Xing Ding-Yu

Spin Chern numbers and time-reversal-symmetry-broken quantum spin Hall effect

The quantum spin Hall (QSH) effect is considered to be unstable to perturbations violating the time-reversal (TR) symmetry. We review some recent developments in the search of the QSH effect in the absence of the TR symmetry. The possibility to realize a robust QSH effect by artificial removal of the TR symmetry of the edge states is explored. As a useful tool to characterize topological phases without the TR symmetry, the spin-Chern number theory is introduced.

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.

Magnetic-Field-Induced Semimetal-Insulator-like Transition in Highly Oriented Pyrolitic Graphite

We report the extraordinarily large positive magnetoresistances (MR, 69400% at 4.5 K under a magnetic field of 8.15 T), de Hass–van Alphen oscillations effect at 10 K and the semimetal-insulator-like transition in a wide range of temperature in highly oriented pyrolitic graphite (HOPG). Besides a dominating ordinary MR (OMR) mechanism in the free-electron mode, it is realized from qualitative analysis that the Coulomb interacting quasiparticles within graphite layers play some roles. However it is difficult to associate the transition with the simple OMR theory. In order to investigate the possible origins of the transition, further analysis is carried out. It is revealed that the magnetic-field-induced behaviour is responsible for the semimetal-insulator-like transitions in HOPG.

Conductance Oscillations in Spin Field-Effect Transistors

Ballistic spin transport in spin field-effect transistors is studied by taking into account the Rashba spin-orbit coupling, interfacial scattering, and band mismatch. It is shown that the spin conductance oscillation with the semiconductor channel length is a superimposition of the Rashba spin precession and spin interference oscillations. They have different oscillation periods π/kR and π/k with kR the Rashba wavevector and k the Fermi wavevector of the semiconductor channel, and play different parts of slow and rapid oscillations, depending upon the relative magnitude of π/kR and π/k. Only at k = kR does the spin conductance exhibit oscillations of a single period. Two types of different behaviors of the tunnelling magnetoresistance are discussed.

Spin-Triplet Andreev Reflection in Ferromagnet/Ferromangnet/s-Wave Superconductor Junctions

Four-component Bogoliubov-de Gennes equations are applied to study the tunnelling conductance spectra G(E) of half-metallic ferromagnet/ferromagnet/s-wave superconductor tunnel junctions. It is found that only for noncollinear magnetizations, there exists nonzero G(E) structure within the energy gap, which is a signature of appearance of the novel Andreev reflection and spin-triplet pairing correlations.

A Three-Josephson-Junction Device with Noise*

A Gaussian white noise model and a symmetric dichotomous noise model for a three-Josephson-junction device are studied. It is shown that the symmetric noise can produce net voltage (Its corresponding dc current is zero), which sterns from the multiplicative noise,the asymmetry of the potential or the asymmetry of the potential itselt; respectively for the Gaussian white noise model or for the dichotomous noise model. The net voltage is negative,and exhibits a peak with increasing the additive noise strength (the stochastic resonance). The dc current-voltage characteristics are calculated for the two models. In addition, we find that when the dc current is not zero, in some scope the absolute value of the dc voltage versus the additive noise strength also presents the stochastic resonance.

Generation and Quantum Interference of Entangled Electron-Hole Pairs in a Hanbury Brown and Twiss Interferometer

The Hanbury–Brown–Twiss interferometer is proposed to serve as a detector of the crossed Andreev reflection and to generate entangled electron-hole pairs. It is shown that the non-local electron and hole induced by the crossed Andreev reflection are entangled. Quantum interference of the entangled electron-hole pairs gives rise to a new measurable effect of the phase difference between two superconductors in the cross correlation. The present theoretical predictions can be experimentally realized within the present-day microelectronic technology.

The Linear Optical Polarizability Spectra of Five C78 Isomers

Effects of anisotropy, symmetry, atom arrangement and Coulomb interaction on the linear optical polarizability spectra of five C78 isomers are theoretically studied by using the extended Su–Schrieffer–Heeger model with and without Coulomb interaction. The main results are as follows. 1) The spectra become anisotropic with respect to the direction of the electric field of light. The property is common in the niorlel with and without Coulomb interaction. 2) The symmetry and atom arrangement have grcat effects on the peak positions, strengths, and number. 3) The Coulomb interaction tends to shift peaks to higher energies and enhances the oscillator strengths at higher energies.

Escape over Fluctuating Barrier for a Stochastic Model of Motor Proteins in the Case of Environmental Perturbation

We study the mean first passage time (MFPT) over the fluctuatingpotential barrier for a stochastic model of motor proteins in the case of environmental perturbation proposed by J.H. LI et al. [Phys. Rev. E57 (1998) 39171. The MFPT is derived for a particle over the fluctuating potential barrier. It is shown that (i) there is resonant activation for the MFPT as a function of the flipping rate of the fluctuating potential barrier; (ii) the additive and multiplicative noises can weaken the resonant activation, but the correlations between them enhance the resonant activation; (iii) the susceptibility of the resonant activation to the multiplicative noise is far larger than that to the additive noise.

Large Piezoresistance and Pressure-Induced Change in Electric Properties in Perovskite-like La0.85Sr0.15MnO3

Uniaxial pressure dependencies of the electric properties of perovskite-like La1-xSrxMnO3 (0.2 ≤ x ≤ 0.6) system have been investigated. A large nonlinear piezoresistance effect and a remarkable pressure-induced change of metal-semiconductor (M-S) transition have been observed. For La0.7Sr0.3MnO3, the piezoresistance value can reach about 86% at 290 K under a pressure of 16.8 MPa, and the pressure coefficient of Tp, the onset of the M-S transition, is as high as 6.4 K/MPa, which is significantly larger than that of known similar pressure effects. These pressure effects suggest that the resistivity of the perovskite compounds is relative not only to double exchange but also to the symmetry of lattice. This phenomenon may provide another route in understanding the colossal magnetoresistance behavior in the perovskites.