Effects of the Order Parameter Anisotropy on the Vortex Lattice in UPt3

Abstract

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 plane 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° 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.

Activated vortex lattice transition in a superconductor with combined sixfold and twelvefold anisotropic interactions

“…( 1) is not based on an ab-initio derivation, but is instead motivated by an expression for the VL free energy obtained from Ginzburg-Landau (GL) theory 50 . As shown by Olszewski et al, this potential reproduces the experimentally observed macroscopic VL phases in MgB 2 10 and UPt 3 [11][12][13] for a suitable choice of parameters K 6 and K 12 25 . It can thus be considered a minimal model for MD simulations of the VL in superconductors with a hexagonal crystal structure.…”

“…For magnetic fields applied along the c axis of superconductors with a hexagonal crystal structure, the VL is empirically found to always have a triangular symmetry, but will often undergo continuous and sometimes even non-monotonic rotations as seen in MgB 2 and UPt 3 [11][12][13] . In addition, the VLs in these materials exhibit significant metastability when cooled or heated across equilibrium phase transitions in a constant field 11,14 .…”