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

TL;DR

This study investigates the superconducting properties of alkaline-earth intercalated FeSe compounds, revealing two-gap s-wave superconductivity and structural features linked to enhanced critical temperatures.

Contribution

It reports the synthesis and characterization of Ba- and Sr-intercalated FeSe superconductors, highlighting their two-gap superconductivity and structural changes related to increased T_c.

Findings

Both compounds are type-II superconductors.

Ba-intercalation induces a two-gap s-wave superconducting state.

Superconducting gaps are Δ₁=6.47 meV and Δ₂=1.06 meV.

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.