Dependence of the magnetic and superconducting characteristics of fine-grained HTSCs YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> on the degree of structural disorder and type of oxygen vacancy ordering in chain planes

The characteristics determining different contributions to the magnetic susceptibility at T > T _C (Pauli susceptibility, coherence length at T = 0, and Curie constant) as functions of the degree of structural disorder have been analyzed for high-temperature superconducting YBa₂Cu₃O _y samples ( y ≈ 6.92, T _C ≈ 92 K) with micrometer and submicron average grain sizes D _av. It is shown that the decrease in these characteristics, which is observed in fine-grained samples with a decrease in D _av, occurs in various ways, depending on the number and type of oxygen vacancy ordering in chain planes.     

Suppression of hole doping in fine-grained HTSC YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> due to structural disordering

It was proven experimentally that the structural disordering inherent to fine-grained high-temper- ature YBa₂Cu₃O _y superconductors (with an average grain size of 〈D〉 < 2 μm) leads to a reduction of the level of hole doping and the creation of features inherent to the pseudogap state (antiferromagnetic correlations and the lowered density of states at the Fermi level) even in samples with optimum oxygen content y ≈ 6.92.     

Features of the superconducting state and structural disorder in ultrafine particles of the HTSC YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>

We study the reasons for which the compound YBa₂Cu₃O _y begins to lose its superconducting ability with a decrease in particle sizes to values of ∼0.1 μm. Our analysis of the results of structural and magnetic studies has allowed us to reveal changes in the parameters of the crystal structure and of the superconducting state in small particles of an HTSC and to show that the main cause of these changes is a special kind of a structural disorder peculiar only to small particles of YBa₂Cu₃O _y and is realized as a consequence of the need for nonequilibrium conditions during their synthesis.     

The influence of interplane oxygen redistribution on the magnetization of fine-grained HTSC YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>

The magnetization of the fine-grained high-temperature superconductor (HTSC) YBa₂Cu₃O _y is experimentally investigated at T < T _c . A distinctive feature of this material is the increased oxygen content in CuO_δ planes. The magnetization decrease with an increase in δ is revealed. This correlation indicates that during interplane oxygen redistribution, which is characteristic of fine-grained samples, the oxygen content in the chain planes increases due to its reduction in the superconducting CuO₂ planes.     

Origin of different contributions to the magnetic susceptibility of the normal-state YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> HTSC

The magnetic susceptibility of the normal-state YBa₂Cu₃O _y high-temperature superconductor has been experimentally investigated as a function of the degree of structural disorder by the example of a series of fine-grained samples. Different coexisting contributions have been revealed and their origin was investigated. A correlation between the changes in the structural parameters and the magnitudes of these contributions to the magnetic susceptibility is established.     

The oxygen content and its ordering in the chain planes in fine-grained HTSC YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>

A comparative analysis of the results of the X-ray and Mösbauer studies of the high-temperature superconductor (HTSC) YBa₂Cu₃O _y and YBa₂Cu_{3 ? x} ⁵⁷Fe _x O _y (x = 0.015, T _c ≈ 91.5 K) samples with different average grain sizes <D> in the micron and submicron ranges has been performed. The regularities in the change in the lattice parameter c and in the degree of occupation of different oxygen sites in the CuO_δ chain planes taking place at the decrease in <D> have been studied. The quantitative interrelation between the parameter c and the oxygen content δ in the CuO_δ planes exceeding the amount of the mobile oxygen due to the interplane oxygen redistribution is established.     

