Time-reversal symmetry breaking in superconducting low-carrier-density quasi-skutterudite Lu3Os4Ge13

TL;DR

This study reveals time-reversal symmetry breaking and unconventional superconductivity in Lu3Os4Ge13, a low-carrier-density quasi-skutterudite, through muon-spin relaxation measurements indicating complex gap structure and magnetic fields in the superconducting state.

Contribution

The paper provides the first microscopic evidence of time-reversal symmetry breaking and unconventional pairing in Lu3Os4Ge13 using $$SR techniques.

Findings

Spontaneous static or quasi-static magnetic fields in the superconducting state.

Indications of a complex, multi-gap superconducting order parameter.

Electron-electron interactions are crucial for pairing, suggesting unconventional superconductivity.

Abstract

The complex structure of the Remeika phases, the intriguing quantum states they display, and their low carrier concentrations are a strong motivation to study the nature of their superconducting phases. In this work, the microscopic properties of the superconducting phase of single-crystalline Lu$_3$Os$_4$Ge$_{13}$ are investigated by muon-spin relaxation and rotation ($\mu$SR) measurements. The zero-field $\mu$SR data reveal the presence of spontaneous static or quasi-static magnetic fields in the superconducting state, breaking time-reversal symmetry; the associated internal magnetic field scale is found to be exceptionally large ($\approx$ 0.18~mT). Furthermore, transverse-field $\mu$SR measurements in the vortex state of Lu$_3$Os$_4$Ge$_{13}$ imply a complex gap function with significantly different strengths on different parts of the Fermi surface. While our measurements do not completely determine the order parameter, they strongly indicate that electron-electron interactions are essential to stabilizing pairing in the system, thus, demonstrating its unconventional nature.