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  • 1
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 87 (2000), S. 2984-2988 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We report on measurements of the surface resistance of YBa2Cu3Ox thin films at frequencies between 0.087 and 2 THz and temperatures between 50 and 120 K by time-domain terahertz-transmission spectroscopy (TDTTS) and resonant microwave spectroscopy. The determination of the surface resistance of superconducting thin films by TDTTS is extended to higher frequencies and thicker films than previously by numerically solving the complex transmission coefficient. The numerical solution also provides the dielectric function of the YBa2Cu3Ox thin films. The temperature and frequency dependence of the surface resistance of YBa2Cu3Ox thin films in the THz range is successfully explained by a weak coupling model of d-wave superconductivity which incorporates inelastic and elastic scattering. The surface resistance of YBa2Cu3Ox thin films at THz frequencies is compared to the surface resistance of gold and niobium. The advantages of YBa2Cu3Ox thin films for superconducting THz electronic devices are discussed. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Springer
    The European physical journal 72 (1988), S. 139-159 
    ISSN: 1434-6036
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The magnetic field penetration depth, surface resistance and far infrared reflectivity are calculated for two anisotropic order parameters having either points or lines of nodes in the energy gap. Resonant impurity scattering is taken into account for a wide range of scattering rates. Comparison with experimental results on heavy Fermion superconductors shows that the order parameter cannot be deduced unambiguously from the temperature dependence of the penetration depth. Fits to surface resistance measurements on UBe13 are best in the Born approximation, rather than the unitarity limit. Experiments on high-T c materials are largely inconclusive, offering little support for the applicability of BCS theory and providing no evidence against novel pairing interactions leading to unusual order parameters.
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  • 3
    Electronic Resource
    Electronic Resource
    Springer
    Journal of superconductivity 12 (1999), S. 385-393 
    ISSN: 1572-9605
    Keywords: High-temperature superconductors ; theory ; nonlocal effects ; surface resistance
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract The low-temperature surface resistance R s of d-wave superconductors is calculated as function of frequency assuming normal state quasiparticle mean free paths l in excess of the penetration depth. Results depend strongly on the geometric configuration. In the clean limit, two contributions to R s with different temperature dependencies are identified: photon absorption by quasiparticles and pair breaking. The size of nonlocal corrections, which can be positive or negative depending on frequency decreases for given l as the scattering phase shift δ N is increased. However, except in the unitarity limit δ N =0.5π, nonlocal effects should be observable.
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  • 4
    Electronic Resource
    Electronic Resource
    Springer
    Journal of low temperature physics 105 (1996), S. 755-760 
    ISSN: 1573-7357
    Keywords: 74.20.−z ; 74-25.Nf ; 78.30.−j
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The electronic Raman spectrum for the experimentally relevant polarizations is calculated within a Hubbard-type model for anisotropic superconductors and compared with the results for the standard Coulomb model. The requirement of gauge invariance is shown to play an especially important role. The Raman A1g response depends rather sensitively on band structure.
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  • 5
    ISSN: 1573-7357
    Keywords: 05.70 Ln ; 05.70 Jk ; 64
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The surface impedance of two 350nm thick YBCO films was measured for temperatures between 4.2K and 150K in a copper cavity at 87 GHz. Both films, one grown by electron beam evaporation on MgO, the other one by high oxygen-pressure dc sputtering from a stoichiometric target on LaAlO3, provide critical temperatures of about 91K, low residual surface resistances of Rs(4.2K) 〈 1mΩ and low specific resistivities in the normal state of ϱ(100K) 〈 85μΩcm. The experimental data obtained on these two films are compared to a d-wave model of superconductivity which incorporates elastic and inelastic scattering. Good agreement between theory and the experimental results for both the surface resistance and the penetration depth in the whole temperature range is achieved for scattering phase shifts near 0.4π and order parameter amplitudes in the range of 2Δ0(0)/kBTc = 6.0 – 7.5 without subtracting an extrinsic residual surface resistance.
