Flux-line lattice anisotropy in V 3Si: Observation and interpretation

Abstract

The high- T c A15 compound V 3Si belongs to a small class of intrinsically high-gk, clean, type-II superconductors. For such a material in the low-field mixed state, the inter-flux-line forces are dominated by the microscopic field interactions. We have observed directly manifestations of anisotropic flux line (FL) interactions by means of small-angle neutron scattering (SANS) from the flux-line lattice (FLL) in a high quality single crystal of V 3Si. Pronounced correlations were found between the FLL morphology (orientation and symmetry) and the symmetry of the real crystal lattice (CL) axis parallel to the applied field. Moreover, for a fixed sample orientation in the field, we observe changes in the FLL symmetry with flux density B. We have interpreted the data in terms of weak nonlocality in the electrodynamic response, and anisotropy in the Fermi surface and electron-phonon interaction of the cubic material. A reciprocal space power series expansion for the mixed state free energy is obtained and fitted to the observe FLL morphology. The results are compaed with of previously measured H c2 and gap anisotropy.