Current-density dependence of the charge-ordering gap in the organic salt θ-(BEDT-TTF) ₂CsM(SCN) ₄ (M = Zn, Co, and Co _0.7Zn _0.3)

We have measured and analyzed the resistivity and voltage-current characteristics of the charge-ordered organic salt θ-(BEDT-TTF) ₂CsM(SCN) ₄ (M = Zn, Co, and Co _0.7Zn _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 lowcurrent 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.