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Aragón Sánchez, J., R. Cortés Maldonado, N. R. Cejas Bolecek, G. Rumi, P. Pedrazzini, M. I. Dolz, G. Nieva, C. J. van der Beek, M. Konczykowski, C. D. Dewhurst et al. "Unveiling the vortex glass phase in the surface and volume of a type-II superconductor." Communications Physics 2, no. 1 (2019): 143.
Abstract: Order-disorder transitions between glassy phases are common in nature and yet a comprehensive survey on the entailed structural changes is challenging since the constituents are in the micro-scale. Vortex matter in type-II superconductors is a model system where some of these experimental challenges can be tackled. Samples with point disorder present a glassy transition on increasing the density of vortices. A glassy yet quasi-crystalline phase, the Bragg glass, nucleates at low densities. The vortex glass stable at high densities is expected to be disordered, however its detailed structural properties remained experimentally elusive. Here we show that the vortex glass has large crystallites with in-plane positional displacements growing algebraically and short-range orientational order. Furthermore, the vortex glass has a finite and almost constant correlation length along the direction of vortices, in sharp contrast with strong entanglement. These results are important for the understanding of disorder-driven phase transitions in glassy condensed matter.
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Bolecek, N. R. C., M. I. Dolz, H. Pastoriza, M. Konczykowski, C. J. van der Beek, A. B. Kolton, and Y. Fasano. "Excess of topological defects induced by confinement in vortex nanocrystals." Physical Review B 96, no. 2 (2017): 024507.
Abstract: We directly image individual vortex positions in nanocrystals in order to unveil the structural property that contributes to the depletion of the entropy jump entailed at the first-order transition. On reducing the nanocrystal size, the density of topological defects increases near the edges over a characteristic length. Within this “healing-length†distance from the sample edge, vortex rows tend to bend, while towards the center of the sample, the positional order of the vortex structure is what is expected for the Bragg-glass phase. This suggests that the healing length may be a key quantity to model confinement effects in the first-order transition of extremely layered vortex nanocrystals.
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Cejas Bolecek, N. R., A. B. Kolton, M. Konczykowski, H. Pastoriza, D. Dominguez, and Y. Fasano. "Vortex matter freezing in Bi2Sr2CaCu28 samples with a very dense distribution of columnar defects." Physical Review B 93, no. 5 (2016): 054505.
Abstract: We show that the dynamical freezing of vortex structures nucleated at diluted densities in Bi2Sr2CaCu2O8 samples with a dense distribution of columnar defects, B∼10−2BΦ with BΦ=5kG, results in configurations with liquidlike correlations. We propose a freezing model considering a relaxation dynamics dominated by double-kink excitations driven by the local stresses obtained directly from experimental images. With this model we estimate the relaxation barrier and the freezing temperature. We argue that the low-field frozen vortex structures nucleated in a dense distribution of columnar defects thus correspond to an out-of-equilibrium nonentangled liquid with strongly reduced mobility rather than to a snapshot of a metastable state with divergent activation barriers as, for instance, expected for the Bose-glass phase at equilibrium.
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Cejas Bolecek, N. R., M. I. Dolz, A. Kolton, H. Pastoriza, C. Jvander Beek, M. Konczykowski, M. Menghini, G. Nieva, and Y. Fasano. "Geometrical Confinement Effects in Layered Mesoscopic Vortex Matter." Journal of Low Temperature Physics 179, no. 1-2 (2015): 35–41.
Abstract: We study the geometrical confinement effect in Bi 2 Sr 2 CaCu 2 O 8+δ mesoscopic vortex matter with edge-to-surface ratio of 7–12 %. Samples have in-plane square and circular edges, 30 μ m widths, and ∼ 2 μ m thickness. Direct vortex imaging reveals the compact planes of the structure align with the sample edge by introducing topological defects. The defect density is larger for circular than for square edges. Molecular dynamics simulations suggest that this density is not an out-of-equilibrium property but rather determined by the geometrical confinement.
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Dolz, M. I., A. B. Kolton, and H. Pastoriza. "Direct determination of the collective pinning radius in the cuprate superconductor Bi2 Sr2 CaCu2 O8+ delta." Physical Review B 81, no. 9 (2010): 092502.
Abstract: We study the onset of the irreversible magnetic behavior of vortex matter in micron-sized Bi2Sr2CaCu2O8+δ single crystals by using silicon micro-oscillators. We find an irreversibility line appearing well below the thermodynamic Bragg-glass melting line at a magnetic field which increases both with increasing the sample radius and with decreasing the temperature, paradoxically implying the existence of a reversible vortex solid. We show that at this irreversibility line, the sample radius can be identified with the crossover length between the Larkin and the random-manifold regimes of the vortex-lattice transverse roughness. Our method thus allows to determine, as a function of temperature and applied magnetic field, the minimum size of a vortex system that can be collectively pinned, or the so-called three-dimensional weak collective pinning Larkin radius, in a direct way.
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Pérez-Morelo, D. J., E. Osquiguil, A. B. Kolton, G. Nieva, I. W. Jung, D. López, and H. Pastoriza. "Thermodynamic evidence for the Bose glass transition in twinned YBa2Cu3O7-δ crystals." Physical Review B 92, no. 2 (2015): 020510.
Abstract: We used a micromechanical torsional oscillator to measure the magnetic response of a twinned YBa2Cu3O7−δ single crystal disk near the Bose glass transition. We observe an anomaly in the temperature dependence of the magnetization consistent with the appearance of a magnetic shielding perpendicular to the correlated pinning of the twin boundaries. This effect is related to the thermodynamic transition from the vortex liquid phase to a Bose glass state.
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Rumi, G., J. Aragón Sánchez, F. Elías, R. Cortés Maldonado, J. Puig, N. R. Cejas Bolecek, G. Nieva, M. Konczykowski, Y. Fasano, and A. B. Kolton. "Hyperuniform vortex patterns at the surface of type-II superconductors." Physical Review Research 1, no. 3 (2019): 033057.
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Sanchez, J. A., G. Rumi, R. C. Maldonado, N. R. C. Bolecek, J. Puig, P. Pedrazzini, G. Nieva, M. I. Dolz, M. Konczykowski, C. J. van der Beek et al. "Non-Gaussian tail in the force distribution: a hallmark of correlated disorder in the host media of elastic objects." Scientific Reports 10, no. 1 (2020): 19452.
Abstract: Inferring the nature of disorder in the media where elastic objects are nucleated is of crucial importance for many applications but remains a challenging basic-science problem. Here we propose a method to discern whether weak-point or strong-correlated disorder dominates based on characterizing the distribution of the interaction forces between objects mapped in large fields-of-view. We illustrate our proposal with the case-study system of vortex structures nucleated in type-II superconductors with different pinning landscapes. Interaction force distributions are computed from individual vortex positions imaged in thousands-vortices fields-of-view in a two-orders-of-magnitude-wide vortex-density range. Vortex structures nucleated in point-disordered media present Gaussian distributions of the interaction force components. In contrast, if the media have dilute and randomly-distributed correlated disorder, these distributions present non-Gaussian algebraically-decaying tails for large force magnitudes. We propose that detecting this deviation from the Gaussian behavior is a fingerprint of strong disorder, in our case originated from a dilute distribution of correlated pinning centers.
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