Crystal structure of a new superconducting high-pressure phase in the YBaCuO-System

A superconducting (T _c=40K) high-pressure phase recently discovered in the system of perovskite type YBaCuO structures is investigated by high-resolution transmission electron microscopy and electron diffraction in order to find its crystal structure. A structure model is proposed on the basis of a comparison between the observed images and image simulations based on crystal chemical considerations. The new phase has aB-centered orthorhombic cell with a monoclinic primitive cell. The primitive cell is composed of two subunits. The first of these is identical with the unit cell of orthorhombic YBa₂Cu₃O_7–x (1-2-3 structure) withT _c=92K. In contrast to the first subunit, the second one contains two adjacent Cu–O chains but is identical otherwise. This second subunit has been observed as one type of planar defect in the 1-2-3 structure. It is therefore concluded, that other stacking polytypes composed of these two different units could exist. The structure of the new phase is compared to the structures of the other known high-T _c superconductors. The chemical formula for the new phase can be written as Y₂Ba₄Cu₇O_14+x, with x0.5±0.2.     

Short-circuit current limiter utilizing a high-Tc superconductor

Experimental data are given on a prototype short-circuit current limiter utilizing a polycrystalline high-T _c superconductor of composition Y₁Ba₂Cu₃O_7−δ . The limiter comprises a seriesconnected dc circuit element immersed in liquid nitrogen. To improve the efficiency, a polycrystalline high-T _c superconductor having an S-shaped current-voltage characteristic is used as the current limiter. Zh. Tekh. Fiz. 68, 129–130 (October 1998)     

A model for the high-T c superconductivity based on electron hopping

A model is proposed based on electron hopping between octahedral Cu²⁺ and Cu³⁺ ions. Exchange interaction is assumed to be an electron-exciton one. Ground-state excitons are formed by freeze-in of the hopping process. The model is able to explain the main phenomena observed in the new high-T _c superconductors.     

The vortex-like excitations detected by Nernst signals in high-Tc superconductors

The nature of the pseudogap state and its relation to the d-wave superconductivity in high-T _c superconductors is still an open issue. The vortex-like excitations detected by the Nernst effect measurements exist in a certain temperature range above superconducting transition temperature T _c, which strongly support that the pseudogap phase is characterized by finite pairing amplitude with strong phase fluctuations and imply that the phase transition at T _c is driven by the loss of long-range phase coherence. We first briefly introduce the electronic phase diagram and pseudogap state of high-T _c superconductors, and then review the results of Nernst effect for different high-T _c superconductors. Related theoretical models are also discussed.     

On the dependence of the critical temperature on pressure in the bisoliton model of high temperature superconductors

Abstract

In the framework of the bisoliton model we have studied the critical temperature T _c, as a function of the pressure P and of the hole concentration δ for the high temperature superconductors YBa₂Cu₃O_x and (La_1-xM_x)₂CuO₄. Our results for δ ln T_c/δ ln V as a function of T _c describe quite satisfactorily the general trend of the experimental data. Furthermore we show that in the bisoliton model the energy gap δ (in units of Jg³/3, where J is the nearest-neighbour exchange integral and g is the nonlinearity parameter) is an universal function of δ/g. An analogous property is valid for T _c By fitting the maximum value of T _c we are able also to reproduce the experimental data for T _c(δ).     

Polar Jahn-Teller centers and the anomalous isotope effect in copper-oxygen high-T c superconductors

We examine the isotope effect in copper-oxygen high-T _c superconductors using the model of polar Jahn-Teller centers, a variant of the local-boson model. The isotope-effect exponents for oxygen and copper depend on the structure of Jahn-Teller centers and its variation due to boson motion. A number of YBa₂Cu₃O_7−δ -based compounds are used to illustrate the feasibility of quantitatively describing the experimentally observed values of α _Cu and α _O and their relationship to T _c. Zh. éksp. Teor. Fiz. 111, 644–653 (February 1997)     

The transition width and critical current density measurements on slow-cooled YBa2Cu3O7−x superconductor

It is shown that by adopting a very gradual programmed cooling procedure in oxygen environment one can obtain a sharp transition (ΔT _c⋍1 K) in YBa₂Cu₃O_7−x while retaining the high-T _c value (⋍105 K) of samples prepared by a semi-wet route. This is attributed to a maximum occupancy of oxygen at 0 1/2 0 sites and a near-perfect ordering of vacancies at 1/2 0 0 sites in the orthorhombic unit cell which maximizes the availability of conduction paths in the form of continuous CuO₄ chains. Critical current densities (J _c) of 204 A cm⁻² are obtained for bulk samples at 77 K. It is suggested that the intergrain coupling is weak and thus limits the transportJ _c-values.     

