Flux-flow resistivity anisotropy in the instability regime of the a-b plane of epitaxial superconducting YBa2Cu3O7-δ thin films

Measurements of the nonlinear flux-flow resistivity ρ and the critical vortex velocity vφ* at high voltage bias close to the instability regime predicted by Larkin and Ovchinnikov (Z. Eksp. Teor. Fiz 68, 1915 (1975) [Sov. Phys. JETP 41, 960 (1976)]) 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 vφ* 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.