent of the lattice temperature. These observations imply that it is hot carriers, not phonons, which dominate ultrafast energy transport. Gate dependent measurements show many interesting features such as pump induced saturation, enhancement, and sign reversal of probe generated PC. These observations reveal that the underlying PC mechanism is a combination of the thermoelectric and built-in electric field effects. Our results enhance the understanding of non-equilibrium electron dynamics, electron-electron interactions, and electron-phonon interactions in graphene. They also determine fundamental limits on ultrafast device operation speeds ($\sim $500 GHz) for graphene-based photodetectors.   

Relaxation dynamics of photoexcited carriers in graphene probed by optical pump-THz probe spectroscopy

The relaxation of hot carriers in graphene is a subject of much current interest. The role of electronic and phonon relaxation channels is a topic of particular focus, with phenomena such as carrier multiplication, in which multiple charge carriers are generated from absorption of a single photon, having been predicted.\footnote{T. Winzer, A. Knorr, and E. Malic, Nano Letts. 10, 4839 (2010).} In this work, we apply the optical pump-THz probe spectroscopy to investigate the relaxation dynamics of photoexcited carriers in large-area single-layer graphene samples grown by chemical vapor deposition (CVD). The complex optical conductivity induced by optical excitation is determined over a broad range of THz frequencies as a function of the pump-probe delay time. To probe carrier relaxation dynamics, we compare the transient optical conductivity to a single-particle model for the intraband response. The effect of pump fluence and the static doping density on the carrier dynamics will be discussed.   

Excitonic Effects and Optical Absorption Spectrum of Doped Graphene

First-principles calculations based on the GW-Bethe-Salpeter Equation (GW-BSE) approach and subsequent experiments have shown large excitonic effects in the optical absorbance of graphene. Here we employ the GW-BSE formalism to probe the effects of charge carrier doping and of having an external electric field on the absorption spectrum of graphene. We show that the absorbance peak due to the resonant exciton exhibits systematic changes in both its position and profile when graphene is gate doped by carriers, in excellent agreement to very recent measurements\footnote{Tony F. Heinz, private communications.}. We analyze the various contributions to these changes in the absorption spectrum, such as the effects of screening by carriers to the quasiparticle energies and electron-hole interactions. This work was supported by National Science Foundation Grant No. DMR10-1006184, the U.S. Department of Energy under Contract No. DE-AC02-05CH11231, and the U.S. DOD - Office of Naval Research under RTC Grant No. N00014-09-1-1066. Computer time was provided by NERSC.   

Magneto-phonon resonance in graphene

Recently, much attention has been paid to electron-phonon coupling in graphene. In particular, significant re-normalization and broadening of long-wavelength optical phonons are predicted to occur through resonant interaction with Landau-quantized Dirac fermions. We report a high-field magneto-Raman spectroscopy study of single-layer graphene in magnetic fields up to 45 T. The Raman G peak exhibits clear splitting at approximately 30 T, which we attribute to the fundamental magneto-phonon resonance associated with (0,1) inter Landau level transitions. The coupled electron-phonon modes exhibit characteristic anti-crossing behavior allowing for an accurate determination of the electron-phonon coupling strength in graphene.   

Coherent photocurrent control in a graphene bilayer in a magnetic field

We consider theoretically the coherent control of a Bernal-stacked graphene bilayer in a perpendicular magnetic field. When the system is exposed to a two-color optical pulse, photocurrents of electrons and holes are induced through interference between one- and two-photon excitation processes. The generated photocurrents are time-dependent as a result of the two processes placing electrons or holes in different Landau levels. The direction and phase of the terahertz current oscillation can be tuned through the polarization and relative phase parameter of the optical pulses. We compare the results to those obtained in the absence of a magnetic field [1] and also to the results for monolayer graphene. \\[4pt] [1] J. Rioux, G. Burkard and J. E. Sipe, Phys. Rev. B 83, 195406 (2011).   

Giant optical nonlinearity of graphene in a strong magnetic field

We demonstrate theoretically that graphene placed in a strong magnetic field possesses by far the highest third-order optical nonlinearity among all known materials. The giant nonlinearity originates from unique electronic properties and selection rules near the Dirac points, which gives rise to resonantly enhanced nonlinear response. We present rigorous and intuitive quantum-mechanical density-matrix formalism for calculating linear and nonlinear optical properties of graphene, valid for arbitrarily strong magnetic and optical field [1]. The calculated magnitude of the third-order nonlinearity is of the order of 0.01 esu for the field of several Tesla in the mid/far-infrared spectral range. Due to this giant nonlinearity, even one monolayer of graphene gives rise to appreciable nonlinear frequency conversion efficiency for incident mid/far-infrared radiation. \\[4pt] [1] X. Yao and A. Belyanin, Giant optical nonlinearity of graphene in a strong magnetic field, Phys. Rev. Lett. submitted; arXiv: 1110.4869.   

Mid-Infrared Magneto-Optical Kerr and Faraday Studies in Gated Monolayer and Rotated Bilayer CVD Graphene

Previous studies of multi-layer graphene grown on C-face SiC have proven that mid-IR (111-135 meV) Polar Magneto-Optical Kerr Effect (PMOKE) measurements provide a unique and sensitive way to probe the Landau Level (LL) structure of graphene. Graphene PMOKE measurements, which are proportional to the Hall conductivity ($\sigma _{xy})$, have revealed large changes in the Kerr angle due to the chiral nature of LL transitions in mono- and multi-layer graphene, as well as the dependence of these Kerr features on the position of the Fermi level. In this work we present new results that extend these measurements to more ideal samples consisting of large area (5x5mm), epitaxial single layer CVD grown graphene that has been deposited onto a Si/SiO$_{2}$ substrate. Unlike epitaxial SiC graphene these new samples can be back-gated and also allow magneto-optical measurements to be made in transmission (Faraday geometry). The ability to tune parameters such as the Fermi energy, probing photon energy, magnetic field strength, and temperature allows us to better understand graphene through its mid-IR Hall conductivity and to test for theoretical predictions of an infrared quantum Hall effect in graphene (Morimoto, PRL, 2009). This work is supported by NSF-DMR1006078 and by the Office of Naval Research.   

Substrate effects and photon energy dependence of ultrafast carrier relaxation in graphene

Ultrafast photo-excitation in graphene creates non-equilibrium carrier distributions and provides an avenue for the measurement of couplings between electronic and lattice degrees of freedom. Previous studies have explored the carrier relaxation dynamics in graphene on various substrates albeit primarily in the linear dispersion regime. We investigate the ultrafast carrier dynamics of graphene both in the linear band regime and near the saddle point. We perform femtosecond-resolved degenerate pump-probe differential transmission experiments to extract the timescales for electronic relaxation from different starting points on the band structure. The use of degenerate pump-probe allows us to obtain exact relaxation timescales corresponding to the local band structure without contributions from other carriers. We use multiple transparent substrates, such as fused silica, quartz, sapphire etc. to probe the substrate-graphene interactions at various points along the local electronic band structure. We find that in the UV regime, the relaxation dynamics in graphene shows remarkable dependence on the substrate, with relaxation timescales ranging from a few to hundreds of picoseconds We will also compare our measurements with those obtained in freestanding graphene samples.   

Excitonic Effects on Optical Absorption Spectra of Doped Graphene

We have performed first-principles calculations to study optical absorption spectra of doped graphene with many-electron effects included. Both self-energy corrections and electron-hole interactions are reduced due to the enhanced screening in doped graphene. However, self-energy corrections and excitonic effects nearly cancel each other, making the prominent optical absorption peak fixed around 4.5 eV under different doping conditions. On the other hand, an unexpected increase of the optical absorbance is observed within the infrared and visible-light frequency regime (1 $\sim $ 3 eV). Our analysis shows that a combining effect from the band filling and electron-hole interactions results in such an enhanced excitonic effect on the optical absorption. These unique variations of the optical absorption of doped graphene are of importance to understand relevant experiments and design optoelectronic applications.   

Dirac Plasmas in Graphene

Collective excitation of Dirac Fermions in graphene has many similarities to that of conventional semiconductor two dimensional electron gas (2DEG). For instance, the resonance frequency is proportional to q1/2, where q is the wave-vector. However, there are some fundamental differences. In this paper, we'll present our far-infrared spectroscopy studies of Dirac plasma in graphene. Localized plasmons in graphene disks and micro-ribbons are excited by far-infrared photons. The resonance frequency scales as n1/4 , where n is the sheet carrier density. This is distinctively different from conventional semiconductor. In addition, plasmons in graphene superlattice are compared to their counterparts in semiconductor superlattice.   

Experimental and theoretical investigations on SERS enhancement mechanism of graphene

Graphene has recently been shown to improve the Surface-enhanced Raman Scattering (SERS) performance of traditional nanostructured metallic substrates. Here we present an experimental and theoretical study on its SERS enhancement mechanism. We observed that SERS enhancement of graphene can be tuned by changing its Fermi level via doping. Both molecular doping and gate doping experiments show that hole-doped graphene yields a larger SERS enhancement in methylene blue (MB) than electron-doped one. The MB-graphene system is then modeled using both a fully quantum mechanical (QM) description as well as a QM-polarizable force field model wherein the graphene is modeled using a Drude-Lorentz function. In the first model, charge transfer (CT) excitations between MB and a graphene cluster can be accounted for, while in the second model we can account for the ``infinite'' size of the graphene sheet. Both of the models confirm the role of graphene Fermi level on its SERS enhancement. Our preliminary results suggest that graphene SERS enhancement would likely be coming from a ground-state chemical enhancement.   

Exciting and probing plasmons in graphene by local defects

The short wavelength of collective excitations, i.e., plasmons, in doped graphene ($\sim$10-20 nm) is very attractive for multiple applications. However, the same short wavelength makes photoexcitation of plasmons in graphene a very challenging task. In this work, we discuss various types of local defects including semiconductor quantum dots, metallic nanoclusters, edges and holes in graphene as means to ``squeeze'' the large wavelength of optical excitation down to the nanometer scale, thus, providing an effective coupling between free photons and plasmons in graphene. In the case of semiconductor quantum dots, we show how plasmons in graphene can be excited and probed by Forster resonance energy transfer from the optically excited quantum dot to the graphene sheet. Specifically, we demonstrate how the calculated dispersion relation of plasmons in graphene as well as of other electronic excitations can be accurately extracted by controlling the backgate voltage and the distance between the quantum dot and graphene [1]. \\[4pt] [1] K. A. Velizhanin, A. Efimov``Probing plasmons in graphene by resonance energy transfer,'' Phys. Rev. B, 085401, 84 (2011).   

Session P7: Focus Session: Computational Design of Materials: Graphene - Electronic Structure and Transport

Electronic transport properties of a graphene monolayer covered by another layer with infinite or finite width

Intrinsic graphene, a zero-gap semiconductor, is one of the most promising materials for nanodevices. In this work, the electronic transport properties of a graphene monolayer covered by another infinite or finite layer were studied. The results of the transmission spectrum and the local density of states (LDOS) showed a weak interaction between the two layers when the top layer is infinite or semi-infinite. Thus the transport properties of the monolayer do not change much. However, when the monolayer is covered by a finite-width nanoribbon, the change in its transmission spectrum is dependent on the width of the ribbon. In order to understand the origin of this phenomenon we calculated the transmission spectrum of one individual channel, and observed that the changes are due to antiresonance in the electronic transmission, which is caused by interlayer interference between the wavefunctions.   

Electronic transport proprieties of graphene few-layers

We study electronic transmission spectrums of graphane few-layers (up to 6 layers) with AA or AB stacking by using of first-principle transport calculations. The results demonstrate flat transmission spectrum for even layers while linear transmission for odd layers, which can be understood by different band structures of graphene few-layers due to the interlayer interaction.   

Quantum Transport Properties of Modified Graphene Nanoribbons with Boron Nitride Domains at the Mesoscopic Scale

Graphene nanoribons are seen as promising building blocks for engineering graphene based field effect transistor (GNR-FET) . The quest for fabricating efficient GNR-FETs requires a trade-off between a sufficiently wide energy gap and a reasonably large charge mobility. The solution might be the chemical codoping of one-atom thick layers of C with B and N atoms. These hybrid systems are attracting much attention as they can provide an efficient way to create new materials with complementary properties to those of graphene and h-BN. I will present a study of charge transport in graphene nanoribbons with BN domains randomly distributed along the ribbon surface. My results describe how the conductance of the hybrid systems is altered as a function of the incident electron energy and the BN domain density which leads to transport band gap opening. We explore the transport regimes comparing different degrees of BN codoping and BN domain size for ribbons of various widths and lengths on the order of the micrometer. A comparison with other types of defects such as oxygen atoms in epoxy configuration and functional groups covalently attached to the ribbon surface will be discussed as well.   

First-principles study of charge transfer doping and electronic transport in single--walled carbon nanotubes

It is well known that charge transfer doping can greatly enhance the conductivity in single-walled carbon nanotube thin films. Recent experiments showed that the tube-tube contact resistance dominates the impedance in the films. To understand effects of doping on the tube-tube contact, we studied the tube-tube distance changes and electron transport properties upon doping (K and Br) using first-principles calculations. Our results suggest that the effect of doping on the tube-tube distance depends not only on the type of dopants but also on the electronic properties of the carbon nanotubes.   

Computational Study of the Thermal and Electronic Transport Properties of Rigidly-Interconnected Carbon Nano Foam

We study the thermal and electronic transport properties of rigidly-interconnected structures having $sp^2$ carbon minimal surface called schwarzites. The system consists of core parts composed of schwarzite and interconnection parts with (4,4) carbon nanotube segments [1]. Using direct molecular dynamics simulations with the Tersoff potential, we compute the thermal conductivity of various configurations to explore the dependence on the number of core parts and on the length of interconnection parts. Our calculations show that each core part plays as a scattering center, which reduces the phonon mean free path and thus the thermal conductivity. We also investigate the electronic transport properties of the system by applying the non-equilibrium Green function approach in combination with density functional theory. We explore the effects of different core connectivity and structural defects introduced near the core parts on the electrical conductance. These thermal and electonic properties may be connected to the thermoelectric properties of the schwarzite system.[1] S. Park, K. Kittimanapun, J. S. Ahn, Y.-K. Kwon and D. Tom\'anek, J. Phys.: Condens. Matter {\bf{22}}, 334220 (2010).   

Klein, anti-Klein tunneling and pseudo-spintronics in graphene heterojunctions

Electrons in monolayer graphene act like massless spin-1/2 Dirac fermions. Backscattering is suppressed due to the pseudospin orthogonality of the forward and reverse scattering modes. The resulting Klein tunneling provides unit transmission for normally incident electrons at a pn junction, regardless of barrier height. By combining voltage gating with a tunnel barrier, we can realize a gate tunable metal insulator transition that promotes subthermal switching [1] and also makes the conduction unipolar. In contrast, bilayer graphene electrons act like parabolic spin-1 systems with perfect reflection for normal incidence (anti-Klein tunneling). For n$^{+}$n or p$^{+}$p junction, the transmission maximizes for normal incidence like single layer, but unlike monolayer graphene, it's barrier-dependent. We also perform atomistic numerical calculation of graphene sheets with experimentally relevant size (hundreds of nanometer) using non-equilibrium Green's function formalism and we show that the conductance can be varied substantially with gate voltage for multiple sequenced pn junctions with smoothly varying potential. \\[4pt] [1] Sajjad and Ghosh, APL 99, 123101 (2011).   

Curvature-induced spin-orbit coupling and spin relaxation in a chmically-clean single-layer graphene

Based on the second-order perturbation theory, we show that curvature induced by corrugations or periodic ripples in single-layer graphenes generates two types of effective spin-orbit coupling. In addition to the spin-orbit coupling reported previously that couples with sublattice pseudospin and corresponds to the Rashba-type spin-orbit coupling, there is an additional spin-orbit coupling that does not couple with the pseudospin. The additional spin-orbit coupling depends on the direction of principal curvature, which is similar with the curvature-induced spin-orbit coupling of carbon nanotubes that depends on the chiral angle. However, the spin-orbit coupling of single-layer graphenes can not be obtained from the trivial extension of the spin-orbit coupling of carbon nanotubes owing to their distinct topological structure. Via the numerical calcualtion, we show that both types of the curvature-induced spin-orbit coupling make the same order of contribution to spin relaxation in chemically-clean single-layer graphene with nanoscale corrugation. The spin relaxation dependence on the corrugation roughness is also investigated.   

Fractional topological phases, triplet superconductivity and spontaneous time reversal symmetry breaking in strained graphene

Despite wide interests to realize fractional time-reversal symmetric phases, an experimentally realizable system with these exotic topological orders is still lacking. Recent experiment has confirmed that strain can be used to control the electronic states of graphene and create flat pseudo-Landau levels in the absence of external magnetic field. In this talk, I show that graphene under strain is a natural playground for the observation of exotic phases such as fractional valley Hall insulator as well as flat band superconductivity. For neutral graphene, we find a competition between the Ising valley ferromagnet and the spin ferromagnet ruled by the value of short range Hubbard and long range Coulomb interactions. At fractional filling with different intervalley and intravalley interactions, a spin triplet superconductor, a valley-polarized Laughlin state or a time-reversal symmetric fractional valley-Hall state could be realized.   

Satellite Structures in Spectral Functions of Silicon and Graphene from ab initio GW and Cumulant Expansion Calculations

The GW approximation is a well-established method for obtaining accurate quasiparticle properties in a wide range of materials. Its suitability for satellite structures (e.g., those measured in photoemission spectroscopies), however, has rarely been addressed in detail for real materials and the fact that GW overestimates the position of the plasmon satellite peaks in the spectral function of silicon indicates the need for an improved method for satellites. One such method is the cumulant expansion. The cumulant expansion is a method that includes, approximately, higher-order processes beyond GW that are important for satellite properties. We present here full-frequency results for the satellite and quasiparticle properties of silicon and doped graphene using the GW and the cumulant expansion methods, and discuss the improvements in satellite properties given by the cumulant expansion. We also compare our results to earlier model calculations on doped graphene.   

Static and dynamical response of graphene

We discuss the static and dynamical response of graphene. First, we show that including the full hexagonal lattice leads to anisotropic Friedel oscillations and paramagnetic orbital susceptibility around the neutrality point [1]. We then apply the dynamical current-current correlation function to discuss graphene's fluorescence quenching including also transverse decay channels and full retardation [2]. We finally discuss the optical properties of double layer graphene. \\[4pt] [1] G. G\'omez-Santos and T. Stauber, Phys. Rev. Lett. 106, 045504 (2011).\\[0pt] [2] G. G\'omez-Santos and T. Stauber, Phys. Rev. B 84, 165438 (2011).   

First-principles study of graphene - carbon nanotube contacts

The electron transport properties of carbon nanotube -- graphene junctions are investigated with first-principles total energy and electron transport calculations. By combining the advantageous material properties of graphene and nanotubes one can create all carbon hybrid architectures with properties that are particularly well suited to applications. The p-type Schottky barrier height is calculated in model junctions with (8,0) and (10,0) nanotubes in a top-contact configuration. Results indicate a lower barrier in carbon nanotube -- graphene junctions than in other carbon nanotue -- metal systems.   

Electronic Band Structures of Graphene Nanomeshes

Owing to its many remarkable properties, graphene is very promising for electronic and opto-electronic applications for size miniaturization and improving performance; however, bulk graphene is a semi-metal with zero band gap, and many methods have been proposed to open up a sizable band gap. In this work, we carry out first-principles calculations based on the density functional theory (DFT) to investigate electronic band structures of graphene nanomeshes (GNMs), the defected graphene containing a high-density array of nanoholes, studying the bandgap opening mechanism and evaluating band gap as functions of structural parameters, including hole size, hole shape, hole-hole distance, and hole arrangement. Our results suggest that while the band gap opening is a result of quantum confinement at nanomesh necks, the size of band gap depends strongly on the detailed GNM structures. For the simplest hexagonal holes, two thirds of GNMs remain semi-metallic and the rest one third are semiconductors.   

Quantum Mechanics on a Mobius Strip: Energy Levels, Symmetries, and Level Splitting in a Magnetic Field

We investigate the energy levels of an electron on a M\"obius strip. Schr\"odinger's equation on this curved surface is shown to have terms that do not have invariance under parity transformation in parameter space for the strip. The double degeneracy of energy levels that exists for flat cylindrical rings is shown to be removed for the pairs of energies in the M\"obius strip due to parity symmetry breaking. The orbital angular momentum is found to have approximately not only integer but also half-integer values of $\hbar$. The splitting of the energy levels in an external magnetic field is displayed. The effects of multiple twists are investigated to further clarify that the parity symmetry breaking is the effect of the curved geometry, while the appearance of half-integer angular momentum states is a topological effect. The implications for twisted rings composed of graphene will be discussed, and carrier transport through the M\"obius strip will be considered. This work was supported by AFLR/DARPA under grant FA8650-10-1-7046.   

QED Kapitza conductance of nano-carbon thermal interconnects

The theory for the near-field Kapitza conductance across the interface of a nano-carbon material and the quartz is thoroughly investigated. The near-field photon tunneling is shown to contribute to the total heat flux between the hot and cold sides of the interface on the order of or even larger than the normal thermal conductance. Quartz is chosen as the most common example of non-conductive and strongly polar substrate material with the well known polarization properties, though the theory is not restricted to quartz only. Our approach allowed us to derive a unified expression for QED Kapitza conductance of the nanocarbon thermal interconnect material, such as graphene, a nanotube, or a nanotube forest and predict thermal phenomena, such as the heat rectification, as a function of the materials properties of the interface.   

Session P8: Focus Session: Spin Liquids I

Quantum Ice : Experimental Signatures

``Quantum Spin Ice'' materials have attracted considerable attention as three-dimensional examples of quantum spin liquids. Recently, we have used zero-temperature Quantum Monte Carlo simulation to explore one possible scenario for these materials, confirming the possibility of a ``quantum ice'' state driven by quantum tunnelling between an extensive number of different spin-ice configurations [1]. Here we address the simple question : what would such a quantum ice look like in experiment ? We focus in particular on the fate of ``pinch point'' singularities seen in neutron scattering experiments on spin ice materials, showing how these are suppressed and ultimately eliminated as the system is cooled to its ground state [1,2]. \\[4pt] [1] N. Shannon et al., arXiv:1105.4196\\[0pt] [2] O. Benton et al., in preparation.   

Quantum Ice : A Quantum Monte Carlo Study

The magnetic ``ice'' state found in spin ice materials has recently generated great excitement for its magnetic monopole excitations. However the deconfined nature of these monopoles depends crucially on the macroscopic degeneracy of the classical ice ground state. And at very low temperatures we might expect this degeneracy to be lifted by quantum tunneling between different ice configurations. Here we present the results of large-scale Green's function Monte Carlo simulation of ice-type models which include quantum tunneling. We find compelling evidence of an extended quantum U(1)-liquid ground state with deconfined monopole excitations in both the quantum dimer model [1,2] and the quantum ice model on the diamond lattice [3]. This quantum U(1) liquid proves to be remarkably robust against the inclusion of long range dipolar interactions. \\[0pt] [1] O. Sikora {\it et al.}, Phys. Rev. Lett. {\bf 103}, 247001 (2009) [2] O. Sikora {\it et al.}, Phys. Rev. B {\bf 84}, 115129 (2011) [3] N. Shannon {\it et al.}, arXiv:1105.4196   

Three-dimensional generalized Kitaev models

We generalize Kitaev's honeycomb lattice spin model to a gamma matrix model on three-dimensional cubic octahedron and pyrochlore lattices. We find the ground state ${Z}_2$ flux configuration, reducing the problem to free Majorana fermion hopping. For the cubic octahedron lattice, which has reflection planes, the ground states must obey Lieb's theorem, i.e. the ${Z}_2$ fluxes are reflection symmetric. By adding flux-flux interaction terms, a variety of interesting phases can be stabilized, including metallic, semimetallic, and both trivial and topological insulating phases.   

High pressure sequence of Ba$_{3}$NiSb$_{2}$O$_{9}$ structural phases: new $S $= 1 quantum spin-liquids based on Ni$^{2+}$

A quantum spin-liquid (QSL) is a ground-state where strong quantum- mechanical ?uctuations prevents a phase-transition towards conventional magnetic order and makes the spin ensemble to remain in a liquid-like state. Most QSL candidates studied to date are two-dimensional frustrated magnets with either a triangular or a kagome lattice composed of $S$ = 1/2 spins. Here, we report the use of a high pressure, high temperature technique to transform the antiferromagnetically ordered ($T_{N}$ = 13.5 K) 6H-A phase of Ba$_{3}$NiSb$_{2}$O$_{9}$ into two new QSL candidates with larger $S$ = 1 (Ni$^{2+})$ moments: the 6H-B phase of Ba$_{3}$NiSb$_{2}$O$_{9}$ which crystallizes in a triangular lattice and the 3C-phase of Ba$_{3}$NiSb$_{2}$O$_{9}$ which forms a three-dimensional edge-shared tetrahedral lattice. Both compounds show no evidence for magnetic order down to $T$ = 0.35 K despite Curie-Weiss temperatures \textit{$\theta $}$_{CW}$ of -75.5 K (6H-B) and -182.5 K (3C), respectively. Below $\sim $25 K the magnetic susceptibility of the 6H-B phase is found to saturate at a constant value $\chi $ = 0.013 emu/mol which is followed below 7 K, by a linear in temperature dependence for the magnetic contribution to the specificheat ($C_{M})$ which displays a giant coefficient $\gamma $ = 168 mJ/mol-K$^{2}$ comparable to values observed in heavy-fermion metallic systems. Taken together, both observations indicate the development of a Fermi-liquid like ground-state characterized by a Wilson ratio of 5.6 in this otherwise insulating material It also points to the formation at finite temperatures of a well defined Fermi surface of $S$ = 1 spin-excitations which behave as charged quasiparticles. For the 3C phase one observes $C_{M} \quad \propto \quad T ^{2}$ indicating a unique $S$ = 1 three-dimensional QSL ground-state as previously reported for Na$_{3}$Ir$_{4}$O$_{8}$ although this later compound is composed of Ir$^{4+ }$ions having $S$ = 1/2. \\[4pt] Work done in collaboration with J. G. Cheng, G. Li, J. S. Zhou, J. B. Goodenough, C Xu and H. D. Zhou.   

Generalizations of the Kitaev-Heisenberg model and connections to experiment

Generalizations of Kitaev-Heisenberg spin models are both interesting on their own right and potentially relevant for the Mott insulating layered Iridates A$_2$IrO$_3$ (A=Na,Li) and other complex oxides of 5d transition metal compounds. To describe Na$_2$IrO$_3$ and Li$_2$IrO$_3$ we propose the Kitaev-Heisenberg-$J_2$-$J_3$ model, a combination of the Kitaev honeycomb model and the Heisenberg model with all three nearest neighbor couplings $J_1$, $J_2$ and $J_3$. A rich phase diagram is obtained at the classical level, including the experimentally suggested \textit{zigzag} ordered phase; as well as the \textit{stripy} phase, which extends from the Kitaev-Heisenberg limit to the $J_1$-$J_2$-$J_3$ one. Combining the experimentally observed spin order with the optimal fitting to the uniform magnetic susceptibility data gives an estimate of possible parameter values, which in turn reaffirms the necessity of including both the Kitaev and farther neighbor couplings in describing the materials. Generalizations for other spin-orbit coupled Mott insulating 5d transition metal oxides with Kitaev-Heisenberg type models exhibit related magnetically ordered phases as well as different spin liquid phases.   

Topological phases of the 2D AKLT like model via study of degenerate singular value spectra

We study the two-dimensional quantum spin-3/2 systems on the honeycomb lattice, which include the 2D AKLT model. We use the infinite time-evolving block decimation (iTEBD) method to numerically solve the ground state wave function by adopting the tensor product states (TPS) ansatz. We then evaluate the singular value spectra, entanglement entropy, and the Neel order by the method of tensor renormalization group (TRG). With all these results we determine the phase diagram of the spin-3/2 model. Our result shows the singular value spectra become doubly degenerate in a belt region including the AKLT point, and indicates the existence of possible topological phases. The double degeneracy of singular value spectra is related to the recent observation in [C.-Y. Huang~and~F.-L. Lin, Phys. Rev. B 84, 125110 (2011)] of using it to characterize the topological phase. We also cross check this statement by calculating the topological Renyi entropy. The 2D AKLT state has been shown to be a universal resource for measurement-based quantum computation. Our results indicate this state is topological in nature and should be robust against local perturbations.   

Finite temperature phase diagram of the classical Heisenberg-Kitaev model.

We study finite-temperature properties of classical version of the Heisenberg-Kitaev model on the honeycomb lattice. This model is a prominent example of anisotropic spin-orbital models, which can possibly describe the low-energy physics of Na$_2$IrO$_3$ and Li$_2$IrO$_3$. In these compounds, Ir$^{4+}$ ions are in a low spin $5d^5$ configuration and form weakly coupled hexagonal layers. Our main result is a finite-temperature phase diagram obtained by classical Monte Carlo simulations. Because of highly anisotropic Kitaev interaction, the spin symmetry of the model is reduced to the discrete symmetry, which may be regarded as $Z_3 x Z_2$. As the discrete symmetry can be broken at finite temperature even at 2D, the model undergoes phase transitions as a function of temperature. At low temperature phase, thermal fluctuations induce order-by-disorder, just as the quantum fluctuations do at zero temperature. As a result, magnetically ordered ground states of the Heisenberg-Kitaev model persist up to a certain critical temperature. Finally, we discuss the relevance of obtained results for experimental findings in real compounds.   

Dynamical Jahn-Teller effect in spin-orbital coupled system

Orbital degree of freedom is one of the most attractive themes in strongly correlated electron system. A coupling between the orbital and the lattice vibration is known as a Jahn-Teller effect (JTE). The dynamical aspect of the Jahn-Teller interaction is often neglected in solid, because it is strongly suppressed by the cooperative JTE. Recently, Ba$_{3}$CuSb$_{2}$O$_{9}$ has been reported as a candidate of the spin liquid. A Cu$^{2+}$ has the $e_{g}$ orbital degree of freedom and is surrounded by the O$^{2-}$ octahedron. The octahedra on the neighboring sites do not have the common O ions. This fact implies that the cooperative JTE is weak, and the dynamical JTE is expected to play some key roles on orbital and magnetic properties. The purpose of this research is to study the dynamical JTE in a spin-orbital coupled system. In particular, we focus on the competitive or cooperative phenomena between the superexchange interaction and the dynamical JTE. The superexchange interactions are derived from the $d-p$ model on a honeycomb lattice. We have confirmed this interaction stabilizes the antiferro-spin and ferro-orbital configurations for the realistic parameters. The dynamical JTE described as the orbital-lattice coupling is obtained by extracting the low energy states of the vibronic Hamiltonian. We analyze the model including the two kinds of interactions by using the Bethe approximation. We find that the magnetic order is unstable in wide parameter region and the spin-dimer state with the orbital order is realized. Furthermore the orbital order is strongly suppressed by the dynamical JTE.   

Quantum spin liquid in frustrated one dimensional LiCuSbO$_4$

A quantum magnet, LiCuSbO$_4$, with chains of edge-sharing $S\!=\!1/2$ CuO$_6$ octahedra is reported. Short-range ordering is observed while no phase transition or spin freezing occurs down to 100 mK in zero magnetic field. Specific heat indicates a distinct low-temperature high-field phase near the 12 T saturation field. Neutron scattering shows incommensurate spin correlations with $q\!=\!0.47\pm0.01~\pi/a$ and places an upper limit of 70~$\mu$eV on a potential spin gap. Exact diagonalization of easy plane $S\!=\!1/2$ chains with competing nearest neighbor ferro- and next-nearest neighbor antiferromagnetic interactions (J$_1\!=\!-75$~K, J$_2\!=\!34$~K) accounts for the $T\!>\!2$~K bulk and neutron data. Close to a quantum critical point, free from long-range order and with an achievable saturation field, LiCuSbO$_4$ is a promising candidate material to test long-standing predictions for chiral and nematic states in quantum spin chains   

Exotic S=1 spin liquid state with fermionic excitations on triangular lattice

Motivated by recent experiments on the material Ba$_3$NiSb$_2$O$_9$ we consider a spin-one quantum antiferromagnet on a triangular lattice with the Heisenberg bilinear and biquadratic exchange interactions and a single-ion anisotropy. Using a fermionic ``triplon'' representation for spins, we study the phase diagram within mean-field theory. In addition to a fully gapped spin-liquid ground state, we find a state where one gapless triplon mode with Fermi surface coexists with $d + id$ topological pairing of the other triplons. Despite the existence of a Fermi surface, this ground state has fully gapped bulk spin excitations. Such a state has linear in-temperature specific heat and constant in-plane spin susceptibility, with an unusually high Wilson ratio.   

Spin Liquid Phases for Spin-1 systems on the Triangular lattice

Motivated by recent experiments on material Ba$_3$NiSb$_2$O$_9$, we propose two novel spin liquid phases (A and B) for spin-1 systems on a triangular lattice. At the mean field level, both spin liquid phases have gapless fermionic spinon excitations with quadratic band touching, thus in both phases the spin susceptibility and C$_v$/T saturate to a constant at zero temperature, which are consistent with the experimental results on Ba3NiSb2O9. On the lattice scale, these spin liquid phases have Sp(4) $\sim$ SO(5) gauge fluctuation; while in the long wavelength limit this Sp(4) gauge symmetry is broken down to U(1)xZ$_2$ in type A spin liquid phase, and broken down to Z$_4$ in type B phase. We also demonstrate that the $A$ phase is the parent state of the ferro-quadrupole state, nematic state, and the noncollinear spin density wave state.   

Stability of the U(1) Dirac spin liquid state on the kagome lattice

The ground state of the spin 1/2 nearest neighbor Heisenberg antiferromagnet on the kagome lattice is still unknown. Recent DMRG calculations[1] have challenged the proposal[2] that this ground state is an algebraic spin liquid with Dirac fermions and photons as elementary excitations. We numerically study all time reversal invariant Gutzwiller projected variational wave functions for this system and find a state with mild symmetry breaking as the lowest energy state. To avoid getting stuck in a local minimum, we begin our Monte-Carlo calculation from all Z2 spin liquid states cataloged in Ref. [3]. Analyzing the resulting wave function, we find it lies very close to the U(1) Dirac state proposed in Ref. [2] in terms of its gauge fluxes, but with lower energy. We believe these results strongly emphasize the dominance and stability of the U(1) Dirac spin liquid state among Gutzwiller projected wave functions. However, our state has higher energy than the DMRG matrix product state, so correlations beyond those captured by it must play a fundamental role in the characterization of the ground state. [1]S. Yan, D.A. Huse, and S.R. White, Science 332, 1173 (2011) [2]Y. Ran, M. Hermele, P.A. Lee, and X.-G. Wen, PRL, 98, 117205 (2007) [3]Y.-m. Lu, Y. Ran, and P. Lee, PRB, 83, 12 (2011)   

Theory of spin liquids in integer spin pyrochlores

Rare earth pyrochlores, with a chemical formula A2B2O7, exhibit many interesting features in A site spin system. Depending on A site rare earth elements, spin ice and magnetically ordered phases are shown in several experiments. Moreover, they have been also focused as possible candidates of U(1) spin liquid. In order to explore such versatile phases, we study the pseudospin-1/2 model, which is quite generic to describe rare earth pyrochlores with integer spins, in the presence of spin-orbit coupling and crystalline electric field. Using a new ``gauge mean field theory,'' we show the possible ground states, corresponding to several phases listed above. We also briefly discuss the experimental suggestions based on our theory.   

Session P9: Focus Session: Magnetic Oxide Thin Films And Heterostructures: Manganite Thin Films

Magnetic non-uniformity in (La$_{0.4}$Pr$_{0.6})_{0.67}$Ca$_{0.33}$MnO$_{3}$ films and measurement of the strain-magnetization coupling coefficient

We have characterized the non-uniformity of chemical and magnetic properties of (La$_{0.4}$Pr$_{0.6})_{0.67}$Ca$_{0.33}$MnO$_{3}$ (LPCMO) films grown on NdGaO$_{3}$ using polarized neutron reflectometry (PNR). Our data indicate that the films exhibit coexistence of different magnetic phases as a function of depth. The variation of magnetism with depth is correlated with a variation of chemical composition with depth. Using PNR we also measured the magnetization depth profile of the LPCMO film as a function of applied bending stress. From these measurements we were able to obtain values for the coupling coefficients relating strain to the variation of the magnetization depth profile. Our results suggest that application of compressive (tensile) bending stress increases (suppresses) magnetization. We will discuss the implications of our results on the prevailing theories of the role of strain on phase separation in manganites.   

The dead layer in La0.67Sr0.33MnO3 thin films

La0.67Sr0.33MnO3 (LSMO) films have an interfacial dead layer that is attributed to either changes in the Mn valence state at the interface, a change in the Mn orbital ordering at the interface or a change in the magnetic exchange interaction due to a structural reconstruction at the interface. We studied the dead layer in LSMO films grown on SrTiO3 (STO) substrates. To directly compare the effect of the polar discontinuity on the dead layer, we removed the polar discontinuity by compositional interface engineering. We also studied films in the (110) direction. We found the presence of the dead layer in all types of films and interface configurations. The LSMO (001) samples with compositional interface engineering have the thinnest dead layer (2 nm). Using electron energy loss spectroscopy, we found no deviations in the Mn valence state at the interface. Using linear dichroism in x-ray absorption spectroscopy, we also found no deviations in the orbital ordering. Therefore, we suggest that the dead layer is caused by the structural reconstruction at the interface. An outlook towards preventing the structural reconstruction and further improving the interfacial properties of the LSMO thin films will be given.   

Angular dependence of the anomalous Hall effect in La$_{0.8}$Sr$_{0.2}$MnO$_3$ films

The anomalous Hall effect (AHE) is an intriguing magnetotransport phenomenon linked to various intrinsic and extrinsic mechanisms. While for some conductors quantitative understanding of this phenomenon has been achieved, the understanding of the AHE in the manganites is far from comprehensive. We measured the transverse resistivity ($\rho_{xy}$) of thin films of La$_{0.8}$Sr$_{0.2}$MnO$_3$ at temperatures between 5 to 200 K and magnetic fields up to 9 T as a function of the angle $\theta$ between the film normal and the magnetic field. We find that for fields above 4 T, for which the magnetization (M) is practically parallel to the magnetic field, $\rho_{xy}=A\cos\theta+B\cos(3\theta)$. The first term is attributed to the ordinary and anomalous Hall effect, and the unexpected $\cos(3\theta)$ term is attributed only to the anomalous Hall effect. We show that the angular dependence of the longitudinal resistivity, $\rho_{xx}$, and of the magnitude of M cannot explain the existence of a $\cos(3\theta)$ term. We discuss the implication of this term on the possible mechanisms of the anomalous Hall effect in this compound.   

Effect of electronic reconstruction on the superconducting properties in high $T_C$ superconducting spin valve structures

We have studied the angular dependence of the magnetoresistance (MR) and magnetization alignment in La$_{0.7}$Ca$_{0.3}$MnO$_3$ (LCMO)/YBa$_2$Cu$_3$O$_{7-\delta}$ (YBCO)/LCMO trilayers in the superconducting transition region. The top and bottom LCMOs have different easy-axis coercivities (H$_C$'s) so that the mutual alignment between the two LCMO's magnetizations is tunable with in-plane rotation of the magnetic field. When the amplitude of the applied field is between the two H$_C$'s, the MR shows a quasi-four-fold symmetry, an angular hysteresis between clockwise and anticlockwise rotations, and a unidirectional offset along the initial saturation direction. We find that the MR is not correlated with the LCMO's magnetization alignment. More interestingly, the angular dependence of the MR is understandable by the alignment between the applied magnetic field and the (exponential tail of the) induced exchange fields in YBCO, the latter of which originate from the electronic reconstruction at the LCMO/YBCO interfaces. Our results support the scenario recently proposed by Salafranca and Okamoto [Phys. Rev. Lett. \textbf{105}, 256804 (2010)], which explains the inverse superconducting spin switch effect in this system.   

STEM-EELS and theoretical analysis of the electronic structure in cuprate-manganite heterostructures

Scanning transmission electron microscopy in combination with electron energy loss spectroscopy allows sub-nanometer scale resolution mapping of the formal oxidation state of the transition metal ions in YBa$_2$Cu$_3$O$_{7-\delta}$/La$_{0.7}$Ca$_{0.3}$MnO$_3$ superlattices. The experiments show an unexpected excess of valence electrons near the interface. We compare these results with tight binding model calculations where Coulomb interactions are included within Hartree approximation. Neither the polar catastrophe mechanism nor the mismatch of chemical potentials between the two materials are sufficient to account for the observed profile. We study the effect of oxygen vacancies near the interface and find that they can explain the measured electronic structure.   

Phase Diagram of Thin Film Oxides Growth by Pulsed Laser Deposition

We present a qualitative analysis of the microscopic thermo-dynamical origin of thin film oxides growth using the pulsed laser deposition technique. A phase diagram containing different growth mechanisms has been established. By tuning growth parameters experimentally in [LaSr]MnO3/SrTiO3 system, we observe an excellent fit of thin film morphologies to our growth phase diagram. Our results offer guidance on controlling morphology, stoichiometry and crystallinity of oxides thin films.   

Domain Structures in Perovskite Oxide Superlattices

Perovskite oxides possess a wide range of technologically relevant functional properties including ferromagnetism, ferroelectricity, and superconductivity. Furthermore, the interfaces of perovskite oxides have been shown to exhibit unexpected functional properties not found in the constituent materials. These functional properties arise due to various structural and chemical changes as well as electronic and/or magnetic interactions occurring over nanometer length scales at the interfaces. In order to understand how these interfacial effects impact the ferromagnetic (FM) properties of the half metal La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (LSMO), we have examined superlattices composed of LSMO sublayers alternating with either the antiferromagnetic (AFM) insulator La$_{0.7}$Sr$_{0.3}$FeO$_{3}$ (LSFO) or the non-magnetic metal La$_{0.5}$Sr$_{0.5}$TiO$_{3}$ (LSTO). A comprehensive approach consisting of bulk magnetization, magneto-transport measurements, and scanning transmission electron microscopy as well as soft x-ray magnetic spectroscopy and microscopy has been used to fully characterize the properties of the interfaces. We find that the nature of the charge transfer across the interfaces affects the FM properties of LSMO, such that at a given sublayer thickness, the LSMO/LSTO system displays a similar Curie temperature but a higher saturation magnetization than the LSMO/LSFO system. For a specific range of sublayer thicknesses, the LSMO/LSFO system displays a unique spin-flop coupling where the FM moments and the AFM spin axis maintain a perpendicular orientation relative to one another. Through this coupling mechanism, the direction of the AFM spin axis can be reoriented with an applied magnetic field. In this talk, I will discuss how these interfacial phenomena contribute to the types of FM and AFM domain patterns observed in the individual layers in the superlattices.   

Temperature dependent optical properties of thin films of the doped manganite La$_{0.67}$Ca$_{0.33}$MnO$_{3}$

Reflectivity as a function of temperature has been measured for thin film samples of the manganite La$_{0.67 }$Ca$_{0.33 }$MnO$_{3 }$across the metal-insulator transition. The optical properties in the infrared and visible range were determined by fits to a Drude-Lorentz model, using exact formulas for the thin film optics and the measured properties of the substrate. The phonon modes were identified and verified with lattice dynamical calculations for distorted orthorhombic crystal structure of the material. The reflectance has a strong temperature dependence in the far infrared and in the region of the phonons, rising as the temperature is lowered and the film becomes metallic. In the near-infrared and visible range, there are conductivity peaks due to electronic band transition shifts to the lower frequencies with decreasing temperature. We also observe the spectral weight shift with temperature.   

Induced ferromagnetism in La$_{0.7}$Sr$_{0.3}$MnO$_{3}$/LaFeO$_{3}$ interfaces and its role on magnetic tunnel junctions

Magnetic tunnel junctions with antiferromagnetic barriers have so far been poorly studied. We have investigated La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (LSMO)/LaFeO$_{3}$ (LFO)/LSMO magnetic tunnel junctions(MTJ) where LFO is an antiferromagnetic tunnel barrier. We examined the tunneling magnetoresistance (TMR) behavior of junctions as a function of temperature finding a 30\% maximum at 100K. If the top ferromagnetic electrode is replaced by a non-ferromagnetic metal (Au) we still observe a TMR effect, which we attribute to spin filtering. We will show that this effect is connected to the presence of an induced (ferro)magnetic moment in the nominally antiferromagnetic LFO barrier at the interface with LSMO, which was detected by XMCD measurements. Finally, we will conclude discussing the new opportunities offered by such type of interfaces to obtain large spin filtering effects.   

Magnetic studies on thin films of La$_{0.65}$Pb$_{0.35}$MnO$_{3}$

Magnetic and structural investigations on thin films of La$_{0.65}$Pb$_{0.35}$MnO$_{3}$ deposited on a LaAlO$_{3}$ substrate are reported. Transmission electron microscopy showed an almost epitaxial growth of the perovskite film, indicating fourfold symmetry for both substrate and thin film. Low Energy Electron Diffraction and Wide Angle X-Ray Scattering support transmission electron microscopy and scanning tunneling microscopy results. Magneto-optical Kerr effect data are consistent with the fourfold symmetry. Ferromagnetic Resonance experiments performed in the X band revealed a more complex structure. The angular dependence of the resonance line width, resonance line intensity, and double integral of the resonance line support a slightly distorted four-fold symmetry whereas the angular dependence of the resonance line position has a two-fold symmetry. This discrepancy was ascribed to the mismatch between the film and the substrate and it is considered as a proof of the sensitivity of ferromagnetic resonance.   

Using ultrafast optical pump-probe spectroscopy to reveal coexisting magnetic orders in epitaxial $R$MnO$_{3}$ films

Recent discoveries of spin-driven ferroelectricity in perovskite manganites, $R$MnO$_{3}$ ($R$=rare-earth ions), have attracted enormous interest in the research of multiferroics. Although extensive experimental and theoretical studies have already been done on single crystal $R$MnO$_{3}$, there are only a few reports describing the properties of $R$MnO$_{3}$ thin films. Here, we choose two typical materials in $R$MnO$_{3}$ manganites as examples: SmMnO$_{3}$ and TbMnO$_{3}$. Previously, ultrafast optical pump-probe spectroscopy has proven to be an ideal technique for unraveling the interplay between different orders in the time domain. In this work, we used this technique to study ultrafast dynamics in epitaxial SmMnO$_{3}$ and TbMnO$_{3}$ films grown on SrTiO$_{3}$ substrates. At low temperatures, we observed an extraordinarily slow rising process, with a timescale of tens of picoseconds, followed by another decay process with a relaxation time of hundreds of picoseconds. Analysis of the time constants associated with these two processes as a function of temperature reveals that antiferromagnetic, ferromagnetic, and ferroelectric orders can coexist in these materials.   

Influence of growth mode \& substrate doping on the reversed remanent magnetic configuration in La$_{0.7}$Sr$_{0.3}$MnO$_3$ films

Charge transfer effects which can occur at oxide interfaces can modify the properties of oxide thin films. In such a system, an unusual reversed orientation of the remanent magnetic state was observed recently for La$_{0.7}$Sr$_{0.3}$MnO$_3$ on Nb-doped SrTiO$_3$(001) deposited via pulsed laser deposition\footnote{J.-S. Lee \emph{et al., Phys. Rev. Lett. } {\bf 105}, 257204 (2010)}. We observe a similar effect for La$_{0.7}$Sr$_{0.3}$MnO$_3$ grown via molecular beam epitaxy, a deposition method with different growth kinetics, onto both Nb-doped and undoped SrTiO$_3$ (STO) substrates. The reversed magnetic state occurrs in both samples, and a region of slightly increased charge density was revealed with x-ray reflectivity. Intriguingly, the onset of the reversed remanent state occurred at different temperatures: $\sim$125K for the Nb:STO substrate and $\sim$240K for the undoped STO substrate. High resolution x-ray diffraction reveals a subtle relationship with the cubic-to-tetragonal structural transition of the STO substrate at $\sim$105 K. Our results point to an additional mechanism for controlling the magnetism in mixed-valence oxide films\footnote{J.-S. Lee \emph{et al., J. Phys. D: Appl. Phys}. {\bf 44}, 245002 (2011)}.   

Magnetic Exchange Between Superconducting and Ferromagnetic Oxide Layers

The origins of high temperature superconductivity and the rich phase diagrams in complex oxides are still a matter of contention that have stimulated many novel experimental studies and observations. Recently the improvement of layer by layer growth techniques of thin films has enabled investigations of both bulk and surface properties. For most common superconductors the order parameter is thought to be antagonistic to that of the exchange mechanism in ferromagnets. Accurately grown thin fllms have enabled these competing interactions to be probed experimentally. In particular, the growth of epitaxial oxide layers, with well-characterized atomically flat interfaces, consisting of superconducting layers of YBa$_2$Cu$_3$O$_7$ (YBCO) and lattice-matched ferromagnetic La$_{2/3}$Ca$_{1/3}$MnO$_3$ (LCMO) has flourished. Using XMCD we demonstrate that the known superexchange between Mn and Cu across the YBCO/LCMO is modified when an apparent critical thickness of the superconducting layer is reduced. All samples show an apparent exchange below the superconducting transition but above it is dependent on the YBCO thickness. Possible origins of this behaviour will be discussed.   

Session P10: Invited Session: Quantum Simulations

Quantum simulations and artificial gauge fields with ultracold atoms

Here I present our experimental work synthesizing gauge fields for Bose-Einstein condensates (BECs). I will first summarize our earlier work creating a scalar (abelian) gauge field (akin to the electromagnetic vector potential) and then focus in detail our current work creating a matrix valued (although still abelian) gauge field. I will discuss this gauge field in the language of spin-orbit coupling where it consists of an equal sum of Rashba and Dresselhaus couplings. Specifically, we couple two internal states of rubidium 87 with a pair of ``Raman'' lasers and load our BEC into the resulting adiabatic eigenstates. In agreement with theory, we observe that below a critical coupling strength our BEC has well defined spin degrees of freedom and acts like a spin-orbit-coupled spin-1/2 Bose gas. As a function of the Raman laser strength, a new exchange-driven interaction between the two dressed spins develops, which drives a (quantum) phase transition from a state where the two dressed spin states spatially mix, to one where they phase separate. Our 3D mean field theory accurately locates the critical laser strength for this transition.   

Circuit QED Simulation of Interacting Bosons with Microwave Polaritons

A polariton is a coherent superposition of a photon and an electronic excitation such as an exciton. Polaritons can have very low mass (associated with the photon component) and repulsive interactions (associated with the exciton component). Recent experimental progress has observed Bose-Einstein condensation and superfluidity in polaritons in semiconductor quantum wells. In this talk I will discuss the possibility that many-body physics and quantum phase transitions of interacting polaritons [1-3] can be observed in arrays of microwave resonators containing superconducting qubits [4-6]. If the qubits are not far-detuned from the cavities, the natural excitations are coherent superpositions of cavity and qubit excitations and they have interactions acquired from the anharmonicity of the qubits. These interactions can lead to quantum phase transitions in the limit of weak dissipation. It may even be possible to simulate the fractional quantum Hall effect for bosons by coupling the polaritons between sites using superconducting structures which act as `circulators' that break time-reversal and charge-conjugation symmetry. In light of recent progress in achieving very long-coherence times for superconducting qubits and strong qubit coupling to microwave photons, experimental prospects for observing quantum phase transitions in microwave resonator lattices will be described. \\[4pt] [1] A. D. Greentree, et al., {\sl Nat. Phys.} {\bf 2}, 856 (2006).\\[0pt] [2] M. J. Hartmann et al., {\sl Nat. Phys.} {\bf 2}, 849 (2006).\\[0pt] [3] D. G. Angelakis, M. F. Santos, and S. Bose, {\sl Phys. Rev. A} {\bf 76}, 031805 (2007).\\[0pt] [4] J. Koch and K. Le Hur, {\sl Phys. Rev. A} {\sl 80}, 023811 (2009).\\[0pt] [5] `Time-reversal symmetry breaking in circuit-QED based photon lattices,'Jens Koch, Andrew A. Houck, Karyn Le Hur, and S. M. Girvin, {\sl Phys. Rev. A} {\bf 82}, 043811 (2010).\\[0pt] [6] `Synthetic gauge fields and homodyne transmission in Jaynes-Cummings lattices,' A. Nunnenkamp, Jens Koch, and S. M. Girvin, {\sl New J. Phys.} {\bf 13} 095008 (2011).   

Quantum Hall physics with photons and its application

Phenomena associated with the topological properties of physical systems can be naturally robust against perturbations. This robustness is exemplified by quantized conductance and edge state transport in the quantum Hall and quantum spin Hall effects. Here we demonstrate how quantum spin Hall Hamiltonians can be simulated with linear optical elements using a network of coupled resonator optical waveguides (CROW) in two dimensions. Key features of quantum Hall systems, including the characteristic Hofstadter butterfly and robust edge state transport, can be obtained in such systems. As a specific application, we show that topological protection can be used to improve the performance of optical delay lines and to overcome some limitations related to disorder in photonic technologies. Furthermore, the addition of an optical non-linearity to our proposed system leads to the possibility of implementing a fractional quantum Hall state of photons, where phenomenon such as fractional statistics may be observable.   

Mixed Bose-Fermi Mott Phases and Phase Transitions

A recent experiment with an ultra-cold mixture of $^174$Yb and $^173$Yb atoms in an optical lattice [S. Sugawa e. al. Nature Physics 7, 642 (2011)] found a remarkable quantum phase that can be described as a mixed Mott insulator. Such a an incompressible state established at integer combined filling of the two species, must have residual low energy Fermionic degrees of freedom associated with relative motion of the two species. I will discuss the novel quantum states formed by the composite Fermions in the mixed Mott insulator as well as the unconventional phase transitions separating these states from the compressible Bose-Fermi mixture established at weak interactions. Finally I will propose to utilize the mixed Mott insulator as a quantum simulator for models of the doped Mott insulator relevant to high Tc superconductivity. The new approach, where the bosonic atoms play the role of doped holes offers significant advantages over direct simulation of the Hubbard model. In particular the mixed Mott plateau naturally provides a flat trap potential to the doped holes, while the hole doping is easily tuned by varying the relative fraction of the bosons.   

Session P11: Focus Session: Graphene Structure, Stacking, Interactions: Strain and Stacking

Non-Abelian gauge potentials in graphene bilayers

We discuss the effect of spatial modulations in the interlayer hopping of graphene bilayers, such as those that arise upon shearing or twisting. We show that their single-particle physics, characterized by charge localization and recurrent formation of zero-energy bands as the pattern period L increases, is governed by a non-Abelian gauge potential arising in the low-energy electronic theory due to the coupling between layers. We find that such gauge-type couplings give rise to a confining potential that, for certain discrete values of L, localizes states at zero energy in particular regions of the Moire patterns. We also draw the connection between the recurrence of the flat zero-energy bands and the non-Abelian character of the potential.   

Electronic Topological Transition in Sliding Bilayer Graphene

We demonstrate theoretically that the topology of energy bands and Fermi surface in bilayer graphene undergoes a very sensitive transition when an extremely tiny lateral interlayer shift occurs in arbitrary directions. The phenomenon originates from a generation of an effective non-Abelian vector potential in the Dirac Hamiltonian by the sliding motions. The characteristics of the transition such as pair annihilations of massless Dirac fermions are dictated by the sliding direction owing to a unique interplay between the effective non-Abelian gauge fields and Berry's phases associated with massless electrons. The transition manifests itself in various measurable quantities such as anomalous density of states, minimal conductivity, and distinct Landau level spectrum.   

Fictitious gauge fields in bilayer graphene

We discuss the effect of elastic deformations on the electronic properties of bilayer graphene membranes. Distortions of the lattice translate into fictitious gauge fields in the electronic Dirac Hamiltonian which are explicitly derived for arbitrary elastic deformations. We include gauge fields associated to intra- as well as inter-layer hopping terms and discuss their effects on the strain-induced Lifshitz transition and on the electron-phonon resistivity. Of special interest is the appearance of a linear coupling for flexural modes which is shown to dominate the temperature-dependent resistivity in suspended samples with low tension.   

Effective theory of rotationally faulted multilayer graphene

The crystal structure of graphene multilayers with an interlayer twist is characterized by Moir\'{e} patterns with various commensurabilities. Also the electronic structure of the material, which grows for instance epitaxially on SiC, is remarkably rich. In this talk an effective low-energy description of such multilayers will be discussed. In certain limits the theory reduces to a (single-layer) Dirac model with space-dependent potentials and mass term. The consequences of this theory will be explored and comparison with experiment will be made. The discussion of experimental consequences will focus on the Landau quantization in a magnetic field, where much experimental data is available. For instance, a spatially modulated splitting of the zeroth Landau level in the material has been observed through scanning tunneling spectroscopy [1]. That splitting finds a natural explanation in the space-dependent mass term of the presented theory [2]. Also a large low-field splitting of higher Landau levels recently observed in graphene multilayers [3] will be shown to follow from that theory. Finally, a theoretically expected [4] amplitude modulation of the Landau level wavefunctions on the top layer of the material will be discussed. \\[4pt] [1] D. L. Miller, K. D. Kubista, G. M. Rutter, M. Ruan, W. A. de Heer, M. Kindermann, P. N. First, and J. A. Stroscio, Nature Physics \textbf{6}, 811 (2010). \\[0pt] [2] M. Kindermann and P. N. First, Phys. Rev. B \textbf{83}, 045425 (2010). \\[0pt] [3] Y. J. Song, A. F. Otte, Y. Kuk, Y. Hu, D. B. Torrance, P. N. First, W. A. de Heer, H. Min, S. Adam, M. D. Stiles, A. H. MacDonald, and J. A. Stroscio, Nature \textbf{467}, 185 (2010). \\[0pt] [4] M. Kindermann and E. G. Mele, Phys. Rev. B \textbf{84}, 161406(R) (2011).   

High-temperature topological insulator states in strained graphene

Recently, it was realized that the electronic properties of graphene can be manipulated via mechanical deformations, which opens prospects for both studying the Dirac fermions in new regimes and for device applications. More specifically, non-uniform strains give rise to pseudomagnetic fields that are opposite in the two valleys of Dirac fermions. Certain natural configurations of strain generate large nearly uniform pseudo-magnetic field, leading to flat spin- and valley-degenerate Landau levels (LL). Here we consider the effect of the Coulomb interactions in strained graphene with nearly uniform pseudo-magnetic field. We show that the spin/valley degeneracies of the LL get lifted, giving rise to topological insulator-like states. We find that both anomalous quantized Hall states and quantum spin Hall states can be realized. These many-body states are characterized by quantized conductance and persist to high temperature scale set by the Coulomb interactions. This work provides a new route to designing robust topological insulator states in mesoscopic graphene and other 2D Dirac materials. \\[4pt] [1] D. A. Abanin, D. A. Pesin, submitted.   

Theory of topological phases in multilayer Graphene

We present microscopic theories of possible topological phases (i.e. quantum anomalous Hall effect, quantum valley Hall effect , and two-dimensional topological insulators) in bilayer graphene systems. We show the phase diagrams as well as the resulting nontrivial edge states in these systems. We further generalize our findings to trilayer graphene systems, where similar topological states may exist. Finally, we give low energy effective models to reveal the underlying physics of these topological states.   

The correct solution for strain-induced pseudo vector potentials in graphene

Prior calculations of the strain-induced pseudo vector potential considered only the change in the nearest neighbor hopping energy in their derivations [1,2]. Here we show that including lattice deformations introduces new terms of the same order in strain. These terms are different at each K point, causing each population of electrons to feel different strain induced pseudo magnetic fields. We use isotropic strain, a situation where lattice deformations solely determine the pseudo vector potential, to illustrate the conceptual importance of these new terms. Finally, we exhibit how the additional terms force us to rethink the strain geometries that were previously thought to generate particular pseudo magnetic fields. [1] A.H. Castro Neto, et al. \textit{Rev. Mod. Phys.} \textbf{81}, 109 (2009). [2] M.A.H. Vozmediano, M.I. Katsnelson, and F. Guinea, Phys. Rep. \textbf{496}, 109 (2010).   

Two-Dimensional Topological Insulator State and Topological Phase Transition in Bilayer Graphene

In this talk, we show that gated AB-stacking bilayer graphene can host a quantum phase transition from a quantum valley Hall (QVH) insulator to a two-dimensional strong topological insulator (TI) as a function of Rashba spin-orbit (SO) coupling. Different from a conventional TI phase, the edge modes of our strong TI phase exhibit both spin and valley filtering, and thus share the properties of both TI and QVH insulators. The strong TI phase remains robust in the presence of weak intrinsic SO coupling.   

Disorder-induced inhomogeneity in bilayer graphene

We describe the effect of charge density inhomgeneity (electron and hole puddles) and a spatially fluctuating band gap caused by charged impurity disorder in bilayer graphene. We derive a phenomenological averaging technique to calculate $\frac{d\mu}{dn}$ in the presence of this disorder and apply it recent experimental measurements in suspended bilayer graphene. This work was done in collaboration with S. Das Sarma, E. Hwang, and H. Min.   

Raman spectra of strained bilayer graphene

In the Raman spectra of strained single layer graphene, modified electron and phonon dispersions result in the splitting of the double resonance 2D Raman band. It originates from significant changes in the resonant conditions owing to both the distorted Dirac cones and anisotropic modifications of the phonon dispersion under uniaxial strains [D. Yoon et al., Phys. Rev. Lett. \textbf{106}, 155502 (2011)]. In unstrained bilayer graphene, the Raman 2D band consists of 4 Lorentzian peaks corresponding to the double resonance Raman scattering processes between the two conduction bands and the two valance bands. Under uniaxial strain, each of the four peaks in the Raman 2D band. We examined the polarization behaviors of the split 2D band and analyzed using a model similar to the one used for single layer graphene.   

Field Effects on the Optical Vibrations in Bilayer Graphene

A first-principles study of the optical phonon modes in bilayer graphene (BLG) under a perpendicular electric field is presented. It is found that the electric field breaks the inversion symmetry of BLG and mixes the eigenvectors of the in-phase and out-of-phase optical modes. Detailed analysis shows that the mixing effect is more evident on the out-of-plane optical modes than on the in-plane modes. The field effects on the electron-phonon coupling in BLG will also be discussed.   

