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van der Beek, C. J., S. Demirdis, M. Konczykowski, Y. Fasano, N. R. Cejas Bolecek, H. Pastoriza, D. Colson, and F. Rullier-Albenque. "Vortex pinning : a probe for nanoscale disorder in iron-based superconductors." Physica B 407, no. 11 (2012): 1746–1749.
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Demirdiş, S., C. J. van der Beek, Y. Fasano, N. R. Cejas Bolecek, H. Pastoriza, D. Colson, and F. Rullier-Albenque. "Strong pinning and vortex energy distributions in single-crystalline Ba(Fe{1−x}Co{x}){2}As{2}." Physical Review B 84, no. 9 (2011): 094517.
Abstract: The interrelation between heterogeneity and flux pinning is studied in Ba(Fe1−xCox )2As2 single crystals with widely varying Co content x.Magnetic Bitter decoration of the superconducting vortex ensemble in crystals with x = 0.075 and x = 0.1 reveals highly disordered vortex structures. The width of the Meissner belt bserved at the edges of the crystals, and above the surface steps formed by leaving, as well as the width of the intervortex distance distribution, indicate that the observed vortex ensemble is established at a temperature just below the critical temperature Tc. The vortex interaction energy and pinning force distributions extracted from the images strongly suggest that the vortex lattice disorder is attributable to strong pinning due to spatial fluctuations of Tc and of the superfluid density. Correlating the results with the critical current density ields a typical length scale of the relevant disorder of 40–60 nm.
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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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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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Rumi, G., L. J. Albornoz, P. Pedrazzini, M. I. Dolz, H. Pastoriza, C. J. van der Beek, M. Konczykowski, and Y. Fasano. "Entropy jump at the first-order vortex phase transition in Bi2Sr2CaCu2O8+δ with columnar defects." Materials Today: Proceedings 14 (2019): 30–33.
Abstract: We study the entropy jump associated with the first-order vortex melting transition (FOT) in Bi2Sr2CaCu2O8+δ crystals by means of Hall probe magnetometry. The samples present a diluted distribution of columnar defects (CD) introduced by irradiation with Xe ions. The FOT is detected in ac transmittivity measurements as a paramagnetic peak, the height of which is proportional to the enthalpy difference entailed by the transition. By applying the Clausius-Clapeyron relation, we quantify the evolution of the entropy jump Δs as a function of the FOT temperature, TFOT, in both pristine crystals and crystals with CD. On increasing the density of CD, Δs decreases monotonically with respect to values found in pristine samples. The Δs versus TFOT dependence in the case of pristine samples follows reasonably well the theoretical prediction of dominant electromagnetic coupling for a model neglecting the effect of disorder. The data for samples with a diluted distribution of CD are not properly described by such a theoretical model.
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Dolz, M. I., N. R. C. Bolecek, J. Puig, H. Pastoriza, G. Nieva, J. Guimpel, C. J. van der Beek, M. Konczykowski, and Y. Fasano. "Enhancement of penetration field in vortex matter in mesoscopic superconductors due to Andreev bound states." Physical Review B 100, no. 6 (2019): 064508.
Abstract: We study the field for the penetration of a first vortex, HP, for vortex matter nucleated in micron-sized samples with edges aligned along the nodal and antinodal directions of the d-wave superconducting order parameter of Bi2Sr2CaCu2O8−δ. Here we present evidence that the HP for vortex matter nucleated in mesoscopic samples with edges parallel to the nodal direction is larger than for the antinodal case, ∼72% at low temperatures. This finding supports the theoretical proposal that surface Andreev bound states appearing in a sample with edges parallel to the nodal direction would produce an anomalous Meissner current that increases the Bean-Livingston barrier for vortex penetration.
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van der Beek, C. J., S. Demirdis, D. Colson, F. Rullier-Albenque, Y. Fasano, T. Shibauchi, Y. Matsuda, S. Kasahara, P. Gierlowski, and M. Konczykowski. "Electron irradiation of Co, Ni, and P-doped BaFe2As2–type iron-based superconductors." In Journal of Physics: Conference Series, 012023. Vol. 449., 2013.
Abstract: High energy electron irradiation is used to controllably introduce atomic-scale point defects into single crystalline Ba(Fe1−xCox)2As2, Ba(Fe1−xNix)2As2, and BaFe2(As1−xPx)2. The appearance of the collective pinning contribution to the critical current density in BaFe2(As1−xPx)2, and the magnitude of its enhancement in Ba(Fe1−xCox)2As2, conform with the hypothesis of quasi-particle scattering by Fe vacancies created by the irradiation. Whereas the insignificant modification of the temperature dependence of the superfluid density in Ba(Fe1−xCox)2As2 and Ba(Fe1−xNix)2As2 points to important native disorder present before the irradiation, the critical temperatures of these materials undergo a suppression equivalent to that observed in the much cleaner BaFe2(As1−xPx)2. This lends credence to the hypothesis of line nodes of the order parameter (at finite kz) in the former two materials.
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Aragón Sánchez, J., R. Cortés Maldonado, M. I. Dolz, N. R. CejasBolecek, C. J. van der Beek, M. Konczykowski, and Y. Fasano. "Direct visualization of local interaction forces in Bi2Sr2CaCu2O8+δ vortex matter." Materials Today: Proceedings 14 (2019): 34–37.
Abstract: We study the local vortex-vortex interaction force fi of the structure frozen during a field-cooling process in an electron-irradiated Bi2Sr2CaCu2O8+δ sample. We compute this magnitude from snapshots of the vortex structure obtained via magnetic decoration experiments at various fields H in the same sample. Since the observed structures correspond to the equilibrium ones frozen at T∼Tirr(H)[1], at this temperature the local modulus of fi roughly equals the local pinning force at the decorated surface of the sample. We estimate the most probable local pinning force from the mode value of the fi(r)distribution, fpm. We found that fpm grows algebraically with H and in electron-irradiated samples is 50-20% smaller than for samples with columnar defects.
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