LONG-RANGE INTERACTION OF FLUCTUATING VORTICES WITH A PARALLEL SURFACE IN LAYERED SUPERCONDUCTORS

In extremely anisotropic layered superconductors randomly fluctuating vortex lines are attracted to a planar specimen surface by a long-rang e interaction decreasing as [u(2)]/x(0)(2) where x(0) much greater tha n lambda is the average vortex distance to the surface oriented perpen dicular to the layers and [u(2)] is the mean square vortex displacemen t, e.g., [u(2)](proportional to)kT for thermal fluctuations. This long -range force exceeds the short-range exponential interaction (proporti onal to)exp(-2x(0)/lambda) of straight vortices with their image vorte x, where lambda=lambda(ab) is the penetration depth of supercurrents i n the layers. The long-range attraction originates from the dipole-dip ole interaction between each displaced pancake vortex and its image. I t is analogous to the Casimir effect, which predicts an attraction bet ween two closely spaced metal plates. The energy contribution of the a dditional stray field generated by the distorted vortex is calculated. For short-wavelength distortions this term decreases more rapidly wit h increasing surface distance xo and may be disregarded; for long-wave length distortions the stray-field contribution is comparable to the e nergy of the dipole-dipole interaction, compensating it partly.