On the resistance maximum in high-T_cK-(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, W_SB (∼0.2 eV), and hence reduces the gap E_G of ( 1 2) (V-W_SB). 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 E_G = 300-500 K at room temperature and negative (band-overlapped) E_G =-(200-300) K at helium temperature.