Direct observation of ferroelectric domains created by Wigner crystallization of electrons in α-[bis(ethylenedithio)tetrathiafulvalene]2I3

The organic conductor alpha-[bis(ethylenedithio)tetrathiafulvalene](2)I-3 undergoes a ferroelectric transition due to the Wigner-crystal-like charge ordering. The present paper reports the direct observation of 180 degrees polar domains growing in the ferroelectric phase by means of optical second-harmonic generation interferometry. Mapping observation of the nonlinear optical interference revealed the development of large polar domains, illustrating a strong tendency of single-domain growth in the ferroelectric compound. The domain structures were varied when the crystal was annealed above the transition temperature and recooled into the ferroelectric phase. The variation indicates the mobile nature of the domains in the ferroelectric organic conductor.