Twofold spontaneous symmetry breaking in a heavy fermion superconductor UPt$_3$

TL;DR

This study investigates the superconducting gap structure of UPt$_3$ across different phases, revealing twofold symmetry breaking and specific nodal configurations through thermal conductivity measurements.

Contribution

It provides the first detailed experimental evidence of twofold symmetry breaking and clarifies the gap structure in UPt$_3$ using thermal conductivity data.

Findings

Twofold oscillation of thermal conductivity in the C phase indicating a line node along the a axis.

Abrupt disappearance of oscillation at the B phase transition, indicating a change in gap symmetry.

Identification of additional line nodes at the tropics and point nodes at the poles, confirming the $E_{1u}$ gap symmetry.

Abstract

The field-orientation dependent thermal conductivity of the heavy-fermion superconductor UPt$_3$ was measured down to very low temperatures and under magnetic fields throughout three distinct superconducting phases: A, B, and C phases. In the C phase, a striking twofold oscillation of the thermal conductivity within the basal plane is resolved reflecting the superconducting gap structure with a line of node along the a axis. Moreover, we find an abrupt vanishing of the oscillation across a transition to the B phase, as a clear indication of a change of gap symmetries. We also identify extra two line nodes below and above the equator in both B and C phases. From these results together with the symmetry consideration, the gap function of UPt$_3$ is conclusively determined as a $E_{1u}$ representation characterized by a combination of two line nodes at the tropics and point nodes at the poles.