Analysis of thermoelectric power of some insulating high-T c superconductors using extended hubbard model

The thermoelectric power (TEP) for a one-dimensional lattice has been studied using the extended Hubbard model in the limitU ≠ ∞, whereU is the on-site Coulomb interaction. A new expression for TEP has been derived in this study. Our study shows that if theV-dependent term in the extended Hubbard model Hamiltonian is considered as attractive, the new expression for TEP could successfully reproduce the TEP results of high-T _c hole-doped insulating systems of Tl₂Ba₂Ca_1−x Y _x Cu₂O_8+y (Tl-2212) and Bi₂Sr₂Ca_−x Y _x Cu₂O_8+y (Bi-2212).     

Correlations betweenT c andn s/m * (carrier density/ effective mass) in high-T c and organic superconductors

We update our muon spin relaxation studies of the magnetic field penetration depth which show the correlations betweenT _c and the relaxation rate σ αn _s/m ^* (carrier density/effective mass) of hole-doped high-T _c cuprate superconductors (La₂, Sr₃)CuO₄, YBa₂Cu₃O₇ (Y_1−xPr_x)Ba₂Cu₃O₇, and other double and triple layer systems. These studies are extended to the organic superconductor (BEDT-TTF)₂Cu(NCS)₂.     

Stoichiometric relations in high-temperature superconducting compounds

Applying the Jing and Gonser model for the high-T _c superconductivity based on electron hopping the Stoichiometric connections and appropriate restrictions are given. It is shown that for the compounds of typeA _2–x B _xCuO₄ andAB ₂Cu₃O_9–x, whereA is a trivalent rare-earth ion andB is a divalent alkali earth ion, the superconductivity is realized only if 0T _c superconducting compounds are propounded and considered, too. The optimum compound which contains fluorine atoms should be of the form YBa₂Cu₃F₂O_5.875. All the relations presented find confirmation in experiments.     

Numerical simulations of a high-T c -superconductive glass model

Numerical simulations for anXY-spin-glass model are presented. The connection to recent experiments on high-T _c superconductors is outlined. In particular, the glass behaviour in experiments is reproduced by a 2-D model. A phase diagram important for future experiments is also provided. A substantial gain inT _c is predicted for enhanced couplings between planes.     

Ground state of spin liquids,hole-hole attraction and high-T csuperconductivity

We develop a notion of high-T _csuperconductivity, which considers that phenomenon as a general property of close to half-filled bands of strongly correlated electrons confined to low-dimensional orbital lattices. Based on Andersons suggestion [1] we initially investigate the Cud electron system of the undoped mother substances of the high-T _c superconductors in the framework of the spin 1/2 Heisenberg model. We derive a new representation of the corresponding Hamiltonian in terms of triplet quasi-particles. The triplet representation leads to simple physical pictures for the spin liquid state and to a hierarchy of nearly exact variational wave functions for the ground state of the linear chain. These wave functions are employed as vacuum states for doping with holes. Holes are shown to form a conduction band embedded in the Hubbard-Mott insulating phase of strongly correlated electrons. The chemical binding forces among these localized electrons entail a strong attractive pair potential acting between the mobile defects. The generality of the hole-hole attraction and its independence of a strict localization of the electrons in the Hubbard-Mott phase is demonstrated by nearly exact solutions of the Hubbard and Pariser-Parr-Pople models of small regular electron chains at various degrees of electron correlation. It is suggested that this exchange-driven attraction leads to an instability of the free hole gas towards Cooper pair formation.     

Electronic structure of high-T c superconductors and related compounds

We study a 5-band Hubbard model for the CuO₂ planes in cuprate super-conductors using Hartree-Fock mean-field theory including spiral spin density waves. For the half-filled case we recover a ZSA-like phase diagrambut with an additional new region characterized by strong covalency effects, which we call a covalent insulator region. We also provide a nonperturbative calculation ofJ _eff, the effective in-plane antiferromagnetic interaction, as a function of parameters of the model. We suggest that the high-T _c cuprates are in or very close to the covalent insulator region and within this we show that a consistent explanation of apparently conflicting high energy spectroscopic and magnetic measurements of the high-T _c cuprates can be given.     

Nb/BiSCCO point conctact junctions

Summary Preparation procedure and sample characterization of 112 BiSCCO pellets exhibiting high-T _c superconductivity are discussed. TypicalI–V and dV/dI curves of Nb/BiSCCO point conctact junctions at various temperatures are presented.     

