Yanxia Xing

Double quantum dot as detector of spin bias

It was proposed that a double quantum dot can be used as a detector of the spin bias. Electron transport through a double quantum dot is theoretically investigated when a pure spin bias is applied on two conducting leads in contact with the quantum dot. It is found that the spin polarization in the left and right dots may be spontaneously induced, while the intradot 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, a charge bias instead of a charge current will be induced at 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.

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

We study the Nernst effect and the spin Nernst effect and show 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 the SOI or the weaker the 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 better than the spin Nernst effect.

Influence of dephasing 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 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 is 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 nonresonant spin Hall current.

Accumulation of opposite spins on the transverse edges of a two-dimensional electron gas in a longitudinal electric field

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.

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 nonuniversal 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. This surprise behavior of the spin Hall conductance is attributed to the fact that the eigenstates of the spin Hall system are extended in the transverse direction and do not form the edge states.

Spin bias measurement based on a quantum point contact

Electron charge transport through a quantum point contact (QPC) driven by an asymmetric spin bias (SB) 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 SB by using a very simple device, a QPC or a STM tip. In addition, with the aid of magnetic field, polarization direction of the SB can also be determined.

Supercurrent and its Fano effect in a Josephson 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 εd. Due to the electron-hole symmetry, the Josephson current I has the property I(εd, Φ) = I(−εd, 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 Ic are identical. Due to the interference between the two paths, the critical current Ic versus the QD’s level εd 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.