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Cluster formation is a focus of interdisciplinary research in both chemistry and physics. Here we discuss the exotic example of this phenomenon in the vortex matter of a thin superconductor. In superconducting films, the clustering takes place because of particular properties of the vortex interactions in the crossover or intertype regime between superconductivity types I and II. These interactions are controlled by the two parameters that are responsible for the crossover, Ginzburg-Landau parameter κ, which specifies the superconducting material of the film, and film thickness d, which controls effects due to stray magnetic fields outside the sample. We demonstrate that their competition gives rise to a complex spatial dependence of the interaction potential between vortices, favoring the formation of chainlike vortex clusters.

This paper describes the Gdx−2BaxCu2x−2Oδ (x = 6, 7) cuprate system synthesis using the conventional solid-state reaction method. Rietveld analysis of X-ray patterns evidences that samples crystallize in Pmm2 (#25) space group at room temperature with lattice parameter a = 3.8807 A, b = 3.8735 A, c = 38.7094 A for Gd4Ba6Cu10Oδ and a = 3.8680 A, b = 3.8763 A, c = 46.5065 for Gd5Ba7Cu12Oδ. Resistivity measurements evidence zero-resistance state, which is a typical behavior of superconducting materials at bulk critical temperature of 15.5 K for Gd4Ba6Cu10Oδ and 22.1 K for Gd5Ba7Cu12Oδ.

Una capa superconductora epitaxial de Y Ba2Cu3 7 es el resultado de un óptimo proceso de crecimiento epitaxial de los granos superconductores. Además, la epitaxia del sustrato tiene una directa repercusión en la microestructura y las propiedades físicas de la capa. Por medio de técnicas de caracterización convencionales, tales como: microscopía electrónica de barrido, y difracción de rayos X, permiten realizar una rápida valoración de la calidad de la epitaxia por medio de la cuantificación de la textura de la capa. Presentamos los resultados de la cuantificación de la textura en capas superconductoras de Y Ba2Cu3O7 .

In this work, we revisit the problem of superconductivity under the influence of boundary effects. By solving the Bogoliubov-de Gennes (BdG) equations for the tight-binding model, we demonstrate that the critical temperature of the nucleation of superconductivity near a sample surface can be considerably enhanced as compared to its bulk value. To bring to light this effect, we investigate different methods to solve numerically the BdG equations, including the continuous and Anderson approximations, and perform the calculations for a wide range of the system parameters. We obtain that all the self-consistent BdG eigenstates are delocalized and occupy the entire volume of the sample. Our results reveal that the enhancement of the surface critical temperature originates from the quantum interference of different BdG states contributing to the order parameter. We also find that the surface enhancement is the largest when the conduction band is symmetric with respect to the Fermi level, particularly, the half filling is an important proviso for the pronounced surface effect on the critical temperature. The approximate continuous model as well as the Anderson approximation do not capture the main feature of the surface effect. In addition, our study of this effect versus surface roughness reveals its fragile character.

The structural and magnetic properties of the Al-doped LaCrO 3 have been investigated. The samples were produced by combustion method using urea as fuel. X-ray diffraction measurement shows that all samples are formed in a single phase. It is observed that increasing the Al content induces a structural phase transition from orthorhombic to a rhombohedral structure. The magnetic measurements indicate that all samples have an antiferromagnetic order with a strong decrease in the magnetic ordering temperature with increasing Al content. The consistent variation of the magnetic moment confirms that the Al 3+ ion replaced the Cr 3+ ion on the B-site of the perovskite.

We have studied the high-temperature magnetic properties of a highly oriented FeSe0.5Te0.5 superconducting sample produced by self-flux method. The sample was characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), electrical resistivity, and magnetic measurements. The XRD pattern revealed the formation of the desired crystalline phase. In addition, two minority impurity phases, a FeSe orthorhombic phase and another Fe3O4 magnetite phase, were observed. SEM micrographs showed that the sample was formed in stacked lamellas. Resistivity measurement showed that the onset superconducting temperature occurs at $T_{\mathrm {c}}^{\text {onset} } \sim $ 14 K with a transition width ΔT = 5.5 K. Magnetic measurements at high temperature showed that the FeSe0.5Te0.5 compound presents a ferromagnetic-paramagnetic (FM-PM) phase transition at ∼ 843 K. An anomaly was also observed at ∼ 122 K, which may be associated with the Verwey transition due to the presence of the magnetite phase in the sample.

