Single-vortex fluctuations in layered superconductors: Electromagnetic coupling and crossover to strong pinning

Abstract

Positional fluctuations of single vortices induced by thermal fluctuations and random point pinning are considered in detail for all length scales fully taking into account the competing effects of Josephson and electromagnetic coupling between vortex elements. The electromagnetic coupling gives rise to a pronounced dispersion of the line stiffness and soft short-wavelength modes which modify the displacement fluctuations of the pinned vortex over a wide range of length scales. Furthermore, we present a detailed analysis of strongly pinned individual pancake vortices in a layered superconductor and study the crossover to the collectively pinned single-vortex line. The line stiffness dispersion leads to sharp increases in the pinning length at characteristic temperatures both for weak and strong pinning and two crossover scales. We calculate as well the corresponding increase in the characteristic pinning energy and the pronounced drop of the critical current at these characteristic temperatures. We predict distinct features in the current-voltage characteristics due to the effects of strong pinning and vortex line tension dispersion.