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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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NASA NTRS1982

Optical conductivity of alpha-Mn

The optical constants were measured at room temperature in the photon-energy range 0.6 to 6.5 eV on evaporated thin films. Evaporation conditions were chosen that gave the alpha-Mn crystal structure with reasonably large grains. The optical conductivity was separated into intraband and interband contributions by fitting to the Drude formula at low energies. The results are anomalous in comparison to other 3d transition metals. The free-electron lifetime is exceptionally sort (in agreement with the large dc resistivity of Mn), and the interband transitions seem unusually weak at the lower energies. Possible explanations related to the complicated crystal structure of alpha-Mn are discussed.

Scoles, K. J.↗
NASA NTRS1982

Optical conductivity of alpha-Mn

The optical constants were measured at room temperature in the photon-energy range 0.6-6.5 eV on evaporated thin films. Evaporation conditions were chosen that gave the alpha-Mn crystal structure with relatively large grains. The optical conductivity was separated into intraband and interband contributions by fitting to the Drude formula at low energies. The results are anomalous in comparison to other 3d transition metals: the free-electron lifetime is exceptionally short (in agreement with the large dc resistivity of Mn), and the interband transitions seem unusually weak at the lower energies. Possible explanations related to the complicated crystal structure of alpha-Mn with its loss of point symmetry at the atom sites are discussed.

Scoles, K. J.↗
NASA NTRS1999

Vortex-State Complex Hall Conductivity of Superconducting YBa2Cu3O7 Epitaxial Films at Radio Frequencies

The first intermediate-frequency measurements of the vortex-state complex Hall conductivity \sigma_xy of YBa_2CU_3O_7 superconducting epitaxial films are reported. A direct transport measurement technique from dc to 7 MHz was used. The results are analyzed in terms of a phenomenological model, generalized from that for the dc Hall conductivity, with the assumptions that: 1) the sign reversal in the vortex-state Hall conductivity is associated with the different carrier densities within and far away from the vortex cores; 2) the Drude approximation Is applicable; and 3) the anomalous sign reversal occurs in the flux-flow Limit. The temperature, frequency, and magnetic field (B) dependencies of our \sigma_xy data are in good agreement with the model. The B-dependence of \sigma_xy reveals that both vortices (\sigma_xyAv \propto BA-1) and quasiparticles (sigma_xyAq \propto B) contribute to the vortex-state Hall conduction. The magnitude of the real part of \sigma_xy, \sigma_xyA\prime, is in good agreement with our model, while that of the imaginary part, \sigma_xyA\prime \prime, is significantly Luger than the theoretical prediction. This may be attributed to the unconventional electronic structures in the vortex core of cuprate superconductors with d-wave or mixed pairing symmetries.

Beam, D. A.↗
NASA NTRS1981

Intraband optical conductivity sigma/omega,T/ of Cu, Ag, and Au - Contribution from electron-electron scattering

The frequency and temperature dependence of the intraband optical conductivity of the noble metals Cu, Ag and Au is measured and contributions of electron-electron scattering are assessed. Optical measurements were performed at temperatures of 77, 295 and 425 K to obtain values of the Drude electron scattering rate with a linear dependence on temperature which may be attributed to electron-phonon scattering, and a quadratic dependence on photon energy, which is suggestive of electron-electron scattering but is a factor of two to three times greater than would be expected. Comparison of the optical data with dc electrical and thermal resistivity data which also show behavior attributed to electron-electron scattering reveals discrepancies of up to an order of magnitude. Other possible mechanisms for the frequency dependence, including absorptance, electron-surface plasmon interactions, a two-carrier model, and a structure dependence are considered, and it is concluded that the frequency dependence in the Drude scattering rates of the noble metals is not yet quantitatively understood

Parkins, G. R.↗