Enhancement of pseudogap anomalies induced by nanoscale structural inhomogeneity in YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>6.93</Subscript> high-<Emphasis Type="Italic">T</Emphasis><Subscript>c</Subscript> superconductor

The magnetization M(H) in the superconducting state, dc magnetic susceptibility χ(T) in the normal state, and specific heat C(T) near the superconducting transition temperature T _c have been measured for a series of fine-crystalline YBa₂Cu₃O _y samples having nearly optimum values of y = 6.93 ± 0.3 and T _c = (91.5 ± 0.5) K. The samples differ only in the degree of nanoscale structural inhomogeneity. The characteristic parameters of superconductors (the London penetration depth and the Ginzburg–Landau parameter) and the thermodynamic critical field H _c are determined by the analysis of the magnetization curves M(H). It is found that the increase in the degree of nanoscale structural inhomogeneity leads to an increase in the characteristic parameters of superconductors and a decrease in H _c(T) and the jump of the specific heat ΔC/T _c. It is shown that the changes in the physical characteristics are caused by the suppression of the density of states near the Fermi level. The pseudogap is estimated by analyzing χ(T). It is found that the nanoscale structural inhomogeneity significantly enhances and probably even creates the pseudogap regime in the optimally doped high-T _c superconductors.     

Mechanisms of interaction of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript> <Emphasis Type="Italic">y</Emphasis> </Subscript> with water vapor at low-temperature annealing

Abstract—The interaction of YBa₂Cu₃O_y (123) with water vapor at temperatures T ≤ 150° has been studied. It has been shown that, with an increase in temperature, the mechanism of its interaction with water changes. Near room temperature, the main process is hydrolytic decomposition. At T ~ 100°C, the absorption of water is significantly reduced, because the role of hydrolysis becomes less important and water penetrates the structure weakly and is incorporated into oxygen vacancies mainly in the form of OH–-groups, which leads to the transition of YBa₂Cu₃O_y from the tetragonal to orthorhombic phase. With an increase in temperature to 150°C, the absorption of water increases again. In this case, the main mechanism is the penetration of water to the 123 structure, which leads to splitting of Cu–O chains and a phase transition from the 123 to pseudo-124 structure. The role of different mechanisms of interaction with water essentially depends on the oxygen content in the 123 structure. At a low oxygen index (y = 6.3), the role of hydrolysis is more important, and, at y ≥ 6.5, the incorporation of water into the structure prevails. It has been revealed that, at T = 150°C, after absorption of water, YBa₂Cu₃O_6.96 becomes a proton conductor.     

Peculiarities in the low-temperature specific heat related to nanoscale structural inhomogeneity in fine-crystalline YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>6.93</Subscript> high-<Emphasis Type="Italic">T</Emphasis><Subscript>c</Subscript> superconductors

A comparative study of the low-temperature specific heat for two types of YBa₂Cu₃O _y high-T _c superconductor samples is performed within the temperature range of 2?10 K. The samples of the first type are fine-crystalline optimally doped ones with different degrees of nanoscale structural inhomogeneity. The second type includes coarse-crystalline equilibrium samples with different hole doping levels. A similarity in the behavior of different contributions to the specific heat for structurally inhomogeneous and underdoped samples is revealed. The samples of both types exhibit a metal-like contribution linear in temperature to the specific heat ~γT, which is not characteristic of the superconducting phase. It is found that this contribution moderately grows with the decrease in the oxygen content, whereas with the increase in the structural inhomogeneity, such growth of the linear contribution (γT) becomes anomalously large. This leads to the conclusion about the coexistence of metallic and superconducting states in the bulk of the samples under study. Such common feature of electron systems could be related to the formation of the pseudogap regime. It is demonstrated that this regime suppresses just the superconducting states, leaving intact the metallic ones.     

Electrophysical properties and the structure of the YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> compound thermally restored after low-temperature decomposition

The electrophysical properties and structure of the nonstoichiometric high-temperature superconductor YBa₂Cu₃O _y restored at T = 930–950°C after low-temperature decomposition (T = 200°C) into phases different in the oxygen content have been studied. It has been shown that, unlike heat treatments at T ≤ 900°C, the superconducting properties are almost completely restored for 3–5 h during grain recrystallization, which is impossible at lower temperatures. After short-term annealing at T = 930–950°C (for 1–2 h), the ceramic material still contains a significant number of structural defects, most likely, in cation sublattices. These defects can contribute to the pinning of magnetic vortices, which substantially increases the critical current density in magnetic fields up to 2 T as compared to ceramic materials produced by the conventional technology.     