There is a tacit assumption that multiband superconductors are essentially the same as multigap superconductors. More precisely, it is usually assumed that the number of excitation gaps in the single-particle energy spectrum of a uniform superconductor determines the number of contributing bands in the corresponding superconducting model. Here we demonstrate that contrary to this widely accepted viewpoint, the superconducting magnetic properties are sensitive to the number of contributing bands even when the corresponding excitation gaps are degenerate and cannot be distinguished. In particular, we find that the crossover between superconductivity types I and II - the intertype regime - is strongly affected by difference between characteristic lengths of multiple contributing condensates. The reason for this is that condensates with diverse characteristic lengths coexisting in one system interfere constructively or destructively, which results in multi-condensate magnetic phenomena regardless of the presence/absence of the multigap structure in the single-particle excitation spectrum., Comment: 13 pages and 2 figures

A combination of strong Cooper pairing and weak superconducting fluctuations is crucial to achieve and stabilize high-Tc superconductivity. We demonstrate that a coexistence of a shallow carrier band with strong pairing and a deep band with weak pairing, together with the Josephson-like pair transfer between the bands to couple the two condensates, realizes an optimal multicomponent superconductivity regime: it preserves strong pairing to generate large gaps and a very high critical temperature but screens the detrimental superconducting fluctuations, thereby suppressing the pseudogap state. Surprisingly, we find that the screening is very efficient even when the inter-band coupling is very small. Thus, a multi-band superconductor with a coherent mixture of condensates in the BCS regime (deep band) and in the BCS-BEC crossover regime (shallow band) offers a promising route to higher critical temperatures., Comment: 8 pages, 1 figure, including supplemental materials

Structural and magnetic properties of LaCrO 3 half-doped with Al are reported in this work. Pure and half-doped samples were prepared by combustion synthesis using urea as fuel. The crystal structure was investigated by X-ray diffraction and Rietveld analysis. A structural phase transition caused by a decrease of the chemical pressure was observed. The scanning electron microscopy (SEM) images show the formation of porous samples with particles of irregular morphologies. A quantitative Energy-dispersive X-ray spectroscopy (EDS) analysis on the surface indicates the inclusion of the Al on the structure. However, a small deficiency of La and Al was observed. Magnetization measurements as a function of temperature reveal an antiferromagnetic order in both samples. A large decrease of T N (Neel temperature) and a reduction of the frustration factor is observed in the sample doped with Al. The magnetic isothermal at 5 K shows a typical antiferromagnetic behavior with a slightly spin canting for the doped sample.

In this work we report the formation of hollow α-Fe2O3 (hematite) microspheres by the gas-bubble template method. This technique is simple and it does not require hard templates, surfactants, special conditions of atmosphere or complex steps. After reacting Fe(NO3)3.9H2O and citric acid in water by sol–gel, the precursor was annealed in air at different temperatures between 180 and 600 °C. Annealing at 550 and 600 °C generates bubbles on the melt which crystallize and oxidizes to form hematite hollow spheres after quenching. The morphology and crystal evolution are studied by means of X-ray diffraction and scanning electron microscopy. We found that after annealing at 250–400 °C, the sample consist of a mixture of magnetite, maghemite and hematite. Single hematite phase in the form of hollow microspheres is obtained after annealing at 550 and 600 °C. The crystallization and crystal size of the hematite shells increase with annealing temperature. A possible mechanism for hollow sphere formation is presented.

Self-duality or matching between the magnetic and condensate characteristic lengths is a fundamental property of isotropic superconductors at the critical Bogomolnyi point (B-point). The self-dual state of the condensate is infinitely degenerate, which is the core reason for the sharp transition between the superconductivity types in the nearest vicinity of the critical temperature T_c. Below T_c non-local interactions in the condensate remove the degeneracy, which leads to the appearance of a finite intertype (IT) domain between types I and II. This domain exhibits the mixed state with exotic field-condensate configurations and non-standard magnetic response, which cannot be understood within the dichotomy of the conventional superconductivity types. At a first glance, this picture does not apply to an anisotropic system because no spatial matching between the condensate and magnetic field can be generally expected for direction-dependent characteristic lengths. However, contrary to these expectations, here we demonstrate that anisotropic superconductors follow the same scenario of the interchange between types I and II. In anisotropic materials the IT domain is governed by the B-point of the effective isotropic model obtained by the appropriate scaling transformation of the initial anisotropic formalism. This transformation depends on the direction of the applied magnetic field, and thus the superconductivity type of strongly anisotropic materials can be dependent on this direction., Comment: 6 pages, 2 figures

We demonstrate that the presence of edges in a superconducting film made of a type-I/type-II bilayer stabilizes type-II/type-I hybrid (inter-type) flux patterns, as vortex clusters, chains, and gel phase. These patterns are very sensitive to primary parameters such as applied magnetic field, layer coupling, and temperature. Thus, the magnetization versus temperature curves, M(T), for many values of coupling were used to estimate the strength of the layer couplings, and also as a guide for obtaining sequentially the flux patterns. We also show that the effect of the borders on the unrestricted states is to shift them to states of higher density, since they introduce extra compression on the vortex matter. For a low layer coupling regime, we observe an unusual magnetic response where few partial vortices (partial in a sense they miss the contribution of the type I part), repelling each other and bounded to the surfaces, populate one layer leaving the other empty. We expect that the predicted flux configurations can stimulate experimentalists in trying to observe them by direct imaging techniques.

