Disorder-induced gap in the normal density of states of the organic superconductor $\kappa$-(BEDT-TTF)${}_2$Cu[N(CN)${}_2$]Br

Abstract

The density of states of the organic superconductor $\kappa$-(BEDT-TTF)${}_2$Cu[N(CN)${}_2$]Br, measured by scanning tunneling spectroscopy on \textit{in-situ} cleaved surfaces, reveals a logarithmic suppression near the Fermi edge persisting above the critical temperature $T_{\mathrm{c}}$. A soft Hubbard gap as predicted by the Anderson-Hubbard model for systems with disorder exactly describes the experimentally observed suppression. The electronic disorder also explains the diminished coherence peaks of the quasiparticle density of states below $T_{\mathrm{c}}$.