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The influence of anharmonic phonons on the isotope effect in high- Tc oxides

Description: Anharmonic phonons are examined to study the unusual isotope effect exponents for the high-{Tc} oxides. Within a simple model of anharmonicity, the mass dependences of the electron-phonon coupling constant {lambda} and the phonon frequency determine the isotope effect exponent {alpha} as a function of coupling strength. A model in which the outer wells of a multiple-well potential deepen as the orthorhombic/low temperature tetragonal phase transition in La{sub 2-x}M{sub x}CuO{sub 4} is approach… more
Date: January 1, 1992
Creator: Crespi, V.H. & Cohen, M.L.
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Partner: UNT Libraries Government Documents Department
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Generation of nonequilibrium optical phonons in GaAs/AlAs quantum wells by intrasubband and intersubband scatterings

Description: The generation of a nonequilibrium population of optical phonons by photoexcited hot electrons in semiconductor quantum wells is investigated theoretically. The microscopic model of electron-phonon interaction proposed by Huang and Zhu has been used to compute the distributions of confined longitudinal optical phonons and interface modes in GaAs/AlAs quantum wells as a function of well width. Experimental tests of the calculated distributions by Raman scattering are discussed.
Date: November 1, 1991
Creator: Wald, K.R.; Kim, Dai-sik & Yu, P.Y.
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Partner: UNT Libraries Government Documents Department
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Interactions of energetic particles and clusters with solids

Description: Ion beams are being applied for surface modifications of materials in a variety of different ways: ion implantation, ion beam mixing, sputtering, and particle or cluster beam-assisted deposition. Fundamental to all of these processes is the deposition of a large amount of energy, generally some keV's, in a localized area. This can lead to the production of defects, atomic mixing, disordering and in some cases, amorphization. Recent results of molecular dynamics computer simulations of energetic… more
Date: December 1, 1990
Creator: Averback, R.S.; Hsieh, Horngming (Illinois Univ., Urbana, IL (USA). Dept. of Materials Science and Engineering); Diaz de la Rubia, T. (Lawrence Livermore National Lab., CA (USA)) & Benedek, R. (Argonne National Lab., IL (USA))
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Partner: UNT Libraries Government Documents Department
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The effect of strong Coulomb correlations on electron-phonon interactions in the copper oxides: Implications for transport

Description: The effects of strong Coulomb correlations on the electron-phonon interactions in the high temperature superconductors are investigated. Very strong Coulomb correlations, which are essential for creating the insulating state at half filling, lead to a suppression of charge fluctuations as the insulator is approached. This thereby significantly reduces the electron-phonon coupling. We present a self-consistent frozen phonon'' scheme which is based on a Coulomb renormalized band structure of the … more
Date: January 1, 1990
Creator: Kim, J.H. (Johns Hopkins Univ., Baltimore, MD (USA). Dept. of Physics and Astronomy); Levin, K. (Chicago Univ., IL (USA). James Franck Inst.); Wentzcovitch, R. (Brookhaven National Lab., Upton, NY (USA)) & Auerbach, A. (Boston Univ., MA (USA). Dept. of Physics)
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Partner: UNT Libraries Government Documents Department
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Competing electron-electron/electron-phonon interactions and polyacetylene

Description: Using Lanczos exact diagonalization, we investigate the effects of the competition between the electro-electron and electron-phonon interactions in the context of the 1-D tight-binding Peierls-Hubbard Hamiltonian, studying various structural, optical, and vibrational properties of strongly correlated systems. We use polyacetylene as our experimental guide, and perform a parameter space search to determine the level at which a unique set of parameters can model this prototypical conducting polym… more
Date: April 8, 1991
Creator: Gammel, J.T. (Los Alamos National Lab., NM (USA) Bayreuth Univ. (Germany, F.R.). Physics Inst.); Campbell, D.K. (Los Alamos National Lab., NM (USA)) & Loh, E.Y. Jr. (Thinking Machines Corp., Cambridge, MA (USA))
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Partner: UNT Libraries Government Documents Department
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Specific heat of La sub 2-x Sr sub x CuO sub 4 : Volume fraction of superconductivity; possible structural transition at 45K