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  • 6
    Electronic Resource
    Electronic Resource
    Springer
    Journal of low temperature physics 117 (1999), S. 1295-1299 
    ISSN: 1573-7357
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Evidence for d-wave superconductivity from measurements of the local conductivity is rather indirect and not very compelling. Very recently, experimental data on the surface resistance taken at five different frequencies on very high quality single crystals of YBa 2 Cu 3 O 7 have become available. Having the frequency as additional variable provides a much more stringent test for the predictive powers of the d-wave model. Because of the high quality of the samples we expect that at low temperatures the nonlocal limit applies to the surface resistance. A nonlocal relation between current and field would allow a direct investigation of the order parameter anisotropy by varying the wave vector q of the radiation relative to the crystal axes.
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  • 7
    Electronic Resource
    Electronic Resource
    Springer
    Journal of low temperature physics 84 (1991), S. 381-464 
    ISSN: 1573-7357
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract A theory of the upper critical field is developed which is sufficiently general to incorporate strong coupling effects, anisotropy of both Fermi surface and pairing interaction, and momentum dependent scattering, including exchange and spin-orbit scattering. It is not necessary to assume that spin-orbit scattering is a small fraction of the total scattering. An important improvement in this theory is the correct treatment of the spatial dependence of the anomalous Green function and the order parameter in the mixed state. In the presence of anisotropy this is, in general, not described with sufficient accuracy by the Abrikosov vortex lattice. Application of this theory to anisotropy effects, with the clean limit taken in most cases, have been published elsewhere and additional studies will appear in the near future. In this paper we concentrate on the effects arising from the momentum dependence of the elastic scattering. If spin-orbit scattering forms a sizeable fraction of the total scattering it is necessary to take its explicit momentum dependence into account. When this is done we find that spin-orbit scattering is actually less effective in reducing paramagnetic limiting.p- andd-wave scattering have a marked effect on the upper critical fieldH c2 at low reduced temperatures, partially cancelling each other.d-wave scattering raisesH c2(O), but for physically acceptable parameter value is not sufficient to explain the apparent absence of paramagnetic limiting in some high-field superconductors. For a complete description of the upper critical field, other effects mentioned above need to be taken into account, but for superconductors of intermediate purity the momentum dependence of the scattering cannot be ignored. The greatest problem remaining in our quest for a complete understanding of the upper critical field appears to be an accurate determination of the relevant normal state properties.
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  • 8
    Electronic Resource
    Electronic Resource
    Springer
    Journal of low temperature physics 84 (1991), S. 465-475 
    ISSN: 1573-7357
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We employ Eliashberg theory to calculate the ratioR=2Δ/k B T c for s-wave pairing for a two dimensional tight-binding band. The pairing is mediated in our model by exchange of phonons and higher frequency “excitons” which, by themselves, would yield aT c of about 200 K. The additional exchange of an antiferromagnetic spin fluctuation (antiparamagnon) is adjusted to reduceT c to the observed 90 K range. We calculate the RPA spin fluctuation propagator self-consistently including the “feedback” effect of the superconductivity and assuming that only wave vectors near “nesting” are important. The addition of the antiparamagnon exchange further enhancesR, which depends strongly on the band-filling. The resulting values should, however, be compared with a weak-coupling BCS value that, depending on the band structure and band-filling, can be significantly less than 3.5.
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  • 9
    Electronic Resource
    Electronic Resource
    Springer
    Journal of low temperature physics 99 (1995), S. 527-529 
    ISSN: 1573-7357
    Keywords: 74.7e.−h ; 74.25. Kc ; 71.27.+a ; 63.20. Kr
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We investigate the effect of electronic correlations on the phonon-mediated part of the pair interaction by including the effect of an exchange interaction U between the electrons in the calculation of the phonon frequencies and the electron-phonon vertices. We employ two RPA models: the first, consisting of a direct Coulomb interaction in addition to U, leads to softer phonons and an enhanced attractive s-wave phonon-induced electron-electron interaction while the second, a pure Hubbard model, yields the opposite effect. The weak-coupling selfconsistency equation for the order parameter in the 2D first Brillouin zone is solved for a tight-binding band. The change of Tc arises primarily from the change of the phonon frequencies rather than from the vertex renormalization.
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  • 10
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