  We have used small-angle neutron scattering to determine the vortex lattice
phase diagram in the topological superconductor UPt3 for the applied magnetic
field along the crystalline c-axis. A triangular vortex lattice is observed
throughout the superconducting state, but with an orientation relative to the
hexagonal basal place that changes with field and temperature. At low
temperature, in the chiral B phase, the vortex lattice undergoes a
non-monotonic rotation with increasing magnetic field. The rotation amplitude
decreases with increasing temperature and vanishes before reaching the A phase.
Within the A phase an abrupt +/-15 degree vortex lattice rotation was
previously reported by Huxley et al., Nature 406, 160-164 (2000). The complex
phase diagram may be understood from competing effects of the superconducting
order parameter, the symmetry breaking field, and the Fermi surface anisotropy.
The low-temperature rotated phase, centered around 0.8 T, reported by Avers et
al., Nature Physics 16, 531-535 (2020), can be attributed directly to the
symmetry breaking field.
