Thermal Conductivity of the Accidental Degeneracy and Enlarged Symmetry Group Models for Superconducting UPt₃

TL;DR

This paper theoretically analyzes the thermal conductivity in UPt3, comparing models with different symmetries and degeneracies, and finds that certain models with specific nodal structures best match experimental anisotropy data.

Contribution

It evaluates various symmetry-based models for UPt3's superconducting order parameter against thermal conductivity measurements, identifying the most consistent ones.

Findings

Accidental degeneracy and enlarged symmetry models without spin-orbit coupling do not match thermal conductivity data.

The 2D E1g and E2u models, with line nodes in the ab-plane and point nodes along c, fit the experimental anisotropy.

The A1g+E1g model predicts spontaneous rotational symmetry breaking and large in-plane anisotropy.

Abstract

We present theoretical calculations of the thermal conductivity for the accidental degeneracy and enlarged symmetry group models that have been proposed to explain the phase diagram of UPt3. The order parameters for these models possess point nodes or cross nodes, reflecting the broken symmetries of the ground state. These broken symmetries lead to robust predictions for the ratio of the low-temperature thermal conductivity for heat flow along the c axis and in the basal plane. The anisotropy of the heat current response at low temperatures is determined by the phase space for scattering by impurities. The measured anisotropy ratio, $\kappa_c/\kappa_b$, provides a strong constraint of theoretical models for the ground state order parameter. The accidental degeneracy and enlarged symmetry group models based on no spin-orbit coupling do not account for the thermal conductivity of UPt3. The models for the order parameter that fit the experimental data for the c and b directions of the heat current are the 2D E1g and E2u models, for which the order parameters possess line nodes in the ab-plane and point nodes along the c axis, and the A1g+E1g model of Zhitomirsky and Ueda. This model spontaneously breaks rotational symmetry in theab-plane below Tc2 and predicts a large anisotropy for the $ab$-plane heat current.