Uniaxial-stress anisotropy of the double superconducting transition in UPt_3

We study the specific heat of single-crystal UPt_3 for uniaxial stress 0≤S≤4 kbar. For S∥c^, the lower and upper superconducting transitions move towards each other in temperature and merge by S=2 kbar and T=485 mK. In the normal state, the Sommerfeld constant γ increases 12% by 2 kbar and then plateaus. By contrast, for S∥a^, γ decreases steadily and the onset T_c's of the two superconducting transitions remain essentially unchanged, although the specific-heat peaks broaden markedly. Residual strain in the basal plane may account for some crystals of UPt_3 showing only one broad superconducting transition.