Abrikosov-Josephson Vortices Research Articles

Magnetic relaxation in (Bi, Pb)-2223 superconducting ceramics doped with α-Al2O3 nanoparticles

A preliminary study of the magnetic relaxation in (Bi, Pb)-2223 superconducting ceramics doped with α-Al2O3 nanoparticles is presented, taking as starting point the measurements of the M(t) dependence, both in samples in the form of powder as in pellet. The relaxation of the three types of vortices that can exist inside these materials is analyzed, emphasizing on the intragranular regionwhere the planar defects exist and, consequently, Abrikosov-Josephson vortices emerge. Finally, interesting results of the comparison between the behavior of the samples in the form of powder and in the form of pellet are shown from the estimation of the pinning energy of the vortices.

Trapping of Magnetic Flux in Bi-2223 Ceramic Superconductors Doped with α-Al2O3 Nanoparticles

By combining experimental results and a simple model, we offer here an explanation for the role played by the low-angle grain boundaries and the doping with α-Al2O3 nanoparticles in the trapping of the magnetic flux in Bi1.65Pb0.35Sr2Ca2Cu3O10+δ (Bi-2223) ceramic samples. Our model correlates the size of the nanoparticles, properties of the superconducting matrix, and the magnetic flux trapped at the intragranular planar defects, the so-called Abrikosov-Josephson vortices. The results indicate that the role played by the doping with α-Al2O3 nanoparticles is to pin the AJ vortices at the grain boundaries with misorientation angles $\theta \sim 10^{\circ }$. We also argue that a similar procedure for identifying conditions for the trapping of the magnetic flux may be applied to other superconducting materials doped with non-magnetic nanoparticles.

Manifestation of granularity in the transport current of coated conductors

The effects of granularity on the angle and field dependence of the transport Jc in coated conductors (CCs) are discussed. The granularity is revealed by scanning Hall probe microscopy of the trapped field profiles in tapes with different architectures. It was found that pulsed laser deposited (PLD) YBCO film on a RABiTS NiW substrate has the most prominent granular morphology in its trapped field profiles. This is complemented by a peculiar behavior of the critical currents when the orientation of the applied field is within a certain angular range around the ab-planes: Jc(θ) becomes independent of the angle and Jc(B) exhibits a peak. These phenomena are explained by the physics of Abrikosov–Josephson vortices at grain boundaries (GB) that leads to a transition from a GB-limited to a grain-limited regime both in Jc(θ) and Jc(B). The effects of granularity are strongly suppressed by the chemical solution deposition route for growing YBCO films on NiW, and are almost absent in PLD-YBCO on ABAD-textured templates. These results identify the collective influence of the GB on the percolative current flow in CCs, which becomes most significant at low temperatures.

Voltage relaxation and Abrikosov–Josephson vortices in Bi-2223 superconductors doped with α-Al2O3 nanoparticles

We have observed a change in the behaviour of voltage-relaxation curves with remnant magnetic flux, when the maximum magnetic field applied before starting the relaxation process, $$H_{am}$$ , reaches 80 Oe in two samples of $$\hbox {Bi}_{1.65}\hbox {Pb}_{0.35}\hbox {Sr}_{2}\hbox {Ca}_{2}\hbox {Cu}_{3}\hbox {O}_{10+\delta }$$ (Bi-2223) doped with $$\alpha $$ - $$\hbox {Al}_{2}\hbox {O}_{3}$$ nanoparticles. This modification consists in the emergence of a maximum in the temporal dependence of the voltage, V(t), which appears in the vicinity of the first 5 s after applying to the sample an excitation current lightly higher than the superconducting critical current of the material. In addition to that, the voltage drop rate increases appreciably with increasing applied magnetic fields. As complementary studies, X-ray diffraction patterns, scanning electron microscopy, measurements of magnetization as a function of applied magnetic field and critical current density are also included. The experimental results corroborate the occurrence of intragranular Abrikosov–Josephson vortices in Bi-2223 ceramic superconductors. Furthermore, the analysis provides a new procedure for detecting penetration of the magnetic flux into intragranular planar defects and clean regions of the grains (crystallites), with increasing applied maximum magnetic field, $$H_{am}$$ .

Imaging of super-fast dynamics and flow instabilities of superconducting vortices

Quantized magnetic vortices driven by electric current determine key electromagnetic properties of superconductors. While the dynamic behavior of slow vortices has been thoroughly investigated, the physics of ultrafast vortices under strong currents remains largely unexplored. Here, we use a nanoscale scanning superconducting quantum interference device to image vortices penetrating into a superconducting Pb film at rates of tens of GHz and moving with velocities of up to tens of km/s, which are not only much larger than the speed of sound but also exceed the pair-breaking speed limit of superconducting condensate. These experiments reveal formation of mesoscopic vortex channels which undergo cascades of bifurcations as the current and magnetic field increase. Our numerical simulations predict metamorphosis of fast Abrikosov vortices into mixed Abrikosov-Josephson vortices at even higher velocities. This work offers an insight into the fundamental physics of dynamic vortex states of superconductors at high current densities, crucial for many applications.

