Realignment of the flux-line lattice by a change in the symmetry of superconductivity in UPt3

In 1957, Abrikosov1 described how quanta of magnetic flux enter the interior of a bulk type II superconductor. It was subsequently predicted that, in an isotropic superconductor, the repulsive forces between the flux lines would cause them to order in two dimensions, forming a hexagonal lattice2. Flux-line lattices with different geometry can also be found in conventional (type II) superconductors3; however, the ideal hexagonal lattice structure should always occur when the magnetic field is applied along a hexagonal crystal direction4. Here we report measurements of the orientation of the flux-line lattice in the heavy-fermion superconductor5 UPt3, for this special case. As the temperature is increased, the hexagonal lattice, which is initially aligned along the crystal symmetry directions, realigns itself with the anisotropic superconducting gap. The superconductivity in UPt3 is unusual (even compared to unconventional oxide superconductors6) because the superconducting gap has a lower rotational symmetry than the crystal structure. This special feature enables our data to demonstrate clearly the link between the microscopic symmetry of the superconductivity and the mesoscopic physics of the flux-line lattice. Moreover, our observations provide a stringent test of the theoretical description of the unconventional superconductivity in UPt3.