Searching for the in-plane anisotropy of the specific heat of UPt3 in rotating fields

In order to identify the gap structure of a spin-triplet superconductor UPt3, we performed field-angle-resolved specific heat (C-phi) measurements in a magnetic field rotated around the [0001] axis. Although various theories predict fourfold symmetry of the gap in the superconducting C phase, we observed no angular variation in C-phi in a temperature range down to 88 mK (similar to 0.17T(c)). This absence of the variation in C-phi implies that the field-angle-dependent quasiparticle excitations, reflecting the gap anisotropy, are averaged out in the bulk of UPt3.