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

We have investigated polarized optical conductivity for the triangular-lattice Mott insulator kappa-(ET)(2)Cu-2(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 kappa-(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 Delta approximate to 110 meV, which was previously reported for the barely insulating kappa-(ET)(2)Cu[N(CN)(2)]Cl [Kornelsen , Solid State Commun. 81, 343 (1992)]. 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 kappa-(ET)(2)Cu-2(CN)(3) is characterized by intermediate strength of electron correlation.