Six-band nearest-neighbor tight-binding model for the $\pi $-bands of bulk graphene and graphene nanoribbons

The commonly used single-p$_{z}$ orbital first nearest-neighbor tight-binding model faces two main problems: (i) it fails to reproduce asymmetries in the bulk graphene bands; (ii) it cannot provide a realistic model for hydrogen passivation of the edge atoms. As a result, some armchair graphene nanoribbons (AGNRs) are incorrectly predicted as metallic. A new nearest-neighbor, three orbital per atom p/d tight-binding model [1] is built to address these issues. The parameters of the model are fit to bandstructures obtained from first-principles density-functional theory and many-body perturbation theory within the GW approximation, giving excellent agreement with the ab initio AGNR bands. This model is employed to calculate the current-voltage characteristics of an AGNR MOSFET and the conductance of rough-edge AGNRs, finding significant differences versus the single-p$_{z}$ model. Taken together these results demonstrate the importance of an accurate and computational efficient band structure model for predicting the performance of graphene-based nanodevices. [1] T. B. Boykin, M. Luisier, G. Klimeck, X. Jiang, N. Kharche, Y. Zhou and S. Nayak, J. Appl. Phys. 109, 104304 (2011)   

Electronic structures of geometrically restricted nanocarbons

We use large scale ab-initio calculations to explore the electronic structures of graphene, graphene nanoribbons, and carbon nanotubes periodically perforated with nanopores. We disclose common features in electronic structures of these porous nanocarbons (PNCs) with nanopores of different size, shapes, and localization. We develop a unified picture that permits to analytically predict and systematically characterize metal-semiconductor transitions in PNCs, allowing mapping of their electronic structures on those in pristine nanocarbons [1]. In contrast to other studies, we show that porous graphene can be metallic for certain arrangements of the pores. When we replace pores by defects (such as SW 55-77), we observe similar features in the electronic structures of the formed nanocarbons. We also study magnetic ordering in these nanocarbons and show that the position of pores/defects can influence the ordering of localized electronic spin states. These periodically modified nanocarbons with highly tunable band structures have great potential applications in electronics and optics. [1] A.I. Baskin and P. Kral, Sci. Rep.1, 36 (2011).   

Session P12: Tutorial for Authors and Referees

Tutorial for Authors and Referees

Editors from Physical Review Letters and Physical Review will provide information and tips for our less experienced referees and authors. This session is aimed at anyone looking to submit to or review for any of the APS journals, as well as anyone who would like to learn more about the authoring and refereeing processes. Topics for discussion will include advice on how to write good manuscripts, similarities and differences in writing referee reports for PRL and PR, and other ways in which authors, referees, and editors can work together productively. Following a short presentation from the editors, there will be a moderated discussion. Refreshments will be served.   

Session P13: Focus Session: Low-Dimensional and Molecular Magnetism - Single-Molecule Magnets - Dynamics, structure and interactions

Quantum Tunneling of Magnetization in Trigonal Single-Molecule Magnets

We perform a numerical analysis of the quantum tunneling of magnetization (QTM) that occurs in a spin $S$ = 6 single-molecule magnet (SMM) with idealized $C_{3}$ symmetry. The deconstructive points in the QTM are located by following the Berry-phase interference (BPI) oscillations. We find that the $\hat {O}_4^3 $ ($=\frac{1}{2}[\hat {S}_z ,\hat {S}_+^3 +\hat {S}_-^3 ])$ operator unfreezes odd-$k$ QTM resonances and generates three-fold patterns of BPI minima in all resonances, including $k$~=~0! This behavior cannot be reproduced with operators that possess even rotational symmetry about the quantization axis. We find also that the $k$ = 0 BPI minima shift away from zero longitudinal field. The wider implications of these results will be discussed in terms of the QTM behavior observed in other SMMs.   

Exploration of the Berry phase interference in a single-molecule magnets of trigonal symmetry

The quantum behavior of single-molecule magnets (SMM) is mainly governed by their molecular composition and crystallographic symmetries, thus playing an essential role in the tunneling dynamics. We present low temperature magnetometry measurements on a trigonal symmetric, low nuclearity Mn3 SMM. The experiments are designed to explore the behavior of the tunnel splittings within the transverse field magnitude/direction phase space, by applying a transverse field (0-1~T) along different directions within the hard anisotropy plane of the molecules. The expected quantum interference pattern can be understood as an outcome of a competition between different intramolecular magnetic interactions. A multi-spin description using non-collinear zero-field splitting tensors and intra molecular dipolar interactions between the manganese ions is employed to explain the symmetry patterns.   

Investigation of the Barriers of Blocking of Magnetization In Strongly Anisotropic SMM By Ab Initio Methods

A large amount of data concerning the blocking barriers of reversal of magnetization in various complexes with strongly anisotropic metal ions (Ln$^{III}$, Co$^{II})$ became recently available. Understanding the mechanisms of formation of these barriers is of primary importance for an efficient design of Ln-based single-molecule magnet (SMM) and represents a challenging task for the theory. Here an ab initio based approach for the investigation of blocking barriers will be presented. The methodology will be applied for the construction of the blocking barriers and the understanding of the variation of SMM properties in the series of mixed 3d-4f trinuclear complexes Co-Ln-Co, Ln=Gd, Tb, Dy. In particular, the reasons for a more pronounced SMM behavior manifested by the gadolinium complex will be elucidated. Another example is a recently synthesized Dy$_{3}$ complex, for which the origin of magnetization steps in the hysteresis loops will be explained. \\[4pt] [1] T. Yamaguchi, J.-P. Costes, Y. Kishima, M. Kojima, Y. Sunatsuki, N. Br\'{e}fuel, J.-P. Tuchagues, L. Vendier, W. Wernsdorfer \textit{Inorg. Chem}. \textbf{2010}, $49$, 9125--9135.   

Decoherence: Intrinsic, Extrinsic, and Environmental

Environmental decoherence times have been difficult to predict in solid-state systems. In spin systems, environmental decoherence is predicted to arise from nuclear spins, spin-phonon interactions, and long-range dipolar interactions [1]. Recent experiments have confirmed these predictions quantitatively in crystals of Fe$_8$ molecules [2]. Coherent spin dynamics was observed over macroscopic volumes, with a decoherence $Q$-factor $Q_{\phi} = 1.5 \times 10^6$ (the upper predicted limit in this system being $Q_{\phi} = 6 \times 10^7$). Decoherence from dipolar interactions is particularly complex, and depends on the shape and the quantum state of the system. No extrinsic ``noise'' decoherence was observed. The generalization to quantum dot and superconducting qubit systems is also discussed. We then discuss searches for ``intrinsic'' decoherence [3,4], coming from non-linear corrections to quantum mechanics. Particular attention is paid to condensed matter tests of such intrinsic decoherence, in hybrid spin/optomechanical systems, and to ways of distinguishing intrinsic decoherence from environmental and extrinsic decoherence sources. \\[4pt] [1] Morello, A. Stamp, P. C. E. \& Tupitsyn, Phys. Rev. Lett. {\bf 97}, 207206 (2006).\\[0pt] [2] S. Takahashi et al., Nature {\bf 476}, 76 (2011).\\[0pt] [3] Stamp, P. C. E., Stud. Hist. Phil. Mod. Phys. {\bf 37}, 467 (2006). \\[0pt] [4] Stamp, P.C.E., Phil. Trans. Roy. Soc. A (to be published)   

The Effect of Uniaxial Pressure on the Spin Hamiltonian of Mn12-Ac Single-Molecule Magnet

We study the effect of uniaxial pressure on the magnetic hysteresis loops of the single-molecule magnet Mn12-Ac. We find that the application of pressure along the easy axis increases the fields at which quantum tunneling of magnetization occurs. Density functional theory (DFT) calculations yield the pressure dependence of the energy barrier for spin reversal that is consistent with the experimental results. The observations, when constrained by the DFT calculations, indicate that the pressure induces changes in both the second-order anisotropy constant $D$ and the fourth-order anisotropy constant $A$.   

Mitigation of decoherence in crystals of a Ho$_{x}$Y$_{1{\-}x}$W$_{10}$ ($x$ = 0.001 to 0.25) single-molecule magnet

Mononuclear lanthanide-based single-molecule magnets (SMMs) have attracted considerable recent attention due to their potential application in quantum information processing devices [Nat. Nanotechnol. \textbf{2}, 312 (2007)]. In these systems, the magnetization is associated with a single rare-earth ion, which facilitates mitigation of spin decoherence due to nuclear hyperfine and electron dipolar interactions via isotope purification and dilution. Their large magnetic moments enable coherent manipulation at low driving fields. We report multi-frequency electron paramagnetic resonance (EPR) studies on a Ho polyoxometalate (POM). Simulations indicate appreciable transverse spin-orbit anisotropy, resulting in a gap in the spectrum of several GHz between pairs of levels having the same nuclear projection (effectively a tunneling gap between excited electron-nuclear spin states). Measurements at 9~GHz reveal electron-spin-echoes at low temperatures. Remarkably, a $T_{2}$ time of several hundred nanoseconds is found for concentrated samples, with much longer values found in diluted samples containing deuterated solvent. We show that these long coherence times are related to the tunneling gap, which results in an insensitivity of the spin dynamics to dipolar field fluctuations.   

Single-molecule magnets ``without'' intermolecular interactions

Intermolecular magnetic interactions (dipole-dipole and exchange) affect strongly the magnetic relaxation of crystals of single-molecule magnets (SMMs), especially at low temperature, where quantum tunneling of the magnetization (QTM) dominates. This leads to complex many-body problems [l]. Measurements on magnetically diluted samples are desirable to clearly sort out the behaviour of magnetically-isolated SMMs and to reveal, by comparison, the effect of intermolecular interactions. Here, we diluted a Fe4 SMM into a diamagnetic crystal lattice, affording arrays of independent and iso-oriented magnetic units. We found that the resonant tunnel transitions are much sharper, the tunneling efficiency changes significantly, and two-body QTM transitions disappear. These changes have been rationalized on the basis of a dipolar shuffling mechanism and of transverse dipolar fields, whose effect has been analyzed using a multispin model. Our findings directly prove the impact of intermolecular magnetic couplings on the SMM behaviour and disclose the magnetic response of truly-isolated giant spins in a diamagnetic crystalline environment.\\[4pt] [1] W. Wernsdorfer, at al, PRL 82, 3903 (1999); PRL 89, 197201 (2002); Nature 416, 406 (2002); IS Tupitsyn, PCE Stamp, NV Prokof'ev, PRB 69, 132406 (2004).   

How Weak Dipole Interactions Can Enhance of the Collective Coupling of an Ensemble of Single-molecule Magnets to a Microwave Cavity

When $N$ identical spins are on resonance with a resonant mode of an electromagnetic cavity, the coupling strength (Vacuum Rabi splitting) is enhanced by $\sqrt{N}$ [1]. Add some inhomogeneity so that the spins' resonant frequencies are distributed around the cavity frequency with width $\sigma_\omega$, and this enhancement will remain as long as $\sigma_\omega < \sqrt{N} g_1$, where $g_1$ is the coupling strength of a single, isolated spin to the cavity [2]. Recent experiments have shown that $\sim 10^{16}$ spins in a crystal of the single-molecule magnet Fe$_8$ nevertheless exhibit the enhanced collective coupling to a cavity, despite substantial inhomogeneous broadening. I present numerical calculations that show that weak dipole interactions between the spins can enhance the coupling of the spins to the cavity, allowing collective coupling even when the inhomogeneous broadening is large (i.e.~when $\sigma_\omega > \sqrt{N} g_1$). \\[4pt] [1] M. Tavis and F. W. Cummings, Phys. Rev. {\bf 170}, 379 (1968).\break [2] R. Houdre, R. P. Stanley and M. Ilegems, Phys. Rev. A {\bf 53}, 2711 (1996).   

Quantum deflagration in Mn$_{12}$-acetate in the presence of a transverse field

Mn$_{12}$-acetate single crystal have been shown to exhibit abrupt reversal of the magnetic moment through propagation of a narrow front at subsonic velocities, termed magnetic deflagration [1]. Experiments where avalanches in Mn$_{12}$-acetate are triggered at a fixed applied field have shown that the velocity of the front has maxima at resonant fields (kH$_{o}$, H$_{o}$ = 0.45 T, k$>$1), due to thermally assisted tunneling of magnetization [2]. Application of a transverse field increases the tunnel splitting, which increases the magnetic relaxation and allows us to explore the deflagration for the first time at small longitudinal fields (k=0 and 1). Using time resolved measurements of local magnetization by an array of micron sized Hall sensors at temperature of 350 mK, we present the measurements on both spontaneously ignited and triggered deflagration for a large transverse field ($>$ 3 T) allowing us to explore directly the effect of a significant tunneling splitting on both the ignition and the velocity of the front. [1] Y. Suzuki, et. al PRL 95, 147201 (2005) \newline [2] A. Hernandez-Minguez, et. al, PRL 95, 217205 (2005)   

Demagnetizing effect in local magnetic measurements

It is well-known that magnetic measurements need to be corrected for the presence of demagnetizing fields that depend on both $\chi$ and the sample shape. Calculated demagnetization factors are generally available in tabular form for standard shapes, such as ellipsoids, spheres, and parallelopipeds, thereby providing corrections for measurements of the magnetization of the entire sample. However, appropriate corrections are not available for measurements obtained by local probes, such as micron-size Hall sensors. In this talk we present calculations of the local demagnetizing field profile and show how these results can be applied to interpret local magnetization measurements in Mn$_{12}$-ac.   

Simulation of AC Susceptibility and Electronic Structure of Mn-containing Molecular Magnets

The family of Mn$_{12}$-Acetate molecular magnets has been recently enhanced by the synthesis of new members with S = 20/2 and S = 19/2 ground states. These closely related systems display similar but not identical AC susceptibility paterns which we have modelled in terms of their real ($\chi^{\prime}$) and imaginary ($\chi^{\prime \prime}$) components. The fits of AC data, as a function of frequency, show subtle differences between the parameters that control the spin dynamics in the S = 20/2 and S = 19/2 systems. To further understand the dynamic parameters we have performed electronic structure calculations based on spin density functional theory (SDFT) on both systems. Results from SDFT calculations, which describe the ground state and magnetic structures, have been correlated to the AC data to gain insight about the subtle differences in their magnetization dynamics.   

Exact and quasi exact numerical methods for giant magnetic molecules

The determination of the energy spectra of large magnetic molecules is a demanding numerical problem. In this contribution we demonstrate that theory has advanced very much in recent years. We first show that it is possible to diagonalize the Heisenberg Hamiltonian by employing the spin-rotational symmetry SU(2) in combination with arbitrary point-group symmetries [1]. This goes far beyond earlier approaches and enables us to evaluate thermodynamic observables such as the magnetization and spectroscopic data for molecules as large as the famous ferric wheel Fe$_{10}$ with a Hilbert space dimension of more than 60 Millions. Then we explain how the finite-temperature Lanczos method can be applied to magnetic molecules in order to determine thermodynamic functions for Hilbert spaces as large as up to 1 Billion [2]. The new method enables us to discuss the magnetic properties of the highly frustrated Keplerate molecule \{$\textrm{W}_{72}\textrm{V}_{30}$\} which behaves like a finite size Kagome lattice antiferromagnet. \\[4pt] [1] R. Schnalle and J. Schnack, Int. Rev. Phys. Chem. 29 (2010) 403; R. Schnalle, J. Schnack, Phys. Rev. B 79 (2009) 104419. \\[0pt] [2] J. Schnack, O. Wendland, Eur. Phys. J. B 78 (2010) 535-541.   

Low temperature magnetic properties of antiferromagnetic rings V6 and V7 studied by NMR

The recent progress in synthesizing odd-member antiferromagnetic (AF) ring molecules gives us the opportunity to investigate spin frustration effects on magnetic properties in systems with small number of magnetic ions. Na$_{7}$[(VO)$_{7}$Na$_{7}$(H$_{2}$O)$_{7}(\beta $-CD)$_{14})_{2}$]59D$_{2}$O (in short, V7) is known to be one of the odd-member AF rings, in which seven V$^{4+}$ (S=1/2) ions make an almost coplanar ring shape. Magnetic susceptibility measured at T=1.8 -300K follows a Curie-Weiss law with a Weiss temperature of -0.5K. This indicates an AF interaction between V$^{4+}$ spins is of order of 0.5K. In order to investigate ground state magnetic properties of the spin frustrated V7 ring, we have carried out proton NMR measurements at low temperatures down to 0.05K using a dilution refrigerator. We also carried out proton NMR in another non-frustrated ring system (V6) which is comprised by six V$^{4+}$ ions for a comparison. NMR spectrum line width in V7 increases with decreasing temperature down to 0.05K. On the other hand, for V6, line width shows a peak around 0.2K and decreases below the temperature. These results clearly indicate that these systems have a different magnetic ground state and the ground state of V6 is a spin singlet state but V7 has a magnetic ground state.   

Session P14: Focus Session: Spins in Semiconductors - Quantum Dots and Qubits

Magnetic Polarons and Bipolarons in Quantum Dots

Magnetically doped (typically by Mn) semiconductor quantum dots (QDs) allow a control of magnetic ordering in ways not available in the bulk. For example, onset of magnetism can be realized by adding a single carrier or changing symmetry of the quantum confinement, even at a fixed carrier number [1]. Recent experiments revisit the concept of magnetic polaron [2], formed when a single carrier added to a QD aligns the Mn spins through exchange interaction. The experiments [3,4] show that the induced magnetization persists at relatively high temperatures. First, we discuss a QD system, in which the experimental magnetic polaron energy, in addition to its relatively high value, shows a surprisingly weak temperature dependence [4]. We explain this effect by magnetic anisotropy of the QD. Next, we turn to the case where a magnetic QD contains two carriers. We find theoretically that Mn spins align, forming a magnetic 'bipolaron', even when the ground state has zero carrier-spin [5]. The corresponding state breaks spatial symmetry, unlike in the case of a single magnetic polaron. We propose experimental tests of our prediction. We also explore the stability of the broken-symmetry state with zero net magnetization versus other patterns of magnetization [6]. Finally, we show some interesting consequences of diffusive coupling of a magnetic QD to a reservoir of carriers [7]. This work was done in collaboration with P. Stano, J. Pientka, A. Petukhov, and I. Zutic.\\[4pt] [1] R. M. Abolfath, et al. Phys. Rev. Lett. 101, 207202 (2008).\\[0pt] [2] Maksimov, A. et al. Phys. Rev. B 62, R7767 (2000), J. Seufert et al., Phys. Rev. Lett. 88, 027402 (2002).\\[0pt] [3] R. Beaulac et al., Science 325, 973 (2009).\\[0pt] [4] I. R. Sellers, et al., Phys. Rev. B 82, 195320 (2010).\\[0pt] [5] R. Oszwaldowski, et al., Phys. Rev. Lett. 106, 177201 (2011)\\[0pt] [6] P. Stano, R. Oszwaldowski, A. G. Petukhov, and I. Zutic, preprint.\\[0pt] [7] J. M. Pientka, R. Oszwaldowski, A. G. Petukhov, J. E. Han and I. Zutic, Reentrant Magnetic Polaron Formation in Quantum Dots, preprint.   

Reentrant Magnetic Polaron Formation in Quantum Dots

Recently, there have been several theoretical studies that show multiple ways of manipulating magnetic ordering in Quantum Dots (QD) [1,2]. Experiments [3,4] display the formation of Magnetic Polarons in both colloidal and self-assembled QDs. We focus on a type-II QD band profile, where the electrons reside in the barrier, while the holes are localized in the QD interior, which contains the magnetic impurities. In our model, photo-excitation of carriers induces a quasi equilibrium. We consider various QD states to describe the carrier-mediated magnetic ordering in QDs. Allowing for different QD states changes the magnetic properties due to different carrier spin density [5], which affects the alignment of the magnetic impurities. [1] R. M. Abolfath, A. G. Petukhov, and I. Zutic, Phys. Rev. Lett. 101, 207202 (2008); [2] I. Zutic and A. G. Petukhov, Nature Mater.4, 623 (2009). [3] R. Beaulac et al., Science 325, 973 (2009). [4] I. R. Sellers, R. Oszwaldowski, et al., Phys. Rev. B 82, 195320 (2010). [5] J. M. Pientka, R. Oszwaldowski, A. G. Petukhov, J. E. Han, and I. Zutic, Reentrant Magnetic Polaron Formation in Quantum Dots, preprint.   

Magnetic anisotropies of quantum dots

Magnetic anisotropies in quantum dots (QDs) doped by magnetic ions are discussed in terms of two frameworks: anisotropic g-factors and magnetocrystalline anisotropy energy [1]. Two examples, related to zinc-blende $p$-doped materials, are given of how these frameworks are utilized: four-level Hamiltonian of a flat QD and a cuboid infinite-well QD containing a single hole. The latter model, despite being an idealization of a real QD, displays a rich phenomenology of anisotropies. We quantify the anisotropy constants for ZnSe and CdTe QDs, confirming that the Ising-like effective Hamiltonians apply to magnetic QDs [2]. Compared to bulk systems, confinement tuning offers a new way to control easy axes in magnetic QDs. [1] K. Vyborny et al., preprint (2011). [2] C. Le Gall et al., Phys. Rev. Lett. 107, 057401 (2011).   

The Interplay of Electronic Properties and Magnetic Anisotropy in Quantum Dots

Tunability of magnetic anisotropy (MA) in nanostructures is a fascinating topic, for both fundamental understanding of nanomagnetism and possible spintronic applications. While there have been preceding efforts to systematically study the MA in bulk [1], we still lack a fundamental understanding of that in magnetic quantum dots (QDs). We first explore electronic properties of nonmagnetic QDs that can be significantly altered from the bulk-state depending upon the material and geometry. Focusing on II-VI materials forming both cubic and non-cubic QDs, we confirm qualitatively different energy spectra between different materials [2]. These findings can guide the control of MA in magnetic QDs. Supported by DOE-BES, NSF-DMR, AFOSR-DCT, U.S. ONR, and NSF-ECCS. \\[4pt] [1] X. Liu, Y. Sasaki and J. K. Furdyna, Phys. Rev. B 67, 205204 (2004). \\[0pt] [2] K. V\'yborn\'y, J.E. Han, R. Oszwadowski, I. \v{Z}uti\'c, and A. G. Petukhov, preprint (2011).   

Theory of collective quantum effects of the nuclear spin bath in a Ge/Si core/shell nanowire quantum dot

We study a quantum-dot spin-valve setup with a uniform hyperfine coupling of the electron spin to the nuclear bath. We propose Ge/Si core/shell nanowire quantum dots as a promising platform in which, through engineering of the nuclear spin positions and of the radial and longitudinal electron confinement, a nearly uniform hyperfine interaction can be realized. The dynamics of this coupled system are exactly soluble in terms of collective nuclear states with fixed total angular momentum. We theoretically show that the quantum-mechanical properties of such collective states of the nuclear spins can be probed through electron transport in this spin-valve setup. The associated transport current shows an enhancement due to coupling to collective modes in the nuclear-spin system directly analogous to the problem of superradiance in quantum optics. This effect is robust to dephasing of the nuclear spins and would provide a demonstration of large-scale collective quantum effects in a nuclear-spin system.   

Magnetotransport in Ge/Si Quantum Dot Molecules Fabricated by Directed Self-Assembly

The electronic states of strained self-assembled Ge quantum dots embedded in silicon provide an attractive system for controlling electron spin interactions via direct exchange\footnote{C. E. Pryor, M. E. Flatte, and J. Levy, Applied Physics Letter \textbf{95}, 232103 (2009)} . Directed self-assembly of sub-10 nm Ge islands are fabricated to produce laterally coupled quantum dot molecules with geometrically-defined spin exchange couplings. Ge islands are coupled to the Si capping layer, and geometries can be defined that are suitable for either vertical or lateral transport. We describe low-temperature vertical magneto-transport measurements on individual and small arrays of Ge islands grown on SOI substrate. Characteristic features in the magnetotransport are observed that correspond to specific geometrical arrangements of the quantum dots.   

Singlet-triplet splitting in double dots due to spin orbit and hyperfine interactions

We analyze the low-energy spectrum of a detuned double quantum dot in the presence of magnetic fields, spin orbit interaction, and nuclear spins, and focus on the regime of spin blockade. Starting from a realistic model for two interacting electrons in a double dot, we derive perturbatively an effective two-level Hamiltonian in the vicinity of an avoided crossing between singlet and triplet levels, which are coupled by the spin-orbit and hyperfine interactions. We evaluate the level splitting at the anticrossing in various parameter regimes, and show that it depends on two controllable parameters: the angle between the external magnetic field and the internal spin orbit field, and on the detuning, as well as on the difference between nuclear fields in the two dots. We identify a parameter regime where spin orbit and hyperfine terms can become of equal strength and propose a protocol for tuning their relative sizes.   

Single-shot correlations and two-qubit gate of solid-state spins

Recent advances in nanotechnology and quantum engineering have made it possible to probe single spins in the solid-state. Here we report independent single-shot read-out of two electron spins in a double quantum dot. The read-out method is all-electrical, cross-talk between the two measurements is negligible, and read-out fidelities are $\sim$ 86\% on average. This allows us to directly probe the anti-correlations between two spins prepared in a singlet state and to demonstrate the operation of the two-qubit exchange gate on a complete set of basis states. Ongoing work focuses on integration with single-spin rotations, scaling and extending coherence times.\\[4pt] K.C. Nowack, M. Shafiei, M. Laforest, G.E.D.K. Prawiroatmodjo, L.R. Schreiber, C. Reichl, W. Wegscheider and L. M. K. Vandersypen, Single-shot correlations and two-qubit gate of solid-state spins, Science 330, 1269 (2011)   

Spin-orbit effects on the full dynamics of double quantum dot qubit states

We study spin-obit interaction (SOI) and relaxation effects on the measurement of the extended singlet state return probability $P(S)$ in a double quantum dot (DQD) system with two electrons, in the presence of hyperfine interaction (HFI) and weak external magnetic fields. Using appropriate pulse cycles to change the detuning between the two quantum dots, we describe the full dynamical behavior of the system taking into account the complete set of states. We find that the mixing of the $m_s =1\;(T_+ )$ triplet with the (0,2) local singlet, induced by SOI via non-spin-conserving tunneling transitions, has an important effect on the measurement of $P(S)$, and a clear experimental signature. The numerical results are also analyzed in terms of a Feshbach projection to the effective low-energy dynamics, which explain the role of SOI on the relaxation and overall dynamics relevant in experiments. We also explore the case of the Landau-Zener-St\"{u}ckelberg interferometry realized via voltage sweeps through the $S-T_+ $anticrossing generated by HFI in the DQD energy spectrum [1]. We focus on studying the effects of SOI and relaxation on the interferometric properties of the system in this regime. [1] J.R. Petta, H. Lu and A.C. Gossard, Science 327, 669 (2010).   

Spin blockade in a triple silicon quantum dot in CMOS technology

We study the spin blockade (SB) phenomenon by quantum transport in a triple quantum dot made of two single electron transistors (SET) on a CMOS platform separated by an implanted multiple donor quantum dot [1]. Spin blockade condition [2] has been used in the past to realize single spin localization and manipulation in GaAs quantum dots [3]. Here, we reproduce the same physics in a CMOS preindustrial silicon quantum device. Single electron quantum dots are connected via an implanted quantum dot and exhibit SB in one current direction. We break the spin blockade by applying a magnetic field of few tesla. Our experimental results are explained by a theoretical microscopic scheme supported by simulations in which only some of the possible processes through the triple quantum dot are spin blocked, according to the asymmetry of the coupling capacitances with the control gates and the central dot. Depending on the spin state, the SB may be both lifted and induced. Spin control in CMOS quantum dots is a necessary condition to realize large fabrication of spin qubits in some solid state silicon quantum device architectures.\\[0pt] [1] Pierre et al., Appl. Phys. Lett., 95, 24, 242107 (2009); [2] Liu et al., Phys. Rev. B 77, 073310 (2008); [3] Koppens et al., Nature 442, 766-771 (2006)   

Spin-orbit effects in triple dot quantum shuttles

Within the framework of a fully quantum mechanical approach we use a generalized density matrix formalism to study the spin-orbit coupling effects in a triple dot quantum shuttle. An interesting feature of this type of nanoelectromechanical systems is that the interplay between the electronic, spin, and mechanical degrees of freedom give rise to novel transport phenomena that has attracted a great deal of interest in both the applied and basic research. In this work, the effect of spin-orbit coupling is incorporated into the system by introducing non spin-conserving tunneling elements between the quantum dots. We explore the features of spin-polarized current by changing the Zeeman-split levels of the dots, and the frequency of the oscillating central dot. We show that the spin-orbit effect manifests itself as sidebands in the spin-polarized current, and that the tunneling channels can be controlled by adequately tuning the relative energies of the Zeeman-split levels, and by manipulating the current contribution from the vibrational modes.   

Session P15: Focus Session: Spins in Metals - Spin Current Generation and Spin Motive Force

Slonczewski-like torque due to Rashba spin-orbit coupling in thin magnetic layers

Recent experiments report that large Rashba spin-orbit coupling (RSOC) can exist for ferromagnetic nanostructure with structural asymmetry. Previous theories of spin-transfer torque (STT) induced by RSOC addressed this problem by introducing an effective field due to RSOC. However, the Rashba field is not sufficient, and another STT perpendicular to the Rashba STT is required to explain experimental results. In this work, we propose the mechanism of such STT based on nonadiabaticity and examine the effect of the torque. By studying the domain wall (DW) motion, we demonstrate that the DW motion is significantly affected by our result. We show that the effect of the RSOC-induced torque on the DW velocity can be so large that the DW can move along the current direction without assuming negative nonadiabaticity or spin polarization. Our result is discussed in comparison with experimental results.   

Spin-orbit coupling of Pt studied by circular dichroism in soft x-ray ARPES

Pt has a large spin-orbit coupling (SOC) and is reported to exhibit the largest spin Hall conductivity among all materials studied to date [1,2]. To establish the role of SOC in the electronic structure, we investigate the bulk electronic structure of Pt(111) using circular dichroism (spin-orbit dichroism) in soft x-ray (SX)-ARPES. We have measured band dispersions along $\Gamma$-K-X, L-W and $\Gamma$-L and the complete set of Fermi surfaces of Pt. Calculated band dispersions including SOC gives a very good match with the experimental results [2,3], thus demonstrating the role of SOC. Our results also show a $k$-dependent suppression of spin-orbit dichroism, implying a $k$-dependent quenching of the spin polarization [3]. [1] T. Kimura {\it et al.}, Phys. Rev. Lett. {\bf 98}, 156601 (2007). [2] G. Y. Guo {\it et al.}, Phys. Rev. Lett. {\bf 100}, 096401 (2008). [3] M. Gradhand {\it et al.}, Phys. Rev. B {\bf 80}, 224413 (2009).   

Spin pumping with coherent elastic waves

The generation and detection of pure spin currents is an important topic for spintronic applications. Spin currents may be generated, e.g., via spin pumping. In this approach, a precessing magnetization relaxes via the emission of a spin current. Conventionally, electromagnetic waves, i.e. microwave photons, are used to drive the magnetization precession. We here show that a spin current can also be pumped by means of an acoustic wave, i.e. microwave phonons. In the experiments, coherent surface acoustic wave (SAW) phonons with a frequency of 1.55 GHz traverse a ferromagnetic thin film/normal metal (Co/Pt) bilayer. The SAW phonons drive the resonant magnetization precession via magnetoelastic coupling [1]. We use the inverse spin Hall voltage in the Pt film as a measure for the generated spin current and record its evolution as a function of time and external magnetic field magnitude and orientation. Our experiments show that a spin current is generated in the exclusive presence of a resonant elastic excitation. This establishes acoustic spin pumping as a resonant analogue to the spin Seebeck effect and opens intriguing perspectives for applications in, e.g., micromechanical resonators. \\[4pt] [1] M. Weiler \textit{et al.}, Phys. Rev. Lett. \textbf{106}, 117601 (2011)   

Giant spin accumulation and long-distance spin precession in metallic lateral spin valves

The non-local spin injection technique in lateral spin valves (LSVs) has provided not only scientific interests to study spin transport and relaxation in a nanowire but also potential spintronic device applications. The non-local method involves no charge-current flow but spin accumulation in the nonmagnetic wire. In order to increase the output signal, related to the spin accumulation splitting for electrochemical potential, efficient spin injection into nonmagnet from ferromagnet as well as high applied current are indispensable. The spin resistance mismatch between the ferromagnet and the nonmagnet needs to be overcome by using high interface resistance such as tunnel barrier for the efficient spin injection, while a low junction resistance is preferred for applying high current. In this talk, I will discuss a guideline to design metallic LSVs with high output signal. The interface resistance of around 0.1 $\Omega$$\mu$m2, several orders of magnitude smaller than that of a typical tunnel junction, could effectively overcome the spin resistance mismatch in the metallic system [1] and leads to the giant spin accumulation signal over 200 $\mu$V in LSVs with NiFe/MgO/Ag junctions [2,3]. The Hanle effect measurements demonstrate a long-distance collective 2$\pi$ spin precession along the 10 $\mu$m long Ag wire. This result will accelerate the development of novel spintronic devices utilizing the pure spin current and the spin accumulation.\\[4pt] [1] Y. Fukuma, L. Wang, H. Idzuchi and Y. Otani, Appl. Phys. Lett. 97, 012507(2010).\par [2] Y. Fukuma, L. Wang, H. Idzuchi, S. Takahashi, S. Maekawa and Y. Otani, Nature Mater. 10, 527(2011).\par [3] L. Wang, Y. Fukuma, H. Idzuchi, G. Yu, Y. Jiang and Y. Otani, Appl. Phys. Exp. 4, 093004(2011).   

Temperature and temporal evolution of nonlocal spin signals

An unusual non-monotonic temperature dependence of spin signals was previously observed for nanoscale metallic nonlocal spin valves (NLSV): The spin signal increases as the temperature decreases from room temperature, reaches a maximum value around 50 K, and then decreases as the temperature approaches 4 K. This has been interpreted as due to a high rate of surface spin-flip scattering in the nonmagnetic channel, but the origins of the high surface spin-flip rate are yet to be understood. In this work, we show that for an as fabricated Py-Cu NLSV device this temperature dependence is clearly observed. The device was then stored in the ambient environment for a period of 5 months. Afterwards, an increase of the spin signals was found, and more interestingly the temperature dependence became monotonic. From room temperature to 50 K the spin signal increases, but from 50 K to 4 K the spin signal levels off instead of decreasing further. We conclude that the surface spin-flip scattering originates from the magnetic impurities embedded in the Cu channel near the side surfaces. The impurities are introduced into the Cu during the fabrication procedure. Upon oxidizing the Cu in the ambient environment, the surface impurities are buried in copper oxide and become less accessible to the conduction electrons. Therefore the surface spin-flip rate is reduced over time, resulting in a larger spin signal and monotonic temperature dependence.   

Continuous dc Spinmotive Force in a Patterned Ferromagnetic Film

Recently, a motive force of spin origin, i.e., a ``spinmotive force", has been theoretically predicted[1] and experimentally observed[2,3]. A spinmotive force reflects the spin of electrons in an essential manner and is a new concept relevant to electronic devices. However, problems remain, one of which is the continuous generation of such a spinmotive force. In this study, we present experiment and theory that demonstrate the continuous generation of a dc spinmotive force by exciting a ferromagnetic resonance of a single comb-shaped ferromagnetic thin film[4]. Experimental results are well reproduced by theoretical calculations, offering a quantitative and microscopic understanding of this spinmotive force. \newline [1] S. Barnes and S. Maekawa, PRL 98, 246601 (2007). [2] S. A. Yang et al. PRL 102, 067201 (2009). [3] P. N. Hai et al. Nature 458, 489 (2009). [4] Y. Yamane, K. Sasage et al., to appear in PRL.   

Theory of anomalous Hall and spin Hall effects in ferromagnetic metals

We give a unified theory of the anomalous Hall effect (AHE) and the spin Hall effect (SHE) in ferromagnetic metals (FM). We do this by extending Kondo's theory of the AHE in FM and including the short range spin-spin correlations. We find a novel relation between the SHE and the second order nonlinear spin fluctuation in FM near Curie temperature Tc, which has been hidden for about 50 years, since Kondo gave a relation between the AHE and the first order nonlinear spin fluctuation in pure FM near Tc in 1962. Our results show an essential difference between the AHE and SHE in terms of the symmetry. Our theory can be applied to the recent SHE experiment in FM near Tc by Y. Otani et al.   

Spin motive force in a Rashba system

Spin motive force (SMF) is the nonconservative spin force induced by magnetization dynamics. In addition to spin-transfer torque (STT), SMF is considered as one of the representative interactions between magnetization and conduction electrons. It has been reported that large Rashba-type spin-orbit coupling (RSOC) can show up in magnetic nanostructures containing thin ferromagnetic layers with structural asymmetry. In this work, we investigate the effect of RSOC on SMF. The additional SMF induced by RSOC is proportional only to time variation of magnetization while the conventional SMF is proportional to both time and space variation of magnetization. This result indicates that SMF can be induced from even homogeneous magnetic structure. The resulting RSOC SMF has so large magnitude that the feedback effect on the magnetization dynamics can be significant. Combining RSOC SMF and the previously known Rashba STT effect, we derived that Gilbert damping is one or two orders of magnitude enhanced by RSOC. Lastly, we show that the symmetry breaking nature of RSOC and the induced SMF greatly help the electrical distinction of magnetization structure. It is expected that RSOC will play a key role in extending applicable area of SMF.   

Light-induced pure spin current

We propose theoretically that a pure spin current without an accompanying charge current can be generated by light in magnetic tunneling junctions. The principle is based on a photovoltaic effect combined with the spin selectivity of the magentic electrodes of the junction. We demostrate this effect in graphene nanostructures by atomic first principles calculation. The results show that appreciable pure spin-currents and open circuit spin bias are generated in pure graphene nanostructures, and it can reach significant values if half metal with high spin polarization is used as the electrodes.   

Perpendicular switching of a single ferromagnetic layer induced by in-plane current injection

We demonstrate switching of a ferromagnetic Pt/Co/AlOx layer with perpendicular anisotropy through lateral current-injection. Magnetization reversal occurs as an unpolarized electric current is injected parallel to an in-plane magnetic field of moderate magnitude. The switching direction depends on the sign of the current with respect to that of the in-plane field. The critical switching current scales with the lateral dimensions of the layer and duration of the current pulse. Our measurements also indicate that the switching efficiency increases with the magnetic anisotropy of the Co layer and oxidation of the top Al layer. The symmetry of this effect corresponds to an in-plane torque perpendicular to the current. We will discuss possible contributions to this torque, including Rashba-induced spin accumulation and the spin Hall effect [1]. \\[0pt] [1] I. M. Miron, K. Garello, G. Gaudin, P.-J. Zermatten, M. V. Costache, S. Auffret, S. Bandera, B. Rodmacq, A. Schuhl, and P. Gambardella, Nature 476, 189 (2011).   

Current induced magnetization dynamics in spin-orbit coupled thin film ferromagnets

We consider the effect of an in-plane current on the magnetization dynamics of a two-dimensional spin-orbit coupled nanoscale itinerant ferromagnet. By solving the appropriate kinetic equation for an itinerant electron ferromagnet, we show that Rashba spin-orbit interaction provides transport currents with a switching action, as observed in a recent experiment (I. M. Miron \textit{et al}., Nature 476, 189 (2011)). The dependence of the effective switching field on the magnitude and direction of an external magnetic field in our theory agrees well with experiment. We comment on the possibility of finding materials in which this spin-orbit switching effect can be achieved at moderate current densities   

Spin-flipping at Sputtered Co/Ag Interfaces

We measured at 4.2K the Current-Perpendicular-to-Plane Magnetoresistances (CPP-MRs) of sputtered ferromagnetically coupled [Co(3 nm)/Ag(1.8 or 2.0 nm)]$_{n}$Co(3 nm) multilayers with $n$ = 0 to 8 that were imbedded in the middle of symmetric, Py-based, double exchange-biased spin-valves. The measurements yielded the parameter for spin-flipping at the Co/Ag interface, $\delta _{Co/Ag}=\mathop {0.30}\nolimits_{-0.1}^{+0.05} $. $\delta $ is related to the probability $P$ of spin-flipping at the Co/Ag interface by $P$ = [(1-exp(-$\delta )$]. Despite the expected larger lattice-mismatch-induced disorder at the Co/Ag interface, this value is similar to that reported earlier for the Co/Cu interface [1], suggesting that such disorder does not play a major role in $\delta _{F/N}$ at ferromagnetic/non-magnetic (F/N) interfaces. \\[4pt] [1] B. Dassonneville et al., Appl. Phys. Lett. 96, 022509 (2010).   

Large non-inverted and inverted spin signals in break-junction-based nonlocal spin valves

A nonlocal spin valve (NLSV) is a lateral structure with a ferromagnetic (F) spin injector, an F spin detector and a nonmagnetic (N) channel. A pure spin current can be generated in the N channel by electrical injection through the injector, and can be detected as a spin signal between the spin detector and the N channel. For a typical metallic NLSV, one expects a regular spin signal meaning that the nonlocal resistance is high for the parallel (P) states of the injector and the detector and low for the antiparallel (AP) states. We investigate spin signals of NLSV's with a break junction formed between the detector and the copper N channel by electrostatic discharge. We observed both non-inverted and inverted spin signals with large magnitudes. An inverted spin signal has low nonlocal resistance values for the P states and high values for the AP states. The magnitude is up to 90 milliohms at 4 K and up to 30 milliohms at 300 K, larger than the typical metallic NLSV with similar dimensions. The large magnitude can be explained by the spin-charge coupling across the highly resistive break-junction. The signs of the spin signals (non-inverted vs. inverted) are determined by the local spin-dependent density of states near the break-junction.   

Session P16: Heavy Fermions- Mostly URu2Si2

Fano resonance and hybridization gap in the Kondo lattice URu$_{2}$Si$_{2}^{\ast }$

The nature of the `hidden' order transition in URu$_{2}$Si$_{2}$ remains puzzling despite intensive research over the past two and half decades. A key question under debate is whether a hybridization gap between the renormalized bands can be identified as the long-sought hidden order parameter. We report on the measurement of a hybridization gap in URu$_{2}$Si$_{2}$ employing a spectroscopic technique based on quasiparticle scattering across a ballistic metallic junction [1]. The differential conductance data exhibit an asymmetric double-peak structure, a signature for a Fano resonance in a Kondo lattice [2]. The extracted hybridization gap opens well above the hidden order transition temperature, indicating that it is not the order parameter for the hidden order phase. Our results place constraints on the origin of the hidden order transition in URu$_{2}$Si$_{2}$.\\[4pt] [1] W. K. Park \textit{et al}., arXiv:1110.5541.\\[0pt] [2] M. Maltseva, M. Dzero, P. Coleman, PRL 103, 206402 (2009).   

Tuning soft point-contact spectroscopy of URu$_{2}$Si$_{2}$ from hidden order to antiferromagnetic state through pressure

We have extended the soft point-contact spectroscopy technique under nearly hydrostatic pressure to make charge-spectroscopy measurements of URu$_{2}$Si$_{2}$ in both hidden order (HO) and large-moment antiferromagnetic (LMAF) states. In the HO state at ambient pressure, the spectroscopy shows two asymmetric peaks around the Fermi energy that emerge below the hidden order temperature T$_{HO}\sim $ 17.5 K. In the LMAF state at higher pressures, the spectra are remarkably similar to those in the HO state, indicating a similar Fermi surface gapping in the HO and LMAF states. The energy scale of this gap is, within experimental uncertainty, consistent with that of the incommensurate spin resonance at Q$_{1}$= (1$\pm $0.4, 0, 0), which also is present in both HO and LMAF states. Our results provide a new clue to unraveling the puzzling HO state.   

Temperature-dependent phonon dispersions in URu$_{2}$Si$_{2}$

The acoustic and low-energy optical phonon modes of a single crystal of URu$_{2}$Si$_{2}$ were studied via inelastic neutron scattering. The temperature dependence of the phonon dispersions will be compared with results of prior studies of phonons in this material. Our measurements were also sensitive to the temperature evolution of magnetic excitations in the hidden order phase. We will reflect on implications for the nature of the hidden order parameter.   

Hidden Order Transition in URu$_2$Si$_2$: Evidence for the Emergence of a Coherent Anderson Lattice from Scanning Tunneling Spectroscopy

The heavy-fermion compound URu$_2$Si$_2$ exhibits an onset of Kondo screening around T $\approx$ 55K and undergoes a second order phase transition at T$_0$ = 17.5K into a state with a still unknown hidden order parameter. Recent scanning tunneling spectroscopy experiments have provided insight into the temperature evolution of the electronic structure. Above the hidden order transition, the differential conductance, dI/dV, exhibits a characteristic Fano lineshape. In contrast, below T$_0$, a soft gap opens up in dI/dV and a quasi-particle interference (QPI) analysis reveals a band structure similar to that expected in a screened Kondo lattice. We demonstrate that the experimental dI/dV and QPI results below T$_0$ are consistent with the formation of a coherent Anderson lattice (CAL). In particular, dI/dV exhibits characteristic signatures of the Anderson lattice band structure, such as an asymmetric gap and a peak inside the gap which arises from the van Hove singularity of the heavy f-electron band. We identify several branches of the QPI pattern arising from intra- and interband scattering. Finally, the temperature evolution of dI/dV suggests that the formation of the CAL below the HOT is primarily driven by a strong increase of the lifetime of the heavy quasi-particles.   

Nuclear Magnetic Resonance Study of the Paramagnetic State of URu2Si2

URu2Si2 is a heavy fermion system that has challenged researchers for many years due to its transition into a hidden order (HO) state at 17.5K. We present new nuclear magnetic resonance (NMR) data in the paramagnetic phase near the HO phase transition. An analysis of the spin-lattice relaxation rate indicates a suppression of the spin fluctuations above the HO phase transition extending up to approximately 30 K. We analyze this data in the context of several different models for the spin lattice relaxation.   

Evidence of New Features in the c-axis Optical Conductivity of URu$_2$S$i_2$

The hidden order state of URu$_2$S$i_2$ remains mysterious despite many years of investigation. High quality, low noise optical data on both cleaved and cut-and-polished faces with in-plane c-axis of the tetragonal structure offer new insight into the electronic behavior at the ordering temperature. As the gap opens in the density of states, a new mode appears in the gap region that is visible in the conductivity only when reflectance is measured with light polarized along the crystal c-direction. A marked anisotropy in the gap energy seen in the optical conductivity between a-axis and c-axis provides further insight into the structure and magnitude of the energy gap and the behavior of the electrons near the greatly-reduced Fermi surface.   

Linear and Nonlinear Susceptibility Measurements in URu2Si2 and UPt3

We will discuss both DC and AC susceptibility measurements in single crystals of URu2Si2 and UPt3. In URu2Si2 we detect a ferromagnetic signature separated only by $\sim $ 1 K from the well established antiferromagnetic signature due to the hidden order at 17.5 K. This ferromagnetic signature appears to be well pronounced only in those samples where a strong ferromagnetic anomaly (also observed by others previously) appears at 35 K. This new ferromagnetic signature is further apparent in AC measurements, with its fingerprint appearing in a pronounced manner in first order, third order and fifth order susceptibility measurements. In contrast at the hidden order transition signatures are seen only in the first order and the fifth order susceptibility with no apparent change in the third order susceptibility. In UPt3, the DC third order susceptibility measurements reveal a broad peak at $\sim $10 K which is at half the temperature where a peak in the linear susceptibility is observed. This proportionality appears thus far to be universal across the f-electron based strongly correlated metals .   

Small Angle Neutron Scattering Studies of the Vortex Lattice of UPt$_3$

Although a paradigm for unconventional superconductivity, the true nature of the superconducting order parameter in UPt$_3$ is still an open question. We present results from small angle neutron scattering (SANS) studies on the vortex lattice (VL) of UPt$_3$, for fields both perpendicular and parallel to the crystal \textbf{c}-axis, that have implications for the superconducting order parameter. For perpendicular fields, an unconventional temperature dependence of the VL form factor -- and thus penetration depth -- is seen, with different dependences in the A and B-phases. This bulk measurement of the penetration depth indicates the presence of nodes in the superconducting gap. For parallel fields, we report the first measurement of a rocking curve from the VL. This is an encouraging result for the viability of using SANS to detect signatures of time reversal symmetry breaking, as the \textbf{c}-axis is taken as the chiral axis in these order parameter theories.   

Hidden order and unconventional superconductivity in URu$_2$Si$_2$

The nature of the so-called hidden order in URu$_2$Si$_2$ and the subsequent superconducting phase have remained a puzzle for over two decades. Motivated by evidence for rotational symmetry breaking seen in recent magnetic torque measurements [Okazaki et al. Science {\bf 331}, 439 (2011)], we derive a simple tight-binding model consistent with experimental Fermi surface probes and ab-initio calculations. From this model we use mean-field theory to examine the variety of hidden orders allowed by existing experimental results, including the torque measurements. We then construct a phase diagram in temperature and pressure and discuss relevant experimental consequences.   

Hastatic Order in $URu_2Si_2$

The hidden order that develops below 17.5K in $URu_2Si_2$ has eluded identification for twenty-five years. Here we show that the recent observation of Ising quasiparticles in $URu_2Si_2$ suggests a novel ``hastatic order'' {(Latin:\sl spear)},with a two-component order parameter describing hybridization between electrons and the Ising $5f^{2}$ states of the uranium atoms. Hastatic order breaks time-reversal symmetry by mixing states of different Kramers parity; this accounts for the magnetic anomalies observed in torque magnetometry and the pseudo-Goldstone mode observed in neutron scattering. Hastatic order is predicted to induce a basal-plane magnetic moment of order $0.01\mu_{B}$, a gap to longitudinal spin fluctuations that vanishes continuously at the first-order antiferromagnetic transition and a narrow resonant nematic feature in the scanning tunneling spectra.   

Staggered spin-orbit coupling induced hidden order state in heavy-fermion metal URu2Si2

The order parameter responsible for a second-order phase transition in the heavy fermion metal URu2Si2 at T = 17.5 K has remained a long-standing mystery. Here we show via ab-initio calculations that an incommensurate Fermi surface ``nesting'' in the partially-filled f-states causes a staggered spin-orbit coupling in the hidden-order state. In this cause neither the spin (S) nor the orbital (L) alone causes ordered state, rather a modulated spin-momentum locked density wave propagates along the unidirectional nesting direction with a polarized total angular momentum $m_J = \pm$ 2, in excellent agreement with experiments. It breaks spontaneous rotational symmetry, but not the time-reversal symmetry and thus gives rise to the recently observed ``nematic order'' in this state. The hidden order state will be immune to any time-reversal invariant perturbation such as pressure, whereas magnetic field will destroy it. Remarkably, these are the hallmark properties of the hidden order state. We also compute the topological quantum number to show that the hidden-order gap opening can causes a trivial to non-trivial topological phase transition, and hence defines a novel ``topological quantum critical point.'' Work is supported by US DOE.   

Emergent Rank-5 ``Nematic'' Order in URu2Si2

In the strongly correlated $f$-electron systems, novel electronic states often appear due to the interplay between electron correlations and entangled spin and orbital degrees of freedom. A spectacular example is the so-called ``hidden-order'' phase in the heavy-electron metal URu$_{2}$Si$_{2}$. The phase transition is characterized by the large amount of entropy loss observed at $T_{HO}=17.5$K, however no evidence of magnetic/structural phase transition below $T_{HO}$ have been reported so far. Despite efforts over a quarter century, the order parameter has remained unidentified. We show here that the hidden order is a rank-5 multipole (dotriacontapole) state with $E^{-}$ symmetry, based on the first-principles theoretical approach. This novel electronic state provides natural explanations of the key features including anisotropic magnetic excitations, nearly degenerate antiferromagnetic state, and spontaneous fourfold symmetry breaking.   

Electronic Structure and Correlation Effects in PuCoIn$_5$ as Compared to PuCoGa$_5$

Since their discovery nearly a decade ago, plutonium-based superconductors have attracted considerable interest, which is now heightened by the latest discovery of superconductivity in PuCoIn$_5$. In the framework of density functional theory (DFT) within the generalized gradient approximation (GGA) together with dynamical mean-field theory (DMFT), we present a comparative study of the electronic structure of superconducting PuCoIn$_5$ with an expanded unit cell volume relative to its PuCoGa$_5$ cousin. Overall, a similar GGA-based electronic structure, including the density of states, energy dispersion, and Fermi surface topology, was found for both compounds. The GGA Pu 5$f$ band was narrower in PuCoIn$_5$ than in PuCoGa$_5$ due to the expanded lattice, resulting in an effective reduction of Kondo screening in the former system, as also shown by our DMFT calculations.   

Upper critical field of $p$-wave superconductors with orthorhombic symmetry

Recent experiments on exotic ferromagnetic superconducting materials such as UCoGe and topological superconductors such as Cu$_{x}$Bi$_{2}$Se$_{3}$, have spawned renewed interest in $p$-wave superconductivity. We present an extension of the Scharnberg-Klemm theory of $H_{c2}$ in $p$-wave superconductors to cases of partially broken symmetry in an orthorhombic crystal. Using a uniaxial anisotropic pairing interaction as is appropriate for the low-field regime of UCoGe, we have shown that a field induced crossover from one $p$-wave state to another can lead to kinks in $H_{c2}(T)$, which can mimic upward curvature in all three crystal axis directions. Reasonably good fits to the low-field UCoGe data are obtained. We have also investigated the angular dependence of the axial $p$-wave state, which might prove useful in identifying the $p$-wave state present in certain materials, and possibly suggest new experiments on well known $p$-wave superconductors.   

Time-resolved quasiparticle dynamics of the itinerant antiferromagnet UPtGa$_{5}$

Time-resolved photoinduced reflectivity is measured in the spin-density-wave phase of the itinerant antiferromagnet UPtGa$_{5}$. Two relaxation components were seen: (a) a slow component whose amplitude appears below $T_{N}$, and relaxation time $\tau_{slow}$ exhibits an upturn near $T_{N}$; (b) the fast component persists at all temperatures, with the relaxation time $\tau_{fast}$ also exhibiting an upturn near $T_{N}$. Comparing with pump-probe data on UNiGa$_{5}$, the differences are explained in the context of UPtGa$_{5}$ having A-type, (rather than G-type) antiferromagnetism, resulting in partial Fermi surface nesting, partial gapping and consequently finite density of states, at the Fermi surface.   

Session P17: Focus Session: Thermoelectrics - TE and Thermal Conductivity

Skutterudite Thermoelectric Generator for Electrical Power Generation from Automotive Waste Heat

Filled skutterudites are state-of-the- art thermoelectric (TE) materials for electrical power generation from waste heat. They have suitable intrinsic transport properties as measured by the thermoelectric figure of merit ZT = S$^{2}\sigma {\rm T}$/$\kappa $ (S = Seebeck coefficient, $\sigma $ = electrical conductivity, T = temperature, and $\kappa $ = thermal conductivity) and good mechanical strength for operation at vehicle exhaust gas temperatures of $>$550\r{ }C. We have demonstrated TE electrical power generation on a production test vehicle equipped with a fully functional prototype TE generator (TEG). It was assembled with TE modules fabricated from filled skutterudites synthesized at GM. Our results and analysis show that improvement in total power generated can be achieved by enhanced thermal and electrical interfaces and contacts. A substantial T decrease along the exhaust gas flow results in a large variation of voltage, current, and power output for each TE module depending on its position in the module array. Total TEG output power depends directly on the position-dependent T profile via the temperature dependence of both ZT and Carnot efficiency. Total TEG power output also depends on how the modules are connected in parallel or series combinations because mismatch in output voltage and/or internal resistance among the modules degrades the performance of the entire array. Uniform T profiles and consistent TE module internal resistances improve overall TEG performance.   

Theoretical Study of the Properties of the Type II Clathrates A$_{x}$Si$_{136}$ and A$_{x}$Ge$_{136}$ (A = Na, K)

Type II clathrate semiconductors have cage-like lattices where atoms are tetrahedrally coordinated and sp$^{3}$ bonded. An observed property of these materials is the variation of unit cell volume as different types of alkali metal atoms are placed in the clathrate cages. Experiments\footnote{M. Beekman, et al, \textit{Inorganic Chem} \underline {49}, 5338 (2010)} on Na$_{x}$Si$_{136}$ reveal that, starting with Si$_{136}$, as x increases (0 $<$ x $<$ 8), the cell volume contracts; where (8 $<$ x $<$ 24), the cell volume expands. This variation with x has been explained\footnote{Ibid.} as due to preferential incorporation of Na into the Si$_{28}$ cages for x $<$ 8, followed by incorporation into the Si$_{20}$ cages for 8 $<$ x (when all Si$_{28}$ cages are full). With this motivation, we have used density functional theory to explore the possibility Type II Si and Ge clathrates with alkali atom guests other than Na may exhibit a similar variation in cell volume with guest inclusion. We present results for the electronic and vibrational properties of the Na$_{x}$Si$_{136}$, Na$_{x}$Ge$_{136}$, K$_{x}$Si$_{136}$, and K$_{x}$Ge$_{136}$ clathrates. These results are compared with experiment and the properties of the materials are compared and contrasted.   

Enhanced Thermoelectric Property of Single Phase MnSi1.75 through Non-equilibrium Synthesis Method

We report thermoelectric properties of the single phase MnSi1.75 using a one-step non-equilibrium synthesis method. Extremely high quenching speed of melt spinning prevents the formation of the second phase MnSi, which is usually found in this class of materials made by using the conventional solid state reaction methods. We found that the phase-pure MnSi1.75 samples exhibit much higher electrical conductivity, as compared with the conventionally prepared samples. Thermal conductivity is measured and analyzed by introducing the Debye model. It is found that the reduced grain sizes after melt spinning play an important role on decreasing lattice thermal conductivity. The combination of enhanced electrical conductivity and reduced lattice thermal conductivity results in large increase of the thermoelectric figure of merit zT at room temperature.   

Investigation of phonon mass-difference scattering model by molecular dynamics

While nanostructuring has been shown to be a promising approach to effectively reduce lattice thermal conductivity and enhance efficiency of thermoelectrics, alloying still remains to be a key process that determines the base material performance. Therefore, understanding lattice thermal conductivity of alloyed crystals, particularly from the viewpoint of mode-dependent phonon transport, holds importance. In this work, we have investigated phonon scattering rates in alloy crystals by focusing on the mass-difference scattering. The phonon mass-difference scattering rates were obtained through spectral analysis of phase-space molecular trajectories computed by molecular dynamics simulations. Obtained results for simple Lennard-Jones crystals show that the mass-difference scattering rates of long-wavelength phonons follow the frequency dependence of Rayleigh scattering regardless of isotope mass and concentration. Meanwhile we obtained the large deviation from scattering models based on the mass perturbation theory. We will report and discuss details of frequency-dependence of mass-difference scattering rates to~clarify validity and limit of the models.~Furthermore, the analysis will be extended to thermoelectric materials, such as lead telluride. This work is supported by Global COE Program, Global Center of Excellence for Mechanical System Innovation.   

Stronger phonon scattering by larger differences in atomic mass and size in p-type half-Heuslers Hf$_{1-x}$Ti$_{x}$CoSb$_{0.8}$Sn$_{0.2}$

High lattice thermal conductivity has been the bottleneck for further improvement of thermoelectric figure-of-merit (ZT) of half-Heuslers (HHs) Hf$_{1-x}$Zr$_{x}$CoSb$_{0.8}$Sn$_{0.2}$. Theoretically the high lattice thermal conductivity can be reduced by exploring larger differences in atomic mass and size in the crystal structure. In this paper, we experimentally demonstrated that lower than ever reported thermal conductivity in p-type HHs can indeed be achieved when Ti is used to replace Zr, i.e., Hf$_{1-x}$Ti$_{x}$CoSb$_{0.8}$Sn$_{0.2}$, due to larger differences in atomic mass and size between Hf and Ti than Hf and Zr. The highest peak ZT of about 1.1 in the system Hf$_{1-x}$Ti$_{x}$CoSb$_{0.8}$Sn$_{0.2}$ (x=0.1, 0.2, 0.3, and 0.5) was achieved with x=0.2 at 800 $^{o}$C.   

Neutron Scattering Study of the Temperature-Dependent Phonon Spectra of AgSbTe$_{2}$

The thermoelectric material AgSbTe$_{2}$ has attracted much attention due to its simple rocksalt structure, high thermoelectric figure-of-merit, and its extremely low thermal conductivity in bulk samples. Previous theoretical studies have suggested that phonons can be scattered by anharmonicity (phonon-phonon coupling) and nano-defects in AgSbTe$_{2}$. However, systematic measurements of phonons in this compound have not been available. We report our results of detailed time-of-flight neutron scattering measurements, as a function of temperature, and departure from stoichiometry. The temperature dependence of the phonon density-of-states is discussed, and compared with the reported thermal conductivity in this system.   

Inelastic Neutron Study of Phonon Lifetime Effects in Thermoelectric Bi$_2$(Se,Te)$_3$ Alloys

One important avenue of optimizing the thermoelectric figure of merit, ZT, is to reduce the thermal conductivity of phonons while preserving the electrical conductivity. Mass disorder caused by alloying provides an avenue of enhancing phonon scattering. In this work, the phonon spectra of different alloys of Bi$_2$(Se,Te)$_3$ are measured using inelastic neutron measurements. The temperature and composition dependence provide information on phonon softening and enhanced phonon scattering of acoustic phonon modes. Measurements on single crystals also reveal the dependence on the polarization of the modes. An additional low energy dispersing mode has been observed.   

Anomalous Lattice Dynamics in PbTe and Its Implications to Low Intrinsic Lattice Thermal Conductivity

Recent experiments on PbTe-based high performance thermoelectric materials raise fundamental questions about the nature of low intrinsic lattice thermal conductivity and underlying lattice dynamics. We show by first-principles calculations that the reported results can be attributed to abnormally large-amplitude thermal vibrations that stem from a delicate competition of dual ionicity and covalency, which puts PbTe near ferroelectric instability. It produces anomalous properties such as partially localized low-frequency phonon modes, a soft transverse optical phonon mode, and a positive temperature coefficient for the band gap. These results account for experimental findings and resolve the underlying atomistic mechanisms. The relation between these anomalies and the low intrinsic lattice thermal conductivity will be discussed.   

Phonon Anharmonicity in PbTe Thermoelectrics

Achieving high thermoelectric conversion efficiency requires limiting the thermal conductivity, through the disruption of phonon propagation. A detailed understanding of phonon dispersions and linewidths is thus critical to develop accurate microscopic theories of thermal conductivity, and design efficient thermoelectric materials. We investigate the phonon dispersions and linewidths in the thermoelectric material PbTe with inelastic neutron scattering experiments. Our measurements indicate that the soft transverse optic mode in PbTe is strongly anharmonic, which could cause a lowering of thermal conductivity by scattering the heat-conducting acoustic modes [1]. We also present results on the effect of alloying. \\[4pt] [1] O. Delaire et al., Nature Materials 10, 614 (2011).   

