RESISTIVITY AND PENETRATION DEPTH MEASUREMENTS OF ORGANIC SUPERCONDUCTORS IN HIGH MAGNETIC FIELDS USING A TUNNEL DIODE OSCILLATOR

摘要

We have made measurements of resistivity and penetration depth in dc-and pulsed-magnetic fields of the organic superconductors alpha-(ET)_2NH_4Hg(SCN)_4, and kappa-(ET)_2Cu(NCS)_2 using a resonant rf circuit powered by a tunnel diode oscillator (TDO)). We compare the critical fields as measured by resistivity, and the TDO. All the superconductors we have studied have anisotropic critical fields with a ratio H_(c2)// to H_(c2)perpendicular of 5-17. In kappa-(ET)_2Cu(NCS)_2, we observed an almost monotonic increase in the parallel critical fields up to 24 T at 390 mK, and a sharp cusp in the critical field as a function of angle when the sample is parallel to the magnetic field. This suggests that the superconducting layers are only Josephson-coupled and the superconductivity is two-dimensional (2D). Another salt, alpha-(ET)_2NH_4HG(SCN)_4, shows similar cusp behavior, but this material shows evidence of extreme Pauli limiting when the field is turned parallel to the conducting layers.