Session J1: Recent Advances in Density Functional Theory IV

Towards accurate density-functional treatment of non-covalent interactions in complex systems

Accurate treatment of intermolecular interactions is an outstanding challenge in density-functional theory. While numerous approaches are now capable of modeling small molecular dimers, it is unclear how applicable these methods are to larger systems. This talk systematically investigates errors in density-functional approximations for non-covalent interactions, applied to both dispersion-bound and hydrogen-bonded clusters. In particular, a combination of the large-gradient behaviour of the exchange functional and delocalization error, rather than three-body dispersion terms, are shown to determine the performance of a given method. Additionally, we attempt to develop an optimal range-separated hybrid functional to pair with the exchange-hole dipole (XDM) dispersion model for molecular clusters.   

Optimization of van der Waals Density Functionals using Data Projection onto Parameter Space (DPPS)

The parameterization and optimization of complex models fitted to reproduce a reference data set is an important part of the development of interatomic potentials. It is an approach that can also be used to design exchange and correlation functionals in density functional theory. Generally, this is a problem that requires choosing functional forms that depend on many parameters. The balance between the number of parameters and the size of the fitted data sets involves difficult and subjective decisions that are nevertheless critical for obtaining good results. We present a general and powerful optimization scheme, data projection onto parameter space (DPPS). The DPPS method tries to find the optimal parameters for a complex model which depends on a scalar function F which is determined by a large number of variables and parameters. The procedure involves the projection a vector of unknown parameters onto the vectors of known data. As an example, we apply DPPS to the optimization of the local exchange in a vdW density functional (vdW-DF). Our goal is to obtain an improved vdW-DF for water. To do so, we use an accurate potential energy surface for the water dimer as our initial data set.   

Electronic Properties of Surfaces and Interfaces with Self-Consistent van der Waals Density Functional

The long-range van der Waals (vdW) energy is only a small part of the total energy, hence it is typically assumed to have a minor influence on the electronic properties. Here, we address this question through a fully self-consistent (SC) implementation of the Tkatchenko-Scheffler (TS) density functional [1]. The analysis of TS-vdW$^{\rm{SC}}$ effects on electron density \textit{differences} for atomic and molecular dimers reveals quantitative agreement with correlated densities obtained from ``gold standard'' coupled-cluster quantum-chemical calculations. In agreement with previous work [2], we find a very small overall contribution from self-consistency in the structure and stability of vdW-bound molecular complexes. However, TS-vdW$^{\rm{SC}}$ (coupled with PBE functional) significantly affects electronic properties of coinage metal (111) surfaces, leading to an increase of up to 0.3 eV in the workfunction in agreement with experiments. Furthermore, vdW interactions visibly influence workfunctions in hybrid organic/metal interfaces, changing Pauli push-back and charge transfer contributions. [1] A. Tkatchenko and M. Scheffler, PRL (2009); [2] T. Thonhauser \textit{et al.}, PRB (2007).   

Van der Waals Interactions: Beyond Energies

The strong and ubiquitous influence of van der Waals (vdW) interactions on the structure and stability of molecules and materials is well established by now. However, much less is known about the role of vdW interactions in electronic and response properties of molecules, solids, and interfaces between them. We have recently developed and coded a fully self-consistent implementation of the Tkatchenko-Scheffler vdW density functional, enabling us to categorically assess the role of long-range vdW interactions beyond trivial energetic stabilization. We demonstrate that vdW interactions have a significant impact (and improve agreement with experiment) for HOMO-LUMO gaps, dipole moments, and polarizabilities of ``chemically bound'' alkali dimers. We rationalize this result based on Feynman's view on vdW interactions arising from electrostatic-like picture [1] rather than from the more conventional electrodynamic model. Finally, the role of vdW interactions on workfunctions and charge transfer in hybrid organic/metal interfaces, as well as elastic properties of molecular materials will be shortly discussed. [1] R. P. Feynman, Phys. Rev. 56, 340 (1939).   

Recent Developments in Fragment-based Density Functional Theory

I describe our progress on the development, implementation, and application of partition density functional theory (PDFT), a formally exact method for obtaining molecular properties from self-consistent calculations on isolated fragments [1]. 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 discuss the behavior of the fragment energies as a function of fragment occupations [2], the different ways in which PDFT can be used to avoid the delocalization and static-correlation errors of approximate density functionals [3], our recent extension to the time-dependent case [4], and future directions. \\[4pt] [1] P. Elliott, K. Burke, M.H. Cohen, and A. Wasserman, Phys. Rev. A 82, 024501 (2010).\\[0pt] [2] R. Tang, J. Nafziger, and A. Wasserman, Phys. Chem. Chem. Phys. 14, 7780 (2012).\\[0pt] [3] J. Nafziger and A. Wasserman, arXiv:1305.4966 \\[0pt] [4] M. Mosquera, D. Jensen, and A. Wasserman, Phys. Rev. Lett. 111, 023001 (2013).   

Ab-initio Charge and Spin Dynamics in Solids using TDDFT

With the advent of ultrafast and high intensity laser pulses, we can probe many new and interesting phenomena. Due to the time-scale of such situations, fully quantum mechanical approaches for the electron dynamics are required. Time-dependent density functional theory (TDDFT) is the natural choice for this problem, as it balances accuracy and efficiency. Here we report on the implementation of real time TDDFT for periodic systems including non-collinear magnetization, in the ELK electronic structure code[1]. This allows us to study situations beyond the usual linear-response, for example ultrafast demagnetization[2] or laser-induced dielectric breakdown. Additionally, we are developing and testing new methods related to time-dependent problems, such as an exchange-correlation magnetic field which is locally non-collinear[3], a time-dependent polarization field, and coupling to Maxwell's equations. [1] elk.sourceforge.org [2] Ab-initio Ultrafast Demagnetization in Solids, K. Krieger, P. Elliott, S. Sharma, J.K. Dewhurst, E.K.U. Gross, in prep (2013). [3] Transverse spin-gradient functional for noncollinear spin-density-functional theory, F.G. Eich and E.K.U. Gross, Phys. Rev. Lett. 111, 156401 (2013).   

Ensemble treatment of fragments within a molecule leads to improved description of dissociation

Approximate XC-functionals in Kohn-Sham (KS) density-functional theory (DFT) often fail when describing dissociation processes. This is due to improper treatment of fractional charges and spins within the dissociating systems. We demonstrate how the alternative framework of partition density-functional theory (PDFT) can correctly describe bond dissociation through its ensemble treatment of fragments within molecules. This method is illustrated through calculations on the dissociation of diatomic molecules.   

Practical methods in time-dependent density functional theory (TDDFT) at elevated temperatures

There is a great need to simulate dynamic material response properties under shock conditions where experimental data is often limited due to the extreme scales involved (MBars, 1000s of K, and manifold compressed solid densities). Knowing materials properties at this scale is vital element of simulations of planetary collisions, inertial confinement fusion experiments, and the surfaces of some stars. Considerable progress has been made using density functional molecular dynamics (DFT-MD) to model thermodynamic properties of material under these conditions; however, the approach is limited to cases in which the electrons are constrained to a thermodynamic distribution within the Mermin formulation. We will explore practical schemes to generalize this method to the time-dependent case. Several challenges come up such as the role of non-adiabatic electron-electron and electron-nuclear physics and the correct choice of initial state. One of the most straightforward choices of initial state is to project the Mermin state since the original Runge-Gross proof does not make explicit choice of occupations. We will present some numerical tests of finite systems to examine this formulation. We will also explore how simple models of non-adiabatic effects might be sufficiently accurate under extreme conditions. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Security Administration under contract DE-AC04-94AL85000.   

Recovering the Integer Discontinuity of Density Functional Approximations

The derivative discontinuity (DD) of the exchange-correlation (XC) energy of density functional theory (DFT) is a consequence of the piece-wise linear dependency of the energy functional on the number of electrons of atoms or fragments that have been separated adiabatically from a molecule. Most approximations to the XC energy functional as the local-density approximation, the generalized-gradient approximation, exact exchange, among others, miss the DD or the piece-wise linear behavior, leading to inconsistencies in the analysis of molecular dissociation. We derive formal properties of the {\slshape exact} XC energy functional that lead to a framework to correct {\slshape any} density-functional approximation to display the required piece-wise linear dependency on the number of electrons and the DD. We will also illustrate how new approximation to the XC energy functionals can be developed for applications in DFT and fragment-based extensions.   

Modeling long-range time-resolved charge-transfer within TDDFT: Insights from a 2-site lattice model

It has been shown that approximate adiabatic TDDFT functionals dramatically fail to reproduce time-resolved long-range charge-transfer dynamics (LR-CTD) [1]. In order to decouple the impact of the adiabatic approximation and the choice of ground state (gs) functional it would be instructive to propagate using the adiabatically-exact (adia-ex) functional. Numerically this involves an iterative process at each time-step to find the gs potential for a given density, which converges badly for CTD due to regions of low density. To circumvent this, we use as model system an asymmetric 2-site Hubbard model with small hopping parameter, its small Hilbert space allows to perform a Levy-Lieb constrained search and find the exact gs Hartree-exchange-correlation (Hxc) functional [2]. The later develops a sharp step feature in the long-range limit (limit of small hopping parameter). Both closed-shell to closed-shell and open-shell to open-shell LR-CT are investigated. By propagating the Kohn-Sham system in the presence of the exact gs Hxc functional under a resonant laser we are able to perform, for the first time, a fully self-consistent adia-ex propagation for CTD. [1] J. I. Fuks, P Elliott, A. Rubio, N. T. Maitra, JPCLett 4, 735 - 739 (2013) [2] J. I. Fuks et al, in preparation   

Time-Resolved Dynamics in Exact TDDFT: Studies of Two-Electron Systems

An exact decomposition of the exchange-correlation potential in time-dependent density functional theory (TDDFT) into kinetic and hole contributions is derived, with the goal of a better understanding of features of the TDDFT functionals, leading eventually to improved approximations. We study the kinetic and hole contributions for a range of dynamical situations in two-electron systems in one-dimension, from models of Rabi oscillations to local excitations, charge-transfer excitations, and resonance energy transfer, and compare them to their adiabatically-exact approximation. We find that dynamical step structures are present in both terms, that require a non-adiabatic functional approximation. In many cases, the kinetic contribution dominates the step structure, but not in all. The adiabatically-exact approximation is generally worse for the kinetic contribution than for the hole contribution.   

Session J2: Focus Session: Surface Chemistry and Catalysis IV

Benzene and Its Derivatives Adsorbed on Metal Surfaces: A Bag Full of Surprises

The study of molecule/metal interfaces is important for fundamental and applied surface science, and the electronic properties of these interfaces can be tuned by controlling their geometries. In this regard, a particular challenge for electronic structure theory is to reliably model the structure and stability of such hybrid interfaces. Here, we demonstrate that our DFT+vdW$\rm^ {surf}$ method [1] is able to describe 25 systems [\textit{e.g.}, benzene on Au(111) and Pt(111), thiophene/Ag(111), and DIP/Ag(111)] with an accuracy of 0.1~{\AA} in adsorption heights and 0.1~eV in binding energies wrt. reliable experimental data. In addition, our DFT+vdW$\rm ^ {surf}$ calculations lead to a few peculiar findings: (1) The vdW energy can contribute more to the binding of covalently bonded systems than it does in physisorbed interfaces [2,3]; (2) the binding energies of similar molecules can be identical, despite significantly different adsorption heights; (3) the physically bound (precursor) state for aromatics on Pt(111) can be prominently stabilized and long-lived, making it potentially useful in molecular switches [4]. [1] Ruiz, \textit{et al.}, PRL (2012). [2] Liu, \textit{et al.}, PRB (2012). [3] Liu, \textit{et al.}, NJP (2013). [4] Liu, \textit{et al.}, Nat. Commun. (2013).   

Selective self-assembly of molecular clusters with designed sizes on metal surfaces

The self-assembly of ``magic'' molecular clusters on various substrates provides a new arena for studies of surface nanocatalysis and molecular electronics. Here we present the self-assembly of phenylacetylene molecules on Cu(100) by a combined low-temperature STM and in-depth density functional theory investigation. We observe the molecules form distinct tetramer clusters on Cu(100) at 40 K. Each cluster has a four-fold symmetry and consists of four molecules. A delicate balance of intramolecular and dipole-dipole interactions between clusters maintains this magic tetramer configuration on Cu(100). The strong interaction between the molecules and the copper surface creates an anchor at each adsorption site. Through comparison with our previous observed hexamer (six-molecule) clusters on Au(111), we conclude that the epitaxial relationship between the molecules and metal surfaces is crucial in defining magic numbers of surface-supported molecular clusters under weak intermolecular interaction.   

One Step Propylene Epoxidation by Size Selected Subnanometer Cluster Silver Catalysts: Structure-Function Relationships Resolved Through in Situ Studies

The selective partial oxidation of propylene at low temperatures is accomplished by soft-landed, size selected subnanometer Ag clusters. The activity and selectivity for the creation of propylene oxide vs. acrolein is found to be size and support dependent, determined through the temperature programmed reactivity (TPRx) investigation of three cluster sizes between 3 and 20 atoms and three supports (Al2O3, TiO2, and ZnO). in Situ synchrotron X-ray characterization including Grazing Incidence Small Angle X-ray Scattering (GISAXS) and Grazing Incidence X-ray Absorption Spectroscopy (GIXAS) were performed to determine structural morphology and oxidation state during catalytic activity. The oxidation state of the Agn clusters (GIXAS) varies significantly due to size and support. At higher temperatures, changes in size due to assembly are observed through GISAXS with marked dependence on support with aggregates presenting distinct chemical properties and activity. Utilizing the presented method of catalyst synthesis and in situ characterization, it is feasible to investigate single active sites without the convolution that occurs in many studies from a range of particles sizes and active sites being present.   

Reaction-driven restructuring of Pt and Pd catalysts: In operando X-ray absorption spectroscopy study

The catalyzed hydrogenation of ethylene on supported metal catalysts has been intensively investigated, mainly because this reaction lies at the heart of many industrial processes. Most previous studies have been performed using surface science techniques in UHV. Therefor little is known about the nature of the active state of the catalyst at ambient pressure where the kinetics is very different. We employed operando X-ray absorption spectroscopy (XAS) to correlate the structural changes of SiO$_{\mathrm{2}}$-supported Pt and Pd catalysts with their activity for ethylene hydrogenation. The XAS experiments were performed at the beamlines X19A and X18B, NSLS, BNL. For both catalysts, strong and largely reversible transformations of the metal bonding were identified at about the maximum ethane conversion. The changes were different for Pt/SiO$_{\mathrm{2}}$ and Pd/SiO$_{\mathrm{2}}$ due to the ability of the latter to form bulk hydride, while the former can only adsorb hydrogen on the surface. As a result, Pt/SiO$_{\mathrm{2}}$ undergoes disordering of the surface, leading to a strong reduction of the Pt-Pt coordination number under H$_{\mathrm{2}}$-deficient conditions, while the main effect for Pd/SiO$_{\mathrm{2}}$ is the hydrogen uptake with concomitant increase in Pd-Pd bond length. The correlation between these different kinds of order transitions and differences in rates for these catalysts will be discussed.   

In-situ observation of sintering of Mono-metallic nanoparticles

Grain growth is detrimental in many applications of nanoparticles, especially for catalysis. During the physical processing of nanoparticles for various applications, they tend to coalesce and sinter. Upon grain growth, the size dependent physical and chemical properties of these nanoparticles undergo complete changes. For example, the nanoparticles need to be thermally activated to function as catalysts. However the thermal treatment renders these catalysts less efficient due to the decrease in electrochemically active area related to sintering. So it is imperative to study growth laws which predict the sizes of nanoparticles as a function of temperature to have better control of the structures and sizes in the nano-scale regime. The grain growth and sintering of Au, Pd and Cu nanoparticles of sizes 2-5nm were monitored using in-situ synchrotron based x-ray diffraction (XRD) in the temperature range from 500C to 800C. The data was compared to the empirical Kolmogorov-Johnson-Mehl-Avrami analysis and the activation energy was estimated. This was complemented with the study of transmission electron microscopy (TEM) data and mass transport analysis using basic sintering laws. The diffusion coefficients predicted from XRD and TEM were compared.   

Exploration of surface chemistry and structure of catalysts under reaction condition and during catalysis with surface-sensitive in-situ techniques

In heterogeneous catalysis, each catalytic event occurs on a catalytic site. The catalytic site typically consists of a couple of or a few atoms of a catalyst which pack into a structure to offer specific electronic state to turn on a catalytic reaction. Surface structure and chemistry are the key for understanding a catalytic mechanism. From thermodynamic point of view, the surface structure of a catalyst depends on the environment of reactant gases or liquid around the catalyst. Thus, the surface chemistry and structure of a catalyst under a reaction condition or during catalysis (in an environment of reactant(s) with certainly pressure) could be different from those from ex-situ studies. In-situ surface science characterization techniques have been developed for disclosing the hidden surface chemistry and structure of catalysts under reaction conditions or during catalysis. In-situ ambient pressure XPS (AP-XPS) and ambient pressure STM (AP-STM) are two of these surface-sensitive techniques appropriate for exploring surface chemistry and structure, respectively. In this talk, I will present the origin of pressure dependent surface chemistry and structure from thermodynamic point of view. AP-XPS and AP-STM techniques will be introduced briefly. I will focus on (1) the evolution of surface composition and oxidation state of a reducible oxide and how the evolution is correlated to the corresponding catalytic performances, (2) the distribution of surface elements on surface of a bimetallic catalyst under a reaction condition and how a restructuring is used to generate a new surface with different catalytic performance, and (3) geometric restructuring of a metal catalyst surface at atomic scale and how it is related to its catalytic performances.   

Acid-Base Interaction Induced Stability of Self-assembled Monolayer at Solid Interface Characterized by Sum Frequency Generation Spectroscopy

Long chain alcohols have been known to form hydrogen bonding with the hydroxyl groups on aluminum oxide surface. We used the interface sensitive technique, sum frequency generation (SFG) spectroscopy, to study the molecular structure of hexadecanol at liquid/sapphire interface and air/sapphire interface. We characterized the hydrocarbon chain conformation and the hydrogen bonding at different temperatures. Peak intensity changes were used to determine order-disorder transition of interfacial molecules. The transition hysteresis will also be discussed.   

First principles calculations of enthalpy and O-H stretching frequency of hydrogen-bonded acid-base complexes

Understanding the acid-base interactions is important in surface science as it helps to rationalize materials properties such as wetting, adhesion and tribology. Quantitative relation between changes in enthalpy ($\Delta$H) and frequency shift ($\Delta\nu$) during the acid base interaction is particularly important. We investigate $\Delta$H and $\Delta\nu$ of twenty-five complexes of acids (methanol, ethanol, propanol, butanol and phenol) with bases (benzene, pyridine, DMSO, Et$_2$O and THF) in CCl$_4$ using intermolecular perturbation theory calculations. $\Delta$H and $\Delta\nu$ of complexes of all alcohols with bases except benzene fall in the range from -14 kJ/mol to -28 kJ/mol and 215 cm$^{-1}$ to 523 cm$^-1$, respectively. Smaller values of $\Delta$H (-2 to -6 kJ/mol) and $\Delta\nu$ (23 to 70 cm$^{-1}$) are estimated for benzene. For all the studied complexes, $\Delta$H varies linearly (R$^2$ ? 0.974) with $\Delta\nu$ yielding the average slope and intercept of 0.056 and 1.5, respectively. Linear correlations were found between theoretical and experimental values of $\Delta$H as well as $\Delta\nu$ and are concurrent with the Badger-Bauer rule.   

Porous Particles: Controlling Molecular Diffusion within Metal-Organic Frameworks

Systematic investigation of molecular diffusion under nanoconfinement is carried out utilizing pore tunability of ionic metal-organic frameworks (MOFs). The translational and rotational diffusion of specially-selected guest dyes is evaluated by fluorescence correlation microscopy (FCS). A curious novel technique is demonstrated of controlling diffusion by switching counterions. Systematically, this study provides generalizable examples of how pore size, guest size, and host-guest interaction affect diffusion within nanopores.   

Manipulating the Band Structure of SrTiO$_{3}$ with Strain

SrTiO$_{3}$, the hydrogen atom of perovskites, is a very stable photocatalyst for water splitting. In this talk we demonstrate that the bandgap of SrTiO$_{3}$ can be altered by $\pm$ 10{\%} (0.3 eV) using biaxial strain in combination with phase transitions. The strain behavior is predicted and experimentally observed to be significantly different for (100) vs. (111) biaxially strained SrTiO$_{3}$ surfaces. In the absence of phase transitions the bandgap of biaxially strained SrTiO$_{3}$ decreases. In contrast, a strain-induced ferroelectric phase transition results in an increase in the bandgap. The band structure can also be morphed from indirect to direct bandgap through an antiferrodistortive phase transition. Both of these phase transitions can be manipulated using experimentally realizable biaxial strains, providing a new means to accomplish bandgap engineering of SrTiO$_{3}$ and related perovskites.   

Halide anion dependence of ionic surfactant adsorption in air/water interface

It was recently proposed that there is surface excess of halide anions at the air/water interface, and more surface excess of I$^{\mathrm{-}}$ than Br$^{\mathrm{-}}$ or Cl$^{\mathrm{-}}$, which cannot be explained by Debye-Huckel theory. In case of charged surfaces such as Gibbs monolayer consisting of cationic surfactant molecules, surface excess of anions can also be expected. In this study, by using surface-sensitive grazing angle X-ray fluorescence in conjunction with surface tension measurement, we investigated adsorption behavior of [C$_{\mathrm{12}}$mim]Cl, [C$_{\mathrm{12}}$mim]Br, [C$_{\mathrm{12}}$mim]I aqueous solutions, in which the surface is first covered by [C$_{\mathrm{12}}$mim]$^{\mathrm{+}}$ cations at low concentrations, and the adsorption of the halide anions to this charged interface would follow with the increase in the concentration of solutes. From the surface tension measurements, it was observed that critical micelle concentration of [C$_{\mathrm{12}}$mim]I solution was 4.6 mM, much smaller than that of [C$_{\mathrm{12}}$mim]Cl (16.7 mM) indicating surface activity of surfactant increases with size of halide anions. From X-ray fluorescence, surface excess of halide anion was measured quantitatively from the interface of these solutions. By putting NaCl and NaI in [C$_{\mathrm{12}}$mim]I and [C$_{\mathrm{12}}$mim]Cl solutions, respectively, competition between Cl$^{\mathrm{-}}$~and I$^{\mathrm{-}}$~adsorption was investigated, to find that I$^{\mathrm{-}}$ has stronger adsorption on the charged surface than Cl$^{\mathrm{-}}$.   

Interaction between M/CuO (M$=$ Ti, V, Cr, Mn) as studied by X-ray Photoelectron Spectroscopy

The technique of x-ray photoelectron spectroscopy has been utilized to investigate the chemical reactivity between metal M (where M is Ti, V, Cr, or Mn) and copper oxide at the M/CuO interface. Thin films of copper (about 20 nm) were deposited on silicon substrates by the e-beam method. Such samples were oxidized in an oxygen environment in a quartz tube furnace at 400$^{\circ}$C. The formation of CuO was checked by the XPS spectral data. Thin films of the metal M were then deposited on these CuO sample. The M 2p, oxygen 1s and copper 2p regions were investigated by XPS. The magnesium anode (energy $=$ 1253.6 eV) has been used for this purpose. The metal 2p peaks shift to the high binding energy side while the satellites associated with the copper core level peaks disappear. The shifting of the metal 2p peaks is associated with the formation of the oxide. The disappearance of the satellites in the copper 2p region is associated with the reduction of copper oxide to elemental copper. The spectral data show chemical reactivity at the M/CuO interface.   

Session J3: History of Physics, Public Policy and National Facilities

Timing and Impact of Bohr's Trilogy

In their article- Genesis of the Bohr Atom Heilbron and Kuhn asked - what suddenly turned his [Bohr's] attention, to atom models during June 1912- they were absolutely right; during the short period in question Bohr had made an unexpected change in his research activity, he has found a new interest ``atom'' and would soon produce a spectacularly successful theory about it in his now famous trilogy papers in the Phil Mag (1913). We researched the trilogy papers, Bohr`s memorandum, his own correspondence from that time in question and activities by Moseley (Manchester), Henry and Lawrence Bragg. Our work suggests that Bohr, also at Manchester that summer, was likely to have been inspired by Laue's sensational discovery in April 1912, of X-ray interference from atoms in crystals. The three trilogy papers include sixty five distinct (numbered) references from thirty one authors. The publication dates of the cited works range from 1896 to 1913. Bohr showed an extraordinary skill in navigating thru the most important and up-to date works. Eleven of the cited authors (Bohr included, but not John Nicholson) were recognized by ten Noble prizes, six in physics and four in chemistry.   

Jakob Narkiewicz-Jodko --Tesla ``Predecessor''

Prof. Jakob Narkiewicz-Jodko (1947--1905) is a bright figure in the history of science of the XIXth century [1,2]. His major discoveries are: Electrography -- the method of the visualization of electric discharge from the bodies due to the application of high strength and high frequency electric fields [3,4], and one of the first observations of the propagation of the electromagnetic waives and information transfer over the distances [5,6]. We review Prof. Jakob Narkiewicz-Jodko's research results and explain our point why we consider him as the predecessor of Nikola Tesla. [1] Decrespe M. La vie et les oeuvres de M. de Narkiewicz-Iodko, member et collaborateur de l'Institut imperial de medecineexperimentale de Saint-Petersbourg, member of correspondent de la Societe de Medecine de Paris, etc./ Marius Decrespe.- Paris, Chamuel, 1896, 51p. [2] Annalen der Physik.- Leipzig, 1896. -- Bd 293, 132 [3] Electrography// The Photographic news for amateur photographers.- 1896.- vol. 40, p.450 [4] Maack F. Elektrographie. Mit besonderer Berucksich-tigung der Versuche Narkiewicz-Jodko/ Ferdinand Maack// Wissenseschaltliche Zeitschrift\textellipsis -- 1898.- Bd 1, 1, 8-22; -1898.- Bd 1, 2/3, 89-99. [5] S\'{e}ances de la societe francaise de physique/ Societe francaise de physique. -- Paris, 1898, p. 77-79. [6] Present condition of wireless telegraphy// Consular reports: Commerce, manufacturers, etc. of their consular districts. Bureau of Foreign Commerce of United States.- Washington 1901, v.66. p. 44.   

Childhood Trauma and Coping through the Science of Physics: An Attachment Perspective

Trauma can be defined as stressful life events that disrupt and/or delay successful transition during childhood developmental stages (Roberts, 2000). In this exploratory study, transitional stressors are defined as: childhood physical, sexual, or emotional abuse; loss of a caregiver or significant relative due to death or abandonment; exposure to physical violence by non-family members (e.g., bullying); or illness resulting in permanent physical disability. Trauma may produce disorganized attachments in childhood, which may lead to emotional and to social impairment in adulthood (Siegel, 1999). Consequently, traumatized individuals, who suffer from disorganized attachments, may seek to engage in activities which are emotionally predictable. An examination of the personal childhood histories from a sample of Nobel Prize winners in the field of physics provides support for the hypothesis that the study of physics may serve as an effective coping method for individuals who have experienced childhood trauma.   

Special course for Masters and PhD students: phase transitions, Landau theory, 1D Ising model, the dimension of the space and Cosmology

Symmetry breaking transitions. The phenomenological (L.D.Landau, USSR, 1937) way to describe phase transitions (PT's). Order parameter and loss of the symmetry. The second derivative of the free energy changes jump wise at the transition, i.e. we have a mathematical singularity and second order PT ($T_{C}$\textgreater 0). Extremes of free energy. A point of loss of stability of the symmetrical phase. The eigenfrequency of PT and soft mode behavior. The conditions of applicability of the Landau theory (A.Levanyuk, 1959, V.Ginzburg, 1960). 1D Ising model and exact solution by a transfer matrix method. Critical exponents in the L.Landau PT's theory and for 1D Ising model. Scaling hypothesis (1965) for 1D Ising model with zero critical temperature. The order of PT in 1D Ising model in the framework of the R.Baxter approach. The anthropic principle and the dimension of the space. Why do we have a three-dimensional space? Big bang, the cosmic vacuum, inflation and PT's. Higgs boson and symmetry breaking transitions.   

Do the Math - The Role of Physicists in the Climate Movement

``It's simple math: we can emit 565 more gigatons of carbon dioxide and stay below 2$^{\circ}$C of warming - anything more than that risks catastrophe for life on earth. The only problem? Burning the fossil fuel that corporations now have in their reserves would result in emitting 2,795 gigatons of carbon dioxide - five times the safe amount.''\footnote{Do The Math Tour, 350.org.} Physicists stand in a powerful position to help the world wiggle out of this circumstance: our profession not only has the technical capacity to work on renewable energy development but also is popularly recognized as a source of scientific authority. This ability to influence public perception and politics is arguably even more important than our technological skills in the fight to stop rapid climate change. I will discuss several strategic campaigns presently underway at universities across the country, such as fossil fuel divestment, and how the physics community can become a valuable asset.   

The NHMFL Pulsed Field Facility at Los Alamos National Lab

National user facilities provide scientists and industrial development companies with access to specialized experimental capabilities to enable development of materials and solve long standing technical problems. Magnetic fields have become an indispensable tool for researchers to better understand and manipulate ground states of electronic materials. As magnetic field intensities are increased the quantum nature of these materials become exponentially more likely to be observed and this is but one of the drivers to go further in high magnetic field generation. At the Los Alamos branch of the National High Magnetic Field Laboratory we have significant efforts in extremely high magnetic field generation and experimentation. In direct opposition with our efforts are the tremendous electro-mechanical forces exerted on our magnets and the electromagnetic interference that couples to the sample under study and the diagnostic equipment. Challenges in magnetic field generation and research will be presented. Various methods of pulsed high magnetic field generation and experimentation capabilities will be reviewed, including our recent ``World Record'' for the highest non-destructive magnetic field.   

Education through the prism of computation

With the rapid development of technology, computation claims its irrevocable place among research components of modern science. Thus to foster a successful future scientist, engineer or educator we need to add computation to the foundations of scientific education. We will discuss what type of paradigm shifts it brings to these foundations on the example of Wolfram Science Summer School [1]. It is one of the most advanced computational outreach programs run by Wolfram Foundation, welcoming participants of almost all ages and backgrounds. Centered on complexity science and physics, it also covers numerous adjacent and interdisciplinary fields such as finance, biology, medicine and even music. We will talk about educational and research experiences in this program during the 12 years of its existence. We will review statistics and outputs the program has produced. Among these are interactive electronic publications at the Wolfram Demonstrations Project [2] and contributions to the computational knowledge engine Wolfram\textbar Alpa [3]. \\[4pt] [1] http://www.wolframscience.com/summerschool\\[0pt] [2] http://demonstrations.wolfram.com\\[0pt] [3] http://www.wolframalpha.com   

Rapidly Slowing Microdrops, Quantized Pixel Detectors and More at Hartnell College in Salinas, CA

In this conference, we describe the Research Scholar Institute program, a unique physics research internship program administered at Hartnell College. Specifically, we present a novel dynamics of microdroplet experiment which develops from the search for fractional charge experiment at SLAC, cosmic ray physics experiments that implements multipixel photon counters (MPPC) which show quantized photon distribution peaks, and other innovative physics experiments. Our internship program has successfully mentored community college STEM students to do research both at Hartnell College and at Department of Energy labs that include Lawrence Berkeley Lab and FermiLab.   

Session J4: Focus Session: Emergent Properties in Bulk Complex Oxides: Iridates II

Time-Reversal Symmetry Breaking and Consequent Physical Responses Induced by All-In-All-Out Type Magnetic Order on the Pyrochlore Lattice

Pyrochlore-type 5d transition-metal oxide compounds Cd$_{2}$Os$_{2}$O$_{7}$ and R$_{2}$Ir$_{2}$O$_{7}$ (R$=$rare earth) undergo a metal-insulator transition accompanied by a magnetic transition. Recently, the magnetic structures of Cd$_{2}$Os$_{2}$O$_{7}$ [1] and Eu$_{2}$Ir$_{2}$O$_{7}$ [2] were investigated by means of resonant x-ray magnetic scattering. The x-ray data indicated the all-in/all-out type magnetic order. The all-in/all-out order breaks the time-reversal symmetry, while the spontaneous magnetization is essentially absent. The magnetic order can be viewed as ferroic magnetic octupolar order. The magnetic order is expected to provide several unique physical properties like quadratic magnetization. linear magneto-capacitance, linear magneto-resistance, linear magneto-mechanical coupling and so on [3]. The symmetry breaking results in two non-equivalent domains, ``all-in/all-out'' and ``all-out/all-in.'' Interestingly, some theoretical works predict that a peculiar metallic state would appear on the domain wall. The observation and control of the domain distribution are essential for studying verious exotic physical responses. We have developed an x-ray technique for domain imaging and started studying the effects of external stimuli on the domain distribution [4]. This work was performed in collaboration with S. Tardif, S. Takeshita, H. Ohsumi, D. Uematsu, H. Sagayama, J. J. Ishikawa, S. Nakatsuji, J. Yamaura, and Z. Hiroi. \\[4pt] [1] J. Yamaura et al., Phys. Rev. Lett. 108, 247205 (2012).\\[0pt] [2] H. Sagayam et al., Phys. Rev. B 87, 100403R (2013).\\[0pt] [3] T. Arima, J. Phys. Soc. Jpn. 82, 013705 (2013).\\[0pt] [4] T. Samuel et al., submitted.   

Characterization of the Structural and Magnetic Symmetries of Sr$_{2}$IrO$_{4}$ via Nonlinear Optical Spectroscopy

The combination of strong electron-electron interactions and large spin-orbit coupling in the iridates provides a unique platform for realizing exotic electronic phases. A characterization of the structural and magnetic symmetries of these systems is important for understanding whether many of the predicted phases can be realized. Here we discuss how measurement of the nonlinear optical susceptibility using a novel rotational anisotropy technique can be used to study the structural and magnetic symmetries of iridates, which provides a complement to neutron and x-ray diffraction based probes. The perovskite iridate Sr$_{2}$IrO$_{4}$ in particular has been intensively studied recently owing to its novel $J_{eff} =$1/2 magnetic Mott insulating ground state, possible unconventional metal-to-insulator transition and potential for high-T$_{\mathrm{c}}$ superconductivity upon doping. We apply our technique to Sr$_{2}$IrO$_{4}$ to examine both its structural and magnetic symmetries across the Neel transition and discuss our observations in the context of the intriguing physics of these systems.   

A Spatially Resolved Optical Second Harmonic Generation (SHG) Study of the Perovskite Iridate Sr$_{2}$IrO$_{4}$ with Bulk Sensitivity

There has been a lot of recent interest in the layered perovskite iridate, Sr$_{2}$IrO$_{4}$, owing to its novel spin-orbital entangled Mott insulator ground state and its potential to realize high-Tc superconductivity upon doping. Although its bulk structural and magnetic point group symmetries have been characterized by resonant x-ray and neutron diffraction, these measurements provide spatially integrated information. In fact, recent neutron diffraction studies on Sr$_{2}$IrO$_{4}$ suggest that such measurements may be averaging over crystallographic domains of reduced symmetry that in turn generate distinct magnetic domains [1]. Therefore, spatial resolution is desirable in order to gain full understanding of the point group symmetries of Sr$_{2}$IrO$_{4}$. Here, we show that optical SHG can provide a bulk sensitive measurement of the point group symmetries. By performing such SHG measurements in an imaging mode, we study the possible microscopic domain structures recently suggested. More generally, our SHG imaging technique provides an alternative way to probe the point group symmetries of iridate crystals, which are not always amenable to neutron scattering due to their small sample sizes and strong neutron absorption cross section. \\[4pt] [1] F. Ye et al., Phys. Rev. B 87, 140406 (R) (2013); C. Dhital et al., Phys. Rev. B 87, 144405 (2013).   

Polaronic absorption in Sr$_{2}$IrO$_{4}$

Sr$_{2}$IrO$_{4}$ has received much attention as a novel $J_{\mathrm{eff}} =$ 1/2 Mott insulator. Many theorists have supposed that exotic novel ground state such as superconductivity, topological insulator, and quantum spin liquid could emerge in $J_{\mathrm{eff}} =$ 1/2 state. However, despite of great interests on Sr$_{2}$IrO$_{4}$, the ground state of this material is elusive up to now. Unlike previous Mott scenario, recent reports support that Sr$_{2}$IrO$_{4}$ can be described as Slater insulator rather than Mott insulator. The origin of temperature evolutions of electronic structure shown in many experiments also remains vague until now. Here, we investigated the detail temperature evolution of electronic structure of Sr$_{2}$IrO$_{4}$ using infrared spectroscopy. We couldn't observe any anomaly in optical conductivity near the $T_{N}$, which is not consistent with recent reports. Instead, we observed the continuous changes in our optical data which can be explained in terms of polaronic behavior, closely related to La$_{2}$CuO$_{4}$.   

Ferromagnetic Resonance of the Weak Ferromagnet Sr$_2$IrO$_4$

We derive a pseudospin model for the strongly spin-orbit coupled 5$d$ oxide Sr$_2$IrO$_4$ that is based on a standard $t/U$ expansion and Slater's theory of atomic multiplets. Using this model, we use linear spin-wave theory to evaluate the material's spin wave spectrum and address the influence of quantum fluctuation on the saturation magnetization. We find that the ferromagnetic resonance (FMR) frequency has an unusual square root dependence on magentic field, with a prefactor related to the strength of the exchange coupling between $j=1/2$ pseudospins. FMR can thus be used as an alternative probe of the nontrivial magnetism of Sr$_2$IrO$_4$.   

Phonon assisted optical excitation in narrow bandgap spin-orbit insulator Sr$_{3}$Ir$_{2}$O$_{7}$

We examined the temperature (T) evolution of the optical conductivity spectra of Sr$_{3}$Ir$_{2}$O$_{7}$ crystal. We found that the features of low energy $d$-$d$ excitation ($\hbar\omega$ $<$ 300 meV) between two $J_{\mathrm{eff}}$ = 1/2 states, evolve drastically in a wide temperature range (4 K $<$ T $<$ 400 K). This large T evolution in the low energy feature is not observed in O K-edge x-ray absorption spectra, suggesting that it is presumably originated from phonon-assisted indirect optical transitions. The results of the simulation in which the phonon-absorbing and phonon-emitting processes is considered, show a consistency with the experimental spectra. The peak energy of the transition between two $J_{\mathrm{eff}}$ = 1/2 bands also decreases apparently by $\sim$100 meV along with the abundance of phonon-assisted charge excitations in the narrow bandgap semiconductor Sr$_{3}$Ir$_{2}$O$_{7}$.   

Static and time-resolved optical spectroscopy on Lithium Iridate

We use FTIR and pump-probe spectroscopy to study lithium iridates. The IR spectrum shows an anomalous peak which emerges as temperature is reduced and is highly anisotropic in the ab-plane polarization. In the time-domain we observe similarly anisotropic reflectivity transients whose multiple dynamic components evolve as temperature is reduced.   

Possible Superconductivity Induced by Strong Spin-Orbit Coupling in Carrier Doped Iridium Oxides Insulators

$5d$ transition metal oxide Sr$_{2}$IrO$_{4}$ and its relevant Iridium oxides have attracted much interest because of exotic properties arising from highly entangled spin and orbital degrees of freedom due to strong spin-orbit coupling (SOC). Sr$_{2}$IrO$_{4}$ crystalizes in the layered perovskite structure, similar to cuprates. Five $5d$ electrons in Ir occupy its $t_{2g}$ orbitals which are split by strong SOC, locally inducing an effective total angular momentum $J_{\textrm{eff}}=1/2$, analogous to a $S=1/2$ state in cuprates. Because of the similarities to cuprates, the possibility of superconductivity (SC) in Iridium oxides has been expected theoretically once mobile carriers are introduced into the $J_{\textrm{eff}}=1/2$ antiferromagnetic insulator [1]. To study theoretically possible SC in carrier doped Sr$_{2}$IrO$_{4}$, we investigate a three-orbital Hubbard model with SOC. By solving the Eliashberg equation in the random phase approximation, we find that $J_{\textrm{eff}}=1/2$ antiferromagnetic fluctuations favor $d_{x^{2}-y^{2}}$-wave SC with a mixture of singlet and triplet Cooper pairings. We will also discuss the particle-hole asymmetry of the SC induced by electron and hole doping. [1] H. Watanabe, et. al., Phys. Rev. Lett. {\bf 110}, 027002 (2013)   

Quantum oscillations in a metallic pyrochlore irridate, Bi2Ir2O7

We report quantum oscillations observed in single crystals of Bi2Ir2O7 using torque magnetometry in high magnetic fields up to 33T and low temperatures to 0.3K. Quantum oscillations allow to determine the extremal areas of the Fermi surface perpendicular to the magnetic field, the quasiparticle masses and also the scattering time. Our results are compared with first-principle band structure calculations that take into account the effects of the spin-orbit coupling. We find evidence both for small and large Fermi surfaces and the effective masses are unexpectedly light. The effect of the magnetic field on the electronic structure of Bi2Ir2O7 will be also discussed.   

Metal-Insulator Transition and Topological Phases of Pyrochlore Iridates

The 4d and 5d transition metal oxides are interesting because these materials incorporate both strong spin-orbit coupling and strong correlations, and consequently display distinct physical properties and the tantalizing possibility of novel topological phases. A prominent family in this class, the rare earth pyrochlore iridates, shows a metal-insulator transition and non-collinear complex magnetic ordering in the insulating state. We carry out magnetic band structure calculations using the GGA+U method, which reproduce the systematic trend that stronger order and larger gaps occur with decreasing rare earth radius. A corresponding paramagnetic band calculation shows that Pr$_2$Ir$_2$O$_7$ is a strong candidate for a nodal quadratic band touching state, in which the doubly degenerate conduction and valence bands touch at the zone center, right at the Fermi level. This suggests that Pr$_2$Ir$_2$O$_7$ is very sensitive to perturbations, such as time reversal symmetry or cubic symmetry breaking terms, giving rise to the possibility of many novel phases. Indeed, we demonstrate using first-principles calculations that uniaxial strain applied along the (111) direction opens a band gap and converts the material to a strong topological insulator.   

Magnetic order in pyrochlore iridate Y$_2$Ir$_2$O$_7$ probed by electron paramagnetic resonance

We performed electron paramagnetic resonance (EPR) studies of the magnetic properties of the pyrochlore iridate Y$_2$Ir$_2$O$_7$ compound. The resonance line in the EPR spectrum shifts towards lower fields when the temperature is below 100 K, suggesting the appearance of a local field generated by the ordered magnetic moments at low temperatures. The temperature dependent local field and magnetization both show hysteresis between zero field cooling and field cooling, however, the magnetic ordering temperature $T_O$ ($\sim$ 100 K) determined by EPR is about 50 K lower than the transition temperature $T_M$ ($\sim$ 150 K) in the $M-T$ measurements, indicating a possible short-range order state at the intermediate temperature. A hyperfine structure was detected below the long-range ordering temperature, suggesting the co-existence of isolated paramagnetic Ir$^{4+}$ moments with the magnetic order state.   

Microscopic Mechanism of Giant Non-reciprocal Directional Dichroism in CuB$_2$O$_4$

CuB$_2$O$_4$ shows giant Non-reciprocal Directional Dichroism (NDD) at 1.405 eV, which corresponds to the intratomic d$_{x^2-y^2 }$-d$_{xy}$ transition of Cu$^{2+}$ hole. To clarify the origin of NDD, we performed measurements of the optical absorption in high magnetic fields up to 15 T. The g-factor estimated from the Zeeman splitting was as large as 2.8. This result suggests that orbital angular momentum contributes to the magnetic moments. To make sure of this estimation, we conducted an exact-diagonalization calculation considering crystal field, spin-orbit coupling, and Zeeman energy. The calculation reveals that d$_{xy}$ hybridizes with d$_{yz}$, d$_{zx}$ to form a spin-orbital coupled state. The calculated g-factor of this state is 2.6, which is comparable with the observed g-factor. We further calculated the oscillator strength of the transition from d$_{x^2-y^2}$ to the spin-orbital coupled state. The resulting oscillator strength successfully explains the experimental results of the magnetic field direction dependence of NDD.   

Multiferroicity with coexisting isotropic and anisotropic spins in Ca$_3$Co$_{2-x}$Mn$_x$O$_6$

We study physical properties in Ca$_3$Co$_{2-x}$Mn$_x$O$_6$ (CCMO) by magnetization, magnetostriction, electric polarization, and magnetocaloric measurements under high magnetic fields. On the controversial topic of the spin state of Co, we find evidence for high spin state with $S$ = 3/2 with no field-induced spin-state crossover up to 97 T. In addition, our data also indicate that Mn spins are quasi-isotropic and develop components in the $ab$-plane in applied magnetic fields of 10 T and cant until saturation at 85 T whereas the Ising Co spins saturate by 25 T. We also find transient steps that appear only within a range of magnetic field sweep rates. This feature resembles the behavior observed in isostructural compound Ca$_3$Co$_2$O$_6$ (CCO) where metastable steps appear in magnetization in non-equilibrium condition. However, CCMO has more complex magnetic interactions due to Mn spins and a different ground state compared to CCO. Our results give a different view to the magnetoelectric coupling in this material, namely the spin-flop of Mn$^{4+}$ destabilizes electric polarization instead of a spin flip. The origin of the the transient behavior in CCMO is also discussed.   

Session J6: Focus Session: Spin-Dependent Physics in Carbon-Based Materials III

Spin Hall Effect Induced by Resonant Skew Scattering in Graphene

The spin Hall effect is the appearance of a transverse spin current in a non-magnetic conductor by pure electrical control; it can originate from the spin-dependent skew scattering of electrons by impurities in the presence of SOI. We consider a monolayer of graphene decorated by a small density of impurities generating a spin-orbit interaction in their surroundings. We show that large spin hall effect develops through skew scattering and it is strongly enhanced in the presence of resonant scattering. Our single impurity scattering calculations show that impurities with either intrinsic or Rashba spin-orbit coupling in a graphene sheet originate robust SHE. Also, the solution of the transport equations for a random distribution of impurities suggests that the spin Hall effect is robust with respect to thermal fluctuations and disorder averaging.   

Alternative generation of spin current in graphene

The manipulation of spin current which can be achieved in various device configurations has been under intense research in recent years. The spin current is typically obtained by injecting electrons from the ferromagnetic electrodes. In this study, we employed alternative methods for the generation of spin current in graphene. The first method we studied is using spin Hall effect. In the spin Hall effect, the charge current generates spin current due to a relativistic spin-orbit coupling. Generally the spin-orbit coupling in graphene is extremely weak to produce substantial spin current. We employed physical doping of heavy atoms on top of the graphene layer for the spin Hall induced spin current in graphene. The second alternative method we investigated is seebeck spin tunneling. The ferromagnetic electrode together with thin tunnel barrier (1-3nm of Al$_{2}$O$_{3}$ layer) was employed to introduce thermally induced spin imbalance in graphene. The gate dependence of generated spin current reflects unique electronic structure of graphene.   

Transport Studies of Exchange Interaction at Magnetic-Insulator/Graphene and Magnetic-Insulator/Topological-Insulator Interfaces

Spintronics, where carrier spin instead of charge serves as the state variable, is a promising candidate for post-CMOS low-voltage logic. An essential component of the spin-FET class of spintronic devices is the electrical modulation of spin. To realize this functionality, we explore the interfacial exchange interaction of quasi-2D systems in proximity to a magnetic insulator (MI). We study the magneto-transport of graphene/MI heterostructures as the model system. In this talk, we will discuss several schemes for probing the interfacial exchange. We demonstrate that the H-bar configuration exhibits strong enhancement in non-local resistance as a result of the exchange interaction. We will also present the magneto-transport results of MI multilayers on topological insulators as another platform for building low-voltage spintronic devices.   

Tuning the Kondo effect in graphene in the presence of point defects

Removing a single carbon atom from the honeycomb lattice in graphene produces a localized state orthogonal to the undisturbed lattice states. According to theory this will give rise to a local magnetic moment when occupied by an electron, but its fate in the presence of conduction electrons is not known. Will it be screened by many body interactions below a critical Kondo temperature to form a singlet state, or will it remain unscreened? Recent studies using transport or magnetic measurements on graphene in the presence of point disorder have reached opposite conclusions 1,2. We addressed this question by combining STM and transport measurements to study the effect of interactions between the electrons in graphene and local magnetic moments as a function of carrier density and dielectric environment. At high density we observe a clear signature of Kondo screening in the form of a Fano resonance in the density of state that is pinned to the Fermi energy and splits in a magnetic field. We further find that the Kondo temperature strongly depends on the carrier density and that it can be tuned in or out with a gate voltage. 1. Chen, J. H., et.al, Nature Physics 7, 535--538 (2011) 2. Nair, R. R., et. al, Nature Physics 8, 199--202(2012)   

Kondo-like magnetism induced by single vacancies in graphene

A new phase for graphene with a single carbon vacancy was found by our first-principles calculation. Single vacancies can be developed by irradiation experiments in graphene and were found to be magnetic.[1,2] The measured Kondo effect also triggered extensive studies.[3] The current understanding of the ground state best supported by density functional theory is that a Stoner instability gives rise to ferromagnetism of $\pi$ electrons aligned with the localized moment of a $\sigma$ dangling bond. The induced $\pi$ magnetic moments were suggested to vanish at low vacancy concentrations. However, the observed Kondo effect suggests that $\pi$ electrons around the vacancy should antiferromagnetically couple to the local moment and carry non-vanishing moments. Here we propose that a phase possessing both significant out-of-plane displacements and $\pi$ bands with antiferromagnetic coupling to the localized $\sigma$ moment is the ground state.[4] With the features we provide, it is possible for spin-resolved STM, STS, and ARPES measurements to verify the proposed phase. [1] M. M. Ugeda et al., Phys. Rev. Lett. 104, 096804 (2010). [2] R. R. Nair et al., Nature Phys. 8, 199 (2012). [3] J.-H. Chen et al., Nature Phys. 7, 535 (2011). [4] C.-C. Lee et al., http://arxiv.org/abs/1311.0609.   

Kondo effect in graphene with Rashba spin-orbit interaction

We study the Kondo screening of a magnetic impurity in monolayer graphene in the presence of Rashba spin-orbit interaction. The host density of states (DOS), with two split bands and particle-hole symmetry, results in a complex hybridization function that suggests interesting phenomena as a function of the chemical potential and the Rashba strenght. Although the Rashba coupling produced by depositing graphene in a conventional substrate is weak, a strong increase of this interaction was shown to occur by intercalation of Au on a Ni substrate [1] or by hydrogenation of the sample [2]. An effective single channel Anderson model sets the ground to analyze the properties of the system, which are obtained by numerical renormalization group calculations. We find a Kosterlitz-Thouless quantum phase transition (QPT) separating free moment and strong-coupling phases at half-filling, whenever the Rashba coupling is present. Tuning the chemical potential close to sharp features of the hybridization function results in an interesting interference of the Kondo peak and a virtual bound state resonance that appears due to a jump in the DOS. All these features would be visible in STM experiments, providing a realistic system in which to study QPTs. [1] Nat. Commun. 3, 1232; [2] Nat. Phys 9, 284.   

Probing magnetic properties of sidewall epitaxial graphene nanoribbons

Epitaxial graphene nanoribbons grown on sidewall SiC have recently emerged as a novel material system enabling single channel room temperature ballistic transport over micrometer distance. In this work, we study the tunnel magnetoresistance (TMR) of sidewall-ribbon-based magnetic tunnel junctions as a function of temperature and magnetic field (both amplitude and tilting angle). We show that the measured TMR exhibits a spin-switch behavior at temperatures below 30 K, indicating that the sidewall ribbons are magnetic and possess a spin component either parallel or antiparallel to the magnetization direction of the magnetic contact. Furthermore, we find that the TMR signal switches the sign at certain negative bias voltages, which has important implications in device applications.   

First principles electronic transport simulations of spin coherence length in disordered graphene nanoribbons due to spin-orbit interaction

Graphene presents high hopes for next-generation electronic applications. In particular, due to the small spin-orbit coupling in carbon one might envision using the electron's spin - instead of its charge - for information processing. In what has been dubbed Spintronics one of the main challenges, as one strives to obtain spin-based devices, is to obtain long spin coherence times (small spin relaxation) during electronic transport. Albeit the spin-coherence length in pristine graphene is deemed to be very large, the presence of defects and impurities can lead to spin-flips due to spin-orbit interactions. The presence of a large number of impurities randomly distributed in the system can, consequently, lead to the loss of spin-coherence. In this talk we will discuss spin-flip processes in disordered graphene nanoribbons containing a number of metal impurties. This will be achieved via a combination of Density Functional Theory - including Spin Orbit effects - with a recursive Green's function method to simulate the electronic transport of disordered systems. This way one is able to atomistically infer the spin-coherence length in graphene nanoribbons in the presence of defects or impurities. As a point in case I will show results for Ni adatoms.   

Spatially Separated Spin Carriers in Spin-Semiconducting Graphene Nanoribbons

A graphene nanoribbon with sawtooth edges has a ferromagnetic ground state. Using first-principles and tight-binding model calculations, we show that, under a transverse electrical field, the sawtooth graphene nanoribbons become a spin semiconductor whose charge carriers are not only spin polarized in energy space but also spatially separated at different edges. Low-energy excitation produces spin-up electrons localized at one edge and spin-down holes at the opposite edge, and the excitation energy of spin carries can be tuned by the electric field to reach a new state of spin gapless semiconductor. Also, the spin semiconducting states are shown to be robust against at least 10{\%} edge disorder. These features demonstrate a good tunability of spin carriers for spintronics applications.   

Large Magnetoresistance in Nanostructured Armchair Graphene Nanoribbon Junctions

The prospect of all-carbon nanoelectronics has motivated significant interest in the transport of electrons through graphene and graphene nanoribbon (GNR) based junctions.... The weak intrinsic spin-orbit coupling in graphene also makes graphene an attractive candidate for replacing conventional materials in spintronics applications. Several interesting spin transport properties, such as giant magnetoresistance and half-metallicity, have been predicted. Most of these predictions have centered on GNRs with zigzag atomic edges (ZGNRs). On the other hand, significant progress has been made in the controlled atomic-scale synthesis of GNRs with \textit{armchair} edges (AGNRs), all with specific widths.... Yet, to date, little is known about the potential of such well-defined AGNRs in electronics or spintronics. In this work, we use first principles transport calculations to predict the electron and spin transport properties of nanostructured AGNR junctions. We predict a large magnetoresistance of $\sim$ 900{\%}, related to resonant transmission channels close to the Fermi energy.   

Charge transport properties of boron/nitrogen binary doped graphene nanoribbons: An ab initio study

Opening a bandgap by forming graphene nanoribbons (GNRs) and tailoring their properties via doping is a promising direction to achieve graphene-based advanced electronic devices. Applying a first-principles computational approach combining density functional theory (DFT) and DFT-based non-equilibrium Green's function (NEGF) calculation, we herein study the structural, electronic, and charge transport properties of boron-nitrogen binary edge doped GNRs and show that it can achieve novel doping effects that are absent for the single B or N doping. For the armchair GNRs, we find that the B-N edge co-doping almost perfectly recovers the conductance of pristine GNRs. For the zigzag GNRs, it is found to support spatially and energetically spin-polarized currents in the absence of magnetic electrodes or external gate fields: The spin-up (spin-down) currents along the B-N undoped edge and in the valence (conduction) band edge region. This may lead to a novel scheme of graphene band engineering and benefit the design of graphene-based spintronic devices.\\[4pt] This work was supported by the Basic Science Research Grant (No. 2012R1A1A2044793), Global Frontier Program (No. 2013-073298), and Nano-Material Technology Development Program (2012M3A7B4049888) of the National Research Foundation funded by the Ministry of Education, Science and Technology of Korea.   

Confinement effect on spin-polarized edge states in graphene nanostructures

One of the most intriguing phenomena in condensed matter physics is the existence of edge states on the boundary of a 2D system. In graphene, the edge states have distinct properties from the bulk states and play important roles in the physicochemical properties of the material. In this work, we show ab-initio results of spin-polarized electronic edge states in graphene quantum dots of different sizes and shape. We found a critical size at which the singlet nonmagnetic ground state becomes singlet open-shell with antiferromagnetic order. We found that the critical size is strongly influenced by the shape of the quantum dot. We discuss this behavior based on energetics and electronic structure of the system under study. The calculations are base on the Density functional Theory (DFT). The Linear Combination of Atomic Orbital (LCAO) method for bases functions it was used. For exchange-correlation functional has been used the Generalized Gradient Approximation (GGA).   

Unravelling the zero-field-splitting parameters in Pt-rich polymers with tuned spin-orbit coupling

Recently pi-conjugated polymers that contain heavy metal Platinum (Pt-polymers, Scientific Reports 3, 2653, 2013) have attracted substantial interest due to their strong and tunable spin-orbit coupling (SOC). The magnetic field effect (MFE), such as magneto-photoluminescence (MPL) is considered to be a viable approach to address the SOC strength in the organics. Alas conventional MFE up to several hundred Gauss is unable to overcome the relative large spin splitting energies in Pt-polymers due to their strong SOC. To overcome this difficulty we study the MPL response in two Pt-polymers at high magnetic field (up to several Telsa). We found that the MPL response is dominated by triplet excitons that are generated in record time, and from the MPL(B) response width we could obtained the triplet zero-field splitting (ZFS) parameters. We found that the ZFS parameters in the Pt-polymers are proportional to the intrachain Pt atom concentration.   

Influence of the phonon-mediated spin-orbit coupling in graphene decorated with adatoms

Graphene covered with certain heavy adatoms has been predicted( Conan Weeks et al., Phys. Rev. X 1, 021001 (2011)) to become a topological insulator by virtue of a proximity-effect induced spin-orbit coupling. In addition, the adatoms also induce a coupling between the electron spin and the phonons. Using group theory and tight-binding models, we systematically investigate the coupling between the electron spin and in-plane lattice phonons. We discuss the consequences of this coupling for the dynamics of electrons on the graphene $\pi$ band.   

Session J7: Focus Session: Molecules on Surfaces & in Devices

Spin interactions in Graphene-Single Molecule Magnets Hybrids

Graphene is a potential component of novel spintronics devices owing to its long spin diffusion length. Besides its use as spin-transport channel, graphene can be employed for the detection and manipulation of molecular spins. This requires an appropriate coupling between the sheets and the single molecular magnets (SMM). Here, we present a comprehensive characterization of graphene-Fe$_{4}$ SMM hybrids. The Fe$_{4}$ clusters are anchored non-covalently to the graphene following a diffusion-limited assembly and can reorganize into random networks when subjected to slightly elevated temperature. Molecules anchored on graphene sheets show unaltered static magnetic properties, whilst the quantum dynamics is profoundly modulated. Interaction with Dirac fermions becomes the dominant spin-relaxation channel, with observable effects produced by graphene phonons and reduced dipolar interactions. Coupling to graphene drives the spins over Villain's threshold, allowing the first observation of strongly-perturbative tunneling processes. Preliminary spin-transport experiments at low-temperature are further presented.   

Effect of electron-vibron coupling on electron transport via a single-molecule magnet Fe4

Recently, single-molecule junctions consisting of individual single-molecule magnets (SMMs) bridged between electrodes have been fabricated in three-terminal devices, and magnetic anisotropy of SMMs has been shown to be affected by electron transport through the SMMs. In such junctions, vibrational modes of the SMM can couple to electronic charge and/or spin degrees of freedom, and the coupling influences magnetic and transport properties of the SMM. An effect of the electron-vibron coupling on transport has been extensively studied in small molecules, but not yet for junctions of SMMs. In this talk, we present our calculations of the electron-vibron coupling in a SMM Fe4 based on density-functional theory, and an effect of the coupling on electron transport. In addition, we compare our results with experimental data.   

First-principles study of a single-molecule magnet $Mn_{12}$ monolayer on the graphene surface.

Electronic structures of single-molecule magnets $Mn_{12}$ on graphene surfaces are studied using spin-polarized density-functional theory. Charge transfer between molecule and graphene, densities of states, and magnetization are fully analyzed. We also report effects of various ligands and strain. Our results suggest that graphene can be p-doped upon $Mn_{12}$ adsorption, and the doping level is closely related to the choice of ligands in molecule. In addition, we find that the strain in graphene plays an important role in modulating the doping level.   

Studies of Mn$_{12}$-Ph Single Molecule Magnets by LT-STM and Modeling of Magnetic Stability Under Perturbation

We study Mn$_{12}$O$_{12}$(C$_6$H$_5$COO)$_{16}$(H$_2$O)$_4$ (Mn$_{12}$-Ph) single-molecule magnets on a Cu(111) surface using low temperature scanning tunneling microscopy, LT-STM. We report the observation of Mn$_{12}$-Ph in isolation and in thin films, deposited through vacuum spray deposition onto clean Cu(111). The local tunneling current observed within the molecular structure shows a strong bias voltage dependency, which is distinct from that of the Cu surface. Furthermore, we identify an internal inhomogeneity in the bias behavior within a single molecule. To further understand the stability of the magnetic properties of the molecules while on the surface, we develop a theoretical model to study the stability of the net magnetic moment under deformation of the spin-spin interaction graph. We develop a spin Hamiltonian-type model to predict magnetic moments that are intrinsically robust under random shape deformations to the spin-graph structure. This spin moment is shown to be a weak topological invariant for molecules with sufficiently many spin centers, approximately 20 to 50.   

Magnetic Properties of Single Cobalt Atoms on Thin MgO Films

Studies of individual magnetic atoms on surfaces have shown magnetic anisotropies that inhibit thermal transitions over the barrier at cryogenic temperatures. To observe stable magnetic moments in single atoms at room temperatures, as would be desirable for technological applications, it is necessary to further increase the magnetic anisotropy barrier. The size of this barrier is usually limited by the quenching of the orbital moment that occurs because of the binding of the atom to its ligands. We use inelastic electron tunneling spectroscopy in a low temperature scanning tunneling microscope to measure the energy splitting between the atomic spin states of single Cobalt atoms deposited on 1 monolayer MgO on top of a metal substrate. We show that in this crystal environment the orbital moment of the magnetic atom is not quenched. This leads to the maximal spin-orbit induced separation between the spin ground and excited state and a lower bound on the magnetic anisotropy energy barrier for Cobalt of 58meV.   

Metallocene Molecular Clusters Studied with Scanning Tunneling Microscopy and Spectroscopy

Atomic spins and molecular magnets have been actively reported using Scanning Tunneling Microscope(STM) in recent studies. One can even assemble an artificial magnet by STM manipulation. Manganocene((C$_{\mathrm{5}}$H$_{\mathrm{5}})_{\mathrm{2}}$Mn), a sandwich complex of metallocene, is composed of one manganese atom and two cyclopentadianyl ligands. This molecule is known to reveal not only high spin number S $=$ 5/2 at room temperature but also two structural states: monomer and molecular chain. In this presentation, we report STM images and spectroscopic results of these monomers and dimers. We try to map the molecular electronic state and the spin texture. The molecule is adsorbed on an insulating layer to decouple the spin state from the metallic substrate. We will present that manganocene can become a basic element of a spin chain.   

Scanning SQUID microscopy with single electron spin sensitivity

Superconducting interference devices (SQUIDs) have been traditionally used for studying fundamental properties of magnetic materials and superconductors. Although widely used in scanning magnetic microscopy, their progress towards detection of small magnetic moments was stagnating of late due to limitations imposed by conventional designs of planar SQUIDs and contemporary lithography techniques, restricting sample-to-sensor distance smaller than $\sim$ 0.5 micron and SQUIDs diameters smaller than $\sim$ 200 nm. These limitations were overcome by the invention of a SQUID-on-tip device [1], subsequent realization of a SQUID-on-tip microscope [2], and by creation of an ultra-small sensor with spatial resolution of 20 nm and sensitivity to a single electron spin per 1 Hz bandwidth [3]. In this talk I will describe the principles of scanning SQUID magnetometry, its applications to study superconductors and its potential for magnetic nano-scale imaging of novel materials.\\[4pt] [1] Self-aligned nanoSQUID on a tip. A. Finkler, Y. Segev, Y. Myasoedov, M. L. Rappaport, L. Ne'eman, D. Vasyukov, E. Zeldov, M. E. Huber, J. Martin and A. Yacoby, Nano Letters 10 (2010) 1046.\\[0pt] [2] Scanning superconducting quantum interference device on a tip for magnetic imaging of nanoscale phenomena. A. Finkler, D. Vasyukov, Y. Segev, L. Ne'eman, E. O. Lachman, M. L. Rappaport, Y. Myasoedov, E. Zeldov and M. E. Huber, Rev. Sci. Instrum. 83 (2012) 073702.\\[0pt] [3] A scanning superconducting quantum interference device with single electron spin sensitivity. D. Vasyukov, Y. Anahory, L. Embon, D. Halbertal, J. Cuppens, L. Ne'eman, A. Finkler, Y. Segev, Y. Myasoedov, M. L. Rappaport, M. E. Huber and E. Zeldov, Nature Nanotechnology 8 (2013) 639; Magnetic sensors: A tip for better sensing. D. Koelle, Nature Nanotechnology 8 (2013) 617.   

Tailoring the Kondo Effect in Composite Magnetic Systems

We manipulate different transition metal atoms and build chains using a sub-Kelvin scanning tunneling microscope (STM). The chains composed of transition metal atoms form a highly correlated spin singlet ground state exhibiting a Kondo resonance. By studying temperature and magnetic field dependence, we confirm the Kondo effect in this composite system. We find that the occurrence of the Kondo resonance sensitively depends on the length of the atomic chain and the spin anisotropy energy of each atom. We construct chains with different elemental composition and obtain various spin ground states. In this way, we can tailor the singlet ground state on and off and also modify its strength. The addition of vector magnetic fields to the atomically assembled nanostructures provides another parameter that can continuously tune the behavior of atomic spins. This opens up a fruitful experimental model spin system to be conquered and makes it possible to engineer many-body effects in prototypical spin structures at atomic dimensions.   

Kondo effect signatures in the electronic transport through a single-ion magnet under an applied transverse magnetic field

We study the low-temperature electronic transport properties of a single magnetic ion molecule (SMIM) in the presence of a rhombic ligand field using the numerical renormalization group method. The rhombic ligand environment induces uniaxial and transverse zero-field spin anisotropies in the ion. We find signatures of a Kondo effect caused by the presence of a transverse (zero-field) anisotropy in the molecule. Upon applying a transverse magnetic field to the SIMM, we observe oscillations of the Kondo effect near the diabolical (degeneracy) points of the energy spectrum of the molecule. The field-induced lifting of the ground state degeneracy competes with the interference modulation, resulting in some situations in a suppression of the Kondo peak.   

A molecular approach to the Kondo problem in Carbon based systems

There has been a great effort in recent years to understand the emerging Kondo-like resonances in different magnetic molecules such as MnPc. Theoretical approaches based on atomic models have proven to be very useful for the study of this phenomenon when the magnetic moment is essentially localized on a magnetic atom [1,2]. Nevertheless the Kondo effect can arise in pure carbon-based systems as has been demonstrated experimentally in fullerenes and carbon nanotubes [3,4]. In this communication we present a multiorbital Anderson model where the orbitals are not atomic but molecular orbitals. This model is fully obtained from Density Functional Theory calculation in combination with Green's functions methodologies [5,6]. Standard impurity solver techniques are used to solve the model which is applied to fullerenes and other nanographene structures [7]. \\[4pt] [1] A. Str\'{o}zecka et. al. Phys. Rev. Lett. 109, 147202 (2012);\\[0pt] [2] D. Jacob et. al. Phys. Rev. B 88, 134417 (2013);\\[0pt] [3] J. J. Parks et. al. Phys. Rev. Lett. 99, 026601 (2007);\\[0pt] [4] P. Jarillo-Herrero et. al. Nature 434, 484. (2005);\\[0pt] [5] ANT.G03. www.alacant.dfa.ua.es;\\[0pt] [6] D. Jacob et. al. Phys. Rev. B. 82, 195115 (2010);\\[0pt] [7] J Fernandez-Rossier et. al. Phys. Rev. Lett. 99, 177204 (2007).   

Spectral evolution of the SU(4) Kondo effect from the single impurity to the two-dimensional lattice

We describe the evolution of the SU(4) Kondo effect as the dimensionality of the system is gradually increased from the zero-dimensional limit (i.e., impurity) to the two-dimensional (2D) lattice. We derive a Hubbard-Anderson model describing a 2D array of atoms or molecules with two-fold orbital degeneracy, acting as magnetic impurities and interacting with a metallic host. We calculate the differential conductance, observed typically in experiments of scanning tunneling spectroscopy, for different arrangements of impurities on a metallic surface: a single impurity, a periodic square lattice, and several sites of a rectangular cluster. Our results point towards the crucial importance of the orbital degeneracy and agree well with recent experiments in different systems of of iron(II) phtalocyanine molecules deposited on top of Au(111) [N. Tsukahara \textit{et al.}, Phys. Rev. Lett. \textbf{106}, 187201 (2011)]. Our results indicate that this would be the first experimental realization of a 2D SU(4) Kondo-lattice system.   

Incorporating isolated molybdenum (Mo) atoms into Bilayer Epitaxial Graphene on 4H-SiC(0001)

The atomic structures and electronic properties of isolated Mo atoms in bilayer epitaxial graphene (BLEG) on 4H-SiC(0001) are investigated by low temperature scanning tunneling microscopy (LT-STM). LT-STM results reveal that isolated Mo dopants prefer to substitute C atoms at $\alpha $-sites, and preferentially locate between the graphene bilayers. First-principles calculations confirm that the embedding of single Mo dopants within BLEG is energetically favorable as compared to monolayer graphene. The calculated bandstructures show that Mo-doped BLEG is n-doped, and each Mo atom introduces a local magnetic moment of 1.81 $\mu_{\mathrm{B}}$. Our findings demonstrate a simple and stable method to incorporate single transition metal dopants into the graphene lattice to tune its electronic and magnetic properties for possible use in graphene spin devices.   

Session J8: Focus Session: Spin-Dependent Phenomena in Semiconductors: Spin Manipulation and Phenomena in Semiconductors

Magnetotransport and Structural Properties of Mn$_{2}$CoAl Thin Film Spin Gapless Semiconductor

Spin gapless semiconductors (SGS) are predicted to have a density of states displaying both half-metallic and zero-gap semiconducting properties. They are being investigated for spintronic devices due to their unique magnetic and electrical properties. Calculations predict several SGS compounds\footnote{S. Skaftouros, K. Ozdogan, E. Sasioglu, and I. Galanakis, App. Phys. Lett. 102, 022402 (2013).}$^,$\footnote{S. Ouardi, G. Fecher, and C. Felser, and J. Kubler, Phys. Rev. Lett. 110, 100401 (2013).} including Mn$_{2}$CoAl, Ti$_{2}$CoSi, V$_{3}$Al, and Ti$_{2}$MnAl. Mn$_{2}$CoAl thin films were grown by MBE on GaAs (100) substrates at 200 $^{\circ}$C.\footnote{M.E. Jamer, B.A. Assaf, T. Devakul and D. Heiman, Appl. Phys. Lett. 103, 142403 (2013).} The as-grown thin films were epitaxial with the substrate, which resulted in a tetragonal distortion. Annealing studies showed that the films lose their epitaxial registration and approach an aligned cubic structure for 325 $^{\circ}$C with a$=$c$=$5.80 {\AA}. The resistivity shows a thermally-activated semiconducting-like negative slope at higher temperatures. The Hall resistivity scales with $\rho _{xx}^{2}$ for all temperatures and magnetic fields, expected for a topological intrinsic anomalous Hall effect computed from the Berry phase curvature. The connection of electrical and spin-gapless properties is discussed.   

Quantifying absolute spin polarization with non-magnetic contacts in FM/$n$-GaAs heterostructures

We report on a novel method of quantifying spin accumulation in Co$_2$MnSi/$n$-GaAs and Fe/$n$-GaAs heterostructures using a non-magnetic probe. The presence of a non-equilibrium spin polarization generates a large electrostatic potential shift relative to the equilibrium state. This is due to the combination of (1) the parabolic (non-constant) density of states and (2) the population imbalance between the two spin sub-bands. We observe this shift as a Hanle effect in a non-local, non-magnetic semiconducting contact. Since this signal depends only on experimentally accessible parameters of the bulk semiconductor, its magnitude may be used to quantify the injected spin polarization in absolute terms. By comparison with the (smaller) spin-valve signal observed with a second ferromagnetic contact, we demonstrate that this electrostatic shift scales quadratically with spin polarization, dephases in the presence of both applied and hyperfine fields, and is observable to higher temperatures than traditional non-local measurements. Quantitative modeling allows extraction of absolute polarizations in excess of 50 \% at low temperatures, and further indicates that this contribution constitutes a large fraction of the three-terminal signal observed in these devices.   

