Superconductivity induced by fluctuations of momentum-based multipoles*

Recent studies of superconductivity have focused on spin fluctuation, instead of electron-phonon coupling, as an origin of attractive interaction between electrons.
On the other hand, a multipole order, which represents electrons' degrees of freedom in strongly spin-orbit coupled systems, is attracting much attention. Therefore, it is helpful for understanding unconventional superconductivity to consider “superconductivity induced by multipole fluctuations,” as an extension of spin-fluctuation-mediated superconductivity. Indeed, previous works have studied superconductivity induced by fluctuations of odd-parity electric multipole orders in isotropic systems.
In this study, we investigate superconductivity induced by fluctuations odd-parity magnetic multipoles, aiming at good understanding of multipole-fluctuation-mediated superconductivity. Furthermore, this study focuses on the effect of crystalline electric fields (CEF) using the classification theory of multipoles, while the previous studies simply assume isotropic systems. As a result, we suggest that a nodal extended s-wave pairing may be favored by an odd-parity magnetic multipole fluctuation under CEF.