Showing papers by "Andre K. Geim published in 1992"

Asymmetric scattering and diffraction of two-dimensional electrons at quantized tubes of magnetic flux.

Abstract: The Hall conductivity of a high-mobility two-dimensional electron gas (2DEG) has been investigated in a distribution of quantized magnetic flux tubes (vortices) formed at a type-II superconducting ``gate'' layer. A pronounced suppression of the Hall effect was observed for long-Fermi wavelengths (as compared to the submicron vortex size) indicating a situation where electrons are diffracted by the flux quanta. In contrast, for shorter Fermi wavelengths the Hall conductivity has been found to be insensitive to the extreme inhomogeneity of the magnetic field and determined by the average field.

Breakdown of universal scaling of conductance fluctuations in high magnetic fields.

Abstract: We have studied universal conductance fluctuations in the nonlocal magnetoresistance of ${\mathit{n}}^{+}$-GaAs wires. The Lee-Stone correlation field \ensuremath{\Delta}${\mathit{B}}_{\mathit{c}}$ increases by a factor of 5 as the magnetic field increases from 0 to 12 T but the amplitude of the fluctuations does not change. It is possible to explain the increase of \ensuremath{\Delta}${\mathit{B}}_{\mathit{c}}$ quantitatively in terms of a variation of the size of the phase coherence length. However, this should also give rise to a large change in the fluctuation amplitude, which is not observed. This implies that the universal scaling of the conductance fluctuations is not valid in high magnetic fields.

High-field magnetoresistance in a periodically modulated two-dimensional electron gas.

Abstract: We have extended earlier measurements of the magnetoresistance in a periodically modulated two-dimensional electron gas to high magnetic fields, where the cyclotron radius, r(c), is much smaller than the period, a, of modulation. A giant nonoscillatory magnetoresistance was found to arise beyond the range of low-field (Weiss) oscillations periodic in 1/B. Its value in a magnetic field of a few tesla may exceed the zero-field resistance value, rho-0, by 1 to 2 orders of magnitude. This result is in agreement with the semiclassical theory by Beenakker, if a modulation of the electron mobility is taken into account.

Quantitative numerical model for nonlocal quantum diffusion in a distribution of magnetic-flux tubes.

Abstract: Recently a number of experiments have investigated a mesoscopic system in which a very inhomogeneous magnetic field distribution is generated at a two-dimensional electron gas by depositing a type-II superconducting film on top. The weak-localization magnetoconductivity in such structures has been shown to be qualitatively different from the homogeneous magnetic field case and in complementary experiments it has been demonstrated that micrometer-sized devices of this type can be sensitive to the dynamics of individual quanta of flux. The huge latent potential of such structures will only be realized if quantitative models of the detector operation can be developed. In this Brief Report we demonstrate that available asymptotic results for the magnetoconductivity [Phys. Rev. B 36, 3135 (1987)] are of limited use in describing real experiments, but show that quantitative agreement can be reached with a numerical model that has been developed here.

Optical suppression of ionized impurity scattering in vertical hot‐electron transport

Abstract: A striking effect of illumination on the vertical nonequilibrium electron transport has been observed in the GaAs‐based tunneling hot electron transfer amplifier (THETA). Weak illumination can considerably increase the transparency of a THETA structure for quasiballistic electrons if the photon energy exceeds the GaAs band gap. The temperature and illumination intensity dependencies indicate that the effect is caused by photoneutralization of ionized impurities which are a major source of hot electron scattering.