Electrical detection of FMR in epitaxial FM/$n$-GaAs heterostructures by tunneling anisotropic magnetoresistance

Electrical detection of ferromagnetic resonance (FMR) is a widely used technique for the detection of spin pumping. We report here a new means for electrical detection of FMR based on tunneling anisotropic magnetoresistance (TAMR). The TAMR is due to the spin-orbit coupling at the FM/$n$-GaAs interface. Because the tunneling resistance in this case depends on the orientation of the magnetization with respect to the crystalline axes, the opening of the cone angle when the FM is driven resonantly produces a dc voltage which is proportional the square of the time-dependent magnetization. Our samples are FM//(001) $n$-GaAs heterostructures grown by MBE, where the FM is Fe, Co$_2$MnSi, or Co$_2$FeSi. All three of these heterostructures show non-local spin transport effects when they are biased. In each case, we observe a symmetric FMR peak with a bias and angular dependence that tracks the TAMR almost exactly. The resonance field depends on the anisotropies as expected, and the linewidth is smallest for Co$_2$MnSi. At low temperatures, the amplitude of the FMR signal is clearly sensitive to the spin accumulation, but this effect is not due to spin pumping.   

Experimental Measurements of $^{69/71}$Ga NMR in Optically-pumped NMR (OPNMR) of AlGaAs/GaAs Quantum Wells

We have conducted photon-energy and helicity-dependent measurements of the $^{69}$Ga and $^{71}$Ga NMR signals that result from optical pumping of states in the conduction band. The sample we have used for these studies is a 60-well multiple quantum well sample of Al$_{0.34}$Ga$_{0.66}$As/GaAs. Our measurements show a particularly strong dependence of the OPNMR signal from the GaAs quantum wells, when irradiating at photon energies consistent with the spin-split light hole within the material. (We use a frequency-stabilized continuous wave Ti:sapphire ring laser, with a very narrow linewidth for these excitation.) The coupling to the light-hole has an important NMR signature which we will discuss in this presesntation. We will show results for multiple external magnetic field strengths (B$_{0})$ and for different laser light intensities. A thorough understanding of the ``fine structure'' observed in the photon energy dependence of these OPNMR signals is afforded through theoretical modeling of these results, which will be shown in a separate presentation.   

Spin-Dependent Band Structure Spectroscopy in a Strained Al$_{0.1}$Ga$_{0.9}$As/GaAs Multiple Quantum Well by Optically Pumped Nuclear Magnetic Resonance

We present the photon energy dependence of optically pumped NMR (OPNMR) signals in a Si-$\delta $-doped Al$_{0.1}$Ga$_{0.9}$As/GaAs multiple quantum well (MQW). Data was acquired at 3.9 T, 4.9 T, and 9.4 T with different polarizations of light. The OPNMR spectra exhibit a strain-induced nuclear quadrupole splitting, caused by differential contraction of the MQW and the Si support to which it is epoxy bonded. The tensile strain in the GaAs quantum well layers, which is estimated from the observed quadrupole splitting, is included in band structure calculations based on the 8-band Pidgeon-Brown model generalized to include the effects of the confinement potential. Our OPNMR photon energy dependence data is compared with calculations of optically pumped electron spin polarization to correlate the OPNMR data with the strained MQW's band structure. Our results demonstrate that the OPNMR photon energy dependence is sensitive to strain-induced light-hole/heavy-hole splitting and quantum confinement effects on the MQW's band structure.   

Modelling Optically Pumped NMR and Spin Polarization in AlGaAs/GaAs Quantum Wells

Optically Pumped NMR (OPNMR) is a combination of the optical pumping of semiconductors to create spin-polarized electrons and the direct detection of an enhanced NMR signal as the electron spin polarization is transferred to the nucleus. We present theoretical calculations for the average electron spin polarization at different photon energies for different values of external magnetic field in both unstrained and strained $Al_{x}Ga_{1-x}As/GaAs$ quantum wells. Comparison is made with the experimental OPNMR signal intensity. We identify the Landau level transitions which are responsible for the peaks in the OPNMR signal intensity. Our calculations are based on the 8-band Pidgeon-Brown model generalized to include the effects of the confinement potential as well as strain. In strained wells, the strain is calculated using a relation that associates the experimental value of the nuclear quadrupole splitting with the strain along a given axis. Optical properties are calculated using Fermi's Golden rule. Results show that the strength and sign the OPNMR signal is related to the average electron spin polarization.   

Mapping the anisotropic Lande g-factor tensor of 1D GaAs holes in all 3 spatial directions

We have studied the Zeeman splitting of 1D holes formed on a (100) GaAs/AlGaAs heterostructure on a single cooldown. The strong spin orbit coupling and 1D confinment give rise to a highly anisotropic spin splitting. By use of the high-symmetry (100) crystal, we eliminate the effects of crystal anisotropy on our measurements. In measuring the spin splitting as a function of angle between the wire and the applied magnetic field, we are able to identify the principle axes of the $g$-tensor. We show that the principle axes are defined by the potential confining the 1D holes, and are not affected by the crystal axes. We find that $g_{\parallel}^{\perp} < g_{\parallel}^{\parallel} < g_{\perp}$, where $g_{\parallel}$ refers to the in-plane $g$-factors parallel and perpendicular to the wire, and $g_{\perp}$ refers to the $g$-factor perpendicular to the 2D well.   

Optical spin injection in GaAs nanowires

Controlling quantum confinement in semiconductor nanowires (NWs) allow desirable spin-dependent properties and enable novel devices, such as spin-interconnects[1], spin-lasers[2,3] or spin-enhanced phonon lasers[4]. Typically, the key element in such applications is the presence of non-equilibrium spin population. Focusing on GaAs NWs of different cross-sectional areas, we analyze their carrier spin polarization based on k.p band structure calculations[5,6]. We show that shining circularly polarized light propagating along the NW axis provides a robust spin injection, reaching $\sim 100 \%$ and switchable by changing the incident photon energy. For optical spin injection in bulk GaAs near the $\Gamma$-point, we recover previously known results[7]. [1] H. Dery, et al., Appl. Phys. Lett. 99, 082502 (2011). [2] J. Lee, et al., Phys. Rev. B 85, 045314 (2012). [3] J. Sinova and I. Zutic, Nature Mater. 11, 368 (2012). [4] A. Khaetskii, et al., Phys. Rev. Lett. 111, 186601 (2013). [5] G. M. Sipahi, et al., Phys. Rev. B 57, 9168 (1998). [6] P. Li and H. Dery, Phys. Rev. Lett. 107, 107203 (2011). [7] F. Nastos, et al., Phys. Rev. B 76, 205113 (2007).   

Properties of InGaAs/InAlAs double quantum wells toward spin-filtering application

We recently determined the intrinsic constant for the Rashba effect in (001)InP-matched InGaAs/InAlAs quantum wells (QW) [1]. We now apply this knowledge to band-engineer a spin-filter based on the double QW (DQW) [2]. Firstly, we studied the subband energy spectra of the InGaAs/InAlAs DQW from the beatings in the Shubnikov de Haas (SdH) oscillations. The basic properties obtained here provide useful information to refine the device structures for the spin-filter devices based on the DQW system [2]. In our DQW samples, a thin barrier layer of In$_{0.52}$Al$_{0.48}$As ($d_{\rm B}=$ 1.5$\sim$5 nm) is sandwiched with non-doped QW layers of In$_{0.53}$Ga$_{0.47}$As ($d_{\rm QW}=$ 10 nm). By carefully tuning the doping densities above and below the DQW part and the top-gate voltage, the potential profile of this DQW should be ideally made symmetric about the middle barrier layer, though not yet realized in our experiment. The experimental results of our not-yet-ideal DQW samples showed clear beatings in the SdH data, which originate from both the subband and Rashba splittings. We report on our successful separation of these two effects based on the proper band-bending assumptions.\\[4pt] [1] Faniel, et.al., Phys. Rev. B {\bf 83}, 115309 (2011).\\[0pt] [2] Souma, et.al, arXiv:1304.6992.   

Spin pumping efficiency in room temperature CdSe nanocrystal quantum dots

To understand and optimize optical spin initialization in room temperature CdSe nanocrystal quantum dots (QDs) we studied the dependence of the pump energy $E$ on the time-resolved Faraday rotation signal in a series of QD samples with different sizes. In $6.1$-nm-diameter QDs, we observe two peaks in the spin signal vs. $E$. The first peak occurs on resonance at $1.95$eV, with the second peak $\sim 0.17$eV higher in energy before the spin signal falls off to near zero. We calculate the spin-dependent oscillator strengths of optical transitions using an 8-band effective mass model to understand these results. The observed $E$ dependence of the spin pumping efficiency (SPE) arises from the competition between the heavy hole (hh), light hole (lh) and split-off (so) band contributions to transitions to the conduction band. The two latter contributions lead to an electron spin polarization in the opposite direction from the former. At lower $E$ the transitions mainly involve the hh band, giving rise to the two main peaks. At higher $E$, the increasing contributions from the lh and so bands lead to a reduction in SPE. In smaller QDs, both peaks merge while moving to higher $E$, and the overall SPE is reduced.   

Proposal for a phonon laser utilizing quantum-dot spin states

We propose [1] a nano-scale realization of a phonon laser utilizing phonon-assisted spin flips in quantum dots to amplify sound. Owing to a long spin relaxation time, the device can be operated in a strong pumping regime, in which the population inversion is close to its maximal value allowed under Fermi statistics. In this regime, the threshold for stimulated emission is unaffected by spontaneous spin flips. Considering a nanowire with quantum dots defined along its length, we show that a further improvement arises from confining the phonons to one dimension, and thus reducing the number of phonon modes available for spontaneous emission. Our work calls for the development of nanowire-based, high-finesse phonon resonators. [1]. A. Khaetskii, V.N. Golovach, X. Hu, and I. Zutic, PRL {\bf 111}, 186601 (2013).   

Two-State Polariton Lasing in a Planar Semiconductor Microcavity

We report room-temperature sequential polariton lasing at two distinct energies in a planar microcavity non-resonantly pumped by a 2-ps pulsed laser. The sample consists of multiple InGaAs/GaAs quantum wells embedded within GaAs/AlGaAs distributed Bragg mirrors. A sub-10-ps high energy (HE) lasing mode with a linewidth $\sim$ 3 meV commences within 10 ps after pump, and is followed by a 20 to 50 ps low energy (LE) mode with a transient linewidth $\sim$ 1 meV. The time-average degree of polarization of both states decrease with increasing photoexcited densities. Near above the lasing threshold, the HE state is spin polarized, resulting in fully circularly polarized radiation under a circularly polarized pump and partially circularly polarized radiation under a linearly polarized pump. In contrast, the LE state is partially linearly polarized with stochastic polarization orientations that are weakly correlated to the [110] crystalline direction. The energy difference between the two lasing modes is controlled by the photoexcited density and pump polarization. With increasing pump flux, the HE state blue-shifts about 5 meV, while the LE state red-shifts less than 1 meV. This two-state lasing effect exemplifies spontaneous symmetry breaking in a microcavity laser.   

Putting Spin into Lasers

Considering circular polarization, an optical analog of electron spin, semiconductor lasers with spin-polarized carriers can open up unexplored possibilities for spin-controlled devices. Once spin-polarized carriers are introduced in the gain region of lasers, by circularly polarized light or electrical spin-injection, the operation of such spin-lasers should be revisited to incorporate their novel properties. Spin-polarized carriers can enhance the performance of lasers for communication and signal processing [1]. In the steady-state, such spin-lasers already demonstrated a lower threshold current for the lasing operation [2] compared to their conventional (spin-unpolarized) counterparts, however, the most exciting opportunities come from their dynamical operation. We reveal that the spin modulation in lasers can lead to an improvement in the two key figures of merit: enhanced bandwidth [3] and reduced parasitic frequency modulation---chirp [4]. Analyses are carried out under generalized modulation regimes we propose. Different mechanisms for quantum dots and quantum wells as a gain medium are also discussed and we provide a mapping between the two gain media. Spin states in quantum dots may also enable elusive phonon lasers [5], which emitts coherent phonons instead of photons. This work was performed in collaboration with R. Oszwa\l dowski, C. G{\o}thgen, G. Boeris, and I. \v{Z}uti\'{c}. \\[4pt] [1] J. Sinova and I. \v{Z}uti\'{c}, Nature Materials 11, 368 (2012); Handbook of Spin Transport and Magnetism, edited by E. Y. Tsymbal and I. \v{Z}uti\'{c} (CRC Press, New York, 2011). \\[0pt] [2] J. Rudolph et al., Appl. Phys. Lett. 87, 241117 (2005); M. Holub et al, Phys. Rev. Lett. 98, 146603 (2007); S. Hovel et al., Appl. Phys. Lett. 92, 041118 (2008); S. Iba, et al., Appl. Phys. Lett. 98, 08113 (2011); M. Holub and B. T. Jonker, Phys. Rev. B 83, 125309 (2011). \\[0pt] [3] J. Lee, W. Falls, R. Oszwa\l dowski, and I. \v{Z}uti\'{c}, Appl. Phys. Lett. 97, 041116 (2010); J. Lee, R. Oszwa\l dowski, C. G{\o}thgen, and I. \v{Z}uti\'{c}, Phys. Rev. B 85, 045314 (2012). \\[0pt] [4] G. Bo\'{e}ris, J. Lee, K. V\'{y}born\'{y}, and I. \v{Z}uti\'{c}, Appl. Phys. Lett. 100, 121111 (2012). \\[0pt] [5] A. Khaetskii, V. N. Golovach, X. Hu, and I. \v{Z}uti\'{c}, Phys. Rev. Lett. 111, 186601 (2013).   

Session J10: Focus Session: Interactions in Biological Systems

Immune cell activation from multivalent interactions with liquid-crystalline polycation-DNA complexes

Microbial DNA can trigger type I interferon (IFN) production in plasmacytoid cells (pDCs) by binding to endosomal toll-like receptor 9 (TLR9). TLR9 in pDCs do not normally respond to self-DNA, but in certain autoimmune diseases self-DNA can complex with the polycationic antimicrobial peptide LL37 into condensed structures which allow DNA to access endosomal compartments and stimulate TLR9 in pDCs. We use x-ray studies and cell measurements of IFN secretion by pDCs to show that a broad range of polycation-DNA complexes stimulate pDCs and elucidate the criterion for high IFN production. Furthermore, we show via experiments and computer simulations that the distinguishing factor for why certain complexes activate pDCs while others do not is the self-assembled structure of the liquid-crystalline polycation-DNA complex.   

High Fc Density Particles Result in Binary Complement Activation but Tunable Macrophage Phagocytosis

Macrophage phagocytosis and complement system activation represent two key components of the immune system and both can be activated through the presentation of multiple Fc domains of IgG antibodies. We have created functionalized micro- and nanoparticles with various densities of Fc domains to understand the modulation of the immune system for eventual use as a novel immunomodulation platform. Phagocytosis assays were carried out by adding functionalized particles to macrophage cells and quantitatively determined using fluorescent microscopy and flow cytometry. Complement system activation by the functionalized particles in human serum was quantified with an enzyme immunoassay. Our phagocytosis assay revealed a strong dependence on particle size and Fc density. For small particles, as the Fc density increased, the number of particles phagocytosed also increased. Large particles were phagocytosed at significantly lower levels and showed no dependency on Fc density. Complement was successfully activated at levels comparable to positive controls for small particles at high Fc densities. However at low Fc densities, there is a significant decrease in complement activation. This result suggests a binary response for complement system activation with a threshold density for successful activation. Therefore, varying the Fc density on micro/nanoparticles resulted in a tunable response in macrophage phagocytosis while a more binary response for complement activation.   

Can correlations among receptors affect the information about the stimulus?

In the context of neural information processing, it has been observed that, compared to the case of independent receptors, correlated receptors can often carry more information about the stimulus. We explore similar ideas in the context of molecular information processing, analyzing a cell with receptors whose activity is intrinsically negatively correlated because they compete for the same ligand molecules. We show analytically that, in case the involved biochemical interactions are linear, the information between the number of molecules captured by the receptors and the ligand concentration does not depend on correlations among the receptors. For a nonlinear kinetic network, correlations similarly do not change the amount of information for observation times much shorter or much longer than the characteristic time scale of ligand molecule binding and unbinding. However, at intermediate times, correlations can increase the amount of available information.   

Three-component homeostasis control

Two reciprocal components seem to be sufficient to maintain a control variable constant. However, pancreatic islets adapt three components to control glucose homeostasis. They are $\alpha$ (secreting glucagon), $\beta$ (insulin), and $\delta$ (somatostatin) cells. Glucagon and insulin are the reciprocal hormones for increasing and decreasing blood glucose levels, while the role of somatostatin is unknown. However, it has been known how each hormone affects other cell types. Based on the pulsatile hormone secretion and the cellular interactions, this system can be described as coupled oscillators. In particular, we used the Landau-Stuart model to consider both amplitudes and phases of hormone oscillations. We found that the presence of the third component, $\delta$ cell, was effective to resist under glucose perturbations, and to quickly return to the normal glucose level once perturbed. Our analysis suggested that three components are necessary for advanced homeostasis control.   

Surface binding of polymer coated nanoparticles: Coupling of physical interactions, molecular organization, and chemical state

A key challenge in nanomedicine is to design carrier system for drug delivery that selectively binds to target cells without binding to healthy cells. A common strategy is to end-functionalize the polymers coating of the delivery device with specific ligands that bind strongly to overexpressed receptors. Such devices are usually unable to discriminate between receptors found on benign and malignant cells. We demonstrate, theoretically, how one can achieve selective binding to target cells by using multiple physical and chemical interactions. We study the effective interactions between a polymer decorated nanosized micelle or solid nanoparticle with model lipid layers. The polymer coating contains a mixture of two polymers, one neutral for protection and the other a polybase with a functional end-group to optimize specific binding and electrostatic interactions with the charged lipid head-groups found on the lipid surface. The strength of the binding for the combined system is much larger than the sum of the independent electrostatic or specific ligand-receptor binding. The search for optimal binding conditions lead to the finding of a non-additive coupling that exists in systems where chemical equilibrium, molecular organization, and physical interactions are coupled together.   

Coherence and energy transport in molecular aggregates: stochastic approach

Recent spectroscopy studies of various molecular assemblies have shown that optically induced quantum coherences in these systems survive much longer than predicted from standard rate equations. This result sparked numerous debates whether the coherent system dynamics are related to the efficiency and/or speed of the excitation energy transfer in such systems. The problem could be addressed by studying coherent excitation dynamics and its relaxation due to interaction with the bath. The reduced density matrix propagation theories provide the reduced/averaged information on the dynamics. Stochastic approaches allow accessing more detailed microscopic picture. Two types of stochastic equations have been derived for a system coupled to the bath of an arbitrary spectral density. The stochastic wave functions allowed to define excitation coherent dynamics, polaron formation dynamics and energy relaxation times together with energy transport pathways in molecular aggregates. Simulations of the energy transport in few model molecular aggregates were performed using both approaches for the system wavefunction. It was demonstrated that the quantum coherences in the system appearing from mixture of vibrational and excitonic resonances significantly affect the energy transport process.   

Radical-pair Based Avian Magnetoreception: Robustness and Optimality

Behavioural experiments suggest that migratory birds possess a magnetic compass sensor able to detect the direction of the geomagnetic. One hypothesis for the basis of this remarkable sensory ability is that the coherent quantum spin dynamics of photoinduced radical pair reactions transduces directional magnetic information from the geomagnetic field into changes of reaction yields, possibly involving the photoreceptor cryptochrome in the birds retina. The suggested radical-pair based avian magnetoreception has attracted attention in the field of quantum biology as an example of a biological sensor which might exploit quantum coherences for its biological function. Investigations on such a spin-based sensor have focussed on uncovering the design features for the design of a biomimetic magnetic field sensor. We study the effects of slow fluctuations in the nuclear spin environment on the directional signal. We quantitatively evaluate the robustness of signals under fluctuations on a timescale longer than the lifetime of a radical pair, utilizing two models of radical pairs. Our results suggest design principles for building a radical-pair based compass sensor that is both robust and highly directional sensitive.   

Influence of thermal light correlations on photosynthetic structures

The thermal light from the sun is characterized by both classical and quantum mechanical correlations. These correlations have left a fingerprint on the natural harvesting structures developed through five billion years of evolutionary pressure, specially in photosynthetic organisms [1]. In this work, based upon previous extensive studies of spatio-temporal correlations of light fields, we hypothesize that structures involving photosensitive pigments like those present in purple bacteria vesicles emerge as an evolutionary response to the different properties of incident light. By using burstiness and memory as measures that quantify higher moments of the photon arrival statistics, we generate photon-time traces. They are used to simulate absorption on detectors spatially extended over regions comparable to these light fields coherence length. Finally, we provide some insights into the connection between these photo-statistical features with the photosynthetic membrane architecture and the lights' spatial correlation.\\[4pt] [1] N. Johnson et al. ``Extreme alien light allows survival of terrestrial bacteria,'' Nature Scientific Reports 3, 2198 (2013) doi:10.1038/srep02198.   

Radical-Ion-Pair Spin Decoherence and the Quantum Efficiency of Photosynthetic Charge Separation

We have pioneered the fundamental quantum dynamics of radical-ion-pair reactions, elucidating the basic spin-decoherence mechanism pertaining to these biochemical reactions. Radical-ion pair reactions appear in the avian magnetic compass, but more importantly, they participate in the cascade of electron-transfer reactions taking place in photosynthetic reaction centers. We will here present new insights on how the fundamental quantum dynamics of radical-ion pair reactions affect the quantum efficiency of charge separation in photosynthetic reaction centers.   

Session J11: Focus Session: Gene Regulatory Networks in Medicine and Biology

Network Approach to Disease Diagnosis

Human diseases could be viewed as perturbations of the underlying biological system. A thorough understanding of the topological and dynamical properties of the biological system is crucial to explain the mechanisms of many complex diseases. Recently network-based approaches have provided a framework for integrating multi-dimensional biological data that results in a better understanding of the pathophysiological state of complex diseases. Here we provide a network-based framework to improve the diagnosis of complex diseases. This framework is based on the integration of transcriptomics and the interactome. We analyze the overlap between the differentially expressed (DE) genes and disease genes (DGs) based on their locations in the molecular interaction network ("interactome"). Disease genes and their protein products tend to be much more highly connected than random, hence defining a disease sub-graph (called disease module) in the interactome. DE genes, even though different from the known set of DGs, may be significantly associated with the disease when considering their closeness to the disease module in the interactome. This new network approach holds the promise to improve the diagnosis of patients who cannot be diagnosed using conventional tools.   

Autonomous Boolean modeling of gene regulatory networks

In cases where the dynamical properties of gene regulatory networks are important, a faithful model must include three key features: a network topology; a functional response of each element to its inputs; and timing information about the transmission of signals across network links. Autonomous Boolean network (ABN) models are efficient representations of these elements and are amenable to analysis. We present an ABN model of the gene regulatory network governing cell fate specification in the early sea urchin embryo, which must generate three bands of distinct tissue types after several cell divisions, beginning from an initial condition with only two distinct cell types. Analysis of the spatial patterning problem and the dynamics of a network constructed from available experimental results reveals that a simple mechanism is at work in this case.   

Construction of A Self-consistent Landscape for Multistable Gene Regulatory Circuits

Cell fate decisions during embryonic development and tumorigenesis pose a major research challenge in modern developmental and cancer biology. Cell fate decisions between different phenotypes are regulated by multistable gene circuits that give rise to the coexistence of several stable states. Internal and external noise play crucial role in determining the transitions between and the relative stability of the coexisting phenotypes. The deterministic dynamics of these circuits is not derivable from a potential. Yet, motivated by Waddington Epigenetic Landscape, many rely on the notion of effective potential to describe cell fate determination in the presence of noise. Here, we present a construction of a self-consistent landscape (effective potential, W $\equiv $ -ln(probability)), utilizing the Eikonal equation approach (WKB approximation of the corresponding Fokker Planck equation) for the cases of white noise and shot noise. The approach is based on utilizing the method of characteristics in a special way. We also devised a numerical method to efficiently calculate the contour of the potential and the optimal path for the transitions from one stable state to another. We tested the method on the bistable and tristable double inhibition circuits, and we showed that the constructed landscape agrees very well with the numerical simulation of the stochastic equations. We expect this method to be valuable to a wide range of multistable gene circuits.   

On the dephasing of genetic oscillators

The digital nature of genes combined with the associated low copy numbers of proteins regulating them is a significant source of stochasticity, which affects the phase of biochemical oscillations. We show that unlike ordinary chemical oscillators the dichotmoic molecular noise of gene state switching in gene oscillators affects the stochastic dephasing in a way that may not always be captured by phenomenological limit cycle based models.Through simulations of a realistic model of the $NFB\kappa B$/$I\kappa B$ network we also illustrate the dephasing phenomena which are important for reconciling single cell and population based experiments on gene oscillators.   

A dynamical network model for frailty-induced mortality

Age-related clinical and biological deficits can be used to build a frailty index that is a simple fraction of observed to possible deficits. As a proxy measure of aging, such a frailty index is empirically a better predictor of human mortality than chronological age. We present a network dynamical model of deficits that allows us to naturally consider causal interactions between deficits, deficit formation and repair, and mortality. We investigate the information provided by various model frailty indices, how they reflect the underlying dynamics of the network, and how well they predict mortality.   

Network Medicine: From Cellular Networks to the Human Diseasome

Given the functional interdependencies between the molecular components in a human cell, a disease is rarely a consequence of an abnormality in a single gene, but reflects the perturbations of the complex intracellular network. The tools of network science offer a platform to explore systematically not only the molecular complexity of a particular disease, leading to the identification of disease modules and pathways, but also the molecular relationships between apparently distinct (patho)phenotypes. Advances in this direction not only enrich our understanding of complex systems, but are also essential to identify new disease genes, to uncover the biological significance of disease-associated mutations identified by genome-wide association studies and full genome sequencing, and to identify drug targets and biomarkers for complex diseases.   

Epigenetic landscape of master regulators with cooperative feedback

A common view is that cell phenotypes can be understood as attracting valleys in a complex epigenetic landscape. On the other hand, cell fates have also been associated with master regulatory genes controlling development, which is particularly important in the context of cellular reprogramming. In this work I describe a simple noisy model of gene regulation in which master transcription regulators are involved in cooperative positive feedback loops with a large number of downstream regulated genes. It is shown that this model can be mapped onto a finite-temperature Hopfield associative memory spin model with effective pairwise Hebbian interactions, thus providing a mechanism for concurrent storage of gene expression patterns representing different cell states, where each cell state is associated with a particular master regulator. The inclusion of simple dynamics then leads to a description in terms of an N-dimensional potential landscape, N being the number of regulators. Within this model I discuss the stability of cell states as well as different mechanisms of switching between states when triggered by an external signal, suggesting possible scenarios for cell differentiation events.   

Discrete-state stochastic simulation of mutant cell dynamics in different environments

Phenotypic heterogeneity among genetically identical cells was shown in a number of experiments.\footnote{E.M. Ozbudak, NATURE 427, (2004)} This non-genetic variability can arise from low copies of molecular components like DNA and protein within the cell. These low copy of molecules then cause cell-to-cell variation in their gene expression.\footnote{M.B. ELOWITZ, SCIENCE 297, (2002)} Gene expression interacts with the environment to give rise to different phenotypes in the population. Thus, the population has sub-populations with different growth rates and different cellular switching rates from one sub-population to others.\footnote{Nevozhay, PLoS 8,(2012)} The dynamics of mutation in such populations is not well understood. So, what is the fate of mutants in such populations? To address this problem, we developed a stochastic discrete-state model which incorporates a fixed number population, different cellular switching rates, and a different growth rate for each sub-populations. We randomly induced a single mutation in the population in various environments and measured the population fitness change and fraction of mutant cells in the population. The model predicts that the induced mutation follows the dynamics consistent with those experiments observed in our lab.   

A variational model for propagation time, volumetric and synchronicity optimization in the spinal cord axon network, and a method for testing it

Most information in the central nervous system in general and the (simpler) spinal cord in particular, is transmitted along bundles of parallel axons. Each axon's transmission time increases linearly with length and decreases as a power law of caliber. Therefore, evolution must find a distribution of axonal numbers, lengths and calibers that balances the various tradeoffs between gains in propagation time, signal throughput and synchronicity, against volumetric and metabolic costs. Here I apply a variational method to calculate the distribution of axonal caliber in the spinal cord as a function of axonal length, that minimizes the average axonal signal propagation time, subject to the constraints of white matter total volume and the variance of propagation times, and allowing for arbitrary fiber priorities and end-points. The Lagrange multipliers obtained thereof can be naturally interpreted as 'exchange rates', e.g., how much evolution is willing to pay, in white matter added volume, per unit time decrease of propagation time. This is, to my knowledge, the first model that quantifies explicitly these evolutionary tradeoffs, and can obtain them empirically by measuring the distribution of axonal calibers. We are in the process of doing so using the isotropic fractionator method.   

Session J12: Focus Session: AFM in Studying Cell Mechanics and Biointerfaces

Mechanical properties of metastatic breast cancer cells invading into collagen I matrices

Mechanical interactions between cells and the extracellular matrix (ECM) are critical to the metastasis of cancer cells. To investigate the mechanical interplay between the cells and ECM during invasion, we created thin bovine collagen I hydrogels ranging from 0.1-5 kPa in Young's modulus that were seeded with highly metastatic MDA-MB-231 breast cancer cells. Significant population fractions invaded the matrices either partially or fully within 24 h. We then combined confocal fluorescence microscopy and indentation with an atomic force microscope to determine the Young's moduli of individual embedded cells and the pericellular matrix using novel analysis methods for heterogeneous samples. In partially embedded cells, we observe a statistically significant correlation between the degree of invasion and the Young's modulus, which was up to an order of magnitude greater than that of the same cells measured in 2D. ROCK inhibition returned the cells' Young's moduli to values similar to 2D and diminished but did not abrogate invasion. This provides evidence that Rho/ROCK-dependent acto-myosin contractility is employed for matrix reorganization during initial invasion, and suggests the observed cell stiffening is due to an attendant increase in actin stress fibers.   

Mechanical Properties of Human Cells Change during Neoplastic Processes

Using an AFM with a spherical probe of 5.3 $\mu$m, we determined mechanical properties of individual human mammary epithelial cells that have progressed through four stages of neoplastic transformation: normal, immortal, tumorigenic, and metastatic. Measurements on cells in all four stages were taken over both the nucleus and the cytoplasm. Moreover, the measurements were made for cells outside of a colony (isolated), on the periphery of a colony, and inside a colony. By fitting the AFM force vs. indentation curves to a Hertz model, we determined the Young's modulus, E. We found a distinct contrast in the influence a cell's colony environment has on its stiffness depending on whether the cells are normal or cancer cells. We also found that cells become softer as they advance to the tumorigenic stage and then stiffen somewhat in the final step to metastatic cells. For cells averaged over all locations the stiffness values of the nuclear region for normal, immortal, tumorigenic, and metastatic cells were (mean +/- sem) 880 +/- 50, 940+/-50, 400 +/- 20, and 600 +/-20 Pa respectively. Cytoplasmic regions followed a similar trend. These results point to a complex picture of the mechanical changes that occur as cells undergo neoplastic transformation.   

Causes of retrograde flow in fish keratocytes

Confronting motile cells with AFM-cantilevers serving as obstacles and doubling as force sensors we tested the limits of the driving actin and myosin machinery. We could directly measure the force necessary to stop actin polymerization as well as the force present in the retrograde actin flow. Combined with detailed measurements of the retrograde flow velocity and specific manipulation of actin and myosin we found that actin polymerization and myosin contractility are not enough to explain the cells behavior. We show that ever-present depolymerization forces, a direct entropic consequence of actin filament recycling, are sufficient to fill this gap, even under heavy loads.   

High-resolution elasticity maps and cytoskeletal dynamics of neurons measured by combined fluorescence and atomic force microscopy

Detailed knowledge of mechanical parameters such as cell elasticity, stiffness of the growth substrate, or traction stresses generated during axonal extensions is essential for understanding the mechanisms that control neuronal growth. Here I present results obtained in my research group, which combine Atomic Force Microscopy and Fluorescence Microscopy measurements to produce systematic, high-resolution elasticity maps for different types of live neuronal cells cultured on glass or biopolymer-based substrates. We measure how the stiffness of neurons changes both during neurite outgrowth and upon chemical modification (disruption of the cytoskeleton) of the cell. We find a reversible local stiffening of the cell during growth, and show that the increase in local elastic modulus is primarily due to the formation of microtubules in the cell soma. We also report a reversible shift in the elastic modulus of the cortical neurons cytoskeleton with temperature, from tubulin dominated regions at 37C to actin dominated regions at 25C. We demonstrate that the dominant mechanism by which the elasticity of the neuronal soma changes in response to temperature is the contractile stiffening of the actin component of the cytoskeleton induced by the activity of myosin II motors.   

Analysis of Load Rate Dependence of Neuronal Soma Using Atomic Force Microscopy

Surfaces of biological cells are covered with a layer of molecules (glycocalyx) and membrane protrusions (microvilli and microridges). This so-called ``brush'' layer plays a distinct role in the measured elastic modulus of cells. We utilize atomic force microscopy (AFM) to study mechanical properties of the soma and brush layer of live rat cortical neurons. The elastic modulus of the soma and brush are measured for cells indented at different AFM probe loading rates, ranging from 1-10 $\mu$m/s. The cells were studied at both 37 $^{\circ}$C (near-physiological temperature at which microtubules dominate high stiffness regions in the soma) and at 25 $^{\circ}$C (reduced temperature state at which actin components dominate high stiffness regions in the soma). If one uses a model with no brush taken into account, the derived elastic modulus shows the rate dependence similar to the one reported previously in the literature. Using the model with brush, we observed no statistically significant rate dependence of the elastic modulus of the soma, whereas the effective brush length demonstrates strong rate dependence. These measurements yield insight into the mechanical reaction of living neurons to externally applied stresses.   

If mechanics of cells can be described by elastic modulus in AFM indentation experiments?

We study the question if cells, being highly heterogeneous objects, can be described with an elastic modulus (the Young's modulus) in a self-consistent way. We analyze the elastic modulus using indentation done with AFM of human cervical epithelial cells. Both sharp (cone) and dull AFM probes were used. The indentation data collected were processed through different elastic models. The cell was considered as a homogeneous elastic medium which had either smooth spherical boundary (Hertz/Sneddon models) or the boundary covered with a layer of glycocalyx and membrane protrusions (``brush'' models). Validity of these approximations was investigated. Specifically, we tested the independence of the elastic modulus of the indentation depth, which is assumed in these models. We demonstrate that only one model shows consistency with treating cells as homogeneous elastic medium, the bush model when processing the indentation data collected with the dull probe. The elastic modulus demonstrates strong depth dependence in all other three models. We conclude that it is possible to describe the elastic properties of the cell body by means of an effective elastic modulus in a self-consistent way when using the brush model to analyze data collected with a dull AFM probe.   

High spatiotemporal resolution imaging of mechanical processes in live cells using T- shaped cantilevers

Mechanical properties of cells are paramount regulators of a plethora of physiological processes, such as cell adhesion, motility and proliferation. Yet, their knowledge is currently hampered by the lack of techniques with sufficient spatiotemporal resolution to monitor the dynamics of such biological processes. We introduce an atomic force microscopy-based imaging platform based on newly-designed cantilevers with increased force sensitivity, while minimizing viscous drag. This allows us to uncover mechanical properties of a wide variety of living cells - including fibroblasts, neurons and Human Umbilical Vein Endothelial Cells - with an unprecedented spatiotemporal resolution. Our mechanical maps approach 50nm resolution and monitor cellular features within a minute's timescale. To identify the counterparts of our mechanical maps' features we perform simultaneous fluorescence microscopy and recognize cytoskeletal elements as the main molecular contributors of cellular stiffness at the nanoscale. Furthermore, the enhanced resolution and speed of our method allows the recognition of dynamic changes in the mechanics of fine cellular structures, which occurred independently of changes within optical images of fluorescently-labeled actin.   

Poking vesicles in silico

The Atomic Force Microscope (AFM) is used to poke cells and study their mechanical properties. Using Coarse-Grained Molecular Dynamics simulations, we study the deformation and relaxation of lipid bilayer vesicles, when poked with a constant force. The relaxation time, equilibrium area expansion, and surface tension of the vesicle membrane are studied over a range of applied forces. The relaxation time exhibits a strong force-dependence. Our force-compression curves show a strong similarity with results from a recent experiment by Schafer et al. (Langmuir, 2013). They used an AFM to ``poke'' adherent giant liposomes with constant nanonewton forces and observed the resulting deformation with a Laser Scanning Confocal Microscope. Results of such experiments, whether on vesicles or cells, are often interpreted in terms of dashpots and springs. This simple approach used to describe the response of a whole cell ---complete with cytoskeleton, organelles etc.--- can be problematic when trying to measure the contribution of a single cell component. Our modeling is a first step in a ``bottom-up'' approach where we investigate the viscoelastic properties of an in silico cell prototype with constituents added step by step.   

Morphology And Local Mechanical Properties Of A Block Copolymer Cell Substrate

Atomic force microscopy (AFM) was applied for the characterization of morphology and mechanical properties of a block copolymer coating designed for biomaterials applications. The material is a block-copolymer with poly(ethylene glycol) as one block and a peptide as second block, which are connected through urethane bonds. The AFM images obtained in amplitude modulation mode revealed the morphology is characterized by micron-scale sheaf-like structures embedded in a more homogeneous and, presumably, amorphous matrix. The self-assembly of the peptide segments is responsible for the formation of the ordered sheaf structures and this phenomenon was common for different variations of the components. Maps of elastic modulus and work of adhesion of the block copolymer, which also differentiate the matrix and ordered regions, were obtained with Hybrid mode at different tip-force levels. The quantitative estimates show that elastic modulus varies in the MPa range and work of adhesion in the hundreds of mJ/m$^{\mathrm{2}}$ range. These data are compared with AFM-based nanoindentation that was performed at higher tip-force level. The results indicate that material surface is more complicated and they suggest in-depth morphology variations. A tentative model of the structural organization is proposed.   

Toolkit for the Automated Characterization of Optical Trapping Forces on Microscopic Particles

Optical traps have been in use in microbiological studies for the past 40 years to obtain noninvasive control of microscopic particles. However, the magnitude of the applied forces is often unknown. Therefore, we have developed an automated data acquisition and processing system which characterizes trap properties for known particle geometries. Extensive experiments and measurements utilizing well-characterized objects were performed and compared to literature to confirm the system's performance. This system will enable the future analysis of a trapped primary cilium, a slender rod-shaped organelle with aspect ratio L/R \textgreater 30, where `L' is the cilium length and `R' the cilium diameter. The trapping of cilia is of primary importance, as it will lead to the precise measurements of mechanical properties of the organelle and its significance to the epithelial cell.   

A Simplified Model for the Optical Force exerted on a Vertically Oriented Cilium by an Optical Trap and the Resulting Deformation

Eukaryotic cilia are essentially whiplike structures extending from the cell body. Although their existence has been long known, their mechanical and functional properties are poorly understood. Optical traps are a non-contact method of applying a localized force to microscopic objects and an ideal tool for the study of ciliary mechanics. Starting with the discrete dipole approximation, a common means of calculating the optical force on an object that is not spherical, we tackle the problem of the optical force on a cilium. Treating the cilium as a homogeneous nonmagnetic cylinder and the electric field of the laser beam as linearly polarized results in a force applied in the direction of polarization. The force density in the polarization direction is derived from the force on an individual dipole within the cilium, which can be integrated over the volume of the cilium in order to find the total force. Utilizing Euler--Bernoulli beam theory, we integrate the force density over a cross section of the cilium and numerically solve a fourth order differential equation to obtain the final deformation of the cilium. This prediction will later be compared with experimental results to infer the mechanical stiffness of the cilium.   

Session J13: Focus Session: ARPES in Fe-Based Superconductors

ARPES study on FeSe films

The record of superconducting transition temperature (Tc) has long been 56 K for the iron-based high temperature superconductors (Fe-HTS's). Recently, in single layer FeSe films grown on SrTiO$_3$ substrate, signs for a new 65 K Tc record are reported. Combining molecular beam epitaxy and in situ angle resolved photoemission spectroscopy (ARPES), we study the ultra thin FeSe films on various substrates. We substantiate the presence of collinear antiferromagnetic order (CAF) in FeSe films, a key ingredient of Fe-HTS that was missed in FeSe before, which weakens with increased thickness or reduced strain. We demonstrate that the superconductivity occurs when the electrons transferred from the oxygen-vacant substrate suppress the otherwise most pronounced CAF in single layer FeSe. We establish the phase diagram of FeSe vs. lattice constant that contains all the essential physics of Fe-HTS's [1]. With first principle calculations, we show that the superexchange interactions across Fe-As-Fe is enhanced with increased lattice constant [2]. Therefore, with heavy electron doping, the FeSe would have been an overdoped non-superconductor likely due to the weakened spin fluctuation; however by expanding its lattice in thin films, the magnetism is enhanced and superconductivity is restored. By fabricating FeSe/STO/KTO heterostructure, we further enhanced the lattice constant of FeSe, and increased the gap-closing temperature to 70K. Two un-hybridized electron Fermi surfaces are resolved, and the superconducting gap exhibits strong anisotropy around the individual Fermi surface. This observation contradicts many existing theories on the pairing symmetry of Fe-HTS's with only electron pockets [3].\\[4pt] [1] S. Tan et al., Nature Materials 12, 634--640 (2013)\\[0pt] [2] Hai-Yuan Cao et al., arXiv:1310.4024\\[0pt] [3] R. Peng et al., arXiv:1310.3060   

Spin resonance in A$_x$Fe$_{2-y}$Se$_2$ with s-wave pairing symmetry

We study spin resonance in the superconducting state of recently discovered alkali-intercalated iron selenide materials A$_x$Fe$_{2-y}$Se$_2$ (A=K,Rb,Cs) with an aim to address the basic issue of pairing symmetry and gap structure in these materials. As the Fermi surface of these materials has only electron pockets, the widely believed $s^{+-}$ symmetry for several iron based superconductors, which implies a sign changing gap between the hole and electron pockets, becomes questionable in case of these materials. While the earlier proposed d-wave symmetry is ruled out in ARPES studies, the observations of a spin-resonance like feature in the inelastic neutron scattering experiments indicate for a sign changing gap in these materials. In this study we demonstrate that the hybridization-induced sign-changing unconventional $s^{+-}$-wave symmetry, where the SC gap changes its sign between the hybridized electron pockets, supports spin resonance, and the dynamical structure factor is consistent with the results of inelastic neutron scattering. This ``other'' s-wave symmetry is also consistent with the recent ARPES studies.   

Small and nearly isotropic hole-like Fermi surfaces in LiFeAs detected through de Haas--van Alphen effect

We show a detailed dHvA study unveiling small and nearly isotropic Fermi surface sheets in LiFeAs single crystals, which is not observed by previous dHvA results, as well as the cylindrical electron-like Fermi surfaces. Our results are in partial agreement with the ARPES results, and the small, nearly isotropic Fermi surface should correspond to the hole-like pocket, suggesting a prominent role for the electronic correlations in LiFeAs. The absence of gap nodes, in combination with the coexistence of quasi-two-dimensional and three-dimensional Fermi surfaces, favor an s-wave pairing symmetry for LiFeAs.   

Interplay between the nematic and SDW states in iron-pnictides and iron-chalcogenides

Utilizing the angle-resolved photoemission spectroscopy (ARPES) and detwinning method, we have studied the electronic structure reconstruction in the nematic and SDW states of iron-based superconductors. The key features associated with the symmetry breaking process across the structure and magnetic transitions have been resolved. In the nematic state, we found that the energy splitting of the $d_{xz}$ and $d_{yz}$ bands is strongly momentum dependent. It is negligible in the zone center and reaches maximum at the zone corner, which is inconsistent with the ferro-orbital ordering scenario. In the SDW state, the reconstruction of the electronic structure is dominated by the SDW gap opening, whose orbital dependence and momentum dependence could be well described by the theoretical calculations. More intriguingly, we found that the coupling between the nematic and SDW states is strong in iron-pnictides, but very weak in iron-chalcogenides. Our findings resolve controversies on the electronic structure reconstruction in the nematic and SDW states of iron-based superconductors, and have strong implications on theory.   

Electronic Structure and High Temperature Superconductivity of the FeSe /SrTiO3 Films

High resolution angle-resolved photoemission measurements have been carried out to study the electronic structure and high temperature superconductivity of the single-layer FeSe films grown on SrTiO3 substrate [1]. Distinct Fermi surface topology and nearly isotropic superconducting gap without nodes are observed in the system [2]. Phase diagram is established and electronic indication of high temperature superconductivity at $\sim$65K is observed in tuning the carrier concentration of the single-layer FeSe film [3]. With a variation of the charge carriers, an insulator-superconductor transition in the single-layer FeSe/SrTiO3 is observed. A dichotomy of electronic structure and superconductivity is revealed between the single-layer and double-layer FeSe/SrTiO3 films. Implications of these results on the superconductivity mechanism of the iron-based superconductors will be discussed. \\[4pt] References:\\[0pt] [1]. Qing-Yan WANG, Zhi LI, Wen-Hao ZHANG, Zuo-Cheng ZHANG, Jin-Song ZHANG, Wei LI, Hao DING, Yun-Bo OU, Peng DENG, Kai CHANG, Jing WEN, CanLi SONG, Ke HE, Jin-Feng JIA, Shuai-Hua JI, Ya-Yu WANG, Li-L WANG i, Xi CHEN, Xu-Cun MA, Qi-Kun XUE, Chinese Physics Letters 29 (2012) 037402.\\[0pt] [2]. Defa Liu*, Wenhao Zhang*, Daixiang Mou*, Junfeng He*, Yun-Bo Ou, Qing-Yan Wang, Zhi Li, Lili Wang, Lin Zhao, Shaolong He, Yingying Peng, Xu Liu, Chaoyu Chen, Li Yu, Guodong Liu, Xiaoli Dong, Jun Zhang, Chuangtian Chen, Zuyan Xu, Jiangping Hu, Xi Chen, Xucun Ma, Qikun Xue, and X. J. Zhou, Nature Communications 3 (2012) 931.\\[0pt] [3]. Shaolong He*, Junfeng He*, Wenhao Zhang*, Lin Zhao*, Defa Liu, Xu Liu, Daixiang Mou, Yun-Bo Ou, Qing-Yan Wang, Zhi Li, Lili Wang, Yingying Peng, Yan Liu, Chaoyu Chen, Li Yu, Guodong Liu, Xiaoli Dong, Jun Zhang, Chuangtian Chen, Zuyan Xu, Xi Chen, Xucun Ma, Qikun Xue, and X. J. Zhou, Nature Materials 12 (2013) 605.   

ARPES observation of strong Mn-pnictide hybridization and small electronic correlations in BaMn$_2$Pn$_2$ (Pn = As, Sb)

ARPES experiments on Fe-based superconductors indicate non-negligible band renormalization due to electronic correlations. The key role attributed to a significant Hund's rule coupling in these materials and in their related isostructural nonferropnictide counterparts in tuning the electronic correlations depends strongly on the filling of the 3$d$ electronic shell and are expected to reach a maximum at half-filling (Mn 3$d^5$). Here we report an ARPES study of BaMn$_2$As$_2$ and BaMn$_2$Sb$_2$, which are isostructural to BaFe$_2$As$_2$. We show the existence of a strongly $k_z$-dependent band gap with a small minimum at the Brillouin zone center, in agreement with the semiconducting properties of these compounds. In contrast to the common expectation, we show that the electronic correlations in these materials are small. Our photon energy dependent study provides evidence for strong Mn-pnictide hybridization, which might play a role in reducing the electronic correlations in these compounds.   

Role of bonding angle on superconductivity in iron pnictide: Comparative electronic structure study on LiFeAs and Sr$_{4}$V$_{2}$O$_{6}$Fe$_{2}$As$_{2}$

The special relation between As-Fe-As bonding angle and superconductivity is the most remarkable universal feature in iron-based supercondutor. The maximum Tc can be achieved only with the certain bonding angle, the optimal angle that makes FeAs$_{4}$ tetrahedron regular. Despite its importance, a hinge behind the relationship between bonding angle and Tc is unclear and less considered so far. In this presentation, we present the comparative electronic structure study on two representative systems, LiFeAs and Sr$_{4}$V$_{2}$O$_{6}$Fe$_{2}$As$_{2}$. LiFeAs has small bonding angle with relatively lower Tc at 18 K and Sr$_{4}$V$_{2}$O$_{6}$Fe$_{2}$As$_{2}$ shows higher Tc at 37 K and its bonding angle is close to the optimal value. Using ARPES, the band dispersion including kz dependence and its orbital characters are explored. Detailed analysis reveals only Sr$_{4}$V$_{2}$O$_{6}$Fe$_{2}$As$_{2}$ electronic structure has orbital mixed nature and relatively strong Fermi surface nesting instability. Both feature indicate strong inter-orbital coupling and its possible role to the superconductivity, which can be the hinge between bonding angle and Tc.   

Insulator to Superconductor Transitions and Strong Electronic Correlations in Single-Layer FeSe/SrTiO$_3$ Films

There has been a debate on whether an appropriate starting point in describing these compounds should go from an itinerant picture or a localized picture. The single-layer FeSe/SrTiO$_3$ films have recently generated great interest for its simplicity of crystal structure and electronic structure, as well as possible high superconducting transition temperature. In this talk, by performing detailed doping-dependent measurements with ARPES, we report an insulator-metal-superconductor transition in the S phase of the single-layer FeSe/SrTiO$_3$ films. The results indicate that strong electronic correlations should be considered in the single-layer FeSe/SrTiO$_3$ films that might be on the verge of Mott physics.   

Electronic anisotropy and orbital ordering in Ru doped BaFe2As2 revealed by ARPES and XLD

A central issue in iron pnictides is the origin of electronic anisotropy. It is considered that its role in iron pnictides is important to the nature of magnetism and superconductivity. It was proposed that orbital ordering may play a key role. Since XLD can observe different occupation numbers for dyz and dzx orbitals, it is regarded to provide an experimental signature for the existence of orbital ordering. The work was motivated by the goal of exploring whether the orbital ordering and its fluctuation would explain the underlying mechanism on magnetism and superconductivity. We performed temperature dependent measurements both ARPES and XLD on Ba(Fe1$-$xRux)2As2 to experimentally verify existence of electronic anisotropy and orbital ordering.   

ARPES observation of isotropic superconducting gaps in isovalent Ru-substituted Ba(Fe$_{0.75}$Ru$_{0.25}$)$_2$As$_2$

We used high-energy resolution angle-resolved photoemission spectroscopy to extract the momentum dependence of the superconducting gap of Ru-substituted Ba(Fe$_{0.75}$Ru$_{0.25}$)$_2$As$_2$ ($T_c = 15$ K). Despite a strong out-of-plane warping of the Fermi surface, the magnitude of the superconducting gap observed experimentally is nearly isotropic and independent of the out-of-plane momentum. More precisely, we respectively observed 5.7 meV and 4.5 meV superconducting gaps on the inner and outer $\Gamma$-centered hole Fermi surface pockets, whereas a 4.8 meV gap is recorded on the M-centered electron Fermi surface pockets. Our results are consistent with the $J_1-J_2$ model with a dominant antiferromagnetic exchange interaction between the next-nearest Fe neighbors.   

Remarkable doping effects beyond altering Fermi surface on the superconductivity of iron-based superconductors

The superconductivity in Fe-based superconductors could be achieved by doping the parent compounds. Previous researches were focusing on the charge carrier density or Fermi surface alteration by doping only. However, the dominating factors based on Fermiology have many inconsistencies, which indicates that some other effects induced by doping are neglected. Using ARPES, we have established the microscopic and more comprehensive picture of doping on the electronic structure beyond altering Fermi surface. We have figured out other two critical effects of doping, scattering and changing correlation. With doping, the dxy-related band around the zone center is found to be much more sensitive than the dxz/dyz-related bands and the strength of the impurity scattering strongly depends on the position of dopants, which resembles the case in cuprates. On the other hand, we observed that the electron correlation decreases with doping, which is universal in various systems of Fe-based superconductors. Moderate electron correlation is critical for the high Tc. The two effects we observed here both are very important for the superconductivity, and explain a lot of previous mysteries and unresolved issues.   

Session J14: Invited Session: Collective Motion Across Scales: From Proteins to Animals

Motile Fluids: Granular, Colloidal and Living

My talk will present our recent results from theory, simulation and experiment on flocking, swarming and instabilities in diverse realizations of active systems. The findings I will report include: flocking at a distance in vibrated granular monolayers; the active hydrodynamics of self-propelled solids; clusters, asters and oscillations in colloidal chemotaxis.   

How Many Insects Does It Take to Make a Swarm?

Aggregations of social animals, such as flocks of birds, schools of fish, or swarms of insects, are beautiful, natural examples of self-organized behavior far from equilibrium. They tend to display a range of emergent properties, from enhanced sensing to the rapid propagation of information throughout the aggregate. Many classes of models have been proposed to describe these systems, including agent-based models that specify explicit social forces between individuals and continuum models that abstract the interactions between individuals into some smooth advecting velocity field. Assessing these various modeling approaches requires comparison with empirical data. We will discuss measurements of laboratory mating swarms of the non-biting midge Chironomus riparius in the context of model assessment. In particular, we focus on the question of the small-number limit: how large must the population be before collective properties emerge?   

Dynamics and Emergent Structures in Active Fluids

In this talk, we consider an active fluid of colloidal sized particles, with the primary manifestation of activity being a self-replenishing velocity along one body axis of the particle. This is a minimal model for varied systems such as bacterial colonies, cytoskeletal filament motility assays vibrated granular particles and self propelled diffusophoretic colloids, depending on the nature of interaction among the particles. Using microscopic Brownian dynamics simulations, coarse-graining using the tools of non-equilibrium statistical mechanics and analysis of macroscopic hydrodynamic theories, we characterize emergent structures seen in these systems, which are determined by the symmetry of the interactions among the active units, such as propagating density waves, dense stationary bands, asters and phase separated isotropic clusters. We identify a universal mechanism, termed ``self-regulation,'' as the underlying physics that leads to these structures in diverse systems.   

Topological and behavioral disorder in collective motion

A major underlying assumption in many studies on the collective motion of self-propelled agents has been that the environment is continuous, isotropic and ordered and agents are all identical. In the natural world there are many examples of disordered environments or heterogeneous swarms where collective motion can exist. Examples include bats that navigate natural caverns via echolocation, schools of fish that maneuver through dark and light areas, microbial colonies that move about in heterogeneous soil, quorum sensing bacteria, crowds of people that are evacuating a building and traffic flow in major cities. In general disorder can arise from two basic sources that inhibit/augment both movement and information flow, those that represent physical obstacles (i.e topological), (\textit{extrinsic}), and those that arise from behavioral heterogeneties within the swarm itself (\textit{intrinsic}). In either case, extrinsic or intrinsic, disorder can be quenched or dynamic in space or time or both. To understand the effect of the various forms of disorder that can be present in the environment of the agents, we study both discrete and continuous $2d$ agent based models that utilize only local interactions and study the transition to the collectively moving state as a function of the amount of disorder or behavioral heterogeneities present in the environment. I will present our recent results and discuss the effect that disorder has on collective motion and the general physical mechanisms that swarms, either real or artificial, could utilize in order to overcome disorder in their environment.   

Biomimetic Phases of Microtubule-Motor Mixtures

We try to determine the universal principles of organization from the molecular scale that gives rise to architecture on the cellular scale. We are specifically interested in the organization of the microtubule cytoskeleton, a rigid, yet versatile network in most cell types. Microtubules in the cell are organized by motor proteins and crosslinkers. This work applies the ideas of statistical mechanics and condensed matter physics to the non-equilibrium pattern formation behind intracellular organization using the microtubule cytoskeleton as the building blocks. We examine these processes in a bottom-up manner by adding increasingly complex protein actors into the system. Our systematic experiments expose nature's laws for organization and has large impacts on biology as well as illuminating new frontiers of non-equilibrium physics.   

Session J15: Focus Session: Phase Transitions and Criticality in Cells

Physical limits of cells and proteomes

The biomass in cells is mostly protein. So, it is natural to expect that some of the physical behaviors of cells, including components of their growth laws, should be explainable in terms of the physical properties of cellular proteomes, the sum total of a cells proteins. We develop simple physical models for how cells respond to temperature, osmotic pressure, and under different growth conditions.   

Stochasticity and universal dynamics in communicating cellular populations

A fundamental problem in biology is to understand how biochemical networks within individual cells coordinate and control population-level behaviors. Our knowledge of these biochemical networks is often incomplete, with little known about the underlying kinetic parameters. Here, we present a general modeling approach for overcoming these challenges based on universality. We apply our approach to study the emergence of collective oscillations of the signaling molecule cAMP in populations of the social amoebae \textit{Dictyostelium discoideum} and show that a simple two-dimensional dynamical system can reproduce signaling dynamics of single cells and successfully predict novel population-level behaviors. We reduce all the important parameters of our model to only two and will study its behavior through a phase diagram. This phase diagram determines conditions under which cells are quiet or oscillating either coherently or incoherently. Furthermore it allows us to study the effect of different model components such as stochasticity, multicellularity and signal preprocessing. A central finding of our model is that \textit{Dictyostelium} exploit stochasticity within biochemical networks to control population level behaviors.   

Morphogenesis at criticality?

Spatial patterns in the early fruit fly embryo emerge from a network of interactions among transcription factors, the gap genes, driven by maternal inputs. Such networks can exhibit many qualitatively different behaviors, separated by critical surfaces. At criticality, we should observe strong correlations in the fluctuations of different genes around their mean expression levels, a slowing of the dynamics along some but not all directions in the space of possible expression levels, correlations of expression fluctuations over long distances in the embryo, and departures from a Gaussian distribution of these fluctuations. Analysis of recent experiments on the gap genes shows that all these signatures are observed, and that the different signatures are related in ways predicted by theory. While there might be other explanations for these individual phenomena, the confluence of evidence suggests that this genetic network is tuned to criticality.   

Cell-cell interactions stabilize emerging collective migration modes

We propose a coarse-grained mechanistic model for simulating the dynamics of the biological model organism \textit{Dictyostelium discoideum}, incorporating gradient sensing, random motility via actin protrusions, persistent random motion and signal relay. We demonstrate that our simple cell model does result in the macroscopic group migration patterns seen in no-flow gradient chambers, namely a transition from individual motion to multi-cell ``streaming'' to aggregation as the external signal is decreased. We also find that cell-cell adhesion further stabilizes the contact network independent of chemical signaling, suggesting no indirect feedback between mechanical forces and gradient sensing. We discuss further modifications to the model and as well as further applications to quantifying dynamics using spatio-temporal contact networks.   

Emergence of Critical Behavior in $\beta $-Cell Network

The $\beta $-cell is a cell type located in the Islet of Langerhans, a micro-organ of the pancreas which maintains glucose homeostasis through secretion of insulin. An electrophysiological process governing insulin release occurs through initial uptake of blood glucose and generation of ATP which inhibits the ATP sensitive potassium channel (K-ATP) causing membrane depolarization (activation). Neighboring $\beta $-cells are electrically coupled through gap junctions which allow passage of cationic molecules, creating a network of coupled electrical oscillators. Cells exhibiting hyperpolzarized (inactive) membrane potential affect behavior of neighboring cells by electrically suppressing their depolarization. Here we observe critical behavior between global active-inactive states by increasing the number of inactive elements with the K-ATP inhibitor Diazoxide and a tunable ATP insensitive transgenic mouse model. We show this behavior occurs due to from cell-cell coupling as mice lacking $\beta $-cell gap junctions show no critical behavior. Also, a computational $\beta $-cell model was expanded to construct a coupled $\beta $-cell network and we show this model replicates the critical behavior seen \textit{in-vitro. }While electrical activity of single $\beta $-cells is well studied these data highlight a newly defined characteristic of their emergent multicellular behavior within the Islet of Langerhans and may elucidate pathophysiology of Diabetes due to mutations in the K-ATP channel.   

Phase transition in Caenorhabditis elegans: A classical oil-water phase separation?

In Caenorhabditis elegans droplets form before the cell divides. These droplets, also referred to as P-granules, consist of a variety of unstructured proteins and mRNA. Brangwynne et al. [Science, 2009] showed that the P-granules exhibit fluid-like behavior and that the phase separation is controlled spatially by a gradient of a component called Mex-5. It is believed that this system exhibits the same characteristics as a classical oil-water phase separation. Here we report the recent experimental investigations on the phase separation in Caenorhabditis elegans and compare our findings with a classical oil-water phase separation. Specifically, we consider the underlying coarsening mechanisms as well as the impact of temperature and species composition. Finally, we present a preliminary model incorporating the characteristics of the phase separation kinetics for Caenorhabditis elegans.   

Anesthetics lower $T_c$ of a 2D miscibility critical point in the plasma membrane

Many small hydrophobic molecules induce general anesthesia. Their efficacy as anesthetics has been shown to correlate both with their hydrophobicity and with their potency in inhibiting certain ligand gated ion channels. I will first report on our experiments on the effects that these molecules have on the two-dimensional miscibility critical point observed in cell derived vesicles (GPMVs). We show that anesthetics depress the critical temperature ($T_c$) of these GPMVs but do not strongly affect the ratio of phases found below $T_c$. The magnitude of this affect is consistent across the n-alcohols only when their concentration is rescaled by the median anesthetic concentration (AC50) for tadpole anesthesia and at AC50 we see a 4K downward shift in $T_c$. I will next present a model in which anesthetics interfere with native allosteric regulation of ligand gated channels by the critical membrane, showing that our observed change in critical properties could lead to the previously observed changes in channel conductance without a direct interaction between anesthetic molecules and their target proteins. Finally, I will discuss ongoing experiments that will clarify the role of this membrane effect in mediating the organism level anesthetic response.   

A Hessian geometric construction that aids analysis of non-monotonic effects in ternary mixture phase separation

Ternary, quaternary, and multi-component phase separations are common in biological systems, and their properties have many physiological and pathological consequences. As one example, understanding the molecular origins of the phase boundaries of eye lens protein solutions can help understand loss of transparency of the eye lens in cataract, a leading cause of blindness. The phase boundaries respond in a sensitive and non-monotonic fashion to small changes in molecular interaction strengths. We show how the geometry of relevant intersections, in the space of the components of the Hessian of the intensive Gibbs free energy with respect to relative compositions, can assist in comprehending the origins of such non-monotonic and sensitive changes of the phase boundaries. We apply this construction to analyze recent results about non-monotonic dependence of the phase boundaries of eye lens protein solutions on interprotein interaction strengths.   

The geometry of the Waddington Landscape

We study the ``landscape'' of cell states that emerge during \textit{in vitro} differentiation of mouse embryonic stem (ES) cells. Profiling the gene expression of cell populations captured at specific locations along different developmental trajectories, we uncover a low-dimensional landscape with an ultrametric distance structure between states; this provide a natural basis (and limit) for reconstructing cell lineages from gene expression profiles. From the correlation spectrum of this landscape, we infer ``directions'' in gene expression along which cells transition from one state to another, as well as signaling pathways that control these transitions. Finally, we study the dynamics of cell movement on this landscape using an ES cell line where yellow fluorescent protein (YFP) has been fused to Otx2, a transcription factor that plays an important role during early development.   

Zipf's law and criticality in multivariate data without fine-tuning

Recently it has become possible to directly measure simultaneous collective states of many biological components, such as neural activities or antibody sequences. A striking result has been the observation that the underlying probability distributions of the collective states of these systems exhibit a feature known as Zipf's law. They appear poised near a unique critical point, where the extensive parts of the entropy and energy exactly cancel. Here we present analytical arguments and numerical simulations showing that such behavior naturally arises in systems with an unobserved random variable (e.g., input stimulus to a neural system) that affects the observed data. The mechanism does not require fine tuning and may help explain the ubiquity of Zipf's law in disparate systems.   

Session J16: Focus Session: Soft Matter Perspectives on Protein Self-Assembly II

Arrest scenarios in concentrated protein solutions - from hard sphere glasses to arrested spinodal decomposition

The occurrence of an arrest transition in concentrated colloid suspensions and its dependence on the interaction potential is a hot topic in soft matter. Such arrest transitions can also occur in concentrated protein solutions, as they exist e.g. in biological cells or are increasingly used in pharmaceutical formulations. Here we demonstrate the applicability of concepts from colloid science to understand the dynamics of concentrated protein solutions. In this presentation we report a combination of 3D light scattering, small-angle X-ray scattering and neutron spin echo measurements to study the structural properties as well as the collective and self diffusion of proteins in highly concentrated solutions on the relevant length and time scales. We demonstrate that various arrest scenarios indeed exist for different globular proteins. The proteins chosen are different bovine lens crystallins. We report examples of hard and attractive glass transitions and arrested spinodal decomposition directly linked to the effective pair potentials determined in static scattering experiments for the different proteins. We discuss these different arrest scenarios in view of possible applications of dense protein solutions as well as in view of their possible relevance for living systems.   

The bacterial cytoplasm has glass-like properties and is fluidized by metabolic activity

In eukaryotes, active transport involves motor proteins and cytoskeletal filaments. In contrast, bacteria (which lack cytoskeletal motor proteins) are thought to rely on diffusion for molecular transport, though the physical properties of the bacterial cytoplasm are poorly understood. Through single particle tracking of foreign particles of different sizes, we have found that the bacterial cytoplasm exhibits striking similarities to glass-forming liquids. Glass-forming liquids are noted for their metastability near the glass transition where their behavior changes from liquid-like to amorphous solid with even small perturbations. Particles of different sizes exhibit distinct dynamics and their mobility changes from fluid-like to glassy with increasing size. This size dependency provides an explanation for previous reports of both normal and anomalous diffusion in the bacterial cytoplasm. Moreover, we find that cellular metabolism attenuates the glassy properties of the bacterial cytoplasm. As a result, components that would otherwise be caged in narrow regions of confinement are able to explore the cytoplasmic space under metabolically active conditions. These findings have broad implications for our understanding of bacterial physiology as the glassy behavior of the cytoplasm impacts all intracellular processes involving large cellular components.   

Dynamic clusters in highly concentrated lysozyme solutions

New biologic drugs need to be highly concentrated to have the required dosage for injection. Such high concentrations pose challenges for solution viscosity and stability. We therefore have studied the viscosity and dynamic clustering behavior of concentrated (up to 500 mg/mL) lysozyme solutions. Cluster dynamics are measured by neutron spin echo scattering experiments, which yield the mutual diffusivity. Viscosity is measured with a miniature capillary viscometer. While static scattering indicates cluster-like organization, the dynamic measurements show that these are momentary and do not survive local diffusion times. At high concentrations, they persist and diffusivity and viscosity dramatically increase.   

An overview of protein phase behavior

A homogenous protein solution can undergo several transformations when the conditions are changed. The proteins can form crystals, dense liquid phases, aggregates and gels. These transformations are central to numerous practical applications, such as protein x-ray crystallography, protein condensation diseases and the industrial purification of proteins. In this talk I review the efforts that have been made over the past twenty years to understand protein phase behavior from a physical perspective with an emphasis on globular proteins in aqueous solution. I also present recent insights and conclude with some ideas for future directions.   

A residue level protein-protein interaction model in electrolyte solutions

The osmotic second virial coefficients $B_{2}$ are directly related to the solubility of protein molecules in electrolyte solutions and can be useful to narrow down the search parameter space of protein crystallization conditions. Using a residue level model of protein-protein interaction in electrolyte solutions $B_{2}$ of bovine pancreatic trypsin inhibitor and lysozyme in various solution conditions such as salt concentration, pH and temperature are calculated using an extended Fast Multipole Methods in combination with the boundary element formulation. Overall, the calculated $B_{2}$ are well correlated with the experimental observations for various solution conditions. In combination with our previous work on the binding affinity calculations of protein complexes it is demonstrated that our residue level model can be used as a reliable model to describe protein-protein interaction in solutions.   

The role of crystal contacts in protein crystallization: soft matter characterization of two protein families

Crystallizing proteins is the bottleneck to systematically determining their structures, which are key to understanding certain biological processes and engineering bio-inspired materials. Identifying the conditions under which proteins crystallize should be equivalent to determining their phase diagram, but one typically resorts to combinatorial rather than physics-based sampling of solution conditions to tackle this difficult problem. Although several soft matter ``patchy particle'' models have been suggested to rationalize the phase behavior of proteins, the interactions that drive crystallization are insufficiently characterized for them to be of much use. We use atomistic simulations of solvated proteins of the rubredoxin family to parameterize patchy models. Their phase diagram is then compared with experimental crystallization conditions. The agreement between model and experiment supports the suitability of patchy models to describe globular proteins crystallization and provides physical guidelines to systematically improve protein crystallization experiments. An analogous analysis of ubiquitin, which crystallizes in multiple crystal forms, further clarifies the role of competing patches in controlling crystal assembly.   

Getting Closer to Real Proteins: Asymmetric and Competing Interactions in Patchy Models

Patchy particle models have been proposed to describe protein crystal assembly. Previous analyses of these models typically assume homogeneous patch interactions and symmetric patch geometry, but recent studies suggest otherwise. Typical protein interactions have a wide range of strengths, and sterically competing interactions are the rule rather than the exception. More complex patchy models are thus needed to guide protein crystallization. We study the phase diagram and assembly kinetics of patchy models with varying interaction strength and spatial distribution asymmetry. The results rationalize George and Wilson's observation that proteins with a second virial coefficient within a specific range are easier to crystallize and provide guidelines to facilitate crystallization of recalcitrant proteins. In models with sterically competing patches we also observe distinct crystal forms (dimeric vs. monomeric) depending on the relative strength of the secondary patches.   

Design rules for the self-assembly of a protein crystal

Theories and models of protein crystallization based on spheres that form close-packed crystals suggest that protein crystallization can be enhanced by metastable liquid-liquid criticality or demixing, and can be predicted by the osmotic second virial coefficient. However, most protein crystals are open structures, stabilized by anisotropic interactions. I will use analytic theory and computer simulations to argue that the self-assembly of open crystal lattices should not in general be best near the metastable liquid-liquid critical point or binodal (although assembly can certainly happen there), and to argue that the second virial coefficient cannot be a fully predictive measure of assembly propensity (although it is a useful starting point). Instead, the conditions that lead to best self-assembly of one particular computer model of a porous protein crystal are closer to the conditions that lead to best self-assembly of certain model viral capsids than they are to the conditions that optimize assembly of close-packed crystals.\\[4pt] References:\\[0pt] Haxton \& Whitelam Soft Matter 2012 \& 2013\\[0pt] Whitelam PRL 2010\\[0pt] Whitelam JCP 2010   

Shifting Phases for Patchy Particles -- Effect of mutagenesis and chemical modification on the phase diagram of human gamma D crystallin

Single mutations in human gamma D crystallin (HGD), a protein found in the eye lens are associated with several childhood cataracts. Phase diagrams for several of these protein mutants have been measured and reveal that phase boundaries are shifted compared with the native protein, leading to condensation of protein in a physiologically relevant regime. Using HGD as a model protein, we have constructed phase diagrams for double mutants of the protein, incorporating two single amino acid substitutions for which phase diagrams are already known. In doing so, the characteristics of each of the single mutations are maintained but both are now present in the same protein particle. While these proteins are not of interest physiologically, this strategy allows the controlled synthesis of nano-scale patchy particles in which features associated with a known phase behavior can be included. It can also provide a strategy for the controlled crystallisation of proteins. Phase boundaries also change after the chemical modification of the protein, through the covalent attachment of fluorescent labels, for example, and this will also be discussed.   

