Open Access

István Kézsmárki, Y. Shimizu, G. Mihály, Y. Tokura, K. Kanoda, G. Saito

• We have investigated polarized optical conductivity for the triangular-lattice Mott insulator κ−(ET)2Cu2(CN)3 over the photon-energy range of E=8meV–5eV. This compound is located far from the metal-insulator phase boundary in the bandwidth-controlled phase diagram of κ−(ET)2X 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≈110meV, which was previously reported for the barely insulating κ−(ET)2Cu[N(CN)2]Cl [Kornelsen et al., 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 κ−(ET)2Cu2(CN)3 is characterized by intermediate strength of electron correlation.