Intragranular defects and Abrikosov–Josephson vortices in Bi-2223 bulk superconductors

The Abrikosov–Josephson (AJ) vortices that emerge at intragranular defects of Bi-2223 bulk superconductors are detected. The study is based on the combination of magnetic and transport characterizations with a brief theoretical formulation that allow estimating the minimum angle of the intragranular planar defects where the AJ vortices emerges for a given applied magnetic field. Measurements of magnetization versus applied magnetic field, $$M(H_{a})$$ , in powder samples reveal that the intrinsic anisotropy effects of the grains in these materials suffer almost a complete attenuation due to their high shape anisotropy. In fact, powder samples behave as composed of isotropic grains with lower critical field ~80 Oe. However, a more detailed analysis of the quasi-linear part of these curves show that the penetration of the magnetic flux into intragranular defects occurs at similar values of applied magnetic fields than those reported in whiskers or well textured ceramics for their two main perpendicular directions which are lower values than ~80 Oe. Finally, taking into account a brief theoretical formulation, we have found that in the case of defects where the magnetic flux penetrates perpendicular to the c-axis of the crystallites (~30 Oe) approximately the stacking faults and defects with misorientation angles higher than ~14 $$^{\circ }$$ can be the main causes of flux-trapping, but the mechanism may change to low misorientation angles defects, less than ~6 $$^{\circ }$$ , when the intergranular magnetic field is approximately parallel to the c-axis (~60 Oe).

Vortex Pinning Properties at Grain Boundary in SmBa2Cu3 ${\text{O}}_{y}$ Superconducting Films With BaHfO3 Nanorods Controlled via Low-Temperature Growth

We investigated the effect of the BaHfO 3 (BHO) nanorods' configuration on transport properties at artificial grain boundaries (GBs) in SmBa 2 Cu 3 O y (SmBCO) superconducting films. To control the nanorods' configuration at the GBs, we deposited 3.0 vol.% BHO-doped SmBCO films on 5° [001]-tilt bicrystal (LaAlO 3 ) 0.3 (SrAl 0.5 Ta 0.5 O 3 ) 0.7 substrates at relatively low growth temperature of 750 °C via the pulsed laser deposition method adopting a seed layer technique. According to the transport results, the critical current density (J c ) at the GBs was improved in the films with the nanorods at low magnetic fields. In addition, field angular dependence of Jc was also improved for a wide range of field directions with a broad peak for B//c. We estimated that the BHO nanorods in the GBs pinned the Abrikosov-Josephson (AJ) vortices with various directions in the GBs. AJ vortex pinning can be controlled by configuration of a pinning center at GB.

Intrinsic and extrinsic pinning in NdFeAs(O,F): vortex trapping and lock-in by the layered structure.

Fe-based superconductors (FBS) present a large variety of compounds whose properties are affected to different extents by their crystal structures. Amongst them, the REFeAs(O,F) (RE1111, RE being a rare-earth element) is the family with the highest critical temperature Tc but also with a large anisotropy and Josephson vortices as demonstrated in the flux-flow regime in Sm1111 (Tc ∼ 55 K). Here we focus on the pinning properties of the lower-Tc Nd1111 in the flux-creep regime. We demonstrate that for H//c critical current density Jc at high temperatures is dominated by point-defect pinning centres, whereas at low temperatures surface pinning by planar defects parallel to the c-axis and vortex shearing prevail. When the field approaches the ab-planes, two different regimes are observed at low temperatures as a consequence of the transition between 3D Abrikosov and 2D Josephson vortices: one is determined by the formation of a vortex-staircase structure and one by lock-in of vortices parallel to the layers. This is the first study on FBS showing this behaviour in the full temperature, field, and angular range and demonstrating that, despite the lower Tc and anisotropy of Nd1111 with respect to Sm1111, this compound is substantially affected by intrinsic pinning generating a strong ab-peak in Jc.

Critical Current Density Through Grain Boundaries in High-Temperature Superconductors

In this paper, a theoretical study is proposed based on the assumption that the vortices on low-angle grain boundaries (GBs) in high-temperature superconductor (HTS) are mixed Abrikosov-Josephson (AJ) vortices. The critical current density through GB is obtained on the basis of the Bean critical model and the assumption that the periods of AJ vortices coincide with the ones of Abrikosov (A) vortices. The model also enables us to calculate J c of HTS with an inclined GB. In addition, the effect of strain on critical current density is also taken into account in this model by considering the strain dependence of deparing current density within GB. There is a good agreement of our results with the classical power-law expression. The model proposed in this work can be used for simultaneous studies of the effects of misorientation angles, GB-inclined angles, and applied fields on the critical current density of polycrystalline HTS.

