arXiv subjects

Yanxia Xing

Publications and source records attributed to Yanxia Xing.

31 records · Page 2Linked to original sources

arXiv2010

Universal Conductance Fluctuations in Mesoscopic Systems with Superconducting Leads: Beyond the Andreev Approximation

We report our investigation of the sample to sample fluctuation in transport properties of phase coherent normal metal-superconductor hybrid systems. Extensive numerical simulations were carried out for quasi-one dimensional and two dimensional systems in both square lattice (Fermi electron) as well as honeycomb lattice (Dirac electron). Our results show that when the Fermi energy is within the superconducting energy gap $Δ$, the Andreev conductance fluctuation exhibits a universal value (UCF) which is approximately two times larger than that in the normal systems. According to the random matrix theory, the electron-hole degeneracy (ehD) in the Andreev reflections (AR) plays an important role in classifying UCF. Our results confirm this. We found that in the diffusive regime there are two UCF plateaus, one corresponds to the complete electron-hole symmetry (with ehD) class and the other to conventional electron-hole conversion (ehD broken). In addition, we have studied the Andreev conductance distribution and found that for the fixed average conductance $,G>$ the Andreev conductance distribution is a universal function that depends only on the ehD. In the localized regime, our results show that ehD continues to serve as an indicator for different universal classes. Finally, if normal transport is present, i.e., Fermi energy is beyond energy gap $Δ$, the AR is suppressed drastically in the localized regime by the disorder and the ehD becomes irrelevant. As a result, the conductance distribution is that same as that of normal systems.

cond-mat.mes-hall↗

arXiv2009

Controllable Andreev retroreflection and specular Andreev reflection in a four-terminal graphene-superconductor hybrid system

We report the investigation of electron transport through a four-terminal graphene-superconductor hybrid system. Due to the quantum interference of the reflected holes from two graphene-superconductor interfaces with phase difference $θ$, it is found that the specular Andreev reflection vanishes at $θ=0$ while the Andreev retroreflection disappears at $θ=π$. This means that the retroreflection and specular reflection can be easily controlled and separated in this device. In addition, due to the diffraction effect in the narrow graphene nanoribbon, the reflected hole can exit from both graphene terminals. As the width of nanoribbon increases, the diffraction effect gradually disappears and the reflected hole eventually exits from a particular graphene terminal depending on the type of Andreev reflection.

cond-mat.mes-hall↗

arXiv2009

New universal conductance fluctuation of mesoscopic systems in the crossover regime from metal to insulator

We report a theoretical investigation on conductance fluctuation of mesoscopic systems. Extensive numerical simulations on quasi-one dimensional, two dimensional, and quantum dot systems with different symmetries (COE, CUE, and CSE) indicate that the conductance fluctuation can reach a new universal value in the crossover regime for systems with CUE and CSE symmetries. The conductance fluctuation and higher order moments vs average conductance were found to be universal functions from diffusive to localized regimes that depend only on the dimensionality and symmetry. The numerical solution of DMPK equation agrees with our result in quasi-one dimension. Our numerical results in two dimensions suggest that this new universal conductance fluctuation is related to the metal-insulator transition.

cond-mat.dis-nn↗

arXiv2009

Transient dynamics of molecular devices under step-like pulse bias

We report first principles investigation of time-dependent current of molecular devices under a step-like pulse.Our results show that although the switch-on time of the molecular device is comparable to the transit time, much longer time is needed to reach the steady state. In reaching the steady state the current is dominated by resonant states below Fermi level. The contribution of each resonant state to the current shows the damped oscillatory behavior with frequency equal to the bias of the step-like pulse and decay rate determined by the life time of the corresponding resonant state. We found that all the resonant states below Fermi level have to be included for accurate results. This indicates that going beyond wideband limit is essential for a quantitative analysis of transient dynamics of molecular devices.

