Third-order Electrical Conductivity of the Charge-ordered Organic Salt α-(BEDT-TTF)₂I₃

We performed third-order electrical conductivity measurements on the organic conductor α-(BEDT-TTF)₂I₃ using an ac bridge technique sensitive to nonlinear signals. Third-order conductance G₃ is clearly observed even at low electric fields, and interestingly, G₃ is critically enhanced above the charge-order transition temperature T_CO = 136 K. The observed frequency dependence of G₃ is incompatible with a percolation model, in which a Joule heating in a random resistor network is relevant to the nonlinear conduction. We instead argue the nonlinearity of the relaxation time according to a phenomenological model on the mobility in materials with large dielectric constants, and find that the third-order conductance G₃ corresponds to the third-order electric susceptibility χ3. Since the nonlinear susceptibility is known as a probe for higher-order multipole ordering, the present observation of the divergent behavior of G₃ above T_CO reveals an underlying quadrupole instability at the charge-order transition of the organic system.