Density Of Columnar Defects Research Articles

Monte Carlo simulation of the crossover from Bose glass to Bragg glass phase in layered BSCCO with columnar defects

Monte Carlo simulations of layered BSCCO samples are used to investigate the behavior of vortex matter at low fields, particularly in connection with the possible occurrence of a Bragg glass (BrG) phase at low density of columnar defects, a phenomenon characterized by the prevalence of short-range over long-range order. In this dislocation-free topological phase the translational order correlation function displays a power law decay. For magnetic induction B = 0.1 kG the analysis of the data for the first Bragg peak of the planar structure factor, the hexatic order parameter, and the Delaunay triangulation shows that, as the density of columnar defects is lowered, a crossover (or transition) from Bose glass to BrG phase takes place in this highly anisotropic high-T superconductor. Most importantly, an analysis of the low-temperature 3D vortex–vortex correlation function in terms of the structure factor, calculated via a saddle point approach and the use of the numerical data as input, provides clear-cut evidence of the power law decay of the divergent Bragg peaks in the BrG phase, a fundamental feature that was inequivocally verified only in isotropic compounds.

Upper critical field of isoelectron substituted SrFe2(As1−xPx)2

The upper critical field ${H}_{c2}$ of optimally doped, iron-based superconductor SrFe${}_{2}$(As${}_{1\ensuremath{-}x}$P${}_{x}$)${}_{2}$ ($x$ $=$ 0.35, ${T}_{c}$ $=$ 25 K) was measured as a function of temperature down to 1.6 K for two principal directions of magnetic field $H\ensuremath{\parallel}c$ and $H\ensuremath{\parallel}a$. Measurements were performed in pulsed magnetic fields up to 65 T using a tunnel-diode resonator technique on as-grown and heavy-ion-irradiated single crystals, with columnar defect density corresponding to a matching field ${B}_{\ensuremath{\phi}}$ $=$ 25 T. The ${H}_{c2,c}(T)$ is close to $T$ linear, while clear saturation is observed for ${H}_{c2,a}(T)$, leading to a strongly temperature-dependent anisotropy parameter $\ensuremath{\gamma}$. The linear shape of ${H}_{c2,c}(T)$ is very similar to that observed in nodal KFe${}_{2}$As${}_{2}$, but very different from full-gap LiFeAs. Irradiation does not introduce any additional features on the ${H}_{c2}(T)$ line corresponding to the matching field. Instead, it suppresses uniformly both ${T}_{c}$ and ${H}_{c2}$, keeping their ratio constant.

Multiple Changes of Order of the Vortex Melting Transition inBi2Sr2CaCu2O8with Dilute Columnar Defects

A low concentration of columnar defects is reported to transform a first-order vortex lattice melting line in Bi2Sr2CaCu2O8 crystals into alternating segments of first- and second-order transitions separated by two critical points. As the density of columnar defects is increased, the critical points shift apart and the range of the intermediate second-order transition expands. The measurement of equilibrium magnetization and the mapping of the melting line down to 27 K was made possible by employment of the shaking technique.

Numerical study of quantum vortex phase diagram in two-dimensional superconductors

Vortex phase diagrams of weakly disordered two-dimensional (2D) superconductors under a magnetic field perpendicular to the plane are numerically studied based on a recent development on the corresponding issue in three-dimensional (3D) systems with a low density of columnar defects at high temperatures. By examining the field dependences of magnetic quantities and the glass correlation, two consecutive first-order transitions (FOTs) are found to occur in weakly disordered cases even at low but finite temperatures, and the lower FOT is identified with a remnant of the melting transition of Bragg-Bose glass at zero temperature. The resulting phase diagram is discussed in relation to a FOT-like behavior found in superconducting thin films.

