S-wave Superconductivity due to Orbital and Spin fluctuations in Fe-pnictides: Self-Consistent Vertex Correction with Self-Energy (SC-VCΣ) Analysis

To understand the amazing variety of the superconducting states of Fe-based superconductors, we analyze the multiorbital Hubbard models for LaFeAsO and LiFeAs going beyond the random-phase approximation (RPA), by calculating the vertex correction (VC) and self-energy correction. Due to the spin+orbital mode coupling described by the VC, both orbital and spin fluctuations mutually develop, consistently with the experimental phase diagram with the orbital and magnetic orders. Due to both fluctuations, the s-wave gap function with sign-reversal ($s_{\pm}$-wave), without sign-reversal ($s_{++}$-wave), and nodal s-wave states are obtained, compatible with the experimental wide variety of the gap structure. Thus, the present theory provides a microscopic explanation of the normal and superconducting phase diagram based on the realistic Hubbard model.