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

• Main Authors: Foronda, F. R, Ghannadzadeh, S, Sedlmaier, S. J, Wright, J. D, Burns, K, Cassidy, S. J, Goddard, P. A, Lancaster, T, Clarke, S. J, Blundell, S. J
• Format: Journal Article
• Language: English
• Published: 07-10-2015
• Subjects: Physics - Superconductivity
• DOI: 10.48550/arxiv.1510.01919

Phys. Rev. B 92, 134517 (2015) 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.