A classical model for the negative dc conductivity of ac-driven 2D electrons near the cyclotron resonance

A classical model for {\em dc} transport of two dimensional electrons in a perpendicular magnetic field and under strong irradiation is considered. We demonstrate that, near the cyclotron resonance condition, and for {\em linear} polarization of the {\em ac} field, a strong change of the diagonal component, $\sigma_d$, of the {\em dc} conductivity occurs in the presence of a {\em weak} nonparabolicity of the electron spectrum. Small change in the electron effective mass due to irradiation can lead to negative $\sigma_d$, while the Hall component of the {\em dc} conductivity remains practically unchanged. Within the model considered, the sign of $\sigma_d$ depends on the relative orientation of the {\em dc} and {\em ac} fields, the sign of the detuning of the {\em ac} frequency from the cyclotron resonance, and the sign of nonparabolic term in the energy spectrum.Comment: 4 pages, 1 figur

Terahertz magneto-spectroscopy of transient plasmas in semiconductors

Using synchronized near-infrared (NIR) and terahertz (THz) lasers, we have performed picosecond time-resolved THz spectroscopy of transient carriers in semiconductors. Specifically, we measured the temporal evolution of THz transmission and reflectivity after NIR excitation. We systematically investigated transient carrier relaxation in GaAs and InSb with varying NIR intensities and magnetic fields. Using this information, we were able to determine the evolution of the THz absorption to study the dynamics of photocreated carriers. We developed a theory based on a Drude conductivity with time-dependent density and density-dependent scattering lifetime, which successfully reproduced the observed plasma dynamics. Detailed comparison between experimental and theoretical results revealed a linear dependence of the scattering frequency on density, which suggests that electron-electron scattering is the dominant scattering mechanism for determining the scattering time. In InSb, plasma dynamics was dramatically modified by the application of a magnetic field, showing rich magneto-reflection spectra, while GaAs did not show any significant magnetic field dependence. We attribute this to the small effective masses of the carriers in InSb compared to GaAs, which made the plasma, cyclotron, and photon energies all comparable in the density, magnetic field, and wavelength ranges of the current study.Comment: 8 pages, 9 figures, submitted to Phys. Rev.

Microwave radiation induced magneto-oscillations in the longitudinal and transverse resistance of a two dimensional electron gas

We confirm the existance of magneto-resistance oscillations in a microwave-irradiated two-dimensional electron gas, first reported in a series of papers by Zhudov et al. and Mani et al. In our experiments, on a sample with a more moderate mobility, the microwave induced oscillations are observed not only in the longitudinal - but also in the transverse-resistance (Hall resistance). The phase of the oscillations is such that the decrease (increase) in the longitudinal resistance is accompanied by an increase (decrease) in the absolute value of the Hall resistance. We believe that these new results provide valuable new information to better understand the origin of this interesting phenomenon.Comment: Accepted for publication in journal of Solid State Comunication