It is well known that superconducting films made of a type-I material can demonstrate a type-II magnetic response, developing stable vortex configurations in a perpendicular magnetic field. Here we show that the superconducting state of a type-I nanowire undergoes more complex transformations, depending on the nanowire thickness. Sufficiently thin nanowires deviate from type I and develop multiquantum vortices and vortex clusters similar to intertype (IT) vortex states in bulk superconductors between conventional superconductivity types I and II. When the nanowire thickness decreases further, the quasi-one-dimensional vortex matter evolves towards type II so that the IT vortex configurations gradually disappear in favor of the standard Abrikosov lattice (chain) of single-quantum vortices. However, type II is not reached. Instead, an ultrathin nanowire re-enters abruptly the type-I regime while vortices tend to be suppressed by the boundaries, eventually becoming one-dimensional phase-slip centers. Our results demonstrate that arrays of nanowires can be used to construct composite superconducting materials with a widely tunable magnetic response.

In this work, we report superconductivity at 5.5 K in the new pseudo-ternary Th0.93Zr0.07B12 compound. We show clearly evidence that appropriate amounts of Zr substitution at the Th site induce the stabilization of the UB12 prototype structure at ambient pressure. The superconducting bulk properties of Th0.93Zr0.07B12 are confirmed by means of magnetization, electronic transport properties and specific heat measurements. The H – T phase diagrams based on magnetization and magnetoresistance measurements yield μ 0 H c 1 ( 0 ) = 6 mT and μ 0 H c 2 ( 0 ) = 98 mT and allow us to estimate the coherence length ξ o ∼ 57.9 nm and the penetration depth λ L ∼ 234 nm at zero kelvin.

Highlights: • We investigated the effect of the dilution of magnetic Ru sub-lattice of RuSr{sub 2}GdCu{sub 2}O{sub 8}. • We synthesized the doped compound Rui{sub x}Re{sub x}Sr{sub 2}GdCu{sub 2}O{sub y}, for 3%, 6%, 9% and 12% Re. • Re would affect the electron coupling: just 3 and 6% samples were superconductor. • Superconductivity emergence strongly affects magnetic properties of 3 and 6% samples. • A weak ferromagnetic component is consistent with a globally antiferromagnetic system. - Abstract: Despite the discovery of new superconductors classes, high-Tc oxides continue to be a current topic, because of their complex phase diagrams and doping-dependant effects (allowing one to investigate the interaction between orbitals), as well as structural properties such as lattice distortion and charge ordering, among many others. Ruthenocuprates are magnetic superconductors in which the magnetic transition temperature is much higher than the critical superconducting temperature, making them unique compounds. With the aim of investigating the dilution of the magnetic Ru sub-lattice, we proposed the synthesis of the Ru{sub 1−x}Re{sub x}Sr{sub 2}GdCu{sub 2}O{sub y} ruthenocuprate-type family, adapting the known two-step process (double perovskite + CuO) by directly doping the double perovskite, thus obtaining the perovskite compound Sr{sub 2}GdRu{sub 1−x}Re{sub x}O{sub y}, which represents amore » new synthesis process to the best of our knowledge. Our samples were structurally characterized through X-ray diffraction, and the patterns were analysed via Rietveld refinement. A complete magnetic characterization as a function of temperature and applied field, as well as transport measurements were carried out. We discuss our results in the light of the two-lattice model for ruthenocuprates, and a relation between RuO{sub 2} (magnetic) and CuO{sub 2} (superconductor) sub-lattices can clearly be observed.« less

We theoretically investigate the local magnetic field, order parameter and supercurrent profiles of a parallelepiped mesoscopic superconductor submersed in an applied magnetic field; this same geometry with a pillar on its top surface is also considered. Our investigation was carried out by solving the three-dimensional time-dependent Ginzburg–Landau (TDGL) equations. We obtain the magnetization curve as a function of the external applied magnetic field for several sample sizes. We have determined an analytical dependence of the thermodynamic fields and the magnetization as functions of the lateral dimension of the superconductor. Finally, a systematic comparative study of the two- and three-dimensional approaches of the Ginzburg–Landau model is carried out.

