Columnar defects and vortex fluctuations in layered superconductors.

The question of the effective dimensionality of the vortex structure in highly anisotropic high-Tc superconductors is the subject of intense experimental and theoretical interest. It was predicted that thermal fluctuations of vortex positions produce strong relative phase fluctuations in adjacent layers and that the phase coherence along the c-axis is destroyed above the decoupling line (DL) in the H −T plane [1]. Above this line an additional transverse field is not screened by Josephson currents and completely penetrates the sample. Several experiments were interpreted [2] as a manifestation of decoupling effects. In this Letter we focus on the influence of columnar defects produced by heavy ion irradiation on the vortex decoupling and the resulting effective dimensionality of the vortex structure. The interest in this question is motivated not only by the technological importance of highly anisotropic compounds such as Bi2Sr2CaCu2Ox (BSCCO), but also by the expected rich variety of nontrivial dynamic and static properties arising from the interplay between the continuous linear geometry of heavy ion tracks and the discrete pancake-like character of the vortex structure. An important question is whether columnar defects can be especially efficient to pin 2D pancakes which are nearly independent in each layers. Early experiments [3] of irradiation of BSCCO failed to show unidirectional pinning along the columns thus suggesting that the linear nature of columnar defects may not be efficient for highly anisotropic materials. However subsequent experiments revealed [4–6] that the relative enhancement of critical current (by several orders of magnitude) and the upward shift of the technologically useful irreversibility region, was even greater for highly anisotropic material such as BSCCO than for the less anisotropic YBCO. We demonstrate that the observed enhanced pinning of pancakes can be explained in terms of the recoupling of the layers by the columnar defects. We show that columnar defects effectively increase the interlayer Josephson coupling by suppressing thermal fluctuations of pancakes positions. This leads to an upward shift in the decoupling line, the maximal shift being expected around the matching field. We also investigate in detail pinning regimes of a single pancake stack on a single column and on the system of columns.