Phase Transitions in Antibody Solutions: from Pharmaceuticals to Human Disease

Antibodies are very important proteins. Natural antibodies play essential role in the immune system of human body. Pharmaceutical antibodies are used as drugs. Antibodies are also indispensable tools in biomedical research and diagnostics. Recently, a number of observations of phase transitions of pharmaceutical antibodies have been reported. These phase transitions are undesirable from the perspective of colloid stability of drug solutions in processing and storage, but can be used for protein purification, X-ray crystallography, and improving pharmokinetics of drugs. Phase transitions of antibodies can also take place in human body, particularly in multiple myeloma patients who overproduce monoclonal antibodies. These antibodies, in some cases, crystallize at body temperature and cause severe complications called cryoglobulinemia. I will present the results of our current studies on phase transitions of both pharmaceutical antibodies and cryoglobulinemia-associated antibodies. These studies have shown that different antibodies have different propensity to undergo phase transitions, but their phase behavior has universal features which are remarkably different from those of spherical proteins. I will discuss how studies of phase behavior can be useful in assessing colloid stability of pharmaceutical antibodies and in early diagnostics of cryoglobulinemia, as well as general implications of the fact that some antibodies can precipitate at physiological conditions.   

Structure of biological graded refractive index materials, and possible routes to self-assembly

For a camera-like eye, a spherical lens with a radially graded refractive index is required for high-quality image formation. Squids have evolved this lens design, and the index gradient results from variation in the density of protein in the lens from the center (70\% packing fraction) to the periphery (2\% packing fraction). However, density fluctuations must also remain low in all regions to maintain lens transparency. Squids have achieved this by an evolutionary radiation of the isoforms of one protein, S-crystallin; different protein isoforms are synthesized in different radial positions of the lens. We studied whether these proteins self-assemble into the observed gradient index material. X-ray scattering was performed on both intact lenses and solubilized lens protein. Our results show that protein packing is organized, and that the organization changes with radial position. We identify possible self-assembled routes to the observed structures via the predicted interactions between the proteins. Our study may provide insights into engineering new self-assembling graded refractive index materials.   

How Single-site Mutation Affects HP Lattice Proteins

We developed a heuristic method based on Wang-Landau\footnote{T. W\"{u}st and D. P. Landau, J. Chem. Phys. \textbf{137}, 064903 (2012).} and multicanonical sampling for determining the ground-state degeneracy of HP lattice proteins \footnote{K. A. Dill, Biochemistry 24, 1501 (1985); K. F. Lau and K. A. Dill, Macromolecules 22, 3986 (1989). }. Our algorithm allowed the most precise estimations of the (sometimes substantial) ground-state degeneracies of some widely studied HP sequences. We investigated the effects of single-site mutation on specific long HP lattice proteins comprehensively, including structural changes in ground-states, changes of ground-state degeneracy and thermodynamic properties of the systems. Both extremely sensitive and insensitive cases have been observed; consequently, properties such as specific heat, tortuosities etc. may be either largely unaffected or may change significantly due to mutation. More interestingly, mutation can even induce a lower ground-state energy in a few cases.   

Session J17: Granular Materials

Developing a Magnetic Resonance Imaging measurement of the forces within 3D granular materials under external loads

Granular materials are comprised of an ensemble of discrete macroscopic grains that interact with each other via highly dissipative forces. These materials are ubiquitous in our everyday life ranging in scale from the granular media that forms the Earth's crust to that used in agricultural and pharmaceutical industries. Granular materials exhibit complex behaviors that are poorly understood and cannot be easily described by statistical mechanics. Under external loads individual grains are jammed into place by a network of force chains. These networks have been imaged in quasi two-dimensional and on the outer surface of three-dimensional granular materials. Our goal is to use magnetic resonance imaging (MRI) to detect contact forces deep within three-dimensional granular materials, using hydrogen-1 relaxation times as a reporter for changes in local stress and strain. To this end, we use a novel pulse sequence to narrow the line width of hydrogen-1 in rubber. Here we present our progress to date, and prospects for future improvements.   

Imaging Forces in a Three-Dimensional Granular Material

We experimentally study the quasi-static deformation of three-dimensional sphere packings subjected to macroscopic deformation. We perform these experiments on slightly polydisperse, nearly frictionless soft hydrogel spheres in a modified tri-axial shear apparatus. We resolve the microscopic force network in a this three dimensional packing of spheres through imaging the entire packing. By resolving particle deformations via custom written image analysis software, we extract all particle contacts and contact forces. In addition, we measure boundary stresses during compression and shear. We address the non-linear force response of a disordered packing under compression, force network dynamics and explore the plastic rearrangements inside cyclically sheared and compressed packings.   

Quantitative DEM of granular packings

We introduce a new model for simulating granular assemblies. This model explicitely accounts for the cross-influence of multiple contacts on grains. It maintains the surface deformations of the grains induced by the contacts, improving on the classical non-deformable interpenetrable spheres model, for a reasonable computational cost. We show that both multiple contacts and surface deformations are necessary for reproducing quantitatively the 3D force measurements we recently demonstrated. We also show that friction has a dramatic effect on the forces and number of contacts, so it cannot be ignored even for very small values.   

Collisional Model for Granular Impact Dynamics

When an intruder collides with a granular material, the grains exert a stopping force which decelerates the intruder. A macroscopic force law, dominated by a $v^2$ drag term, is often used to characterize this decelerating force. However, a description which connects this drag force to grain-scale dynamics is still lacking, due in part to difficulty in obtaining sufficiently fast data at the grain scale. We present experiments using photoelastic particles and a high-speed camera, which capture the intruder dynamics and local granular force response at fast time scales. This allows us to analyze our experiments using both the macroscopic force law, and microscopically, where we observe large fluctuations at small space and time scales. Thus the intruder deceleration is not smooth and steady, but dominated by intermittent collisions with clusters of grains. Based on this, we present a model for the velocity-squared drag force in terms of these intermittent collisions we observe. We show that this model captures the shape-dependence of the $v^2$ drag force, as well as off-axis rotation. Therefore the microscopic assumptions of our model are confirmed, and may provide insight into other dense, driven granular flows.   

Effect of Mach number on granular impacts

When an object strikes a granular material, its momentum and energy are transferred to the grains and dissipated. An important dimensionless parameter in such impacts is $M$, the ratio of the intruder speed, $v_0$, to a typical granular sound speed, $c$. In many previous studies, $M$ has been very small, $M\sim 10^{-2}$. In this regime, the granular force on the intruder is dominated by a $v^2$ drag term, leading to a smooth, monotonic deceleration of the intruder. To probe the regime closer to $M\sim 1$, we perform experiments (and matching simulations) with granular materials comprised of photoelastic disks of varying stiffness, where softer particles allow us to reduce the granular sound speed. As we increase $M$, we reach a regime for which the intruder dynamics are no longer described by $v^2$ drag, but rather show a shock-like front which behaves elastically in response to the impact. Surprisingly, for the higher $M$ impacts ($M\sim 10^{-1}$), penetration depth is greatly reduced compared to the smaller $M$ impacts ($M\sim 10^{-2}$), and the intruder typically rebounds temporarily, before coming to rest. We understand the transition from $v^2$ drag to damped elastic behavior in terms of grain-grain collision time compared to the time for the intruder to move one grain size.   

Velocity Regimes for Sphere Penetration of Granular Materials

Penetration of granular materials as a function of velocity is made complex by transitions where one or another physical process is dominant. At the lowest velocity, bearing resistance (which depends on friction and depth) is dominant, then dynamic Coulomb friction, then inertial resistance, then particle crushing. There is also a special regime where resistance is very high during the first radius of penetration, probably due to shock wave effects. These transitions are very evident in penetration of dry sand, between 0 and 300 m/s, as revealed by measurements of deceleration and the final depth of penetration. With crushed quartz particles, the particle crushing regime is not observed. Additionally, in saturated sand, the crushing regime appears to be suppressed. The regime where particles are crushed corresponds to an increase in penetration resistance, and this plays a large role in the relative difficulty in penetration of dry as opposed to wet materials. Measurements of deceleration give rise to estimates of average stress in the granular materials. For the case of sand, the threshold for comminution is at about 100 MPa, and this is also where significant crushing of sand is seen in triaxial compression experiments.   

Impact and crater formation in an inhomogeneous granular medium

Non-circular impact crater shapes, including polygons, have been observed on many terrestrial planets as well as moons and asteroids. In this talk, we investigate how the impact of a spherical projectile on a granular bed (sand) is affected by inhomogeneity of the bed. To create inhomogeneity, we locally inject nitrogen gas beneath the bed to balance the hydrostatic pressure of the sand. Our experimental results show that in low energy impacts and when the inhomogeneity is within two ball diameters of the impact, the sphere is deflected and rotated, and the resulting crater is non-circular. We characterize these behaviors as a function of drop height and location of the impact relative to the inhomogeneity, and we relate our findings to a model of forces in granular impact.   

Dense Suspension Splash

Upon impact onto a solid surface at several meters-per-second, a dense suspension plug splashes by ejecting liquid-coated particles. We study the mechanism for splash formation using experiments and a numerical model. In the model, the dense suspension is idealized as a collection of cohesionless, rigid grains with finite surface roughness. The grains also experience lubrication drag as they approach, collide inelastically and rebound away from each other. Simulations using this model reproduce the measured momentum distribution of ejected particles. They also provide direct evidence supporting the conclusion from earlier experiments that inelastic collisions, rather than viscous drag, dominate when the suspension contains macroscopic particles immersed in a low-viscosity solvent such as water. Finally, the simulations reveal two distinct routes for splash formation: a particle can be ejected by a single high momentum-change collision. More surprisingly, a succession of small momentum-change collisions can accumulate to eject a particle outwards.   

Quasi-2D dynamic jamming of cornstarch suspensions

A dense suspension of cornstarch in water has the extraordinary behavior that, when perturbed lightly, it behaves like a liquid, but, when impacted at high velocities, the material solidifies. Waitukaitis et al. (Nature, 2012) have shown that this behavior is due to a dynamic jamming front that propagates through the system. The details of this jamming front, however, are obscured by the surrounding suspension in a 3-dimensional system. In our current experiment, we prepare a layer (thickness order 1 cm) of the cornstarch suspension, which floats on a dense, low-viscosity liquid. This setup provides a stress-free boundary condition on the bottom and upper surface of the suspension. The floating suspension is bounded at three sides by solid walls, and on one side by a thin rubber sheet. We perturb the system by impacting an object horizontally on one side at a controlled velocity using a linear actuator. Tracer particles sitting on the top surface of the suspension allow us to perform PIV on the perturbed suspension. From the PIV analysis we determine the shape of the jammed region, the growth rate, shear rates, and the expected force response due to the added mass. We compare this to direct force measurements and determine which components make up the total force response.   

Transiently Jammed State in Shear Thickening Suspensions under Shear

We examine the response of a suspension of cornstarch and water under normal impact at controlled velocities. This is a model system to understand why a person can run on the surface of a discontinuous shear thickening fluid. Using simultaneous high-speed imaging of the top and bottom surfaces along with normal force measurements allows us to investigate whether the force response is a result of system spanning structures. We observe a shear thickening transition where above a critical velocity the normal force increases by orders of magnitude. In the high force regime the force response is displacement dependent like a solid rather than velocity dependent like a liquid. The stresses are on the order of $10^6$ $Pa$ which is enough to hold up a person's weight. In this regime imaging shows the existence of a solid like structure that extends to the bottom interface.   

Dilation dynamics of granular suspensions during the shear thickening transition

We experimentally investigate the dilation dynamics of dense granular (non-Brownian) suspensions under shear. We focus on the scenario where the packing fraction is close to the dynamic jamming point and combine oscillatory rheological measurements with \emph{in situ} high-speed imaging to study the particle dynamics throughout the shear-thickening (ST) transition. By visualizing the shear profile at different strain amplitudes, we show that, although frustrated dilation is the dominant factor for ST in granular suspensions, viscous hydrodynamic stress $\tau_\mu$ still plays an important role in determining the velocity profile and shear localization during the dilation process. Moreover, when the suspending liquid becomes highly viscous, $\tau_\mu$ affects the magnitude of the stress increment. By imaging the air-suspension boundary during shear, we demonstrate that the upper stress limit of the observable ST regime in suspensions of hard particles corresponds to the point where the confining pressure due to capillary forces is exceeded, as signaled by movement of the contact line between suspension and substrate.   

Particle-Laden Liquid Bridge: Simulation and Experiment

Particle-laden fluids like pastes are important in many industries, but they are not well understood. We developed coordinated experimental and computational techniques to explore the flow behavior of these systems. Due to surface tension fluids can be suspended between the flat ends of two cylinders in a ``liquid bridge.'' In experiments, we vary the gap height and measure the forces and bridge shape to determine the response of the particle-laden fluid. We simulate the liquid bridge using a new hybrid technique combining direct particle trajectory calculations with a grid based model for the surface of the fluid. The model is appropriate for flows that are slow compared to the speed of sound in the fluid and flows in which the fluid can move freely between the particles. By combining experiments and simulation we have unprecedented access to information on both particle details and overall fluid response to external stress.   

Onset of motion at the surface of a porous granular bed by a shearing fluid flow

We will discuss an experimental investigation of the onset of particle motion by a fluid flow over an unconsolidated granular bed. This situation arises in a number of natural and industrial processes including wind blowing over sand, sediment transport in rivers, tidal flows interacting with beaches and flows in slurry pipelines and mixing tanks. The Shields criteria given by the ratio of the viscous shear and normal stresses is used to understand the onset of motion. However, reviews reveals considerable scatter while noting broad trends with Reynolds Number. We discuss an idealized model system where fluid flows with a prescribed flow rate through a horizontal rectangular pipe initially fully filled with granular beads. The granular bed height decreases and reaches a constant height when the shear stress at the boundary decreases below a critical value. We compare and contrast the values obtained assuming no-slip boundary conditions with those observed with PIV using florescent tracer particles to measure the actual fluid flow profile near the porous interface. We will also report the observed variation of the Shields criteria with particle Reynolds Number by varying particle size and fluid flow rates.   

A Microstructural View of Burrowing with Roboclam

Roboclam is a burrowing technology inspired by \textit{Ensis directus}, the Atlantic razor clam. The organism only has sufficient strength to burrow a few centimeters into the soil, yet razor clams dig to over 70 cm. The animal uses motions of its valves to contract and thereby locally fluidize the surrounding soil and reduce burrowing drag. Roboclam technology is valuable for subsea applications that could benefit from efficient burrowing, such as anchoring, mine detonation, and cable laying. We directly visualize the movement of soil grains during the contraction of Roboclam, using a novel index-matching technique along with particle tracking. We show that a previously developed mechanical theory for \textit{E. directus} describes the size of the failure zone around contracting Roboclam, provided that the timescale of contraction is sufficiently large. We also show that the nonaffine motions of the grains are a small fraction of the motion within the fluidized zone, affirming the relevance of a continuum model for this system, even though the grain size is comparable to the size of Roboclam.   

Shear alignment and orientational order of shape-anisotropic grains

Granular matter research was focused for a long time mainly on ensembles of spherical or irregularly shaped grains. In recent years, interest has grown in the study of anisometric, i.e. elongated or flattened particles [see e. g. B\"{o}rzs\"{o}nyi, Soft Matter 9, 7401 (2013)]. However, many related phenomena are still only little understood, quantitative experiments are scarce. We investigate shear induced order and alignment of macroscopic shape-anisotropic particles by means of X-ray computed tomography. Packing and orientation of individual grains in sheared ensembles of prolate and oblate objects (ellipsoids, cylinders and similar) are resolved non-invasively [T. B\"{o}rzs\"{o}nyi PRL 108, 228302 (2012)]. The experiments show that many observations are qualitatively and even quantitatively comparable to the behavior of well-understood molecular liquid crystals. We establish quantitative relations between aspect ratios and shear alignment. The induced orientational order influences local packing as well as macroscopic friction properties.   

Session J18: Liquid Crystals III: Mostly Nematics and Cholesterics

Twist-bend nematic liquid crystals in high magnetic fields

We present magneto-optic measurements on two odd-numbered dimer molecules that form the recently discovered twist-bend nematic (N$_{TB}$) phase, which represents a new type of 3-dimensional anisotropic fluid with about 10 nm periodicity and accompanied optical stripes. We show that B $=$ 25T shifts downward the N-N$_{TB}$ phase transitions by almost 1$^{\circ}$C, and explain it quantitatively. We also show that the optical stripes can be unwound by a temperature and material dependent magnetic induction in the range of B $=$ 5-25T. Finally, we propose a Helfrich-Hurault type mechanism for the optical stripe formation. Based on this model we calculate the unwinding magnetic field, and find agreement with our experimental results.   

Second harmonic light scattering from a bent-core nematic liquid crystal

We study the angular distribution of second harmonic (SH) light scattered from the aligned nematic phase of a bent-core liquid crystal. Throughout the nematic range and for certain combinations of polarizations of the fundamental and second harmonic fields, we detect peaks in the SH signal at non-zero scattering vectors ($\pm$q) along the nematic director, while the signal for q = 0 (forward direction) remains at the background level. The value of q, which corresponds to a length scale in the micron range, decreases with temperature when heating toward the nematic to isotropic transition. We will present a model to explain the major aspects of our results and indicate their significance in terms of the structure of bent-core nematics.   

Direct Observation of Heliconical Pitch in the Twist-bend Nematic Liquid Crystal Phase of Bent Molecular Dimers

Nanometer-scale modulation of the director field is directly observed using freeze-fracture transmission electron microscopy (FFTEM) in the heliconical twist-bend nematic (N$_{\mathrm{TB}})$ phase, a periodic mesophase with no detectable modulation of the electron density [Chen, D., \textit{et al}., PNAS, 2013, 110(40):15931--15936]. A homologous series of achiral odd-methylene-linked~dimers CB$m$CB ($m =$ 5, 7, 9, and 11) and binary mixtures with simple cyanobiphenyl $n$CBs ($n =$ 5, 6, 7, and 8) in the N$_{\mathrm{TB}}$ phase has been studied. The helix pitch is found to vary between 6 and 11 nm. Increase the m or n value increases the helix pitch. Meanwhile, surprisingly, the helix pitch becomes shorter as the monomer concentration in the mixtures increases. FFTEM images show homogenous phases and preliminary measurements of the transition temperature versus concentration indicate that the binary mixtures are close to ideal. Polarized optical microscopy and calorimetry are carried out to study the nature of the N-N$_{\mathrm{TB}}$ transition in detail.   

Distortion of cholesteric helical structures by in-plane fields and effect on the efficiency of second-order Bragg reflections

When an electric field is applied perpendicular to the helical axis of a chiral nematic liquid crystal, as at the center of gap regions in cells with interdigitated electrodes, materials with positive dielectric anisotropy and in a planar conformation not only experience an elongation of the pitch, but also undergo a deformation of their helical structure so that the refractive index deviates from exhibiting a sinusoidal variation in any direction perpendicular to the axis. Under these conditions, Bragg reflections of all orders are in principle active even for light at normal incidence. We will show that for various chiral nematic mixtures with the main (first-order) reflection in the near-infrared range, second and third-order reflections can easily be observed in the visible range when an electric field is applied and the material optical properties are probed selectively in the center of gap regions. The efficiency of the second-order reflections increases with the magnitude of the applied field and can become comparable to that of the first order reflection at high fields. This process can be exploited in the design of switchable reflective filters whose reflection band can be electrically tuned.   

Colloidal Transport and Periodic Stick-Slip Motion in Cholesteric Finger Textures

We have investigated the transport of colloidal particles within cholesteric finger textures formed by mixtures of the nematic liquid crystal 4-cyano-4'-pentylbiphenyl (5CB) and the chiral dopant4-(2-methylbutyl)-4-cyanobiphenyl (CB15) with cholesteric pitches between 24 and 55 micrometers. Spherical silica colloids (radius 5-10 micrometers) moving under the force of gravity through the texture translated strictly perpendicular to the cholesteric axis and had no measurable mobility parallel to the axis. Thus, when the applied force was oriented at an oblique angle to the axis, the spheres moved at an angle to the force. Nickel disks, 20 micrometers in radius and 300 nanometers thick, driven by gravity similarly showed no mobility parallel to the cholesteric axis for small pitch. For larger pitch, the disks displayed a periodic stick-slip motion caused by elastic retardation followed by yielding of the finger texture. Effective drag viscosities obtained from the sphere and disk motion were anomalously large compared with those of pure 5CB.   

Deformations in chiral liquid crystals

Deformations and their relaxation in chiral liquid crystals are studied experimentally and theoretically in planar geometry for liquid crystalline mixtures of varying viscosities. It is shown by both methods that shear deformation in liquid crystals results in the inclination and extension of cholesteric helix in samples with high viscosity [1,2]. Stretching deformation results in shrinking cholesteric helix. This leads to a possibility of detecting deformations on a nanometer scale by observing changes in selective reflection spectra. Theoretical model takes into account elastic strain of physical network formed by the entanglements between components of liquid crystalline mixture, viscosity of the matrix and elasticity of the liquid crystalline subsystem. This allows to model mechanical response of the matrix with different viscosities to stretching and shear of various amplitudes. It is shown that relaxation of the cholesteric helix takes much shorter time than mechanical relaxation of the mixtures. The model perfectly agrees with experimental data. The model is compared with theoretical model describing behavior of elastomers. \\[4pt] [1] P.V. Shibaev, L. Newman, \textit{Liquid Crystals}, 40, 428 (2013);\\[0pt] [2] P.V. Shibaev,C. Schlesier, \textit{Applied Physics Letters, 101, 193503 (2012)}   

Electrooptic response of chiral nematic with conical helicoidal director deformation

Electrically induced reorientation of liquid crystal (LC) director caused by dielectric anisotropy is a fundamental phenomenon widely used in modern technologies. We report on the experimental observation of an electrooptic effect with a distinct conical helicoid deformation of the director. The effect is observed in a chiral nematic in which the ground state of the director represents a right-angle helicoid. Application of the electric field along the helicoid axis transforms it into an oblique-angle helicoid. Further increase of the field causes complete unwinding into a uniaxial nematic state, in agreement with the theory proposed by R.B. Meyer in 1968. The effect is observed in a dimer nematic material in which the bend elastic constant is smaller than its twist counterpart.   

A Simple Procedure for The Determination of Refractive Indices of Liquid Crystals from High-Resolution Birefringence Measurements

We present a simple procedure to determine the temperature dependence of the extraordinary and ordinary refractive indices of liquid crystals based on the high precision birefringence measurements. We show that the procedure needs only a single value for refractive index, namely n$_{\mathrm{I}}$ being the value of the refractive index in the isotropic phase just above the nematic-isotropic (NI) transition temperature apart from the birefringence. In most studies the calculation of the order parameter is based on the Haller approximation known to be inconsistent with the weakly first-order character of the N-I transition and to lead systematically lower values for the critical exponent. Here, we revisit the methodology for the determination of the orientational order parameter in the N phase We have calculated the order parameter by applying Vuks and Neugebauer models and the procedure by Kuczynski \textit{et al.} We conclude that the approximation for the average refractive index \textless n\textgreater $\approx $ n$_{\mathrm{I}}^{2}$ is plausible to determine the temperature dependence of the refractive indices together with the birefringence data. This procedure allows one to obtain the normalised polarizabilities of the extraordinary and ordinary rays without addressing density measurements.   

Simulated Textures of Toroidal Nematic Liquid Crystal Droplets

Nematic liquid crystals under confinement by curved surfaces can produce complex hierarchical structures whose design principles and properties have yet to be unraveled. Here we focus on toroidal geometries and perform computer simulations of the nematic textures seen between crossed-polarizers. We find agreement with experiments using director fields that exhibit pronounced twist deformations with contributions from bend and splay.   

Liquid crystals confined inside toroids

We generate stable toroidal droplets of liquid crystals stabilized inside a yield stress material. In this talk we discuss aspects of both nematic and cholesteric tori: from double twist to low pitch organization.   

Magnetic field-induced suppression of the amorphous Blue Phase

We present magneto-optical measurements on two liquid crystals that exhibit a wide temperature-range amorphous blue phase (BPIII). Magnetic fields up to 25T are found to suppress the onset of BPIII in both materials by almost 1 $^{\circ}$C. This effect appears to increase non-linearly with the field strength. The effect of high fields on established BPIII's is also reported, in which we find significant hysteresis and very slow dynamics. Possible explanations of these results are discussed.   

Observation of NanoDNA Liquid Crystal Phases from Four Base Pair Duplexes at Subambient Temperatures

Based upon conventional Onsager model considerations, liquid crystal (LC) formation in DNA-water mixtures was originally thought to be impossible for DNA polymers of very short length (\textless 100 bases). We originally reported the discovery of chiral nematic (N*), columnar C$_{\mathrm{U}}$ and C$_{2}$ LC phases in NanoDNA oligomers as short as 6 bases in length and have since described additional LC phases involving DNA with random sequences and various blunt or sticky-end duplex architecture, all in the regime of \textless 20 bases. These results suggested a self-assembly motif where hydrophobic forces or hydrogen bond mediated base-pairing enable unusually short polymers to stack into functionally longer units that permit them to exhibit LC phase behavior. We report now the existence of LC phases of ultra short duplexed NanoDNA, 4 bases in length, in blunt-end, sticky-end and random sequence configurations, all observed at temperatures of $\sim$ 5 $^{\circ}$C and not stable \textgreater 13-15 $^{\circ}$C. These oligomers demonstrate an unusual wealth of phase behavior, including the typical N*, C$_{\mathrm{U}}$ and C$_{2}$ phases as well as higher order dark and bright phases, including what we believe to be a Blue Phase.   

Direct observation of lens-shaped nematic tactoids and micro-phase separation in aqueous suspensions of $\alpha $-zirconium phosphate nanosheets

We study the ordering of monolayer $\alpha $-zirconium phosphate nanosheets in aqueous suspensions. As the concentration increases, we confirm that the inter-plate spacing decreases, the X-ray correlation increases, and there is nematic ordering even at the highest concentrations. The inter-plate spacing shows linear swelling behavior, but not as expected for a uniform swelled system. The micro-phase separation is proposed and further demonstration by centrifugation, optical microscope, confocal fluorescent microscope and freeze fracture TEM. Self-assembling of stacks of the platelets are proposed to form liquid crystal phases. Lens-shaped tacoids with radial director field are observed, and the quantitative analysis of the tactoid properties gives estimates of ratios of the bulk elastic constant, anchoring strength to the bare surface tension.   

Modulated liquid-crystal phases induced by polarity: Twist-bend, splay-bend, and blue phases

Nematic liquid crystals exhibit flexoelectric couplings between polar order and gradients in the director field. When the couplings become strong enough, the uniform nematic phase can become unstable to the formation of a modulated polar phase. The question is then: What is the structure of the modulated polar phase? Classic work by Meyer and further studies by Dozov predicted two possible structures, known as twist-bend and splay-bend. One of these predictions, the twist-bend phase, has recently been identified in experiments on bent-core liquid crystals. Here, we investigate modulated polar phases through a combination of Landau theory and lattice simulations. We find a range of possibilities, including the twist-bend and splay-bend phases as well as polar blue phases, with 2D or 3D modulations of the director field and the polar order. We compare these polar blue phases with chiral blue phases, and discuss opportunities for observing them experimentally.   

Session J19: Focus Session: Theory and Simulations of Macromolecules VI - Block Copolymers

Nucleation of ordered microphases in fluctuation-induced first-order phase transitions

The Landau-Brazovskii model is a field-based Hamiltonian describing a variety of systems which exhibit ordered microphases defined by characteristic periodicity and symmetries (e.g., lamellar, hexagonal, body-centered cubic). Interestingly, this model can undergo a fluctuation-induced first-order phase transition: for the symmetric model, the disorder-to-lamellar transition is second-order at the mean-field level but takes on first-order character when fluctuations are added. A disordered phase supercooled to within the resulting metastable region will then transition to the stable lamellar phase via nucleation. We demonstrate it is possible to discover the critical nucleus' size and geometry by applying the numerical string method\footnote{Weinan E et al, J. Chem. Phys. \textbf{126}, 164103 (2007)} to a renormalized Landau-Brazovskii Hamiltonian which incorporates the effects of fluctuations. We find good agreement with predicted nucleus size and shape obtained by analytic approximation. Hohenberg and Swift\footnote{P. C. Hohenberg and J. B. Swift, Phys. Rev. E \textbf{52}, 1828 (1995)} predict that for this transition, certain defect structures in the critical nucleus might act to lower the nucleation free energy barrier; we present a search for these structures.   

Universal phenomenology of the order-disorder transition in symmetric diblock copolymers

The order-disorder transition (ODT) in melts of symmetric diblock copolymers has been precisely identified by a free-energy based technique in several simulation models over a wide range of experimentally relevant values of the invariant degree of polymerization $\overline{N}$. To compare results of different models, we determine the parameter dependence of $\chi$ for each model from a fit of disordered phase data for the structure function $S(q)$ to the renormalized one-loop theory. The value of $\chi N$ at the transition obtained using this estimate of $\chi$ is found to exhibit a universal dependence on $\overline{N}$. Simulation results for the both the ODT and strength of order appear to slowly converge above a crossover value of $\overline{N}$ of order 10,000. This value corresponds to a crossover between strongly segregated low-N regime, in which the center of each domain is nearly pure even at the ODT, and the beginning of the more weakly segregated regime for which the FH theory was originally designed.   

Molecular Interaction Control in Diblock Copolymer Blends and Multiblock Copolymers with Opposite Phase Behaviors

Here we show how to control molecular interactions via mixing AB and AC diblock copolymers, where one copolymer exhibits upper order-disorder transition and the other does lower disorder-order transition. Linear ABC triblock copolymers possessing both barotropic and baroplastic pairs are also taken into account. A recently developed random-phase approximation (RPA) theory and the self-consistent field theory (SCFT) for general compressible mixtures are used to analyze stability criteria and morphologies for the given systems. It is demonstrated that the copolymer systems can yield a variety of phase behaviors in their temperature and pressure dependence upon proper mixing conditions and compositions, which is caused by the delicate force fields generated in the systems.   

Computational study of solvated block-copolymer microphases

Using field-theoretic simulations, we study the equilibrium self-assembly of solvated block copolymer microphases with different chain architectures and block selectivities. Initially within the mean-field approximation (self-consistent field theory), we employ unit-cell calculations to determine the phase diagram by comparing the free energy of candidate phases. We find good agreement with prior computational and experimental reports in the literature. We subsequently move beyond the mean-field approximation using Complex Langevin sampling to investigate the effect of fluctuations on relative phase stabilities.   

Structure and Phase Behavior of Tapered Diblock Copolymers from Self-Consistent Field Theory

Tapered block copolymers are like AB diblock copolymers with a ``tapered block'' inserted between the A and B endblocks. This tapered block sequence is random with its average composition changing linearly from pure A to pure B (or B to A for inverse-tapered systems). Depending on the fraction of A monomers and the quantity $\chi N$, the blocks microphase separate to form various ordered morphologies. Increasing $N$ (such as to improve mechanical properties) simultaneously affects the microphase separated state. Tapering adds another adjustable parameter, taper length, that can be used to control the microphase separated state. We map the phase diagrams of model tapered and inverse tapered polymers using self-consistent field theory (SCFT). The ordered phases shift to higher $\chi N$ for tapered systems, and the shift increases as the taper length increases. Inverse tapers shift the phase diagram to even higher $\chi N$. Direct tapered systems' phase diagrams are like those of diblocks, but with a larger gyroid region. For large inverse tapered systems, the polymer appears like an ABAB tetrablock, and it folds across the interface or bridges between domains. In this case some of the ordered structures show reversed A and B domains where the majority phase is relatively impure.   

Microphase Separation and Interfacial Behavior of Model Tapered Diblock Copolymers

We use a combination of theoretical and simulation methods to understand the microphase separated structure and dynamics of model copolymers. Tapered diblock copolymers, containing pure A and B blocks separated by a region with an A/B composition gradient, are of particular interest: the length of the tapered region can be adjusted to modify the system's phase and interfacial behavior. Experimentally, tapered diblocks have been found to form the bicontinuous gyroid phase, which is of interest for transport applications and can be difficult to access using typical diblocks. Phase diagrams from self-consistent field theory (SCFT) do show a larger gyroid region for certain tapered systems versus diblocks. To further understand the detailed microphase separated structure, we employ fluids density functional theory (fDFT) and molecular dynamics (MD) simulations together. These both capture monomer scale packing effects and are implemented for very similar models so that the fDFT results can be used as a guide to ensure the appropriate equilibrium state is formed in the MD simulations. Density profiles from SCFT, fDFT, and MD are in qualitative and sometimes quantitative agreement; tapers widen the interfacial region and large tapers decrease the maximum purity of the microphases.   

Block Copolymer Compatibilizers for Morphological Control on the Equilibrium Structural Characteristics of Polymer/Fullerene Blends

We develop a single chain in mean field model for the equilibrium morphologies of solar cells based on the homopolymer/block copolymer/fullerene blend. Using our model, we study the ability of the block copolymer compatibilizer to provide morphological control on the domain and interfacial characteristics of the equilibrium structures. We focus our efforts on the case of a semiflexible homopolymer and a semiflexible/flexible diblock copolymer as these are emblematic of the kinds of molecules used in photovoltaic applications. Our results reveal a novel progression of morphologies in transitioning the ternary composition space, the rigidity of the semiflexible chains, and the flexible block ratio of the diblock copolymer. To elucidate the morphologies, we first present a series of ternary phase diagrams and then use a simple morphological characterization scheme to evaluate the domain sizes and interfacial quantities characterizing our equilibrium structures.   

Self-Consistent Field Theoretical Study on the Crossed Cylinder Morphology of Block Copolymers

Cylindrical morphologies are commonly observed for various block copolymer systems. Both theory and experiment confirm that the stable bulk morphology is hexagonally packed parallel cylinders. However, it does not necessarily mean that other types of arrangements are impossible. In this work, we explore a few possible strategies to promote the formation of crossed cylinder geometry. Our self-consistent field theoretical calculations along with experiments of our collaborators demonstrate that such crossed cylinder morphology is obtainable if the system is well designed and prepared. One strategy is to use surface interaction energy. If a block copolymer thin film resides on a substrate with stripe shaped chemical patterns which prefer one block, cylinders parallel to the pattern is energetically favorable on the stripe. However, on the neutral region, thin film confinement promotes cylinders vertical to the substrate. Another strategy is to use locally inclined substrates. In general, cylinders vertical to the substrate have difficulty in fitting themselves on a non-flat substrate and they prefer to lie down on the substrate. These strategies are applicable for the formation of other non-traditional crossed geometries such as the crossed lamellar morphology.   

Session J20: Focus Session: Microfluidics and Nanofluidics IV: Hydrodynamics, Separations and Slip

Drainage and Stratification Kinetics of Foam Films

Baking bread, brewing cappuccino, pouring beer, washing dishes, shaving, shampooing, whipping eggs and blowing bubbles all involve creation of aqueous foam films. Foam lifetime, drainage kinetics and stability are strongly influenced by surfactant type (ionic vs non-ionic), and added proteins, particles or polymers modify typical responses. The rate at which fluid drains out from a foam film, i.e. drainage kinetics, is determined in the last stages primarily by molecular interactions and capillarity. Interestingly, for certain low molecular weight surfactants, colloids and polyelectrolyte-surfactant mixtures, a layered ordering of molecules, micelles or particles inside the foam films leads to a stepwise thinning phenomena called stratification. Though stratification is observed in many confined systems including foam films containing particles or polyelectrolytes, films containing globular proteins seem not to show this behavior. Using a Scheludko-type cell, we experimentally study the drainage and stratification kinetics of horizontal foam films formed by protein-surfactant mixtures, and carefully determine how the presence of proteins influences the hydrodynamics and thermodynamics of foam films.   

Hydrodynamically enforced entropic trapping of Brownian particles

In small systems on length scales spatial confinement causes entropic forces that in turn implies spectacular consequences for the control for mass and charge transport. In view of its importance, recent efforts in theory triggered activities which allow for an approximate description that involves a reduction of dimensionality; thus making detailed predictions tractable. Up to present days, the focus was on the role of conservative forces and its interplay with confinement. Within the presented work, we overcome this limitation and succeeded in considering also non-conservative forces that derive from a vector potential [S. Martens et al., PRL 110, 010601 (2013)]. A relevant application is the fluid flow across microfluidic structures where a solute of Brownian particles is subject to both, an external bias and a pressure-driven flow. Then a new phenomenon emerges; namely, the intriguing finding of identically vanishing average particle flow which is accompanied by a colossal suppression of diffusion. This entropy-induced phenomenon, which we termed hydrodynamically enforced entropic trapping, offers the unique opportunity to separate particles of the same size in a tunable manner [S. Martens et al., Eur. Phys. ST 222, 2453-2463 (2013)].   

First clues to understand red blood cell interactions: numerical studies of vesicle suspensions

The scientific community started raising questions on blood flow for nearly two centuries, a period traced back to the pioneering work of Poiseuille. This topic has known a considerable upsurge of interest during the past decade. Vesicles capture several essential features shared with red blood cells. A single vesicle is now fairly understood, whereas study of suspensions is still unclear. We conduct bidimensionnal numerical studies by mean of the boundary integral method. Confinement plays a major role in that it introduces an interaction cut-off length. I will present results on the behavior of relative viscosity as function of the viscosity contrast between the fluid encapsulated by vesicles and the ambient fluid. This viscosity contrast is a key parameter: it triggers transition from tank-treading to tumbling regimes. Historical characterization of blood have led to the discovery of the Fahraeus-Lindqvist effect. I will introduce some results on this effect with a rheological study as function of concentration and confinement. I will report on non-standard behavior induced by a subtle spatio-temporal organization of the suspension.   

Separation in microfluidics using periodic structures

We investigate the complex behavior that takes place during the motion of suspended particles in periodic systems, both when Brownian motion is important as well as when transport is nearly deterministic. We are interested in the development of separation devices that rely on the unique features of transport in periodic structures. A typical system in our studies consists of suspended particles moving either through a periodic array of posts or on top of a periodic pattern fabricated in the bottom surface of a microfluidic channel. In all cases, we investigate how to take advantage of the selective and repetitive effects present in periodic systems to promote and amplify the separation of a mixture of suspended particles. In particular, we focus on vector separation systems in which different species move in different directions within the device. We present analytical and experimental results that show the potential that periodic systems have to induce the spontaneous fractionation of a mixture of particles.   

Slip effects in dewetting polymer microdroplets

Spherical caps on a substrate with less than equilibrium contact angles contract as a result of capillary forces. Applying the classical no-slip condition at the liquid-substrate interface results in diverging stress at the contact line. This divergence can be alleviated, however, by allowing finite flow velocity at the substrate, corresponding to the slip boundary condition. Experiments have been conducted in which glassy polystyrene microdroplets are placed upon, as substrates, different self-assembled monolayers (SAMs). The spherical caps are prepared such that initial contact angles are much less than the equilibrium contact angle. Above the glass transition temperature, a capillary induced flow is observed; the droplet radii shrink while their heights grow. Furthermore, the intermediate height profiles are highly non-spherical. Different SAMs give rise to differing slip lengths, resulting in dramatic changes to the temporal and morphological path these tiny droplets take toward their equilibrium spherical cap shapes.   

Imbibition dynamics on surfaces of legs of a small animal and on artificial surfaces mimicking them

Recently, imbibition of textured surfaces covered with homogeneous micro-pillar arrays has been actively studied partly because of the potential for transport of a small amount of liquids. In most cases, the dynamics is described by the Washburn law, in which the imbibition distance scales with the square root of elapsed time, while a different scaling law has been recently found [1]. In this study, we studied imbibition on legs of a small animal that absorbs water via its legs [2] to find yet another scaling law. Furthermore, imbibition of artificial surfaces mimicking the leg surface was found to be described well by a composite theory.\\[4pt] [1] Obara and Okumura, Phys. Rev. E 86, 020601R (2012).\\[0pt] [2] Ishii, Horiguchi et al. Sci. Rep. 3, 3024 (2013).   

Wall Driven Cavity Approach to Slug Flow Modeling In a Micro channel

Slug flow is a commonly observed stable regime and occurs at relatively low flow rates of the fluids. Wettability of channel decides continuous and discrete phases. In these types of biphasic flows, the fluid -- fluid interface acts as a barrier that prohibits species movement across the interface. The flow inside a slug is qualitatively similar to the well known shallow cavity flow. In shallow cavities the flow mimics the ``fully developed'' internal circulation in slug flows. Another approach to slug flow modeling can be in a moving reference frame. Here the wall boundary moves in the direction opposite to that of the flow, hence induces circulations within the phases which is analogous to the well known Lid Driven Cavity. The two parallel walls are moved in the opposite directions which generate circulation patterns, equivalent to the ones regularly observed in slug flow in micro channels. A fourth order stream function equation is solved using finite difference approach. The flow field obtained using the two approaches will be used to analyze the effect on mass transfer and chemical reactions in the micro channel. The internal circulations and the performance of these systems will be validated experimentally.   

Some scaling laws for fluid dynamics in a confined space

This talk is composed of several topics of fluid dynamics in confined spaces. Scaling laws and simple physical understanding is stressed in this talk. Wetting on textured surfaces where array of pillars of micron scale are arranged is one of the topics, which include wetting transition of a drop on the surfaces and capillary rise on textured surfaces at the micron scale. For the capillary rise, we discuss a new type of scaling law resulting from competition of three effects: capillary drive, viscous and gravitational drags [1]. We also discuss drag frictions acting on fluids in confined spaces, in which liquid film is confined on the micron scale and the role of the film is important for understanding the physics of the drag [2]. Other topics include the coalescence of liquid drops in a confined space in different situations. Especially, we discuss the effect of high electric field in the coalescence phenomena on the system studied in [3]. In addition, formation of liquid thin film and bursting of the film during liquid-drop coalescence are also discussed.\\[4pt] [1] Noriko OBARA and K.O., Phys. Rev. E 86, 020601R (2012).\\[0pt] [2] Ayako ERI and K.O., Soft Matter, 7, 5648 (2011).\\[0pt] [3] Maria YOKOTA and K.O., Proc. Nat. Acad. Sci. (USA), 108 (2011) 6395.   

First-Principles Investigation on Water dynamics at Functionalized Silicon surface

Interfacial water behavior at semiconductor surfaces is one of the most important areas of investigation for diverse industrial applications such as crystal growth, lubrication, catalysis, electrochemistry and sensors. Although the hydrophobicity at surface is widely recognized to be important in determining the behavior of water molecules near the surface, we show that subtle molecular details may also play a role in determining the dynamical behavior of water by employing first principles molecular dynamics simulations. By comparing water diffusivity at three non-polar surfaces, we find that water diffusivity is significantly faster near the H-terminated surface as compared to either CH3- or CF3-terminated surfaces. By examining the interfaces in detail, we find that the specific surface corrugation that is characteristic of the H-terminated surface leads to a suppression of hydrogen bond network ring structures by enhancing hexagonal spatial distribution of water molecules near the surface. Such a distinct molecular dependent behavior of the interfacial water was found to persist well into the liquid, while the most structural properties are noticeably influenced in only the first water layer ($\sim$5 {\AA}).   

Session J21: Focus Session: Polymers for Energy Storage and Conversion I- Capacitors and Fuel Cells

Effect of Dipolar Orientational Polarization on Electronic Conductivity in Ferroelectric Polymer Electrets

The leakage current, ion migration, and dipolar orientational polarization are major losses in ferroelectric polymers. The loss from the leakage current originates from electronic conduction and its behavior could be significantly affected by the internal electric field, which is induced by the dipolar orientational polarization. In this work, the leakage current in the corona charged PVDF electrets is studied under different external electric fields. Under low applied electric field, when no or very few dipoles could flip, the conductivity from the leakage direct current increases upon increasing the electric field. Under higher electric field, the aligned dipole-induced internal field would prevent the electrons from going through so that the conductivity decreases. After all the dipoles are aligned with the external electric field, the conductivity can increase again. This study will help us better understand the interplay between electronic conduction and dipolar orientation in ferroelectric materials.   

Structural and Interfacial Effects on the Dielectric Properties of PVDF and its Composites for Energy Storage

High energy density capacitors based on dielectric polymers are a focus of increasing research effort motivated by the possibility to realize compact and flexible energy storage devices. Multilayered ferroelectric polyvinylidene fluoride (PVDF) systems are fabricated using enabling technology in co-extrusion for increased energy storage efficiency. These micro- and nano-layered polymeric systems result in much improved device performance and a three-time enhancement of capacitive electrical energy density has been demonstrated. PVDF thin film nanocomposites with ZnO nanofillers have also been fabricated and evaluated for further enhancement of energy density storage. To understand the physics of why these multilayered and nanocomposite systems perform better than single layer PVDF we are developing characterization techniques using confocal second harmonic generation (SHG), electric field induced second harmonic (EFISH) and Raman laser spectroscopy. Our results have shown that the combination of Raman and SHG is a very sensitive, non-destructive and versatile technique that can be used to study the ferroelectric and structural properties of these systems. The addition of the EFISH technique allows the interrogation of structural and dielectric properties within individual layers and at the interfaces.   

Multiscale simulations of polyurea-based dielectrics for capacitive energy storage

In high energy capacitors, bi-axially oriented polypropylene is the preferred the state-of-art low loss dielectric. However, its energy density only reaches 4 J/cm$^3$ at 600 MV/m with 85\% efficiency, while the recently synthesized polythiourea reaches 8 J/cm$^3$ with 95\% efficiency under the same conditions [1]. Members of the aromatic polyurea family [2] have also been reported to have similarly high energy densities. We have performed multiscale simulations to investigate several members of the polyurea/ polythiourea family, focusing on their structural and dielectric properties. Antiparallel packing of urea/thiourea units is found to be preferred energetically, but the energy surfaces are remarkably flat overall, with several distorted and disordered structures being energetically close. Nevertheless, microscopic geometries are found to be critical for the ionic response. Local disorder leads to larger permittivities, but also increased losses. [1] Wu et al, Advanced Material, 25, 1734 (2013). [2] Wang et al, APL 94, 202905 (2009).   

Effect of Organic Blocking Layer on the Energy Storage Characteristics of High-Permittivity Sol-Gel Thin Film Based on Neat 2-Cyanoethyltrimethoxysilane

Organic-inorganic hybrid sol-gel materials with polar groups that can undergo reorientational polarization provide a potential route to dielectric materials for energy storage. We have investigated the influence of nanoscale polymeric layer on dielectric and energy storage properties of 2-cyanoethyltrimethoxysilane (CNETMS) films. Two polymeric materials, fluoropolymer (CYTOP) and poly(p-phenylene oxide, PPO), are examined as potential materials to control charge injection from electrical contacts into CNETMS films by means of a potential barrier, whose width and height are defined by thickness and permittivity. Blocking layers ranging from 20 nm to 200 nm were deposited on CNETMS films by spin casting and subjected to thermal treatment. Polarization-electric field measurements show 30\% increase in extractable energy density with PPO/CNETMS bilayers, relative to CNETMS alone, due to improved breakdown strength. Conduction current of the bilayers indicate that onset of charge conduction at high field is much delayed, which can be translated into effective suppression of charge injection and probability of breakdown events. The results will be discussed in regards to film morphology, field partitioning, width and height of potential barrier, charge trapping and loss of bilayers.   

Structured block copolymer thin film composites for ultra-high energy density capacitors

Development of high energy density capacitors is essential for future applications like hybrid vehicles and directed energy weaponry. Fundamentally, energy density is governed by product of dielectric permittivity $\varepsilon $ and breakdown strength V$_{bd}$. Hence, improvements in energy density are greatly reliant on improving either $\varepsilon $ or V$_{bd}$ or a combination of both. Polymer films are widely used in capacitors due to high Vbd and low loss but they suffer from very low permittivities. Composite dielectrics offer a unique opportunity to combine the high $\varepsilon $ of inorganic fillers with the high V$_{bd}$ of a polymer matrix. For enhancement of dielectric properties, it is essential to improve matrix-filler interaction and control the spatial distribution of fillers for which nanostructured block copolymers BCP act as ideal templates. We use Directed Self-assembly of block copolymers to rapidly fabricate highly aligned BCP-TiO2 composite nanostructures in thin films under dynamic thermal gradient field to synergistically combine the high $\varepsilon $ of functionalized TiO2 and high V$_{bd}$ of BCP matrix. The results of impact of BCP morphology, processing conditions and concentration of TiO2 on capacitor performance will be reported.   

Development of In-situ Resonant Soft X-ray Scattering for Soft Materials at Advanced Light Source

Resonant Soft X-ray Scattering was developed at ALS over the past a few years. It combines soft x-ray spectroscopy with x-ray scattering and offers statistical information for 3D chemical morphology over a large sample area. Its unique chemical sensitivity, large accessible size scale, polarization control and high coherence make it a powerful tool for mesoscale chemical/morphological structure characterization for many classes of materials. However, in order to study sciences in naturally occurring conditions, we need to overcome the sample limitations set by the low penetration depth of soft x-rays and requirement of high vacuum. Adapting to the evolving environmental cell designs utilized increasingly in the Electron Microscopy community, we will report our development of customize design liquid/gas environmental cells that will enable soft x-ray scattering experiments on biological, electro-chemical, self-assembly, and hierarchical functional systems in both static and dynamic fashion. Initial RSoXS result of solar fuel membrane assembly/fuel-cell membrane structure in wet cell will be presented.   

Structure and Water Transport in Nafion Nanocomposite Membranes

Perfluorinated ionomers, specifically Nafion, are the most widely used ion exchange membranes for vanadium redox flow battery applications, where an understanding of the relationship between membrane structure and transport of water/ions is critical to battery performance. In this study, the structure of Nafion/SiO$_{2}$ nanocomposite membranes, synthesized using sol-gel chemistry, as well as cast directly from Nafion/SiO$_{2}$ nanoparticle dispersions, was measured using both small-angle neutron scattering (SANS) and ultra-small-angle neutron scattering (USANS). Through contrast match studies of the SiO$_{2}$ nanoparticles, direct information on the change in the structure of the Nafion membranes and the ion-transport channels within was obtained, where differences in membrane structure was observed between the solution-cast membranes and the membranes synthesized using sol-gel chemistry. Additionally, water sorption and diffusion in these Nafion/SiO$_{2}$ nanocomposite membranes were measured using in situ time-resolved Fourier transform infrared-attenuated total reflectance (FTIR-ATR) spectroscopy and dynamic vapor sorption (DVS).   

Role of Substrate/Film interactions in Controlling Structure and Swelling of Nafion Thin Films

Nafion is the prototypical ionomer in electrochemical energy devices due to its good ionic conductivity and permselectivity. In most devices, bulk ionomers are in contact with the catalysts. When confined to nanometer-thick 'thin' films, Nafion's structure/property relationship deviate from bulk, resulting in a complex polymer behavior dependent on thickness, environmental and casting conditions, and substrate material. In this talk, results of a systematic investigation on the substrate/film interactions of Nafion will be presented. The nanostructure of hydrated films is studied by Grazing-incidence X-Ray Scattering and analyzed along with swelling and water uptake measured by ellipsometry and QCM. Overall, films exhibit phase-separation with 4 to 6nm water-domain spacing and 10 to 15{\%} swelling. Film thickness has a universal impact on properties such that thicker films (ca. 100nm) behave like bulk, whereas thin films (20 to 100nm) exhibit confinement effects with reduced swelling, regardless of the substrate. However, thin(ner) films (ca. 20nm) have no separated-structure and demonstrate significant swelling. Moreover, metallic substrates induce more ordered and anisotropic structure accompanied by additional reduction in swelling.   

Analytical model describes ion conduction in fuel cell membranes

Many fuel cell designs employ polyelectrolyte membranes, but little is known about how to tune the parameters (water level, morphology, etc.) to maximize ion conductivity. We came up with a simple model based on a random, discrete water distribution and ion confinement due to neighboring polymer. The results quantitatively agree with molecular dynamics (MD) simulations and explain experimental observations. We find that when the ratio of water volume to polymer volume, $V_{w}/V_{p},$ is small, the predicted ion self-diffusion coefficient scales roughly as $D_{w}\left(T\right)\sqrt{V_{w}/V_{p}}\exp(-\cdots V_{p}/V_{w}),$ where $D_{w}\left(T\right)$ is the limiting value in pure water at temperature $T.$ At high water levels the model also agrees with MD simulation, plateauing to $D_{w}\left(T\right).$ The model predicts a maximum conductivity at a water level higher than is typically used, and that it would be beneficial to increase water retention even at the expense of lower ion concentration. Also, membranes would conduct better if they phase-separated into water-rich and polymer-rich regions.   

Thermal-induced changes in Transport Properties of PFSA Ionomers

Perfluorosulfonic-acid ionomers are widely used as the solid-electrolyte in electrochemical energy applications due to their remarkable conductivity and chemical/mechanical stability. Driven by achieving even higher conductivities, it is of interest to increase ion-exchange capacities without deteriorating the mechanical stability. Heat-treatments are commonly employed to change the balance between chemical and mechanical properties, where the latter can be enhanced by annealing-induced crystallinity at the expense of reduced conductivity. In this talk, we focus on how the annealing time membrane undergoes results in non-monotonic changes in its nanostructure, crystallinity and ion conductivity. Hydrophilic domains and crystallinity of the annealed samples, studied by Small- and Wide-angle X-Ray scattering, are correlated to their swelling and conductivity. Our results suggest that the conductivity can be enhanced by optimizing the annealing procedure for the ionomer. However, over a long period of annealing, conductivity and crystallinity of the ionomer appear to decrease and increase, respectively, although by preserving the overall chemical/mechanical balance. Our findings provide new insights into the thermal treatments in altering the structure/function relationship of ionomers due to their non-equilibrium state.   

Session J22: Focus Session: Biological and Bio-Inspired Adhesive Polymers II

Switchable adhesion of liquid crystalline elastomers

Liquid crystal elastomers (LCEs) are rubbery materials that composed of liquid crystalline polymers (LCPs) crosslinked into a network. The rod-like mesogens incorporated into the LCPs are have random orientations in the high temperature isotropic phase, but can adopt the canonical liquid crystalline phases as the temperature is lowered. LCEs have not yet found a key application, however, these materials are highly dissipative. I will describe a proposed application of reversibly switchable pressure sensitive adhesives (PSAs). The quality of their adhesion can be measured by the tack energy. To investigate their performance as switchable PSAs we compare the tack energy for the director aligned parallel, and perpendicular to the substrate normal, with that for the isotropic state using a finite element model that incorporates cavitation within the adhesive layer. The constitutive properties of the LCE are modelled using the nematic dumbbell model. We find that the tack energy depends on the director orientation, with parallel orientation of the nematic having higher tack energy than both the isotropic and the perpendicular director orientation [1]. I will report on how this model compares with recent experiments on LCE PSAs. [1] Soft Matter, 2013,9, 1151-1163   

Measurement of depletion-induced force in microtubule bundles

Microtubule (MT) bundles formed in the presence of non-adsorbing polymers - poly-ethylene glycol (PEG) or Dextran - are widely used in experimental active matter systems. However, many properties of such MT bundles have not been studied experimentally. In this work, we combine optical trapping techniques with an umbrella sampling method in order to measure the depletion force acting on individual microtubule in the axial direction within the bundle. We find depletion force is independent of bundle overlap length and measure its magnitude to be on the order of tens of $\frac{k_{B}T}{\mu m}$. We explore the dependence of the depletion force on concentration of depletant (PEG 20K) as well as $K^{+}$ ions (necessary for screening electrostatic repulsion between MT filaments). We also verify additivity of depletion interaction and confirm that force is increased by a factor of two for three-MT bundles. Additionally, our experimental technique allows us to probe interactions between MTs within the bundle. Experimental data suggests that filaments in the bundle interact only hydrodynamically when depletant concentrations are low enough; however, we observe onset of solid-like friction when osmotic pressure is increased above a certain threshold.   

Hybrid metal-coordinate transient networks: using bio-inspired building blocks to engineer the mechanical properties of physical hydrogels

Recently, metal-coordinate complex crosslinks have been suggested to contribute to the self-healing properties of mussel byssi. Two specific amino acid derivatives - 3,4 dihydroxy-L-phenylalanine (dopa) and histidine (his) - are known to form coordinate complexes with trivalent and divalent ions (respectively) in aqueous solutions. We show here that, by functionalizing poly(ethylene glycol) polymers with dopa and his we are (1) able to characterize the fundamental kinetics and energetics of each specific metal-ligand pair using small amplitude oscillatory shear rheology and (2) create hybrid networks using various mixtures of metals and ligands. From this information, we can design gels with specific target mechanical properties by tailoring the amounts and types of metal-ligand crosslinks present in the gel network, resulting in the ability to engineer the mechanical relaxation spectrum. This work provides basic understanding necessary to intelligently design materials which incorporate metal-ligand crosslinks in more complex architectures.   

Predictive relationships between crosslinker unbinding kinetics, gel stiffness, and plasticity in adhesive biopolymers

We determine the viscoelastic responses of rigid rod polymer networks that have been strongly bonded by labile crosslinkers. Experimentally, we use microtubules, extremely stiff biopolymers that play important roles in maintaining the strength and organization of cells. We generate controllable adhesive bonds using well-characterized protein chemistries, such as biotin-streptavidin bonds, or using recombinant microtubule-associated proteins. Networks are visualized using confocal scanning fluorescence microscopy or transmission electron microscopy, and custom-built, high-force magnetic tweezers devices are used to apply localized forces to the gels. For rigid crosslinkers, we find that at short time scales, the networks respond nonlinearly to applied force, with stiffening at small forces, followed by a softening regime, which we attribute to the force-induced unbinding of crosslinkers. At long time scales, force-induced bond breakage leads to local network rearrangement and significant bead creep. Interestingly, the material retains its elastic modulus even under conditions of significant plastic flow, suggesting that crosslinker breakage is balanced by the formation of new bonds. These results provide important insight into the determinants of gel toughness, elasticity, and plastic deformation in rigid networks, but also suggest new avenues for materials optimization based on modulation of crosslinker kinetics. In particular, the incorporation of crosslinkers that break under force, but are competent to reform when the force is removed, significantly enhance gel toughness while minimizing material fatigue under cyclic loading.   

DNA Gel with dynamic cross-links

The mechanical properties of a living cell are strongly related to the cytoskeletal network, which is comprised of diverse protein filaments connected by cross-linking proteins, some of which are dynamic. Gels comprised of dynamic cross-linkers exhibit unique mechanical properties not seen in those using permanent cross-linkers [1,2]. To investigate the effect of a dynamic cross-linker on mechanical properties of a material, we have synthesized biopolymer gels with a well-known semi-flexible biopolymer, DNA, and probed the mechanics of the system using microrheological techniques. We discuss these results in comparison to cytoskeletal systems, and seek to establish universal principles of dynamic cross-link based gels. - References 1. S. M. V. Ward, A. Weins, M. R. Pollak, D. a Weitz, Dynamic viscoelasticity of actin cross-linked with wild-type and disease-causing mutant alpha-actinin-4., Biophys. J. 95, 4915--23 (2008). 2. C. P. Broedersz et al., Cross-Link-Governed Dynamics of Biopolymer Networks, Phys. Rev. Lett. 105, 238101 (2010).   

Bio-inspired adhesion: local chemical environments impact adhesive stability

3,4-dihydroxyphenylalanine (Dopa) is an amino acid that is naturally synthesized by marine mussels and exhibits the unique ability to strongly bind to surfaces in aqueous environments. However, the Dopa functional group undergoes auto-oxidation to a non-adhesive quinone form in neutral to basic pH conditions, limiting the utilization of Dopa in biomedical applications. In this work, we performed direct surface force measurements with in situ electrochemical control across a Dopa-rich native mussel foot protein (mfp-5), as well as three simplified model peptide sequences. We find that the neighboring peptide residues can significantly impact the redox stability of Dopa functional groups, with lysine residues imparting a substantial degree of Dopa redox stabilization. Surprisingly, the local chemical environments only minimally impact the magnitude of the adhesion forces measured between molecularly-smooth mica and gold surfaces. Our results provide molecular level insight into approaches that can be used to mitigate the detrimental impact of Dopa auto-oxidation, thus suggesting new molecular design strategies for improving the performance of Dopa-based underwater adhesives.   

Multi-scale models for cell adhesion

The interactions of membrane receptors during cell adhesion play pivotal roles in tissue morphogenesis during development. Our lab focuses on developing multi-scale models to decompose the mechanical and chemical complexity in cell adhesion. Recent experimental evidences show that clustering is a generic process for cell adhesive receptors. However, the physical basis of such receptor clustering is not understood. We introduced the effect of molecular flexibility to evaluate the dynamics of receptors. By delivering new theory to quantify the changes of binding free energy in different cellular environments, we revealed that restriction of molecular flexibility upon binding of membrane receptors from apposing cell surfaces (trans) causes large entropy loss, which dramatically increases their lateral interactions (cis). This provides a new molecular mechanism to initialize receptor clustering on the cell-cell interface. By using the subcellular simulations, we further found that clustering is a cooperative process requiring both trans and cis interactions. The detailed binding constants during these processes are calculated and compared with experimental data from our collaborator's lab.   

Semiflexible networks with labile crosslinkers: Bundling, rheology, ripping, and healing

Networks of semiflexible filaments may be cross-linked by molecules that unbind and then rebind in different places throughout the network. The structure of such networks in equilibrium is dynamic. That structure will also evolve in time either in the relaxation towards equilibrium, or in response to external perturbations such as applied stress. Cross linker mobility leads to new rheological features that depend on e.g., the degree of filament bundling, and allows for new dissipative mechanisms related to cross linker unbinding and rebinding in the networks under applied mechanical load. In this talk, I present the results of analytic calculations and numerical simulations exploring the effect of labile cross linkers on the rheology and structural evolution of semiflexible networks. Specifically, I discuss the fluctuation-induced or Casimir interactions between cross linkers in a semiflexible filament network. I also report on the linear response of such network to applied shear, particularly for the case where the cross linkers induce filament bundling. In that case, there is a universal high-frequency bundle rheology distinct from that of semiflexible filament networks. Cross linker unbinding leads to new dissipative mechanisms, and there is a new low frequency, non-Newtonian rheological regime associated with bundle dissolution. Finally, I comment on the nonlinear response of these networks to applied stress, examining the role of cross linker unbinding (and rebinding) on the energy dissipation in, and the plastic deformation of the network under a time-independent applied load.   

Session J23: Invited Session: Industrial Physics Forum: Frontiers of Physics

Probing the Last 13.8 Billion Years in the Universe with the Atacama Cosmology Telescope

The Atacama Cosmology Telescope (ACT) is a 6 m special purpose telescope designed to measure the cosmic microwave background (CMB) at millimeter wavelengths. ACT has an angular resolution of better than 1.4', which means it measures not only the primordial fluctuations in the CMB, but is also sensitive to the intervening universe in several ways. ACT observes from a site at 5300 m in the Atacama Desert in Chile. This midlatitude site allows ACT to map regions of the sky in which there exist substantial data from surveys at other wavelengths. ACT detects clusters of galaxies through their Sunyaev-Zeldovich effect (a spectral effect due to scattering off the hot electrons in the clusters). ACT measures clusters directly, in blind surveys, and also makes statistical measurements based on stacking analyses and by measuring the 3-point function in the maps. Furthermore, gravitational lensing by all the intervening matter from the primordial epoch to now leads to signatures in the 4-point functions in the ACT maps. Cross-correlating the ACT lensing deflection field with other optical surveys in the same region is a particularly fruitful way of deriving cosmological information on the expansion history of the universe.   

Power blackouts, sudden death, and flash crashes: The physics of interdependent networks

Recent disasters ranging from abrupt financial ``flash crashes'' and large-scale power outages to sudden death among the elderly dramatically exemplify the fact that the most dangerous vulnerability is hiding in the many interdependencies among different networks. This talk reports recent work quantifying failure mechanisms in interconnected networks, and demonstrates the need to consider mutually dependent network properties in designing resilient systems. Specifically, we have uncovered new laws governing the nature of switching phenomena in coupled networks, and found that phenomena that are continuous ``second order'' phase transitions in isolated networks become discontinuous abrupt ``first order'' transitions in interdependent networks [J. Gao, S. V. Buldyrev, H. E. Stanley, and S. Havlin, ``Novel Behavior of Networks Formed from Interdependent Networks,'' Nature Physics {\bf 8}, 40 (2012)]. We also report parallel efforts to understand the phenomenon of spontaneous recovery in dynamical networks as occurs, e.g., immediately after a flash crash [A. Majdandzic, B. Podobnik, S. V. Buldyrev, D. Y. Kenett, S. Havlin, and H. E. Stanley, ``Spontaneous Recovery in Dynamic Networks,'' Nature Physics {\bf 9}, No. 1 (2014)].   

Exploring the Habitability Potential of Mars with Mars Science Laboratory

Curiosity has been roving Gale Crater since landing on Mars August 5, 2013. The investigations that comprise the Mars Science Laboratory payload have interrogated the environment in as comprehensive an approach as has ever been attempted on the surface of another planet, using a variety of approaches to characterize both the surface materials and the atmosphere. This talk will summarize the Curiosity's progress at Gale Crater.   

Cryogenic Semiconductor Detectors in Search of Dark Matter

Dark Matter dominates the matter content in the Universe and is believed to be made up of Weakly Interacting Massive Particles (WIMP) that rarely interact with ordinary matter. Cryogenic Dark Matter Search (CDMS) has been a leader among more than 30 experiments worldwide, which are attempting to detect tiny vibrations from the recoil of WIMPs in terrestrial detectors. It uses sophisticated photo-lithographically patterned cryogenically cooled large mass Germanium and Silicon. Help from the semiconductor industry has been crucial in reducing the cost 20 fold from half-million/kg, while simultaneously improving the quality and throughput of fabrication, essential for large ton-scale experiments capable of making such a discovery possible.   

The Physics of Cosmic Rays

Cosmic rays, mostly relativistic protons, comprise one billionth of interstellar particles by number, but have as much energy as the rest of the interstellar gas combined. They are probably accelerated in supernova remnants, and are confined to the Galaxy by the interstellar magnetic field. Through interacting with the field, they exchange energy and momentum with the interstellar gas, driving magnetic turbulence and outflows, and generating significant heat. An even smaller minority of cosmic rays, those with the highest energies, probably originate outside the Galaxy, and challenge all existing theories of how they are accelerated.   

Session J24: Photovoltaics, including Thermal, LSC, Oxides and Device Physics

Exploring Near-Field Radiative Heat Transfer for Thermo-photovoltaic Applications

Understanding near-field radiative heat transfer (NFRHT) is critical for developing efficient thermo-photovoltaic devices. Theoretical predictions suggest that when the spatial separation of two parallel planes at different temperatures is less than their Wien's thermal wavelength, thermal transport via radiation can be greatly enhanced. The radiative heat flow across nanoscale gaps is predicted to be orders-of-magnitude higher than that given by Stefan-Boltzmann law, due to contribution of evanescent waves. In order to test these predictions, a novel experimental platform was designed and built enabling parallelization of two planar surfaces (50 $\mu$m~by~50 $\mu$m) with 500 microradian resolution in their relative orientation. This platform was used to probe NFRHT between two planes and also between a plane and a sphere. It was found that, when a 50 $\mu$m diameter silica sphere was approximately 20 nm away from a 50 by 50 $\mu$m$^{2}$~silica plane, a significant increase in radiative heat transfer coefficient was observed. This increase is 3 orders of magnitude higher than the value predicted by the blackbody limit. Other setups, including Au spheres and planes, and the plane-plane geometries are currently being investigated.   

Heterostructure designs for photon-enhanced thermionic emission

Photon-Enhanced Thermionic Emission (PETE) is a promising method of solar energy conversion that relies on photoexcitation and thermionic emission into vacuum, combining quantum and thermal approaches into a single mechanism. We have previously reported a heterostructure design that separates the PETE process from the process of vacuum emission, which resulted in a large improvement in quantum efficiency to 1-2{\%}, compared to 10\textasciicircum 4 electrons per photon in proof of concept measurements. In addition to this performance improvement, the heterostructure architecture also creates the opportunity to separately target internal PETE and vacuum emission. In this talk, we describe designs which can be used to independently study these mechanisms.   

An Isothermal Device Configuration for Diamond Based Photon-Enhanced Thermionic Solar Energy Conversion

Diamond can obtain a negative electron affinity (NEA) after hydrogen termination. With NEA and n-type doping, a low effective work function and efficient thermionic emission has been observed from these diamond films. Photo-induced electron emission from nitrogen doped diamond with visible light illumination has also been established by our group. Recently several reports have described efficient energy conversion based on the photon-enhanced thermionic emission (PETE) mechanism. This study proposes a multi-layer emitter and collector structure for an isothermal PETE converter. The emitter structure is based on an n-type NEA diamond film deposited on a p-type Si substrate to enable electron emission across a vacuum gap. In this structure the above-bandgap light is absorbed in the Si and establishs an enhanced electron population for emission through the low work function surface, while sub-bandgap light is absorbed in the collector for transfer to a heat engine. Spectroscopy measurements of the n-type diamond on Si indicate strong electron emissivity with photon illumination, and the emission intensity is significantly increased at elevated temperatures. A simplified model describing the efficiency and performance of an isothermal PETE device is presented.   

Nanoscale Radiative Heat Transfer between a Scanning Probe and a Flat Surface

Fluctuational electrodynamics based calculations predict a significant increase in the efficiency of thermophotovoltaic devices when an emitter is placed in the close proximity of an appropriately designed photovoltaic (PV) cell. The enhancement is expected to be further increased if the emissive properties of the emitter are matched to the band gap of the PV cell via nanostructuring. However, before this can be accomplished, it is necessary to better understand the underlying physics. This is especially true given the discrepancies seen between published experimental and theoretical studies. Here we present our measurements of nanoscale radiative heat transfer between the tip of scanning probes and an atomically flat surface spatially separated by very small gaps (1-10 nm). The experiments were performed in a UHV environment using custom-developed scanning probed with picowatt heat-flow resolution. Current measurements show significant deviations from computational predictions. We are currently studying radiative thermal transport between a range of materials to reveal the contribution of important effects such as non-locality and eddy currents.   

Non-tinted Transparent Luminescent Solar Concentrators Employing Both UV and NIR Selective Absorbers

Luminescent solar concentrators are a potentially low-cost solar harvesting solution that additionally offer opportunities for integration around buildings and windows. However, the visible absorption and emission of previously demonstrated chromophores hamper their widespread applications including solar windows. Here, we demonstrate non-tinted transparent luminescent solar concentrators (TLSC) that employ both ultraviolet (UV) and near-infrared (NIR) selective absorbing luminophores that create an entirely new paradigm for power-producing transparent surfaces and enhances the potential over UV-only TLSCs. We have previously designed UV-harvesting systems composed of metal halide phosphorescent luminophore blends that enable absorption cutoff positioned at the edge of visible spectrum (430nm) and massive-downconverted emission in the near-infrared (800nm) with quantum yields for luminescence of 75{\%}. Here, we have developed a complimentary TLSC employing fluorescent organic salts with both efficient NIR absorption and deeper NIR emission. We will discuss the photophysical properties of these luminophores, the impact of ligand-host control, and optimization of the TLSC architectures.   

Particle scattering applications in solar panels

The focus of this work is to apply the scattering characteristics of particles to model particle assisted solar concentrators. In this work, the scattering patterns of particles of different shapes, sizes, and refractive indices are computationally studied using Discrete Dipole Approximation (DDA). The study investigates the optical behavior of different particle ensembles. The simulated results are used to explain the characteristic behavior seen in [1]. The computational methodology can be used to determine the ideal ensemble of particles to produce the most efficient energy yield in a scattering-based photovoltaic concentrator.\\[4pt] [1] J. Wen, M. J. Berg, M. Steed, ``Scattering-based solar concentrator,'' \emph{Opt. Express} (submitted in review, 2013).   

Wavelength-Selective Photovoltaics for Power-generating Greenhouses

While photovoltaic (PV) technologies are being developed that have the potential for meeting the cost target of \$0.50/W per module, the cost of installation combined with the competition over land resources could curtail the wide scale deployment needed to generate the Terrawatts per year required to meet the world's electricity demands. To be cost effective, such large scale power generation will almost certainly require PV solar farms to be installed in agricultural and desert areas, thereby competing with food production, crops for biofuels, or the biodiversity of desert ecosystems. This requirement has put the PV community at odds with both the environmental and agricultural groups they would hope to support through the reduction of greenhouse gas emissions. A possible solution to this challenge is the use of wavelength-selective solar collectors, based on luminescent solar concentrators, that transmit wavelengths needed for plant growth while absorbing the remaining portions of the solar spectrum and converting it to power. Costs are reduced through simultaneous use of land for both food and power production, by replacing the PV cells by inexpensive long-lived luminescent materials as the solar absorber, and by integrating the panels directly into existing greenhouse or cold frames. Results on power generation and crop yields for year-long trials done at academic and commercial greenhouse growers in California will be presented.   

