Observation of broken time-reversal symmetry in the heavy fermion superconductor UPt$_3$

TL;DR

This study provides experimental evidence that the low-temperature superconducting phase of UPt$_3$ breaks time-reversal symmetry, indicating a complex order parameter in this heavy fermion superconductor.

Contribution

It demonstrates the first direct detection of broken time-reversal symmetry in UPt$_3$ using polar Kerr effect measurements.

Findings

Broken TRS observed below the lower superconducting transition temperature.

Polar Kerr effect appears only in the low-temperature phase.

Supports the existence of a complex two-component order parameter.

Abstract

The symmetry properties of the order parameter characterize different phases of unconventional superconductors. In the case of the heavy-fermion superconductor UPt$_3$, a key question is whether its multiple superconducting phases preserve or break time-reversal symmetry (TRS). We tested for asymmetry in the phase shift between left and right circularly polarized light reflected from a single crystal of UPt$_3$ at normal incidence, finding that this so-called polar Kerr effect appears only below the lower of the two zero-field superconducting transition temperatures. Our results provide evidence for broken TRS in the low-temperature superconducting phase of UPt$_3$, implying a complex two-component order parameter for superconductivity in this system.