Time-reversal symmetry breaking and multigap superconductivity in the noncentrosymmetric superconductor La$_{7}$Ni$_{3}$

TL;DR

This study reveals that La$_{7}$Ni$_{3}$ exhibits both multigap superconductivity and time-reversal symmetry breaking, making it the first in its family to show these combined unconventional properties, confirmed by thermodynamic and muon spin measurements.

Contribution

The paper provides the first evidence of simultaneous multigap superconductivity and time-reversal symmetry breaking in La$_{7}$Ni$_{3}$, expanding understanding of unconventional superconductivity in the Th$_{7}$Fe$_{3}$ family.

Findings

Evidence for two isotropic superconducting gaps.

Presence of internal magnetic fields indicating time-reversal symmetry breaking.

Large gap value suggests strong electron correlations.

Abstract

The Th$_{7}$Fe$_{3}$ family of superconductors provides a rich playground for unconventional superconductivity. La$_7$Ni$_3$ is the latest member of this family, which we here investigate by means of thermodynamic and muon spin rotation and relaxation measurements. Our specific heat data provides evidence for two distinct and approximately isotropic superconducting gaps. The larger gap has a value slightly higher than that of weak-coupling BCS theory, indicating the presence of significant correlations. These observations are confirmed by transverse-field muon-rotation measurements. Furthermore, zero-field measurements reveal small internal fields in the superconducting state, which occur close to the onset of superconductivity and indicate that the superconducting order parameter breaks time-reversal symmetry. We discuss two possible microscopic scenarios -- an unconventional $E_{2}(1,i)$ state and an $s+i\,s$ superconductor, which is reached by two consecutive transitions -- and illustrate which interactions will favor these phases. Our results establish La$_{7}$Ni$_{3}$ as the first member of the Th$_{7}$Fe$_{3}$ family displaying both time-reversal-symmetry-breaking and multigap superconductivity.