Temperature dependent local atomic displacements in ammonia intercalated iron selenide superconductor

Abstract

Recently, ammonia-thermal reaction has been used for molecular intercalation in layered FeSe, resulting a new Lix(NH3)yFe2Se2 superconductor with Tc ~ 45 K. Here, we have used temperature dependent extended x-ray absorption fine structure (EXAFS) to investigate local atomic displacements in single crystals of this new superconductor. Using polarized EXAFS at Fe K-edge we have obtained direct information on the local Fe-Se and Fe-Fe bondlengths and corresponding mean square relative displacements (MSRD). We find that the Se-height in the intercalated system is lower than the one in the binary FeSe, suggesting compressed FeSe4 tetrahedron in the title system. Incidentally, there is hardly any effect of the intercalation on the bondlengths characteristics, revealed by the Einstein temperatures, that are similar to those found in the binary FeSe. Therefore, the molecular intercalation induces an effective compression and decouples the FeSe slabs. Furthermore, the results reveal an anomalous change in the atomic correlations across Tc, appearing as a clear decrease in the MSRD, indicating hardening of the local lattice mode. Similar response of the local lattice has been found in other families of superconductors, e.g., A15-type and cuprates superconductors. This observation suggests that local atomic correlations should have some direct correlation with the superconductivity.

Emergence of superconductivity in (NH3)yMxMoSe2 (M: Li, Na and K)

“…Here, it is worth noting that instead of a polycrystalline powder, in this study, an agglomeration of MoSe 2 was used as the starting material for metal-intercalation. This is based on the successful synthesis of metal-doped FeSe from an agglomeration of FeSe 20 .…”

Intermittent dynamics of antiferromagnetic phase in inhomogeneous iron-based chalcogenide superconductor

“…Indeed, the measurements across the superconducting transition temperature show that the AFM phase goes through an anomalous atomic dynamics across the superconducting transition temperature reflecting involvement of complex energy landscape to establish the superconducting quantum state. It is worth recalling that superconductivity is accompanied by a hardening in local atomic modes, that has been seen in a series of superconducting families [42][43][44]. Therefore, the behavior of the AFM phase could be a result of the superconducting transition that can be mediated by lattice fluctuations (Fe-Fe lattice) or spin fluctuations in the AFM phase.…”

EXAFS spectroscopy: a powerful tool for the study of local vibrational dynamics

“…The results show how the isotope substitution has a negligible effect on the interatomic distances but a strong influence on the local vibrations of the Fe-As and Fe-Fe bonds, thus confirming that phonons play an important role in the superconductive mechanism of iron-based superconductor materials. Paris et al [122] investigated the local atomic displacements in the new Li x (NH 3 ) y Fe 2 Se 2 [119] . © APS Publishing.…”