Flux-pinning characteristics as a function of density of columnar defects comprised of self-assembled nanodots and nanorods in epitaxial YBa 2Cu 3O 7− δ films for coated conductor applications

Pure YBa 2Cu 3O 7− δ (YBCO) and YBCO films with extended columnar defects comprised of self-assembled columns of nanodots and nanorods of BaZrO 3 (BZO), CaZrO 3 (CZO) and 8 mol.% Y 2O 3 stabilized ZrO 2 (YSZ) were prepared by pulsed laser deposition on RABiTS templates of configuration CeO 2/YSZ/Y 2O 3/Ni–W. Effects on increasing the size and density of columnar defects by varying the total volume fraction BZO, CZO or YSZ incorporations within the YBCO film are reported. BZO volume fraction was varied between 0% and 8%, CZO between 0% and 2% and YSZ 0–2%. The critical transition temperature ( T c) was found to linearly decrease with increase in vol.% of BZO, CZO and YSZ, with the steepest decline for CZO additions. Nevertheless, YBCO films with these columnar defects exhibit significantly reduced J c dependence on applied field. At 77 K, best properties are obtained for 1–2 vol.% BZO incorporation in the form of columns of self-assembled nanodots and nanorods in fields of 0.1–1.5 T.

Effect of inclined magnetic field on pinning parameters in YBaCuO thin films with columnar defects

The effect of inclined magnetic field on pinning parameters in YBCO thin films with columnar defects was investigated. The columnar defects were produced by the heavy-ion irradiation with 200 MeV Xe ions parallel to c-axis. The magnetic field was applied inclined to c-axis with angle θ B which was 0°, 10°, and 20°. One of the pinning parameters z, which is also called the dynamic critical exponent, extremely increased near the certain field related to the density of columnar defects in the case of θ B = 0°, that is, a peak in the magnetic field dependence of z appeared near B/ B ϕ = 1/3, while such peak was not observed before irradiation. On the other hand, the peak did not appear and the values of z decreased with the tilting angle θ B . According to the percolation model, the appearance of the peak of z near B/ B ϕ = 1/3 for θ B = 0° originates from the matching effect. When the magnetic field is inclined away from columnar defects, the vortices are not pinned along those defects so that the matching effect is not achieved and the columnar defects take the various values of pinning force to flux lines.

Effect of controlled densities of columnar defects on pinning parameters in YBa2Cu3O7−δ thin films

We have studied the effect of the densities of columnar defects on pinning parameters in YBa2Cu3O7−δ thin films prepared by the PLD method with the transport measurements. In order to produce the defects artificially in a controlled manner, the heavy-ions were irradiated onto the films. The critical current density Jc increases with the doses of the irradiation due to the introduction of the strong pinning sites into samples. The dynamic critical exponent z, which is also one of the pinning parameters in the percolation model, increases with the densities of the irradiation defects. It should be noted that the values of z abruptly increase near a third part of the matching magnetic field, which is independent on the fluence of the irradiation. These results could be described by the percolation model as a variation of pinning strength distribution affected from the induced defects.

Effects of Non-Uniform Columnar Defect Distribution on the Microwave Properties of Y-Ba-Cu-O Coplanar Resonators

This paper reports on nonuniform heavy ion irradiations of YBCO coplanar resonators aimed at controlling vortex-induced microwave dissipation and nonlinearity. The adopted moving-target irradiation technique allows producing tailored profiles of columnar defect density to get an optimal compromise between defect-induced enhancement of vortex pinning and addition of dissipative scattering. The same samples were characterized before and after irradiation in order to evaluate the surface impedance, by means of data analysis suitable to account also for the substrate properties. In the case of the modulated dose profile, matching the fundamental-mode field distribution along the resonator, the onset of nonlinearity is shifted toward higher circulating power, without increasing dissipation. Finally, the possibility to locally modify by irradiation the properties of the material to obtain the confinement of external perturbations, while maintaining the current continuity throughout the whole device, has been preliminarily tested.

Non-uniform columnar defect implantation in YBCO coplanar resonators for the control ofvortex-induced microwave dissipation and nonlinearity

This paper reports on 4.2 GeV Au ion irradiation of YBCO coplanar resonators aimed atcontrolling vortex-induced microwave dissipation and nonlinearity by means of asuitable distribution of columnar defects. Two procedures were used: (i) uniformirradiation at a given dose and (ii) non-uniform irradiation, obtained by means of amoving-target technique, producing a tailored profile of columnar defect density. The lastconfiguration is needed in order to attain an optimal compromise between defect-inducedenhancement of vortex pinning and increase of dissipative scattering. The resonatorswere characterized before and after irradiation in order to evaluate penetrationdepth and surface impedance, by means of a data analysis also suitable to accountfor the substrate properties. It turns out that in the case of uniform irradiationthe induced damage, attributed to the addition of dissipative regions, alwaysprevails over the improvement due to vortex pinning. In contrast, in the case of thetailored dose profile, significant damage is absent, while the onset of non-linearity isshifted towards higher circulating power. The need to control vortex-inducedeffects is confirmed by magneto-optical analysis that puts in evidence that vortexpenetration occurs even at low values of transverse dc magnetic field and temperature.