Anharmonic vibrational effects of thermoelectric Cu-Sb-Se ternary semiconductors: Density-functional theory studies

Strong anharmonicity can lead to intrinsically minimal thermal conductivity even in defect-free single crystals. In an effort to understand this behavior, we have investigated two Cu-Sb-Se ternary semiconductors, Cu$_3$SbSe$_4$ and Cu$_3$SbSe$_3$, by both experimental measurements and density functional theory (DFT) calculations. The experimental lattice thermal conductivity measurements show that while Cu$_3$SbSe$_4$ exhibits classical behavior, the lattice thermal conductivity in Cu$_3$SbSe$_3$ is anomalously low and nearly temperature independent. The vibrational properties of these two semiconductors are calculated by DFT phonon calculations within the quasi-harmonic approximation. The average of the Gr\"{u}neisen parameters of the acoustic mode in Cu$_3$SbSe$_3$ is larger than that of Cu$_3$SbSe$_4$, which theoretically confirms that Cu$_3$SbSe$_3$ has a stronger lattice anharmonicity than Cu$_3$SbSe$_4$. Using our DFT-determined longitudinal and transverse Gr\"{u}neisen parameters, Debye temperatures, and phonon velocities, we calculate the lattice the lattice thermal conductivity using the Debye-Callaway model (without the use of any adjustable parameters). The calculated thermal conductivity is in good agreement with the experimental measurements.   

First-principles calculation of anharmonicity induced phonon lifetimes in FeSi

FeSi has attracted a lot of interest as a promising thermoelectric material for refrigeration applications. We present a first principle calculation of phonon lifetimes of FeSi based on our newly developed computational technique which combines first-principles density functional theory (DFT) and quantum scattering theory. Phonon lifetimes are calculated within the Fermi's golden rule using third order lattice anharmonicity tensors and vibrational phonon frequencies as inputs. Second order force constant matrices and third order lattice anharmonicity tensors are extracted using a real space supercell technique within the local density approximation (LDA). We compare our calculated phonon spectrum and lifetimes with recent neutron scattering measurements of phonon dispersions and linewidths. Finally we use the calculated phonon lifetimes to estimate the thermal conductivity of FeSi using kinetic transport theory.   

Resistivity and Specific Heat under Localized Anharmonic Motion in Type-I Ba$_{8}$Ga$_{16}$Sn$_{30}$ Clathrate

Anharmonic guest atom oscillation has direct connection to the thermal transport and thermoelectric behavior of type-I Ba$_{8}$Ga$_{16}$Sn$_{30}$ clathrates. This behavior can be observed through several physical properties, with for example the heat capacity providing a measure of the overall excitation level structure. In addition the nuclear magnetic resonance (NMR) relaxation behavior provides a sensitive probe for the oscillator dynamics, as we have recently reported. Localized anharmonic excitations also influence the low-temperature resistivity, as we show in this paper. By combining heat capacity and transport measurements we address the distribution of local-oscillators in this material, as well as the shape of the confining potential for Ba ions in the cages. Analyzed along with NMR relaxation measurements for the same sample, a two phonon Raman process is used to extract information about the excitation energies, which along with a quantum computational solver we have used to address the potential structure. We also compare to the soft-potential model and other models used for this system. The results indicate that a single confining potential cannot describe the system properly, whereas a distribution of local oscillators provides a more reasonable fit to the data.   

The Debye-Waller factor and its application to anharmonic vibrations

The Debye-Waller factor relates the intensities of the Bragg peaks to the mean square displacements of the atoms. In the structural refinement of diffraction data it is standard practice to use the harmonic expression for Debye-Waller factor. For most materials and conditions the phonons are only mildly anharmonic, thus the harmonic assumption is a good one. For some materials and conditions, however, the phonons can be strongly anharmonic, and thus the harmonic assumption is physically unrealistic. As examples we cite the rattling atoms in clathrates and skutterudites, and atoms participating in displacive phase transitions. In the present study we investigate the error associated with using the harmonic Debye--Waller factor to analyze anharmonic vibrations. We find that even for strongly anharmonic potentials, such as a double well, the mean square displacements deduced using the harmonic approximation are at most 15$\%$ larger than those deduced using a full anharmonic analysis. Furthermore, the quasi-harmonic and anharmonic values have nearly the same temperature dependences. We conclude that the error introduced by using the harmonic approximation is comparable to or smaller than the usual errors associated with measurement and refinement of diffraction patterns.   

Minimum thermal conductivity in superlattices: A first-principles formalism

In certain superlattice systems such as silicon-germanium superlattices a minimum in thermal conductivity with increase in period thickness has been observed. This minimum has been reported at a relatively short superlattice period of a few nanometers and cannot be explained by existing formalisms. An accurate prediction of this minimum thermal conductivity holds importance for thermoelectrics where low thermal conductivity is desired. We develop a first-principles formalism based on use of harmonic and anharmonic force-constants derived from density-functional perturbation theory and single-mode relaxation time approximation to predict the thermal conductivity of superlattices. The phonon relaxation times are computed based on scattering due to both anharmonicity and interface roughness. We show that the formalism leads to an excellent agreement between predicted and measured values and also explains the observed minimum in thermal conductivity.   

Thermal transport across perovskite superlattices

Understanding thermal transport across interfaces is useful for designing materials for thermal management, thermoelectricity etc. Despite years of investigation, there are several open questions on the nature of thermal transport across solid-solid interfaces. The ability to control materials synthesis down to a monolayer has enabled study of interface phonon scattering in systems such as superlattices, heterostructures etc. We chose perovskite oxides, which are excellent thermoelectric materials, as model systems to study interfacial thermal transport. Superlattices of SrTiO$_3$, CaTiO$_3$ and CaMnO$_3$, with period thicknesses ranging from 1-176 monolayers were grown using pulsed laser deposition, monitored by in-situ RHEED. We measured temperature dependent (100 - 400 K) cross plane thermal conductivity of these superlattices using the time domain thermoreflectance (TDTR). The lowest thermal conductivity measured is below the alloy limit at room temperature. The period thickness dependent thermal conductivity shows signs of zone folding for short period superlattices.   

Session P18: Focus Session: Electron, Ion, and Exciton Transport in Nanostructures - Quantum Transport I

Electrostatic tuning between 1-dimensional and 2-dimensional electron gases

Although low dimensional systems such as 1-D and 2-D electron gases have been separately studied in details, a system enabling co-existence of both dimensions is still challenging to achieve. Such a system where the dimensionality can be tuned between 1-D and 2-D electrons can be an extremely promising platform to explore new phenomena. Here we investigate a novel system based on vicinal GaN-based heterostructure where we exploit its polarization charges to demonstrate for the first time, direct electrostatic tuning of the dimensionality of electrons between 1-D and 2-D. This tuning is achieved by adjusting the Fermi level with applied gate bias. A capacitance-voltage profiling to probe the Fermi occupation function of electron gas was used to demonstrate distinct signatures of the density of states for both the dimensions at room temperature. We developed a 2-sub-band model consisting of 1-D and 2-D sub-bands to describe the behavior of the electron gas, which is in excellent agreement with our experimental data, confirming the co-existence of electrons of both dimensions. This demonstration of co-existence of 1-D and 2-D electrons and the ability to tune between their dimensions at room temperature could open new research paths for low-dimensional physics besides enabling devices with added functionalities.   

Quantum coherence phenomena in bismuth thin film nanostructures

We present low temperature quantum magnetotransport measurements on bismuth nanostructures. The large spin-orbit interaction and prominent surface states in Bi films are expected to produce non trivial mesoscopic quantum transport. Bi thin films are grown by thermal evaporation onto SiO$_2$ with a two-step process to optimize the film mobility and ensure oriented growth along the [111] direction. The optimization leads to a minimum thickness of 25 nm for continuous films. Structures as small as 100 nm are subsequently patterned into the film via lithographic techniques. Phase and spin coherence lengths are obtained by analyzing electron interference phenomena in mesoscopic wires and rings. We show that the phase coherence shows a decreasing trend with decreasing channel width. The decrease in phase coherence in Bi wires ranging in width from 1200 nm to 100 nm cannot readily be accounted for by increased boundary scattering. A width dependence of the spin coherence is also detected. The observed confinement dependence of the phase and spin coherence in Bi nanostructures will be discussed (DOE DE-FG02-08ER46532).   

Resistance oscillations and dephasing in ring patterns on InGaAs/InAlAs two-dimensional electron systems

In InGaAs/InAlAs heterostructures with spin-orbit interaction, patterned into mesoscopic rings, we experimentally investigate low-temperature Aharonov-Bohm resistance oscillations. The oscillation amplitude was studied as function of applied current and temperature, to obtain the dependence of these parameters on dephasing. Previous results on mesoscopic quantum interferometers have shown a lobe structure in the dependence of the oscillation amplitude on measurement bias, in some models ascribed to Coulomb interaction, and have surmised a universal behavior with an energy scale dependent on interferometer size. The ring interferometers studied here were of typical radius of 700 nm, and lithographic arm width of 300 nm, similar-sized but of higher carrier density than in the previous studies, and used for similar bias- and temperature-induced dephasing studies. The measurements so far show the expected temperature dependence, but have not revealed the lobe structure in the bias dependence. We discuss the results in the light of dephasing phenomena expected in mesoscopic quantum interferometers (DOE DE-FG02-08ER46532, NSF DMR-0520550).   

Transport through double quantum dot with electron-LO-phonon interaction

We theoretically study the transport through a serial double quantum dot (DQD) in the presence of electron-LO-phonon interaction. In contrast to the case of acoustic phonon,\footnote{P.\ Roulleau {\it et al}., Nat.\ Commun.\ {\bf 2}, 239 (2011).} the coherent coupling between an electron and an optical phonon, so-called polaron formation, has a small dissipation, which influences the transport properties markedly. We calculate the current through the DQD using the Keldysh Green function, as a function of the tuning of energy levels between the quantum dots, when the bias voltage is sufficiently large. The electron-phonon interaction is considered by the perturbation expansion. We find a subpeak structure of the current due to the polaron formation and evaluate elastic and inelastic components of the current.   

Imaging Spatial Current Eigenmodes in Nanoscopic Quantum Networks

Using the Keldysh Green's function formalism, we study the non-equilibrium charge transport in nanoscopic quantum networks [1]. Due to quantum confinement, charge transport takes place via current eigenmodes that possess characteristic spatial patterns of current paths. In the ballistic limit, these patterns exhibit unexpected features such as current backflow and closed loops of circulating currents. These current eigenmodes are the non-equilibrium analogue of eigenmodes in the local density of states of confined systems [2]. Moreover, we demonstrate that dephasing leads to a smooth evolution of the current patterns, and ultimately reproduces the charge transport in a classical resistor network. Finally, we propose a new method using scanning tunneling spectroscopy to image the spatial current patterns associated with individual current eigenmodes in nanoscopic quantum networks.\\[4pt] [1] T. Can, H. Dai, and D.K. Morr, in preparation\\[0pt] [2] M. Crommie, C. Lutz, and D. Eigler. Confinement of electrons to quantum corrals on a metal surface. Science, 262, 1993.   

Microscopic Theory for the 0.7 Anomaly in Quantum Point Contacts - the Role of Geometry- and Interaction-Enhanced Spin-Fluctuations

We present a detailed microscopic analysis of some local observables of a quantum point contact (QPC) to gain better understanding of the origin of the 0.7 conductance anomaly. We model the system by a one-dimensional tight binding model with local interactions, a smooth potential barrier and a homogeneous magnetic field. We calculate conductance $G$, local density $n$ and local magnetization $m$ as a function of magnetic field at zero temperature, using the functional Renormalization Group (fRG). Our potential can be tuned to describe the smooth crossover from a single barrier, representing a QPC, to a double barrier, modelling a quantum dot (QD) exhibiting the Kondo effect. We find that both geometries show interaction-enhanced spin-fluctuations, manifested via an enhanced local spin susceptibility, for gate voltages that lead to an anomalously large negative magnetoconductance, characterized by an anomalously small low-energy scale $B_*$. This finding explains why both the Kondo effect and the 0.7-anomaly exhibit a very similar conductance behavior at sufficiently low magnetic fields and temperatures ($T,B \ll T_*$), amenable to a similar Fermi-liquid description. We also show that at high fields ($B \gg B_*$) the analogy between Kondo effect and 0.7-anomaly breaks down.   

Towards a Fermi-Liquid description of the 0.7 Anomaly in Quantum Point Contacts

In addition to plateaus in integer values of $G_0 = \frac{2e^2}{h}$, the linear conductance of a quantum point contact (QPC) shows an anomalous shoulder at around $0.7 G_0$ that evolves in a characteristic fashion with rising magnetic field and temperature. We present a microscopic theory for the $0.7$ conductance anomaly, based on a one-dimensional tight binding model with a local interaction, a smooth potential barrier and a homogeneous magnetic Zeeman field. We calculate the conductance as a function of magnetic field and temperature using standard second order perturbation theory. Furthermore we use a more sophisticate method, the functional Renormalisation Group (fRG), to obtain a more reliable description at zero temperature and finite magnetic field. We analyze the leading temperature $T$ and magnetic field $B$ dependence of the conductance, which define, respectively, low-energy scales $T_\star$ and $B_\star$ and find that both $T_\star$ and $B_\star$ depend exponentially on gate voltage, whereas the ratio $\frac{B_\star}{T_\star}$ is almost independent of gate voltage. This result indicates that the low-energy behavior of the $0.7$ anomaly displays Fermi-liquid behavior. We present new experimental data that corroborate this conclusion.   

Quantum Wires with Electrons and Cold Atoms

Advances in atom trapping and atom chip techniques allow the controlled preparation and manipulation of clouds of cold, neutral atoms, in optical lattices as well as on lithographically patterned gate structures. This opens up many new possibilities to study quantum transport in systems other than semiconductor quantum dots or nanowires, with the exciting perspective of future ``atomtronic'' systems. Inspired by these developments, we study transport through two different interacting systems as a function of externally controllable parameters: Electrons in a nanowire, and cold atoms in a one-dimensional finite well. For a nanowire system we can demonstrate that Wigner crystallization occurs, when the length of the wire is changed in the experiment [1]. Distinct diamond patterns appear, when conductance is plotted against source-drain bias and the gate voltage, as common for quantum dot systems. The same holds for more general types of ``interaction blockade,'' and we demonstrate that such phenomena can also be studied by cold atom systems. \\[4pt] [1] Kristinsd\'ottir, L.H., J. Cremon, H. Nilsson, H. Xu, H. Linke, L. Samuelson, A. Wacker and S.M. Reimann, Phys. Rev. B \textbf{83}, 041101, (2011).   

Functional dependence of the exact time-dependent Kohn-Sham potential for quantum transport

We present methods for determining exact steady-state and time-dependent Kohn-Sham potentials from known charge and current densities. Applying these methods to cases of a single electron added to the conduction band of a model semiconductor, we calculate the exact Kohn-Sham potentials and discuss their meaning in the context of describing quantum transport. We show that the inclusion of a longitudinal Kohn-Sham vector potential is quite unavoidable in describing steady-state current-carrying systems, whereas the addition of an electron in a wavepacket state necessitates, inter alia, the appearance of an exchange-correlation electric field. We also present our findings on the functional dependence of the exact Kohn-Sham scalar and vector potentials on the charge and current density.   

Waiting time distributions of mesoscopic conductors

Electronic transport through mesoscopic structures is stochastic due to the quantum nature of the charge carriers. In full counting statistics the interest is in the number of particles that are detected in a given time interval. Another important and fundamental question concerns the waiting time between consecutive carriers. Recently, waiting time distributions (WTD) have been calculated for periodically driven systems described by master equations and shown to clearly distinguish random charge emissions from charge transfer processes which are frequency-locked to the period of the external drive [1]. In this work we investigate the WTD of a biased quantum point contact (QPC) with one channel [2]. The WTD clearly reflects the fermionic statistics of the elementary charges: with increasing transmission probability of the QPC, the WTD changes from that of a Poisson process for a nearly closed QPC to a Wigner-Dyson distribution, known from the analogous problem of free fermions described by random matrix theory. [1] M. Albert, C. Flindt and M. Buettiker, Phys. Rev. Lett. 107, 086805 (2011). [2] M. Albert, G. Haack, C. Flindt and M. Buettiker, \textit{in preparation}.   

Magnetic field induced mixed level Kondo effect in two-level quantum dots

Semiconductor quantum dots provide an easily tunable environment in which to investigate the Kondo effect. As it is known, Kondo correlations are suppressed by magnetic fields, showing e.g. a drop in the conductance of a quantum dot device. However, certain systems may exhibit an increasing conductance as a function of an applied magnetic field [1]. In this work we use the numerical renormalization group method to study a two-level quantum dot system with on-level and interlevel Coulomb repulsion, coupled to a single channel. When there is a finite detuning between levels, and a local singlet develops in one of them, the linear conductance of the device shows a maximum structure as a function of an in-plane magnetic field, which depends on the temperature of the system. This maximum occurs at a magnetic field strength such that the spin up state of one of the levels and spin down of the other are degenerate, allowing a ``mixed level'' Kondo effect. The respective spectral functions feature a resonance at the Fermi energy, commensurate with the Kondo physics. We discuss the properties of this mixed level Kondo state in terms of the detuning and the other parameters of the system. \\[4pt] [1] R. Sakano and N. Kawakami, PRB 73, 155332 (2006)   

Time dependent quantum transport in a vibrating quantum dot in Kondo regime

We employ the time dependent non-crossing approximation to investigate the effect of strong electron-phonon coupling on the instantaneous conductance and thermopower of a single electron transistor which is abruptly shifted into the Kondo regime via a gate voltage. We find that the instantaneous conductance exhibits decaying sinusoidal oscillations on the long timescale for infinitesimal bias [1]. The ambient temperature and electron-phonon coupling strength influence the amplitude of these oscillations and the frequency of oscillations is equal to the phonon frequency. We discuss the origin of these oscillations and the effect of finite bias on them. On the other hand, we argue that measurement of the decay time of thermopower to its steady state value in linear response might be an alternative tool in determination of the Kondo temperature and the actual value of the electron-phonon coupling strength in an experiment. \\[4pt] [1] A. Goker, J. Phys.: Condens. Matter, 23 (2011) 125302   

Dynamic response of a single-electron transistor in the ac Kondo regime

A single-electron transistor (SET) consisting of a small conducting island contacted by macroscopic conductors can be used to study strongly correlated electrons in artificial systems. SETs also provide possibilities of exploring non-equilibrium Kondo phenomena by applying source-drain voltage. In the context of recent measurements, we will discuss the non-trivial dynamics that emerges when externally imposed energy scales compete with the Kondo correlations. The response of the system to a simultaneous application of a magnetic field and a high-frequency modulation of the SET voltages will also be reported.   

Double Quantum Dot Kondo Effect in a Magnetic Field

Conventionally the Kondo effect is thought of as describing how conduction electrons screen a localized spin. More generally, it describes how itinerant electrons screen a degenerate degree of freedom of a localized site. A double quantum dot (with negligible inter-dot tunneling) can have both spin degeneracy, as well as a degeneracy associated with an electron being on dot 1 or dot 2. The latter degeneracy corresponds to a pseudo-spin degree of freedom that can also be screened by the Kondo effect [A. H\"{u}bel, et al. PRL 101, 186804 (2008)]. Applying a finite magnetic field can split the spin degeneracy of the dots, which should allow the realization of a purely pseudo-spin Kondo effect. We present conductance measurements of this double dot Kondo effect in a magnetic field. We compare our measurements to theoretical predictions for SU(2) Kondo to check whether we have realized a purely pseudo-spin Kondo effect in a double dot.   

Theory of anomalous magnetotransport in triple quantum dots

Magneto-transport measurements on a triple quantum dot ring have recently shown anomalous quantum oscillations with dominant frequencies separated by a factor of three in magnetic flux [1]. Such oscillations, suggestive of a one-third periodicity in the flux quantum, are usually not observed in larger mesoscopic rings in which only larger periods are observed. We develop a microscopic transport model for the triple dot and show that the anomalous oscillations can dominate the transport behavior under certain conditions. Furthermore, we discuss the range of validity of our model by studying dephasing due to broadening and electric dipole interactions. \\[4pt] [1] L. Gaudreau et al., Phys. Rev. B 80, 075415 (2009)   

Session P19: Invited Session: Dynamics of Strongly Correlated Systems: Control and Ultrafast X-Ray Probes

Controlling Quantum Condensed Matter With Light

In this talk I will discuss some of our recent work aimed at controlling the properties of quantum condensed matter with light, for instance in High-Tc cuprates or in Mott Insulators. The focus is on the use of high-field THz radiation rather than near-visible excitations, thus using photon energies that do not destroy the broken symmetry state of the solid, for example by breaking Cooper pairs. Rather we study the non-linear electrodynamics of the solid, for example by manipulating low-lying lattice vibrations coherently or by driving the phase excitations in superconducting condensates. A straightforward conceptual analogy can be found with experiments that that study driven dynamics of strongly correlated atomic gases in optical lattices. Due to the short lengthscales and the fast timescales involved in condensed matter, femtosecond x-ray scattering and spectroscopy experiments with the LCLS Free Electron Laser are necessary to interrogate the microscopic non-equilibrium paths explored by the stimulated solid.   

Light induced meltdown of quasiparticles in high temperature superconductors

Ultrafast \textit{time}- and angle- resolved photoemission spectroscopy (tr-ARPES) is emerging as a powerful tool to access the non-equilibrium quasiparticles dynamics and Cooper pairs formation in unconventional superconductors. In a tr-ARPES experiment a non-equilibrium transient state is created by pumping with an infrared pulse and is then measured via photoemission with an ultraviolet probe pulse. By varying the time delay between pump and probe we can directly access the recovery of the superconducting gap and the non --equilibrium quasiparticles population decay. Here we present a detailed momentum, temperature, doping and density dependent study of the response of a high temperature Bi2212 superconductor to a femtosecond pump-probe. In particular, through systematic pump fluence dependence we have induced the meltdown of quasiparticles and have driven the system normal by inducing a collapse of the superconducting gap. Interestingly we observed that only quasiparticles beyond a particular boson mode respond to the pump laser excitations, while the others remain untouched and that both quasiparticles recombination and gap dynamics are a density, momentum and doping dependent process, showing a crossover from a weakly perturbed to a strongly perturbed regime. These results point to a new dichotomy between the ultrafast gap and quasiparticles response within and beyond the Fermi arc and reveal a new window into the nature of the pairing interaction in high Tc superconductors.   

Shooting the electronic structure movie: Femtosecond time-resolved photoemission of layered charge-density-wave systems

Charge-density waves (CDWs) are broken-symmetry states of low-dimensional materials that are brought about by strong electron-phonon interaction. Yet surprisingly, a universal microscopic understanding beyond this statement has not really evolved for this classical paradigm of condensed matter physics. In quasi-two-dimensional systems, for example, the common approaches based on ARPES band structure results--looking for nested sections of the Fermi surface or for a peak in the electronic susceptibility--have almost no predictive power. Apparently, more successful explanations have to take into account the delicate balance between several factors including not only electronic and phononic structure, but also electron-electron and electron-phonon interactions. Here, we will show that femtosecond time-resolved XPS and ARPES using pulsed extreme ultraviolet radiation generated by the free-electron laser FLASH [1] as well as by a table-top high-harmonic-generation source [2] can provide novel insights into the relative roles that the various factors play in CDW formation. We will focus on three conspicuous CDWs in prominent members of the family of layered transition-metal dichalcogenides: the $(\sqrt{13}\times\sqrt{13})$ CDW in the Mott insulator $1T$-TaS$_2$, the $c(2\sqrt{3}\times4)rect.$ CDW in the Peierls insulator Rb$_x$TaS$_2$, and the $(2\times2\times2)$ CDW in the possible excitonic insulator $1T$-TiSe$_2$. The specific program is to reveal the relative importance of electronic and phononic contributions to the various CDW transitions by relating measured melting and relaxation times of CDW-induced spectral features to typical elementary time scales in layered materials. \\[4pt] [1] S. Hellmann {\it et al.}, Phys. Rev. Lett. {\bf 105}, 187401 (2010). \\[0pt] [2] T. Rohwer {\it et al.}, Nature {\bf 471}, 490 (2011).   

Theoretical description of photo-doping in Mott and charge-transfer insulators

Many aspects of photo-excited insulator-to-metal transitions in Mott and charge-transfer systems are theoretically not well understood: How is the photo-doped state related to a chemically doped state? On what timescale do we expect the formation of quasiparticles? To describe the electronic dynamics of Mott insulators, we have used nonequilibrium dynamical mean-field theory (DMFT) in combination with Quantum Monte Carlo and various weak and strong-coupling [1] techniques. In the talk, I will briefly present the current status of this approach and of related cluster approaches for nonequilibrium. I will then discuss results for the photo-doping in the Hubbard model, and in a in a p-d model for charge-transfer insulators. When the onsite Coulomb repulsion U is much larger than the hopping, rapid thermalization of the pump-excited Mott insulator is inhibited by the energetic stabilization of doublon-hole pairs [2], and various types of non-thermal states can arise. Immediately after the excitation process, the system of doublons and holes is too hot to form quasiparticle states, but coupling to a heat-bath of phonons can drive the system into a metallic state with well developed doublon and hole bands. Close to the metal-insulator transition, on the other hand, when U is of the order as the hopping, doublons and holes rapidly thermalize due to the electron-electron interaction, which makes the system a bad metal rather than a Fermi liquid. \\[4pt] [1] M. Eckstein and Ph. Werner, Phys. Rev. B {\bf 82}, 115115 (2010).\\[0pt] [2] M. Eckstein and Ph. Werner, Phys. Rev. B {\bf 84}, 035122 (2011).   

High frequency properties of individual metallic carbon nanotubes

We study the electrical and electrothermal dynamics of individual metallic single-walled carbon nanotubes (SWNT). Using Johnson noise thermometry, we characterize the dependence of the electron temperature on the dc bias current. This allows us to determine the thermal conductance associated with cooling of the nanotube electron system as a function of both temperature and nanotube length [1]. This thermal conductance can be used to predict the measured radio frequency (rf) bolometric response. At low temperatures and low bias current, an additional rf response is observed from the (non-thermal) electrical nonlinearity of the contacts [2]. Finally, we compare these rf measurements with measurements of terahertz (THz) detection. The THz measurements are used as a probe of plasmon standing wave resonances on the SWNT. \\[4pt] [1] D.F. Santavicca, J.D. Chudow, D.E. Prober, M.S. Purewal, and P. Kim, Nano Lett. 10, 4538 (2010). \\[0pt] [2] D.F. Santavicca, J.D. Chudow, D.E. Prober, M.S. Purewal, and P. Kim, Appl. Phys. Lett. 98, 223503 (2011).   

Session P20: Invited Session: Scientific Reasoning in a Physics Course

Empowering the crowd: faculty discourse strategies for facilitating student reasoning in large lecture

Oregon State University (OSU) has restructured its introductory calculus-based sequence including reformed curriculum modeled after the Interactive Science Learning Environment (ISLE). ISLE is driven by an experimental cycle roughly summarized as: observe phenomena, find patterns and devise explanations, test explanations, develop a model, apply the model to new observations. In implementing ISLE at OSU we have chosen to focus on student scientific reasoning, specifically student ability to develop and test models, make explicit judgments on how to approach open-ended tasks, and take an authoritative role in knowledge development. In order to achieve these goals, the lecture course heavily utilizes social engagement. During large-lecture group work, emphasis is placed on facilitating student discourse about issues such as what systems to choose or how to define an open-ended problem. Instructional strategies are aimed at building off the group discourse to create a full-class community where knowledge is developed through collaboration with peers. We are achieving these goals along with an increase in measured student conceptual knowledge and traditional problem solving abilities, and no loss of content coverage. It is an ongoing effort to understand ``best'' instructional strategies and to facilitate new faculty when they teach the curriculum. Our research has focused on understanding how to facilitate activities that promote this form of discourse. We have quantitative analysis of engagement based on video data, qualitative analysis of dialogue from audio data, classroom observations by an external researcher, and survey data. In this session we share a subset of what we have learned about how to engage students in scientific reasoning discourse during large lecture, both at the group-work and full-class level.   

Designing and using multiple-possibility physics problems in physics courses

One important aspect of physics instruction is helping students develop better problem solving expertise. Besides enhancing the content knowledge, problems help students develop different cognitive abilities and skills. This presentation focuses on multiple-possibility problems (alternatively called ill-structured problems). These problems are different from traditional ``end of chapter'' single-possibility problems. They do not have one right answer and thus the student has to examine different possibilities, assumptions and evaluate the outcomes. To solve such problems one has to engage in a cognitive monitoring called epistemic cognition. It is an important part of thinking in real life. Physicists routinely use epistemic cognition when they solve problems. I have explored the instructional value of using such problems in introductory physics courses.   

Assessing high-level scientific reasoning in a physics exam: Pipe-dream or reality?

What do we want students to be able to do when they have finished their introductory physics course? In addition to learning the physics content, we want students to learn to think like physicists. We want students to develop specific scientific reasoning abilities that are the hall-mark of scientific thinking. These include, analyzing and interpreting experimental data, designing an experiment to test different hypotheses, identifying assumptions in a physical model amongst many others. Physics courses such as the Investigative Science Learning Environment (ISLE) have been developed to focus specifically on developing students' scientific reasoning abilities. Research has shown that ISLE is successful in achieving its goal. We would like our assessments to directly reflect our learning goals for our students. In order to measure higher-level scientific reasoning, we can, for example, require students to participate in a laboratory practical exam in which they have to engage in experimental design and analysis. However, this assessment method could become very difficult to administer and grade in a large-enrollment class. Is it possible to assess scientific thinking abilities of students using traditional formats such as paper and pencil exams? In this talk I will present some of our latest ideas about how to re-design traditional exam questions to measure a range of scientific reasoning abilities.   

Using mathematics to make sense in undergraduate physics

Physics courses involve the study of physical quantities constructed to facilitate the characterization of nature, and the study of the connections between these quantities. These connections are often ratios or products of more familiar quantities. Learning to use the predictive power these relationships provide is an important part of learning to make sense of the physical world. Mathematically inspired reasoning is foundational to the way physicists make sense of the natural world and math is often referred to as the language of physics. Students rarely understand the relationships between the physical quantities in the way their instructors hope they will. There is often a disconnect between the specialized way we use mathematics in physics and the broad spectrum of processes that students learn to master as they progress through the pre-college mathematics curriculum. We are often surprised by how little math our students are able to use in physics, despite successful performance in their previous math classes. Much of the reasoning used in introductory physics is borrowed from mathematics that is taught in middle school and early high school (facility and practice with integers, fractions and ratios, multiplication and division using symbolic representations, manipulation of linear equations, analyzing right triangles.) But physics is a very different context, with confounding factors that often render the mathematics opaque to the learner. In this talk, I will discuss the specific ways in which physicists' use of mathematics differs from what many students acquired in their math classes. I will discuss how a weak mastery of conceptualizing fundamental mathematical operations interferes with students' ability to make sense in physics, and can carry over into difficulties with subsequently more abstract reasoning at higher levels. I will also offer suggestions for ways in which instructors can be more cognizant of (and transparent about) their specialized use of mathematics, thereby helping their students to effectively use mathematics for making sense of the physics they are learning.   

Student reasoning about ratio and proportion in introductory physics

To many students, introductory physics must seem a fast-moving parade of abstract and somewhat mysterious quantities. Most such quantities are rooted in proportional reasoning. Using ratio, physicists construct the force experienced by a unit charge, and attach the name electric field, or characterize a motion with the velocity change that occurs in a unit time. While physicists reason about these ratios without conscious effort, students tend to resort to memorized algorithms, and at times struggle to match the appropriate algorithm to the situation encountered. Although the term ``proportional reasoning'' is prevalent, skill in reasoning with these ratio quantities is neither acquired nor applied as a single cognitive entity. Expert ability seems to be characterized by the intentional use of a variety of components, or elements of proportional reasoning, by a fluency in shifting from one component to another, and by a skill in selecting from among these components. Based on this perspective, it is natural to expect students to develop proportional reasoning ability in fits and starts as various facets are acquired and integrated into existing understandings. In an ongoing collaboration between Western Washington University, New Mexico State University, and Rutgers, we are attempting to map the rich cognitive terrain of proportional reasoning, and to use our findings to guide the design of instruction that develops fluency. This talk will present a provisional set of proportional reasoning components, along with research tasks that have been developed to measure student ability along these components. Student responses will be presented as evidence of specific modes of thinking. The talk will conclude with a brief outline of our approach to improving student understanding.   

Session P21: Superconductivity: Electronic Structure of Cuprates

Co-existence of spin fluctuation and superconductivity in electron doped cuprate Pr$_{1-x}$LaCe$_{x}$CuO$_{4}$

Even though spin fluctuation has been proposed to be as the pairing glue in the cuprate high temperature superconductivity, there is lack of a clear evidence for its coupling to electron. One of the reasons is that, for hole doped cuprates, both anti-ferromagnetism (AFM) and recently proposed charge ordering effects due to Fermi surface nesting occur in the same region of the momentum space (anti-nodal region). On the other hand, electron doped cuprates are known to have the pseudo gap effect at hot spots from AFM band renormalization. This fact makes it advantageous to investigate electron doped cuprates for the spin fluctuation issue. We performed ARPES studies on superconducting electron doped cuprates PLCCO (x=0.1, 0.15, 0.18) to investigate the relation between the spin fluctuation and superconductivity. We observe pseudo gap for all the dopings, which indicates that the short range AFM ordering survives far away from the AFM phase boundary. This coincidence of the short range AFM and superconductivity even in the over doped state may support the spin fluctuation scenarios at least in the electron doped side.   

Evidence for the coexistence of antiferromagnetism and superconductivity in the electron-doped infinite-layer cuprate Sr$_{1-x}$La$_x$CuO$_2$ from ARPES

The asymmetry between hole doping and electron doping of cuprates has major implications for theories of high-$T_c$ superconductivity, yet the vast majority of our knowledge of electron-doped cuprates originates from the Re$_{2-x}$Ce$_x$CuO$_4$ (RCCO) materials. One salient feature of electron doping is the robustness of antiferromagnetism, but at present it has not been established whether this is intrinsic to the electron-doped CuO$_2$ plane or idiosyncratic to the RCCO family. Here we report high-resolution \emph{in situ} angle-resolved photoemission spectroscopy measurements of superconducting Sr$_{1-x}$La$_x$CuO$_2$ (SLCO) thin films grown by molecular beam epitaxy. The observed electronic structure exhibits many features consistent with ($\pi$,$\pi$) scattering, and the clear observation of such scattering in this material demonstrates that strong antiferromagnetism is generic to the electron-doped CuO$_2$ plane. Furthermore, the ($\pi$,$\pi$) order in SLCO is sufficiently strong to fully gap the nodal portion of the Fermi surface, leaving only electron pockets at ($\pi$,0); these pockets are gapped by the coexisting (presumably \emph{d}-wave) superconductivity. This offers a simple explanation for the many reports of \emph{s}-wave superconductivity in this material.   

Electronic structure modulations in the stripe phase of La$_{1.475}$Nd$_{0.4}$Sr$_{0.125}$CuO$_4$

A prevailing description of the stripe phase in underdoped cuprate superconductors is that the charge carriers (holes) phase segregate into hole rich regions that form anti-phase boundaries between regions of anti-ferromagnetic order. I will present resonant elastic x-ray scattering measurements of stripe-ordered La$_{1.475}$Nd$_{0.4}$Sr$_{0.125}$CuO$_4$ at the Cu $L_{3,2}$ and O $K$ absorption edges that point to an alternate interpretation of the stripe phase. Analysis of the energy dependence of the scattering intensity reveals that the dominant feature of the charge density wave state is a spatial modulation in the energies of Cu $3d$ and O $2p$ states rather than the large modulation of the valence (charge density) envisioned in the common stripe paradigm. These energy shifts are interpreted as a spatial modulation of the electronic structure, possibly involving a modulation of the Cu $3d$ -- O $2p$ hopping, $t_{pd}$, the onsite Coulomb repulsion, $U_{dd}$, and other local electronic structure parameters. This result may point towards a valence-bond-solid interpretation of the stripe phase, where translational symmetry can be broken with minimal modulation of the charge density.   

Quantum Oscillations in a $\pi$-Striped Superconductor

Within Bogoliubov-de Gennes theory, a semiclassical approximation is used to calculate the Fermi surface area associated with quantum oscillations in a model of a $\pi$-striped superconductor, where the d-wave superconducting order parameter oscillates spatially with zero average value. This system has a non-zero density of particle-hole states at the Fermi energy, which form Landau-like levels in the presence of a magnetic field. The oscillation frequency found for large pairing interaction, for $\pi$-stripes with period 8, is close to that reported for experimental measurements in the cuprates. A comparison is made of this theory to data for quantum oscillations in the specific heat measured by Riggs et al.   

Density-functional theory calculation of Fermi surface in stripe ordered YBa$_{2}$Cu$_{3}$O$_{6.5}$

High temperature superconductors (HTSC) attract a lot of interests since their discovery in 1986. More recently, observations of quantum oscillations in underdoped YBa$_{2}$Cu$_{3}$O$_{6.5}$ (YBCO6.5) at a low frequency suggested a small pocket constitute the Fermi surface (Doiron-Leyraud et al. Nature 447, 565 (2007)). In this work, we present results of density-functional theory (DFT) calculations of YBCO electronic structure. In order to better represent the electron-electron interaction, we add an on-site repulsion term (Hubbard term) on the copper d-orbitals (DFT+U). This method is known to improve DFT calculations for Mott insulators like La$_{2}$CuO$_{4}$ and YBCO$_{6.0}$ since the Hubbard term favors an anti-ferromagnetic ground state. Using this method, we compare various magnetic states calculated with different values of the Hubbard term U. Our results suggest that an atom-centered stripe, similar to the one found in La$_{1.875}$Sr$_{0.125}$CuO4 (Tranquada et al. Nature 375 561 (1995)), is consistent with the presence of a Fermi pocket of the size reported in the experiments. We further show that the size of the pocket and the nature of the carriers (electrons or holes) can be varied with pressure suggesting a way to test this hypothesis.   

Kinetics-Driven Superconducting Gap in Underdoped Cuprate Superconductors Within the Strong-Coupling Limit

A generic theory [1] of the quasiparticle superconducting gap in underdoped cuprates is derived in the strong-coupling limit, and found to describe the experimental ``second gap'' in absolute scale. In drastic contrast to the standard pairing gap associated with Bogoliubov quasiparticle excitations, the quasiparticle gap is shown to originate from anomalous kinetic (scattering) processes, with a size unrelated to the pairing strength. Consequently, the k dependence of the gap deviates significantly from the pure $d_{x^2-y^2}$ wave of the order parameter. Our study reveals a new paradigm for the nature of the superconducting gap, and is expected to reconcile numerous apparent contradictions among existing experiments and point toward a more coherent understanding of high-temperature superconductivity. \\[4pt] [1] Y. Yildirim and Wei Ku, PRX 1, 011011 (2011).   

Recent ARPES study on extremely underdoped LSCO system

It has been widely accepted that Mott physics plays an important role in how superconductivity develops in high Tc cuprates. However, only a few in the family can reach to deeply underdoped region where Mott physics truly dominates, where the ``pseudogap'' is entirely disentangled from superconducting gap. Being an important compound to tackle this issue, extremely underdoped La$_{2-x}$Sr$_x$CuO$_4$ samples were systematically investigated through ARPES experiments. The doping dependence of Gaussian envelope at higher binding energy suggests strong polaronic contribution to the near Fermi level coherent feature. In complementary to previous observation of nodal gap in semiconducting LSCO, we found the Fermi surface to be fully gapped over wide range in the phase diagram in accordance with transport measurements. By comparing the marginal Fermi liquid model with momentum/temperature dependent MDC width analysis, we will extend our discussion on the intriguing connections between the nodal gap, polaronic excitation and ``pseudogap'' in this system.   

One gap, two gaps, and universality in high temperature superconductors

A dramatic change in energy gap anisotropy upon reducing carrier concentration has often been observed in the cuprate high temperature superconductors (HTSC). A simple d-wave gap in materials with the optimal Tc evolves with underdoping into a ?two-gap? structure, with different dependences in different regions of momentum space. It is tempting to associate the large antinodal gap with a second order parameter distinct from d-wave superconductivity. We use angle-resolved photoemission spectroscopy (ARPES) to show that this two-gap behavior, and the concomitant destruction of well defined electronic excitations, are not universal features of HTSC, and depend sensitively on how the underdoped materials are prepared. Depending on cation substitution, underdoped crystals either show two-gap behavior or not. In contrast, many characteristics of HTSC like the superconducting dome (Tc versus doping), nodal quasiparticles, antinodal gap that decreases monotonically with doping, and the pseudogap, are present in all samples, irrespective of whether they exhibit two-gap behavior or not. Our results imply that universal aspects of high Tc superconductivity are insensitive to differences in the electronic states in the antinodal region of the Brillouin zone.   

Imaging Doped Holes in a Cuprate Superconductor with High-Resolution Compton Scattering

The high-temperature superconducting cuprate La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) shows several phases ranging from antiferromagnetic insulator to metal with increasing hole doping. To understand how the nature of the hole state evolves with doping, we have carried out high-resolution Compton scattering measurements at room temperature together with first-principles electronic structure computations on a series of LSCO single crystals in which the hole doping level varies from the underdoped (UD) to the overdoped (OD) regime [1]. Holes in the UD system are found to primarily populate the O 2px/py orbitals. In contrast, the character of holes in the OD system is very different in that these holes mostly enter Cu d orbitals. High-resolution Compton scattering provides a bulk-sensitive method for imaging the orbital character of dopants in complex materials. \\[4pt] [1] Y. Sakurai \textit{et al.} Science {\bf 332}, 698 (2011).   

Anisotropic quasiparticle scattering in overdoped La-based cuprate superconductors: An angle resolved photoemission study

High-temperature superconductivity remains one of the outstanding unresolved problems of condensed matter physics. In the cuprate superconductors, a central part of the problem is to understand the unusual normal state properties. Here we present a new angle resolved photoemission spectroscopy (ARPES) study on the normal state of overdoped La-based cuprates. The pseudogap phase is found to vanish in a quantum critical point $x_c$ inside the superconducting dome. Momentum dependent quasiparticle scattering have been studied for doping concentrations larger than $x_c$. Comparison to angle-integrated bulk probes such as electrical transport measurements will be made.   

ARPES evidence for a unidirectional bond-direction order in La-based curates

Lattice translational symmetry breaking has wide implications with a variety of emergent quantum phases of condensed matters. In cuprate superconductors, various types of shadow bands have been observed in ARPES as band replicas that are displaced from the main bands by specific wave vectors in momentum space, suggestive of the breaking of lattice translational symmetry in different forms. Shadow bands associated with a wave vector along the Cu-O bond direction, were recently observed in a Y-based cuprate, which are ascribed to the unique occurrence of oxygen ordering in the CuO chains of the material. In contrast, the shadow bands reported so far in other cuprates without CuO chains in their crystal structures are associated with some wave vectors all along the bond-diagonal direction and it remains controversial, in most cases, whether they are due to some orderings of primarily structural or electronic origins. Here we report the first clear observation of shadow bands in ARPES associated with a bond-direction wave vector in the La-based cuprates (without CuO chains). These shadow bands were observed under various polarization conditions over a wide doping range in La2-xSrxCuO4 and are associated with a unidirectional order. We will present a doping and temperature dependence study of this order and discuss the results in relation to the complementary bulk-sensitive scattering experiments on the same system.   

Synchrotron X-ray study reveals oxygen chains in HgBa$_{2}$CuO$_{4+\delta }$

X-ray scattering work shows that the double-layer high-$T_{c}$ superconductors YBa$_{2}$Cu$_{3}$O$_{8+d }$and Bi2Sr$_{2}$CaCu$_{2}$O$_{6+\delta }$ are intrinsically inhomogeneous [1-3], with short-range lattice modulations driven by oxygen dopants. HgBa$_{2}$CuO$_{4+\delta }$(Hg1201) has a simpler (tetragonal) structure and the highest $T_{c}$ (at optimal doping) among all single-layer cuprates. It is thus a very good candidate system to address the issue of charge modulations. Using synchrotron X-ray scattering and high-quality single crystals, we have observed a short-range lattice modulations in Hg1201. Careful analysis of the diffuse intensity pattern, and a study of the doping and temperature dependence, point toward the formation of local one-dimensional order in the form of uncorrelated oxygen chains in the charge-reservoir layer. The chains exist at intermediate and high doping, form along [100], and have typical lengths of 15-30 lattice constants [4]. \begin{enumerate} \item Z. Islam \textit{et al}. PRL \textbf{93} 157008 (2004) \item J. Strempfer \textit{et al}. PRL \textbf{93}, 157007 (2004) \item J. P. Castellan \textit{et al}. PRB \textbf{73}, 174505 (2006) \item G. Chabot-Couture, W. Tabis, J. N. Hancock Z. Islam, L. Lu, G. Yu, Y. Li, X. Zhao, Y. Ren, A. Mehta, M. Greven, (unpublished) \end{enumerate}   

Doping dependent intrinsic line width of the Cu-O bond-stretching phonon with q=(0.25 0 0) in $La_{2-x}Sr_xCuO_4$

We have recently found that the charge inhomogeneities provide significant broadening in the Cu-O bond stretching phonon of $La_{2-x}Sr_xCuO_4$, and the line shape of the phonon at zone boundary is well reproduced by the simple model which takes charge inhomogeneous effect into account [1]. The question is, now, how large intrinsic line width of the phonon at q=(0.25 0 0), where the giant phonon softening and broadening exist [2], is apart from the charge inhomogeneous effect on the line width. In this talk, we will show the doping dependence of the intrinsic line width of the phonon from x=0.05 to x=0.30. Interestingly, the intrinsic line width as a function of doping follows the superconducting transition temperature. We will discuss relationship between the phonon and the superconductivity in $La_{2-x}Sr_xCuO_4$. \\[4pt] [1] S. R. Park at al., accepted for publication in PRB (2011).\\[0pt] [2] D. Reznik et al., Nature {\bf 440}, 1170 (2006).   

Investigation of Fermi {\&} Luttinger surfaces in Ca$_{2-x}$Na$_{x}$CuO$_{2}$Cl$_{2}$ using ARPES

The electronic structure {\&} occupancy of doped cuprate superconductors Ca$_{2-x}$Na$_{x}$CuO$_{2}$Cl$_{2}$ of various doping levels is probed using angle-resolved photoemission spectroscopy (ARPES). The Fermi surface investigated shows that the Luttinger sum rule involving only the Fermi surface fails to account for the particle sum rule. Instead, the Luttinger sum rule that involves both the Fermi surface and the Luttinger surface seems necessary. We argue that such a generalized sum rule indicates the importance of very strong electron correlations.   

The electronic structure of tetragonal CuO

The cupric oxide CuO exhibits an insulating ground state with a correlation-induced charge-transfer gap and antiferromagnetism. It is, in principle, the most straightforward parent compound of the doped cuprates, and therefore has been theoretically studied as a model material for high temperature superconductivity. Bulk CuO crystallizes in a low-symmetry monoclinic form, in contrast to the rocksalt structure typical of late 3d transition metal monoxides. It was recently synthesized by epitaxial growth on SrTiO$_3$ substrates in a higher symmetry tetragonal structure with elongated c-axis (Siemons \emph{et} al. PRB 79, 2009). Extrapolating the behavior of other 3d transition metal monoxides, this phase of CuO is predicted to have a much higher Neel temperature than its bulk counterpart. At beamline 7 of the Advanced Light Source, we have grown tetragonal CuO thin films by pulsed laser deposition and investigated their electronic structure by angle-resolved photoelectron spectroscopy (ARPES). These measurements represent the first mapping of the band structure of this new material, not available in bulk phase, and will serve as a reference point for future doping experiments.   

Session P22: Focus Session: Fe-based Superconductivity - Fe(Te,Se)

Electronic disorder and magnetic-field-induced superconductivity enhancement in Fe$_{1+y}$(Te$_{1-x}$Se$_{x})$

The iron chalcogenide Fe$_{1+y}$(Te$_{1-x}$Se$_{x})$ superconductor system exhibits a unique electronic and magnetic phase diagram distinct from those seen in iron pnictides: bulk superconductivity does not appear immediately following the suppression of long-range ($\pi $,0) AFM order. Instead, an intermediate phase with weak charge carrier localization appears between AFM order and bulk superconductivity (Liu \textit{et al.}, Nat. Mater. \textbf{9}, 719 (2010)). In this talk, we report our recent studies on the relationship between the normal state and superconducting properties in Fe$_{1+y}$(Te$_{1-x}$Se$_{x})$. We show that the superconducting volume fraction $V_{SC}$ and normal state metallicity significantly increase while the normal state Sommerfeld coefficient \textit{$\gamma $} and Hall coefficient $R_{H}$ drop drastically with increasing Se content in the underdoped superconducting region. Additionally, $V_{SC}$ is surprisingly enhanced by magnetic field in heavily underdoped superconducting samples. The implications of these results will be discussed. Our analyses suggest that the suppression of superconductivity in the underdoped region is associated with electronic disorder caused by incoherent magnetic scattering arising from ($\pi $,0) magnetic fluctuations.   

The structure of the oxygen-annealed Fe$_{1.08}$Te$_{0.55}$Se$_{0.45}$O$_{x}$ superconductor

Effect of oxygen annealing on the iron chalcogenide superconductor with excess Fe is studied and structure change is investigated by using electron microscopy. The as-grown single crystal Fe$_{1.08}$Te$_{0.55}$Se$_{0.45}$ with the tetragonal PbO-type structure is non-superconducting owing to the excess Fe. Superconductivity is induced after oxygen annealing with an onset and zero resistance transition temperature around 14.5 K and 11.5 K, respectively. The oxygen doping is evidenced by electron energy loss spectroscopy, and accompanied by improved homogeneity in the remaining PbO-type phase as well as an increase in the L$_{3}$/L$_{2}$ intensity ratio of the Fe-L$_{2,3}$ edge, indicating an increase in Fe valence. Local phase transformation from the tetragonal PbO-type phase to the hexagonal NiAs-type phase is also observed after oxygen annealing.   

Unusual Persistence of Superconductivity Against High Magnetic Fields in the Strongly-Correlated Iron-Chalcogenide Film FeTe:O$_{x}$

We report an unusual persistence of superconductivity against high magnetic fields in the iron chalcogenide film FeTe:O$_{x}$ below 2.52 K. Instead of saturating like a mean-field behavior with a single order parameter, the measured low-temperature upper criticial field increases progressively, suggesting a large supply of superconducting states accessible via magnetic field or low-energy thermal fluctuations. We demonstrate that superconducting states of finite momenta can be realized within the conventional theory, despite its questionable applicability. Our findings reveal a fundamental characteristic of superconductivity and electronic structure in the iron-based superconductors.   

Growth conditions for pulsed laser deposited FeTeO$_{y}$ superconducting thin films and observation of a low temperature structural transition

FeTe thin films were grown by pulsed laser deposition and oxygen was incorporated to make superconducting FeTeO$_{y}$. The Te concentration of the films was highly dependent on growth temperature, due to the evaporation of Te at higher temperatures. Films grown using a porous, unreacted Fe/Te target showed islanding, an open structure, and a strong c axis texture. Oxygen could be easily added or removed by low temperature anneals. Films grown using a dense, polycrystalline conglomerate for a target and a short target-substrate distance had better epitaxy, and were dense, continuous and smooth. Post processing of oxygen on these films was difficult, but we were able to control oxygen concentration during growth using a small oxygen partial pressure. Initial low temperature diffraction studies of these films using both x-rays and neutrons indicate a sudden change in c axis lattice parameter at the superconducting T$_{C}$, but continued exposure of the beam drives out oxygen. Dense epitaxial films should allow for studying this phase change.   

Growth and oxygen doping of thin film FeTe by Molecular Beam Epitaxy

FeTe is isomorphic to FeSe, a representative of the 11 family of iron based superconductors. While not a superconductor itself, FeTe, particularly in thin film form, undergoes a superconducting transition when doped with oxygen. In this presentation, we will discuss the growth of FeTe by MBE and various schemes we used to dope the samples. Evidence from our investigation suggests that FeTe films are doped via an oxygen diffusion process which is strongly activated by temperature.   

Suppression of superconductivity in Fe chalcogenides by annealing: A reverse effect to pressure

Superconductivity in FeTe$_{1-x}$Se$_{x}$ can be controlled by annealing, in the absence of extrinsic influences. Using neutron diffraction, we show that T$_{C}$ sensitively depends on the atomic configurations of the Te and Se ions. Low temperature annealing not only homogenizes the Te and Se ion distribution as previously observed, it suppresses T$_{C}$ because of changes in the chalcogen ions' z-parameter. In particular, the height of Te from the Fe basal plane is much reduced while that for Se shows a modest increase. These trends are reverse of the effects induced by pressure.   

Phase diagram and oxygen annealing effect of FeTe1-xSex iron-based superconductor

Phase diagrams of as-grown and O$_{2}$-annealed FeTe$_{1-x}$Se$_{x}$ decided by magnetic susceptibility measurement were obtained. For as-grown samples, the antiferromagnetic order was fully suppressed in the region of $x \quad \ge $ 0.15 and superconductivity appeared at $x \quad \ge $ 0.1. However, bulk superconductivity emerged at only $x$ = 0.5. Interestingly, for O$_{2}$-annealed samples, complete suppression of the magnetic order and bulk superconductivity was observed at $x \quad \ge $ 0.1. We found that O$_{2}$ annealing induces the bulk superconductivity for FeTe$_{1-x}$Se$_{x}$. The O$_{2}$ probably play a key role of a suppression of the magnetic order and appearance of bulk superconductivity.   

Magnetism and superconductivity in Pd$_{1-x}$Fe$_{x}$Te

PdTe is a long-known superconductor but its physical properties are almost unknown. We have recently studied its basic physical properties in both normal and superconducting states. While FeTe forms different crystallographic structure and is known to form spin density wave below T$_{N}$ = 70 K, we have successfully synthesized Pd$_{1-x}$Fe$_{x}$Te with x from 0 to 1. By measuring its electrical and magnetic properties, we establish the phase diagram of Pd$_{1-x}$Fe$_{x}$Te for the first time. With increasing x, we found that T$_{c }$is quickly suppressed. Ferromagnetism appears for the samples with x $\ge $ 0.02. For 0.25 $\le $ x $\le $ 1.0, the system exhibits antiferromagnetic ordering with T$_{N}$ increasing with x. This is a prototype system for studying the interplay between superconductivity and magnetism.   

FeSe$_{0.5}$Te$_{0.5}$ thin films with critical current density above 1MA/cm$^{2}$

High quality FeSe$_{0.5}$Te$_{0.5}$ thin films have been prepared on various substrates, such as SrTiO$_{3}$, LaAlO$_{3}$ and YSZ, some with buffer layers. $T_{c}$'s as high as 20K with superconducting transition widths of about 1K were obtained. These $T_{c}$'s are much higher than those of bulk FeSe$_{0.5}$Te$_{0.5}$ ($\sim $15K). Our films carry high critical current densities $J_{c}$'s (above 1MA/cm$^{2})$ at liquid helium temperature. These films hold $J_{c}$'s above 1$\times$10$^{5}$A/cm$^{2}$ and very low $J_{c}$ anisotropies ($<$ 3) under magnetic fields as high as 30T at 4.2K. We have also prepared textured FeSe$_{0.5}$Te$_{0.5}$ thin films on a buffered metal template with results similar to the ones mentioned above. This shows that iron chalcogenides have a very promising future for high-field applications at liquid helium temperatures. Pinning force analysis indicates the presence of a point defect flux-pinning mechanism, suggesting a straightforward approach to conductor optimization.   

London penetration depth measurements of Fe$_{1+y}$(Te$_{1-x}$Se $_{x})$ single-crystals at ultra-low temperatures

The evolution of the superconducting properties as derived from the in-plane penetration depth measurements as a function of temperature in single crystals of Fe$_{1.02}$(Te$_{1-x}$Se $_{x})$, with Se concentration spanning from 25{\%} to 45{\%}, was studied using a tunnel diode oscillator technique in a dilution refrigerator down to a temperature of 30mK. By using a set of two mutually coupled planar inductors parallel to the ab plane of the samples, the probing ac field is uniform across the sample along the c axis making the variation in susceptibility solely due to in-plane currents while significantly increasing the signal to noise ratio compared to usual inductors used in similar experiments The evolution of the topology of the superconducting gap from underdoped to optimally doped samples, as derived from exponential and power law behavior of $\lambda _{ab}$ at low temperatures, is presented.   

Electron-phonon coupling in layered FeSe compounds

Iron-chalcogenide superconductors, showing many characteristic physical properties, can serve as a model materials to study the electron-pairing mechanism for all iron-based superconductivity. Layered iron-chalcogenide systems including single layer FeSe, bulk FeSe, K-intercalated FeSe, were studied using first principle pseudopotential density functional based approach. Electronic structure, vibrational properties and electron-phonon coupling strength were studied for the cases with and without iron magnetic moment ordering. The latter is incorporated using local spin density approximation. Our results show significant changes to electronic structure resulting in much higher electron-phonon coupling for spin-resolved configurations. Electron-phonon matrix elements for particular phonon mode of A1g symmetry are showing dramatic increase. Superconducting transition temperature estimates based on McMillan's equation are showing values significantly higher then previously reported, but still not high enough to account for the experimental results.   

Magnetism and superconductivity in modulated FeTe systems.

We examined the interplay between magnetism and superconductivity by monitoring the non-superconducting chalcogenide FeTe. We studied its transitions under insertion of oxygen, iron and vacancies of iron using spin-polarized band structure methods (LSDA with GGA) starting from the collinear and bicollinear magnetic arrangements. A supercell with 8-Fe and 8-Te atoms was used so that it can capture local changes in magnetic moments. The calculated values of magnetic moments agree well with available experimental data while some of the modulations lead to significant changes in the bicollinear or collinear magnetic moments/arrangements. The total energies of these systems indicate that the collinear-derived structure is more favorable prior to a possible superconducting transition.   

Revealing the dual nature of magnetism in iron pnictides and iron chalcogenides using x-ray emission spectroscopy

We present a Fe K$\beta$ x-ray emission spectroscopy study of local magnetic moments in various iron-based superconductors in their paramagnetic phases. Our findings show that a local magnetic moments exists in all samples studied: PrFeAsO, $\rm Ba(Fe,Co)_2As_2$, LiFeAs, Fe$_{1+x}$(Te,Se), and $\rm A_2Fe_4Se_5$ (where A = K, Rb, and Cs). The moment size is independent of temperature or carrier concentration, but varies significantly across different families. Specifically, all iron pnictides samples have local moments of about 1$\mu_B$/Fe, while FeTe and $\rm K_2Fe_4Se_5$ families have much larger local moments of $\sim\!\!2 \mu_B$/Fe and $\sim\!\! 3.3 \mu_B$/Fe, respectively. Our results illustrate the importance of multi-orbital physics in describing magnetism of these compounds.   

Electronic Structure of A$_y$Fe$_{2-x}$Se2 from First Principles

The newly discovered A$_y$Fe$_{2-x}$Se2 iron-selenide material family was studied in a series of first-principles simulations. The electronic structure and magnetic properties of possible vacancy-superstructure phases A$_2$Fe$_3$Se$_4$ and A$_2$Fe$_4$Se$_5$, as well as the possible parental phase of superconductivity AFe2Se2 were examined. It was discovered that AFe2Se2 ground state is a SDW-AFM metal without hole Fermi-surface, thus FS-nesting is absent and its magnetism is very likely to be local moment; A$_2$Fe$_3$Se$_4$ is a Mott insulator with SDW-AFM magnetism; A$_2$Fe$_4$Se$_5$ shows block-spin AFM ground state with 400$\sim$600 meV band gap. Under high pressure, the A$_2$Fe$_4$Se$_5$ phase exhibits rich and exotic physical properties. \\[4pt] [1] Chao Cao and Jianhui Dai, Block Spin Ground State and Three-Dimensionality of (K,Tl)$_y$Fe$_{1.6}$Se$_2$, \textit{Phys. Rev. Lett.} \textbf{107}, 056401 (2011)\\[0pt] [2] Chao Cao and Jianhui Dai, Electronic structure and Mott localization of iron-deficient TlFe$_{1.5}$Se$_2$ with superstructures, \textit{Phys. Rev. B} \textbf{83}, 193104 (2011)\\[0pt] [3] Chao Cao and Jianhui Dai, Electronic Structure of KFe$_2$Se$_2$ from First-Principles Calculations, \textit{Chin. Phys. Lett.} \textbf{28}, 057402 (2011)   

Session P23: Superconductivity Theory II: p-wave, chiral, and topological

Spin-orbit induced mixed-parity pairing in Sr$_2$RuO$_4$: a self-consistent quantum many-body analysis

The unusual superconducting state in Sr$_2$RuO$_4$ has long been viewed as being analogous to a superfluid state in liquid $^3$He. Nevertheless, calculations based on a pure odd-parity state are presently unable to completely reconcile the properties of Sr$_2$RuO$_4$. Using a self-consistent quantum many-body scheme that employs realistic parameters, we are able to model several signature properties of the normal and superconducting states of Sr$_2$RuO$_4$ such as the weak temperature dependence of the spin susceptibility below $T_c$. However, we find that the dominant component of the model superconducting state is of even parity and closely related to superconducting state for the high-$T_c$ cuprates although a smaller odd-parity component is induced by spin-orbit coupling. This mixed parity pairing state provides an alternative scenario for understanding the complex phenomena measured in Sr$_2$RuO$_4$.   

Spin and charge collective excitations in the multiband superconductivity of Sr2RuO4

Multiband superconductors with weak interband pair scattering can support soft collective modes. In the case of spin-singlet superconductivity, interband pair scattering leads to fluctuations of the relative phase of the order parameter on the different bands. However, when the superconductivity is spin-triplet, the interband pair scattering gives rise to fluctuations of both the relative phase and the relative spin on the different bands. One possible example of a multiband triplet superconductor is a recently proposed model [1] of the superconductivity in Sr$_2$RuO$_4$ in which the pairing occurs primarily on the two quasi-1D bands. We show that the collective excitations arising from relative spin fluctuations can lead to a double resonance peak in the presence of an oscillating magnetic field. We discuss how the presence or absence of such collective modes can yield clear information concerning the precise microscopic structure of the order parameter. \\ \\ $[1]$ S. Raghu, A. Kapitulnik, and S. Kivelson, Phys. Rev. Lett. {\bf 105}, 136401 (2010)   

Dynamics of domain walls in chiral p-wave superconductors

Two-fold degeneracy of the ground state of chiral $p$-wave superconductors or superfluids with $k_x\pm ik_y$ order parameter makes it possible for domain walls separating regions of opposite chirality to exist. In addition to affecting the scattering spectrum in the bulk, the domain walls also carry localized low-energy fermionic quasiparticles, whose energy essentially depends on the Josephson phase difference between the domains. Dynamical properties of the domain walls are determined by the transitions between different quasiparticle states induced by the domain wall motion. We present a microscopic calculation of the viscous friction coefficient and the effective mass of the domain walls.   

