Visualization of a nonlinear conducting path in an organic molecular ferroelectric by using emission of terahertz radiation

Nonlinear electric transport and switching to a negative resistance state are typical electric-field-induced phenomena in correlated electron materials, while their mechanisms are generally difficult to solve. In the present study, we apply the terahertz-radiation imaging method to an organic molecular ferroelectric, \ensuremath{\alpha}-type bis(ethylenedithio)tetrathiafulvalene iodide salt, and investigate the nature of its negative resistance phenomenon. When the negative resistance state is produced, the ferroelectric order is melted in an elongated region with the width of \ensuremath{\sim}100 \ensuremath{\mu}m and that region grows along the direction inclined by about 40\ifmmode^\circ\else\textdegree\fi{} from the $b$ axis with the increase of nonlinear current. A comparison of the terahertz radiation intensity with the current magnitude revealed that the melted region forms a conducting path. We interpreted the diagonal growth of the conduction path by taking into account the anisotropy of the intermolecular transfer integrals.