Neutron-Inelastic-Scattering Peak by Dissipationless Mechanism in the s++ -wave State in Iron-based Superconductors

We investigate the neutron scattering spectrum in iron pnictides based on the random-phase approximation in the five-orbital model with a realistic superconducting (SC) gap, Delta=5meV. In the normal state, the neutron spectrum is suppressed by large inelastic quasi-particle (QP) scattering rate gamma* ~ Delta. In the fully-gapped s-wave state without sign reversal (s++), a hump-shaped enhancement appears in the neutron spectrum just above 2Delta, since the inelastic QP scattering is prohibited by the SC gap. That is, the hump structure is produced by the dissipationless QPs for Ek<3Delta. The obtained result is more consistent with experimental spectra, compared to the results of our previous paper with Delta=50meV. On the other hand, both height and weight of the resonance peak in the fully-gapped s-wave states with sign reversal (s+-) are much larger than those observed in experiments. We conclude that experimentally observed broad spectral peak in iron pnictides is created by the present "dissipationless mechanism" in the s++ -wave state.Comment: 9 pages, 6 figures, note added in conclusion following an editor's suggestio

Surface density of states of s+-wave Cooper pairs in a two-band model

We calculate surface density of state (SDOS) of s+-wave Cooper pair in two-band superconductor model, where gap functions have different signs between two bands. We find that Andreev bound state appears at surface due to the sign change in the gap function in the interband quasiparticle scattering. However, we do not obtain the zero-energy peak of SDOS in contrast to the d-wave case. The tunneling spectroscopy of s+-wave is much more complex as compared to the d-wave case realized in high-Tc cuprates.Comment: 7 pages, 10 figure

Orbital Fluctuation Theory in Iron Pnictides: Effects of As-Fe-As Bond Angle, Isotope Substitution, and Z2Z^2-Orbital Pocket on the Superconductivity

We study the pairing mechanism in iron pnictide superconductors based on the five-orbital Hubbard-Holstein model. Due to Fe-ion oscillations, the s-wave superconducting (SC) state without sign reversal (s_{++}-wave state) is induced by orbital fluctuations by using realistic model parameters. The virtue of the present theory is that the famous empirical relation between Tc and the As-Fe-As bond angle is automatically explained, since the electron-phonon (e-ph) coupling that creates the orbital fluctuations is the strongest when the As$_4$-tetrahedron is regular. The negative iron isotope effect is also reproduced. In addition, the magnitude of the SC gap on hole-pockets is predicted to be rather insensitive to the corresponding d-orbital (xz/yz- or z^2-orbital), which is consistent with the recent bulk-sensitive angle-resolved photoemission spectroscopy (ARPES) measurement for (Ba,K)Fe$_2$As$_2$ and BaFe$_2$(As,P)$_2$. These obtained results indicate that the orbital-fluctuation-mediated s_{++}-wave state is a plausible candidate for iron pnictides.Comment: 11 pages, 10 figures, New discussions and figures had been adde

Structure of Neutron-Scattering Peak in both s++ wave and s+- wave states of an Iron pnictide Superconductor

We study the neutron scattering spectrum in iron pnictides based on the random-phase approximation in the five-orbital model, for fully-gapped s-wave states with sign reversal (s+-) and without sign reversal (s++). In the s++ wave state, we find that a prominent hump structure appears just above the spectral gap, by taking account of the quasiparticle damping gamma due to strong electron-electron correlation: As the superconductivity develops, the reduction in gamma gives rise to the large overshoot in the spectrum above the gap. The obtained hump structure looks similar to the resonance peak in the s+- wave state, although the height and weight of the peak in the latter state is much larger. In the present study, experimentally observed broad spectral peak in iron pnictides is naturally reproduced by assuming the s++ wave state.Comment: 5 pages, 4 figures, to be published in Phys. Rev. B (Rapid Communication

Emergence of Fully-Gapped s++s_{++}-wave and Nodal d-wave States Mediated by Orbital- and Spin-Fluctuations in Ten-Orbital Model for KFe2_2Se2_2

We study the superconducting state in newly discovered high-Tc superconductor K$_x$Fe$_2$Se$_2$ based on the ten-orbital Hubbard-Holstein model without hole-pockets. When the Coulomb interaction is large, spin-fluctuation mediated d-wave state appears due to the nesting between electron-pockets. Interestingly, the symmetry of the body-centered tetragonal structure in K$_x$Fe$_2$Se$_2$ requires the existence of nodes in the d-wave gap, although fully-gapped d-wave state is realized in the case of simple tetragonal structure. In the presence of moderate electron-phonon interaction due to Fe-ion optical modes, on the other hand, orbital fluctuations give rise to the fully-gapped $s_{++}$-wave state without sign reversal. Therefore, both superconducting states are distinguishable by careful measurements of the gap structure or the impurity effect on Tc.Comment: 5 pages, 4 figures, to be published in Phys. Rev. B (Rapid Communication