arXiv subjects

M. I. Dolz

Publications and source records attributed to M. I. Dolz.

13 recordsLinked to original sources

arXiv2026

Healing of topological defects while crystallizing nanocrystals

Understanding the role of confinement while crystallizing nanocrystals is very relevant for predicting their structure and physical properties. With this aim we perform Langevin dynamics simulations of nanocrystals of the model system of few hundred vortices nucleated in micron-sized superconductors. We study the crystallization dynamics and the low-temperature structural properties of vortex nanocrystals nucleated in field-cooling conditions when changing vortex density or elasticity of the system and physical size of the samples. The low-temperature snapshots obtained in simulations present a healing effect at the edges that is in quantitative agreement with experimental data in Bi2Sr2CaCu2O8+δ micron-sized samples. We show that the low-temperature radial distribution of topological defects is a stationary profile frozen at a temperature below the melting line tuned by intrinsic properties of the vortex structure and on the confinement effect. These findings on the dynamics and spatial profile of topological defects can be applied to describe the physical properties of confined soft condensed matter nanocrystals in general.

cond-mat.supr-con↗

arXiv2020

Magnetic hysteresis behavior of granular manganite La$_{0.67}$Ca$_{0.33}$MnO$_3$ nanotubes

A silicon micromechanical torsional oscillator is used to measure the hysteresis loops of two manganite La_0.67Ca_0.33MnO_3 nanotubes at different temperatures, applying an external field along its main axes. These structures are composed of nanograins with a ferromagnetic core surrounded by a dead layer. Micromagnetic calculations based on the stochastic Landau-Lifshitz-Gilbert equation, are performed to validate a simple model that allows for quantitatively describing the ferromagnetic behavior of the system. Further simulations are used to analyze the experimental data more in depth and to calculate the coercive field, the saturation and remanent magnetizations, and the effective magnetic volume for single nanotubes, over a wide temperature range.

cond-mat.mes-hall↗

arXiv2019

Enhancement of penetration field in vortex nanocrystals due to Andreev bound states

We study the penetration field $H_{\rm P}$ for vortex nanocrystals nucleated in micron-sized samples with edges aligned along the nodal and anti-nodal directions of the d-wave superconducting parameter of Bi$_2$Sr$_2$CaCu$_2$O$_{8 - δ}$. Here we present evidence that the $H_{\rm P}$ for nanocrystals nucleated in samples with edges parallel to the nodal direction is larger than for the antinodal case, $\sim 72$\,\% 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.This has been detected thanks to the nucleation of vortex nanocrystals with a significant surface-to-volume ratio.

cond-mat.supr-con↗

arXiv2019

Direct visualization of local interaction forces in Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ vortex matter

We study the local vortex-vortex interaction force $\textbf{f}_i$ of the structure frozen during a field-cooling process in an electron-irradiated Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ 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$\sim$T$_{irr}$(H), at this temperature the local modulus of $\textbf{f}_i$ 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 f$_i$(r) distribution, f $_{p}^{m}$. We found that f $_{p}^{m}$ grows algebraically with H and in electron-irradiated samples is 50-20% smaller than for samples with columnar defects.

cond-mat.supr-con↗

arXiv2019

Entropy jump at the first-order vortex phase transition in Bi2Sr2CaCu2O8+δ with columnar defects

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.

cond-mat.supr-con↗

arXiv2018

Design and implementation of a micro-coil induction magnetometer

We present a micron-sized induction magnetometer designed to measure the magnetic response of superconducting mesoscopic samples. The device is manufactured using the Memscap PolyMUMPs process and consists of two octagonal planar parallel micro-coils covering an area of 240 microns X 240 microns, which are separated by only 2.75 microns. We show that this design is sufficiently sensitive to detect the Meissner transition at zero dc field, of a high-Tc superconductor Bi2Sr2CaCu2O8 disk of 40 microns in diameter and 1 micron in thicknesses.

cond-mat.supr-con↗

arXiv2017

Excess of topological defects induced by confinement in vortex nanocrystals

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 the entropy-jump depletion in the first-order transition of extremely-layered vortex nanocrystals.

cond-mat.supr-con↗

arXiv2014

Geometrical confinement effects in layered mesoscopic vortex-matter

We study geometrical confinement effects 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 $\sim 2\,μ$m thickness. Direct vortex imaging reveals the compact planes of the structure align with the sample edge by introducing topological defects. The defects density is larger for circular than for square edges. Molecular dynamics simulations suggest this density is not an out-of-equilibrium property but rather determined by the geometrical confinement.