Description: Specific heat measurements on La{sub 1.85}Sr{sub 0.15}CuO{sub 4} show a linear correlation of the zero-field low-temperature linear term with the discontinuity at {Tc}, each of which provides a measure of the superconducting volume fraction. The correlation leads to an estimate of the normal-state density of electronic states which, together with the band structure calculated value, gives an electron-phonon interaction parameter that is too small to account for the superconductivity. For a samp… more
Date: March 1, 1990
Creator: Amato, A.; Fisher, R. A.; Phillips, N. E. (Lawrence Berkeley Lab., CA (USA)) & Torrance, J. B. (IBM Research Div., San Jose, CA (USA). Almaden Research Center)
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Partner: UNT Libraries Government Documents Department
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Theory of normal and superconducting properties of fullerene-based solids

Description: Recent experiments on the normal-state and superconducting properties of fullerene-based solids are used to constrain the proposal theories of the electronic nature of these materials. In general, models of superconductivity based on electron pairing induced by phonons are consistent with electronic band theory. The latter experiments also yield estimates of the parameters characterizing these type H superconductors. It is argued that, at this point, a standard model'' of phonons interacting wi… more
Date: October 1, 1992
Creator: Cohen, M.L.
Item Type: Refine your search to only Report
Partner: UNT Libraries Government Documents Department
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Specific heat evidence for strong coupling in YBa sub 2 Cu sub 3 O sub 7

Description: Specific heat data for YBa{sub 2}Cu{sub 3}O{sub 7} are consistent with {gamma} = 15 {plus minus} 3 mJ/mole{center dot}K{sup 2} and 2{Delta}{sub 0}/k{Tc} = 6.8 {plus minus} 0.6. These results indicate that strong-coupling effects are present but that the coupling is unlikely to be predominantly a conventional electron-phonon interaction. 8 refs., 1 fig.
Date: August 1, 1990
Creator: Gordon, J. E.; Fisher, R. A.; Kamin, S. & Phillips, N. E.
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Partner: UNT Libraries Government Documents Department
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Local states in one-dimensional CDW (charge density wave) materials: Spectral signatures for polarons and bipolarons in MX chains

Description: We have undertaken a combined theoretical and experimental effort directed toward the examination of both the ground and defect states in halide-bridged mixed-valence metal linear chains materials as they are tuned within and between broken symmetry phases. Novel low-dimensional highly correlated electronic materials offer a difficult theoretical challenge as we must span from a description of electronic structure on a molecular scale to the meso scale structure that is intrinsic to these solid… more
Date: January 1, 1990
Creator: Swanson, B. I.; Donohoe, R. J.; Worl, L. A.; Bulou, A. D.F.; Arrington, C. A.; Gammel, J. T. et al.
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Partner: UNT Libraries Government Documents Department
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High pressure optical studies of semiconductors and heterostructures. Final report

Description: The authors have studied the effects of hydrostatic pressure on the confined transitions in quantum well heterostructures, using lattice matched GaAs/Al{sub x}GaAs{sub 1{minus}x}As, strained layer narrow band gap GaSb/AlSb and In{sub x}Ga{sub 1{minus}x}As/GaAs, and strained layer wide gap Zn{sub 1{minus}x}Cd{sub x}Se/ZnSe as examples. Precise values of the energies, pressure coefficients and band alignments are determined. In strained epilayers the interfacial strains, deformation potential con… more
Date: February 1, 1995
Creator: Chandrasekhar, H. R.
Item Type: Refine your search to only Report
Partner: UNT Libraries Government Documents Department
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Design analysis of a novel hot-electron microbolometer

Description: We propose a novel antenna coupled microbolometer which makes use of the weak coupling between electrons and phonons in a metal at low temperatures. The radiation is collected by a planar lithographed antenna and thermalized in a thin metal strip. Resulting temperature rise of the electrons is detected by a tunnel junction, where part of the metal strip forms the normal electrode. All components are deposited directly on a substrate so that arrays can be conveniently produced by conventional li… more
Date: August 1, 1992
Creator: Nahum, M.; Richards, P. L. & Mears, C. A.
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Partner: UNT Libraries Government Documents Department
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Experimental evidence for lattice effects in high temperature superconductors

