Revealing the anisotropy effects on the critical magnetic field in CaC6 superconductor

Abstract

The calcium intercalated graphite [Formula: see text] is considered to be a representative material of the graphite intercalated superconductors, which exhibits sizable anisotropy of the Fermi surface (FS). Herein, the influence of the FS anisotropy on the critical magnetic field [Formula: see text] in [Formula: see text] superconductor is comprehensively analyzed within the Migdal–Eliashberg (M–E) formalism. To precisely account for the mentioned anisotropy effects, the analysis is conducted in the framework of the six-band approximation, hitherto not employed for calculations of the [Formula: see text] function in [Formula: see text] material. For convenience, the obtained results are compared with the available one- and three-band estimates reported by using the M–E theory. A notable signature of the increased number of bands is observed for the temperature-dependent [Formula: see text] functions. In particular, the [Formula: see text] function decreases at T = 0 K as the number of the considered bands is higher. Moreover, it is argued that the six-band formalism yields the most physically relevant shape of the [Formula: see text] function among all considered approximations. Therefore, the six-band model introduces not only quantitative but also qualitative changes to the results, in comparison to the other discussed FS approximations. This observation supports postulate that the six-band model constitutes minimal structure for FS of [Formula: see text], but also magnifies importance of anisotropy effects for the critical magnetic field calculations.