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423 results on '"Draxl, C."'

159. Ballistic Electron Emission Spectroscopy Used as a Tool for Determining Accurate Hot-Electron Lifetimes in Metals.

Author

De Pablos, P. F., De Andres, P. L., Ladstädter, F., Hohenester, U., Puschnig, P., Ambrosch-Draxl, C., García-Vidal, F. J., and Flores, F.

Subjects
ELECTRON emission, SPECTRUM analysis, ELECTRON spectroscopy, METAL analysis, ELECTRIC properties of metals, SEMICONDUCTORS
Abstract

The main goal of this paper is to show how BEES technique can be used as an accurate tool for determining reliable hot-electron lifetimes in metals. This is achieved by presenting a first-principles transport theory of BEES, except for the metal lifetimes, that are treated as parameters to be fitted to the BEES current. We have illustrated this ability by analyzing Au/Si and Pd/Si systems. Good agreement is also found between our fitted lifetimes and theoretical hot-electron lifetimes calculated within an ab initio framework based on a density functional theory using a LAPW basis. © 2003 American Institute of Physics [ABSTRACT FROM AUTHOR]

Published
2003
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165. On the structure of oligophenylenes

Author

Heimel, G., primary, Cai, Q., additional, Martin, C., additional, Puschnig, P., additional, Guha, S., additional, Graupner, W., additional, Ambrosch-Draxl, C., additional, Chandrasekhar, M., additional, and Leising, G., additional

Published
2001
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190. Lattice vibrations in CaV2O5 and their manifestations: a theoretical study based on density functional theory.

Author

Spitaler, J., Ambrosch-Draxl, C., and Ya Sherman, E.

Subjects
*LATTICE theory, *FUNCTIONAL analysis, *VECTOR spaces, *DENSITY functionals, *ELECTROMAGNETIC interactions
Abstract

We present a comprehensive ab initio study of all non fully symmetric Γ point lattice vibrations in CaV2O5. The characteristics of CaV2O5 are a half-filled upper electron band and a ladder-like crystal structure. Adopting the frozen-phonon approach we calculate and analyze in detail the phonon eigenfrequencies and eigenvectors. The corresponding spectra of inelastic light scattering are computed for all Raman-active modes, including their resonance behavior as a function of incoming light. Our results allow for a qualitative and quantitative understanding of the phonon-related features in experimental optical excitation spectra. The theoretical data are employed to discuss available measurements and provide an improved assignment of the observed modes with respect to symmetry and displacement patterns. Electron hopping parameters and spin-spin exchange matrix elements are evaluated from the electronic bands. The changes in these parameters upon atomic displacements along the phonon eigenvectors are analyzed semiquantitatively, yielding the effect of lattice vibrations on charge and spin degrees of freedom. It is discussed how all the obtained results differ from the ones in the quarter-filled NaV2O5 compound due to differences in the electron band structure and in the chemistry. [ABSTRACT FROM AUTHOR]

Published
2009
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191. The role of doping and pressure in Hg based high T c cuprates: A theoretical study

Author

Ambrosch-Draxl, C., Thonhauser, T., and Sherman, E.Ya.

Subjects
*ELECTRIC conductivity, *SUPERCONDUCTIVITY, *CRITICAL currents, *SUPERFLUIDITY
Abstract

Abstract: We present a series of first-principles calculations for Hg based high T c cuprates investigating the effect of pressure, doping, and composition on the electronic and crystalline structure. In particular, the total and site-projected hole concentration in the CuO2 planes and the density of states are studied in detail. We discuss effects of inhomogeneity introduced by doping and the limitations on creating holes by either doping, pressure, or the number of CuO2 layers per unit cell. From an analysis and comparison of our results to available experimental data on the pressure dependence of T c, we conclude that the effective coupling constant to the boson mediating the Cooper pairing is of the order of 1 ruling out the weak coupling approaches. [Copyright &y& Elsevier]

Published
2008
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194. Hole Doping and Inhomogeneous Charge Distribution in High Tc Cuprates Investigated from First Principles.

Author

Ambrosch-Draxl, C., Sherman, E., Auer, H., and Thonhauser, T.

