Depressed charge gap in the triangular-lattice Mott insulator κ- (ET)2 Cu2 (CN)3

Abstract

We have investigated polarized optical conductivity for the triangular-lattice Mott insulator κ- (ET)2 Cu2 (CN)3 over the photon-energy range of E=8 meV-5 eV. This compound is located far from the metal-insulator phase boundary in the bandwidth-controlled phase diagram of κ- (ET)2 X salts and consequently strong charge correlations are expected. However, its ground-state optical conductivity spectrum for polarizations within the bc plane shows no distinct charge gap in the energy range investigated, i.e., it is considerably smaller than Δ≈110 meV, which was previously reported for the barely insulating κ- (ET)2 Cu [N (CN)2] Cl. We attribute the pseudo-gap-like nature of the optical spectra to strong spin fluctuations emerging in an isotropic triangular lattice. The midinfrared peak is analyzed in terms of Coulomb interaction and κ- (ET)2 Cu2 (CN)3 is characterized by intermediate strength of electron correlation. © 2006 The American Physical Society.