This work investigates interactions of vortices in superconducting materials between standard types I and II, in the domain of the so-called intertype (IT) superconductivity. Contrary to common expectations, the many-body (many-vortex) contribution is not a correction to the pair-vortex interaction here but plays a crucial role in the formation of the IT vortex matter. In particular, the many-body interactions stabilize vortex clusters that otherwise could not exist. Furthermore, clusters with large numbers of vortices become more stable when approaching the boundary between the intertype domain and type I. This indicates that IT superconductors develop a peculiar unconventional type of the vortex matter governed by the many-body interactions of vortices.

Multiple condensates in a superconducting material can interfere constructively or destructively and this leads to unconventional effects not inherent in single-band superconductors. Such effects can be pronounced when the spatial scales (healing lengths) of different band condensates deviate from each other. Here we show that, contrary to usual expectations, this deviation can be considerable even far beyond the regime of nearly decoupled bands, being affected by difference between the band Fermi velocities. Our study is performed within the extended Ginzburg-Landau (GL) formalism that goes to one order beyond the GL theory in the perturbation expansion of the microscopic equations over the proximity to ${T}_{c}$. The formalism makes it possible to obtain closed analytical results for the profiles of the band condensates and for their healing lengths and, at the same time, captures the difference between the healing lengths which does not appear in the standard GL domain.

We demonstrate that in a weak-coupled two-band superconducting slab the interaction between vortices penetrating the sample and its boundaries leads to the phenomenon of vortex splitting, which divides composite vortices and creates fractional ones. The interaction between vortices, attractive for different bands and repulsive for the same band, which is controlled by the electric current density flowing through the system, leads to an ordered alternating arrangement of the vortices. This arrangement creates nonequilibrium interband phase textures or domains with different signs of the Josephson energy of the interaction between the band condensates. Such phase textures have a significant effect on the dissipation caused by the vortex motion. In particular, in the phase-texture regime the onset of the dissipation is shifted to higher current densities.

The appearance of a shallow band(s) drives a superconductor towards the BCS-BEC crossover, conventionally associated with notable changes in single-particle properties and an elevated critical temperature. Here we demonstrate that the proximity to the crossover induced by a shallow band has also a dramatic effect on the phase diagram of the superconducting magnetic properties. When the system passes from the BCS to BEC regime, the intertype domain between superconductivity types I and II enlarges systematically, being inversely proportional to the square of the Cooper-pair radius, the main parameter that controls the BCS-BEC superconductivity. We also show that despite the presence of a shallow band, the condensate fluctuations are suppressed when it coexists in one material with standard deep bands, as in recent iron chalcogenides ${\mathrm{Fe}\mathrm{Se}}_{x}{\mathrm{Te}}_{1\ensuremath{-}x}$ and $\mathrm{FeSe}$.

We report the synthesis and characterization of graphene nanoribbons (GNRs) decorated with iron oxide (Fe3O4) nanoparticles to obtain the GNR_Fe3O4 nanocomposite and its use as a DNA sensor. Characterization results confirm the successful synthesis of a nanocomposite based on reduced GNRs and mostly Fe3O4 nanoparticles distributed randomly and homogeneously on the ribbon's surface and whose specific surface area (766 m2 g−1) is higher compared to pure GNRs (588 m2 g−1). These characteristics make this nanocomposite suitable for effective DNA immobilization and hybridization in sensor applications. Taking advantage of the latter, the electrochemical analysis demonstrated that GNR_Fe3O4-based electrodes amplify the electrochemical signal by more than one order of magnitude compared to bare carbon electrodes, and 70% more compared to pristine GNRs-based electrodes. The capability of the GNR_Fe3O4 nanocomposite as a DNA sensor was evaluated in terms of the electrochemical response by monitoring the cathodic peak in DNA immobilization and hybridization through a redox process. The electrochemical current was measured in immobilized single-stranded DNA and double-stranded DNA to be 92 and 49 μA, respectively, for GNR_Fe3O4-based electrodes; these values are indicative of an effective discrimination between the immobilization and hybridization of DNA. The present work demonstrates the viability of a DNA sensor based on the facile synthesis of GNRs decorated with Fe3O4 nanoparticles.

In this paper, we analyze the structural, morphological, and magnetic properties of CaMn1−xMoxO3 (x = 0.08, 0.10, 0.12) system prepared by the usual solid-state reaction method. The results obtained by scanning electron microscopy (SEM) show an appropriate sintering process, and energy dispersive spectroscopy (EDS) shows that there is not a presence of strange elements. The X-ray diffraction analysis (XRD), through the Rietveld method, reveals that samples crystallize in a Pnma (62) orthorhombic structure, with an increase in the unit cell volume as a function of Mo concentration. Curves of magnetization zero field cooling (ZFC)-field cooling (FC) in a temperature range of 50 to 350 K show similar paramagnetic behavior of every applied field above 110 K in each one of the Mo doping. Below this temperature, the magnetic response becomes ferromagnetic. The antiferromagnetic transition was analyzed by fitting of the Curie-Weiss law. An increase in total effective magnetic moment and Neel temperature, ranging from 200 to 295 K, is observed when Mo concentration increases in CaMn1−xMoxO3 (x = 0.08, 0.10, 0.12) system.

