MAGNETOTRANSPORT STUDIES OF THE ORGANIC SUPERCONDUCTOR KAPPA-(BEDT-TTF)2CU(NCS)2 UNDER PRESSURE - THE RELATIONSHIP BETWEEN CARRIER EFFECTIVE-MASS AND CRITICAL-TEMPERATURE

Magnetotransport measurements have been carried out on the organic sup erconductor kappa-(BEDT-TTF)2Cu(NCS)2 at temperatures down to 500 mK a nd in hydrostatic pressures up to 16.3 kbar. The observation of Shubni kov-de Haas and magnetic breakdown oscillations has allowed the pressu re dependences of the area of the closed pocket of the Fermi surface a nd the carrier effective masses to be deduced and compared with simult aneous measurements of the superconducting critical temperature T(c). The effective mass measured by the temperature dependence of the Shubn ikov-de Haas oscillations is found to fall rapidly with increasing pre ssure up to a critical pressure P(c) congruent-to 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 superc onducting behaviour are directly connected in this organic superconduc tor. A simplified model of the bandstructure of kappa-(BEDT-TTF)2Cu(NC S)2 has been used to derive the bare band masses of the electrons from optical data. Comparisons of these parameters with cyclotron resonanc e data and the effective masses measured in the present experiments in dicate that the greater part of the enhancement of the effective mass necessary for superconductivity in this material is due to quasipartic le interactions, with the electron-phonon interactions playing a secon dary 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.