Anisotropic viscous flux motion in high-T_c superconducting films

We have studied the microwave transmission through thin Y-Ba-Cu-O films in the presence of magnetic field and for different field orientation with respect to the crystallographic c - axis. We demonstrate that such measurements yield the anisotropy of the viscosity coefficient for fluxon motion, η. We found experimentally that the ratio η_a^(c)/η_c^(a) is temperature independent and equals to ≈4-6 forT_c> T> 0.8T_c (here η_j⁽ⁱ⁾ represents motion along the j- axis of a fluxon oriented along the i -axis). This allows an estimate of the effective mass ratio γ = (m_c m_a) ^{1 2} ≈ 5-8, using recent theories.