ON THE CROSSOVER TO THE SO-CALLED 2-DIMENSIONAL FLUXOIDS IN LAYERED HIGH-T-C SUPERCONDUCTORS

We investigated theoretically the effect of flux pinning on the correl ation of fluxoids in applied magnetic fields, B, parallel to the c-axe s of layered-superconductor single crystals. It is shown that the stre ngth of the correlation between neighboring layers of thermally vibrat ing fluxoids along the c-axis, normalized by the auto-correlation of f luxoids within the same layer, is given by (B-cr/B)(1/2) above a cross over field, B-cr, and hence begins to decrease as B increases, while i t is almost unity below B-cr. Thus, B-cr may be regarded as the crosso ver held, above which even the pinned fluxoids begin to have a tendenc y of pseudo-two-dimensional (''2D'') character in terms of the correla tion on the spatial scale of the order of the layer thickness, d. For the medium-range correlation of the spatial scale much larger than d, on the other hand, the interlayer correlation due to flux pinning stil l keeps even at B > B-cr the same order of magnitude as in the isotrop ic superconductors. From this view point on the correlation due to flu x-pinning, therefore, the fluxoids can be regarded as three-dimensiona l even above the crossover field, B-cr, when each fluxoid is locally d iscontinuous in units of the spatial scale of d.