Yu-Xian Li

Transport in a magnetic field modulated graphene superlattice

Using the transfer matrix method, we study the transport properties through a magnetic field modulated graphene superlattice. It is found that the electrostatic barrier, the magnetic vector potential, and the number of wells in a superlattice modify the transmission remarkably. The angular dependent transmission is blocked by the magnetic vector potential because of the appearance of the evanescent states at certain incident angles, and the region of Klein tunneling shifts to the left. The angularly averaged conductivities exhibit oscillatory behavior. The magnitude and period of oscillation depend sensitively on the height of the electrostatic barrier, the number of wells, and the strength of the modulated magnetic field.

Quantum transport through the system of parallel quantum dots with Majorana bound states

We study the tunneling transport properties through a system of parallel quantum dots which are coupled to Majorana bound states (MBSs). The conductance and spectral function are computed using the retarded Greens function method based on the equation of motion. The conductance of the system is 2e2/h at zero Fermi energy and is robust against the coupling between the MBSs and the quantum dots. The dependence of the Fermi energy on the spectral function is different for the first dot (dot1) than for the second dot (dot2) with fixed dot2-MBSs coupling. The influence of the Majorana bound states on the spectral function was studied for the series and parallel configurations of the system. It was found that when the configuration is in series, the Majorana bound states play an important role, resulting in a spectral function with three peaks. However, the spectral function shows two peaks when the system is in a parallel configuration. The zero Fermi energy spectral function is always 1/2 not only in series ...

Controllable valley polarization using graphene multiple topological line defects

The electron transport of different conical valleys is investigated in graphene with extended line-defects. Intriguingly, the electron with a definite incident angle can be completely modulated into one conical valley by a resonator which consists of several paralleling line-defects. The related incident angle can be controlled easily by tuning the parameters of the resonator. Therefore, a controllable 100% valley polarization, as well as the detection of the valley polarization, can be realized conveniently by tuning the number of line-defects and the distance between two nearest neighbouring line-defects. This fascinating finding opens a way to realize the valley polarization by line-defects. With the advancement of experimental technologies, this resonator is promising to be realized and thus plays a key role in graphene valleytronics.

Transport through quantum wells and superlattices on topological insulator surfaces

We investigate electron transmission coefficients through quantum wells and quantum superlattices on topological insulator surfaces. The quantum well or superlattice is not constituted by general electronic potential barriers but by Fermi velocity barriers which originate in the different topological insulator surfaces. It is found that electron resonant modes can be renormalized by quantum wells and more clearly by quantum superlattices. The depth and width of a quantum well and superlattice, the incident angle of an electron, and the Fermi energy can be used to effectively tune the electron resonant modes. In particular, the number N of periodic structures that constitute a superlattice can further strengthen these regulating effects. These results suggest that a device could be developed to select and regulate electron propagation modes on topological insulator surfaces. Finally, we also study the conductance and the Fano factor through quantum wells and quantum superlattices. In contrast to what has been reported before, the suppression factors of 0.4 in the conductance and 0.85 in the Fano factor are observed in a quantum well, while the transport for a quantum superlattice shows strong oscillating behavior at low energy and reaches the same saturated values as in the case of a quantum well at sufficiently large energies.

Topological Anderson insulator induced by inter-cell hopping disorder

We have studied in detail the influence of same-orbit and different-orbit hopping disorders in HgTe/CdTe quantum wells. Intriguingly, similar to the behavior of the on-site Anderson disorder, a phase transition from a topologically trivial phase to a topological phase is induced at a proper strength of the same-orbit hopping disorder. For different-orbit hopping disorder, however, the phase transition does not occur. The results have been analytically verified by using effective medium theory. A consistent conclusion can be obtained by comparing phase diagrams, conductance, and conductance fluctuations. In addition, the influence of Rashba spin-orbit interaction (RSOI) on the system has been studied for different types of disorder, and the RSOI shows different influence on topological phase at different disorders. The topological phase induced by same-orbit hopping disorder is more robust against the RSOI than that induced by on-site Anderson disorder. For different-orbit hopping disorder, no matter wheth...

