539 research outputs found
Non-Fermi-Liquid-Like Behaviors and Superconductivity Driven by Orbital Fluctuations in Iron Pnictides: Analysis by Fluctuation-Exchange Approximation
We study the five-orbital Hubbard-Holstein model for iron pnictides with
small electron-phonon interaction due to Fe-ion Einstein oscillators. Using the
fluctuation-exchange (FLEX) approximation, orbital fluctuations evolve
inversely proportional to the temperature, and therefore the resistivity shows
linear or convex T-dependence for wide range of temperatures. We also analyze
the Eliashberg gap equation, and show that s-wave superconducting state without
sign reversal (s_{++}-wave state) emerges when the orbital fluctuations
dominate the spin fluctuations. When both fluctuations are comparable, their
competition gives rise to a nodal s-wave state. The present study offers us a
unified explanation for both the normal and superconducting states.Comment: 5 pages, 4 figure
Violation of Anderson's Theorem for 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.Comment: 4 pages, 4 figure
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