Flux Lattice Symmetry in $V_3\mathrm{Si}$: Nonlocal Effects in a High- $\mathit{\kappa }$ Superconductor

In the high-κ cubic superconductor ${\mathrm{V}}_3\mathrm{Si}$, phase transitions of the flux lattice structure occur as a function of applied field and temperature. With the field parallel to the fourfold [001] axis, the flux lattice transforms from triangular to square symmetry at approximately 1 T. With the field parallel to the twofold [110] axis, the lattice, which is a nearly perfect hexagonal array at the lowest fields, distorts as the field is increased; the rate of increase in this distortion changes abruptly at 1.3 T. As $T_c$ is approached, the system tends towards a more isotropic hexagonal array of the flux lines. These transitions are largely but not completely in agreement with a recent theory of the effects on the flux line arrangements of nonlocal electrodynamics in the London limit.