Raman spectroscopy was used to examine the structure of barium titanium oxide thin films grown by metal-organic chemical vapor deposition (MOCVD) and laser-assisted deposition. The spectra were compared with the spectra of a ceramic specimen and a single crystal. Raman peaks specific to the tetragonal ferroelectric phase of BaTiO3 were seen in the spectra of several films. Other Raman peaks were ascribed to impurity (non-BaTiO3) phases in the films or to the substrates (fused quartz, MgO). Some of the Raman peaks showed a strong polarization dependence. The MOCVD films were also characterized by x-ray diffraction, energy-dispersive x-ray spectroscopy, and transmission electron microscopy. The film-to-film variation of the strength of BaTiO3 features in the Raman spectrum, relative to impurity-phase features, was qualitatively consistent with the x-ray diffraction and electron microscopy results. Spatially resolved Raman measurements showed that the structure of the laser-deposited film varies significantly over the deposited area. The temperature dependencies of the Raman spectra of two MOCVD films were examined in the 25–175 °C range. Raman peaks due to the tetragonal phase of BaTiO3 were observed at temperatures well above the Curie temperature of bulk single-crystal BaTiO3 (132 °C). This observation suggests that the tetragonal ferroelectric phase is stabilized by an anisotropic film-substrate interaction that gives rise to a two-dimensional stress in the plane of the film.

The resistivity of single-crystal YBa2Cu307-jc has been found to be anisotropic, with magnitude and temperature dependence similar to ceramic samples in directions parallel to the Cu-O planes, and with a 30 times larger room-temperature value and a much smaller temperature dependence in the orthogonal direction. The Hall coefficient, with a magnetic field applied parallel to the Cu-O planes, is negative (electronlike) and essentially temperature independent in these crystals, in direct contrast to the behavior of other types of samples.PACS numbers: 74.70.YaThe report of superconductivity at unprecedented high temperatures in La2x Ba JC Cu04 by Bednorz and Miiller 1 led to the discovery of superconductivity at temperatures above 90 K in YE^C^C^-* and numerous related compounds. 2 The layered perovskite structure 3 of these materials indicates that they should be quite anisotropic. Transport measurements on single crystals are thus an important step towards gaining an understanding of these materials. Anisotropic magnetic properties (critical fields, critical current density) have been reported in single crystals of both La 2x Ba x Cu04 and YBa 2 -Cu 3 07x -type materials, 4 " 7 as has an anisotropy in resistivity in La2x Sr x Cu04 single crystals 8 and in oriented films 9 of YBa2Cu 3 07-Jc . In addition to being anisotropic, these new superconductors represent a state of matter significantly different from those which are presently understood. A fundamental question is whether or not the normal state from which the superconducting one condenses is a Fermi liquid, as is believed to be the case with all other known superconductors. One signature of the Fermi-liquid state is a quadratic dependence of electrical resistivity on temperature below the range in which phonon scattering dominates. The linear temperature dependence of the resistivity characteristic of these materials in ceramic form 3 appears, in this context, to be rather mysterious, implying a breakdown of the Fermi-liquid description. For example, Lee and Read 10 infer both nonphonon and d-wave pairing from this linear dependence.In this paper we report measurements of the anisotropic resistivity and Hall coefficient in bulk singlecrystal YBa2Cu 3 0 7 -Jc . We find that the resistivity in the a and b directions (i.e., parallel to the Cu-O planes) is approximately 450 fid cm at room temperature, decreasing linearly with temperature above the superconducting transition with a slope of 1.3 //O cm/K. This is similar to the resistivity behavior of ceramic samples, for which slopes of 1.7-2.5 fiCt cm/K are typical. 3 Thus we can state that the relatively large magnitude and the linear temperature dependence of resistivity are bulk properties and not artifacts of the granular nature of the ceramic materials. In the c direction (i.e., orthogonal to the Cu-O planes), we find a room-temperature resistivity 30 times larger than in the in-plane direction, with only a weak temperature dependence, confirming the highly anisotropic nature of the material. The Hall coef...

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Crystals of orthorhombic, superconductingYBa2Cu307have been prepared. They are twinned in the nearly tetragonal ab plane. The resistivity and Hall effect have been measured in the normal state in the ab plane, the highly conducting plane of the Cu-0 layers. We find that the resistivity in the ab plane, measured with currents exclusively in the plane, is strictly linear in temperature from room temperature down to the fluctuation regime near the superconducting transition. This result confirms previous less direct measurements in which the currents flowed in the much more resistive c direction as well. We find the Hall constant for the magnetic field parallel to c and the currents in the ab plane, to be p type and inversely proportional to temperature. This latter behavior is extremely unusual and unexplained, but similar to that found in polycrystalline YBa2Cu307-, material. It is in contrast to our previous result for the magnetic field in the ab plane. In that case, the Hall constant is n type and nearly temperature independent. Both the Hall effect and the resistivity demonstrate the extremely anisotropic nature of this system.