Quantum and classical phase transitions in double-layer quantum Hall ferromagnets

We consider the problem of quantum and classical phase transitions in double-layer quantum Hall systems at $\nu=1/m$ (m odd integers) from a long-wavelength statistical mechanics viewpoint. We derive an explicit mapping of the long-wavelength Lagrangian of the quantum Hall system into that of a three-dimensional isotropic classical XY model whose coupling constant depends on the quantum fluctuation in the original quantum Hall Hamiltonian. Universal properties of the quantum phase transition at the critical layer separation are completely determined by this mapping. The dependence of the Kosterlitz-Thouless transition temperature on layer separation, including quantum fluctuation effects, is approximately obtained by simple finite-size scaling analyses.Comment: 12 pages, RevTeX, figures include

Towards Fine-Grained Prosody Control for Voice Conversion

In a typical voice conversion system, prior works utilize various acoustic features (e.g., the pitch, voiced/unvoiced flag, aperiodicity) of the source speech to control the prosody of generated waveform. However, the prosody is related with many factors, such as the intonation, stress and rhythm. It is a challenging task to perfectly describe the prosody through acoustic features. To deal with this problem, we propose prosody embeddings to model prosody. These embeddings are learned from the source speech in an unsupervised manner. We conduct experiments on our Mandarin corpus recoded by professional speakers. Experimental results demonstrate that the proposed method enables fine-grained control of the prosody. In challenging situations (such as the source speech is a singing song), our proposed method can also achieve promising results

Coulomb drag between disordered two-dimensional electron gas layers

We derive and evaluate expressions for the frictional Coulomb drag between disordered two-dimensional electron gas layers. Our derivation is based on the memory-function formalism and the expression for the drag reduces to previously known results in the ballistic limit. We find that Coulomb drag is appreciably enhanced by disorder at low temperatures when the mean-free-path within a layer is comparable to or shorter than the layer separation. In high mobility two-dimensional electron gas systems, where the drag has been studied experimentally, the effect of disorder on the drag is negligible at attainable temperatures. We predict that an enhancement due to disorder and a crossover in the temperature-dependence and layer-separation dependence will be observable at low temperatures in moderate and low mobility samples.Comment: 17 pages, revtex, iucm93-00

The Hofstadter Spectrum and Photoluminescence

The observability of the Hofstadter spectrum generated by a Wigner crystal using photoluminescence techniques is studied. Itinerant hole geometries are examined, in which a hole may combine directly with electrons in the lattice. It is found that when the effect of lattice distortions of the WC due to interactions with the hole are accounted for, only the largest Hofstadter gaps are observable. To overcome the problems of lattice distortion, a novel geometry is proposed, involving a two layer system with electrons in one layer forming a WC and in the other a full Landau level. It is found that recombination of electrons in the full Landau level with {\it localized} holes reflects the full Hofstadter spectrum of the lattice.Comment: 13 pages in RevTex, figures available upon reques

Wigner crystal states for the two-dimensional electron gas in a double quantum well system

Using the Hartree-Fock approximation, we calculate the energy of different Wigner crystal states for the two-dimensional electron gas of a double quantum well system in a strong magnetic field. Our calculation takes interlayer hopping as well as an in-plane magnetic field into consideration. The ground The ground state at small layer separations is a one-component triangular lattice Wigner state. As the layer separation is increased, the ground state first undergoes a transition to two stacked square lattices, and then undergoes another transition at an even larger layer separation to a two-component triangular lattice. The range of the layer separation at which the two-component square lattice occurs as the ground state shrinks, and eventually disappears, as the interlayer hopping is increased. An in-plane magnetic field induces another phase transition from a commensurate to a incommensurate state, similar to that of $\nu=1$ quantum Hall state observed recently. We calculate the critical value of the in-plane field of the transition and find that the anisotropy of the Wigner state, {\it i.e.,}, the relative orientation of the crystal and the in-plane magnetic field, has a negligible effect on the critical value for low filling fractions. The effect of this anisotropy on the low-lying phonon energy is discussed. A novel exerimental geometry is proposed in which the parallel magnetic field is used to enhance the orientational correlations in the ground state when the crystal is subject toa random potential.Comment: RevTex 3.0, 22pages, 3figures available upon request. ukcm-xxx

Unusual temperature dependent resistivity of a semiconductor quantum wire

We calculate the electronic resistivity of a GaAs-based semiconductor quantum wire in the presence of acoustic phonon scattering. We find that the usual Drude-Boltzmann transport theory leads to a low temperature activated behavior instead of the well-known Bloch-Gr\"uneisen power law. Many-body electron-phonon renormalization, which is entirely negligible in higher dimensional systems, has a dramatic effect on the low temperature quantum wire transport properties as it qualitatively modifies the temperature dependence of the resistivity from the exponentially activated behavior to an approximate power law behavior at sufficiently low temperatures.Comment: 11 pages, RevTex, 4 figures. Also available at http://www-cmg.physics.umd.edu/~lzheng/preprint