Stability of topological defects in chiral superconductors: London theory

We examine thermodynamic stability of chiral domain walls and vortices -- topological defects which can exist in chiral superconductors. Using London theory it is demonstrated that at sufficiently small applied and chiral fields the existence of domain walls and vortices in the sample is not favored and the sample's configuration is a single domain. The particular chirality of the single-domain configuration is neither favored nor disfavored by the applied field. Increasing the field leads to an entry of a domain wall loop or a vortex into the sample. Formation of a straight domain wall is never preferred in equilibrium. Values of the entry (critical) fields for both types of defects, as well as the equilibrium size of the domain wall loop, are calculated. The applicability of these results to $\rm Sr_2RuO_4$ -- a tentative chiral superconductor -- is discussed.   

Intrinsic Hall effect in a multiband chiral superconductor

We identify an intrinsic, dissipationless Hall effect in multiband chiral superconductors in the absence of a magnetic field (i.e., an \emph{anomalous} quantum Hall effect). Similarly to its analog in ferromagnets, this effect arises from inter-band transitions when time-reversal symmetry is spontaneously broken. We discuss the implications of this effect for the putative chiral $p$-wave superconductor, $\mathrm{Sr}_2\mathrm{RuO}_4$, and show that it can be of the right order of magnitude to contribute significantly to the polar Kerr rotation observed in experiments, depending on the structure of the order parameter across the bands.   

Topologically protected surface Majorana arcs and bulk Weyl fermions in ferromagnetic superconductors

A number of ferromagnetic superconductors have been recently discovered which are believed to be in the so-called ``equal spin pairing'' (ESP) state. In the ESP state the Cooper pairs condense forming order parameters $\Delta_{\uparrow,\uparrow}, \Delta_{\downarrow\downarrow}$, which are decoupled in the spin-sector. We show that these 3D systems generically support topologically protected surface Majorana arcs and bulk Weyl fermions as gapless excitations. Similar protected low-energy exotic quasiparticles should also appear in the recently discovered non-centrosymmteric superconductors in the presence of a Zeeman field. The protected surface arcs can be probed by angle-resolved photoemission (ARPES) as well as fourier transform scanning tunneling spectroscopty (FT-STS) experiments.   

Thermodynamic signatures of half-quantum vortices in $p+ip$ Josephson junction arrays

A very interesting type of excitation in a chiral p-wave superconductor is a half-quantum vortex. As the name suggests, they carry half of a superconducting flux quantum, and are only possible in superconductors with spin-triplet pairing. An astonishing feature of these excitations is the presence of topologically protected Majorana zero modes. Single half-quantum vortices were recently discovered (J. Jang et al, Science \textbf{331}(6014): 186-188) in superconducting mesoscopic rings made of Sr$_2$RuO$_4$, yet to this date they have not been observed in macroscopic samples. We propose a method for detecting half-quantum vortices in Josephson junction arrays, which could host a large number of these vortices. Contrary to a 3D setting, we argue that half-quantum vortices can be energetically preferable in quasi-2D chiral spin-triplet superconductors. As a result, half-quantum vortices rather than full vortices could drive a Berezinskii-Kosterlitz-Thouless transition (which manifests itself as a resistive transition). We propose to look for their signatures by comparing transition temperatures in $p+ip$ Josephson junction arrays in a transverse magnetic field in both unfrustrated and frustrated cases.   

Scattering theory of chiral Majorana fermion interferometry

Using scattering theory, we investigate interferometers composed of chiral Majorana fermion modes coupled to normal metal leads. We advance an approach in which also the basis states in the normal leads are written in terms of Majorana modes. As a consequence the scattering at the junctions of the lead to the Majorana mode couples modes effectively only pair-wise irrespective of the number of normal scattering modes. We demonstrate that the charge current can also be expressed in terms of interference between pairs of Majorana modes. These two basic facts permit a treatment and understanding of current and noise signatures of chiral Majorana fermion interferometry in an especially elegant way. As a particular example of applications, in Fabry-Perot-type interferometers where chiral Majorana modes form loops, resonances (anti-resonances) upon such loops always lead to peaked (suppressed) Andreev differential conductances and negative (positive) cross-correlations originate purely from two-Majorana-fermion exchange. These investigations are intimately related to current and noise signatures of Majorana bound states.   

Probing Majorana fermions in one-dimensional topological superconductors using non-equilibrium transport: an open-quantum-system study

We study one-dimensional topological superconductors using an open-quantum-system approach based on Langevin equations. We go beyond a low-energy effective model of Majorana fermions, to derive different non-equilibrium transport properties exactly in these systems. The role of the coupling between the superconducting wire and metallic leads in physical experiments to detect Majorana bound states is also examined.   

Majorana End States of Au Wires in Proximity to $d_{x^2-y^2}$-wave Superconductors

We propose a one dimensional DIII class Hamiltonian which respects time-reversal symmetry and supports zero-energy double Majorana end states. Single Majorana end states can appear if time-reversal symmetry is broken. Majorana fermions survive in the quasi-1D regime when multiple transverse sub-bands of the wire are occupied. More importantly, this model can be realized by inducing $d_{x^2-y^2}$-wave superconductivity on a quantum wire with strong spin-orbit coupling. We suggest Au wires deposited on doped LSCO realize this topological superconducting phase. The energy scales of this set-up, an induced proximity gap of 10meV and the Rashba energy of 60meV, are two orders of magnitude larger than the corresponding energy scales in semiconductor-based proposals.   

Topological Josephson effect for arbitrary values of the tunnel coupling in the Kitaev model

We investigate the Josephson effect for a setup with two lattice quantum wires featuring fused Majorana boundary modes at the tunnel junction. We show exactly that additional degeneracies occur when the size of the Josephson coupling attains a certain critical value, thus introducing additional energy level crossings. The physical consequences of these additional level crossings are discussed. It is shown that for this critical coupling the Andreev levels can be cast in the form $E_{m\sigma}=2\sigma\sqrt{2}w\cos(\phi/6-\pi m/3)$, where $m=-1,0,1$ and $\sigma=\pm 1$. The exact Josephson current exhibits the characteristic $4\pi$ periodicity along with additional features related to the extra crossings of Andreev levels at the critical value of the tunnel coupling.   

Single-particle off-diagonal long range order in $|Pf|$ wave function for topological superconductors

We ask what can be glimpsed about a fermion system from a boson wave function obtained as the absolute value of the fermion wave function in the particle number basis. We consider examples of spinless $p$-wave superconductors in one and two dimensions and find that thus constructed wave function shows single-boson off-diagonal long range order when the fermion system is in the weak-coupling topological phase, but only pair-boson order in the strong-coupling non-topological phase. We discuss implications when such fermion states are used as factors in trial wave functions e.g. from slave-fermion construction.   

Topological Superconductivity in Spin-Orbit Coupled Bands

Topological superconductors have nontrivial winding of their order parameter phase and are expected to support Majorana Fermions in their vortex cores. For this reason they have been sought after in the past couple of decades. Over the past few years, a new and promising route for realizing topological superconductors has opened due to recent advances in the field of topological insulators. The current proposals are based on semiconductor heterostructures. In the proposed devices, spin-orbit coupled bands are Zeeman split by a magnetic field and superconductivity is induced by proximity to a conventional superconductor. This leads to heterostructures of two or three layers. This talk will focus on the prospect of realizing a topological superconductor in materials with inherent spin-orbital coupling and an intrinsic tendency for superconductivity. The proposed device will allow simplification of recently suggested devices as the need for a superconducting layer will be eliminated.   

Theory of Superconductivity in Mesoscopic Systems

By using Bogoliubov-de Gennes (BdG) equations, we study superconducting (SC) states in a quasi 2-dimensional system of radius $R$. It is shown that no vortices exist in $s$-wave SC samples with $R < R_c\sim\xi(0)$, the $T=0$ coherence length. We predict that chiral $p$-wave states exhibit superconductivity for $R < R_c$ only in the presence of a vortex with opposite chirality. This {\it reentrant} SC phase is a consequence of non-zero chirality of the pairing order parameter and implies the presence of chiral edge currents. Our study may be applied to sharply probing the pairing symmetry of unconventional superconductors.   

Session P24: Fractional Quantum Hall Effect I

Exploring the possibility of Universal Edge Physics in the Fractional Quantum Hall States

The edge of a fractional quantum Hall (FQH) droplet is described by the chiral Luttinger liquid theory which predicts a universal power-law behavior in the current-voltage ($I$-$V$) characteristics when electrons tunneling into the FQH edge through a barrier, e.g., from a three-dimensional Fermi liquid. However, this university has not been unambiguously observed in transport experiments in two-dimensional electron gases based on GaAs/GaAlAs heterostructures or quantum wells. One plausible cause is reconstruction of the fractional quantum Hall edge, which introduces non-chiral edge modes. The coupling between counterpropagating edge modes can modify the exponent of the $I$-$V$ characteristics. By comparing the fractional quantum Hall states at the filling factor $\nu=1/3$ in modulation-doped semiconductor devices and in graphene devices, we show that the GaAs-based FQH experiments are always in the edge reconstruction regime, whereas graphene-based systems have an experimental accessible parameter region where edge reconstruction can be avoided. This regime offers the possibility of the exploration of the universal edge tunneling exponent predicted by the chiral Luttinger liquid theory.   

Exploration of the Limits to Mobility in Two-Dimensional Hole Systems in C-Doped (001) GaAs/AlGaAs Quantum Wells

We report on the growth of a series of high mobility two-dimensional hole systems (2DHSs) in 20 nm (001) oriented GaAs/AlGaAs quantum wells and the analysis of possible scattering mechanisms. The hole density was controlled by changing the delta-doping setback and Al mole fraction and was measured at low temperature (T = 300 mK) after illumination with a red LED. We varied the density over a range from 2.0 x 10$^{10}$ cm$^{-2}$ to 1.9 x 10$^{11}$ cm$^{-2}$, and the mobility was observed to peak at an intermediate density of 6.5 x 10$^{10}$ cm$^{-2}$ where we report a new record T = 300 mK mobility of 2.3 x 10$^{6}$ cm$^{2}$/Vs . We find that even when the density dependent effective mass is taken into account, remote and background impurity scattering cannot qualitatively explain the behavior of the mobility, in contrast with comparable 2DEGs. We discuss possible mechanisms leading to the observed non-monotonic density dependence of the mobility and the factors leading to our new record mobility.   

Effective Field Theory of Fractional Quantized Hall Nematics

We present a Landau-Ginzburg theory for a fractional quantized Hall nematic state and the transition to it from an isotropic fractional quantum Hall state. This justifies Lifshitz-Chern-Simons theory -- which is shown to be its dual -- on a more microscopic basis and enables us to compute a ground state wave function in the symmetry-broken phase. In such a state of matter, the Hall resistance remains quantized while the longitudinal DC resistivity due to thermally-excited quasiparticles is anisotropic. We interpret recent experiments by Xia et al. (cond-mat/1109.3219) at Landau level filling factor $\nu =7/3$ in terms of our theory.   

Dipolar Bogolons: From Superfluids to Pfaffians

We study neutral fermionic `Bogolons' which are quasiparticle excitations of gapped phases that arise due to fermion (BCS) pairing, such as superfluids, superconductors, and paired quantum Hall states. As we demonstrate, a na\"{i}ve construction of a quasiparticle wavepacket by solving the mean-field BCS equations leads to a contradiction: there is a net electrical current {\it even when the group velocity vanishes}. Resolution of this paradox requires the computation of supercurrents in the wavepacket state, typically a complicated exercise in self-consistency. In this Letter we demonstrate that these corrections may be approximately calculated from correlations in the mean-field ground state, and lead to a divergence-free, dipolar current pattern associated with the quasiparticle. When Maxwell electrodynamics is included, as appropriate to a superconductor, this pattern is confined over a penetration depth. For paired quantum Hall states of composite fermions, the Maxwell term is replaced by a Chern-Simons term, which leads to a dipolar {\it charge} distribution, paralleling Read's observation that composite fermions are neutral dipoles.   

Coulomb Oscillations in Antidots in the Integer and Fractional Quantum Hall Regimes

We present measurements of Coulomb oscillations as a function of both top gate and magnetic field in gate-defined, micron-scale antidots in the integer and fractional quantum Hall regimes. We find resistance oscillations at filling factors $\nu=2,\nu=1,\nu=2/3,$ and $\nu=1/3$. At $\nu=1$, we find the tunneling charge to be $e$ and the presence of one edge. At $\nu=2$, we also find the tunneling charge to be $e$ and the presence of two edges. A generalized picture of Coulomb oscillations in the fractional quantum Hall regime suggests the presence of one charged edge at both $\nu=1/3$ and $\nu=2/3$. We find the tunneling charge at $\nu=1/3$ to be $e/3$ but unexpectedly find the tunneling charge at $\nu=2/3$ to be $(2/3)e$.   

Lande's g values and the quantum Hall effect

It is reported that the Lande's formula for the g values of the electron as a function of L, S and J does not have the particle-hole symmetry needed for the understanding of high magnetic field data of quantum Hall effect. Hence it is modified to yield the particle-hole symmetry by means of the two signs before S in J= L$\pm $S. The correct g value is then given by g=(2J+1)/(2L+1). Since the Bohr magneton involves the charge of the particles, the corrected g value formula explains the data of quantum Hall effect. The value L/2L+1 gives 1/3 and ( L+1)/(2L+1) gives 2/3. The consideration of Landau levels gives many values in agreement with the data. In electron clusters the spin need not be $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $. Hence a lot of data is explained by means of spin greater than $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $. Some of the clusters show the formation of spin waves so that there is a finite spin deviation which is characteristic of electron lattices. It is found that Laughlin's wave function is the ground state of $\nabla ^2\delta (r_i -r_j )$ type Hamiltonian which is not equivalent to Coulomb's Hamiltonian. K. N. Shrivastava, Intl. J. Mod. Phys. B 25, 1301-1357(2011).   

Numerical Study of Realistic Models of the $\nu=5/2$, 7/3, 8/3 Hamiltonians: Effects of Landau-level mixing and Finite Well-width

We construct a realistic effective Hamiltonian for electrons in the first excited Landau level, taking into account the effects of both Landau-level mixing and the finite width of the GaAs quantum well. The latter includes both short-distance softening of the Coulomb interaction as well as sub-band mixing. Through exact diagonalization, we find a rich phase diagram as a function of the Landau level mixing parameter $\kappa$ and quantum well width $d$. In particular, small changes in either parameter can drive phase transitions between states in the universality classes of the Moore-Read Pfaffian, anti-Pfaffian, and exotic compressible metallic states.   

Thermopower and the Fractional Quantized Hall Effect in the N=1 Landau Level

Having recently eliminated an issue involving long thermal time constants [1], we are now able to resolve diffusion thermopower deep into the fractional quantized Hall effect (FQHE) regime. In this talk we report measurements of thermopower in the first excited (N=1) Landau level as a continuous function of magnetic field down to temperatures as low as 30mK. Above 50mK we can clearly resolve the $\nu$ = 5/2 as well as $\nu$ = 7/3, 8/3, and 14/5 FQHEs in both the electrical and thermoelectrical transport. Below 50mK a prominent feature of the electrical transport in the first excited Landau level is the Re-entrant Integer Quantized Hall Effect (RIQHE) which is associated with insulating collective phases [2]. In this temperature regime the thermopower exhibits a series of intriguing sign reversals that are as yet not fully understood. We will conclude with a brief discussion of the connection between thermopower and the entropy of the 2D electron system. This connection is invoked by a recent prediction [3] of the thermopower at $\nu$ = 5/2, which assumes the ground state is the non-Abelian Moore-Read paired composite fermion state.\\[4pt] [1] Chickering, Phys. Rev. B 81, 245319 (2010)\\[0pt] [2] Eisenstein, Phys. Rev. Lett. 88, 076801 (2002)\\[0pt] [3] Yang, Phys. Rev. B 79, 115317 (2009)   

Tilt Magnetic Field Dependence of the 12/5 Fractional Quantum Hall State

The 12/5 state has attracted growing interest due to its superior potential in performing universal topological quantum computation. Up to date, except for the observation of a well developed quantum Hall plateau at this filling, much less experimental work has been carried out and there is no experimental evidence to support this state being a paraferminoic or non-Abelian state. Here, we present our tilt magnetic field dependence results in examining its spin-polarization. It was observed that the diagonal resistance R$_{xx}$ at $\nu $=12/5 shows a non-monotonic dependence on tilt angle ($\theta )$. It first increases sharply with increasing $\theta $, reaches a maximal value of $\sim $ 60 $\Omega $ around $\theta \sim $14$^{o}$, and then decreases with $\theta $ further increased. Correlated with this R$_{xx}$ dependence, the 12/5 activation energy gap ($\Delta _{12/5})$ also shows a non-monotonic $\theta $ dependence. $\Delta _{12/5 }$first decreases. Around 14$^{o}$, R$_{xx}$ becomes non-activated and a true activation energy gap is not obtainable. With further increasing $\theta $, R$_{xx}$ becomes activated again and $\Delta _{12/5 }$increases with $\theta $. This tilt dependence in R$_{xx}$ and $\Delta _{12/5 }$is similar to the composite fermion states at $\nu $=2/5 and 8/5 in the lowest Landau level, which was interpreted as a spin transition. Our results thus call for more investigations on the nature of the 12/5 ground state.   

The temperature-dependences of the dissipative conductance in quantum Hall states

We discuss how to estimate the saddle point gap of quantized Hall states from the temperature dependence of the longitudinal response, $\log \sigma_{xx}$. The dissipative response is assumed to be the result of thermally activated quantum tunneling through saddle points in the long-range impurity potential set up by the ionized donors (see Phys. Rev. Lett. 106, 126804 (2011)). We apply the method to published data on states at $\nu=5/2$, as well data at other integer and fractional filling fractions, and compare the gap estimates and the typical parameters of the saddle points with expectation from microscopic calculations. Even in the case of very weak quantum Hall states, we find that the analysis suggests saddle point gaps which are consistently around 50\% of the gap predicted for the homogeneous system.   

Fractionalization in spontaneous integer quantum Hall systems

We show that electron fractionalization can occur in quantum Hall liquids even in the absence of strong correlations. Focusing on a Kondo lattice model that exhibits spontaneous integer Hall effect due to non-coplanar magnetic ordering, we find that $Z_2$ vortices in the magnetic order parameter can bind fractional quantum numbers. The vortices have anyonic exchange statistics.   

Exact soliton solutions in a many-body system in non-trivial background

Soliton solutions are usually described as lumps of density propagating without changing their shape. They usually occur in integrable systems. We consider the classical Calogero model in an external harmonic potential. Due to the presence of the external potential, momentum in the system is not conserved and the aforementioned description of solitons is inapplicable, nevertheless the system remains integrable. Naturally, the question about the existence and the form of soliton solutions arises. I will explain what a soliton solution of this model is and I will show how to find these solutions in the case of finite number of particles and in the hydrodynamic limit. In the latter limit the model is described by hydrodynamic equations on continuous density and velocity fields. Soliton solutions in this case are finite dimensional reductions of the hydrodynamic model and describe the propagation of lumps of density and velocity in the nontrivial background. These solutions of Calogero model were previously qualitatively described by A. Polychronakos in the context of the quantum Hall effect.   

On the hydrodynamic description of FQHE fluid

A simple classical two-dimensional hydrodynamic model for FQHE fluid has been constructed. Hydrodynamics of this model is Hamiltonian, nonlinear and has many features of fractional quantum Hall states. The effective classical fluid is incompressible and has a correct static structure factor. The model incorporates FQHE relation between the density and the vorticity of the fluid, gives the correct value for the Hall viscosity and for the Hall conductivity at finite wavevectors. The model can serve as a starting point in deriving the boundary theory of FQHE states beyond chiral Luttinger liquid. Indeed, in the leading approximation the model is reduced to chiral waves with nonlinearities defined by a curvature of the confining potential.   

Anomalous resistance across a quantum point contact in the integer and fraction quantum Hall regimes

A $\sim $600 nm quantum point contact (QPC) is studied in various integer and fractional quantum Hall states. An anomalous increase of the differential resistance across the QPC from the expected quantized value is observed at nonzero DC bias. The onset of this increased differential resistance occurs sharply at DC bias values which vary continuously with magnetic field and gate voltage. Dependence of the increased resistance on filling factor, magnetic field, gate voltage, and temperature is presented. Of particular interest is the observation that the onset DC bias shows opposite gate voltage dependence for integer and fractional quantum Hall states.   

Improved Measurements of Quasi-Particle Tunneling in the nu = 5/2 Fractional Quantum Hall State

It is predicted that the nu = 5/2 fractional quantum Hall state may potentially exhibit novel non-abelian quasi-particle statistics, which would make it a candidate for implementation of topological quantum computation. We present measurements of quasi-particle tunneling between edge channels, which provide information about the wave function of the nu = 5/2 state. Weak tunneling is investigated as a function of temperature and DC bias and fit to the theoretical tunneling conductance. We improve on previous quasi-particle tunneling measurements by reducing measurement noise and studying two different quantum point contact (QPC) geometries. For both QPCs the best fits give e*, the quasi-particle effective charge, close to the expected value of e/4 and g , the strength of the interaction between quasi-particles, close to 3/8. Here we show that fits corresponding to the various proposed wave functions, along with qualitative features of the data, strongly favor the abelian 331 state.   

Session P25: Focus Session: Simulation of Matter at Extreme Conditions - Static Pressure

A new metastable phase of silicon in the \textit{Ibam} structure

In a study aimed at finding new useful forms of silicon, we use an \textit{ab initio} random structure searching (AIRSS) method to identify a new phase of silicon in the \textit{Ibam} structure. The \textit{Ibam} phase is found to be semimetallic within density functional theory with a small band overlap, and it is expected that quasiparticle corrections using the GW approximation would yield a semiconducting state with a small band gap. Calculation of the lattice dynamics reveals that the structure is locally stable. Enthalpy-pressure relations are calculated for the \textit{Ibam} structure as well as all other known Si structures, including the previously predicted phases st12 and bct. These results indicate that \textit{Ibam} silicon is metastable over the pressure range considered. Calculated coexistence pressures of the other known phase transitions are in good agreement with experimental observation.   

Ultra-incompressible Three Dimensional Long-range Ordered Amorphous Carbon Clusters

Here we report the synthesis of a long range ordered material constructed from units of amorphous carbon clusters and solvent molecular. This material has super-incompressibility which can make indents on diamonds. It was synthesized by crushing the fullerenes cages at high pressure. Using high pressure x-ray diffraction and Raman spectroscopy, we observed that the fullerenes cages collapse as the pressure higher but the sample remains in crystalline phase even at 60 GPa. The high pressure phase is ultraincompressible, quenchable and much denser than the starting material. The discovery of the existence of such a unique phase should lead to a great deal of interest for design and synthesis of materials with this characteristic.   

Anharmonicity and bonding electrons in silicon under high pressures

Electron density distributions have been measured for silicon at high pressures by single crystal diffraction using a diamond anvil cell. An abrupt change in charge density distribution is observed at 10.1 GPa, a pressure close to a phase transition from diamond structure to beta-tin structure at 12.5 GPa. Our results show a strong anharmonicity effect in silicon in a pressure range of 2.5 GPa before the phase transition to beta-tin.   

Formation of superconducting platinum hydride under pressure: an ab initio approach

Noble metals such as Pt, Au, or Re are commonly used for electrodes and gaskets in diamond anvil cells for high-pressure research because they are expected to rarely undergo structural transformation and possess simple equation of states. Specifically Pt has been used widely for high-pressure experiments and has been considered to resist hydride formation under pressure. Pressure-induced reactions of metals with hydrogen are in fact quite likely because hydrogen atoms can occupy interstitial positions in the metal lattice, which can lead to unexpected effects in experiments. In our study, PRL 107 117002 (2011), we investigated crystal structures using {\it ab initio} random structure searching (AIRSS) and predicted the formation of platinum mono-hydride above 22 GPa and superconductivity T$_{c}$ was estimated to be 10 -- 25 K above around 80 GPa. Furthermore, we showed that the formation of fcc noble metal hydrides under pressure is common and examined the possibility of superconductivity in these materials.   

New ultrahigh pressure phases of H2O ice predicted using an adaptive genetic algorithm

We propose three new phases of H$_{2}$O under ultrahigh pressure. Our structural search was performed using an adaptive genetic algorithm which allows an extensive exploration of crystal structure at density functional theory(DFT) accuracy. The new sequence of pressure-induced transitions beyond ice X at 0 K should be ice X $\to$ Pbcm $\to$ Pbca $\to$ Pmc2$_{1}$ $\to$ P2$_{1} \to$ P2$_{1}$/c phases. Across the Pmc2$_{1}$-P2$_{1}$ transition, the coordination number of oxygen increases from 4 to 5 with a significant increase of density. All stable crystalline phases have nonmetallic band structures up to 7 TPa.   

Quantum Monte Carlo applied to Solids under Pressure

Diffusion quantum Monte Carlo (DMC) has been applied to solids under pressure in several different contexts a high degree of success.\footnote{J. Kolorenc and L. Mitas. Rep. Prog. Phys. 74 026502 (2011)} All of these calculations must address three errors present in DMC calculations of solids: the fixed node approximation, the pseudopotential approximation and the finite size approximation. Due to the varying approximations to address these errors, these calculations suffer from an uncertainty that is almost comparable to that introduced by the choice of functional in density functional theory (DFT). In this presentation, we present lattice constants and bulk moduli of more than fifteen solids under compression performed with a consistent approach to these three approximations. These results help establish the general accuracy that may be expected from DMC calculations of solids under pressure and also provide a reference from which improvements to DMC methods may be judged. \\[4pt] 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. Department of Energy's National Nuclear Security Administration under Contract No. DE-AC04-94AL85000.   

First-principles simulations on bonding pathways of chemical transformations under hydrostatic compression

High pressure as a thermodynamic parameter provides a strong structural constraint to lead chemical transformations with selective ways. Thus, chemical transformations under pressure can create novel materials which may not be accessible by covalent synthesis. However, bonding evolution toward high pressure chemical transformations can be a complex process and may happen over widely different pressures. To understand bonding evolution pathways of high pressure chemical transformations, first-principles simulations were performed following hydrostatic compression enthalpy minimization paths to obtain experimentally and theoretically established phase transitions of carbon. The results showed that the chemical transformations from hydrostatic compression carbon to single-bonded phases were characterized by a sudden decrease in principal stress components, indicating the onset of chemical transformation. On this basis, a number of hydrostatic compression chemical transformations from molecular precursors to novel materials were predicted, such as hydrocarbon graphane, a hydrogenated carbon nitride sheet, and carbon nitrides. All predicted hydrostatic compression transformations are featured as a sudden change in principal stress components, representing chemical bonding destruction and formation reactions with a cell volume collapse.   

Dielectric constant of water under deep Earth pressures and temperature conditions

The knowledge of the dielectric constant of water as a function of pressure (P) and temperature (T) plays a critical role in understanding the chemistry of aqueous systems, and in particular of fluids in the Earth mantle, where water is stored in hydrous minerals. By using first-principles molecular dynamics, we have computed the dielectric constant of water at T = 1000 K, between 1 and 10 GPa, under conditions of the Earth upper mantle. We present a detailed comparison of our results with available experimental data and empirical models, and we discuss how the liquid dielectric constant is affected by the changes in the hydrogen-bond network and molecular dipole moment observed upon compression.   

Hydrogen-Helium Mixtures at High Pressures

We extend our previous work on hydrogen-helium mixtures (Morales, M. A., et. al. PNAS 106, 1324 (2009).) to lower pressures and lower temperatures, across the molecular dissociation regime in hydrogen, to the low pressure molecular liquid. Using density functional theory-based molecular dynamics together with thermodynamic integration techniques, we calculate the Gibbs free energy of the dense liquid as a function of pressure, temperature, and composition. Our work focuses on the mixing properties of the liquid, the optical properties including conductivity and reflectivity, and the creation of accurate mixing models for thermodynamic properties, including pressure and entropy. The resulting models will provide the basis for accurate first-principles equations of state for planetary modeling. Prepared by LLNL under Contract DE-AC52-07NA27344.   

Pressure makes mercury a transition metal: a first-principles study of HgF4 solid phases

Mercury is considered as a post-transition metal, because its d shell is filled and does not involve in forming chemical bonds. Yet, because the large relativistic effect pushes up the outmost d level, there is a high expectation that Hg can be stabilized in a higher oxidation state. The HgF4 molecule has been predicted by calculations, and an evidence of such molecule is shown by IR absorption recently. However, there is neither computation nor experiment report on possible high oxidation state of Hg in solid. By using first-principles density functional theory and a structure-searching method, we studied the structural change of a solid system of Hg and F under pressures from 0 to 300 GPa. We found that at lower pressure, the stable structure consists of HgF2 and F2 molecules. At about 25 GPa, the system undergoes a structural change and forms HgF4 planar molecules featuring d8 configuration. The calculations show that the d orbitals of Hg involve in chemical bonding, which is the signature of a transition metal.   

Ab-initio parametrization of a fully polarizable and dissociable force field for water

A novel all-atom, dissociative, and polarizable force field for water is presented. The force field is parameterized based on forces, stresses and energies obtained form ab-initio calculations of liquid water at ambient conditions. The accuracy of the force field is tested by calculating structural and dynamical properties of liquid water and the energetics of small water clusters. The transferability of the force field to dissociated states is studied by considering the solvation a proton and the ionization of water at extreme conditions of pressure and temperature. In the case of the solvated proton the force field properly describes the presence of both Eigen and Zundel configurations. In the case of the pressure-induced ice VIII / ice X transition and the temperature-induced transition to a superionic phase, the force field is found to describe accurately the proton symmetrization and the melting of the proton sublattice,   

Spin states and hyperfine interactions of iron incorporated in MgSiO$_3$ post-perovskite

Using density functional theory + Hubbard $U$ (DFT+$U$) calculations, we investigate the spin states and nuclear hyperfine interactions of iron incorporated in magnesium silicate (MgSiO$_3$) post-perovskite (Ppv), a major mineral phase in the Earth's D'' layer, where the pressure ranges from about 120 to 135 GPa. In this pressure range, ferrous iron (Fe$^{2+}$) substituting for magnesium at the dodecahedral (A) site remains in the high-spin (HS) state; intermediate-spin (IS) and low-spin (LS) states are highly unfavorable. As to ferric iron (Fe$^{3+}$), which substitutes magnesium at the A site and silicon at the octahedral (B) site to form (Mg,Fe)(Si,Fe)O$_3$ Ppv, we find the combination of HS Fe$^{3+}$ at the A site and LS Fe$^{3+}$ at the B site the most favorable. Neither A-site nor B-site Fe$^{3+}$ undergoes a spin-state crossover in the D'' pressure range. The computed iron quadrupole splittings are consistent with those observed in M{\"o}ssbauer spectra. The effects of Fe$^{2+}$ and Fe$^{3+}$ on the equation of state of Ppv are found nearly identical, expanding the unit cell volume while barely affecting the bulk modulus.   

Effects of aluminum on spin-state crossover of iron in the Earth's lower mantle

Using density functional theory + Hubbard $U$ (DFT+$U$) calculations, we investigate how aluminum affects the spin-state crossover of iron in MgSiO$_3$ perovskite and post-perovskite, the major mineral phases in and at the bottom of the Earth's lower mantle. We find that aluminum does not change the response of iron spin state to the increasing pressure, namely, only the ferric iron (Fe$^{3+}$) residing the octahedral (B) site undergoes a crossover from high-spin to low-spin state, same as aluminum-free iron-bearing MgSiO$_3$. The presence of aluminum, however, does affect the population of B-site ferric iron significantly -- the majority of Fe$^{3+}$ reside the dodecahedral (A) site at lower pressures, and the population of B-site Fe$^{3+}$ increases with pressure at higher pressure range. Therefore, in the Earth's lower mantle, the amount of B-site Fe$^{3+}$ and the degree of elastic anomalies (and thus the possible seismic anomalies) associated with spin-state crossover is directly affected by the concentration and configuration of aluminum.   

Phase stability of mixtures at extreme conditions: implications for the interior structure of the Outer Planets

The unusual magnetic fields of the planets Uranus and Neptune represent important observables for constraining and developing deep interior models. Models suggests that the non-dipolar and non-axial magnetic fields of these planets originate from a thin convective and conducting shell of material around a stratified fluid core. We present a computational study of the physical properties of a fluid compositionally similar to what is expected in the interior of Uranus and Neptune. Our diffusivity and conductivity results suggest that the core cannot be well mixed if it is to generate non-axisymmetric magnetic fields. The simulations highlight the importance of chemistry on the properties of this complex mixture, including the possible formation of carbon and nitrogen clusters. We present results concerning the overall phase stability of the mixture under conditions relevant to the planetary interiors.   

Session P26: Focus Session: Friction, Fracture and Deformation Across Length Scales - Friction and Geometric Effects

Effect of Atomic Scale Geometry on Contact and Friction Between Rough Solids

Microscopic roughness on real surfaces is known to have a profound influence on macroscopic contact mechanics. It has previously been reported that surfaces that differ only at the atomic level can show different relationships between load, stiffness, and friction. Here we use molecular dynamics simulations to study contact properties of self-affine rough surfaces that are identical at continuum scales, but differ at the atomic scale. We compare surfaces that have atomic positions displaced to a self affine surface to ``stepped'' surfaces that have been cut from a lattice. The stepped surfaces exhibit more plasticity, contributing to a larger contacting area at a given load. A unified framework captures the relation between roughness, system size, surface separation, stiffness, and contact area.   

Breakdown of Amontons' Law of Friction in Sheared-Elastomer with Local Amontons' Friction

It is well known that Amontons' law of friction i.e. the frictional force against the sliding motion of solid object is proportional to the loading force and not dependent on the contact area, holds well for various systems. Here we show, however, the breakdown of the Amontons' law for the elastic object which have local friction obeying Amontons' law and is under uniform pressure by FEM calculation The external shearing force applied to the trailing edge of the sample induces local slip. The range of the slip increases with the increasing external force adiabatically at first. When the range reaches the critical magnitude, the slips moves rapidly and reaches the leading edge of the sample then the whole system slides. These behaviors are consistent with the experiment by Rubinstein et.al. (Phys. Rev. Lett. 98, 226103). The static frictional coefficient, the ratio between the static frictional force for the whole system and the loading force, decreases with the increasing pressure. This means the breakdown of Amontons' law. The pressure dependence of the frictional coefficient is caused by the change of the critical length of the local slip. The behaviors of the local slip and the frictional coefficient are well explained by the 1 dimensional model analytically.   

Geometry and elasticity for detachment fronts in friction

Mesoscale friction bridges nanoscale contact mechanics to the macroscopic Amontons laws and is relevant for several mechanical problems including seismology. Recent experiments on polymeric blocks show that the onset of friction occurs by nucleation of detachment fronts and that frictional properties vary along the sample surface. The earthquake-like dynamics found at the millimeter scale is in contrast with the usual assumption of uniform detachment without a coherent pattern in the front formation. Simulating the quasi-equilibrium dynamics of an elastic sample sliding on a rough surface under a shear force, we show that the dynamics of detachment fronts depends on the sample geometry. In particular, we study the effect of the sample aspect ratio by computing the elastic Green function for a finite three-dimensional slider. Our model allows to study the onset of friction in different geometries, from the thin slabs used in the aforementioned experiments to more general samples shapes.   

Local friction at a rubber/glass multicontact interface

When rubber is squeezed against a hard, rough surface contact only occurs at localized spots between surface asperities. Friction thus involves the shearing of a myriad of micro-contacts which are distributed over length scales ranging from micrometers down to nanometers. In order to get more insights into this widely debated problem, spatially resolved measurements of frictional stresses are much needed. We recently proposed a method to measure local friction of rubbers by means of a contact imaging approach. Silicon rubber substrates marked on their surface are prepared in order to measure the displacement field induced by the steady state friction of a glass lens. Then, the deconvolution of this displacement field provides a spatially resolved measurement of the actual shear stress and contact pressure distributions within the contact interface. As a result, the local friction law, i.e. the relationship between the actual shear stress and normal pressure, is obtained. The effect of roughness are analyzed from experiments using statistically rough surfaces differing in their roughness power density spectrum. Experimental results are discussed in the light of theoretical contact models for the friction of multi-contact interfaces.   

Adhesive contact of randomly rough surfaces

The contact area, stiffness and adhesion between rigid, randomly rough surfaces and elastic substrates is studied using molecular statics and continuum simulations. The surfaces are self-affine with Hurst exponent 0.3 to 0.8 and different short $\lambda _{s}$ and long $\lambda _{L}$ wavelength cutoffs. The rms surface slope and the range and strength of the adhesive potential are also varied. For parameters typical of most solids, the effect of adhesion decreases as the ratio $\lambda _{L}$/$\lambda _{s}$ increases. In particular, the pull-off force decreases to zero and the area of contact A$_{c }$becomes linear in the applied load L. A simple scaling argument is developed that describes the increase in the ratio A$_{c}$/L with increasing adhesion and a corresponding increase in the contact stiffness [1]. The argument also predicts a crossover to finite contact area at zero load when surfaces are exceptionally smooth or the ratio of surface tension to bulk modulus is unusually large, as for elastomers. Results that test this prediction will be presented and related to the Maugis-Dugdale [2] theories for individual asperities and the more recent scaling theory of Persson [3]. [1] Akarapu, Sharp, Robbins, Phys. Rev. Lett. 106, 204301 (2011) [2] Maugis, J. Colloid Interface Sci. 150, 243 (1992) [3] Persson, Phys. Rev. Lett. 74, 75420 (2006)   

The Effect of Surface Topography on Interface Stresses During Peeling

Surface topography can have a large impact on the adhesive strength of soft interfaces. While previous experiments have revealed some of the underlying mechanisms, there has been no direct measurement of interface stresses during adhesive failure. We use traction force microscopy to measure the microscopic distribution of interface stresses during peeling. We focus on the relationship between local stresses and topography near the peeling front.   

Direction dependence of static friction for commensurate and moderately incommensurate surfaces

We present results from our calculations of quasi-static sliding of two atomically flat surfaces in dry, wearless contact using the Density Functional Theory package \emph{VASP}. The main focus of our work was to determine to which extent commensurability of the surfaces and the sliding direction effects the friction force. The examined systems include commensurable fcc (111) Aluminium slabs and moderately incommensurate surfaces like bcc (110) Titanium on hcp (001) Titanium. A model consistent with stick-slip friction was devised to calculate the friction forces along sliding paths of up to 1 $\mu$m on a quantum mechanical basis. To map all forces and energies for rigid and relaxed atomic positions the top slab was scanned over the bottom one on a properly fine grid, which covers the entire unit cell. In this manner, it is shown that the mean friction force depends on the sliding direction and that due to relaxations incommensurate paths may result, counter-intuitively, in higher friction then commensurate ones.   

Laser induced projectile impact test (LIPIT): A micron-scale ballistic test for high-strain rate mechanical study of nano-structures

We present a method to apply a highly localized deformation at a high-strain-rate for the study of mechanical characteristics of micro- and nano-structures. In the technique, Laser Induced Projectile Impact Test (LIPIT), micro-projectiles (solid silica spheres of 3.7$\mu $m diameter) are accelerated to a supersonic speed (up to 4 km/s) in air by a micro-explosion created by laser ablation of polystyrene and impact a sample target. The velocity information of the micro-projectiles is explicitly determined by two consecutive high-speed images during the flight of the projectiles. For demonstration, a glassy-rubbery nanocomposite consisting of a periodic self-assembled stack of 20 nm thick layers of polystyrene and polydimethylsiloxane blocks (PS-b-PDMS) is tested by LIPIT at the extremely high-strain rate of 10$^{8}$ s$^{-1}$. The polymer nanocomposite demonstrates new orientation dependent deformation and failure mechanisms including a surprising order to disorder transition fluidization, and the energy absorbing ability of a layered nanocomposite through plastic deformation leading to a melting of the layered structure.   

Probing the sliding interactions between bundled actin filaments

Assemblies of filamentous biopolymers are hierarchical materials in which the properties of the overall assemblage are determined by structure and interactions between constituent particles at all hierarchical levels. For example, the overall bending rigidity of a two bundled filaments greatly depends on the bending rigidity of, and the adhesion strength between individual filaments. However, another property of importance is the ability for the filaments to slide freely against one another. Everyday experience indicates that it is much easier to bend a stack of papers in which individual sheets freely slide past each other than the same stack of papers in which all the sheets are irreversibly glued together. Similarly, in filamentous structures the ability for local re-arrangement is of the utmost importance in determining the properties of the structures observed. We have developed a method to directly measure the frictional interactions between a pair of aligned filaments in a well-defined and controllable configuration. This enables us to systematically investigate the role of adhesion strength, filament orientation, length, and surface structure.   

Harnessing polymer gels to regulate friction between sliding surfaces

We examine the microscale tribological behavior of a pair of gel coated surfaces separated by a thin layer of lubricant. The soft, elastic gel is modeled using a bond-bending lattice spring model that captures the micromechanics of a random network of interconnected filaments. We couple this model with the dissipative particle dynamics that explicitly models the hydrodynamics of a viscous fluid. We probe how elasticity and internal structure of compliant gels affect the tribological behavior and examine how gel elasticity can be harnessed to regulate friction between sliding surfaces. We also study the effect of lubricant composition and the inclusion of nanoscopic particles of different shapes on the friction forces between wet compliant surfaces. Our findings could be useful for developing new methods for regulating friction and reducing wear of lubricated surfaces and also for understanding the micro-mechanics of friction in biological systems.   

On the Equilibrium of an Heavy Elastic Cylinder on Horizontal Nondeformable Plane

The plane static contact problem of the equilibrium of a homogeneous heavy elastic cylinder on fixed non-deformable horizontal plane is discussed. In the vicinity of the initial contact between these bodies, there is some contact area, which covers the central angle and which are unknown contact stresses. The elastic equilibrium of a cylinder under the influence of its gravity and some unknown contact stress is discussed. To determine the distribution of contact stress, the size of the contact area of the cylinder and immersion in the horizontal plane is required.   

Internal friction peak in silicon revealed by moderate temperature annealing

In order to maximize of the quality factor of a mechanical resonator, one must minimize energy loss mechanisms. We have identified a new internal friction (IF) peak that is present in as-fabricated ultra-high $Q$ silicon resonators known as Double Paddle Oscillators. The IF peak can be removed (and thus its presence revealed) by annealing at moderate ($300\,^{\circ}$C) temperatures in both inert (Argon) and reactive (H${}_2$) atmospheres, and does not re-appear after aging for $10^7\,$s. The success of a relatively low temperature operation in eliminating this mechanism indicates that the phenomenon is surface-, as opposed to bulk- related. We compare this loss mechanism to other known loss mechanisms in silicon.   

Investigation of stability for an electrostatically actuated flexible electrode

An argument for employing dimensional analysis to explore stability in an electrostatically actuated flexible electrode is presented theoretically and experimentally. The electrode is configured as a cantilever beam, as many applications in MEMs, medical devices, and sensing devices have been studied for years. This study investigates a macro scale beam (length = 100mm - 150mm), for applications in cooling fan and flapping micro air vehicle devices. The influence of scale is validated, voltage potential and frequency contributions are quantitatively measured, and a comparison of input signal (analog versus digital) is discussed using dynamical systems analysis. Based on experimental data and numerical models, characteristics of stability are presented that could influence design considerations for various micro- and macro-scale devices.   

Session P27: Invited Session: Transport Studies of Topological Insulators

Towards the quantum anomalous Hall effect in HgMnTe

Although there are plenty of theoretical and experimental studies of the anomalous Hall effect in the ferromagnetic materials, the quantum version, namely ``quantum anomalous Hall effect'', has never been observed in the realistic materials. In this work, we report the experimental evidence of the quantum anomalous Hall effect in the Mn doped HgTe quantum wells. We observe a long quantized Hall plateau from 0.2 T to $>$ 25 T with the Hall conductance e$^{2}$/h in the p-doped regime. By carefully analyzing the gate voltage and temperature dependence of the experimental data, we find the long plateau origins from the fact that the two zero Landau levels in HgMnTe doped system have the same slope, which is exactly the required condition for the quantum anomalous Hall effect. Theoretical \textbf{k}$\cdot$\textbf{p} calculation is carefully compared with the experimental data to identify the influence from the magnetic impurities. A HgMnTe-ferromagnet hybrid structure is proposed for the possible future device applications.   

Quantum Spin Hall Insulator Made of InAs/GaSb

Quantum Spin Hall Insulator (QSHI) is a two-dimensional variant of a novel class of materials characterized by topological order, whose unique properties have recently triggered much interest and excitement in the condensed matter community. Most notably, topological properties of these systems hold a great promise in mitigating the difficult problem of decoherence in implementations of quantum computers. Although QSHI has been theoretically predicted in a few different materials, so far only the semiconductor systems of HgTe/CdTe and, more recently, inverted InAs/GaSb, have shown direct evidence for the existence of this phase. Ideally insulating in the bulk, QSHI is characterized by one-dimensional channels at the sample perimeter, which have helical property, with carrier spin tied to the carrier direction of motion, and protected from back-scattering by time-reversal symmetry. Here we present low temperature transport measurements of inverted InAs/GaSb quantum wells, showing strong evidence for the existence of proposed helical edge channels. Edge modes persist in spite of conductive bulk, which is of non-trivial origin but highly tunable via electrostatic gates, and show only a weak magnetic field dependence. This is a direct consequence of a gap opening away from the zone center leading to effective decoupling of edge to bulk states due to the Fermi velocity mismatch. Low Schottky barrier of this material system and good interface to superconductors allows us to further probe topological properties of helical channels in Andreev reflection measurements and opens a promising route in realization of exotic Majorana modes.   

Transport Studies of Topological Insulators and Superconductors

A topological state of matter is characterized by a topological feature of the quantum-mechanical wavefunction in the Hilbert space. In 3D topological insulators (TIs), a non-trivial Z$_2$ topology of the bulk valence band leads to the emergence of Dirac fermions on the surface. Similarly, in 3D topological superconductors (TSCs), a non-trivial winding number of the superconducting wavefunction leads to the appearance of Majorana fermions on the surface. The Dirac or Majorana fermions in those topological states of matter are of fundamental interest, but there have been significant materials problems that hindered their experimental studies so far: In the case of TIs, most of the materials identified as such are poor insulators in the bulk, making it difficult to probe the peculiar surface transport properties; in the case of TSCs, no concrete example has yet been discovered. In this talk, I will present our recent contributions to address these issues. For TIs, we discovered that the chalcogen-ordered tetradymite TI material Bi$_2$Te$_2$Se presents a high bulk resistivity, allowing one to observe clear surface quantum oscillations [Z. Ren {\it et al.}, PRB {\bf 82}, 241306(R) (2010)]; more recently, we discovered that the bulk-insulating nature can be further improved in the solid-solution system Bi$_{2-x}$Sb$_x$Te$_{3-y}$Se$_y$, making it possible to achieve the surface-dominated transport in a bulk crystal and observe both Dirac holes and electrons via Shubnikov-de Haas oscillations [A. A. Taskin {\it et al.}, PRL {\bf 107}, 016801 (2011)]. For TSCs, we developed a new synthesis technique for a candidate TSC, Cu$_x$Bi$_2$Se$_3$, to obtain single-crystal samples with a high shielding fraction [M. Kriener {\it et al.}, PRL {\bf 106}, 127004 (2011); PRB {\bf 84}, 054513 (2011)]. Using these crystals, we have recently succeeded in observing the surface Andreev bound state which gives evidence for an unconventional superconductivity; since the unconventional superconductivity in Cu$_x$Bi$_2$Se$_3$ can only be topological (thanks to the simple and peculiar band structure), one can conclude with confidence that this material is the first concrete example of a TSC [S. Sasaki {\it et al.}, PRL {\bf 107} (2011)]. Work in collaboration with A. A. Taskin, Z. Ren, S. Sasaki, and K. Segawa.   

Quantum anomalous Hall effect in the shin film of magnetic topological insulators and semimetals

The great interests on Hall effects come with their quantization under certain conditions.By now all five types of the Hall effects have been discovered, and the only remaining one is the quantized anomalous Hall effect, which is the quantized Hall effect without external magnetic field and the formation of Landau levels. In the present talk, I will summarize two possible ways proposed by our group to reach such an effect, which are thin films of magnetically doped topological insulators and topological semimetals. I will mainly focused on the latter proposal, which is important in the following sense. First the proposal is based on the stoichiometric material, which is very good for obtaining large mobility. Second, the exchange coupling energy between the magnetization and the valence electrons is of the order of eV, which makes QAHE more easy to be realized.   

Session P28: Optical Applications: Nonlinear Optics, Waveguides, and Novel Structures

An optical antenna for high-count-rate single-photon-sensitive superconducting transition edge sensors

There are number of promising applications for a GHz count-rate, energy-resolving single-photon detector in the near-infrared. However, such a detector has not yet been perfected. For thermal detectors, this is partly due to the difficulty of coupling relatively large ($\sim $1 micron) photons into the necessarily small ($\sim $100 nm) thermal sensor element. We report on the simulation, fabrication, and preliminary measurements of an antenna-coupled superconducting transition edge sensor. The optical antenna is designed to directly couple incident near-infrared photons into much shorter wavelength surface plasmons, which are then delivered to a nanoscale superconducting niobium detector element at the antenna feed. This detector is inherently energy resolving, unlike the superconducting nanowire single-photon detector (SNSPD) or the single-photon avalanche photodiode (SPAD), and it operates at the relatively convenient temperature of 4 K.   

Nanocrystal-based Optoelectronic Devices

Optoelectronic devices capable of detecting and emitting light on a scale well below its wavelength could have a profound impact on basic and applied experimental research in light-based electronics, on-demand photon generation, and for studying poorly understood quantum phenomena such as blinking and spectral wandering. We present a fabrication procedure for ultrasmall, nanocrystal optoelectronic devices based on self-assembled layers of quantum dots in plasmonically-active gold nanogaps. We provide preliminary experimental results which examine the possibility for surfaced-enhanced fluorescence, subwavelength detection and emission of light as well as plasmon-based optical trapping in these systems.   

Frequency and Intensity Stabilization of Planar Waveguide-External Cavity Lasers

Planar Waveguide External Cavity Lasers (PW-ECL) show an immense potential for use in precision measurement tasks and space missions because of its compactness and simple design. We show the techniques used to frequency and intensity stabilize a PW-ECL 1550nm laser system with the goal of achieving a frequency stability of 30~Hz/sqrt(Hz) and a RIN of less than 10$^{-6}$. These PW-ECL systems are a potential replacement for Non-Planar Ring Oscillator (NPRO) laser systems, which have become a standard for low-noise interferometric applications, if the PW-ECL can meet the required stability. We present the initial experimental results of the intensity and frequency stabilization setup and we show a comparison between PW-ECL lasers and NPRO lasers with respect to measurements and applications requiring a high frequency and intensity stability.   

Experimental setup to demonstrate low-frequency high-precision frequency stabilization of 1550 nm ECL Lasers

Advances in fiber and waveguide technologies have brought about a new type of laser: the Planar Waveguide External Cavity Laser (PW-ECL) that shows a great potential for precision interferometric measurements. We show an experimental setup based on a 1550nm PW-ECL which was designed to achieve a frequency stabilization of 30 Hz/sqrt(Hz) or less at 10 mHz. The presented design makes use of thermal shielding to suppress temperature fluctuations at low frequencies as well as a vacuum system, high finesse cavity and low-noise electronics to reduce the frequency noise. A description of the components used in the design is given and initial results are presented.   

Experimental demonstration waveguide with arbitrary bending angles in hyperuniform disordered photonics materials

Contradicting to the long standing intuition that long-range translational order is required in photonic band gap formation, recently a new class of disordered hyperuniform materials was predicted to possess sizeable photonic band gaps. We report the first experimental demonstration of complete and isotropic photonic band gap for all polarizations in such disordered hyperuniform structures made of alumina with a dielectric constant of 8.7. In periodic structures there are only a limited number of allowed rotational symmetries; hence bending angles of waveguiding channels are greatly limited. In isotropic hyperuniform disordered structures there are no preferential symmetry directions and waveguiding channels can be constructed with arbitrary bending angles. In our study, near 100 percent transmission of electromagnetic waves around sharp corners of arbitrary angles with bending radii smaller than one wavelength are observed experimentally. The hyperuniform disordered structures also enable the realization of isotropic confinement of radiation in cavities and can be used as flexible optical insulator platforms.   

Kerr Nonlinear Modes in Photonic Crystal Waveguide with Off-channel Features

A theoretical method using nonlinear difference equation approach has been developed for investigating guided modes of photonic crystal waveguide for cases in which the guided modes interact with multiple bound electromagnetic modes localized on off-channel impurity features of Kerr nonlinear media. The interest is on the properties of resonant scattering of the modes exhibited by the system formed of both linear and nonlinear media sites in the photonic crystal lattice. The scattering is treated and compared with results of the scattering of the modes in the linear limit of the Kerr media, i.e. in the absence of intensity $|E|^{2}$ term in the Kerr dielectrics derived in recursive difference equation formulation. The equations of the system are simple and can be quickly solved to demonstrate the wide and interesting varieties of behavior present in the system, including among others, optical bistability and induced transparency. Additionally, the method is applied for a case in which the field dependence of Kerr dielectric properties allows two different frequency waveguide modes to interact with one another by a modulation of the off-channel site dielectric properties. In this interaction, the one mode is used to model numerically the transmission characteristics of the other.   

The Phase Effect in perturbative nonlinear optics

Waveform control is essential for ultrafast nonlinear optical processes such as high-order harmonic generation (HHG). For example, a sawtooth-like waveform can enhance the kinetic energies of the electrons such that the cutoff of HHG is extended. In this work, we show that relative phase of the two-color driving laser can affect the outcome of perturbative nonlinear optical processes such as lower-order harmonic generation. Consider the third-harmonic signal generated in argon by the fundamental and second-harmonics of a pump laser with frequencies of $\omega _{1}$, and $\omega _{2}$. A cross-term emerges due to interference of four-wave mixing signals of ($\omega _{1}+\omega _{1}+\omega _{1})$ and ($\omega _{2 + }\omega _{2}-\omega _{1})$. When the intensities of two-color pump at $\omega _{1}$ and $\omega _{2}$ are equal, the modulation in the third-harmonic signal by the cross-term is about 30{\%} of the DC term. As the relative phase between $\omega _{1}$ and $\omega _{2}$ varies, a sinusoidal modulation in output intensity at 3$\omega _{1 }$ is expected. We have also calculated the phase effects for fifth, seventh and ninth harmonic generation, which show more complicated behavior.   

Micro-Photoluminescence for Optoelectronic Material Characterization

A hardware environment and software bundle have been developed for measuring the absolute value of and the variance in photoluminescence (PL) intensity across samples of optoelectrical materials. The fully automated assembly uses a dual-axis translation stage to allow for ``micro-PL'' measurements of the sample surface with a resolution of 10 microns on either axis. The user is given the option to digitally adjust the boundaries of the area being mapped, and set the measurement resolution to produce coarse or fine detailed PL maps of the sample surface. By using a monochromator, the system can perform preliminary measurements of PL at wavelengths ranging from 400nm to 1.7um, and determine the optimal spectral operation settings for detailed mapping. Since the system is a modular design, components can be switched to operate in other spectra ranges as well. As all components are digitally controlled by a PC, a universal user interface and integration module has been created to allow for simultaneous operation of all components with minimal user interaction, and intuitive representations of final data for material quality assessment. Various materials are characterized and discussed to demonstrate the utility.   

Rapid adiabatic passage in nonlinear optics for complete power transfer between ultrabroadband optical pulses

Rapid adiabatic passage is a central tool for full transfer of level populations in an atomic system. Recent work showed the equation of motions describing three-optical-wave mixing in a dielectric medium through a nonlinear electric susceptibility are isomorphic to the optical Bloch equations for a two-level atom (neglecting radiative losses) when the middle frequency wave is strong. We have exploited this analogy for the first time to prove the principle of complete Landau-Zener adiabatic transfer in nonlinear wave mixing. Using intense laser pulses and a specially designed poled potassium titanyl phosphate nonlinear crystal with an adiabatic longitudinal variation of the poling period, we demonstrate complete energy transfer of a near-IR pulse to the mid-IR. Moreover, we find the principle is upheld for huge laser bandwidths. In our experiment the power transfer covers two-thirds of an octave of bandwidth with preservation of the power spectral density profile. Control of optical power transfer through rapid adiabatic passage thus can serve to optimize sources for coherent control and strong-field physics.   

Nano-Gap Embedded Plasmonic Gratings for Surface Plasmon Enhanced Fluorescence

Plasmonic nanostructures have been extensively used in the past few decades for applications in sub-wavelength optics, data storage, optoelectronic circuits, microscopy and bio-photonics. The enhanced electromagnetic field produced at the metal/dielectric interface by the excitation of surface plasmons via incident radiation can be used for signal enhancement in fluorescence and surface enhanced Raman scattering studies. Novel plasmonic structures on the sub wavelength scale have been shown to provide very efficient and extreme light concentration at the nano-scale. The enhanced electric field produced within a few hundred nanometers of these structures can be used to excite fluorophores in the surrounding environment. Fluorescence based bio-detection and bio-imaging are two of the most important tools in the life sciences. Improving the qualities and capabilities of fluorescence based detectors and imaging equipment has been a big challenge to the industry manufacturers. We report the novel fabrication of nano-gap embedded periodic grating substrates on the nanoscale using micro-contact printing and polymethylsilsesquioxane (PMSSQ) polymer. Fluorescence enhancement of up to 118 times was observed with these silver nanostructures in conjugation with Rhodamine-590 fluorescent dye. These substrates are ideal candidates for low-level fluorescence detection and single molecule imaging.   

Micro-manufacturing with Three-dimensional Intensity Patterns: Practical Limits and Considerations

It is possible to generate 3D intensity patterns for micro-manufacturing using a phase hologram displayed on a liquid-crystal spatial light modulator (SLM). Knowledge of the phase and amplitude of a field in a single plane (the SLM plane) allows for calculation of the phase and amplitude in any set of subsequent planes. Iterative phase retrieval algorithms can take a 3D target intensity pattern and generate a hologram for display on the SLM; however, arbitrary 3D intensity patterns are not necessarily achievable because the light field must obey the wave equation. Furthermore, these algorithms have not been discussed from the point of view of lithographic micro-manufacturing. This paper presents one method for making a 3D intensity pattern that can be used to cure resist in a single-shot, and discusses the limits to patterning due to power, resolution, and SLM-related issues. The relationship between the resolution in the patterning volume and physical specifications of the SLM is more restrictive than if the patterning were being done in a single plane, but with sufficient laser power, this algorithm could be used in high-throughput 3D micro-manufacturing.   

High Finesse Microcavity for Gas Sensing

We report our progress in using optical Fabry-Perot microcavities for multiatomic gas sensing. The microcavities consist of one micromirror at a fiber tip and another micromirror on a planar silica substrate, each with a diameter near 50 microns. The micromirrors were fabricated by an improved CO2 laser ablation process that uses feedback from the light emitted during ablation to control the mirror dimensions. A cavity finesse in excess of 50 000 was obtained in the near-infrared. Our goal is to make use of the Purcell effect of cavity quantum electrodynamics to obtain an enhancement of Raman scattering when a double resonance condition is achieved.   

Control of transverse optical patterns in semiconductor quantum well microcavities

Recently, a low intensity all optical switching scheme exploiting directional optical instabilities in a semiconductor quantum well microcavity was proposed. It was demonstrated numerically that a sufficiently strong laser (pump) beam normally incident on the microcavity can suffer directional instabilities, generating new beams in oblique directions. These off-axis beams form a pattern when projected onto a plane in the far field placed transverse to the pump. Furthermore, the numerical results showed that the azimuthal orientation of the transverse optical pattern can be reversibly switched by applying a control beam much weaker than the pattern intensity. These phenomena are mediated by the nonlinear interactions among elementary excitations of the microcavity --- polaritons formed from strong coupling between the quantum well excitons and photons in a cavity mode. In this Contribution, we provide an overview of the rich parametric dependencies of pattern selection, the time scale of pattern formation, and the switching process. We also present an analysis of system's dynamics based on the contributing polariton wave-mixing processes.   

MEMS Fabricated MM-Wave Slow Wave Structure

We report on the fabrication and test of a MEMS slow wave structure designed for a $>$ 40 GHz bandwidth centered on 220 GHz operation, that slows radiation down to group velocity of 8.16 x 10$^{7}$ ms$^{-1}$ where the velocity matches the speed of electrons from a 20 keV source. The slow wave device uses a 40 mm long staggered interdigitated vane structure within a waveguide [1]. Ultimately, such a device will be integrated with an electron beam to become part of a sheet beam travelling wave tube (SBTWT) amplifier. A gold coated deep reactive ion etched (DRIE) silicon test structure was fabricated to test the RF properties of the design. This MEMS structure was coupled to WR-4 waveguide in a metal fixture and the S-parameters measured using a vector network analyzer, allowing extraction of the insertion loss and signal delay as a function of frequency. A further MEMS structure with just 10 cells of the vane structure within a cavity were fabricated which allows points on the dispersion curve to be directly measured as resonances of the structure. Extraction of the dispersion curve verifies the group velocity measurement of the device. \\[4pt] [1] Y-M. Shin {\&} L.R. Barnett, \textit{Appl.Phys. Lett. 2008,} 92 pp. 091501.   

Session P29: Focus Session: Superconducting Qubits: Multiple Qubits and Entanglement

Prospects for a prototype quantum processor based on three-dimensional superconducting cavities and qubits

Superconducting qubits embedded in three-dimensional waveguide resonators (recently pioneered by Paik, et al., arXiv:1105.4652) have shown excellent coherence properties and an ease of implementation that make them an enticing core component for small prototype quantum processors. The relatively large physical dimensions of the microwave modes of these 3D-cQED systems may lead one to discount their prospects for scaling. We argue that the larger characteristic dimensions, among others factors, in fact facilitate scaling for currently practicable prototype systems of $\sim $10 to 1,000 qubits. This emerges from significantly reduced fabrication complexity and costs, larger tolerances to parameter deviations, a more prevalent role for off-the-shelf components, and greater amenability to full-device electromagnetic simulation. At IBM we are working towards a modular quantum processor prototype based on 3D-cQED. Multi-qubit cavities, each implementing an artificial NMR molecule, are to be connected in an array by non-linear elements which double as tunable couplers between qubits in adjacent cavities and as single-shot readout circuitry. The resulting lattice of physical qubits provides a fabric on which surface code error correction can take place, implying a fault-tolerant threshold error rate for this architecture of $\sim $1{\%}. We describe recent experiments demonstrating a two-qubit gate with a two qubit/one cavity device and progress toward tunable coupling of qubits in adjacent cavities.   

