No abstract

No abstract

Flux pinning in Y-Ba-Cu-O crystals is studied as a function of fluence of 3-MeV protons, which create random local defects. Order-of-magnitude increases in the critical current density are deduced from magnetic hysteresis loops, with values up to 2x 10 5 A/cm 2 observed at 77 K and 1 T. However, the irreversibility line in the field-temperature plane and the pinning potentials deduced from flux-creep studies are hardly changed. These results are compared to melting and pinning models.

We construct a "phase diagram" of the vortex-solid phase of Y-Ba-Cu-O crystals by making a first overall connection of the shape of the magnetic hysteresis M(H,T) with the single-vortex (ID) and collective (3D) pinning regimes. The crossovers between different regimes are visualized from contours of constant J c in the H-T plane. We identify the transition from ID to 3D pinning, and from the nonlocal into a local behavior of vortex bundles in the collective pinning regime. A direct correlation between M(H) and the thermal relaxation rate is demonstrated. We also identify a signature of the thermal softening boundary at which thermal fluctuations on the scale of the coherence length £ are relevant. PACS numbers: 74.70.Vy, 74.60.Ge, 74.60.JgIt is well established now that in the mixed state of high-temperature superconductors, the conventional Abrikosov vortex lattice is replaced by a melted vortex liquid over large regions of the magnetic (H-T) phase diagram [l]. The behavior of vortices in the diminished solid phase below the melting transition is undoubtedly controlled by the numerous weak randomly distributed defects, such as oxygen vacancies [2], and it has been argued that the collective pinning theory [3], describing the critical current density J c , should be relevant. Recently, FeigePman and Vinokur [4] explored the vortex-solid phase in the framework of collective pinning by weak disorder. Considering thermal harmonic fluctuations of vortices, but neglecting thermally activated creep, they proposed a "phase diagram" of nonequilibrium regimes in the H-T plane and derived boundaries for the singlevortex and various collective pinning regimes. Specific regions of the vortex-solid phase, however, are complicated by creep effects (thermally activated jumps of vortices or bundles) [5] and the full picture has not yet been tested experimentally.In this Letter, we construct the "phase diagram" of the vortex-solid regimes below the melting line for Y-Ba-Cu-O single crystals, by tracing the boundaries for the transitions from the single-vortex pinning (ID) to the collective pinning (3D) predicted in the theory of collective pinning [4]. We present a first direct association of the shape of dc magnetization M(H y T) with various regimes in the H-T plane, which we show to be strongly influenced by the field-dependent thermal relaxation consistent with the collective creep [5] idea. The novel analysis technique allows us to confirm low-field anomalies which we have recently reported in the ac response [6] and associated with the thermal softening transition [4].The dc magnetization M(H) of single crystals of Y-Ba-Cu- O [7] was measured with a Quantum Design SQUID magnetometer up to 5.5-T fields at temperature intervals of 1 K. The magnetic hysteresis loops for a roughly millimeter size and 20-/im-thick crystal, with

No abstract

No abstract

No abstract

No abstract