In this work we report the surface morphology of amorphous germanium (a-Ge) thin films (140 nm thickness) following thermal outgassing of SiO2/Si substrates. The thermal outgassing was performed by annealing the samples in air at different temperatures from 400 to 900 °C. Annealing at 400 °C in slow (2 °C/min) and fast (10 °C/min) modes promotes the formation of bubbles on the surface. A cross sectional view by transmission electron microscope taken of the sample slow annealed at 400 °C reveals traces of gas species embedded in the a-Ge film, allowing us to propose a possible mechanism for the formation of the bubbles. The calculated internal pressure and number of gas molecules for this sample are 30 MPa and 38 × 108, respectively. Over an area of 22 × 10−3 cm2 the density of bubbles obtained at slow annealing (9 × 103 cm−2) is smaller than that at rapid annealing (6.4 × 104 cm−2), indicating that the amount of liberated gas in both cases is only a fraction of the total gas contained in the substrate. After increasing the annealing temperature in the slow mode, bubbles of different diameters (from tens of nanometers up to tens of micrometers) randomly distribute over the Ge film and they grow with temperature. Vertical diffusion of the outgas species through the film dominates the annealing temperature interval 400–600 °C, whereas coalescence of bubbles caused by lateral diffusion is detected after annealing at 700 °C. The bubbles explode after annealing the samples at 800 °C. Annealing at higher temperatures, such as 900 °C, leads to surface migration of the film to the exploded bubbles and eventually forming islands of polycrystalline GeO2.

Superconducting films are usually regarded as type II superconductors even when they are made of a type I material. The reason is the presence of stray magnetic fields that stabilize the vortex matter by inducing long-range repulsive interactions between vortices. While very thin films indeed reach this limit, there is a large interval of thicknesses where magnetic properties of superconducting films cannot be classified as either of the two conventional superconductivity types. Recent calculations revealed that in this interval the system exhibits spontaneous formation of magnetic flux-condensate patterns and superstructures appearing due to the interplay between the long-range stray field effects and proximity to the Bogomolnyi self-duality point. These calculations were based on the periodic in-plane boundary conditions which, as is well known from classical electrodynamics, for systems with long-range interactions can lead to field distortions and considerable discrepancies between results of different calculation methods. Here we demonstrate that similar spontaneous patterns are obtained for superconducting films with open in-plane boundary conditions (vanishing in-plane currents perpendicular to the edges of the finite film) and thus the phenomenon is not an artefact of chosen boundary conditions.

In this work we report the preparation of fucan-coated magnetite (Fe3O4) nanoparticles by the co-precipitation method. These nanoparticles were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, Mossbauer spectroscopy and magnetic measurements. The nanoparticles showed quasi-spherical morphology with mean sizes around 10 nm. XRD and FT-IR confirmed the functionalization of the Fe3O4 nanoparticles with the fucan polysaccharide. Room temperature magnetization measurements and Mossbauer spectroscopy showed that the nanoparticles exhibited superparamagnetic behavior at 300 K and the magnetic properties of the Fe3O4 are partly screened by the coating preventing aggregation.

The vortex matter in a superconducting disk with a linear configuration of metallic and superconducting defects is studied. Effects associated to the pinning (anti-pinning) force of the metallic (superconducting) defect on the vortex configuration and on the thermodynamic critical fields are analyzed in the framework of the Ginzburg Landau theory. We calculate the loop of the magnetization, vorticity and free energy curves as a function of the magnetic field for a thin disk. Due to vortex–defect attraction for a metallic defect (repulsion for a superconducting defect), the vortices always (never) are found to be sitting on the defect position.

Fucan-coated magnetite (Fe3O4) nanoparticles were synthesized by the coprecipitation method and studied by Mossbauer spectroscopy and magnetic measurements. The sizes of the nanoparticles were 8–9 nm. Magnetization measurements and Mossbauer spectroscopy at 300 K revealed superparamagnetic behavior. The magnetic moment of the Fe3O4 is partly screened by the Fucan coating aggregation. When the magnetite nanoparticles are capped with oleic acid or fucan, reduced particle-particle interaction is observed by Mossbauer and TEM studies. The antitumoral activity of the fucan-coated nanoparticles were tested in Sarcoma 180, showing an effective reduction of the tumor size.

