Cosine bands, flat bands and superconductivity in orthorhombic iron selenide

Abstract

Electronic band structures (EBSs) for orthorhombic beta FeSe1-x at less than 16 K and up to 23 GPa using experimentally determined cell dimensions are evaluated for cosine-shaped bands near, or crossing, EF. Cosine shaped bands are present in reciprocal directions parallel to the c axis at all pressures. Calculations using a P1 cell derived from Cmma symmetry with a 2c superlattice moderates the effect of intersecting bands to 9.0 GPa. This approach enables determination of a superconducting gap consistent with experimentally determined values. Key influences on charge distribution and transfer in the interplanar region of beta FeSe1-x are lone pair electrons which feature as flat bands (FBs) near EF along GZ in an EBS. FBs also influence the topology of Fermi surfaces as pressure increases and in directions parallel to the c* direction (i.e. offset along ky) within the Brillouin zone. At the Fermi surface along b*, cosine bands split and align favorably for electron-hole pairing with nodal inflection points located at EF. For P greater than 12.0 GPa, FBs interact with folded cosine bands invoking additional band dispersions. These calculations suggest that FBs participate in, and with increased pressure, enhance and sustain the superconducting properties of beta FeSe1-x to 23 GPa.