Current-Density Dependence of the Charge-Ordering Gap in the Organic Salt θ-(BEDT-TTF)₂Cs<i>M</i>(SCN)₄ (<i>M</i>=Zn, Co, and Co_0.7Zn_0.3)

Abstract

We have measured and analyzed the resistivity and voltage–current characteristics of the charge-ordered organic salt θ-(BEDT-TTF) 2 Cs M (SCN) 4 ( M =Zn, Co, and Co 0.7 Zn 0.3 ). At 4.3 K, all the samples show giant nonlinear conduction, in which the c -axis resistivity markedly decreases with external current by a factor of 10 against a small electric field of 1 V/cm. The temperature dependence of the resistivity is well described by an activation law, the activation energy of which is evaluated to be 2 meV in the low-current limit, and linearly decreases with external current. The activation energy and x-ray intensity near q 2 =(0, k ,1/2) are found to show the same dependence on current density. This coincidence strongly suggests that the energy gap of the charge order indexed by q 2 is an order parameter of this phenomenon.