Seiichiro Onari

Unconventional pairing originating from the disconnected Fermi surfaces of superconducting LaFeAsO1-xFx.

For a newly discovered iron-based high T_{c} superconductor LaFeAsO1-xFx, we have constructed a minimal model, where inclusion of all five Fe d bands is found to be necessary. The random-phase approximation is applied to the model to investigate the origin of superconductivity. We conclude that the multiple spin-fluctuation modes arising from the nesting across the disconnected Fermi surfaces realize an extended s-wave pairing, while d-wave pairing can also be another candidate.

Orbital-fluctuation-mediated superconductivity in iron pnictides: Analysis of the five-orbital hubbard-holstein model

In iron pnictides, we find that the moderate electron-phonon interaction due to the Fe-ion oscillation can induce the critical d-orbital fluctuations, without being prohibited by the Coulomb interaction. These fluctuations give rise to the strong pairing interaction for the s-wave superconducting (SC) state without sign reversal (s(++)-wave state), which is consistent with experimentally observed robustness of superconductivity against impurities. When the magnetic fluctuations due to Coulomb interaction are also strong, the SC state shows a smooth crossover from the s-wave state with sign reversal (s(+/-)-wave state) to the s(++)-wave state as impurity concentration increases.

Violation of Anderson's theorem for the sign-reversing s-wave state of iron-pnictide superconductors.

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.

Superconductivity in Ca1-xLaxFeAs2: A Novel 112-Type Iron Pnictide with Arsenic Zigzag Bonds

We report superconductivity in the novel 112-type iron-based compound Ca1-xLaxFeAs2. Single-crystal X-ray diffraction analysis revealed that the compound crystallizes in a monoclinic structure (space group \(P2_{1}\)), in which Fe2As2 layers alternate with Ca2As2 spacer layers such that monovalent arsenic forms zigzag chains. Superconductivity with a transition temperature (\(T_{\text{c}}\)) of 34 K was observed for the \(x = 0.1\) sample, while the \(x = 0.21\) sample exhibited trace superconductivity at 45 K. First-principles band calculations demonstrated the presence of almost cylindrical Fermi surfaces, favorable for the high \(T_{\text{c}}\) in La-doped CaFeAs2.

Structure of neutron-scattering peaks 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

Orbital fluctuation theory in iron pnictides: Effects of As-Fe-As bond angle, isotope substitution, and Z2 -orbital pocket on superconductivity

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Theory of Superconductivity of Carbon Nanotubes and Graphene

-wave states with sign reversal

Nematicity and Magnetism in FeSe and Other Families of Fe-Based Superconductors

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Theory of odd-frequency pairings on a quasi-one-dimensional lattice in the Hubbard model

and without sign reversal

Emergence of fully gapped s++-wave and nodal d-wave states mediated by orbital and spin fluctuations in a ten-orbital model of KFe 2Se2

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