Unusual electronic structure near ${\mathit{E}}_{\mathit{F}}$ in the organic superconductor κ-[bis(ethylenedithio)tetrathiafulvalene${]}_2$Cu[N(CN${)}_2$]Br

We report on photoemission studies of the quasi-two-dimensional organic superconductor κ-[bis(ethylenedithio)tetrathiafulvalene${]}_2$Cu[N(CN${)}_2$]Br (${\mathit{T}}_{\mathit{c}}$=11.6 K). Finite photoelectron-emission intensities were observed in the region close to the Fermi level (${\mathit{E}}_{\mathit{F}}$). However, the spectral line shape displays a broad onset from the Fermi level with a near zero emission intensity at exactly ${\mathit{E}}_{\mathit{F}}$. 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.