Effect of BaHfO3 introduction on the transport current at the grain boundaries in SmBa2Cu3Oy films

We report the effect of BaHfO3 (BHO) introduction into SmBa2Cu3Oy (SmBCO) films on the transport properties at artificial grain boundaries (AGBs) by comparing pure SmBCO and BHO-doped SmBCO films deposited on bicrystal substrates. Improvement of the critical current density (Jc) at the AGB of BHO-doped SmBCO films was confirmed under the magnetic fields at which Jc was restricted at the AGB. By investigating the superconducting properties and from TEM observations, we found that the in-field Jc value for BHO-doped SmBCO films was improved through flux pinning of the Abrikosov–Josephson vortices by BHO nanorods.

Vortex Molecules in Thin Films of Layered Superconductors

Both the equilibrium and transport properties of the vortex matter are essentially affected by the behavior of the intervortex interaction potential. In isotropic bulk superconductors this potential is well known to be repulsive and is screened at intervortex distances R greater than the London penetration depth λ. As a result, in perfect crystals quantized Abrikosov vortices form a triangular lattice. In thin films of anisotropic superconductors this standard interaction potential behavior appears to be strongly modified because of the interplay between the long-ranged repulsion predicted in the pioneering work by J. Pearl and the attraction caused by the tilt of the vortex lines with respect to the anisotropy axes. This interplay results in a new type of vortex arrangement formed by finite-size vortex chains, i.e., vortex molecules. Tilted vortices with such unusual interaction potential form clusters with the size depending on the field tilting angle and film thickness or/and can arrange into multiquanta flux lattice. The magnetic flux through the unit cells of the corresponding flux line lattices equals to an integer number N of flux quanta. Thus, the increase in the field tilting (or varying temperature) should be accompanied by the series of the phase transitions between the vortex lattices with different N. A similar scenario should be realized in strongly anisotropic BSCCO high-T c superconductors where in tilted field a crossing lattice of Abrikosov vortices (the stacks of pancakes in this case) and Josephson vortices appears. This crossing leads to the zigzag deformation of the pancakes stacks which is responsible for the attraction interaction competing with the long-ranged Pearl’s repulsion.

Attraction between pancake vortices and vortex molecule formation in the crossing lattices in thin films of layered superconductors

We study the intervortex interaction in thin films of layered superconductors for the magnetic field tilted with respect to the $c$ axis. In such a case, the crossing lattice of Abrikosov vortices (AVs) and Josephson vortices appears. The interaction between pancake vortices, forming the AVs, with Josephson ones, produces the zigzag deformation of the AV line. This deformation induces a long-range attraction between Abrikosov vortices and, in thin films, it competes with another long-range interaction, i.e., with Pearl's repulsion. This interplay results in the formation of clusters of Abrikosov vortices, which can be considered as vortex molecules. The number of vortices in such clusters depends on field tilting angle and film thickness.

Vortex pinning regimes in YBa2Cu3O7−x bulk boundaries investigated by quantitative magnetic Hall microscopy

Magnetic Hall probe microscopy in combination with an inverse problem solver was used tosimultaneously determine the intragranular and intergranular critical current density dependenceson the applied field of YBCO melt-textured welds with [001]-misorientation angles from0° to30°. This methodology enabled us to investigate the pinning mechanisms of grain boundaryvortices, which evolve from Abrikosov vortices (AVs) to Abrikosov–Josephson vortices(AJVs) and then to Josephson vortices (JVs) as the angle increases. We present a magneticfield versus grain boundary angle phase diagram at 77 K in which different intergrain vortexmotion regimes can be identified.

Tilt angle dependences of vortex structure and critical current density at low-angle grain boundaries inYBa2Cu3O7−xfilms

A tilt angle dependence of vortex structure was studied at low-angle grain boundaries (LAGBs) in $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$ films. The 2\ifmmode^\circ\else\textdegree\fi{} tilt LAGB did not degrade ${J}_{c}$ of the film in a self-field, demonstrating the existence of Abrikosov vortices at the 2\ifmmode^\circ\else\textdegree\fi{} LAGB. The magnetic field dependence of the flux flow resistance indicated the existence of Abrikosov Josephson vortices at the 5\ifmmode^\circ\else\textdegree\fi{} tilt LAGB. At the 10\ifmmode^\circ\else\textdegree\fi{} tilt LAGB, a Josephson junction behavior was observed in a self-field, indicating the existence of Josephson vortices. The influence of the LAGBs on critical current density was understood based on these vortices.