Thermopower and conductance for a graphene p-n junction.

The thermopower and conductance in a zigzag graphene p-n junction are studied by using the nonequilibrium Greens function method combined with the tight-binding Hamiltonian. Our results show that the conductance and thermopower of the junction can be modulated by its width, the potential drop, and the applied perpendicular magnetic fields. A narrow graphene p-n junction shows insulating characteristics, and its thermopower is much larger than that of the wider one around the Dirac point. The insulating characteristic of the junction decreases as the width increases. In particular, with increasing junction width or the potential drop, the first conductance plateau is strongly enhanced and the thermopower is inverted around the Dirac point. A perpendicular magnetic field strongly suppresses the conductance and enhances the thermopower in the p-n region. The influence of edge vacancy defects on the conductance and thermopower is also discussed. Our results provide theoretical references for modulating the electronic and thermal properties of a graphene p-n junction by tuning its geometry and working conditions.

Andreev reflection and shot noise in a quantum dot with phonon modes

Building on the nonequilibrium Greens function technique and a canonical transformation of the electron-phonon interaction, this paper focuses on the study of the Andreev reflection conductance and the shot noise in a single quantum dot coupling with local phonon modes. The effect of the intradot spin-flip interaction on the transport properties is considered. We pay attention to the effects of the phonon on the Andreev reflection conductance and the shot noise. It is found that splits due to spin-flip scattering appear not only in the main Andreev reflection peaks but also in the new satellite peaks. The electron-phonon interaction leads to new satellite resonant peaks that are located exactly on the integer number of the phonon frequency. Moreover, the peak height is sensitive to the electron-phonon coupling. Even if the electron-phonon coupling is weak, the shot noise spectrum shows the phonon mode peaks rather clearly, but in the Andreev reflection conductance the phonon mode peaks weakly.

Quantum interference in topological insulator Josephson junctions

Using non-equilibrium Greens functions, we studied numerically the transport properties of a Josephson junction, superconductor-topological insulator-superconductor hybrid system. Our numerical calculation shows first that proximity-induced superconductivity is indeed observed in the edge states of a topological insulator adjoining two superconducting leads and second that the special characteristics of topological insulators endow the edge states with an enhanced proximity effect with a superconductor but do not forbid the bulk states to do the same. In a size-dependent analysis of the local current, it was found that a few residual bulk states can lead to measurable resistance, whereas because these bulk states spread over the whole sample, their contribution to the interference pattern is insignificant when the sample size is in the micrometer range. Based on these numerical results, it is concluded that the apparent disappearance of residual bulk states in the superconducting interference process as described in Ref. [\onlinecite{HartNautrePhys2014f}] is just due to the effects of size: the contribution of the topological edge states outweighs that of the residual bulk states.

Andreev reflection and Aharonov-Bohm oscillations through a parallel-coupled double quantum dot with spin-flip scattering.

Using nonequilibrium Greens function techniques, we investigate Andreev reflection and Aharonov-Bohm oscillations through a parallel-coupled double quantum dot connected with a ferromagnetic lead and a superconductor lead. The possibility of controlling Andreev reflection and Aharonov-Bohm oscillations of the system is explored by tuning the interdot coupling, the gate voltage, the magnetic flux, and the intradot spin-flip scattering. When the spin-flip scattering increases, Fano resonant peaks resulting from the asymmetrical levels of the two quantum dots begin to split, and Aharonov-Bohm oscillations are suppressed. Due to the interdot coupling, one strongly and one weakly coupled state of the system can be formed. The magnetic flux can exchange the function of the two states, which leads to a swap effect.

The effect of magnetic field on the ground and excited states of the two-dimensional D- centre

The ground and excited state energies of the two-dimensional D- centre have been calculated, respectively, as a function of magnetic field. The critical magnetic field values at which the excited states change from unbound to bound have been found. The optical transition between two bound states is discussed. Numerical results with the hyperspherical approach are in good agreement with those obtained through other intensive numerical methods and those obtained through experiments.