Resistivity and Penetration Depth Measurements of Organic Superconductors in High Magnetic Fields Using a Tunnel Diode Oscillator

We have made measurements of resistivity and penetration depth in dc-and pulsed-magnetic fields of the organic superconductors α-(ET)₂NH₄Hg(SCN)₄, and κ-(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⊥ of 5-17. In κ-(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, α-(ET)₂NH₄HG(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.