Ferroelectric nature and real-space observations of domain motions in the organic charge-transfer compound tetrathiafulvalene-p-chloranil

Ferroelectricity in an organic charge-transfer compound, tetrathiafulvalene-p-chloranil (TTF-CA), originating from the one-dimensional valence and lattice instabilities, has been investigated by an electroreflectance (ER) method. Microscopic ER spectroscopy in the visible region enables real-space observations of both ferroelectric domain structures with a few hundred micrometers in size and depinning of the domain walls under strong electric fields. In addition, from ER spectroscopy in the infrared molecular-vibration region, we demonstrate that field-induced changes in the dimeric molecular displacement as well as charge transfer between TTF and CA molecules play an important role in the large dielectric response in TTF-CA.