Pressure-induced Crossover from Variable Range Hopping to Electron–Electron Interaction in the Organic Conductor (DOET)₂BF₄

Abstract

This paper reports the pressure effect on resistivity in the quasi two-dimensional (2D) organic conductor (DOET)₂BF₄ with orientational disorder of BF₄ anions. At ambient pressure, the resistivity shows metal–insulator transition at 80 K and 2D variable range hopping (VRH) conduction below 80 K. The insulating behavior is suppressed by applying pressure and the temperature dependence of the resistivity shows the crossover from the 2D VRH to the logarithmic divergence as applied pressure increases. From the observation of the positive magnetoresistance, the logarithmic divergence of the resistivity is not attributed to the Anderson localization, but to the effect of the electron–electron interaction. This crossover is simply understood in terms of the screening effect on the random potential by increase of bandwidth due to applied pressure. It is found that a value of sheet conductance at low temperatures above 8 kbar, where the insulating state is almost suppressed, approaches to the minimum conductivity for a metallic state, σ_min=e²⁄h=1⁄(25.8kΩ), first proposed by Mott.