Absence of minimum metallic conductivity inGd(3−x)vxS4at very low temperature and evidence for a Coulomb gap

Abstract

${\mathrm{Gd}}_{\mathrm{}(3-x)\mathrm{}}{v}_x{\mathrm{S}}_4$ provides a convenient analog of a compensated semiconductor in which, for $x\simeq 0.3$, the mobility edge can be tuned smoothly through the Fermi energy by the application of a magnetic field. The results of a search for a minimum metallic conductivity demonstrate that, down to $T=6\mathrm{}$ mK, the metal-insulator transition is smooth. In the insulating regime, the temperature dependence of the conductivity was more consistent with the theory of mutual interactions than with the theory of pure localization.