Nodeless multigap superconductivity in organic-ion-intercalated (tetrabutyl~ammonium)${}_{0.3}$FeSe

Abstract

We probe the superconducting order parameter of the organic-ion-intercalated FeSe-based superconductor (tetrabutyl ammonium)${}_{0.3}$FeSe [(TBA)${}_{0.3}$FeSe] using muon-spin relaxation/rotation ($\mu$SR). Zero-field $\mu$SR measurements show only a weak temperature dependence with no evidence for magnetic ordering or broken time-reversal symmetry in the superconducting state. The temperature dependence of the superfluid density is deduced from transverse-field $\mu$SR measurements with fields applied both parallel and perpendicular to the $c$~axis axis, and can be well described by a nodeless two-gap $s+s$ wave model. These properties are reminiscent of those of (Li${}_{1-x}$Fe${}_x$)OHFe${}_{1-y}$Se, which also has a comparably enhanced $T_c$, suggesting that such a gap structure is a common feature of quasi-two-dimensional intercalated FeSe-based superconductors.

8 pages, 5 figures

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