Robert Kouba

Force calculation and atomic-structure optimization for the full-potential linearized augmented plane-wave code WIEN

Abstract Following the approach of Yu, Singh, and Krakauer [Phys. Rev. B 43 (1991) 641], we extended the linearized augmented plane wave code WIEN of Blaha, Schwarz, and coworkers by the evaluation of forces. In this paper we describe the approach, demonstrate the high accuracy of the force calculation, and use them for an efficient geometry optimization of poly-atomic systems.

Investigation of A 1 g phonons in YBa 2 Cu 3 O 7 by means of linearized-augmented-plane-wave atomic-force calculations

We report first-principles frozen-phonon calculations for the determination of the force-free geometry and the dynamical matrix of the five Raman-active

A1g phonons in YBa2Cu3O7 by first-principles atomic-force calculations

{A}_{1g}

Theory of isotope effects in Raman spectra of YBa2Cu3O7

modes in

First-principles Raman studies on YBa2Cu3O7

{\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}

Pressure study of structural properties and phonons in YBa{sub 2}Cu{sub 3}O{sub 7}

. To establish the shape of the phonon potentials atomic forces are calculated within the linearized-augmented-plane-wave method. Two different schemes\char22{}the local-density approximation (LDA) and a generalized gradient approximation (GGA)\char22{}are employed for the treatment of electronic exchange and correlation effects. We find that in the case of LDA the resulting phonon frequencies show a deviation from experimental values of approximately -10%. Invoking GGA the frequency values are significantly improved and also the eigenvectors are in very good agreement with experimental findings.

Structure optimization of YBa 2 Cu 3 O 7 and its influence on phonons and Fermi surface

We report on ab-initio frozen-phonon calculations for the five A1g Raman-active-modes of YBa2Cu3O7 atq=0. The determination of the equilibrium positions of the atoms Ba, Cu(2), O(2), O(3), O(4) as well as of phonon frequencies and respective eigenvectors is based on the calculation of atomic forces within the LAPW-method. The general agreement between our results and experimental structural data and phonon frequencies is good. Deviations are discussed in connection with the local-density approximation.

Raman scattering in YBa2Cu3O7: A comprehensive theoretical study in comparison with experiments

We investigate the effect of site-selective isotope substitution on the Raman spectra of the yttrium-barium cuprate by means of first-principles bandstructure calculations. Upon oxygen isotope substitution we find a dramatic change in the Raman scattering intensities which is much more pronounced than the shift of the phonon frequencies. This effect is due to large changes in the eigenvectors caused by the site-selective substitution.

First-principles band-structure calculations as a tool for the quantitative interpretation of Raman spectra of high temperature superconductors

Abstract We have investigated all Raman-active phonon modes (A 1 g , B 2 g , and B 3 g symmetry) of YBa 2 Cu 3 O 7 by first-principles LAPW calculations based on density functional theory. We demonstrate that an excellent overall agreement between measured and calculated phonon frequencies can only be achieved on the basis of a fully optimized crystal structure. The Raman scattering intensities for the A 1 g modes are determined. Due to a strongly decreased admixture of apical oxygen in the eigenvector of the O2-O3 mode with respect to previously published results the discrepancy between theory and experiment in the resonance profile of this mode is lifted.

Phonons and electron-phonon interaction by linear-response theory within the LAPW method

The pressure dependence of crystal structure and vibrational properties of YBa2Cu3O7is investigated by first-principles calculations based on density functional theory. We have determined the optimized geometry for different unit cell volumes by relaxing the atoms to their force-free positions. The tendency of altered bond strength with pressure is consistent with experimental investigations and previous c-axis strain calculations. The pressure effect on the three A1gphonon frequencies dominated by oxygen vibrations is in very good agreement with experimental investigations showing constant linear pressure coefficients dω/dp over a wide range of pressure values.