Two-gap superconducting properties of alkaline-earth intercalated A_x(NH₃)Fe₂Se₂ (A = Ba or Sr)

Superconducting properties were studied on high quality superconductors Bax(NH₃)Fe₂Se₂ (T_c= 39 K) and Srx(NH₃)Fe₂Se₂ (Tc = 44 K) prepared by intercalating Ba/Sr atoms into tetragonal β-FeSe by liquid ammonia. The elongated c-axis and almost unchanged a-axis of Bax(NH₃)Fe₂Se₂, compared with β-FeSe, suggests an unchanged intra- Fe₂Se₂-layer structure and the T_c enhancement is due to a 3D to 2D-like Fermi surface transformation. The superconducting coherent lengths (0), the Ginzburg-Landau parameters κ and penetration depths (0) obtained from the extrapolated lower and upper critical fields B_c1 (0) and B_c2 (0) indicate that both compounds are typical type-II superconductors. The temperature dependence of 1/l2(T) of Bax(NH₃)Fe₂Se₂ deduced from the low-field magnetic susceptibility shows a twogap s-wave behavior with superconducting gaps of δ₁ = 6.47 meV and δ₂= 1.06 meV.