Phase transformation and microtwinning in crystals of the high-T c superconductor YBa2Cu3O8−x,x≅1.0

We employed transmission electron microscopy (TEM) and single crystal X-ray diffraction techniques, supplemented by differential thermal and thermo-gravimetric analysis (DTA, TG) and high temperature powder X-ray diffraction, to study microtwinning in orthorhombic crystals of the high-T _c superconductor YBa₂Cu₃O_8–x (x1.0). This twinning is associated with a structural phase transition at 750°C from a tetragonal high temperature phase (s.g.P4/mmm) to the orthorhombic ambient temperature phase (s.g.Pmmm) and seems to be inherent to virtually all orthorhombic crystals of YBa₂Cu₃O_8–x . The domain size ranges from typically 100 Å to 1000 Å. All our observations are compatible with a twin law where the tetragonal (110)-mirror plane is the twin element.     

High-T c superconducting Bi−Sr−Ca−Cu−O thin films prepared by laser-induced plasma deposition

High-T _c superconducting thin films of Bi–Sr–Ca–Cu oxides were prepared by laser-induced plasma deposition of high-T _c superconducting Bi₂Sr₂Ca₁Cu₂O_x and Bi₂Sr₂Ca₂Cu₃O_x targets in vacuum and a short post-annealing in air at 875°C. Thin films (thickness <500 nm) with a critical temperatureT _c -onset of 95 K can be prepared on silicon substrate material with a SrTiO₃ interface layer. The thin films were completely superconducting between 80 and 90 K. The stoichiometry transfer of superconducting target material by laser-induced plasma deposition was investigated.     

Specific heat anomaly in Bi1.6Pb0.4Sr2Ca2Cu3Oy superconductor

The specific heat anomaly ΔC of Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O_y was observed in the temperature range 80–100 K. It is estimated from ΔC that γ15 mJ/mol·K² and H_{c2(T=0)100 T} in this sample. The specific heat anomaly confirmed the occurrence of bulk superconductivity of the high-T_c phase in Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O_y superconductor.     

Interlayer pairing in high temperature superconductors: effect of nonmagnetic impurities

The influence of impurity scattering is studied theoretically in a two-layer model for the high-T _c superconductor Y₁Ba₂Cu₃O_7– with intra- and inter-layer pairing. Two types of impurities are considered: (I) impurities which conserve the reflection symmetry of the two layers and (II) impurities which break it. Impurities of type (I) have no influence on the critical temperature. Type (II) impurities have strong influence onT _c as well as onH _c2 if there is a pairing interaction between carriers of different layers. The treatment of type (II) impurities is generalized to a periodic layer model appropriate for La_2–x Ba _x CuO₄. Available experiments on impurities in Y₁Ba₂Cu₃O_7– and La_2–x Ba _x CuO₄ are interpreted with our theory.     

Crystal structures of high-T c oxides

A comprehensive review of structure work on high-T _c oxides as reported during the years 1987 and 1988 is given. Thirteen structures are refined from X-ray single-crystal and/or neutron powder diffraction data:I. (Ba_1–x ,K _x )BiO₃ (T _c =30 K),II. (La_2–x , Sr _x )CuO₄ (T _c =40 K),III. (Nd, Ce, Sr)₂CuO₄ (T _c =28 K),IV. (Nd_2–x , Ce _x )CuO₄ (T _c =24 K),V. YBa₂Cu₃O₇ (T _c =90 K),VI. YBa₂Cu₄O₈ (T _c =80 K),VII. Y₂Ba₄Cu₇O₁₄ (T _c =40 K),VIII. Pb₂Sr₂NdCu₃O₈ (T _c =70 K),IX. TlBa₂CaCu₂O₇ (T _c =103 K),X. TlBa₂Ca₂Cu₃O₉ (T _c =120 K),XI. Tl₂Ba₂CuO₆ (T _c =90 K),XII. Tl₂Ba₂CaCu₂O₈ (T _c =112 K),XIII. Tl₂Ba₂Ca₂Cu₃O₁₀ (T _c =125 K). Except forI (perovskite type),II (K₂NiF₄ type) andIV (Nd₂CuO₄ type) they represent new structure types. Structure data, bond distances, structure drawings and calculated X-ray powder diffraction patterns are given for each compound. Structural features and correlations with superconductivity are discussed. The review contains 301 citations.     

Possible transformation of YBa2Cu3O7 under pressure

Crystal structure measurements on the high-T _c material YBa₂Cu₃O₇ were carried out under pressure. Several X-ray reflections do not reveal a uniform shift with increasing pressure. Two models are presented, which show the experimental data to be consistent with an extensivec-axis orb-axis decrease, respectively.