Anomalous variation of the normal state resistance of (kappa)-(BEDT-TTF)(sub 2)CuN(CN)(sub 2)Br with temperature and hydrostatic pressure. Small polaron

摘要

Several radical organic cation conductors have resistances, R(T)'s which attain maxima as functions of temperature. In some cases, (kappa)-(BEDT-TTF)(sub 2)CuN(CN)(sub 2)Br for instance (Sushko et al., J. Phys. I. France, 1 (1991) 1375), the maxima are suppressed by hydrostatic pressure, and with sufficient pressure R(T) becomes classified as metallic in the sense that dR/dT > 0. These variations with temperature and pressure can be understood quantitatively in terms of two equivalent models: (a) one in which the electronic density of states g((epsilon)) is constant except that it is zero for E(sub f)(= or