Topological Phonon Modes in a Two-Dimensional Wigner Crystal

We investigate the spin-orbit coupling effect in a two-dimensional(2D)Wigner crystal.It is shown that sufficiently strong spin-orbit coupling and an appropriate sign of g-factor could transform the Wigner crystal to a topological phonon system.We demonstrate the existence of chiral phonon edge modes in finite size samples,as well as the robustness of the modes in the topological phase.We explore the possibility of realizing the topological phonon system in 2D Wigner crystals confined in semiconductor quantum wells/heterostructure.It is found that the spin-orbit coupling is too weak for driving a topological phase transition in these systems.It is argued that one may look for topological phonon systems in correlated Wigner crystals with emergent effective spin-orbit coupling.     

Effect of quantum fluctuations on the dipolar motion of bose-einstein condensates in optical lattices

We reexamine dipolar motion of condensate atoms in one-dimensional optical lattices and harmonic magnetic traps including quantum fluctuations within the truncated Wigner approximation. In the strong tunneling limit we reproduce the mean field results with a sharp dynamical transition at the critical displacement. When the tunneling is reduced, on the contrary, strong quantum fluctuations lead to finite damping of condensate oscillations even at infinitesimal displacement. We argue that there is a smooth crossover between the chaotic classical transition at finite displacement and the superfluid-to-insulator phase transition at zero displacement. We further analyze the time dependence of the density fluctuations and of the coherence of the condensate and find several nontrivial dynamical effects, which can be observed in the present experimental conditions.     

First-principles prediction of a decagonal quasicrystal containing boron

We interpret experimentally known B-Mg-Ru crystals as quasicrystal approximants whose deterministic decoration of tiles by atoms can be extended quasiperiodically. Experimentally observed disorder corresponds to phason fluctuations. First-principles total energy calculations find many distinct tilings close to stability and suggest a phase transition from a crystalline state at low temperatures to a high temperature state characterized by tile fluctuations. We predict B38Mg17Ru45 forms a metastable decagonal quasicrystal that may be thermodynamically stable at high temperatures.     

Wigner crystal in snaked nanochannels: Outlook

We study properties of Wigner crystal in snaked nanochannels and show that they are characterized by a conducting sliding phase at low charge densities and an insulating pinned phase emerging above a certain critical charge density. We trace parallels between this model problem and the Little suggestion for electron transport in organic molecules. We also show that in the presence of periodic potential inside the snaked channel the sliding phase exists only inside a certain window of electron densities that has similarities with a pressure dependence of conductivity in organic conductors. Our studies show emergence of dynamical glassy phase in a purely periodic potential in the absence of any disorder that can explain enormously slow variations of resistivity in organic conductors. Finally we discuss the KAM concept of superfluidity induced by repulsive Coulomb interaction between electrons. We argue that the transition from the sliding KAM phase to the pinned Aubry phase corresponds to the superfluid-insulator transition.     

Persistent currents for Coulomb interacting electrons on 2d disordered lattices

A Wigner crystal structure of the electronic ground state is induced by strong Coulomb interactions at low temperature in clean or disordered two-dimensional (2d) samples. For fermions on a mesoscopic disordered 2d lattice, being closed to a torus, we study the persistent current in the regime of strong interaction at zero temperature. We perform a perturbation expansion starting from the Wigner crystal limit which yields power laws for the dependence of the persistent current on the interaction strength. The sign of the persistent current in the strong interaction limit is independent of the disorder realization and strength. It depends only on the electro-statically determined configuration of the particles in the Wigner crystal. Received 14 March 2000     

Distribution of the delay time and the dwell time for wave reflection from a long random potential

We re-examine and correct an earlier derivation of the distribution of the Wigner phase delay time for wave reflection from a long one-dimensional disordered conductor treated in the continuum limit. We then numerically compare the distributions of the Wigner phase delay time and the dwell time, the latter being obtained by the use of an infinitesimal imaginary potential as a clock, and investigate the effects of strong disorder and a periodic (discrete) lattice background. We find that the two distributions coincide even for strong disorder, but only for energies well away from the band-edges. Received 11 June 2001 and Received in final form 30 July 2001     

Anderson localization in a two-dimensional random gap model

We study the properties of the spinor wavefunction in a strongly disordered environment on a two-dimensional lattice. By employing a transfer-matrix calculation we find that there is a transition from delocalized to localized states at a critical value of the disorder strength. We prove that there exists an Anderson localized phase with exponentially decaying correlations for sufficiently strong scattering. Our results indicate that suppressed backscattering is not sufficient to prevent Anderson localization of surface states in topological insulators.     

