Viscous flux motion in anisotropic type-II superconductors in low fields

The Bardeen-Stephen model of viscous flux motion in isotropic type-II superconductors is extended to the anisotropic case characterized by a phenomenological effective mass tensor m/sub ij/. When the magnetic field is low and the vortex lines are aligned along one of the three principal axes, simple expressions for the viscosity tensor eta /sub ij/ of the viscous flux motion are obtained as functions of m/sub ij/ and the normal state conductivity tensor sigma /sub ij/ for temperature T close to the critical temperature T/sub c/. For the high-temperature oxide superconductors, the theory predicts that eta /sub b//sup (a)/: eta /sub b//sup (c)/: eta /sub c//sup (a)/ approximately=1:4 gamma :3 gamma /sup 2/, where eta /sub i//sup (i)/ is the viscosity for the motion along the i-axis of a vortex parallel to the j-axis and gamma = square root m/sub c//m/sub a/ is the anisotropy parameter (m/sub i/, i=a, b, c, are the principal values of the mass tensor satisfying m/sub a/ approximately=m/sub b/<