Third-order Electrical Conductivity of the Charge-ordered Organic Salt alpha-(BEDT-TTF)(2)I-3

We performed third-order electrical conductivity measurements on the organic conductor alpha-(BEDT-TTF)(2)I-3 using an ac bridge technique sensitive to nonlinear signals. Third-order conductance G(3) is clearly observed even at low electric fields, and interestingly, G(3) is critically enhanced above the charge-order transition temperature T-CO = 136 K. The observed frequency dependence of G(3) 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 G(3) corresponds to the third-order electric susceptibility chi(3). Since the nonlinear susceptibility is known as a probe for higher-order multipole ordering, the present observation of the divergent behavior of G(3) above T-CO reveals an underlying quadrupole instability at the charge-order transition of the organic system.