Absence of minimum metallic conductivity in Gd(3-x)vxS4 at very low temperature and evidence for a Coulomb gap

Gd(3-x)vxS4 provides a convenient analog of a compensated semiconductor in which, for x0.3, 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 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. © 1984 The American Physical Society.