Effect of Y-site and Ba-site substitution on thermal properties of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−δ</Subscript>

The effect of partial replacement of Y site with Ca or Ba site with Sr on the thermal and electrical properties of YBa₂Cu₃O_7?δ superconductors was investigated. For Y(Ba_1?XSr_X)₂Cu₃O_7?δ system, the characteristic enhancement of the thermal conductivity κ(T) below the transition temperature T_c was decreased more slowly by substitution than that for (Y_1?XCa_X)Ba₂Cu₃O_7?δ system. Electron-phonon interaction in these systems were discussed based on the phonon heat conduction model under the weak coupling d-wave energy gap.     

Superconductivity and electronic liquid-crystal states in twin-free YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>6+x</Subscript> studied by neutron scattering

In this paper, we first give a concise overview of recent experimental and theoretical work dealing with “electronic liquid-crystal states” which spontaneously break different symmetries of the CuO₂ layers of high-T _c cuprates, with an emphasis on evidence in the spin excitation spectrum. Then we describe the importance of using twin-free samples to look for evidence for fourfold symmetry breaking in the spectrum and explain the preparation procedure to obtain such samples. We present inelastic neutron scattering results for moderately underdoped YBa₂Cu₃O_6.6(T _c = 61  K) and nearly optimally doped YBa₂Cu₃O_6.85(T _c = 89  K). In YBa₂Cu₃O_6.6, the dispersion topology changes when heating above T _c from an hourglass shape with constricted, commensurate resonance peak to a “Y”-shape without resonance anomaly. This change, and the fact that the low-energy signal above T _c can be described by an incommensurate, quasi-one-dimensional distribution, indicates a competition of superconductivity with an electronic liquid-crystal state. We then show a striking analogy between the difference signal I(5  K) − I(70  K) and the downward dispersing resonance mode in YBa₂Cu₃O_6.85. We therefore argue that a resonance mode only emerges below T _c, irrespective of the doping level. We finally discuss the implications of our results for the different scenarios invoked to explain the electronic liquid-crystal state in cuprates.     

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.     

Reluctance of the YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>x</Subscript> oxygen-deficient ceramics

Influence of an external magnetic field on the reluctance of the YBa ₂ Cu ₃ O _x ceramics is investigated. A significant reluctance of the oxygen-deficient ceramics (with critical temperature T_c < 77 K) is established for a sample in the normal state at T < 160 K. It is demonstrated that after ceramics annealing that restores the oxygen content to a nearly optimum value, the magnetic field has essentially no effect on the sample reluctance at temperatures exceeding T_c. To explain the revealed mechanisms, a model involving ferromagnetic clusters effectively decreasing the free carrier density is used. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 68–71, April, 2007.     

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.     

Helical magnetic structure in a quasi-one-dimensional LiCu<Subscript>2</Subscript>O<Subscript>2</Subscript> multiferroic crystal according to <Superscript>63,65</Superscript>Cu NMR studies

The NMR spectra of ⁶³Cu and ⁶⁵Cu natural copper isotopes in a LiCu₂O₂ multiferroic single crystal compound have been measured above and below the temperature of magnetic phase transition (T _c = 23 K) in zero magnetic field and in applied magnetic field H ₀ = 94 kOe parallel to the c axis of the crystal. In LiCu₂O₂ below T _c, a complicated helical magnetic structure with the magnetic moment of copper ions Cu²⁺ varying along the chain according to the harmonic law with the wave vector being incommensurate to the crystal lattice constants has been revealed. The experimental results have been successfully interpreted using the model based on the planar helical magnetic structure. It has been found that the plane of rotation for Cu²⁺ magnetic moments in LiCu₂O₂ does not coincide at H ₀ = 0 with the ab plane. The high magnetic field (H ₀ = 94 kOe) applied along the c axis of the single crystal does not affect the spatial orientation of the plane of rotation.     

Effect of the nanoscale structural inhomogeneity on the magnetic and superconducting characteristics of fine-grained YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> HTSCs

It has been experimentally established that the nanoscale structural inhomogeneity, inherent in fine-grained (0.4 ≤ 〈D〉 ≤ 2μm) high-temperature superconductors YBa₂Cu₃O _y (y ≈ 6.92, T _C ≈ 92 K) and manifesting itself in partial interplane redistribution of oxygen [1, 2], changes the density of states near the Fermi level and decreases the coherence length and density of superconducting carriers in CuO₂ planes. The revealed relationship between the changes in these characteristics with respect to their equilibrium values corresponds to the relationship that might occur for conventional superconductors.     

