ON THE RESISTANCE MAXIMUM IN HIGH-TC K-(BEDT-TTF)2CU(NCS)2

To explain the resistance maximum around 100 K observed in a title superconductor of highly correlated Fermi liquid, we present a quasi-particle band model. The band is assumed to be symmetrically split with respect to the Fermi level, due to the inter-site Coulomb interaction V (typically 0.2-0.5 eV). As the temperature is lowered or the pressure is applied, the transfer integrals increase as a consequence of the lattice contraction. This leads to an increase in the width of each split subbands, WSB (∼0.2 eV), and hence reduces the gap EG of ( 1 2) (V-WSB). The resistance maximum is thus understood as an insulator-to-metal transition and numerical calculations of the number density of carriers well reproduce the experiments with positive EG = 300-500 K at room temperature and negative (band-overlapped) EG =-(200-300) K at helium temperature. © 1990.