Induced in-plane order in the vortex liquid by periodic pinning arrays

The vortex phase diagram of high-T_c superconductors in the field parallel to the c-axis have been already extensively explored. It has been commonly observed that pinning effects are quite weak at high temperatures and high fields in high quality single crystals. Such weak pinning phases, like the vortex liquid and the Bragg glass, are good candidate to study the influence of additional artificial pinning arrays on vortex phases, because the original quenched disorders do not work well as effective pinning centers in these phases. We have introduced several types of hole arrays in Bi₂Sr ₂CaCu₂O_8+y single-crystal thin film using a focused ion beam. To preserve the quality of the crystals we have made film samples from bulk crystals by the cleaving. In samples with triangular and square pinning arrays of 1μm lattice spacing, the flow resistance of pancake vortices exhibit a matching effect at multiples of the matching fields. This matching effect occurs in the vortex liquid phase. It is difficult to explain these results by the idea of the normal vortex liquid because of the lack of in-plane symmetries. There is a possibility that the in-plane order is induced in the vortex liquid by the introduction of the periodic pinning arrays.