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...

Externally driven glassy systems may undergo nonequilibrium phase transitions (NEPTs). In particular, acdriven systems display special features, such as those observed in the vortex matter of NbSe 2 , where oscillatory drives reorganize the system into partially ordered vortex lattices. We provide experimental evidence for this dynamic reorganization and we show an unambiguous signature of its connection with an NEPT driven by ac forces. We perform a scaling analysis and we estimate critical exponents for this transition. Our results share similarities with some glass-to-viscous-liquid NEPTs and invite a search for similar physics in other elastic disordered media.

Vortex matter in type-II superconductors is a model complex system belonging to the vast ensemble of disordered elastic media. In all these systems, depinning occurs when an external drive is increased beyond a critical force. The nature of the depinning transition, the connection with a non-equilibrium phase transition, and its relation with proliferation or annealing of topological defects has been long and thoroughly studied. When driven by alternating forces these systems display particular characteristics, which are not a trivial extension of the corresponding DC regimes, and may be dynamically reorganized into different configurations. In fact, thermal history effects and dynamic reorganization are generally found in the vicinity of an order-disorder transition, whose fingerprint is the sudden rise of the effective pinning, known as Peak Effect, whose origin remained controversial for a while. In this review we reassess together a set of results presented during the last two decades, which allows building a comprehensive picture describing thermal and dynamic history effects in vortex matter, the oscillatory dynamics, the associated dynamic reorganization, and its connection with the AC depinning dynamic phase transition.

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The in-plane distance between atomic defects is much larger than the superconducting coherence length and, for typical magnetic fields (∼ kOe), it is even larger than the inter-vortex distance. The surface-density of atomic defects on a growth surface is generally arXiv:1807.10397v2 [cond-mat.supr-con]

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1. Information on resource selection by a species is essential for understanding the species' ecology, distribution and requirements for survival. Research on habitat selection frequently relies on animal detection at point locations to determine which resource units are used. A variety of approaches and statistical tools can be employed for assessing selection based on habitat variables measured in those units. The aim of this work is to evaluate the reliability of common sampling designs and statistical methods in detecting habitat selection at fine scales based on point data 2. We reviewed literature on microhabitat selection to determine characteristics of typical studies and analysed simulated small mammal live-trapping data as a case study. We considered various scenarios differing in the number of sampled units and sampling duration. For each scenario, a set of simulated surveys was analysed through two univariate Accepted ArticleThis article is protected by copyright. All rights reserved.tests (Welch's t-and Mann-Whitney U-test), generalised linear models (GLMs), mixed-effect models (GLMMs) and occupancy models (OMs). 3.Analysis of simulated data revealed that overall performance of all statistical methods improved with increased trapping effort. Univariate tests were especially sensitive to the number of sampling units, while modelling methods took also advantage of longer trapping sessions. Univariate tests and GLMs provided partially correct information in most cases, whereas GLMMs and OMs offered higher probabilities of fully describing simulated habitat preferences. 4.With typical sampling efforts, appropriate statistical analysis of point data is able to provide a moderately accurate description of habitat selection at small scales, in spite of the violation of closure and independence assumptions of applied models. Modelling approaches are proliferating; we encourage using models that can deal with multiple sources of variability, such as GLMMs and OMs, when data are hierarchically structured.There is no a priori best survey design; it should be chosen according to the scope and goals of the study, environment heterogeneity, species characteristics and practical constraints.Researchers should realise that sampling design and statistical methods likely affect conclusions regarding habitat selection.