Abstract
The author argues that the observed phase transition in the UPt3 superconductor in applied magnetic field indicates the unconventional superconductivity in this heavy-fermion compound. It is shown that if the superconducting state corresponds to the two-dimensional representation and exhibits spontaneous anisotropy in the basal plane then the applied magnetic field splits the superconducting state into two phases. Each phase has a different structure of the Abrikosov vortex lattice: the high-field state has the vortex lattice with the hexagonal symmetry, while in the low-field state this symmetry is spontaneously broken. The transition between these two vortex states may be identified with the observed transition.

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