Angle-Dependent Performance in Thin-Film and Transparent Photovoltaics

Understanding the angle dependent performance is an important consideration for building integrated photovoltaics (PVs), such as transparent PV windows, where illumination angles are rarely at normal incidence. While the transfer matrix model (TMM) has been widely utilized to model optical interference and quantum efficiency in thin-film PVs at normal incidence, self-consistent simulations for PVs under oblique illumination have not yet been demonstrated. We derive an updated model that is self-consistent for all angles, light polarizations, and electrical / optical configurations, and experimentally verify the predicted angular quantum efficiency response of planar heterojunction (PHJ) transparent PVs. We subsequently use this model to optimize PHJ transparent PVs for maximum short circuit photocurrent density (J$_{sc})$ and transparency as a function of the multivariable landscape under a variety of optical and electrical configurations, showing that it is possible to greatly reduce the angle-dependent roll-off in efficiency by moving in this multi-parameter space. We will provide insights into the lesson learned for designing devices that can reduce this roll-off and increase overall yearly power output.   

Absorption and emission of NIR fluorophores for use in Wavelength Selective Solar Concentrators

Wavelength Selective Solar Concentrators (WSSCs) offer a variety of applications as compared to traditional solar panels. Exploiting the property of power generation with transmission, we have turned our attention to the greenhouse industry. Our current design employs an organic dye (Lumogen Red 305) with an excitation peak in green wavelengths and an emission peak in red wavelengths, specifically targeting wavelengths unused in photosynthesis. To increase the efficiency of the WSSC without disrupting either existing function, we explore the addition of NIR dyes. Presented are the absorption and emission peaks of the dyes deposited into poly-vinyl butyral (PVB), polymethyl methacrylate (PMMA), and (TPU); the quantum yields of these films; and the combined spectra of the dyes with LR305.   

Detailed Balance Comparison of the Series and Non-Series Tandem Solar Cell

Using thin film multijunction photovoltaics to achieve higher power conversion efficiency has been proposed as a means to lower costs in next generation solar cells. As they are typically constructed, each cell is connected in series to the next, with each cell having a band-gap optimized to more efficiently harvest energy from a subset of the solar spectrum. However, the series-constrained solar cell limits the range of materials compatible because any excess current generated by any cell in the structure is lost. In this work, we use the detailed balance analysis developed by Shockley and Queisser to show how inserting a third transparent electrode between two active layers can dramatically increase the range of materials for which a tandem structure can break present efficiency limits. We show that the non-series architecture exceeds 40{\%} power conversion efficiency for five times as many band-gap combinations as the series tandem, significantly expanding the materials phase-space available to researchers. We apply this analysis to the case study of thin film structures built on the ubiquitous silicon platform.   

Computational characterization of optical and thermodynamic properties of bulk zinc stannate (Zn2SnO4)

$Zn_2SnO_4$ (ZTO) is an important material with a wide band gap. It is often used in novel device designs such as quantum dot- and die-sensitized solar cells. The crystal structure of ZTO is inverse spinel, with the general formula $AB_2O_4$. In inverse spinel A and B atoms share the occupation of octahedral sites 0.5/0.5, while the exact occupation is often unknown. Here we study configuration space of ZTO with DFT and derive cluster expansion model. We find temperature dependence for the occupation of octahedral sites and demonstrate that the lowest energy ground state configuration is stable at the normal range of temperatures. Because of the large unit cell (56 atoms) the calculation of optical properties with many-body methods appeared to be impractical and we compute band structure with DFT using Tran-Blaha correlation functional. The optical band gap we obtain with this method matches experimental value.   

Zinc stannate as a solar cell material

Semiconducting ferroelectric materials are attractive as solar absorbers because they have a built in polarization that facilitates electron-hole separation and can drive carriers to opposite ends of the device. ZnSnO$_3$ is an exciting material that has recently been shown to be ferroelectric with a relatively large (~50 $\mu$C/cm$^2$) remnant polarization. It holds potential as a solar absorber because it is a direct gap material composed of relatively cheap, abundant, and non-toxic elements. The bandgap of ideal ZnSnO$_3$ is too large to make it an efficient solar absorber. However, like many semiconducting oxides containing tin, the bulk bandgap is an extremely strong function of the lattice constant. In fact, just a few percent change in the lattice constant of ZnSnO3$_3$ can alter its bandgap by as much as a factor of 2-4. This opens the possibility of tuning the bandgap by applying a slight epitaxial strain, which can be accomplished by affixing a ZnSnO$_3$ film to a substrate with a modest lattice mismatch. In this work we use sophisticated methods (DFT and GW) to identify materials that can be affixed to a ZnSnO$_3$ film, modifying its bandgap to a near optimal value. Attention will be paid to the bandgap, band alignments, and the thermodynamics of the interfaces.   

First-principles materials design of high-performing bulk photovoltaics with the LiNbO3 structure

The bulk photovoltaic effect describes the ability of inversion symmetry breaking materials to produce intrinsic photocurrents and photovoltages. Recently, we have previously demonstrated the ability to compute, from first principles, the bulk photocurrent using the theory of ``shift current,'' and have successfully reproduced experimental results. This ability has allowed for understanding of the structural and chemical properties generating large bulk photovoltaic response and the design of high-performing materials. In this talk we present three polar oxides with the LiNbO$_3$ structure that we predict to have band gaps in the 1-2 eV range and very high bulk photovoltaic response: PbNiO$_3$, Mg$_{1/2}$Zn$_{1/2}$PbO$_3$, and LiBiO$_3$. The first is has already been synthesized, and the others are very similar to known materials. All three have band gaps determined by cations with $d^{10}s^0$ electronic configurations, leading to conduction bands composed of cation $s$-orbitals and O $p$-orbitals. This both dramatically lowers the band gap and increases the bulk photovoltaic response by as much as an order of magnitude over previous materials, demonstrating the potential for high-performing bulk photovoltaics.   

Quantitative high-resolution mapping of built-in fields in polycrystalline photovoltaic devices using electron beams: effects of surface band bending and recombination

Thin film solar cells are based on polycrystalline materials that are structurally and electronically non-uniform. The power conversion efficiency of these inhomogeneous devices is currently well below theoretical limits. To effectively mitigate the recombination sources and further boost the efficiency in such systems, it is highly desirable to understand how the grain cores (GCs), grain boundaries (GBs), and other local variations of composition affect the overall photoelectronic properties of devices. Electron beam induced current (EBIC) is a powerful technique which directly measures the local collection efficiency of excited charge carriers. To achieve the desired high spatial resolution, the size of the electron-hole bulbs has to be minimized, which, in turn, means that the carriers are created within an immediate proximity of the exposed surface. We systematically examine the surface contribution to EBIC by comparing different solar cell devices varying surface preparation and passivation methods, and by analyzing the injection level-dependence of EBIC. We discuss new approaches to quantify the surface field and recombination including EBIC measurements in thin lamella geometry, beam injection parallel to the surface, and in-situ gating.   

Session J25: Focus Session: Materials for Electrochemical Energy Storage: Layered Materials and Capacitors

Origins of Lithium-Carbon Binding in Carbon-based Lithium-ion Battery Anodes

Many key performance characteristics of carbon-based lithium-ion battery anodes are determined by the strength of binding between lithium (Li) and $sp^2$ carbon (C). Using extensive density functional theory calculations, we investigate the detailed interaction of Li with a wide variety of $sp^2$ C substrates, including pristine, defective, and strained graphene; planar C clusters; nanotubes; C edges; and multilayer stacks. We find that in almost all cases, the Li-C binding energy scales is determined largely by the work required to fill unoccupied carbon states, suggesting that intrinsic quantum capacitance is important for predicting Li capacity. This allows the binding energy and capacity to be estimated based solely on the electronic structure of the substrate. It also provides a connection to carbon-based supercapacitors, and underscores the role of electronic structure in interfacial electrochemical systems. Implications for improving the effective capacity of carbon-based anodes will be discussed. This work was performed under the auspices of the U.S. DOE by LLNL under Contract DE-AC52-07NA27344.   

Fe-catalyzed carbon nanotubes for high-energy density carbon-based supercapacitors

Carbon nanotubes (CNTs) are one of the most suitable supercapacitor electrode materials due to their high mechanical strength, electrical conductivity, and surface area. Albeit these unique properties of CNTs, energy density of carbon-based double layer capacitors is limited by the inability of CNTs to actively participate in redox processes. Here, we show that electrochemical characteristics of CNTs can be improved by activating the residual Fe catalyst to participate in Faradaic charge storage via Fe$^{\mathrm{2+}}$ -\textgreater Fe$^{\mathrm{3+}}$ redox process. By using traditional liquid injection chemical vapor deposited CNTs which contains 5.7 wt.{\%} residual Fe catalyst (R. Andrews et al.,, \textit{Chem. Phys. Letters},~\textbf{303}, 467-474 (1999)), the capacitance of CNT electrodes can be increased from 20 F/g to 150 F/g, in the range of -0.2 to 1.2 V. The use of Fe containing CNTs to manufacture supercapacitor electrodes with increased energy density and charge capacity of with high charge/discharge rates with extremely long-term cycle stability will be discussed.   

Performance of Liquid Phase Exfoliated Graphene As Electrochemical Double Layer Capacitors Electrodes

We will present the results of our investigations of electrochemical double layer capacitors (EDLCs) or supercapacitors (SC) fabricated using liquid-phase exfoliated graphene. Several electrolytes, such as aqueous potassium hydroxide KOH (6M), ionic 1-Butyl-3-methylimidazolium hexafluorophosphate [BMIM][PF$_{6}$], and ionic 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate[BMP][FAP] were used. These EDLC's show good performance compared to other carbon nanomaterials based EDLC's devices. We found that the liquid phase exfoliated graphene based devices possess specific capacitance values as high as 262 F/g, when used with ionic liquid electrolyte[BMP][FAP], with power densities ($\sim$ 454 W/kg) and energy densities ($\sim$ 0.38Wh/kg). Further, these devices indicated rapid charge transfer response even without the use of any binders or specially prepared current collectors. A detailed electrochemical impedance spectroscopy analysis in order to understand the phenomenon of charge storage in these materials will be presented.   

Roll-to-Roll production of carbon nanotubes based supercapacitors

Carbon nanomaterials provide an excellent platform for electrochemical double layer capacitors (EDLCs). However, current industrial methods for producing carbon nanotubes are expensive and thereby increase the costs of energy storage to more than {\$}10 Wh/kg. In this regard, we developed a facile roll-to-roll production technology for scalable manufacturing of multi-walled carbon nanotubes (MWNTs) with variable density on run-of-the-mill kitchen Al foils. Our method produces MWNTs with diameter (heights) between 50-100 nm (10-100 $\mu$m), and a specific capacitance as high as $\sim$ 100 F/g in non-aqueous electrolytes. In this talk, the fundamental challenges involved in EDLC-suitable MWNT growth, roll-to-roll production, and device manufacturing will be discussed along with electrochemical characteristics of roll-to-roll MWNTs.   

Biopolymer-nanocarbon composite electrodes for use as high-energy high--power density electrodes

Supercapacitors (SCs) address our current energy storage and delivery needs by combining the high power, rapid switching, and exceptional cycle life of a capacitor with the high energy density of a battery. Although activated carbon is extensively used as a supercapacitor electrode due to its inexpensive nature, its low specific capacitance (100-120 F/g) fundamentally limits the energy density of SCs. We demonstrate that a nano-carbon based mechanically robust, electrically conducting, free-standing buckypaper electrode modified with an inexpensive biorenewable polymer, viz., lignin increases the electrode's specific capacitance ($\sim$ 600-700 F/g) while maintaining rapid discharge rates. In these systems, the carbon nanomaterials provide the high surface area, electrical conductivity and porosity, while the redox polymers provide a mechanism for charge storage through Faradaic charge transfer. The design of redox polymers and their incorporation into nanomaterial electrodes will be discussed with a focus on enabling high power and high energy density electrodes.   

Structure of Room Temperature Ionic Liquids on Charged Graphene: An integrated experimental and computational study

Electrical double layer capacitors (EDLCs) with room temperature ionic liquid (RTIL) electrolytes and carbon electrodes are promising candidates for energy storage devices with high power density and long cycle life. We studied the potential and time dependent changes in the electric double layer (EDL) structure of an imidazolium-based room temperature ionic liquid (RTIL) electrolyte at an epitaxial graphene (EG) surface. We used \textit{in situ} x-ray reflectivity (XR) to determine the EDL structure at static potentials, during cyclic voltammetry (CV) and potential step measurements. The static potential structures were also investigated with fully atomistic molecular dynamics (MD) simulations. Combined XR and MD results show that the EDL structure has alternating anion/cation layers within the first nanometer of the interface. The dynamical response of the EDL to potential steps has a slow component (\textgreater 10 s) and the RTIL structure shows hysteresis during CV scans. We propose a conceptual model that connects nanoscale interfacial structure to the macroscopic measurements.   

MoS$_{2}$/graphene Composite Paper Electrodes for Na-ion Battery Applications

We study the synthesis, electrochemical and mechanical performance of large area layered freestanding papers composed of acid functionalized few layer molybdenum disulfide (MoS$_{2})$ and reduced graphene oxide (rGO) flakes for use as a self-standing flexible electrode in sodium ion batteries. Synthesis was achieved through vacuum filtration of homogenous dispersions consisting of varying wt. {\%} of exfoliated MoS$_{2}$ flakes in GO in DI water, followed by thermal reduction. The electrochemical behavior of the composite paper was evaluated as counter electrode against pure Na foil in a half-cell configuration. The papers showed good Na cycling ability with charge capacity of approx. 225 mAh.g$^{-1}$ with respect to total weight of the electrode and coulombic efficiency reaching 99{\%}.   

First Principles Studies for Lithium Intercalation and Diffusion Behaviors in MoS2 treated with the Compressive Sensing Cluster Expansion

Molybdenum disulfide (MoS2) is a good candidate electrode material for high capacity energy storage applications, such as lithium ion batteries and supercapacitors. In this work, we investigate lithium intercalation and diffusion kinetics in MoS2 by using first-principles density-functional theory (DFT) calculations. Two different lithium intercalation sites (1-H and 2-T) in MoS2 are found to be stable for lithium intercalation at different van der Waals' (vdW) gap distances. It is found that both thermodynamic and kinetic properties are highly related to the interlayer vdW gap distance, and that the optimal gap distance leads to effective solid-state diffusion in MoS2. Additionally, through the use of compressive sensing, we build accurate cluster expansion models to study the thermodynamic properties of MoS2 at high lithium content by truncating the higher order effective clusters with significant contributions. The results show that compressive sensing cluster expansion is a rigorous and powerful tool for model construction for advanced electrochemical applications in the future.   

Quasiparticle Energies in Pristine and Oxygen Depleted MoO3 for Pseudocapacitor Applications

Alpha-MoO3 is a promising electrode material for pseudocapacitors, devices that store electrical energy faradaically, but feature fast reactions/intercalations enabling high power applications [1]. Electrical conductivity and optical properties in alpha-MoO3 are strongly affected by defects, such as oxygen vacancies, which affect the electronic structure. Utilizing self-consistent GW calculations in the quasiparticle picture, along with G0W0 calculations with starting orbitals from HSE06 and DFT$+$U, we calculate the electronic structure of pristine and oxygen depleted alpha-MoO3. We focus on the sensitivity of our results to the calculated description of the localized d-electron states and compare with band gap values determined by measurements on optical properties, electrical conductivity, and photoemission spectroscopy from the literature.\\[4pt] [1] T. Brezesinski, J. Wang, S. H. Tolbert and B. Dunn, Nature Materials 9, 146 (2010)   

Charge storage in $\beta$-FeSi$_2$ nanoparticle layers

We report on the observation of a surprisingly high specific capacitance of $\beta$-FeSi$_2$ nanoparticle layers. Lateral, interdigitated capacitor structures were fabricated on silicon dioxide and covered by FeSi$_2$ particles [1] in the size range 10-30 nm. Compared to the bare electrodes, the nanoparticle-coated samples exhibit a 3-4 orders of magnitude increased capacitance. Time-resolved current-voltage measurements show that for short times (seconds to minutes), the material is capable of storing up to 1 As/g at voltages of around 1 V. The devices are rugged and exhibit long-term stability under ambient conditions. The specific capacitance is the highest for a relative humidity of \~95\%, while for a relative humidity below 40\% the capacitance is almost indistinguishable from the bare electrodes. This strongly suggests that the storage mechanism is not purely geometric and that a -yet unexplored- electrochemical process may be responsible for the observed high specific capacitance. Our findings may also be of technological interest: The devices work without the need of a fluid phase, the charge storing material is earth abundant and cost-effective, and the sample design is easy to fabricate. \\ (1) Robert Bywalez et al., J. Nanopart. Res. 15, 1878 (2013).   

Simulation of electric double-layer capacitors: evaluation of constant potential method

Atomistic simulations can play an important role in understanding electric double-layer capacitors (EDLCs) at a molecular level. In such simulations, typically the electrode surface is modeled using fixed surface charges, which ignores the charge fluctuation induced by local fluctuations in the electrolyte solution. In this work we evaluate an explicit treatment of charges, namely constant potential method (CPM)\footnote{Reed et al. J. Chem. Phys., \textbf{126}, 084704 (2007)}, in which the electrode charges are dynamically updated to maintain constant electrode potential. We employ a model system with a graphite electrode and a LiClO$_4$/acetonitrile electrolyte, examined as a function of electrode potential differences. Using various molecular and macroscopic properties as metrics, we compare CPM simulations on this system to results using fixed surface charges. Specifically, results for predicted capacity, electric potential gradient and solvent density profile are identical between the two methods; However, ion density profiles and solvation structure yield significantly different results.   

Improved modeling of electrified interfaces using the effective screening medium method

The effective screening medium (ESM) method has been developed as a way to simulate electrified interfaces within a first principles framework using periodic boundary conditions. Given a slab geometry standing for the interface, the ESM method allows filling the region away from the slab with a dielectric screening medium-- the ESM per se--as a simple way to include electrostatic screening effect of the environment. In the original version of the ESM method, the relative permittivity changes discontinuously from $\epsilon=1$ to $\epsilon > 1$ at the boundary located between the molecular system and the ESM, which causes numerical instability when the electron density of the molecular system touches the boundary. Here we improve upon the description of the screening medium by imposing a smooth transition of the dielectric permittivity between the molecular system and the ESM (smooth ESM), thus precluding numerical instabilities when molecules come in contact with the ESM. Moreover, at short distances, the smooth ESM acts as a repulsive wall, and thus the simulation cell can serve as a natural container for molecules in molecular dynamics simulations. Consequently, the smooth ESM method is a substantial advancement in modeling solid-liquid interfaces under electric bias.   

Direct Observation of Virtual Electrode Formation Through a Novel Electrolyte-to-Electrode Transition

Novel electrochemical solutions to problems in energy storage and transportation can drive renewable energy to become an economically viable alternative to fossil fuels. In many electrochemical systems, the behavior of a device can be fundamentally limited by the surface area of a triple phase boundary, the boundary region where a gas-phase species, electrode, and electrolyte coincide. When the electrode is an ionic insulator the triple phase boundary is typically a one-dimensional boundary with nanometer-scale thickness: ions cannot transport through the electrode, while electrons cannot be transported through the electrolyte. Here we present direct experimental measurements of a novel electrolyte-to-electrode transition with photoemission electron microscopy, and observe that the surface of an ionically conductive, electronically insulative solid oxide electrolyte undergoes a transition into a mixed electron-ion conductor in the vicinity of a metal electrode. Our direct experimental measurements allow us to characterize this system and address the mechanisms of ionic reactions and transport through comparisons with theoretical modeling to provide us with a physical picture of the processes involved. Our results provide insight into one of the mechanisms of ion transport in an electrochemical cell that may be generalizable to other systems.   

Session J26: Focus Session: Materials in Extremes: Energetic Materials

Shock induced chemistry in liquids on picosecond timescales

While great progress has been made in theory and simulation of shock induced chemical reactivity, there are few experiments sensitive to the time and length scales necessary to validate these theories. In this talk, we will report the results of experiments on liquids exposed to ultrafast laser driven shocks observed by interferometry and spectroscopy with picosecond time resolution. These time and length scales correspond to those accessible to reactive molecular dynamics simulations, and are often required to observe chemical kinetics using optical methods prior to sample opacity caused by product formation. We will report interferometric and transient absorption data for times up to 300 ps on nitromethane, carbon disulfide, phenylacetylene, acrylonitrile, and several other liquids shocked to initial states between 5 and 22 GPa. Indications of volume increasing and decreasing chemical reactions are observed interferometrically. Chemical products are observed via transient absorption signatures. Efforts to identify these products with vibrational spectroscopies will be reported. We will also compare the results observed in these small scale experiments with literature results from experiments acquired on time and length scales larger by orders of magnitude.   

The Raman spectrum of ammonium nitrate at high pressures from first principles calculations

The pressure induced phase transitions in sodium azide, which include a potential polymeric nitrogen phase transition, are investigated using evolutionary crystal structure prediction methods coupled with density functional theory calculations. Two new phases are predicted to be stable above 53 GPa that have an inequivalent ratio of sodium to nitrogen atoms as compared to sodium azide. The Raman spectrum is calculated from 0-100 GPa using these newly predicted structures, as well as the newly discovered I4/mcm phase of sodium azide. The predicted Raman spectrum is shown to give good agreement to experimental data above 30 GPa and below 15 GPa.   

Toward a reaction rate model of condensed-phase RDX decomposition under high temperatures

Shock ignition of energetic molecular solids is driven by microstructural heterogeneities, at which even moderate stresses can result in sufficiently high temperatures to initiate material decomposition and the release of the chemical energy. Mesoscale modeling of these ``hot spots'' requires a chemical reaction rate model that describes the energy release with a sub-microsecond resolution and under a wide range of temperatures. No such model is available even for well-studied energetic materials such as RDX. In this presentation, I will describe an ongoing effort to develop a reaction rate model of condensed-phase RDX decomposition under high temperatures using first-principles molecular dynamics, transition-state theory, and reaction network analysis.   

Optical characterization of shock-induced chemistry in the explosive nitromethane using DFT and time-dependent DFT

With continual improvements in ultrafast optical spectroscopy and new multi-scale methods for simulating chemistry for hundreds of picoseconds, the opportunity is beginning to exist to connect experiments with simulations on the same timescale. We compute the optical properties of the liquid phase energetic material nitromethane (CH$_{\mathrm{3}}$NO$_{\mathrm{2}})$ for the first 100 picoseconds behind the front of a simulated shock at 6.5km/s, close to the experimentally observed detonation shock speed. We utilize molecular dynamics trajectories computed using the multi-scale shock technique (MSST) for time-resolved optical spectrum calculations based on both linear response time-dependent DFT (TDDFT) and the Kubo-Greenwood (KG) formula within Kohn-Sham DFT. We find that TDDFT predicts optical conductivities 25-35{\%} lower than KG-based values and provides better agreement with the experimentally measured index of refraction of unreacted nitromethane. We investigate the influence of electronic temperature on the KG spectra and find no significant effect at optical wavelengths. With all methods, the spectra evolve non-monotonically in time as shock-induced chemistry takes place. We attribute the time-resolved absorption at optical wavelengths to time-dependent populations of molecular decomposition products, including NO, CNO, CNOH, H$_{\mathrm{2}}$O, and larger molecules.   

Micron-scale Reactive Atomistic Simulation of Void Collapse and Hotspot Growth in PETN

Material defects and heterogeneities such as dislocations, grain boundaries, and micro-porosity play key roles in the shock-induced initiation of detonation in energetic materials. Non-equilibrium molecular dynamics simulations (NEMD) with the ReaxFF force field (ReaxFF) in LAMMPS were performed to explore the effect of nanoscale voids on hotspot growth and initiation in pentaerythritol tetranitrate (PETN) crystals under weak shock conditions. Previously, we have performed reactive NEMD simulations of weak shocks in a $(20nm)^3$ PETN crystal containing a spherical void. We observed hotspot formation and an exothermic reaction zone. To observe growth of the hotspot, we have now greatly extended the time and lengthscale of the simulation. We created a cylindrical pore in a $0.3\times0.2\times0.001\mu m^3$ crystal. Once the shockwave reached the free surface we continued the simulation using the shock-front absorbing boundary condition. Results show steadily increasing axial and lateral spatial extent of the hotspot and a complex coupling of exothermic chemistry to hotspot growth.   

Unreacted Equations of State of Shocked Single Crystal PETN and Beta-HMX

We report results from ultrafast shockwave experiments conducted on single crystal high explosives. Ultrafast shock studies can enable high throughput characterizations of unreacted equations of state to higher pressures than previously reported and also quantify the magnitude of anisotropic mechanical response to shock waves. Our ultrafast results yield --as of this writing- [110] PETN data up to a pressure of 26 GPa, which is 1.6x higher than published mid-scale gun results. Published HMX shock data are strikingly sparse; seven points up to approximately 10 GPa are reported from shocked solvent-pressed beta-HMX and Robert Craig reported three single crystal points (undisclosed crystal orientation) between 34 and 42 GPa. Two nonhydrostatic cold-compression diamond-anvil cell studies, u-Raman $+$ u-XRD, and u-Raman $+$ deflagration rates, report a transition in HMX, possibly shear induced, beginning at 26-27 GPa. A previously posed question is whether Craig's data are affected by this transition.$\backslash $pard An analysis of our results for [010] beta-HMX indicate it is less compressible than portrayed by the commonly accepted Hugoniot, which is based on a parameterized third-order Birch-Murnaghan model EoS using the ten before mentioned shock wave measurements and the more recent cold-compression u-XRD study by Yoo et al.   

Structural diversity in the ammonium azide molecular crystal at high pressures

Ammonium azide (NH$_{4}$N$_{3})$ seems to undergo phase transitions under compression as indicated by the experimentally measured Raman spectrum. However, X-ray diffraction studies of the NH$_{4}$N$_{3}$ crystal beyond the known first phase transition at $\sim$ 3 GPa have yet to be performed. Additionally, first-principles density functional perturbation theory calculations of the known phase of NH$_{4}$N$_{3}$ have been unsuccessful at reproducing Raman spectral evolution with pressure seen in experiment, while no evidence has been found that NH$_{4}$N$_{3}$ transitions to hydronitrogen solid at the predicted pressure of 36 GPa. This may indicate that the true lowest enthalpy configuration has yet to be discovered. Here, evolutionary structure prediction method coupled with density functional calculations are employed to calculate the lowest enthalpy phases of ammonium azide as a function of pressure. Novel structures are predicted, and ground state enthalpies and the Raman spectra are calculated as a function of pressure and compared with the experimental Raman spectra.   

Effect of subcritical damage on sensitivity of a plastic bonded explosive

As energetic materials are subjected to increasingly extreme environments, a more thorough understanding of the relationships between mechanical insult and changes in explosive sensitivity is desired. To that end, a Shock Wave Apparatus, originally developed at TDW (Schrobenhausen, Germany), has been employed to induce subcritical shocks of up to 0.7 GPa in a plastic bonded explosive sample while preserving the sample for further study. Changes in density due to the subcritical shocks are measured, and the sensitivity of the damaged explosive is determined through a TDW/AFRL Modified Gap Test configuration that allows the run-to-detonation (RTD) to be determined for a given shock loading. Changes in sensitivity are determined by comparing the RTD for each damaged sample with corresponding RTD for pristine (i.e. undamaged) samples. Confined Split-Hopkinson Pressure Bar experiments are also conducted in order to understand the effects of damage at lower strain-rates and pressures. Finally, the effects on sensitivity due to multiple shocks are also investigated in this study.   

A comparison of reactive burn models to gap/rate-stick experiments

We present a numerical study of shock initiation for the case when a detonation wave in a donor PBX 9502 passes through an inert polymer material (epoxy) and then into an acceptor of PBX 9502. The pressure gradient behind the detonation wave causes the lead shock in the epoxy to decay and strongly influences whether the acceptor detonates. We compare four reactive burn models and discuss difference in their predictions.   

Investigations of PBX 9502 relight phenomena using a modified gap test

We present a series of experimental results on PBX 9502 relight gap tests where epoxy gaps of varying thickness and material were placed within between equal lengths of PBX 9502. Piezo pins were used to record velocity before and after the gap. Relight location was measured and subsequent velocity calculated. These results were used to validate and improve models, and support gas-gun shock initiation experiments. The design for these tests utilized a modified gap test where the donor and the acceptor explosives are the same, and separated by an epoxy gap of varying thickness. The epoxy used was comprised of Epon-828 and Jeffamine T-403. The explosive studied was PBX 9502. The goal of the experiment was to initially reach steady state detonation behavior, and then retard it with the gap, and measure the velocity and re-initiation behavior. The results were then compared to existing models. Other gap materials were studied as well, and the approach and results of all materials will be discussed.   

Multi-physics Meso-scale Finite Element Simulation of HMX-based Solid Propellant Subjected to Thermal Insults

A large strain chemo-thermo-mechanical numerical framework has been developed to model the coupled chemical, thermal and mechanical behavior of solid propellant at the meso-scale. The mechanical behavior is modeled using a hyperelastic material model with viscous damage and J2 plasticity. The model admits a general nonlinear coefficient of thermal expansion to capture the thermo-mechanical behavior. The chemical model considers a system of chemical reactions with the rate kinetics being governed by a modified Arrhenius law. The thermal model considers thermodynamically consistent energy contributions from the inelastic mechanical deformations and the chemical reactions. The finite element method has been employed to discretize the continuum equations. Some simulation results will be presented to demonstrate the use of the developed framework in modeling the behavior of HMX-based solid propellant under thermal loads. The developed framework captures the large volumetric strains that are a characteristic of the $\beta$-$\delta$ phase transition of the HMX crystals and is able to predict locations of potential cracks in the binder. Such a simulation tool may prove to be useful in determining optimal conditions for the safe storage of such materials.   

Near Absolute Equation of State Measurements of CH using Velocimetry and Radiography

The OMEGA EP laser was used to conduct absolute near equation of state measurement along the principal Hugoniot of CH to 6 Mbar. A 6 ns long, 3700 J laser pulse in direct drive was used to launch a cylindrical shock in a multi-layered aluminum/CH target which was imaged using a Fe backlighter. The technique presented here incorporated VISAR shock velocity measurements with shock compression measured using side-on radiography to determine the Hugoniot. Experimental uncertainties of less than 10{\%} in density were obtained in these experiments. The measured Hugoniot values of this study are consistent with previous measurements that were impedance matched to quartz (Barrios et al. PoP 2010). These experiments were conducted, as proof of principle, for future absolute EOS measurements on the NIF. Future experimental work will be discussed.   

Modeling Shock Desensitization of Composition B Explosive

The NOBEL multimaterial adaptive grid Eulerian hydrodynamic code was used to model a shaped charge jet formation, its interaction with a steel plate, and shock formation of a bow shock in front of the jet that shocks and desensitizes a cylinder of Composition B (60/40 RDX/TNT at 1.715 g/cc) explosive so that when the jet arrives it fails to initiate detonation in the desensitized explosive. The jet passes through the Composition B explosive cylinder, an air gap, and then initiates propagating detonation in a second Composition B explosive cylinder that has not been desensitized by a preshock. The experimental arrangement was studied using X-ray radiography at the Material Research Laboratory in Melbourne, Australia.   

X-ray induced mobility of molecular oxygen at extreme conditions

We report an in situ Raman study of KClO4 irradiated with x-rays in a diamond anvil cell. Decomposition via KClO4 $+$ hv $\to $ KCl $+$ 2O2 was monitored via the O2 vibron at 2 GPa, 6 GPa and 9 GPa. For all pressures, the vibron grew in intensity and then diminished after successive irradiation suggesting that O2 was diffusing away from the irradiated region. Surprisingly, the diffusion rate accelerated with pressure increase, indicating that the nonhydrostatic pressure gradient was likely driving molecular diffusion of oxygen. At 9 GPa, the vibron bifurcated suggesting that O2 exists as two forms: interstitial and bulk solid. This method can be employed to study molecular diffusion under extreme conditions.   

Session J27: Focus Session: Computer Simulations of Interactions of Electromagnetic Fields and Nanostructures

First-Principles Description of Strong Electromagnetic Fields in Solids

Interactions between light and matter are usually described by two theories: Macroscopic Maxwell equations describe propagation of a light in a medium, while quantum calculation of susceptibilities such as dielectric function is one of central issues in the first-principles calculation. Recent progresses of laser technologies, however, require theories beyond it. Strong electromagnetic fields of a laser pulse induce extremely nonlinear electron dynamics which no more allows perturbative separation between macroscopic electromagnetic fields and microscopic electron dynamics. We have recently developed a multi-scale theory for this problem, describing electron dynamics in solid using real-time time-dependent density functional theory. This theory provides a quite general and computationally feasible basis for the problem, including ordinary macroscopic electromagnetism in a weak field limit and being applicable to problems involving arbitrarily intense fields. In my presentation, I will discuss the basic theory, computational implementation, and physics applications of our new approach.   

First Principles Real-Space GW+BSE Calculations for Confined Systems

We investigate the performance of various levels of GW theories for electronic excitations as well as the resulting solutions of the Bethe-Salpeter-Equation (BSE) for optical excitations in a wide range of confined systems including atoms, ions, diatomic molecules, and organic molecules relevant for photovoltaic applications. Starting with solutions of the Kohn-Sham equations for ground state properties computed via the real-space {\em ab initio} pseudopotential code PARSEC, we perform the GW calculations in the space of single-particle transitions at various levels of theory, and compare the results with photoemission data. The levels of theory include such approximations as $G_0W_0$ with RPA screening, $G_0W_f$ that includes vertex corrections through the use of a dielectric screening within the time-dependent-local-density approximation (TDLDA), the $GW_0$, and the self-consistent GW. The resulting quasiparticle energies and wave functions from the GW calculations are used to solve the BSE for optical excitations, which are then compared with experiments and results from calculations performed within the TDLDA. The effects of the vertex corrections, self-consistency in GW, and core-valence partitioning are discussed.   

Kadanoff-Baym-Keldysh-Ehrenfest dynamics of correlated materials responding to ultrafast laser pulses

In our many earlier simulations of the response of materials and molecules to laser pulses, one-electron states were determined by the time-dependent Schr\"{o}dinger equation with an instantaneous one-electron Hamiltonian. These states were then used with Ehrenfest's theorem in a semiclassical treatment of the coupled dynamics of electrons and nuclear coordinates. For strongly-correlated materials, however, true nonequilibrium self-energies are required. Here we describe a practical numerical procedure for employing the Kadanoff-Baym/Keldysh equations together with Ehrenfest's theorem. In preliminary work we treat the simplest possible model of a system in which there is the potential for both a Peierls structural transition (involving doubling of the unit cell) and a Mott-Hubbard electronic transition (involving electron correlations). These calculations are relevant to understanding the dynamics of insulator-metal phase transitions in VO$_{2}$, which has been studied in ultrafast pump-probe measurements [1-3] and nanoscale imaging [4]. \\[4pt] [1] C. K\"{u}bler, Phys. Rev. Lett. 99, 116401 (2007).\\[0pt] [2] M. van Veenendaal, Phys. Rev. (in press).\\[0pt] [3] A. Cavalleri et al., Phys. Rev. B 70, 161102(R) (2004).\\[0pt] [4] M. M. Qazilbash et al., Phys. Rev. B 83, 165108 (2011).   

Effective transient states for nonequilibrium systems under ultrafast control pulses

We investigate the transient states in nonequilibruim time-dependent systems. Intense ultrafast laser pulses allow the preparation of transient states of matter exhibiting strong non-equilibrium between electrons and lattice. By controling the laser pulse, we are able to change the transient states of these quantum systems. The optical and structural properties as well as the temporal evolution of such states provide insight into the mutual dependence of electronic and atomic structure. We approach the problem by showing examples from charge-density-wave systems and model two level systems. In both of these, nonequilibrium techniques can be used to qualitatively describe the common short-time experimental features. Through simulations based on non-equilibrium Green's function formalism and time dependent master equations approaches we show how to achieve effective transient states for nonequilibrium systems under ultrafast control pulses.   

Dynamics of irradiation: from molecules to nano-objects and from material science to biology

We discuss microscopic mechanisms of irradiation in clusters and molecules. We consider the case of isolated molecules/clusters [1] possibly in contact with an environment [2]. We use Time Dependent Density Functional Theory (for electrons) coupled to Molecular Dynamics (for ions) and follow explicitly in time both irradiation and response of the system. Examples are taken from free metal clusters, from fullerenes, from molecules of biological interest and from clusters deposited on a surface or embedded in a matrix [3,4]. We analyse in particular the properties of emitted electrons (photo electron spectra, angular distributions\textellipsis ), which constitute a key tool of analysis of the properties of irradiated clusters and molecules [5]. We also discuss the possibility of pump and probe scenarios (opening the road to manipulation at the molecular scale) with help of dedicated laser pulses, exploring high laser frequencies towards the FEL regime and very short times scales down towards the attosecond domain.\\[4pt] [1] F. Calvayrac et al, Phys. Reports (2000)4932\\[0pt] [2] P. M. Dinh et al, Phys. Reports (2009) 433\\[0pt] [3] Z.P. Wang et al, Int. J. Mass Spect. (2009) 14304\\[0pt] [4] U. F. NdongmuoTaffoti et al, Eur. Phys. J. D (2010) 1315\\[0pt] [4] Th. Fennel et al, Rev. Mod. Phys. (2010) 1793.   

The time-dependent particle-hole map: one-dimensional benchmark studies

The time-dependent particle-hole map (TD-PHM) is a computational tool for visualization and interpretation of electronic excitation processes in many-body systems, in particular for molecular systems that are used in organic photovoltaics. In practice, the TD-PHM is obtained from time-dependent Kohn-Sham calculations, which implies three types of approximations: approximation to the exchange-correlation (xc) potential, replacement of exact many-body wave function with the corresponding Kohn-Sham Slater determinant, and, for molecules, the reduction of the six-dimensional TD-PHM to a 2-dimensional object suitable for representation in real space. Here, we focus on the first two approximations, and study one-dimensional lattice systems with several electrons interacting via soft-Coulomb potentials. We carry out benchmark calculations and to assess the validity of approximate xc potentials and the replacement of the exact wave functions with Kohn-Sham Slater determinants.   

Time-dependent density functional theory of magneto-optical response of periodic insulators

Though the linear response theory has been successfully used for molecular systems for a long time, the extension of this theory to solids is not straightforward since the position operator is ill defined in extended periodic systems. The theoretical description of homogeneous static magnetic field in periodic systems is particularly challenging as the corresponding vector potential breaks the translational invariance of the Hamiltonian. We present a unified approach to calculation of all-order response to arbitrary electromagnetic fields both for periodic and molecular systems within the formalism of non-equilibrium Green functions. The approach is applied to derive the expression for the magneto-optical response of insulating solids in the approximation of non-interacting electrons. The formula obtained is completely identical to the expression for molecular systems if the proper position and orbital magnetization operators are chosen. The terms corresponding to changes in the optical response due to the orbital magnetization of Bloch states and due to the modified density of Bloch states in the magnetic field are identified. A computational scheme based on the density matrix-perturbation theory is developed for practical calculations of the magneto-optical response.   

\emph{Ab initio} study of optical excitations in VO$_2$

Motivated by recent experimental efforts to fabricate p-n junctions from transition metal oxides (TMOs) and a recent theoretical study claiming TMOs to be good absorbers and promising materials for efficient carrier multiplication, we study the optical properties of a prototypical TMO, the insulator $M_1$ phase of vanadium dioxide (VO$_2$), by \emph{ab initio} methods. We applied the Bethe-Salpeter equations (BSE) to calculate the optical properties, starting from self-consistent GW quasi-particle energy levels and states. In contrast to expectations, the exciton binding energy obtained by BSE is in good agreement with the experiment. We find that the electron-electron interaction is very strong which makes this material promising for efficient carrier multiplication that might lead to an enhanced efficiency in photo-voltaics applications. To illustrate this more quantitatively, we calculated the impact ionization rate within the independent quasiparticle approximation, and find that the rate is significantly higher than silicon in the region of highest solar intensity, due to the strong multiple carrier excitations.   

Nanoscope based on nanowaveguides

The far field spatial resolution of conventional optical lenses is of the order of the wavelength of light due to loss in the far field of evanescent, near electromagnetic field components. We show that subwavelength details can be restored in the far field with an array of divergent nanowaveguides, which map the discretized, subwavelength image of an object into a magnified image observable with a conventional optical microscope. We demonstrate that metallic nanowires, nanocoaxes, and nanogrooves can be used as such nanowaveguides. Thus, an optical microscope capable of subwavength resolution --- a nanoscope --- can be produced, with possible applications in a variety of fields where nanoscale imaging is of value, including living systems.   

The Anderson-Condon-Shortley Site in X-ray Spectroscopies of Solids

Electronic structures of compounds involving open d- and f- shell are studied frequently by X-ray and electron spectroscopies. The excitation, especially core excitation, is localized on a single site makes this the problem of impurity site states interacting with the continuum of bands. on the other hande, the electron-electron interaction whithin the d- or f- shell leads to a multiplet problem as addressed long ago for isolated atoms. Building on our easy to use program multiX (*), which treats an atom in a general crystal field environment without symmetry analysis, we now address the interaction of this atomic entity with the band continuum. The crossover from atomic to bandlike spectra is the focus of interest. We discuss experimental examples where available and accessible to our methods.   

Opto-electronic properties of silicon nanoparticles: Excitation energies, sum rules, and Tamm-Dancoff approximation

We present an ab initio study of the excited state properties of silicon nanoparticles (NPs) with diameters of 1.2 and 1.6 nm. Quasiparticle corrections were computed within the GW approximation. The absorption spectra were computed by time-dependent density functional theory (TDDFT) using the adiabatic PBE approximation, and by solving the Bethe-Salpeter equation (BSE). In our calculations we used recently developed accelerated methods that avoid the explicit inversion of the dielectric matrix and summations over empty states [1]. We found that the scissor approximation reliably describes quasiparticle corrections for states in the low energy part of the spectra. We also found good agreement between the structure and positions of the absorption peaks obtained using TDDFT and the BSE. We discuss the effect of the Tamm-Dancoff approximation on the optical properties of the NPs and present a quantitative analysis in terms of sum rules. In the case of the BSE we found that, even in the absence of the Tamm-Dancoff approximation, the f-sum rule is not fully satisfied due to the inconsistency between the approximations used for the BSE kernel and for the quasiparticle Hamiltonian. [1] D. Rocca, D. Lu,G. Galli, J. Chem. Phys. 133, 164109 (2010).   

Session J28: Superconducting Qubits: Topological Effects, Entanglement and EIT

The merge of superconducting qubits with topological superconductors: microwave transitions as a signature of coherent parity mixing effects

In this talk we will discuss the light-matter effects that could arise if Majorana fermions are added to a superconducting charge qubit. Coupling Majorana fermion excitations to coherent external fields is an important stepping stone towards their manipulation and detection. We argue that such a device could contribute to the spectroscopic detection of topological-superconductor Majorana excitations. We analyse the charge and transmon regimes of a topological nano-wire embedded within a Cooper-Pair-Box, where the superconducting phase difference is coupled to the zero energy parity states that arise from Majorana quasi-particles. We show that at special gate bias points, the microwave photon-qubit coupling can be switched off via quantum interference, and in other points it is exponentially dependent on the control parameter $E_J/E_C$. We propose that this type of device could perform as a high coherence four-level system in the superconducting circuits architecture with tunability of the coupling to photons, a coveted property which is difficult to achieve with regular devices.   

Topological Quantum Espionage

Can one transfer information encoded in Majorana modes between two distinct platforms? Or must one read out the information before transferring it to a new medium? We explore this question, and find that not only can information be transfered, but in some cases a fermionic occupation number can be stored non-locally by Majorana modes localized in two distinct p-wave superconductors with opposite chirality, as long as some tunneling contact between the two exists.   

Flux-controlled quantum computation with Majorana zero modes

Majorana zero modes, exotic quasiparticles which are their own antiparticles, can be constructed out of electron and hole excitations in topological superconductors. Because widely separated Majorana zero modes can store quantum information nonlocally and their non-Abelian braiding statistics allows accurate quantum gates, Majorana zero modes offer a promise for topological quantum computation. The coupling of Majorana zero modes to superconducting transmon qubits permits braiding of Majoranas and readout operations by external variation of magnetic fluxes. We identify the minimal circuit for the demonstration of the non-Abelian Majorana statistics and discuss the possible limitations which might hinder the braiding operation. A key benefit of our approach is that the whole operation is performed at the electrical circuit level, without requiring local control of microscopic parameters. Finally, we take a longer term perspective and introduce the Random Access Majorana Memory, a scalable circuit that can perform a joint parity measurement on Majoranas belonging to a selection of topological qubits. Such multi-qubit measurements allow for the efficient creation of highly entangled states and simplify quantum error correction protocols by avoiding the need for ancilla qubits.   

Exploring the Effect of Noise on the Berry Phase using circuit QED

The Berry phase is independent of both energy and time: it solely depends on the trajectory of the quantum system in state space, and is equipped with a certain degree of robustness against slow fluctuations [1,2]. By introducing artificial distortions in the path in state space, we measure the geometric contributions to the dephasing of an effective two-level system. Our experiments, realized with a microwave-driven superconducting qubit, demonstrate that only those fluctuations which deform the path cause geometric dephasing. A direct comparison with the path-independent dynamic phase reveals that the Berry phase is less affected by noise-induced dephasing in the adiabatic limit of long evolution times. \newline [1] G.~De Chiara and G.~M.~Palma, \emph{Phys.~Rev.~Lett.~}\textbf{91}, 090404 (2003) \newline [2] S.~Filipp \emph{et al.}, \emph{Phys.~Rev.~Lett.~}\textbf{102}, 030404 (2009)   

Measuring the Chern Number of a Superconducting Qubit from Nonadiabatic Response

The accumulation of Berry's phase in superconducting qubits under cyclical evolution has been well studied,\footnote{P. J. Leek \emph{et al.}, Science \textbf{318}, 5858 (2007)}$^,$\footnote{S. Berger \emph{et al.} PRA \textbf{87}, 060303(R) (2013)} typically requiring fully adiabatic evolution. We demonstrate an alternative means of accessing the topology of a qubit, based on nonadiabatic manipulation.\footnote{V. Gritsev and A. Polkovnikov, PNAS \textbf{109}, 6457 (2012)} Integrating the measured Berry curvature over the Bloch sphere yields the Chern number ($\mathrm{ch}_{\mathrm{I}}$), which we find to be quantized. By adding an effective static field to the qubit, we demonstrate a topological transition from $\mathrm{ch}_{\mathrm{I}} = 1$ to $\mathrm{ch}_{\mathrm{I}} = 0$. This simple example of extracting the Chern number may be easily scaled to larger systems.   

Theory of implementation of an impedance-matched $\Lambda$ system in circuit QED

In one-dimensional optical setups, light-matter interaction is drastically enhanced by the interference between the incident and scattered fields. Particularly, in an impedance-matched $\Lambda$-type three-level system, which has two identical radiative decay rates from the top level and interacts with a semi-infinite one-dimensional field in reflection geometry, a single photon deterministically induces the Raman transition and switches the electronic state of the system. Here we theoretically investigate a circuit QED system composed of a driven superconducting qubit and a resonator in the dispersive regime. We show that the dressed states of this system constitute an impedance-matched $\Lambda$ system under a proper choice of the frequency and power of the qubit drive. When we apply a resonant probe field to this system, it is down-converted nearly perfectly after a single reflection as long as the probe power is sufficiently weak. This indicates a deterministic quantum dynamics induced by single photons, which is applicable, for example, to the detection of single microwave photons and the bidirectional quantum memory (swapping) between a microwave photon and a superconducting qubit.   

Realization of an Impedance-Matched $\Lambda$ System Using an Artificial Atom

We realize an impedance-matched $\Lambda$-type three-level system using dressed states in a circuit QED system, where a superconducting flux qubit and a coplanar waveguide resonator are coupled capacitively. Under an appropriate choice of microwave frequency and intensity for the qubit drive, two radiative decay rates from the upper level to the lower two levels in the $\Lambda$ system become identical, and the impedance-matched $\Lambda$ system is realized. Perfect absorption and nearly deterministic down-conversion of the incident microwave photons are theoretically expected in this system. We experimentally observe that the incident microwave is perfectly absorbed and is down-converted by 64~MHz corresponding to detuning between the qubit transition and the qubit drive. The down-converted signal is amplified by a Josephson parametric amplifier, and the power spectrum is directly detected. The conversion efficiency of $\sim 75\%$ has been obtained.   

Demonstration of Geometric Landau-Zener Interferometry in a Superconducting Phase Qubit

Geometric quantum manipulation and Landau-Zener interferometry have been separately explored in many quantum systems. Here we fill this gap by combining these two approaches in the study of the dynamics of a superconducting phase qubit. We propose and then experimentally demonstrate Landau-Zener interferometry based on pure geometric phases in this solid-state qubit. We observe the interference due to geometric phases accumulated in the evolution between two consecutive Landau-Zener transitions, while the dynamical phase is eliminated by a spin-echo pulse. Our numerical simulation results using measured energy relaxation and dephasing times agree well with the experimental results. The full controllability of the qubit population as a function of intrinsically fault-tolerant geometric phases provides a promising approach to fault-tolerant quantum computation.   

Preparing Schrodinger cat states by parametric pumping

Maintaining a quantum superposition state of light in a cavity has important applications for quantum error correction. We present an experimental protocol based on parametric pumping and Josephson circuits, which could prepare a Schrodinger cat state in a cavity. This is achieved by engineering a dissipative environment, which exchanges only pairs or quadruples of photons with our cavity mode. The dissipative nature of this preparation would lead to the observation of a dynamical Zeno effect, where the competition between a coherent drive and the dissipation reveals non trivial dynamics. Work supported by: IARPA, ARO, and NSF.   

Stabilization of entanglement between remote transmon qubits

Entanglement between remote qubits can be a valuable resource for scalable quantum computation and other quantum technologies. Here, we discuss non-unitary methods for generating and stabilizing such entanglement between remote superconducting qubits. While joint measurement of the qubits using a sequential probe allows for post-selected entanglement, adding feedback during the measurement conditioned on the outcome allows for deterministic entanglement. This can be supplemented or substituted for with reservoir engineering techniques, which allow for non-zero concurrence in the steady state even in the presence of dephasing. Both the dispersive and near-resonant regimes of circuit QED are analysed.   

The dynamical Casimir effect generates entanglement

The existence of vacuum fluctuations, i.e., the presence of virtual particles in empty space, represents one of the most distinctive results of quantum mechanics. It is also known, under the name of dynamical Casimir effect, that fast-oscillating boundary conditions can generate real excitations out of the vacuum fluctuations. Long-awaited, the first experimental demonstration of this phenomenon has been realized only recently, in the framework of superconducting circuits [C. M. Wilson \textit{et al.} Nature 479, 376-379 (2011)]. In this contribution, we will discuss novel theoretical results, showing that the dynamical Casimir effect can be exploited to generate bipartite and multipartite entanglement among qubits. We will also present a superconducting circuit design which can feasibly implement the model considered with current technology. Our scheme is composed of a SQUID device side-coupled to two transmission line resonators, each one interacting with a superconducting qubit. Such proposal can be straightforwardly generalized to the multipartite case, and it can be scaled up to build strongly correlated cavity lattices for quantum simulation and quantum computation.   

Observation of Autler-Townes effect in a dispersively dressed Jaynes-Cummings System

We report on the spectrum of a superconducting Al/AlOx/Al transmon qubit coupled to a planar superconducting resonator in the strong dispersive limit. We resolve discrete peaks in the transition spectrum, each corresponding to a different number of photons. At a base temperature of 30 mK and in the absence of a coherent drive on the resonator, we find a weak n = 1 photon peak along with the n = 0 photon peak in the qubit spectrum, corresponding to a population of 5.474 GHz photons at an effective resonator temperature of T = 120mK. Two-tone spectroscopy using independent coupler and probe tones reveals an Autler-Townes splitting in the thermal n = 1 photon peak. The observed effect is explained accurately using the four lowest levels of the dispersively dressed qubit-resonator system and compared to results from numerical simulations of the steady-state master equation for the coupled system.   

Towards Electromagnetically Induced Transparency in a Transmon

We have observed the Autler-Townes (AT) doublet in a superconducting Al/AlO$_{\mbox{x}}$/Al transmon qubit that acts as an artificial atom embedded in a three-dimensional Cu microwave cavity at a temperature of 22 mK.\footnote{S. Novikov \textit{et al.}, Phys. Rev. B 88, 060503(R) (2013).} The long coherence time ($\sim$40 $\mu$s) of the transmon enables us to observe a clear AT splitting, such that three-level density matrix simulations with no free parameters provide excellent fits to the data. Due to specifics of inter-level transition rates in the transmon, the regime of electromagnetically induced transparency (EIT) was not achievable. We will discuss our progress towards engineering the decay rates of the system with the goal of crossing over from the AT to EIT regime.   

A Quantum Simulation on the Emergence of Lorentz Invariance

Lorentz invariance (LI) is one of the best tested symmetries of Nature. It is natural to think that LI is a fundamental property. However, this does not need to be so. In fact, it could be an emergent symmetry in the low energy world. One motivation on Lorentz-violating theories may come from consistent non-relativistic models of gravity, where LI appears at low energies. The basic approach is by taking two interacting quantum fields. The bare (uncoupled fields) have different light velocities, say v1 and v2. The coupling tends to ``synchronize'' those velocities providing a common light velocity: the LI emergence. So far, only perturbative calculations are available. In this perturbative regime the emergence of LI is too slow. Therefore it is mandatory going beyond perturbative calculations. In this talk I will discuss that such models for emergent Lorentz Invariance can be simulated in an analog quantum simulator. In 1+1 dimensions two transmission lines coupled trough Josephson Junctions do the job. We show that the emergence can be checked by measuring photon correlations. Everything within the state of the art in circuit QED. We show that our proposal can provide a definite answer about the LI emergence hypothesis in the strong coupling regime.   

Session J29: Glassy & Amorphous Systems, including Quasicrystals

Medium-range order in Al$_{90}$Sm$_{10}$ liquids revealed by a pre-peak in the structure factor

Aluminum (Al) alloyed with about 10 at.{\%} rare-earth metals such as samarium (Sm) can display promising mechanical properties upon rapid quenching from the liquid state. Knowledge about the structure of the liquid phase is an important starting point for understanding the profound phase selection during the rapid solidification process, which plays a key role in materials performance. We have performed ab-initio molecular dynamics (AIMD) calculations on the liquid Al$_{90}$Sm$_{10}$ system with 500 atoms per unit cell at T $=$ 1300 K, which is well above the melting temperature of the system. The AIMD simulations show that the liquid system develops a medium-range order by segregating into nanometer-sized regions with different atomic compositions. This segregation ultimately gives rise to a distinct pre-peak in the structure factor observed both in experiments and in AIMD calculations. Our results are in good agreement with experimental measurements using three-dimensional atom probe on the amorphous Al$_{90}$Sm$_{10}$ structure rapidly quenched from the liquid state.   

Atomic structural evolution in metallic liquids and glasses: A measure of fragility

The glass forming ability (GFA) of metallic alloys is widely varied. Bulk metallic glasses (BMGs) have been identified in a number of alloy systems but far more compositions can be vitrified only when their liquids are rapidly quenched. Understanding the structural evolution of metallic liquids as they are supercooled and quenched into glasses is critically important, not only for providing insight into the nature of the glass transition, but also for understanding technical aspects of glass formation and the thermal stability of the glassy solid. In this talk, we discuss the results of viscosity and high energy X-ray diffraction studies on a range of transition metal-based liquids and glasses. The temperature dependence of the X-ray structure factor has been measured in the glass by means of stationary diffraction and in the equilibrium and supercooled liquid state using Beamline Electrostatic Levitation. As will be shown, the temperature dependence of the structure factor above the glass transition shows anomalous acceleration. The degree of this acceleration has a strong correlation with liquid fragility as measured from non-contact viscosity data. These results suggest a \textit{structural fragility} metric distinguishing good glass formers from poor ones.   

Two-level tunneling systems in amorphous alumina

The decades of research on thermal properties of amorphous solids at temperatures below 1 K suggest that their anomalous behaviour can be related to quantum mechanical tunneling of atoms between two nearly equivalent states that can be described as a two-level system (TLS) [1]. This theory is also supported by recent studies on microwave spectroscopy of superconducting qubits [1]. However, the microscopic nature of the TLS remains unknown. To identify structural motifs for TLSs in amorphous alumina we have performed extensive classical molecular dynamics simulations. Several bistable motifs with only one or two atoms jumping by considerable distance $\sim$ 0.5 {\AA} were found at T=25 K. Accounting for the surrounding environment relaxation was shown to be important up to distances $\sim$ 7 {\AA}. The energy asymmetry and barrier for the detected motifs lied in the ranges 0.5 - 2 meV and 4 - 15 meV, respectively, while their density was about 1 motif per 10 000 atoms. Tuning of motif asymmetry by strain was demonstrated with the coupling coefficient below 1 eV. The tunnel splitting for the symmetrized motifs was estimated on the order of 0.1 meV. The discovered motifs are in good agreement with the available experimental data. \\[4pt] [1] G. J. Grabovskij et al. Science 338, 232 (2012)   

Fragility, network adaptation, rigidity- and stress- transitions in homogenized binary Ge$_{x}$S$_{100-x}$ glasses

Binary Ge$_{x}$S$_{100-x}$ glasses reveal elastic and chemical phase transitions driven by network topology. With increasing Ge content x, well defined rigidity (x$_{c}$(1)$=$19.3{\%}) and stress(x$_{c}$(2)$=$24.85{\%}) transitions and associated optical elasticity power-laws are observed in Raman scattering. Calorimetric measurements reveal a square-well like minimum with window walls that coincide with the two elastic phase transitions. Molar volumes show a trapezoidal-like minimum with edges that nearly coincide with the reversibility window. These results are signatures of the isostatically rigid nature of the elastic phase formed between the rigidity and stress transitions. Complex C$_{p}$ measurements show melt fragility index, m(x) to also show a global minimum in the reversibility window, underscoring that \textit{melt dynamics encode the elastic behavior of the glass} formed at T$_{g}$. The strong nature of melts formed in the IP has an important practical consequence; they lead to slow homogenization of non-stoichiometric batch compositions reacted at high temperatures. Homogenization of chalcogenides melts/glasses over a scale of a few microns is a \textit{pre-requisite} to observe the intrinsic physical properties of these materials.   

Consequences of the superstrong nature of chalcogenide glass-forming liquids at select compositions

Growth of homogeneous melts of stoichiometric compositions of chalcogenides is facilitated by underlying crystalline phases. Such is not the case for non-stoichiometric melt compositions in which, for example, variation of fragility (m) from complex specific heat measurements show global minimum [1] at an extremely low value (m$=$14.8(0.5)) in the 21.5{\%} \textit{\textless }x \textit{\textless }23{\%} range of Ge in homogenized Ge$_{x}$Se$_{100-x} $melts. This has unwittingly led to variability of results in physical properties of \textit{non-stoichiometric} melts/glasses due to their heterogeneity. By directly mapping melt stoichiometry variation along a quartz tube as a function of reaction time of starting materials at a fixed temperature T\textit{\textgreater }T$_{g}$ over days, we have observed a slowdown [1] of melt-homogenization by the super-strong melt compositions, 21.5{\%} \textless x \textit{\textless }23{\%}. This range, furthermore, appears to be correlated to the one observed between the ?exible and stressed rigid phase in network glasses.\\[4pt] [1] K.Gunasekera et al. , J.Chem Phys. 139, 164511 (2013).   

Fragility and super-strong character of non-stoichiometric chalcogenides: implications on melt homogenization

The kinetics of homogenization of binary As$_{x}$Se$_{100-x}$ melts in the As concentration range 0\%$<$x$<$50\% are followed using Raman profiling, and show that 2 gm sized melts in the range 20\%$<$x$<$30\% take nearly two weeks to homogenize when the starting materials are reacted at 700$^{\circ}$C. The enthalpy of relaxation at T$_{g}$ - $\Delta$ H$_{nr}$(x) - shows a minimum in 27\%$<$x$<$37\% in aged samples. In such homogeneous glasses, molar volumes vary non-monotonically with composition and the fragility index $\textit{m}$ displays a broad global minimum in 20\%$<$x$<$40\% where $\textit{m}$$<$20. The super-strong nature of melt compositions in 20\%$<$x$<$30\% hinders melt diffusion at high temperatures, leading to the observed slow kinetics of melt homogenization. In comparing these results with earlier reports, there is evidence that fragility decreases as melts are homogenized. Furthermore, a clear scaling of $\textit{m}$ vs. T$_{g}$ is observed with a negative slope for Flexible glasses and a positive slope for Rigid and Stressed-rigid ones. The absence of a melting endotherm in non-stoichiometric As-Se compositions is reported. Fragilities of the Ge-As-Se are reported and a correlation observed with fragilities of As-Se and Ge-Se.   

Modeling amorphous thin films: Kinetically limited minimization

Amorphous materials become increasingly attractive components of thin film devices such as thin film displays or solar cells. They are typically prepared using physical vapor deposition (PVD) techniques at temperatures well below the melting point of deposited material ($< 0.2T_m$). Computational models of amorphous structures, however, are almost elusively constructed from a high temperature equilibrated crystal melt using simulated annealing (SA) protocol. While such procedure imitates the quench form melt preparation of bulk glasses, its applicability to modeling low temperature synthesized amorphous thin films is unclear. To account for low T growth conditions we propose a new method. The method, kinetically limited minimization (KLM), starts from a randomly initialized structure and minimizes the total energy in a number of local structural perturbation-relaxation events. We compare KLM and SA with quench rates ranging from 64 K/ps to 2500 K/ps using two prototypical ionic and covalent materials: In2O3 and Si, respectively. While both methods provide qualitatively similar structures, we find that, compared to KLM, slow quench SA provides stronger medium range order in a-In2O3 and fast quench SA provides weaker short range order in a-Si.   

Optical spectroscopy of RE-Cd (RE = Gd, Y) quasicrystals and approximants

To date, the optical conductivity of icosahedral quasicrystals, and their approximants, either have lacked an intraband Drude peak altogether or have shown an optical conductivity that, at best, can be described as an unresolved Drude peak with significant broadening, due to intense scattering, which is difficult to separate from interband transitions. We have measured the optical conductivity of the new family of RE-Cd (RE $=$Gd, Y) icosahedral quasicrystals and their approximant and found that the approximants show a well-defined peak at low frequency that cannot be fit with standard Drude theory. We will discuss our findings in terms of Mayou's generalized Drude theory of anomalously diffusing electrons.   

Physical properties of i-R-Cd quasicrystals(R = Y, Gd-Tm)

Detailed characterization of recently discovered i-R-Cd (R = Y, Gd-Tm) binary quasicrystals by means of room-temperature powder x-ray diffraction, dc and ac magnetization, resistivity and specific heat measurements will be presented. i-Y-Cd is weakly diamagnetic. The dc magnetization of i-R-Cd (R = Gd, Ho-Tm) shows typical spin-glass type splitting between field-cooled (FC) and zero-field-cooled (ZFC) data. i-Tb-Cd and i-Dy-Cd do not show a clear cusp in their ZFC dc magnetization. ac magnetization measured on i-Gd-Cd indicates a clear frequency-dependence and the third-order non-linear magnetization, $\chi_{3}$, is consistent with a spin-glass transition. The resistivity for i-R-Cd is of order 100 $\mu \Omega$ cm and weakly temperature-dependent. No feature that can be associated with long-range magnetic order was observed in any of the measurements. Characteristic freezing temperatures for i-R-Cd (R = Gd-Tm) deviate from ideal de Gennes scaling.   

Complex antiferromagnetic order in the Cd$_6R$ approximants to the i-$R$-Cd quasicrystals

The observation of antiferromagnetic order in the Cd$_6R$ ($R$ = rare earths) approximants [1-2] to the recently discovered related i-$R$-Cd quasicrystals [3] provides new and exciting opportunities to unravel the nature of magnetism in these materials. We present single-crystal studies employing x-ray and neutron scattering that revealed complex antiferromagnetism in the Cd$_6R$ approximants. Resolution-limited magnetic Bragg peaks have been observed at lattice points forbidden by the center-symmetry and at incommensurate positions demonstrating long-range antiferromagnetic correlations between the $R$ moments. The work at the Ames Laboratory was supported by US DOE, Office of Basic Energy Sciences, DMSE, contract DE-AC02-07CH11358. Work at the Tokyo University of Science was supported by KAKENHI (Grant No. 20045017). \\[4pt] [1] M. G. Kim et al., Phys. Rev. \textbf{B 85} (2012) 134442. \\[0pt] [2] A. Kreyssig et al., Phil. Mag. Lett. \textbf{93} (2013) 512. \\[0pt] [3] A. I. Goldman et al., Nature Mater. \textbf{12} (2013) 714.   

Session J30: Physical Review X Q&A Session

Physical Review X Q\&A Session

Physical Review X (PRX) is already being recognized as a top-quality journal in physics. What are its current standards and strengths? How will it grow and evolve in the coming years? Why is PRX a journal for you? PRX editors and the Editorial Board invite you to a Q & A session, where we will answer these questions and others you have about the journal. Bring your questions and learn more about PRX. Light refreshments will be provided.   

Session J31: Focus Session: Beyond Graphene: Synthesis, Defects, Structure, and Properties III

Effect of monolayer substrates on the electronic structure of single-layer MoS$_{2}$

We have performed first-principles calculations based on density functional theory (DFT) to study structural and electronic properties of a single layer of MoS$_{2}$ deposited on single-layer substrates of hexagonal boron nitride (BN), graphene and silicene. All have a honeycomb structure; hence the formation of heterostructures is expected. Since the lattice mismatch between MoS$_{2}$ and these substrates is large, we have considered different periodicities among layers to reduce as far as possible the incommensurability between lattices. Our results show that BN barely affects the electronic structure of isolated single-layer MoS$_{2}$; the DFT gap remains $\sim$1.8 eV. Graphene and silicene severely modify the electronic structure introducing additional states within the optical gap. Adsorption on graphene turns the system into a zero band gap semiconductor bringing the conduction bands of MoS$_{2}$ down to the Fermi level of graphene. Adsorption on silicene shifts both MoS$_{2}$ bands, valence and conduction, towards the silicene Fermi level, in addition to inducing a gap of 55 meV in the silicene itself. We present analysis of possible charge transfer in these systems and discuss the relevance of these hetero structures for practical applications.   

Phonon Properties in 2D Transition Metal Dichalcogenide Crystals: Symmetry and Dimensionality Matter

2D transition-metal dichalcogenide (TMD) crystals possess lower symmetry compared to their bulk counterparts and thus have different phonon modes and behaviors. While in general the Raman active modes in 3D are also Raman active in 2D, the reverse is not always true due to the reduced symmetry in 2D. Here, using both Raman spectroscopy and first principles calculations, we uncover the ultra-low frequency (5 $\sim$ 55 cm$^{-1})$ interlayer breathing and shear modes in few-layer MoS$_{2}$, MoSe$_{2}$, WS$_{2}$ and WSe$_{2}$, prototypical layered TMDs. The interlayer breathing modes correspond to an optically inactive mode in the bulk, and thus only exist in the 2D case. Remarkably, the frequencies of these modes can be perfectly described using a simple linear chain model. Besides, two new Raman peaks located at 176 cm$^{-1}$ and 310 cm$^{-1}$ were observed ONLY in few-layer WSe$_{2}$, as a result of the lower symmetry in 2D. Our results shed light on a general understanding of the Raman/IR activities of the phonon modes in layered TMD materials and their evolution behaviors from 3D to the 2D.   

Outstanding mechanical properties of monolayer MoS$_2$ and its application in elastic energy storage

The structural and mechanical properties of graphene-like honeycomb monolayer structures of MoS$_2$(g-MoS$_2$) under various large strains are investigated using density functional theory (DFT). g-MoS$_2$ is mechanically stable and can sustain extra large strains: the ultimate strains are 0.24, 0.37, and 0.26 for armchair, zigzag, and biaxial deformation, respectively. The in-plane stiffness is as high as 120 N/m (184 GPa equivalently). The third, fourth, and fifth order elastic constants are indispensable for accurate modeling of the mechanical properties under strains larger than 0.04, 0.07, and 0.13 respectively. The second order elastic constants, including in-plane stiffness, are predicted to monotonically increase with pressure while the Poisson ratio monotonically decreases with increasing pressure. With the prominent mechanical properties including large ultimate strains and in-plane stiffness, g-MoS$_2$ is a promising candidate of elastic energy storage for clean energy. It possesses a theoretical energy storage capacity as high as 8.8 MJ/L and 1.7 MJ/kg, or 476 Wh/kg, larger than a Li-ion battery and is environmentally friendly.   

Watching Silica's Dance: Imaging the Structure and Dynamics of the Atomic (Re-) Arrangements in 2D Glass

Even though glasses are almost ubiquitous---in our windows, on our iPhones, even on our faces---they are also mysterious. Because glasses are notoriously difficult to study, basic questions like: ``How are the atoms arranged? Where and how do glasses break?'' are still under contention. We use aberration corrected transmission electron microscopy (TEM) to image the atoms in a new two-dimensional phase of silica glass -- freestanding it becomes the world's thinnest pane of glass at only 3-atoms thick, and take a unique look into these questions. Using atom-by-atom imaging and spectroscopy, we are able to reconstruct the full structure and bonding of this 2D glass and identify it as a bi-tetrahedral layer of SiO$_{2}$ [1]. Our images also strikingly resemble Zachariasen's original cartoon models of glasses, drawn in 1932. As such, our work realizes an 80-year-old vision for easily understandable glassy systems and introduces promising methods to test theoretical predictions against experimental data. We image atoms in the disordered solid [1] and track their motions in response to local strain [2]. We directly obtain ring statistics and pair distribution functions that span short-, medium-, and long-range order, and test these against long-standing theoretical predictions of glass structure and dynamics. We use the electron beam to excite atomic rearrangements, producing surprisingly rich and beautiful videos of how a glass bends and breaks, as well as the exchange of atoms at a solid/liquid interface. Detailed analyses of these videos reveal a complex dance of elastic and plastic deformations, phase transitions, and their interplay. These examples illustrate the wide-ranging and fundamental materials physics that can now be studied at atomic-resolution via transmission electron microscopy of two-dimensional glasses. Work in collaboration with: S. Kurasch, U. Kaiser, R. Hovden, Q. Mao, J. Kotakoski, J. S. Alden, A. Shekhawat, A. A. Alemi, J. P. Sethna, P. L. McEuen, A.V. Krasheninnikov, A. Srivastava, V. Skakalova, J. C. Meyer, and J.H. Smet. \\[4pt] [1] P. Y. Huang, et al., \textit{Nano Lett.}, \textbf{12} 1081--1086 (2012).\\[0pt] [2] P. Y. Huang et. al, \textit{Science} \textbf{342}, 224-227 (2013)   

The formation and pinning of folds in two-dimensional materials

The isolation of two-dimensional materials such as graphene, hexagonal boron nitride and transition metal dichalcogenides from their bulk counterparts allows a new kind of crystal defect - a fold. Using density functional theory simulations, we characterize the geometry, energetics and formation mechanism of different kinds of folds in fluorinated graphene and modified molybdenum disulfide sheets with sulfur vacancies and selenium substitutions. Furthermore we demonstrate two methods of pinning a fold once it is formed: by the preferential adsorption of adatoms along the fold-line, or by forcing the registry of the folded sheets via selective p- or n-type doping. The latter can be applied to the design and self-assembly of two-dimensional sheets into more complex geometries.   

Possible Structural Phase Transitions in Transition Metal Dichalcogenides

Most of the the transition metal dichalcogenides (TMD) have graphene-like hexagonal crystal structure which are composed of metal atom layers (M) sandwiched between layers of chalcogen atoms (X) and these structures have MX$_2$ stoichiometry. Chalcogen layers can be stacked on top of each other in two different forms: H phase made of trigonal prismatic holes for metal atoms and T phase that consists staggered chalcogen layers forming octahedral holes for metals. Among the TMDs that have been reported to be stable, individual layers of MoS$_2$, MoSe$_2$, WS$_2$ and WSe$_2$ have 1H structure in their ground state while dichalcogens of Ti, V and Ta prefer the 1T phase. In our study we investigate the physical mechanisms underlying for the possible phase transitions in TMDs. Our calculations based on first-principles techniques reveal that in addition to H and T phases various distorted H and T phases can be also stabilized by point defects. These new phases have entirely different electronic properties.   