cond-mat.mes-hall↗

arXiv2008

Spin Bias Measurement Based on a Quantum Point Contact

Electron charge transport through a quantum point contact (QPC) driven by an asymmetric spin bias is studied. A large charge current is induced when the transmission coefficient of the QPC jumps from one integer plateau to the next. Furthermore, for an open external circuit, the induced charge bias instead of the charge current is found to be quite large. It provides an efficient and practical way to detect spin bias by using a very simple device, a QPC or a STM tip. In addition, with the aid of magnetic field, polarization direction of the spin bias can also be determined.

cond-mat.mes-hall↗

arXiv2008

Double quantum dot as detector of spin bias

It was proposed that a double quantum dot can be used to be a detector of spin bias. Electron transport through a double quantum dot is investigated theoretically when a pure spin bias is applied on two conducting leads contacted to the quantum dot. It is found that the spin polarization in the left and right dots may be induced spontaneously while the intra-dot levels are located within the spin bias window and breaks the left-right symmetry of the two quantum dots. As a result, a large current emerges. For an open external circuit an charge bias instead of a charge current will be induced in equilibrium, which is believed to be measurable according to the current nanotechnology. This method may provide a practical and whole electrical approach to detect the spin bias (or the spin current) by measuring the charge bias or current in a double quantum dot.

cond-mat.mes-hall↗

arXiv2008

Spin Nernst effect and Nernst effect in two-dimensional electron systems

We study the Nernst effect and the spin Nernst effect, that a longitudinal thermal gradient induces a transverse voltage and a spin current. A mesoscopic four-terminal cross-bar device having the Rashba spin-orbit interaction (SOI) under a perpendicular magnetic field is considered. For zero SOI, the Nernst coefficient peaks when the Fermi level crosses the Landau Levels. In the presence of the SOI, the Nernst peaks split, and the spin Nernst effect appears and exhibits a series of oscillatory structures. The larger SOI is or the weaker magnetic field is, the more pronounced the spin Nernst effect is. The results also show that the Nernst and spin Nernst coefficients are sensitive to the detailed characteristics of the sample and the contacts. In addition, the Nernst effect is found to survive in strong disorder than the spin Nernst effect does.

cond-mat.mes-hall↗

arXiv2008

Josephson current transport through a Quantum Dot in an Aharonov-Bohm Ring

The Josephson current through an Aharonov-Bohm (AB) interferometer, in which a quantum dot (QD) is situated on one arm and a magnetic flux $Φ$ threads through the ring, has been investigated. With the existence of the magnetic flux, the relation of the Josephson current and the superconductor phase is complex, and the system can be adjusted to $π$ junction by either modulating the magnetic flux or the QD's energy level $\varepsilon_d$. Due to the electron-hole symmetry, the Josephson current $I$ has the property $I(\varepsilon_d,Φ)=I(-\varepsilon_d,Φ+π)$. The Josephson current exhibits a jump when a pair of Andreev bound states aligns with the Fermi energy. The condition for the current jump is given. In particularly, we find that the position of the current jump and the position of the maximum value of the critical current $I_c$ are identical. Due to the interference between the two paths, the critical current $I_c$ versus the QD's level $\varepsilon_d$ shows a typical Fano shape, which is similar to the Fano effect in the corresponding normal device. But they also show some differences. For example, the critical current never reaches zero for any parameters, while the current in the normal device can reach zero at the destruction point.

cond-mat.mes-hall↗

arXiv2008

Influence of dephasing process on the quantum Hall effect and the spin Hall effect

We study the influence of the phase relaxation process on Hall resistance and spin Hall current of a mesoscopic two-dimensional (2D) four-terminal Hall cross-bar with or without Rashba spin-orbit interaction (SOI) in a perpendicular uniform magnetic field. We find that the plateaus of the Hall resistance with even number of edge states can survive for very strong phase relaxation when the system size becomes much longer than the phase coherence length. On the other hand, the odd integer Hall resistance plateaus arising from the SOI are easily destroyed by the weak phase relaxation during the competition between the magnetic field and the SOI which delocalize the edge states. In addition, we have also studied the transverse spin Hall current and found that it exhibits resonant behavior whenever the Fermi level crosses the Landau band of the system. The phase relaxation process weakens the resonant spin Hall current and enhances the non-resonant spin Hall current.

