Superconducting mechanism due to the orbital and spin fluctuations in Fe-based superconductors

The rich variety of the phase diagrams in Fe-based superconductors, such as the nonmagnetic/magnetic nematic phase in FeSe/LaFeAsO, is not able to be explained by the mean-field level approximations. Recently, we explained the phase diagrams of FeSe and LaFeAsO in term of the orbital$+$spin fluctuation theory, by including the Aslamazov-Larkin vertex correction (AL-VC). The nematic orbital order without magnetization in FeSe is well explained [1]. In the present study, we analyze the superconducting states in FeSe and LaFeAsO, by applying the orbital$+$spin fluctuation theory. Rich variety of the superconducting gap structures are induced by the strong orbital and spin fluctuations driven by the AL-VC ($=$orbital-spin interplay). We find that the pairing interaction due to the orbital fluctuations is strongly enlarged by the AL-VC for the anomalous self-energy, so the Migdal theory is seriously violated in the orbital-fluctuation pairing mechanism. [1] Y. Yamakawa, S. Onari and H. Kontani, arXiv:1509.01161.