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

Abstract

We have made measurements of resistivity and penetration depth in dc-and pulsed-magnetic fields of the organic superconductors α- (ET) 2 NH 4 Hg(SCN) 4, and κ- (ET) 2 Cu(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 ⊥ of 5-17. In κ- (ET) 2 Cu(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, α- (ET) 2 NH 4 Hg(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.