2PO5-30 The Investigation of Phase Evolution in Composite Ceramic Superconductors using Raman Microscopy Techniques*

V.A. Maroni 1, A.K. Fischer 1, and K.T. Wu 2. 1 Argonne National Laboratory, Argonne, IL 60439, USA. 2 State University of New York, Old Westbury Campus, Old Westbury, NY 11568, USA.

Presenting Author: V.A. Maroni

Raman microspectroscopy and imaging Raman microscopy offer unique opportunities for studying the evolution and spatial distribution of chemical phases in the bismuth-based (BSCCO) and thallium-based (TBCCO) families of high-critical-temperature (high-T_c) superconducting ceramics. These techniques have been applied to compressed/sintered powders and silver-clad composite conductors in conjunction with powder x-ray diffraction, scanning electron microscopy, and energy dispersive x-ray spectroscopy. Many seminal insights have been gained about the identity, size, shape, orientation, and spatial distribution of the various nonsuperconducting secondary phases (NSPs) that form and dissipate during heat treatment of the BSCCO and TBCCO silver-clad composite tapes. The results have permitted us to elucidate key mechanistic features that influence the formation of the layered superconducting phases as heat treatment progresses, including the location of lead-rich NSPs and the identification of the constituent phases in certain NSP agglomerations that tend to resist dissipation as high-T_c phase formation proceeds to completion.

*Work sponsored by the U.S. Department of Energy, Energy Efficiency and Renewable Energy, as part of a DOE program to develop electric power technology, under Contract W-31-109-ENG-38.

2PO5-31 First-Principles Raman Studies on YBa₂Cu₃O₇

Heinrich Auer, Robert Kouba, and Claudia Ambrosch-Draxl. Institute of Theoretical Physics, University Graz, Universitätsplatz 5, 8010 Graz, Austria

Presenting Author: H. Auer

We have investigated all Raman-active phonon modes of YBa₂Cu₃O₇ by first-principles calculations based on density functional theory. We have obtained frequencies and eigenvectors of the A_1g, B_2g, and B_3g modes by carrying out total-energy and atomic-force calculations using the full-potential LAPW method. We demonstrate that an overall excellent agreement between theoretical and experimental phonon frequencies is only achievable on basis of a fully optimized crystal structure. The absolute Raman scattering intensities for the A_1g modes are determined and their resonance behavior is investigated. With respect to previously published data, the atomic displacements are less coupled. In particular, the apical oxygen contribution to the eigenvector of the 340 cm^-1 mode is very small. Consequently, the discrepancy between theory and experiment in the resonance profile for this mode [1] is lifted.

[1] T. Heyen et. al, Phys. Rev.Lett. 65, 3048 (1990).

2PO5-32 Theory of Isotope Effects in Raman Spectra of YBa₂Cu₃O₇

Heinrich Auer, Claudia Ambrosch-Draxl, Eugene Ya. Sherman, and Robert Kouba , Institute of Theoretical Physics, University Graz, Universitätsplatz 5, 8010 Graz, Austria

Presenting Author: H. Auer

On the basis of first-principles calculations of phonon spectra of YBa₂Cu₃O₇ we investigated the influence of the isotope content on frequencies, eigenvectors, and Raman intensities of Raman-active phonon modes. First, we considered isotopically pure crystals: YBa₂⁶⁵⁽⁶³⁾Cu₃O₇ and Y^138(134)Ba₂Cu₃O₇. Second, site-selective isotope substitutions ¹⁶O ¹⁸O in the CuO₂ plane, and the apical oxygen site were investigated. The strongest influence on the calculated Raman spectra was obtained for the oxygen substitution. The eigenvector and Raman intensity of every oxygen-related vibration show large changes under both types of oxygen substitution. This behavior cannot be observed under the complete substitution at all the oxygen sites. The intensity effects arise due to the strong influence of the site-selective substitution on the mixing of the vibrational modes [1]. Several predictions for experimental observations are discussed.

[1] O.V. Misochko et. al, Phys. Reports 194, 387 (1990).

2PO5-33 A Study of c-axis Electronic Raman Scattering in YBa₂Cu₄O₈

J.W. Quilty 1, S. Adachi 1, S. Tajima 1, and A. Yamanaka 2. 1 Superconductivity Research Laboratory, International Superconductivity Technology Center, 1-10-13 Shinonome, Koto-ku, Tokyo 135-0062, Japan. 2 Research Institute for Electronic Science, Hokkaido University, Sapporo 060, Japan.

