Coulomb Gap and the Metal–Insulator Transition

Critical collapsing of the recently discovered multielectron Coulomb gap (CG) in moderately compensated neutron transmutation doped (NTD) Ge:Ga and the metal–insulator (MI) transition in this system had been under investigation. The results obtained are compared with those received for strongly and weakly compensated Ge, where single electron CG takes place far from the MI transition. Independently of the single- or multielectron origin all the gaps, as deduced by the variable range hopping (VRH) spectroscopy procedure, turned out to be collapsing in the critical point of the MI transition just the same where the low temperature metallic conductivity vanishes. One can consider such MI transition as the CG collapsing phenomenon and describe it in the frame of scaling theory as a second-order phase transition. In compensated semiconductors it is characterised by the critical index for a correlation length nearly equal to unity.