Tunneling measurements were applied to investigate the density of states of the high Tc superconductor in the temperature range from 4.2 K to the transition temperature. Several structures were found in conductance curves in higher bias region than ever reported, and they vanished around Tc. Temperature dependence of the bias voltages at which these structures appeared showed the BCS-like behavior.

Zn substitution for Cu in YBa2Cu3O7 rapidly reduces the superconducting transition temperature, Tc. Superconductivity is quenched between x=0.08 (T =30 K) and x=0.10. The normal state paramagnetism grows with Zn substu-tition, presumably due to increased localization on the Cu sublattice. Susceptibility studies of oxygen depleted (nonsuperconducting) Zn-substituted samples support this. Strong non-linear isothermal magnetization suggesting an internal magnetic field is found at T=4.2 K in samples with Zn concentration near to the critical value for suppression of superconductivity. The results are discussed in terms of increased localization of d-electrons on the Cu sites with increasing Zn concentration, which is consistent with recent EPR data.

The recent discovery of the YBa2Cu3O7-δ type high Tc superconductors stimulated worldwide R&D interest in this field. However, the relatively low critical current density (Jc) in the polycrystalline, bulk superconductors (as well as its significant deterioration in weak magnetic fields) has been a major roadblock to the rapid technical advancement toward applications. In this paper, we investigated the effect of processing and microstructural control on Jc of the superconductor. Improved Jc values of -3100 A/cm2 at 77K with somewhat reduced field dependence have been obtained through appropriate microstructural modifications.

Optical reflectance spectra in the range 30–35000 cm-1 and 4–300 K are presented on several YBa2CuxOy phases, as well as different forms of the superconducting YBa2CuxO7−x (polycrystalline pellets, textured pellets with ab-plane oriented surface, and single crystals).

Polarized Raman spectra have been measured on single crystals of the high Tc materials A2BCu3Ox with A=Ba and Sr, B=Y and La and 6<x<7. The dependence of the 500 cm-i peak position (αzz)on chemical composition is in agreement with its assignment to the O(1) axial stretching vibration. The intensity of the 600 cm1 band, which has been ascribed to a defect induced Cu-O mode, strongly depends on the crystal preparation and resulting purity.

Ag-doping was found to enhance the critical current density of the ceramic superconductors by as much as a factor of 15. This Jc enhancement is due to both the increased amount of the superconducting phase and the resulting preferred crystal orientation. The optimum level of dopant may vary according to the chemical forms of Ag dopants.

Magnetization measurement was carried out for Ba2Y1Cu3O7-y bulk and powder samples at 77K and 4.2K in order to evaluate intrinsic Jc. Excellent Jc values above 10 A/cm were obtained for powder samples at 77K and lOkOe. These Jc values were somewhat increased by substiution of Er for Y. High temperature sintering at 1030°C brings about drastic increase in magnetization which can be explained by the grain coarsening.

We have used electron energy loss spectroscopy (EELS) in a transmission electron microscope (TEM) to probe the L23 excitation of the 930 eV copper 2p to 3d transition in YBa2Cu3O7−x. Unlike UPS and XPS which are surface sensitive, EELS permits analysis of bulk properties from submicron regions of a single crystal. In addition, EELS probes the lowest unoccupied states and is particularly suited to the analysis of Cu valance states in YBa2Cu3O7−x We observe a splitting of the L33 peak of ∼2.7 eV, which we attribute to contributions from both Cu 2+ and Cu 3+. The intensity contribution Cu 3+ is maximized when a two beam condition is used to enhance channeling of the electrons between the {033} planes of the crystal near a [100] orientation. Upon prolonged exposure of the compound to the incident electron beam, the splitting disappears and the only contribution to the L3 white line is from divalent copper.

Calculations are reported of the perfect lattice and defect properties of the two phases of La2−x(ba,sr)xCuO4 [1] there has been intense interest, both experimental and theoretical, in materials of this type. To date, however, there appears to be no detailed account of the defect structure of La2CuO4 or La2−x(Ba,Sr)xCuO4, despite the evident importance of defect-related properties associated with the processing of these materials and the role that hole states, oxygen vacancies and impurities play in determining Tc. Accordingly, the present paper reports what are believed to be the first atomistic lattice calculations of the point defect properties of La2CuO4 related to high TC superconductivity.

Based on very recent experimental evidence about magnetic transitions in high-Tc superconductors the role of superconductivity and magnetism is analyzed. A model of interacting electrons in a one-dimensional system is constructed, taking into account that antiferromagnetic interactions in the underlying lattice are present. The origin of this antiferromagnetism could be due to local Y d-electrons or rare earth f-electrons, related to the structure. Self-consistent calculations in this model show the possibility of a new state, with a gap, in which there are electron pairs. The characteristics of this new state are discussed in relation to superconductivity. The results of this model suggest that an extension of it could be applied to the new superconducting ceramics.