Presenting Author: J.W. Quilty

Previous reports of electronic Raman scattering in the Y124 system show a strong temperature dependence of the normal-state a-b plane Raman continuum which is well accounted for by the Bose-Einstein thermal factor - behaviour which differs from the nearly temperature independent normal state continuum seen in other high-T_c cuprate superconductors. We have performed electronic Raman scattering measurements on single crystals of YBa₂Cu₄O₈ with the incident light polarised both parallel and perpendicular to the c-axis. In the normal state the characteristic broad featureless continuum is observed in both polarization conditions, the c-axis and a-b plane continua showing comparable intensities. The temperature dependence of the normal-state continuum, however, differs between the in-plane and out-of-plane polarizations and we compare our results to theoretical predictions of the c-axis Raman continuum and Raman measurements of other high-T_c cuprates. Superconducting state spectra for in-plane and out-of-plane photon polarizations show no strong renormalization of the electronic continuum below T_c, in agreement with prior measurements on underdoped cuprate superconductors. This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) as collaborative research and development of fundamental technologies for superconductivity applications.

2PO5-34 Raman scattering in YBa₂Cu₄O₈ and PrBa₂Cu₄O₈ - indications of pseudogap effects in non-superconducting PrBa₂Cu₄O₈

J. Backstrom 1, M. Rubhausen 1, M. Kall 1, L. Borjesson 1, A.P. Litvinchuk 2, M. Kakihana 3, M. Osada 4, and B. Dabrowski 5. 1 Applied Physics, Chalmers University of Technology, S-412 96 Göteborg, Sweden. 2 Texas Center for Superconductivity, University of Houston, Houston, TX 77204-5932, USA. 3 Materials and Structures Laboratory, Tokyo Institute of Technology, Nagatsuta 4259, Midori-ku, Yokohama 226-8503, Japan. 4 Division of Surface Characterization, The Institute of Physical and Chemical Research, 2-1 Hirosawa, Wako-shi, Saitama 351-0198, Japan. 5 Argonne National Laboratory, 9700 S. Cass. Avenue, Argonne, IL 60439, USA.

Presenting Author: J. Bäckström

We have performed temperature dependent inelastic light scattering experiments on a superconducting single crystal of YBa₂Cu₄O₈ (Y-124) and a polycrystalline sample of non-superconducting PrBa₂Cu₄O₈ (Pr-124). At temperatures below 150 K in Y-124, a large loss of spectral weight in the electronic continuum scattering is observed. We attribute this to an opening of a pseudogap, as earlier studies also indicates a pseudogap opening at this temperature. The Ba phonon mode around 100 cm-1 shows an anomalous decrease in the linewidth that, in agreement with expectations, correlates well with the supression of electronic scattering intensity. A similar anomaly in the Ba phonon line profile at the same temperature is observed for Pr-124. We argue that this indicates that pseudogap effects can take place without a following transition to a superconducting state at lower temperatures.

2PO5-35 Raman scattering on parasitic phases related to the mercurates compounds

A. Sacuto 1, D. Colson 2, J. Cayssol 1, and P. Monod 1. 1 Laboratoire de Physique de la Matière Condensée, Ecole Normale Supérieure, 24 rue Lhomond, 75231 Paris cedex 05 France. 2 Service de Physique de l'Etat Condensé, DRECAM, CEA, Saclay, 91191 Gif sur Yvette, France.

Presenting Author: A. Sacuto

Raman measurements on parasitic phases related to the mercurates compounds have been performed. We carried out measurements under the same experimental conditions than the ones undertaken in order to obtain the electronic Raman spectra of the Hg-1223 and Hg-1201 single crystals. [1] Among various phases, the phonons of the BaCuO₂ and HgO phases are detected on the Raman spectra. The BaO₂ and CaO phases provide a frequency linear background which is able to compete with the electronic Raman spectra of the superconducting state (even with minute quantities) and drastically modifies the experimental data.

[1] A. Sacuto et al. abstract in this proceeding.

2PO5-36 Raman scattering study of Nd_1.85Ce_0.15Cu_1+d O_y with controlled oxygen content

A. Fainstein 1,2, A. Serquis 2, R.G. Pregliasco 2, and A. Caneiro 1. 1 Centro Atómico Bariloche, CNEA, 8400, Argentina. 2 CONICET, Argentina.

Presenting Author: A. Fainstein

The superconducting properties of the electron doped materials of general formula Ln_2-x(Ce,Th)_xCuO_y (Ln = Nd, Sm, Pr and Eu) are extremely sensitive to small variations of the "x" and "y" contents. Superconducting samples are obtained after an additional heat treatment "reduction step" at high temperatures (850< T < 1150°C) under slightly reduced atmospheres (10^-6 < pO₂ < 10^-3 atm.) Recently, it has been reported that superconductivity only appears in samples quenched under oxygen pressures below a critical value of pO₂ (Physica C, 313 (1999) 271).

We present a Raman scattering investigation of Nd_1.85Ce_0.15Cu_1+d O_y samples prepared under controlled oxygen content above and below this critical oxygen partial pressure. The Raman scattering spectra show that the B1g Raman active O(2) mode, which is single peaked for superconducting samples, splits into two lines for the non-superconducting compounds.

This result suggests that a very subtle modification of the crystalline structure separates superconducting from non-superconducting samples.