Finite-temperature Wigner solid and other phases of ripplonic polarons on a helium film

Electrons on liquid helium can form different phases depending on density, and temperature. Also the electron-ripplon coupling strength influences the phase diagram, through the formation of so-called “ripplonic polarons”, that change how electrons are localized, and that shifts the transition between the Wigner solid and the liquid phase. We use an all-coupling, finite-temperature variational method to study the formation of a ripplopolaron Wigner solid on a liquid helium film for different regimes of the electron-ripplon coupling strength. In addition to the three known phases of the ripplopolaron system (electron Wigner solid, polaron Wigner solid, and electron fluid), we define and identify a fourth distinct phase, the ripplopolaron liquid. We analyse the transitions between these four phases and calculate the corresponding phase diagrams. This reveals a reentrant melting of the electron solid as a function of temperature. The calculated regions of existence of the Wigner solid are in agreement with recent experimental data.     

Mass-asymmetry effects in coupled electron-hole quantum wire system

The effect of mass-asymmetry on the ground-state of coupledelectron-hole quantum wire system is investigated within thequantum version of the self-consistent mean-field approximationof Singwi, Tosi, Land, and Sjölander. The pair-correlationfunctions, static density susceptibility, and correlation energy arecalculated over a range of wire parameters.We find that themass-asymmetry affects appreciably both the intra- and inter-wire correlations,which in turn bring in a marked change in the e-h ground-state.Below a critical density, the e-h correlations now favor theliquid-Wigner crystal phase transition at a sufficiently large wirespacing. This result is in striking difference with the correspondingstudy on the mass-symmetric e-h wire model since here transition to theWigner crystal phase occurs in the adequately closeproximity of two wires at a much lower density, and there also occurs acrossover from Wigner to a charge-density-wave phase at relatively higherdensities. We find that for a GaAsbased e-h wire the critical density for Wignercrystallization is enhanced by a factor of about 2.6.As an important result, our theory captures nicely the recentexperimental observation of Wigner crystallization in anun-equal density GaAs based e-h wire by Steinberg et al.[Phys. Rev. B 73, 113307 (2006)].     

Universal aspects of coulomb-frustrated phase separation

We study the consequences of Coulomb interactions on a system undergoing a putative first order phase transition. In two dimensions (2D), near the critical density, the system is universally unstable to the formation of new intermediate phases, which we call "electronic microemulsion phases," which consist of an intermediate scale mixture of regions of the two competing phases. A corollary is that there can be no direct transition as a function of density from a 2D Wigner crystal to a uniform electron liquid. In 3D, if the strength of the Coulomb interactions exceeds a critical value, no phase separation occurs, while for the weaker Coulomb strength electronic microemulsions are inevitable. This tendency is considerably more pronounced in anisotropic (quasi-2D or quasi-1D) systems, where a devil's staircase of transitions is possible.     

Freezing of Classical Fluids in a Quenched Random Potential

The phase diagram of a hard-sphere fluid in the presence of a random pinning potential is studied analytically and numerically. In the analytic work, replicas are introduced for averaging over the quenched disorder, and the hypernetted chain approximation is used to calculate density correlations in the replicated liquid. The freezing transition of the liquid into a nearly crystalline state is studied using a density-functional approach, and the liquid to glass transition is studied using a phenomenological replica symmetry breaking approach. In the numerical work, local minima of a discretized version of the Ramakrishnan-Yussouff free-energy functional are located and the phase diagram in the density-disorder plane is obtained from an analysis of the relative stability of these minima. Both approaches lead to similar results for the phase diagram. The first-order liquid to crystalline solid transition is found to change to a continuous liquid to glass transition as the strength of the disorder is increased above a threshold value.     

Moving Wigner glasses and smectics: dynamics of disordered Wigner crystals

We examine the dynamics of driven classical Wigner solids interacting with quenched disorder from charged impurities. For strong disorder, the initial motion is plastic, in the form of crossing winding channels. For increasing drive, there is a reordering into a moving Wigner smectic with the electrons moving in separate 1D channels. These different dynamic phases can be related to the conduction noise and I(V) curves. For strong disorder, we show criticality in the voltage onset just above depinning. We obtain the dynamic phase diagram for driven Wigner solids and demonstrate a finite threshold of force for transverse sliding, recently observed experimentally.     

Multiple first-order metamagnetic transitions and quantum oscillations in ultrapure Sr3Ru2O7

We present measurements on ultraclean single crystals of the bilayered ruthenate metal Sr3Ru2O7, which has a magnetic-field-tuned quantum critical point. Quantum oscillations of differing frequencies can be seen in the resistivity both below and above its metamagnetic transition. This frequency shift corresponds to a small change in the Fermi surface volume that is qualitatively consistent with the small moment change in the magnetization across the metamagnetic transition. Very near the metamagnetic field, unusual behavior is seen. There is a strong enhancement of the resistivity in a narrow field window, with a minimum in the resistivity as a function of temperature below 1 K that becomes more pronounced as the disorder level decreases. The region of anomalous behavior is bounded at low temperatures by two first-order phase transitions. The implications of the results are discussed.     