Abstract

To understand the link between doping and electronic properties in the high-temperature superconductors, we have performed first-principles calculations for several representatives of high Tc compounds. In the single-layer cuprate HgBa2CuO4 the excess oxygen attracts electrons from the CuO2 plane leading to an increase of the hole concentration in this building block, where the maximum amount of holes is reached when the dopant oxygen shell is closed. The usage of supercells allows to study the inhomogeneous charge distribution as a function of doping, i.e. from the underdoped up to the overdoped regime. Comparison is made with other compounds like Ba-doped La2CuO4 and oxygen-deficient YBa2Cu3O7− x. The effects of our findings on superconductivity are discussed. [ABSTRACT FROM AUTHOR]

Published
2004
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195. Probing electron-phonon coupling in high-Tc superconductors by site-selective isotope substitution.

Author

Sherman, E.Ya. and Ambrosch-Draxl, C.

Abstract

We consider changes in the electron-phonon coupling in high- T c cuprates caused by site-selective oxygen isotope substitution. Contrary to the total substitution, the site-selective replacement influences the coupling constant for each phonon mode due to the induced changes in the phonon eigenvectors. The relative changes for some modes can be larger than 100%. The measured properties sensitive to these changes are discussed. [ABSTRACT FROM AUTHOR]

Published
2002
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196. Ab-initio Study of the Vacancy Ordering in Yba2Cu3O7− x.

Author

Korzhavyi, P., Ambrosch-Draxl, C., and Johansson, B.

Abstract

With the aim of establishing a relationship between the electronic structure and the site occupancy within the Cu-O chains, we perform total energy calculations for the sub-stoichiometric compound Yba2 Cu3 O6.5 for various patterns of vacancy ordering. Supercells containing up to four formula units are considered. The effective interactions of O(1) atoms are extracted from the total energies by means of the cluster expansion technique. The most stable configuration is found to be the experimentally observed ortho-II structure. This finding is confirmed by Monte-Carlo simulations for a rigid lattice using the pair interactions. Thermal disorder effects are studied. The influence of vacancy ordering on the total and partial densities of states is discussed. [ABSTRACT FROM AUTHOR]

Published
1999
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197. Resonant Raman scattering in the superconducting cuprates: Frozen-phonon versus perturbational approach

Author

Sherman, E.Ya. and Ambrosch-Draxl, C.

Abstract

Abstract:: We consider in detail Raman scattering by vibration of the apical oxygen ions in the RBa2Cu3O7 superconducting cuprates. The scattering intensity is very sensitive to the ratio of diagonal and off-diagonal matrix elements of electron-phonon coupling, bandstructure, and carrier concentration. Our results show a large quantitative difference between the results of frozen-phonon and perturbational approach to the Raman process. The discrepancy becomes especially large when interband transitions to the states near the Fermi level are close to resonance with the incident light. The calculation of phonon-induced ion charge fluctuations shows an analogous discrepancy. The reason for these effects is the possibility of carrier redistribution between different parts of the Fermi surface arising in the frozen-phonon approximation. Our results show that Raman scattering in superconducting superlattices is very sensitive to the properties of the states near the Fermi level. For this reason experiments performed on the superlattices can help to resolve the discrepancy.

Published
2000
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198. Ab-initio Study of the Vacancy Ordering in Yba2Cu3O7−x

Author

Korzhavyi, P., Ambrosch-Draxl, C., and Johansson, B.

Abstract

With the aim of establishing a relationship between the electronic structure and the site occupancy within the Cu-O chains, we perform total energy calculations for the sub-stoichiometric compound Yba2Cu3O6.5for various patterns of vacancy ordering. Supercells containing up to four formula units are considered. The effective interactions of O(1) atoms are extracted from the total energies by means of the cluster expansion technique. The most stable configuration is found to be the experimentally observed ortho-II structure. This finding is confirmed by Monte-Carlo simulations for a rigid lattice using the pair interactions. Thermal disorder effects are studied. The influence of vacancy ordering on the total and partial densities of states is discussed.

Published
1999
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199. Calculation of EFGs in high Tc superconductors

Author

Ambrosch-Draxl, C., Blaha, P., and Schwarz, K.

Abstract

Abstract: The electric field gradients (EFGs) at all atomic positions of YBa2Cu3O7 and YBa2Cu3O6 are calculated on the basis of full potential linearized augmented plane wave calculations by an ab initio method. Cood agreement with experimental EFGs is found for all oxygen positions in YBa2Cu3O7 and the Cu1 position in both compounds, while for the Cu2 position in the superconductor the symmetry of the EFG agrees with experiment but the magnitude of our theoretical EFG is only half the experimental value. A small transfer of 0.07 electrons from d(x2−y2) to d(z2) symmetry would be sufficient to achleve agreement with experiment. Thus the underlying local density approximation is not perfect, but remains a very accurate method for describing ground state properties.

Published
1990
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