UNUSUAL ELECTRONIC-STRUCTURE NEAR E(F) IN THE ORGANIC SUPERCONDUCTOR KAPPA-[BIS(ETHYLEHEDITHIO)TETRATHIAFULVALENE]CU-2[N(CN)(2)]BR

We report on photoemission studies of the quasi-two-dimensional organic superconductor -[bis(ethylenedithio)tetrathiafulvalene]2Cu[N(CN)2]Br (Tc=11.6 K). Finite photoelectron-emission intensities were observed in the region close to the Fermi level (EF). However, the spectral line shape displays a broad onset from the Fermi level with a near zero emission intensity at exactly EF. This is in contrast to a sharp Fermi edge that is observed in most three- and two-dimensional metallic systems. The midpoint of the broad emission onset is shifted toward the higher binding-energy side by about 0.1 eV, resulting in an apparent gap. Strong electron-electron correlations and the excitation of phonons corresponding to molecular-vibration modes are considered as possible explanations for this unusual spectral behavior. The possible existence of a one-dimensional-like open Fermi surface in this material is discussed. © 1995 The American Physical Society.