SPE-LEEM Studies on the Surface and Electronic Structure of 2-D Transition Metal Dichalcogenides (Part II)

In this work, we studied the surface and electronic structure of monolayer and few-layer exfoliated MoS$_{\mathrm{2}}$ and WSe$_{\mathrm{2}}$, as well as chemical-vapor-deposition (CVD) grown MoS$_{\mathrm{2}}$, using Spectroscopic Photoemission and Low Energy Electron Microscope (SPE-LEEM). LEEM measurements reveal that, unlike exfoliated MoS$_{\mathrm{2}}$, CVD-grown MoS$_{\mathrm{2}}$ exhibits grain-boundary alterations due to surface strain. However, LEEM and micro-probe low energy electron diffraction show that the quality of CVD-grown MoS$_{\mathrm{2}}$ is comparable to that of exfoliated MoS$_{\mathrm{2}}$. Micrometer-scale angle-resolved photoemission spectroscopy (ARPES) measurement on exfoliated MoS$_{\mathrm{2}}$ and WSe$_{\mathrm{2}}$ single-crystals provides direct evidence for the shifting of the valence band maximum from $\Gamma $ to ${\rm K}$, when the layer number is thinned down to one, as predicted by density functional theory. Our measurements of the k-space resolved electronic structure allow for further comparison with other theoretical predictions and with transport measurements.   

Characterization of large area molybdenum disulphide by low energy electron microscopy

Molybdenum disulphide (MoS$_{2}$) is a new 2D direct-bandgap semiconductor material which has recently attracted substantial interest due to its potential applications in electronics, optics and energy storage. One of the challenges that needed to be overcome is in the large scale synthesis of high quality single crystal MoS$_{2}$. Recently, it is shown that chemical vapor deposition (CVD) is a promising way of in the production of single layer MoS$_{2}$. Here we report our study using low energy electron microscopy (LEEM) of large area MoS$_{2}$ synthesized by CVD technique. The MoS$_{2}$ samples are grown on Si/SiO$_{2}$ substrates and then transferred onto n-doped Si substrates. In the LEEM images, we observe large triangular shaped MoS$_{2}$ flakes along with irregular shaped flakes. Using low energy electron diffraction (LEED) and dark field imaging technique, we identify the triangularly shaped flakes as MoS$_{2}$ single crystal while the irregular ones contain multiple domains orientations. These studies provide insight into the growth of large area single domain MoS$_{2}$ crystals using CVD technique and the transfer process onto different substrates for potential device applications.   

SPE-LEEM Studies on the Surface and Electronic Structure of 2-D Transition Metal Dichalcogenides

In this work, we studied the surface and electronic structure of monolayer and few-layer exfoliated MoS$_{\mathrm{2}}$ and WSe$_{\mathrm{2}}$, as well as chemical-vapor-deposition (CVD) grown MoS$_{\mathrm{2}}$, using Spectroscopic Photoemission and Low Energy Electron Microscope (SPE-LEEM). LEEM measurements reveal that, unlike exfoliated MoS$_{\mathrm{2}}$, CVD-grown MoS$_{\mathrm{2}}$ exhibits grain-boundary alterations due to surface strain. However, LEEM and micro-probe low energy electron diffraction show that the quality of CVD-grown MoS$_{\mathrm{2}}$ is comparable to that of exfoliated MoS$_{\mathrm{2}}$. Micrometer-scale angle-resolved photoemission spectroscopy (ARPES) measurement on exfoliated MoS$_{\mathrm{2}}$ and WSe$_{\mathrm{2}}$ single-crystals provides direct evidence for the shifting of the valence band maximum from $\Gamma $ to K, when the layer number is thinned down to one, as predicted by density functional theory. Our measurements of the k-space resolved electronic structure allow for further comparison with other theoretical predictions and with transport measurements.   

Inorganic nanotubes and fullerene-like nanoparticles at the crossroad between materials science and nanotechnology

This presentation is aimed at underlying the principles, synthesis, characterization and applications of inorganic nanotubes (INT) and fullerne-like (IF) nanoparticles (NP) from 2-D layered compounds. While the high temperature synthesis and study of IF materials and INT from layered metal dichalcogenides, like WS$_{2}$ and MoS$_{2}$ remain a major challenge, progress with the synthesis of IF and INT structures from various other compounds has been realized, as well. Intercalation and doping of these nanostructures, which lends itself to interesting electronic properties, has been realized, too. Core-shell nanotubular structures, like PbI$_{2}$@WS$_{2}$ and SnS/SnS$_{2}$ and PbS/NbS$_{2}$ nanotubes from ``misfit'' compounds have been recently reported. Re doping of the IF and INT endow them with interesting electrical and other physio-chemical properties. Major progress has been achieved in elucidating the structure of INT and IF using advanced microscopy techniques, like aberration corrected TEM and electron tomography. Also recently, scaling up efforts in collaboration with ``NanoMaterials'' resulted in multikilogram production of (almost) pure multiwall WS$_{2}$ nanotubes phases. Extensive experimental and theoretical analysis of the mechanical properties of individual INT and more recently IF NP was performed casting light on their behavior in the macroscopic world. IF-MS$_{2}$ (M$=$W,Mo, etc) were shown to be superior solid lubricants in variety of forms, including an additive to various lubricating fluids/greases and for various self-lubricating coating. Full commercialization of products based on this technology is taking place now.   

Session J32: Focus Session: Physics of Proteins I

Copper and Zinc Chelation as a Treatment of Alzheimer's Disease

Alzheimer's disease (AD) is a neurodegenerative disorder affecting millions of people in the U.S. The cause of the disease remains unknown, but amyloid-$\beta$ (A$\beta$), a short peptide, is considered causal its pathogenesis. At cellular level, AD is characterized by deposits mainly composed of A$\beta$ that also contain elevated levels of transition metals ions. Targeting metals is a promising new strategy for AD treatment, which uses moderately strong metal chelators to sequester them from A$\beta$ or the environment. PBT2 is a chelating compound that has been the most promising in clinical trials. In our work, we use computer simulations to investigate complexes of a close analog of PBT2 with Cu$^{2+}$ and Zn$^{2+}$ ions. The calculations employ KS/FD DFT method, which combines Kohn-Sham DFT with the frozen-density DFT to achieve efficient description of explicit solvent beyond the first solvation shell. Our work is based on recent experiments and examines both 1:1 and 2:1 chelator-metal stochiometries detected experimentally. The results show that copper attaches more strongly than zinc, find that 1:1 complexes involve water in the first coordination shell and determine which one of several possible 2:1 geometries is the most preferable.   

Structural Transitions and Aggregation in Amyloidogenic Proteins

Amyloid fibrils are a common component in many debilitating human neurological diseases such as Alzheimer's and Parkinson's. A detailed molecular-level understanding of the formation process of amyloid fibrils is crucial for developing methods to slow down or prevent these horrific diseases. Alpha-helix to beta-sheet structural transformation is commonly observed in the process of fibril formation. We performed replica-exchange molecular dynamics simulations of structural transformations in an engineered model peptide cc-beta. Several sets of simulations with different number of cc-beta monomers were considered. Conversion of alpha-helix monomers to beta strands and the aggregation of beta strand monomers into sheets were analyzed as a function of the system size. Hydrogen bond analysis was performed and the beta-aggregate structures were characterized by a nematic order parameter.   

Computational stability ranking of mutated hydrophobic cores in staphylococcal nuclease and T4 lysozyme using hard-sphere and stereochemical constraints

Molecular dynamics methods have significantly advanced the understanding of protein folding and stability. However, current force-fields cannot accurately calculate and rank the stability of modified or \textit{de novo} proteins. One possible reason is that current force-fields use knowledge-based corrections that improve dihedral angle sampling, but do not satisfy the stereochemical constraints for amino acids. I propose the use of simple hard-sphere models for amino acids with stereochemical constraints taken from high-resolution protein crystal structures. This model can enable a correct consideration of the entropy of side-chain rotations, and may be sufficient to predict the effects of single residue mutations in the hydrophobic cores of staphylococcal nuclease and T4 lysozyme on stability changes. I will computationally count the total number of allowed side-chain conformations $\Omega $ and calculate the associated entropy, S $=$ k$_{\mathrm{B}}$ln($\Omega )$, before and after each mutation. I will then rank the stability of the mutated cores based on my computed entropy changes, and compare my results with structural and thermodynamic data published by the Stites and Matthews groups. If successful, this project will provide a novel framework for the evaluation of entropic protein stabilities, and serve as a possible tool for computational protein design.   

Single-Molecule Ion Channel Conformational Dynamics in Living Cells

Stochastic and inhomogeneous conformational changes regulate the function and dynamics of ion channels that are crucial for cell functions, neuronal signaling, and brain functions. Such complexity makes it difficult, if not impossible, to characterize ion channel dynamics using conventional electrical recording alone since that the measurement does not specifically interrogate the associated conformational changes but rather the consequences of the conformational changes. Recently, new technology developments on single-molecule spectroscopy, and especially, the combined approaches of using single ion channel patch-clamp electrical recording and single-molecule fluorescence imaging have provided us the capability of probing ion channel conformational changes simultaneously with the electrical single channel recording. By combining real-time single-molecule fluorescence imaging measurements with real-time single-channel electric current measurements in artificial lipid bilayers and in living cell membranes, we were able to probe single ion-channel-protein conformational changes simultaneously, and thus providing an understanding the dynamics and mechanism of ion-channel proteins at the molecular level. The function-regulating and site-specific conformational changes of ion channels are now measurable under physiological conditions in real-time, one molecule at a time. We will focus our discussion on the new development and results of real-time imaging of the dynamics of gramicidin, colicin, and NMDA receptor ion channels in lipid bilayers and living cells. Our results shed light on new perspectives of the intrinsic interplay of lipid membrane dynamics, solvation dynamics, and the ion channel functions.   

Thermal response of alpha-synuclein structure with knowledge-based residue-residue interactions

Structure and dynamics of alpha-synuclein (140 residues) are studied via a coarse-grained Monte Carlo simulation as a function of temperature. Knowledge-based residue-residue [1] interactions are used as input to a generalized LJ potential. We analyze a number of local and global physical quantities such as residue mobility profiles, contact map, radius of gyration, structure factor, etc. We find that the radius of gyration ($R_{g})$ of the protein increases on increasing the temperature within a range. Although the thermal response of the gyration radius shifts with the type of knowledge-based interaction potential, general feature of linear response is retained. These findings are consistent with the NMR measurements [2] on the variation of the gyration radius with the temperature. Detailed analysis of structure factor reveals the thermal response of multi-scale segmental conformations.\\[4pt] [1] S. Miyazawa and R.L. Jernigan, Macromolecules 18,534 (1985); M.R. Betancourt and D. Thirumalai, Protein Sci. 2,361 (1999). \\[0pt] [2] J.R. Allison et al. JACS 131, 18314 (2009).   

Simulation Model of Protein Transport and Stabilization

In a previous communication (Linhananta et al., Biophys. J., 2011, 100, 459), we reported results of a simulation model of a protein in solvents with protein-solvent contact energy parameter $\varepsilon _{ps}$, which mimics the effects of osmolytes ( $\varepsilon _{ps}>0$) and denaturants ($% \varepsilon _{ps}<0$ ). Here a model three-helix-bundle (THB) protein in solvents is confined in cylindrical cavity to mimic GroEL/ES. The interior wall is characterized by the protein-wall energy $\varepsilon _{pw}$ , and solvent-wall energy, $\varepsilon _{sw}$. Simulations found a substantial increase in the folding temperature from $T^{\ast }=4.2$, in scaled unit, for THB in vacuum, to $T^{\ast }>6.0$ for confined THB in osmolytes. The model is generalized to THB and solvents confined in two connected cylindrical segments. The bottom segment represents the interior of a GroEL/ES, with the sidewall characterized by the parameters $\varepsilon _{pw}$ and $\varepsilon _{sw}$. The upper segment represents the exterior surrounding the GroEL/ES, with periodic boundary condition on the sidewall. The protein and solvents can move through the channel connecting the two segments. Simulation data reveals new insights on the transport of unfolded proteins into GroEL/ES.   

The heat released in single catalytic events locally enhances enzyme diffusion

Recent experiments have shown that some enzymes catalyzing highly exothermic reactions exhibit increased diffusion with rising substrate concentration. We present a stochastic theory linking increased enzyme diffusion to reaction rate, discuss other possible origins for diffusion coefficient increases and finally provide a mechanistic interpretation showing how the heat released by the reaction perturbs the enzyme.   

Measuring Conformational Dynamics of Single Biomolecules Using Nanoscale Electronic Devices

Molecular motion can be a rate-limiting step of enzyme catalysis, but motions are typically too quick to resolve with fluorescent single molecule techniques. Recently, we demonstrated a label-free technique that replaced fluorophores with nano-electronic circuits to monitor protein motions. The solid-state electronic technique used single-walled carbon nanotube (SWNT) transistors to monitor conformational motions of a single molecule of T4 lysozyme while processing its substrate, peptidoglycan. As lysozyme catalyzes the hydrolysis of glycosidic bonds, two protein domains undergo 8 {\AA} hinge bending motion that generates an electronic signal in the SWNT transistor. We describe improvements to the system that have extended our temporal resolution to 2 $\mu s$. Electronic recordings at this level of detail directly resolve not just transitions between open and closed conformations but also the durations for those transition events. Statistical analysis of many events determines transition timescales characteristic of enzyme activity and shows a high degree of variability within nominally identical chemical events. The high resolution technique can be readily applied to other complex biomolecules to gain insights into their kinetic parameters and catalytic function.   

Observation of Protein Structural Vibrational Mode Sensitivity to Ligand Binding

We report the first measurements of the dependence of large-scale protein intramolecular vibrational modes on ligand binding. These collective vibrational modes in the terahertz (THz) frequency range (5-100 cm$^{-1})$ are of great interest due to their predicted relation to protein function. Our technique, Crystals Anisotropy Terahertz Microscopy (CATM), allows for room temperature, table-top measurements of the optically active intramolecular modes. CATM measurements have revealed surprisingly narrowband features [1]. CATM measurements are performed on single crystals of chicken egg-white lysozyme (CEWL) as well as CEWL bound to tri-N-acetylglucosamine (CEWL-3NAG) inhibitor. We find narrow band resonances that dramatically shift with binding. Quasiharmonic calculations are performed on CEWL and CEWL-3NAG proteins with CHARMM using normal mode analysis. The expected CATM response of the crystals is then calculated by summing over all protein orientations within the unit cell. We will compare the CATM measurements with the calculated results and discuss the changes which arise with protein-ligand binding. \\[4pt] [1] G. Acbas, K.A. Niessen, E. Snell, and A.G. Markelz, ``Optical Measurements of Long-Range Protein Structural Motions,'' Nature Communications, In Press.   

Using engineered intra-molecular disulfide bonds to identify FIMs that matter

The realization that proteins are not the static structures derived from crystallographic structure determination, but instead undergo conformational dynamics on a wide range of length- and time-scales started a novel field of research that has remained active to the present day. Protein dynamics have been shown to occur on a complex energy landscape, and can be divided into equilibrium fluctuations (EFs) and non-equilibrium functionally important motions (FIMs). Much effort has been spent on the complex task of discovering and describing such FIMs. However, if a region of a protein is found to undergo conformational changes during function, this is not sufficient to conclude that this motion is important for protein function. We use engineered intra-molecular disulfide bonds as an experimental tool to examine functionally critical conformational changes. In this approach the effect of preventing large-scale motions in a specific region of the protein by the introduction of an intramolecular covalent crosslink on protein functional dynamics is examined. We will report results of the application of this approach to photoactive yellow protein, a bacterial photosensor.   

Live cell FLIP: anomalous protein diffusion and its fluctuation

Macromolecular crowding in the cell modulates protein structure and stability, as well as protein diffusion and transportation in cytoplasm. This crowded environment limits the protein diffusion in a confined space and gives rise to anomalous subdiffusion at long time and distance scales. The anomalous diffusion in living cells have been sufficiently studied with fluorescence recovery after photobleaching (FRAP). However, this method focuses on local diffusion, giving too little information about the global cellular environment. Fluorescence loss in photobleaching (FLIP), though giving up details about short distance behavior, provides a better view on the larger scale of anomalous diffusion. We use this powerful tool combined with numerical simulation to study the temperature and protein conformation dependence of diffusion in living cells. We compared the different anomalous behaviors of protein diffusion between cells. The fluctuation of diffusion in cellular microenvironment is also studied.   

Molecular Dynamic Study to Determine the Ammonia Conduction Mechanisms in Human RhCG and Bacterial Homoloques

The transport of Ammonia is provided by Amt/MEP/Rh protein superfamily. The x-ray structures of AmtB from Escherichia coli, Rh50 from Nitrosomonas europaea, and human RhCG show only few differences on periplasmic vestibules. After more than microsecond simulation on three models, we determined the striking difference on conduction mechanism between bacterial AmtB and Human RhCG proteins. In AmtB the backbone carbonyl groups at the periplasmic vestibule direct charged ammonia to the conserved aromatic cage at the bottom of the vestibule. Furthermore, two partially stacked phenyl rings of F107 and F215, separating the periplasmic vestibule from the hydrophobic lumen, flip open and closed \textit{simultaneously }with a frequency of approximately 108 flipping events per second. During the passage from the phenyl gates charged ammonia releases its proton and becomes gas. However, the absence of an aromatic cage on Rh proteins and a strongly conserved E166 residue in the vicinity hints different conduction mechanism. Our studies confirm the conserved E166 emerges as a strong charged ammonia recruitment site for Human RhCG. The conserved phenyl gate behaves different for Rh proteins and the synchronized motion is not observed. These findings suggest a different deprotonation mechanism than bacterial AmtB.   

Conformational Transition Mechanism of Adenylate Kinase: A Comparison of All-Atom Molecular Dynamics Simulation to Coarse-Grained Methods

Adenylate kinase (ADK) performs a large conformational transition between its open and closed conformations. In this transition, order of conformational events can be investigated by molecular dynamics (MD) method. However, MD method requires large-scale computational resources and a significant amount of time to observe a full conformational transition. On the other hand, coarse-grained methods can produce transition pathways in a short amount of time with a questionable accuracy. To assess accuracy of coarse-grained methods, they need to be compared with all-atom models. Due to this reason, we produced a full conformational transition of ADK from closed state to open state by using all-atom classical molecular dynamics. This conformational transition has been compared with 7 coarse-grained methods in terms of order of conformational events. In the end, we evaluated successes and failures of each coarse-grained method.   

Conformational Analysis of Single Polymer Chain by Super-resolution Fluorescence Microscopy

The direct observation of individual polymer chains would provide valuable information to understand the fundamental properties of polymer materials. Fluorescence imaging is the most effective method to detect a single molecule embedded in a bulk medium; however, the imaging of the conformation of a single chain has been impossible because of the diffraction-limited spatial resolution ($\sim$ 200 nm). In the current study, we developed a super-resolution fluorescence microscopy technique, photo-activated localization microscopy (PALM), for the direct observation of the conformation of a single polymer chain. For the PALM observation, a trace amount of poly(butyl methacrylate) (PBMA) labeled by rhodamine spiroamide was dispersed in the unlabeled PBMA matrix. The conformation of the individual PBMA chain was observed in three dimensions with the lateral spatial resolution of 20 nm and the depth resolution of 50 nm. The nanometric imaging by the super-resolution technique was applied to the conformational analysis of single polymer chain under macroscopic deformation.   

Session J33: Few-body and Cold Molecule Physics

Manipulation of p-wave scattering of cold atoms in low dimensions using the magnetic field vector

It is well known that the magnetic Feshbach resonances of cold atoms are sensitive to the magnitude of the external magnetic field. Much less attention has been paid to the \textit{direction} of such a field. In this work we calculate the scattering properties of spin polarized fermionic atoms in reduced dimensions, near a $p$-wave Feshbach resonance. Because of spatial anisotropy of the $p$-wave interaction, the scattering has nontrivial dependence on both the magnitude and the direction of the magnetic field. In addition, we identify an inelastic scattering process which is impossible in the isotropic-interaction model; the rate of this process depends considerably on the direction of the magnetic field. Significantly, an EPR entangled pair of identical fermions may be produced during this inelastic collision. This work opens a new method to manipulate resonant cold atomic interactions.   

Pairing of few Fermi atoms in one dimension

Experimental advances allow us to confine a chosen number of few quantum degenerate Li6 atoms in a trap with unit precision down to the empty-trap limit. The Heidelberg group recently observed an even-odd oscillation of the ``ionization'' energy required to subtract an atom from a one-dimensional trap in the presence of moderate attractive interactions, which was attributed to pairing [PRL 111, 175302 (2013)]. Naively, one would expect pairing to be strongly suppressed in one dimension, due to the lack of orbital degeneracies. Here we address theoretically the pairing behavior of a few Fermi atoms in a one-dimensional harmonic trap through the exact diagonalization of the fully interacting Hamiltonian. From the analysis of exact ground- and excited-state energies and wave functions we extract both the pairing gap and the Cooper pair size, reproducing the observed even-odd behavior. Our results demonstrate that pairing in one dimension is a strongly cooperative effect that significantly deviates from the behavior predicted by perturbation theory at interaction strengths within experimental reach.   

Efimov physics in an ultracold Bose-Fermi gas of ~${ }^{40}K$ and ${ }^{87}Rb$ atoms

We present measurements of Efimov physics in an ultracold Bose-Fermi gas of ${ }^{40}K$and ${ }^{87}Rb$ atoms near an interspecies Feshbach resonance. ~In particular, we measure loss rate coefficients for the trapped gas and find a resonance in the inelastic collisions of Feshbach molecules with~${ }^{87}Rb$ atoms.~However, we do not observe any E?mov-related resonances in the rates of inelastic collisions between three atoms.~   

High fidelity pseudopotentials for ultracold atomic gases

An accurate computational description of ultracold atoms interacting with effective repulsive interactions has proved elusive. Previous computational work has either used a truly repulsive potential (most often, a hard sphere potential), which fails to recover the very short-range nature of the interactions, or the excited states of an attractive potential. In this presentation, we propose a new pseudopotential, inspired by those used in electronic structure work. This potential is carefully tuned to recover the correct scattering length and effective range for a broad range of atomic energies, and does not have an undesirable bound state. This will greatly facilitate quantum Monte-Carlo and exact diagonalization simulations of cold atoms. We have used this potential to study the zero-temperature phase diagram of ultracold atomic gases at $k_Fa \ge 0$, obtaining accurate values for the ferromagnetic phase transition at different polarizations.   

Universality in $s$-wave and higher partial wave Feshbach resonances: an illustration with a single atom near two scattering centers

It is well-known that cold atoms near $s$-wave Feshbach resonances have universal properties that are insensitive to the short-range details of the interaction. What is less known is that atoms near higher partial wave Feshbach resonances also have remarkable universal properties. We will illustrate this with a single atom interacting resonantly with two fixed static centers. At a Feshbach resonance point with orbital angular momentum $L\ge1$, we find $2L+1$ shallow bound states whose energies behave like $1/R^{2L+1}$ when the distance $R$ between the two centers is large. This sheds additional light on the fundamental question whether Efimov effect exists for higher partial wave resonances. The effects of nonresonant partial-wave channels and the shape parameters in the effective range expansions enter as correction terms. Near $p$-wave and higher partial wave resonances, the energies can be described by a simple universal formula in terms of a parameter called ``proximity parameter.'' We will also discuss modifications of the low energy physics due to the long range Van der Waals potential.   

Averaged collision and reaction rates in a two-species gas of ultracold fermions

Reactive or elastic two-body collisions in an ultracold gas are affected by quantum statistics. We study ensemble-averaged collision rates for a two-species fermionic gas, where the two species may have different masses, densities, and temperatures. It is shown in what way Fermi-averaged collision rates deviate from Boltzmann-averaged ones, particularly for a gas with strong imbalance of masses or densities. The results are independent of the details of the collision process.   

A two-dimensional pseudospectral multi-configuration Hartree-Fock method for low-Z atoms in intense magnetic fields

We present here the very first two-dimensional multi-configuration Hartree-Fock studies of low-Z atoms in intense magnetic fields. The first few low-lying states are calculated in this study. The method described herein is applicable to calculations of atomic structure in magnetic fields of arbitrary strength as it exploits the natural symmetries of the problem without assumptions of any basis functions for expressing the wave functions of the electrons, or the commonly employed adiabatic approximation. A pseudospectral formulation is employed which affords considerable computational speed-up and the results obtained here are significant improvements upon earlier pseudospectral single-configuration calculations and are consistent with findings elsewhere. We also present new data for some of the states of the low-Z atoms considered here.   

Hyperfine structure of OH molecules in electric and magnetic fields

Ultracold polar molecules offer a unique opportunity in table-top experiments to study quantum phenomena originating from strong dipole-dipole interactions and incorporating internal degrees of freedom controllable by external electric and magnetic fields. Recently, a gas of OH molecules was evaporatively cooled at JILA to milliKelvin temperatures. However, in the presence of electric and magnetic fields, the energy spectra of OH were calculated only to energy scales of mK, far from the sub-microKelvin temperatures at which OH molecules will become quantum degenerate. We investigate single-particle energy spectra of the OH radical in the lowest rovibrational and electric ground states under combined electric and magnetic fields. In addition to the fine-structure interactions, the hyperfine interactions and centrifugal distortion effects are taken into account, yielding the zero-field spectrum of the lowest ${}^2\Pi_{3/2}$ manifold to an accuracy of less than 2kHz$\sim$100nK. We also examine level crossings and repulsions in hyperfine structures induced by applied electric and magnetic fields. We will mention many-body applications of ultracold OH molecules to simulate quantum dipolar systems.   

Efimov trimers under confinement: From discrete to continuous scaling symmetry

The effect of dimensionality and confinement on the interactions between particles is key to understanding the behaviour of many quantum systems. Classic examples range from the fractional quantum Hall effect and high temperature superconductivity to the adsorption of molecules on a surface. As a general rule, one expects confinement to favour the binding of particles. However, attractively interacting bosons apparently defy this expectation: while three identical bosons in three dimensions can support an infinite tower of Efimov trimers, only two universal trimers exist in the two dimensional (2D) case. Here we reveal how these two limits are connected by investigating the problem of three bosons confined by a harmonic potential along one direction. We show that the confinement breaks the discrete Efimov scaling symmetry and destroys the weakest bound trimers. However, the deepest bound Efimov trimer persists under strong confinement and hybridizes with the quasi-two-dimensional trimers, yielding a superposition of trimer configurations that effectively involves tunnelling through a short-range repulsive barrier. Our results have immediate impact on the ongoing efforts to observe Efimov scaling in an ultracold atomic gas.   

Tunneling and two particle interference with laser cooled bosons in optical tweezers

We report on experiments realizing coherent tunneling of laser cooled atoms between precisely tunable optical tweezers. To verify the degree of indistinguishability achieved via laser cooling, we perform Hong-Ou-Mandel interferometry of massive bosons in tunnel-coupled tweezers. This marks the first direct observation of quantum indistinguishability with independently prepared laser cooled atoms. Our results demonstrate the viability of the tweezer plus laser cooling platform for studying few-body systems in the quantum regime, with highly tunable parameters of tunneling and interaction.   

Using STM tip as electrochemical sensor for the characterization of bond vibration frequencies of a chemical analyte

Traditional electrochemical interfaces are comprised of an electrically biased electrode-electrolyte interface, where charge exchange occurs between electronic energy levels of the electrode and a redox-active ion in the electrolyte. Much of the recent progress in electrochemical sensing technology has focused on enhancing the detection limit of such sensing platforms. However, much of the molecular-level chemical information describing the non-redox active species that may also be present in the electrolyte, which is encoded in the acquired current/voltage signal, is lost as background information. In this talk, a design methodology is proposed for electrochemical interfaces that are engineered from STM tips specifically to transduce information about the intra-molecular bond vibrational frequencies of non-redox active molecular analytes. A quantum statistical model of a generalized charge transfer process, developed by the authors, will be presented as the underpinning for the design method. Minimization of electronic and nuclear entropy will be derived from the presented model, as the necessary condition required for resolving vibrational frequency information, and we will also describe select experimental strategies that may be implemented for total entropy minimization.   

Electron Energy Deposition in Fast-Shock Ignition

Calculations of fast electrons penetration and energy stopping power in dense fuel show that about 25{\%} of the initial electron energy effectively reaches to the central part of the fuel if the initial electron energy is of the order $\sim$6.5 MeV. To evaluate more realistically the performance of FSI approach, we have used a quasi-two temperature electron energy distribution function of Strozzi (2012) and fast ignitor energy formula of Bellei (2013) that are consistent with 3D PIC simulations. In terms of figure of merit and for fuel mass \textgreater 1 mg, the general advantages of fast-shock ignition in comparison with shock ignition can be estimated to be better than 1.3.   

Testing excited-state energy-density functionals and potentials with the ionization potential theorem

The modified local spin density functional and the related local potential for excited-states are tested by employing the ionization potential theorem. The functional is constructed [1] by splitting k-space. Since its functional derivative cannot be obtained easily, the corresponding potential is given by analogy to its ground-state counterpart. Further, to calculate the highest occupied orbital energy $\epsilon_{max}$ accurately, the potential is corrected for its asymptotic behavior by employing the van Leeuwen-Barends correction [2] to it. The highest occupied orbital energy $\epsilon_{max}$ thus obtained is then compared with the $\Delta$SCF ionization energy calculated using the excited-state functional. It is shown that the two match quite accurately, demonstrating thereby that our approach of constructing excited-state functional is on sound footing. \\[4pt] [1] P. Samal and M.K. Harbola, J. Phys. B: At. Mol. Opt. Phys. 39, 4065 (2006); M. Hemanadhan and M.K. Harbola, J. Mol. Struct. Theochem \textbf{943}, 152 (2010).\\[0pt] [2] R. van Leeuwen and E.J. Baerends, Phys. Rev. A \textbf{49}, 2421 (1994).   

Session J34: Focus Session: Hybrid Quantum Devices

Quantum Hybrid Systems: New Interface between Quantum Optics and Nanoscience

We will discuss recent developments involving the use of quantum optical techniques in a combination with nanoscale localization of light and atoms inside or near solid-state systems. Specially, realization of quantum optical phase switch operating at a single photon level will be presented. In addition, new applications involving nanoscale magnetic and temperature sensing in living cells will be discussed.   

Characterization of Spin-Lattice Relaxation Times of Impurity Centers in Diamond with Superconducting Circuits

We present measurements of the T$_1$ relaxation times of impurity centers in diamond below 4 K obtained with both a nanobridge SQUID magnetometer and a superconducting resonator with an independent broadband excitation line. We show temperature, field and power dependence of the P1 center relaxation times, and discuss how these dependences fit within current models of spin-lattice relaxation and spin diffusion. These measurements are a step toward identifying the dominant spin-lattice relaxation processes in diamond at cryogenic temperatures, and optimizing such solid-state devices for quantum information processing.   

Fourier space encoding techniques applied to magnetic resonance imaging using NV centers in diamond

Nitrogen-vacancy (NV) centers in diamond have demonstrated a number of properties that make them viable candidates for detecting nearby external spins with sub-nanometer resolution at ambient conditions. So far, they have been used to image dark electron spins on the diamond surface and resolve few spins[1], as well as detect ensembles of nuclear spins external to the diamond[2, 3]. A promising direction for improving spatial resolution, signal-to-noise ratio, as well as stability of the detector over time is to integrate variable pulsed DC and RF magnetic field gradients on-chip. Spatial information about the target spins can then be obtained by using Fourier imaging techniques[4]. We present preliminary results in depositing silver wires on the diamond surface above the NV center. We flow time-dependent currents through the wires to produce controllable magnitudes of magnetic fields at the NV, which can be used to both address the NV and provide field gradients for imaging techniques. Our approach has the potential to enable highly-resolved nuclear spin imaging. \\[4pt] [1] Grinolds \textit{et al.}, submitted (2013)\\[0pt] [2] Staudacher \textit{et al.}, Science \textbf{339}, 561 (2013)\\[0pt] [3] Mamin \textit{et al.}, Science \textbf{339}, 557 (2013)\\[0pt] [4] Nichol \textit{et al.}, Phys. Rev. X \textbf{3}, 031016 (2013)   

Coherent Coupling of a Nitrogen Vacancy Center in Diamond to Lattice Strain

Nitrogen-vacancy (NV) centers in diamond are a versatile resource in the development of hybrid quantum systems due to their excellent quantum properties and their ability to strongly couple to several external degrees of freedom. Recent theoretical studies indicate that a system composed of single-crystal diamond (SCD) mechanical resonators may be used to form a hybrid quantum network in which an embedded NV spin forms a quantum memory, and the strain-coupled extended phonon modes of the resonator serve as a quantum data bus. However, experimental investigations of the strain interaction with NV centers are prominently lacking. Here, we use high quality SCD cantilevers to quantitatively measure the NV ground state strain sensitivity in directions both parallel and perpendicular to the NV symmetry axis. Furthermore, we demonstrate strain-mediated coherent coupling of the NV spin evolution to the mechanical motion of the resonator.   

Atomic clock transitions in NV centers in diamond

Progress towards a spin-ensemble based quantum memory for superconducting qubits has been made over the past few years, involving reversible coherent storage and retrieval of a single microwave photon from a qubit into the spin ensemble [1]. In this experiment, the storage time is however limited to few 100ns by inhomogeneous broadening of the ensemble, and refocusing techniques like Hahn echo have to be applied to benefit from their long coherence times [2]. First experimental results with refocusing will be presented demonstrating the storage and retrieval of a few-photon field into an ensemble of electronic spins (NV centers in diamond) with storage time up to 40$\mu$s. Of particular importance in this adaptation of Hahn echo techniques for quantum memory are so-called `clock transitions' where the spin frequency is insensitive to first order magnetic field fluctuations, leading to longer coherence times [3]. We study the spectrum and coherence time of an ensemble of $^{14}$NV centers, and reveal the existence of three such clock transitions. \\[4pt] [1] Y. Kubo, PRL 107, 220501 (2011).\\[0pt] [2] B. Julsgaard, PRL 110, 250503 (2013).\\[0pt] [3] G. Wolfowicz, Nature Nano 8, 561-564 (2013).   

Electron Spin Resonance Detected by a Superconducting Qubit

We have realized a highly sensitive electron spin resonance (ESR) spectrometer. We use a superconducting qubit as a single-microwave-photon detector for the microwave signal emitted by the spins. We implement such an ESR spectrometer in a hybrid quantum circuit [1] where an ensemble of electron spins is coherently coupled to a superconducting qubit via a frequency tunable ``quantum bus'' cavity [2,3]. The electron spins are nitrogen-vacancy (NV) centers in a diamond crystal. A very weak excitation microwave pulse is first applied to a spin ensemble, during which the quantum bus cavity is far detuned from the resonance frequency of the spins. Immediately after the excitation pulse, the quantum bus cavity is rapidly tuned at resonance with the spins for a certain time such that the weak excitation is transferred to the cavity. Finally, the excitation in the cavity is swapped to the qubit; then the excited state probability of the qubit is measured. Small values of the magnetization, $\sim$ 15 m$_{\mathrm{B}}$, can be detected out of 10$^{11}$ spins by this spectrometer [4]. [1] Kubo et al., PRL, \textbf{107}, 220501 (2011). [2] Kubo et al., PRA, \textbf{85}, 012333 (2012). [3] Kubo et al., PRL, \textbf{105}, 140502 (2010). [4] Kubo et al., PRB, \textbf{86}, 064514 (2012).   

CPHASE gate for two distant NV center spins using optical cavity QED

We propose and analyze a controlled-phase (CPHASE) gate for the spins of two nitrogen-vacancy (NV) centers in diamond embedded in a common optical cavity and driven by two off-resonant lasers. In combination with previously shown single-qubit gates, CPHASE allows for arbitrary quantum computations with the NV $|m_s=0\rangle$ and $|m_s=-1\rangle$ ground states. The coupling of the NV center spin to the cavity mode is based upon Raman transitions via the excited states of the NV center and can be controlled with the intensity or the relative phase of the lasers. We find a characteristic laser frequency at which a laser photon is only scattered into the cavity mode if the NV center spin is $|m_s=0\rangle$, and not in the case $|m_s=-1\rangle$. The scattered photon can then be reabsorbed by another selectively driven NV center and give rise to the conditional phase shift required for the CPHASE gate. Selectivity of NV centers within a larger array could be achieved using electrical control of the zero-field splittings or strain engineering in the crystal. We estimate a gate time of below 10 ns, several orders of magnitude shorter than typical NV electron spin coherence times. The separation between the two NV centers is only limited by the extension of the cavity.   

Entanglement transfer from microwaves to diamond NV centers

Strong candidates to create quantum entangled states in solid-state environments are the nitrogen-vacancy (NV) defect centers in diamond. By the combination of radiation from different wavelength (optical, microwave and radio-frequency), several protocols have been proposed to create entangled states of different NVs. Recently, experimental sources of non-classical microwave radiation have been successfully realized. Here, we consider the entanglement transfer from spatially separated two-mode microwave squeezed (entangled) photons to a pair of NV centers by exploiting the fact that the spin triplet ground state of a NV has a natural splitting with a frequency on the order of GHz (microwave range). We first demonstrate that the transfer process in the simplest case of a single pair of spatially separated NVs is feasible. Moreover, we proceed to extend the previous results to more realistic scenarios where $^{13}$C nuclear spin baths surrounding each NV are included, quantifying the degradation of the entanglement transfer by the dephasing/dissipation effects produced by the nuclear baths. Finally, we address the issue of assessing the possibility of entanglement transfer from the squeezed microwave light to two nuclear spins closely linked to different NV center electrons.   

Strong coupling of ferromagnetic magnons to a microwave resonator

Coherent coupling between paramagnetic spin ensembles and superconducting quantum circuits is now widely studied for quantum memories and microwave-to-optical quantum transducers. Since those applications require strong coupling and sufficiently long coherence time simultaneously, collective excitation (magnon) in yttrium iron garnet (YIG), a typical ferromagnetic insulator, is an alternative promising candidate. The material known to have high spin density (2$\times 10^{21} \mathrm{cm}^{-3}$) and narrow ferromagnetic resonance (FMR) linewidth ($\sim$ 45 kHz at 4.2 K). As a first step towards quantum state transfer from superconducting qubits to YIG magnons, we demonstrate strong selective coupling between a 3D microwave resonator and the uniform magnon mode. In the experiment, we used a YIG sphere with a diameter of 0.75 mm, mounted in a copper resonator and cooled down to 10 mK. We clearly observed the normal mode splitting between the magnon mode and the resonator in the transmission spectrum. It survived even at the weakest power level where the average photon number is below one. The coupling strength, cavity and FMR linewidth were 82 MHz, 2.2 MHz, and 11.5 MHz, respectively. A coupling scheme of a superconducting qubit to the YIG magnons is also discussed.   

Mechanical driving of nitrogen-vacancy center spins in diamond

We demonstrate direct coupling between nitrogen-vacancy (NV) center spins and cavity phonons by driving spin transitions with mechanically-generated harmonic strain, without mediation by a magnetic field. Using a bulk-mode acoustic resonator fabricated from single-crystal diamond, we exert $\sim7$~MPa of non-axial ac stress on the NV centers within the substrate. When we tune the $m_{s}=+1\leftrightarrow m_{s}=-1$ spin state splitting into resonance with a $\sim1$~GHz mechanical mode, we observe a $\Delta m_{s}=\pm2$ spin transition, which is forbidden by the magnetic dipole selection rule. Additionally, we find that the amplitude of the spin signal varies with the spatial periodicity of the stress standing wave, verifying that NV center spins are directly driven by mechanical oscillations. These direct spin-phonon interactions provide a new opportunity for quantum control and enable studies of spin-phonon interactions at the nanoscale.   

Interfacing rare earth spin ensembles with superconducting circuits

Interfacing photonic and solid-state qubits within a hybrid quantum architecture offers a promising route towards large scale distributed quantum computing. Ensembles of optically active rare earth spins embedded in a crystalline matrix are promising candidates for realizing such an interface. Among these, Er ions are distinct from other spin ensembles due to their optical transitions inside the telecom C-band at 1.54 $\mu$m [1]. We report on single photon on-chip ESR spectroscopy of Er spin ensembles strongly coupled to superconducting and non superconducting microwave resonators [2]. The maximum coupling strength was measured to be 45 MHz at 200 ppm, and the minimum linewidth was 4 MHz at 50 ppm Er concentration, respectively. The strong anisotropy of Er:YSO prevents us from reaching the strong coupling regime at low field transitions. However, with crystals of higher symmetry such as YAP, strong coupling can be reached at relatively small magnetic fields of 30 mT at 5 GHz. In addition, we measured T$_2$ of the spins at millikelvins of about 40 $\mu$s. The experiments demonstrate the potential of rare earth ion doped crystals for their application in quantum information processing and communication. [1] Phys. Rev. B 84, 06051 (R) (2011) [2] Phys. Rev. Lett. 110, 157001 (2013)   

Coherent manipulation of Rydberg states above surfaces at cryogenic temperatures

The integration of atom optics on a chip presents new possibilities for manipulation and readout of atomic internal and external degrees of freedom. In particular, strong fields and field gradients achievable with microstructured electrodes facilitate the manipulation of Rydberg atoms because of their large dipole moments. This is expected to allow for a strong coupling to microwave photons contained in superconducting coplanar resonators. However, the large dipole moment also makes Rydberg atoms susceptible to stray electric fields, which broaden and shift the atomic transitions [1]. We have developed methods to reduce stray fields by reducing surface adsorption and compensating residual fields. Using an external microwave source, we have recorded Rydberg-Rydberg transitions of a 0.5~mm sized ensemble at a mean distance of $250~\mu$m above gold and superconducting chip surfaces at 3~K. Finally, we have observed coherent Rabi oscillations and have extracted information on residual dc and ac fields in the vicinity of the surface. These techniques may be used for coherent chip-based interfaces between Rydberg atoms and microwave photons. \\[4pt] [1] S.D. Hogan, J.A. Agner, F. Merkt, T. Thiele, S. Filipp and A. Wallraff, PRL {\bf 108}, 063004 (2012)   

Engineering of micron-sized electron trap in a superconducting tuning-fork resonator

Electrons on helium is a unique two-dimensional electron gas system formed at the interface of a quantum liquid (superfluid helium) and vacuum. The motional and spin states of single-electron quantum dots defined on such systems have been proposed for hybrid quantum computing [1,2]. Here, we will present experiments in which an ensemble of electrons are trapped above a tuning fork superconducting resonator and describe their coupling with both the differential and common mode. Next, we will discuss the design of superconducting resonators with a micron-sized trapping area and a reduced number of trapped electrons, and the experimental progress towards a single trapped electron regime.\\[4pt] [1] S. Lyon, Phys. Rev. A. 74, 5 (2006)\\[0pt] [2] D.I. Schuster, et al. Phys. Rev. Lett. 105, 040503 (2010)   

Session J35: Focus Session: Quantum Computing Architectures and Algorithms: Adiabatic Quantum Computation

Symmetry-Protected Quantum Adiabatic Transistors

An essential development in the history of computing was the invention of the transistor as it allowed logic circuits to be implemented in a robust and modular way. The physical characteristics of semiconductor materials were the key to building these devices. We aim to present an analogous development for quantum computing by showing that quantum adiabatic transistors (as defined by Flammia et al.) are built upon the essential qualities of symmetry-protected (SP) quantum ordered phases in one dimension. Flammia et al. and Renes et al. have demonstrated schemes for universal adiabatic quantum computation using quantum adiabatic transistors described by interacting spin chain models with specifically chosen Hamiltonian terms. We show that these models can be understood as specific examples of the generic situation in which all SP phases lead to quantum computation on encoded edge degrees of freedom by adiabatically traversing a symmetric phase transition into a trivial symmetric phase. This point of view is advantageous as it allows us to readily see that the computational properties of a quantum adiabatic transistor arise from a phase of matter rather than due to carefully tuned interactions.   

Graph isomorphism and adiabatic quantum computing

In the Graph Isomorphism (GI) problem two N-vertex graphs G and G' are given and the task is to determine whether there exists a permutation of the vertices of G that preserves adjacency and maps G $\to $ G'. If yes (no), then G and G' are said to be isomorphic (non-isomorphic). The GI problem is an important problem in computer science and is thought to be of comparable difficulty to integer factorization. We present a quantum algorithm that solves arbitrary instances of GI, and which provides a novel approach to determining all automorphisms of a graph. The algorithm converts a GI instance to a combinatorial optimization problem that can be solved using adiabatic quantum evolution. Numerical simulation of the algorithm's quantum dynamics shows that it correctly distinguishes non-isomorphic graphs; recognizes isomorphic graphs; and finds the automorphism group of a graph. We also discuss the algorithm's experimental implementation and show how it can be leveraged to solve arbitrary instances of the NP-Complete Sub-Graph Isomorphism problem.   

Improved Bounds for Eigenpath Traversal

We present an improved bound on the length of the path defined by the ground states of a continuous family of Hamiltonians in terms of the spectral gap $\Delta$. We use this bound to obtain a better cost of recently proposed methods for quantum adiabatic state transformations and eigenpath traversal. In particular, we prove that a method based on evolution randomization, which is a simple extension of adiabatic quantum computation, has an average cost of order $1/\Delta^2$, and a method based on fixed-point search has a maximum cost of order $1/\Delta^{3/2}$. Additionally, if the Hamiltonians satisfy a frustration-free property, such costs can be further improved to order $1/\Delta^{3/2}$ and $1/\Delta$, respectively. Our methods offer an important advantage over adiabatic quantum computation when the gap is small, where the cost is of order $1/\Delta^3$.   

Period Finding with Adiabatic Quantum Computation

We outline an efficient quantum-adiabatic algorithm that solves Simon's problem, in which one has to determine the ``period,'' or xor-mask, of a given black-box function. We show that the proposed algorithm is exponentially faster than its classical counterpart and has the same complexity as the corresponding circuit-based algorithm. Together with other related studies, this result supports a conjecture that the complexity of adiabatic quantum computation is equivalent to the circuit-based computational model in a stronger sense than the well-known, proven polynomial equivalence between the two paradigms. We also discuss the importance of the algorithm and its implications for the existence of an optimal-complexity adiabatic version of Shor's integer factorization algorithm and the experimental implementation of the latter.   

Distinguishing graphs with a quantum annealer using susceptibility measurements

Recently it has been proposed~[1] that the Graph Isomorphism (GI) problem could be solved using a quantum annealer. This is done by encoding the graphs into Ising Hamiltonians, identifying the vertices with spins and the edges with antiferromagnetic interactions. The idea is that measurements of simple observables during and at the end of the annealing process should distinguish non-isomorphic graphs. The first experimental study of the GI problem using D-Wave's quantum computer has been carried out by Vinci et al.~[2], utilizing measurements taken at the end of the annealing process. Here, we will present preliminary evidence that measurements taken part way through the annealing process, now obtainable using state-of-the-art devices, may offer better distinguishing capabilities.\\[4pt] [1] I. Hen and A. P. Young, Physical Review A \textbf{86} (2012).\\[0pt] [2] W. Vinci et al., arXiv:1307.1114.   

Assessing claims of quantum annealing: Does D-Wave have a quantum computer?

There has recently been much publicity surrounding D-Wave's so-called quantum computing machines. Though there is little reason to expect that their highly decoherent devices actually perform quantum computation, they have persisted in making strong claims about the performance of their system in the scientific literature and to the press. Here we focus on showing that the evidence of quantumness given in the literature is extremely weak and is consistant with an effective classical description. We further show by comparing to the performance of classical simulated annealing that the idea that their current machines outperform a classical computers is unfounded.   

Error Corrected Quantum Annealing with Hundreds of Qubits

Physical realizations of quantum computing are always threatened by decoherence, especially as the scale of the device increases. This makes the implementation of error correction crucial. We have developed error correction techniques tailored to quantum annealing using superconducting adiabatic quantum optimization processors. I will show experimental results for computations using up to 448 physical qubits on the D-Wave Two device. Scaling of code performance on both antiferromagnetic chains and random 2D Ising problems will be addressed, along with insights into device error mechanisms and choices of decoding strategy. The error correction substantially enhances the observed success probabilities.   

Thermal Quantum Annealing on the D-Wave device

We report on new experimental results supporting previous work concluding that the D-Wave processor implements quantum annealing. We introduce techniques adopted to the D-Wave programmable annealer to correct for systematic fabrication and control errors. Correcting for systematic errors allows us to explore the behavior of the annealer at low energy scales, which were previously inaccessible. We describe the behavior of the annealer as we investigate the effect of thermal noise on the programmed Ising Hamiltonian. Thermal noise becomes dominant when we scale down the overall energy of the final-time Ising Hamiltonian, or increase the total annealing time. We found three qualitatively different thermal noise regimes; a high energy scale where ground state statistics dominates, a moderate noise regime regime where low lying excited states contribute, and a high thermal noise regime where the system dynamics are dominated by thermalization effects. The qualitative results are robust to increasing the size (number of qubits) of the benchmark Hamiltonian. We additionally investigated auto-correlations in the final state statistics.   

Simulations of Thermal Quantum Annealing on the D-Wave Device

We report on classical and quantum simulations to model the open-system dynamics of the D-Wave programmable annealer as we increase the thermal noise level on the device. We consider three models for the device: (1) the evolution is described by a classical simulated annealer acting on the final-time Ising Hamiltonian; (2) the evolution is described by an O(3) model with a time-dependent Hamiltonian; (3) the evolution is described by a quantum adiabatic Markovian master equation with a time dependent Hamiltonian. We increase the thermal noise level by either decreasing the overall energy scale of the final-time Ising Hamiltonian or by increasing the total annealing time. Using a benchmark Ising Hamiltonian, we show that all three models give distinct predictions for the behavior of the system as the noise level on the device is increased. The only model that captures the results of the device over the entire range of noise levels studied is the quantum master equation, ruling out the two classical models considered here.   

Performance of quantum annealing on random Ising problems implemented using the D-Wave Two

Detecting a possible speedup of quantum annealing compared to classical algorithms is a pressing task in experimental adiabatic quantum computing. In this talk, we discuss the performance of the D-Wave Two quantum annealing device on Ising spin glass problems. The expected time to solution for the device to solve random instances with up to 503 spins and with specified coupling ranges is evaluated while carefully addressing the issue of statistical errors. We perform a systematic comparison of the expected time to solution between the D-Wave Two and classical stochastic solvers, specifically simulated annealing, and simulated quantum annealing based on quantum Monte Carlo, and discuss the question of speedup.   

Benchmarking the D-Wave Two

We report on experimental work benchmarking the performance of the D-Wave Two programmable annealer on its native Ising problem, and a comparison to available classical algorithms. In this talk we will focus on the comparison with an algorithm originally proposed and implemented by Alex Selby. This algorithm uses dynamic programming to repeatedly optimize over randomly selected maximal induced trees of the problem graph starting from a random initial state. If one is looking for a quantum advantage over classical algorithms, one should compare to classical algorithms which are designed and optimized to maximally take advantage of the structure of the type of problem one is using for the comparison. In that light, this classical algorithm should serve as a good gauge for any potential quantum speedup for the D-Wave Two.   

Performance of quantum annealing under different Ising mappings and graph embeddings

To apply quantum annealing (QA) to solve optimization problems, an Ising Hamiltonian must be derived that encodes the sought solution in its ground state, while respecting QA hardware constraints. In general, there are a variety of ways to obtain a Hamiltonian with quadratic spin interactions from a given problem (mapping), and to derive from it a Hamiltonian that fits the hardware graph (embedding). The probability quantum annealing finds the ground state depends on the spectral and relaxation properties of the quantum spin model derived from these mapping and embedding procedures. In this work, we classify mappings and embeddings into categories and we perform a rigorous study of these families on the performance of QA on the D-Wave II quantum annealer to check the validity of theoretical hypotheses. We also discuss the effect of noise on this performance under different choices. Our test set includes a collection of hard combinatorial optimization problems arising in the field of operational planning. Our results illustrate ways in which the efficacy of quantum annealing depends on both the mapping and the embedding, and paves the way to the formulation of general prescriptions for how to encode fruitfully combinatorial optimization problems into quantum annealers.   

Probing for quantum speedup on D-Wave Two

Quantum speedup refers to the advantage quantum devices can have over classical ones in solving classes of computational problems. In this talk we show how to correctly define and measure quantum speedup in experimental devices. We show how to avoid issues that might mask or fake quantum speedup.   

Session J36: Focus Session: Semiconductor Qubits: Multi-Spin Gates & Sequence Optimization

Composite Sequences for Triple-dot Qubits that Compensate for Miscalibration and Hyperfine Gradients

Exchange-only qubits defined in triple quantum dots form a promising means for all-electrical semiconductor quantum control, but they suffer from both charge noise and random magnetic field gradients. Low-frequency noise sources can be compensated using composite sequences, but the development of such sequences is constrained by the fact that exchange energies are always positive and the control axes are non-orthogonal. Here, we present the results of both analytical approaches and computational searches for composite pulse sequences, which compensate for simultaneous low-frequency miscalibration (due to fixed random electric fields) and hyperfine effects (due to nuclear magnetic fields) in a single triple-dot qubit. We also present compensation sequences for multi-qubit gates. These results can substantially improve the working fidelity of quantum operations in semiconductor quantum dot devices. Sponsored by United States Department of Defense. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressly or implied, of the United States Department of Defense or the U.S. Government.   

Robust Two-Qubit Gates for Exchange-Coupled Exchange-Only Qubits

We show how to perform dynamically corrected two-qubit gates, with the leading hyperfine error term cancelled, for various geometries of an exchange-only qubit network. These sequences are designed to obey the realistic experimental constraint of strictly non-negative couplings. Moreover, we show that these corrected sequences lead to substantial improvement in the gate fidelity. Together with single-qubit dynamically corrected gates, our results facilitate universal and robust multi-qubit quantum operations and pave the way towards scalable fault-tolerant quantum computation on the exchange-only qubit platform.   

Optimizing efficiency of noise cancelling in a singlet-triplet spin-qubit array

Singlet-triplet qubits are a very good candidate for use in quantum computing and quantum operations due to their long coherence time and rapid gate operations. The fluctuations of the background nuclear spin bath and fluctuations in electrostatic quantum dot confinement potential affects the precise manipulation of the qubit. Recently, a method was developed to create an identity operation that corrects both sources of errors to the first order [1]. We now consider how much the compensating identity pulse sequence can be shortened in cases where only one dominant source of error needs correction. [1] J.P. Kestner et al., Phys. Rev. Lett. 110, 140502 (2013).   

High-Fidelity Single-Qubit Gates for Two-Electron Spin Qubits

High fidelity gate operations for manipulating individual and multiple qubits in the presence of decoherence are a prerequisite for fault-tolerant quantum information processing. However, the control methods used in earlier experiments on semiconductor two-electron spin qubits are based on unrealistic approximations which preclude reaching the required fidelities. An attractive remedy is to use control pulses found in numerical simulations that minimize the infidelity from decoherence and take the experimentally important imperfections and constraints into account. Using this approach and experimentally determined noise spectra, we find pulses for singlet-triplet qubits in GaAs double quantum dots with fidelities as high as 99.9{\%}. Fully eliminating systematic pulse errors will likely require a calibration of the pulses on the experiment using some form of self-consistent approach. Starting with inaccurate control pulses we show that elimination of individual systematic gate errors is possible by applying a modification of the bootstrap protocol proposed by Dobrovitski et al. (PRL 105, 2010) while still retaining the pulses' high fidelities.   

Dynamically Corrected Quantum Gates for Two-Electron Spin Qubits

Two-electron spin qubits in double quantum dots offer the possibility of fast and fully electrical manipulation via the exchange interaction. Arbitrary single-qubit gates have been demonstrated while maintaining a magnetic field gradient. However, simple gate constructions are extremely sensitive to noise in the Hamiltonian and thus incur considerable decoherence. Dynamically corrected gates are first-order insensitive to disturbances and present an appealing solution if slow noise sources are dominant. Using a numerical model that reflects the experimentally important imperfections and hardware constraints, we find control pulses for singlet-triplet qubits in GaAs double quantum dots which decouple in both the electrical control and the hyperfine magnetic field gradient. Additionally, dephasing effects from fast noise sources are minimized by favoring operating points close to a sweet spot. For experimentally determined noise levels the resulting gates feature fidelities as high as 99.9\% and are mainly limited by high-frequency noise and nonlinearities.   

Quantum optimal local control of coherent dynamics in custom-made nanostructures

We apply quantum optimal control theory to establish a local voltage-control scheme that operates in conjunction with the numerically exact solution of the time-dependent Schr\"odinger equation. The scheme is demonstrated for high-fidelity coherent control of electronic charge in semiconductor double quantum dots. We find tailored gate voltages in the viable gigahertz regime that drive the system to a desired charge configuration with $>99\%$ yield. The results could be immediately verified in experiments and would play an important role in applications towards solid-state quantum computing.   

Coherent Manipulation of a Silicon Spin-Charge Hybrid Qubit

The recently proposed quantum dot hybrid qubit enables fast, coherent quantum operations [1, 2]. We demonstrate rotations of a hybrid qubit in a three-electron Si/SiGe double quantum dot about two axes of the Bloch sphere (X and Z). We perform Larmor oscillations (x-rotations on the Bloch sphere) between the 0 and 1 hybrid states, demonstrating a T2* time of 2.1 ns at the charge degeneracy point [3,4]. Using tailored pulse gating sequences, we perform fast (\textgreater 10GHz) phase (z-axis) rotations of the hybrid qubit states. We measure a lower bound of the coherence time T$_{2}$* of 10 ns and high figure of merit \textgreater 150. This work was supported in part by ARO (W911NF-12-0607) and the United States Department of Defense. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressly or implied, of the US Government. \\[4pt] [1] Z. Shi, et al., Phys. Rev. Lett. 108, 140503 (2012).\\[0pt] [2] Teck Seng Koh, et al, Phys. Rev. Lett. 109, 250503 (2012)\\[0pt] [3] Z. Shi, et al., Phys. Rev. B 88, 075416 (2013) \\[0pt] [4] Z. Shi, et al., e-print: http://arxiv.org/abs/1308.0588   

Statistical benchmarking for orthogonal electrostatic quantum dot qubit devices

Quantum dots in semiconductor systems have emerged as attractive candidates for the implementation of quantum information processors because of the promise of scalability, manipulability, and integration with existing classical electronics. A limitation in current devices is that the electrostatic gates used for qubit manipulation exhibit strong cross-capacitance, presenting a barrier for practical scale-up. Here, we introduce a statistical framework for making precise the notion of orthogonality. We apply our method to analyze recently implemented designs at the University of Wisconsin-Madison that exhibit much increased orthogonal control than was previously possible. We then use our statistical modeling to future device designs, providing practical guidelines for devices to have robust control properties. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy Nuclear Security Administration under contract DE-AC04-94AL85000. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressly or implied, of the US Government.   

Enhancing the performance of exchange-only qubits in triple-quantum-dots

The exchange-only qubit has several potential advantages for quantum computation: all-electrical control, fast gate operations, and robustness against global magnetic noise. Such a device has recently been implemented in a GaAs triple-quantum-dot. In this talk, we discuss theoretical simulations of the fidelity of pulsed gate operations of the exchange-only qubit, based on a master equation approach. Our model accounts for several different dephasing mechanisms, including hyperfine interactions and charge noise arising from double-occupation errors and fluctuations of the detuning parameter. Our investigations indicate the optimal working regimes and maximum gate fidelities for these devices, in terms of experimentally tunable parameters. This work was supported by the Army Research Office, the National Science Foundation, and the United States Department of Defense. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressly or implied, of the US Government.   

Universal Set of Quantum Gates for Double-Dot Exchange-Only Spin Qubits Under Realistic Conditions

We report on a universal set of quantum logic gates for hybrid qubits. In a hybrid qubit the information is encoded in the spin state of three electrons elettrostatically confined in a silicon double quantum dot (QD), in (2,1) filling [1]. All electrical operations, reduced fabrication complexity and high scalability are the strengths of this technology. Schrieffer-Wolff effective models for both one [2] and two coupled hybrid qubit [3] are developed including the inescapable exchange interaction between electrons in the same QD. Optimal sequences of exchange interactions creating a complete set of quantum operations, namely Hadamard, $\pi$/8 and CNOT gates [4], are obtained by using a search algorithm, based on simplex and genetic ones. Silicon devices have been designed by SDFT-based program and efforts in its fabrication have produced in-plane inter-QDs distances down to 100 nm by means of electron beam lithography. Double QDs devices operating in few electron filling regime have been preliminary characterized at 4.2 K. Ref: [1] T.S. Koh et al., PRL 109, 250503 (2012) [2] E. Ferraro et al., quant-ph/arXiv, 1304.1800 (2013) [3] In preparation [4] M. De Michielis et al., submitted (2013)   

Quantum Interference between Three Spin Qubits

Recently both hyperfine and exchange based qubits based on three spin states in triple quantum dot circuits have been individually demonstrated. The effective targeting of a specific qubit species required a carefully designed pulse shape and measurement sequence. We discuss results where pulses are chosen to activate both three spin qubit species simultaneously. In our results two novel coherent behaviors have been identified which are related to quantum interference effects involving an interplay between the two qubits types. Such experiments are important to gain an understanding of critical leakage paths which drive the system away from the intended qubit states. Certain features of the data are analyzed in terms of a breakdown of the usual spin blockade spin to charge conversion technique for three spin experiments and the consequences of charge noise on the measurements.   

Maximal Rabi frequency of an electrically driven spin in a disordered magnetic field

We present a theoretical study of the spin dynamics of a single electron confined in a quantum dot. Spin dynamics is induced by the interplay of electrical driving and the presence of a spatially disordered magnetic field, the latter being transverse to a homogeneous magnetic field. We focus on the case of strong driving, i.e., when the oscillation amplitude $A$ of the electron's wave packet is comparable to the quantum dot length $L$. We show that electrically driven spin resonance can be induced in this system by subharmonic driving, i.e., if the excitation frequency is an integer fraction (1/2, 1/3, etc) of the Larmor frequency. At strong driving we find that (i) the Rabi frequencies at the subharmonic resonances are comparable to that at the fundamental resonance, and (ii) at each subharmonic resonance, the Rabi frequency can be maximized by setting the drive strength to an optimal, finite value. Our simple model is applied to describe electrical control of a spin-valley qubit in a weakly disordered carbon nanotube. Reference: http://arxiv.org/abs/1310.7350   

Deterministic photonic cluster state generation from quantum dot molecules

Currently, the most promising approach for photon-based quantum information processing is measurement-based, or one-way, quantum computing. In this scheme, a large entangled state of photons is prepared upfront and the computation is implemented with single-qubit measurements alone. Available approaches to generating the cluster state are probabilistic, which makes scalability challenging. We propose to generate the cluster state using a quantum dot molecule with one electron spin per quantum dot. The two spins are coupled by exchange interaction and are periodically pulsed to produce photons. We show that the entanglement created by free evolution between the spins is transferred to the emitted photons, and thus a 2D photonic ladder can be created. Our scheme only utilizes single-spin gates and measurement, and is thus fully consistent with available technology.   

Session J37: Focus Session: Graphene Growth on Cu Substrates

Novel Growth Mechanism of Low-Temperature-Grown Graphene by Plasma-Enhanced Chemical Vapor Deposition (PECVD)

We show a one-step method that employs PECVD for rapidly producing superior quality, large-area ($\sim$ 1 cm$^{2})$, monolayer graphene on Cu at low temperature (LT). The key to our approach is that deposition of high-quality graphene on Cu can be achieved through balancing carbon deposition by methyl radicals with etching of amorphous carbon by atomic hydrogen, while concurrently preparing the Cu surface for growth by cyano radicals. We find that removal of Cu always accompanies graphene growth, as evidenced by the presence of Cu deposits on the quartz tube and sample holder for each successful growth. We are also able to fabricate monolayer graphene by PECVD growth in 3 minutes. Even if the growth time is increased to 20 minutes, we still observe monolayer instead of multilayer graphene, suggesting that the growth mechanism differs from high-temperature CVD grown graphene. Electrical mobility determined by the field-effect-transistor configuration exhibits consistently high values, up to 60,000 cm$^{2}$/V-s on BN at 300K, exceeding the best values reported for thermal-CVD graphene on BN. Our findings suggest a promising pathway to large-scale, superior-quality and one-step inexpensive graphene fabrication for scientific research and technological applications.   

Electronic and Atomic-Scale Properties of Ultraflat CVD Graphene

Chemical vapor deposition (CVD) growth on copper foils has proven to be a reliable and cost-effective method for the production of graphene. However, most films grown by this method suffer from misoriented graphene grains as well as topographic roughness due to the polycrystallinity of the underlying copper foil substrate. Recent methods of copper foil treatment have allowed for the growth of graphene predominantly on large single crystal Cu(111) facets. In this talk we discuss scanning tunneling microscope (STM) measurements on such samples that reveal large terraces and atomically-resolved images that allow us to analyze the graphene-copper interaction during the growth. Scanning tunneling spectroscopy (STS) measurements and mapping are further employed to probe the electronic interaction between the graphene and copper substrate.   

Influence of Substrate Orientation on the Growth of Graphene on Cu Single Crystals

A systematic study of graphene growth on on-axis Cu(100) and Cu(111) single crystals oriented within 0.1$^{\circ}$ from the surface normal and a vicinal Cu(111) crystal oriented 5$^{\circ}$ off-axis has been performed. Initial attempts to grow graphene by heating each crystal to 900 $^{\circ}$C in UHV, followed by backfilling the chamber with C$_{2}$H$_{4}$ at pressures up to 5 x 10$^{-3}$ Torr did not result in graphene formation on either the on-axis Cu(100) or on-axis Cu(111) surfaces. For the vicinal Cu(111) surface, epitaxial graphene was formed under the same growth conditions. By backfilling the chamber with C$_{2}$H$_{4}$ before heating to the growth temperature, epitaxial graphene was formed on both the on-axis Cu(100) and off-axis Cu(111) surfaces, but not the on-axis Cu(111) surface. By using an argon overpressure, epitaxial overlayers could be achieved on all three Cu substrates. These results indicate that the most catalytically active sites for the dissociation of ethylene are the step edges, followed by the Cu(100) terraces sites and the Cu(111) terrace sites. The need for an argon overpressure to form graphene the on-axis Cu(111) surface indicates that the Cu sublimation rate is higher than the graphene growth rate for this surface.   

Chemical vapor deposition growth of large grapheme single crystal from ethanol

Ethanol as a precursor has proven effective in the chemical vapor deposition (CVD) synthesis of graphene on both Ni foils and Cu capsule substrates. For applications of graphene in field effect transistors or as transparent conducting electrodes, larger singe-crystal graphene without any grain boundaries shows superior electrical performance and has attracted enormous interests. Here we report a protocol to synthesize large graphene single crystals (up to 600 $\mu $m) using ethanol as precursor on commercially-available polycrystalline Cu foils. We explored the mechanism by studying the influences of different growth parameters such as pressure, flow rate and temperature. Low partial pressure and low flow rate of ethanol is essential in achieving low nucleation density over the metal surface and therefore large graphene grains can be obtained. We found that growth temperature dramatically affects the crystallinity and the growth rate of graphene grains. Moreover, this CVD growth of large graphene single crystals involves no electro-polishing or annealing treatments to the metal surface, presenting a significant simplification to the current graphene synthesis process.   

Large Single-Crystal Graphene Growth on Copper: The Role of Oxygen

Graphene grown by CVD on Cu is enabling fundamental studies and applications. However, growth of high quality single crystals with controlled domain size and morphology has not been achieved, implying unknown or uncontrolled growth parameters. We discovered that oxygen on the Cu surface not only decreases the graphene nucleation density but also accelerate graphene domain growth and affect the domain shapes. SEM, EBSD, Raman, and LEED were used to characterize and analyze the graphene domains under the effects of oxygen. First-principles calculations and phase-field simulations provide deeper insight into the proposed growth mechanisms. Finally, electric- and magneto-transport measurements show that the graphene quality is comparable to mechanically exfoliated graphene, in spite of being grown in the presence of oxygen.   

Influence of Chemisorbed Oxygen on the Growth of Graphene on Cu(100) by Chemical Vapor Deposition

The growth of graphene by catalytic decomposition of ethylene in a UHV chamber on both a clean Cu(100) surface and a Cu(100) surface predosed with a layer of chemisorbed oxygen has been studied. The crystal structure of the graphene films was characterized with in-situ LEED. By heating the clean Cu(100) substrate from room temperature to the growth temperature in ethylene, epitaxial graphene films were formed. The crystal quality was found to depend strongly on the growth temperature. At 900 $^{\circ}$C, well-ordered two-domain graphene films were formed. Predosing the Cu(100) surface with a chemisorbed layer of oxygen before graphene growth was found to adversely affect the crystal quality of the graphene overlayer by inducing a much higher degree of rotational disorder of the graphene grains with respect to the Cu(100) substrate. The growth morphology of the graphene islands during the initial stages of nucleation was monitored with ex-situ SEM. The nucleation rate of the graphene islands was observed to drop by an order of magnitude by predosing the Cu(100) surface with a chemisorbed oxygen layer before growth. Therefore, the presence of oxygen during graphene growth affects both the relative orientation and average size of grains within the films grown on Cu(100) substrates.   

Physics and applications of novel structures with CVD graphene: edges, grain boundaries, twisted bilayers, and hybrids

In this talk, I will discuss experimental studies (including electronic transport, optical/Raman, and STM) of physical properties of various novel synthetic graphene structures formed in CVD graphene grown on Cu, including edges of graphene single crystals, grain boundaries between such single crystals, and twisted bilayer graphene. Such synthetic graphene structures could be used as playground to explore novel physics and engineer new functionalities in graphene based electronic devices. Furthermore, I will discuss graphene based ``hybrid'' materials combining CVD graphene with semiconductor and metallic nanostructures for potential optoelectronic and plasmonics applications.   

Steps in Cu(111) thin films affect graphene growth kinetics

The kinetics of chemical vapor deposition of graphene on Cu substrates depend on the relative rates of C diffusion on the surface, C attachment to graphene islands, and removal of C from the surface or from graphene islands by etching processes involving H atoms. Using Cu(111) thin films with centimeter-sized grains [1], we have grown graphene under a variety of conditions and examined the edges of graphene islands with SEM and AFM. The Cu surface shows a series of regular steps, roughly 2~nm in height, and the graphene islands are diamond-shaped with faster growth along the edges of Cu steps. In contrast, growth on polycrystalline Cu foils under the same conditions shows hexagonal graphene islands with smooth edges. \\[4pt] [1] D. L. Miller, M. W. Keller, J. M. Shaw, K. P. Rice, R. R. Keller, and K. M. Diederichsen, ``Giant secondary grain growth in Cu films on sapphire,'' AIP Advances, vol. 3, p. 082105, 2013.   

Synthesis of Bilayer and Trilayer Graphene with Different Stacking Orders

We report the growth of bilayer and few layer graphene with different multilayer morphologies and stacking orders. The synthesis was performed by ambient pressure chemical vapor deposition at low methane concentrations. The shape of the monolayer graphene region was hexagon. The few layer graphene regions had different shapes either in the center or at the edge of the monolayer. The grain size of the hexagonal graphene was enlarged with Cu foil pretreatment and annealing. Raman spectra and selected area electron diffraction at the few layer graphene regions revealed the stacking order. Under different growth conditions, both Bernal and twisted stacking order were observed, and different growth mechanism was proposed.   

A Comparative Study of Graphene and h-BN Growth on Cu(100) based on DFT Calculations

Using density functional theory (DFT) calculations, we carry out a comparative study of the epitaxial growth of graphene and hexagonal boron nitride (h-BN) on Cu(100) foils. We first show that van der Waals interactions play an important role in stabilizing the h-BN islands as they are nucleated on the metal substrate, similar to the physical picture from our previous study of graphene nucleation on Cu(111). By exploring the atomic structures of characteristic graphene and h-BN islands, we reveal the contrasting behavior between the orientations of graphene and h-BN clusters on the Cu(100) substrate, and correlate the differences with the differences in bond strength. We further advance the understanding on the spatial orientation at the nucleation stage to grain boundary (GB) formation, and provide insights in explaining the characteristic angle distribution of the GBs in graphene growth on Cu foils. The present theoretical study clearly explains our experimental observations, and may prove to be instrumental in modifying growth methods for large-scale fabrication of high-quality graphene and h-BN without GBs.   

