Extrinsic spin Nernst effect in two-dimensional electron systems

The spin accumulation due to the spin current induced by the perpendicular temperature gradient (the spin Nernst effect) is studied in a two-dimensional electron system (2DES) with spin-orbit interaction by employing the Boltzmann equation. The considered 2DES is confined within a symmetric quantum well with delta doping at the center of the well. A symmetry consideration leads to the spin-orbit interaction which is diagonal in the spin component perpendicular to the 2DES. As origins of the spin current, the skew scattering and the side jump are considered at each impurity on the center plane of the well. It is shown that, for repulsive impurity potentials, the spin-Nernst coefficient changes its sign at the impurity density where contributions from the skew scattering and the side jump cancel each other out. This is in contrast to the spin Hall effect in which the sign change of the coefficient occurs for attractive impurity potentials.Comment: 8 pages, 1 figur

Polaron Masses in CH₃NH₃PbX₃ Perovskites Determined by Landau Level Spectroscopy in Low Magnetic Fields

次世代太陽電池材料 ペロブスカイト半導体中の「電子の重さ」の評価に成功 --太陽電池やLED応用へ向けてさらなる期待--. 京都大学プレスリリース. 2021-06-14.We investigate the electron-phonon coupling in CH₃NH₃PbX₃ lead halide perovskites through the observation of Landau levels and high-order excitons at weak magnetic fields, where the cyclotron energy is significantly smaller than the longitudinal optical phonon energy. The reduced masses of the carriers and the exciton binding energies obtained from these data are clearly influenced by polaron formation. We analyze the field-dependent polaronic and excitonic properties, and show that they can be quantitatively reproduced by the Fröhlich large polaron model

Excitonic enhancement of optical nonlinearities in perovskite CH₃NH₃PbCl₃ single crystals

Metal halide perovskites have emerged as versatile photonic device materials because of their outstanding band structure and excellent optical properties. Here, we determined the excitation wavelength dependences of the two-photon absorption coefficient and the Kerr-effect-induced nonlinear refractive index of CH₃NH₃PbCl₃ perovskite single crystals by means of the Z-scan method. From theoretical analysis, we found that the electron-hole interaction, so-called exciton effect, significantly enhances the nonlinear optical responses even for the interband transitions. This interaction explains the universal relation between the exciton reduced mass and the bandgap for lead halide perovskites

Gate-voltage-induced switching of the Rashba spin-orbit interaction in a composition-adjusted quantum well

The coefficient alpha of the Rashba spin-orbit interaction is calculated in an asymmetric quantum well consisting of Ga0.47In0.53As (well), Al0.48In0.52As (left barrier), and AlxGa1-xAsySb1-y (right barrier) as a function of the external electric field perpendicular to the well E-z(ex) which is controlled by the gate voltage. This coefficient alpha, which depends on the band offset, can be tuned to be zero by adjusting the Al fraction x in the right barrier layer to the optimum value x(0) in the case where the wave function vanishes at the left heterointerface. Such a composition-adjusted asymmetric quantum well is proposed as a structure in which the magnitude of a can be switched by changing the polarity of E-z(ex). The calculation shows that, when vertical bar x - x(0)vertical bar 40 for a large enough vertical bar E-z(ex)vertical bar (vertical bar E-z(ex)vertical bar > 10(7) V/m for a well width of 20 nm), which results in the on/off spin-relaxation-rate ratio exceeding 10(3) in the Dyakonov-Perel mechanism

Spin relaxation in a quantum well by phonon scatterings

The spin relaxation due to the spin-orbit interaction (SOI) is studied theoretically in a quantum well with electrons occupying only the ground subband. First, it is shown that the coefficient of the Rashba SOI is proportional to b(off) - 1, in which the parameter boff, determined by the band offsets and the band gaps, passes through unity, for example, by changing x in Ga0.47In0.53As(well)/AlxGa1-xAsySb1-y (barrier). Second, it is derived that the transition matrix element of each spin-flip phonon scattering has the same proportionality factor b(off) - 1, in addition to the impurity scattering previously studied by the same authors [Phys. Rev. B 89, 075314 (2014)]. These findings suggest the possibility of strongly suppressing the spin-relaxation rate by choosing appropriate materials

Cross-polarized excitons in double-wall carbon nanotubes

Optical absorption in double-wall carbon nanotubes for light polarized perpendicular to the tube axis is studied by taking into account exciton effects and depolarization effects within an effective-mass theory. The Coulomb interaction is suppressed by not only intrawall screening effects but also interwall screening, leading to the reduction of exciton binding energies and band gaps. When two tubes are both semiconducting, a clear exciton peak still survives even under depolarization effects for the outer tube, but the exciton peak of the inner tube has an asymmetric Fano line shape due to the coupling with continuum states of the outer tube. When a double-wall nanotube contains a metallic tube, either inner or outer, the exciton of the semiconducting tube loses its peak structure under depolarization effects

Interwall screening and excitons in double-wall carbon nanotubes

Exciton properties of double-wall carbon nanotubes are studied in the static screened Hartree-Fock approximation within a k・p scheme. The intrawall electron-hole interaction is largely suppressed by interwall screening effects. The suppression is sensitive to the effective interwall distance between the inner and outer tubes and reduces the exciton binding energy as well as the band gap. As a result, the exciton energy levels are redshifted from those in the single-wall tube with the same diameter. The energy shift of the ground exciton has little dependence on the tube diameter, in contrast to that of the excited excitons. In the case of a metallic outer and inner tube, excited exciton states in the semiconducting tube disappear due to strong screening