Vortex Pinning Research Articles

Hydromagnetic waves in a superfluid neutron star with strong vortex pinning

Neutron star cores may be the hosts of a unique mixture of a neutron superfluid and a proton superconductor. Compelling theoretical arguments have been presented over the years that if the proton superconductor is of type II, then the superconductor fluxtubes and superfluid vortices should be strongly coupled and hence the vortices should be pinned to the proton-electron plasma in the core. We explore the effect of this pinning on the hydromagnetic waves in the core, and discuss two astrophysical applications of our results. (i) We show that, even in the case of strong pinning, the core Alfven waves thought to be responsible for the low-frequency magnetar quasi-periodic oscillations (QPO) are not significantly mass loaded by the neutrons. The decoupling of ∼0.95 of the core mass from the Alfven waves is, in fact, required in order to explain the QPO frequencies, for simple magnetic geometries and for magnetic fields not greater than 10 15 G. (ii) We show that in the case of strong vortex pinning, hydromagnetic stresses exert stabilizing influence on the Glaberson instability, which has recently been proposed as a potential source of superfluid turbulence in neutron stars.

Vortex Bose glass in ErBa2Cu3Oy films with size-controlled nanorods

Anomalous changes in irreversibility lines (ILs) and critical current properties depending on growth conditions were demonstrated in ErBa2Cu3Oy films with Ba–(Nb,Er)–O nanorods. Despite the formation of nanorods in the matrix at all growth temperatures (Ts), Bose-glass-like vortex pinning was strengthened with decreasing Ts. In particular, the behavior of ILs can be understood in terms of the morphology of introduced nanorods, and Ts strongly affects the vortex states. Our present study suggests the importance of optimization of growth conditions in the production process of rare-earth-based coated conductors containing nanorods as vortex-pinning centers.

Fishtail effect and the vortex phase diagram of single crystal Ba0.6K0.4Fe2As2

By measuring the magnetization hysteresis loops of superconducting Ba0.6K0.4Fe2As2 single crystals, we obtained the high upper critical field and large current carrying ability, which point to optimistic applications. The fishtail (or second peak) effect is also found in the material, and the position of the vortex pinning force shows a maximum at 1∕3 of the reduced field, being consistent with the picture of vortex pinning by small size normal cores in the sample. Together with the resistive measurements, for the first time the vortex phase diagram is obtained for superconductor Ba0.6K0.4Fe2As2.

Peak Effect in the Critical Current of Type II Superconductors with Strong Magnetic Vortex Pinning

We perform 2D Langevin simulations studying the peak effect (PE) of the critical current taking into account the temperature dependence of the competing forces. We observe and report that the PE results from the competition of vortex-vortex interactions and vortex-pin interactions which have different temperature dependencies. The simulations reveal that the PE can take place only for certain pinning strengths, densities of pinning centers, and driving forces, which is in good agreement with experiments. No apparent vortex order-disorder transition is observed across the PE regime. In addition, the PE is a dynamical phenomenon, and thermal fluctuations can speed up the process for the formation of the PE.

Vortex–nucleus interaction in the inner crust of neutron stars

The structure of a vortex in the inner crust of neutron stars is calculated up to density equal to one fourth of the nuclear saturation density, within the framework of quantum mean field theory, taking into account the interaction with the nuclei composing the Coulomb lattice. Vortices are associated with Coopers pairs formed out of single-particle levels of opposite parity and, due to (quantal-size) shell effects, their formation is hindered within the nuclear volume, by an amount that depends on the Fermi energy and on the effective mass associated with the adopted nuclear two-body interaction. When the vortex axis goes through the center of a nucleus, the typical linear rise of the pairing gap away from the vortex axis is delayed by about 8 fm, as compared to the case of a vortex in uniform matter. Also the velocity field is suppressed in a large region close to the interface between the nucleus and the neutron gas. As a consequence, pinning of a vortex on a nucleus leads to a loss of condensation energy, contrary to the predictions of all previous models. This result strongly influences the density dependence of the pinning energy, relevant in the study of glitches. We find that pinning of vortices on nuclei is favoured at low density.