Low Temperature Synthesis of Graphene on Cu(111) from CH4 via Chemical Vapor Deposition

We report the low temperature CVD synthesis of high-quality, monolayer graphene on epitaxial Cu(111) thin films. The growth temperature of 750 $^{\circ}$ C used in this work is around 150 $^{\circ}$C lower than previous reports of continuous graphene growth using CH4 as the carbon precursor. Conditions that yield continuous films on Cu(111) result in sub-monolayer coverage on Cu(110) and Cu(100). This demonstrates that Cu(111) is a more effective graphene catalyst than Cu(100), which is predominately used in literature. The single crystal orientation of the Cu(111) thin films allows us to control the graphene orientation over large areas. Field effect measurements show ambipolar carrier behavior and electron and hole mobilities of 2500 cm$^{2}$/Vs. The Dirac point is near 0 V and the sheet resistance is 2 k$\Omega $/$\Box$. Raman imaging reveals a negligible D:G ratio, indicating a low level of defects in these samples. We find that the graphene becomes more defective on all Cu facets with increasing growth rate. Optical transmittance of 97{\%} over the visible spectrum confirms that the graphene is monolayer. This low temperature synthesis will help enable industrial scale fabrication of high-quality, continuous graphene films.   

Atomistic Processes in Self-Assembly of Millimeter-Sized Conducting Graphenne on Cu(111)

In a latest study, we have fabricated a new two-dimensional material which we call graphenne, consisting of perfectly ordered N dopants in a graphene matrix. Due to the doped electrons and the ordered nature of the N dopants, the newly discovered graphenne is a highly conducting crystal possessing superb electronic properties. Using density functional theory calculations, here we investigate the atomistic processes in the fabrication of graphenne on Cu(111) using molecular precursors, and reveal the elegant concerted roles played by the London dispersion, chemical, and screened Coulomb repulsive forces in enhancing molecule-substrate binding, facilitating easy detachment of the terminating atoms, and dictating the overall orientation of the remaining radicals, respectively. Furthermore, in contrast to graphene growth, the ordered N atoms anchor the selection of a single orientation relative to the substrate, effectively suppressing the creation of orientational disorders such as grain boundaries as the islands coalesce to form graphenne samples as large as the millimeter-sized Cu(111) substrate. These findings are directly compared with experimental observations.   

Session J38: Invited Session: Research and Opportunties at the DOE Nanoscaled Science and Research Centers

Nanoscale Engineering of Structures and Devices on Surfaces

The relentless increase in both density and speed that has characterized microelectronics, and now nanoelectronics, will require a new paradigm to continue beyond current technologies. One proposed such paradigm shift demands the ultimate control over the number and position of dopants in a device, which includes quantum information processing and variety of semiconductor device materials and architectures aimed at solving end-of-Moore's law issues. Such a work requires the development of a tool for the design of atomically precise devices on silicon and other surfaces, in hope of studying the effect of local interactions between atomic-scale structures, their microscopic behavior, and how quantum mechanical effects might influence nano-device behavior in very small structures. Demonstrations of remarkable 2D nanostructures down to single atom devices are reported here thanks to the development of scanning tunneling microscopy (STM) as an imaging and patterning tool. These include the formation of molecular chiral superstructures on metallic surfaces, as well as the atomic-scale depassivation of a hydrogen terminated surface with an STM, toward the incorporation of dopants in silicon. I will spend some time at the end, talking about my experience working at a national laboratory.\\[4pt] Acknowledgments: This work was performed, in part, at the Center for Integrated Nanotechnologies, a U.S. DOE, Office of Basic Energy Sciences user facility. The work was supported by the Sandia National Laboratories LDRD Program. Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, a Lockheed-Martin Company, for the U. S. Department of Energy under Contract No. DE-AC04-94AL85000. Use of the Center for Nanoscale Materials at Argonne National Laboratory was supported by the U.S. DOE, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.   

Computing Correlated Electrons: Roadmap and Roadblocks

Starting with a reminder of the motivations and key concepts, I will present an overview of computational strongly correlated electron research, its challenges and its current applications and directions. I will explain methods to obtain information from the theory, given the roadblocks one encounters due to strong quantum correlation. The focus will be on the density matrix renormalization group, one of the preferred methods to extract information from the low dimensional theories. A roadmap to study time evolution, temperature dependence, and spectral functions will be discussed. These three topics are of interest for experiments at Oak Ridge National Laboratory's CNMS nanocenter and elsewhere. The talk will conclude with a summary of our efforts to program and make available free and open source codes to compute correlated electrons in models for functional materials.   

The Role of Nanoscience and Nanotechnology in Addressing the World's Energy Challenges

The Center for Functional Nanomaterials (CFN) at Brookhaven National Laboratory in the United States provides state-of-the-art capabilities for the fabrication and study of nanoscale materials, with an emphasis on atomic-level tailoring to achieve desired properties and functions. The CFN is a science-based user facility, simultaneously developing strong scientific programs while offering broad access to its capabilities and collaboration through an active user program. The overarching scientific theme of the CFN is the development and understanding of nanoscale materials that address the Nations' challenges in energy security, consistent with the Department of Energy mission. The CFN is one of five Nanoscale Science Research Centers (\underline {NSRCs}) funded by the Office of Science of the United States Department of Energy. The CFN supports Brookhaven's goal of leadership in the development of advanced materials and processes for selected energy applications. In my presentation, I will highlight the role that the CFN, through its scientific staff and this scientific user community, is playing in addressing the world's energy challenges. I will focus on several trajectories of research that are being executed at CFN, including work on photovoltaics, novel nanostructured materials for catalysis, soft and biological materials, and our state-of-the-art electron microscopy and proximal probe microscopy facilities.   

Nanoscience and nanofabrication at Argonne National Laboratory: The art of making small

Over a decade ago the Department of Energy started the design, and construction of five Nanoscale Science Research Centers at different national laboratories with the objective to provide research opportunities in Nanoscience for the scientific community worldwide. The Center for Nanoscale Materials (CNM) at Argonne National Laboratory was constructed in 2006, and opened its doors to the user community in 2007. Currently the CNM hosts over 400 user proposals a year. There are six research groups at the CNM that do work in nanophotonics, electronic and magnetic materials and devices, nanobio interfaces, nanofabrication and devices, x-ray nanoscale microscopy and theory and modeling. I work in the Nanofabrication and Devices group and my research career has covered the use of x-rays, electrons and ions in the pursuit of making the smaller and smaller structures and devices. At the CNM I have been able to push the limits of electron beam lithography, and expand the use of ion beams to large area nanofabrication. Some of our accomplishments include determining liquid-polymer interactions as a function of temperature, redefining proximity effect correction at the nanoscale (NanoPEC), measuring to less than 0.5{\%} error the backscatter range for 100 KV electron beams and finding that the range is a function of the density of the substrate, fabrication of plasmonic slit waveguides, and using ions to create complex three dimensional structures for use in fluidics. None of these accomplishments are possible without detailed understanding of the physics and chemistry mechanisms involved during fabrication. This requires extensive theory and simulation work to validate our experimental results. The fruit of our work then is a full understanding of ``why'' we use certain processes for nanofabrication and not just a simple set of process recipes. A summary of all these activities will be discussed at the presentation.   

Plasmonic Smart Windows: A New Invention from Berkeley's Molecular Foundry

In the United States, roughly 20{\%} of the annual energy consumption comes from lighting and thermal management within buildings. By adjusting to the surrounding environment, dynamic ``smart'' window coatings minimize the need for heating and artificial lighting through solar gain optimization. Current dynamic windows can only operate through a visible tint, which reduces natural light during thermal management. This talk will focus on discussing a near infrared plasmonic electrochromic coating developed at Berkeley's Molecular Foundry that dynamically modulate solar heat without affecting visible light. Use of this new class of dynamic coating can improve energy consumption by minimizing artificial lighting during solar gain optimization.   

Session J39: Invited Session: Transport, Superconductivity, and Magnetism at the LAO/STO Interface

Understanding oxide interfaces: From microscopic imaging to electronic phases

In the last decade, the advent of complex oxide interfaces has unleashed a wealth of new possibilities to create materials with unexpected functionalities. A notable example is the two-dimensional electron system formed at the interface between LaAlO$_{\mathrm{3}}$ and SrTiO$_{\mathrm{3}}$ (LAO/STO), which exhibits ferromagnetism, superconductivity, and a wide range of unique magneto-transport properties. A key challenge is to find the microscopic mechanisms that underlie these emergent phenomena. While there is a growing understanding that these phenomena might reflect rich structures at the micro-scale, experimental progress toward microscopic imaging of this system has been so far rather limited due to the buried nature of its interface. In this talk I will discuss our experiments that study this system on microscopic and macroscopic scales. Using a newly-developed nanotube-based scanning electrometer we image on the nanoscale the electrostatics and mechanics of this buried interface. We reveal the dynamics of structural domains in STO, their role in generating the contested anomalous piezoelectricity of this substrate, and their direct effects on the physics of the interface electrons. Using macroscopic magneto-transport experiments we demonstrate that a universal Lifshitz transition between the population of d-orbitals with different symmetries underlies many of the transport phenomena observed to date. We further show that the interactions between the itinerant electrons and localized spins leads to an unusual, gate-tunable magnetic phase diagram. These measurements highlight the unique physical settings that can be realized within this new class of low dimensional systems.   

Chiral magnetism at oxide interfaces

There are tantalizing hints of magnetism at the n-type LaAlO$_3$/SrTiO$_3$ interface, but the experimental evidence remains controversial in view of some of the differences between different samples and probes. I will argue that if magnetism exists at interfaces, symmetry arguments imply chiral interactions [1] that lead to a spiral ground state in zero external field and skyrmion crystals for $H \neq 0$. I will next present a microscopic model that provides a possible mechanism for the formation of local moments. I will show that the coupling of these moments to itinerant electrons leads to ferromagnetic double exchange together with Dzyaloshinskii-Moriya (DM) interactions and an easy-plane ``compass'' anisotropy, which arise from Rashba spin-orbit coupling (SOC) due to the lack of inversion symmetry at the interface. The compass term, often ignored in the literature on chiral magnetism, is shown to play a crucial role in determining the magnetic ground state. I will compare our results with existing torque magnetometry data on LAO/STO and try to reconcile it with scanning SQUID magnetometry. Finally, I will present the phase diagram in a field and show that easy-plane anisotropy stabilizes an unexpectedly large skyrmion crystal phase and describe its properties. (Work done in collaboration with Sumilan Banerjee, Onur Erten, Daniel Kestner and James Rowland). \\[4pt] [1] S. Banerjee, O. Erten and M. Randeria, Nature Physics 9, 626 (2013).   

Magnetism, Superconductivity and Pseudogap at the LaAlO$_{3}$-SrTiO$_{3}$ Interface

The electron liquid at the LaAlO$_{3}$-SrTiO$_{3}$ interface is a two-dimensional superconductor and simultaneously displays magnetic order. To experimentally explore the fundamental properties of this state, we developed a planar tunnel junction technology that allows to measure the spectral density-of-states of the superconducting liquid while its carrier density can be altered by the electric-field effect. These studies yield surprising results, as key features of the superconducting electron liquid at the LaAlO$_{3}$-SrTiO$_{3}$ interface are found to be analogous to features deemed characteristic for the high-$T_{\mathrm{c}}$ cuprates. This work was performed in collaboration with C. Richter, H. Boschker, W. Dietsche, E. Fillis-Tsirakis, R. Jany, F. Loder, L.F. Kourkoutis, D.A. Muller, J.R. Kirtley, and C.W. Schneider.   

Session J40: Invited Session: Superconductivity and Magnetism of Iron-based Superconductors II

Magnetism and its interplay with Superconductivity in the Doped Iron Chalcogenide Fe$_{\mathrm{1+y}}$Te$_{\mathrm{1-x}}$Se$_{\mathrm{x}}$

I examine the relationship of iron superconductivity with impurities, and low energy magnetic excitations in the structurally simple iron superconductor, (Fe$_{\mathrm{1+y}}$Te$_{\mathrm{1-x}}$Se$_{\mathrm{x}})$. In the first part of the talk, the pivotal role played by interstitial iron impurities in the microscopic origin of the quasi-static magnetism at (1/2,0) is demonstrated in Fe$_{\mathrm{1+y}}$Te$_{0}$ 0.38 [1]. We used polarized and unpolarized neutron scattering together with simulations of the scattering function based on structural data and a semi-metallic 5-band model with super-exchange interactions with the interstitial iron, to show that the formation of magnetic polarons around the interstitial iron atoms seeds the observed (1/2,0) magnetism. Though the quasi-static magnetism occurs at (1/2,0), the low energy spin dynamics are dominated by fluctuations at (1/2,1/2), like other iron based superconductors. In the second part of the talk, I will discuss these fluctuations and in particular the so-called spin resonance -- the signature feature in the low energy inelastic neutron scattering spectrum. We show that this scattering is quasi two dimensional and largely isotropic. Further, the first moment sum-rule for the dynamic correlation function is applied to the inelastic data in the normal and superconducting states to quantitatively determine the magnetic component of the superconducting condensation energy [2]. This method is sensitive to changes in the inter-site magnetic correlation energy, $\Delta $Eij, associated with superconductivity. We find that the length scale over which $\Delta $Eij is appreciable is similar to the superconducting coherence length, as determined by Scanning Tunneling Microscopy. Comparison of the inter-site magnetic correlation energy to the superconducting condensation energy determined through specific heat measurements indicates a significant role of magnetic fluctuations in stabilizing superconductivity. \\[4pt] [1] V. Thampy et al, Phys. Rev. Lett. 108, 107002 (2012).\\[0pt] [2] J. Leiner et al, Manuscript under preparation.   

ARPES Study on the Strongly Correlated Iron Chalcogenides Fe$_{1+y}$Se$_x$Te$_{1-x}$

The level of electronic correlation has been one of the key questions in understanding the nature of iron-based superconductivity. Using Angle Resolved Photoemission Spectroscopy (ARPES), we systematically investigated the correlation level in the iron chalcogenide family Fe$_{1+y}$Se$_x$Te$_{1-x}$. For the parent compound Fe$_{1.02}$Te, we discovered ``peak-dip-hump'' spectra with heavily renormalized quasiparticles in the low temperature antiferromagnetic (AFM) state, characteristic of coherent polarons seen in other correlated materials with complex electronic and lattice interactions. As the temperature (or Se ratio x) increases and Fe$_{1.02}$Se$_x$Te$_{1-x}$ is in the paramagnetic (PM) phase, we observed dissociation behavior of polarons, suggestive of connection between the weakening electron-phonon coupling and AFM [1]. Further increase of x leads to an incoherent to coherent crossover in the electronic structure, indicating a reduction in the electronic correlation as the superconductivity emerges. Furthermore, the reduction of the electronic correlation in Fe$_{1+y}$Se$_x$Te$_{1-x}$ evolves in an orbital-dependent way, where the d$_{xy}$ orbital is influenced most significantly [2]. At the other end of the phase diagram (FeSe) where the single crystal is not stable, we have studied the MBE-grown thin film which also reveals orbital-dependent strong correlation in the electronic structure [3]. Our findings provide a quantitative comprehension on the correlation level and its evolution on the phase diagram of Fe$_{1+y}$Se$_x$Te$_{1-x}$. We discuss the physical scenarios leading to strong correlations and its connection to superconductivity.\\[4pt] [1] Z. K. Liu, et al., Physical Review Letters 110, 037003(2013);\\[0pt] [2] Z. K. Liu, et al., submitted;\\[0pt] [3] M. Yi, et al., submitted.   

Electron delocalization, orbital order, magnetism, and emergent superconductivity in Fe$_{1+y}$Te and Fe$_{1+y}$(Te,S/Se)

Neutron scattering [1] reveales an unusual enhancement, on warming, of dynamical magnetism in iron telluride, Fe$_{1+y}$Te, the non-superconducting parent material of the chalcogenide family of iron-based superconductors, and in nearly critical Fe$_{1+y}$Te$_{1-x}$(S,Se)$_{x}$, where bulk measurements show the presence of filamentary superconductivity [2]. While these findings are consistent with both Kondo-like screening of local spins by conduction electrons, or a delocalization, on cooling, of one of the electrons, our more recent results shed light on this issue, favoring the latter scenario. Investigation of the magneto-structural phase diagram of the Fe$_{1+y}$Te series revealed that the low-temperature phase, which in the nearly stoichiometric (y $\approx $ 0) material is attained via the first order phase transition at T$_{N} \quad \approx $ 70 K, is characterized not only by antiferromagnetic and structural order, but also by a peculiar type of orbital order. By combining results of bulk characterization of electronic behavior and the diffraction data on the microscopic structural changes for samples with y $\approx $ 0.05 to 0.13, we were able to disentangle different low-temperature orders and identify new, electronically driven ferro-orbital ordering transition. The newly discovered orbital ordering is characterized by the formation of zigzag Fe-Fe chains similar to those in manganites, and is associated with the delocalization of one of the electrons. This has profound effect on magnetic and electronic properties, including marked decrease of resistivity and magnetic susceptibility.\\[4pt] In collaboration with D. Fobes, Z. Zhu, R. Zhong, G. Gu, J. Tranquada, C. Petrovic, V. Solovyov, Condensed Matter Physics and Material Science Department, Brookhaven National Laboratory, Upton, NY 11973-5000, USA; A. Savici, M. Lumsden, M. Stone, and B. Winn, NSSD, Oak Ridge National Laboratory, Oak Ridge, TN. This work was supported by the US DOE under Contract DE-AC02-98CH10886. \\[4pt] [1] I. A. Zaliznyak, Z. J. Xu, J. M. Tranquada, G. D. Gu, A. M. Tsvelik, M. B. Stone, Phys. Rev. Lett. 107, 216403 (2011). \\[0pt] [2] Rongwei Hu, E. S. Bozin, J. B. Warren, C. Petrovic, Phys. Rev. B 80, 214514 (2009).\\[0pt] [3] I. A. Zaliznyak, Z. J. Xu, J. S. Wen, J. M. Tranquada, G. D. Gu, V. Solovyov, V. N. Glazkov, A. I. Zheludev, V. O. Garlea, M. B. Stone, Phys. Rev. B 85, 085105 (2012); also unpublished (2014).   

High temperature superconductivity in single unit-cell FeSe films on SrTiO$_{3}$

High transition temperature ($T_{C})$ superconductivity was discovered in single unit-cell thick FeSe films grown on a SrTiO$_{3}$(001) substrate by molecular beam epitaxy. \textit{In situ} scanning tunneling microscopy revealed a superconducting gap as large as 20 meV in single unit-cell thick FeSe films [1]. By \textit{ex situ} transport measurements on single unit-cell thick FeSe films protected with FeTe layer, we demonstrated an onset $T_{C}$ above 40 K and a critical current density $J_{C}$ $\sim$ 1.7 $\times$ 10$^{6}$ A/cm$^{2}$ at 2 K, which are much higher than $T_{C}$ $\sim$ 8 K and $J_{C}$ $\sim$ 10$^{4}$ A/cm$^{2}$ for bulk FeSe [2,3], and that the characteristics of the transition are consistent with a two-dimensional superconductor undergoing a Berezinskii-Kosterlitz-Thouless transition. The superconductivity is further confirmed by measuring Meissner effect. The simple structure of the current system provides an ideal platform for understanding the underlying physics of high-$T_{C}$ superconductivity.\\[4pt] [1] Wang, Q. Y. \textit{et al.}, Interface-induced high-temperature superconductivity in single unit-cell FeSe films on SrTiO$_{3}$. \textit{Chinese Physics Letters}, \textbf{29}, 037402 (2012).\\[0pt] [2] Hsu, F. C. \textit{et al.}, Superconductivity in the PbO-type structure $\alpha $-FeSe. \textit{Proc. Natl. Acad. Sci. USA}\textbf{ 105}, 14262 (2008).\\[0pt] [3] Lei, H. C.\textit{ et al.}, Critical fields, thermally activated transport, and critical current density of $\beta $-FeSe single crystals. \textit{Phys}.\textit{Rev. B} \textbf{84}, 014520 (2011).   

Effects of electron-phonon coupling on the superconductivity of FeSe/SrTiO3 interface

The maximal Tc in iron-based high temperature superconductors has remained around 55K since 2008. In 2012 a Chinese group reported STS evidences of enhanced superconductivity of one-unit-cell FeSe film on SrTiO3 substrate, with an estimate of Tc over 77K [1]. Similarly large gaps were later observed in ARPES experiments [2,3] and a recent transport measurement directly confirmed the superconductivity at this FeSe/STO interface [4]. These exciting progresses call for a better understanding of the mechanism of high Tc in this and other iron-based materials. In this talk I will discuss our work on the possible role of electron-phonon coupling in the FeSe/STO system [5]. We propose that electron-phonon coupling, which is largely overlooked in the studies of bulk Fe-based superconductors, can play a significant role here due to the soft ferroelectric phonon modes in SrTiO3. We generalize the phenomenological Eliashberg theory to this multiple-band case, and obtain generalized McMillan formula of Tc for conventional and unconventional s-wave pairing states. We can therefore demonstrate that moderate electron-phonon coupling will be able to produce the observed large enhancement of pairing gap. This result is further confirmed by a microscopic functional renormalization group calculation. We will also discuss the experimental signatures of electron-phonon coupling, and propose other substrate materials to utilize this mechanism. This work could foster further experimental and theoretical studies of Fe-based superconductivity, and may eventually lead to the discovery of even higher Tc systems.\\[4pt] [1] Q.-Y. Wang et al. Chin. Phys. Lett. 29, 037402 (2012). \\[0pt] [2] D. Liu et al. Nat. Commun. 3, 931 (2012). \\[0pt] [3] S.Y. Tan et al. Nat. Mater. 12, 634 (2012). \\[0pt] [4] Jian Wang et al. unpublished. \\[0pt] [5] Y.-Y. Xiang, F. Wang, D. Wang, Q.-H. Wang, D.-H. Lee, Phys. Rev. B 86, 134508 (2012).   

Session J41: Topological Insulators: Experimental Tuning and Extreme Conditions

In situ monitoring of resistivity and carrier concentration during molecular beam epitaxy of topological insulator Bi$_2$Se$_3$

Bismuth selenide (Bi$_2$Se$_3$) is a three-dimensional strong topological insulator of particular interest due to its relatively large bulk band gap (300 meV) and single set of topologically non-trivial surface states. However, there are outstanding problems in isolating the surface state from the bulk (trivial) conduction: this problem is frequently attributed to doping from selenium vacancies and atmospheric exposure. To address this question of doping, we have constructed a molecular beam epitaxy system with the additional capability of doing real time, in situ measurement of resistivity and Hall carrier density. Bismuth selenide has a micaceous crystal structure of quintuple layer units weakly bonded to one another making it well suited to this growth (van der Waals epitaxy) and measurement technique. Cooling to 15 K and controlled exposure to atmospheric dopants is additionally possible without breaking vacuum. We have been able to achieve direct measurement of mobilities on the order of 300 cm$^2/$Vs and carrier densities of 3$\times$10$^{13}$cm$^{-2}$ measured at growth temperatures of 200 to 300 $^\circ$C. The latest results of carrier density and mobility as a function of film thickness and growth parameters will be reported.   

Ambipolar magnetotransport of topological insulator thin film in the extreme quantum limit

Topological insulators (TIs) are quantum materials with insulating bulk and topologically protected metallic surfaces. An outstanding challenge in the field of TIs is to reveal the intrinsic quantum transport properties of the topological surface states. Here, we investigate the transport properties of (Bi$_{\mathrm{1-x}}$Sb$_{\mathrm{x}})_{\mathrm{2}}$Te$_{\mathrm{3}}$ TI thin film in 60 T pulsed magnetic fields. A complex and systematic evolution of magnetoresistance (MR) is observed when the Fermi level is tuned across the Dirac point by gate voltage. In particular, an unusual negative MR prevails at the charge neutral point and gradually becomes positive at higher band filling. This intriguing phenomenon is related to the exotic property of surface states that is only shown in the extreme quantum limit. Our results approach the regime necessary to access the half quantum Hall effect in gated topological insulators.   

Transport in Quantum Confined Sb(111)

Sb is a topological semi-metal with a negative bandgap of 180meV, however it is anticipated that in ultra-thin films, quantum confinement will open the bulk gap such that transport is dominated by topological surface states. We have studied the magneto-transport of ~1.5 nm to 3.2 nm films of Sb(111) grown via molecular beam epitaxy on nearly lattice matched epilayers of GaSb(111). SEM shows the Sb growth yielded smooth and continuous films that show significantly reduced bulk conduction at low temperatures. $\rho_{xx}$ displays a linear dependence at high magnetic fields that increases with decreasing film thickness in good agreement with calculations for a linearly dispersing systems with overlapping Landau levels.. At lower fields the films display positive magneto-resistance, well-described by the weak anti-localization (WAL) theory of Hikami, Larkin and Nagaoka for strong spin orbit coupling, yielding phase breaking lengths ~1$\mu$m at 300mK. Projects to investigate quantum interference in lithographically defined wires and to seek proximity-induced superconductivity are ongoing.   

Evidence of the two surface states of (Bi$_{0.53}$Sb$_{0.47}$)$_{2}$Te$_{3}$ films grown by van der Waals

The discovery of topological insulators (TIs) has led to numerous exciting opportunities for studying topological states of quantum physics and for exploring spintronic applications due to the new physics arising from their robust metallic surface states. Here, we report the growth of high-quality topological insulator (Bi$_{\mathrm{x}}$Sb$_{\mathrm{1-x}}$)$_{2}$Te$_{3}$ thin films using a single van der Waals GaSe buffer layer by molecular beam epitaxy. Ultra-low surface carrier density of 1.3 $\times$ 10$^{12}$ cm$^{-2}$ and a high Hall mobility of 3100 cm$^{2}$/Vs have been achieved for (Bi$_{0.53}$Sb$_{0.47}$)$_{2}$Te$_{3}$. The high-quality films enable us to observe quantum oscillations associated with the top and bottom surface states and to manipulate the Dirac electrons and bulk holes' conduction properties.   

Anomalous nuclear magnetic resonance spectra in powdered Bi$_2$Se$_3$

We present $^{209}$Bi NMR spectra and relaxation rate data on single crystal and powder samples of the topological insulator material Bi$_2$Se$_3$, including data on nanoscale powders with percentages of surface nuclei on the order of 2\%. Powder samples are measured as-prepared, annealed to relieve mechanical strains, and fixed in epoxy to prevent alignment of grains with the applied magnetic field of 9 T. Our results reveal anomalous behavior in both the angular dependence of the single crystal spectra and in the powder spectra. All powder spectra display features not accounted for by summation of spectra of single crystal orientations.   

Two-carrier transport and multi-channel weak antilocalization in SnTe thin films grown by MBE

SnTe has recently been identified as a four-fold degenerate topological system where each (001) surface accommodates four identically-chiral Dirac surface states [1]. The surface states were successfully studied by ARPES and STM [2,3]. We perform magnetotransport measurements at 2K and up to 5T on epitaxial SnTe thin films grown by MBE on BaF$_{\mathrm{2}}$ (001). We report the observation of a two-carrier contribution to the magnetotransport as well as a multi-channel weak antilocalization (WAL) correction to the low field magnetoresistance. Analysis of the WAL using the HLN model [4] yields 0.75\textless $\alpha $\textless 2.5 suggesting evidence of intervalley coupling on the surface of SnTe. Films grown under different growth conditions are discussed and compared. [1] T.H. Hsieh et al. \quad Nature Commun$.$ \textbf{3, }982 (2012). [2] S.X. Yu et al. Nature Commun$. $\textbf{3, }1192 (2012). Tanaka, Y. et al. Nature Phys$. $\textbf{8, }800 (2012). [3] Okada, Y. et al. Science 1239451 (2013). [4] Hikami et al. Prog. Theor. Phys. \textbf{63} 707 (1980). Supported by NSF-DMR-0907007, partly by NSF-DMR-1207469, ONR-N00014-13-1-0301, and MIT MRSEC through NSF-DMR-0819762 and partly by DOE under Contract DE-AC02-06CH11357.   

Compensation of intrinsic charge carriers in topological insulators using high energy electron beams

One of the main challenges in probing charge transport of the topological Dirac surface states is a non-vanishing conductivity of the bulk. With the techniques employed thus far, reaching the charge neutrality point (CNP) has proved difficult. Here we demonstrate that we can reach CNP by compensating intrinsically \textit{p}-type topological insulators (TIs) by irradiation with high energy (2.5 MeV) electrons, and increase bulk resistivity by orders of magnitude. Irradiations, performed at liquid hydrogen, create Frenkel (vacancy-interstitial) pairs in the bulk, with donor-type vacancies that remain stable up to room temperature. The conversion of conductivity type (from \textit{p}- to \textit{n}-type) in Bi$_2$Te$_3$ and Sb$_2$Te$_3$ occurs at the resistivity maximum obtained for the beam fluence $\phi \cong 30-35~mC/cm^2$. The 2D character of longitudinal conductance $\sigma_{xx}$ near CNP is indicated by the appearance of weak anti-localization (WAL) cusp that scales with $H_\perp = H cos \theta$. The coherence length extracted from the fits to 2D Hikami-Larkin-Nagaoka theory of WAL is $\sim 210~\textrm{nm}$, comparable to that obtained in thin MBE films.   

Quantum-interference effects in single- and poly-crystalline topological insulator Bi$_{2-x}$Te$_{3}$

We have studied the carrier transport properties of both single- and poly-crystalline topological insulator (TI) Bi$_{2-x}$Te$_{3}$ samples. Single-crystalline microflakes were made by exfoliation from a single-crystalline Bi$_{2}$Te$_{3}$ bulk. Polycrystalline samples were made by flash evaporation of 5N purity Bi$_{2}$Te$_{3}$ sheets. In single-crystalline Bi$_{2}$Te$_{3}$ microflakes, temperature dependent resistances revealed two-dimensional (2D) electron-electron interaction effect. The extracted Coulomb screening parameter is negative, in accord with the situation of strong spin-orbit coupling in the TI materials. Positive magnetoresistances (MRs) originated from 2D weak-antilocalization (WAL) effect were measured in low magnetic fields, and satisfactorily described by a multichannel-conduction model. Especially, as T below 1 K and under high positive backgate voltages, signature of two coherent conduction channels was found. We discuss our results in terms of Dirac fermion states on the bottom surface, in addition to the bulk states. Polycrystalline Bi$_{2-x}$Te$_{3}$ thin films were patterned by electron-beam lithography. In low perpendicular magnetic fields, positive MRs due to the 2D WAL effect were observed. In parallel magnetic fields, Aharonov-Bohm oscillations were measured, suggesting the presence of metallic surface states.   

Shubnikov-de Haas oscillations and electrical transport properties of Bi based topological insulators

The discovery of the anomalous superconductivity in Cu$_{x}$Bi$_{2}$Se$_{3}$ has attracted much attention because of the relation between superconducting state and topological surface state. In this study we present electronic transport properties in Cu$_{x}$Bi$_{2}$Se$_{3}$. We have grown various kinds of single crystals of topological insulators based on Bi related compounds and the transition metal doped compounds and have studied basic transport phenomenon in order to characterize them. The pronounced quantum oscillations in the magnetoresistance were observed in both doped and non-doped Cu$_{x}$Bi$_{2}$Se$_{3}$, which provide the precise information about their electronic structures. We will discuss the results as topologically interesting properties.   

Robust ferromagnetism in V doped ultrathin three dimensional topological insulator Bi$_{2}$Te$_{3}$ films

Motivated by the discovery of quantum anomalous Hall effect in Cr doped (BiSb)2Te3 ferromagnetic topological insulator (TI) films,1 high quality vanadium (V) doped three dimensional (3D) TI Bi2Te3 films were successfully grown via molecular beam epitaxy on etched Si(111) and heat treated insulating SrTiO3 (111) substrates. Anomalous Hall effect measurements and magnetization studies found that a robust long range out-of-plane ferromagnetic order occurs in ultrathin Bi2-xVxTe3 films down to 5QLs. There was systematic dependence of ferromagnetism on the concentration of V. However, the ferromagnetic order was observed to be insensitive to the carrier type and density. Upon the application of a bottom gate electric field to reduce the carrier density, the anomalous Hall resistance increased, while the coercivity was unaffected. These observations are in contrast to that seen in conventional dilute magnetic semiconductors (DMSs). Our observation might lead to the carrier independent control of anomalous Hall voltage in ferromagnetic TIs and that could form the basis for magneto-electronics and spintronics applications. References:[1] Cui-Zu Chang et al. Science. 340, 167 (2013).   

Linear magnetoresistance in disordered magnetically doped topological insulator Bi2Te3

First-principle calculations predict that certain topological insulators (TIs) can turn ferromagnetic (ferro-TIs) when doped with magnetic ions such as Fe or Cr. These ferro-TIs support anomalous Hall effect (AHE) that becomes quantized in the thin film limit. In the absence of disorder, doping with vanadium (SP = 0.7 eV per V, comparable to Fe) is not expected to produce a ferro-TI due to the position of vanadium 3D bands. Here we show that disorder introduced by doping vanadium into Bi$_{2}$Te$_{3}$ thin films has three remarkable effects:(i) it shows unusual AHE as seen in the hysteretic behavior of Hall conductance that does not scale with magnetization M, (ii) it forms a donor band that turns conductivity type from p- to n- and turns R vs. T from metallic to semiconducting-like, and (iii) it results in a large region below 100 K that has negative linear magnetoresistance (MR) in high magnetic fields. A large positive linear MR was observed in silver chalcogenides Ag$_{2+\delta}$Se and Ag$_{2+\delta}$Te, consistent with the predicted quantum linear MR in disordered semimetals. We will discuss this mechanism in a TI, including the MR sign reversal arising from frustrated magnetic doping. * Supported in part by NSF-DMR-1122594, NSF-DMR-1312483-MWN, and DOD-W911NF-13-1-0159   

Enhanced surface state of topological insulators by optimal magnetic doping

Topological insulators (TIs) attract attentions both for fundamental science and potential applications because of their bulk band inversion arising from the strong spin orbital coupling. In addition, magnetic impurities doped into TIs can lead to opening of energy gap and induce some interesting fundamental physical phenomena such as the quantum anomalous Hall effect and magnetoelectric effect. In this work, we investigate the manipulation of the Fermi level, the band structure, and related surface states of (Sb$_{\mathrm{x}}$Bi$_{\mathrm{1-x}})_{\mathrm{2}}$Te$_{\mathrm{3}}$ by Cr doping. We will show the magnetic dopants in the TI films are necessary to sustain an insulating bulk while simultaneously keeping the Dirac point of the surface in the bulk gap of (Sb$_{\mathrm{x}}$Bi$_{\mathrm{1-x}})_{\mathrm{2}}$Te$_{\mathrm{3}}$ thin film. It will be also shown that Cr doping in the films will both increase the magnetic response of the TI films by increasing the permeability and result in the opening a surface band gap. As a result, the materials can be made more suitable for spintronics and electronic devices such as magnetic sensors.   

Electrically tuned magnetic order and magnetoresistance in a topological insulator

Topological insulators (TIs) possess spin-polarized, Dirac-like surface states protected by time reversal symmetry (TRS). Introducing magnetism into TI, which breaks the TRS, is expected to create exotic topological magnetoelectric effects. In particular, it may lead to highly unconventional magnetoresistance (MR) behavior that can find unique applications in magnetic sensing and data storage. In this talk, we present magneto transport studies of Cr doped (Bi,Sb)2Te3 ferromagnetic TI thin film fabricated into a field effect transistor device. We observe an unusually complex evolution of MR when the Fermi level is tuned across the Dirac point by gate voltage. The MR behavior cannot be explained by the simple localization picture, but is closely related to the gate-tuned ferromagnetic order. The underlying physics is the competition between the broken TRS and topological protection in magnetic TI. The simultaneous electrical control of magnetic order and magneto transport facilitates future TI-based spintronic devices.   

Pressure evolution of electrical transport in the 3D topological insulator (Bi,Sb)2(Te,Se)3

The group V-VI compounds---like Bi$_{2}$Se$_{3}$, Sb$_{2}$Te$_{3}$, or Bi$_{2}$Te$_{3}$---have been widely studied in recent years for their bulk topological properties. The high-Z members of this series form with the same crystal structure, and are therefore amenable to isostructural substitution studies. It is possible to tune the Bi-Sb and Te-Se ratios such that the material exhibits insulating behavior, thus providing an excellent platform for understanding how a topological insulator evolves with applied pressure. We report our observations of the pressure-dependent electrical transport and compare that behavior with other binary V-VI compounds under pressure.   

Session J42: Majorana Fermions in Nanowires

Topological Superconductivity and Majorana Fermions in RKKY Systems

We consider quasi one-dimensional RKKY systems in proximity to an s-wave superconductor [1]. We show that a $2k_F$ -peak in the spin susceptibility of the superconductor in the one-dimensional limit supports helical order of localized magnetic moments via RKKY interaction, where $k_F$ is the Fermi wavevector. The magnetic helix is equivalent to a uniform magnetic field and very strong spin-orbit interaction (SOI) with an effective SOI length $1/2k_F$ [2,3] . We find the conditions to establish such a magnetic state in atomic chains and semiconducting nanowires with magnetic atoms or nuclear spins. Generically, these systems are in a topological phase with Majorana fermions. The inherent self-tuning of the helix to $2k_F$ eliminates the need to tune the chemical potential [3-6]. [1] J. Klinovaja, P. Stano, A. Yazdani, and D. Loss, Phys. Rev. Lett. 111, 186805 (2013). [2] B. Braunecker, G. I. Japaridze, J. Klinovaja, and D. Loss, Phys. Rev. B 82, 045127 (2010). [3] J. Klinovaja, P. Stano, and D. Loss, Phys. Rev. Lett. 109, 236801 (2012). [4] J. Klinovaja and D. Loss, Phys. Rev. B 86, 085408 (2012). [5] J. Klinovaja and D. Loss, Phys. Rev. X 3, 011008 (2013). [6] J. Klinovaja and D. Loss, Phys. Rev. B 88, 075404 (2013).   

Self-Organized Topological State with Majorana Fermions

A topological superconductor phase with a pair of localized spatially separated Majorana fermions can be achieved in semiconductor wires with strong spin-orbit interactions, however, it requires subtle fine tuning of the chemical potential of the order of 1meV. This makes it difficult to access the desired topological phase in experiments. We find that, remarkably, this fine tuning is not required for a magnetic chain of adatoms placed on top of an s-wave superconductor. Using a simple model, we show that for a wide range of the chemical potential the magnetic moments self-organize into a spiral state with a wave-vector that corresponds to the perfect configuration to achieve the topological superconductor phase for electrons. The local coupling between magnetic moments and electronic spins effectively plays the role of the spin-orbit interaction required for the topological phase and the phase remains stable against spin fluctuations at experimentally accessible temperatures.   

Majorana fermions in hybrid superconductor-semiconductor nanowire devices

Recently the first experimental signatures of Majorana fermions were reported. Experiments are now focusing on more rigorous ways to identify Majorana's. Since Majorana's should come in pairs, further experimental evidence could be given by measuring the correlated emergence of two Majorana's at both ends of the topological superconductor. Additionally, recent developed theories show that interacting Majorana's lead to an oscillation between a splitted zero bias peak and a single zero bias peak in both gate and magnetic field space. We perform our experiments in three terminal normal-superconductor-normal InSb nanowire devices. This enables us to simultaneously probe both Majorana fermions by using tunneling spectroscopy from the two normal contacts into the superconducting contact. An improved gate design enhances gating of Majorana's makes it possible to observe the oscillatory peak splitting behavior. Our preliminary results are in line with the expected behavior of interacting Majorana bound states at the ends of a topological superconductor.   

Measuring fermion parity correlations in 1D topological superconducting wires

Zero energy Majorana fermion states (Majoranas) can arise at the ends of a semiconducting wire in proximity with a superconductor. A first generation of experiments has detected a zero bias conductance peak in these systems that strongly suggests these Majoranas do exist; however, a definitive demonstration of the long-ranged entanglement that is crucial for potential applications in quantum computing has yet to be carried out. We will discuss a possible measurement scheme to detect this long-ranged entanglement in a wire system with two coupled pairs of Majoranas, by varying the coupling between one pair and measuring the effect this has on the state of the second pair.   

Nonequilibrium transport between helical Luttinger liquids leads or helical Majorana modes

We study a steady state non-equilibrium transport between (i) two interacting helical edge states of a two dimensional topological insulator, described by helical Luttinger liquids, through a quantum dot [1] or tunneling junction [2]. (ii) one Luttinger liquids lead and a helical Majorana modes lead connected by tunneling junction(s). We find the metal-to-insulator quantum phase transition for attractive or repulsive interactions in the leads when the magnitude of the interaction strength characterized by a charge sector Luttinger parameter goes beyond a critical value.\\[4pt] [1] S. P. Chao, S. A. Silotri, C. H. Chung, Phys. Rev. B 88, 085109 (2013)\\[0pt] [2] Y. W. Lee, Y. L. Lee, C. H. Chung, Phys. Rev. B 86, 235121 (2012)   

Conductance plateau due to Majorana bound state in a quantum dot coupled to a topological quantum wire

The search for Majorana bound state (MBS) is topological superconductor nanowires is currently a topic of great interest. Despite the various theoretical proposals and the experimental results, the question of whether the possible signatures of MBS can be distinguished from those arising from other phenomena such as the Kondo effect is still under debate. A recent proposal for detecting MBS using a quantum dot coupled to normal two leads and to a topological quantum wire has proven to be very appropriate structure to investigate this problem. In this system, the presence of MBS in the wire is marked as a $e^2/2h$ conductance through the dot. In this work we find, that the $e^2/2h$ conductance peak is not per se an distinct signature of a MBS in the wire. We show instead that it results from a leaking of the Majorana state into the dot [1]. Moreover, by gating the dot level ($\varepsilon_d$) far away below and above the Fermi level of the leads ($\varepsilon_F$), the conductance remains at $e^2/2h$. The surviving of the conductance plateau for $\varepsilon_d>\varepsilon_F$ contrasts with Kondo effect plateau known to emerge only for $\varepsilon_d<\varepsilon_F$.\\[4pt] [1] E. Vernek, P. H. Penteado, A. C. Seridonio and J. C. Egues, arXiv:1308.0092 (2013).   

Capacitive Signal of Majorana States in a Finite 1D wire

We propose a new measurement technique for the observation of Majorana fermion end states in finite-length semiconductor-superconductor hybrid nanowire systems. We demonstrate how a charge measurement, say by an external single-electron transistor, as a function of external magnetic field and chemical potential, could reveal the presence -- or lack -- of localised Majorana end states. Whereas existing experimental proposals require direct contact to the wire for tunneling measurements, our proposal avoids this issue and provides an orthogonal measurement to confirm recent experimental developments. Furthermore, we shed light on a new parameter regime for nanowire-superconductor hybrid systems.   

New topological types of Majorana modes at ends of one-dimensional topological superconductors

As being known, topological insulators/superconductors are completely classified into various topological types with respect to their anti-unitary symmetries and dimensions, and for a certian dimension different topological types correspond to different boundary gapless modes, which is quantitatively described as a general index theorem. Based on this and Kitaev's model in class D, we construct models for all the other types of D1 topological superconductors and analyze their topologically protected Majorana zero-modes at ends. We highlight that: 1)The two kinds of $\mathbf{Z}_2$ topological numbers imply distinct forms of Majorana zero-modes. 2) The two-fold degenerate ground state of the DIII model with Majorana fermions can be effectively regarded as a spin when the model is coupled to a weak external magnetic field. 3)The BDI model with $\mathbf{Z}$-type unit topological number can be assigned topological charges $\pm1$ to its Majorana zero-modes at two ends in agreement with the general index theorem. 4)The CII model with $\mathbf{Z}$-type topological number $2$ may be regarded as two copies of the BDI model with certain spin-pairing patterns, and consistently the topological charge of its Majorana zero-modes is defined in the same sense of that of the BDI model.   

Realistic models for Majorana wires

We construct realistic effective models to theoretically facilitate the experimental search for Majorana modes in quantum wires. Starting with an accurate first-principles calculation, we provide a detailed discussion of finite size and multiband effects, and of the spin-orbit splitting. We also present a thorough consideration of proximity induced superconductivity, extensively supporting it with numerical evidence. The comparison of our results to previously used models and actual experiments is done.   

Majorana zero modes through domain wall wires in inhomogeneous gated silicene

We report a new way to realize Majorana zero-modes in one-dimensional (1D) domain wall wires generated in inhomogeneous gated silicene sheet. By applying inhomogeneous perpendicular electric field to the gapped silicene sheet, 1D domain walls, which can host either propagating spinful fermions or spin-polarized fermions (in the presence of a Zeeman field), can be created at the desired positions with great flexibility. Since the appreciable spin-orbit couplings (SOC) due to the buckled structure of silicene are present, such domain wall propagating channels can be a good alternative of 1D semiconducting quantum wires with strong SOC, usually taken as an essential starting point to generate the end-point Majorana zero modes. By the proximity with a conventional s-wave superconductor and a modest magnetic field applied on the domain wall, our approach provides a clean way in sharp contrast to using the semiconducting wires, where the complexity of the sub-band issue could be significant, to realize the 1D Kitaev's chain with Majorana zero modes at the ends.   

Possibility of disorder-induced sub-gap states near Majorana modes in topological insulator edges

We study the effects of multiple channel and disorder in the topological insulator(TI)-superconductor(SC)-ferromagnetic insulator(FI) hybrid structure, which has been proposed to realize Majorana modes. According to Anderson's theorem, proximity-induced SC in a TI is robust to all non-magnetic impurities. This however cannot be applied to the SC/FI interface where the end Majorana is located, since the time-reversal symmetry is locally broken. In this paper we study the spectrum near a SC/FI interface on a disordered TI edge. While we find that only the Majorana mode is induced single-channel case, inter-channel scatterings in a multichannel TI can induce extra localized states. We shalll comment on its effects in the detection and manipulations of the Majorana.   

Majorana Fermions on Zigzag Edge of Monolayer Transition Metal Dichalcogenides

Majorana fermions, quantum particles with non-Abelian exchange statistics, are not only of fundamental importance, but also building blocks for fault-tolerant quantum computation. Although certain experimental breakthroughs for observing Majorana fermions have been made recently, their conclusive detection is still challenging due to the lack of proper material properties of the underlined experimental systems. Here we propose a new platform for Majorana fermions based on edge states of certain non-topological two-dimensional semiconductors with strong spin-orbit coupling, such as monolayer group-VI transition metal dichalcogenides (TMD). Using first-principles calculations and tight-binding modeling, we show that zigzag edges of monolayer TMD can host well isolated single edge band with strong spin-orbit coupling energy. Combining with proximity induced \textit{s}-wave superconductivity and in-plane magnetic fields, the zigzag edge supports robust topological Majorana bound states at the edge ends, although the two-dimensional bulk itself is non-topological. Our findings points to a controllable and integrable platform for searching and manipulating Majorana fermions.   

Boosting Majorana zero modes

When Majorana bound states are driven at high speeds, competing processes may destroy quantum coherence. To study this regime we exploit an effective Lorentz invariance of a generic Majorana supporting Hamiltonian to obtain an exact solution of the domain wall bound states for arbitrary velocities. An effective 'speed of light' emerges, which acts as an absolute speed limit for braiding Majorana states. We also use our exact solutions to study further restrictions on the domain wall motion due to the presence of static impurities in the system. Looking beyond the context of topological quantum computing, our insights and analysis can also be viewed as theoretical basis for creating extreme relativistic phenomena, such as the Unruh effect, in solid state systems.   

Majorana mode in vortex core of Bi$_{2}$Te$_{3}$/NbSe$_{2}$ topological insulator-superconductor heterostructure

Majorana fermions have been intensively studied in recent years for their importance to both fundamental science and potential applications in topological quantum computing. Majorana fermions are predicted to exist in a vortex core of superconducting topological insulators. However, they are difficult to be distinguished experimentally from other quasiparticle states for the tiny energy difference between Majorana fermions and these states, which is beyond the energy resolution of most available techniques. Here, we overcome the problem by systematically investigating the spatial profile of the Majorana mode and the bound quasiparticle states within a vortex in Bi$_{2}$Te$_{3}$/NbSe$_{2}$. While the zero bias peak in local conductance splits right off the vortex center in conventional superconductors, it splits off at a finite distance $\sim$ 20nm away from the vortex center in Bi$_{2}$Te$_{3}$/NbSe$_{2}$, primarily due to the Majorana fermion zero mode. While the Majorana mode is destroyed by reducing the distance between vortices, the zero bias peak splits as a conventional superconductor again. This work provides strong evidence of Majorana fermions and also suggests a possible route to manipulating them.   

Mapping the topological phase diagram of multiband semiconductors with supercurrents

We show that Josephson junctions made of multiband semiconductors with strong spin-orbit coupling carry a critical supercurrent $I_c$ that contains information about the non-trivial topology of the system. In particular, we find that the emergence and annihilation of Majorana bound states in the junction is reflected in strong even-odd effects in $I_c$ under specific conditions. This effect allows for a mapping between $I_c$ and the topological phase diagram of the junction, thus providing a dc measurement of its topology.   

Session J43: Interaction Effects in Topological Insulators: Theory

Lattice construction of pseudopotential Hamiltonians for Fractional Chern Insulators

Fractional Chern insulators (FCIs) are new realizations of fractional quantum Hall states in lattice systems without orbital magnetic field. These states can be mapped onto conventional fractional quantum Hall states through the Wannier state representation (WSR) (Phys. Rev. Lett. 107, 126803 (2011)). In this talk, I shall show how the WSR can be used to construct FCIs pseudopotential Hamiltonians that are interaction Hamiltonians with certain ideal model wavefunctions as exact ground states. These pseudopotential Hamiltonians can be approximated by short-ranged interactions in FCIs, with the range minimized by an optimal gauge choice for the Wannier states. I will illustrate this lattice construction by showing the explicit form of the lowest pseudopotential for a few commonly used FCI models like the lattice Dirac model and the checkerboard model with Chern number C=1, and the d-wave model and triangular lattice model with C=2. The proposed pseudopotential Hamiltonians have the 1/3 Laughlin state as their ground-state when C=1, and a topological nematic (330) state as their groundstate when C=2. The proposed states can be verified by future numerical works, and in particular provide a model Hamiltonian for topological nematic states that have not yet been realized numerically.   

Electron-Hole Entanglement in a Quantum Spin Hall Insulator

We demonstrate that entangled electron-hole pairs can be produced and detected in a quantum spin Hall insulator with a constriction that allows for a weak inter-edge tunneling. A violation of a Bell inequality, which can be constructed in terms of low-frequency nonlocal current-current correlations, serves as a detection of the entanglement. We show that the maximum violation of a Bell inequality can be naturally achieved in this setup, without a need to fine tune tunneling parameters. This may provide a viable route to producing spin entanglement in the absence of any correlations and pairing, where spin-to-charge conversion is enabled by the helical edge structure of a quantum spin Hall insulator.   

correlation effects in topological phase transitions

We study topological insulators/trivial insulators topological phase transitions in the Kane-Mele-Hubbard model using the projective quantum Monte Carlo method. We numerically compute the topological invariants and study topological phase transitions under correlation. We find that quantum fluctuation effects from interactions can act both to stabilize and destabilize topological phases, depending on the details of the model. When the one-body terms break the lattice symmetry, e.g. bond dimerization breaks the rotational symmetry in the Kane-Mele model, the Hubbard interaction destabilizes the topological insulator phase. On the other hand, when the one-body terms (e.g. the third-nearest neighbor hopping) preserves the lattice symmetry, the interaction stabilizes the topological phase.   

Gravitational responses and entanglement for 2-dimensional chiral topological states

Chiral topological phases in 2+1D, which have a gapped bulk and gapless chiral edges, can be characterized by their response to deformations of spacetime. The leading order `gravitational response' is encoded in the gravitational and torsional Chern-Simons terms, which result in a chiral central charge and Hall viscosity respectively; however, it is not clear which aspects of these responses remain universal for a model without microscopic Lorentz invariance. We demonstrate how the chiral central charge and Hall viscosity may be extracted via an entanglement measure in the bulk, giving evidence for the correctness of a subset of the predicted responses. We also discuss some physical interpretations of the thermal responses. As a concrete example, we explicitly calculate the relevant bulk thermal responses and entanglement properties of a p+ip superconductor.   

Numerical studies of band geometry of fractional topological insulators

One of the chief motivations behind the current interest in topological insulators is the possibility that they may offer an alternate, more experimentally accessible venue for studying fractional quantum Hall effect (FQHE) physics, as well as possible novel states arising from the larger phase space relative to the lowest Landau level. Roy [1] identified several sufficient conditions on the single-particle Berry curvature and quantum metric for the algebra of projected density operators to be isomorphic to the Girvin-MacDonald-Platzman algebra of the FQHE. Here, we study the influence of these conditions in determining the stability of topological phases arising from density-density interactions in fractionally filled Chern bands. We present numerical results on the correlations between single-particle band geometry and the gaps in the energy and entanglement spectra for the corresponding interacting many-body state in a variety of lattice models exhibiting fractional Chern insulator phases. There is a key geometrical distinction between two- and multiple-band models. We also discuss extensions of the $W_{\infty}$ algebra and explore connections between the quantum metric and Hall viscosity.\\[0pt] [1] R. Roy, arXiv:1208.2055.   

A symmetry-protected many-body Aharonov-Bohm effect

It is known as a purely quantum effect that a magnetic flux affects the real physics of a particle, such as the energy spectrum, even if the flux does not interfere with the particle's path - the Aharonov-Bohm effect. We examine an Aharonov-Bohm effect on a many-body wavefunction. Specifically, we study this many-body effect on the gapless edge states of a two dimensional bulk gapped phase protected by a global symmetry - the symmetry-protected topological (SPT) states. The many-body analogue of spectral shifts, the twisted wavefunction and the twisted boundary realization are identified in this SPT state. An explicit lattice construction of SPT edge states is derived, and a challenge of gauging its non-onsite symmetry is overcome. Agreement is found in the twisted spectrum between a numerical lattice calculation and a conformal field theory prediction. Talk based on arXiv:1310.8291   

Classification and Description of Bosonic Symmetry Protected Topological Phases with semiclassical Nonlinear Sigma models

Symmetry protected topological (SPT) phases are a new type of quantum disordered phases with certain symmetry G, which is intrinsically different from a trivial direct product state. Well-known examples include topological insulators, topological superconductors and the Haldane phase of spin-1 chain. We focus on the field theory description of Bosonic SPT phases in all physical spatial dimensions. We propose that many bosonic SPT phases with different symmetries on a d-dimensional lattice can be described and classified by the same O(d+2) Nonlinear Sigma Model (NLSM) of a semiclassical Landau order parameter field in (d+1)-dimensional space-time, with a topological $\Theta$-term. Our classification based on topological NLSMs is completely identical to the Group Cohomology Classification of bosonic SPT phases. Besides that, NLSMs formalism also allow us to describe explicit physical properties of SPT phases, such as the bulk wave functions and boundary theories.   

Gapped symmetric boundaries of topological insulators

Topological insulators (TIs) are gapped quantum phases which host symmetry-protected gapless boundary excitations. On the other hand, the boundary states can be gapped by spontaneously breaking symmetry. We show that topological defects on the symmetry-broken boundary cannot proliferate due to their fractional statistics. A gapped symmetric boundary, however, can be achieved between a TI phase and certain fractionalized phase by condensing the bound state of a topological defect and an anyon. Such a hybrid structure containing TI and fractionalized phase generally support ground state degeneracy on torus.   

Chasing the Hofstadter Butterfly

The experimental observation of the Hofstadter butterfly, the fascinating quantum fractal that also encodes the Chern numbers associated with quantum Hall state, continues to remain a challenging task. It may be possible to observe the fine structure of the butterfly, consisting of small gaps of the spectrum characterized by topological invariants greater than unity, with a resolution matching that of the Chern-$1$ gaps that form the skeleton of the butterfly. The tiny gaps of the butterfly emanating from a rational flux $p/q$ are found to be associated with infinity of possible solutions (of Diophantine equation )for the rational flux. Not supported by the simple square lattice nearest-neighbor hopping model of the Hofstadter system, these solutions are found to be hiding in neighborhood of these fluxes. By perturbing this simple system, it is possible to ``amplify'' these small gaps corresponding to higher Chern states where they replace the Chern $1$ gap of the Hofstadter butterfly. In other words, by tuning a parameter, it is possible to induce topological quantum phase transitions where the finer gaps become the new major gaps that dominate the spectrum. This may provide a possible pathway to see the topological landscape of the Hofstadter butterfly fractal in its entirety.   

Precision of the quantum anomalous Hall effect in magnetic topological insulators

The quantum Hall effect normally refers to quantized Hall conductivity due to Landau quantization, as observed in 2D electron systems. To produce a Hall effect, one has to break time-reversal symmetry which is conveniently accomplished by applying an external magnetic field. The precision of the quantized Hall effect which occurs near integer Landau level filling factors has been verified to more than 8 figures. There are no known limitations to the accurary of the effect in the limit of zero temperature. The internal magnetization of a system in combination with spin-orbit coupling can also break time-reversal symmetry \emph{without} a magnetic field and can lead to a quantum anomalous Hall effect (QAHE). Recently, the QAHE has been observed experimentally in thin films of chromium-doped (Bi,Sb)$_2$Te$_3$, a magnetic topological insulator, where at zero magnetic field the Hall resistance reaches the predicted quantized value of $h/e^2$ [1]. We address the precision of the QAHE focussing on the role of quantum and thermal fluctuations of the magnetization.\\[0.2em] \noindent [1] C. Chang \textit{et al.}, Science \textbf{340}, 167--170 (2013).   

Phases of correlated spinless fermions on the honeycomb lattice

We use exact diagonalization and cluster perturbation theory to address the role of strong interactions and quantum fluctuations for spinless fermions on the honeycomb lattice. We find quantum fluctuations to be very pronounced both at weak and strong interactions. A weak second-neighbor Coulomb repulsion $V_2$ induces a tendency toward an interaction-generated quantum anomalous Hall phase, as borne out in mean-field theory. However, quantum fluctuations prevent the formation of a stable quantum Hall phase before the onset of the charge-modulated phase predicted at large $V_2$ by mean-field theory. Consequently, the system undergoes a direct transition from the semimetal to the charge-modulated phase. For the latter, charge fluctuations also play a key role. While the phase, which is related to pinball liquids, is stabilized by the repulsion $V_2$, the energy of its low-lying charge excitations scales with the kinetic energy $t$, as in a band insulator.   

Helical topological exciton condensates

We investigate a bilayer system of critical HgTe quantum wells each featuring a spin-degenerate pair of massless Dirac fermions. In the presence of an electrostatic inter-layer Coulomb coupling, we determine the exciton condensate order parameter of the system self-consistently. Calculating the bulk topological Z2 invariant of the resulting mean field Hamiltonian, we discover a novel time reversal symmetric topological exciton condensate state coined the helical topological exciton condensate. We argue that this phase can exist for experimentally relevant parameters. Interestingly, due to its multi-band nature, the present bilayer model exhibits a nontrivial interplay between spontaneous symmetry breaking and topology: Depending on which symmetry the condensate order parameter spontaneously picks in combined orbital and spin space, stable minima in the free energy corresponding to both trivial and nontrivial gapped states can be found.   

Ferromagnetism and quantum anomalous Hall effect in half-saturated germanene

Owing to the buckled structure of germanene, saturating one sublattice of atoms is workable. After studying cases of different percentages of saturation, we confirm that a narrow band always exist at the chemical potential which makes flat-band ferromagnetism possible. As the vacancy density increases, ferromagnetism becomes weaker. The magnetization of the ferromagnetism is directly relates to the saturation percentage, which makes ferromagnetic gap controllable. Importantly, we observe quantum anomalous Hall (QAH) states with Chern number one or two depending on the magnetization in the 1/4-saturation system. Our finding provides a potent method in the pursuit of room-temperature QAH effect.   

Fractional Topological Phases in Generalized Hofstadter Bands with Arbitrary Chern Numbers

We examine similarities and differences between topological flat bands with Chern numbers $C > 1$ and conventional quantum Hall multi-layers. By constructing generalized Hofstadter models that possess ``color-entangled'' flat bands, we provide an intuitive understanding of certain puzzling properties of $C > 1$ flat bands, which can effectively be mapped either to a single-layer or to a multi-layer model depending on the lattice configuration. We identify interacting systems in which the ground state degeneracy depends on whether the system consists of an even or odd number of unit cells along one particular direction, and discuss the relation between these observations and the previously proposed ``topological nematic states.'' Our study also provides a systematic way of stabilizing various fractional topological states in $C > 1$ flat bands.   

Weak symmetry breaking in two dimensional topological insulators

We show that there exist 2D time reversal invariant fractionalized insulators with the property that both their boundary with the vacuum and their boundary with a topological insulator can be fully gapped without breaking any symmetries. This result leads us to an apparent paradox: we consider a geometry in which a disk-like region made up of a topological insulator is surrounded by an annular strip of a fractionalized insulator, which is in turn surrounded by the vacuum. If we gap both boundaries of the strip, we naively obtain an example of a gapped interface between a topological insulator and the vacuum that does not break any symmetries -- an impossibility. The resolution of this paradox is that this system spontaneously breaks time reversal symmetry in an unusual way, which we call weak symmetry breaking. In particular, we find that the only order parameters that are sensitive to the symmetry breaking are nonlocal operators that describe quasiparticle tunneling processes between the two edges of the strip; expectation values of local order parameters vanish exponentially in the limit of a wide strip. Also, we find that the symmetry breaking comes with a ground state degeneracy, but the degeneracy is topologically protected, rather than symmetry protected.   

Session J44: MEMS, NEMS, and Mechanical Properties

All-thin-film PZT/FeGa Multiferroic Cantilevers and Their Applications in Switching Devices and Parametric Amplification

We are investigating the characteristics of microfabricated PZT/FeGa multiferroic cantilevers. The cantilevers can be driven by AC or DC magnetic and electric field, and the device response can be read off as a piezo-induced voltage. We can use the multiple input parameters to operate the devices in a variety of manners for different applications. They include electromagnetic energy harvesting, pulse triggered nonlinear memory devices, and parametrically amplified ME sensors. Due to the competition of anisotropy and Zeeman energies, the mechanical resonant frequency of the cantilevers was found to follow a hysteresis behavior with DC bias magnetic field applied in the cantilever easy axis. We can also control and tune the occurrence of nonlinear bifurcation in the frequency spectrum. The resulting hysteresis in the frequency spectrum can be used to make switching devices, where the input can be DC electric and magnetic fields, as well as pulses of AC fields. We have also demonstrated parametric pumping of the response from an AC magnetic field using frequency-doubled AC electric field. The enhanced equivalent ME coefficient is as high as 10 million V/(cm*Oe), when the pumping voltage is very close to a threshold voltage. The quality factor also increases from 2000 to 80000 with pumping.   

Rotational Modes in Phononic Crystals

We propose a lumped model for the rotational modes in two-dimensional phononic crystals comprised of square arrays of solid cylindrical scatterers in solid hosts. The model not only can reproduce the dispersion relations in a certain range with one fitted parameter, but also gives simple analytical expressions for the frequencies of the eigenmodes at the high symmetry points in the Brillouin zone. These expressions provide physical understandings of the rotational modes as well as certain translational and hybrid mode, and predict the presence of accidental degeneracy of the rotational and dipolar modes, which leads to a Dirac-like cone in the Brillouin zone center.   

Fracture and Failure of Nanoparticle Monolayers and Multilayers

We present the first systematic investigation of fracture in self-assembled gold nanoparticle mono- and multilayers, attached to elastomer substrates and subjected to tensile stress. Imaging the fracture patterns down to the scale of single particles provides detailed information about the crack width distribution and allows us to compare the scaling of the average crack spacing as a function of strain with predictions by shear-lag models. With increasing particle size, the fracture strength is found to increase while it decreases as the film thickness is built up layer by layer, indicating stress inhomogeneity and crack propagation in the thickness dimension.   

Elastic Properties of Graphene Nanomeshes

We report results on the elastic properties of graphene nanomeshes following a systematic analysis based on molecular-statics and molecular-dynamics simulations of uniaxial tensile deformation tests according to reliable bond-order classical interatomic potentials. Elastic properties are determined as a function of the nanomesh architecture, including the regular arrangement of pores in the nanomesh (pore lattice structure), pore morphology, nanomesh density ($\rho$), and pore edge passivation. We report scaling laws for the density dependence of the elastic modulus $M$ and find that $M$ scales with the square of the density, consistently with other cellular materials, for circular unpassivated pores over the range of temperature and nanomesh architectural parameters examined. We find that pore edge passivation strengthens the elastic moduli. The effects of passivation and pore morphology, namely, the aspect ratio of elliptical pores, on the $M(\rho)$ scaling laws are analyzed in detail.   

Scattering of Gigahertz Coherent Acoustic Phonons by Nanoporous Structures in Hypersonic Crystals

A gigahertz acousto-optic modulation technique, based on a mechanism in which the perturbation of the photonic band gap is caused by the coherent oscillation of the phonon modes in the hypersonic crystal, is demonstrated. We present the measurement results of the coherent acoustic vibrations of the hypersonic crystals comprised of SiO$_{2}$ or nanoporous SiO$_{2}$ spheres. Our transient reflection spectroscopy results identify the different transport behaviors of acoustic waves in the hypersonic crystals. While the bulk phonon waves are heavily damped by the nanoporous structures in the hypersonic crystal, the decay time of the surface phonon wave is barely affected.   

High-density aerogels with ultralow sound velocity: Microstructure is a key parameter determining the sound velocity

Aerogels are more and more regarded as a new state of matter nowadays because of its diverse chemical compositions and unique properties which could fill the gap between condensed matter and gas-state matter. Among the properties, the ultralow sound velocity in the aerogels (lower than that in the air) is of great interests. J. Fricke's group studied many kinds of aerogels with different compositions and found that the sound velocity was mainly influenced by the density. Thus they obtained the lowest sound velocity result ($\sim$ 100 m/s) in a low-density silica aerogel medium ($\sim$ 0.05 g.cm$^{-3})$. Here we studied the acoustical properties of the aerogels with the similar high density (about 1.3 g.cm$^{-3})$ but different skeleton structure (nano-, micro- or nano-/micro- structured) by adjusting the phase separation mode. The sound velocities of all the aerogels are below 300 m.s$^{-1}$, among which micro-/nano- structured aerogel exhibits lowest longitudinal wave velocity (below 80 m.s$^{-1})$. Further structural studies indicated that the hierarchical arrangement of microstructure is the key parameter determining the sound velocity besides the density.   

Absolute surface energies, fracture toughness, and cracking in nitrides

Growth of high quality single crystals and epitaxial layers of GaN is critical for producing high-efficiency optoelectronic and power electronic devices. One of the fundamental material properties that govern growth of single crystals is the absolute surface energy of the crystallographic planes. Knowledge of these energies is required to understand and optimize growth rates of different facets in GaN, and provide fracture toughnesses for brittle fracture. By means of hybrid functional calculations, we have determined absolute surface energies for the non-polar \{11-20\} $a$ and \{10-10\} $m$ planes, and approximated values for polar (0001) $+c$ and (000-1) $-c$ planes in wurtzite GaN. For all surfaces, we consider low-energy bare and hydrogenated reconstructions under a variety of conditions relevant to experimental growth techniques. We find that the energies of the m and a planes are similar, and constant over the range of conditions studied. In contrast, the energies of the polar planes are strongly condition dependent. Even so, we find that the $+c$ polar plane is systematically lower in energy than the $-c$ plane. We have used our surface energies to determine brittle fracture toughnesses in AlN and GaN, as well as the critical thickness for cracking of AlGaN on GaN.   

Cryogenic Nano-Fabrication using the Fab on a Chip approach

The Fab on a Chip approach is a novel fabrication technique that leverages the control and stability of MEMS machines to fabricate structures on the nano-scale. This contrasts to standard deep-UV and e-beam lithography methods typically used today. We present how a fully functional nano-fabrication system can be operated in a cryostat to enable novel physics experiments. To this end MEMS based machines are built that mimic typical macroscopic tools found in a modern nano-fabrication facility. We demonstrate functioning film thickness monitors, heaters, shutters and atom flux sources that can all be integrated on a single silicon chip. At the heart of the fab is a dynamic shutter-aperture system that functions as a programmable stencil which guides atoms to specific locations at precise times. It is argued that this method has the potential to obtain single atom control of the deposited materials. The low power and small footprint enables the setup to function in a cryogenic environment. We demonstrate basic functionality of the elements at liquid helium temperatures. The advantage of resist free lithography and the deposition being the final fabrication step is the ability to pattern materials incompatible with standard techniques. Furthermore, the ultra-clean environment is suited for high purity fabrication of structures made of exotic materials such as lithium, with the intent to enable novel electron transport experiments.   

Integrated MEMS mass sensor and atom source for a ``Fab on a Chip''

``Fab on a Chip'' is a new concept suggesting that the semiconductor fabrication facility can be integrated into a single silicon chip for nano-manufacturing. Such a chip contains various MEMS devices which can work together, operating in a similar way as a conventional fab does, to fabricate nano-structures. Here we present two crucial ``Fab on a chip'' components: the MEMS mass sensor and atomic evaporation source. The mass sensor is essentially a parallel plate capacitor with one suspended plate. When incident atoms deposit on the suspended plate, the mass change of the plate can be measured by detecting the resonant frequency shift. Using the mass sensor, a mass resolution of 3 fg is achieved. The MEMS evaporation source consists of a polysilicon plate suspended by two electrical leads with constrictions. By resistively heating the plate, this device works as a tunable atom flux source. By arranging many of these devices into an array, one can build a multi-element atom evaporator. The mass sensor and atom source are integrated so that the mass sensor is used to monitor and characterize the atomic flux. A material source and a sensor to monitor the fabrication are two integral components for our ``Fab on a Chip.''   

Measurement of Casimir Force between Monolithic Silicon Microstructures

We present measurements of the Casimir force between silicon components in a near-planar geometry. We create the device from a silicon-on-insulator wafer using microfabrication. It contains a force-sensing micromechanical beam and an electrostatic comb actuator for controlling the distance. The two lithographically-defined micromechanical components are on the same silicon substrate and are automatically aligned after fabrication. Thus, we can achieve a high degree of parallelism between the two interacting surfaces. We employ a magneto-motive technique to measure the shift in the resonance frequency of the force sensing beam. Periodic Lorentz forces are exerted on the beam when an ac current is applied in a perpendicular magnetic field. As the movable electrode is pushed towards the silicon beam by the comb drives, the Casimir force increases. The force gradient is proportional to the resonance frequency shift of the beam. After the calibration using electrostatic forces and balancing the residual voltage, we measure the Casimir force gradient. Our results are in reasonable agreement with theoretical calculations, considering possible contributions of patch potentials. Apart from providing a compact platform for Casimir force measurements, this scheme also opens new opportunities for the measurement of Casimir force in complex geometries.   

Silicon Nanomembrane Bipolar Junction Transistors for Microwave Frequency Applications

Silicon nanomembranes (SiNMs) are a promising material for flexible semiconductor devices due to their high carrier mobility and compatibility with standard CMOS processing. Previous studies have reported SiNM field-effect transistors with operating frequencies as high as 12 GHz. In order to expand the utility of SiNM devices, a method for the fabrication of monocrystalline microwave frequency silicon bipolar junction transistors (BJTs) will be presented. High-temperature processing of SiNM BJT devices is performed on a Silicon-on-Insulator (SOI) wafer. Using angled ion implantation, conformal chemical vapor deposition and anisotropic reactive ion etching, a poly-silicon sidewall spacer is formed. This spacer defines a base region approximately 200nm wide without the use of electron beam lithography. Devices are then released using selective wet etching in HF and transferred to alternate flexible substrates. Microwave frequency data will be presented, and the effects of the transfer process on device performance will be discussed.   

Microelectromechanical Systems (MEMS) for Tunable Plasmon Coupling

The plasmonic response of metallic nanoparticles depends upon the particle composition, size, shape, surrounding medium, and electromagnetic field coupling to neighboring particles. We present schemes for using MEMS to tune the separation of plasmonic elements and thereby alter the plasmonic response. One of our devices can move two nanoparticles along three axes, creating a dimer with a tunable separation. Localized surface plasmon resonances are sensitive to changes in the surrounding dielectric medium, a phenomenon that has been used in sensing applications [1]. We will use the dimer as a tunable sensor by scanning it through a region of interest and extrapolating changes in the local dielectric properties from the shift in the plasmonic resonance. Another MEMS device actuates arrays of micron-sized gold antennas relative to one another. Changing the separation between elements in an array of plasmonic particles can lead to electromagnetically induced transparency (EIT) and absorption (EIA) [2]. We will shift the arrays relative to one another and measure the spectral response using Fourier transform infrared spectroscopy to demonstrate EIT and EIA. [1] Mock, J., Et al. Nano Lett. 3 (4), 485-491 (2003). [2] Adato, R., et. al., Nano Lett. 13 (6), 2584-2591 (2013).   

