Incoherent-Coherent Crossover Behavior in κ-(BEDTTTF)2 X, Probed by 13C-NMR and Optical Studies

We have measured 13C-NMR and reflection spectrum in κ-(BEDTTTF)2 Ag(CN)2·H2O. No increase of the AF magnetic fluctuation as in κ-(BEDT-TTF)2Cu[N(CN)2]C was observed. Results of 13C-NMR suggested the emergence of superconductivity does not require the increase of the AF magnetic fluctuation, but a crossover to the Fermi liquid regime. Optical studies suggested the crossover corresponds to the development of the coherence of conduction electrons.