Flux-flow resistivity anisotropy in the instability regime in the a-b plane of epitaxial YBCO thin films

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Measurements of the nonlinear flux-flow resistivity ρ and the critical vortex velocity v*ϕ at high voltage bias close to the instability regime predicted by Larkin and Ovchinnikov LO are reported along the node and antinode directions of the d-wave order parameter in the a-b plane of epitaxial YBa₂Cu₃O₇₋δ films. In this pinning-free regime, ρ and v*ϕ are found to be anisotropic with values in the node direction larger on average by 10% than in the antinode direction. The anisotropy of ρ is almost independent of temperature and field. We attribute the observed results to the anisotropic quasiparticle distribution on the Fermi surface of YBa₂Cu₃O₇₋δ.