# Signature of multigap nodeless superconductivity in CaKFe4As4

## TL;DR

This study reveals that CaKFe4As4 exhibits multigap nodeless superconductivity with preserved time-reversal symmetry, characterized by two distinct superconducting gaps, advancing understanding of unconventional superconductivity in Fe-based materials.

## Contribution

It provides the first muon spectroscopy evidence of multigap s + s-wave superconductivity in CaKFe4As4, with detailed gap values and superfluid density analysis.

## Key findings

- No magnetic anomaly at Tc, indicating preserved time-reversal symmetry.
- Superfluid density fits a two-gap s + s-wave model with specific gap values.
- Penetration depth at zero temperature is 289 nm.

## Abstract

A newly discovered family of high-Tc Fe-based superconductors, AeAFe4As4 (Ae = Ca, Sr, Eu and A = K, Rb, Cs), offers further opportunities to understand unconventional superconductivity in these materials. In this Rapid Communication, we report on the superconducting and magnetic properties of CaKFe4As4, studied using muon spectroscopy. Zero-field muon spin relaxation studies carried out on the CaKFe4As4 superconductor do not show any detectable magnetic anomaly at Tc or below, implying that time-reversal symmetry is preserved in the superconducting ground state. The temperature dependence of the superfluid density of CaKFe4As4 is found to be compatible with a two-gap s + s-wave model with gap values of 8.6(4) and 2.5(3) meV, similar to the other Fe-based superconductors. The presence of two superconducting energy gaps is consistent with theoretical and other experimental studies on this material. The value of the penetration depth at T = 0 K has been determined as 289(22) nm.