We studied thermal and dynamic history effects in the vortex lattice (VL) near the order-disorder transition in clean NbSe2 single crystals. Comparing the evolution of the effective vortex pinning and the bulk VL structure, we observed metastable superheated and supercooled VL configurations that coexist with a hysteretic effective pinning response due to thermal cycling of the system. A novel scenario, governed by the interplay between (lower) elastic and (higher) plastic energy barriers, is proposed as an explanation for our observations: Plastic barriers, which prevent the annihilation or creation of topological defects, require dynamic assistance to be overcome and to achieve a stable VL at each temperature. Conversely, thermal hysteresis in the pining response is ascribed to low energy barriers, which inhibit rearrangement within a single VL correlation volume and are easily overcome as the relative strength of competing interactions changes with temperature.
We report structural evidence of dynamic reorganization in vortex matter in clean NbSe2 by joint small angle neutron scattering and ac-susceptibility measurements. The application of oscillatory forces in a transitional region near the order-disorder transition results in robust bulk vortex lattice configurations with an intermediate degree of disorder. These dynamically-originated configurations correlate with intermediate pinning responses previously observed, resolving a long standing debate regarding the origin of such responses.PACS numbers: 74.25. Uv, 61.05.fg, 64.60.Cn In a wide variety of complex systems, competing interactions promote an order-disorder transition (ODT). Ordered phases are characterized by spatial correlations decaying weakly over distances larger than the relevant system scale, whereas disordered configurations are characterized by correlation lengths ζ of the order of the mean inter-particle distance a 0 . Configurations with intermediate degrees of disorder, strong enough to affect the system response, but still with ζ a 0 , have received attention recently [1]. Vortex matter in superconductors provides an ideal model system for the experimental study of the topic [2]: vortex-vortex interactions favoring an ordered vortex lattice (VL) compete with both thermal fluctuations and pinning interactions that tend to disorder the system.In very low-pinning superconductors, such as clean NbSe 2 single crystals, most of the vortex field-temperature phase diagram is properly described by an ordered dislocation-free Bragg Glass (BG) phase [3][4][5], which undergoes an ODT near the normal-superconductor transition [6][7][8]. In practice, when a superconductor is cooled from the normal state in an external magnetic field (fieldcooled/FC), energy barriers may trap the VL in highly disordered metastable configurations [8]. Even so, high transport current densities [9,10] or large oscillatory "shaking" magnetic fields [11,12] may anneal the VL into the ordered low-temperature BG. Once in the BG, as temperature or magnetic field is increased the VL softens and accommodates to the random pinning potential more efficiently. Eventually, vortex entanglement and the proliferation of VL dislocations increase the effective pinning, producing a sudden rise of the critical current J c known as the Peak Effect (PE) [7,8], which is the fingerprint of the ODT in vortex matter. While both the highpinning disordered phase and the low-pinning ordered phase, above and below the ODT, are widely accepted in the literature, intermediate responses have been ascribed to surface contamination and partial reordering by the probing transport current [9,10,13,14]. However, in the narrow transitional region adjacent to the PE, for which a multidomain phase has been theoretically proposed [15], non-invasive techniques have shown that pinning can be partially decreased or even increased by applying dc currents [16,17] To address this question, we performed an experiment in a clean NbSe 2 single crystal, combining small angle...
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