Impurity-induced in-gap state and Tc in sign-reversing s-wave superconductors: analysis of iron oxypnictide superconductors

The sign-reversing fully gapped superconducting state, which is expected to be realized in oxypnictide superconductors, can be prominently affected by nonmagnetic impurities due to the interband scattering of Cooper pairs. We study this problem based on the isotropic two-band BCS model: In oxypnictide superconductors, the interband impurity scattering $I'$ is not equal to the intraband one $I$. In the Born scattering regime, the reduction in Tc is sizable and the impurity-induced density of states (DOS) is prominent if $I\sim I'$, due to the interband scattering. Although impurity-induced DOS can yield a power-law temperature dependence in $1/T_1$, a sizable suppression in Tc is inevitably accompanied. In the unitary scattering regime, in contrast, impurity effect is very small for both Tc and DOS except at $I=I'$. By comparing theory and experiments, we expect that the degree of anisotropy in the $s_\pm$-wave gap function strongly depends on compounds.Comment: 16 pages, 5 figures, to be published in New. J. Phy

Distinct transport behaviors of LaFe1-yCoyAsO1-xFx (x=0.11) between the superconducting and nonsuperconducting metallic y regions divided by y ~ 0.05

Electrical resistivities, Hall coefficients and thermoelectric powers have been measured for polycrystalline samples of LaFe1-yCoyAsO1-xFx (x=0.11) with various values of y. The results show that there exists clear distinction of these transport behaviors between the superconducting and nonsuperconducting metallic regions of y divided by the boundary value yc~0.05. We have found that the behaviors in both regions are very similar to those of high-Tc Cu oxides in the corresponding phases. If they reflect, as in the case of Cu oxides, effects of strong magnetic fluctuations, the energy scale of the fluctuations is considered to be smaller than that of the high Cu oxides by a factor of ~1/2. Arguments on the electronic nature and superconducting symmetry are presented on the basis of the observed small rate of the Tc suppression rate by the Co doping.Comment: 8 pages, 4 figures, submitted to J. Phys. Soc. Jp

Nuclear magnetic relaxation and superfluid density in Fe-pnictide superconductors: An anisotropic \pm s-wave scenario

We discuss the nuclear magnetic relaxation rate and the superfluid density with the use of the effective five-band model by Kuroki et al. [Phys. Rev. Lett. 101, 087004 (2008)] in Fe-based superconductors. We show that a fully-gapped anisotropic \pm s-wave superconductivity consistently explains experimental observations. In our phenomenological model, the gaps are assumed to be anisotropic on the electron-like \beta Fermi surfaces around the M point, where the maximum of the anisotropic gap is about four times larger than the minimum.Comment: 10 pages, 8 figures; Submitted versio

Gap symmetry and structure of Fe-based superconductors

The recently discovered Fe-pnictide and chalcogenide superconductors display low-temperature properties suggesting superconducting gap structures which appear to vary substantially from family to family, and even within families as a function of doping or pressure. We propose that this apparent nonuniversality can actually be understood by considering the predictions of spin fluctuation theory and accounting for the peculiar electronic structure of these systems, coupled with the likely 'sign-changing s-wave' (s\pm) symmetry. We review theoretical aspects, materials properties and experimental evidence relevant to this suggestion, and discuss which further measurements would be useful to settle these issues.Comment: 86 pages, revie

Orbital-Selective Superconductivity and the Effect of Lattice Distortion in Iron-Based Superconductors

The superconducting (SC) state of iron-based compounds in both tetragonal and orthorhombic phases is studied on the basis of an effective Hamiltonian composed of the kinetic energy including the five Fe 3d-orbitals, the orthorhombic crystalline electric field (CEF) energy, and the two-orbital Kugel'-Khomski\u{i}-type superexchange interaction. Our basic assumption is that the antiferromagnetic (AF) state in the parent compounds can be described by the $d_{xz}$ and $d_{yz}$ orbitals, and that the electrons in these orbitals have relatively strong electron correlation in the vicinity of the AF state. In order to study the physical origin of the structure-sensitive SC transition temperature, the effect of orthorhombic distortion is taken into account as the energy-splitting, $\Delta_{\textrm{ortho.}}$, between the $d_{xz}$ and $d_{yz}$ orbitals. We find that the eigenvalue of the linearized gap equation decreases accompanied with the reduction of the partial density of states for the $d_{xz}$ and $d_{yz}$ orbitals as $\Delta_{\textrm{ortho.}}$ increases, and that the dominant pairing symmetry is an unconventional fully gapped $s_{+-}$-wave pairing. We also find large anisotropy of the SC gap function in the orthorhombic phase. We propose that the CEF energy plays an important role in controlling $T_{\textrm{c}}$ and the SC gap function, and that orbital-selective superconductivity is a key feature in iron-based superconductors, which causes the structure-sensitive $T_{\textrm{c}}$.Comment: 11 pages, To appear in J. Phys. Soc. Jp