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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 to50 mK, where phi and theta denote the azimuthal and polar angles of themagnetic field H, respectively. For T = 88 mK, C(H, theta=90) increasesproportionally to H^{1/2} up to nearly the upper critical field Hc2, indicatingthe presence of line nodes. By contrast, C(H, theta=0) deviates upward from theH^{1/2} dependence for (H/Hc2)^{1/2} > 0.5. This behavior can be related to thesuppression of Hc2 along the c direction, whose origin has not been resolvedyet. Our data show that the unusual Hc2 limit becomes marked only when theta issmaller than 30. In order to explore the possible vertical line nodes in thegap 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 theaccuracy of dC/C~0.5%. This result implies that field-induced excitations ofthe heavy quasiparticles occur isotropically with respect to phi, which isapparently contrary to the recent finding of a twofold thermal-conductivityoscillation.

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