Mechanisms for enhanced supercurrent across meandered grain boundaries in high-temperature superconductors

It has been well established that the critical current density Jc across grain boundaries (GBs) in high-temperature superconductors decreases exponentially with misorientation angle θ beyond ∼2°–3°. This rapid decrease is due to a suppression of the superconducting order parameter at the grain boundary, giving rise to weakly pinned Abrikosov-Josephson (AJ) vortices. Here we show that if the GB plane meanders, this exponential dependence no longer holds, permitting greatly enhanced Jc values: up to six times at 0T and four times at 1T at θ∼4°–6°. This enhancement is due to an increase in the current-carrying cross section of the GBs and the appearance of short AJ vortex segments in the GB plane, confined by the interaction with strongly pinned Abrikosov (A) vortices in the grains.

Grain and grain boundary vortex dynamics in YBa2Cu3O7-δ coated conductor by ac susceptibility

ac-susceptibility measurements have been performed on YBa2Cu3O7−δ (YBCO) epitaxial thin films and coated conductors (CCs) in order to determine and compare the vortex motion regimes and dynamic responses induced at different temperatures and magnetic fields. A general phase diagram in the (Hac,T) plane has been built up, where three different dynamic regimes could be identified and compared for epitaxial films and CCs with different textures. Moreover, we have performed a comprehensive study of the dc-magnetic field and current density dependencies of the vortex activation energies associated with Abrikosov vortices inside the grains and Abrikosov-Josephson vortices at the grain boundaries and have built up a complete magnetic phase diagram valid for CCs in the (Hdc-T) plane. We present a full description of the different vortex motion regimes identified in the diagram. With this study we have been able to analyze and describe the general behavior of vortex dynamics at high temperatures, close to the irreversibility line, for YBCO thin films and CCs.

Vortex deformation and breaking in superconductors: a microscopic description

Vortex breaking has traditionally been studied for non-uniform critical current densities, although it may also appear due to non-uniform pinning force distributions. In this article we study the case of a high-pinning/low-pinning/high-pinning layered structure. We have developed an elastic model for describing the deformation of a vortex in these systems in the presence of a uniform transport current density J for any arbitrary orientation of the transport current and the magnetic field. If J is above a certain critical value, Jc , the vortex breaks and a finite effective resistance appears. Our model can be applied to some experimental configurations where vortex breaking naturally exists. This is the case for YBa2Cu3O7−δ (YBCO) low-angle grain boundaries and films on vicinal substrates, where the breaking is experienced by Abrikosov–Josephson vortices (AJV) and Josephson string vortices (SV), respectively. With our model, we have experimentally extracted some intrinsic parameters of the AJV and SV, such as the line tension ϵ l and compared it to existing predictions based on the vortex structure.

Ac susceptibility of a melt-textured YBa2Cu3Ox ring closed by a strong-coupling contact

The complex ac susceptibility χ=χ′−jχ″ of a melt-textured YBa2Cu3Ox ring closed by a strong-coupling contact is measured after zero-field cooling to 77K as a function of the ac field amplitude Hm and frequency f. The resulting χ(Hm,f) is similar to that derived from a power-law relation between the local current density and electric field but with maximum χ″(Hm) increasing steadily with increasing f, which is explained by the creep of Abrikosov-Josephson vortices along the contact driven by the London force of supercurrents and the Lorentz force of the induced normal currents.

Magnetic-field-induced crossover from flux-flow to Josephson-junction behavior in a highly transparent weak link

Magnetic-field-induced Josephson-junction (JJ) behavior in a highly transparent weak link was observed at the 5\ifmmode^\circ\else\textdegree\fi{} tilt low angle grain boundary (LAGB) in a $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ film. The magnetic field dependence of current density-voltage curves showed that Abrikosov Josephson (AJ) vortices exist in the LAGB. Both JJ and flux-flow (FF) behaviors were observed in a single LAGB depending on the temperature and magnetic field. The crossover from FF to JJ arose from the spread of the phase variation along the junction when the AJ vortex cores overlapped at ${B}^{*}={\ensuremath{\phi}}_{0}∕{(4.4l)}^{2}$, where $l$ is the characteristic length of AJ vortex.

Pinning regimes of grain boundary vortices inYBa2Cu3O7−xcoated conductors

A comprehensive analysis of the processes limiting the magnetic and temperature dependences of the critical currents of $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$ coated conductors is presented. We conclude that an unexpected heterogeneous vortex matter associated with two interacting vortex species characterizes the flux dynamics in the $H\text{\ensuremath{-}}T$ space: The Abrikosov-Josephson vortices (AJVs) nucleated on the grain boundaries and the Abrikosov vortices (AVs) localized in the grains. A crossover field separating a plastic motion of strongly interacting AVs- and AJVs from an individual pinning regime of AJVs has been identified.