Electronic band structure and superconducting transition of κ-(BEDT–TTF)2I3

Abstract

Crystals of κ-(BEDT–TTF)₂I₃ have been prepared electrochemically from a tetrahydrofuran (THF) solution containing the mixed supporting electrolytes {[(C₄H₉)₄N]I₃(95%)–[(C₄H₉)₄N]AuI₂(5%)} and also from a THF solution without [(C₄H₉)₄N]Aul₂. A resistivity drop indicating a superconducting transition was observed at 4.2 K (mid-point). The T_c decreased with increasing pressure (dT_c/dP=–0.8 K kbar^–1). The resistivity exhibited an anomaly around 170 K, which was coupled with the structural phase transition accompanied by the change of the space group (P2₁/c→P2₁). The crystal structure was determined at 295, 150 and 10 K. The extinction rule of the X-ray intensity data collected at 295 K confirmed the space group to be P2₁/c as reported previously. The (h 0 l) reflections at 150 and 10 K revealed the absence of c-glide symmetry. The low-temperature crystal structure analyses showed conformational change of the ethylene groups. The low-temperature band structure calculation based on a simple extended Hückel approximation gave Fermi surfaces consistent with de Haas–van Alphen experiments.