Prime factoring using a Josephson phase-qubit architecture: $15 = 3*5$

We demonstrate a compiled version of Shor's algorithm using a quantum processor. The processor consists of ``off-the-shelf" components: qubits and resonators arranged in the ReZQu architecture. We have performed the algorithm for N=15 and the period r=2. The required two and three qubit entanglement is observed during the computation, which exemplifies the quantum nature of the algorithm.   

Building Transmon Qubits in the ReZQu Architecture

Transmon qubits are promising candidates for use in a superconducting quantum computer because of their long coherence times, but traditionally involve a difficult measurement scheme. By reading out each transmon through a phase qubit, we are able to take advantage of the single shot and multiplexed readout technologies already in use. This allows us to drop transmons into the ReZQu architecture. However, fabricating a transmon and phase qubit on the same chip comes with its own set of challenges. We present fabrication techniques and preliminary data as we move toward our next generation of qubits.   

High-fidelity CZ gate for the quantum Von Neumann architecture

The building block of a scalable superconducting quantum computer has recently been demonstrated [M. Mariantoni et al., Science 334, 61 (2011)]. This architecture consists of superconducting phase qubits capacitively coupled both to individual memory resonators as well as a common bus. In this work we investigate the fidelity of a CZ logic gate between a qubit and bus in a multi-qubit device. Our results show that it is possible to implement the CZ gate with 99.99\% intrinsic fidelity in 30ns with a simple two-parameter pulse profile (plus two Z rotations). An analytical error model is also developed to explain and extend this result.   

High-fidelity gates towards a scalable superconducting quantum processor

We experimentally explore the implementation of high-fidelity gates on multiple superconducting qubits coupled to multiple resonators. Having demonstrated all-microwave single and two qubit gates with fidelities $> 90\%$ on multi-qubit single-resonator systems, we expand the application to qubits across two resonators and investigate qubit coupling in this circuit. The coupled qubit-resonators are building blocks towards two-dimensional lattice networks for the application of surface code quantum error correction algorithms.   

Driving two-qubit entanglement with an enhanced ZZ interaction in circuit QED

The quantum bus architecture is fast becoming a popular approach for coupling superconducting qubits [1,2]. With two fixed-frequency qubits coupled by a resonator, it is possible to engineer the system's frequencies such that the qubits experience a strong ZZ interaction. This interaction can be used as a resource for creating entanglement when needed, but can also be suppressed at will using appropriate decoupling sequences. We will show measurements of a device where this ZZ interaction is enhanced by interactions with higher-levels of superconducting transmon qubits. To achieve high-fidelity control in this regime, we employ robust composite pulses and optimal control methods to decouple the two-qubit interaction during single-qubit operations. The resulting system serves as a testbed for adapting control techniques from liquid-state NMR to fixed-frequency superconducting qubits.\\[4pt] [1] L. DiCarlo {\it el al}. Nature {\bf 460}, 240-244 (2009).\\[0pt] [2] Matteo Mariantoni {\it et al}. Science {\bf 334}, 61-65 (2011).   

Controlling superconducting qubits in the presence of a strong, constant ZZ interaction

We look at the problem of optimal gate design in a system of two superconducting qubits coupled by a cavity. The system is designed to have a strong ``always on'' ZZ interaction, which is essential for two-qubit entangling gates, but presents a challenge for single qubit manipulation. Using pulse shaping ideas from NMR we are able to analytically derive single qubit gates that remove this unwanted coupling to 3rd order in the Magnus expansion, and applying DRAG corrections prevents leakage to higher oscillator levels. In the limit of strong cross-talk these pulses break down when the qubit being manipulated has a resonance close to the higher transition frequencies of the other, however we are still able to find high fidelity pulses numerically.   

Implementation of a Toffoli Gate with Superconducting Circuits

The Toffoli gate is an important primitive in many quantum circuits and quantum error correction schemes. Here we demonstrate the implementation of a Toffoli gate with three superconducting transmon qubits coupled to a microwave resonator [1]. Following Ralph \emph{et~al.}~[2] we used the third energy level of the transmon qubit to significantly reduce the number of elementary gates needed to implemente the Toffoli gate in comparison to approaches using two-level systems only. A similar scheme to realize a Toffoli-class gate has independently been devised on a system of three logical qubits encoded in the states of two qubits and a resonator [3]. Our gate fidelity evaluated by both full process tomography and Monte Carlo process certification is $68.5\pm0.5$\%. The results reinforce the potential of macroscopic superconducting qubits for implementation of complex quantum operations and point at the possibility to implement quantum error correction schemes~[4]. \newline [1] A. Fedorov \emph{et~al.}, arXiv:1108.3966. \newline [2] T.~C.~Ralph, K.~J.~Resch, A.~Gilchrist, Phys. Rev. A \textbf{75}, 022313 (2007).\newline [3] M. Mariantoni, \emph{et~al.} Science \textbf{334}, 61 (2011).\newline [4] M.~D. Reed, \emph{et~al.}, arXiv:1109.4948.   

Multiple photon effects in coupled anharmonic oscillators for circuit QED architectures

In recent years many superconducting qubits have emerged that are essentially weak anharmonic oscillators. When these systems are coupled, due to the weak anharmonicity the major source of error in this system is leakage. In this talk I will present a method for reducing leakage when implementing a two qubit gate. I will also show that due to this weak anharmonicity new multi-photon transitions emerge that can be used for implementing new types of two qubit gates.   

Observing Resonant Entanglement Dynamics in Circuit QED

We study the resonant interaction of up to three two-level systems and a single mode of an electromagnetic field in a circuit QED setup. Our investigation is focused on how a single excitation is dynamically shared in this fourpartite system. The underlying theory of the experiment is governed by the Tavis-Cummings-model, which on resonance predicts dynamics known as vacuum Rabi oscillations. The resonant situation has already been studied spectroscopically with three qubits [1] and time resolved measurements have been carried out in a tripartite system [2]. Here we are able to observe the coherent oscillations and their $\sqrt{N}$- enhancement by tracking the populations of all three qubits and the resonator. Full quantum state tomography is used to verify that the dynamics generates the maximally entangled 3-qubit W-state when the cavity state factorizes. The $\sqrt{N}$-speed-up offers the possibility to create W-states within a few ns with a fidelity of 75\%. We compare the resonant collective method to an approach, which achieves entanglement by sequentially tuning qubits into resonance with the cavity.\\[4pt] [1] J.~M.~Fink, Physical Review Letters \textbf{103}, 083601 (2009)\\[0pt] [2] F. Altomare, Nature Physics \textbf{6}, 777--781 (2010)   

Benchmarking a Teleportation Protocol realized in Circuit QED

Teleportation of a quantum state may be used for distributing entanglement between distant qubits in quantum communication and for realizing universal and fault-tolerant quantum computation. Here we demonstrate the implementation of a teleportation protocol, up to the single-shot measurement step, with superconducting qubits coupled to a microwave resonator [1]. Using full quantum state tomography and evaluating an entanglement witness, we show that the protocol generates a genuine tripartite entangled state of all three-qubits. Calculating the projection of the measured density matrix onto the basis states of two qubits allows us to reconstruct the teleported state. Repeating this procedure for a complete set of input states we find an average output state fidelity of 86\% for the teleported state.\newline [1] M.~Baur, A.~Fedorov, L.~Steffen, S.~Filipp, M.P.~da~Silva, and A.~Wallraff, arXiv:1107.4774.   

Characterization of a two-transmon processor with individual single-shot qubit readout

We report the characterization of a two-qubit processor implemented with two capacitively coupled tunable superconducting qubits of the transmon type, each qubit having its own non-destructive single-shot readout. The fixed capacitive coupling yields the $\sqrt{iSWAP}$ two-qubit gate for a suitable interaction time. We reconstruct by state tomography the coherent dynamics of the two-bit register as a function of the interaction time, observe a violation of the Bell inequality by 22 standard deviations after correcting readout errors, and measure by quantum process tomography a gate fidelity of 90\%.   

Demonstrating quantum speed-up in a superconducting two-qubit processor

We operate a superconducting quantum processor consisting of two tunable transmon qubits coupled by a swapping interaction, and equipped with non destructive single-shot readout of the two qubits [1]. With this processor, we run the Grover search algorithm among four objects and find that the correct answer is retrieved after a single run with a success probability between 0.52 and 0.67, significantly larger than the 0.25 achieved with a classical algorithm. This constitutes a proof-of-concept for the quantum speed-up of electrical quantum processors [2].\\[4pt] [1] arXiv:1109.6735v1 \\[0pt] [2] arXiv:1110.5170v1   

Session P30: Semiconductor Qubits - Communication and Hybridization

Circuit Quantum Electrodynamics with Semiconductor Quantum Dots

Research on semiconductor quantum dots has tremendously contributed to the understanding of the physics of individual charges and spins in a solid state environment. Typically, quantum dots are investigated by direct current transport measurements or using quantum point contacts for charge sensing. Instead, we have realized a novel device in which a semiconductor double quantum dot is dipole coupled to a GHz-frequency high-quality tranmission line resonator. This approach allows us to characterize the properties of the double dot by measuring both its dispersive and dissipative interaction with the resonator [1]. In addition to providing a new readout mechanism, this architecture has the potential to isolate the dots from the environment and to provide long distance coupling between spatially separated dots. These features are expected to improve the potential for realizing a quantum information processor with quantum dots as previously demonstrated for superconducting circuits making use of circuit quantum electrodynamics. \newline [1] T. Frey et al., arXiv:1108.5378v1 (2011)   

Dispersive microwave readout of a silicon double quantum dot

Microwave resonators coupled to quantum systems have been used for fast dispersive measurement in several different architectures in solid state and atomic physics. The electronic states of a semiconductor quantum dot represent a promising candidate for quantum information processing. Our work is geared toward developing a fast, non-demolition readout of a semiconductor qubit as realized in silicon by coupling to a superconducting resonant circuit. We report progress on a novel design of a lateral double quantum dot with a unique accumulation gate that allows for control of the spatial location of the 2DEG on the device, allowing the lossy 2DEG to be decoupled from the resonator.   

Coupling a single charge in a nanowire double quantum dot to a high-Q superconducting microwave resonator

In the field of circuit quantum electrodynamics (cQED), superconducting microwave cavities have provided an important tool with which to probe single superconducting qubits and mediate interactions between distant qubits [1,2]. In view of applying the cQED architecture to semiconductor qubits [3,4], we have integrated a tunable InAs nanowire double quantum dot (DQD) into a high-Q ($>2000$), high frequency ($\sim6$ GHz) niobium microwave resonator. Our design enables a dipole coupling of $\sim40$ MHz between the DQD and cavity. We will present experimental results demonstrating how the microwave cavity can be used to explore coherence in nanowire DQDs and discuss prospects for achieving the strong coupling regime. \\[4pt] [1] A. Wallraff et al., Nature 431, 162 (2004)\\[0pt] [2] L. DiCarlo et al., Nature 467, 574 (2010)\\[0pt] [3] J.R. Petta et al., Science 309, 2180 (2005)\\[0pt] [4] K.D. Petersson et al., Phys. Rev. Lett. 105, 246804 (2010)   

Communication between spin qubits with microwave photons

We consider possibilities of quantum state transfer between spin qubits (e.g. electrons in quantum dots) utilizing a microwave transmission line and a tunable coupler. We outline the possibility for optimal quantum transfer between qubit nodes depending on qubit-resonator coupling strengths and tunability, and in the presence of imperfections. Implications of such systems to practical quantum computing in silicon and/or GaAs quantum dots are considered.   

Spin echo measurements for hybrid qubit systems

We have performed electron spin echo measurements on donor spins in silicon samples using micro resonator structures fabricated in thin superconducting niobium films on sapphire substrates. The ability to implement refocusing pulses is an important constituent towards the realization of spin memories for hybrid superconducting/spin qubit applications. The resonators consist of quarter wavelength sections of coplanar waveguide with one end shorted and the other end capacitively coupled to a common feed line. These devices are suitable for pulsed ESR experiments on a commercial spectrometer at microwave frequencies between 9 and 10GHz, and in-plane magnetic fields of about 0.35T, which is well below the critical field of the thin film superconductor. Samples are flip-chip mounted on the resonator, and the magnetic component of the microwave radiation that extends into the sample is used for ESR excitation and detection. The high filling factor due to the small resonator size and the high quality factors that can be obtained with superconductors lead to high sensitivity to small numbers of spins, making our devices an attractive alternative to conventional resonators for certain ESR applications. The low microwave power requirements are appealing for low temperature measurements.   

Long-distance spin-spin coupling via floating gates

The electron spin is a natural two level system that allows a qubit to be encoded. When localized in a gate defined quantum dot, the electron spin provides a promising platform for a future functional quantum computer. The essential ingredient of any quantum computer is entanglement---between electron spin qubits---commonly achieved via the exchange interaction. Nevertheless, there is an immense challenge as to how to scale the system up to include many qubits. Here we propose a novel architecture of a large scale quantum computer based on a realization of long-distance quantum gates between electron spins localized in quantum dots. The crucial ingredients of such a long-distance coupling are floating metallic gates that mediate electrostatic coupling over large distances. We show, both analytically and numerically, that distant electron spins in an array of quantum dots can be coupled selectively, with coupling strengths that are larger than the electron spin decay and with switching times on the order of nanoseconds.   

Quantum gates between non-neighboring spin qubits

In quantum circuits involving many qubits, we usually need to perform gates such as CNOT between qubits that are not proximal. For spin qubits, this requires intermediate gate operations because the exchange interaction is very short ranged. Here, we consider three quantum dots in a linear array. We explore the effective coupling between the two outer spins mediated by the central spin. By using the central spin as a ``bus", we show how to efficiently perform gates such as CNOT between the outer spins. We find that arbitrary two-qubit gates can be achieved by applying the bus operations repetitively, with additional single-qubit rotations.   

Resonant Adiabatic Passages of Spin Qubits

We study adiabatic quantum teleportation through a spin chain with XX and Heisenberg couplings. We show that an adiabatic quantum teleportation protocol on a three-spin chain can be mapped exactly onto two parallel and coherent adiabatic passage channels, one for each spin orientation. When the time evolution is non-adiabatic, the information transfer displays a series of resonances where perfect transmission fidelity is achieved. This resonant operation is both fast and robust, indicating a possible new route for implementing robust quantum gates between spin qubits.   

Quantum phase transitions in a spin bus system

A spin chain can be used as a quantum data bus to enable long distance interactions and to create multi-qubit entanglement between spin qubits. The operation of the spin bus strongly depends on its ground state properties. When the ground state changes abruptly, quantum phase transitions (QPTs) occur and affect the bus operation. Here, we describe the theory of QPTs induced by an external magnetic field in a Heisenberg spin chain which acts as a spin bus. We study the non-analytic behavior of the entanglement between qubits connected to the spin bus and its scaling properties near a quantum critical point. In some cases, we find the entangling properties actually \emph{grow} with the length of the chain. We also analyze the magnetically induced anisotropy and disorder effects on the effective interactions between qubits.   

Coupling semiconductor qubits to phonons via a nanomechanical phoniton system

We explore the possibility of strongly coupling semiconductor qubit states to nanomechanical resonators (phonons) in silicon. These systems may be relevant to qubit transduction schemes, as supporting technology for quantum information processing, for qubit characterization, and for quantum-enabled devices. Specifically, we consider systems where cavity phonons can interact with suitable qubit states in the 1-10 GHz (and higher) regime (tunable using strain, electric and/or magnetic fields). These results may be useful for several solid-state devices as well as being of interest to the optomechanics community.   

Phonon Mediated Off-resonant Quantum Dot-Cavity Interaction

Optically controlled quantum dot (QD) spins coupled to semiconductor microcavities constitute a promising platform for robust and scalable quantum information processing devices. In recent experiments on coupled QD optical cavity systems a pronounced interaction between the dot and the cavity has been observed even for detunings of many cavity linewidths. This interaction has been attributed to an incoherent cavity enhanced phonon-mediated scattering process and is absent in atomic systems. We demonstrate that despite its incoherent nature, this process preserves the signatures of coherent interaction between a QD and a strong driving laser, which may be observed via the optical emission from the off-resonant cavity. Under bichromatic driving of the QD, the cavity emission exhibits spectral features consistent with optical dressing of the QD transition, namely Rabi side-bands. These cavity emission measurements are more akin to absorption measurements of a strongly driven QD rather than resonance fluorescence measurements. In addition to revealing new aspects of the off-resonant QD-cavity interaction, this result provides a new, simpler means of coherently probing QDs and opens the possibility of employing off-resonant cavities to optically interface QD-nodes in quantum networks.   

Rabi-Vibronic resonance at large number of vibrational quanta

Rabi oscillations of a resonantly driven two-level system (qubit) which is linearly coupled to a cavity (vibrational mode) with frequency, $\omega_0$, much smaller than the driving frequency, are studied theoretically. We show that, for small coupling constant $\lambda \ll 1$, Rabi oscillations are strongly modified in the vicinity of the {\em Rabi-vibronic resonance} $\Omega_R=\omega_0$, where $\Omega_R$ is the Rabi frequency proportional to the amplitude of the driving field. The width of the resonance is shown to be $(\Omega_R-\omega_0) \sim \lambda^{4/3}\omega_0$, and is much larger than the polaronic frequency shift, $\lambda^2\omega_0$. We show that within the resonant domain of $\Omega_R$ the actual frequency of the Rabi oscillations exhibits bistable behavior as a function of $\Omega_R$. Most importantly, within the resonant domain, the oscillator is highly excited, which allows one to treat it classically. Decay of the Rabi oscillations due to losses in the cavity and spontaneous emission of two-level system are also studied.   

Photon heralded entanglement between radiatively mismatched matter qubits

Photon heralded entanglement usually requires well matched radiative characteristics of two matter-based qubits. In practice, we may want to entangle two solid-state or atomic qubits with different radiative lifetime, or to entangle a solid-state qubit with an atomic qubit that has significant mismatch in lifetime and transition frequency. We propose a protocol that can effectively shape the emitted photon pulses of two matter qubits to a common analytically known function by simply tuning the classical laser fields applied to each qubit. Entanglement fidelity and success rates are found to be promising under current experimental conditions.   

Fast optically-controlled two-qubit operation for cavity-coupled semiconductor quantum dots

Electron spin qubit systems based on charged InAs/GaAs quantum dots have demonstrated long coherence times and the capability of ultra fast single- and two-qubit operations in a configuration when two dots are tunnel-coupled. Designing fast two- and multi-qubit gates for spatially-separated quantum dots is currently an important challenge. We propose fast optically controlled design where a two-qubit gate is mediated by a photonic crystal cavity mode. The design addresses the challenge of scalability and does not require quantum dots to have the same energies. The proposed gate scheme is also compatible with available optically induced single qubit rotations.   

H1 Photonic Crystal Microcavities for Quantum Information

Semiconductor quantum dots coupled to photonic crystal microcavities show promise for quantum information processing in solid-state systems. For many applications, the cavity mode needs to have high extraction efficiency and unpolarized emission. Here we describe a possible implementation using the two orthogonally-polarized, spectrally-degenerate dipole modes of the H1 photonic crystal microcavity. By modifying the shape of the far-field profile, high collection efficiency from the H1 cavity modes can be achieved while maintaining a high cavity quality factor. We optimize and experimentally measure the far-field profiles of our cavities, which show good agreement with simulations. We also implement techniques to minimize the energy splitting of the two dipole modes due to fabrication imperfections, which are compatible with the far-field optimization.   

Session P31: Topological Insulators: Disorder

Disordered topological conductor

A topological conductor, like a topological insulator is a system in which the bands are characterized by non-trivial topological invariants such as the Chern numbers. However, unlike a topological insulator, in this system the Fermi energy does not lie in an energy gap but instead intersects at least one of the bulk bands. Although not an insulator the topological conductor supports chiral edge modes. In this work we consider a disordered topological conductor and analyze its properties. In particular we find that moderate disorder reduces the edge conductivity from its quantum value and stronger disorder increases it before the whole system is localized and the conductivity drops to zero. This effect is seen numerically on a lattice system and analytically in a disorder averaged continuum model.   

Dislocations and their braiding in topological insulators

We demonstrate the fundamental importance of crystal lattice dislocations in two-dimensional topological insulators. These defects characterize the topological state through the appearance of electronic localized midgap states. The states turn out to be robust even for the class of materials where they are not protected. At the same time, these localized electronic states have interesting quantum properties. We show that adiabatic braiding of dislocations, which can be achieved using lattice shear induced dislocation glide, brings out the quantum statistics of the electronic bound states.   

Study of the robustness of two dimensional topological insulators

Two dimensional topological insulators present gapless spin filtered edge states which are topologically protected against backscattering. As long as disorder does not mix the states of opposite edges or with bulk ones, these states contribute to the two terminal conductance as a single quantum channel regardless of the amount of non-magnetic disorder present in the sample. We address this problem studying the effect of different types of disorder: constrictions and Anderson disorder, for two different materials that have been predicted to present the quantum spin Hall insulator phase, graphene and a bilayer of Bi(111). We also study the effect of the zigzag edge reconstruction of graphene over the robust behavior of the edge states. We describe their electronic structure using an orthogonal tight-binding model in the Slater-Koster approximation including the intra-atomic spin-orbit interaction. The conductance is computed using the Landauer formula making use of the ALACANT transport package.   

Investigating Impurities on 3D Topological Insulators with Multiple Scattering Theory

Recently a new class of materials, three-dimensional topological insulators (3DTI), have generated significant theoretical and experimental interest due to surface states (SS) exhibiting linear dispersion at a Dirac pont that are protected by time-reversal symmetry (TRS). Using magnetism to break TRS is of particular interest, especially via the deposition of magnetic impurities (MI) on the 3DTI surface. Experimental studies are in the early stages and consensus on the effect of MI of 3DTI SS has yet to be reached. Multiple scattering theory (MST) has proven useful in investigating surfaces, impurities and disordered systems and provides an ideal framework for first-principles, computational study. This presentation will report on progress in adapting MST to 3DTI systems with impurities.   

LDOS Multifractal Hunter's Guide to Dirty Topological Insulators

We compute the multifractal spectra associated to local density of states (LDOS) fluctuations due to weak quenched disorder, for a single Dirac fermion in two spatial dimensions. Our results are relevant to the surfaces of $Z_2$ topological insulators such as Bi$_2$Se$_3$ and Bi$_2$Te$_3$, where LDOS modulations can be directly probed via scanning tunneling microscopy. We find a qualitative difference in spectra obtained for magnetic versus non-magnetic disorder. Randomly polarized magnetic impurities induce quadratic multifractality at first order in the impurity density; by contrast, no operator exhibits multifractal scaling at this order for a non-magnetic impurity profile. For the time-reversal invariant case, we compute the first non-trivial multifractal correction, which appears at two loops (impurity density squared). We discuss spectral enhancement approaching the Dirac point due to renormalization, and we survey known results for the opposite limit of strong disorder.   

Effects of Strong Disorder in a 3-Dimensional Topological Insulators: Phase Diagram and Mapping of the Z2 Invariant

We study the effect of strong disorder in a 3-dimensional topological insulators with time-reversal symmetry and broken inversion symmetry. Using level statistics analysis, we demonstrate first the persistence of delocalized bulk states even at large disorder. The delocalized spectrum displays the levitation and pair annihilation effect, indicating that the delocalized states continue to carry the Z2 invariant after the onset of disorder. The Z2 invariant is computed via twisted boundary conditions using a novel and efficient numerical algorithm. We demonstrate that the Z2 invariant remains well defined and quantized even after the spectral gap closes and becomes filled with dense localized states. In fact, our results indicate that the Z2 remains quantized until the mobility gap closes or until the Fermi level touches the mobility edge. Based on such data, we compute the phase diagram as function of disorder strength and position of the Fermi level.   

Strong potential impurities on the surface of a three-dimensional topological insulator

Topological insulators (TIs) are said to be stable against non-magnetic impurity scattering due to suppressed backscattering in the Dirac surface states. We solve a lattice model of a three-dimensional TI in the presence of strong potential impurities on the surface and find that both the Dirac point and low-energy states are significantly modified: low-energy impurity resonances are formed that produce a peak in the density of states near the Dirac point, which is destroyed and split into two nodes that move off-center. The impurity-induced states penetrate up to 10 layers into the bulk of the TI. These findings demonstrate the importance of bulk states for the stability of TIs and how they can destroy the topological protection of the surface. Extensions to sub-surface and extended defects, as well as direct comparisons to recent experimental results are also made.   

Disorder induced quantized conductance with fractional value and universal conductance fluctuation in three-dimensional topological insulators

We report a theoretical investigation on the conductance and its fluctuation of three-dimensional topological insulators (3D TI) in \textit{Bi}2\textit{Se}3 and \textit{Sb}2\textit{Te}3 in the presence of disorders. Extensive numerical simulations are carried out. We find that in the diffusive regime the conductance is quantized with fractional value. Importantly, the conductance fluctuation is also quantized with a universal value. For 3D TI connected by two terminals, three independent conductances $G$\textit{zz}, $G$\textit{xx} and $G$\textit{zx} are identified where z is the normal direction of quintuple layer of 3D TI. The quantized conductance are found to be $\left\langle {G_{zz} } \right\rangle $ = 1, $\left\langle {G_{xx} } \right\rangle $ = 4/3 and $\left\langle {G_{zx} } \right\rangle $ = 6/5 with corresponding quantized conductance fluctuation 0.54, 0.47, and 0.50. The quantization of average conductance and its fluctuation can be understood by theory of mode mixing. The experimental realization that can observe the quantization of average conductance is discussed.   

Conducting state of GeTe by defect-induced topological insulating order

Topological insulating order protected by time-reversal symmetry is robust under structural disorder. Interestingly, recent studies on phase change materials like GeSbTe showed that their topological insulating order is sensitive to atomic stacking sequences. It was also shown that their structural phase transition is correlated with topological insulating order. GeTe, a well-known phase change material, is trivial insulator in its equilibrium structure. In this study, we discuss how atomic defects such as Ge tetrahedral defect observed in amorphous GeTe can change its topological insulating order based on first-principles calculations and model Hamiltonian. We also investigated the critical density of such tetrahedral defects to induce topological insulating order in GeTe. Our study will help explore hidden orders in GeTe.   

Mapping the conductivity tensor of disordered topological insulators in the presence of magnetic fields

Transport measurements on topological insulators revealed extremely interesting effects and generated data that contain extremely valuable information. It is now possible to control the carriers' concentration using finely tuned gate voltages and to do the transport measurements under controlled applied magnetic fields. As such, accurate maps of the conductivity tensor are now available, as function of the Fermi level and the magnetic field strength. To extract useful information, we need a quantitative theory of charge transport for aperiodic systems in presence of magnetic fields. Such theory has been developed in the past using C*-Algebras and Non-Commutative calculus, the result being closed and exact formulas for the conductivity tensor. In this talk we explain how to manipulate the algebras and how to implement the non-commutative calculus on a computer, in order to compute the conductivity tensor of topological insulating materials in the presence of disorder and magnetic fields. Quantitative simulations of the transport experiments on 2D (and possibly 3D) topological insulators will be presented. Since the methodology can treat disorder and magnetic fields in the same time, it enable us to reproduce, for example, the quantization and the plateaus of the Hall conductance.   

Magnetic field induced Quasi Helical Liquid state in a disordered 1D electron system with strong spin-orbit interaction

We study the crossover from a Luttinger liquid to a quasi-helical liquid state in a one-dimensional system of interacting electrons with strong spin-orbit interaction in the presence of a transverse magnetic field, which leads to a gap for one-half of the conducting modes. In particular, we study the effect of gap opening by electron localization in the presence of non-magnetic disorder. We show that the localization length has a nonuniform behavior as a function of the magnetic field. With increasing field, the localization length grows from its zero-field Luttinger-liquid value to a maximum, after which it crosses over to again smaller values corresponding to the localization length of spinless fermions.   

Delocalized States and Topological Edge States of Quantum Walks in Random Environment

The quantum walk (QWs) describe quantum mechanical time evolution of particles, which is identified as random walks when systems are brought to classical limit. QWs can be applied for efficient algorithms of quantum computation and have been realized in experiments. Remarkably, QWs realized by many experiments possess chiral symmetry. Thereby, QWs in a one dimensional (1D) space possibly have non-trivial topological phases and show edge states near boundaries of the system. In this work, we consider QWs interacting with spatial and temporal disorders and study how the edge states of QWs are influenced. Even by introducing the weak spatial disorder to the QWs, the edge states are robust. However, in the strong disorder limit, the energy gap vanishes and the edge states disappear. We found that critical states due to the Anderson transition in the 1D chiral class alternatively appear at energy $\omega=0$ in this case. Significantly, these critical states also appear at $\omega=\pm \pi/2$ for any strength of static disorder, since the extra sublattice symmetry of the time-evolution operator $U$ makes $\omega=\pm \pi/2$ singular. Consequently, for the QWs with relatively weak spatial disorder, the edge states, critical states, and Anderson localized sates are coexist.   

The strong side of weak topological insulators

Three-dimensional topological insulators are classified into ``strong'' (STI) and ``weak'' (WTI) according to the nature of their surface states. While the surface states of the STI are topologically protected, in the WTI they are believed to be very fragile to disorder. In this work we show that the WTI surface states are actually protected from any random perturbation which does not break time-reversal symmetry, and does not close the bulk energy gap. Consequently, the conductivity of metallic surfaces in the clean system will remain finite even in the presence of strong disorder of this type. In the weak disorder limit the surfaces are perfect metals, and strong surface disorder only acts to push them inwards. We find that WTI's differ from STI's primarily in their anisotropy, and that the anisotropy is not a sign of their weakness but rather of their richness.   

Session P32: Focus Session: Dielectric, Ferroelectric, and Piezoelectric Oxides - BiFeO3

Inelastic neutron scattering study of spin-wave from single crystal BiFeO3

BiFeO$_3$ is one of the most promising multiferroic materials for device applications in spintronics and memory devices. There have been a number of studies on electric field tuning of antiferromagnetic domains, as well as possible E-field control of spin-waves in this material. The potential of controlling spin dynamics using electric field is extremely appealing. However, so far there have been very limited work on the direct measurements of spin-waves in BiFeO$_3$, mostly due to lack of large size single crystals. We will present our recent inelastic neutron scattering studies on a single crystal BiFeO$_3$, showing the full spin-wave spectrum in three-dimensions. A classical spin-wave model can be used to describe the results in details. The coupling parameters and spin-wave velocities have been obtained, and are in good agreements with those obtained in Raman measurements.   

Local Raman spectroscopic study of BiFeO$_3$ strained states

Among single-phase multiferroic materials, BiFeO$_3$ (BFO) has relatively high Curie and N$'{e}$el temperatures, which possesses ferroelectric and antiferromagnetic couplings at room temperature, so is motivated for novel device applications. Recent studies had shown piezoelectric and magnetic properties of BFO in strained states varied significantly. For BFO epitaxial films grown on LaAlO$_3$ substrate, high piezoelectric coefficient and spontaneous ferromagnetic moments had been demonstrated in a new kind of morphotropic tetragonal-rhombohedral phase boundary driven by substrate strain. In this study, we used Raman spectrum to investigate the local BFO distorted structure under substrate strain or strain caused by external electric fields. The crystal structure of BFO under compressive substrate strain is monoclinically distorted. The ordering of the monoclinic structures could also be controlled by electric field. These two kinds of strained states were locally studied by atomic force microscopy (AFM) equipped with on-axis Raman measurement. This study provided the basic physical insight of unique physical properties depended on distorted structures.   

Unambiguous phonon mode assignment in multiferroic BiFeO$_3$ single crystals

In Bismuth ferrite (BiFeO$_3$) antiferromagnetic and ferroelectric order parameters coexist at room temperature, making this material an excellent candidate for new functionalities, such as electrical control of magnetism. Despite extensive reports on Raman scattering experiments on single crystals and thin films, controversy still remains in the observation and assignment of the phonon mode symmetries. We present polarized micro-Raman spectroscopy of single crystals ((1 0 0)$_{cubic}$ surface) with uniform ferroelectric polarization. Careful examination of the Raman spectra upon crystal rotation enables us to unambiguously assign the (A$_1$, E$_x$ and E$_y$) modes. We will show that ambiguity is easily introduced by slight misalignment of the crystal and that the crystal rotation is necessary to reach unambiguous mode assignment. Our method not only results in proper Raman mode assignment, which is necessary to describe the phonons critical for the multiferroic behavior, it also allows study of symmetry breaking and may provide a way to non-invasively check the ferroelectric polarization direction.   

Multiple Phase Transitions in the model multiferroic BiFeO3

Bismuth ferrite BiFeO3 (BFO) is commonly considered a model system for multiferroics, and is perhaps the only material that is both magnetic and a ferroelectric with a strong electric polarization at 300K [1]. Despite numerous investigations, the crystal structures of BFO as a function of temperature and pressure are still not established and lead to ongoing controversial reports in the literature [1,3]. Besides being a model multiferroic, BFO is also one of the very few materials that present both octahedra tilts and strong cation displacements at room temperature. Here we report the high-pressure phase transitions in BFO by both synchrotron x-ray diffraction and Raman spectroscopy, namely a surprising richness of six phase transitions in the 0--60 GPa range [2-3]. At low pressures, 4 transitions are evidenced at 4, 6, 7 and 11 GPa. In this range, the crystals display in that range unusual large unit cells and complex domain structures, which suggests a competition between complex tilt systems and possibly off-center cation displacements. The non polar Pnma phase remains stable over a large pressure range between 11 and 38 GPa. The two high pressure phase transitions at 38 and 48 GPa are marked by the occurrence of larger unit cells and an increase of the distortion away from the cubic parent perovskite cell. The previously reported insulator-to-metal transition appears to be symmetry breaking. Finally, we will present a new schematic P-T phase diagram for BFO and discuss the recently reported phase transition in highly strained BFO films [4,5] in the light of our high-pressure findings. \\[4pt] [1] G. Catalan, J. F. Scott, Advanced Materials 21, 1 (2009).\\[0pt] [2] R. Haumont et al., Phys. Rev. B 79, 184110 (2009).\\[0pt] [3] M. Guennou et al., Phys. Rev. B 2011, accepted http://arxiv.org/abs/1108.0704.2011\\[0pt] [4] J. Kreisel et al. J. Phys.: Cond. Matt. 23, 342202 (2011).\\[0pt] [5] W. Siemons et al. Appl. Phys. Express 4 (2011).   

Phase Transitions in Epitaxial (-110) BiFeO$_3$ Films from First Principles*

The effect of misfit strain on properties of epitaxial BiFeO$_3$ films that are grown along the pseudo-cubic [$\bar{1}$10] direction, rather than along the ``usual'' [001] direction, is predicted from density functional theory. These films adopt the monoclinic $Cc$ space group for compressive misfit strains smaller in magnitude than $\simeq$1.6\% and for any investigated tensile strain. In this $Cc$ phase, both polarization and the axis about which antiphase oxygen octahedra tilt rotate {\it within} the epitaxial plane as the strain varies. Surprisingly and unlike in (001) films, for compressive strain larger in magnitude than $\simeq$1.6\%, the polarization vanishes and two orthorhombic phases of $Pnma$ and $P2_12_12_1$ symmetry successively emerge via strain-induced transitions. The $Pnma$-to-$P2_12_12_1$ transition is a rare example of a so-called pure ``gyrotropic'' phase transition, and the $P2_12_12_1$ phase exhibits original interpenetrated arrays of ferroelectric vortices and antivortices. This work is mostly supported by ONR Grants N00014-08-1-0915 and N00014-07-1-0825 (DURIP). *S. Prosandeev, Igor A. Kornev, and L. Bellaiche, Phys. Rev. Lett. 107, 117602 (2011).   

Nonlinearity in the high-electric-field piezoelectric response of epitaxial BiFeO3

The multiferroic material BiFeO3 provides the means to understand the piezoelectric coupling between lattice strain and ferroelectric polarization. Little is known about the piezoelectric properties of BiFeO3 under high electric fields. In our study, the transient high-electric-field piezoelectricity of BiFeO3 was measured at electric fields up to about 300 MV/m using time-resolved x-ray microdiffraction. A linear strain-electric field response with a piezoelectric coefficient of 55 pm/V was observed at electric fields up to 150 MV/m. At higher electric fields, the strain is larger than the value anticipated from the low-field regime, reaching 0.02 at 281 MV/cm. The integrated intensity near the BiFeO3 (002) Bragg reflection is unchanged in large electric fields, showing that the enhanced piezoelectricity occurs without producing a field-induced phase transition. We also observe a relative increase of diffuse x-ray intensity at high electric fields. We will discuss a model in which the increase of piezoelectricity and diffuse scattering originates from the softening of phonon modes at high electric fields.   

Ultrafast photostriction in BiFeO$_{3}$ thin films

The ultrafast dynamics of BiFeO$_{3}$ (BFO) thin films was studied by dual-color transient reflectivity measurements ($\Delta R/R$) from 80 K to room temperature. Based on the thickness-dependent propagating time of the photoinduced strain pulse, the sound velocity along [110] direction of BFO is 4.76 km/s. Anisotropic photostriction effect in BFO generated within short time scale of 5 ps is enhanced by the optical rectification effect. Furthermore, the anomalous changes of the temperaure-dependent $\Delta R/R$ at 130 K and 210 K may reveal the spin-orbital coupling and magnetoelastic effect in BFO.   

Photoinduced structural dynamics in BiFeO$_3$ thin films studied by ultrafast x-ray diffraction

We have used time-resolved x-ray diffraction to study the temporal response of multiferroic BiFeO$_3$ to laser excitation. Above-bandgap light pulses, with 400 nm central wavelength and 50 fs duration, were used to photoexcite 35-nm thick BiFeO$_3$ films grown by molecular beam epitaxy on SrTiO$_3$ (001) substrates. The angular shifts of BiFeO$_3$ Bragg peaks vs.\ time were recorded with $\sim$100 ps resolution and used to determine the out-of-plane strain in the film. Observed strains range up to several tenths of a percent after excitation and relax on a several-ns timescale. Strains of such magnitude are too large to be explained by thermal expansion alone, but rather appear to be due to screening of the depolarization field by photoexcited carriers. At higher laser fluences, the integrated intensity of the Bragg peak decreases due to transient rearrangement of the atomic lattice on the scale of $\sim$0.1 \AA.   

The valence electronic structure of multiferroic BiFeO$_{3}$ from high energy X-ray photo-electron spectroscopy and first principles theory

BiFeO3 (BFO) is a multi-functional material with high ferroelectric and magnetic ordering temperature. Here we have investigated the electronic structure of (001) oriented 100nm rhombohedral BFO thin films using high energy X-ray photoelectron spectroscopy (XPS). By making use of the energy dependence of the relative cross sections for different states, we were able to selectively probe the elemental contributions to the valence band . At high energies, states with high main quantum number will have a higher relative probability for photo-ionization, i.e., the Bi 6s and 6p contributions in the valence region are enhanced relative to the Fe 3d and O 2p. We find that the Bi 6p states hybridize strongly with the valence band dominated by the Fe 3d and O 2p states, resulting in a splitting of the 3d states due to bonding and anti-bonding combinations with the Bi 6p. Our results thus suggest that a previously relatively ignored electronic interaction needs to be considered for BFO and related Bi-TMOs. Ab initio calculations indicate the importance of screened Coulomb correlations to describe Bi and Fe electronic states.   

Direct characterization of the surface layer of BiFeO3 Single Crystals

A surface layer different from the bulk was found in single crystals of BiFeO3, and was analyzed with different techniques such as surface impedance and grazing incidence x-ray diffraction, showing an specific phase transition at T* $\sim $ 275 $^{\circ}$C. Local physical characterization studies have been performed with different AFM techniques, such as Piezoelectric Force Microscopy (PFM), Scanning Kelvin Probe Microscopy (SKPM) and Force Modulated Microscopy (FMM) at different temperatures up to 300$^{\circ}$C. The thin superficial skin layer is found to be an electrically ``dead'' layer with a thickness of 6 +/- 1 nm and different thermal expansion coefficient with respect to the bulk. The sharp thermal expansion of the surface layer at T* facilitates its mechanical declamping from the underlying crystal at the transition temperature, enabling direct access to the sub-surface region using scanning probe microsocopy techniques. A distribution of near-surface ferroelectric domains is found in a region of less than 200 nanometers deph under the surface. These nanodomains organize in a hierarchical metastructure on top of the existing bulk domains. The symmetry and properties of these sub-surface nanodomains will be discussed.   

First-principles investigation of phase-change effects in multiferroics

Using first-principles calculations we have characterized new phases of bulk multiferroic materials that are close in energy to the ground state, but that display very different properties. This suggests that the application of electric fields could induce phase changes that would involve large effects of different kinds. In particular, (i) we have found stable supertetragonal bulk phases for the prototype multiferroic bismuth ferrite [Di\'eguez {\em et al}, Phys.~Rev.~B {\bf 83}, 094105 (2011)], and (ii) we propose to use a solid solution of bismuth ferrite and bismuth cobaltite to create a material where it is possible to switch between two very different phases in a way that involves strong piezoelectric, electric, and magnetoelectric effects [Di\'eguez and \'I\~niguez, Phys.~Rev.~Lett. {\bf 107}, 057601 (2011)].   

Magnetic and structural properties of BiFeO$_{3}$ thin films grown epitaxially on SrTiO$_{3}$/Si substrates

Commensurate growth of SrTiO$_{3}$ (STO) on Si using molecular beam epitaxy (MBE) has been achieved. STO on Si is used as a virtual substrate to enable the growth of BiFeO$_{3}$ (BFO). Having a crystalline oxide surface on Si is an enabler for deposition of various functional oxides that would not have been possible directly on silicon. A systematic study of the dependence of the magnetic and structural properties of BFO on the growth conditions, such as O$_{2}$ plasma pressure and film thickness, is performed. The crystalline nature of the BFO film has been confirmed by X-Ray diffraction showing the expected peak positions for (100) oriented oxide films with no additional, unidentified peaks. The BFO/STO/Si films exhibit antiferromagnetic behavior with high transition temperatures, thus leading to the possibility of room temperature magnetoelectric coupling-based devices integrated onto Si CMOS circuitry. Thinner films at lower O$_{2}$ plasma pressures exhibit stronger magnetic characteristics.   

Magnetoelectric and Ferroelectric Properties of BiFeO3/Ni film Deposited by Pulsed Laser Deposition

To fabricate a layer by layer (2-2) magnetoelectric (ME) sensor, ferroelectric (FE) BiFeO3 film was directly deposited on ferromagnetic (FM) nickel foil by pulsed laser deposition (PLD) without oxide or noble metal buffer layer, which significantly lowers the cost of ME and FE devices, and makes it possible to deposit longer ME and FE bendable band by PLD. X-ray diffraction and transmission electron microscopy analysis confirmed that the BiFeO3 film was successfully deposited on the top of nickel foil. The BiFeO3 film had a saturation polarization and a piezoelectric d33 coefficient of 69 $\mu $C/cm2 and 52 pm/V respectively. The ME coefficient of the sample was 4mV/cmOe which was measured under 1 Oe AC magnetic field at 1 kHz frequency.   

Session P33: Physics of Photovoltaics and other Electricity Production

Light Reflection and Absorption by Free Standing Titania Nanotube Arrays

Newly discovered titania nanotube arrays fabricated by anodization have become the main interest of many research groups around the world, mainly due to their potential use for solar energy harvesting. Light conversion to electron-hole pairs can occur in the surface area of these nanotubes, resulting from their very high aspect ratio and the low recombination probability in the titian surface. In our work we have investigated the transmission of light though the nanotubes for different tube lengths, to explore the penetration depth for various wavelengths in the nanotube array. Specifically, we have investigated the reflection and transmission of light incident from both the open and closed sides of the nanotubes. We find that the reflectivity is generally noticeably smaller for light incident on the open end, but the transmission is about the same, implying greater absorption for light incident on the open side, although for wavelengths close to 400nm, the reflectivity from the open side becomes larger than that from the closed side. We will present a theoretical model to explain our experimental results. The model treats wave propagation along the tubes in the eikonel approximation, and wave propagation transverse to the tubes as Bloch waves.   

A quantum-mechanical study of ZnO and TiO$_{2}$ based DSC

Since the pioneering work of Graetzel, Dye Sensitized Cells (DSCs) have attracted great attention as cheap and effective solar power devices based on wide bandgap metal oxide electrode. Optimization of the DSC is a challenging task as it is a highly complex interacting molecular system. Surface properties of the metal-oxide and proper sensitization with dyes may strongly affect the efficiency. Optimizated DSCs based on TiO2 photoanodes and organic dye have reached conversion efficiency of about 10{\%} whereas the efficiency of ZnO based DSC has been found to be much lower, although this material has photochemical properties similar to TiO2, in general due to the nature of the binding between sensitizer and semiconductor. For this reason understanding how anchoring groups interact with the metal-oxide is fundamental to shed light on the different behaviour of these materials in DSC. Aim of this work is to address the binding of small organic sensitizers, such as catechol and isonicotinic acid molecules, to TiO2 and ZnO surfaces, in terms of geometry, stability, electronic structure and absorption properties. To this end, we employed quantum-mechanical simulations based on hybrid DFT and hybrid TDDFT.   

Band gap engineering and optical properties of tungsten trioxide

Tungsten trioxide (WO3) is a good photoanode material for water oxidation but it is not an efficient absorber of sunlight because of its large band gap (2.6 eV). Recently, stable clathrates of WO3 with interstitial N2 molecules were synthesized [1], which are isostructural to monoclinic WO3 but have a substantially smaller bang gap, 1.8 eV. We have studied the structural, electronic, an vibrational properties of N2-WO3 clathrates using ab-initio calculations and analyzed the physical origin of their gap reduction. We also studied the effect of atomic dopants, in particular rare gases. Substantial band gap reduction has been observed, especially in the case of doping with Xe, due to both electronic and structural effects. Absorption spectra have been computed by solving the Bethe-Salpeter Equation [2] to gain a thourough insight into the optical properties of pure and doped tungsten trioxide. [1] Q. Mi, Y. Ping, Y. Li., B.S. Brunschwig, G. Galli, H B. Gray, N S. Lewis (preprint) [2]D. Rocca, D. Lu and G. Galli, J. Chem. Phys. 133, 164109 (2010)   

Aluminum nanoparticles for plasmonic enhancement of light absorption in organic semiconductors

The energy conversion efficiency in organic photovoltaic (OPV) devices is low, primarily due to the short exciton diffusion length (~10 nm) in organic semiconductors. Much effort is currently devoted to improving the optical absorptivities in OPVs by incorporating plasmonic nanostuctures in or near the active layer, which would allow for significantly thinner absorption layers. We suggest that aluminum nanoparticles are a better choice than gold or silver particles for embedded plasmonic structures in OPV active layers. This is due to the high plasmon frequency of Al, which makes it easy to create good overlap between plasmon resonance and semiconductor absorption band in a random mixture of polymer and nanoparticles. We will present both modeling and experimental data to support this hypothesis.   

Si-SiGe hetero-structure thin-film solar cells using integrated electro-optical modeling

Hetrostructure Si-SiGe thin film solar cells have been designed and optimized using advance electro-optical theoretical modeling and simulation. Some of the key characteristics such as short-circuit current density (Jsc), open circuit voltage (Voc), and fill factor (FF) have been studied for varying concentration of Ge in SiGe buffer layer. The effect of thickness variation of alloyed layer for varying Ge composition $\sim $0.1---10{\%} has been performed. The improvement in the conversion efficiency of these cells was calculating by tailoring the thickness of p+ doped layer. An approach relying on phenomena of improved absorption of the alloys which leads to a gain in the current was explored. Improved infrared response with higher short circuit current has been obtained for about 25 $\mu $m thick structures. With the optimized Ge concentration, and the incorporated structure design parameters, as much as 4-6{\%} enhancement in the overall efficiency of the solar cells has been calculated compared to that of the conventional single crystal Si solar cells. Moreover, the efficiency of these cells can further be improved because Si-SiGe based solar cells have improved absorption characteristics and offer minimum operating temperature sensitivity. It is believed that with better understanding of the band-gap engineering of SiGe when used as buffer and junction layers, the overall conversion efficiency of such devices can further be improved and could play a critical role to develop low cost and high efficiency solar cells technology. This work is supported by DOE grant DE-FG02-10ER46709 and the state of South Dakota.   

Multiscale Simulation on a Light-Harvesting Molecular Triad

We have investigated the effect of solvation and confinement on an artificial photosynthetic material, carotenoid-porphyrin-C$_{60}$ molecular triad, by a multiscale approach and an enhanced sampling technique (Replica Exchange Method). We have developed a combined approach of quantum chemistry, statistical physics, and all-atomistic molecular dynamics simulation to determine the partial atomic charges of the ground-state triad. The confinement effects on the triad were modeled by imposing three sizes of spherocylindrical nanocapsules. The triad is structurally flexible under ambient conditions and its conformation distribution is manipulated by the choice of water models and confinement. Two types of water models (SPC/E and TIP3P) are used for solvation. We have shown that a slight structural difference in the two water models with the same dipole moment can have great distinction in water density, water orientation and the number of hydrogen bonds in the proximity of a large flexible compound such as the triad. Subsequently, it has direct impact on the position of the triad in a confinement as well as the distribution of conformations at the interface of liquid and solid in a finite-size system.   

The Potential for Hot Carrier Collection from an Amorphous Semiconductor

The quest for clean, inexpensive sources of energy has produced intense interest in low-cost methods for dramatically increasing the efficiencies of solar cells. One such method is to collect carriers before they lose energy to heat. Here we present strong evidence for such hot carrier transfer in an unlikely place, between the amorphous and crystalline regions of nanocrystalline Si. Nanocrystalline Si is a thin film photovoltaic material formed of Si nanocrystallites imbedded in a hydrogenated amorphous Si matrix. Using a combination of photoluminescence quenching and electron spin resonance measurements as a function of nanocrystalline fraction, we observe clear evidence that above a critical fraction carriers excited in the amorphous region transfer to the nanocrystals rather than relaxing to band tail states of the amorphous silicon matrix. The average nanocrystallite spacing is consistent with estimates of the distance hot carriers can transfer in amorphous silicon before thermalization. This result has implications that extend from improving the stability of amorphous silicon under optical illumination to the development of a new paradigm in solar cell design using nanostructured amorphous absorbers.   

Enhanced photocatalytic H$_2$ production by sub-nanometer Au nanoparticles

CdS surfaces were found to exhibit a dramatically enhanced photo-catalytic activity for the water splitting process when loaded with sub-nanometer Au particles, compared to that of bare CdS or CdS modified with other noble metal particles (Pt, Ru, Rh, Pd) with similar co-catalyst loading. As a first step towards understanding the striking photocatalytic activity of Au/CdS, we conducted a detailed characterization of the Au particle samples by combining mass spectroscopy, (scanning) transmission electron microscopy, UV-vis spectroscopy and first-principle calculations. In particular, we carried out systematic studies of the influence of particle size, surface termination and charge state on the structural, electronic and spectroscopic properties of ligand-protected Au nanoparticles using density functional theory and time-dependent density functional theory. The structural and electronic stability of diphosphine-protected Au$_9$ and Au$_{11}$ cations with composition determined from mass spectroscopy, were confirmed from total energy and electronic structure calculations, while the computed optical absorption spectra were found to be in excellent agreement with UV-vis data. Work is underway to study the interaction between Au particles and Cds.   

First-principles study of III-V electrode interfaces for photoelectrochemical hydrogen production

Photoelectrochemical (PEC) cells promise clean, sustainable production of hydrogen fuel using water and sunlight. However, combining solar conversion efficiency with durability in electrolyte solution has proven difficult, in part because the complex chemistry active at the electrode-electrolyte interface remains poorly understood. We use first-principles molecular dynamics simulations and model density-functional calculations to study the structure, stability, and chemical activity of GaP/InP semiconductor electrodes in contact with water. We find that a local bond-topological model is able to capture much of the basic surface chemistry. Interpretation of our results points to the particular importance of surface-adsorbed oxygen in determining the available reaction pathways for photocorrosion and water dissociation. Electronic signatures of the local bond topologies are compared to data from X-ray absorption and emission spectroscopy for insight into actual electrode structure.   

Photoresistance of Li$_{0.2}$Zn$_{0.8}$O thin films and Li$_{0.2}$Zn$_{0.8}$O/Al$_{0.15}$Zn$_{0.85}$O multilayers

Thin films of Li$_{0.2}$Zn$_{0.8}$O and Li$_{0.2}$Zn$_{0.8}$O/Al$_{0.15}$Zn$_{0.85}$O multilayers have been grown on sapphire substrates by pulsed-laser deposition. P-type Li$_{0.2}$Zn$_{0.8}$O is an insulator with the band gap of 3.26eV and n-type Al$_{0.15}$Zn$_{0.85}$O is a transparent conductor. Under the ultraviolet light irradiation, the Li$_{0.2}$Zn$_{0.8}$O films show photoresistance (PR) (PR=(R$_{dark}-$R$_{irradiation})$/R$_{irradiation})$ effect of $\sim $340{\%} at room temperature. In bilayers of Li$_{0.2}$Zn$_{0.8}$O($\sim $110nm)/Al$_{0.15}$Zn$_{0.85}$O($\sim $90nm) where a p-n junction is formed, the photoresistance increases to $\sim $8600{\%}. The photoresistance dependence on wavelength (300-700nm) measurement shows that the photoresistance effect is observed when the light wavelength is below $\sim $380nm. The dependence of the light intensity and the response time of the resistance switching have also been measured and will be discussed. The largely enhanced PR effect in bilayers is probably due to the enhancement of the PR effect in p-n junctions. The increased photosensitivity indicates that the Li$_{0.2}$Zn$_{0.8}$O/Al$_{0.15}$Zn$_{0.85}$O multilayers are promising for UV photodetection applications.   

Materials considerations for the efficiency of photon-enhanced thermionic emission

Photon-Enhanced Thermionic Emission (PETE) is a promising method of solar energy conversion that is based on thermal emission of photoexcited electrons from a high-temperature semiconductor, making it attractive for use in tandem with solar thermal systems. Theoretical efficiencies of PETE devices can exceed those of single junction photovoltaics, but experimental tests of the PETE process have displayed low efficiencies.[1] Here we examine this disconnect between experimental results and theoretical promise. We analyze the effects of real semiconductor parameters on the PETE process and relate them to the ultimate performance of a PETE device, directly translating non-idealities of practical materials into constraints on conversion efficiency. The analysis identifies fundamental challenges to efficient conversion based on PETE and establishes design rules that direct the search for semiconductor systems to form the basis of real devices. We also review experimental work guided by these insights that shows increased emission efficiency due to reduced surface recombination, one of the key challenges for realistic solar energy conversion based on PETE.\\ $\left[1\right]$ J.W. Schwede, {\it et al.}, Nat. Mater. {\bf 9}, 762-767 (2010)   

Designing long wavelength barrier based thermophotovoltaic cells

This work describes the process of designing low source-temperature thermophotovoltaic cells to harvest energy from thermal sources in the 7-10 micron range. Simulations of the bandstructure are performed for strained layer superlattices (SLS) to act as the p, B, and n regions in a TPV barrier diode. Simulations of the band-alignment of these regions are then performed to ensure a high enough barrier in one band, and a smooth transition in the other. The process is performed iteratively to find an ideal match of specific SLS material and doping concentration in each region. Both conduction band barriers and valence band barriers have been investigated. The balance of system and test apparatuses will also be discussed, as well as preliminary results from samples grown via MBE.   

Thought waves remotely affect the performance (output voltage) of photoelectric cells

In our experiments, thought waves have been shown to be capable of changing (affecting) the output voltage of photovoltaic cells located from as far away as 1-3 meters. There are no wires between brain and photoelectric cell and so it is presumed only the thought waves act on the photoelectric cell. In continual rotations, the experiments tested different solar cells, measuring devices and lamps, and the experiments were done in different labs. The first experiment was conducted on Oct 2002. Tests are ongoing. Conclusions and assumptions include: 1) the slow thought wave has the energy of space-time as defined by C1.00007: The mass, energy, space and time systemic theory- MEST. Every process releases a field effect electrical vibration which be transmitted and focussed in particular paths; 2) it has a information of order of tester; 3) the brain (with the physical system of MEST which like a hardware) and consciousness (with the spirit system of the mind, consciousness, emotion and desire-MECD! which like a software) build up a life-informational computer, through some algorithms of DNA and RNA, produce the life-information (include the Genetic code). The Life-Information is a female parent of any information; 4) human can optimize the information. This function is the intelligence; 5) In our experiments, not only thought waves can affect the voltage of the output pho toelectric signals by its energy, but they can also selectively increa! se or decrease those photoelectric currents through remote consciousness interface by a life-information technology.   

Session P34: Focus Session: Nano III: New Nanoscale Fabrication and Sensing

Computational nanomaterials for novel desalination membrane design: Nanoporous graphene

We describe a novel approach for desalination based on nanoporous graphene. Our molecular dynamics calculations show that freestanding graphene patterned with nanometer-sized pores can act as an ultra-thin filtration membrane. Due to size exclusion and chemical interactions with the confining pores, salt ions can be blocked from permeating the membrane at sufficiently small pore diameters. Notably, the pore diameter and the chemical interactions at the water-membrane interface are most important criteria for this system's desalination performance. We will share insights from Molecular Dynamics calculations regarding the theoretical performance of this membrane system and the effects of chemical passivation of the graphene pores on the filtration dynamics. Although the narrow range of acceptable pore sizes suggests that further design innovations will be necessary at the molecular scale before large-scale applications are possible, our existing results predict that pressure requirements for this system can be made roughly competitive with commercial Reverse Osmosis.   

Structural and electronic properties of bare and capped CdnSen/CdnTen nanoparticles (n = 6, 9)

Relationships between structures and properties (energy gaps, vertical ionization potentials (IP$_{v})$, vertical electron affinities (EA$_{v})$, and ligand binding energies) in small capped CdSe/CdTe nanoparticles (NPs) are poorly understood. We have performed the first systematic density functional theory study of the structures and electronic properties of Cd$_{n}$Se$_{n}$/Cd$_{n}$Te$_{n}$ NPs (n = 6, 9), both bare and capped with NH$_{3}$-, SCH$_{3}$, and OPH$_{3}$-ligands. NH$_{3}$- and OPH$_{3}$-ligands cause HOMO/LUMO energy \textit{destabilization} in capped NPs, more pronounced for the LUMOs than for the HOMOs. Orbital destabilization drastically reduces both the IP$_{v}$ and EA$_{v}$ of the NPs compared with the bare NPs. For SCH$_{3}$-capped Cd$_{6}$X$_{6}$ NPs, formation of expanded structures was found to be preferable to crystal-like structures. SCH$_{3}$-groups cause \textit{destabilization} of the HOMOs of the capped NPs and \textit{stabilization} of their LUMOs, which indicates a reduction of the IP$_{v}$ of the capped NPs compared with the bare NPs. For the Cd$_{9}$X$_{9}$ NPs, similar trends in stabilization/destabilization of frontier orbitals were observed.   

New Directions in Plasmonics: Pushing the Space-Time Limit

The lecture will begin with the discussion of our efforts to provide a robust existence proof for SMSERS (Single Molecule Surface Enhanced Raman Spectroscopy). Further, fundamental questions such as: (1) what is the largest possible enhancement factor (EF) and (2) what nanostructure produces the largest EF, will be addressed. Our approach to answering these questions involved the development of new tools such as single nanoparticle SERS and single nanoparticle LSPR spectroscopy spatially correlated with high resolution transmission electron microscopy (HRTEM). Recent results using LSPR biosensors to detect molecular binding events and conformation changes will be presented, including discussions of: (1) pushing the sensitivity of plasmonic biosensors towards the single-molecule detection limit, (2) combining LSPR with complementary molecular identification techniques such as matrix assisted laser desorption ionization mass spectrometry (MALDI-MS), and (3) the development of new instrumentation for high throughput plasmonic biosensing, and gas sensing with plasmonic nanosensors. Finally, recent developments showing that for the first time, the revolutionary techniques of surface enhanced Raman spectroscopy and femtosecond stimulated Raman spectroscopy (FSRS) can be combined and substantial progress in tip-enhanced Raman spectroscopy (TERS) will be presented. A UHV-TERS instrument has been constructed with atomic resolution of the surface and sub-molecular resolution of the adsorbate, as illustrated with the copper phthalocyanine (CuPc)/Ag(111) system. We can now foresee the day when it will be possible to combine UHV-TERS and surface enhanced FSRS to enable single-molecule spectroscopy with simultaneous nanometer spatial resolution and femtosecond time resolution.   

Charge Distributions in Polar Semiconductor Nanorods explored with Linear-Scaling DFT Calculations

Binary polar semiconductors in the wurtzite structure can be grown as nanorods along $\pm$[0001]. In such structures, large dipole moments have been observed. We have studied the distribution of charge in GaAs and ZnO nanorods to elucidate the origin of the dipole moments. To make contact with realistic experiments, rods containing thousands of atoms are simulated using Linear-Scaling DFT calculations with ONETEP [1]. From our calculations we show that both the direction and magnitude of the dipole moment of a nanorod, and its electric field, depend sensitively on how its surfaces are terminated, not on the spontaneous polarization of the underlying lattice. Furthermore, we observe that the Fermi level for an isolated nanorod always coincides with significant density of electronic surface states on its polar surfaces (either mid-gap states or band-edge states). These states pin the Fermi level, and therefore fix the potential difference along the rod. We provide evidence that this effect has a determining influence on the polarity of nanorods, with consequences for the response to changes in surface chemistry, scaling of dipole moment with size, and dependence of polarity on composition.\\[4pt] [1] C. Skylaris et al, JCP 122, 084119 (2005).\\[0pt] [2] P. Avraam et al, PRB 83 241402(R) (2011).   