Description: We present an overview of the experimental evidence for a role of the lattice in the mechanism of high temperature superconductivity. It appears unlikely that a solely conventional electron-phonon interaction produces the pairing. However, there is ample evidence of strong electron and spin to lattice coupling and observations of a response of the lattice to the electronic state. We draw attention to the importance of the local structure in discussions of lattice effects in high-{Tc} supercondu… more
Date: January 18, 1994
Creator: Billinge, S. J. L.; Kwei, G. H. & Thompson, J. D.
Item Type: Refine your search to only Article
Partner: UNT Libraries Government Documents Department
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Theory of normal and superconducting properties of fullerene-based solids

Description: Recent experiments on the normal-state and superconducting properties of fullerene-based solids are used to constrain the proposal theories of the electronic nature of these materials. In general, models of superconductivity based on electron pairing induced by phonons are consistent with electronic band theory. The latter experiments also yield estimates of the parameters characterizing these type H superconductors. It is argued that, at this point, a ``standard model`` of phonons interacting … more
Date: October 1992
Creator: Cohen, M. L.
Item Type: Refine your search to only Report
Partner: UNT Libraries Government Documents Department
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Polaron tunneling in high-temperature superconductors

Description: Nonlinear self-trapping effects are frequently proposed in the nonlinear science community -- in contexts from excitons, magnons, and polarons in solid state materials, to molecular crystals and polypeptides, to nonlinear optics. The general phenomena of self-trapping involves the self-consistent response of one field to one or more with which it is coupled, and provides excellent examples of coexisting influences of nonlinearity, lattice discreteness, quantum and thermal fluctuations, disorder… more
Date: July 1, 1993
Creator: Bishop, A. R.; Mustre de Leon, J. & Salkola, M. I.
Item Type: Refine your search to only Article
Partner: UNT Libraries Government Documents Department
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Pairing on small clusters in the Peierls-Hubbard model: Implications for C{sub 60}

Description: Inclusion of electron-phonon interactions can substantially modify the conclusions about pairing obtained from models including only electron-electron interactions. The authors study pairing within the Peierls-Hubbard model for electron- and hole-doped analogs of C{sub 60} accessible to exact diagonalization techniques (cube, truncated tetrahedron, etc.), and discuss the extrapolation from these small system calculations to C{sub 60}.
Date: August 1, 1994
Creator: Gammel, J. T.; Ung, K. C. & Mazumdar, S.
Item Type: Refine your search to only Report
Partner: UNT Libraries Government Documents Department
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The influence of anharmonic phonons on the isotope effect in high-{Tc} oxides

Description: Anharmonic phonons are examined to study the unusual isotope effect exponents for the high-{Tc} oxides. Within a simple model of anharmonicity, the mass dependences of the electron-phonon coupling constant {lambda} and the phonon frequency determine the isotope effect exponent {alpha} as a function of coupling strength. A model in which the outer wells of a multiple-well potential deepen as the orthorhombic/low temperature tetragonal phase transition in La{sub 2-x}M{sub x}CuO{sub 4} is approach… more
Date: January 1, 1992
Creator: Crespi, V. H. & Cohen, M. L.
Item Type: Refine your search to only Article
Partner: UNT Libraries Government Documents Department
open access

Generation of nonequilibrium optical phonons in GaAs/AlAs quantum wells by intrasubband and intersubband scatterings

Description: The generation of a nonequilibrium population of optical phonons by photoexcited hot electrons in semiconductor quantum wells is investigated theoretically. The microscopic model of electron-phonon interaction proposed by Huang and Zhu has been used to compute the distributions of confined longitudinal optical phonons and interface modes in GaAs/AlAs quantum wells as a function of well width. Experimental tests of the calculated distributions by Raman scattering are discussed.
Date: November 1, 1991
Creator: Wald, K. R.; Kim, Dai-sik & Yu, P. Y.
Item Type: Refine your search to only Article
Partner: UNT Libraries Government Documents Department
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Ion-channeling study of anomalous atomic displacements at the superconducting transition in high-T sub c materials

