Vicinity Of Metal-insulator Transition Research Articles

Metal-Insulator Transition in Doped Semiconductors

A survey is given of different kinds of metal—insulator transitions (MIT) in doped semiconductors. The role of electron—electron Coulomb interactions and of disorder is discussed vis-a-vis millikelvin experimental results for semimagnetic semiconductors (SMSC) in the vicinity of MIT. Critical behavior of conductivity tensor components and dielectric susceptibility at the magnetic field-induced MIT in p-type Hg1-xMnxTe is compatible with the model in which the MIT is a result of quantuni localization driven by disorder-modified electron—electron interactions. At the same time the critical behavior of the Hall coefficient suggests that, in addition to electrons forming the Fermi liquid (FL) and undergoing localization at the MIT, there is certain a concentration of local electron moments, even on the metallic side of the MIT. The formation of these moments can presumably be described in terms of a disordered Hubbard-Mott model.

Superconductivity of LiTi2O4 and related systems

New experimental results are reported on spinel LiTi2O4, and the related mixed valent systems Li1+xTi2−xO4 and Li1−xMg x Ti2O4 characterized by distinct Ti3+/Ti4+ ratios ( 1 in the latter). Specific heat data and transport properties show the high quality of the samples prepared by new routes. It is emphasized that, as far the specific heat data are concerned, the stoichiometric compound (withT c =12.4 K) satisfies BCS predictions in the weak coupling limit. Conversely, the mechanism of superconductivity is shown to be more complex when off-stoichiometric compouds are considered, due to the vicinity of metal-insulator transition. It is to be noted that the disappearance of sizable superconductivity occurs for 0.12<x<0.18, but the insulator features are only reached forx=1/3.