Theory of the vortex matter transformations in high-Tc superconductor YBCO

Flux line lattice in type II superconductors undergoes a transition into a "disordered" phase such as vortex liquid or vortex glass, due to thermal fluctuations and random quenched disorder. We quantitatively describe the competition between the thermal fluctuations and the disorder using the Ginzburg-Landau approach. The following T-H phase diagram of YBCO emerges. There are just two distinct thermodynamical phases, the homogeneous and the crystalline one, separated by a single first order transition line. The line, however, makes a wiggle near the experimentally claimed critical point at 12 T. The "critical point" is reinterpreted as a (noncritical) Kauzmann point in which the latent heat vanishes and the line is parallel to the T axis. The magnetization, the entropy, and the specific heat discontinuities at melting compare well with experiments.     

Random anisotropy nematic model: Connection with experimental systems

We study theoretically the phase behavior of the continuum Random Anisotropy Nematic model. A domain-type pattern is assumed to appear in a distorted nematic liquid crystal (LC) phase. We map the model parameters to physical quantities characterizing LCs confined to Controlled-Pore Glasses and LC-aerosil dispersions. The domain size dependence on the disorder strength is obtained in accordance with the Imry-Ma prediction. The model estimates for temperature shifts of the paranematic-nematic phase transition and for the critical point, where this transition ceases to exist, are compared to the available experimental results.Received: 28 March 2004, Published online: 29 June 2004PACS: 61.30.-v Liquid crystals - 61.30.Dk Continuum models and theories of liquid crystal structure - 61.30.Gd Orientational order of liquid crystals; electric and magnetic field effects on order - 61.30.Hn Surface phenomena: alignment, anchoring, anchoring transitions, surface-induced layering, surface-induced ordering, wetting, prewetting transitions, and wetting transitions     

Quantum criticality between topological and band insulators in 3+1 dimensions

Four-component massive and massless Dirac fermions in the presence of long range Coulomb interaction and chemical potential disorder exhibit striking fermionic quantum criticality. For an odd number of flavors of Dirac fermions, the sign of the Dirac mass distinguishes the topological and the trivial band insulator phases, and the gapless semimetallic phase corresponds to the quantum critical point that separates the two. Up to a critical strength of disorder, the semimetallic phase remains stable, and the universality class of the direct phase transition between two insulating phases is unchanged. Beyond the critical strength of disorder the semimetallic phase undergoes a phase transition into a disorder controlled diffusive metallic phase, and there is no longer a direct phase transition between the two types of insulating phases.     

硼酸锂系列晶体的高压拉曼散射研究

本文进行了硼酸锂系列晶体的高压拉曼散射及其压致相变的研究。对于三硼酸锂（ＬｉＢ３Ｏ５），我们发现在５．０ＧＰａ有一可逆的晶态到晶态的相变，在２７．０ＧＰａ有一不可逆的晶态到非晶态的相变。二硼酸锂（Ｌｉ２Ｂ４Ｏ７）不可逆压致非晶相变发生在３２．０ＧＰａ附近。对于一硼酸锂，我们研究了０—５５．８ＧＰａ范围内的高压拉曼光谱，只在２．０ＧＰａ发现了一个晶态到晶态的相变，但未发现不可逆压致非晶化现象。在硼酸锂系列晶体中，不可逆压致非晶化的压力随Ｌｉ２Ｏ的含量的增加而升高。硼酸锂晶体中Ｌｉ２Ｏ的含量越高，压致非晶化越不容易发生，这与熔体急冷法制备硼酸锂玻璃的规律是一致的。     

Continuous paranematic-to-nematic ordering transitions of liquid crystals in tubular silica nanochannels

The optical birefringence of rodlike nematogens (7CB, 8CB), imbibed in parallel silica channels with 10 nm diameter and 300 microm length, is measured and compared to the thermotropic bulk behavior. The orientational order of the confined liquid crystals, quantified by the uniaxial nematic ordering parameter, evolves continuously between paranematic and nematic states, in contrast to the discontinuous isotropic-to-nematic bulk phase transitions. A Landau-de Gennes model reveals that the strength of the orientational ordering fields, imposed by the silica walls, is beyond a critical threshold, that separates discontinuous from continuous paranematic-to-nematic behavior. Quenched disorder effects, attributable to wall irregularities, leave the transition temperatures affected only marginally, despite the strong ordering fields in the channels.     

Wigner crystal in snaked nanochannels

We study the properties of a Wigner crystal in snaked nanochannels and show that they are characterized by a conducting sliding phase at low charge densities and an insulating pinned phase above a critical charge density. The transition between these phases has a devil’s staircase structure typical for the Aubry transition in dynamical maps and the Frenkel-Kontorova model. We discuss the implications of this phenomenon for charge density waves in quasi-one-dimensional organic conductors and for supercapacitors in nanopore materials.     

Coupling-matrix approach to the Chern number calculation in disordered systems

The Chern number is often used to distinguish different topological phases of matter in two-dimensional electron systems. A fast and efficient coupling-matrix method is designed to calculate the Chern number in finite crystalline and disordered systems. To show its effectiveness, we apply the approach to the Haldane model and the lattice Hofstadter model, and obtain the correct quantized Chern numbers. The disorder-induced topological phase transition is well reproduced, when the disorder strength is increased beyond the critical value. We expect the method to be widely applicable to the study of topological quantum numbers.