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We propose a framework for the theory of the "normal" metallic state of the CuO2 planes of high Tc superconductors. This state is closely analogous to the known state of the one-dimensional Hubbard model, with spin excitations which can be thought of as chargeless (Z = 0) Fermions occupying the interior of the conventional Fermi surface, and charged excitations which have zero energy near the spanning vectors 2kF of that Fermi surface. The electron spectrum is the composite spectrum of two of these excitations, and can be fitted to angle-resolved photoemission data. When we do so we can calculate or estimate many properties of the normal state in excellent agreement with experiment, and show that the pair susceptibility is anomalously large and temperature-dependent, explaining the high Tc and the specific heat behavior.
A brief review is given of some historical aspects of theoretical research on superconductivity including a discussion of BCS theory and some theoretical proposals for mechanisms which can cause superconductivity at high temperatures.
Strong coupling models for the electronic structure of La2CuO4 are derived in two successive stages of renormalization. First, a three-band Hubbard model is derived using a constrained density functional approach. Second, exact diagonalization studies of finite clusters within the three band Hubbard model are used to select and map the low energy spectra onto effective one-band Hamiltonians. At each stage, some observables are calculated and found to be in quantitative agreement with experiment. The final results suggest the following models to be adequate descriptions of the low energy scale dynamics: (1) a spin 1/2 Heisenberg model for the insulating case with nearest neighbor J≈130 meV; (2) a "t–t'–J" model with nearly identical parameters for the electron and hole doped cases.
A model of continuous two-dimensional melting in the mixed state of high temperature superconductors is proposed. Two-dimensional melting sets in at a cross-over temperature Tx(H) below the three-dimensinal phase transition Tx(H) due to finite size effects, and Tx(H) is a function of the sample thickness (lc), applied magnetic field (H), and k(= λ/ξ) For a given zero-field transition temperature Tc0 and material properties, (such as defect density), the onset temperature of 2D-melting (Tx(H)) decreases with decreasing sample thickness and increasing magnetic field. In transport studies, thermally induced melting is further complicated by the depinning effect of high current densities.
An approach to the theory of high‐Tc superconductivity is discused in which the semiconductor‐like layers between the copper‐oxide planes are more active participants in the superconductivity. Two types of states appear in the theory, a lattice of negative‐U sites and single particle (positive‐U) dispersive states. The negative‐U sites originate in oxygen vacancies and the single particle states in the copper‐oxide planes. The coupling between these two types of states lead to a two‐fluid superconductivity where the negative‐U sites give rise to a charged Bosonic fluid and the single particle states are a Cooper‐paired superconductor. The coupled Fermion‐Boson model for superconductivity is related both to theories of superconductivity in semiconductor‐metal eutectic alloys, and recent suggestions of s‐channel superconductivity of R. Freidberg and T. D. Lee. Basic aspects of general phenomenology are described. The breakdown of Fermi liquid theory in the normal state results from mixing of Fermions and Bosons at the Fermi energy. When the two‐particle states are filled, anti‐ferromagnetism arises from the positive‐U of the single particle states with a contribution from virtual excitation of two‐particle states. Prediction is made of a large class of superconducting materials based on metal‐semiconductor layering.
One potential objection to virtual “sandwich excitons” as a pairing mechanism is that the transition densities may not be large enough to overcome the inherent disadvantage of a large energy denominator . In model calculations, we find that some of the transition densities and matrix elements can be large even if the dielectric constant is sizeable for, e.g., LaO excitons in La2‐xSrxCuO4
Molecular ab initio seIf-consistent calculations on clusters simulating the
copper-oxygen layers in the Yba2Cu3O6;δ are reported. The electronic structure, of this layer, was computed for different sets of values of the lattice parameters (a,b,c), according to their dependence on the oxygen stiochiometry. For the molecular orbitals , two different electronic occupations are considered, a closed shell and an open shell. For the open shell, an electron has been excited to the first virtual molecular orbital. It is found that this excited state has lower energy than the closed shell configuration for 0 < δ < 1. Molecular energies an electronic population are reported.
The band structures of YBa2Cu307 superconductors in which one of the Cu atoms was replaced by Ga or Zn have been studied by the first‐principles orthogonalized LCAO method. Substitutions involving Cu on the chain site and Cu on the plane site were investigated. Results show significant modifications in the electronic structure due to the dopant‐specific and site‐specific substitutions. These results are discussed in light of some recent experimental observations.
The average structure of superconducting La2CuO4.032 has been determined by single crystal neutron diffraction data. The excess oxygen is located between two adjacent LaO layers. Its presence distorts the apical‐oxygen sublattice in such a way that a short O‐O bond is formed (1.64Å). By scanning several hkl reflexions, we have confirmed that a phase separation occurs below room temperature. The peaks of the phases are in agreement with the unit cells proposed by Jorgensen et al. and Zolliker et al. However, Cmca seems to be the correct space group for both phases. La2CuO4+მ and the Ni counterpart are, thus, not isostructural.
La2‐xRExCuO4 (RE = Nd‐Y) solid solution systems have been investigated to determine the factors stabilizing the T (La2CuO4), T’ (Nd2CuO4) and hybrid T*‐type structures. A simple ionic model with a perovskite‐like tolerance factor is found to accurately define the existence field of each. Metastable T* phases are observed for the larger RE cations Nd, Gd, Eu. The structure is quite stable for RE = Dy, Tb, but does not occur at all for the smallest rare earths. Oxygen activity plays a role in T’ and T* phase formation.