Reduction of graphene oxide to graphene, A study of changes in the atomic structure

An economic method for large scale production of graphene is based on exfoliation of graphite into 1-atom thick sheets by oxidation, creating graphene oxide (GO) and subsequent reduction of GO into graphene. Reduced GO sheets approach the highly desired properties of graphene, such as electrical conductivity and mechanical strength, to various degrees, but not completely. To understand why, we must understand the nanostructure of the sheets. Different methods of reduction result in products that are similar to graphene, but these products retain some oxidized areas or contain regions with sp$^{3}$ bonded carbon. The concentration and distribution of these defects on the reduced GO sheet affect the properties of the 2D material. Here, we have characterized the atomic structure of GO and reduced GO via high resolution transmission electron microscopy, electron diffraction, and electron energy loss spectroscopy. Spectroscopic data taken during thermal reduction of GO shows changes in the fine structure of carbon K-edge as the carbon changes from an oxidized form to elemental amorphous carbon to graphite like form, clearly delineating the process of reduction of GO to graphene. Products of several other reduction methods are also characterized revealing information on electronic environment surrounding carbon atoms, distribution of crystalline areas, and oxygen removal from GO.   

Atomic-scale mapping of cerium valence in ceria-zirconia-supported Pd model planar catalysts

Cerium-based oxides have long been regarded as an important class of catalyst support materials. It is also recognized that the interaction between precious metal and ceria-based support material enhances the reducibility of the ceria. The combination of scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) can provide an atomic-scale picture of the interaction between precious metal particles and their support material. In our work, aberration corrected STEM-EELS is used to study the valence of cerium in the vicinity of palladium nanoparticles supported on a ceria-zirconia (CZO) thin film. A monolayer-equivalent of Pd was deposited onto a 50nm-thick CZO thin film, which was then subjected to different thermal treatments. The EELS spectra extracted from the top several atomic layers of the CZO film exhibit typical 3+ character following a low-temperature reduction treatment, indicating the formation of oxygen vacancies. A variety of control experiments have also been performed to exclude possible artifacts caused by electron beam irradiation.   

Correlated Percolation Model of Graphene Hydrogenation

Hydrogenation of graphene by exposure to an atomic hydrogen plasma is a random process. However, the presence of hydrogen already attached to the plane increases the sticking probability of incoming adatoms. We simulate this process as a correlated percolation model where the hydrogen occupation probability of a carbon site is increased or decreased depending on the hydrogenation of the nearest neighboring carbon atoms. This enhancement modifies the cluster distribution on the surface and consequently the electronic structure of the system. We study these effects with a tight binding model and find that, although the density of states at the Fermi level is greatly increased by hydrogenation, the inverse participation ratio shows that not all of these states will contribute to conduction. In fact, for hydrogenation levels of greater than 40\% of the lattice, localized states begin to dominate at the Fermi level. A realistic set of values for the sticking probabilities is determined by analysis of the STM images of this system. Through the modeling of this mesoscopic process, we gain a better understanding of how chemically modified graphene is produced and what its transport properties are.   

Effect of charged impurities and morphology on oxidation reactivity of graphene

Chemical reactivity of single layer graphene supported on a substrate is observed to be enhanced over thicker graphene. Possible mechanisms for the enhancement are Fermi level fluctuations due to ionized impurities on the substrate, and structural deformation of graphene induced by coupling to the substrate geometry. Here, we study the substrate-dependent oxidation reactivity of graphene, employing various substrates such as SiO$_{2}$, mica, SiO$_{2}$ nanoparticle thin film, and hexagonal boron nitride, which exhibit different charged impurity concentrations and surface roughness. Graphene is prepared on each substrate via mechanical exfoliation and oxidized in Ar/O$_{2}$ mixture at temperatures from 400-600 $^{\circ}$C. After oxidation, the Raman spectrum of graphene is measured, and the Raman D to G peak ratio is used to quantify the density of point defects introduced by oxidation. We will discuss the correlations among the defect density in oxidized graphene, substrate charge inhomogeneity, substrate corrugations, and graphene layer thickness. This work has been supported by the University of Maryland NSF-MRSEC under Grant No. DMR 05-20471 with supplemental funding from NRI, and NSF-DMR 08-04976.   

Numerical Study of Carrier Multiplication in Nanocrystalline form of PbSe and PbS and the bulk

Using previously developed Exciton Scattering Model, we report on systematic numerical study of the carrier multiplication (CM) dynamics in spherically symmetric PbSe and PbS nanocrystals (NCs) and bulk. The quantum efficiency (QE) associated with the photogeneration and population relaxation processes are calculated. It is found that the photogeneration event provides small, about $5\%$, contribution to the total QE compared to the contribution from the population relaxation process. The analysis shows that the impact ionization dynamics is the main mechanism responsible for the CM during {\em both} the photogeneration and the population relaxation events. Furthermore, the calculated photogeneration and total QEs for various size NCs are found never to exceed the calculated limiting values for bulk. This observation is explained in terms of the quantum-confinement induced weak effective Coulomb enhancement whose contribution to the impact ionization rate is fully suppressed by the reduction in the biexciton density of states. We also find weak dependence of the total QE on the transform limited pump pulse duration. Comparison of the calculated QEs to published experimental data shows that our calculations well reproduce the experimentally observed trends.   

Session P35: Focus Session: DFT V: Partitioning and Embedding Theories; Finite-Temperature

Exactly Embedded Density Functional Theory for Modeling Chemical Reactions

We will describe embedded density functional theory methods for performing accurate and scalable electronic structure theory calculations in large molecular systems [1,2], with application to clusters, liquids, and electrode interfaces. \\[4pt] [1] Goodpaster JD, Ananth N, Manby FR, and Miller TF, J. Chem. Phys., 133 (2010) 084103. \\[0pt] [2] Goodpaster JD, Barnes TA, and Miller TF, J. Chem. Phys., 134 (2011) 164108.   

Exactly embedded DFT for the efficient simulation of large systems

Although standard wavefunction-based approaches to electronic structure problems have experienced great success in the study of small systems, their poor size scaling prohibits their application to larger systems. One promising technique for overcoming this scaling problem is embedded Density Functional Theory (e-DFT), in which a large system is divided into many smaller subsystems, with individual wavefunction-based calculations being performed on each subsystem. Using our newly developed Exactly Embedded (EE) technique, we demonstrate highly accurate e-DFT calculations on aqueous systems consisting of hundreds of atoms. Furthermore, these calculations are shown to exhibit excellent size scaling and to be massively parallelizable, allowing for efficient calculations of condensed-phase systems.   

A unified quantum mechanics embedding theory for materials and molecules

It is essentially impossible to apply highly accurate quantum mechanics methods to large material samples, creating a need for a sophisticated embedding theory that can locally refine the accuracy of predicted properties. Here, we present a new ab-initio embedding theory that can treat different regions in the material with quantum mechanics methods of appropriately varying levels of accuracy in a seamless way. We first remove the non-uniqueness of embedding potential definitions that exists in most previous embedding theories by introducing a physical constraint that all regions share a common embedding (interaction) potential. We then introduce a key step to achieve seamless embedding: reformulating the system's total energy solely in terms of the embedding potential, i.e., we construct a potential-functional embedding theory (PFET). We demonstrate how to efficiently solve PFET for molecules and materials and give an outlook for how to perform seamless ``multi-physics'' material simulations with PFET.   

Multi-level partitioning using embedded density functional theory

Embedded density functional theory (e-DFT) methods are typically limited to the description of weakly interacting systems, because an exact form of the kinetic energy (KE) density functional is unknown. We have developed a method that avoids approximations to the KE functional and provides a formally exact approach to performing electronic structure calculations in the e-DFT framework. This framework allows systems to be divided into smaller subsystems which can be treated at different levels of theory with the inter-subsystem potential calculated using our e-DFT protocol. Therefore, in regions of large systems where DFT is known to perform poorly, such as van der Waals interactions and strongly correlated electrons, wavefunction calculations can be used. We discuss density partitioning strategies for embedded density functional theory and the accuracy of this multi-level method.   

Partition Density Functional Theory

Partition Density Functional Theory (PDFT) is a formally exact method for obtaining molecular properties from self-consistent calculations on isolated fragments [1,2]. For a given choice of fragmentation, PDFT outputs the (in principle exact) molecular energy and density, as well as fragment densities that sum to the correct molecular density. I describe our progress understanding the behavior of the fragment energies as a function of fragment occupations, derivative discontinuities, practical implementation, and applications of PDFT to small molecules. I also discuss implications for ground-state Density Functional Theory, such as the promise of PDFT to circumvent the delocalization error of approximate density functionals. \\[4pt] [1] M.H. Cohen and A. Wasserman, J. Phys. Chem. A, 111, 2229(2007).\\[0pt] [2] P. Elliott, K. Burke, M.H. Cohen, and A. Wasserman, Phys. Rev. A 82, 024501 (2010).   

Finding Partition Potentials

Partition Density Functional Theory is a formally exact approach to partitioning molecules into fragments via functional minimization and constraints on fragment densities. Cohen and Car proposed a Dynamical Optimization Algorithm for Partition Theory inspired by the Car-Parrinello Method of electronic structure [1]. We modify this algorithm to incorporate a reference HOMO wave-function calculation as a guide to obtain the partition potential, a global quantity arising as the Lagrange multiplier that guarantees satisfaction of the density constraint. We report on the implementation of this procedure for one-dimensional systems, and possible implications for linear-scaling electronic-structure calculations.\\[4pt] [1] M. H. Cohen, and R. Car, J. Phys. Chem. A 2008, 112, 571-575   

Temperature dependence of Thomas-Fermi errors

Finite temperature Thomas-Fermi theory, in addition to its success in systems dominated by classical behavior, can also form the basis for development of a finite temperature local density approximation and its leading corrections. It is therefore imperative that we fully understand its limitations and strengths. To this end, the temperature dependence of Thomas-Fermi errors in densities and integrated quantities for simple models is explored. Behavior of finite-temperature Thomas-Fermi theory in limiting cases will be discussed in the contexts of traditional DFT and its semiclassical foundations. Analysis of finite temperature Thomas-Fermi as a potential functional will be presented.   

Exact Conditions in Finite-Temperature Density-Functional Theory

Density-Functional Theory (DFT) for electrons at finite-temperature is increasingly important in condensed matter and chemistry. The exact conditions that have proven crucial in constraining and constructing accurate approximations for ground-state DFT are generalized to finite-temperature, including the adiabatic connection formula [1]. We discuss consequences for functional construction. \\[4pt] [1] S. Pittalis, C. R. Proetto, A. Floris, A. Sanna, C. Bersier, K. Burke, and E. K. U. Gross, Phys. Rev. Lett, 107, 163001 (2011)   

Systematic Results in Finite Temperature DFT

An exact representation for the non-interacting free energy density functional is identified from the thermodynamics of a non-uniform, finite temperature system of particles in an external potential. A formally exact functional density expansion whose leading term is the Thomas-Fermi approximation is described to second order in the density non-uniformity. The familiar Perrot form [Phys. Rev. A 20, 586 (1979)] is recovered from a subsequent smooth gradient expansion. A related formal expansion about the Thomas-Fermi plus von Weizsacker functional to second order is also described and discussed.   

Finite Temperature Scaling of the Free Energy Density Functional

Recently, we reported exact scaling relations and related bounds and inequalities for the non-interacting (``Kohn-Sham'') free-energy density functional [Phys. Rev. B \textbf{84}, 125118 (2011)]. Here we extend the analysis to obtain similar results for the full,interacting functional. The extension is obtained by dimensionless analysis for the case of $v$-representable densities. Connection with the corresponding ground-state scaling is made.   

Construction of Generalized Gradient Approximation Free Energy Density Functionals

By analyis of the second-order gradient approximation (SGA) for the non-interacting electron free energy density, we propose a finite-temperature generalized gradient approximation (ftGGA) for the noninteracting free energy density (both kinetic and entropic contributions). By analogy we also introduce a ftGGA for the exchange free energy density functional. We have implemented the finite-temperature Thomas-Fermi (ftTF), SGA, and a new finite-temperature GGA free-energy functional in the orbital-free density functional theory (OFDFT) code PROFESS. We compare self-consistent OFDFT results with standard Kohn-Sham data. The local pseudopotentials used in the OFDFT calculations are validated by comparison between Kohn-Sham results obtained with standard non-local pseudopotentials and with the same local pseudopotentials.   

Session P36: Focus Session: Environment III: Nanoparticles, Surfaces and Catalysis

Nanominerals, Mineral Nanoparticles, and Earth Processes: Details on How Nanoparticles Work in the Environment

Naturally occurring inorganic nanoparticles have been one of the principal catalytic components of Earth throughout its history. Yet these ubiquitous materials have largely escaped our close scrutiny until very recently. They are illusive and difficult to study. They have properties that change significantly with their exact size, shape, aggregation state, and surrounding environment. In the past, it has not even been clear how they accumulate, disperse, and move around the planet, nor even for sure what their major sources and sinks are. We have now compiled and derived a global budget for naturally occurring inorganic nanoparticles, including an assessment of their sources and sinks, as well as their fluxes between various Earth compartments (atmosphere, continents, continental shelves, and open oceans). In addition, these kinds of budgets provide a basis for fundamental understanding such as residence and transfer times between compartments. Specific findings include the following: 1) The primary producer of Earth's inorganic nanoparticles is soil through terrestrial weathering processes; 2) rivers, and to a lesser extent glaciers, bring 0.1\% to 0.01\% of the Earth's continental nanomaterial reservoir to the continental edge/ocean margins each year; 3) only about 1.5\% of this material makes it to the deep oceans due to aggregation and settling in saline ocean margins; and 4) the airborne and waterborne inputs of nanominerals and mineral nanoparticles to the open oceans are very similar. These kinds of results, along with a much better understanding of the characteristics of naturally occurring inorganic nanoparticles via direct observation in the field coupled with laboratory studies, provide a useful foundation upon which to predict the behavior and fate of manufactured nanoparticles, many of which are very similar to naturally occurring varieties. Even when specific correlations between naturally occurring and manufactured nanoparticles cannot be made, important clues in manufactured nanoparticle behavior in complex environments can be obtain by observing natural systems.   

Surface Reactivity of Core Shell Iron-Iron Oxide Nanoclusters towards Breakdown of Carbon Tetrachloride

Zero-valent iron (ZVI) is one of the technologies for groundwater remediation to reduce contaminants by removal of mobile chlorinated hydrocarbons. Iron-Iron oxide (Fe/Fe$_{3}$O$_{4})$ nanoclusters (NCs) made in our laboratory using cluster deposition technique have enhanced reactivity towards targeted contaminants due to the presence of ZVI protected by a passivated oxide shell. Here, we investigate the effectiveness of the Fe/Fe$_{3}$O$_{4 }$NCs in reducing carbon tetrachloride (CT) under laboratory conditions. The reactivity of the NCs was investigated by conducting unbuffered aqueous batch experiments to reduce CT at room temperature. Initial results show that 80{\%} of the degradation of CT resulted in the formation of dichloromethane (DCM) and chloroform (CF); the remainder likely followed a competing pathway to yield nonhazardous products such as CO. The production of undesirable hydrogenated products such as DCM and CF suggests that the dominant reaction pathway occurs through hydrogen (H) atom transfer via H atoms generated by corrosion of the iron. Comparative experiments with ZVI NCs prepared by other methods are underway and the results will be reported. Future work is to analyze and understand factors that control the reaction pathways between desirable and undesirable products.   

Environmental Catalysis at the Boundary between Metals and Metal Oxides

Growing theoretical and computational evidence points to the participation of partially to completely oxidized catalyst surfaces during catalytic oxidations under realistic conditions. These catalytic oxidations are of both fundamental scientific interest and of practical importance in a variety of contexts, including in particular environmental NOx remediation, yet fundamental understanding of the coupling between surface structure and composition, reactive environment, particle size, and catalytic reactivity is still in its infancy. In this work we use density functional theory (DFT) models to consider the coupling between catalytic oxidation activity and the transformation of metal to oxide surface, taking as our model CO and NO oxidation on oxygen-covered to oxidized Pt surfaces. We describe DFT-parameterized cluster expansions (CEs) of O on Pt that capture the transition from metal to oxide, spectroscopic signatures of these transformations, and the incorporation of the metal-to-oxide transition into kinetic models of surface reactivity.   

Effect of substrate-catalyst interaction on Spin-dependent chemical reactions: CO oxidation

First-principles calculations have been performed to investigate and compare the catalytic reactivity of Ni(Pd)$_{1}$/TiO$_{2}$(110) and Ni(Pd)$_{2}$/TiO$_{2}$, for CO oxidation. A recent experiment showed that the catalysis of small Ni(Pd)$_{2}$ clusters deposited on rutile TiO$_{2}$(110) surface exhibit very different performance for CO oxidation, compared with the single atom cases, Ni(Pd)/TiO$_{2}$(110). However, the underlying mechanism of this interesting phenomenon is still unclear. Our calculations show that the catalyst-substrate interaction plays a key role in both the thermodynamic and kinetic process of the catalytic reactions. Particularly, the spin degree of freedom of the complex oxide is found to dominate the reaction rate. Essentially, the oxidation of CO on the single atom cases is a spin-forbidden reaction, while it is spin-permitting for the dimer cases. This work provides valuable guidance for high efficient catalyst design at the atomic scale.   

Assessing Actinide-Oxygen Covalency by K-edge X-ray Absorption Spectroscopy.

The development of many essential nuclear technologies requires a comprehensive grasp of the electronic ground and valence states of molecular actinide bonding interactions. Identifying itinerant or delocalized electrons -- in molecular nomenclature, ionic or covalent bonds -- is a longstanding problem in actinide science. Recent advances have shown that the transition intensities measured by ligand K-edge X-ray absorption spectroscopy (XAS) directly relate to coefficients of covalent orbital mixing. Ligand K-edge XAS has been employed successfully to describe the valence states of materials containing predominantly ionic metal--Cl and metal--S bonds, however, it remains experimentally challenging to obtain quantitative intensity information at the K-edge for light atoms such as C, O, N, and F. Insights regarding the nature and extent of orbital mixing in actinide--O bonds are now within reach through a combination of XAS with a scanning transmission X-ray microscope (STXM) and hybrid density functional theory calculations (DFT). A new effort to employ these techniques for non-conducting molecular systems containing interactions between actinide and oxygen-based ligands will be discussed. Oxygen K-edge XAS measurements and DFT for a series of six structurally related transition metal oxides suggest that metal nd and O 2p orbital mixing increases with increasing Z. The actinyl ions were chosen for the first O K-edge XAS measurements with actinides because they represent the most important high-valent actinide species in the environment. Features in the polarized XAS of the actinyls follow anticipated trends based on the 5f and 6d orbital energies and occupancies. Results from an ongoing collaboration with theorists ties these experimental trends in actinide--O orbital mixing to changes in 3d, 4d, 5d, and 6d/5f valence orbital occupancies.   

Submonolayers of Au/Pd on the hematite (0001) and magnetite (111) surfaces

Ultra-thin films and nanostructures formed by noble metals on oxide surfaces exhibit enhanced catalytic activity for CO oxidation. We used the spin-polarized density functional theory (DFT) and the DFT+U method, accounting for the strong on-site Coulomb correlations, to study the submonolayer adsorption of Au/Pd atoms on two stable iron-oxide surfaces: hematite (0001) and a magnetite (111). For each surface, adsorption on two terminations has been studied: one terminated with iron and the other with oxygen. Both Au and Pd bind strongly to hematite and magnetite surfaces and induce large changes in their geometry. DFT and DFT+U provide qualitatively similar surface geometries but they differ much in the prediction of the surface energetics and the electronic and magnetic properties of the oxides. Pd binds stronger than Au both to hematite and magnetite surfaces and the Au/Pd bonding to the O-terminated surface is distinctly stronger than that to the Fe-terminated one. For hematite, the DFT+U bonding is by 0.3-0.6 eV weaker than DFT on the Fe-terminated surface and about 2 eV stronger on the O-terminated one. For magnetite, in each case, DFT+U gives stronger bonding than DFT. The differences between DFT and DFT+U results are discussed based on the calculated electronic structure.   

Session P37: CDW Materials - Chalcogenides and other

Temperature- and pressure-dependent Raman scattering study of phase transitions in ZrTe$_{3}$

Pressure-induced superconductivity has been discovered in many classes of materials, such as the iron pnictides and transition metal chalcogenides. ZrTe$_{3}$ is a representative member of the latter whose ground state can be tuned between charge density wave and superconducting phases via pressure or intercalation. Microscopic information about the structural evolution of ZrTe$_{3}$ in response to applied pressure is lacking at present. In this talk, we describe a temperature- and pressure-dependent Raman scattering study of the structural evolution of ZrTe$_{3}$ through its temperature- and pressure-dependent phase transitions.   

Ultrafast Dynamics of a Charge Density Wave via Time-Resolved Resonant Diffraction

Understanding the emergence of collective behavior in correlated electron systems remains at the forefront of modern condensed matter physics. The key to such an understanding is unraveling the contributions from the coupling degrees of freedom in exotic many body states. Density waves, both of charge and spin, have been studied for decades and a wealth of information and insight has been gained. However, there are still open questions that need to be solved for a complete description of the phenomena as there are several existing density wave systems that exhibit prototypical behavior while violating traditional theory. Ultrafast dynamics of such a system, TbTe3, has been investigated via time-resolved resonant diffraction at the SXR endstation at LCLS. Oscillations of the amplitude mode and coherent phonons have been observed previously in time resolved photoemission and reflectivity measurement but, here we reveal a direct observation of the lattice response via resonant diffraction. Watching dynamics of the two dimensional Te plane density wave diffraction peak at a resonant energy of a bystander Tb atom reveals new insights into the coupling responsible for the formation of the state. Results and comparison with previous time resolved measurements will be discussed.   

Experimental Observation of Chiral Order in TiSe$_2$

Recent STM measurements on TiSe$_2$ were interpreted as evidence of chirality in the charge-density-wave order parameter, \textit{i.e.}, a rotation in the phase of the three in-plane components of the CDW order from one layer to the next. Recently, J. van Wezel has shown theoretically how a chiral state can arise from the onset of orbital order of the Ti 3$d$ and Se 4$p$ states in conjunction with the charge order at a temperature at or below the CDW transition temperature [arXiv:1106.1930v1 (2011)]. This theory predicts a lowering of the symmetry of the ordered phase from $P\bar{3}c1$ to $P2$. We present the results of synchrotron x-ray diffraction measurements on a single crystal of TiSe2 that provide evidence of a second structural phase transition 15K below the CDW transition, consistent with the proposed space group.   

Pressure-induced CDW suppression in 1T-TiSe$_2$

1T-TiSe$_2$ is a prototypical transition-metal dichalcogenide showing a commensurate CDW phase transition. It has been shown that hydrostatic pressure suppresses the CDW order and induces superconductivity. Here we present a high-pressure x-ray scattering study of the CDW order parameter and its fluctuations in TiSe$_2$. The integrated intensity at the base temperature as a function of pressure shows a positive curvature, indicating deviation from mean-field behavior. The ratio between the lattice distortion and the critical temperature is pressure-dependent, indicating a cross-over from strong to weak coupling limits. Using a Monte Carlo simulation of the three-component Potts model, we argue that the amount of anisotropy and the effective dimension must be taken into account to explain the properties of the phase transition.   

Chiral charge and orbital order in 1T-TiSe2

Helical arrangements of spins are common among magnetic materials. The first material to harbor a corkscrew pattern of charge density on the other hand, was discovered only very recently [1,2]. The nature of the order parameter is of key relevance, since rotating a magnetic vector around any propagation vector trivially yields a helical pattern. In contrast, the purely scalar charge density cannot straightforwardly support a chiral state. Here we resolve this paradox by identifying the microscopic mechanism underlying the formation of the chiral charge density wave in {\it 1T}-TiSe$_2$. It is shown that the emergence of chirality is accompanied by the simultaneous formation of orbital order [3] We show that this type of combined orbital and charge order may in fact be expected to be a generic property of a broad class of charge ordered materials and discuss the prerequisites for finding chiral charge order in other materials. \\[4pt] [1] J. Ishioka, Y. H. Liu, K. Shimatake, T. Kurosawa, K. Ichimura, Y. Toda, M. Oda and S. Tanda, {\it Phys. Rev. Lett.} \textbf{105}, 176401 (2010). \newline \noindent [2] J. van Wezel and P. B. Littlewood, {\it Physics} \textbf{3}, 87 (2010). \newline \noindent [3] J. van Wezel, arXiv:1106.1930v1 (2011).   

Evolution of the Charge Density Wave state in Cu$_x$TiSe$_2$

1T-TiSe$_2$ is a quasi two-dimensional CDW material showing a 2x2x2 superlattice modulation below 200 K. Upon Cu doping the CDW is suppressed and superconductivity is induced. We present scanning tunneling microscopy and spectroscopy measurements of the charge-density wave state in 1T-TiSe$_2$, Cu$_{0.05}$TiSe$_2$ and Cu$_{0.06}$TiSe$_2$ single crystals. Topography images at 4.2 K reveal that the charge density waves are present in all samples studied, although the amplitude of the charge modulation decreases with the Cu-doping. Moreover, the chiral phase of the charge density wave is preserved also in Cu-doped samples. Tunneling spectroscopy shows that there is only a partial gap in the pure compound, with bands crossing the Fermi surface. In the Cu-doped samples the system becomes more metallic due to the increase of the chemical potential.   

Effect of dimensionality on charge density wave instabilities in TaS$_2$ and TaSe$_2$

Recent successes in making exfoliated single-layer transition-metal dichalcogenides has brought new interest to these materials, particularly with respect to the effects of dimensionality. As layered bulk materials, the 1T and 2H polymorphs of TaS$_2$ and TaSe$_2$ undergo a number of charge-density-wave (CDW) transitions. However, recent experiments have found that the CDW instability does not survive in nanopatches of 2H-TaS$_2$. Here we present a density-functional theory investigation of the CDW instability in single- and few-layer TaS$_2$ and TaSe$_2$, focusing on the role of the interlayer interactions. The effects of dimensionality on structure, electronic structure, and electronic-phonon coupling will be discussed.   

Disorder Induced Melting of Charge Density Wave Order in doped 2H-NbSe2 systems

Using a combination of Angle Resolved Photoemission Spectroscopy (ARPES), X-ray diffraction, transport and Scanning Tunneling Microscopy (STM) measurements on pristine as well as disordered 2H-NbSe2 samples, we have found that the onset Temperature Tcdw for Long Ranged Charge Density Wave (CDW) order gets quickly suppressed with concentration of disorder ions (X) and at certain critical concentration (Xc) it undergoes a quantum melting. Our STM measurements provide the evidence for local CDW ordering in doped samples for temperatures way above Tcdw. On the other hand, our ARPES measurements have found evidences for the presence of energy gap for both T$>$Tcdw {\&} X$>$Xc. We argue, all these experimental observations from completely different probes hint towards phase fluctuations of the order parameter as the mechanism behind the destruction of CDW order in quasi 2-d systems, such as 2H-NbSe2.   

Spectroscopic imaging of the charge density wave in $2H$-NbSe$_2$

Transition metal dichalcogenides are an ideal playground to study the interplay between charge density waves (CDWs) and superconductivity. We perform atomically resolved scanning tunneling microscopy and spectroscopy at cryogenic temperatures on the chalcogenide polytype $2H$-NbSe$_2$ to study the energy, temperature and spatial dependence of the CDW. By comparing our results with a tight-binding model, we disentangle the spectral behavior of the CDW phase in the material.   

Strong Periodic Lattice Distortion in Transition Metal Dichalcogenides

The charge density wave (CDW) instability was initially proposed to be the result of the Peierls mechanism in which a divergence in electronic response function results in a periodic charge redistribution; i.e. the electron gas itself is unstable with respect to the formation of a periodically varying electron charge density. However, the mechanism of CDW in many 2D Transition Metal Dichalcogenide (TMD) is still under debate. Fermi surface nesting was originally believed to act as the driving mechanism of CDW transitions in these materials; however, recent reports from both theoretical and experimental studies are not quite within this simple model. We use Spectroscopic Imaging Scanning Tunneling Microscope (SI-STM) to study the surfaces of 2H-TaSe$_{2}$, 2H-TaS$_{2}$, and 2H-NbSe$_{2}$ at various temperatures from 6K to above 100K. Topographic images and differential conductance data were recorded and analyzed in order to help understanding the underlying physics of CDW phases. Our results shows that Periodic Lattice Distortion (PLD) likely plays a more important role than the charge modulation in 2D TMD.   

Low frequency resistance fluctuations in nanoribbons of charge density wave (CDW) conductor NbSe$_{3}$

We investigate finite size effects in the low-frequency (1 mHz $<$ f $<$ 10 Hz) resistance fluctuations of individual nanoribbons of single-crystalline NbSe$_{3}$ (cross sections of 10$^{4}$ nm$^{2})$ across the two CDW transitions ($\sim $ 59 K and $\sim $ 141 K). This ultra sensitive frequency-dependent study of the electrical noise is crucial in improving our understanding of the mechanisms that generate noise around CDW transitions. The power spectral density, S$_{R}$, of the resistance noise has a generic form, S$_{R} \quad \sim $ 1/f$^{\alpha }$, typical of a diffusive metallic conductor. Below the CDW transition at 59 K, where the CDW is pinned by disorder, S$_{R}$ (at 1 Hz) shows a non-monotonic behavior with a maximum magnitude around 45 K. A similar peak in S$_{R}$ is also observed at 125 K, below the second CDW transition. Also, it is well known that the CDW state can be depinned by an application of a high bias voltage or current and S$_{R}$ is measured as a function of current across the pinning-depinning of CDW. S$_{R}$ shows a complex, non-monotonic dependence and is extremely sensitive to temperature below the CDW transition. In contrast, S$_{R}$ is bias independent above the CDW transitions as expected from a metal. The implications of these noise behaviors in understanding the pinning and depinning of CDW in NbSe$_{3}$ will be discussed.   

Extrinsic control of collective transport in quasi-1D materials with end contacts geometry

End contacts to mesowires of NbSe$_{3}$ and TaS$_{3}$ were nanofabricated and tested with transport, noise and X-ray microdiffraction measurements. We measured unusual and unexpected weak dependence of collective current on temperature in the [70K, 90K] range, close to 2/3T$_{P1}$ point, indicating a modification of CDW condensate transport due to the end contact geometry. This is accompanied with modifications to the temperature dependence to of the phase slip voltage. We also report a partial control of the threshold field (E$_{T})$ for CDW sliding, below T$_{P2}$, with the decrease in E$_{T}$ by as much as one order of magnitude in a limited temperature range below 2/3T$_{P2}$. These changes can be also seen in electric field modified X-ray topography images performed with sub-micron focused synchrotron X-rays (X13B beamline at NSLS). The most likely causes of these phenomena when end contacts are applied, are in modifications of: (a) carrier injection efficiency and, (b) the phase loop formation mechanism.   

Spectroscopic evidence of the Peierls transition in three-dimensional charge density wave solids

We studied the ultrafast photoinduced charge-density-wave to metal phase transition in a complex solid, namely Lu5Ir4Si10. After melting the charge ordering using infrared laser pulses, the consequent charge redistribution is probed with fs time resolution through the spectral weight analysis of the transient optical response over a broad energy range. The time-dependent spectral weight reveals a signature of the CDW melting and the time-scale of this photo-induced phase transition. This new kind of analysis allows us to show that the charge order remains preserved until the lattice distorts sufficiently to induce the phase transition. These results are completed by ab-initio modeling of the electronic band structure, identifying the orbitals involved in the CDW and the electronic transitions leading to the photo-induced melting of the charge order. This allows us to reveal the Peierls origin of multiple CDW in this three-dimensional solid.   

Gapped sliding phononic modes in the incommensurate structure of the ladder-chain system Sr$_{14}$Cu$_{24}$O$_{41}$

We report on low energy Raman and infra-red (IR) excitations in Sr$_{14}$Cu$_{24}$O$_{41}$. Two modes are observed starting from room temperature in the 1-2 meV range. One is a fully symmetric Raman mode and the other, observed in c-axis reflectance, is an excitation carrying a dipole moment along the chain/ladder direction. We associate these modes with the gapped c-axis sliding motions of the charged, incommensurate CuO$_{2}$ chains and Sr$_{2}$Cu$_{2}$O$_{3}$ ladder layers. This approach is able to quantitatively explain the range and relative energies of these excitations which are sensitive probes of the charge and spin density-wave ordering in the ``14-24-41'' systems.   

Dynamics of Sr$_{14}$Cu$_{24}$O$_{41}$ in a transient high intensity THz field

Over the past few years new techniques have become available to generate electromagnetic radiation with high electric field values in the THz range [1], allowing the dynamic study of materials whose excitations lie in the far infrared frequency range. This is the case of many charge density wave compounds, which exhibit a collective response due to pinning. Sr$_{14}$Cu$_{24}$O$_{41}$ is a charge density wave compound of particular interest, given its quasi one-dimensional structure consisting of alternating layers of Cu$_{2}$O$_{3}$ chains and CuO$_2$ ladders [2]. Understanding the dynamics of Sr$_{14}$Cu$_{24}$O$_{41}$ excitations in the far infrared has the potential not only to shed light onto the complex nature of charge ordering in this material, but also to help provide a better understanding of the nature of superconducting behavior in two-dimensional high temperature superconducting cuprates. In our work, THz pulses generated using a LiNbO$_3$ crystal interact with single crystal Sr$_{14}$Cu$_{24}$O$_{41}$ samples grown by the traveling solvent floating zone method. We will present preliminary results of a high field THz study of the dynamics of Sr$_{14}$Cu$_{24}$O$_{41}$. [1] Jepsen et al., Laser Phot Rev 5 124 (2011) [2] Gorshunov et al., Phys Rev B 66 060508 (2002)   

Session P40: Protein Fluctuations and Conformation Changes

Atomistic simulations of the MS2 coat protein conformational transition

During the replication of many viruses, hundreds to thousands of proteins self-assemble to form a protective protein coat, called a capsid, around the viral nucleic acid. Often these proteins have identical amino acid sequences with slightly different, or quasi-equivalent, conformations, which join in precise spatial arrangements. Although the structure of completed capsids is known to atomic resolution, little is known about the assembly intermediates and how protein conformations are selected during assembly. In this talk, we will use all-atom simulations to investigate how protein-RNA interactions guide conformational transitions of capsid proteins from the single-stranded RNA bacteriophage MS2. Since conformational changes occur on timescales which are not accessible to all-atom simulations, we use enhanced sampling methods to sample probable transition pathways and corresponding free energy profiles. Specifically, we will present free energy profiles associated with the MS2 capsid protein conformation in the presence and absence of RNA.   

Conformational changes of surface immobilized proteins studied by combined Atomic Force Microscopy and Fluorescence Spectroscopy

Atomic Force Microscopy (AFM) and Fluorescence Spectroscopy techniques have provided unique methods for characterizing conformational changes in proteins. Here we are using a technique called nanografting to immobilize proteins at well defined locations on atomically flat surfaces. In nanografting the AFM tip is used to shave alkanethiol molecules from a prescribed patch on a surface coated with an alkanethiol monolayer. Thiol-linked proteins in the surrounding solution are then able to self assemble on the newly exposed surface patch in a highly ordered array of the order of 100nm. Stable and meta-stable conformations of fluorescently tagged proteins and other molecules assembled in this manner can then be characterized using a combination of AFM and Fluorescent Resonance Energy Transfer (FRET). Due to the high spatial, temporal and force resolution provided by both AFM and FRET, a free energy landscape of the protein may be determined using this technique.   

The Glassy State of Crambin and the THz Time Scale Fluctuations Related to Protein Function

THz experiments have been used to characterize the picosecond time scale fluctuations taking place in the model protein crambin. Using both hydration and temperature as an experimental parameter, we have successfully identified the collective fluctuations ($\le $ 200 cm$^{-1})$ in the protein (Figure 1). Observation of the transition of the protein dynamics in the THz spectrum from both below and above the glass transition temperature (T$_{g})$ provides unique insight into microscopic interactions and modes that allow the solvent to couple with the protein dynamics (Figure 2). Our findings suggest that the solvent dynamics on the picosecond time scale not only contribute to the flexibility of the protein but also provides a dynamical parameter that allows the protein to modulate local regions of its structure that are distinct from the protein whole. These distinct dynamical regions of the protein may be important for energy transport and hence, linked with protein function.   

Stabilization of peptide helices by length and vibrational free energies: \emph{Ab initio} case study of polyalanine

Helices are one of the most abundant secondary structure ``building blocks" of polypeptides and proteins. Here, we explore helix stabilization as a function of peptide length and temperature [harmonic approximation to the vibrational free energy (FE)], for the alanine-based peptide, Ac-Ala$_n$-LysH$^+$ $n$=4-15, in the gas phase. For $n$=4-8, we predict the lowest energy structures in density-functional theory, using the van der Waals (vdW) corrected[1] PBE exchange-correlation potential. $\alpha$-helices become the lowest energy structures at $n\approx$7-8 on the potential energy surface, but only barely and if including vdW interactions. At finite temperatures, the helices are further stabilized over compact conformers. While the vibrational entropy is the leading stabilizing term at 300 K, also the zero-point-energies favor the helical structures. For $n\geq$8, the $\alpha$-helix should be the only accessible conformer in the FE surface at 300 K, in agreement with experiment[2] and with our own comparison[3] of calculated \emph{ab initio} anharmonic IR spectra to experimental IR multiple photon dissociation data for $n$=5, 10, and 15. [1] Tkatchenko and Scheffler, PRL 102, 073055 (2009); [2] Kohtani and Jarrold, JACS 108, 8454 (2004); [3] Rossi et al., JPCL 1, 3465 (2010).   

A Physics-based Approach of Coarse-graining the cytoplasm of E. coli

We have investigated protein stability in an environment of E. coli cytoplasm using coarse-grained computer simulations. To coarse-grain a small slide of E. coli's cytoplasm consisting of over 16 million atoms, we developed a self-assembled clustering algorithm (CGCYTO). CGCYTO uses a tunable resolution parameter ($\lambda $, ranging from 0 to 1) to justify the resolution of a cytoplasm, depending on the size of a test protein for the computation of covolumes and the volume of a macromolecule in the cytoplasm. We compared the results from a polydisperse cytoplasm model (PD model) from CGCTYO with two other coarse-grained hard-sphere cytoplasm models: (1) F70 model, macromolecules in the cytoplasm were modeled by homogeneous hard spheres with a radius of 55{\AA} and (2) HS model, each macromolecule in the cytoplasm is modeled by hard spheres of various sizes. It was found that the folding temperature Tf of a test protein (apoazurin) is $\sim $5 degrees higher in a PD model than that in a F70 model. In addition, there is a deviation of 1.7 degrees on Tf when an apoazurin is randomly placed at different voids formed by particle fluctuations in a PD model, 0.7 degrees higher than that in a HS model.   

Investigation of flexibility in Myosin V using a new 3D mechanical model

This paper presents the development of a three dimensional rigid multibody model for the simulation and analysis of motor protein locomotion. The interesting aspect of this model is that it retains the mass properties, in contrast to the commonly used models which omit mass properties at the nano scale. The disproportionate size of the small mass of Myosin V relative to the large viscous friction forces requires a small integration step size that leads to a long simulation run time; however, the proposed model can be numerically integrated in a reasonable amount of time. This paper discusses modeling flexibility in the protein as an extension of the original rigid body model. Empirical studies have shown that Myosin V's neck domain can be considered as three pairs of tandem elements called IQ motifs which can bending at junctures between them. Therefore, each neck is modeled by three rigid bodies connected by ball-and-socket joints together, rather than single rigid body has been used in the previous works. Euler parameters are used to model the orientation of bodies in order to eliminate singularities in the description of orientation. In order to accomplish this, the equations of motion are reduced to minimal form using changing holonomic and non-holonomic constraints applied to the model which represent the normalization of the Euler parameters as well as contact and impact non-penetration conditions. The differences between the dynamic behavior of the new mechanical model, with flexible neck domains, and the original rigid body model are compared using simulation results.   

Barrel fluctuation and oxygen diffusion pathways in the monomeric fluorescent proteins

Fluorescent proteins are valuable tools as biochemical markers for studying cellular processes. Improving the photostability of the FPs is highly desirable in biochemical, biomedical and cell biology. Oxygen is necessary for the proper maturation of the chromophore in fluorescent proteins (FPs), but photobleaching of FPs is also oxygen sensitive. The photobleaching of the monomeric variant of RFPs has been attributed to the lack of proper shielding against oxygen or other small molecules, ions or halides. We use molecular dynamics simulation to investigate the protein barrel fluctuations in mCherry, one of the most useful monomeric mFruit variant of RFPs. We also employ oxygen diffusion simulations to search for possible pathways of oxygen to the chromophore. The ultimate goal is to use the results of these calculations to propose amino acid substitutions that will block the oxygen pathways and prevent photobleaching in the engineered proteins.   

Protein Dynamics Studied by Quasi-elastic Neutron Scattering

The biological function and activities of proteins are intimately related to their structures and dynamics. Nowadays, neutron scattering is one of the most powerful tools to study the protein dynamics. In this study, we use quasielastic neutron scattering (QENS) at the Spallation Neutron Source, ORNL, to study relaxational dynamics of two structurally different proteins --- hen egg white lysozyme and an inorganic pyrophosphatase from a hyperthermophile, in the time range of 10ps to 1ns. We experimentally prove that the slow dynamics of globular proteins can be described by the mode-coupling theory (MCT) that was originally developed for glass-forming molecular liquids. The MCT predicts the appearance of a logarithmic decay for a glass-forming liquid. Such dynamic behavior is also observed by recent molecular dynamics (MD) simulations on protein molecules. In addition, we compare the temperature dependence of the dynamics of the two proteins with completely different activity profiles. Our results greatly help understanding the relation between protein dynamics and their biological functions.   

Intrinsic Mean Square Displacement in Proteins

The dynamics of biological molecules is investigated in neutron scattering experiments, in molecular dynamics simulations, and using analytical theory. Specifically, the mean square displacement (MSD), $\langle r^2\rangle _{exp}$, of hydrogen in proteins is determined from measurements of the incoherent elastic neutron scattering intensity (ENSI). The MSD, $\langle r^2\rangle _{exp}$, is usually obtained from the dependence of the ENSI on the scattering wave vector $Q$. The MSD increases with increasing temperature reaching large values at room temperature. Large MSD is often associated with and used as an indicator of protein function. The observed MSD, however, depends on the energy resolution of the neutron spectrometer employed. We present a method, a first attempt, to extract the intrinsic MSD of hydrogen in protein from measurements, one that is independent of the instrument resolution. The method consists of a model of the ENSI that contains (1) the intrinsic MSD, (2) the instrument resolution width and (3) a parameter describing the motional processes that contribute to the MSD. Several examples of intrinsic MSDs $\langle r^2\rangle$ in proteins obtained from fitting to data in the existing literature will be presented.   

Scaling Behavior in Twisted, Helical and Undulating Lysozyme Amyloid Fibrils

We combine atomic force microscopy single-molecule statistical analysis with polymer physics concepts to study the molecular conformations of lysozyme amyloid fibrils. We use different denaturation conditions to yield amyloid fibrils of different types. At 90\r{ }C and pH2, highly laminated twisted and helical ribbons are found, in which as many as 17 protofilaments pack laterally for a total width approaching 180 nm. In the case of 60\r{ }C and pH2, we find thin, wavy fibrils, in which the scaling behavior varies at multiple length scales. We use bond and pair correlation functions, end-to-end distribution and worm-like chain model to identify 3 characteristic length scales. At short length scales there is a first bending transition of the fibrils, corresponding to a bending length Lb. At slightly larger length scales ($>$2Lb), the fibrils become pseudoperiodic and start to undulate. Finally, at length scales larger than the persistence length Lp, the fibrils become flexible and are well described by a 2D self-avoiding random walk. We interpret these results in terms of the periodic fluctuations of the cross-section orientation of the fibrils (twisting) and the impact these have on the area moment of inertia and the corresponding propensity of the fibrils to bend.   

Probing Volume Changes and the Intracellular Water in Single Erythrocytes

In the living cell, water is one of the most abundant substances, and cells have developed very efficient machinery for transporting water in and out. Erythrocytes can undergo large, but reversible, volume changes under hydrostatic pressure and a possible mechanism may involve transport of water. We employ confocal micro-Raman spectroscopy over the frequency range from 150 to 4000 cm$^{-1}$ to probe both the intracellular hemoglobin and water in individual red blood cells under physiological conditions. We investigate changes in the OH stretch bands near 3400 cm$^{-1}$ due to the cellular water. Results of experiments that employ variations in external parameters such as pressure will be presented.   

Effects of pressure on the protein barrel and the chromophore interactions in mCherry

Fluorescent proteins (FP) can be attached to proteins of interest, which makes it possible to study the movement, localization and many other physiological processes of the tagged proteins. A number of fluorescent proteins have been genetically engineered to enhance their intensity, photo-stability, pH-stability etc. The structural fluctuations of FPs determine the ease of access of small molecules like oxygen and may be an important consideration for their fluorescence spectrum and stability. A protein's response to pressure perturbations provides useful insights for understanding their folding and dynamics. Experiments have shown that application of pressure affects both the fluorescence peak as well as the quantum yield of the protein. We report on molecular dynamics computational investigations of the effect of pressure on the fluctuations of the beta barrel and the structure of the chromophore of a well characterized Red Fluorescent Protein, mCherry. We discuss our results on how pressure affects the ability of water and other ions to penetrate the barrel to reach the chromophore, as well as the effect on the time dependent hydrogen bonding network in the chromophore's cavity region.   

Thermal response of proteins (histone H2AX, H3.1) by a coarse-grained Monte Carlo simulation with a knowledge-based phenomenological potential

Using a coarse-grained bond fluctuating model, we investigate structure and dynamics of two histones, H2AX (143 residues) and H3.1 (136 residues) as a function of temperature ($T)$. A knowledged based contact matrix is used as an input for a phenomenological residue-residue interaction in a generalized Lennard-Jones potential. Metropolis algorithm is used to execute stochastic movement of each residue. A number of local and global physical quantities are analyzed. Despite unique energy and mobility profiles of its residues in a specific sequence, the histone H3.1 appears to undergo a structural transformation from a random coil to a globular conformation on reducing the temperature. The radius of gyration of the histone H2AX, in contrast, exhibits a non-monotonic dependence on temperature with a maximum at a characteristic temperature ($T_{c})$ where crossover occurs from a positive (stretching below $T_{c})$ to negative (contraction above$ T_{c})$ thermal response on increasing $T$. Multi-scale structures of the proteins are examined by a detailed analysis of their structure functions.   

Session P41: Drops, Bubbles and Interfacial Fluid Mechanics

The Life and Death of the Air Film Beneath an Impacting Drop

Droplet impact is ubiquitous in our everyday experience; yet many mysteries associated with the phenomenon remain, including the role played by air during the impact process. When a liquid meets a solid surface in an atmosphere, it must drain the air beneath it before initiating contact. In spite of the relatively low viscosity of the air, recent experiments and simulations suggest that this drainage dominates the dynamics of drop impact. Here I present recent experimental work, wherein Total Internal Reflection (TIR) microscopy is used to directly observe the thin air films that develop above the impact surface. We find that the formation of the thin air film is insensitive to liquid viscosity over a range of impact velocities, confirming prior theoretical predictions of thin air film formation. Going beyond this, the viscous response of the drop is also found to be important - high viscosity liquids maintain a steep front that progresses outward as the breadth of the air film increases, whereas lower viscosity liquids broaden without a steep front, suggesting a transition in the kinematics of the air liquid interface.   

Manipulating Leidenfrost temperature with surface modification

When a drop contacts a surface that is at a sufficiently high temperature, the drop can float on it's own vapor in a process known as the Leidenfrost effect. ~Although it is known that the critical temperature needed to achieve this effect depends on the properties of the drop and its vapor, often these parameters are fixed for a particular process.~Here, we demonstrate a new way to control the critical temperature through the surface structure.   

Reducing contact time of drops on superhydrophobic surfaces

When water drops impact on to a superhydrophobic surface, the drops can recoil to such an extent that they completely bounce off the solid material. The time it takes for the drop to spread and recoil – the contact time – scales with the hydrodynamic inertial-capillary timescale. However, there is evidence that the coefficient of this scaling depends on surface-structure interactions, such as pinning. Here we investigate how surface interactions can influence droplet contact time, and we compare our results to existing models. We highlight an assumption in the current theory that imposes a lower-bound on the contact time. By designing around this constraint, we demonstrate novel superhydrophobic surfaces on which water droplets impact with shorter contact times than previously thought possible.   

Hydrodynamic self-rectification: A novel mechanism for generating uniform static droplet arrays

Microfluidic static droplet arrays are a powerful means to simultaneously monitor many biochemical reactions in individual drops. We report a new mechanism for generating exceptionally monodisperse microfluidic droplet arrays. When a train of surfactant-free confined droplets are introduced into a fluidic network with hydrodynamic traps, the droplets are immobilized in the traps due to collective hydrodynamic resistive interactions. In the event, that an immobilized drop either under-fills or overfills the trap, we find that subsequent drops rectify its volume through coalescence, followed by break-up. This self-rectification mechanism thus yields highly monodisperse static droplet arrays. We map the phase space in terms of drop size, spacing and capillary number and find a broad window where this mechanism operates. Because this mechanism alleviates the need to control drop size and spacing in the train to create arrays, we demonstrate its capability to create static arrays with tuneable drop volumes and variable composition.   

In Situ Observation of Liquid Capillary Bridges on Superhydrophobic Surfaces

We describe a new technique for observing the dynamic behavior of the contact line of a liquid droplet on a superhydrophobic surface using environmental scanning electron microscopy. We find that on a surface patterned with an array of superhydrophobic micropillars, the receding contact line exhibits discrete hierarchical de-pinning events. As the macroscopic contact line recedes across the pillars, capillary bridges are formed along with microscale contact lines, thus perpetuating a self-similar wetting condition. We are able to measure the local receding angle and find that it follows the Gibbs criterion of depinning. By considering the line density of the microscale features and the pinning strength of each of those features, we relate the macroscopic adhesion force to that derived from a model based on pinning of the capillary bridges.   

Two particle microrheology of quasi-2D viscous systems in the limit of shallow bulk layer

Human serum albumin (HSA) protein molecules at an air-water interface is a model system for which it is difficult to decouple the properties of the 2D interfacial film from those of the 3D fluid. Here we focus on the influence of the bulk confinement (i.e. the thickness of the layer of water) on the dynamics of HSA at an air-water interface. To do so, we have developed a setup which allows us to control the depth of the water layer over which HSA protein molecules are dispersed. In particular, we investigate the limit of shallow layers, for which we report measurements of the spatially correlated motion of colloidal particles embedded at the interface, for different surface viscosities. We describe the influence of the bulk finite size on the behaviour of the spatial correlation functions of the particle motion, and extend the description of the correlation functions in terms of a master curve first obtained for large bulk volumes, to the limit of shallow layers.   

Measuring interfacial viscosity using macro- and micro-rheology

We measure the viscous moduli of thin films using two different methods. First, we use a magnetic needle viscometer. Our apparatus employs Helmholtz coils to control the position and orientation of the needle in the film. By driving the needle we can produce a response in the film which allows us to probe the bulk viscous properties of the film. Second, we use single particle microrheology to probe the local properties of the film. Tracking the mean-squared displacement of particles as they undergo Brownian motion probes the local viscous properties of any heterogeneous domains. Coupling this technique with the magnetic needle viscometer provides information on the effect local viscous properties have on the bulk properties.   

Molecular Modeling of Three Phase Contact for Static and Dynamic Contact Angle Phenomena

Interfacial phenomena arise in a number of industrially important situations, such as repellency of liquids on surfaces, condensation, etc. In designing materials for such applications, the key component is their wetting behavior, which is characterized by three-phase static and dynamic contact angle phenomena. Molecular modeling has the potential to provide basic insight into the detailed picture of the three-phase contact line resolved on the sub-nanometer scale which is essential for the success of these materials. We have proposed a computational strategy to study three-phase contact phenomena, where buoyancy of a solid rod or particle is studied in a planar liquid film. The contact angle is readily evaluated by measuring the position of solid and liquid interfaces. As proof of concept, the methodology has been validated extensively using a simple Lennard-Jones (LJ) fluid in contact with an LJ surface. In the dynamic contact angle analysis, the evolution of contact angle as a function of force applied to the rod or particle is characterized by the pinning and slipping of the three phase contact line. Ultimately, complete wetting or de-wetting is observed, allowing molecular level characterization of the contact angle hysteresis.   

Nonlinear electrohydrodynamics of a viscous droplet

A classic result due to G.I.Taylor is that a drop placed in a uniform electric field adopts a prolate or oblate spheroidal shape, the flow and shape being axisymmetrically aligned with the applied field. We report an instability and transition to a nonaxisymmetric rotational flow in strong fields, similar to the rotation of solid dielectric spheres observed by Quincke in the 19th century. Our experiments reveal novel droplet behaviors such as tumbling, oscillations and chaotic dynamics even under creeping flow conditions. A phase diagram demonstrates the dependence of these behaviors on drop size, viscosity ratio and electric field strength. The theoretical model, which includes anisotropy in the polarization relaxation, elucidates the interplay of interface deformation and charging as the source of the rich nonlinear dynamics.   

Breakup of Bubbles or Drops by Capillary Waves Induced by Coalescence or Other Excitations

Capillary breakup of a bubble or drop by various excitations is ubiquitous in both nature and technology. Examples include coalescence with another bubble or drop, wetting on a solid surface, impact on a solid surface, detachment from a nozzle, or vibrations driven by acoustic, electrical, or magnetic fields. When the excitation ceases, capillary forces on the surface naturally drive the deformed bubble or drop to recover its spherical shape. However, when the viscosity is small, this recovery can lead to nonlinear oscillations of the interface and a singularity in the flow. Here we use high-speed imaging to investigate the coalescence of bubbles and drops of various sizes. In many cases, coalescence leads to pinch-off events and the formation of the satellite and sub-satellite. Our experiments use pressured xenon gas in glycerol/water mixtures so that the density ratio and viscosity ratio can be varied over many orders of magnitude. We characterize the generation, propagation, and convergence of capillary waves, the formation time and sizes of satellites, and the dynamics of two-fluid pinch-off as a function of the density ratio and viscosity ratio. The work shall benefit the wide-spread applications and fulfill the scientific and public curiosities.   

Thermo-actuated migration in a micro-system

Digital microfluidics require element displacement by simple means featuring high integration rates. Within this context, the transport and handling of elements constitutes a problem [Squires and Quake, 2005]. This context has rekindled interest in the Marangoni surface effect, which refers to tangential stresses along an interface. Producing a surface tension gradient by imposing a temperature gradient is especially efficient and easy to control. In a recent paper, we have shown [Selva et al., Phys. Fluids (2011)] that a bubble undergoing a constant temperature gradient is indeed set into motion. However, the direction of motion (toward the cooler side) is in contradiction with experiments performed at the millimetre scale in which bubble migration is driven towards hoter regions. We believe this observation is due to the PDMS deformability. Indeed, PDMS expands when the temperature increases. A temperature gradient inside a microsystem results in a cavity thickness gradient, and thus leads to the bubble travelling towards the thicker part of the cavity. The physical phenomena involved in such a system are multifaceted (PDMS dilation, thermocapillarity, solutocapillarity) and may have either complementary or opposite effects depending on the experimental conditions.   

Towards an effective surface tension at a foam/water interface

Foams are defined as assemblies of gas bubbles immersed into a continuous liquid phase. Depending on the ration between the total volume occupied by the foam and the amount of liquid inside the foam, different rheological behaviors are observed. Beside the numerous studies on the foam's bulk behavior, poor is known concerning the interface between a foam and the liquid bath it has been generated from. This interface is however separating two identical liquids where, one of these, also contained a dispersed phase. Our studies aim in describing this interface in terms of an effective surface tension, while considering the foam as a continuous medium. Monodisperse foams are produced in Hele-Shaw cells and the features of the boundary between the foam and the baliquid pool is studied by means of hydrodynamical instabilities. Namely, Faraday waves, Rayleigh-Taylor instability and Saffmann-Taylor fingering are considered. Among these instabilities, the shearing of the interface is studied within a rotating drum experiment, similarily to the granular case.   

Continuous dielectrophoretic centering of compound droplets

Compound droplets, or droplets-within-droplets, are traditionally key components in applications ranging from drug delivery to the food industry. Presently, millimeter-sized compound droplets are precursors for foam shell targets in inertial fusion energy work. A key constraint is a uniform foam shell thickness, which in turn requires a centered core in the compound droplet precursor. Previously, Bei et al. (2009, 2010) have shown that compound droplets could be centered in a static fluid using an electric field of 0.7 kV/cm at 20 MHz. To apply centering to existing or future applications, it is imperative to develop a continuous droplet centering process by overcoming the additional complications from motion. Here, we present analysis and experimental data of a continuous droplet centering device that uses an electric field to force a core droplet to the center of a moving compound drop. Our analysis focuses on how interfacial rheology and electrohydrodynamic flows affect the centering dynamics and droplet deformation. Proof-of-principle experiments are performed in a vertical channel using buoyancy to drive a solution of compound droplets stabilized with phospholipid and protein emulsifiers through a kV/cm electric field.   

The Dynamics of Coupled Droplets Under Gravity Condition

The dynamics of a two-dimensional, incompressible, and two coupled spherical-cap water droplets pinned in a straight channel is investigated under gravity condition through the use of CFD. Since the capillary length is three times as large as the channel width, the effect of gravity is small but not negligible. In this simulations FLUENT with a 2-D pressure based solver is utilized. The suspended droplet states are measured by the location of the center of mass of the droplet. Under no gravity condition we find that there is a critical volume, Vc, where a bifurcation of asymmetric states occurs. However, gravity changes the pitchfork bifurcation diagram of coupled droplets systems into two separate branches of equilibrium states. The primary branch describes a gradual and stable change of the droplet states from symmetric to non-symmetric as V is increased across Vc. The secondary branch appears at a certain modified critical volume, Vmc, and describes two additional non-symmetric states for V$>$Vmc. CFD demonstrated that the large-amplitude state along the secondary branch is stable whereas the small-amplitude states are unstable.   

Session P42: Focus Session: Evolutionary Systems Biology II - From molecules to cells

Statistical Physics Approaches to RNA Editing

The central dogma of molecular Biology states that DNA is transcribed base by base into RNA which is in turn translated into proteins. However, some organisms edit their RNA before translation by inserting, deleting, or substituting individual or short stretches of bases. In many instances the mechanisms by which an organism recognizes the positions at which to edit or by which it performs the actual editing are unknown. One model system that stands out by its very high rate of on average one out of 25 bases being edited are the \textit{Myxomycetes}, a class of slime molds. In this talk we will show how the computational methods and concepts from statistical Physics can be used to analyze DNA and protein sequence data to predict editing sites in these slime molds and to guide experiments that identified previously unknown types of editing as well as the complete set of editing events in the slime mold \textit{Physarum polycephalum}.   

Biophysical Models of Evolution: Application to Transcription Factor Binding Sites in Yeast

There has been growing interest in understanding the physical driving forces of evolution at the molecular scale, in particular how biophysics determines the fitness landscapes that shape the evolution of DNA and proteins. To that end we study a model of molecular evolution that explicitly incorporates the underlying biophysics. Using population genetics, we derive a steady-state distribution of monomorphic populations evolving on an arbitrary fitness landscape. Compared to previous studies, we find this result is universal for a large class of population models and fully incorporates both stochastic effects and strong natural selection. This distribution can then be used to infer the underlying fitness landscape from genomic data. This model can be applied to a variety of systems, but we focus on transcription factor binding sites, which play a crucial role in gene regulatory networks. Since these sites must be bound for successful gene regulation, we consider a simple thermodynamic model of fitness as a function of the free energy for binding a transcription factor at the site. Using empirical energy matrices and genome-wide sets of binding sites from the yeast Saccharomyces cerevisiae, we use this model to infer the role of DNA-protein interaction physics in evolution.   

The Fitness of Genomic Order

Most bacteria have a single circular chromosome that can range in size from 160,000 to 12,200,000 base pairs. Considering the typical gene density, i.e. 1 gene per 1,000 base pairs, both the number of genes and the ways to arrange are huge. Intuitively, the arrangement of genes on the circle is not important if all of them can be replicated. However, there is typically one origin of replication, and when bacteria is attacked by genotoxic stress during replication, the whole replication process can not be finished. As a result, which gene is replicated first, which is second, ..., becomes very important. Experimentally, we found a broad increase of DNA copy number near the origin of replication (OriC) of bacteria E.coli ($\sim$3200 genes) under genotoxic stress. Since the genes near OriC are mostly efflux pump genes, we propose that there is fitness advantage for those rapid stress response genes got replicated first, because they can facilitate the replication of the rest of genome. Similar to bacterial evolution to present genomic order, in the somatic evolution of cancer, genomic shuffling was also frequently observed, especially under genotoxic chemotherapy. Such re-arrangement of genome can be viewed as a journey to optimal point in the rugged fitness landscape of genomic order.   

Plasticity of metabolic networks and the evolution of C4 photosynthesis

Over 50 groups of plants have independently developed a common mechanism (C4 photosynthesis) for increasing the efficiency of photosynthetic carbon dioxide assimilation. Understanding the high degree of evolvability of the C4 system could offer useful guidance for attempts to introduce it artificially to other plants. Previously, the nonlinear relationship between carbon dioxide levels and rates of carbon assimilation and photorespiration has prevented the application of genome-scale metabolic models to the problem of the evolution of the pathway. We apply a nonlinear optimization method to find feasible flux distributions in a plant metabolic model, allowing us to explore the plasticity of the metabolic network and characterize the fitness landscape of the transition from C3 to C4 photosynthesis.   

Uncovering principles of cellular decision-making

Cells can cope with unpredictable environmental conditions by differentiating into appropriate states. In this talk, I will present our recent attempts to understand the role of genetic circuits in regulating the underlying process of cellular decision-making. Specifically, we are interested in how interactions within and across genetic circuits enable cells to choose among alternative fates. To address this question my laboratory is employing systems and synthetic biology approaches. Our ultimate goal is to uncover possible evolutionary pressures that may have selected for specific gene circuit architectures, dynamics and noise properties.   

Role of Multisite Phosphorylation in Timing of a Yeast Cell Cycle Event

We study the biochemical network that triggers the S phase in yeast cell cycle. Key components of this network are three proteins: two kinases and an inhibitor. First kinase, Cln1/2-Cdk, acts as an input signal, phosphorylating the inhibitor, Sic1. The second kinase, Clb5/6-Cdk, is sequestered into an inactive complex by Sic1. Clb5/6-Cdk is the output signal of the circuit. Sic1 has nine phosphorylation sites, and phosphorylation of six or more of them causes it to degrade rapidly, leading to a sharp rise of free Clb5/6-Cdk. Our experiments indicate that multisite phosphorylation (MSP) is responsible for the timing robustness of this sharp transition. We study the role of MSP in timing using computer simulations. Preliminary results indicate that, MSP does not bring timing robustness when each kinase can phosphorylate each site with identical specificity. We employ in silico evolution to find the specificity configuration for the phosphorylation sites that leads to most robust timing under extrinsic noise.   

