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

TL;DR

This study investigates the superconducting gap structure of (tetrabutyl ammonium)$_{0.3}$FeSe, revealing nodeless multigap superconductivity with no magnetic order, using muon-spin relaxation/rotation techniques.

Contribution

It provides the first muon-spin relaxation/rotation evidence for nodeless multigap superconductivity in organic-ion-intercalated FeSe, highlighting a common gap structure among similar intercalated FeSe superconductors.

Findings

No magnetic ordering or time-reversal symmetry breaking detected.

Superfluid density fits a nodeless two-gap s+s wave model.

Similar gap structure to other intercalated FeSe superconductors.

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.