2PO5-37 Raman scattering from superconducting hybrid ruthenate-cuprate compounds

V.G. Hadjiev 1, C. Bernhard 1, C.T. Lin 1, T. Ruf 1, M. Cardona 1, and J.L. Tallon 2. 1 Max-Planck für Festkörperforschung, Heisnbergstr. 1, D-70569 Stuttgart, Germany. 2 Industrial Research Ltd., P.O. Box 13130, Lower Hutt, New Zealand.

Presenting Author: V.G. Hadjiev

We report on Raman scattering from magnetic and coupled magnetic-phonon excitations in RSr₂RuCu₂O₈ (R=Gd, Eu) and Gd_1.4Ce_0.6Sr₂RuCu₂O_10-x in (up to 14 T) and without applied magnetic field. These compounds are superconducting ferromagnets with Curie temperature (Tm) in the range of 90-140 K, and superconducting transitions at T_c=40-45 K. The low temperature spectra of RSr₂RuCu₂O₈ (Tm~140 K) reveal strong Raman scattering from magnetic excitations peaked at 150 cm^-1 that persists in the superconducting state. Similar excitations, with lower energy, have also been observed in Gd_1.4Ce_0.6Sr₂RuCu₂O_10-x (Tm~90 K). The frequency of the phonons involving vibrations of the CuO₂ planes shows a definite, though weak, anomaly around Tm. An oxygen vibration mode, forbidden in the paramagnetic phase, with frequency close to 260 cm^-1, becomes Raman active below Tm apparently due to the magnetic superstructure. It shows strong coupling to the magnetic excitations resulting in a hardening and an enhancement of the Raman intensity with decreasing temperature. We discuss the possible origin of the intensity of the 150 cm^-1 excitation, unusually strong for ferromagnets, and the phonon anomalies. The discussion also includes the effect of external magnetic field on the magnetic excitations.

*2PO5-38 Phonons and Spin-Phonon Interaction in RuSr₂GdCu₂O₈

M.N. Iliev, A.P. Litvinchuk, R.L. Meng, M. Dezaneti, and C.W. Chu, Texas Center for Superconductivity, University of Houston, Houston, TX 77204-5932, USA

Presenting Author: M.N. Iliev

The Raman spectra of polycrystalline samples of Ru₂Sr₂GdCu₂O₈, annealed under high pressure (3 Gpa, 1000° C), were studied between 10 and 420 K. The ferromagnetic ordering has a little (if any) effect on the variations with temperature of the position and width of the Raman lines at 158, 318, 440 and 659 cm^-1. In contrast, for the line near 270 cm^-1 one observes a local maximum of the halfwidth near the magnetic transition and strong hardening upon cooling below T_c. The observation of the latter line, which has no analog in the YBCO-type compounds, supports I4/mcm crystal structure, where, due to the tilting of the RuO₆ octahedra, the vibrations of the oxygen atoms in the Ru-O planes become Raman active. The 270 cm^-1 line is tentatively assigned to the A_1g mode involving rotations of RuO₆ octahedra around the c-axis. The effect of spin-phonon interaction is discussed in close comparison with the Raman spectra of ferromagnetic SrRuO₃, where the corresponding A1g mode exhibits similar variations near T_c.

2PO5-39 Raman Monitoring of Dynamical Jahn-Teller Distortions in Rhombohedral Antiferromagnetic LaMnO₃ and Ferromagnetic Magnetoresistive La_0.93Mn_0.98O₃

M.N. Iliev 1, A.P. Litvinchuk 1, M.V. Abrashev 2, V.G. Ivanov 2, W.H. McCarrol 3, and R.L. Meng 1, and C.W. Chu 1. 1 Texas Center for Superconductivity, University of Houston, Houston TX 77204-5932. 2 Faculty of Physics, University of Sofia, 1164 Sofia, Bulgaria. 3 Department of Chemistry and Biochemistry, Rider University, Lowrenceville, NJ 08648-3099.

Presenting Author: M.N. Iliev

The dynamical Jahn-Teller lattice distortions in rhombohedral LaMnO₃ result in appearance of characteristic bands in the Raman specta. The intensity of these bands were used as a measure of the distortion amplitudes and their variations through the antiferromagnetic transition in stoichiometric LaMnO₃ (T_N=160K) and concomitant ferromagnetic and insulator-metal transitionsin cation-deficient magnetoresistive La_0.93Mn_0.98O₃ (T_c » 200 K).

Experimental results show that the antiferromagnetic ordering in LaMnO₃ has a little effect on the Jahn-Teller distrotions and their amplitude remains virtually unchanged through the temperature range between 10 and 300 K. In contrast, the Jahn-Teller-induced bands in the Raman spectra of magnetoresistive La_0.93Mn_0.98O₃, and hence the corresponding distortions of the MnO₆ octahedra, increase upon cooling in the paramagnetic region, pass over a maximum near T_c and then strongly decrease or disappear in the ferromagnetic metallic phase. The anomaly in the width of the Jahn-Teller bands near T_c is also discussed.