Anisotropy of the vortex structure and resistivity in the basal plane of YBa<Subscript>2</Subscript>Cu<Subscript>4</Subscript>O<Subscript>8</Subscript> single crystals

The vortex lattice of the YBa₂Cu₄O₈ high-temperature superconductor is studied in the basal plane of monocrystalline samples using the decoration technique in a field interval of 40–600 Oe. Vortex lattice anisotropy (field-independent “compression” of a regular hexagonal vortex cell in the poorly conducting direction by a factor of about 1.3) is detected. Resistivity anisotropy ρ _a /ρ _b measured at temperatures from T _c to room temperature is 16–9. Possible reasons for the discrepancy between our results and the available data are discussed.     

Manifestation of pseudogap features in structurally inhomogeneous optimally doped HTSC YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>6.92</Subscript>

Magnetization M(H,T) in magnetic fields H up to 90 kOe and at temperatures 2 K ≤ T < T _c (where Tc is the superconducting transition temperature), along with magnetic susceptibility χ(T) in the normal state T _c < T < 400 K for optimally oxygen-doped samples of YBa₂Cu₃O_6.92 with varying degrees of defects in the crystal structure, are studied to determine the influence of structural inhomogeneity on the electron systems characteristics of cuprate superconductors. It is shown that the existence of structural inhomogeneity of samples leads to the manifestation of peculiarities appropriate to pseudogap regime in their properties.     

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.     

Band structure of (Sr<Subscript>3</Subscript>Sc<Subscript>2</Subscript>O<Subscript>5</Subscript>)Fe<Subscript>2</Subscript>As<Subscript>2</Subscript> as a possible basis phase of new FeAs superconductors

The results of the ab initio FLAPW-GGA computations of the band structure of the recently synthesized layered tetragonal (space group I4/mmm) arsenide (Sr₃Sc₂O₅)Fe₂As₂ as a possible basis phase of a new group of FeAs superconductors are presented. For (Sr₃Sc₂O₅)Fe₂As₂, the energy bands, electron state density distributions, Fermi surface topology, low-temperature electron specific heat, molar Pauli paramagnetic susceptibility, and effective atomic charges have been determined. These results are discussed compared to similar data for the layered tetragonal crystals LaFeAsO, SrFeAsF, SrFe₂As₂, and LiFeAs that are the basis phases of the recently discovered high-temperature (T _C ～ 26–56 K) 《1111》, 《122》, and 《111》 FeAs superconductors.     

Magnetic susceptibility of fine crystalline high-temperature YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>6.93</Subscript> superconductors. The role of nanoscale structural disordering

Static magnetic susceptibility χ(T) in the normal state (T_c ≤ T ≤ 400 K) and specific heat C(T) near temperature T_c of the transition to the superconducting state are experimentally studied for a series of fine crystalline samples of high-temperature YBa₂Cu₃O_y superconductor, having y and T_c close to optimal but differing in the degree of nanoscale structural disordering. It is shown that under the influence of structural disordering, there is enhancement of anomalous pseudogap behavior of the studied characteristics and a significant increase in the width of the pseudogap.     

Detection of vortex excitations in a Bi<Subscript>2</Subscript>Sr<Subscript>2</Subscript>Ca<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>10</Subscript> crystal at temperatures above the critical temperature by EPR of the surface layer

Vortex excitations have been detected at temperatures both below and above the critical temperature when investigating local magnetic fields on the surface of a Bi₂Sr₂Ca₂Cu₃O₁₀ single crystal by means of an electron paramagnetic resonance (EPR) probe. A thin layer of a diphenyl picrylhydrazyl organic radical deposited on the crystal surface is used as the EPR probe. A narrow EPR signal makes it possible to detect weak distortions of the magnetic field appearing at T ≿ T _c. The analysis of the temperature dependences of the resonance field and the EPR linewidth is thebasis of the assumption of the vortex nature of magnetic excitations in this temperature range.     

Electronic structure of novel multiple-band superconductor SrPt<Subscript>2</Subscript>As<Subscript>2</Subscript>

The electronic structure of the recently discovered superconductor SrPt₂As₂ with T _c = 5.2 K has been calculated in the local-density approximation. Despite its chemical composition and crystal structure are somehow similar to FeAs-based high-temperature superconductors, the electronic structure of SrPt₂As₂ is very much different. The crystal structure is orthorhombic (or tetragonal if idealized) and has layered nature with alternating PtAs₄ and AsPt₄ tetrahedra slabs sandwiched with Sr ions. The Fermi level is crossed by Pt-5d states with rather strong admixture of As-4p states. Fermi surface of SrPt₂As₂ is essentially three-dimensional, with complicated sheets corresponding to multiple bands. We compare SrPt₂As₂ with 1111 and 122 representatives of FeAs-class of superconductors, as well as with isovalent (Ba,Sr)Ni₂As₂ superconductors. Brief discussion of superconductivity in SrPt₂As₂ is also presented.     

