We have investigated the field-angle variation of the specific heat C(H, φ, θ) of the heavy-fermion superconductor UPt3 at low temperatures T down to 50 mK, where φ and θ denote the azimuthal and polar angles of the magnetic field H, respectively. For T = 88 mK, C(H, θ = 90°) increases proportionally to √H up to nearly the upper critical field Hc2, indicating the presence of line nodes. By contrast, C(H, θ = 0°) deviates upward from the √H dependence for √H/Hc2 ≥ 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 θ is smaller than 30°. In order to explore the possible vertical line nodes in the gap structure, we measured the ′ 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 ΔC/C ~ 0.5%. This result implies that field-induced excitations of the heavy quasiparticles occur isotropically with respect to ′, which is apparently contrary to the recent finding of a twofold thermal-conductivity κ(′) oscillation.
ASJC Scopus subject areas
- Physics and Astronomy(all)