Two-dimensional variable-range hopping conductivity: Influence of the electron-ectron interaction

Abstract 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 hoping pre-factor po equal to the quanta of two-dimensional resistance R Q = h/e 2 = 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.