Investigation of the phase diagram and superconductivity of Hg−Pb−Bi−Ba<Subscript>2</Subscript>CaCu<Subscript>2</Subscript>O<Subscript>6+δ</Subscript>

Compounds of the series Hg_(1?x?y) Bi_xPb_yBa₂CaCu₂O_6+δ (0≤x,y≤0.5) are synthesized directly from the metal oxides without using a precursor. Structural, and morphological investigations are made by x-ray diffraction, SEM and EDXA techniques. Bi and Pb substitution suppresses or enhances the simultaneous formation of Hg-1201 and Hg-1223 phase. Multiphase Hg_0.8Bi_0.2Ba₂CaCu₂O_6+δ has a maximum T_c of 134K. For Bi and Pb concentrations higher than 0.5 mole, non-superconducting phases are formed.     

Mechanism of thermal decomposition of the nonstoichiometric compound YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>6.8</Subscript>

Thermal decomposition of the nonstoichiometric high-temperature superconductor YBa₂Cu₃O_6.8 at a temperature of 200°C in air has been investigated using the full-profile analysis of X-ray diffraction lines. Two mechanisms of decomposition are revealed. The first mechanism, i.e., separation into two phases with a different oxygen content, occurs continuously. The second mechanism, i.e., disordering of the heavy atoms Y, Ba, Ba, Y along the crystallographic axis c, begins to occur after a 20- to 35-h annealing and progresses with a further annealing.     

Microemulsion-mediated in-situ synthesis and magnetic characterization of polyaniline/Zn<Subscript>0.5</Subscript>Cu<Subscript>0.5</Subscript>Fe<Subscript>2</Subscript>O<Subscript>4</Subscript> nanocomposite

Polyaniline/Zn_0.5Cu_0.5Fe₂O₄ nanocomposite was synthesized by a simple, general and inexpensive in-situ polymerization method in w/o microemulsion. The effects of polyaniline coating on the magnetic properties of Zn_0.5Cu_0.5Fe₂O₄ nanoparticles were investigated. The structural, morphological and magnetic properties of as-prepared samples were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectra, scanning electron microscopy (SEM) and magnetic measurements. The morphology analysis confirmed that polyaniline was deposited on the porous surface of magnetic Zn_0.5Cu_0.5Fe₂O₄. It was shown that the saturation magnetization and coercivity of Zn_0.5Cu_0.5Fe₂O₄ decreased after polyaniline coating, which can be interpreted by the interparticle dipole–dipole interactions that contributed to magnetic anisotropy and changed the magnetic properties of the nanoparticles. PACS  74.25.Ha; 81.05.-t; 81.05.Lg     

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.     

On synthesis of BaFe<Subscript>12</Subscript>O<Subscript>19</Subscript>, SrFe<Subscript>12</Subscript>O<Subscript>19</Subscript>, and PbFe<Subscript>12</Subscript>O<Subscript>19</Subscript> hexagonal ferrite ceramics with multiferroid properties

We analyze the possibility of obtaining M-type hexagonal ferrites of barium, strontium, and lead with multiferroid properties with the help of ceramic technology. Using the modified ceramic technology (especially pure initial raw materials, admixture of B₂O₃, and sintering in the oxygen atmosphere), we obtained for the first time the BaFe₁₂O₁₉ and SrFe₁₂O₁₉ samples with intense multiferroid properties at room temperature. At the same time, the employed technology does not make it possible to obtain PbFe₁₂O₁₉ samples exhibiting ferroelectricity. The multiferroid characteristics of experimental samples are compared with the characteristics of classical high-temperature multiferroic BiFeO₃ and with the characteristics of BaFe₁₂O₁₉, SrFe₁₂O₁₉, and PbFe₁₂O₁₉ ferrite ceramics obtained in accordance with polymer precursor technology. We propose a mechanism explaining multiferroid properties of the hexagonal ferrite ceramic samples and note the importance of our results for applications.     

The dependence of critical temperature on oxygen concentration in YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>6+<Emphasis Type="Italic">x</Emphasis></Subscript> in terms of the fragmented chain model

An extended ASYNNNI model, that beside nearest-neighbour and next-nearest neighbour O-O interactions in the basal plane also includes interactions between the three nearest oxygen atoms, is used to describe the statistics of CuO chain fragmentation and to calculate doping and T _c in YBa₂Cu₃O_6+x . Calculations were made by the Monte Carlo method employing the recently proposed charge transfer model that assumes only chains whose length is equal to, or exceeds, a characteristic (critical) length, l _cr , can provide holes to the layers and contribute to doping p. The obtained p(x) is then combined with a universal T _c versus p relation to yield T _c (x) characteristics that correlate remarkably with those reported in recent experiments. The best coordination between theoretical and experimental T _c (x) characteristics has been achieved for l _cr = 2, implying that only isolated basal plane oxygen atoms (trivial chains) do not contribute holes to CuO₂ layers.