Annealing and quenching experiments in a thermogravimetric analysis apparatus are used to vary the oxygen content in La2Ni1‐yCuy04+δ, followed by magnetic susceptibility and x‐ray diffraction experiments. These studies show that for certain δ, transition to antiferromagnetism occurs with TN < 20 K. The effect of Cu doping (y = 0.01 and 0.05) on TN is negligible.
The perovskite related La2‐xMxCuO4‐y oxides substituted with alkaline earth metals were one of the first classes of high temperature superconductors discovered. Determining the thermodynamic properties is important to understand the stability and superconducting mechanism of these structures. High temperature solution calorimetry, using a molten lead borate solvent, has been performed on La2CuO4 and the related Sr substituted oxides. Calorimetric measurements on CuO, La2O3 and SrCO3 yield heats of formation. A change in the trend of the heats of formation appears at Sr content 0.1, the reported orthorhombic to tetragonal transition, the onset of superconductivity and loss of oxygen.
La2‐xSrxCuO4‐δ has been studied to determine the role of dopants and oxygen stoichiometry on the transport properties and defect chemistry of this system. The conductivity was found to reach a maximum for 0.2 < x < 0.3 which reflects a change from electronic to ionic compensation of the Sr dopant at large values of x. A metal to semiconductor transition is observed for x > 0.6 for temperatures below ∼ 600°C. However, the source of this behavior is not clear, given that an increasing amount of second phase was observed for x > 0.4. The conductivity and thermoelectric power (TEP) of La2CuO4 show a Po2⅙ dependence and a temperature independence, implying that the hole mobility is not thermally activated and the enthalpy of oxidation is nearly zero. This last conclusion is confirmed by TGA data.
In order to grow large single crystals of the La2‐xSrxCuOy and Bi2+xSr2‐zCuOy families of high Tc superconductors, we have studied the relationships between nominal Sr composition in the starting materials and the Sr compositions of the resulting products. We present growth, x‐ray, and composition data for these samples.
We report the synthesis of Cm2CuO4. The lattice constants of this material, determined by x‐ray diffraction, show it to be a new member of the isostructural series R2CuO4 (R=Pr, Nd, Sm, Eu, and Gd). Analysis of magnetic measurements is consistent with a free‐ion effective moment for Cm3+, with no contribution to the susceptibility from Cu‐ions.
We have investigated the Y‐Sr‐Cu‐O system which has been reported to form a K2NiF4‐type superconducting phase (TC∼40K) and a “123”‐type phase (Tc‐80K). Difficulties in preparing single phase materials by standard solid state reaction of carbonates and oxides have compelled us to explore other methods. A two‐stage solid state processing technique in addition to a coprecipitation method will be discussed along with the relative advantages and disadvantages of each. Using data obtained from XRD and EDS, we have mapped some of the YO1.5‐SrO‐CuO ternary phase diagram in anticipation of continued efforts at single crystal growth.
The influence of the ionic size of the lanthanides R on melting relations of Ba2RCu3O6+x, where R=Y, Eu and Nd, was studied and compared with that of a high Tc superconductor mixed‐lanthanide phase Ba2(Y.75Eu.125Nd 125)Cu3O6+xThese materials have been characterized by a variety of methods including differential thermogravimetric analysis (DTA), scanning electron microscopy (SEM) with energy dispersive X‐ray spectroscopy (EDX) and X‐ray powder diffraction. Single phase samples of Ba2(Y.75Eu.125Nd.125)Cu3O6+x were annealed at 1004, 1040, 1052, 1060, 1078, 1107 and 1160°C and quenched into a helium gas container cooled by liquid nitrogen. The SEM micrographs of these samples showed the progressive chnages in features of the microstructures from sintering and grain growth through melting and then recrystallization from the melt. The addition of the SEM technique in conjunction with X‐ray diffraction has been helpful in the study of phase equilibria in this system.
Using crystallographic data on HTSC's and a recently developed description of many HTSC's as intercalation compounds, we review briefly the structural evidence and develop some structural correlations involving the dimensions, coordination and connectivities of functional units.
Auger electron spectroscopy (AES) and core‐level x‐ray photoelectron spectroscopy (XPS) have been used to study the compositional and electronic‐state variations from the contaminated surface layer to the inner region of YBa2Cu3Ox and Bi2(Sr,Ca)n+1Cun02n+4 compounds. The results showed that the carbon‐rich contamination layer in BSCCO is thin and easier to be removed by Ar+ sputtering, indicating a much more stable surface than that of YBCO. This layer is oxygen deficient and contains higher Cu2+ satellites ( 2p3d9 final states) than in the bulk materials. Line‐shape analysis suggests three‐Gaussian features for both Cu 2p3/2 and O Is lines. The 529 eV signal is observed in both YBCO and BSCCO O Is spectra.
Considerations of the in‐plane Cu‐0 bond distance in a variety of cuprate‐based superconducting systems indicate: (1) that an optimum level of hole doping exists in these systems, and (2) that this optimum value is roughly the same for Sr‐ and Ba‐classes of superconductors when one normalizes for structural effects due to the difference in ionic size. Structural adjustment at the optimum level of doping leads to variations in Tc between structure‐types.