Precision of sensing, memory and fluctuating environments

Multiple cell types were recently shown to sense their chemical environment with astonishing accuracy, crucial for nutrient scavenging, mating, immune response, and development. It is currently unknown if this sensing near the single-molecule detection limit is due to highly precise single measurements, or due to learning over time. In this work, we analyze if cell memory can allow cells to sense beyond the current estimates of the fundamental physical limit. By merging Bayesian inference with information theoretic concepts, we derive analytical formulas which show that memory improves sensing in correlated fluctuating environments, but not in strongly uncorrelated environments. Despite many analogies with problem solving strategies in engineering, our theory shows fundamental differences in interpreting noisy stimuli in the microscopic and macroscopic world.   

Rapidly evolving microorganisms with high biofuel tolerance

Replacing non-renewable energy sources is one of the biggest and most exciting challenges of our generation. Algae and bacteria are poised to become major renewable biofuels if strains can be developed that provide a high,consistent and robust yield of oil. One major stumbling block towards this goal is the lack of tolerance to high concentrations of biofuels like isobutanol. Using traditional bioengineering techniques to remedy this face the hurdle of identifying the correct pathway or gene to modify. But the multiplicity of interactions inside a cell makes it very hard to determine what to modify a priori. Instead, we propose a technology that does not require prior knowledge of the genes or pathways to modify. In our approach that marries microfabrication and ecology, spatial heterogeneity is used as a knob to speed up evolution in the desired direction. Recently, we have successfully used this approach to demonstrate the rapid emergence of bacterial antibiotic resistance in as little as ten hours. Here, we describe our experimental results in developing new strains of micro-organisms with high oil tolerance. Besides biofuel production, our work is also relevant to oil spill clean-ups.   

A Cahn-Hilliard model of vascularized tumor growth in a complex evolving confinement using a diffuse domain approach

Understanding the spatiotemporal evolution of tumor growth is essential for developing effective strategies to treat cancers. Various studies have suggested that spatial heterogeneity during tumors growth is a key factor associated with subsequent tumor invasion and the effectiveness of chemotherapy. Spatial heterogeneity may arise due to morphological instability of the tumors and the complex tissue structure surrounding the tumors. In previous works, we have used a Cahn-Hilliard tumor growth model to study the morphological instability for tumors in non-resisting tissues. However, most tumors are surrounded by complex tissue structures and confined in the capsules of some organs or between certain basement membranes. The capsules and basement membranes may be distorted by interacting with the evolving tumors, affecting the morphological instability. Here we adopt a novel diffuse domain approach to adapt our previous Cahn-Hilliard model for tumor growth in such complex evolving environments. As an example, we apply the model to simulate the evolution of lymphoma in a lymph node, incorporating also the tumor-induced angiogenesis.   

Emergence of Information Transmission in a prebiotic RNA Reactor

A poorly understood step in the transition from a chemical to a biological world is the emergence of self-replicating molecular systems. We study how a precursor for such a replicator might arise in a hydrothermal RNA reactor, which accumulates longer sequences from unbiased monomer influx and random ligation. In the reactor, intra- and intermolecular base pairing locally protects from random cleavage. Analyzing stochastic simulations, we observe a strong bias towards long sequences with complex secondary structures, which would facilitate the emergence of ribozymes. Further, we find temporal sequence correlations that constitute a signature of information transmission, weaker but of the same form as in a true replicator.   

Towards molecular evolution with thermal traps

Live evolves by replication and selection of nucleotide polymers. Our experiments aim to drive molecular replication and selection with physical nonequilibrium boundary conditions. We discuss three approaches. Replication Trap. Molecules are exponentially accumulated by a combination of thermophoresis and convection, driven both by the same thermal gradient [1]. We have shown last year that with the help of a polymerase protein, concurrent replication and accumulation is possible [2]. Convection is melting and annealing the DNA in an oscillatory pattern, doubling the DNA in each cycle. Trapped polymerization. The chemical equilibrium of polymerization is expected to shift in the thermal trap. As the trap accumulates the monomers, polymerization yields longer polymers. However, since the trap is exponentially length selective, distributions beyond exponential tails are predicted. Replication by selective degradation. Replication typically is discussed as template directed polymerization. We showed that selective degradation and a thermal trap leads to replication-like behavior using only non templated polymerization [3]. The progression of information is given by the faster degradation of single stranded over double stranded RNA. [1] PNAS 104, 9346 (2007) [2] PRL 104, 188102 (2010) [3] PRL 107, 018101 (2011)   

Session P43: Invited Session: High Content Biophysical Data for Dynamic Studies in Cancer

Tissue Refractive Index Fluctuations Report on Cancer Development

The gold standard in histopathology relies on manual investigation of stained tissue biopsies. A sensitive and quantitative method for \textit{in situ} tissue specimen inspection is highly desirable, as it will allow early disease diagnosis and automatic screening. Here we demonstrate that \textit{quantitative phase imaging} of entire unstained biopsies has the potential to fulfill this requirement. Our data indicates that the refractive index distribution of histopathology slides, which contains information about the molecular scale organization of tissue, reveals prostate tumors. These optical maps report on subtle, \textit{nanoscale morphological properties} of tissues and cells that cannot be recovered by common stains, including hematoxylin and eosin (H{\&}E). We found that cancer progression significantly alters the tissue organization, as exhibited in our refractive index maps. Furthermore, using the quantitative phase information, we obtained the spatially resolved scattering mean free path and anisotropy factor g for entire biopsies and demonstrated their direct correlation with tumor presence. We found that these scattering parameters are able to distinguish between two adjacent grades, which is a difficult task and relevant for determining patient treatment. In essence, our results show that the tissue refractive index reports on the nanoscale tissue architecture and, in principle, can be used as an intrinsic marker for cancer diagnosis. \\[4pt] [1] Z. Wang, K. Tangella, A. Balla and G. Popescu, \textit{Tissue refractive index as marker of disease,} Journal of Biomedical Optics, in press).\\[0pt] [2] Z. Wang, L. J. Millet, M. Mir, H. Ding, S. Unarunotai, J. A. Rogers, M. U. Gillette and G. Popescu, \textit{Spatial light interference microscopy (SLIM),} Optics Express, 19, 1016 (2011).\\[0pt] [3] Z. Wang, D. L. Marks, P. S. Carney, L. J. Millet, M. U. Gillette, A. Mihi, P. V. Braun, Z. Shen, S. G. Prasanth and G. Popescu, \textit{Spatial light interference tomography (SLIT),} Optics Express, 19, 19907-19918 (2011).\\[0pt] [4] Z. Wang, H. Ding and G. Popescu, \textit{Scattering-phase theorem,} Optics Letters, 36, 1215 (2011).\\[0pt] [5] G. Popescu \textit{Quantitative phase imaging of cells and tissues} (McGraw-Hill, New York, 2011).\\[0pt] [6] H. F. Ding, Z. Wang, F. Nguyen, S. A. Boppart and G. Popescu, \textit{Fourier Transform Light Scattering of Inhomogeneous and Dynamic Structures,} Physical Review Letters, 101, 238102 (2008).   

Session P44: Focus Session: Block Copolymer Micelles and Polymersomes

Modeling of Interactions Between Spherical Micelles for Diblock Copolymers in Selective Solvents

The self-assembly of spherical micelles formed in systems with a diblock copolymer AB, consisting of a solvent-philic block (B) and a solvent-phobic block (A), in selective solvents (S) is studied here. Effective interactions between spherical micelles for a model system are quantified using self-consistent field modeling in real space, for the dilute regime $\phi_{\rm AB} < 0.2$, as well as using a pseudo-spectral implementation of SCFT, for the concentrated regime $\phi_{\rm AB} >0.2$. We show that the free energy of BCC, and FCC phases can be described in terms of a single effective pair potential that depends on micelle aggregation number, but the aggregation number changes significantly with concentration as well as temperature.   

Dissipative particle dynamics study of relationship between wall thickness and size in polymer vesicles

Vesicles and membrane properties have long been thought to be essential for reproducing the natural environment of living cells. By using dissipative particle dynamics method, we have studied the relationship between wall thickness and size of vesicles obtained from A1BnA1 block copolymers, where block A is hydrophilic and block B is hydrophobic. Our findings suggest that, the wall thickness is sensitive to the size of vesicles at a low block length ratio of B/A, but insensitive to the size at a large ratio. It shows both weak and strong effects with a crossover point in between. These behaviors are consistent with the experimental results of Eisenberg and co-workers. Besides, an additional crossover point also has been observed. With the B/A ratio increases, the relationship goes from strong to weak behavior, and this transformation first appears to affect the outer area for large sized vesicles, and then to the inner area for small sized vesicles. These results may also be useful in delivery applications through controlling the hydrophobic membrane and the hydrophilic coronas.   

Effect of Hydrophobic Block Length on Vesicle Formation of Block Copolymers

Self-assembly of amphiphilic block copolymers result in a wide range of aggregates in aqueous solution, including spheres, rods, lamellae, vesicles, and large compound vesicles with an inverted core-shell structure surrounded by hydrophilic surface. In the work, we carried out a systematic investigation of A1BnA1 triblock copolymers with different hydrophobic block length by using the dissipative particle dynamics (DPD). As the B/A ratio increase further, no well-defined disk-like micelles (bilayer structures) are seen. Spherical micelles rearrange internally and swell the hydrophilic group in the center until vesicles are formed. Because the longer chains appear to drift slowly, it is clear that the rearrangement process becomes slower in the system with longer hydrophobic block, which needs more time to form vesicles. The segregation of A segments (hydrophilic group) leaves in the internal layer when the two small spherical micelles merge into a larger one. We observe that the existence of the two pathways of vesicles formation based on the hydrophobic block length using DPD particle models.   

Nanostructured assemblies from amphiphilic ABC multiblock polymers

Amphiphilic AB diblock copolymers containing a water compatible segment can self-assemble in aqueous media to give supramolecular structures that include simple spherical micelles and macromolecular vesicles termed polymersomes. Amphiphilic ABA triblocks with hydrophobic end blocks can adopt analogous structures but can also form gels at high polymer concentrations. The structural and chemical diversity demonstrated in block copolymer micelles and gels makes them attractive for applications ranging from drug delivery to personal care products to nanoreactors. The inclusion of a third block in amphiphilic ABC triblock systems can lead to a much wider array of self-assembled structures that depend not only on composition but also on block sequence, architecture and incompatibility considerations. I will present our recent efforts on tuning micelle and gel structure and behavior using controlled architecture ABC triblocks. The combination of diverse polymer segments into a single macromolecule is a powerful method for development of self-assembled structures with both new form and new function.   

The Morphology of Lipid Aggregates based on the Interplay among Molecular Architectures, Hydrophobic-Hydrophilic and Coulombic Interactions and their Kinetics

Lipids resemble amphiphilic diblock copolymers in the many ways in terms of rich resultant structures, which are controlled by spontaneous curvature and molecular interaction, except that the molecular length scale of interest is smaller. One of the most commonly studied systems is so-called ``bicelle'' (``bilayered micelle''), which is composed of a long- and a short- chain lipid, spontaneously forming discoidal micelles in aqueous solutions under certain conditions. The lamellae of ``bicellar mixtures'' have been used as magnetically alignable templates for structural study of membrane-associated proteins since they provide a native bilayered environment for the proteins in study. In this talk, I will summarize how and why a variety of morphology can be obtained from the ``bicellar'' system based on the molecular architectures (spontaneous curvatures) of the lipids, inter-particle Coulombic interaction and hydrophobic-hydrophilic interaction. Most interestingly, many of these structures are kinetically controlled but have robust formation mechanisms and high stability. Currently, we are particularly interested in how the exchange rate of the lipid molecules affects the kinetics.   

Phase Behavior and Kinetics of Diblock Copolymer in Selective Solvent

Synchrotron based time-resolved small angle x-ray scattering (SAXS) was used to study the kinetics of the formation of a gyroid phase in solutions of a poly (styrene -isoprene) (SI) diblock copolymer in dimethyl phthalate, a selective solvent for the polystyrene block. Temperature ramp measurements over the range of 70-130C show the transition from hexagonally-packed cylinders (HEX) to Gyroid phase for 75{\%} and 80{\%} (w/v) samples to be 117C and 96C, respectively. Results of temperature jump experiments to different jump depths to examine the kinetics of this transition will be presented. In addition to the Bragg scattering from the ordered phases, we were able to observe the temperature dependence of the diffuse scattering near q=0. The temperature dependence of the correlation length shows a crossover from $\surd $T near the glass transition for polystyrene to linear in T near the HEX to Gyroid transition. The effect of adding low molecular weight linear homopolymer PS to the samples on the phase behavior will be discussed.   

Phase Behavior of Gradient Copolymer Solution

We study the behavior of amphiphilic linear gradient copolymer chains under poor solvent conditions. Using Bond Fluctuation model and parallel tempering algorithm, we explore qualitative behavior of this class of polymers with varying gradient strength; which is the largest difference in the instantaneous composition along the polymer chain. Under poor solvent conditions, the chains collapse to form micelles. We find a linear dependence of hydrophilic to hydrophobic transition temperature on gradient strength. Systematic analysis of these clusters reveals a strong dependence of micelle properties on gradient strength. Also, we discuss our results with reference to recent experiments on synthesis and cloud point depression in gradient copolymers confirming gradient strength as key parameter in tuning micelle properties.   

Self-Assembly of Novel Amphiphilic 21-Arm, Star-Like Coil-Rod Diblock Copolymers at Interfaces

A series of novel amphiphilic 21-arm, star-like diblock copolymers, poly(acrylic acid)-$b$-poly(3-hexylthiophene) (PAA-$b$-P3HT) based on $\beta $-cyclodextrin ($\beta $-CD) with well defined molecular architectures and ratio of two chemically distinct blocks were prepared, for the first time, via a combination of quasi-living Grignard metathesis method (GRIM), click reaction, and atom transfer radical polymerization (ATRP). The star-like PAA-$b$-P3HT diblock copolymers consist of hydrophilic coil-like PAA cores and hydrophobic rod-like P3HT shells with narrow molecular weight distribution and controllable molecular weight of each block. Owing to the compact structure, the amphiphilic star-like PAA-$b$-P3HT formed a unimolecular micelle. Vesicles based on these novel amphiphilic star-like, coil-rod diblock copolymers were readily produced at the oil/water interface by crosslinking hydrophilic coil-like PAA cores with a bifunctional crosslinker, ethylenediamine. They also self-assembled into a nanotubular structure at the air/water interface.   

Formation and Characterization of Anisotropic Block Copolymer Gels

Cylindrical micelles formed from block copolymer solutions closely mimic biological fibers that are presumed to guide mineral formation during biosynthesis of hard tissues like bone. The goal of our work is to use acrylic block copolymers as oriented templates for studying mineral formation reactions in model systems where the structure of the underlying template is well characterized and reproducible. Self-consistent mean field theory is first applied to investigate the thermodynamically stable micellar morphologies as a function of temperature and block copolymer composition. Small-angle x-ray scattering, optical birefringence and shear rheometry are used to study the morphology development during thermal processing. Initial experiments are based on a thermally-reversible alcohol-soluble system that can be converted to an aqueous gel by hydrolysis of a poly(t-butyl methacrylate) block to a poly(methacrylic acid) block. Aligned cylindrical domains are formed in the alcohol-based system when shear is applied in an appropriate temperature regime, which is below the critical micelle temperature but above the temperature at which the relaxation time of the gels becomes too large. Processing strategies for producing the desired cylindrical morphologies are being developed that account for both thermodynamic and kinetic effects.   

Stimuli-responsive block copolypeptides

Stimuli-responsive polypeptide block copolymers are appealing systems due to the morphological polymorphic states they can exhibit in selective solvents, including micelles, vesicles, fibrils, and more complex supramolecular aggregates. Their morphologies can be engineered a-priori by the relative block lengths, the solvent composition, and their concentration. However controlling the morphology upon external stimuli offers clear benefits, since changes in structure and morphology can be induced on demand. In this talk, we present two examples of stimuli-responsive block copolypeptides: in a first case, a photoresponsive PLGA-PEO diblock is discussed capable to reversibly undergo micellization-dissolution-micellization upon visible or Uv light exposure, due to spiropyrans units decorating the PLGA block; in the second case a PBLG-PDMS-PBLG triblock copolymer undergoing reversible thermal-induced organogelation is also presented. The changes in morphology are correlated, in both cases to the variations in molecular conformations of the polypeptide blocks.   

Session P45: Surfaces, Interfaces, and Polymeric Thin Films - Surface Instabilities

Buckling in a particle film

When a thin rigid plate is adhered to a soft substrate and compressed, the plate will buckle out of plane to accommodate the applied stress. The out of plane bending is resisted by the substrate and the result is a sinusoidal topography (wrinkles). When the plate is replaced by a collection of closely packed particles similar phenomena results -- the positions of the particles move out of plane and follow a roughly sinusoidal curve. Due to the similarity of the end state of each system, the same continuum theory is often applied to model the behaviour. Here, we use a carefully constructed experimental system consisting of micron-scale polymer and silica spheres on a PDMS elastomer substrate to demonstrate the physical differences between a continuum plate and a discrete set of particles. In particular, because we can easily track the position of each particle in three dimensions with confocal microscopy, we have access to all of the particle motion. We note that the wrinkling is independent of particle modulus, and highly dependent on particle packing. This leads us to suggest that the underlying physics is granular (and not continuum) in nature. This result may have implications in biology, where elastic continua are often made of discrete building blocks (e.g. cells).   

Defect induced shape instabilities in textured Membranes

We study the dynamics of defect annihilation and quasi-equilibrium configurations in flexible textured membranes suffering a symmetry breaking phase transition. The phase separation process and relaxational properties are described through a Brazovskii-Helfrich-Canham Hamiltonian. Topological defects favor the development of local curvature to geometrically screen out the intrinsic stress field generated by the perturbations to the low symmetry phase. While in hexagonal systems the unbinding of dislocations and Carraro-Nelson interactions between disclinations slow down the dynamics, in smectic systems we found a wrinkled state where the bending forces are balanced by the out-of plane hoop stretching generated by positive disclinations. The wrinkled configuration found in the smectic systems show features that resemble those found in flexible thin films under small external loads.   

Dewetting Dynamics from Polymer Interfaces

The dewetting dynamics from metastable polymer/polymer interfaces is studied using X-ray Photon Correlation Spectroscopy (XPCS). In addition to more ``usual'' situations where XPCS correlation functions are used to measure out-of-plane height fluctuations (e.g. capillary waves), we show how this novel method can be used to measure in-situ in-plane motion of the dewetting film. The experimental correlation functions associated with this motion are fitted remarkably well by a simple model considering the growth of dewetting rims in the beam footprint.   

Nanoscale Confinement Induced Control of Polymer Thin Film Instabilities

Control of stability and wettability of polymer thin films is invaluable to a range of functional coatings with applications from electronics to biomedical coatings, yet simple non-chemical modification strategies to accomplish this are generally lacking. We demonstrate a novel route to effectively control instabilities in model polystyrene films on partially wetting and non-wetting solid substrates that would otherwise lead to film dewetting. The method involves top down confining capillary force lithography at various length scales. Systematic experimental studies on silicon and silicon oxide substrates supported by analytical theory shows that for confining pattern wavelengths less than $\sim $ 10 times film thickness, stabilizing surface tension forces dominate the overall energy balance of the system giving rise to stable films under confinement. Interestingly, thermal annealing at elevated temperatures after removal of confinement does not revert to growth of longer instability modes and stability of PS films is retained. These results pave the way for important new technological applications of otherwise unstable polymer films.   

The influence of the solid/liquid interface on the dewetting of ultra thin polymer films

In recent years, many studies showed that a thin liquid film on a solid surface in air bears more complexity than expected from a simple three-layer-system: e.g. a highly mobile surface layer in case the liquid is an unentangled polystyrene (PS) melt (Yang et al.,\textit{ Science} 2010; Seemann et al., \textit{J. of Polym. Sci. }2006) or the PS melt can slip over the solid substrate (Baeumchen et al., \textit{PRL} 2009). Our study focuses on such phenomena and explores their influence on dewetting (speed, morphology, etc.). We use hydrophilic and -phobic Si wafer (either covered by a highly ordered silane layer or by a thin layer of an amorphous fluoropolymer, AF 1600). On each of the substrates, one expects for a certain set of parameters spinodal dewetting for the PS melt. Yet experimentally, a much higher hole density is observed for both types of hydrophobic wafers than is theoretically expected. Moreover, the two hydrophobic coatings induce different dewetting speeds: the PS melt dewets faster on the silane covered Si wafer. The difference is attributed to slip (silane) or to no slip (AF 1600) conditions at the PS/substrate interface, which is also observable in the type of liquid front profile, which in turn changes the dewetting morphology.   

Viscoelastic properties of ultrathin polystyrene films by dewetting from liquid glycerol

There is considerable interest in studying the behaviors of polymers at the nanoscale. A liquid dewetting device originally proposed by Bodiguel and Fretigny[1] was built in our lab to study the size effect on the viscoelastic behaviors of ultrathin polystyrene (PS) films. PS with molecular weight of 278 kg/mol and 984 kg/mol and various thermal treatments were examined. The glass transition temperature (T$_{g})$ reduction and film stiffening were observed in films less than 20 nm in thickness and the properties of ultrathin PS films are different from the bulk PS. The value of the plateau compliance changes linearly with film thickness. No molecular weight effect was found on the liquid dewetting behaviors of these PS films. Interestingly, even though the dewetting occurs on liquid glycerol, the apparent T$_{g}$ reductions are less than observed in SiO$_{2}$ substrate supported films. \\[4pt] [1] H. Bodiguel and C. Fretigny, ``Viscoelastic dewetting of a polymer film on a liquid substrate,'' \textit{Eur.Phys. J. E}., 19, 185-193 (2006).   

Evolution of non-equilibrium entanglement networks in spincast thin polymer films

Measuring the rheology of non-equilibrium thin polymer films has received significant attention recently. Experiments are typically performed on thin polymer films that inherit their structure from spin coating. While the results of several rheological experiments paint a clear picture, details of molecular configurations in spincast polymer films are still unknown. Here we present the results of crazing measurements which demonstrate that the effective entanglement density of thin polymer films changes as a function of annealing toward a stable equilibrium value. The effective entanglement density plateaus with a time scale on the same order as the bulk reptation time.   

Evolution of Chitosan Film Thickness Profiles During Spincoating

Many hygroscopic biopolymers can be processed using aqueous solutions. For example, biopolymer films can be prepared by spincoating from dilute biopolymer solutions. We have spincoated ultrathin films of chitosan onto silicon substrates under controlled relative humidity (\textit{RH}) using acetic acid solutions. Since the solvent is much less volatile than organic solvents that are typically used to spincoat thin films of synthetic polymers, the dynamics associated with the spincoating process can be several orders of magnitude slower. Because of the slow evaporation of the solvent, it is possible to control the thickness by controlling the \textit{RH} value in the spincoating chamber. To gain insight into the spincoating process, we have collected images of the film during spincoating using a high-speed camera. This has allowed us to determine the evolution of the radial profile of the chitosan film thickness, which can be correlated with the final film thickness values measured using ellipsometry. We compare the measured film thickness profiles with those predicted by theoretical models of spincoating.   

Spreading of polymer droplets on thin polymer films

We present experimental results of small (r$\sim $10 $\mu $m) polystyrene droplets spreading on thin polystyrene films. Previous experimental work has extensively explored droplets spreading on various types of solid substrates. However, to our knowledge, micron-sized liquid droplets spreading on the same liquid substrate have not been previously studied due to the difficulty of preparing such a geometry. Initially a glassy droplet is placed atop a glassy thin film and a distinct interface exists, upon heating the interface heals quickly. During spreading we must thus consider not only the flow of the polymer droplet but also that of the supporting film. This droplet-on-liquid geometry is fundamentally different from previous studies and allows us to probe the nanorheology of thin polymer films. We observe a characteristic power law spreading that is dependent on the size of the droplet as well as the height of the substrate film. The preparation and study of such samples provides many opportunities because the droplet and supporting film may be of the same polymer, the same polymer but differing molecular weight, or two different polymers.   

Experimental Study of Particle Behavior on a Curved Polymer Interface

A spherical particle bound to an anisotropically curved liquid interface, such as a cylinder or catenoid, cannot maintain a constant contact angle without deforming the interface. Theory predicts that because of this deformation, individual particles will experience a capillary force toward lower negative Gaussian curvature. To test this prediction, particles are deposited from suspension onto interfaces of non-uniform shape. Melted polystyrene (PS) confined on chemically patterned surfaces creates semi-cylinders a few hundred microns in diameter. Microscopic catenoids are created by placing a melted PS film in an electric field. After cooling, PS vitrifies and the particles are frozen in place. The location of particles is observed by optical, scanning electron, and scanning force microscopy (SFM). Particles are observed to migrate to the rims of the catenoids while particles on semi-cylinders cluster, but show no preference for location. At these size scales and particle concentrations, the predicted single-particle behavior is not observed. SFM is used to determine the validity of the assumed constant contact angle boundary condition at the particle's surface. The implication of these results on curvature induced particle assembly will be discussed.   

Tension amplification in branched macromolecules

A molecule's topology can greatly affect the distribution of tension within its bonds. Pom-pom molecules consist of a short linear spacer linking two star polymers, each containing z long arms. The striking ability of this molecular architecture to magnify spacer tension in solution by several orders of magnitude, from the pN to the nN level is due to the steric repulsion between densely packed side chains. In fact, the tension can increase to values that significantly alter the molecule's chemical properties, or even initiate the scission of a carbon-carbon covalent bond in the spacer chain. We study the tension distribution in the spacer and in side branches using molecular dynamics simulations and scaling theory. The dependences of observables such as spacer tension and root-mean-square spacer length on the number of side chains, chemical spacer length, and length of side branches have been quantified. Scaling models are used to explain the interrelated phenomena of tension amplification and spacer elongation and to interpret the results of molecular dynamics simulations.   

Viscous memory effects on the generation of hierarchical morphologies at an emulsified oil/water interface

A defining feature of biological materials is their fractal morphology. Cancellous bone, pulmonary alveoli, small intestine villi, neural networks, and bladder epithelium are just a few examples of biological structures with hierarchically organized topographies spanning multiple length scales. Herein we present a self-assembly method that faithfully reproduces the topographic features of these biomaterials. The system consists of a photocurable monomer and water. To this quasi-two-component system we add surfactants that sculpt the interface into the desired shape. The resulting structures are then solidi?ed by crosslinking with UV light. Drawing from the rich phase behavior of oil/water/surfactant systems, we demonstrate complex fractal morphologies over many length scales ranging from several mm down to 100 nm. Quantitative image analysis reveals fractal morphologies with at least four distinct levels of hierarchy. Increasing viscosity, in particular, shows a strong correlation with the number of hierarchical levels.   

Effect of Molecular Architecture on the Wetting Properties of Polymers

We show that the wetting properties of star-shaped polystyrene (PS) macromolecules possessing sufficiently high functionality, f higher than 4, differ significantly from their linear analogues of otherwise identical chemical structure. The equilibrium contact angles of macroscopic droplets composed of star-shaped macromolecules on silicon oxide substrate, are as much as one order of magnitude smaller than their linear analogues, provided that f is equal to or greater than 8 and the degree of polymerization per arm length, Narm, is sufficiently small. The corresponding line tensions of the star polymers are as much as two orders of magnitude smaller. The dewetting characteristics of the linear and star polymers also differ. Linear PS chains dewet leaving an adsorbed layer of nanoscopic dimensions; this layer is also structurally unstable and breaks up into nano-droplets, leaving a precursor layer at the boundary of the macroscopic droplets. The thickness of this layer is consistent with expectations based on the shape of the effective interface potential. The corresponding nanoscopic layer of star-shaped polymers, of sufficiently large f and sufficiently small Narm, remains structurally stable. These effects are discussed in terms of the role of molecular architecture and entropic effects on the structure and dynamics of macromolecules at interfaces.   

Surface Segregation of Well-Defined Comb Polymers

Blending polymers with different chain architectures may prove useful in controlling interfacial properties by controlling interfacial segregation. A linear response theory by Wu \textit{et al.} predicts that a long-chain branched polymer blended with its linear analog will be preferentially segregated to the surface and interface of the blend film. The comb architecture is particularly promising for achieving substantial surface segregation. In particular, its high degree of branching provides a substantial driving force for surface segregation when chain ends prefer the surface. Comb polystyrenes with well-defined architectural details were prepared by living anionic polymerization via the ``grafting-through'' approach. Neutron reflectivity (NR) and secondary ion mass spectrometry (SIMS) analyses reveal that the comb polymers that are still miscible with linear analogs in the bulk segregate so strongly to the surface that the surface concentration is nearly 100 vol{\%}. The effect on the surface segregation of bulk concentration and chain end chemistry will be discussed.   

Size Dependant Nucleation of Confined 2-Decanol

We have studied freezing and melting of physically confined 2-decanol in nano porous silica using a Differential Scanning Calorimeter (DSC). Both melting and freezing temperatures are suppressed for physically confined 2-decanol. In the presence of bulk, freezing of the confined system is triggered by freezing of the bulk where nucleation is heterogeneous. There is, however, a cutoff size between 100 nm and 300 nm where phase transition is no longer initiated through heterogeneous nucleation. Below the cutoff size, nucleation is homogeneous where the confined system has to be supercooled further before any phase transition can occur. Melting of the confined system, on the other hand, is not influenced by the presence or absence of the bulk.   

Session P46: Invited Session: DNA-Programmable Particle Assembly

Self-Replication of Nanoscale tiles and patterns

We want to make a ``non-biological'' system which can self-replicate. The idea is to design particles with specific and reversible and irreversible interactions, introduce seed motifs, and cycle the system in such a way that a copy is made. Repeating the cycle would double the number of offspring in each generation leading to exponential growth. Using the chemistry of DNA either on colloids or on DNA tiles makes the specific recognition part easy. In the case of DNA tiles we have in fact replicated the seed at least to the third generation. The DNA linkers can also be self-protected so that particles don't interact unless they are held together for sufficient time -- a nano-contact glue. Chemical modification of the DNA allows us to permanently crosslink hybridized strands for irreversible bonds and a new type of photolithography. We have also designed and produced colloidal particles that use novel ``lock and key'' geometries to get specific and reversible physical interactions.\\[4pt] With Tong Wang, Ruojie Sha, Remi Dreyfus, Mirjam E. Leunissen, Corinna Maass, David J. Pine, and Nadrian C. Seeman.   

Inverse Problem in Self-assembly

By decorating colloids and nanoparticles with DNA, one can introduce highly selective key-lock interactions between them. This leads to a new class of systems and problems in soft condensed matter physics. In particular, this opens a possibility to solve inverse problem in self-assembly: how to build an arbitrary desired structure with the bottom-up approach? I will present a theoretical and computational analysis of the hierarchical strategy in attacking this problem. It involves self-assembly of particular building blocks (``octopus particles''), that in turn would assemble into the target structure. On a conceptual level, our approach combines elements of three different brands of programmable self assembly: DNA nanotechnology, nanoparticle-DNA assemblies and patchy colloids. I will discuss the general design principles, theoretical and practical limitations of this approach, and illustrate them with our simulation results. Our crucial result is that not only it is possible to design a system that has a given nanostructure as a ground state, but one can also program and optimize the kinetic pathway for its self-assembly.   

Structural flexibility of DNA-Nanoparticle Assemblies

Encoding interactions between nanoparticles using DNA allows for creation of new classes of materials in which particles arrange in superlattices with the structure mainly defined by particle geometry and interactions between DNA shells. The phase behavior in these systems quite often can be rationalized using the interaction energy maximization argument for DNA provided key-lock recognition. However, a polymeric nature of DNA connections can bring about an unexpected phase behavior with structures typically not observed for non-directional interactions. In addition, DNA sensitivity to various specific and non-specific stimuli provides for precise lattice tunability within a given phase. We will provide several examples of phase change in systems of DNA interacting nanoparticles, where unusual, low dimensional structures form due to collective behavior of DNA chains. We will also discuss various ways to dynamically change superlattice parameters using physical variables such as electrostatic interactions or external osmotic pressure for continuous lattice tunability or using DNA machinery to program a step-wise change in the lattice parameter of the assemblies.   

Nanoparticle Superlattice Engineering with DNA

Recent developments in strategies for assembling nanomaterials have allowed us to draw a direct analogy between the assembly of solid state atomic lattices and the construction of nanoparticle superlattices. Herein, we present a set of six design rules for using DNA as a programmable linker to deliberately stabilize nine distinct colloidal crystal structures, with lattice parameters that are tailorable over the 25-150 nm size regime. These rules are analogous to those put forth by Pauling decades ago to explain the relative stability of lattices composed of atoms and small molecules. It is ideal to use DNA as a nanoscale bond to connect nanoparticles to achieve colloidal superlattice structures in this system, since its programmable nature allows for facile control over nanoparticle bond length and strength, and nanoparticle bond selectivity. This assembly method affords simultaneous and independent control over nanoparticle structure, crystallographic symmetry, and lattice parameters with nanometer scale precision. Further, we have developed a phase diagram that predicts the design parameters necessary to achieve a lattice with a given symmetry and lattice parameters a priori. The rules developed in this work present a major advance towards true materials by design, as they effectively separate the identity of a particle core (and thereby its physical properties) from the variables that control its assembly.   

Real-space crystallography and transformations of DNA-directed particle superlattices

DNA is a versatile tool for directing the controlled self-assembly of nanoscopic and microscopic objects. The interactions between microspheres due to the hybridization of DNA strands grafted to their surface have been measured and can be modeled in detail, using well-known polymer physics and DNA thermodynamics. Knowledge of the potential, in turn, enables the exploration of the complex phase diagram and self-assembly kinetics in simulation. In experiment, at high densities of long grafted DNA strands, and temperatures where the binding is reversible, these system readily form colloidal crystals having a diverse range of symmetries. For interactions that favor alloying between two same-sized colloidal species, our experimental observations compare favorably to a simulation framework that predicts the equilibrium phase behavior, crystal growth kinetics and solid-solid transitions. We will discuss the crystallography of the novel alloy structures formed and address how particle size and heterogeneity affect nucleation and growth rates.   

Session P47: Biopolymers: Molecules, Solutions, Networks, and Gels

Sub-nm porous membrane based on cyclic peptide nanotubes

Porous thin films containing subnanometer channels oriented normal to surface exhibit unique transport and separation properties and can serve as selective membranes for different applications. Generating flexible nanoporous films with densely packed vertical channels over large areas remains a significant challenge. We developed a new approach where the growth of cyclic peptide nanotubes can be directed in a structural framework set by the self-assembly of block copolymers. Conjugating polymers to cyclic peptides enables the nanotube subunits be selectively solubilized in one copolymer microdomain. Conjugated polymers mediate nanotube-polymer interaction to guide nanotube growth. This led to subnanometer porous membranes containing high-density arrays of through channels. In parallel, we also studied how to modify the interior of nanotubes with controlled geometry. Artificial amino acid is introduced in the primary sequence of cyclic peptide with a functional group presented in the nanotube interior without disrupting the high aspect ratio nanotubes. The new design of such a cyclic peptide enables one to modulate the nanotube growth process to be compatible with the polymer processing window, hence opening a viable way of fabricating polymeric membranes for different application   

Gel-on-Brush Model of Airway Surface of the Lung: its Predictive Role in Chronic Pulmonary Disease

Clearance of mucus is the primary defense mechanism that protects airways from inhaled infectious and toxic agents. The current two-layer Gel-on-Liquid model in which a gel-like mucus is propelled on top of a ``watery'' periciliary layer (PCL) surrounding the cilia does not adequately describe efficient mucociliary clearance in health nor properly predict failure of mucus clearance in disease. We propose and provide evidence for a qualitatively different Gel-on-Brush model with a gel-like mucus layer in contact with a ``brush-like'' periciliary layer, composed of macromolecules tethered to the airway surface. The relative osmotic moduli of the mucus layer to the ``brush-like'' PCL layer explain both the stability of mucus clearance in health and its failure in airway disease. Our Gel-on-Brush model of airway surface layer opens important new directions for treatments of airway disease.   

Experimental and Computational Analysis of Structural and Mechanical Properties of Fibrin Gels

An important area in coagulation research is the study of the structural stability of a blood clot, which has important medical consequences. The stability of clots is closely related to the fibrin networks, which provides the structural support for a blood clot. We synthesized, studied, and compared fibrin networks, with and without cells, formed under wild type and hemophilic conditions. The 3D structure of each fibrin network was reconstructed from z-stacks of 2D confocal microscopy sections by implementing novel image analysis algorithms. These 3D images were utilized to establish microstructure-based models for studying the relationship between the structural features and the mechanical properties of the fibrin networks. The mechanical properties were assessed by analyzing the networks' responses to uniaxial tensile and shear stresses, simulating the impact of blood flow on the fibrin network. We will show in this talk that the elasticity of the fiber network predicted by the model agrees well with prior experimental data for small networks. We will also show that a nonlinear worm-like chain model correctly predicts both the alignment of the fibers and the elastic properties of the networks as the clot sample is stretched.   

Block copolymer self-assembly for the responsive reinforcement of injectable protein hydrogels

Shear-thinning injectable protein hydrogels are emerging as important biomaterials for the minimally-invasive implantation of scaffolds for tissue engineering and drug delivery. In this work, responsive block copolymer self-assembly is employed to trigger nanostructure formation in hydrogels made from artificial associative proteins, producing hydrogels with resistance to shear-thinning post injection, reduced erosion rate, higher toughness, and dramatically reduced creep compliance. Polymer-protein-polymer triblock copolymers have been synthesized by conjugating poly(N-isopropylacrylamide) to either end of a protein midblock decorated with self-associating pentavalent sticker domains. Self-assembly at elevated temperatures introduces a second physical network into the protein hydrogel with an independent and tunable relaxation time. The phase behavior of these dual-network hydrogels has been explored, revealing the ability to access nanostructured morphologies, and the effect of self-assembled polymer domains on the linear mechanics and toughness of injectable hydrogels has been investigated.   

Electrophoresis of DNA-protein conjugates: hydrodynamic end effects and electrostatic interactions

DNA fragments can be separated by length in free solution by attaching them to neutral or positively charged ``drag-tags'' (e.g., proteins), a technique known as end-labeled free-solution electrophoresis (ELFSE). We first extend a previous theory of ELFSE for neutral drag-tags to the case of weakly charged drag-tags. The simplest variant of the theory assumes that all parts contribute equally to the mobility (no end effects) and that both the DNA and the drag-tag are fully flexible and do not interact. We analyze the influence of these assumptions. We obtain the exact (within the Kirkwood-Riseman approximation) form of the function describing the end effects for flexible polymers. The main significance of the end effects is the $N^{-3/4}$ (instead of $N^{-1}$) form of the correction to the mobility for large DNA lengths $N$. We also show that the end effects are weaker for semiflexible and stiff polymers. Using a simple model, we study how the conformation of the drag-tag changes due to the electrostatic interaction with the DNA and how this influences the mobility.   

The local dynamics of unfolded versus folded tRNA in comparison to synthetic polyelectrolytes and the role of electrostatic interactions

The local dynamics of RNA is strongly coupled to biological functions such as ligand recognition and catalysis. We have used quasielastic neutron scattering spectroscopy to follow the local motion of RNA and a synthetic polyelectrolyte as a function of Mg2+ concentration. We have observed that increasing Mg+2 concentration increases the picosecond to nanosecond dynamics of hydrated tRNA while stabilizing the tRNA folded structure. Analyses of the atomic mean-squared displacement, relaxation time, persistence length, and fraction of mobile atoms showed that unfolded tRNA is more rigid than in the folded state. This same behavior was observed for sulfonated polystyrene indicating that the increased dynamics in arises from charge screening of the polyelectrolyte rather than specific interactions. These results are opposite to what is observed for proteins for the relationship between the unfolded/folded states and the internal dynamics where the folded state is observed to be more rigid than the unfolded state. We conclude that the local dynamics for both bio- and synthetic polymers are strongly influenced by the electrostatic environment.   

Physical Structure of Methylcellulose Hydrogels

Methylcellulose (MC) is a chemically modified polysaccharide in which there is a partial substitution of hydroxyl groups with methoxy moieties. This results in a polymer that is water soluble at low temperatures and displays lower critical solution temperature (LCST) phase behavior at elevated temperatures. As such, aqueous solutions of MC have long been employed and studied due to their ability to form gels as temperature is increased. Currently, there is no consensus on the detailed mechanism of the gelation process, so a precise determination of the physical structure present in these materials may lead the way to new mechanistic understanding. Transmission electron microscopy (TEM) performed under cryogenic conditions is a powerful tool for the study of hydrogels as it allows direct imaging of the network while preserving the structure in the gel. Cryo-TEM investigations suggest that the hydrogel is composed of fibril-like aggregates comprising multiple polymer chains. Small-angle neutron scattering (SANS) provides a complimentary method to establish the detailed structure of the hydrogel network. We will report the effects of molecular weight, concentration, and temperature on the resultant physical structures within the gel.   

Mesoscopic Simulations of Free Solution Electrophoresis of Polyelectrolytes with Finite Debye Length

The mobility of charged oligomers results from a complex interplay between electrostatic and hydrodynamic interactions leading to a competition between counter-ion condensation and cooperative shearing within the diffuse layer. Simulations of polyelectrolytes that include explicit ions are computationally expensive, while algorithms that couple infinitely thin Debye layers to mesoscopic fluid models are only useful in the long chain limit because they fail to predict the rise and non-monotonicity of the mobility. We present a hybrid mesoscale simulation technique that utilizes multi-particle collision dynamics (MPCD) to simulate surrounding solvent molecules and the Debye-H\"{u}ckel approximation to assign effective charge to the MPCD particles. This hybrid scheme can capture the electro-hydrodynamics without having to explicitly include counter-ions or make costly electrostatic calculations. This MPCD-MD Debye-H\"{u}ckel method shows great potential for simulating electrophoretic behavior of polyelectrolytes in novel microfluidic devices.   

Interfacial adsorption and aggregation of amphiphilic proteins

The adsorption and aggregation on liquid interfaces of proteins is important in many biological contexts, such as the formation of aerial structures, immune response, and catalysis. Likewise the adsorption of proteins onto interfaces has applications in food technology, drug delivery, and in personal care products. As such there has been much interest in the study of a wide range of biomolecules at liquid interfaces. One class of proteins that has attracted particular attention are hydrophobins, small, fungal proteins with a distinct, amphiphilic surface structure. This makes these proteins highly surface active and they recently attracted much interest. In order to understand their potential applications a microscopic description of their interfacial and self-assembly is necessary and molecular simulation provides a powerful tool for providing this. In this presentation I will describe some recent work using coarse-grained molecular dynamics simulations to study the interfacial and aggregation behaviour of hydrophobins. Specifically this will present the calculation of their adsorption strength at oil-water and air-water interfaces, investigate the stability of hydrophobin aggregates in solution and their interaction with surfactants.   

Viscoelastic and poroelastic relaxations of polymer-loaded gels

Gel layers are prevalent in many applications including water purification, fuel cells, tissue engineering and drug delivery. In these materials, their performance is closely linked to controlling transport of solutes such as solvent or polymer. Thus, understanding the critical time- and length-scale that regulate solute transport will enable development of membrane materials with the desired performance. In this contribution, we present the Poroelastic Indentation Relaxation (PRI) approach in quantifying the viscoelastic and poroelastic relaxations of geometrically-confined hydrogel layers. We demonstrate this indentation-based measurement approach in characterizing several materials properties including diffusion coefficient, shear modulus, and average pore dimensions of the hydrogel independent of the extent of geometric confinement. Additionally, we present a relaxation model that accounts for the viscoelastic and poroelastic contributions to the total relaxation process. Finally, we show that the PRI approach can quantify diffusion of solvent and polymer solution in a single test simply by changing the extent of deformation.   

Mechanical Characterization of Photo-crosslinkable Hydrogels with AFM

Stimuli-responsive hydrogel films formed from photo-crosslinkable polymers are versatile materials for controlled drug delivery devices, three-dimensional micro-assemblies, and components in microfluidic systems. For such applications, it is important to understand both the mechanical properties and the dynamics responses of these materials. We describe the use of atomic force microscope (AFM) based indentation experiments to characterize the properties of poly($N$-isopropylacrylamide) copolymer films, crosslinked by activation of pendent benzophenone units using ultraviolet light. In particular, we study how the elastic modulus of the material, determined using the Johnson, Kendall, and Roberts model, depends on UV dose, and simultaneously investigate stress relaxation in these materials in the context of viscoelastic and poroelastic relaxation models.   

Tough hydrogels from hydrophobically associating polymers

Physical gels can be formed by interchain associations involving hydrophobic interactions. The viscoelastic and mechanical behavior of physically crosslinked copolymer hydrogels synthesized from $N$, $N$-dimethylacrylamide (DMA) and 2-(N-ethylperfluorooctane sulfonamido) ethyl acrylate (FOSA), with varying FOSA content, were studied by rheology and static tensile testing. The strong hydrophobic association of the FOSA moieties in an aqueous environment produced core-shell nanodomains (6 nm diameter) that provided the physical crosslinks. The PDMA-FOSA hydrogels exhibited excellent mechanical properties: modulus of 80 -- 130 kPa, elongation at break of 1000 -- 1600 {\%}, tensile strength of 500 kPa, and toughness of 4 --6 MPa, depending on the FOSA concentration. The physical hydrogels were much more efficient at dissipating stress than the chemical hydrogels. Dynamic viscoelastic and stress relaxation experiments of the physical hydrogels and a chemically crosslinked PDMA hydrogel showed that the physical gel was more viscous than chemical gel and displayed much greater stress relaxation. That result was attributed to the extra energy dissipation mechanism provided by the reversible, hydrophobic crosslinks.   

Characterization of the Interfacial Adhesion for Responsive Hydrogels on Substrates

In recent years a growing collection of responsive hydrogels have been developed which are capable of reversibly responding to a range of stimuli. These hydrogels operate in a hydrated environment and respond with significant volumetric reversible transformation through absorption or release of water within the polymeric network. Polymeric hydrogels in this class of shape memory materials have been successfully implemented in microfluidic and biomedical devices as components such as microchannels, micro-orifices, microvalves, micropumps and microcompressors. In practice, the hydrogel component is often polymerized on a substrate or within the confines of a channel in order to create responsive components. It is well known that the adhesion of a swelling hydrogel with its substrate varies with substrate properties. We show an experimental method to quantify the interfacial adhesion of responsive hydrogels with a substrate. This work provides a valuable method and theoretical basis for characterizing and optimizing the adhesion of responsive hydrogels.   

Resilient Synthetic PEG/PDMS Hydrogels Inspired by Natural Resilin

Novel synthetic hydrogels are developed by incorporating hydrophobic polydimethylsiloxane (PDMS) into hydrophilic poly(ethylene glycol) (PEG)-based network using thiol-norbornene chemistry. The properties of these hydrogel are comparable to natural resilin, which is an elastic protein, existing in many insects, such as the tendons of flea and the wings of dragonfly, with extraordinary ability of mechanical energy storage. The energy storage efficiency (resilience) of the hydrogels is more than 97{\%} even at tensile strains up to 170{\%}. In addition, the Young's modulus of the hydrogels ranges from 3 kPa to 300 kPa by increasing the volume fraction of the PDMS in the network. These unique properties are attributed to the well-defined network and negligible secondary structure, provided by the versatile and efficient chemistry.   

pH-dependent and pH-independent self-assembling behavior of surfactant-like peptides

Self-assembly of amphiphilic peptides designed during the last years by several research groups leads to a large variety of 3D-structures that already found applications in stabilization of large protein complexes, cell culturing systems etc. In this report, we present synthesis and characterization of two novel families of amphiphilic peptides KA$_{n}$ and KA$_{n}$W (n=6,5,4) that exhibits clear charge separation controllable by pH of the environment. As the pH changes from acidic to basic, the charge on the ends of the peptide molecule varies eventually leading to reorganization of KA$_{n}$ micelles and even micellar inversion. On contrary, the bulky geometry of the tryptophan residue in KA$_{n}$W limits the variation of the surfactant parameter and hence largely prevents assembly into spherical or cylindrical micelles while favouring flatter geometries. The studied short peptide families demonstrate formation of ordered aggregates with well-defined secondary structure from short unstructured peptides and provide a simple system where factors responsible for self-assembly can be singled out and studied one by one. The ability to control the shape and structure of peptide aggregates can provide basis for novel designer pH sensitive materials including drug delivery and controlled release systems.   

Session P48: Focus Session: Crystallization in Single-, Multicomponent, and Hybrid Systems II

Shear-induced homogeneous crystallisation in confined columns of polymer

In previous work, we have studied the crystallisation of a system of dewetted poly(ethylene oxide) (PEO) droplets. We have shown that this defect-free system nucleates homogeneously within the droplet volume. By capping this droplet system, a parallel plate geometry is obtained with the droplets forming isolated PEO columns (a capillary bridge) between the substrate and the cap. With this geometry we are able to investigate how the application of an oscillatory shear mediates the crystal nucleation step in small, defect-free volumes of material. These studies are typically carried out in bulk systems which are dominated by heterogeneous defects, here we will present our first results on the effect of shear on homogeneous nucleation.   

Chain Folding in Polymer Crystals Detected by Solid-state NMR

Polymer crystallization induces transitions from random coils in the melt states to bilayer structures consisting of chain-folded crystals and disordered amorphous regions. Although the concept of chain folding is well recognized, there have been continuous debates about adjacent re-entry fractions of polymer chains crystallized at different physical conditions. To understand chain-folding structures in complex systems, spatial selectivity to access short-range polymer-polymer interactions is necessary. In this talk, we will propose a novel approach using solid-state NMR and selective isotope labeling for characterizing chain-folding of polymer chains. Spatial selectivity in double quantum NMR reveals adjacent re-entry fractions in the bulk crystals at different conditions.   

Forming hierarchically ordered hybrid materials using polymer crystallization

Hierarchically ordered hybrid materials are of great interest for next generation advanced materials research. In this presentation, we will present our recent results on employing polymer single crystals (PSC) to template nanoparticle assembly, leading to hierarchically ordered hybrid materials. Tailor-made, free-standing NP frames and wires containing single or multiple types of NPs have been obtained by using an in-situ polymer crystallization method. End functionalized poly(ethylene oxide) and polycaprolactone single crystals were used as the templates. Gold and magnetite NPs were successfully patterned as evidenced by transmission electron microscopy experiments. Interaction between NPs, free polymer chain as well as PSC has been systematically monitored using Surface Plasmon Resonance. It has been found that NP-free polymer chain interaction holds the key to forming ordered NP patterns in solution.   

Understanding Molecular Epitaxial Mechanism of the $\gamma $-form Crystal and Chain Tilt in the $\alpha $-form Single Crystal of Isotactic Polypropylene

We attempt to investigate how the epitaxial domination of the crystal morphologies takes place in the $\gamma $-form of the chain-folded crystals using high molecular weight isotactic polypropylene (i-PP) samples with a controlled number of stereodefects. Two different morphologies were identified \textit{via} transmission electron and atomic force microscopies (TEM and AFM). One is needle-like and the other is ``flat''. Based on the tilted selected area electron diffraction (SAED) results from TEM, the microscopic formation mechanism of the ``needle'' and ``flat'' morphology was discussed and it revealed that in the ``flat'' $\gamma $-form crystal, the initial $\alpha $-form single crystal had to have a stem orientation tilted away from the thin film normal within the ac-plane around the b-axis. Elongated $\alpha _{2}$-form lath-like single crystals were grown from thin film melt at $T_{x}$ = 145 $^{\circ}$C -155 $^{\circ}$C using commercial sample. SAED experimental results show that the stems in these lath-like single crystals were tilted at an unusual 17$^{\circ}$ angle around the $b$-axis. This 17$^{\circ}$-stem tilt in the $\alpha _{2}$-form single crystals favors the ($10\bar {2})$ fold surface and appears to depend upon both conformational and chain folding constraints.   

Kinetic Control of the Halogen Distribution in Crystals of Precisely Substituted Polyethylenes

Isothermally crystallized polyethylenes with precise chlorine substitution on each and every 21, 19, 15 or 9 backbone carbon display a drastic change in crystalline structure in a narrow interval of crystallization temperatures. The structural change occurs within one degree of undercooling and is accompanied by a change in WAXD patterns, a sharp increase in melting temperature, an increase in TG conformers and a decrease in SAXS periodicity. These changes correlate with a different distribution of Cl atoms in the crystallites. Under fast crystallization kinetics, the Cl distribution in the crystallites is disordered, while slower crystallization rates favor intermolecular staggering of chlorines and a herringbone structure. The drastic change in morphology is enabled by the precise halogen placement in the chain and driven by the selection of the nucleus stem in the initial stages of the crystallization. Exquisite kinetic control of the crystallization in novel polyolefins of this nature allows strategies for generating nanostructures at the lamellar and sub-lamellar level not feasible in classical branched polyethylenes.   

On the Width of the Crystallization Process

Differential Scanning Calorimetry (DSC) is the most frequently used experimental techniques for the study of crystallization process in polymers and polymer-based nanocomposites. The experimental data are discussed within various theoretical approaches. Isothermal crystallization studies are typically discussed within the Avrami theory of phase change. However, the as recorded experimental data represent the time dependence of the derivative of the crystallinity degree (versus time) rather than the crystallinity degree. From the experimental point of view, the DSC parameters that are considered are the DSC peak position, the area under the DSC curve, and an empirical not very well defined onset temperature. However, all experimental data points are involved in the Avrami analysis after integration (in the Avrami analysis of the dependence representing the degree of crystallinity versus time). However, no effort has been done to understand the significance of the width of the as recorded DSC spectrum. A detailed analysis of the significance of the width of the DSC spectrum is presented. The connection between the width of the DSC spectrum and the exponent of the Avrami equation is analyzed in detail for the case in which nucleation is negligible. Tentatively, the as obtained results are extended to non-isothermal crystallization kinetics, recorded for various heating rates and to the Ozawa treatment of crystallization of polymers in non isothermal conditions.   

Crystallization and Ordering of Giant Molecules based on Nano-atoms

To create new functional materials for advanced technologies, control over their hierarchical structures and orders is vital for obtaining the desired properties. We utilized and functionalized fullerene (C60) and polyhedral oligomeric silsesquioxane (POSS), and assembled these particles with polymers to form those hierarchical structures. The structures of these assemblies along with the resulting ordered structures were analyzed to determine their structure-property relationships. One of the most illustrating examples is a series of novel giant surfactants and lipids possessing a well-defined amphiphilic head and polymeric tails. Various architectures of this class of materials have been constructed and their self-assembly processes in solution, in the condensed bulk and thin films have been investigated. Another set of examples are ``nano-atoms.'' These classes of molecules are designed to possess features of molecular Janus particles with various symmetry breakings. When specific interactions are introduced, these ``nano-atoms'': are functioned as building blocks to construct different amplified molecules and further to self-assemble into hierarchical ordered structures. Their thermodynamic phase diagrams and kinetic pathways are explored to understand this new class of materials and their potential applications in modern technologies.   

Morphologies in Semi-Crystalline Precise Acid- and Ion-Containing Polymers

For over a decade we have endeavored to quantify the morphologies in ion-containing polymers using a combination of X-ray scattering and electron microscopy methods. We are motivated by the promise that when we know the polymer physics connecting chemical structure, nanoscale morphology, and physical properties, new strategies for improving polymer properties will be evident. This is particularly challenging in semi-crystalline polymers. Recently, we have been exploring the morphologies in precise acid copolymers and in the ionomers made neutralizing these precise copolymers. The precise copolymers are synthesized by Prof. K. Wagener's group at the University of Florida using acyclic diene metathesis and have functional groups attached to every 9th, 15th or 21st carbon along a linear polyethylene molecule. The precision of these molecules gives rise to well-defined hierarchical structures including acid-decorated polyethylene lamellae and acid or ionic aggregates arranged on cubic lattices. The temperature dependence of these polyethylene-based copolymers is dominated by the crystallization of the polymer. Stretching these materials can induce anisotropy that has facilitated our determination of these hierarchical morphologies and plans are underway to initiate in situ tensile deformation and X-ray scattering to probe the dynamic morphology.   

Nanohybrid shish kebab paper: Crystal growth and film properties

Polyethylene single crystals were uniformly grown heterogeneously from carbon nanotubes (CNTs) in solution, forming the nanohybrid shish kebab (NHSK) structure. We demonstrate that highly uniform, free-standing nanohybrid buckypaper with high CNT contents (13-70\%) could be produced from vacuum-filtrated polymer single crystal-decorated CNTs. In this way, polymer crystals served as unique spacers for CNTs so that uniform hybrid buckypaper films could be obtained without CNT agglomeration. Wetting techniques, thermal analysis, and scanning electron microscopy were used to elucidate the effect of polymer single crystals on the resultant structure. Surface roughness of NHSK paper could be controlled by tuning the polymer single crystal size (CNT separation distance). Superhydrophobic NHSK papers were obtained with high surface adhesion, which mimics the rose petal effect. Conductivity of the NHSK papers also varied with polymer crystal size. Great enhancement of important properties could be achieved through the formation of ternary hybrids. To that end, initiated- and oxidative chemical vapor deposition methods extend NHSK buckypaper applicability by providing functional polymer surfaces. NHSK papers may find applications in sensors, electrochemical devices and coatings.   

Role of lamellar thickness in the kinetics of polymer crystal growth

The lamellar thickness of polymer crystals reflects their thermodynamic metastability and meanwhile decides the linear crystal growth rates on their lateral sides. The traditional theories about the growth kinetics of polymer crystals, like Lauritzen-Hoffman theory and Sadler-Gilmer theory, attributed the effect of lamellar thickness to the free energy barrier at the lateral growth front, in order to explain the slower growth of thicker crystals observed at higher temperatures. We studied the linear growth rates of flat-on-oriented polymer crystals in ultra-thin films, by means of dynamic Monte Carlo simulations of lattice polymers. We found that at the same temperatures, the thicker crystals are actually growing faster. The effect of lamellar thickness has no relation with the free energy barrier; rather, it is only related with the driving force for crystal growth. On the basis of the intramolecular crystal nucleation model, we discussed a reasonable microscopic image on the growth kinetics of polymer crystals.   

Isothermal crystallization kinetics of Poly (lactic acid) studied by ultrafast chip calorimeter

Poly (lactic acid) (PLA) is a biocompatible, biodegradable polymer which has attracted much attention. The crystallization ability, as one of the most factors influencing the physical properties of the biomaterials such as thermal, mechanical, and biodegradable properties, has been widely studied mainly by differential scanning calorimeters. However, although the crystallization of PLA is relatively slow, it's difficult to avoid the crystallization from the nuclei or the structure reorganization of the metastable crystalline formed during the annealing process when we use the normal DSC with the heating rate on the level of tens of K/min. With the chip calorimeter whose scanning rate can go up to 1000 K/s, we can avoid the structure reorganization of metastable crystalline during the heating. In this case we annealed the PLA sample in the 80-120$^{o}$C temperature range and found the relationship between the onset the melting temperature T$_{m}$ and crystallization temperature T$_{c}$ is T$_{m}$= 0.53T$_{c}$+ 213.5 and the equilibrium melting temperature is T$_{m,f }$=179.6$^{o}$C.   

Session P49: Focus Session: Organic Electronics and Photonics - Interfaces and Contacts

Charge transfer and polarization at interfaces with conjugated molecules

The function and efficiency of organic electronic devices is determined to a significant extent by the electronic properties of organic/organic heterojunctions and interfaces between electrodes and organic semiconductors. The energy level alignment between metal electrodes and active organic layers can be adjusted over wide ranges by employing interlayers of strong molecular acceptors and donors that undergo charge transfer reactions with the metal. It will be shown that such interlayers lead to lower charge injection barriers than pristine metals, even when the work function is the same. It is argued that the molecularly modified electrodes are electronically more rigid than their pristine metal counterparts, i.e., the electron spill-out at the organic-terminated surface is less pronounced compared to metal surfaces. The energy levels at organic/organic heterojunctions comprising donors and acceptors as used in organic photovoltaic cells are essentially independent of deposition sequence, as long as supporting electrodes to not induce energy level pinning. When a high work function electrode is used, the energy levels may become Fermi-level pinned and an electric field drops right at the heterojunction. This effect is exemplified for the donor diindenoperylene and the acceptor C60. The electric field distribution within an organic opto-electronic device may thus be adjusted locally by employing interfacial energy level pinning, even at weakly interacting organic/organic interfaces.   

Tuning Contact Recombination and Open-Circuit Voltage in Polymer Solar Cells via Self-Assembled Monolayer Adsorption

We adsorbed fluoro-alkyl and hydrogenated-alkyl phosphonic acid derivatives onto indium tin oxide (ITO) for forming self-assembled monolayers (FSAMs and HSAMs, respectively) to tune the open-circuit voltage (V$_{oc})$ of polymer solar cells. The adsorption of FSAM and HSAM alters the work function of ITO from 4.3 eV to 5.5 eV and 4.0 eV, respectively, as verified by ultraviolet photoemission spectroscopy. Polymer solar cells having FSAM-, HSAM-treated ITO, and bare ITO as anodes display V$_{oc}$ of 0.58V, 0.48V, and 0.31V, respectively. Yet, the hole injection barrier from the anode to the active layer is the same for all three devices. Inverse photoemission spectroscopy measurements indicate that the energy barrier for minority carrier transport to the anode is largest for solar cells comprising FSAM-treated ITO and lowest for devices with bare ITO as anode. Since a high energy barrier for minority carrier transport results in lower contact recombination at the anode, it is this energy barrier that is responsible for differences in the V$_{oc}$s observed in polymer solar cells having anodes that have been pre-treated with SAMs.   

Electronic Structure of Organic-Metal Interfaces

Organic self-assembled monolayers (SAMs) are important to the understanding of molecular electronics as well as the study of charge transfer in photovoltaic applications. We use two-photon photoemission (2PPE) to investigate the interfacial electronic structure of SAMs on metals. In particular we study the unoccupied states of thiolates and fluorinated thiolates on Cu(111) as a function of molecular coverage using both monochromatic and time-resolved bichromatic 2PPE. While accurate measurements have been made on mostly full monolayer systems, similar effort on detailed coverage-dependence has not yet been reported. We track the formation of the interfacial dipole layer as well as the emergence of intermediate and final states vs. coverage.   

