Penetration depth studies of organic and heavy fermion superconductors in the Pauli paramagnetic limit

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Abstract We report penetration depth and resistivity measurements on a number of organic and heavy fermion superconductors in the Pauli paramagnetic limit using a self resonant tank circuit based on a tunnel diode oscillator (TDO). We call a superconductor Pauli limited when the interaction of the applied magnetic field with the electron spin limits the superconducting state, in contrast to orbital limiting, the traditional effect where vortices eventually destroy the superconducting state. In the Pauli limit, Hc2 can change from a second order to a first order phase transition at low temperatures, or in a very clean sample a new superconducting state with a spatially varying order parameter has been predicted, which we refer to as the FFLO state. We have done extensive experiments that show a range of Pauli limited behavior in anisotropic superconductors. Our results for α-(ET)2NH4Hg(SCN)4, κ-(ET)2Cu(NCS)2, and CeCoIn5 show that the nature of the Pauli limiting state is sensitive to the mean freepath in the sample. In CeCoIn5 we have observed clear evidence of the FFLO state using our TDO method, and our data agrees with specific heat and magnetization measurements. We also show that the application of pressure will suppress the Pauli limiting in κ-(ET)2Cu(NCS)2.