Evolution of Two-Gap Behavior of the Superconductor ${\mathrm{FeSe}}_{1-x}$

The superfluid density, ${\rho }_s$, of the iron chalcogenide superconductor, ${\mathrm{FeSe}}_{1-x}$, was studied as a function of pressure by means of muon-spin rotation. The analysis of ${\rho }_s(T)$ within the two-gap scheme reveals that the effect on both, the transition temperature $T_c$ and ${\rho }_s(0)$, is entirely determined by the band(s) where the large superconducting gap develops, while the band(s) with the small gap become practically unaffected.

Figure 1

TF $\mu \mathrm{SR}$ time spectra (${\mu }_{0}H=10\mathrm{mT}$) of ${\mathrm{FeSe}}_{0.94}$ measured below ($T=0.24\mathrm{K}$) and above ($T=20\mathrm{K}$) the superconducting transition temperature ($T_c\simeq 13\mathrm{K}$) at $p=0.76\mathrm{GPa}$. The stronger damping in the superconducting state is due to the formation of the vortex lattice.

Figure 2

Temperature dependence of ${\lambda }_{ab}^{-2}\propto {\rho }_s$ of ${\mathrm{FeSe}}_{0.94}$ measured at $p=0.0$, 0.28, 0.42, 0.58, 0.76, and 0.84 GPa. The solid and the dashed lines are the theoretical curves obtained within the framework of the two-gap model described in the text. The insets show the temperature dependences of the large (${\Delta }_1$) and the small (${\Delta }_2$) gap. The dash-dotted lines on $p=0.28$ and 0.58 GPa panels represent the results of the fit by means of the empirical $\alpha$ model [33]. A pure agreement between the theoretical curves and the experimental data is caused by the limitations of the $\alpha$ model as discussed by Kogan et al. in 27.

Figure 3

(a) Dependence of ${\lambda }_{ab}^{-2}(T=0)$ on $T_c$. The inset is the “Uemura relation” for Fe-based HTS with some of data obtained to date (see Refs. 16, 17, 25, 32). (b), (c), and (d) Dependence of the parameters obtained in analysis of ${\lambda }_{ab}^{-2}(T,p)$ within the framework of two-gap model (see text for details).