Field-dependent specific heat and multiple superconducting phases inUPt3

We have measured the specific heat, C, of single-crystal ${\mathrm{UPt}}_3$ in the superconducting regime as a function of temperature, T, and magnetic field, H, parallel to the c axis. We find that C(T) at fixed H<${\mathit{H}}_{\mathit{c}2}$ shows no evidence for different superconducting states. In contrast, our field-sweep data, C(H) at fixed T, have sharp changes in slope at H≊${\mathit{H}}_{\mathit{c}2}$/2. The phase diagram deduced from these features agrees with neutron-scattering and torsional-oscillator results on the same samples. These thermodynamic measurements as a function of magnetic field constrain theories of exotic superconductivity in ${\mathrm{UPt}}_3$.