We discuss aspects of a two-dimensional variable-range hopping (VRH) conductivity determined by electron-electron interaction (EEI). The first aspect is connected with existence of a soft 'Coulomb gap' in the density of states (DOS) at the Fermi level. We show that, in the δ-doped GaAs/AlxGa1-xAs heterostructure, a quantitative analysis of the crossover from VRH conductivity governed by the Coulomb gap to VRH conductivity of non-interacting carriers allows one to determine the DOS near the Fermi level. The other phenomena consist in observation of a universal hopping pre-factor ρ0 equal to the quanta of two-dimensional resistance RQ = h/e2 = 25.8 kΩ. This is interpreted as evidence that, at electron densities near the metal-insulator transition, the conductivity is determined by EEI-assisted transitions rather than by the conventional mechanism of phonon-assisted hopping. The EEI mechanism is destroyed in strong parallel magnetic fields, which shows the important role of spin-spin correlations in the above effect.
|Number of pages||11|
|Journal||Philosophical Magazine B: Physics of Condensed Matter; Statistical Mechanics, Electronic, Optical and Magnetic Properties|
|State||Published - 2001|
Bibliographical noteFunding Information:
ACKNOWLEDGEMENTS In conclusion, we would like to emphasize the crucial role of Mike Pollak’s ideas in the study of influence of the EEI on hopping conductivity. This paper reviews the studies made in the Cavendish Laboratory, University of Cambridge, with support of the UK Engineering and Physical Sciences Research Council. We thank our coworkers J. T. Nicholls, S. I. Khondaker and D. A. Ritchie for their contribution to these studies.