Two-gap behaviors of layered superconductors Ax(NH3)Fe2Se2 (A = Ba, Sr, Ca)
Yung-Yuan Hsu1*, Yu-Bo Lee1, Gu-Kuei Li1, Shou-Ting Jian1, Ming-Cheng Yang1
1Department of Physics, National Taiwan Normal University, Taipei, Taiwan
* presenting author:徐鏞元,
Superconducting properties of high quality superconductors Ax(NH3)Fe2Se2 (A = Ba, Sr, Ca with Tc = 39 K, 44 K and 45 K) were reported. The upper critical fields at zero temperature Bc2(0) were estimated by extrapolating magnetic field dependence of Tc. An unexpected low Bc2Ba(0) = 13.4 T was obtained for Bax(NH3)Fe2Se2 in contrast to a seemingly normal Bc2Sr(0) = 60.7 T for Srx(NH3)Fe2Se2. The temperature dependence of superfluid density ρsBa(T) reviled a nodeless 2-gap behavior with small gap values of Δ1 ~ 4.1 meV and Δ2 ~ 0.6 meV which are similar to those of β-FeSe. The small gap values of Bax(NH3)Fe2Se2, un-matching to its high Tc, suggests the enhancement of Tc was due to a more 2D-like electronic structure induced by intercalating Ba and NH3 molecules. Anisotropic superconducting properties and superconducting parameters of the c-axis aligned powders will also be discussed in details.

++ This work was supported by NSC101-2112-M-003-008.

Keywords: iron-chalcogenides superconductors, superfluid density, 2-gap, s-wave, c-axis powder alignment