Two-gap superconducting properties of alkaline-earth intercalated $A_{x}(NH_{3})Fe_{2}Se_{2}$ (A = Ba or Sr)

Abstract

Superconducting properties were studied on high quality superconductors $Ba_{x}(NH_{3})Fe_{2}Se_{2}$ ($T_{c}$ = 39 K) and $Sr_{x}(NH_{3})Fe_{2}Se_{2}$ ($T_{c}$ = 44 K) prepared by intercalating Ba/Sr atoms into tetragonal $\beta$-FeSe by liquid ammonia. The elongated c-axis and almost unchanged a-axis of $Ba_{x}(NH_{3})Fe_{2}Se_{2}$, comparing with $\beta$-FeSe, suggested an unchanged intra-$Fe_{2}Se_{2}$-layer structure and the $T_{c}$ enhancement is due to a 3D to 2D-like Fermi surface transformation. The superconducting coherent lengths $\xi$(0), Ginzburg-Landau parameters $\kappa$ and penetration depths $\lambda$(0) obtained from the extrapolated lower and upper critical fields $B_{c1}$(0) and $B_{c2}$(0) indicates that both compounds are typical type-II superconductors. The temperature dependence of 1/$\lambda^{2}$(T) of $Ba_{x}(NH_{3})Fe_{2}Se_{2}$ deduced from the low field magnetic susceptibility shows a two-gap s-wave behaviour with superconducting gaps of $\Delta_{1}$ = 6.47 meV and $\Delta_{2}$ = 1.06 meV.