Orbital+Spin Multimode Fluctuation Theory in Iron-based Superconductors

Abstract

In the iron-based superconductors, experimental evidences of orbital order and fluctuations have been accumulated, such as by the angle-resolved photoemission spectroscopy, softening of elastic modulus, enhancement of Raman quadrupole susceptibility, and electronic nematic order. These phenomena in the normal state, which cannot be described in the framework of the mean-field approximation (as well as the random phase approximation, the fluctuation exchange approximation, the dynamical mean field theory, etc.), are important in understanding the mechanism of superconductivity. Recently, we have found that strong orbital and spin fluctuations mutually develop in multiorbital systems due to the vertex corrections, which describe the many-body effects beyond the mean-field approximation. Based on the present theory, a comprehensive understanding of the electronic states, such as the coexistence of the structural and magnetic quantum criticalities, has been achieved. The orbital fluctuations favor the S ++-wave superconducting state, and a variety of gap function can be reproduced by the competition and cooperation among the orbital and spin fluctuations.