Robustness of superconductivity to structural disorder in Sr$_{0.3}$(NH$_2$)$_{y}$(NH$_3$)$_{1-y}$Fe$_2$Se$_2$

TL;DR

This study investigates the robustness of superconductivity in Sr-ammonia-intercalated FeSe despite intrinsic structural disorder caused by random layer stacking, revealing that high critical fields and transition temperatures are maintained.

Contribution

It demonstrates that superconductivity in intercalated FeSe is resilient to structural disorder, challenging the notion that perfect coherence is necessary for high $T_c$ and critical fields.

Findings

Upper critical field ${ m H_{c2}}$ reaches 33 T extrapolated to zero temperature.

Superconducting transition observed at approximately 36.3 K.

Superconductivity persists despite stacking disorder along the c-axis.

Abstract

The superconducting properties of a recently discovered high $T_{\rm c}$ superconductor, Sr/ammonia-intercalated FeSe, have been measured using pulsed magnetic fields down to $4.\,\rm {K}$ and muon spin spectroscopy down to $1.5\,\rm K$. This compound exhibits intrinsic disorder resulting from random stacking of the FeSe layers along the $c$-axis that is not present in other intercalates of the same family. This arises because the coordination requirements of the intercalated Sr and ammonia moieties imply that the interlayer stacking (along $c$) involves a translation of either ${\bf a}/2$ or ${\bf b}/2$ which locally breaks tetragonal symmetry. The result of this stacking arrangement is that the Fe ions in this compound describe a body-centred tetragonal lattice in contrast to the primitive arrangement of Fe ions described in all other Fe-based superconductors. In pulsed magnetic fields the upper critical field $H_{\text{c2}}$ was found to increase on cooling with an upwards curvature that is commonly seen in type-II superconductors of a multi-band nature. Fitting the data to a two-band model and extrapolation to absolute zero gave a maximum upper critical field ${\mu_0H_{\rm c2}(0)}$ of ${33(2)\,\rm T}$. A clear superconducting transition with a diamagnetic shift was also observed in transverse-field muon measurements at ${T_{\text c}\approx36.3(2)\,\rm K}$. These results demonstrate that robust superconductivity in these intercalated FeSe systems does not rely on perfect structural coherence along the $c$-axis.