Vortex nanoliquid in high-temperature superconductors.

Using a differential magneto-optical technique to visualize the flow of transport currents, we reveal a new delocalization line within the reversible vortex liquid region in the presence of a low density of columnar defects. This line separates a homogeneous vortex liquid, in which all the vortices are delocalized, from a heterogeneous "nanoliquid" phase, in which interconnected nanodroplets of vortex liquid are caged in the pores of a solid skeleton formed by vortices pinned on columnar defects. The nanoliquid phase displays high correlation along the columnar defects but no transverse critical current.

Dose effect of heavy-ion irradiation on the critical exponents in YBa 2Cu 3O 7− x thin films

We investigated the influence of densities of columnar defects on the critical exponents estimated from the critical scaling analysis of current–voltage characteristics in YBa 2C 3O 7− x thin films. To avoid the difference of background pinning between samples, the densities of columnar defects were controlled in one sample successively. The dynamic critical exponent z increased with increasing the densities of columnar defects from B φ=0.5–1.0 T. It was also found that the installed defects led to the appearance of peak in the magnetic field dependence of z, while such peak did not appear before irradiation. These results are explained by the percolation model considering the statistical distribution of the pinning strength rather than the vortex glass–liquid transition model. The various changes of z due to the heavy-ion irradiation are related to the change of the pinning strength distribution due to the introduction of columnar defects, which act as the strong pinning centers.

Porous vortex matter

Structure, dynamics, and thermodynamic properties of vortex matter in the presence of a low density of columnar defects (CDs) were studied in BSCCO crystals. Magnetic decorations show that when vortices outnumber CDs a heterogeneous vortex matter is formed consisting of two populations of vortices: vortices residing on CDs form a matrix of pinned vortices, whereas the interstitial vortices form ordered crystallites within the `pores' of the matrix. Differential magneto-optical studies reveal that at elevated fields this porous phase melts in two stages, a first-order melting of the crystallites at a temperature considerably higher than the pristine melting, and a continuous melting of the matrix at still higher temperature. At low fields the two transitions occur simultaneously, giving rise to a sharp kink in the observed melting line.

Vortex solid-liquid transition inBi2Sr2CaCu2O8+δwith a high density of strong pins

The introduction of a large density of columnar defects in %underdoped Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ crystals does not, at sufficiently low vortex densities, increase the irreversibility line beyond the first order transition (FOT) field of pristine crystals. At such low fields, the flux line wandering length $r_{w}$ behaves as in pristine %Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ crystals. Next, vortex positional correlations along the $c$--axis in the vortex Bose glass at fields above the FOT are smaller than in the low--field vortex solid. Third, the Bose-glass-to-vortex liquid transition is signaled by a rapid decrease in c-axis phase correlations. These observations are understood in terms of the ``discrete superconductor'' model.

The pinning property of Bi-2212 single crystals with columnar defects

It is qualitatively understood that the condensation energy density in oxide superconductors,which is one of the essential parameters for determining their pinning strength,becomes large with increasing dimensionality of the superconductor. However, thecondensation energy density has not yet been evaluated quantitatively. Its value canbe estimated from the elementary pinning force of a known defect. Columnardefects created by heavy ion irradiation are candidates for being such defects. Thatis, the size and number density of columnar defects can be given. In addition,it is known that two-dimensional vortices like those in Bi-2212 are forced intothree-dimensional states by these defects in a magnetic field parallel to the defects. Thus,the condensation energy density can be estimated from the pinning propertyof the columnar defects even for two-dimensional superconductors. A similaranalysis was performed also for three-dimensional Y-123. A discussion is given ofthe relationship between the condensation energy density and the anisotropyparameter estimated from measurements of anisotropic resistivity and peak field.

Melting of "porous" vortex matter.

Bitter decoration and magneto-optical studies reveal that in heavy-ion irradiated superconductors, a "porous" vortex matter is formed when vortices outnumber columnar defects. In this state ordered vortex crystallites are embedded in the "pores" of a rigid matrix of vortices pinned on columnar defects. The crystallites melt through a first-order transition while the matrix remains solid. The melting temperature increases with density of columnar defects and eventually turns into a continuous transition. At high temperatures a sharp kink in the melting line is found, signaling an abrupt change from crystallite melting to melting of the rigid matrix.

