Optical studies of κ-(ET)2Cu[ N(CN)2] Br0.85Cl0.15

Abstract

The low-temperature ground states of the quasi two-dimensional layered organic charge transfer salts κ-(ET) 2 Cu[N(CN) 2 ]Br 1-x Cl x are known to be strongly dependent on the Br/Cl content of the anions. At low temperatures, pure Cl compounds undergo a phase transition into an antiferromagnetic ground state while the pure Br compound is a organic superconductor. Here we present polarized infrared reflectivity measurements on the compound with 85% bromine and 15 % chlorine. Our measurements were performed within the highly conducting a-c plane. From room temperature down to 90 K, we observe the typical signatures of a semiconductor in our spectra. When the sample is cooled down further, a significant amount of spectral weight is shifted towards lower frequencies. At 6 K, a Drude-like optical conductivity is observed in the far infrared.