V. Ginodman

Longitudinal conductivity in Si/SiGe heterostructure at integer filling factors

(Received 3 February 2003; revised manuscript received 13 May 2003; published 29 August 2003) We have investigated temperature dependence of the longitudinal conductivity σ x x at integer filling factors v= i for Si/SiGe heterostructure in the quantum Hall effect regime. It is shown that for odd i, when the Fermi level E F is situated between the valley-split levels, Δσ x x is determined by quantum corrections to conductivity caused by the electron-electron interaction: Δσ x x (T) ∼ ln T. For even i, when E F is located between cyclotron-split levels or spin-split levels, σ x x ∼exp[-Δ i /T for i=6,10,12 and ∼exp[-(T 0 i /T)] 1 / 2 for i = 4,8. For further decrease of T, all dependences σ x x (T) tend to almost temperature-independent residual conductivity σ i (0). A possible mechanism for σ i (0) is discussed.

Influence of disorder in compensation-doped germanium on the critical indices of the metal-insulator transition

We present a critical review of the present state of the critical exponent puzzle of the metal-insulator transition of doped semiconductors with emphasis on the role of meso-and macroscopic inhomogeneity caused by the disorder of intended or unintended acceptors and donors in crystals. By using both isotopic engineering and neutron transmutation doping (NTD) of germanium we found for low compensations (at K=1.4 and 12%) that the critical exponents of the localization length and the dielectric constant are nearly ν=1/2 and ζ=1, which double for medium compensations (at K=38 and 54%) to ν=1 and ζ=2, respectively.

Determination of the impurity concentration in heavily doped inhomogeneous semiconductors from the measurement of the low-temperature conductivity

A method has been developed for determining the effective concentration of shallow impuritiesN* reponsible for the low-temperature conductivity in the vicinity of the metal-insulator transition for inhomogeneous samples of n-Ge:As. The method is based on the measurement of two ratios of sample resistance γ-R(4.2 K)/R(300 K), β=R(2.0 K)/R(4.2 K) and the conductivity σ(4.2 K). The next step consists of plotting and σ(4.2) vs β. Assuming that β is the most reliable parameter, one can calculate, after an averaging procedare, the corrected values ofγ*,π*(4.2) and the resistivity at room temperatureρ*(300)=[γ*π*(4.2)]−1. Finally, using the known dependence ρ(300)=f(N) for homogeneous samples, one can obtain the values ofN*. The dependences ofN* on β and on γ are plotted. The scaling behavior of the conductivity of the Ge:As samples with corrected values ofπ*(4.2) andN* has been observed down toT=100 mK.

Interplay of bulk pinning and surface barriers in a current-carrying superconducting film

Abstract We show, both experimentally and theoretically, that the inhomogeneous distribution of transport current produced by two contacts located at the opposite corners of a square film is significantly changed as the film becomes superconducting (the total current I flowing through the sample is kept constant). We analyze two possible sources for such a redistribution of transport current: (1) the nonlinear dependence of the resistivity ρ on the current density j, and (2) the effect of surface barriers. In our geometry these sources have the opposite effect and compete with each other. This technique can be easily modified for various sample and contact geometries and is useful for the analysis of pinning and creep of vortices in superconductors.