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Pseudogap and Incommensurate Magnetic Fluctuations in YBa{sub 2}Cu{sub 3}O{sub 6.6}

Description: Unpolarized inelastic neutron scattering is used to study the temperature and wave vector dependence of the dynamical magnetic susceptibility, {xi}`` (q,{omega}), of a well characterized single crystal YBa{sub 2}Cu{sub 3}O{sub 6.6} (T{sub c} = 62.7 K). We find that a pseudogap opens in the spin fluctuation spectrum at temperatures well above T{sub c}. We speculate that the appearance of the low frequency incommensurate fluctuations is associated with the opening of the pseudogap. To within the error of the measurements, a gap in the spin fluctuation spectrum is found in the superconducting state.
Date: July 24, 1997
Creator: Dai, P., Mook, H.A.,
Partner: UNT Libraries Government Documents Department

Filled skutterudite antimonides: Validation of the electron-crystal phonon-glass approach to new thermoelectric materials

Description: After a brief review of the transport and thermoelectric properties of filled skutterudite antimonides, we present resonant ultrasound, specific heat, and inelastic neutron scattering results that establish the existence of two low-energy vibrational modes in the filled skutterudite LaFe{sub 3}CoSb{sub 12}. It is likely that at least one of these modes represents the localized, incoherent vibrations of the La ion in an oversized atomic {open_quotes}cage.{close_quotes} These results support the usefulness of weakly bound, {open_quotes}rattling{close_quotes} ions for the improvement of thermoelectric performance.
Date: April 1, 1997
Creator: Mandrus, D.; Dai, P. & Keppens, V.
Partner: UNT Libraries Government Documents Department

Neutron scattering measurements on YBa{sub 2}Cu{sub 3}O{sub 7-{delta}}

Description: Polarized and unpolarized neutrons were used to study excitations in YBa{sub 2}Cu{sub 3}O{sub 7-d}. The magnetic excitations are found to be centered at ({pi},{pi}) in the reciprocal lattice and are sharply peaked in energy at 41 MeV in the superconducting state and broad in energy in the normal state. The intensity is found to result from coupled bilayers in both the normal and superconducting state. A new set of incommensurate one-dimensional phonon excitations was discovered in scans that integrate over energy along planes in reciprocal space. These are found to be very sharply defined in momentum space and are thought to stem from 2k{sub F} effects for the Cu-O chains.
Date: May 1996
Creator: Mook, H. A.; Dai, P.; Mostoller, M. E.; Dogan, F.; Salama, K. & Aeppli, G.
Partner: UNT Libraries Government Documents Department

Neutron scattering studies of the magnetic fluctuations in YBa{sub 2}Cu{sub 3}O{sub 7-{delta}}

Description: Neutron scattering measurements have been made on the spin fluctuations in YBa{sub 2}Cu{sub 3}O{sub 7-{delta}} for different oxygen doping levels. Incommensurability is clearly observed for oxygen concentrations of 6.6 and 6.7 and is suggested for the 6.93. Measurements of the resonance for the O6.6 concentration show that it exists in a broadened and less intense form at temperatures much higher than Tc.
Date: November 5, 1997
Creator: Mook, H.A.; Dai, P.; Hunt, R.D. & Dogan, F.
Partner: UNT Libraries Government Documents Department

Absolute measurements of the high-frequency magnetic dynamics in high-{Tc} superconductors

Description: The authors review recent measurements of the high-frequency dynamic magnetic susceptibility in the high-T{sub c} superconducting systems La{sub 2{minus}x}Sr{sub x}CuO{sub 4} and YBa{sub 2}Cu{sub 3}O{sub 6+x}. Experiments were performed using the chopper spectrometers HET and MARI at the ISIS spallation source. The authors have placed their measurements on an absolute intensity scale, this allows systematic trends to be seen and comparisons with theory to be made. They find that the insulating S = 1/2 antiferromagnetic parent compounds show a dramatic renormalization in the spin wave intensity. The effect of doping on the response is to cause broadenings in wave vector and large redistributions of spectral weight in frequency.
Date: August 7, 1997
Creator: Hayden, S.M.; Aeppli, G.; Dai, P.; Mook, H.A.; Perring, T.G.; Cheong, S.W. et al.
Partner: UNT Libraries Government Documents Department

Recent neutron scattering results on high-temperature superconductors

Description: Triple-axis spectrometry has been used to determine the magnetic excitations in YBa{sub 2}Cu{sub 3}O{sub 7}. Polarized measurements at 100 K show the scattering consists of a peak near 40 meV superposed on a small relatively flat background. The 40-meV peak is found to be sharp in energy and anisotropic in momentum occupying a square portion of the Cu-O reciprocal lattice plane. The temperature dependence of the scattering is identical at the square side and corner. Polarized measurements show that the small background scattering observed at 100 K disappears below the superconducting transition. Resonant absorption measurements made to examine the Cu phonons show that no changes in the phonons occur at the superconducting transition.
Date: September 16, 1994
Creator: Mook, H.A.; Dai, P.; Aeppli, G.; Mason, T.E.; Hecker, N.E.; Harvey, J.A. et al.
Partner: UNT Libraries Government Documents Department