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Register a new userThe inelastic relaxation time due to electron-electron collisions in high-mobility two-dimensional systems under microwave radiations
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In some theoretical analyses of microwave-induced magnetoresistance oscillations in high-mobility two-dimensional systems, the inelastic relaxation time $tau_{in}$ due to electron-electron scattering is evaluated using an equilibrium distribution function $f^0$ in the absence of radiation, and it is concluded that $tau_{in}$ is much larger than $tau_{q}$, the single-particle relaxation time due to impurity scattering. However, under the irradiation of a microwave capable of producing magnetoresistance oscillation, the distribution function of the high-mobility electron gas deviates remarkably from $f^0$ at low temperatures. Estimating $tau_{in}$ using an approximate nonequilibrium distribution function rather than using $f^0$, one will find the system to be in the opposite limit $1/tau_{in}ll 1/tau_{q}$ even for T=0 K. Therefore, models which depend on the assumption $1/tau_{in}gg 1/tau_{q}$ may not be justifiable.
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We present a detailed experimental and theoretical analysis of the spin dynamics of two-dimensional electron gases (2DEGs) in a series of n-doped GaAs/AlGaAs quantum wells. Picosecond-resolution polarized pump-probe reflection techniques were applied in order to study in detail the temperature-, concentration- and quantum-well-width- dependencies of the spin relaxation rate of a small photoexcited electron population. A rapid enhancement of the spin life-time with temperature up to a maximum near the Fermi temperature of the 2DEG was demonstrated experimentally. These observations are consistent with the Dyakonov-Perel spin relaxation mechanism controlled by electron-electron collisions. The experimental results and theoretical predictions for the spin relaxation times are in good quantitative agreement.
In a high mobility two-dimensional electron gas (2DEG) in a GaAs/AlGaAs quantum well we observe a strong magnetoresistance. In lowering the electron density the magnetoresistance gets more pronounced and reaches values of more than 300%. We observe that the huge magnetoresistance vanishes for increasing the temperature. An additional density dependent factor is introduced to be able to fit the parabolic magnetoresistance to the electron-electron interaction correction.
We have observed cyclotron resonance in a high-mobility GaAs/AlGaAs two-dimensional electron gas by using the techniques of terahertz time-domain spectroscopy combined with magnetic fields. From this, we calculate the real and imaginary parts of the diagonal elements of the magnetoconductivity tensor, which in turn allows us to extract the concentration, effective mass, and scattering time of the electrons in the sample. We demonstrate the utility of ultrafast terahertz spectroscopy, which can recover the true linewidth of cyclotron resonance in a high-mobility ($>{10}^{6} mathrm{cm^{2} V^{-1} s^{-1}}$) sample without being affected by the saturation effect.
Effects of microwave radiation on magnetoresistance are analyzed in a balance-equation scheme that covers regimes of inter- and intra-Landau level processes and takes account of photon-asissted electron transitions as well as radiation-induced change of the electron distribution for high mobility two-dimensional systems. Short-range scatterings due to background impurities and defects are shown to be the dominant direct contributors to the photoresistance oscillations. The electron temperature characterizing the system heating due to irradiation, is derived by balancing the energy absorption from the radiation field and the energy dissipation to the lattice through realistic electron-phonon couplings, exhibiting resonant oscillation. Microwave modulations of Shubnikov de Haas oscillation amplitude are produced together with microwave-induced resistance oscillations, in agreement with experimental findings. In addition, the suppression of the magnetoresistance caused by low-frequency radiation in the higher magnetic field side is also demonstrated.
Understanding the spin dynamics in semiconductor heterostructures is highly important for future semiconductor spintronic devices. In high-mobility two-dimensional electron systems (2DES), the spin lifetime strongly depends on the initial degree of spin polarization due to the electron-electron interaction. The Hartree-Fock (HF) term of the Coulomb interaction acts like an effective out-of-plane magnetic field and thus reduces the spin-flip rate. By time-resolved Faraday rotation (TRFR) techniques, we demonstrate that the spin lifetime is increased by an order of magnitude as the initial spin polarization degree is raised from the low-polarization limit to several percent. We perform control experiments to decouple the excitation density in the sample from the spin polarization degree and investigate the interplay of the internal HF field and an external perpendicular magnetic field. The lifetime of spins oriented in the plane of a [001]-grown 2DES is strongly anisotropic if the Rashba and Dresselhaus spin-orbit fields are of the same order of magnitude. This anisotropy, which stems from the interference of the Rashba and the Dresselhaus spin-orbit fields, is highly density-dependent: as the electron density is increased, the kubic Dresselhaus term becomes dominant and reduces the anisotropy.
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