Electronic Band Structure and Point-Contact Spectroscopy of the Organic Superconductor β-(BEDT-TTF)₂I₃

Abstract

Several band structure calculations of organic metals have been carried out, employing the extended Huckel (EH) method [1,2,3]. The molecular states are calculated using some quantum-chemistry method like CNDO, MINDO, etc. The overlap between states of neighboring molecules is then calculated in some approximation, and transfer integrals are evaluated from the overlap integrals using the Huckel approximation. From these transfer integrals the bandwidth and anisotropy are determined. This is illustrated in Fig. 1 for a case of one molecule per unit cell (a) and two molecules per unit cell (b). The EH method has been applied to β-BEDT-TTF₂I₃ (ET) by Mori et al (5) and Whangbo et al (3). They obtain an (essentially) 2-dimensional band structure (Fig. lc), with a closed, nearly cylindrical Fermi surface of holes around point Г $(\overrightarrow k = (0,0,0))$.