Critical current density of YBa 2Cu 3O y containing inclined columnar defects

Effects of columnar defects on the critical current density have been investigated by the in situ measurement of current–voltage characteristics for a c-axis oriented YBa 2Cu 3O y (YBCO) thin film after the irradiation with 200 MeV Au ions from the direction θ irr=45° off the c-axis. The dependences of the critical current density, J c, on the magnetic field, the magnetic field angle, and the density of columnar defects are discussed. With increasing the density of the columnar defects, B φ , the contribution of the columnar defects to the pinning force per unit volume, ΔF p, increases monotonically. The peak field for ΔF p as a function of the magnetic field seems to be almost independent on B φ .

Mixed-state hall effect in Hg- and Tl-based superconducting thin films containing columnar defects

We have investigated the mixed-state Hall resistivity ρ xy and longitudinal resistivity ρ xx in HgBa 2CaCu 2O 6, HgBa 2Ca 2Cu 3O 8, and Tl 2Ba 2CaCu 2O 8 thin films for various parameters, such as densities of columnar defects, number of CuO 2 planes, and anisotropy ratios, as a function of the magnetic field up to 18 T. The scaling exponent β in ρ xy = Aρ xx β is 1.9 ± 0.1 in the clean limit (CL) at high fields and low T whereas β is 1 ± 0.1 in the moderately clean limit (MCL) at low fields and high T. We also find the triple sign reversal in the HgBa 2CaCu 2O 6 films containing high-density columnar defects. These results can be interpreted within the context of a recent theory based on the midgap states in the vortex core.

Transition from elastic to plastic vortex dynamics in YBa 2Cu 3O 7−δ single crystals: the effect of columnar defects

We have performed partial magnetization loops in pure YBa 2Cu 3O 7−δ single crystals with point and columnar defects. These results have demonstrated that the critical current density in detwinned samples depends strongly on the magnetic history of the superconductor. As shown recently in Ref. [1], this effect is due to metastable topological disorder present in the vortex lattice and is a precursor to the transition from a dislocation-free Bragg glass to a disordered vortex phase. However, even low densities of columnar defects have been found to suppress this transition and eliminate the memory effects.

Surface nanostructuring and damage morphologies along 2 GeV gold-ion-implanted tracks

Abstract Good-quality melt-textured YBa2Cu3O7-x materials with Y2BaCuO5 non-superconducting inclusions were irradiated with 2GeV gold ions in order to produce a surface layer with columnar defects. The finaL goal is to gain a basie understanding of the role of the surface columnar defects in determining those magnetic properties of the materials that are related to the vortex confinement. The particie energy and the sample thickness were chosen in such a way that the percentage of irradiated volume is about 10%. In the paper the intrinsic structure and the damage morphology were analysed by means of transmission electron microscopy studies in both cross-section and plan-view geometry. As a result, the columnar defect density, size and shape were obtained. The irradiation produces parallel heavily damaged tracks whose diameter is between 3 and 6nm. The overlapping of some columnar defects was highlighted and had to be taken into account to understand the superconducting properties of irradiated samples. Th...

Surface nanostructuring and damage morphologies along 2GeV gold-ion-implanted tracks

Abstract Good-quality melt-textured YBa2Cu3O7-x materials with Y2BaCuO5 non-superconducting inclusions were irradiated with 2GeV gold ions in order to produce a surface layer with columnar defects. The finaL goal is to gain a basie understanding of the role of the surface columnar defects in determining those magnetic properties of the materials that are related to the vortex confinement. The particie energy and the sample thickness were chosen in such a way that the percentage of irradiated volume is about 10%. In the paper the intrinsic structure and the damage morphology were analysed by means of transmission electron microscopy studies in both cross-section and plan-view geometry. As a result, the columnar defect density, size and shape were obtained. The irradiation produces parallel heavily damaged tracks whose diameter is between 3 and 6nm. The overlapping of some columnar defects was highlighted and had to be taken into account to understand the superconducting properties of irradiated samples. The defects affect an implanted layer of about 50 um. The measured thickness coincides, within the experimental errors, with the thickness calculated by means of the Monte Carlo TRIM simulations.