‘‘Phase diagram’’ of the vortex-solid phase in Y-Ba-Cu-O crystals: A crossover from single-vortex (1D) to collective (3D) pinning regimes

Abstract

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

Nodeless superconductivity in the presence of spin-density wave in pnictide superconductors: The case ofBaFe2−xNixAs2

“…The second peak effect has been studied extensively and its origin may be attributed to various mechanisms. It has been well established that the second peak effect is strongly influenced by the oxygen deficiency in cuprates [42,43]. In the case of Fe-based superconductors, the local magnetic moments may form the small size normal cores, and may be a possible reason of the second peak effect [44].…”

Disordering transitions in vortex matter: peak effect and phase diagram

“…This peak can be interpreted as occurring when the increasing field weakens the interlayer coupling of vortex pancakes due to geometric constraints, and a transition occurs from weakly pinned 3D line vortices to decoupled 2D pancake vortices which can more easily adjust their positions to maximize the pinning [3]. The peak has also been proposed to arise from plasticity, proliferation of in-plane defects, dynamical effects, or matching effects [4][5][6].There is mounting experimental evidence that the peak effect is associated with a sharp transition in the vortex lattice from an ordered state to a disordered state. In BSCCO, neutron scattering [7] and muon lifetime [8] experiments provide evidence that a transition from an ordered 3D vortex arrangement to a disordered or decoupled arrangement is associated with the peak effect.…”

Static and Dynamic Coupling Transitions of Vortex Lattices in Disordered Anisotropic Superconductors

“…The lattice appears ordered at fields below the transition and disordered above. There seems to be no widely accepted agreement on the mechanism behind this transition, although numerous scenarios have been suggested, including vortex entanglement [13], dislocation proliferations [12], dynamic effects [14], or a 3D to 2D transition in the vortex pancake lattice [4,7,15,16,18]. The effect of strong disorder on a possible 3D-2D transition is unclear, and also it is not known how the transport properties would be affected by a 3D-2D transition.…”