Single phase orthorhombic YBa2(Cuy1−xZnx)3O7 samples were formed for 0<x<0.16. The high T superconductivityXfor x=6 (T =90 K) is rapidly depressed with increasing x, and is quenched for x>0.08. Low field (<100 G) cooled magnetization studies show that the superconducting component decreases as x approaches the critical value for suppression of superconductivity, and this is supported by high resolution specific heat measurements in the vicinity of T. Temperature dependent electrical resistivity studies for x<0.08 show metallic behavior; for x>0.10 semiconducting behavior. The electrical resistance was studied at high quasihydrostatic pressures also, and for x=0.08 showed that T is depressed with increasing pressure: T → 0 K for P >10 GPa. This is in°contrast to YBa2(Cuy1−xZnx)3O7 where dT /dP>0. The data support evidence for the high sensitivity to chemical and ice perturbations of the physical properties of samples near the superconducting-normal transition region.

We present here a study of the effect of Fe doping and hydration on the NMR, EPR spectra and Tc of slowly and rapidly cooled Y-1 Ba2Cu3O6+x Superconductor. The g-values of the EPR signals are related to Cu2+ ions in six-fold coordination. Tc is suppressed 10–20°C due to changes in processing conditions and the addition of Fe. The 89Y MAS-NMR spectra show that Y is more shielded (more ionically bonded) in the slowly cooled sample than in the rapidly cooled sample and the hydrated slowly cooled sample is less shielded. The effect of hydration on 89Y chemical shift is consistent with the presence of protons near the Y.

Infrared reflectivity results of YBa2Cu3O7x and SmBa2Cu3O7−x, in the tetragonal semiconducting phase, are reported. At least 10 of the 11 group theoretically allowed modes are observed, and the 11th may be observed in the Sm-material. Due to the fact that the modes are clearly resolved from each other, and, in some cases, have large shifts when Y is replaced by Sm, severial these modes can be assigned to definate vibrations. This and other aspects of the data are discussed.

In order to better understand the phonon spectra obtained from ceramic (La,Sr)2CuO4 superconductors, we have studied the infrared and Raman spectra of the isomorphic material Sr2TiO4 (as well as related materials). As a consequence, various aspects of the spectra of both materials are better understood, and this is discussed.

The grain boundary segregation of Sr in the La1.85Sr0.15CuO4-y system, and its effects on polycrystalline diamagnetic properties, have been characterized. Equilibration at 1000°C reduces grain boundary segregation compared to that which is present at 700°C. Results indicate that critical currents in randomly oriented polycrystalline materials will be improved if annealing treatments can be optimized to minimize grain boundary segregation without degrading lattice oxygen stoichiometry.

Using a non-empirical ionic model, we find harmonically unstable, double well potential surfaces for both La2CuO4 and YBa2Cu3O7 which may be related to the high temperature superconductivity. We have applied the Potential Induced Breathing (PIB) Model to La2CuO4 and YBa2Cu3O7 and find that the ionic description gives semiquantitative agreement with structural and vibrational observations. Furthermore, comparison with self-consistent linearized augmented plane wave [1] (LAPW) calculations shows that ionic contributions to the bonding and lattice dynamics of oxide superconductors are large. For La2CuO4 we find that an ionic description predicts the observed tetragonal to orthorhombic distortion, predicts a lower symmetry ground state, and predicts a stable oxygen breathing mode. We find harmonically unstable phonon branches that we relate to phase transitions. Unstable (anharmonic) modes will couple via ionic forces to the charge density in the metallic planes. This ionic coupling has not been included in previous lattice dynamics calculations. For YBa2Cu3O7 we find similar unstable phonon branches that involve oxygen motions perpendicular to the Cu-O bonds. We find the crystalline charge density is best approximated by overlapping ions if Cu and O ions of charge +1.6 and -1.8, respectively, are used.

Static magnetic susceptibilities have been measured, following the Faraday method, on a disc of home-made YBa(2)Cu(3)O(x). The superconducting ceramic disc was oriented perpendicular to the applied magnetic induction field < 10 mT. On cooling the sample in a constant field from above to below the critical temperature a Meissner-Ochsenfeld effect is observed. The interpretation of the data yields partial flux expulsion. That the flux is not completely expelled from the sample is understood on the basis of the flatness of the disc. Part of the flux which had not been expelled by the Meissner-Ochsenfeld effect remains trapped in the cooled superconducting material.

We report far infrared reflectance measurements of RBa2Cu3Ox, where R-Y, Er and Nd and 6<×<7 for YBa2Cu3Ox. Based on these measurements we assign the observed phonon features. In particular, one vibration is clearly identified as an R vibration, while the lowest energy mode (near 151 cm-1) is associated with the linear Cu-O chains.

We report ultra-low-field (5 mOe - 1 Oe) magnetic measurements on a ceramic sample of YBaCuO. A positive remanent moment is observed which accurately equals the difference of the field-cooled and zero-field-cooled moments throughout the temperature range. At higher fields this relationship breaks down. A reversible region is observed near the superconducting transition temperature which is independent of field. These results are discussed in the context of recent models.

By examining the reflectance spectra of polycrystalline La1.85Sr0.15CuO4-y and La2NiO4-y in relation to the results of ir studies on single-crystal La2NiO4-y, we have determined the normal-state (room temperature) optical anisotropy of La1.85Sr0.15CuO4-y in the frequency range 40 cm-1 to 4800 cm-1. Vibrational and electronic properties corresponding to this anisotropy are examined, and a method is presented for extracting the optical properties of the superconducting state of La1.85Sr0.15CuO4-y.