DFT+ U study on the multipolar nematic state of nonmagnetic FeSe

In this study, we investigate the electronic structure of FeSe in the normal and nematic states based on the DFT+U method in combination with the multipole analyses, and discuss their microscopic properties and origin. In the normal state, the topological change of the Fermi surface occurs before the nematic transition with increasing the on-site Coulomb interaction U. The resulting nematic ground state is a multipolar state having both hexadecapoles in the E-representation and multipoles in the B 2-representation on each Fe site, where the appearance of the E-type multipoles induces non-trivial spontaneous orbital hybridizations between dxy, dx2-y2 and dxz /dyz orbitals. Such the spontaneous orbital mixings due to E-type multipoles is inherent in the FeSe and related systems, and is expected to play an important role in the formation of the nematic states in other iron-based superconductors.