Structure and orientation of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> fibers used in YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript> <Emphasis Type="Italic">y</Emphasis> </Subscript> sputtering

The internal structure and orientation of thin (150–300 μm) flexible Al₂O₃ fibers used as substrates for third-generation high-temperature superconducting wires are studied by different methods. It is shown that using scanning electron microscopy, electron backscatter diffraction, transmission electron microscopy, and X-ray diffraction, one can reliably determine the position of the \((1\bar 102)\) plane, on which good YBa₂Cu₃O_y films can be grown.     

Features of the low-temperature specific heat in underdoped YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>6 + <Emphasis Type="Italic">x</Emphasis></Subscript> single crystals

The behavior of the low-temperature specific heat C(T) for YBa₂Cu₃O_{6 + x} single crystals with the doping level corresponding to the normal phase has been studied by the relaxation technique at different values of the applied magnetic field. It has been found that the C(T)/T plot exhibits such an anomaly as a steep increase with decreasing temperature from T about 4 K down to T ≤ 2 K (the minimum temperature value accessible in the experiment). The applied magnetic field as high as 9 T inverts this anomaly and leads to a sharp drop in C(T)/T during cooling within the same temperature range. A model involving the Schottky-type centers formulated in this work and the data on spin correlation functions has allowed us to calculate the temperature dependence of the specific heat, which fits the experimental curves quite well.     

Effect of low-temperature annealing on the magnetization curve of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>6 + <Emphasis Type="Italic">x</Emphasis></Subscript> ceramics

The effect of low-temperature annealing on the magnetization curve of YBa₂Cu₃O_{6 + x} ceramics in the superconducting state (x ≈ 0.9) is investigated. When the annealing time is fairly long, the field dependence of magnetic moment M exhibits a feature in the form of a plateau, where the value of M remains almost constant. The evolution of this feature in the magnetization curves of annealed samples with annealing time and temperature is studied. It is assumed that low-temperature annealing gives rise to metastable ferromagnetic clusters in YBa₂Cu₃O_{6 + x} ceramics, the contribution of which to the magnetic moment accounts for the feature in the magnetization curves of the annealed samples.     

Fermion Localization on the <Emphasis Type="Italic">orbifold</Emphasis><Emphasis Type="Italic">S</Emphasis><Subscript>1</Subscript>/<Emphasis Type="Italic">Z</Emphasis><Subscript>2</Subscript>

The hierarchical structure of fermion masses of the Standard Model is explained in split fermion models by localizing the fermions at different points in an extra dimension. We consider split fermion models with two bulk scalars compactified on an orbifold. In the static case we find analytical expression for the localizer. We also address the issue of stability of the localizer. We also find exact solutions for the fermion zero modes. We explore the parameter space of the model. We find ample opportunity for construction of phenomenologically viable theories exist.     

Formation energy and geometry of vacancies at BN and B<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>C<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>N<Subscript><Emphasis Type="Italic">z</Emphasis></Subscript> nanocones

We have studied, through ab initio calculations, the stability of 60° and 120° boron nitride nanocones containing mono and multiple boron, nitrogen, and carbon vacancies. The stability of the vacancies as well as the structures reconstruction mechanism have been investigated. Our results indicate that the stability of the cones presenting such vacancies strongly depends on growth conditions. We have also found that multiple vacancies display formation energies that are comparable, and in some cases, even lower to the ones presented by monovacancies. Therefore, our results allow us to conclude that the formation energy does not depend on the vacancy size. Finally, for 120° cones, we can verify that the stability of the boron and nitrogen vacancies depends on the position where the atom has been removed.     

