Crossover Behavior of Variable Range Hopping in Bi-Doped C60

Mott variable-range hopping (VRH) is characterized by an exp [1/T (1/4)] behavior in the temperature dependence of resistivity and is generally observed near the metal-insulator transition as electron screening increases and the Coulomb gap disappears near the metallic phase, while Efros-Shklovskii VRH, characterized by exp [1/T (1/2)], is found deeper in the insulating region. We have investigated the transport properties of Bi-doped fullerene C-60. Samples were prepared via solid state reaction in a sealed quartz tube near 600A degrees C. The resistivity data can be fit with a single function exp [1/T (upsilon) ] (upsilon=1/4 or 1/2, depending on the Bi concentration) over the entire temperature range below 300 K and over 5-6 orders of magnitude in resistivity. upsilon changes from 1/4 to 1/2 as the Bi concentration increases, suggesting a crossover from Mott VRH to intergranular tunneling at higher Bi concentration. The thermoelectric Seebeck coefficient was also measured and is about 20 mu V/K at room temperature. It decreases with decreasing temperature. The thermal conductivity of the doped samples is extremely low.