Anomalous Field-Angle Dependence of the Specific Heat of Heavy-Fermion Superconductor UPt3

TL;DR

This study investigates the angle-dependent specific heat of heavy-fermion superconductor UPt3 at very low temperatures, revealing line nodes and isotropic quasiparticle excitations, with implications for understanding its unconventional superconductivity.

Contribution

The paper provides detailed measurements of the field-angle dependence of specific heat in UPt3, showing the absence of in-plane anisotropy and contributing to the understanding of its gap structure.

Findings

C(H, theta=88mK) ~ H^{1/2} up to Hc2

No in-plane angular oscillation of C observed

Unusual Hc2 suppression along c-axis

Abstract

We have investigated the field-angle variation of the specific heat C(H, phi, theta) of the heavy-fermion superconductor UPt3 at low temperatures T down to 50 mK, where phi and theta denote the azimuthal and polar angles of the magnetic field H, respectively. For T = 88 mK, C(H, theta=90) increases proportionally to H^{1/2} up to nearly the upper critical field Hc2, indicating the presence of line nodes. By contrast, C(H, theta=0) deviates upward from the H^{1/2} dependence for (H/Hc2)^{1/2} > 0.5. This behavior can be related to the suppression of Hc2 along the c direction, whose origin has not been resolved yet. Our data show that the unusual Hc2 limit becomes marked only when theta is smaller than 30. In order to explore the possible vertical line nodes in the gap structure, we measured the phi dependence of C in wide T and H ranges. However, we did not observe any in-plane angular oscillation of C within the accuracy of dC/C~0.5%. This result implies that field-induced excitations of the heavy quasiparticles occur isotropically with respect to phi, which is apparently contrary to the recent finding of a twofold thermal-conductivity oscillation.