Ben-Ling Gao

First-principles study on the electric structure and ferroelectricity in epitaxial CsSnI3 films

Ferroelectricity is a potentially crucial issue in inorganic halide perovskites, which are breakthrough materials in photovoltaic research. However, to date, conclusive evidence for ferroelectricity in all-inorganic halide perovskites is still lacking. Herein, using density functional theory simulations and symmetry analysis, it is found that two stable ferroelectric phases, P4bm and Pmc21-(I), can be induced by compressive and tensile strain in all-inorganic halide perovskite CsSnI3 films. More importantly, the calculated polarization value is as high as several μC cm−2 which is desirable for the separation of photo-excited carriers in these materials. In addition, it is found that the P4bm phase has an indirect band gap and the Pmc21-(I) phase has a suitable direct band gap for the absorption of visible light. In particular, tensile strain can alter the band gaps by several tenths of an electronvolt in the Pmc21-(I) phase. Overall, these results give useful insight for strained CsSnI3 films and hopefully provide a new route to design ferroelectric semiconductors for photovoltaic materials in the less-explored inorganic halide perovskites.

Controllable persistent spin-polarized charge current in a Rashba ring

We theoretically predict the appearance of a persistent charge current in a Rashba ring with a normal and a ferromagnetic lead under no external bias. This charge current is the result of the breaking of the time inversion symmetry in the original persistent pure spin current induced by the Rashba spin-orbit coupling (RSOC) in the ring due to the existence of the ferromagnetic lead. With the Keldysh Greens function technique, we find that not only the magnitude and sign but also the spin polarization of the generated charge current is determined by the system parameters such as the magnetization direction of the ferromagnetic lead, the tunneling coefficient, the strength of the RSOC and the exchange energy of the ferromagnetic lead, which are all tunable in experiments, that is, a controllable persistent spin-polarized charge current can be obtained in such a device.

Magnetic flux assisted thermospin transport in a Rashba ring

The electron transport through a Rashba ring with a magnetic flux and driven by a temperature difference is investigated. It is found that the spin interference effect induced by the Rashba spin-orbit interaction and by the magnetic flux can break the balance between the spin-up and spin-down component currents in the thermally driven charge current and thus result in a spin current. The analytical derivation and numerical calculations reveal that the magnitude, sign, peaks and spin-polarization of the generated spin current can be readily modulated by the system parameters. In particular, with some choices of the parameters, the spin polarization of the generated spin current can reach 100%, that is, a fully spin-polarized thermospin current can be produced. These results may help the use of the spin-dependent Seebeck effect to generate and manipulate a spin current.

Detecting Spin Bias with Circularly Polarized Light

We theoretically study the spin transport through a two-terminal quantum dot device under the influence of a symmetric spin bias and circularly polarized light. It is found that the combination of the circularly polarized light and the applied spin bias can result in a net charge current. The resultant charge current is large enough to be measured when properly choosing the system parameters. The resultant charge current can be used to deduce the spin bias due to the fact that there exists a simple linear relation between them. When the external circuit is open, a charge bias instead of a charge current can be induced, which is also measurable by present technologies. These findings indicate a new approach to detect the spin bias by using circularly polarized light.

TRANSPORT THROUGH INTERACTING AHARONOV–BOHM–CASHER INTERFEROMETERS

We investigate the transport properties of an interacting ring threaded by a magnetic flux and with Rashba spin-orbit coupling, based on a recently developed functional renormalized group technique. In the calculations of the electronic transport processes, the Coloumb On-site interactions are taken into account. For an interacting ring connected to two leads, we find that (i) for ΦAC = 0, the behavior of transmission zero at ΦAB = π is generic for the universal regime; (ii) for certain ΦAC and ΦAB, one can use the mesoscopic ring as spin filter even in the presence of the local interaction in the ring.

COUNTING STATISTICS OF NONEQUILIBRIUM TUNNELING PROCESS USING FUNCTIONAL RENORMALIZATION GROUP

We develop a scheme to investigate the flux of nonequilibrium transport based on the full counting statistics and nonequilibrium functional renormalization group method. As an illustrative example, we study the charge transfer in the system of an interacting quantum dot connected to two noninteracting reservoirs via tunneling. Within the lowest approximation in functional renormalization group, we obtain the cumulant generating function analytically.