Critical field and Shubnikov-de Haas Oscillations of κ-(BEDT-TTF)2Cu(NCS)2 under pressure

We have used a novel experimental method to study the crossover of an anisotropic superconductor from a possible Pauli limited superconducting state to a vortex limited superconducting state by applying pressure. The new apparatus combined a tuned tank circuit with a nonmetallic diamond anvil cell to measure the change in critical field with angle in kappa-(BEDT-TTF) Cu-2(NCS)(2) at pressures up to 1.75 kbar and at temperatures down to 70 mK. The critical fields ( in the perpendicular or parallel orientation to the conducting planes) have been found to decrease by more than 90% within less than 2 kbar of pressure. In the parallel orientation, at 1.75 kbar, we have seen a clear change from the ambient pressure behavior of the critical field with temperature at low temperatures. Up to P = 1.75 kbar, the H-c2(theta) phase diagram is in good agreement with the theoretical prediction for weakly coupled layered superconductors. We have also succeeded in measuring oscillations in the resistivity of the normal state at higher magnetic field which could be used to find the effective quasi-particle mass. The alpha-orbit Shubnikovde Haas frequency was found to increase at a rate of 44 T/kbar. Our experiment opens the possibility for further investigations of the effective mass with pressure, especially because the setup is suitable for pulsed fields as well.