Mechanical Measurements of the Flux Lattice in the Heavy-Fermion Superconductor UPt3

We report mechanical measurements of the flux lattice in UPt3. In the superconducting state the flux lattice that is formed in the sample when a magnetic field is applied contributes a restoring force and a dissipation term to a high-Q mechanical oscillator, when the oscillator and sample are tilted with respect to the static magnetic field. In addition to the characteristic response for a type-II superconductor that gives Hc2(T), we observe an unexpected dissipation peak along two lines in the (H, T) plane within the superconducting state. The evidence suggests transitions between three different superconducting phases, and hence a complex phase diagram for UPt3 is proposed.