ELECTRONIC TRANSPORT AND THE LOCALIZATION LENGTH IN THE QUANTUM HALL-EFFECT

We report on recent experimental results from transport measurements w ith large Hall bars made of high-mobility GaAs/AlxGa1-xAs heterostruct ures. Thermally activated conductivities and hopping transport were in vestigated in the integer quantum Hall regime. The predominant transpo rt processes in two dimensions are discussed. The implications of tran sport regime on prefactor universality and on the relation between rho (xx) and rho(xy) are studied. Particularly in the Landau-level tails, a strictly linear dependence delta rho(xy)(rho(xx)) was found, with pr onounced asymmetries with respect to the plateau center. At low temper atures, Ohmic (temperature-dependent) as well as non-Ohmic (current-de pendent) transport were investigated and analyzed on the basis of vari able-range hopping theory. The non-Ohmic regime could successfully be described by an effective electron temperature model. The results from either the Ohmic transport or from a comparison of Ohmic and non-Ohmi c data allowed us to determine the localization length xi in two diffe rent ways. The observed divergence of xi(v) with the filling factor v approaching a Landau-level center, is in qualitative agreement with sc aling theories of electron localization. The absolute values of xi far from the rho(xx) peaks are compared with theoretical predictions. On one hand, discrepancies between the xi results obtained from the two e xperimental methods are attributed to an inhomogeneous electric-field distribution. Extrapolation yields an effective width of dominant pote ntial drop of about 100 mu m. On the other hand, our analysis suggests a divergence of the dielectric function epsilon(r) proportional to(be ta) with beta similar or equal to 1.