Classification of multiorbital superconducting state based on augmented multipoles

Abstract: The pairing interactions between electrons play an essential role in determining the properties in superconducting states. Recently, a plethora of unconventional superconducting states has been extensively explored, which often emerge owing to multipole fluctuations in the vicinity of multipole orders. We classify such superconducting states from the viewpoint of the multipole degrees of freedom by extending its representation to Nambu space. We clarify that under the crystallographic point group, arbitrary Cooper pairs between electrons with any angular momenta are systematically classified by four types of multipoles: electric, magnetic, magnetic toroidal, and electric toroidal. As examples, we apply our formulation to an $sp$-orbital electron system, which potentially exhibits exotic Cooper pairs under polar and axial point groups. Our systematic classification will be useful in characterizing unconventional superconducting states in multiorbital systems.