Dynamic Reorganization of Vortex Matter into Partially Disordered Lattices

Abstract

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...

Memory formation and evolution of the vortex configuration associated with random organization

“…All the data were taken at 4.1 K in 3.5 T corresponding to the peak-effect regime at equilibrium [29][30][31][32][33][34][35][36][37] where pinning is effective [13,24]. The average inter-vortex distance is a few tens nm.…”

Competition between dynamic ordering and disordering for vortices driven by superimposed ac and dc forces

“…To realize random organization, it is favorable to prepare the highly disordered initial vortex configuration. Thus, all the data in this work were acquired in the so-called peak-effect regime [46][47][48][49][50][51][52] at 3.5 T and 4.1 K, where the pinning is very effective [4,23]. From the field value of 3.5 T, the mean intervortex spacing is estimated to be 26 nm.…”

Thermal hysteresis of the Campbell response as a probe for bulk pinning landscape spectroscopy

“…The necessary tuning knobs-temperature and ac field shaking-to exploit this spectroscopic tool have recently been used [24][25][26][27] to investigate the linear ac susceptibility of NbSe 2 single crystals; a superconductor with a scarce distribution of structural defects * and where dynamic and thermal effects in vortex matter have been studied extensively [24][25][26][27][29][30][31][32] . In Refs.…”

“…In Refs. [26,27], joint small-angle neutron scattering (SANS) and in-situ ac susceptibility measurements have been used to investigate the vortex lattice structure and its response to ac magnetic perturbations. Various cooling and warming protocols have revealed a strong hysteresis in the ac response of this material.…”