Mechanical and Dynamic Stability of Complete and Nonstoichiometric 3<Emphasis Type="Italic">C</Emphasis>-Si<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>C<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> from Ab Initio Calculations

The energies of formation of vacancies in the carbon and silicon sublattices, the independent elastic constants, the all-round compression, shear and Young’s moduli, and the anisotropy coefficients are determined for the complete and nonstoichiometric cubic phases of 3C-Si_xC_y (x, y = 1.0–0.75) by ab initio methods of the band theory. In the formalism of the density functional perturbation theory (DFPT), the phonon dispersion dependences are obtained for these phases (the comparison with the experiment is given for the complete phase). It is shown that the mechanical characteristics of the phases become strongly anisotropic upon the transition from 3C-SiC_0.875 to 3C-SiC_0.75. It is established from the analysis of the phonon dispersion curves that the 3C-SiC_0.875 and 3C-SiC_0.75 phases, in contrast to the complete 3C-SiC phase, are dynamically unstable at T = 0 K.     

Radiative <Emphasis Type="Italic">K</Emphasis><Subscript><Emphasis Type="Italic">e</Emphasis>3</Subscript> decays revisited

Motivated by recent experimental results and ongoing measurements, we review the chiral perturbation theory prediction for decays. Special emphasis is given to the stability of the inner bremsstrahlung-dominated relative branching ratio versus the K _e3 form factors, and on the separation of the structure-dependent amplitude in differential distributions over the phase space. For the structure-dependent terms, an assessment of the order p ⁶ corrections is given, in particular, a full next-to-leading order calculation of the axial component is performed. The experimental analysis of the photon energy spectrum is discussed, and other potentially useful distributions are introduced.Received: 9 December 2004, Published online: 21 February 2005PACS: 13.20.Eb, 11.30.Rd, 12.39.Fe     

Enhancement of the intrinsic pinning by a magnetic field in a single crystal of high-temperature superconductor TmBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>

This paper presents a comparative study of the complex permeability μ* of single crystals of hightemperature superconductors RBa₂Cu₃Oy (R = Y, Tm) as a function of the magnetic field applied along the crystallographic ab-plane. Contributions to μ* from the oscillatory motion of vortices perpendicular to layers of the crystal lattice, μ_v, and realization of the critical state along the layers are obtained. It is found that, as the temperature approaches T _c, the behavior of the field dependences of μ_v and the critical current is substantially different for the given samples. This effect is related to the manifestation of an additional unusual mechanism of intrinsic pinning, which arises when Y₃₊ is replaced with the Tm³⁺ magnetic ion. The revealed specificities of the interaction of vortices with the magnetic ion layer suggest that, most probably, they have a magnetic nature and are not related to the variation in the condensation energy in the core of the vortex.     

Magnetic M<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>@N-C as heterogeneous catalysts for the catalytic oxidation of aniline solution with sulfate radicals

Metal nanoparticles have been combined with magnet metal–organic frameworks (MOFs) to afford new materials that demonstrate an efficient catalytic degradation, high stability, and excellent reusability in areas of catalysis because of their exceptionally high surface areas and structural diversity. Magnetic M _x O _y @N-C (M = Fe, Co, Mn) nanocrystals were formed on nitrogen-doped carbon surface by using 8-hydroxyquinoline as a C/N precursor. The Co@N-C, MnO@N-C, and Fe/Fe₂O₃@N-C catalysts were characterized by X-ray diffraction (XRD), Raman, scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), N₂ adsorption/desorption, and X-ray photoelectron spectroscopy (XPS). The catalytic performances of catalysts were thoroughly investigated in the oxidation of aniline solution based on sulfate radicals (SO₄ ^?.) toward Fenton-like reaction. Magnetic M _x O _y @N-C exhibits an unexpectedly high catalytic activity in the degradation of aniline in water. A high magnetic M _x O _y @N-C catalytic activity was observed after the evaluation by aniline degradation in water. Aniline degradation was found to follow the first-order kinetics, and as a result, various metals significantly affected the structures and performances of the catalysts, and their catalytic activity followed the order of Co > Mn > Fe. The nanoparticles displayed good magnetic separation under the magnetic field.

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