Evidence for two distinct scales of current flow in polycrystalline Sm and Nd ironoxypnictides

Early studies have found quasi-reversible magnetization curves in polycrystallinebulk rare-earth iron oxypnictides that suggest either widely spread obstacles tointergranular current or very weak vortex pinning. In the present study of polycrystallinesamarium and neodymium iron oxypnictide samples made by high pressure synthesis,the hysteretic magnetization is significantly enhanced. Magneto-optical imagingand study of the field dependence of the remanent magnetization as a functionof particle size both show that global currents over the whole sample do existbut that the intergranular and intragranular current densities have distinctivelydifferent temperature dependences and differ in magnitude by about 1000. Ifthe highest current density loops lie only within grains, their magnitude is∼5 × 106 A cm−2 at 5 K and self-field. Whole sample current densities, though two orders of magnitude lower at1000–10 000 A cm−2, are some two orders of magnitude higher than in random polycrystalline cuprates. Wecannot yet be certain whether this large difference in global and intragrain current densityis intrinsic to the oxypnictides or due to extrinsic barriers to current flow, because thesamples contain a significant second phase, some of which wets the grain boundaries andmay cause a superconducting–normal–superconducting proximity effect in the wholesample critical current.

Analysis of Electromagnetic Phenomena in HTc Superconductors

Unique electromagnetic properties, especially concerning recent observations of the giant remanent magnetic moment in HTc superconductors, require new approach to modeling these phenomena in the oxide ceramics. The present paper is devoted to the analysis of the influence of flux pinning on critical current and flux trapping in HTc superconductors. Modeling of the interaction between nanosized pinning centers and pancake vortices is presented, allowing to determine critical current density, taking into account elasticity forces of the vortices lattice. Ceramic structure of oxide superconductors was considered in trapped flux analysis. The mechanism of trapped flux generation was regarded as strongly related to vortex pinning and critical current phenomena.

Vortex Dynamics in Ferromagnetic/Superconducting Bilayers

The dependence of vortex dynamics on the geometry of magnetic domain pattern is studied in the superconducting/ferromagnetic bilayers, in which niobium is a superconductor, and Co/Pt multilayer with perpendicular magnetic anisotropy serves as a ferrromagnetic layer. Magnetic domain patterns with different density of domains per surface area and different domain size, w, are obtained for Co/Pt with different thickness of Pt. The dense patterns of domains with the size comparable to the magnetic penetration depth (w ≥ λ) produce large vortex pinning and smooth vortex penetration, while less dense patterns with larger domains (w A λ) enhance pinning less effectively and result in flux jumps during flux motion.

Island formation in disordered superconducting thin films at finite magnetic fields

It has been predicted theoretically and observed experimentally that disorder leads to spatial fluctuations in the superconducting (SC) gap. Areas where SC correlations are finite, coined SC islands, were shown experimentally to persist into the insulating side of the superconductor-insulator transition. The existence of such (possibly weakly coupled) SC islands in amorphous thin films of superconducting material accounts for numerous experimental findings related to superconductor-insulator transition and nonmonotonic magnetoresistance behavior in the insulating region. In this work, a detailed analysis pertaining to the occurrence of SC islands in disordered two-dimensional superconductors is presented. Using a locally self-consistent numerical solution of the Bogoliubov--de Gennes equations, the formation of SC islands is demonstrated, and their evolution with an applied perpendicular magnetic field is studied in some detail, along with the disorder-induced vortex pinning. While mean-field theory cannot, in principle, explore phase correlations between different islands, it is demonstrated that, by inspecting the effect of a parallel magnetic field, one can show that the islands are indeed uncorrelated SC domains. Experimental predictions based on this analysis are presented.

Inhomogeneous superconducting state in the overdoped regime of La 2−xSr xCuO 4: Comparison with the superconducting state of NbSe 2

We have measured the temperature dependence of the magnetic susceptibility, χ vs. T, and the magnetization curve, M vs. H, for NbSe 2 single crystals, in order to compare the superconducting (SC) state in the overdoped regime of La 2− x Sr x CuO 4 (LSCO) with the SC state of the layered conventional superconductor NbSe 2. While a plateau in χ vs. T in a moderate magnetic field and a so-called second peak in M vs. H, which is due to the marked enhancement of vortex pinning, have been observed in the overdoped regime of LSCO, these behaviors have not been observed in NbSe 2. The present results indicate that the anomalously marked enhancement of vortex pinning is a characteristic feature in the overdoped LSCO where a microscopic phase separation into SC and normal-state regions takes place.

Current-driven dynamics in Josephson junction networks with an asymmetric potential

Current-driven dynamics of Josephson junction networks (JJNs) is studied using numerical simulations. We consider a JJN with an asymmetric and periodic potential of vortices, which is realized by saw-tooth modulation of junction critical currents. When external ac currents are applied to the JJN in a magnetic field, there appears a ratchet effect, and then directed motion of vortices is induced in certain system parameter regimes. A ratchet behavior is observed even for JJNs with weak structural disorder. We clarify the vortex pinning and dynamics in the JJN as a ratchet system.