Description: Ion channeling along the (001) direction in high-quality single crystals of (Y/Er)Ba{sub 2}Cu{sub 3}O{sub 7-x} revealed an abrupt change in displacements in the a-b plane of the Cu and O atoms at the superconducting transition, {Tc}; normal Debye-like'' vibrations were found for the Y/Er and Ba atoms. The anomalous change in Cu-O displacements was found to shift directly with stoichiometry-induced changes in {Tc}, implying a direct link between the observed phonon anomaly and the superconductin… more
Date: June 1, 1990
Creator: Rehn, L. E.; Sharma, R. P. & Baldo, P. M.
Item Type: Refine your search to only Article
Partner: UNT Libraries Government Documents Department
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Elementary excitations and phase transitions in crystals

Description: The unique method of measuring elementary excitations in solids over a wide range of energy and momentum transfers is inelastic scattering of neutrons. Elementary excitations are defined as a correlated motion of atoms or spins in a solid which include phonons, magnons, rotons, or crystal field excitations. These excitations play a fundamental role in a wide variety of structural and magnetic phase transitions and provide the information in understanding the underlying microscopic mechanism of … more
Date: December 31, 1993
Creator: Shapiro, S.M.
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Partner: UNT Libraries Government Documents Department
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Dynamic Jahn-Teller Effect in Fullerene C{sub 60}

Description: Based on the Bogoliubov-de Gennes formalism, the linear multi-mode T{sub 1u} {circle_times} 8h{sub g} static Jahn-Teller problem is analyzed for the charged C{sub 60}{sup {minus}}, C{sub 60}{sup 2{minus}}, the lowest triplet and singlet self-trapped exciton (STE) of C{sub 60}. The dynamic Jahn-Teller effect due to quantum tunneling between the six degenerate D{sub 5d} configurations is studied within a perturbation approach. The tunneling induced level splittings are determined to be {sup 2}T{s… more
Date: May 1, 1995
Creator: Wang, W. Z.; Bishop, A. R.; Wang, C. L.; Yu, L. & Su, Z. B.
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Partner: UNT Libraries Government Documents Department
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Phase transformation and phonon anomalies in Ni{sub 2}MnGa

Description: Inelastic neutron scattering experiments and transmission electron microscopy have been used to study a single crystal of the Ni{sub 2}MnGa shape memory Hustler alloy in a wide temperature range covering the parent phase (T>T{sub 1}=265 K), a recently discovered pemartensitic (T{sub 1}T>T{sub M}) and martensitic (T<T{sub M}=220 K) phase regions. A temperature-dependent anomaly in the TA{sub 2} phonon dispersion in the parent phase was observed and related to the phase transformations. The prema… more
Date: July 1, 1995
Creator: Zheludev, A.; Shapiro, S.M.; Wochner, P.; Schwartz, A.; Wall, M. & Tanner, L.E.
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Partner: UNT Libraries Government Documents Department
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Electron-phonon driven spin frustration in multi-band Hubbard models: MX chains and oxide superconductors

Description: We discuss the consequences of both electron-phonon and electron-electron couplings in 1D and 2D multi-band (Peierls-Hubbard) models are discussed. After a brief discussion of various analytic limits, we focused on (Hartree-Fock and exact) numerical studies in the intermediate regime for both couplings, where unusual spin-Peierls as well as long-period, frustrated ground states are found. Doping into such phases or near the phase boundaries can lead to further interesting phenomena such as sepa… more
Date: January 1, 1992
Creator: Gammel, J.T. (Naval Command, Control and Ocean Surveillance Center, San Diego, CA (United States)); Yonemitsu, K.; Saxena, A.; Bishop, A.R. (Los Alamos National Lab., NM (United States)) & Roeder, H. (Bayreuth Univ. (Germany). Physics Inst.)
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Partner: UNT Libraries Government Documents Department
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