Optical spectroscopy of the neutral–ionic phase transition in a charge-transfer complex: TTF and tetrahalo-p-benzoquinones

Abstract Far-infrared reflectivity spectra have been investigated for a charge-transfer complex of tetrathiafulvalene and 2-bromo-3,5,6-trichloro- p -benzoquinone, which undergoes a neutral–ionic (N–I) transition at T c =68 K. With decrease in temperature toward T c , the far-infrared reflectivity spectrum is gradually transformed into a Drude-like high reflectance band edging around 100 cm −1 , and the optical conductivity shows an anomalous peak around 10 cm −1 . The far-infrared anomaly is related to the gigantic dielectric response just above T c , which are both discussed in terms of dynamics of N–I domain walls with fractional charge.