In this work, we report the mechanical reorientationof thiolated ferromagnetic microspheres bridging a pairof gold electrodes under an external magnetic field. Whenan external magnetic field (7 kG) is applied during themeasurement of the current–voltage characteristics of a carboxylferromagnetic microsphere (4 μm diameter) attachedto two gold electrodes by self-assembled monolayers(SAMs) of octane dithiol (C8H18S2), the current signal isdistorted. Rather than due to magnetoresistance, this effectis caused by a mechanical reorientation of the ferromagneticsphere, which alters the number of SAMs between thesphere and the electrodes and therefore affects conduction.To study the physical reorientation of the ferromagneticparticles, we measure their hysteresis loops while suspendedin a liquid solution.Keywords Magnetic microparticles

Cobalt ferrite has attracted considerable attention in recent years due to its unique physical properties such as high Curie temperature, large magnetocrystalline anisotropy, moderate saturation magnetization, large magnetostrictive coefficient, excellent chemical stability and mechanical hardness. In this work we present the preparation, of fucan coated cobalt ferrite nanoparticles by a modified coprecipitation method and the study of their structural, microstructural and magnetic characteristics for their application as a solid support for enzymes immobilization and other biotechnology applications. Aqueous suspensions of magnetic particles were prepared by coprecipitation of Fe(III) and Co(II) in the presence of NaOH, acid oleic and fucan polymer. The X-ray diffraction indicates that the funtionalization does not degrade the core cobalt ferrite. The infrared (FTIR) bands, indicate the functional characteristics of the coating on the cobalt ferrite. Mossbauer spectra at room temperature indicate the presence of a broadened sextet plus a doublet which is typical of superparamagnetic relaxation. For the Co-ferrite uncoated and coated with fucan the doublets have areas of 36.1 % and 40.3 % respectively, indicating the presence of non-interacting particles and faster relaxation time. The Co-ferrite coated with oleic acid and oleic acid plus fucan have areas around 17.5%and 17.1% respectively which indicate a weak superparamagnetic relaxation due to a slow relaxation time. The magnetization measurements of the cobalt ferrite nanoparticles with and without coating confirm that they are superparamagnetic and this behavior is produced by the core nanoparticles rather than the coatings. The cobalt ferrite nanoparticles coated with oleic acid presented the highest magnetization than when coating with fucan.

By solving the time dependent Ginzburg–Landau equations, we investigated the influence of an internal triangular arrangement of point-like defects on the vortex configurations in a thin mesoscopic sample. The effect of the number of internal defects and their nature on the entrance position of the vortex is studied for a very thin circular sample. We found that the interplay between the vortex–vortex repulsion, the vortex–defect interaction and the interaction with the sample border leads to non-commensurate vortex configurations.

We present structural, magnetic, electrical, thermal transport, Hall coefficient, and pressure-dependent resistivity measurements on $\mathrm{C}{\mathrm{u}}_{x}\mathrm{ZrT}{\mathrm{e}}_{2\ensuremath{-}y}$ compounds with $x=0.05$, 0.1, 0.15, 0.2, and 0.3, and $y$ varied between $0\ensuremath{\le}y\ensuremath{\le}0.8$. In order to calculate the ground state, ab initio calculations of the electronic structure of these materials were performed. Our results show that copper intercalation in $\mathrm{ZrT}{\mathrm{e}}_{2}$ induces superconductivity in the $\mathrm{ZrT}{\mathrm{e}}_{2}$ system. For the $\mathrm{C}{\mathrm{u}}_{0.3}\mathrm{ZrT}{\mathrm{e}}_{1.2}$ sample, Hall and Seebeck coefficient measurements show that the system is predominantly negatively charged with carrier density close to ${10}^{19}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$. The temperature dependence of the Hall coefficient, the Seebeck coefficient, and the lower critical field indicates that this material presents multiband character. Pressure-dependent resistivity vs temperature measurements reveal that while the normal-state resistivity decreases with increasing applied pressure, the superconducting transition temperature is completely insensitive to the applied pressure (for pressure in the range 0--1.3 GPa). This suggests that the Fermi gas is intrinsically degenerate under very high pressure, and therefore does not change much with varying external pressure. Finally the band structure calculation shows a dispersion curve containing a bulk three-dimensional Dirac conelike feature at the L point in the Brillouin zone, which is gapless in the absence of spin-orbit coupling, but develops a gap when this coupling is considered. Altogether, the results indicate that the superconducting compound $\mathrm{C}{\mathrm{u}}_{x}\mathrm{ZrT}{\mathrm{e}}_{2\ensuremath{-}y}$ presents a signature of multiband behavior and may possibly be a new example of a topological superconductor.

