arXiv subjects

Xuean Zhao

Publications and source records attributed to Xuean Zhao.

8 recordsLinked to original sources

arXiv2013

The numerical operator method to the real time dynamics of currents through the nanostructures with different topologies

We present the numerical operator method designed for the real time dynamics of currents through nanostructures beyond the linear response regime. We apply this method to the transient and stationary currents through nanostructures with different topologies, e.g., the flakes of square and honeycomb lattices. We find a quasi-stationary stage with a life proportional to the flake size in the transient currents through the square flakes, but this quasi-stationary stage is destroyed in the presence of disorder. However, there is no quasi-stationary stage in the transient currents through the honeycomb flakes, showing that the transient current depends strongly upon the topologies of the nanostructures. We also study the stationary current by taking the limit of the current at long times. We find that the stationary current through a square flake increases smoothly as the voltage bias increasing. In contrast, we find a threshold voltage in the current-voltage curve through a honeycomb flake, indicating a gap at the Fermi energy of a honeycomb flake.

cond-mat.mes-hall↗

arXiv2007

Nonvanishing spin Hall currents in the presence of magnetic impurities

The intrinsic spin Hall conductivity in a two dimensional electron gas with Rashba spin-orbit coupling is evaluated by taking account of anisotropic coupling between magnetic impurities and itinerant electrons. In our calculation Kubo's linear response formalism is employed and the vertex correction is considered. In the semiclassical limit $μ\gg 1/τ$, a non-vanishing spin Hall conductivity $σ^{}_{sH}$ is found to depend on the momentum relaxation time $τ$, spin-orbit splitting $Δ$ and the anisotropic coefficient of interaction between itinerant electrons and magnetic impurities. The clean limit of $σ^{}_{sH}$ is in the region of $e/8π\sim e/6π$, depending on the anisotropic coefficient.

cond-mat.mes-hall↗

arXiv2006

Spin precession caused by spin-spin interaction between bounded electrons in quantum dots

In this paper, we study the spin-spin interaction between two electrons bounded in a quantum dot. The result shows that spin-spin interaction will cause a pair of spins precessing synchronously. If the two spins are parallel at initial time, the total spin oscillates as a cosine function. If the two spins are antiparallel at initial time, the total spin keeps zero. The precessing period is proportional to the cube of quantum dot radius. For a 2D round quantum dot with radius $R=50{\text{nm}}$, the precessing period equals nearly $12μ{\text{s}}$.

cond-mat.mes-hall↗

arXiv2005

Correlations of spin-polarized and entangled electrons with Berry phase

The correlation and fluctuation of both entangled electrons and spin-polarized pairs affected by two rotating magnetic fields in a setup proposed by J. Carlos Egues etc. (Phys. Rev. Lett. {\bf 89}(2002) 176401) are studied theoretically by using scattering approach. Differing from polarized pair, the entangled electron pairs are shown to behave like a composite particle with the total spins and its $z$ components. The singlet and triplet states exhibit different bunching and antibunching features, which can be easily adjusted by the magnetic fields. The correlations and variances can show up distinguish output signals for the four incident states. Our results are expected to be tested by using coincident technique.

cond-mat.mes-hall↗

arXiv2005

Spin-flip process through a quantum dot coupled to ferromagnetic electrodes

We study the spin-dependent transport through a quantum dot coupled to two ferromagnetic electrodes using the equation of motion method for the nonequilibrium Green's functions. Our results show that the conductance and the density of states (DOS) are strongly dependent on the configurations of the magnetic electrodes. In parallel configuration of magnetic electrodes the conductance is affected by the spin-flip process and the Coulomb repulsion on the dot. The Kondo peak for spin-dependent transport is splitted into two peaks by the spin-flip process. The locations of the two peaks are symmetric about no spin-flip peak and the separation of the splitting is dependent on the strength of the spin-flip parameter $R$. This effect may be useful to realize the spin-filter device.

cond-mat.mes-hall↗

arXiv2005

Shot Noise for Entangled Electrons with Berry Phase

We study shot noise for entangled electrons in a 4-lead beam-splitter with one incoming lead driven by adiabatically rotating magnetic fields. We propose a setup of an adiabatically rotating magnetic field therefor, which is appropriate for an electron beam to transport through. Using the scattering matrix approach, we find that shot noise for the singlet and that for the entangled triplet oscillates between bunching and antibunching due to the influence of the Berry phase. It provides us a new approach for testing the Berry phase in electron transport on the basis of entanglement.

cond-mat.mes-hall↗

arXiv1999

Electrochemical capacitance of a leaky nano-capacitor

We report a detailed theoretical investigation on electrochemical capacitance of a nanoscale capacitor where there is a DC coupling between the two conductors. For this ``leaky'' quantum capacitor, we have derived general analytic expressions of the linear and second order nonlinear electrochemical capacitance within a first principles quantum theory in the discrete potential approximation. Linear and nonlinear capacitance coefficients are also derived in a self-consistent manner without the latter approximation and the self-consistent analysis is suitable for numerical calculations. At linear order, the full quantum formula improves the semiclassical analysis in the tunneling regime. At nonlinear order which has not been studied before for leaky capacitors, the nonlinear capacitance and nonlinear nonequilibrium charge show interesting behavior. Our theory allows the investigation of crossover of capacitance from a full quantum to classical regimes as the distance between the two conductors is changed.

cond-mat↗

arXiv1999

Nonlinear Quantum Capacitance

We analyze the nonlinear voltage dependence of elelctrochemical capacitance for nano-scale conductors. This voltage dependence is due to finite density of states of the conductors. We derive an exact expression for the electrochemical capacitance-voltage curve for a parallel plate system. The result suggests a quantum scanning capacitance microscopy at the nano-scale: by inverting the capacitance-voltage expression one is able to deduce the local spectral function of the nano-scale conductor.

cond-mat↗