cond-mat.mes-hall↗

arXiv2007

Symmetry and transport property of spin current induced spin-Hall effect

We study the spin current induced spin-Hall effect that a longitudinal spin dependent chemical potential $qV_{s=x,y,z}$ induces a transverse spin conductances $G^{ss'}$. A four terminal system with Rashba and Dresselhaus spin-orbit interaction (SOI) in the scattering region is considered. By using Landauer-B$\ddot u$ttiker formula with the aid of the Green function, various spin current induced spin-Hall conductances $G^{ss'}$ are calculated. With the charge chemical potential $qV_c$ or spin chemical potential $qV_{s=x,y,z}$, there are 16 elements for the transverse conductances $G^{μν}_p=J_{p,μ}/V_ν$ where $μ,ν=x,y,z,c$. Due to the symmetry of our system these elements are not independent. For the system with $C_2$ symmetry half of elements are zero, when the center region only exists the Rashba SOI or Dresselhaus SOI. The numerical results show that of all the conductance elements, the spin current induced spin-Hall conductances $G^{ss'}$ are usually much greater (about one or two orders of magnitude) than the spin Hall conductances $G^{sc}$ and the reciprocal spin Hall conductances $G^{cs}$. So the spin current induced spin-Hall effect is dominating in the present device.

cond-mat.mes-hall↗

arXiv2007

Response time of a normal-superconductor hybrid system under the step-like pulse bias

The response of a quantum dot coupled with one normal lead and a superconductor lead driven by a step-like pulse bias $V_L$ is studied using the non-equilibrium Green function method. In the linear pulse bias regime, the responses of the upwards and downwards bias are symmetric. In this regime the turn-on time and turn-off time are much slower than that of the normal system due to the Andreev reflection. On the other hand, for the large pulse bias $V_L$, the instantaneous current exhibits oscillatory behaviors with the frequency $\hbarΩ=qV_L$. The turn on/off times are in (or shorter than) the scale of $1/V_L$, so they are faster for the larger bias $V_L$. In addition, the responses for the upwards and downwards bias are asymmetric at large $V_L$. The turn-on time is larger than the turn-off time but the relaxation time \cite{note1} depends only on the coupling strength $Γ$ and it is much smaller than the turn-on/off times for the large bias $V_L$.

cond-mat.mes-hall↗

arXiv2006

Nature of Spin Hall Effect in a finite Ballistic Two-Dimensional System with Rashba and Dresselhaus spin-orbit interaction

The spin Hall effect in a finite ballistic two-dimensional system with Rashba and Dresselhaus spin-orbit interaction is studied numerically. We find that the spin Hall conductance is very sensitive to the transverse measuring location, the shape and size of the device, and the strength of the spin-orbit interaction. Not only the amplitude of spin Hall conductance but also its sign can change. This non-universal behavior of the spin Hall effect is essentially different from that of the charge Hall effect, in which the Hall voltage is almost invariant with the transverse measuring site and is a monotonic function of the strength of the magnetic field. These surprise behavior of the spin Hall conductance are attributed to the fact that the eigenstates of the spin Hall system is extended in the transverse direction and do not form the edge states.

cond-mat.mes-hall↗

arXiv2006

Opposite spin accumulations on the transverse edges by the confining potential

We show that the spin-orbit interaction induced by the boundary confining potential causes opposite spin accumulations on the transverse edges in a zonal two-dimensional electron gas in the presence of external longitudinal electric field. While the bias is reversed, the spin polarized direction is also reversed. The intensity of the spin accumulation is proportional to the bias voltage. In contrast to the bulk extrinsic and intrinsic spin Hall effects, the spin accumulation by the confining potential is almost unaffected by impurity and survives even in strong disorder. The result provides a new mechanism to explain the recent experimental data.

cond-mat.mes-hall↗