The Bogomolnyi point separates superconductivity types I and II while itself hiding infinitely degenerate magnetic flux configurations, including very exotic states (referred to here as flux ``monsters''). When the degeneracy is removed, the Bogomolnyi point unfolds into a finite, intertype domain in the phase diagram between types I and II. One can expect that in this case the flux monsters can escape their ``prison'' at the Bogomolnyi point, occupying the intertype domain and shaping its internal structure. Our calculations reveal that such exotic flux distributions are indeed stable in the intertype regime of thin superconductors made of a type-I material, where the Bogomolnyi degeneracy is removed by stray magnetic fields. They can be classified into three typical patterns that are qualitatively different from those in types I and II: superconducting islands separated by vortex chains; stripes/worms/labyrinths patterns; and mixtures of giant vortices and vortex clusters. Our findings shed light on the problem of the interchange between types I and II, raising important questions on the completeness of the textbook classification of the superconductivity types.

Flux-flow phenomena in a superconducting mesoscopic stripe submitted to an appliedcurrent and external magnetic field is studied. The time-dependent Ginzburg-Landauequations are solved numerically to obtain the electric and magnetic response of thesystem. It is shown that the I-V curves, for the wider strips, present a universalbehaviour. The dependence of the flux-flow resistivity on the magnetic field and widthallow us to propose a criterion characterizing, both, the macroscopic and mesoscopicregimes. The power spectrum of the average voltage permits identifying the effect ofsurface currents in vortices movement. Based on the maximum value of the power spectrumfirst harmonic we propose a geometric condition for matching between the sample dimensionsand the vortex lattice parameter. Fil: Sanchez Lotero, Pedro Nel. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidade Federal de Pernambuco; Brasil Fil: Domínguez, Daniel. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina Fil: Albino Aguiar, J.. Universidade Federal de Pernambuco; Brasil

In the nearest vicinity of the critical temperature, types I and II of conventional single-band superconductors interchange at the Ginzburg-Landau parameter $\ensuremath{\kappa}=1/\sqrt{2}$. At lower temperatures this point unfolds into a narrow but finite interval of $\ensuremath{\kappa}$'s, shaping an intertype (transitional) domain in the $(\ensuremath{\kappa},T)$ plane. In the present work, based on the extended Ginzburg-Landau formalism, we show that the same picture of the two standard types with the transitional domain in between applies also to multiband superconductors. However, the intertype domain notably widens in the presence of multiple bands and can become extremely large when the system has a significant disparity between the band parameters. It is concluded that many multiband superconductors, such as recently discovered borides and iron-based materials, can belong to the intertype regime.

This work presents some results of the synthesis and structural, microstructural, and magnetic characterization of fucan coated cobalt ferrite nanoparticles prepared by using a modified coprecipitation method. Aqueous suspensions of magnetic particles were prepared by coprecipitation of Fe(III) and Co(II) in the presence of NaOH, acid oleic and fucan polymer. The samples were characterized by X-ray diffraction (XRD), electron scanning microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), accelerated surface area, and porosimetry (ASAP/BET-Brunauer-Emmet-Teller) determination and magnetization measurements. Our results reveal that both uncoated and fucan polysaccharide coated CoFe2O4 nanoparticles were successfully obtained. The nanoparticles present sizes between 7 to 20 nm and saturation magnetization of the order of 40 emu/g. The nanoparticles thus obtained are suitable for future applications as a solid support for enzymes immobilization and other biotechnology applications.

In this paper we study the formation of vortices in a mesoscopic superconducting segment ring. We evaluate the magnetization, superconducting to normal magnetic field transition and the superconductor density of electrons. Our results point out that, as we reduce the superconducting segment width, remaining its area constant, both the first and third critical fields increases. Also, the third critical field exhibits a typical behavior as function of the angular width Θ of the segment ring, namely, the curve H C 3 ( Θ ) decreases monotonically with increasing the angle width and diverges in the limit Θ → 0 . In addition, we have found an analytical expression from the magnetization as a function of the segment width.

In this paper we study some superconducting properties of a flat disk by taking into account the roughness of the surface. Also, the sample is surrounded by a thin layer of a metallic material and submitted to and external magnetic field applied perpendicular to its plane. The boundary condition is taking into account via the de Gennes extrapolation length b. We evaluate the Gibbs free energy, magnetization, vorticity, the first and second critical thermodynamical fields as a function of the external magnetic field and b parameter. We also evaluate the first critical field as a function of the surface rugosity.

We performed a molecular dynamics study of vortex-antivortex motion in a superconducting disk with a magnetic dot on top, in the Corbino disk geometry. In this system, vortices and antivortices are forced to move in opposite azimuthal directions by a radially applied current. The dot is magnetized out of plane in order to stabilize composite vortex-antivortex configurations, with vortices closer to the center of the disk and antivortices near to the disk edge. We observe that the interplay between the spatially inhomogeneous current distribution, the screening currents induced by the dipole, and the attractive vortex-antivortex (v-av) interaction result in different dynamical phases. At low current values, antivortices which are distributed at outer rings - remain bounded to vortices at inner rings and the whole configuration rotates rigidly. Above a threshold current, vortices and antivortices unbind and move at different angular velocities in a highly correlated way. Finally, at very strong drive, vortex-antivortex attraction is overhelmed by the external current Lorentz force, causing them to move in opposite directions. (C) 2011 Elsevier B.V. All rights reserved.

