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 Physics , 2008, DOI: 10.1143/JPSJ.77.113710 Abstract: To understand the impurity effect on Tc in FeAs superconductors, we analyze a simple two-band BCS model with repulsive interband interaction. The realized fully-gapped superconducting state with sign reversal, which is predicted by spin fluctuation theories in this compound, is suppressed by impurities due to the interband hopping of Cooper pairs, if the interband impurity scattering $I'$ is equal to the intraband one $I$. When $|I'/I| < 1$, in highly contrast, Tc is almost unchanged by strong impurity scattering since the interband scattering is almost prohibited by the multiple scattering effect. Since $|I'/I| \sim 0.5$ is expected, the robustness of superconductivity against impurities in FeAs superconductors is naturally understood in term of the sign reversing fully-gapped state.
 Physics , 2009, DOI: 10.1103/PhysRevB.82.174507 Abstract: We investigate the impurity scattering rates for quasi-particles in vortex cores of sign-reversing s-wave superconductors as a probe to detect the internal phase difference of the order parameters among different Fermi surfaces. The impurity scattering rates and coherence factors are related to quasiparticle interference effect by the scanning tunneling microscopy and spectroscopy technique. With use of the Born and Kramer-Pesch approximations for the Andreev bound states, we show that the sign-reversed forward scatterings are dominant in vortex cores. Owing to the coherence factor in vortex cores of \pm s-wave superconductors, the impurity scattering rate of the Andreev bound states has a characteristic distribution on the Fermi surfaces. For comparison, the impurity scattering rates in vortex cores of s-wave and d-wave superconductors are also discussed.
 Physics , 2010, Abstract: We study the impurity effects on the transition temperature Tc with use of the T-matrix approximation. We propose a way to visualize the multi-orbital effect by introducing the hybridization function characterizing the multi-orbital effect for the impurity scattering. Characterizing function does not depend on the superconducting pairing symmetry, since this function is defined by the eigenvectors in normal states. The result indicates that an impurity-robust superconductivity does not necessarily imply a sign- preserving pairing. Visualizing the hybridization effect in the effective five-band model for LaFeAsO, we show that the impurity effect on Tc is relatively weaker than that in single-band models.
 Physics , 2009, DOI: 10.1143/JPSJ.78.084718 Abstract: A single impurity problem is investigated for multiband s-wave superconductors with different sign order parameters (+-s-wave superconductors) suggested in Fe-pnictide superconductors. Not only intraband but also interband scattering is considered at the impurity. The latter gives rise to impurity-induced local boundstates close to the impurity. We present an exact form of the energy of the local boundstates as a function of strength of the two types of impurity scattering. The essential role of the impurity is unchanged in finite number of impurities. The main conclusions for a single impurity problem help us understand effects of dense impurities in the +-s-wave superconductors. Local density of states around the single impurity is also investigated. We suggest impurity site nuclear magnetic resonance as a suitable experiment to probe the local boundstates that is peculiar to the +-s-wave state. We find that the +-s-wave model is mapped to a chiral dx2-y2+-idxy-wave, reflecting the unconventional nature of the sign reversing order parameter. For a quantum magnetic impurity, interband scattering destabilizes the Kondo singlet.
 Physics , 2009, DOI: 10.1103/PhysRevLett.103.177001 Abstract: Based on the five-orbital model, we study the effect of local impurity in iron pnictides, and find that the interband impurity scattering is promoted by the d-orbital degree of freedom. This fact means that the fully-gapped sign-reversing s-wave state, which is predicted by spin fluctuation theories, is very fragile against impurities. In the BCS theory, only 1% impurities with intermediate strength induce huge pair-breaking, resulting in the large in-gap state and prominent reduction in Tc, contrary to the prediction based on simple orbital-less models. The present study provides a stringent constraint on the pairing symmetry and the electronic states in iron pnictides.
 Yunkyu Bang Physics , 2010, DOI: 10.1103/PhysRevLett.106.259701 Abstract: We pointed out that the main claim of Ref.[1] that the presence of the orbital degree of freedom is essential to describe the correct impurity effects for Iron-based superconductors is incorrect and demonstrated the equivalent results of Tc suppression due to impurities by both the band basis model and the orbital basis model.
 Physics , 2011, DOI: 10.1103/PhysRevB.85.092505 Abstract: We perform large-scale numerical calculations self-consistently solving the Bogoliubov-de Gennes (BdG) equations in the magnetic field together with random impurities to directly demonstrate the typical quasi-particle interference (QPI) in the presence of vortices as observed by scanning tunneling microscopy/spectroscopy experiments in unconventional superconductors. The calculations reveal that vortex itself never works as a scatter causing the QPI pattern but vortex core containing impurity brings about the enhancement of the sign-preserving QPI peaks. Its origin is Andreev bound-states distorted by impurity, and all the measurement findings are consistently explained by the scenario based on the numerical results.
 Physics , 2008, DOI: 10.1103/PhysRevB.79.060503 Abstract: We have performed a temperature-dependent angle-integrated laser photoemission study of iron oxypnictide superconductors LaFeAsO:F and LaFePO:F exhibiting critical transition temperatures (Tc's) of 26 K and 5 K, respectively. We find that high-Tc LaFeAsO:F exhibits a temperature-dependent pseudogap-like feature extending over ~0.1 eV about the Fermi level at 250 K, whereas such a feature is absent in low-Tc LaFePO:F. We also find ~20-meV pseudogap-like features and signatures of superconducting gaps both in LaFeAsO:F and LaFePO:F. We discuss the possible origins of the unusual pseudogap-like features through comparison with the high-Tc cuprates.
 Degang Zhang Physics , 2009, DOI: 10.1103/PhysRevLett.103.186402 Abstract: The low energy band structure of the FeAs based superconductors is fitted by a tight binding model with two Fe ions per unit cell and two degenerate orbitals per Fe ion. Based on this, superconductivity with extended s-wave pairing symmetry of the form $\cos k_x +\cos k_y$ is examined. The local density of states near an impurity is also investigated by using T-matrix approach. For the nonmagnetic scattering potential, we found that there exist two major resonances inside the gap. The height of the resonance peaks depends on the strength of the impurity potential. These in-gap resonances are originated in the Andreev's bound states due to the quasiparticle scattering between the hole Fermi surfaces around $\Gamma$ point with positive order parameter and the electron Fermi surfaces around $M$ point with negative order parameter.
 Physics , 2009, DOI: 10.1016/j.physc.2009.10.043 Abstract: We discuss the surface Andreev bound states in Fe-based superconductors with the use of an effective five-band model and investigate the surface-angle dependence of the tunneling spectroscopy by a quasiclassical approach for an isotropic and an anisotropic /pm s-wave gap superconductivity. We show that information on the normal state is important for the Andreev bound state and its peak positions do not depend on the gap amplitude anisotropy.
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