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, critically enhanced above charge-order transition temperature TCO = 136 K. The frequency dependence of incompatible with a percolation model, in which Joule heating random resistor network relevant conduction. instead argue nonlinearity relaxation time according phenomenological model mobility materials large dielectric constants, find that corresponds susceptibility χ3. Since known as probe for higher-order multipole ordering, present observation divergent behavior reveals underlying quadrupole instability system.