Electrical resistivity measurements on a single crystal of the heavy-fermion superconductor CeCoIn5 at pressures to 4.2 GPa reveal a strong crossover in transport properties near P(*) approximately 1.6 GPa, where T(c) is a maximum. The temperature-pressure phase diagram constructed from these data provides a natural connection to cuprate physics, including the possible existence of a pseudogap.

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We studied the impurity scattering on the Ϯs-wave superconductor ͑SC͒, with realistic parameters for the Fe pnictide SCs. Using the T-matrix method, generalized for the two bands, we found that the strong scattering limit of impurities forms an off-centered resonance state inside the superconducting ͑SC͒ gap, which modifies, surprisingly, the density of states ͑DOS͒ of a fully opened gap to a V-shaped DOS as if in the case of a d-wave SC. This behavior provides coherent explanations to the several conflicting experiments of the Fe-based SC: ͑1͒ the V-shaped DOS observed in photoemission and tunneling spectroscopy but with an isotropic gap; ͑2͒ the power-law behavior of the nuclear-spin-lattice relaxation rate ͑1 / T 1 ϳ T ␣ with ␣ ϳ 3͒ down to very low temperatures. We also extended the same T-matrix method to study the impurity suppression of the critical temperature T c of the Ϯs-wave pairing state. We found that both magnetic and nonmagnetic impurities suppress T c with a rate that is practically indistinguishable from the standard d-wave case despite a possibly large difference of the positive and the negative s-wave order parameter magnitudes.

We consider the minimal two-band model for the Fe-based superconductors with a phenomenological pairing interaction which mimics short-range antiferromagnetic (AFM) fluctuations. Two superconducting (SC) gap solutions are found to exist with the model: sign-changing s-wave gap ($\pm$s-wave) and double d-wave gap states. Both solutions hold the approximate relation $\Delta_{h} ^{max} N_h \approx \Delta_{e} ^{max} N_e$, a generic feature of two band model with a dominant interband pairing interaction. We carried out the calculations of the SC properties of the both SC states such as the density of states, temperature dependencies of spin-lattice relaxation rate $1/T_1$, Knight shift, and penetration depth, particularly taking into account of the interband coherence factors. The results are discussed in comparison with the currently available experimental data.Comment: Published version in Phys. Rev. B 78, 134523 (2008