Fermi level pinning by integer charge transfer at electrode-organic semiconductor interfaces

The atomic structure of interfaces between conducting electrodes and molecular organic materials varies considerably. Yet experiments show that pinning of the Fermi level, which is observed at such interfaces, does not depend upon the structural details. In this work [1], we develop a general model to explain Fermi level pinning, and formulate simple expressions for the pinning levels, based upon integer charge transfer between the conductor and the molecular layer. In particular, we show that DFT calculations give good values for the pinning levels. \\[4pt] [1] Appl. Phys. Lett 98, 113303 (2011).   

Analysis of interface states in blend of polythiophene and polyselenophene: experiments and theory

Organic photovoltaic devices are presently the subject of intense research since they could eventually propose solar energy solutions at a much reduced cost compared to inorganic devices. Presently, electron transport in organic photovoltaic devices is achieved with a fullerene derivative (PCBM) but this solution has some disadvantages. First, the ratio of PCBM to polymer has to be quite high to assume good electronic transport, and second, the relatively high cost of PCBM is not ideal with the goal to reduce the cost of the device. For these reasons, a replacement for PCBM is desirable and an all polymer device solution is viewed as the best avenue. Since polythiophene (P3HT) is ideal for hole transport, its isovalent polyselenophene (P3HS) where sulfur atoms are replaced with selenium atoms might offer an interesting alternative to PCBM. The physical processes in organic devices are quite different from those of inorganic devices. Charge separation in organic devices is achieved by forming an interface between two organic materials with type 2 level alignment. However, because of the large binging energies observed in organic compounds, there is often the presence of H-aggregate states at the junction between the two organic materials. In this presentation, we will report the results of interface states in a blend of polythiophene and polyselenophene. Photoluminescence spectra will be presented along with calculations of these states with the help of the time-dependent density-functional theory.   

Controlling Side Chain Density of Electron Donating Polymers for Improving $V_{OC}$ in Polymer Solar Cells

The ability to tune the LUMO/HOMO levels of electroactive materials in active layer of polymer solar cells is critical in controlling their optical and electrochemical properties because the HOMO and LUMO offsets between the polymer donor and the electron acceptor strongly affect charge separation and the open circuit voltage ($V_{OC})$ of a solar cell. Here, we developed two series of electroactive materials for improving $V_{OC}$ in polymer solar cells. First, we enable facile control over the number of solubilizing groups ultimately tethered to the fullerene by tuning the molar ratio between reactants from 1:1 to 1:3, thus producing $o$-xylenyl C$_{60}$ mono-, bis-, and tris-adducts (OXCMA, OXCBA, and OXCTA) as electron acceptors with different LUMO levels. As the number of solubilizing groups increased, $V_{OC}$ values of the P3HT-based BHJ solar cells increased from 0.63, 0.83, to 0.98 V. Second, we present a series of novel poly[3-(4-n-octyl)phenylthiophene] (POPT) derivatives (POPT, POPTT, and POTQT) as electron donors with different side-chain density. As a result of lower HOMO levels by decrease in the side-chain density of the polymers, the devices consisting of POPT, POPTT, and POPQT with PCBM showed increased $V_{OC}$ values of 0.58, 0.63, and 0.75 V, respectively.   

Charge Injection Mechanism at Carbon Nanotube-Organic Semiconductor Interface

One of the major challenges in the fabrication of high performance organic electronic devices is to overcome the inefficient charge injection from metal electrodes into organic semiconductors caused by large interfacial contact barrier. One potential key step of improving the charge injection is to employ single-walled carbon nanotubes as electrodes. Towards this end, we fabricated pentacene field effect transistor using densely aligned carbon nanotube array electrodes with open-ended and parallel tips. The room temperature electronic transport measurements of the devices show excellent transistor properties with field effect mobility of up to 0.65 cm$^{2}$/Vs and current on-off ratio of up to 1.7$\times $10$^{6}$, which are higher than that of the control devices fabricated with gold electrodes. The high-performance of the devices is attributed to lower charge injection barrier at carbon nanotubes and pentacene interface. In order to find the direct evidence of the low injection barrier, we carry out low temperature electron transport measurement of our devices. We will present the detailed analysis of the low temperature data.   

Does Organic Field Effect Transistors (OFETs) Device Performance using Single-walled Carbon Nanotubes (SWNTs) Depend on the Density of SWNT in the Electrode?

Carbon nanotubes as an electrode material for organic field effect transistors (OFETs) have attracted significant attention. One open question is that whether the density of the Single-walled carbon nanotubes (SWNTs) in the electrode has any influence in the device performance of OFETs. In order to address this issue, we fabricated OFETs using SWNT aligned array electrode, where we varied the linear density of the nanotubes in the array of the electrodes during dielectrophoretic assembly of high quality surfactant free and stable aqueous SWNT solution. The source and drain of SWNT electrodes have been formed by electron beam lithography (EBL) and oxygen plasma etching. The OFETs were fabricated by depositing a thin film of poly (3-hexylthiophene) on the SWNT electrodes. We will present detailed result of our study.   

Work function recovery of air exposed molybdenum oxide thin films with vacuum annealing

We report substantial work function (WF) recovery of air exposed molybdenum oxide thin films with vacuum annealing. The high WF ($\sim $6.8 eV) of thermally evaporated MoO$_{x}$ thin film was observed to decrease sharply to $\sim $5.6 eV with an air exposure of one hour. The drop in the WF was accompanied with a very thin layer of oxygen rich adsorbate on the MoO$_{x}$ film. The WF of the exposed MoO$_{x}$ film started to gradually recover with increasing annealing temperature in a vacuum chamber having base pressure of 8 x 10$^{-11}$ torr. The saturation in the WF recovery was observed around 460 $^{\circ}$C, with WF $\sim $6.4 eV. The adsorb layer was found to be removed after the vacuum annealing. We further studied the interface formation between the annealed MoO$_{x}$ and copper pthalocynine (CuPc). The highest occupied molecular orbital (HOMO) level of CuPc was observed to be almost pinned to the Fermi level, strongly suggesting an efficient hole injection through the vacuum annealed MoO$_{x}$ film.   

Ab-inito calculation of energy level alignment and vacuum level shift at CuPc/C60 interfaces

The alignment of the donor and acceptor enegy levels is of crucial importance for organic photovotaic performance. We investigate the interfaical electronic structure and energy level alignment of copper phthalocyanine (CuPc)/fullerene (C60) using ab-inito density functional theory calculations including van der Waals interactions and hybrid density functionals. We show that energy level alignment critically depends on the standing-up and lying-down orientation of the CuPc molecules relative to C60 at the interface. We calculate the magnitude of the interface dipole at different molecular orientations and compare them to the vacuum level shift observed in photoemission spectroscopy. The validity of existing theoretical models which invoke charge transfer on this organic interface will be discussed in light of our predictions. 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. Deparment of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.   

Solution based cathode deposition for polymer light emitting devices

We report on a method for deposition of cathodes for polymer light-emitting devices (PLEDs), which is fundamentally different than the widely used thermal evaporation of metals. The thermal evaporation is well established in the industry but is very different than the solution processing of the rest of the PLEDs. It requires much more sophisticated equipment and much longer processing time than the solution processing. The metal evaporation requires temperatures around 1000$^{\circ}$C with all following requirements for materials handling. Proposing a method alternative to the already well-established thermal evaporation technique is a challenging task, and we are demonstrating only the principal feasibility of a solution based deposition of the cathodes. It is based on electroless deposition of silver. The process is compatible with the solution processing of the rest of the device and allows to finalize the entire device using solution based processes. We demonstrate the most representative current-voltage characteristics, emission spectra, stability tests, and microstructure of the newly developed electrode. These initial experiments demonstrate the feasibility of the proposed method and point avenues for further improvement.   

Organic solar cells: How can the theory guide the experience?

Research in organic photovoltaic applications are receiving a great interest in the last few years as they offer an environmentally clean and low-cost solution to the world's rising energy needs. One of the main problems limiting the efficiency of an organic solar cell device is the strong binding energy of the excitons, typically of a few hundreds of meV, which is ten to one hundred times more than in inorganic devices. Another limiting factor, persistent in P3HT:PCBM devices, can be the misalignment of the the HOMO (Highest Occupied Molecular Orbital) and the LUMO (Lowest Unoccupied Molecular Orbital) energy levels of the different components of the solar cell. Scharber's model (Scharber, M.C., Adv. Mater. 18, 789) is a simple yet interesting approach for predicting the efficiency of those devices, mainly based on the values of the HOMO and the LUMO and reasonable assumptions for the exciton binding energy. In this presentation, we will discuss how theoretical calculations based on density-functional theory can provide a guide to find promising polymers for photovoltaic cells. The accuracy, limits and possible expansions of Scharber's model will be examined, and a number of interesting polymer candidates to reach and perhaps break the well-known 10 {\%} efficiency will be presented.   

Session P50: Focus Session: Dynamics of Polymers: Phenomena due to Confinement; Glass Transition

Theoretical prospects for understanding the glass transition in polymer thin films

Observations of confinement effects on the glass transition in thin polymer films remain vexing to theorists. Many experiments at sufficiently low frequencies consistently find that the free surfaces of many polymer films, whether freely suspended or supported, have anomalously low glass transition temperatures. This surface effect appears to be independent of molecular weight, and persists 10-20nm into the film. In contrast, freely suspended films have been observed by ellipsometry and other bulk-sensitive techniques to have anomalously low film-averaged $T_g$ values, depending strongly on both film thickness and molecular weight for Mw above about $4 \times 10^5$g/mol. The dependence on Mw persists to astonishingly large values ( $\sim 10^7$g/mol). Recent work of Pye and Roth strongly suggests the presence of two distinct mechanisms in these films, one operative at the free surface and Mw-independent, one depending on Mw and giving a weak second glass transition at considerably lower temperatures. In this talk, I will review the status of various theoretical proposals we have explored for explaining some of these disparate phenomena, including the effects of interrupted percolation at the free surface and the ``sliding mechanism'' originally suggested by de Gennes for Mw-dependent lowering of $T_g$. I will describe the present prospects for a successful theory, as well as suggest some potentially helpful experiments and analysis.   

Cooperative Length Scale and Fragility of Polystyrene under Confinement

While thin films are an attractive model system to investigate the impact of confinement on glassy behavior, extending studies beyond thin films to geometries of higher dimensionalities is vital from both scientific and technological viewpoints. In this talk, we present the impact of confinement on the characteristic length at the glass transition as well as the fragility for confined polystyrene (PS) nanoparticles under isochoric conditions. We measure the glass transition temperature ($T_{g})$, fictive temperature ($T_{f})$ and isochoric heat capacity of silica-capped PS nanoparticles as a function of diameter via differential scanning calorimetry. From the measurement of $T_{f}$, we obtain the isochoric fragility, and via the fluctuation formula, the characteristic length at the glass transition. We illustrate that confinement under isochoric conditions for PS nanoparticles leads to a significant increase in the isochoric fragility while the characteristic length is reduced with size. At the minimum the results demonstrate a relationship between fragility and the characteristics length of isochorically-confined polymer that is not intuitive from the Adam-Gibbs theory.   

Impact of systematic chain architecture changes on the glass transition and modulus of thin polymer films

We will discuss two systems that significantly impact the thin film behavior with minor changes in chemistry and chain architecture. First, two polymers based on 5-(2-phenylethylnorbornene) are examined. Depending on the polymerization route chosen, the resulting polymer backbone is comprised of either bicyclic (norbornyl) units, which leads to a relatively rigid polymer with a high bulk T$_{g}$, or monocyclic (cyclopentyl) units, which leads to a more flexible structure with a lower bulk T$_{g}$. The modulus and T$_{g}$ of the rigid bicyclic polymer is thickness independent down to $<$10 nm, whereas the modulus of the more flexible monocyclic polymer decreases with decreasing thickness. By hydrogenation of the pendant phenyl ring to the cyclohexyl counterpart, we illustrate that minor changes in the relative flexibility of the side chain do not impact the observed thin film behavior. Second, a series of polystyrene with controlled branching including linear, comb, 6-arm star and centipede. Based upon the molecular mass of the arms, the comb polymer has a significantly larger persistence length and interestingly exhibits only a modest decrease in T$_{g}$ (9 K) at 5 nm, while the moduli is thickness independent.   

A Direct Relationship between Enhanced Surface Mobility and Tg Reduction in Thin Polymer Films

Cooling rate dependent Tg measurements of thin polymer films show a strong correlation between the cooling rate and the confinement effects. It is observed that the Tg is more strongly dependent on the cooling rate as the film thickness is decreased. The confinement effects also become weaker at higher cooling rates and it appears that both for thick films and high cooling rates the confinement effect vanishes. The results can be plotted on an Arrhenius plot by assuming that the cooling rate is inversely related to the relaxation time as the temperature that the system falls out of equilibrium (Tg). The surface relaxation times can be independently measured using nano-hole relaxation and nanoparticle embedding techniques and the results can be plotted on the same Arrhenius plot. It is observed that the surface relaxation has much weaker temperature dependence, with an activation energy that matches the limit of zero film thickness for the rate-dependent Tg measurements. The strong correlation between surface properties obtained by direct mechanical measurements with the Tg measurements obtained by ellipsometry suggest that these two phenomena are from the same origin and one cannot be explained without the other.   

Reduced Glass Transition Temperatures of Thin Polymer Films - Confinement Effect or Artifact?

For two decades there have been reports of measurements of reduced glass transition temperatures ($T_{\mathrm{g}}$) in polymer, and in particular poly-styrene, films. These results have motivated theoretical models and a variety of sophisticated experiments probing interfacial polymer properties. While the much larger reductions in $T_{\mathrm{g}}$ for free standing films have suggested the importance of the free surface, a significant concern has been raised about a possible correlation between anomalous dynamics and incomplete equilibration of the sample. Here, we present new ellipsometry measurements which unambiguously address this concern. The glass transition in free standing and supported films can be changed by many 10's of degrees by manipulating the interfacial properties. Taken together with previous work the results clearly reveal the importance of free interfaces as we transition from two, to one, to zero free interfaces.   

Confinement in thin polymer films near Tg leads to factors of 10 to 1000 reductions in dye translational diffusion

A breakthrough time/fluorescence resonance energy transfer method is used to measure out-of-plane translational diffusion coefficients of small-molecule dyes in thin polymer films near the glass transition temperature, Tg. The bulk translational diffusion coefficient is a strong function of dye size, increasing by a factor of 100 in polystyrene when dye molecular volume decreases by 25{\%}. Reduction in PS film thickness below 500 nm leads to a factor of 1000 decrease in Disperse Red 1 diffusion coefficient while reduction below 140 nm leads to slightly more than a factor of 10 decrease in decacyclene diffusion coefficient. At a thickness less than 100 nm, the diffusions coefficients for the two dyes are identical with error. Similar effects have been observed in poly(methyl methacrylate) and polysulfone films at Tg + 3 K. These effects are not directly correlated with the Tg-confinement effect in these polymers as the length scales for confinement effects are much smaller in the case of Tg and in some of the polymers both diffusivity and Tg decrease with confinement.   

Measuring the glass transition of polymer nanodroplets

Despite almost 2 decades of measuring the glass transition temperature (T$_{g})$ in thin polymer films, there is no consensus on the magnitude (or even the existence) of observed T$_{g}$ reductions. Recent suggestions that reduced T$_{g}$~values in thin polystyrene films may be nothing more than a complicated artifact of sample preparation must be taken very seriously. Reduced T$_{g}$~values are reported only for polymer films prepared on a non-wetting substrate where the film is unstable with respect to dewetting. In all cases only kinetic metastability allows one to measure a T$_{g}$ value The fundamental problem is that an unambiguous measure of T$_{g~}$can only be obtained as the sample is cooled from the equilibrium liquid. No experiments involving thin polymer films have yet satisfied this condition. Our approach is to use samples prepared in the equilibrium state atop a non-wetting substrate (a collection of spherical caps). Since the droplets are already in the equilibrium state, they can be annealed for arbitrarily large times without evolution of the structure. We describe dilatometric measurements of~T$_{g}$ for nanometer sized spherical caps of polystyrene~This technique can be used to investigate the effect of annealing history and to extend the studies of T$_{g}$ in high surface area to volume systems beyond polymers and into other molecular glass forming materials   

Molecular motion in polymer thin films exhibits fast and slow subpopulations.

The reorientation of dilute fluorescent probes in thin polymer films was studied using a photobleaching technique. The existence of two subsets of probe molecules with different dynamics was revealed by temperature-ramping and isothermal anisotropy measurements. For freestanding polystyrene films, the slow subset shows bulk-like dynamics while the more mobile subset reorients 4 orders of magnitude faster at Tg - 5 K. The difference in dynamics becomes larger as temperature decreases and disappears as temperature approaches Tg. We interpret the fraction of the sample with fast dynamics as a high mobility layer at the film surfaces. The thickness of this mobile surface layer increases with temperature and does not depend on the molecular weight of the polymer and total film thickness. The mobile surface layer exists in various freestanding polymer thin films including polystyrene and poly(methyl methacrylate) and is also present in supported films of these polymers.   

Confinement Effects of Neighboring Polymer Domains on the Tgs of Infinitely Dilute Blend Components and Ultrathin Film Layers

Using fluorescence, we study the glass transition temperature (Tg) of a polymer species near the limit of infinite dilution. In blends with very dilute polystyrene (PS), intrinsic (unlabeled) and extrinsic dye-labeled fluorescence exhibit quantitative agreement for PS component Tgs. Dilute (0.1 wt{\%}) PS Tg is strongly perturbed towards the matrix Tg; e.g., 38 C in PnBMA and 119 C in PMMA. These dilute component Tgs yield a range of self-concentrations (0.18-0.47) in the framework of the Lodge-McLeish model. We also study multilayer, nanoconfined films with neighboring polymer domains by fluorescence. For PS supported on a bulk underlayer, the Tg of sub-100 nm PS is perturbed towards the underlayer Tg. For example, the 45 C Tg of a 14-nm PS layer on bulk PnBMA approaches the Tg for 0.1 wt{\%} PS in PnBMA. These results underscore the role of neighboring polymer domains on the dynamics of an infinitely dilute species or an ultrathin polymer film layer and indicate that Tg-confinement effects in blends and thin films can be viewed as variations of the same physical phenomenon.   

Molecular Weight Dependent and Independent Glass Transition Temperature Reductions Coexisting in High MW Free-Standing Polystyrene Films

Using transmission ellipsometry, we have measured the thermal expansion of ultrathin, high molecular weight (MW), free-standing polystyrene films over an extended temperature range. For two different MWs, we observed two distinct reduced glass transition temperatures (Tgs), separated by up to 60 K, within single films with thicknesses h less than 70 nm. The lower transition follows the previously seen MW dependent, linear Tg(h) behavior, while we also observe the presence of a much stronger upper transition that is MW independent and exhibits the same Tg(h) dependence as supported and low MW free-standing films. This represents the first experimental evidence indicating that two separate mechanisms can act simultaneously on thin free-standing polymer films to propagate enhanced mobility from the free surface into the material. The change in thermal expansion through the transitions indicate that $\sim $90{\%} of the film (matrix) solidifies at the upper transition with only $\sim $10{\%} of the material remaining mobile, freezing in at the lower transition. Surprisingly, when we compare our results to the existing literature, and especially the low MW free-standing film data, we conclude that the upper transition encompasses the free surface region and associated gradient in dynamics. This leaves open the question about where the small ($\sim $10{\%}) fraction of material that has ultrafast, MW dependent dynamics resides within the film.   

Glass Transition and Physical Aging in Thin Polymer Films: A Unified Picture of Confinement

The effect of confinement on the glass transition, structural relaxation, and other material properties has garnered a great deal of attention during the past two decades. Despite the common misperception, there is considerably more agreement than disagreement (or ``controversy'') of the phenomena in the existing literature. This talk will summarize current experimental findings in the field, focusing on the author's recent work that links physical aging in thin polymer films to Tg changes near a free surface. We also demonstrate that the intricate molecular weight dependence of the film thickness dependent Tg reductions in free-standing films result from two separate mechanisms acting simultaneously on the films. From these results a universal picture is starting to emerge with some effects common to colloidal and small molecule glasses as well.   

Session P51: Colloids III: Shear and Hydrodynamics

Confined colloidal suspensions under simple shear

Here we report a study of a simple model system, a colloidal suspension of near hard spheres in an otherwise Newtonian fluid using Stokesian Dynamics (SD) simulations in combination with Non-Equilibrium Umbrella Sampling (NEUS) techniques. The suspension is confined by an external potential in the y direction and is driven far out of equilibrium with a simple shear flow. At moderate shear rate, the suspension forms layers normal to the flow gradient direction, in contrast to equilibrium. In addition to that, novel anisotropic structures (strings in vorticity direction at low density for example) are observed within each layer. We use Non-Equilibrium Umbrella Sampling to explore the relationship between this string structure and the strength of the layer formation. Furthermore we have also studied the relationship between the non-Newtonian behavior of the suspension and the strength of the layer structure.   

The nonlinear structural response of colloidal suspensions under large amplitude oscillatory shear

When a colloidal suspension is under oscillatory shear, the particle configuration has a flow-induced anisotropy. While these structural rearrangements have been intensively studied in the linear regime where the amplitude of the applied shear is small, the nonlinear structural response of suspensions under large amplitude oscillatory shear is poorly understood. Using a shear cell coupled to a fast confocal microscope, we directly measured the microscopic structure of colloidal suspensions under large amplitude oscillatory shear. To quantify the structural response, we integrated the pair correlation function over all contact positions; this quantity is proportional to the entropic stress of the suspension. We investigated the structural/stress response of colloidal suspensions systematically with increasing shear amplitudes. We observed strong nonlinear responses in both dense and dilute suspensions under large amplitude oscillatory shear. At even higher amplitudes, we found an overshoot of the stress response in dense suspensions. Our results provide insight on the microscopic structural origin of the nonlinear response of sheared colloidal suspensions.   

Imaging the microscopic structure of shear thinning and thickening colloidal suspensions

The viscosity of colloidal suspensions can vary by orders of magnitude depending on how quickly they are sheared. Although this non-Newtonian behavior is believed to arise from the arrangement of suspended particles and their mutual interactions, microscopic particle dynamics in such suspensions are difficult to measure directly. Here, by combining fast confocal microscopy with simultaneous force measurements, we systematically investigate a suspension's structure as it transitions through regimes of different flow signatures. Our measurements of the microscopic single-particle dynamics unambiguously show that shear thinning results from the decreased relative contribution of entropic forces and that shear thickening arises from particle clustering induced by inter-particle hydrodynamic lubrication forces. Furthermore, we explore out-of-equilibrium structures of sheared colloidal suspensions and report a novel string phase, where particles link into log-rolling strings normal to the plane of shear. Our techniques illustrate an approach that complements current methods for determining the microscopic origins of non-Newtonian flow behavior in complex fluids.   

Shear induced diffusion in hard sphere glasses

The response of dense hard sphere suspensions is examined during the application of steady and non-linear oscillatory shear using Brownian Dynamics (BD) simulations and experimental Light Scattering echo coupled with rheology. At rest, volume fractions around the glass transition exhibit long or infinite relaxation times. However, non-linear shear induces out of cage motions of comparable time scale to the applied rates. We found two distinct regimes in terms of stresses and dynamic response under shear. One regime for lower rates or frequencies of oscillation, governed by Brownian activated diffusion, and a second for higher rates related to shear activated diffusion. A linear dependence with rate was found for the diffusivity in the high rate regime, mirroring the viscous loss due to shear activated particle rearrangements, while diffusivities in the Brownian activated regime showed a power law exponent of less than unity. The exponent was found to increase with volume fraction. For applied rates inducing diffusivities above the in-cage diffusivity at rest, we find a time window of super-diffusive behavior, between the short time (in-cage) and long time (out-of cage) diffusivities under shear, a signature of a dynamic breaking and reforming of the cage.   

Diffusion in sheared athermal soft-particle suspensions: the role of inertia and dissipation mechanism

We perform numerical simulations to study diffusion in a model bi-disperse frictionless athermal soft-particle suspension of disks in two dimensions (2D). To model athermal shear, we damp the motion of a particle \emph{either} with respect to the globally imposed flow \emph{or} with respect to its near neighbors. We study shear flows at various rate $\dot{\gamma}$, system size $L$, and damping strength $b$ at packing fractions well above the random close packing point. At low $\dot{\gamma}$, we find a quasi-static effective transverse diffusion co-efficient, $D_{\rm eff}$, which has very weak dependence on $\phi$, $b$, or the damping mechanism yet has a pronounced linear dependence on $L$ in agreement with what is observed in conventional models of bulk metallic glasses. Away from the quasi-static regime, $D_{\rm eff}$ no longer depends on $L$, and $b$ has a profound impact on the scaling behavior of $D_{\rm eff}$ with $\dot{\gamma}$.   

Shear thinning in soft particle suspensions

Suspensions of soft deformable particles are encountered in a wide range of food and biological materials. Examples are biological cells, micelles, vesicles or microgel particles. While the behavior of suspenions of hard spheres - the classical model system of colloid science - is reasonably well understood, a full understanding of these soft particle suspensions remains elusive. The relation between single particle properties and macroscopic mechanical behavior still remains poorly understood in these materials. Here we examine the surprising shear thinning behavior that is observed in soft particle suspensions as a function of particle softness. We use poly-N-isopropylacrylamide (p-NIPAM) microgel particles as a model system to study this effect in detail. These soft spheres show significant shear thinning even at very large Peclet numbers, where this would not be observed for hard particles. The degree of shear thinning is directly related to the single particle elastic properties, which we characterize by the recently developed Capillary Micromechanics technique. We present a simple model that qualitatively accounts for the observed behavior.   

Anisotropic Diffusion of Colloidal Particles in a Shear Flow

Asymmetrically-shaped particles show anisotropic diffusive behavior along different particle axes. This anisotropic diffusion, however, is averaged out on long time scales due to the rotational diffusion of the particles. Here we report on an experimental study of anisotropic colloidal dimers suspended in an oscillatory shear flow. A preferred orientation of the dimers arises due to the applied oscillatory shear. This results in anisotropic particle diffusion that is persistent at long time scales. We compare our results to a simple model of diffusing particles in a shear flow, and comment briefly on the possibility of using this result for assembling out-of-equilibrium colloidal structures.   

Pattern formation in colloidal explosions: Theory and experiment

We study the nonequilibrium pattern formation that emerges when magnetically repelling colloids, trapped by optical tweezers, are abruptly released, forming colloidal explosions [EPL 94, 48008 (2011)]. For multiple colloids in a single trap, we observe a pattern of expanding concentric rings. For colloids individually trapped in a line, we observe explosions with a zigzag pattern that persists even when magnetic interactions are much weaker than those that break the linear symmetry in equilibrium. Theory and computer simulations quantitatively describe these phenomena both in and out of equilibrium. An analysis of the mode spectrum allows us to accurately quantify the nonharmonic nature of the optical traps. Colloidal explosions provide a new way to generate well-characterized nonequilibrium behavior in colloidal systems.   

Universal Scaling Law of Diffusion and Hydrodynamic Corrections in Colloidal Monolayers

Using dense monolayers of colloidal particles and the techniques of optical microscopy and particle tracking, we tested the universal scaling law of the diffusion constant of colloidal particles as a function of excess entropy. By varying the area fraction of the colloidal monolayer, we measured the diffusion constant and the corresponding pair correlation function of the colloidal particles, from which the excess entropy can be calculated. It is found that the universal scaling law applies to a monolayer of latex suspensions at an air-water interface when the inter-particle repulsions are dominant over the hydrodynamic interactions. For colloidal monolayers of hard spheres at the air-water interface and near a solid wall, the universal scaling law starts to deviate from its original form as the short-ranged hydrodynamic interactions increase.   

Long Range Hydrodynamic Correlations in Quasi-One-Dimensional Circular and Linear Geometries

We report the results of studies of the collective and pair diffusion coefficients of particles in two quasi-one-dimensional geometries: straight 2 mm long channels and rings with radii between 3 and 35 $\mu $m. We investigate, for both geometries, the observed density dependence in the collective diffusion coefficient as predicted by Frydel and Diamant (Phys. Rev. Letts. 104, 248302 (2010). The origin of this density dependence is the nonvanishing q = 0 component of the Green's function of the linearized one-dimensional hydrodynamic equation, which is indicative of the hydrodynamic coupling resulting from collective motion of particles in periodic or infinite quasi-one-dimensional geometries.   

Using artificial microswimmers for particle separation

Microscopic self-propelled swimmers capable of autonomous navigation through complex environments provide appealing opportunities for localization, pick-up and delivery of micro-and nanoscopic objects. Inspired by motile cells and bacteria, man-made microswimmers have been created, and their motion was studied experimentally in patterned surroundings [1]. We propose to use artificial microswimmers -- Janus spheres [2] illuminated by light -- as ``driving agents'' that move through a binary mixture of colloidal particles. We demonstrated [3] that binary mixtures can be effectively separated in this way. We analyzed the main features of the particle separation and explained mechanisms of different regimes including the one with a velocity inversion. Our finding can be readily verified in experiments with colloidal binary mixtures and could be of use for various biological and medical applications. \\[4pt] [1] G.~Volpe et al., Soft Matter {\bf 7}, 8810 (2011).\\[0pt] [2] Q.~Chen et al., Science {\bf 331}, 199 (2011).\\[0pt] [3] W.~Yang, V.R.~Misko, K.~Nelissen, M.~Kong, and F.M.~Peeters, arXiv:1109.5099 (2011).   

Diffusion in Dense Inhomogeneous Colloid Suspensions in Narrow Channels

We report the results of a study of single particle diffusion in dense colloid fluids confined in a ribbon channel geometry that is intermediate between quasi-one-dimensional (q1D) and quasi-two-dimensional (q2D). In all of the systems studied the colloid density distribution transverse to the ribbon channel is stratified with peak amplitudes that depend on the colloid density. Although the virtual walls that confine a stratum are structured with a scale length of the colloid diameter, that structure does not have an apparent influence on the single particle diffusion, which shows the characteristic features of diffusion in a q1D channel with smooth walls. We find that for all channel widths and packing fractions studied the single particle transverse diffusion coefficient in a stratum is smaller than the single particle longitudinal diffusion coefficient in the same stratum, and that the single particle longitudinal diffusion coefficient varies very little from stratum to stratum, being only slightly smaller in the dense strata next to the walls than in central strata. The lack of variation of the longitudinal diffusion coefficient with apparent stratum density is explained by application of the Fischer-Methfessel approximation to the local density in an inhomogeneous liquid. The ratio of the transverse to longitudinal diffusion coefficients varies very slowly with ribbon width, implying a very slow transition from q1D to q2D behavior.   

Flow of concentrated emulsion in a microchannel: walls effects and roughness impact

Soft glassy materials have ubiquitous rheological properties. At small stress they deform elastically. For stress above a threshold they flow like liquids. At microscale, they are composed of highly disordered particles caged by the neighborhood. Flow happens by successive cage-jumps -or rearrangement. We study concentrated emulsion as a model fluid. When it flows in confined geometry, the viscosity does not correspond to the rheometer measurements but obeys to a non-local relation (Goyon-2008) due to rearrangement's correlation. As they impose viscosity's boundary conditions, walls modify the flow. We study carefully the conditions imposed by the walls and the impact of the roughness. In a microchannel, we create a Poiseuille flow. Using a fast confocal microscope we visualize the droplets and measure the velocity with high spatial resolution. At high stress, we observe one or two discontinuities of the velocity at respectively one and two droplets' diameters. They are due to stratification of the first droplets' layers. Far from them the non-local model remains valid. We create roughness by adding controlled size patterns. The roughness modifies the apparition of the stratifications and the limit conditions on the viscosity. We will compare these results with theory.   

Non-equilibrium dynamics in particle-interface systems

When a particle is at equilibrium at a fluid-fluid interface, its position can be calculated with Young's law (which has been used since 1805). The non-equilibrium behavior of particles at fluid-fluid interfaces, however, is only just beginning to be studied. In this talk, we will discuss the behavior of colloidal particles as they approach and meet an oil-water interface. A variety of different systems, such as approach from both the aqueous and oil phases and using aqueous phases of various salt concentrations will be compared. The motion of the polymer microspheres is captured using digital holographic microscopy in real time. As the holograms are simply two-dimensional images, the frame rate is limited only by the CMOS sensor and frame rates of up to 2000fps are used in this study. We then analyze the high frame rate data to recover the three-dimensional trajectory and fluctuations of the particles.   

Granular Fluid Kinetics Approach to Modeling Soft Colloid and Polymer Materials

The objective of this study is to understand the fundamental laws governing the Brownian motion of viscoelastic particles suspended in solvent at macroscopic equilibrium. Our hypothesis is that the internal degrees of freedom of the particles couple to their translational Brownian motion and affect their mean square displacement. Our system is similar to granular fluids with the important distinction that the energy absorbed by the particles during a collision is returned back thus maintaining a global thermodynamic equilibrium. We propose a new Molecular Dynamics model system that consists of tracer Brownian particles, solvent, and a virtual third component that serves as a thermal bath. The energy that is lost in an inelastic collision between Brownian and solvent particles is returned to the bath. The bath particles are undergoing elastic collisions among themselves and also with the solvent and Brownian particles. This provides a mechanism to restore and maintain an overall thermal equilibrium in the whole system. We report data on the effect of particle inelasticity on the translational diffusion.   

Session P52: Focus Session: Extreme Mechanics - Structures for Form and Function

Soft Robots: Manipulation, Mobility, and Fast Actuation

Material innovation will be a key feature in the next generation of robots. A simple, pneumatically powered actuator composed of only soft-elastomers can perform the function of a complex arrangement of mechanical components and electric motors. This talk will focus on soft-lithography as a simple method to fabricate robots--composed of exclusively soft materials (elastomeric polymers). These robots have sophisticated capabilities: a gripper (with no electrical sensors) can manipulate delicate and irregularly shaped objects and a quadrupedal robot can walk to an obstacle (a gap smaller than its walking height) then shrink its body and squeeze through the gap using an undulatory gait. This talk will also introduce a new method of rapidly actuating soft robots. Using this new method, a robot can be caused to jump more than 30 times its height in under 200 milliseconds.   

Low-Dimensional Generalized Coordinate Models of Large-Deformation Elastic Joints

In the field of robotics, it is increasingly common to use elastic elements such as rods, beams or sheets to allow motion between the rigid links of a robot, rather than conventional sliding mechanisms such as pin joints. Although these elastic joints are simpler to manufacture, especially at meso- and micro-scales, representational simplicity is sacrificed. It is far easier to compute the Lagrangian of a robot using joint angles as generalized coordinates, rather than by considering the large-deformation continuum behavior of elastic joints. In this talk, we will discuss our work toward finding accurate, low-dimensional discretizations of elastic joint mechanics, suitable for use in generalized coordinate models of robot kinematics and dynamics. We use modally parameterized backbone curves to describe the kinematic configuration of the elastic joints, and compute the energy associated with deformation using rod and shell theory. In the plane, only three smooth deformation modes are sufficient to describe Euler-Bernoulli bending of 90 degrees to within 1 percent. Parametric models for the three-dimensional motion of sheet hinges are more complex, but can be simplified significantly using boundary conditions and constraints imposed by ruled surface assumptions.   

Liquid-Embedded Elastomer Electronics

Hyperelastic sensors are fabricated by embedding a silicone rubber film with microchannels of conductive liquid. In the case of soft tactile sensors, pressing the surface of the elastomer will deform the cross-section of underlying channels and change their electrical resistance. Soft pressure sensors may be employed in a variety of applications. For example, a network of pressure sensors can serve as artificial skin by yielding detailed information about contact pressures. This concept was demonstrated in a hyperelastic keypad, where perpendicular conductive channels form a quasi-planar network within an elastomeric matrix that registers the location, intensity and duration of applied pressure. In a second demonstration, soft curvature sensors were used for joint angle proprioception. Because the sensors are soft and stretchable, they conform to the host without interfering with the natural mechanics of motion. This marked the first use of liquid-embedded elastomer electronics to monitor human or robotic motion. Finally, liquid-embedded elastomers may be implemented as conductors in applications that call for flexible or stretchable circuitry, such as robotic origami.   

Extreme Mechanics in Soft Pneumatic Robots and Soft Microfluidic Electronics and Sensors

In the near future, machines and robots will be completely soft, stretchable, impact resistance, and capable of adapting their shape and functionality to changes in mission and environment. Similar to biological tissue and soft-body organisms, these next-generation technologies will contain no rigid parts and instead be composed entirely of soft elastomers, gels, fluids, and other non-rigid matter. Using a combination of rapid prototyping tools, microfabrication methods, and emerging techniques in so-called ``soft lithography,'' scientists and engineers are currently introducing exciting new families of soft pneumatic robots, soft microfluidic sensors, and hyperelastic electronics that can be stretched to as much as 10x their natural length. Progress has been guided by an interdisciplinary collection of insights from chemistry, life sciences, robotics, microelectronics, and solid mechanics. In virtually every technology and application domain, mechanics and elasticity have a central role in governing functionality and design. Moreover, in contrast to conventional machines and electronics, soft pneumatic systems and microfluidics typically operate in the finite deformation regime, with materials stretching to several times their natural length. In this talk, I will review emerging paradigms in soft pneumatic robotics and soft microfluidic electronics and highlight modeling and design challenges that arise from the extreme mechanics of inflation, locomotion, sensor operation, and human interaction. I will also discuss perceived challenges and opportunities in a broad range of potential application, from medicine to wearable computing.   

Macrocomposite mechanical design, modeling, and behavior of physical models of bioinspired fish armor

The macrocomposite design of flexible biological exoskeletons, consisting of overlapping mineralized armor units embedded in a compliant tissue, is a key determinant of their mechanical function (e.g penetration resistance and biomechanical flexibility). Here, we investigate the role of macrocomposite structure, composition, geometric orientation, and spatial distribution in a flexible model natural armor system present in the majority of teleost fish species. Physical multi-material composite models are fabricated using a combination of 3-D printing and molding methods. Mechanical experiments using digital image correlation enable measurement of both the macroscopic response and underlying deformation mechanisms during various loading scenarios. Finite element-based mechanical models yield detailed insights into the roles and the tradeoffs of the composite structure providing constraint, shear, and bending mechanisms to impart protection and flexibility.   

Periodic Structural Solids: Mechanics and Multifunctional Applications

Triply periodic minimal surfaces have been of great interest to mathematicians, physical scientists, material scientists, and biologists. Close physical approximations to triply periodic minimal surfaces arise in a few material systems, such as block copolymers, nanocomposites, and biological exoskeletons. Here, we demonstrate the potential to design and fabricate two-component periodically ordered structures which correspond to the level set structures associated with triply periodic minimal surfaces. These structures are shown to have a unique combination of stiffness, strength, and energy absorption, as well as damage tolerance. The results provide guidelines for engineering and tailoring the nonlinear mechanical behavior and energy absorption of cocontinuous composites for a wide range of applications and further creating multifunctional materials. For example, polymeric materials which can change shape and material properties in response to external stimuli (temperature or electric field) can provide additional functionality when used as one of the phases, such as 3D shape memory. The periodic and multiphase nature of the structures also enables mechanically tunable band gap (phononic or photonic) materials, and tunable sensors in tissue engineering.   

Honeycombs with hierarchical organization

We investigated the mechanical behavior of two-dimensional hierarchical honeycomb structures using analytical, numerical and experimental methods. Hierarchical honeycombs were constructed by replacing every three-edge vertex of a regular hexagonal lattice with a smaller hexagon. Repeating this process builds a fractal-appearing structure. The resulting isotropic in-plane elastic properties (effective elastic modulus and Poisson's ratio) of this structure are controlled by the dimension ratios for different hierarchical orders. Hierarchical honeycombs of first and second order can be up to 2.0 and 3.5 times stiffer than regular honeycomb at the same mass (i.e., same overall average density). The Poisson's ratio varies from nearly 1.0 (when planar ``bulk'' modulus is considerably greater than Young's modulus, so the structure acts ``incompressible'' for most loadings) to 0.28, depending on the dimension ratios. The work provides insight into the role of structural organization in regulating the mechanical behavior of materials, and new opportunities for developing low-weight cellular structures with tailorable properties.   

Buckling-induced Tunable Chirality in Rationally-Designed Surface-Attached Cellular Structures

Chirality is crucial in understanding and controlling the behavior of living and non-living systems since the presence or absence of chirality in the structures plays important roles in their interactions with molecules, enzymes, light, and mechanical stress. Processes that induce chirality have been extensively studied at the molecular and macroscopic scales, but are relatively unexplored at the mesoscale. By rational design based on modeling, we experimentally demonstrate the controlled reversible switching between achiral and chiral configurations using swelling/de-swelling of surface-attached cellular structures. Importantly, the buckling patterns and the associated symmetry reduction of the initially achiral centrosymmetric structures could be tuned, simply by changing their dimensions. This approach opens the way to deterministically select to select the appearance of either mixed (racemic) or chiral phases. In the case of chiral transformations, spontaneous symmetry breaking resulted in the formation of large uniform areas of structures of single handedness. The fundamental understanding of this process provides a general route to designing deterministically deformable structures with dynamically switchable mechanical and/or optical properties.   

Buckling-induced Planar Chirality of Porous Elastic Structure

We present two periodic elastomeric structures which develop planar chirality induced by buckling under uniaxial/biaxial loading. The geometry of the structure comprises a 2-D plate patterned with a regular array of circular voids. Two specific circular void arrangements are obtained by investigating buckling-induced pattern transformations for void closure. Beyond the critical load, the thin ligaments between two adjacent voids buckle leading to a cooperative buckling cascade within the 2-D plate. Both micro-scale swelling experiments and finite element simulations are used to explore the underlying mechanics in detail and to show a proof of concept of the proposed structures. During swelling, the initial non-chiral pattern of the circular voids is transformed to a deformed pattern which exhibits planar chirality through buckling-induced symmetry breaking. In order to explore the effect of planar chirality, we perform an acoustic band structure calculation at different level of deformation. The planar chirality is found to strongly affect the in-plane phononic band gaps, providing opportunities for tunable phononic band structures.   

Anisotropy-induced wave steering in periodic linear and nonlinear lattices

Structural lattice configurations can be designed with tailored topologies which provide them with unusual behaviors, such as negative bulk modulus, negative Poisson's ratios, or extreme anisotropy\footnote{M. Ruzzene et al. Phisica Status Solidi B, \textbf{242}, 665 (2005)}. The latter is of particular relevance to explore the inherent anisotropic behavior of periodic lattices as a design paradigm for wave guiding and steering applications. The equivalent material anisotropy of square and skew periodic lattices is investigated through the application of Bloch's theorem\footnote{Bloch F., Z. Physik \textbf{52}, 555 (1928)} to the finite element discretization of the representative unit cell. The in-plane directions of wave propagation are determined through detailed analysis of the longitudinal and shear wave velocities, and verified through full-field wave propagation simulations. Similar wave behaviors are investigated analytically and experimentally for multilayer composite panels with anisotropic lay-ups in order to demonstrate the feasibility of micro structural design as an effective approach for wave management.   

2-D Phononic Crystals -- Unraveling the Effect of Void Distribution in Porous Structures

Phononic crystals are periodic materials consisting of different constituents with the capability to control the propagation of elastic waves. In this study, the dispersion relations of two-dimensional phononic crystals with \textit{circular voids} are investigated using Bloch-wave analysis. Porous patterns are derived from the \textit{Laves tilings}, which are duals of the eleven \textit{convex uniform tilings} of the \textit{Euclidean plane}. Numerical simulations are performed on the microstructures using finite element method. Frequency band-gaps are calculated and compared among different geometric configurations, void-volume fractions, and material properties, providing valuable insight into the behavior of phononic crystals. The predictive technical procedure developed here offers opportunities for the design of mechanical wave filters that have many potential applications such as noise-cancelling devices, acoustic wave guides and vibration isolators.   

Guiding of High Amplitude Stress Waves Through Stress-Induced Domain Switching in Multiphase Materials

Periodic and graded Multiphase Materials (MMs) are of great interest to scientists and engineers because of their unique static and dynamic mechanical properties, and the design flexibility they provide. In the linear range operation, MMs can be designed to attenuate vibrations over wide frequency bands and in specified directions, as defined by topology, geometry and material of the unit cell. Similarly, unit cell design and topology can be selected to obtain a desired anisotropy in the material, which can be exploited to alter the path of propagation of elastic and high amplitude stress waves. Specifically, steering of waves in preferential directions can be achieved through the proper arrangement of periodic hard inclusions within a matrix. Such a capability is extremely important for the design of materials capable of guiding stress waves to propagate along specified paths. In the present work, we explore the use of periodic metamaterials for wave management in force protection applications. We define topologies which adapt to high amplitude mechanical inputs, and study through numerical simulations and experiments local and global instabilities which lead to adaptive mechanical behavior through topological and structural modifications.   

Soft Modes and Deformations of Three-Dimensional Isostatic Periodic Lattices

Each particle in a three-dimensional isostatic lattice is connected by springs on average to six nearest neighbors, a condition obtained by J.C. Maxwell for marginal stability. The cubic and pyrochlore lattices satisfy this condition. By calculating the dispersion relations and the density of states for phonons in these lattices, we expand on previous studies of isostatic periodic structures [1], which have largely been focused on the simpler two-dimensional cases. The low energy phonon spectrum of these lattices exhibits features common to isostatic systems in any dimension, such as the presence of floppy modes and the scaling of a divergent length and a vanishing critical frequency. However, the allowed symmetries of an elasticity theory and the number of floppy modes depend on dimension and play a crucial role in the structure of the low-frequency response. We relate these findings to the isostatic transition in systems of close-packed athermal spheres and look at an analogy with three-dimensional crystals with zeolite structure. \\[4pt] [1] A. Souslov, A. J. Liu, and T. C. Lubensky, Phys. Rev. Lett. 103, 205503 (2009)   

Session P53: Disordered Systems: Packing

Analytical Construction of A Dense Packing of Truncated Tetrahedra and Its Melting Properties

Dense polyhedron packings are useful models of a variety of condensed matter and biological systems and have intrigued scientists and mathematicians for centuries. Here, we analytically construct the densest known packing of truncated tetrahedra with a remarkably high packing fraction 207/208=0.995 192, which is amazingly close to unity and strongly implies its optimality. This construction is based on a generalized organizing principle for polyhedra that lack central symmetry that we introduce here. The packing characteristics and equilibrium melting properties of the putative optimal packing as the system undergoes decompression are discussed.   

Packing fraction of dimers and anisotropic objects

We present a statistical theory and computer simulations for the calculation of the average volume in jammed assemblies of dimer shaped objects and other anisotropic particles like spherocylinders. The theory predicts the volume fraction as a function of the coordination number of the particles.   

Statistical Mechanics of Athermal Packings: Incorporating Basin Volumes

We present a first principles formalism for the statistical mechanics of athermal packings subject to driving beyond the weak limit. Edwards hypothesized a statistical mechanics of flat measure associated with packings explored at fixed density. This ensemble has been found to work well in the limit of very weak (but non-zero) driving. Beyond the weak driving limit, the probability measures associated with jammed states become proportional to the volume of basins of attractions associated with the packings on the density landscape. We propose here, a statistical mechanics which takes into consideration the volume of basins of attraction under certain approximations. Further, the statistical mechanics takes into account the protocol by writing the partition function in terms of an integral over protocol dependent generalized coordinates. This will allow an extremum principle to determine states, in these out of equilibrium systems.   

Polydisperse sphere packing in high dimensions, a search for an upper critical dimension

The recently introduced granocentric model for polydisperse sphere packings has been shown to be in good agreement with experimental and simulational data in two and three dimensions. This model relies on two effective parameters that have to be estimated from experimental/simulational results. The non-trivial values obtained allow the model to take into account the essential effects of correlations in the packing. Once these parameters are set, the model provides a full statistical description of a sphere packing for a given polydispersity. We investigate the evolution of these effective parameters with the spatial dimension to see if, in analogy with the upper critical dimension in critical phenomena, there exists a dimension above which correlations become irrelevant and the model parameters can be fixed \textit{a priori} as a function of polydispersity. This would turn the model into a proper theory of polydisperse sphere packings at that upper critical dimension. We perform infinite temperature quench simulations of frictionless polydisperse sphere packings in dimensions 2-8 using a parallel algorithm implemented on a GPGPU. We analyze the resulting packings by implementing an algorithm to calculate the additively weighted Voronoi diagram in arbitrary dimension.   

Constraint percolation on hyperbolic lattices

Constraint percolation models include constraints on the occupation of sites to, for example, better understand the onset of glassiness in glass-forming liquids. The dynamical glass transition in the Fredrickson-Andersen model simplifies to the study of the percolation transition in $k$-core percolation where every occupied site must have at least $k$ occupied neighbors. Other constraint percolation models, such as force-balance percolation, have been introduced to begin to account for mechanical equilibrium on each particle arising during the onset of jamming. To study a mean-field-like version of force-balance percolation in which the directionality of forces becomes important, we consider clusters with occupied particles satisfying the $k=3$-core condition and lying inside a triangle determined by three of its occupied neighbors. The model is constructed on a tessellation of the Poincar\'e disk, thus, bearing a hyperbolic structure. Models on such spaces exhibit mean-field-like behavior and also play an important role in generating geometric frustration in glassy systems. We analytically investigate the conditions under which there exists a transition as well as the underlying nature of the transition. We also present numerical results to compare with our analytical results.   

Cavity method for Edwards ensemble of jammed matter

Two theoretical frameworks have emerged to investigate the problem of random close packings: the Edwards statistical mechanics and the cavity method for glassy systems. Here we propose a model that combines both approaches into a single Hamiltonian imposing force balance constraints and minimization of the volume of the system. The formalism can be put into the framework of constraint optimization problems as recently proposed. The cavity method then solves the problem of force balance providing a prediction of the coordination number of the jammed packing. The model can be applied to spherical frictionless and frictional particles as well as non-spherical particles providing a prediction of the coordination number as a function of the aspect ratio of the particles.   

Glass Transition and Random Close Packing above Three Dimensions

Motivated by a recently identified severe discrepancy between a static and a dynamic theory of glasses, we numerically investigate the behavior of dense hard spheres in spatial dimensions 3 to 12. Our results are consistent with the static replica theory, but disagree with the dynamic mode-coupling theory, indicating that key ingredients of high-dimensional physics are missing from the latter. We also obtain numerical estimates of the random close packing density, which provides new insights into the mathematical problem of packing spheres in large dimension.   

Packing Squares in a Torus

We study the densest packings of N unit squares in a torus (i.e., using periodic, square boundary conditions in 2D) using both analytical methods and simulated annealing. We find a rich array of dense packing solutions: density-one packings when N is the sum of two square integers, a family of ``gapped bricklayer'' Bravais lattice solutions with density N/(N+1), and some surprising non-Bravais lattice configurations -- including lattices of holes, as well as a configuration for N=23 in which not all squares share the same orientation. We assess the entropy of some of these configurations, as well as the frequency and orientation of density-one solutions as N goes to infinity.   

Characterization of Basin Volumes in Mechanically Stable Packings

There are a finite number of distinct mechanically stable (MS) packings in granular systems composed of frictionless particles. For typical packing-generation protocols employed in experimental and numerical studies, the probabilities with which the MS packings occur are highly nonuniform and depend strongly on preparation protocol. Despite intense work, it is extremely difficult to predict {\it a priori} the MS packing probabilities. We describe a novel computational method for calculating the MS packing probabilities by directly measuring the volume of the MS packing `basin of attraction', which we define as the collection of initial points in configuration space at {\it zero packing fraction} that map to a given MS packing by following a particular dynamics in the density landscape. We show that there is a small core region with size $l_c$ surrounding each MS packing in configuration space in which all initial conditions map to a given MS packing. However, we find that the MS packing probabilities are not strongly correlated with $l_c$ and thus they are determined by complex geometric features of the landscape that are distant from the MS packing.   

Hierarchical freezing in a lattice model with a nonperiodic ground state

A recent result in tiling theory provides a two-dimensional lattice model with nearest and next-nearest neighbor interactions that has a limit-periodic ground state. During a slow quench from the high temperature, disordered phase, the ground state emerges through an infinite sequence of phase transitions, all related by renormalizations of the temperature scale with the sequence of critical temperatures approaching zero. As the temperature is decreased, sublattices with increasingly large lattice constants become ordered. Quenching at any finite rate eventually results in glass-like state due to kinetic barriers created by simultaneous freezing on sublattices with different lattice constants.   

Degenerate Quasicrystal of Hard Triangular Bipyramids Stabilized by Entropic Forces

The assembly of hard polyhedra into novel ordered structures has recently received much attention. Here we focus on triangular bipyramids (TBPs)- i.e.~dimers of hard tetrahedra- which pack densely in a simple triclinic crystal with two particles per unit cell [1]. This packing is referred to as the TBP crystal. We show that hard TBPs do not form this densest packing in simulation. Instead, they assemble into a different, far more complicated structure, a dodecagonal quasicrystal, which, in the level of monomers, is identical to the quasicrystal recently discovered in the hard tetrahedron system [2], but the way that tetrahedra pair into TBPs in the nearest neighbor network is random, making it the first degenerate quasicrystal reported in the literature [3]. This notion of degeneracy is in the level of decorating individual tiles and is different from the degeneracy of a quasiperiodic random tiling arising from phason flips [4]. The $(3.4.3^2.4)$ approximant of the quasicrystal is shown to be more stable than the TBP crystal at densities below $79.7\%$.\\[4pt] [1] Chen ER, Engel M, Sharon SC, Disc. Comp. Geom. 44:253 (2010).\\[0pt] [2] Haji-Akbari A, Engel M, et al.~Nature 462:773 (2009).\\[0pt] [3] Haji-Akbari A, Engel M, Glotzer SC, arXiv:1106.5561 [PRL, in press].\\[0pt] [4] Elser V, PRL 54: 1730 (1985)   

A Novel Decomposition of the Structure of Jammed Packings

We use simulations of 2D bidisperse disks to determine the properties of jammed packings and investigate the statistical mechanics of these systems. We have created a novel method for classifying structural subunits of a packing, using the structures to calculate relevant physical quantities. The classification scheme is based on a 20 type decomposition of the Delaunay triangles extracted from the centers of the particles in the packing. We find that the distribution of each type has a universal form, independent of total number of particles N in the packing for N=8-10,000, and that the parameters describing this form saturate as N is increased beyond N=20. We measure the distribution of the particle connections, the area distributions of the different structures, and nearest neighbor distributions. We explore the extent to which the nearest-neighbor distributions can predict the properties of the entire packing.   

8x8 and 10x10 Hyperspace Representations of SU(3) and 10-fold Point-Symmetry Group of Quasicrystals

In order to further elucidate the unexpected 10-fold point-symmetry group structure of quasi-crystals for which the 2011 Nobel Prize in chemistry was awarded to Daniel Shechtman, we explore a correspondence principle between the number of (projective) geometric elements (points[vertices] + lines[edges] + planes[faces]) of primitive cells of periodic or quasi-periodic arrangement of hard or deformable spheres in 3-dimensional space of crystallography and elements of quantum field theory of particle physics [points ( particles, lines ( particles, planes ( currents] and hence construct 8x8 =64 = 28+36 = 26 + 38, and 10x10 =100= 64 + 36 = 74 + 26 hyperspace representations of the SU(3) symmetry of elementary particle physics and quasicrystals of condensed matter (solid state) physics respectively, As a result, we predict the Cabibbo-like angles in leptonic decay of hadrons in elementary-particle physics and the observed 10-fold symmetric diffraction pattern of quasi-crystals.   

Competition of Bergman-type approximants with other packing motifs in the Cu-Zr system

Knowledge about the topological and chemical ordering in metallic liquids and glasses is essential in predicting phase selection and understanding glass formation dynamics. Taking the Cu-Zr system as an example, previous studies have established Bergman-type medium-range ordering (MRO) from a structural analysis with cluster alignment methods [1]. In this study, we examine the thermodynamic stability of a crystalline approximant of Bergman-type quasicrystals [2] against packing geometries existing in other intermetallic compounds for a wide range of Cu compositions. The most stable structures for each structural motif at each Cu composition are obtained using an efficient genetic-algorithm search. Our results show that the Bergman-type approximant structure is thermodynamically favored over other packing geometries at the glass-forming region with Cu compositions around 65{\%}, reaffirming the Bergman-type MRO is the lowest energy in Cu-Zr glasses.\\[4pt] [1] X. W. Fang, C. Z. Wang, Y. X. Yao, Z. J. Ding, and K. M. Ho, Phys. Rev. B 82, 184204 (2010).\\[0pt] [2] G. Bergman J. L. T. Waugh, and L. Pauling, Acta Cryst. 10, 254 (1957).   

Session P54: Superconductivity: Thermodynamics and Phases

Magnetic field scaling of the specific heat of La$_{1.92}$Sr$_{0.08}$CuO$_{4 }$up to 30 Teslas

The catalogue of experimental data on cuprate superconductivity has thoroughly characterized multiple different energy scales. Despite this, the complicated relationships between these energy scales remain poorly understood. New specific heat data on the underdoped cuprate La$_{1.92}$Sr$_{0.08}$CuO$_{4}$, performed in magnetic fields up to 30 Teslas demonstrates how the specific heat scaling behavior evolves as the cyclotron energy approaches the thermal energy at the superconducting transition temperature, T$_{c}$, and the mean-field calculated upper critical field, known as H$_{c2}$. This scaling behavior gives new insight into the complicated relationship of the various energy scales of the cuprate superconductor in the vortex state.   

High Magnetic Field Specific Heat in Cuprate Superconductors

We present new high magnetic field specific heat results for both underdoped YBCO 6.51 and overdoped LSCO. With these measurements we show that across different families of cuprates and across the superconducting-doping phase diagram, 45T does very little to suppress the d-wave superconducting gap. We discuss possible interpretations of the specific heat data through the use of bandstructure, previous high field magnetization measurements, and proposed Fermi surface reconstruction scenarios.   

Evolution of the phase diagrams in the pseudoternary system Pr$_{1-x}$Nd$_x$Os$_4$Sb$_{12}$

The pseudo ternary system Pr$_{1-x}$Nd$_x$Os$_4$Sb$_{12}$ has been used as a model system to investigate the effect of ferromagnetism (FM) on the unconventional superconductivity (SC), the high field ordered phase (HFOP), and quantum critical behavior [1], that was observed in PrOs$_4$Sb$_{12}$. SC in this system disappears near the Nd concentration x $\sim$ 0.58. Between x $\sim$ 0.33 and $\sim$ 0.58, weak FM, confirmed by the $\mu$SR experiments [2], was found to coexist with SC. In order to further inspect the possible quantum critical behavior, a power-law analysis of the temperature dependence of the electrical resistivity data was performed. Upon suppression of SC, for samples in the range 0.33 $<$ x $<$ 0.58, the power-law exponent decreases from $\sim$ 1.8 toward 1 in the temperature region below 2.5\,K, resembling non-Fermi liquid behavior. Detailed T-x, H-x, and H-T phase diagrams for various x will be discussed. \\[4pt] [1] Ho, et. al., PRB 83, 024511 (2011).\\[0pt] [2] Ho, et. al., 2010 APS March Meeting, A38.00005 (2010).   

Resonant Ultrasound study of underdoped single crystal YBCO cuprates

We measure elastic constants and ultrasound attenuation in ultra-high quality underdoped YBCO cuprate single crystal samples. We observe a phase transition at the pseudogap temperature and reveal additional phase structure inside the pseudogap phase.   

A Pre-Formed Pair Approach to the Dynamical (THz) Complex Conductivity in the Cuprates

In this talk we present a theory for $\sigma(\omega)=\sigma_1(\omega)+i\sigma_2(\omega)$ in the underdoped cuprates. Our work presumes that the pseudogap is associated with preformed pairs. We demonstrate how the puzzling extended range of finite $\sigma_2(\omega)$ above $T_c$ (implying a ``dynamical superfluid density'') arises from a new form of pair breaking contribution to $\sigma_2(\omega)$. This is only present in these moderately clean superconductors because of stronger than BCS attraction (BCS-BEC). Sum rule compatibility and good semiquantitative agreement with experiment is found.   

Origin of multiple quantum oscillation frequencies from single carrier nodal pocket in the underdoped cuprate superconductor YBCO

Quantum oscillations are measured in the underdoped high $T_{\rm c}$ cuprate superconductor YBa$_2$Cu$_3$O$_{6+x}$, revealing multiple frequencies. An extended study over a broad angular range and magnetic field range up to 95 T accompanied by detailed harmonic analysis is presented, establishing the multiple frequencies to arise from Fermi surface reconstruction yielding a single carrier nodal pocket. Scenarios are presented for the origin of multiple frequencies from this nodal pocket.   

Synthesis, $P-T$ phase diagram, and $T_c$ of $R_2Ba_4Cu_7O_{15}$ (R= Dy,Y, DyY, GdY )

The oxygen pressure - temperature phase diagrams of $R_2Ba_4Cu_{15}$ (R=Dy,Y, Er, DyY, GdY, and EuY) superconductors have been investigated in the temperature range between $850$ and $1025^{\circ}C$ and the pressure range between $1\,$ and $50\,atm.O_2$. The relative fraction of the phases: 123, 124, and 247, was determined by studying the intensity of the xray diffraction peaks of each phase. The condition at which the phase pure 247 exists was determined to be $P=10 \;atm$ and $T=1025^{\circ} C$. Under these conditions samples with larger size R = LaY, NdY, SmY, and Eu fail to form pure 247 phase. Annealing at $P=200 \; atm. O_2$ and $400 ^{\circ} C$ was used to increase oxygen content of the as-synthesized materials and to induce superconductivity. The highest transition temperatures of $70 K$ were observed for R=Y compositions.   

Pseudogap temperature along the Widom line of a first-order transition in doped Mott insulators

A state of matter with unusual physical properties, dubbed ``the pseudogap'', appears below a characteristic temperature $T^*$ in hole-doped high-temperature superconductors. In many cases, it is not even clear whether $T^*$ is a crossover or a phase transition. We construct the normal-state phase diagram of the two dimensional Hubbard model using cellular dynamical mean-field theory. We find that $T^*$ is a crossover line above the critical endpoint of a first-order phase transition between two metallic phases, one with a pseudogap and one without. Thus $T^*$ appears in a new light: it is an unexpected example of a phenomenon observed in fluids, namely a sharp crossover between different dynamical regimes along a line of thermodynamic anomalies that appears above a first-order phase transition, the Widom line. Our findings thus suggest that the critical point of a first-order transition, and not a quantum critical point, can be the organizing principle for the rich behavior of the normal state of the cuprates. Refs: G. Sordi et al., PRL 104, 226402 (2010); G. Sordi et al., PRB 84, 075161 (2011); G. Sordi et al., arXiv:1110.1392 (2011).