Detection of the paraconductivity anisotropy of two-dimensional superconductors in magnetic field

The excess conductivity σ′ has been found to be anisotropic in the fluctuation region near the phase-transition line Tc(H) in thin films of transition metals. It has been established that when the magnetic field H and the current I are parallel the value of σ′ is greater than for the H ⊥ I orientation (in both cases the field is parallel to the surface of the film). The anisotropy is due to the nonisotropy of the energy fluctuation spectrum of the order parameter. It depends essentially on the ratio of the film thickness d to the coherence length ξ (T) and reaches its maximum in the region of the field at which a single chain of vortices enters the film, i.e., at d ≈ 2ξ(T). Our experimental data are in qualitative agreement with the results of fluctuation theory for the film in a parallel field.

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