Pressure dependence of the superconducting phases inUPt3

We report measurements of the specific heat of the heavy-fermion compound ${\mathrm{UPt}}_3$ under hydrostatic pressure p up to 4.5 kbar. The two superconductive transitions at temperatures ${\mathit{T}}_{\mathit{c}}^{+}$ and ${\mathit{T}}_{\mathit{c}}^{\mathrm{-}}$ observed previously under zero pressure decrease at a rate of ${\mathit{dT}}_{\mathit{c}}^{+}$/dp=-24 mK/kbar and ${\mathit{dT}}_{\mathit{c}}^{\mathrm{-}}$/dp=-5 mK/kbar. The transitions merge at ${\mathit{p}}^{\mathrm{*}}$=3.7 kbar and ${\mathit{T}}^{\mathrm{*}}$=419 mK. These results suggest that the weak antiferromagnetic order, which is suppressed on the same pressure scale, acts as a symmetry-breaking field in splitting the superconductive transition, thus yielding convincing evidence for a non-BCS order parameter for this system.