The Distance Coloring of Graphs

Let $G$ be a connected graph with maximum degree $\Delta \ge 3$. We investigate the upper bound for the chromatic number $\chi_\gamma(G)$ of the power graph $G^\gamma$. It was proved that $\chi_\gamma(G) \le\Delta\frac{(\Delta-1)^{\gamma}-1}{\Delta-2}+1=:M+1$ with equality if and only $G$ is a Moore graph. If $G$ is not a Moore graph, and $G$ holds one of the following conditions: (1) $G$ is non-regular, (2) the girth $g(G) \le 2\gamma-1$, (3) $g(G) \ge 2\gamma+2$, and the connectivity $\kappa(G) \ge 3$ if $\gamma \ge 3$, $\kappa(G) \ge 4$ but $g(G) >6$ if $\gamma =2$, (4) $\Delta$ is sufficiently large than a given number only depending on $\gamma$, then $\chi_\gamma(G) \le M-1$. By means of the spectral radius $\lambda_1(G)$ of the adjacency matrix of $G$, it was shown that $\chi_2(G) \le \lambda_1(G)^2+1$, with equality holds if and only if $G$ is a star or a Moore graph with diameter 2 and girth 5, and $\chi_\gamma(G) < \lambda_1(G)^\gamma+1$ if $\gamma \ge 3$

Canted antiferromagnetic and spin singlet quantum Hall states in double-layer systems

We present details of earlier studies (Zheng et al, Phys. Rev. Lett. 78, 310 (1997) and Das Sarma et al, ibid 79, 917 (1997)) and additional new results on double-layer quantum Hall systems at a total filling \nu = 2 \nu_1, where a single layer at filling \nu_1 forms a ferromagnetic, fully spin-polarized, gapped incompressible quantum Hall state. For the case \nu_1 = 1, a detailed Hartree-Fock analysis is carried out on a realistic, microscopic Hamiltonian. Apart from the state continuously connected to the ground state of two well separated layers, we find two double-layer quantum Hall phases: one with a finite interlayer antiferromagnetic spin ordering in the plane orthogonal to the applied field (the `canted' state), and the other a spin singlet. The quantum transitions between the various quantum Hall states are continuous, and are signaled by the softening of collective intersubband spin density excitations. For the case of general \nu_1, closely related results are obtained by a semi-phenomenological continuum quantum field theory description of the low-lying spin excitations using a non-linear sigma model. Because of its broken symmetry, the canted phase supports a linearly dispersing Goldstone mode and has a finite temperature Kosterlitz-Thouless transition. We present results on the form of the phase diagram, the magnitude of the canted order parameter, the collective excitation dispersions, the specific heat, the form of the dynamic light scattering spectrum at finite temperature, and the Kosterlitz-Thouless critical temperature. Our findings are consistent with recent experimental results.Comment: 60 pages, RevTex, 16 figure

Spin-Excitation-Instability-Induced Quantum Phase Transitions in Double-Layer Quantum Hall Systems

We study intersubband spin density collective modes in double-layer quantum Hall systems at $\nu=2$ within the time-dependent Hartree-Fock approximation. We find that these intersubband spin density excitations may soften under experimentally accessible conditions, signaling a phase transition to a new quantum Hall state with interlayer inplane antiferromagnetic spin correlations. We show that this novel canted antiferromagnetic phase is energetically stable and that the phase transition is continuous.Comment: Revised final version to appear in Phys. Rev. Let

Investigation of Multimodal Features, Classifiers and Fusion Methods for Emotion Recognition

Automatic emotion recognition is a challenging task. In this paper, we present our effort for the audio-video based sub-challenge of the Emotion Recognition in the Wild (EmotiW) 2018 challenge, which requires participants to assign a single emotion label to the video clip from the six universal emotions (Anger, Disgust, Fear, Happiness, Sad and Surprise) and Neutral. The proposed multimodal emotion recognition system takes audio, video and text information into account. Except for handcraft features, we also extract bottleneck features from deep neutral networks (DNNs) via transfer learning. Both temporal classifiers and non-temporal classifiers are evaluated to obtain the best unimodal emotion classification result. Then possibilities are extracted and passed into the Beam Search Fusion (BS-Fusion). We test our method in the EmotiW 2018 challenge and we gain promising results. Compared with the baseline system, there is a significant improvement. We achieve 60.34% accuracy on the testing dataset, which is only 1.5% lower than the winner. It shows that our method is very competitive.Comment: 9 pages, 11 figures and 4 Tables. EmotiW2018 challeng