Guiding superconducting vortices with magnetic domain walls

We demonstrate a unique prospect for inducing anisotropic vortex pinning and manipulating the directional motion of vortices by using the stripe domain patterns of a uniaxial magnetic film in the superconducting/ferromagnetic hybrid. Our observations can be described by a model, which considers interactions between magnetic charges of vortices and surface magnetic charges of domains resulting in the enhanced pinning of vortices on domain walls.

Current progress in ternary LREBa 2Cu 3O y materials and their application

The J c and H irr values at 77 K of ternary light-rare-earth compounds, LREBa 2Cu 3O y “LRE-123”, are usually high enough to serve in various applications. Several sources of vortex pinning can be in these composites tailored to fit the needs of the particular application. The list comprises LRE/Ba solid solution, oxygen vacancies, large particles of secondary phases, twin planes, nanoscale lamellas, etc. By means of the latter defects one can achieve a very high irreversibility field. Refinement of secondary phase particles and the optimal choice of their amount enhance the electromagnetic performance in a broad temperature range, up vicinity of T c, allowing levitation at liquid oxygen, 90.2 K. An optimum content of MoO 3 doubles the self-field super-current at 77 K, H||c-axis. Altogether, the pinning tailoring in ternary LRE-123 materials provides a flexible and reliable way to fit the electromagnetic performance with the needs of sophisticated high-temperature and high-magnetic-field applications.

Pinning performance of (Nd,Eu,Gd)-123 superconductors: Comparison of melt-textured pellet and single crystal

We studied pinning homogeneity and the content fluctuation of light rare earth ions in a (Nd 0.33Eu 0.38Gd 0.28)Ba 2Cu 3O y melt-textured (MT) pellet 16 mm in diameter, doped by 0.035 mol% ZnO. The pinning performance was compared to that of the recently grown (Nd 0.33Eu 0.2Gd 0.47)Ba 2Cu 3O y single crystal. While magnetic properties of most test samples cut from the MT pellet were surprisingly uniform, two samples from the pellet center behaved significantly different. This anomaly was found to correlate with the significant departure of the Eu:Nd ratio from the average. Nearly all MT samples showed a high-field shoulder on J c( B) curve that was attributed to the pinning effect of a nanoscopic correlated lamellar substructure filling channels between regular twin boundaries. As a result, the irreversibility field at 77 K reached in all 20 measured MT samples 9 T. In the single crystal no high-field J c( B) shoulder was observed, and the irreversibility field at 77 K was about 6 T. The strip-like structure observed on the crystal surface was evidently of another origin than the lamellar substructure in the MT samples and did not significantly contribute to vortex pinning. The secondary peak in J c( B) of the single crystal was quite strong, showing a well set point-like disorder. All magnetic characteristics of the single crystal were strongly affected by twin-plane channeling effect.

Transport and magnetic critical current in superconductingMgB2 wires

Direct comparison of the magnetic and transport critical current density(Jc) for the same piecesof copper-sheathed MgB2 wires shows a large discrepancy in magnitude and field dependence of the two. The value of magneticJc can differ from thevalue of transport Jc by a factor of 10 or more. This discrepancy does not occur merely becauseof the difference in the voltage at which the magnetic and transportJc are measured, but mainly because of the specific microstructure ofMgB2. Such microstructure results in superconducting screening on atleast two different length-scales, despite the absence of weak links inMgB2, leading toerroneous magnetic Jc if a simple critical state model is applied to such a system. Consequently, the magneticJc cannot be used for analysis of physical processes where the accurate field dependence ofJc needs to be known, such as vortex pinning. The magneticJc can still be used for qualitative comparison ofMgB2 samples if they are all of the same size and have the samemicrostructure, or if their size is large enough so that the magneticJc does not depend on the sample size.

Vortex pinning in superconductors laterally modulated by nanoscale self-assembled arrays

Being the exponent of the so-called “bottom-up” approach, self-assembled structures are now-a-days attracting a lot of attention in the fields of science and technology. In this work, we show that nanoscale self-assembled arrays used as templates can provide periodic modulation in superconducting thin films by studying their vortex pinning properties. In this work advantage was made of the fact that self-organized assemblies of identical units such as colloidal crystals and anodic aluminum oxide provide extended periodic topographic surfaces. By directly growing Nb on top of these self-assembled arrays, the templating effect was exploited in order to achieve triangular and honeycomb arrays of pinning centers in thin superconducting films. We show experimentally that periodic matching is achieved in both systems at magnetic fields, well above those present in lithographically prepared pinning arrays (up to 1T!). Furthermore, we demonstrate in the case of anodic aluminum oxide that the presence of porous antidots in Nb not only provides strongly increased critical currents but also conserves matching at temperatures well below the critical temperature. The studies conducted on these systems indicate that the method of template growth might be considered as a viable alternative for the incorporation of periodic pinning arrays in superconducting applications of today and the future.

