Direct determination of the collective pinning radius in the cuprate superconductor Bi2Sr2CaCu2O8+ δ

Abstract

We study the onset of the irreversible magnetic behavior of vortex matter in micron-sized Bi_2Sr_2CaCu_2O_{8+delta} single crystals by using silicon micro-oscillators. We find an irreversibility line appearing well below the thermodynamic Bragg-glass melting line at a magnetic field which increases both with increasing the sample radius and with decreasing the temperature, paradoxically implying the existence of a reversible vortex solid. We show that at this irreversibility line, the sample radius can be identified with the crossover length between the Larkin and the random-manifold regimes of the vortex-lattice transverse roughness. Our method thus allows to determine, as a function of temperature and applied magnetic field, the minimum size of a vortex system that can be collectively pinned, or the so-called three-dimensional weak collective pinning Larkin radius, in a direct way.