Theory of Infrared Conductivity from Density Waves: (TMTSF)₂CIO₄

Abstract

In Fig. 1 I show schematically the real part of the conductivity below about 50 wavenumbers obtained by NG, TIMUSK, and BECHGAARD [1] for (TMTSF)₂ClO₄ at 2K and zero pressure. Similar results were obtained by CHALLENER, RICHARDS and GREENE [2]. When I first saw these results late in 1982, we believed that at this temperature the specimen was in a spin density wave (SDW) state. The sharp peak at 7 cm⁻¹ and the asymmetric peak commencing slightly above 20 cm⁻¹ suggest the sharp phonon peaks and asym-metric single-particle peak in Reσ(ω) for TEA(TCNQ)₂ explained by the theory of Michael RICE and colleagues [3,4]. In the Rice theory, intramolecular vibrational modes which are infrared inactive for a normal metallic or insulating state become active in the presence of a charge density wave (CDW), and the phonon states with wavevector equal to the CDW wavevector Q and with frequency in the gap for electron states are very sharp. The problem appeared then to be that we required a theory for a similar effect arising from a SDW in (TMTSF) ₂ClO₄.