Change of strength of vortex pinning in YBCO due to BaZrO3 inclusions

We probe the short-range pinning properties with the application of microwave currents at very high driving frequencies (47.7GHz) on YBa2Cu3O7−δ films with and without sub-micrometer BaZrO3 inclusions. We explore the temperature and field ranges 60 K <T<Tc and 0<μ0H<0.8T, with the field applied along the c-axis. The magnetic field induces a much smaller increase of the microwave resistivity, Δρ1(H)+iΔρ2(H), in YBa2Cu3O7−δ/BaZrO3 with respect to pure YBa2Cu3O7−δ. Δρ1(H) is slightly superlinear in pure YBa2Cu3O7−δ (suggesting a possible contribution of thermal activation), but linear or sublinear in YBa2Cu3O7−δ/BaZrO3 (suggesting a possible suppression of thermal activation as a consequence of BaZrO3 inclusions). These features persist up to close to Tc. We discuss our data in terms of the ratio r=Δρ2(H)/Δρ1(H) in the framework of the models for the microwave surface impedance in the mixed state. Large r are found in YBa2Cu3O7−δ/BaZrO3, with little field dependence. By contrast, smaller values and stronger field dependences are found in pure YBa2Cu3O7−δ. We discuss the different field dependence of the pinning constant.

Doping in MgB 2 superconductors using a high-pressure furnace

Since their discovery, there have been many attempts at modifying MgB 2 superconductors by doping them to improve their pinning properties. Although doping was successful, the superconducting transition temperature decreased. Conversely, Ti doping at Mg sites did not work. Here, we tried to make (Mg,Ti)B 2 using a high-pressure furnace. The transport and low-field magnetization measurements showed that Ti could be doped in a uniform manner using the high-pressure synthesis technique. The superconducting transition temperature of the doped material was lower. Magnetic hysteresis curves showed that vortex pinning decreased markedly, even for Ti doping levels as low as 10%.

Vortex pinning in chemical solution nanostructured YBCO films

A methodology of general validity to study vortex pinning inYBa2Cu3O7−δ (YBCO) is described. It is based on measuring the angular-dependent critical currentdensity in a large range of temperatures and magnetic fields and it permits one to identify,separate and quantify the three basic vortex pinning contributions associatedto anisotropic–strong, isotropic–strong and isotropic–weak pinning centers. Thecorresponding vortex pinning phase diagrams for these three contributions arebuilt up. This methodology is applied to the recently discovered solution-derivedYBCO–BaZrO3 nanocomposites which display a maximum vortex pinning force of78 GN m−3 at 65 K, being five times larger than that of NbTi at 4.2 K. These nanocomposites show aneffective anisotropy tending to unity and the best superconducting properties observed sofar in YBCO. We present the richness and complexity of their microstructure and thecorrelation with the vortex pinning properties. The application of the vortex pinningmethodology and comparison with a standard solution-derived YBCO film enable us toidentify the isotropic–strong defect contribution being at the origin of their uniqueproperties.

Epitaxial thick film high-Tc SQUIDs

Low-noise operation of superconducting quantum interference devices (SQUIDs) in magnetic fields requires high critical current and strong pinning of vortices in the superconducting electrodes and in the flux transformer. Crack-free epitaxial high-Tc dc-SQUID structures with a total thickness ?5 μm and a surface roughness determined by 30 nm high growth spirals were prepared with YBa2Cu3O7-x (YBCO) films on MgO substrates buffered by a SrTiO3/BaZrO3-bilayer. HRTEM demonstrated a high quality epitaxial growth of the films. The YBCO films and SQUID structures deposited on the buffered MgO substrates had a superconducting transition temperature Tc exceeding 91 K and critical current densities Jc > 3 MA/cm2 at 77 K up to a thickness ~5 μm. The application of thicker superconducting and insulator films helped us to increase the critical current and dynamic range of the multilayer high-Tc flux transformer and improve the insulation between the superconducting layers. An optimization of SQUID inductance allowed us to fabricate 8 mm SQUID magnetometers with SQUID voltage swings of ~60 μV and a field resolution of ~30 fT/√Hz at 77 K.