對雙能隙超導體砷化鈮的理論研究

Abstract

NbSe2 has been recognized as a multiband superconductor recently by many experiments, including the reported Fermi-sheet-dependent superconductivity in angle-resolved photoemission spectroscopy, thus strongly suggesting the multiband nature in NbSe2. It appears that a second energy gap involved can give a plausible explanation for previous experiments. Viewing NbSe2 as a two-gap superconductor, we proposed two possible scenarios to investigate how to locate the large gap and the small gap on the three bands in NbSe2. The two-gap Ginzburg-Landau theory is used to calculate the upper critical field and the corresponding gap ratio near Tc. Also, we apply the two-band Eilenberger equations and the two-gap BCS theory to calculate the superfluid density and the specific heat. The obtained results are fitted to previous experimental data, and it appears that the more plausible scenario is to respectively locate the large gap and the small gap on Nb-derived antibonding band and the Nb-derived bonding band. While previous studies have suggested that the Se-derived band serves as the isotropic small gap, we exclude such role of the Se-derived band for two reasons. One is its bad fit to the superfluid density, and the other is that the gap ratio obtained in the fit to Hc2(ab) is inconsistent with previous ARPES studies. Additionally, we discuss how the upper critical field is affected by the intraband Fermi-velocity anisotropy, interband Fermi-velocity anisotropy, density of states, interband coupling and the transition temperature of the large gap.