Far-infrared optical response of neutral-ionic phase transition in an organic charge-transfer complex

Infrared optical 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 the decrease of 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.