Session J45: Semiconductor Electronic Structure: Thermodynamic & Optical Properties

Optical properties of AlGaAsSb thin films lattice-matched to InP(100)

AlGaAsSb quaternary compounds lattice-matched to InP are of interest for applications in InP-based high-efficiency multi-junction solar cells and surface normal opto-electronic devices operating in the wavelength range from 1.3 to 1.55 $\mu $m. Knowledge of optical properties of constituent layers in photonic and photovoltaic devices plays an important role in designing the device structure and modeling the device performance. However, only a limited number of theoretical and experimental studies have been done on AlGaAsSb lattice-matched to either GaSb or InAs, and no systematic optical study is available for AlGaAsSb lattice-matched to InP. Here we apply spectroscopic ellipsometry to study the optical properties of AlGaAsSb thin films grown by MOVPE on InP substrates. The fundamental optical functions such as dielectric function, refractive index, reflectivity, and absorption coefficient are determined by modeling the data. Energies for several critical points and their compositional dependence are obtained from standard lineshape analysis. The results from our study can be used (1) to improve our understanding of the electronic structure of AlGaAsSb and related compounds and (2) to provide optical information for the design of InP-based multi-junction solar cell structures.   

Precise Determination of the Direct-Indirect Band Gap Energy Crossover In Al$_{\mathrm{x}}$Ga$_{\mathrm{1-x}}$As

Al$_{\mathrm{x}}$Ga$_{\mathrm{1-x}}$As is a technologically important semiconductor material system for optoelectronic applications due to its type I band alignment with GaAs under nearly lattice-matched conditions. Heterostructure design often relies on exactly controlling the relative positions of the $\Gamma $ and X conduction band edges, yet despite over three decades of research on this alloy, the precise energy and composition of the direct-indirect band gap crossover is still not well resolved. We report the results of our most recent investigation of Al$_{\mathrm{x}}$Ga$_{\mathrm{1-x}}$As (0.28 \textless $x$\textless 0.42) epitaxial films, in which the observation of concurrent photoluminescence (PL) emission peaks from the direct and indirect band gaps combined with time-resolved PL information yields a precise determination of the direct-indirect band gap crossover energy and composition.   

Anisotropic optical properties of Fe/GaAs nanolayers from first principles

We investigate the anisotropy of the optical properties of few-monolayer Fe films on GaAs from first principles calculations. Both intrinsic and magnetization-induced anisotropy are covered by studying the systems in the absence or presence of external magnetic fields. We use the linearized augmented plane wave (LAPW) method, as implemented in the WIEN2k density functional theory code, to show that the $C_{2v}$ symmetric anisotropy of the spin-orbit coupling fields at the Fe/GaAs interface manifests itself in an analogous anisotropy of the optical properties of the system, such as its optical conductivity and its reflectivity. We find that the optical properties vary significantly when the direction of the external magnetic field is changed. This suggests that the anisotropic spin-orbit coupling fields in experimentally relevant Fe/GaAs slabs can be studied by purely optical means.   

In situ tuning biexciton antibinding-binding transition and fine structure splitting through hydrostatic pressure in single InGaAs quantum dots

We demonstrate that the exciton and biexciton emission energies as well as exciton fine structure splitting (FSS) in single (In,Ga)As/GaAs quantum dots (QDs) can be efficiently tuned using hydrostatic pressure \textit{in situ} in an optical cryostat at up to 4.4 GPa. The maximum exciton emission energy shift is up to 380 meV, and the FSS is up to 150 $\mu$eV. We successfully produce a biexciton antibinding-binding transition in QDs, which is the key experimental condition that generates color- and polarization-indistinguishable photon pairs from the cascade of biexciton emissions and that generates entangled photons via a time-reordering scheme. We also perform the atomistic pseudopotential calculations on realistic (In,Ga)As/GaAs QDs to understand the physical mechanism underlying the hydrostatic pressure-induced effects.   

X-ray and optical pulse interactions via electron trapping in GaAs

A highly excited state of GaAs is created by the absorption of an extremely intense focused 80 ps pulse of hard x-rays at the Advanced Photon Source synchrotron. This state is probed by 2 ps laser pulses with photon energies near the semiconducting band gap, which has previously revealed x-ray induced optical transparency. Two unexpected results are found: x-ray induced luminescence is dramatically enhanced when a high intensity laser pulse precedes the x-ray pulse, and the decay of the induced transparency becomes much slower when the intensity of the subsequent probe laser is increased. Both results require that energy be stored in GaAs by the first pulse, and then released by the second pulse. We describe how this can be explained by electron trapping centers in GaAs with trapping lifetimes of a few nanoseconds. We compare these results with lifetime measurements of other excitations produced by ultrafast optical absorption. We also show how minor improvements in focusing will lead to single-pulse x-ray induced temperature jumps of thousands of Kelvin, allowing new x-ray excited dense matter states to be explored.   

Individually Contacted Electron-Hole Bilayers of InAs/GaSb

Electron-hole bilayers made of InAs/GaSb semiconductors are promising quantum structures in realizing novel condensed phases of excitons. Using low temperature transport we have measured a InAs/GaSb composite quantum well with a AlGaSb tunneling barrier between the layers, and have been able to adjust the Fermi energy of the electron or hole layers independently by double gates. In order to study the interactions between the two layers, we processed devices with a flip-chip technique, where gates were placed on both sides of the wafer within a micrometer distance from respective layers. Additional gates placed on top of the contact leads to facilitate independent contacts to the individual layer. We will present preliminary data for standard and flip-chip devices measured by low temperature transport. The work in Rice is supported by a grant of DOE-BES.   

Mechanism of excitonic dephasing in layered InSe crystals

The dephasing and lifetime of excitons in InSe layered crystals has been carefully measured using three pulse four-wave mixing and two-dimensional Fourier transform (2DFT) spectroscopy. We obtain a complete and detailed picture of the mechanism of excitonic dephasing in this layered material. The temperature dependence provides a detailed description of the phonon-exciton interactions and the zero Kelvin limit of the homogeneous linewidth. The excitation density dependence reveals strong excitation induced dephasing due to exciton-exciton scattering.   

Exploring layered GaSe crystals with 2DFT spectroscopy

The dephasing and lifetime of excitons and biexcitonic effects in GaSe layered crystals have been carefully measured using three pulse four-wave mixing and two-dimensional Fourier transform (2DFT) spectroscopy. Strong biexciton signatures are observed in the 2DFT spectra. Excitation density and temperature dependence 2DFT spectra provide new insights into the many-body interactions in this material.   

Electronic Structure of Ge$_{1-y}$Sn$_{y}$ and Ge$_{1-x-y}$Si$_{x}$Sn$_{y}$ Alloys from Optical and Electro-Optical Measurements

Optical transitions in Ge$_{1-y}$Sn$_{y}$ and Ge$_{1-x-y}$Si$_{x}$Sn$_{y}$ alloys have been studied in detail using spectroscopic ellipsometry, photocurrent, and photoluminescence experiments on films grown on Si, Ge, and Ge-buffered Si platforms using CVD and gas-source MBE reactions of germanes, silanes, and deuterated stannane. The compositional, $x$ and $y$, dependence of the lowest direct and indirect band gaps, as well as other transitions, are determined through these techniques. This has enabled mapping the direct-indirect gap crossover in composition space to reveal the potential of these alloys for optoelectronic applications via band gap engineering. All the measured transition energies can be described by second-order polynomials as functions of composition whose quadratic coefficients (bowing parameters) show systematic chemical trends. The transferability of these parameters between binary and ternary alloys is studied in detail. Due to a larger negative bowing for the direct than for the indirect gap, the crossover to direct gap behavior occurs for Sn concentrations much lower than predicted from a simple linear interpolation between the corresponding elemental semiconductors.   

Structure band-gap correlations in semiconductors: Implications for computational band gap prediction

Large scale structure prediction for novel materials requires computationally inexpensive lattice relaxation methods, which are typically based on density functional theory (DFT) using a semi-local approximation for the exchange-correlation functional. These methods provide structural parameters accurate to within a few percent, but cannot predict band-gaps. Band-gap calculations, require much more computationally expensive methods such as hybrid functionals or the GW approximation. Such an accuracy-tiered method fails dramatically for Cu3PSe4. When the generalized gradient approximation (GGA) is used to relax the lattice and ions, band-gaps calculated using both the single shot GGA+GW method and the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional method are a full 0.5 eV lower than the band gaps calculated for the unrelaxed, experimental structure. The GW and HSE methods predict accurate band gaps only when used with the correct experimental structure. We show that in Cu3PSe4, the calculated band-gap depends strongly on the P-Se bondlength, which can be explained by the P-Se* anti-bonding character of the lowest conduction band state. We show this effect for different lattice relaxation methods including recently developed meta-GGAs.   

Carrier lifetimes in group-IV semiconductors

We have demonstrated that electron-phonon coupling in semiconductors shows a variation over the Brillouin zone and is not constant as considered traditionally within the long wavelength approximation. In group IV semiconductors, the variation in the electron-phonon coupling can range from $50-400$~meV. We evaluate electron-phonon coupling matrix elements over the entire Brillouin zone and use this information to calculate the electron lifetimes in group IV semiconductors C, Si, and Ge. Our results, within the framework of the single particle approximation, allow us to evaluate the lifetimes for different initial electron momenta. For our calculations, we have used the methodolgy developed by Giustino et. al for evaluating the electron-phonon coupling employing the Wannier-Fourier interpolation (Phys. Rev. B {\bf 76}, 165108 (2007)).   

All-electron GW quasiparticle band structures of group 14 nitride compounds

We have investigated the group 14 nitrides (M$_{3}$N$_{4})$ in both the spinel phase (with M$=$C, Si, Ge and Sn) and the beta phase (with M$=$Si, Ge and Sn) using density functional theory (DFT) with the local density approximation (LDA). The Kohn-Sham energies of these systems are first calculated within the framework of full-potential LAPW and then corrected using single-shot G$_{0}$W$_{0}$ calculations, which we have implemented in the Exciting-Plus code. Direct bands gap at the $\Gamma $ point are found for all spinel-type nitrides. The calculated band gaps of Si$_{3}$N$_{4}$, Ge$_{3}$N$_{4}$ and Sn$_{3}$N$_{4}$ agree with experiment. We also find that for all systems studied, our GW calculations with and without the plasmon-pole approximation give very similar results, even when the system contains semi-core 3d electrons. These spinel-type nitrides are novel materials for potential optoelectronic applications.   

Attosecond time-resolved studies of band-gap excitations in semiconductors

Attosecond time-resolved spectroscopy is an emerging technique that has proven to be powerful in investigating electron dynamics in atoms, molecules and dielectrics. Attosecond transient absorption spectroscopy is used to follow electron dynamics in semiconductors. In these experiments, a few-cycle visible pulse excites electrons across the band gap of a semiconductor, followed in time by an attosecond extreme ultraviolet pulse to probe changes to the core level transitions, which report on the electronic structure of the material. Using the few-cycle strong field visible pulse to excite the direct band gap of silicon and an attosecond pulse to probe the Si L$_{\mathrm{2,3}}$ edge, we have resolved ultrafast carrier generation in the conduction band as well as band gap renormalization. These experiments have revealed transient shifts and broadening to the L$_{\mathrm{2,3}}$ edge of Si on a few femtosecond timescale, reflecting an instantaneous response of the electronic structure to ultrafast excitation. The studies will be extended to probe electron dynamics in heteroatom semiconductors such as copper oxide and three-band materials such as tellurium-doped zinc oxide.   

Dipoles in III-V MOSFETs: Scattering and Threshold shifts

A scattering mechanism arising from the charge dipoles at the oxide/III-V semiconductor interface with different crystallographic orientations is identified. We quantitatively evaluate the effect of interface charge dipoles with angular distributions taking into account the effect of crystallographic orientations on electron transport in semiconductor channels of III-V field-effect transistors. The time-independent distribution of dipoles leads to two effects relevant to transistor operation. They cause shifts in the threshold voltage of III-V MOSFETs. The dipoles also scatter conducting charges at the III-V/oxide interface due to their long-range Coulomb potential. The dipole-scattering-limited mobility decreases with increasing dipole parameters such as the dipole length and dipole density, and dipole angle. Higher electron mobility is obtained for aligned dipoles relative to the angular-oriented random dipoles at the interface. A smaller threshold voltage shift is generated for angular-oriented dipoles over the aligned dipoles at the interface. The charge dipole scattering mechanism can be applied to ALD/InGaAs MOSFETs and also to ALD/Nanowire FETs.   

High resolution EELS study of novel semiconductor alloys: Ge$_{1-x-y}$Si$_{x}$Sn$_{y}$ and AlPSi$_{3}$

Metastable alloys play a fundamental role in modern semiconductor science and technology as a major tool for band gap and strain engineering. When these alloys incorporate highly dissimilar materials, such as Si and Sn in Ge$_{1-x-y}$Si$_{x}$Sn$_{y}$ alloys or III-V pairs in group-IV matrices, as in the new (III-V)$_{x}$(IV)$_{5-2x}$ systems synthesized by our group, the atomic distribution at the sub-nanometer scale is of paramount concern, since even slight deviations from randomness or from predicted ordered structures can have a dramatic impact on the electronic structure. Aberration-corrected microscopes provides the opportunity to generate atom-selective images with unprecedented structural and chemical detail. For this work, we used Electron Energy Loss Spectroscopy (EELS) to map the Sn distribution in Ge$_{1-y}$Sn$_{y}$ and Ge$_{1-x-y}$Si$_{x}$Sn$_{y}$ alloys, as well as the distribution of Al and P atoms in AlPSi$_{3}$ to elucidate local bonding configurations and atom substitutionality. The EELS measurements also provide information on the electronic structure, which is compared with optical results and theoretical calculations.   

Session J46: Ce- and Yb-based Heavy-Fermion and Non-Fermi Liquid Systems

Reentrant quantum oscillations in CeIn$_3$

Heavy fermion conductor CeIn$_3$ is a low temperature antiferromagnet with a N\'eel transition at 10\,K, 2.6\,GPa, and 62\,T. We show that N\'eel and Lifshitz transitions both introduce Fermi surface reconstruction, and the difference in temperature dependence between them leads to surprising reentrant behavior of certain Shubnikov de Haas orbits as a function of field, temperature, and pressure. We use a diamond anvil cell in a helium-3 fridge in pulsed magnetic fields to cover almost the entire phase space of the antiferromagnetic state. Our high resolution resistivity probe is a Tunnel Diode Oscillator that is mounted adjacent to the pressure cell within the bore of the magnet. We confirm the extraordinary sensitivity of the technique by observing the 69 kT Shubnikov de Haas orbit in the resistivity of polycrystalline copper.   

Fermi liquid scaling of the optical conductivity in MnSi

We present measurements of the low-frequency optical conductivity of MnSi thin films made with time-domain terahertz spectroscopy. At low temperatures and low frequencies, the conductivity is consistent with the prediction of Fermi liquid theory, $\rho(\omega,T)\equiv [\sigma(\omega,T)]^{-1} = \rho_0 + A[(\hbar\omega)^2 + (2\pi k_B T)^2]$. As the temperature increases, the system loses quasi-particle coherence: deviations from Fermi liquid behavior appear, while the plasma frequency inferred from a Drude fit decreases dramatically. Above T$\approx 50$ K, $\sigma_2(\omega)$ develops a negative slope that indicates a sharp pseudogap in the conductivity spectrum.   

A THz spectroscopic study of the heavy fermion CeFe$_2$Ge$_2$

We present time-domain THz spectroscopy data on a thin film of the heavy fermion compound CeFe$_2$Ge$_2$. Non-Fermi liquid behavior and a metamagnetic anomaly have been observed in CeFe$_2$Ge$_2$ pointing to its proximity to a quantum critical point, much like the well studied compounds CeNi$_2$Ge$_2$ and CeRu$_2$Si$_2$. A $T^{1.5}$ dependence of resistivity has been reported in the temperature range of 2K-15K. Our measurements to obtain the complex conductivity as a function of frequency were taken from room temperature down to 1.5K. Using this data in addition to DC resistivity measurements, we calculate the frequency dependent scattering rate using an extended Drude model analysis. The power law dependence of the scattering rate on frequency will be discussed in its relation to the anomalous transport properties that have been reported in this material.   

The Field-Temperature Phase Diagram of the Heavy Fermion Compound Ce$_2$Ge$_2$Mg

The heavy fermion metal Ce$_2$Ge$_2$Mg has a layered structure with the Ce nearest neighbor pairs arranged orthogonally to one another in the tetragonal $a$-$b$ plane, a structure topologically equivalent to the Shastry-Sutherland lattice (SSL). This material is thought to be more two dimensional than other $R_2T_2X$ SSL compounds such as Yb$_2$Pt$_2$Pb, due to the relatively long distance along the $c$-axis between Ce atoms in adjacent SSL planes. The magnetic phase diagram of Ce$_2$Ge$_2$Mg has been determined for magnetic fields in the SSL plane and along the $c$-axis, for temperatures from the antiferromagnetic transition at $T = 9.4$ K in zero applied field down to $T = 1.8$ K and fields as high as 14 T using magnetization, resistance, and heat capacity. Our measurements show a complex phase diagram with field suppressing the antiferromagnetic transition and the emergence of several ordered phases. These phases are possible evidence for singlet-to-triplet excitations in the Ce dimers. \\ This project was supported by NSF-DMR-1310008   

Strongly-disordered hybridization and non-Fermi liquid behavior in CePt$_4$Ge$_{12-x}$Sb$_x$ studied with thermoelectric power

Non-Fermi liquid (NFL) behavior is commonly associated with the presence of a nearby quantum critical point, but can also be observed in other scenarios. In a clean system, hybridization between localized and itinerant electron states can be characterized by a single Kondo temperature $T_K$, but introducing chemical disorder can lead to a wide distribution of $T_K$ values. Given sufficient disorder, the resulting distribution will tend to include an appreciable number of localized electron states which are characterized by $T_K \sim$ 0 K, and NFL behavior emerges. A Kondo-disorder type of NFL behavior was recently reported in the filled skutterudite system CePt$_4$Ge$_{12-x}$Sb$_x$ in the vicinity of $x = 1$. We performed a study of the thermoelectric power $S(T)$ for this system and observed an evolution of $S(T)$ with $x$ that is dramatic and broadly consistent with the boundaries of the proposed phase diagram. The effect of disordered hybridization is clearly observed in a low-temperature feature in $S(T)$ in the range $0.5 \le x \le 1.5$ and NFL behavior is also observed at x = 1. These results clearly demonstrate how sensitively $S(T)$ is able to probe a Kondo disorder system.   

A percolation description of the critical behavior in quantum critical Ce(Ru0.24Fe0.76)Ge2

Quantum critical points (QCP) arise when the magnetic ordering of local moments that are embedding in a metallic system are just suppressed by Kondo shielding in the absence of thermal energy. Pure stoichiometric systems tend not to occur at QCPs so that an external tuning parameter is typically required to drive the system to the QCP. Often this external tuning parameter takes the form of chemical pressure by way of chemically substituting smaller (larger) ions at certain lattice positions such that the average interatomic spacing become smaller (larger). One unintended consequence of this approach, however, is the formation of a distribution of local interatomic---hence, also inter-moment---distances, which necessarily translates into a distribution of Kondo temperatures. In this case, the magnetic lattice fragments upon cooling such that percolation theory is necessary to describe the system, though---as we will demonstrate---certain modifications are required because of finite-size effects of the clusters. We argue that a complete QCP theory of heavily doped systems must begin by separating out the effects caused by the presence of magnetic clusters from those resulting from non-Fermi liquid behavior associated with the QCP.   

Superconductivity in LaPd$_{1-x}$Bi$_{2}$ and moderate heavy fermion behavior in antiferromagnetic CePd$_{1-x}$Bi$_{2}$

Superconductivity at 2.1 K is observed in LaPd$_{1-x}$Bi$_{2}$. A small residual resistance ratio indicates a strong scattering effect induced by Pd vacancies. Hall effect measurements reveal electron-like carriers and single-band transport behavior in LaPd$_{1-x}$Bi$_{2}$. Band structure calculations support the possibility of Fermi surface nesting near the fully stoichiometric case in LaPd$_{1-x}$Bi$_{2}$. By creating Pd vacancies the Fermi surface nesting is avoided which suppresses any potential CDW on the Bi net. CePd$_{1-x}$Bi$_{2}$ is non-superconducting but shows antiferromagnetic ordering below 6 K. A Sommerfeld coefficient of 0.199 J.molCe$^{-1}$K$^{-2}$ reveals a moderate heavy fermion behavior in CePd$_{1-x}$Bi$_{2}$. The resistivity curve shows the presence of Kondo and crystalline-electric-field effects. Magnetoresistance and Hall effect measurements show the interplay between Kondo and crystalline-electric-field effects obviously reconstructs the Fermi surface topology of CePd$_{1-x}$Bi$_{2}$ around 75 K. With the help of band structure calculations, we argue the f-d hybridization in CePd$_{1-x}$Bi$_{2}$ quenches the superconductivity.   

Anomalous Hall Effect in Frustrated Kondo Lattices and Implications for Heavy Fermion System Pr$_2$Ir$_2$O$_7$

Pr$_2$Ir$_2$O$_7$ is a frustrated Kondo system that has a large zero field anomalous Hall Effect, which hints for a chiral spin liquid ground state of the local moments. Recently, thermodynamic measurements reveal a divergent Gr\"uneisen ratio, which indicates a nearby quantum critical point [1]. Motivated by these findings, we study the prototype chiral spin liquid state on the J1-J2 square lattice with Kondo coupling, and use it as a probe of the Kondo destruction quantum phase transition. For the Kondo screened phase, we study the topological properties of the hybridized heavy quasi-particle bands, and show that they yield a zero-field anomalous Hall effect. For the Kondo destroyed phase, we derived such Hall response by showing that the chiral spin liquids state mediates an effective chiral interaction among the electrons. The behavior of the Hall response on approach, and across, the quantum critical point is discussed based on these calculations. \\[4pt] [1] Y. Tokiwa et al, to be published (2013).   

Magnetic structure of frustrated heavy fermion Yb$_2$Pt$_2$Pb

Different realisations of frustrated magnetic systems attract a lot of interest due to the emergence of many exotic ground states where interplay of frustration and quantum effects is present. Among these systems, tetragonal Yb$_2$Pt$_2$Pb is believed to be a 4$f$ electron realisation of frustrated Shastry-Sutherland lattice (SSL), with antiferromagnetic order emerging below $T_N=$ 2.07 K [1] and where series of subsequent plateaus in the magnetisation suggests a complex magnetic phase diagram [2,3]. With neutron diffraction experiments we have found a solution of the magnetic structure of Yb$_2$Pt$_2$Pb that reproduces the magnetic intensities observed both in powder and single crystal data below $T_N$. The paramount finding is that this magnetic structure shows some features of classical 2D SSL lattice, where magnetic moments are confined within the tetragonal plane, as well as the presence of distinct antiferromagnetic dimers. There are indications as well that spin chain physics may be important. \\ This project was supported by NSF-DMR-1310008.\\ 1) Kim et al., Phys. Rev. B 77, 144425 (2008) \\ 2) Kim et al., PRL 110, 017201 (2013) \\ 2) Shimura et al., J. Phys. Soc. Jpn. 81, 103601 (2012)\\   

Wiedemann-Franz law and non-vanishing temperature scale across the field-tuned quantum critical point of YbRh$_2$Si$_2$

The in-plane thermal conductivity $\kappa$ and electrical resistivity $\rho$ of the heavy-fermion metal YbRh$_2$Si$_2$ were measured down to 50 mK for magnetic fields $H$ parallel and perpendicular to the tetragonal $c$ axis, through the field-tuned quantum critical point, $H_c$, at which antiferromagnetic order ends. The thermal and electrical resistivities, $w \equiv L_0 T/\kappa$ and $\rho$, show a linear temperature dependence below 1~K, typical of the non-Fermi liquid behaviour found near antiferromagnetic quantum critical points, but this dependence does not persist down to $T=0$. Below a characteristic temperature $T^\star \simeq 0.35$~K, which depends weakly on $H$, $w(T)$ and $\rho(T)$ both deviate downward and converge as $T \to 0$. We propose that $T^\star$ marks the onset of short-range magnetic correlations, persisting beyond $H_c$. By comparing samples of different purity, we conclude that the Wiedemann-Franz law holds in YbRh$_2$Si$_2$, even at $H_c$, implying that no fundamental breakdown of quasiparticle behaviour occurs in this material. The overall phenomenology of heat and charge transport in YbRh$_2$Si$_2$ is similar to that observed in the heavy-fermion metal CeCoIn$_5$, near its own field-tuned quantum critical point.   

Using Monte Carlo Ray tracing to Understand the Vibrational Response of UN as Measured by Neutron Spectroscopy

Recently neutron spectroscopy measurements, using the ARCS and SEQUOIA time-of-flight chopper spectrometers, [1] observed an extended series of equally spaced modes in UN that are well described by quantum harmonic oscillator behavior of the N atoms. Additional contributions to the scattering are also observed. Monte Carlo ray tracing simulations with various sample kernels have allowed us to distinguish between the response from the N oscillator scattering, contributions that arise from the U partial phonon density of states (PDOS), and all forms of multiple scattering. These simulations confirm that multiple scattering contributes an $\sim Q$-independent background to the spectrum at the oscillator mode positions. All three of the aforementioned contributions are necessary to accurately model the experimental data. These simulations were also used to compare the T dependence of the oscillator modes in SEQUOIA data to that predicted by the binary solid model [2].\\[4pt] [1] A. A. Aczel, {\it et al.}, Nat. comm. {\bf 3}, 1124 (2013).\\[0pt] [2] M. Warner, {\it et al.} Z. Phys. B {\bf 51}, 109 (1983)   

$^{195}$Pt NMR measurements in the superconducting state of the nearly heavy Fermion metal U$_2$PtC$_2$

AC susceptibility and $^{195}$Pt nuclear magnetic resonance measurements of the nearly heavy Fermion metal U$_2$PtC$_2$, with $T_c(H_0=0) = 1.6$ K and $\gamma = 15$ mJ/mol$\cdot$ K$^2$,[1] were taken at temperature down to 10 mK and in magnetic fields up to 12 T. From AC susceptibility, an anisotropic $H_{c2}(T\approx 0)$ of 8 and 10 T has been measured with the external magnetic field $H_0\parallel c$ and $H_0\perp c$, respectively. Below $T_c$, the spectral shift is independent of temperature consistent with spin triplet electron pairing or large spin orbit coupling. Spin-lattice relaxation measurement at temperatures $T_{c} < T < 20$ K follow a modified Korringa relation which indicate ferromagnetic fluctuations, while below $T_c$, $1/T_1\propto T^2$ signifying unconventional superconductivity. [1] G. P. Meisner et al., PRL 53, 1829 (1984).   

Two-band model of ferromagnetic $p$-wave superconductors

We present results on the full angular and temperature dependencies of the upper critical induction, $B_{c2}(\theta,\phi,T)$, for a ferromagnetic $p$-wave completely broken symmetry state (CBS), in which parallel-spin electron pairing is pinned to only one crystal axis direction, $V(\hat{k},\hat{k}^{\prime})=V_{0}k_{z}k_{z}^{\prime}$, normal to the spontaneuous magnetization direction. We show that the angular dependence of $B_{c2}$ exhibits an anomalous peak at $\theta^{\star}<90^{\circ}$, due to a competition between order parameter anisotropy and effective mass anisotropy. We also propose a two-band model for ferromagnetic superconductors, where we have two ellipsoidal Fermi surfaces (FSs). Using this model, and by assuming a field-dependent anisotropic effective mass on one of the FSs, $m_{\uparrow,i}(B)$, we can fit the experimental data for the low-temperature specific heat $\gamma(B)$ of URhGe, which exhibits a peak at $\mu_0H\sim12$T in the $b$-axis direction. We provide quantitative fits to experiment, and propose that this model of a field-dependent effective mass can help to understand the reentrant phase of the ferromagnetic superconductor URhGe, and the upward curvature observed in $B_{c2}$ of UCoGe   

Session J47: Metal-Insulator and Other Electronic Phase Transitions: Computational

A dual fermion approach for disordered interacting systems: Application to the Anderson-Hubbard model

We have recently generalized the dual fermion approach to the disordered interacting fermionic systems. Here it is applied to the Anderson-Hubbard model at finite temperature. With both disorder and Coulomb interaction treated on equal footing, and non-local correlations taken into account, we analyze the underlying competing physics related to metal-insulation transitions and anti-ferromagnetic transition by looking into both one- and two-particle quantities.   

Towards the Realization of Self-Consistent Effective Medium Theory for Anderson Disorder Model

A mean-field theory that properly characterizes the Anderson localization transition in three dimensions has remain elusive. Here, we present a systematic typical medium dynamical cluster approximation that provides a proper description of this phenomenon. Our method accurately provides a proper way to treat the different energy scales (close to the criticality) such that the characteristic re-entrant behavior of the mobility edge is obtained. This allows us to study the localization in different momenta cells, which renders the discovery that the Anderson localization transition occurs in a \textit{momentum cell-selective fashion}. As a function of cluster size, our method systematically recovers the re-entrance behavior of the mobility edge and obtains the correct critical disorder strength with great improvement on the critical exponent of the order parameter ($\beta > 1.4$).   

Dual-fermion approach to interacting disordered fermion systems

We generalize the recently introduced dual fermion (DF) formalism for disordered fermion systems by including the effect of interactions. For an interacting disordered system the contributions to the full vertex function have to be separated into elastic and inelastic scattering processes, and addressed differently when constructing the DF diagrams. By applying our approach to the Anderson-Falicov-Kimball model and systematically restoring the nonlocal correlations in the DF lattice calculation, we show a significant improvement over the Dynamical Mean-Field Theory and the Coherent Potential Approximation for both one-particle and two-particle quantities.   

Dynamical cluster approximation and typical medium analysis of systems with off-diagonal disorder

A proper theoretical description of realistic disordered materials requires the inclusion of both diagonal and off-diagonal randomness. The single-site self-consistent approximation for systems with off-diagonal disorder was constructed by Blackman, Esterling and Berk (BEB) [1]. Being a single-site approximation, the BEB theory neglects all disorder induced non-local correlations. In order to take into account such non-local effects and the effect of off-diagonal disorder,we extend BEB formalism using the dynamical cluster approximation scheme [2]. Also to address the question of electron localization, we generalize our recently developed typical medium dynamical cluster approximation to systems with off-diagonal randomness. In our numerical analysis we perform a systematic study of the effect of non-local correlations and of off-diagonal disorder on the density of states and electron localization. The results of our calculations are compared with the results obtained using the exact diagonalization and the transfer-matrix method. [1] J. A. Blackman, D. M. Esterling, and N. F. Berk, Phys. Rev. B 4, 2412 (1971). [2] M. Jarrell and H. R. Krishnamurthy, Phys. Rev. B 63, 125102 (2001)   

Benchmarking Mobility Edge Calculations for a Cluster Typical Medium Theory of Off-diagonal Disordered Systems

We apply the transfer matrix method and exact diagonalization to electronic lattice systems with substitutional off-diagonal disorder. Established effective medium methods for studying realistic metallic alloy systems have enjoyed much success calculating the properties of disordered systems, but are criticized for inaccurate predictions (for example, of charge transfer and phase evolution) caused by reliance on the Coherent Potential Approximation (CPA) which neglects nonlocal environmental effects. It has been shown that such non-local correlations can be incorporated with the Dynamical Cluster Approximation (DCA). Furthermore, localization effects have been demonstrated with a local order parameter approach that is used to define a typical medium. The validity of the predicted mobility edge from an effective cluster typical medium theory that extends the Blackman, Esterling and Berk formalism to DCA is explored with finite size scaling of transfer matrix data. We demonstrate it as a promising effective medium theory to incorporate into present ab initio methods for realistic disordered systems.   

Vertex function representation in non-uniform frequency grids

The proper computer representation of many-body vertex functions is a central issue in computational many body methods such as the parquet formalism, a self-consistent two-particle field theory. Despite the great effort over the past two decades, its application is very limited. This is predominately due to two crucial factors - the stability of the iteration and the size of the memory allocation for the vertices. We previously demonstrated that the stability problem can be alleviated by explicitly restoring the crossing symmetry, making simulations beyond weak coupling for the Hubbard model feasible. The next step for the practical applications of the parquet formalism is to compress the memory required to represent the vertex. In this talk, we first demonstrate the problem of perturbation theory off the Matsubara frequency grids. This problem is avoided by working on the so-called decimation grids, which are non-uniform grids on Matsubara frequency. We then use this scheme in the parquet method, for solving an Anderson impurity problem. The results show substantial improvement compared to using the same number of uniform frequency grids. This may represent a crucial step towards practical applications of the parquet formalism for large clusters.   

Revisit of Orbital Selective Phase Transition Induced by Different Orbitals with Different Band Dispersions

Orbital selective phase transition (OSPT) was first suggested to explain a possible coexistence of localized and itinerant electrons in a multi-orbital system, Ca$_{2-x}$Sr$_x$RuO$_4$, as interaction increases. Recently, this scenario was applied to the iron-based superconductors. Up to now, several mechanisms have been proposed, such as different orbitals with different bandwidth, different orbitals with different degeneracies, different orbitals with different magnetic states, and different orbitals with different band dispersions, etc. Unlike other mechanisms which were investigated under a constraint of paramagnetic solution, different orbitals with different band dispersions was only studied with magnetic order. Therefore, here we investigate the mechanism of different orbitals with different band dispersions in paramagnetic state by dynamical mean field theory with exact diagonalization as an impurity solver in order to reveal whether OSPT can still happen. Possible indications of our results will also be discussed.   

Electron Localization in $Fe_3O_4$: an Ab Initio Wannier Study

Magnetite, $Fe_3O_4$, is an unusual ferrimagnetic oxide with emergent physical properties that are not yet fully understood. Among these are the metal-insulator transition at the Verwey Temperature $T_V$ (123K) and a spin-glass-like transition at about twice $T_V$. The ``extra'' fully spin-polarized 3d electrons that span the $t_{2g}$ bands of the B sublattice show strong electron correlation effects and are mainly responsible for conduction above $T_V$. We perform a DFT+U calculation to obtain a set of Bloch orbitals describing the $t_{2g}$ bands. We then use the gauge invariance of Wannier functions to transform the Bloch orbitals into a set of Maximally Localized Wannier Functions (MLWFs). The MLWFs are a real space description of the ``extra'' 3d electrons allowing us to describe their spatial localization and determine the mechanism of conduction above $T_V$. Wannier studies of $Fe_3O_4$ may also allow us to determine the extent of electronic coupling to lattice vibrations, which may provide us substantial quantitative clues on the physical mechanism of the Verwey Transition.   

Localization phase diagram of two-dimensional quantum percolation

We examine two dimensional quantum percolation on a square lattice with random dilution up to $q=38\%$ and energy $0.001 \le E \le 1.6$ (in units of the hopping matrix element), using numerical calculations of the transmission coefficient for finite size systems of up to about 900x900. We extended previous work to determine the phase diagram in $(E,q)$ space, confirming the existence of a localization-delocalization transition. The localized region splits into an exponentially localized and power-law localized regions for energies $E \ge 0.1$. We also examine the scaling behavior of the residual transmission coefficient in the delocalized region, the power law exponent in the power-law localized region, and the localization length in the exponentially localized region. Our results suggest that the residual transmission at the delocalized to power-law localized phase boundary may be discontinuous, and that the localization length is likely not to diverge with a power-law at the exponentially localized to power-law localized phase boundary.   

Numerical studies of a many-body localized system coupled to a bath

We use exact diagonalization to study the breakdown of localization in a many-body localized system coupled to a non-integrable bath. Signatures of incomplete localization survive even when the coupling to the bath is non-zero. In particular, we examine (i) level statistics, (ii) eigenstate thermalization, (iii) zero- and finite temperature spectral functions, (iv) correlation functions, and (v) transport properties. We find a continuous change from localized to ergodic behaviour in these quantities as the coupling to the bath increases.   

Universal Conductivity in a Two-dimensional Superfluid-to-Insulator Quantum Critical System

We compute the universal conductivity of the (2+1)-dimensional XY universality class, which is realized for a superfluid-to-Mott insulator quantum phase transition at constant density. Based on large-scale Monte Carlo simulations of the classical (2+1)-dimensional $J$-current model and the two-dimensional Bose-Hubbard model, we can precisely determine the conductivity on the quantum critical plateau, $\sigma(\infty)=0.359(4)\sigma_Q$ with $\sigma_Q$ the conductivity quantum.The universal conductivity curve is the textbook example of where the AdS/CFT correspondence from string theory can be tested and made to use. For the first time, the shape of the $\sigma(i\omega_n)- \sigma(\infty)$ function in the Matsubara representation is accurate enough for a conclusive comparison and establishes the particle-like nature of charge transport.We find that the holographic gauge/gravity duality theory for transport properties can be made compatible with the data if temperature of the horizon of the black brane is different from the temperature of the conformal field theory. The requirements for measuring the universal conductivity in a cold gas experiment are also determined by our calculation.   

Finite Temperature Phase Diagram of the Disordered Extended Bose-Hubbard Model

The disordered extended Bose-Hubbard model exhibits a very rich phase diagram at low temperatures because of the competition between disorder and interactions. Depending on the parameter regime, it can show superfluid, supersolid, Bose glass, solid and disordered solid phases. We will discuss quantum Monte Carlo calculations used to estimate various physical quantities, such as, superfluid density, charge structure factor, compressibility, etc, which we use to classify these phases at finite temperatures.   

Area laws and topological order in a many-body localized state

The question whether Anderson insulators can persist to finite-strength interactions - a scenario dubbed many-body localization - has recently received a great deal of interest. In this talk, I will discuss our recent work on defining such a many-body localized phase and exploring it through its entanglement properties. We formulate a precise sense in which a many-body localized system can be connected adiabatically to an Anderson insulator. The most striking consequence of our definition is an area law for the entanglement entropy of highly excited states in such a system. We present the results of numerical calculations for a one-dimensional system of spinless fermions, which are consistent with an area law and, by implication, many-body localization for weak enough interactions and strong disorder. Furthermore, we discuss the implications that many-body localization may have for topological phases and self-correcting quantum memories. We find that there are scenarios in which many-body localization can help to stabilize topological order at non-zero energy density, and we propose potentially useful criteria to confirm these scenarios.   

Thermal disorder, entropy and the $\alpha - \gamma$ transition in Ce from density-functional theory

There are many recent theoretical efforts to describe the $\gamma - \alpha$ transition in fcc Cerium. The large volume $\gamma$-phase is magnetic, while the low-volume non-magnetic $\alpha$- phase can be reached at high pressure or low T. It has been recognized that real T-dependent lattice disorder can be important for the electronic structure and properties in some materials with sharp density-of-state variations near $E_F$. This might also be the case for Ce, because of its narrow f-band at the Fermi level, and its relatively soft lattice. Here are presented results for fcc Ce at different volumes from first principles GGA-DFT band-structure calculations for large supercells with different degrees of T-dependent disorder. Local disorder, local density-of-states and magnetic moments are all connected. It is shown that structural disorder at large temperature has a direct influence on the magnetic $\gamma$-phase, and its corresponding entropy. The results corroborate the earlier findings that standard DFT band-theory can describe the T-dependent transition if all entropy contributions are included. In addition, thermal disorder is important for the properties of fcc Ce.   

$\gamma$-$\alpha$ iso-structural Transition in Cerium

We present zero-temperature first-principle calculations of elemental cerium, and we compute its pressure-volume phase diagram within a theoretical framework able to describe simultaneously both the $\alpha$ and the $\gamma$ phase. A surprising result revealed by our study is the presence of a clear signature of the transition at zero temperature, and that this signature can be observed if and only if the spin-orbit coupling is taken into account. Our calculations indicate that the transition line in the pressure-temperature phase diagram of this material has a low-$T$ critical point at negative pressures, placed very close to zero temperature. This suggests that cerium is very close to being ``quantum critical'', in agreement with recent experiments.   

Session J48: Focus Session: Spin Transport and Magnetization Dynamics in Metal-Based Systems: Skyrmion II

Helicity and size dependence of skyrmions in Mn$_{1-x}$Fe$_x$Ge mediated by the Dzyaloshinskii-Moriya interaction

We carry out first-principles electronic structure calculations for bulk alloys of Mn$_{1-x}$Fe$_x$Ge and MnSi in the B20 compound where skyrmions are seen to vary in size and chiral order. We utilize the Virtual Crystal Approximation to vary the concentration of Fe/Mn atoms within the unit cell. Using a first order perturbation approach with spin-orbit coupling applied to spin-spiral calculations we observe the Dzyaloshinski-Moriya vector changes sign and magnitude with the concentration of the transition metal ion.   

The size and helicity of skyrmions in B20-type chiral magnet Mn$_{1-x}$Fe$_x$Ge

A magnetic skyrmion is a topologically-stable spin vortex structure observed in chiral-lattice helimagnets. Skyrmions and their crystallized state, skyrmion crystal, have been attracting much attention because of the emergent electromagnetic properties. However, crystal engineering in terms of controlling the skyrmion crystal structure itself is not well established. Here, we report on the correlation between skyrmion helicity and crystal chirality in alloys of $B$20-type chiral-lattice helimagnet Mn$_{1-x}$Fe$_x$Ge with varying compositions by Lorentz transmission electron microscopy and convergent-beam electron diffraction over a broad range of compositions ($x$ = 0.3 - 1.0)$^{[1]}$. The skyrmion lattice constant or the skyrmion size shows non-monotonous variation with the composition $x$, with a divergent behavior around $x$ = 0.8, where the correlation between magnetic helicity and crystal chirality changes sign. This originates from continuous variation of the spin-orbit coupling strength and its sign reversal in the metallic alloys as a function of $x$. Controllable spin-orbit coupling may offer a promising way to tune skyrmion size and helicity. [1] K. Shibata et al., Nat. Nanotech. 8, 723 (2013).   

Resonant x-ray scattering from a skyrmion lattice

Topologically protected novel phases in condensed matter systems are a current research topic of tremendous interest due to both the unique physics and their potential in device applications. Skyrmions are a topological phase that in magnetic systems manifest as a hexagonal lattice of spin-swirls. We report the first observation of the skyrmion lattice using resonant soft x-ray diffraction in Cu$_2$OSeO$_3$, a cubic insulator that exhibits degenerate helical magnetic structures along <100> axes in zero magnetic field. Within a narrow window of temperature and applied magnetic field we observed the six fold symmetric satellite peaks due to the skyrmion lattice around the (001) lattice Bragg peak. As a function of incident photon energy a rotational splitting of the skyrmion satellite peaks was observed that we ascribe to the two Cu sublattices of Cu$_2$OSeO$_3$, with different magnetically active orbitals. The splitting implies a long wavelength modulation of the skyrmion lattice.   

Tailoring Artificial Skyrmions in single-crystalline Co/Ni/Cu(001) system

Magnetic Skyrmions, which correspond to a topological spin texture pattern, were recently realized in several experimental systems as a result of Dzyaloshinsky-Moriya interactions (DMI). An alternative approach is to produce non-collinear spins in magnetic vortex states. With this motivation, we fabricated single crystalline Co disks on perpendicularly magnetized Ni/Cu(001) film to create artificial Skyrmions whose topology can be tailored by changing the relative orientation between the vortex core polarity and the surrounding perpendicular magnetization. In this way, we studied the topological effect of the Skyrmion using Photoemission Electron Microscopy (PEEM). By applying an in-plane magnetic field of various strength, we find strong evidence that the annihilation of the vortex core depends on the Skyrmion number of the system.   

Dynamic Phases of Skyrmions in Chiral Magnets Driven over Random and Periodic Pinning Arrays

Skyrmions in chiral magnets have been generating tremendous excitement since their recent discovery, both for the intrinsic science and for possible applications of skyrmions. Skyrmions can be driven with an applied spin-polarized current and appear to have many similarities to vortices in type-II superconductors. Here we numerically simulate skyrmions driven over random and periodic arrays of defects or pinning using a combination of particle-based models and continuum models. We find that for weak pinning, the skyrmions depin elastically, while for strong pinning, the skyrmions depin plastically. In both cases, there are distinct features in the resulting transport curves and we show that in the presence of pinning the Hall angle continuously changes as a function of drive. In samples where plastic depinning occurs, at high drives there is a transition to a dynamically ordered state which we compare to the dynamical reordering observed for driven vortices in type-II superconductors. With periodic pinning, the Hall angle changes in discrete steps for increasing drive as the skyrmion motion locks to different symmetry directions of the underlying pinning array.   

Dynamics of an Insulating Skyrmion under a Temperature Gradient

Skyrmion is a topological spin texture in which local magnetic moments wrap the unit sphere an integer number of times. The study of Skyrmion dynamics is not only an important physics issue, but also application oriented. On the other side, dynamics of the insulating Skyrmions is also an interesting subject. In this talk, I will briefly review my previous work on current driven Skyrmion motion based on an emergent gauge field. We study the Skyrmion dynamics in thin films under a temperature gradient. Our numerical simulations show that both single and multiple Skyrmions in a crystal move towards the high temperature region, which is contrary to particle diffusion. Noticing a similar effect in the domain wall motion, we employ a magnon pulling mechanism to explain this counterintuitive phenomenon. Unlike the temperature driven domain wall motion, the Skyrmion's topological charge plays an important role, and a transverse Skyrmion motion is observed. Our theory turns out to be in agreement with numerical simulations, both qualitatively and quantitatively.\\[4pt] [1]Jiadong Zang, Maxim Mostovoy, Jung Hoon Han, and Naoto Nagaosa, Phys. Rev. Lett. \textbf{107}, 136804 (2011). \\[0pt] [2] Lingyao Kong, and Jiadong Zang, Phys. Rev. Lett. \textbf{111}, 067203 (2013).   

Spirals and skyrmions in two dimensional oxide heterostructures

A symmetry-based general free energy governing long-wavelength magnetism in two-dimensional oxide heterostructures will be presented. This leads, in the relevant regime of weak but non-negligible spin-orbit coupling, to a rich phase diagram containing in-plane ferromagnetic, spiral, cone, and skyrmion lattice phases, as well as a nematic state stabilized by thermal fluctuations. The general conclusions are vetted by a microscopic derivation for a simple model with Rashba spin-orbit coupling.   

Orbital Dzyaloshinskii-Moriya Exchange Interaction

A superexchange calculation is performed for multi-orbital band models with broken inversion symmetry. Orbital-changing hopping terms allowed by the symmetry breaking electric field lead to a new kind of orbital exchange interaction closely resembling the Dzyaloshinskii-Moriya spin exchange. Inversion symmetry breaking as present in surfaces and interfaces and a strong on-site repulsion, but not the spin-orbit interaction, are the requirements to observe the proposed effect. Mean-field phase diagram exhibits a rich structure including anti-ferro-orbital, ferro-orbital, and both single and multiple spiral-orbital phases in close analogy with the Skyrmion spin crystal phase recently discovered in thin-film chiral magnets.   

Spin-orbit coupling, compass anisotropy and skyrmions in 2D chiral magnets

Spin-orbit coupling (SOC) gives rise to the chiral Dzyaloshinskii-Moriya (DM) interaction in systems that lack inversion symmetry like non-centrosymmetric helimagnets, and two-dimensional magnetism at surfaces and interfaces. We explore here the role of SOC in several microscopic exchange mechanisms -- superexchange, double exchange and RKKY -- in insulating and itinerant electron systems. We show that, in addition to giving rise to the DM interaction, SOC generically leads to compass anisotropy terms. Although seemingly negligible, the compass terms are energetically comparable to DM and play a crucial role in deciding the fate of the magnetic ground state. We demonstrate that the compass terms act as an effective easy-plane anisotropy in 2D chiral magnets and lead to extremely large region of stable skyrmion crystal (SkX) phase in a perpendicular magnetic field. We discuss the electronic properties of SkX in this hitherto unexplored region of the anisotropy-field plane for itinerant systems. We also comment on the possibility of realizing such SkX phase in the oxide interfaces [1]. [1] S. Banerjee, O. Erten and M. Randeria, Nature Physics 9, 626 (2013).   

Easy-plane anisotropy stabilizes skyrmions in 2D chiral magnets

Experiments on two-dimensional (2D) chiral magnetic materials, like thin films of non-centrosymmetric helimagnets and metallic magnetic layers, have revealed interesting spatially modulated spin textures such as spirals and skyrmions. Motivated by this we study the ground-state phase diagram for a 2D chiral magnet in a magnetic field using a Ginzburg-Landau model, with Dzyaloshinskii-Moriya (DM) term, anisotropic exchange and single-ion anisotropy. The easy-axis anisotropy region of the phase diagram has been well-studied [1], whereas the easy-plane region has not been discussed. In the easy-plane region, we find an unexpectedly large stable skyrmion crystal (SkX) phase in a perpendicular magnetic field. We find re-entrant transitions between ferromagnetic and SkX phases, and intriguing internal structure of the skyrmion core with two-length scales. We argue that such an easy-plane anisotropy arises naturally from the compass terms induced by spin-orbit coupling that is also responsible for the DM term, as proposed recently in the context of oxide interfaces [2]. We also discuss the phase diagram in a tilted field configuration, relevant for torque magnetometry experiments. [1] Robler et. al. Phys.: Conf. Ser. 303, 012105 (2011). [2] Banerjee et. al. Nat. Phys. 9, 626 (2013).   

Controlling the dynamical modes of the chiral magnetic structures by spin Hall effect

Recently, pure spin currents generated due to spin Hall effect have been proved as an efficient approach to reverse the magnetization, modify the dynamical relaxation rates, and excite magnetization oscillations in the heavy metal/ferromagnetic heterostructures. In addition, the Dzyaloshinskii-Moriya interaction (DMI) can also induce chiral magnetization configurations and rich dynamics in these asymmetrical heterostructures$.$ We controllably excited several distinct dynamical modes in spin Hall oscillator based on Pt/ [CoNi] magnetic multilayer with perpendicular anisotropy. At low current, a quasi-linear Slonczewski-like propagating spin wave mode was excited. This mode transforms to a localized soliton mode above a certain threshold current. At large fields, this mode can be identified as the spin wave `bullet' mode. At small fields, the localized mode is transformed to the topological structure of the `droplet' mode, which comes from the oscillations of the chiral domain walls forming the boundary of the bubble domain due to DMI. Our measurements demonstrate a straightforward route for emission of spin waves by nano-oscillators controlled either by current or by the applied magnetic field.   

Chirality of symmetry broken spin-orbit systems

Recently, structures consisting of an ultrathin magnetic layer adjacent to a heavy metal layer with strong atomic spin-orbit coupling have received a considerable attention. Their unexpected behavior not only stimulates scientific interest but also makes them promising candidates for spintronic devices. Strong spin-orbit coupling of two kinds, bulk spin Hall effect and interfacial spin-orbit coupling, play important roles on magnetization dynamics. In this work, we propose a unified theory of magnetic systems with interfacial spin-orbit coupling up to linear order starting from a two dimensional Rashba model which includes structural inversion symmetry breaking and time reversal symmetry breaking. The combination of both broken symmetries makes the system chiral. In our theory, conventional terms in the equation of motion each give rise to a linear chiral effect; this relationship is captured by replacing the usual spatial derivative with a chiral derivative. Introducing the chiral derivative not only captures previously reported results but also reveals previously unreported chiral aspects of the Rashba model such as the Dzyaloshinskii-Moriya interaction. It also clarifies the one-to-one correspondence between interfacial spin-orbit effects and conventional effects without spin-orbit coupling.   

Tuning Interfacial Dzyaloshinskii-Moriya Interactions in Ta/CoFe/MgO through Annealing

Out-of-plane magnetized ultrathin ferromagnets interfaced between a heavy metal and an oxide exhibit anomalously efficient current-induced domain wall (DW) motion. In these ultrathin ferromagnets, the interfacial Dzyaloshinkii-Moriya interaction (DMI) stabilizes N\'{e}el DWs with a fixed chirality [1] which permits the Spin Hall Effect (SHE) to drive the DWs uniformly. The magnitude and direction of DMI is a strong function of the material composition and thickness of the heavy metal underlayer, sharpness of the interface, temperature and other processing parameters [2,3]. Here we quantify the DMI effective field in Ta/CoFe/MgO films by studying the asymmetry of expansion [4] of a circular domain in the presence of in-plane bias fields. We show that while the DMI is relatively weak, it can be enhanced by annealing, and we describe the correlation between DMI and interfacial perpendicular magnetic anisotropy as a function of annealing conditions. These results provide new insights into the interfacial origin of the DMI. [1] A. Thiaville, et al., EPL \textbf{100} 57002 (2012) ;[2] S. Emori, et al.,~arXiv:1308.1432v1 (2013) ; [3] S. Emori, et al., Nat. Mat.~\textbf{12}, 611 (2013) ; [4] S.-G. Je, et al., \underline {arXiv:1307.0984v1}~ (2013)   

Spin to charge conversion using Rashba coupling at the interface between non-magnetic materials

The Rashba effect is an interaction between the spin and the momentum of electrons induced by the spin-orbit coupling (SOC) in surface or interface states. Its potential for conversion between charge and spin currents has been theoretically predicted but never clearly demonstrated for surfaces or interfaces of metals. Here we present experiments evidencing a large spin-charge conversion by the Bi/Ag Rashba interface. We use spin pumping to inject a spin current from a NiFe layer into a Bi/Ag bilayer and we detect the resulting charge current. As the charge signal is much smaller (negligible) with only Bi (only Ag), the spin to charge conversion can be unambiguously ascribed to the Rashba coupling at the Bi/Ag interface. This result demonstrates that the Rashba effect at interfaces can be used for efficient charge-spin conversion in spintronics   

Magnetotransport Properties of the Highly Anisotropic Helimagnet Cr$_{1/3}$NbS$_2$

Unusual electrical transports properties such as the topological Hall Effect in non-trivial spin textures have demonstrated great potential for controlling electrical properties via underlying spin degree of freedom. In particular, magnetic systems with no-inversion symmetry in their crystal structure are promising candidates to search for these effects due to their tendency to support non-collinear spin configurations, a requirement for non-trivial spin texture. Here, we study the in-plane magnetotransport properties in the chiral helimagnet Cr$_{1/3}$NbS$_2$, which falls in such a category and has larger crystalline anisotropy relative to other known systems (e.g. MnSi). At low temperature ($T \ll T_C$), we find that the in-plane magnetoresistance with applied field perpendicular to plane is suppressed up to three times more than with the field in-plane. Concurrently, Hall voltage, which is also taken with B field perpendicular to the plane, displays unique B field dependence. We discuss these results in the light of the role of the anisotropy in Cr$_{1/3}$NbS$_2$'s magnetic structure and band structure.   

Session J49: Focus Session: Metal-Metal Bonding: Vanadates and Niobates

Evolution of correlated electron behavior from the surface to the bulk in Sr$_x$Ca$_{1-x}$VO$_3$

Understanding correlated electron behaviour remains one of the most important, and challenging, problems in modern condensed matter physics. In correlated electron systems, the interaction between electrons is of the order of, or larger than, the electron kinetic energy, and the concept of a well-defined quasiparticle is restricted to a narrow region of energies near the Fermi level, beyond which our strict understanding of a quasiparticle with a defined dispersion relation, easily accessible through band theory, breaks down. In the last few decades, the discovery of unusual and promising behavior in strongly correlated materials has yielded effects as diverse as high temperature superconductivity, colossal magnetoresistance and multiferroics. Indeed, the functionalisation of strongly correlated materials, either in bulk crystalline form or as artificial layered heterostructures, is fast emerging as one of the most promising avenues for future advanced technologies, and key to unlocking the potential of such designed materials is a firm grasp of how electron correlations evolve at surfaces and interfaces. Here, we investigate Sr$_x$Ca$_{1-x}$VO$_3$ as a prototypical example of a strongly correlated material, exhibiting both strong Hubbard subbands and appreciable quasiparticle peaks. Using a variety of ultraviolet, soft, and hard x-ray spectroscopies, we present a detailed depth-sensitive study of the evolution in the effects of electron correlations from the sample surface to its bulk. Our results illustrate the intrinsic enhancement of the effects of electron correlations at the surface, which has important implications for the designed properties at the interface of heterostructures. Strong incoherent subbands are found to lie $\sim$ 20\% closer in energy to the coherent features in the most surface-sensitive measurements, accompanied by a $\sim$ 10\% narrowing in the overall bandwidth. Secondly, we demonstrate that resonant soft x-ray emission spectroscopy is a sensitive probe of correlated electron behavior, capable of providing complementary information to photoemission spectroscopy from a truly bulk perspective.   

Gate-tunable gigantic changes in lattice parameters and optical properties in VO$_{2}$

The field-effect transistor provides an electrical switching function of current flowing through a channel surface by external gate voltage (VG). We recently reported that an electric-double-layer transistor (EDLT) based on vanadium dioxide (VO2) enables electrical switching of the metal-insulator phase transition, where the low-temperature insulating state can be completely switched to the metallic state by application of VG [1]. Here we demonstrate that VO2-EDLT enables electrical switching of lattice parameters and optical properties as well as electrical current. We performed in-situ x-ray diffraction and optical transmission spectroscopy measurements, and found that the c-axis length and the infrared transmittance of VO2 can be significantly modulated by more than 1{\%} and 40{\%}, respectively, by application of VG. We emphasize that these distinguished features originate from the electric-field induced bulk phase transition available with VO2-EDLT. \\[4pt] [1] M. Nakano et al., Nature 487, 459 (2012).   

Evidences of nonthermal optically induced insulator-to-metal switching in VO$_{2}$

Strongly cooperative structural and electronic phase transitions at near room temperature make VO$_{2}$ a promising material for an array of high-speed applications in electronics and photonics. The critical step that limits the ultrafast performance is the structural barrier, which is result of subtle interplay between the Mott and Peierls physics. Using femtosecond electron crystallography, we examine the sequence of events resulted from this interplay and show that the cooperative behavior induced by optically induced charge doping may provide an alternative pathway for efficient ultrafast switching bypassing the high thermodynamic barrier required in the temperature-driven phase transition.   

Crystal structure change accompanying insulator-metal phase transition in VO$_{2}$ field-effect transistor

The insulator-metal transition induced by the carrier accumulation in VO$_{2}$ field-effect transistor (FET) gated by electric double layers of ionic liquid has been extensively studied. To clarify the origin of this transition, we performed simultaneous measurements of in-situ synchrotron x-ray diffraction and resistivity on VO$_{2}$ FET at BL19LXU, SPring-8, Japan. By using micro-beam x-ray, the diffraction only on the carrier-accumulated channel of VO$_{2}$ FET can be measured. By applying a gate voltage, the VO$_{2}$ film becomes metallic. The $c$-lattice length estimated from the peak position of (0 0 2) diffraction on the channel of VO$_{2}$ film shows an increase of 1.4{\%} at 150 K. The $c$-lattice length in the metallic state hardly depends on the temperature, which is consistent with the temperature-independent-metallic resistivity. The changes of $c$-lattice length and resistivity by a gate voltage are reversible. This structural change is quite different with those of thermally-, x-ray-, and pressure-induced metallic phases. The crystal structure with elongated $c$-lattice length is realized only in the metallic state induced by the carrier accumulation.   

Low loss millimeter-wave switches based on the Vanadium Dioxide Metal - Insulator - Transition

A new ultra-low-loss and broad band millimeter wave switch technology based on the reversible metal / insulator phase transition of vanadium dioxide has been developed. We report having fabricated series configured, single-pole single-throw (SPST) switches having measured S-parameters from DC to 110 GHz. The on-state insertion loss is 0.2 dB and off-state isolation is 21 dB at 50 GHz. The resulting impedance contrast ratio, \textit{ZOFF / ZON}, is greater than 500:1 at 50 GHz (i.e. cut-off frequency \textit{fc} $\sim$ 40 THz). As a demonstration of the technology's utility, we also present the results of a 2-bit real time delay phase shifter incorporating a pair of VO2 SP4T switches. This switch technology's high impedance contrast ratio combined with its compactness, ease of integration, and low voltage operation make it an enabler of previously unachievable high-performance millimeter wave FPGAs.   

Correlation between charges and phonons in the phase transition of VO$_{2}$

Interplay among the microscopic degrees of freedom in transition-metal oxides can generate macroscopic quantum phenomena that provide functionality in electronic and photonic devices. Here, we report density functional calculations and molecular dynamics simulations of VO$_{2}$, which undergoes a semiconductor-to-metal phase transition accompanied by a monoclinic-to-rutile structural change at 68 $^{\circ}$ C. We find that the lattice vibration at the critical temperature generates a metallic state in the monoclinic structure, which may explain the observed metallic intermediate phase in experiments. Moreover, we find that the electron/hole doping strongly couples with the lattice vibration causing collapse of one particular phonon mode and stimulating the structural phase transition. Molecular dynamics simulations show a temperature-dependence of the required carrier density for the phonon collapse, that is, at higher temperature, fewer free carriers are required. We show that the abrupt change of the vibration results from the weakening of the V-V bonds induced by the hole doping.   

Ru-Ru Dimers in honeycomb-layered Li$_{2}$RuO$_{3}$

Dark-field transmission electron microscopy and sub-{\AA} aberration-corrected scanning transmission electron microscopy (STEM) have been used to investigate the local structural properties of Li$_{2}$RuO$_{3}$ We found intriguing Ru-Ru dimerization in the Ru honeycomb skeletons associated with the spin-orbital coupling of the 4$d$ electrons below 540 K. Furthermore, we demonstrated that the Ru-Ru dimers can be delicately broken through various antiphase boundaries and chemical doping. Soliton-like walls in the Ru-Ru dimer lattice are unambiguously observed in real space, and are found to order in a periodic manner for particular situations. The correlation between macroscopic physical properties and local structural distortions in the Li$_{2}$RuO$_{3}$ will be discussed in detail.   

Electrical field induced Metal-Insulator Transition in NbO$_{2}$ thin films at room temperature

Highly correlated oxides that exhibit a metal to insulator transition (MIT) are of great interest because of their potential application to high performance switches. NbO$_{2}$ exhibits a MIT at 1081K accompanied by a structural transformation from rutile to a distorted variant, which makes it a potential candidate for the switching applications. By a reactive bias target ion beam deposition (RBTIBD) growth technique, we have obtained crystalline single phase NbO$_{2}$ thin films grown on Al$_{2}$O$_{3}$ (0001), Au/Al$_{2}$O$_{3}$(0001), and Pt/Al$_{2}$O$_{3}$(0001) substrates. AFM, XRD and Raman spectroscopy were used to characterize the morphology and microstructure of the NbO$_{2}$ films. We have observed electrically induced transitions from the insulating to the metallic state with two orders of magnitude change in the resistivity at room temperature. This transition occurred at an electric field between 30-100 kV/cm. We will discuss the possible mechanisms for this induced MIT.   

Properties of MBE-grown NbO$_{2}$ thin films

Niobium dioxide or NbO$_{2}$ a sister compound of the more celebrated VO$_{2}$, belongs to the class of transition metal oxides that undergo a temperature-driven metal-to-insulator transition. Using density functional theory, we explore the electronic properties of both the high-temperature metallic rutile and the low-temperature insulating distorted rutile phases. We investigate the nature of the transition and predict a large carrier concentration change even at the high transition temperature of 1080 K. We also grew thin NbO$_{2}$ films on LSAT(111) single crystal substrates using molecular beam epitaxy. The films show very good crystallinity with a single out-of-plane orientation by x-ray diffraction, and exhibit a smooth surface with the presence of three epitaxial domains as observed by reflection high energy electron diffraction. The NbO$_{2}$ stoichiometry is confirmed by x-ray photoemission measurements of the Nb 3d core level as well as the valence band.   

Cryogenic Infrared Nano-Imaging of the Metal-Insulator Transition in V$_{2}$O$_{3}$

We report on temperature-dependent (18K-300K) near-field infrared imaging of the canonical Mott insulator V$_{2}$O$_{3}$ across its temperature-driven metal-insulator transition. This was accomplished using a home-built s-SNOM (scattering-type scanning near-field optical microscope) affording unprecedented spatial sensitivity ($\approx $20 nm) to surface optical properties with simultaneously acquired AFM topography at \textit{cryogenic temperatures}. Our V$_{2}$O$_{3}$ thin film is found to exhibit extreme nano-scale electronic heterogeneity near the Mott transition (170K) from paramagnetic metal to antiferromagnetic insulator. Through a sequence of near-field infrared images acquired across the transition, we resolve dynamic spatial correlations and competition between electronic phases, offering a direct probe of the metal/insulator fill fraction in strong agreement with macroscopic transport, magnetic susceptibility, and X-ray diffraction measurements of the same film. A statistical and tomographic analysis of our near-field images supports the interpretation of a complex 3-dimensional network of phases propagating across the Mott transition.   

Tuning the charge ordering transitions in single nanobeams of Cu-doped vanadium pentoxide

Vanadium oxide bronze phases derived from the intercalation of metal ions (K, Pb, Cu, Ag) within V$_{2}$O$_{5}$ frameworks exhibit remarkable physical properties such as charge ordering, quantum spin phenomena and metal-insulator transitions. The intercalated ions typically alter the electronic and geometric structure of these strongly correlated materials and this opens up avenues to observe interesting phases and to tune transitions between them using a variety of external parameters. In this work, results from electrical transport measurements on single crystalline, individual nanobeams of Cu-doped vanadium pentoxide will be presented. The nanobeams undergo a metal to an insulator transition (MIT), possibly due to charge ordering, below room temperature (T$_{C}$ values depend on Cu doping levels). The charge ordered state can be progressively altered by applying an electric field and/or ionic liquid gating. The role of oxygen migration in the presence of ionic liquid and its effect on the transport characteristics will be discussed. The work is supported by NSF DMR 0847324, 0847169 and Intel Corporation.   

Properties of the correlated metal phase induced by electrolyte gating of insulating vanadium dioxide nanobeams

Vanadium oxide (VO$_{2}$) undergoes a first order metal to insulator transition (MIT) and a structural phase transition (monoclinic insulator to rutile metal) near 340 K. Over the past few years, several attempts are made to trigger the MIT in VO$_{2}$ using ionic liquids (IL). Parkin's group has recently showed that IL gating leads to the creation of oxygen vacancies in VO$_{2}$ and stabilizes the metallic phase. Our goal is to study the electronic properties, changes in the stoichiometry and structure of this metallic phase created by oxygen vacancies. Electrical transport measurements on single crystal nanobeams show that the metallic phase has a higher resistance while IL gating is applied and results from Raman spectroscopy studies on any structural change during IL gating will be presented. The role of substitutional dopants (such as W, Mo) on the creation of oxygen vacancies and subsequent stabilization of metallic phase in IL gated experiments will also be discussed. The work is supported by NSF DMR 0847324 and 0847169.   

Session J50: Focus Session: Graphene Plasmonics

Nano-plasmonic phenomena in graphene

Infrared nano-spectroscopy and nano-imaging experiments have uncovered a rich variety of optical effects associated with the Dirac plasmons of graphene [Fei et al. Nano Lett. 11, 4701 (2011)]. We were able to directly image Dirac plasmons propagating over sub-micron distances [Fei et al. Nature 487, 82 (2012)]. We have succeeded in altering both the amplitude and wavelength of these plasmons by gate voltage in common graphene/SiO2/Si back-gated structures. Scanning plasmon interferometry has allowed us to visualize grain boundaries in CVD graphene. These latter experiments revealed that the grain boundaries tend to form electronic barriers that impede both electrical transport and plasmon propagation. Our results attest to the feasibility of using these electronic barriers to realize tunable plasmon reflectors: a precondition for implementation of various metamaterials concepts [Fei et al. Nature Nano 8, 821 (2013)]. Finally, we have carried out pump-probe experiments interrogating ultra-fast dynamics of plasmons in exfoliated graphene with the nano-scale spatial resolution [Wagner et al. (under review)].   

Controlling and Creating Plasmonic Absorption Processes in Graphene Nanostructures

Graphene has been recently shown to support electronically tunable ,Mid-IR plasmons with optical mode volumes that are 10\textasciicircum 7 times smaller than freespace, and plasmon wavelengths more than 100 times shorter. In this talk we will demonstrate how the plasmonic absorption of graphene resonators is enhanced and perturbed in controllable ways by varying the thickness and permittivity of the supporting substrate. We will show the results of recent experiments where 17.5{\%} absorption is achieved in a sheet of graphene resonators by carefully selecting the properties of an underlying silicon nitride substrate. We also demonstrate how additional absorption pathways can be created by modifying the surrounding dielectric environment to have optical resonances that can couple to the graphene plasmons. By placing graphene nanoresonators on a monolayer boron nitride (BN) sheet new surface phonon plasmon polariton (SPPP) modes arise due to coupling between the graphene plasmon and BN optical phonon. We map the dispersion relations of these modes, and show that the high quality factor of the BN phonon leads to epsilon near zero (ENZ) behavior in the SPPP mode. These experimental observations are compared to a theoretical model that has been developed to explain optically active graphene devices.   

Plasmonic Resonant Absorption in Mid-Infrared in Graphene Nanoresonators

We experimentally demonstrated polarization-sensitive, tunable plasmonic resonant absorption in the mid-infrared range of 5-14 um by utilizing an array of graphene nanoribbon resonators. By tuning resonator width and charge density, we probed graphene plasmons with $\lambda_{\mathrm{p}}$ $\le \lambda $/100 and plasmon resonance energy as high as 0.26 meV (2100 cm$^{-1})$ for 40 nm wide nanoresonators. Resonant absorption spectra enabled us to map the wavevector-frequency dispersion for graphene plasmons at mid-IR energies and revealed a modified plasmon dispersion as well as plasmon damping due to intrinsic optical phonons of graphene and graphene plasmon interaction with the surface polar phonons in SiO$_{2}$ substrates. Additionally, we studied spectra further by introducing intrinsic defect phonons and doping by direct electron beam irradiation of graphene nanoresonators   

Strong Plasmon Reflection at Nanometer-Size Gaps in Monolayer Graphene on SiC

Tip-enhanced infrared near-field microscopy is used to study propagating plasmons in epitaxial quasi-free-standing monolayer graphene on silicon carbide. We observe that plasmons are strongly reflected at graphene gaps at the steps between the substrate terraces. For the step height of only 1.5 nm, which is two orders of magnitude smaller than the plasmon wavelength, the reflection signal reaches 20 percent of its value at graphene edges, and it approaches 0.5 for steps of 5 nm. We support this observation with extensive numerical simulations and give physical rationale for this intriguing phenomenon. Our work suggests that plasmon propagation in graphene-based circuits can be controlled using ultracompact nanostructures. J. Chen et al., Nano Lett., DOI: 10.1021/nl403622t (2013).   

Infrared nano-imaging and nano-spectroscopy of surface plasmons in bilayer graphene

Bernal stacking bilayer graphene (BLG) has demonstrated its capability for application in a wide range of fields including electronics, photonics and energy engineering. So far, plasmonic properties of BLG have not been fully explored experimentally despite broad interests. Here, we report infrared nano-imaging and nano-spectroscopy of surface plasmons (SPs) in BLG. We found that BLG also supported gate-tunable SPs in the mid-infrared range with nevertheless smaller wavelength compared to equally doped single-layer graphene (SLG) and randomly stacked double-layer graphene (DLG). In addition, the coupling between BLG plasmons and SiO2 phonons appeared much weaker compared to SLG plasmons. Further analysis indicated that these observations about SPs in BLG were attributed to interlayer coupling that affects strongly the electronic structure. Our work uncovered all the essential characteristics of BLG plasmons, and suggested the possibility of developing carbon-based plasmonic circuits where SLG, BLG and DLG are all functioning building blocks.   

Exploring two-dimensional electron gases with 2DFT spectroscopy

The dephasing of excitons in a modulation doped single quantum well was carefully measured using time integrated four-wave mixing (FWM) and two-dimensional Fourier transform (2DFT) spectroscopy. The excitonic linewidths were obtained from the diagonal and cross diagonal profiles of the 2DFT spectra. The laser excitation density and temperature were varied and 2DFT spectra were collected. This work was performed, in part, at the Center for Integrated Nanotechnologies, a U.S. Department of Energy, Office of Basic Energy Sciences user facility. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.   

