Magnetotransport studies of the organic superconductor kappa -(BEDT-TTF)2Cu(NCS)2under pressure: the relationship between carrier effective mass and critical temperature

Abstract

Magnetotransport measurements have been carried out on the organic superconductor kappa -(BEDT-TTF)₂Cu(NCS)₂ at temperatures down to 500 mK and in hydrostatic pressures up to 16.3 kbar. The observation of Shubnikov-de Haas and magnetic breakdown oscillations has allowed the pressure dependences of the area of the closed pocket of the Fermi surface and the carrier effective masses to be deduced and compared with simultaneous measurements of the superconducting critical temperature T_c. The effective mass measured by the temperature dependence of the Shubnikov-de Haas oscillations is found to fall rapidly with increasing pressure up to a critical pressure P_c approximately=5 kbar. Above P_c a much weaker pressure dependence is observed; T_c also falls rapidly with pressure from 10.4 K at ambient pressure to zero at around P_c. This strongly suggests that the enhanced effective mass and the superconducting behaviour are directly connected in this organic superconductor. A simplified model of the band structure of kappa -(BEDT-TTF)₂Cu(NCS)₂ has been used to derive the bare band masses of the electrons from optical data. Comparisons of these parameters with cyclotron resonance data and the effective masses measured in the present experiments indicate that the greater part of the enhancement of the effective mass necessary for superconductivity in this material is due to quasiparticle interactions, with the electron-phonon interactions playing a secondary role. The dependence of T_c on the effective mass may be fitted satisfactorily to a suitably parametrized weak-coupling BCS expression, although this cannot be taken as a definitive proof of the nature of superconductivity in organic conductors.