We report an experimental study of the crystallographic lattice, morphologic characteristics and magnetic feature of Sr 2 RuHoO 6 complex perovskite, which is used as a precursor in the fabrication process of the superconducting ruthenocuprate RuSr 2 HoCu 2 O 8 . The samples were produced through the standard solid state reaction. A Rietveld refinement of experimental X-ray diffraction patterns shows that the material crystallizes in a monoclinic structure, which belongs to the P21/ n (#14) space group, with lattice parameters a= 5.7719(6) A, b =5.8784(5) A , c =8.1651(9) A, and tilt angle β =90.200°. Magnetic susceptibility measurements reveal the occurrence of an antiferromagnetic ordering for a Neel temperature T N =10.1 K . From the Curie-Weiss fitting of the paramagnetic regime we obtain an effective magnetic moment of 11.31 μ B .

In this work, we study the crystallization and electrical resistivity of the formed oxides in a Cu/SiO2/Si thin film after thermal oxidation by ex-situ annealing at different temperatures up to 1000 °C. Upon increasing the annealing temperature, from the X ray diffractogram the phase evolution Cu → Cu + Cu2O → Cu2O → Cu2O + CuO → CuO was detected. Pure Cu2O films are obtained at 200 °C, whereas uniform CuO films without structural surface defects such as terraces, kinks, porosity or cracks are obtained in the temperature range 300–550 °C. In both oxides, crystallization improves with annealing temperature. A resistivity phase diagram, which is obtained from the current–voltage response, is presented here. The resistivity was expected to increase linearly as a function of the annealing temperature due to evolution of oxides. However, anomalous decreases are observed at different temperatures ranges, this may be related to the improvement of the crystallization and crystallite size when the temperature increases.

Magnetic levan was synthesized by co-precipitating D-fructofuranosyl homopolysaccharide with a solution containing Fe2+ and Fe3+ in alkaline conditions at 100°C. The magnetic levan particles were characterized by scanning electron microscopy (SEM), magnetization measurements, X-ray diffractometry (XRD) and infrared spectroscopy (IR). Afterwards, magnetic levan particles were functionalized by NaIO4 oxidation and used as matrices for trypsin covalent immobilization. Magnetite and magnetic levan particles were both heterogeneous in shape and levan–magnetite presented bigger sizes compared to magnetite according to SEM images. Magnetic levan particles exhibited a magnetization 10 times lower as compared to magnetite ones, probably, due to the coating layer. XRD diffractogram showed that magnetite is the dominant phase in the magnetic levan. Infrared spectroscopy showed characteristics absorption bands of levan and magnetite (O–H, C–O–C and Fe–O bonds). The immobilized trypsin derivative was reused 10 times and lost 16% of its initial specific activity only. Therefore, these magnetic levan particles can be proposed as an alternative matrices for enzyme immobilization.

LaCaBaCu3O7 is a high critical temperature superconductor (HTS) below the Tc = 80 K and its structure is similar to that of YBCO. In this work we study the effect in the irreversibility line and the Curie-Weiss temperature of this superconductor after borate (BO3) 3- doping (LaCaBaCu3-x(BO3)xO7-δ x=0.2 and 0.3). Interestingly, borate doping enhances the irreversibility line and this effect is probably caused from alteration the arrangement of the oxygen atoms in the crystalline structure. Moreover, the Curie-Weiss temperature (□p) in the normal state increases with doping concentration. Calculated □p for pure LaCaBaCu3O7 is around -98K, this value increases to -75 and 49 K for x=0.2 and 0.3 respectively and indicating transition from antiferromagnetic to ferromagnetic coupling between ions in the structure. This study provides additional data for the magnetic characteristics of doped like-123 crystalline structures with oxyanions.

We report on magnetization studies in YBa 2 Cu 3 O 7− δ thin films with dispersed Ba 2 YTaO 6 nanoparticles. The magnetization measurements were made using a superconducting quantum interference device (SQUID) and a vibrating sample magnetometer (VSM). Magnetic moments were measured as functions temperature using zero-field cooling (ZFC) and field-cooled (FC) prescriptions for magnetic fields up to 10 T applied parallel and perpendicular to the ab planes. A paramagnetic response related to the superconducting state was observed during the FC experiments. This effect, known as paramagnetic Meissner effect (PME), apparently increases when the magnetic field is increased. We discuss our PME results in terms of the strong pinning scenario modulated by Ba 2 YTaO 6 nanoparticles dispersed into the superconducting matrix.