Plasmon-Phonon Interaction and Phonon Induced Transparency in Graphene Plasmonic~Nanostructures

Electrons in graphene interact via Coulomb forces but also via an optical phonon-mediated interaction. As a result of the chiral nature of electrons, these two interactions are additive. This phonon-mediated interaction results in strong coupling between the~plasmons~and phonons.~Plasmons~in graphene also interact strongly with the substrate optical phonons. In this talk we will present experimental results on~plasmon-phonon interactions. We patterned disc-shaped~plasmon resonators in CVD grown graphene with radii varying from 16-80 nm and studied plasmon~resonances using IR spectroscopy. Sharp features appear in the~plasmon absorption spectra when the~plasmon~frequencies are close to the phonon frequencies. When the~plasmon~frequency matches the zone-center optical phonon frequency, a narrow transparency dip appears in the~plasmon~absorption spectra. This transparency, which resembles EIT in optics, can be explained in terms of the cancellation between the Coulomb and the phonon-mediated electron-electron interactions. Our theoretical model, based on the eigenvalue equation for confined plasmon~modes, explains the data well and enables us to extract parameters related to the~plasmon-phonon interaction in graphene.   

Quantum Plasmonics with Graphene

Graphene has emerged as a powerful platform for plasmonics due to its high mobility, versatile fabrication, and the ability to tune the plasmon properties via external gate voltages. We consider several applications of graphene plasmonics to quantum information science. First, we show that one can take advantage of the strong electromagnetic field confinement and long lifetime of the plasmons to realize significant nonlinear optical interactions at the few photon level in graphene nanostructures. Such systems can be used to realize a single photon transistor. Second we consider a quantum network of graphene coupled to the hydrogen-like excited states of group-V donors in Silicon. The strong coupling of these dipole transitions to the graphene plasmons results in long-range interactions and superradiant transitions. We consider entanglement that can be generated using this collective decay process.   

Graphene plasmonic THz detectors

Frequency and strength of the plasmonic resonance can be tuned in THz range continuously by doping and discretely by changing the width of a single layer graphene strip. It creates strong detector response in the gap between Drude and interband transitions present in infinite graphene sheet. We have working detectors for a FIR laser with power of mW level and we are working toward microWatt sensitivity necessary for a Fourier spectrometer. We will present our devices, optical methods, and our progress in THz detectors.   

Tunable phonon-indued transparency in bilayer graphene plasmonic structures

Electromagnetically induced transparency (EIT) has been extensively studied in atomic systems. EIT-like phenomena have also been demonstrated in classical systems, such as plasmonic and opto-mechanical systems. Here we present an EIT-like behavior in AB stacking bilayer graphene nanoribbons, where the destructive interference of an infrared active phonon mode and a plasmon mode induces an absorption transparency in the vicinity of the phonon frequency. More importantly, this phonon-induced transparency can be tuned by electrostatic or chemical doping. This kind of tunability is lacking in many of the systems with EIT-like property. The phonon-induced transparency is also accompanied by the slow light effect and a light group index as large as 500 has been inferred from our data. Bilayer graphene provides us a unique opportunity to explore EIT-like phenomena involving phonons and plasmons.   

Ultrafast plasmonic behavior of graphene probed by infrared nanoscopy

Recent experiments using near-field spectroscopy (s-SNOM) have revealed the spectroscopic (Z. Fei et al., Nano Lett. 11, 4701 (2011)) and real-space characteristics (Z. Fei et al., Nature 487, 82 (2012)) of graphene plasmons and show that this technique is ideal for their investigation. Here, we discuss the time-dependent plasmonic behavior of graphene. Combining s-SNOM with ultrafast laser excitation we were able to perform near-infrared pump mid-infrared probe spectroscopy beyond the diffraction limit on exfoliated samples. We show picosecond ultrafast plasmon modulation by optical means with an efficiency comparable to electrostatic gating and also to other plasmonic materials such as metals. Modeling of our results reveals that pump-induced heating of carriers is responsible for the ultrafast change in Drude weight that s-SNOM is probing.   

Imaging the full optical response of graphene surface plasmon polaritons

The full realization of electronic devices based on graphene requires the complete characterization of defects and their effect on local electronic properties. Using infrared scattering-type scanning near-field optical microscopy (IR \emph{s}-SNOM) surface plasmon polariton propagation in graphene can be imaged with nanometer spatial resolution, providing information on the local electronic properties. Here we use \emph{s}-SNOM imaging to provide full infrared optical characterization of graphene SPPs by studying both the amplitude and phase of the near-field scattered light. We develop a simple phenomenological model based on SPP reflection from boundaries and defects that provides semi-quantitative agreement for both amplitude and phase simultaneously. These results provide insight into nanometer scale variations in the electronic structure of graphene and thus inform future device development.   

Nano-rods of zinc oxide in nano-graphene

It's of great interest to study the devices based on nano-ZnO and graphene, for their electromagnetic and optical properties to increase the efficiency of solar cells. The graphene multilayers synthesis was done by mechanosynthesis, grinding in a mechanical agate mortar. The zinc oxide nano-rods were synthesized from zinc acetate dihydrate, Ace, (Sigma Aldrich) and ethylene diamine, En, (Sigma Aldrich) with a 1:2 ratio of reagents En/Ace. The ZnO nano-rods in nano-tubes graphene were obtained by mechanosynthesis. The X-ray powder diffraction, shows the shift of C with PDF 12-0212 and ZnO, Zincite PDF 36-1451, both with hexagonal unit cell. The grain size and morphology of graphene (multilayers and nano-tubes), ZnO nano-rods and ZnO-graphene mixture (multilayers, nano-tubes) were observed by scanning electron microscope. Transmission electron microscope, corroborates shown in SEM. Raman spectroscopy, shows the shift of multilayer graphene and the ZnO nano-rods. In photoluminescence measurements, observe the change in intensity in the band defects. Magnetic properties characterization was carried out by Vibrating Sample Magnetometry. We conclude that graphite multilayers dislocated by cutting efforts, forming graphene nano-tubes and encapsulated ZnO nano-rods within graphene.   

Session J51: Focus Session: Beyond Graphene Devices: Function, Fabrication, and Characterization II

Horizontally aligned single walled carbon nanotube arrays on quartz for electrochemical biosensing

We have fabricated and characterized a simple and high performance electrochemical sensor using horizontally aligned single walled carbon nanotube arrays on transparent single crystal quartz substrates grown by chemical vapor deposition. The electrochemical activities of redox probes Fe(CN)$_{6}^{3-/4-}$, Ru(NH$_{3})_{6}^{3+}$ and protein cytochrome c on these pristine SWCNT thin films have been studied. Because the surface coverage of CNTs is extremely low and aligned, the shape of cyclic voltammograms of these molecules in stationary solution is strongly affected by the mass transport rate of molecules and the interactions between molecules and the SWCNT surface. We also studied the electrochemical flow sensing capability of the device for detecting neurotransmitter dopamine at physiological conditions with the presence of \textit{Bovine serum albumin}. Good sensitivity, fast response, high stability and anti-fouling capability are observed. Therefore, this device shows great potential for sensing applications in complex solution.   

Transition from Coulomb Blockade to Resonant Transmission in a MoS$_{2}$ Nanoribbon

We have measured a side-gated nanoribbon of MoS$_{2}$ at low temperature, and observed the transition from Coulomb blockade to resonant transmission when the Fermi level is tuned with a gate. We show that near the crossover between these regimes, the entire nanoribbon acts as a single quantum dot. Our findings may shed light on quasi-ballistic transport in the material. We also discuss the quantum dot formation in terms of a substrate-induced disorder potential, and consider other possible origins of disorder.   

Spin-orbit coupling, quantum dots, and qubits in transition metal dichalcogenides

We derive an effective Hamiltonian describing the dynamics of electrons in the conduction band of transition metal dichalcogenides (TMDC) in the presence of perpendicular electric and magnetic fields [1]. We discuss both the intrinsic and Bychkov-Rashba spin-orbit coupling (SOC) induced by an external electric field. We identify a new term in the Hamiltonian of the Bychkov-Rashba SOC which does not exist in III-V semiconductors. Due to the strong intrinsic SOC is an effective out-of-plane $g$-factor for the electrons which differs from the free-electron $g$-factor $g\simeq 2$. Using first-principles calculations, we estimate the various parameters appearing in the theory. Finally, we consider quantum dots (QDs) in TMDC materials and derive an effective Hamiltonian allowing us to calculate the magnetic field dependence of the bound states in the QDs. We find that all states are both valley and spin split, which suggests that these QDs could be used as valley-spin filters. We explore the possibility of using spin and valley states in TMDCs as qubits, and conclude that, due to the relatively strong intrinsic SOC in the conduction band, the most realistic option appears to be a combined spin-valley (Kramers) qubit at low B fields.\\[4pt] [1] A. Korm\'{a}nyos et al., arXiv:1310.7720.   

Valley relaxation dynamics in monolayer semiconductors studied by transient absorption and multidimensional spectroscopies

Single layer transition metal dichalcogenides are 2D semiconducting systems with unique electronic band structure. Two-valley energy bands along with strong spin-orbital coupling lead to valley dependent career spin polarization, which is the basis for recently proposed valleytronic applications. These systems also exhibit unusually strong many body affects, such as strong exciton and trion binding, due to reduced dielectric screening of Coulomb interactions. Recently observed large photoluminescence helicity suggests beyond ns hole spin and valley lifetimes. But there is not much known about the impact of strong many particle correlations on spin and valley polarization dynamics. Here we report direct measurements of ultrafast valley specific relaxation dynamics in single layer MoS$_{2}$. We found that excitonic many body interactions significantly contribute to the relaxation process.. Biexciton formation reveals hole valley/spin relaxation time. Our results suggest that initial fast intervalley electron scattering and electron spin relaxation leads to loss of valley polarization for holes through an electron-hole exchange mechanism.   

Magnetoelectric effects and valley dependent spin resonances in transition metal dichalcogenide bilayers

In addition to spin, valley is an internal degrees of freedom of carriers in monolayer group-VI transition metal dichalcogenides (TMDCs). In bilayer, carrier is also characterized by the layer pseudospin, which is associated with the electrical polarization. Here we show in TMDC bilayers, the spin, valley and layer pseudospins of carriers are strongly coupled to each other. Because of this coupling, most of the spin physics in TMDC monolayer such as the spin Hall effect and spin-dependent selection rule for optical transitions are inherited in TMDC bilayers. The strong coupling between spin and layer pseudospin also gives rise to a variety of magnetoelectric effects that make possible quantum manipulation of these electronic degrees of freedom. For example, electric polarization will oscillate in a magnetic field, while electric field can be used to tune the spin precessions. Moreover, the coupling between spin, valley and layer pseudospins makes possible valley dependent spin resonances such that spin rotations can be selectively addressed in the two valleys.   

Exction-related electroluminescence from monolayer MoS$_{2}$

Excitons in MoS$_{2}$ dominate the absorption and emission properties of the two-dimensional system. Here, we study the microscopic origin of the electroluminescence from monolayer MoS$_{2}$ fabricated on a heavily $p$-type doped silicon substrate. By comparing the photoluminescence and electroluminescence of a MoS$_{2}$ diode, direct-exciton and bound-exciton related recombination processes can be identified. Auger recombination of the exciton-exciton annihilation of bound exciton emission is observed under a high electron-hole pair injection rate at room temperature. We expect the direct exciton-exciton annihilation lifetime to exceed the carrier lifetime, due to the absence of any noticeable direct exciton saturation. We believe that our method of electrical injection opens a new route to understand the microscopic nature of the exciton recombination and facilitate the control of valley and spin excitation in MoS$_{2}$.   

Spin-valley coupling in atomically thin tungsten dichalcogenides

The monolayers of group VI transition metal dichalcogenides feature a valence band spin splitting with opposite sign in the two valleys located at corners of 1st Brillouin zone. This spin-valley coupling, particularly pronounced in tungsten dichalcogenides, can benefit potential spintronics and valleytronics with the important consequences of spin-valley interplay and the suppression of spin and valley relaxations. In this talk we discuss the optical studies of WS2 monolayers and multilayers. The PL spectra and first-principle calculations consistently reveal a spin-valley coupling of 0.4 eV which suppresses interlayer hopping and manifests as a thickness independent splitting pattern at valence band edge near K valleys. This giant spin-valley coupling, together with the valley dependent physical properties, may lead to rich possibilities for manipulating spin and valley degrees of freedom in these atomically thin 2D materials.   

Electrically Tunable Excitonic Light Emitting Diodes based on Monolayer WSe2 p-n Junctions

Light-emitting diodes (LEDs) are of vital importance for lighting, displays, optical interconnects, logic and sensors. The development of new systems that allow improvements in their efficiency, spectral properties, compactness and integrability could have dramatic ramifications. Monolayer transition metal dichalcogenides have recently emerged as interesting candidates for optoelectronic applications due to their unique optical properties. Electroluminescence (EL) has already been observed from monolayer MoS2 devices. However, the EL efficiency was low and the linewidth broad due both to the poor optical quality of MoS2 and ineffective contacts. In this talk, we present EL from lateral p-n junctions in monolayer WSe2 induced electrostatically using a thin boron nitride dielectric layer with multiple metal gates beneath. This structure allows effective injection of electrons and holes, and combined with the high optical quality of WSe2 it yields bright EL with 1000 times smaller injection current and 10 times smaller linewidth than in MoS2. Further, by increasing the injection bias we can tune the EL between regimes of impurity-bound, charged, and neutral excitons. This system has the required ingredients for new kinds of optoelectronic devices such as spin- and valley-polarized LEDs, on-chip lasers, and two-dimensional electro-optic modulators.   

Enhanced valley polarization in 3R-MoS$_{2}$

Transition-metal dichalcogenides (TMDs) are growing interest as graphene-like layered materials and have rich physical properties including spin/valley physics. Since the spin/valley degeneracy is resolved, inversion symmetry breaking is necessary, and then researchers mainly focused on monolayer TMDs, which has NO inversion symmetry. The size of monolayer, however, is 30um at maximum in the lateral size. This is not sufficient for deepening spin/valley physics. On the other hand, 2H bulk crystal, a famous bulk polytype, has inversion symmetry. Hence, noncentrosymmetric bulk crystals are desired. Here we introduce 3R bulk crystal of MoS$_{2}$ as a new promising material for spin/valley physics. 3R polytype is composed of a trilayer stacking in such a way that the inversion symmetry is kept broken in the bulk form. Using 3R-phase, we observed the enhanced valley polarization through circular polarized photoluminescence measurements in the bilayer form. The degree of polarization $P$ of 3R bilayer reached 68{\%} at 4K, whereas the 2H form showed P $=$ 33 {\%} at most. The temperature dependency also indicated $P$ of 3R is about twice as large as 2H of all temperature range (4K $\sim$ 300K). The noncentrosymmetric 3R MoS$_{2}$ is promising for pushing forward valley/spin-tronics based on 2D crystals.   

Observation of out-of-plane spin-polarization in bulk 3R-MoS$_2$

Transition metal dichalcogenide of noncentrosymmetric 3R-type structure, 3R-MoS$_2$, is investigated by spin- and angle-resolved photoemission spectroscopy. The top of valence bands of 3R-MoS$_2$ at the Brillouin zone corners ($K$- and $K'$-points) are found to show huge spin splitting with $z$-oriented (Zeeman-type) spin-polarization, in contrast to the nearly spin-degenerate centrosymmetric 2H-MoS$_2$. The observed spin-polarizations reach $P_z \sim\pm 1$ at $\overline{K}$- and $\overline{K'}$-points, corresponding well with the relativistic first-principles band calculations. It provides the direct evidence of spin-valley coupling realized through the broken inversion symmetry and the strong spin-orbit interaction of Mo $4d$-orbitals, leading to the spin-valley polarized state in MoS$_2$.   

Carrier and polarization dynamics in monolayer MoS$_2$: temperature and power dependence

In monolayer (ML) MoS$_2$ optical transitions across the direct bandgap are governed by chiral selection rules, allowing optical k-valley initialization [1,2,3]. Here we present the first time resolved photoluminescence (PL) polarization measurements in MoS$_2$ MLs [4], providing vital information on the electron valley dynamics. Using quasi-resonant excitation of the A-exciton transitions, we can infer that the PL decays within $\tau \simeq$4ps. The PL polarization of P$_c \approx 60\%$ remains nearly constant in time for experiments from 4K - 300K, a necessary condition for the success of future Valley Hall experiments [1]. $\tau$ does not vary significantly over this temperature range. This is surprising when considering the decrease of P$_c$ in continuous wave experiments when going from 4K to 300K reported in the literature [2,3]. By tuning the laser following the shift of the A-exciton resonance with temperature we are able to recover at 300K $\sim80\%$ of the polarization observed at 4K. For pulsed laser excitation, we observe a decrease of P$_c$ with increasing laser power at all temperatures.\\[4pt] [1] Xiao et al, PRL 108, 196802 (2012).\\[0pt] [2] Mak, et al Nat. Nanotech. 7, 494 (2012).\\[0pt] [3] Sallen et al, PRB 86, 081301 (2012).\\[0pt] [4] Lagarde et al, arXiv:1308.0696.   

Linearly dispersing plasmons in monolayer transition metal dichalcogenides

We describe the collective excitations of the electronic liquid in monolayer transition metal dichalcogenides such as MoS$_2$ in a strong Zeeman field. The combination of the Dresselhaus type spin-orbit coupling and the Zeeman field lifts the valley degeneracy, and this manifests in the appearance of an additional plasmon mode that has linear dispersion. There is a well-defined quasiparticle peak in the spectral function which corresponds to this second mode.   

Curvature-controlled valley polarization and band-gap tuning in few-layer MoS$_{2}$

Monolayer transition-metal dichalcogenides (TMDCs) display valley-selective circular dichroism due to time-reversal symmetry and lack of inversion symmetry, making them promising candidates for valleytronics. In contrast, few-layer TMDCs possess both time-reversal and inversion symmetry and hence, lose these desirable valley-selective properties. Here, by using density-functional tight-binding electronic structure simulation and revised periodic boundary conditions, we show that bending of multilayer MoS$_{2}$ sheets breaks band degeneracies and localizes states on separate layers due to bending-induced strain-gradients across the sheets. We propose a strategy for employing bending deformations as a simple yet effective means of dynamically and reversibly tuning band gaps while simultaneously tuning coupling between spin, valley, and layer pseudospin of charge carriers in few-layer TMDCs.   

Session J52: Superconductors Phonons and Electrons

Superconducting properties in Li$_x$ZrNCl from first principles

ZrNCl is a layered band insulator. Upon Li intercalation it undergoes a metal-superconductor transition at 5$\%$ Li content. The superconducting critical temperature displays a very peculiar behaviour as it is maximal (T$_C$ =15.2K) for $\approx 5\%$ intercalation, at the edge of the insulator-superconductor transition. When the Li concentration is increased the critical temperature decreases monotonically, stabilizing at T$_C$ =11.5 K for x=0.3. We present calculations beyond DFT on Li$_x$ZrNCl as a function of doping. We show that an ultradense sampling of the Brillouin zone (both for phonon and electron momentum) is necessary to describe the phonon spectra and the electron-phonon coupling in this material. We show that marked Kohn-anomalies, overlooked in all previous calculations, occur in the phonon spectrum at different energy scales. Finally, we discuss the role of correlation effects in determining the superconducting properties of Li$_x$ZrNCl.   

Ambipolar Transport and Gate-Induced Superconductivity in Layered Transition Metal Dichalcogenides

Transition metal dichalcogenides (TMDs) are well known van der Waals layered materials that are easy to be exfoliated into atomically flat nano scale flakes. Owing to high efficiency of electrical double layer (EDL) dielectrics, thin flakes of TMDs have achieved high performance ambipolar transistor operation and established metallic states with high mobility, which are ideal for inducing superconductivity. Here, we report a comprehensive study of ambipolar transport behaviors in the EDL transistors (EDLTs) of MoS$_{2}$, MoSe$_{2}$ and MoTe$_{2}$ thin flakes down to 2 K. In comparison, MoSe$_{2}$ EDLT displayed a well-balanced ambipolar transistor operation while the other two showed opposite predominance in electron and hole accumulation, respectively. By modulation of carrier densities, the metal insulator transition (MIT) was observed in both electron and hole transport measurements. Particularly, superconducting transitions were reached after the formation of metallic states in the electron side. The phase diagram of transition temperature-carrier density was established and a dome-shaped structure was confirmed, revealing a universal feature of gate-induce superconductivity in layered band insulators.   

Intertwined Order Parameters in a Charge-Ordered Superconductor

We present a spectroscopic study of the low temperature state in NbSe$_2$, exhibiting charge-density wave and superconductivity orders. Raman scattering reveals that the spectrum of quasi-particle excitations out of the condensate is characterized by an energy gap derived from both order parameters in a way that suggests intertwining between them. Supported by a calculation of NbSe$_2$ Raman vertices, and by earlier photoemission studies, we conclude that in NbSe$_2$ an isotropic superconductivity interplays with a strongly anisotropic charge-density wave order on selected parts of the Fermi surface, as characterized by admixture of particle-particle and particle-hole excitations.   

Structural transition and doping induced superconductivity in IrTe$_2$

Doped IrTe$_2$ compounds are of current interest as they offer the opportunity to investigate the relationship between a structural transition and the appearance of superconductivity. Here we present the results of an investigation of the structural transition of Ir$_{1-x}$Pt$_x$Te$_2$ (x=0, 0.03, and 0.05) by X-ray and neutron diffraction. In IrTe$_2$ a structural modulation was observed with a wave vector of $k$ = (1/5, 0, 1/5) below 285 K, accompanied by a structural transition from a trigonal to a triclinic lattice. First principles calculations suggest the local bonding instability associated with the Te 5$p$ states is likely the origin of the structural phase transition. Pt-doping (x=0.03) suppresses the structural transition down to 70 K and the superconductivity appears at 3 K. No response to onset of superconductivity was observed in the structural parameters suggesting that strong electron-lattice coupling does not play a role in IrTe$_2$.   

Raman scattering of IrTe$_2$

IrTe$_2$ presents a layered compound with a triangular lattice. It is known to exhibit a first order structural phase transition at approximately 260~K which is of a first order, corresponding to a formation of a superstructure with a period of five unit cells. Using polarized Raman spectroscopy we have studied the temperature dependence of 14 observed Raman allowed phononic modes. These phonons couple strongly to this transition and one additional first order transition at approximately 170~K. In the high-temperature phase only 3 modes are observed, while below approximately 280~K all 14 modes become visible. Below approximately 170~K only 11 modes are observed. Our results shed light on the possible mechanism driving the transitions.   

Synthesis of the $Ba_{1-x}Na_{x}Ti_{2}Sb_{2}O_{z}$ system with x= 0.00, 0.15, 0.25 and 1.00, by solid-state reaction method

The present study of this system is a superconductor without Cu-O planes using solid-state reaction synthesis with slow cooling. The samples used were considered with an initial weight of 1 g after the losses to $CO_2$ owing to the reaction. Used ThermoGravimetric Analysis, we determined the reaction temperatures. The samples were characterized using X-Ray powder Diffraction and Scanning Electron Microscopy, expose the formation to secondary and ternary compounds, also the changes in the oxidation numbers of some initial reagents in each reaction temperature. A new contribution of this work is obtain the conditions of the monophase, $NaSbO_3$ with PDF 42-0223 in x=1.00 at 550 $^\circ$C/62 h 50 min. not reported in the literature.We observed in XRD an amorphous phase at the same temperature, which not observed at 700 $^\circ$C. By SEM we distinguish that the amorphous phase continue exist, but using XRD is not perceptible because it is in less proportion and showing a grains grow in the samples. We determine the thermodynamic conditions to obtain a $NaSbO_3$ monophase at x= 1.00, different that was be reported. This system exhibit an amorphous phase between 550 - 700 $^\circ$C. To optimize the connection between polycrystals grain pellets were manufactured at 800 $^\circ$C.   

Pressure-induced superconductivity in $n$-type bismuth telluride

Stoichiometric bismuth telluride (Bi$_2$Te$_3$), which is a $p$-type semiconductor, has the rhombohedral structure with space group $R$-3$m$ at ambient condition. We have previously reported that pressure-induced superconductivity of stoichiometric $p$-type Bi$_2$Te$_3$ occurs in the high-pressure phases which appear above 8 GPa. However Bi$_2$Te$_3$ shows the variations of the carrier types, the carrier density, and the transport properties with the atomic composition. In this study, we performed the x-ray diffraction study and the electrical resistivity measurement of Bi$_{35}$Te$_{65}$ as the $n$-type Bi$_{2}$Te$_{3}$ sample under high pressure to investigate structural phase transition and pressure-induced superconductivity. At ambient condition, Bi$_{35}$Te$_{65}$ has also the $R$-3$m$ structure. It remains stable up to 8 GPa at room temperature. The superconducting transition is observed at 6 GPa below 2.9 K. The electrical resistivity at room temperature decreases rapidly at pressures from 7 to 8 GPa, indicating the occurrence of structural phase transition. It suggests that the superconducting transition at 6 GPa occurs at the ambient pressure phase with the $R$-3$m$ structure.   

Photon energy and carrier density dependent dynamics of the coherent A1g phonon in bismuth

We investigate the dynamics of the coherent A1g phonon as a function of photon energy and carrier density for photo-excited single-crystal thin-film bismuth. Previous experimental and theoretical studies on group V semimetals such as bismuth show strong softening of the mode with photo-excitation associated with electronic softening and a reduction in the Peierls distortion; however, theoretical models differ on the detailed dependence for how the carriers populate the conduction band states immediately following excitation [Murray et al., Phys. Rev. B \textbf{72, }060301 (2005); Zijlstra et al., Phys. Rev. B \textbf{74, }220301 (2006); Sheu et al., Phys. Rev. B \textbf{87, }075429 (2013)]. By carefully controlling the total energy deposition and the incident photon number as a function of different pump wavelengths, we are able to test two different models for the filling near the Fermi surface: a one-chemical potential model whereby the carrier density depends on electronic temperature and a two-chemical potential model whereby the carrier density depends on the number of photons absorbed. We find evidence that neither model suffices, likely due to different relaxation mechanisms depending on which bands are involved in the initial excitation.   

First-principles calculation of the polarization-dependent force driving the E$_\textrm{g}$ mode in bismuth under optical excitation.

Using first principles electronic structure methods, we calculate the induced force on the E$_\textrm{g}$ (zone centre transverse optical) phonon mode in bismuth immediately after absorption of polarized light. When radiation with polarization perpendicular to the c-axis is absorbed in bismuth, the distribution of excited electrons and holes breaks the three-fold rotational symmetry and leads to a net force on the atoms in the direction perpendicular to the axis. We calculate the initial excited electronic distribution as a function of photon energy and polarization and find the resulting transverse and longitudinal forces experienced by the atoms. Using the measured, temperature-dependent rate of decay of the transverse force\footnote{J.J. Li et al, Phys. Rev. Lett. 110, 047401 (2013)}, we predict the approximate amplitude of induced atomic motion in the E$_\textrm{g}$ mode as a function of temperature and optical fluence.   

Intrinsic anisotropy of critical current in c-tilted MgB$_{2}$ thin films

The intense investigations of MgB$_{2}$ in recent years have revealed many unusual effects of the two-band superconductivity on the properties of this material. However, given the size and geometry of available single crystals, the critical current along the c-direction, mostly affected by the anisotropy of the two bands, has not been thoroughly investigated. To address the issue of the c-axis current transport, we succeeded to measure J$_{\mathrm{c}}$ along the c axis by growing epitaxial films with tilted c-axis, which offers a unique possibility of probing both the in-plane and the out-of-plane critical currents. We show that anisotropic band parameters determine the anisotropy of vortex pinning with respect to the current direction. This yields a temperature and field dependent anisotropy of the critical current density J$_{\mathrm{c}}$ related to the anisotropic penetration depths at low temperature and low field and to the ratio of $\sigma $ band effective masses along the c and ab crystalline axes at temperatures close to T$_{\mathrm{c}}$ or at applied fields larger than the virtual upper critical field of the $\pi $ band H $\approx $ 0.2T. Our results suggest that the out-of-plane current transport could set the ultimate J$_{\mathrm{c}}$ limit in randomly oriented MgB$_{2}$ polycrystals.   

THz pump-THz probe Cooper pair dynamics in MgB$_{2}$

THz pump-THz probe spectroscopy is used to study non-equilibrium superconducting carrier dynamics in MgB$_{2}$ thin films. An intense THz pump pulse resonantly breaks Cooper pairs, exciting quasiparticles across the lowest energy superconducting gap (2$\Delta _{\mathrm{0}}\approx $3.6meV corresponding to $\sim$ 0.8THz). The condensate and quasiparticle dynamics are temporally resolved by measuring the real and imaginary part of THz conductivity. Following THz excitation, the condensate density is strongly suppressed on a picosecond time scale coinciding with the emergence of a Drude peak. These results are compared with optical pump-THz probe experiments where, consistent with previous measurements, much slower Copper pair-breaking dynamics are observed.   

Superconducting graphene sheets in CaC$_{6}$ enabled by phonon-mediated interband interactions

The superconducting mechanism of graphite intercalation compounds has been under intense debate. To reveal this mechanism, we studied a prototypical compound CaC$_{6}$ using angle-resolved photoelectron spectroscopy. Both the calcium-derived and graphene-derived bands were clearly resolved. We performed analysis on the superconducting gaps and electron-phonon coupling constants. We will also discuss the important implications in fabricating superconducting graphene devices.   

Dynamical Jahn-Teller Effect and Antiferromagnetism in insulating Cs$_3$C$_{60}$

The dynamical Jahn--Teller effect on fullerene sites in insulating Cs$_3$C$_{60}$ is investigated fully {\it ab initio} [1]. The vibronic excitations of rotational type are at $\ge$ 65 cm$^{-1}$, while the net kinetic contribution to the Jahn--Teller stabilization energy constitutes ca 90 meV. This means that no localization of distortions by intermolecular interactions is possible in these fullerides, therefore, free rotations of deformations take place independently on each C$_{60}$. The latter destroy the orbital ordering and establish a conventional exchange interaction between $S=1/2$ on fullerene sites. The corresponding exchange model is derived and predicts the N\'{e}el temperature for A15 Cs$_3$C$_{60}$ close to experiment. \\[4pt] [1] N. Iwahara and L. F. Chibotaru, Phys. Rev. Lett. {\bf 111}, 056401 (2013)   

Theory of Jahn Teller signatures in the infrared absorption of C$_{60}^{3-}$

Among the molecular superconductors, trivalent fullerides such as Cs$_{3}$C$_{60}$, with three folded degenerate HOMO and a fully ordered pressure induced superconductor-insulator are still intriguing. The orbital degeneracy of the fulleride ion C$_{60}^{-3}$ implies that besides a Jahn-Teller distorted state with S=1/2 and high-lying spin (S=3/2) excitation known from NMR, another undetected orbital excitation with S=1/2 should exist. Building upon accurate density hybrid functional theory calculations where properties such as the infrared (IR) spectrum and its Jahn-Teller features are well described, we extracted the {\sc ab-initio} orbital and spin spectrum of a C$_{60}^{-3}$ ion in different spin and orbital states including a new low lying L=2 S=1/2 excitation. Despite a Jahn-Teller distortion so small to be observable in its IR spectrum, this state is found to gain a large zero-point energy, placing it just above the L=1, S=1/2 ion ground state, and way below the L=0, S=3/2 high lying excitation. We can now elegantly explain the surprising early thermal disappearance of the low-temperature Jahn-Teller IR spectral features and splitting without a concurrent rise of spin susceptibility that would instead be required by population of the high spin S=3/2 excitation.   

Phonons in solid picene at high pressures

Intercalated hydrocarbons have attracted considerable interest as a new class of superconductors. Calculations of the $e-ph$ interaction in different approximations yield conflicting results on the role of inter and intra-molecular vibrations in the pairing. We present an experimental and theoretical study of the phonon spectrum of solid picene under high pressure. We introduce a new theoretical analysics, based on the projection of phonon eigenvectors, to quantify the increase of intermolecular character under pressure. F. Capitani et al., Phys. Rev. B 88, 144303 (2013).   

Session J53: Structure and Properties of Oxide Surfaces

Sub-Monolayer Water Adsorption on Alkaline Earth Metal Oxide Surfaces: A First-Principles Study

In the present work, we predict atomic structures of adsorbed complexes that should appear on alkaline earth metal oxide (001) terraces in thermodynamic equilibrium with water and oxygen gases. Density-functional theory with the hybrid exchange-correlation functional HSE06 combined with the self-consistent many-body dispersion approach [1] is used to calculate total energies. The choice of this functional is validated by renormalized second-order perturbation theory [2]. An unbiased search for global minima of H$_x$O$_y$ adsorption is performed using first-principles genetic algorithm for periodic models. $x$ and $y$ as a function of temperature and pressure are determined using {\em ab initio} atomistic thermodynamics. We find a range of H$_2$O chemical potentials where one-dimensional adsorbed water structures are thermodynamically stable on CaO(001). On MgO(001) and SrO(001), such structures are not found. The formation of the one-dimensional structures is explained by the balance between water-water and water-surface interactions.\\[4pt] [1] A. Tkatchenko, R. A. DiStasio, Jr., R. Car and M. Scheffler, Phys. Rev. Lett. \textbf{108}, 236402 (2012);\\[0pt] [2] X. Ren, P. Rinke, G. E. Scuseria, M. Scheffler, Phys. Rev. B \textbf{88}, 035120, (2013).   

Atomic and electronic structure of polar oxide-wide band gap semiconductor interfaces: MgO(111)/SiC(0001)

Atomically sharp polar oxide/semiconductor heterostructures are characterized by the abrupt change of the electrostatic potential across these junctions. This inherent property provides opportunities to tailor the functional properties of these heterojunctions by engineering their atomic interface structure. In this work we present a combined experimental and theoretical study on model MgO(111)/SiC(0001) polar interface. Thin MgO(111) films with rock salt structure were grown by molecular beam epitaxy on two surface reconstructions of SiC(0001). Atomic imaging of the film/substrate interface(s) reveals that chemically abrupt interfaces determined by either O or Mg can be formed, depending on the substrate surface preparation. The density functional theory calculations show that screening of the interfacial dipole moment is the driving force for atomic stacking at the interface, which in turn determines the electronic properties of the MgO(111)/SiC(0001) interfaces. The electronic structure calculations show that O and Mg terminated interface have valence band offsets of 1.0 eV and 3.5 eV, respectively. These results demonstrate the large electronic tunability of this heterostructure, and indicate potentials for number of other polar oxide/semiconductor interfaces.   

Structure-Property relationship for H covered Fe3O4(001)

Magnetite(Fe3O4), the oldest permanent magnet, is still attracting intense studies due to its fascinating surface physical properties. Previous LEED and STM experiments have reported reconstruction on Fe3O4(001) clean surface, but the origin of this reconstruction is still under debate. Hydrogen removes this reconstruction. LEED, HREELS, XPS are used to study the hydrogen induced surface properties. Surface geometry change was investigated by LEED-I(V) calculation and the structure refinements are being done now. XPS experiments show evidences of OH species on the hydrogen-covered surface, which is consistent with HREELS data, proving H adatoms are bonded to the surface oxygen sites. This suggested the ratio of Fe2$+$/Fe3$+$ on the surface is increased. This change is observed by Fe 2p peak shift in XPS spectra. We will discuss the role hydrogen played in the surface structure and physical properties change induced by reconstruction.   

Comparing measurements of iron oxide nano-particle monolayer structure using GISAXS, Langmuir-Schraefer Method and SEM

Iron oxide nanoparticles coated with an oleic acid ligand readily form self-assembled monolayers when drop cast onto water's surface. For low particle densities, upon drop casting, particles form into hexagonally close-packed islands, which then merge together when laterally compressed to higher densities. Using Grazing Incidence Small Angle X-Ray Scattering (GISAXS) off the liquid surface we were able to measure the first through fifth order diffraction peaks. By analyzing the peaks' position and shape we investigated the in-plane structure of these monolayers. Alternatively, using the Langmuir-Schraefer method this film can be transferred to a solid substrate for imaging using SEM. Subsequently, applying a Fourier Transform analysis to the SEM images we demonstrate a comparable measurement to that made using GISAXS - thereby obtaining similar in-plane structural information. Correcting for instrumental factors, we can account for some of the differing features between the data taken by the two techniques. By way of this technique comparison we also demonstrate that the Langmuir-Schraefer method is capable of preserving the nano-particle film structure during transfer from liquid surface to solid substrate.   

Distinct Electronic Structure of the Electrolyte Gate Induced Conducting Phase in VO$_{2}$ Revealed by Photoelectron Spectroscopy

Vanadium dioxide (VO$_{2})$, a strongly correlated material, exhibits a temperature-driven metal to insulator transition (MIT), which is accompanied by a structural transformation from rutile (high-temperature metallic phase) to monoclinic (low-temperature insulator phase). Recently, it was discovered that a low-temperature conducting state emerges in VO$_{2}$ thin films upon gating with a liquid electrolyte. In this talk, photospectroscopy measurements of the core electronic states and valence band of electrolyte gated VO$_{2}$ thin films will reveal electronic features in the gate-induced conducting phase that are distinct from those of the temperature-induced rutile metallic phase. The electronic characteristics of the gated metallic state can be accounted for by oxygen vacancy formation and a consequent reduction in V-V dimerization without lifting the orbital ordering. An electronic bandstructure taking into account these modifications will be discussed.   

Effect of strain on the dynamics of optically induced metal-insulator transition of VO$_2$ thin films

Vanadium Dioxide undergoes a first order metal-insulator transition (MIT) when heated to 340K or when stimulated with a strong optical pulse. There is much interest in Vanadium dioxide due to its convenient transition temperature, and the many potential applications. In addition to the dramatic change in conductivity, the advent of ultrafast lasers has made it possible to induce the MIT in the femtosecond regime. Thin films of VO$_2$ are able to robustly undergo reversible MITs over many cycles. Changes in the microstructure can be used to tune the MIT. We are studying the dynamics of the phase transition, specifically how different amount of strain can affect both the quick transition to the metallic phase and the slower relaxation back to the insulating phase. We have found noticeable differences in the threshold fluence needed to optically induce the MIT in films on different substrates, as well as the longevity of the metallic state. We will be discussing the implications of these differences regarding the mechanisms responsible for the optically induced phase transition.   

Optical anisotropy in the metal-to-insulator transition in VO$_2$ thin films

The metal-to-insulator transition in vanadium dioxide (VO$_2$) is being explored for a variety of uses, ranging from opto-electronic switches to nanoparticle coatings for smart windows. The mechanisms behind this transition as well as methods for altering the transition properties are the focus of continuing studies. In particular, the properties of VO$_2$ thin-films are affected by the structure of the underlying substrate material that can influence the temperature, width, and other characteristics of the phase transition. We investigate the anisotropy of the time-resolved optical measurements in an ultrafast photo-induced MIT transition in VO$_2$ on different substrates, including rutile (TiO$_2$) and sapphire (Al$_2$O$_3$). We observe that the optical anisotropy varies with the fluence of the pump used to induce the phase transition on the TiO$_2$ substrate.   

High resolution structural and compositional mapping of the SrTiO$_{3}$/LaFeO$_{3}$ interface using chromatic aberration corrected energy filtered imaging

Interfaces between insulating polar perovskites have demonstrated a wealth of electronic and magnetic properties. Understanding and predicting the properties of a specific interface requires atomic level knowledge of interface structure and chemistry. Electron microscopy is capable of this task, and has been frequently applied to oxide interfaces using a combination of high-angle angular dark field scanning transmission electron microscopy (HAADF-STEM) and electron energy-loss spectroscopy (EELS). Energy-filtered TEM (EFTEM) captures a full image for a given energy losses, allowing a larger field of view than typical for STEM-EELS in far less time. However, EFTEM has not, to date, demonstrated the spatial resolution of STEM-EELS due to the limits set by chromatic aberration C$_{c}$. This study of LaFeO$_{3}$/SrTiO$_{3}$ demonstrates that C$_{c}$ correction enhances the resolution of EFTEM for elemental mapping, allowing a unit cell-by-unit cell analysis of the concentration gradients across the SrTiO$_{3}$/LaFeO$_{3}$ interface. The charge distribution at the interface will be discussed.   

A first principles study of interstitial transition metals in the SrTiO3 bulk and at the surface

Recent advances in oxide thin film deposition and growth have given a new momentum to the field of oxide electronics. For example, it was found that La-doped SrTiO$_3$ shows high charge carrier mobilities and conductivity was observed near the interface with LaAlO$_3$. However, forming low-resistance Ohmic metal/oxide constants has proven to be a challenge. Recently, the interface formed by a Cr film grown on SrTiO$_3$ (001) surface has been shown to have such properties. This raises the question of whether Cr or another metal provides the lowest resistance possible. To this end, we perform ab initio calculations using the VASP package and the PBE and PBE+U functionals and examine the electronic properties of interstitial transition metal atoms (M) in the bulk SrTiO$_3$ and near SrTiO$_3$ (001) surface. We show that for surface doping, the variation in the formation energy linearly depends on the number of d-electrons in the valence shell of the M atoms. This contrasts with the formation energies of M atoms in the bulk SrTiO$_3$, where no such trend is observed and, instead, the formation energy depends on the occupancy of the e$_g$ and t$_{2g}$ sub-bands.   

Scale-bridging simulations of surfaces and defects in BaTiO3

BaTiO3 (BTO) is a ferroelectric perovskite oxide that is of particular interest in thin-film form for its technological application in tunable nanoelectronic devices. The dielectric properties of BTO thin films depend on many different factors, among which the presence of oxygen vacancies is believed to be one of the most important. Oxygen vacancies, however, are difficult to characterize directly in experiments and usually even their concentration is unknown. On the one hand, first-principles simulations based on density-functional theory (DFT) are invaluable for providing insight into the role of defects in materials and, in principle, could be used for the study of oxygen vacancies in BTO thin films. On the other hand, the large length and time-scales associated with structures and processes in realistic surfaces are well beyond the scope of DFT calculations. To overcome some of these limitations we use DFT in conjunction with a computationally efficient classical interatomic force field that has been fitted to DFT energies, forces and stresses in bulk BTO. We assess the transferability of this potential to defects and surfaces. We then use it to study the prevalence of oxygen vacancies and their structures, both in bulk BTO and near BTO surfaces.   

Density Functional Theory studies of metal supported thin Zirconia films

We present the results of DFT calculations of the interface between a single layer thin ZrO$_2$ film and the supporting metals Pt$_{3}$Zr [1] and Pd$_{3}$Zr [2]. Both are very stable and are present when thin ZrO$_2$ films are grown by oxidation. Using the Vienna Ab-initio Simulation Package (VASP) employing standard PBE and van-der-Waals density functionals a thorough investigation of the structural properties was done for both small model cells and the large experimentally found super cells. Both substrates have a similar crystal structure but the binding mechanism differs: the Pt$_{3}$Zr substrate shows a pure Pt surface layer, leading to weak bonding between the film and the substrate with large nonlocal contributions. The Pd$_{3}$Zr substrate is stoichiometric and a Zr-O bond forms between substrate and oxide film, leading to higher adsorption energies. The large buckling found experimentally could be attributed to the closer distance resulting from nonlocal contributions for the Pt$_{3}$Zr substrate and to the Zr-O bond for the Pd$_{3}$Zr substrate. Furthermore we present results on the adsorption of atomic hydrogen and water molecules on the surface generated by the thin Zirconia film.\\[4pt] [1] Antlanger, M. et al, Phys Rev B 86, 035451 (2012).\\[0pt] [2] Choi, J et al, submitted.   

Density-functional study of the La$_{2}$Zr$_{2}$O$_{7}$ low-index faces

The (001), (011), and (111) faces of the catalytic host lanthanum zirconate (La$_{2}$Zr$_{2}$O$_{7})$ are studied at the level of density-functional theory (DFT), including all surfaces formed by cleaving a perfect crystal (``bulk-truncated'') and a subset of defective surfaces. Surface free energies are computed, dependent on O chemical potential for (001) and (011) surfaces and O and La or Zr chemical potentials for (111) surfaces, with vibrational contributions obtained from computed atomic frequencies and/or phonon dispersions. Taking into account error in determining the conditions where a given surface is thermodynamically preferred, a relaxed defective (001) or (011) surface is shown to be preferred to any relaxed bulk-truncated (001) or (011) surface over a subset of temperatures and oxygen gas partial pressures. Thus, the existence is proven of (001) and (011) faces that cannot be relaxed bulk-truncated, e.g., are defective or reconstructed. Extending this work, the existence of a (111) face that is not relaxed bulk-truncated is addressed. The main finding, of faces that are not relaxed bulk-truncated, is discussed in the context of comparing predicted versus experimental crystallite thicknesses obtained from a data analysis.   

Short-Range Charge Transfer Between Oxide Based Superconductor-Ferromagnetic Metal Interfaces

Unlike the conventional superconductor (S) and ferromagnetic metal (F) interface, the understanding of the proximity effect between oxide-based S and F is still unclear. One particular question relates to the charge transfer length scale between S and F layers, which resulted from the lack of an appropriate experimental tool. In this talk, we show that by combining the cross-sectional scanning tunneling microscopy and spectroscopy (XSTM/S) along with scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS), the charge transfer length scale at the interfaces between YBa Cu O $_{\mathrm{-\delta }}$(YBCO) and La$_{3}$Ca$_{3}$MnO (LCMO) was revealed to have upper limit of 1 nm.\\[4pt] [1] Teyu Chien, et al., Nature Commun.4, 2336 (2013).   

The Origin of Giant Electrostriction in Gd-Doped Ceria as Studied by Modulation Excitation X-ray Absorption Spectroscopy

Electromechanical materials, such as piezoelectrics and electrostrictors, are ubiquitous. Recently, an unusually large electrostriction effect, which exceeds that of most common electrostrictors, was found in gadolinium-doped ceria thin films. It is likely to be explained by the dynamic response of oxygen vacancies to external electric field. Verifying this hypothesis is very challenging, as it is required to detect local atomic rearrangement at the 0.01 {\AA} scale. Conventional structural methods have neither elemental specificity nor spatial sensitivity to such structural changes. We applied Quick Extended X-Ray Absorption Fine Structure used in the Modulation Excitation mode to directly observe the dynamic response of the Ce and Gd local environments to the electric field. While using periodic stimulation of the films by electric field in situ, we detected X-ray absorption spectra at the Ce and Gd absorption edges, thus enhancing the sensitivity to electro-active species. Our model of electromechanical activity in this system attributes it to a relatively small population of Ce ions with anomalously short Ce-O bonds formed near the oxygen vacancies. This finding suggests that other oxides with a large concentration of vacancies may exhibit even larger electrostriction.   

Session J54: Mott Insulators and Fermi Fluids

Probing the World of Correlated electron systems: Materials Characterization and Neutron Scattering studies

An important field of experimental Condensed Matter Physics focuses on studying correlated electron systems including unconventional superconductors, iron-based superconductors and multifunctional systems such as multiferroic compounds. The use of various bulk measurement techniques to characterize the physical properties in these systems is an essential first step in revealing the novel electronic and magnetic ground states. Further studies using powerful microscopic probes such as neutron scattering methods are crucial in advancing the central theme in understanding correlated electron systems, which is to make the correlation between structure, magnetism and physical properties. As is common across correlated electron systems, highly degenerate ground states abound which are readily disturbed by chemical dopants and perturbing fields such as an applied magnetic field or pressure. Thus, studying these systems, using neutron scattering techniques in particular, in various extreme conditions reveals new emergent ground states with tunable magnetic, electronic or ferroelectric order parameters.   

Novel Itinerant Antiferromagnet With Nonmagnetic Constituents

While many systems exhibit both local and itinerant magnetism, only two are known to display magnetism while being composed of non-magnetic elements - Sc$_3$In and ZrZn$_2$. Drastic differences in dimensionality, critical scaling and susceptibility to perturbations suggest that more systems like this would be useful in identifying the origin of their magnetic properties. In this talk, the properties of a new itinerant antiferromagent with no magnetic constituents, are presented. Specific heat, resistivity and magnetization data indicate magnetic ordering below $T_N \approx$ 36 K. Above this temperature, the susceptibility displays Curie-Weiss-like behavior, with an unexpectedly large paramagnetic moment $\mu_{PM}\approx 0.8~\mu_B~F.U.^{-1}$. The magnetism is confirmed by band structure calculations, which suggest a spin-density wave ground state with a modulation wavevector $Q$ = $(0, 2\pi/3b, 0)$.   

Phase interference and sub-femtosecond time dynamics of resonant inelastic X-ray scattering from Mott insulators

Resonant inelastic X-ray scattering (RIXS) is a powerful technique for observing the energy states of many-body quantum materials. The core hole resonance states that make RIXS possible are strongly correlated, and undergo complex time evolution that shapes scattering spectra. However, current inelastic scattering measurements cannot be converted to a time resolved picture, because techniques that determine relative phase information from elastic scattering have not been adapted to the greater complexity of inelastic spectra. We will show that inelastic scattering phases can be identified from quantum interference in sharply resolved (dE $<$ 35meV) M-edge RIXS spectra of Mott insulators (e.g. SrCuO$_2$ and NiO), and provide new information for identifying excitation symmetries and many-body time dynamics.   

Novel magnetic state in $d^4$ Mott insulators

We show that the interplay of strong Hubbard interaction $U$ and spin-orbit coupling $\lambda$ in systems with the $d^4$ electronic configuration leads to several unusual magnetic phases. While in the atomic limit the system is in a non-magnetic $J=0$ singlet state, we find that the competition between superexchange and atomic spin-orbit coupling dramatically changes the local moment, which challenges the conventional wisdom that local moments are well-defined in a Mott insulator. Most notably, we find that in the Mott limit at strong $U$ there is a phase transition from a non-magnetic insulator of uncoupled $J=0$ singlets to an orbitally entangled ferromagnetic insulator. We identify candidate materials and present predictions for Resonant X-ray Scattering (RXS) signatures for the unusual magnetism in $d^4$ Mott insulators and contrast them with the well-studied $d^5$ case.   

Mott Glass Phase in the Disordered Quantum Spin Systems

We use quantum Monte Carlo method to study the disordered $S=1/2$ quantum spins on the square lattice with three different nearest neighbor interactions $J_{1}$, $J_{2}$ and $J_{3}$. Here $J_{1}$ represents weak bonds, and $J_{2}$ and $J_{3}$ correspond to stronger bonds which are randomly distributed on columnar rungs forming coupled 2-leg ladders. By tuning the average value of $J_{2}$ and $J_{3}$, the system undergoes N$\acute{e}$el-glass-paramagnetic quantum phase transition. A wide range of Mott glass phase has been found. We notice that its uniform susceptibility in the glass phase follows $ \chi\sim\exp(-b/T^{\alpha})$, where $0.5<\alpha<1$. Furthermore, this dimerized disordered quantum spin system shows the violation of Harris criterion.   

Fractionalized liquid states in doped Mott insulators

We study Mott insulator phases at specifically chosen doping levels on several lattices. Using analytical and numerical techniques we identify interesting quantum liquid phases with fractionalized excitations.   

Ultrafast dynamics of the nanoscale metal-insulator transition in VO$_2$

The metal-insulator transition (MIT) of vanadium dioxide (VO$_2$) exhibits a rich phase behavior involving two monoclinic (M1, M2), a triclinic, and a tetragonal phase that form a complex domain structure and lead to spatial inhomogeneities in the electronic transition. The interplay of these different phases with strain can affect the progress of both the thermal and photoinduced MIT. We report on nano-optical imaging of the MIT in individual microcrystals and thin films of VO$_2$, in order to probe the effects of substrate and crystallite strain, morphology, and orientation on the emergent metallic phase. We find a large variation in transition rates and dynamical behavior among single crystal microrods, indicating intrinsic inhomogeneities in the MIT.   

Iridate spin models: magnetism, 3D spin liquids and an infinite-D entanglement approximation

We present three-dimensional threefold-coordinated structures for iridates which may generate Kitaev-type magnetic exchanges. The resulting solvable 3D quantum spin liquid exhibits the uniquely 3D property of stability to finite temperature ($T_c \sim J_k/100$). Adding Heisenberg couplings spoils exact solubility; however, the large loop length $\ell$ of the lattice suggests an approximation with large $\ell \rightarrow \infty$. The Kitaev-Heisenberg model can be solved on the resulting Bethe lattice using tensor product states; we present the phase diagrams, finding multiple magnetic order parameters and identifying gapped spin liquid phases by an entanglement fingerprint.   

Ferromagnetism in flat bands and Pauli-correlated percolation

Flat-band ferromagnetism is an exotic case of itinerant-electron magnetism in a wide class of geometrically frustrated lattices. We develop an exact mapping between the ground state of the many-body problem and a novel site-percolation problem. This allows us to study the ferromagnetic transition using tools from equilibrium statistical physics. In the case of Hubbard model on the Tasaki lattice, we provide a complete and exact solution in 1D and show that for D $>$ 1, the paramagnetic phase persist beyond the uncorrelated percolation point, with a transition in the form of a first-order jump to an unsaturated ferromagnetic phase.   

Theory of spin density wave glasses

We study the effects of non-magnetic impurities on an easy-plane spin density wave at $\vec Q$ accompanied by a charge density wave at $2\vec Q$ in two and three dimensions. Even though any amount of disorder dramatically reduces both spin and charge correlations, spin nematic correlations remain essentially unaffected. This is due to the proliferation of only certain kinds of defects, leading to a uniaxial spin glass. The presence of a Goldstone mode distinguishes this phase from a conventional spin glass, and can serve as an experimental signature. Similarly, in superconductors with finite momentum pairing, a charge-4 condensate persists in the presence of weak disorder, while pair density wave order (the FFLO state) is lost due to impurities.   

Excitations of a Fermi fluid with coupled magnetic and nematic order parameters

We study possible stable phases of 2D and 3D Fermi fluids with coupled magnetic and nematic order parameters by considering appropriate Ginzburg-Landau free energies, and performing necessary minimizations. In Fermi liquid language, nematic order corresponds to L=2 distortions of the Fermi surface, so here we consider such distortions in a magnetically ordered Fermi fluid. We use Landau kinetic equation to study propagating collective modes and corresponding dispersions of the modes.   

Low-temperature evolution of the spectral weight of a spin-up carrier moving in a ferromagnetic background

The motion of a charged particle in a magnetically ordered background determines the electronic behavior of many weakly doped, magnetically ordered insulators and semiconductors. This problem can be solved exactly for a single charge carrier in a ferromagnetic background at T=0. There are two different cases to be considered (i) the carrier spin is oriented antiparallel with respect to the FM background and (ii) the carrier spin is aligned with the background. For the former case the solution is a spin-polaron, a dressed quasiparticle consisting of a charged particle and a bound magnon. For the latter case, on the other hand, the formation of a spin-polaron is impossible at T=0 due to the absence of spin-flip excitations. The T=0 spectrum of the spin-up carrier is therefore identical to that of a free carrier shifted by the $z$-part of the magnetic interaction. This changes at finite-T where thermal magnons are present in the system. To study this change, we derived the lowest-T correction to the self-energy of the spin-up carrier. This allows us to investigate how the T=0 quasiparticle peak broadens into a continuum at finite-T. Furthermore we find that spectral weight is shifted to energies outside of this continuum, which can be associated with the spin-polaron state.   

Plaquette ordered phase and quantum spin liquid in the spin-1/2 J1-J2 square Heisenberg model

We study the spin-1/2 Heisenberg model on the square lattice with first- and second-neighbor antiferromagnetic interactions $J_1$ and $J_2$. We use the density matrix renormalization group with implementing $SU(2)$ spin rotation symmetry and study the model accurately on open cylinders with different boundary conditions. With increasing $J_2$, we find a N\'{e}el phase, a plaquette valence-bond (PVB) phase with a finite spin gap, and a possible spin liquid in a small region of $J_2$ between these two phases. From the finite-size scaling of the magnetic order parameter, we estimate that the N\'{e}el order vanishes at $J_2/J_1\simeq 0.44$. For $0.5 < J_2/J_1 < 0.61$, we find dimer correlations and PVB textures whose decay length grows strongly with increasing system width, consistent with a long-range PVB order in the two-dimensional limit. The dimer-dimer correlation function reveals the s-wave character of the PVB order. For $0.44 < J_2/J_1 < 0.5$, both spin and dimer orders are weak on finite-size systems and appear to scale to zero with increasing system width, which is consistent with a possible SL or a near-critical behavior. We compare and contrast our results with earlier numerical studies.   

Fragile antiferromagnetic order in the heavy-fermion YbBiPt

YbBiPt is a heavy-fermion compound ($\gamma\approx$ 8 J/molK$^2$) possessing antiferromagnetic order below a N\'{e}el temperature of $T_{\rm{N}}$= 0.4 K, and a proposed quantum critical point at a magnetic field of $H_{\rm{c}}\approx$ 0.4 T. We report results from neutron scattering experiments on single crystals which characterize the antiferromagnetic order. The magnetic scattering is described in terms of two components: a narrow component that appears below $T_{\rm{N}}$ which corresponds to the onset of antiferromagnetic order observed in bulk thermodynamic and transport measurements, and a broad component corresponding to antiferromagnetic correlations extending over $\approx$ 20 $\rm{\AA}$ that persists up to $T^{\rm{*}}\approx$ 0.7 K. These results illustrate the unconventional nature of the magnetism in YbBiPt and may be a consequence of its competing low-energy magnetic interactions and proximity to a quantum critical point.\\ Work at the Ames Laboratory was supported by the Department of Energy, Basic Energy Sciences under Contract No. DE-AC02-07CH11358.   

Session J55: Invited Session: Buckley / McGroddy / Adler / IUPAP YSP/ Nicholson Prize Session

Oliver E. Buckley Prize: Graphene and Beyond

Graphene, a single atomic layer of graphite, has been provided physicists opportunities to explore an interesting analogy to the relativistic quantum mechanics. The unique electronic band structure of graphene lattice yields a linear energy dispersion relation where the Fermi velocity replaces the role of the speed of light. The exotic quantum transport behavior discovered in these materials including unusual half-integer, fractional and fractal quantum Hall effect owing to approximate SU(4) symmetry from spin and valley spin degree of freedom combined with the quasi relativistic dispersion relation. In this presentation I will discuss the exotic quantum transport behavior discovered in graphene and its nanostructures. In addition, I will discuss the new type of material classes based on 2-dimensional van der Waal materials and their heterostructures extending the graphene based research into quasi 3-dimensional systems.   

James C. McGroddy Prize: Piezotronics of ZnO Nanomaterials

Piezoelectricity, a phenomenon known for centuries, is an effect that is about the production of electrical potential in a substance as the pressure on it changes. Wurtzite structures such as ZnO, GaN, InN and ZnS, due to the polarization of ions in a crystal that has non-central symmetry, a piezoelectric potential (\textit{piezopotential}) is created in the crystal by applying a stress. The effect of piezopotential to the transport behavior of charge carriers is significant due to their multiple functionalities of piezoelectricity, semiconductor and photon excitation. By utilizing the advantages offered by these properties, a few new fields have been created. Electronics fabricated by using inner-crystal piezopotential as a ``gate'' voltage to tune/control the charge transport behavior is named \textit{piezotronics}, with applications in strain/force/pressure triggered/controlled electronic devices, sensors and logic units. \textit{Piezo-phototronic effect} is a result of three-way coupling among piezoelectricity, photonic excitation and semiconductor transport, which allows tuning and controlling of electro-optical processes by strain induced piezopotential. The objective of this talk is to introduce the fundamentals of piezotronics and piezo-phototronics and to give an updated progress about their applications in energy science (LED, solar) and sensors (photon detector and human-CMOS interfacing).\\[4pt] [1] W.Z. Wu, X.N. Wen, Z.L. Wang ``Pixel-addressable matrix of vertical-nanowire piezotronic transistors for active/adaptive tactile imaging,'' Science, 340 (2013) 952-957.\\[0pt] [2] C.F. Pan, L. Dong, G. Zhu, S. Niu, R.M. Yu, Q. Yang, Y. Liu, Z.L. Wang* ``Micrometer-resolution electroluminescence parallel-imaging of pressure distribution using piezoelectric nanowire-LED array,'' Nature Photonics, 7 (2013) 752-758.\\[0pt] [3] Z.L. Wang ``Piezopotential Gated Nanowire Devices: Piezotronics and Piezo-phototronics,'' Nano Today, 5 (2010) 540-552.   

David Adler Lectureship Award: A Chance to Grow

Having a chance to grow has been a vital, key, aspect to my research career. A successful condensed matter, new materials group thrives when it can have multiple make-measure-think cycles running in parallel and series. The ability to explore phase space and design, discover and grow new compounds is the starting point for many research projects and, sometimes, new fields. In this talk I want to provide an overview of several of the motivations that can lead to sample growth and also provide some examples of how new materials can lead to the intellectual / technical growth of a group as well. Examples will be drawn, as time allows, from work on magnetic, non-magnetic, low-Tc, and high Tc superconductors as well as heavy Fermions, spin-glasses and quasicrystals.   

IUPAP Young Scientist Prize: Neutron scattering in magnetism

.   

Dwight Nicholson Medal Lecture: Science and Society

I will present some background as to the current ``scientific state'' of our society and some ideas of how we got into the fix we are in. I will then describe The Physics Force a program we developed to popularize physics. It has proven to be a very successful and entertaining outreach program of the College of Science and Engineering in the University of Minnesota developed to make science exciting and fun for students of all ages, from 6 to 106. The Force performed variations of The Physics Circus, our most popular show, at Disney's Epcot Center, parts of it were shown on Newton's Apple and several of us have performed demonstrations on the Knoff-Hoff Show, a very successful German T.V. science program. The goal of The Physics Force is to show students and the public Science is Fun, Science is Interesting, and Science is Understandable. By all measures we have available, we are extremely successful in reaching our goals. In the last three year cycle of our University support about 110,000 residents of Minnesota (or about 2{\%} of the total population) saw a Physics Force performance; over the last decade the total is around 250,000!   

Session J56: Invited Session: Dillon Medal Symposium

John H. Dillon Medal Lecture: Buckling Instabilities of Polymer Multilayers

Soft polymer networks, such as gels and elastomers, can undergo a wide variety of geometry-dependent mechanical shape instabilities when subjected to compressive stress, providing opportunities to tailor the structure and properties of stimuli-responsive materials. These include global buckling modes of unsupported sheets as well as local surface modes such as wrinkling and creasing. The introduction of two or more elastic layers provides a multi-dimensional parameter space in terms of the contrasts in stiffness, geometry, and pre-strain between the layers, yielding a rich landscape of behaviors. Our group has recently focused on two examples. In the first case, we consider the role of mismatch strain in creasing and post-wrinkling bifurcations, which allows for fine control over both the types of surface modes that appear and their hysteretic behavior. In the second case, we consider the buckling of unsupported elastic trilayers as a route to define self-folding and responsive three-dimensional objects.   

Light-induced sequential self-folding of pre-strained polymer sheets

Self-folding is a self-assembly process that causes a predefined 2D template to fold into a desired 3D structure with high fidelity.$^{\, \, }$We have developed a simple method of self-folding that uses predefined ink ``hinges'' printed onto pre-strained polymer sheets via a desk top printer. The ink absorbs external light, causing the area underneath the hinge to heat up and relax the strain in the hinge regions gradually across the sheet thickness. This process results in folding the sheet at the hinge region. We will demonstrate that sequential folding of multiple hinges on the same sample can be programmed by changing the light source and ink color of the hinge. We have successfully employed this strategy to produce complex origami shapes.   

Snap-through instabilities of curved folds on curved, polymer shells

Snap-through instabilities are commonly associated with the catastrophic failure of an elastic structure; yet in nature, snap-through instabilities are also used to execute fast motions. Inspired by origami, the ancient art of paper folding, we show that ``folds'' can be introduced on shells by the local thinning of material. These folds can either be activated continuously or can snap-through to a geometrically determined angle, depending on the delicate interplay between the curvature of the shell and the shape of the fold. We describe how geometry can (and sometimes cannot) be used to control the dynamics of foldable shells.   

Capillary leveling of stepped films with inhomogeneous molecular mobility

The simple geometry of a polymer film on a substrate with a step at the free surface is unfavourable due to the excess interface induced by the step. Above the glass transition Laplace pressure gradients will drive flow, thus providing an excellent probe for nano-rheology. Here we recap some of our recent progress on the capillary leveling of stepped films. In particular, we present new studies on polymeric samples with precisely controlled, spatially inhomogeneous molecular weight distributions.   

Wrinkling vs. scarring: Stress collapse in surface-confined assemblies

Confining assemblies to surfaces possessing Gaussian curvature frustrates the microscopic order of the packing, thus introducing mechanical costs for assemblies in such diverse contexts as viral capsids and particle-coated drops. The structure and stability of these systems is complicated by the non-trivial competition between distinct modes of stress relaxation, including ``elastic" shape deformation of the surface-bound assembly; defect-mediated ``plastic" reorganization of packing. We consider the interplay between these shape-deformation and defect-relaxation for a model of crystalline patch bound to an adhesive and deformable sphere, where the distinct patterns of relaxation become, respectively, radial chains of dislocations, or ``scars", and radial wrinkles. Analysis of highly-wrinkled and defect-riddle states reveals remarkably that both modes achieve the {\it identical} mechanical state in the limits of vanishing thickness and lattice spacing, and further, that the degeneracy between these modes is lifted only by the microscopic and sub-dominant energetics that select their optimal symmetry. We present a structural relaxation phase diagram that predicts a wrinkle-to-scar transition driven both by increasing substrate stiffness and substrate curvature.   

Using theory and simulation to link molecular features of nanoscale fillers to morphology in polymer nanocomposites

Polymer nanocomposites are a class of materials that consist of a polymer matrix embedded with nanoscale fillers or additives that enhance the inherent properties of the matrix polymer. To engineer polymer nanocomposites for specific applications with target macroscopic properties (e.g. photovoltaics, photonics, automobile parts) it is important to have design rules that relate molecular features to equilibrium morphology of the composite. In the first part of the talk I will present our recent theory and simulation work on composites containing polymer grafted nanoparticles, showing how polydispersity in graft and matrix polymers (physical heterogeneity) can be used to stabilize dispersion of the nanoparticles within a polymer matrix. In the second part of the talk I will present our recent work linking block-copolymer functionalization to the nanoparticle location in a polymer matrix consisting of homopolymer blends.   

Entropically-Driven Destabilization of Nanoparticle Crystals

Detailed computer simulations show that polymer-induced depletion forces can cause nanoparticles (NPs) in the athermal limit to crystallize, but only for short polymers at melt-like densities. For long chains, this depletion-induced attraction is in competition with the entropy loss associated with confining a polymer chain within the cavities in the NP crystal. For chains larger than these voids, these crystals are unstable and the NPs form an aggregated, but non-crystalline structure. The experimental results of Mackay et al. [\textit{Science }311:1740, 2006], who find immiscibility for chains smaller than the NP radii but miscibility for larger chains, thus probably reflect this physics, which we show to only be important when the osmotic pressure driving the polymer chains into the NP crystal is relatively large.   

Anomalous Kinetics in Reactive Polymer Glasses

Image formation in modern lithographic processes is based on the acid-catalyzed deprotection of glassy polymer films. It is well-established that slow acid diffusion controls the reaction kinetics, but models based on Fickian transport coupled with a first-order reaction cannot describe experimental data. We studied the acid-catalyzed deprotection of glassy poly(hydroxystyrene-co-tertbutyl acrylate) films using infrared absorbance spectroscopy and stochastic simulations. Experimental data were interpreted with a model that explicitly accounts for acid transport, where heterogeneities at local length scales are introduced through a non-exponential distribution of waiting times between successive hopping events. Subdiffusive behavior predicts key attributes of the observed deprotection rates, such as fast reaction at short times, slow reaction at long times, and a non-linear dependence on acid loading. This transport model is consistent with other literature studies of probe diffusion in inert glasses. We highlight the complex behavior in photoresists by changing the size of acid-counterion pairs, incorporating plasticizer, and reducing film thickness. These data can facilitate the development of predictive lithography models that reflect the behavior of confined polymers.   

Regular and Irregular Mixing in Hydrocarbon Block Copolymers

Since hydrocarbon polymers interact through relatively simple (dispersive) interactions, one might expect them to be described by simple models of mixing energetics, such as regular mixing. However, the pioneering work of Graessley on saturated hydrocarbon polymer blends showed that while regular mixing is obeyed in some cases, both positive and negative deviations (in the magnitude of the mixing enthalpy) from regular mixing are observed in other cases. Here, we describe the mixing energetics for two series of hydrocarbon polymers wherein the interaction strengths may be continuously tuned, and which can be readily incorporated into block copolymers. Random copolymers of styrene and medium-vinyl isoprene, in which either the isoprene or both the isoprene and styrene units have been saturated, obey regular mixing over the entire composition range and for both hydrogenated derivatives. Well-defined block copolymers with arbitrarily small interblock interaction strengths can be constructed from these units, permitting the interdomain spacing to be made arbitrarily large while holding the order-disorder transition temperature constant. However, block copolymers of hydrogenated polybutadiene with such random copolymers show very strong positive deviations from regular mixing when the styrene aromaticity is preserved, and sizable negative deviations when the styrene units are saturated to vinylcyclohexane. Both of these cases can be quantitatively described by a ternary mixing model.   

Thermally Switchable Aligned Nanopores by Magnetic-Field Directed Self-Assembly of Block Copolymers

Magnetic fields provide a facile approach to direct the self-assembly of magnetically anisotropic block copolymer nanostructures in a scalable manner. Here we combine such field-based processing with materials design to enable the fabrication of polymer films with highly aligned stimuli-responsive nanopores. Etch removal of a poly(D,L-lactide) (PLA) brush that is the minority component of a liquid crystalline block copolymer is used to produce nanopores of $\sim$ 8 nm diameter. The pores can be reversibly closed and opened while retaining their alignment by appropriate heating and cooling. We present TEM and temperture resolved scattering data during pore closure and re-opening to explore the mechanism and kinetics of pore collapse.   

Tuning the theta temperature and critical micellization temperature of polymers in ionic liquids

Ionic liquids feature a combination of properties that make them very interesting solvents for polymers, but questions remain regarding the thermodynamics of polymer/ionic liquid solutions. In this work, the lower-critical-solution-temperature (LCST) phase behavior of poly($n$-butyl methacrylate) (PnBMA) in mixtures of the ionic liquids 1-butyl-3-methylimidazolium: bis(trifluoromethylsulfonyl)imide ([BMIm][TFSI]) and 1-ethyl-3-methylimidazolium:TFSI ([EMIm][TFSI]) is characterized by transmittance, light scattering, and small-angle neutron scattering measurements. Relevant thermodynamic parameters are readily tuned by varying the ionic liquid composition. In particular, the cloud point, spinodal, and theta temperatures are all found to increase linearly with [BMIm] content. The interaction parameters are determined as a function of temperature and concentration using three different methods, and the results from each method are compared. The theta temperatures are then compared quantitatively to the critical micellization temperatures (CMTs) for PnBMA-poly(ethylene oxide) diblocks, to test the proposition that the CMT corresponds to a fixed value of chi.   

Design of P3HT-g-P2VP Graft Copolymers as Efficient Compatibilizers for Stable Operation of Polymer Solar Cells (PSCs)

Fabrication of ordered structures from block copolymers of conjugated polymers has been limited due to the strong rod$-$rod interactions between the conjugated blocks. In this work, we developed a molecular design of conjugated polymer-based graft copolymers to control the rigidity of the copolymers and to produce a variety of ordered nanostructures. A series of well-defined poly(3-hexylthiophene)-graft-poly(2-vinylpyridine) (P3HT-g-P2VP) copolymers in which the P2VP chains had different molecular weights (M$_{n})$ was prepared. As the M$_{n}$ of the grafted P2VP chains increased, the crystallinity of the P3HT block in the copolymers decreased. Therefore, we produced well-ordered, non-fibrillar nanostructures of P3HT-based copolymers. In addition, P3HT-g-P2VP polymers can be used as efficient compatibilizers in the active layer of PSCs. P3HT-g-P2VP polymers can modify the sharp interface between polymer donors and fullerenes, resulting in dramatic enhancement in the thermal stabilities and mechanical properties of PSCs. The effectiveness of the graft copolymers as compatibilizers will be demonstrated by comparing them with the P3HT-b-P2VP block copolymers.