cond-mat.supr-con↗

arXiv2014

Effect of quenched disorder in the entropy-jump at the first-order vortex phase transition of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8 + δ}$

We study the effect of quenched disorder in the thermodynamic magnitudes entailed in the first-order vortex phase transition of the extremely layered Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8 + δ}$ compound. We track the temperature-evolution of the enthalpy and the entropy-jump at the vortex solidification transition by means of AC local magnetic measurements. Quenched disorder is introduced to the pristine samples by means of heavy-ion irradiation with Pb and Xe producing a random columnar-track pins distribution with different densities (matching field $B_Φ$). In contrast with previous magneto-optical reports, we find that the first-order phase transition persists for samples with $B_Φ$ up to 100\,Gauss. For very low densities of quenched disorder (pristine samples), the evolution of the thermodynamic properties can be satisfactorily explained considering a negligible effect of pinning and only electromagnetic coupling between pancake vortices lying in adjacent CuO planes. This description is not satisfactory on increasing magnitude of quenched disorder.

cond-mat.supr-con↗

arXiv2014

Latent-heat and non-linear vortex liquid at the vicinity of the first-order phase transition in layered high-Tc superconductors

In this work we revisit the vortex matter phase diagram in layered superconductors solving still open questions by means of AC and DC local magnetic measurements in the paradigmatic Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$ compound. We show that measuring with AC magnetic techniques is mandatory in order to probe the bulk response of vortex matter, particularly at high-temperatures where surface barriers for vortex entrance dominate. From the $T_{\rm FOT}$-evolution of the enthalpy and latent-heat at the transition we find that, contrary to previous reports, the nature of the dominant interlayer coupling is electromagnetic in the whole temperature range. By studying the dynamic properties of the phase located at $T \gtrsim T_{\rm FOT}$, we reveal the spanning in a considerable fraction of the phase diagram of a non-linear vortex phase suggesting bulk pinning might play a role even in the liquid vortex phase.

cond-mat.supr-con↗

arXiv2012

Persistence of the first-order transition lines in mesoscopic Bi$_2$Sr$_2$CaCu$_2$O$_8$ vortex matter with less than hundred vortices

The persistence of the first-order transition line in the phase diagram of mesoscopic Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$ vortex matter is detected down to a system size of less than hundred vortices. Precise and highly-sensitive to bulk currents AC magnetization techniques proved to be mandatory in order to obtain this information. The location of the vortex matter first-order transition lines are not altered by decreasing the sample size down to 20 $μ$m. Nevertheless, the onset of irreversible magnetization is affected by increasing the sample surface-to-volume ratio producing a noticeable enlargement of the irreversible vortex region above the second-peak transition.

cond-mat.mes-hall↗

arXiv2011

Anisotropic response of the moving vortex lattice in superconducting Mo$_{(1-x)}$Ge$_{x}$ amorphous films

We have performed magnetic susceptibility measurements in Mo$_{(1-x)}$Ge$_x$ amorphous thin films biased with an electrical current using anisotropic coils. We tested the symmetry of the vortex response changing the relative orientation between the bias current and the susceptibility coils. We found a region in the DC current - temperature phase diagram where the dynamical vortex structures behave anisotropically. In this region the shielding capability of the superconducting currents measured by the susceptibility coils is less effective along the direction of vortex motion compared to the transverse direction. This anisotropic response is found in the same region where the peak effect in the critical current is developed. On rising temperature the isotropic behavior is recovered.

cond-mat.supr-con↗

arXiv2008

Dependence of vortex phase transitions in mesoscopic BSCCO superconductor at tilted magnetic fields

A micron sized single crystal of the superconductor BSCCO was studied using silicon mechanical micro-oscillators at various tilt angles of the dc magnetic field with respect to the c axis of the sample. Different phases of the vortex matter were detected by measuring changes in the value and sign of the oscillator resonant frequency variation with temperature. We could explain the change in the sign of this variation at high temperatures as the transition from the 2D liquid of decoupled pancakes to a reversible 3D vortex lattice. The data indicates that this transition only depends on the magnetic field perpendicular to the superconducting layers while the dissipation involved in this process depends on the component parallel to them.

cond-mat.supr-con↗