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

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### 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.

### Most cited references12

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### High-temperature superconductivity in iron-based materials

(2010)
The surprising discovery of superconductivity in layered iron-based materials, with transition temperatures climbing as high as 55 K, has lead to thousands of publications on this subject over the past two years. While there is general consensus on the unconventional nature of the Cooper pairing state of these systems, several central questions remain - including the role of magnetism, the nature of chemical and structural tuning, and the resultant pairing symmetry - and the search for universal properties and principles continues. Here we review the progress of research on iron-based superconducting materials, highlighting the major experimental benchmarks that have been so far reached and the important questions that remain to be conclusively answered.
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### Extreme Sensitivity of Superconductivity to Stoichiometry in FeSe (Fe1+dSe)

(2008)
The recently discovered iron arsenide superconductors, which display superconducting transition temperatures as high as 55 K, appear to share a number of general features with high-Tc cuprates, including proximity to a magnetically ordered state and robustness of the superconductivity in the presence of disorder. Here we show that superconductivity in Fe1+dSe, the parent compound of the superconducting arsenide family, is destroyed by very small changes in stoichiometry. Further, we show that non-superconducting Fe1+dSe is not magnetically ordered down to low temperatures. These results suggest that robust superconductivity and immediate instability against an ordered magnetic state should not be considered as intrinsic characteristics of iron-based superconducting systems, and that Fe1+dSe may present a unique opportunity for determining which materials characteristics are critical to the existence of superconductivity in high Tc iron arsenide superconductors and which are not.
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### Density functional study of FeS, FeSe and FeTe: Electronic structure, magnetism, phonons and superconductivity

(2008)
We report density functional calculations of the electronic structure, Fermi surface, phonon spectrum, magnetism and electron-phonon coupling for the superconducting phase FeSe, as well as the related compounds FeS and FeTe. We find that the Fermi surface structure of these compounds is very similar to that of the Fe-As based superconductors, with cylindrical electron sections at the zone corner, cylindrical hole surface sections, and depending on the compound, other small hole sections at the zone center. As in the Fe-As based materials, these surfaces are separated by a 2D nesting vector at ($$\pi$$,$$\pi$$). The density of states, nesting and Fermi surface size increase going from FeSe to FeTe. Both FeSe and FeTe show spin density wave ground states, while FeS is close to an instability. In a scenario where superconductivity is mediated by spin fluctuations at the SDW nesting vector, the strongest superconductor in this series would be doped FeTe.
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Condensed matter