Nodeless multigap superconductivity in organic ion intercalated (tetrabutyl ammonium)0.3FeSe

We probe the superconducting order parameter of the organic-ion-intercalated FeSe-based superconductor (tetrabutyl ammonium)0.3FeSe [(TBA)0.3FeSe] using muon-spin relaxation/rotation (μSR). Zero-field μSR 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 μSR 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 (Li₁₋ₓFeₓ)OHFe1-ySe, which also has a comparably enhanced Tc, suggesting that such a gap structure is a common feature of quasi-two-dimensional intercalated FeSe-based superconductors.