Dynamics of Charge Ordering in the Nonlinear Regime of θ-(BEDT-TTF)2CsZn(SCN)4

We present thermoelectric power measurements in the nonlinear-resistivity regime of θ-(BEDT-TTF)2CsZn(SCN)4 [BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene]. The AC Seebeck coefficient is found to change its sign from positive to negative with increasing DC current, which coincides with a drop in resistivity, implying that the nonlinear regime in θ-(BEDT-TTF)2CsZn(SCN)4 is not due to self-heating. Three different mechanisms are put forward to explain the origin of the sign change of the thermoelectric power with increasing electric current.