Investigation of A1g phonons in YBa2Cu3O7 by means of LAPW atomic-force calculations (original) (raw)
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Physical Review B, 1997
We report first-principles frozen-phonon calculations for the determination of the force-free geometry and the dynamical matrix of the five Raman-active A 1g modes in YBa 2 Cu 3 O 7. To establish the shape of the phonon potentials atomic forces are calculated within the LAPW method. Two different schemes-the local density approximation (LDA) and a generalized gradient approximation (GGA)-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.
Composition dependence of the in-plane Cu-O bond-stretching LO phonon mode in YBa2Cu3O6+x
Physical Review B, 2008
An inelastic pulsed neutron scattering study was performed on the dependence of the dispersion and spectral intensity of the in-plane Cu-O bond-stretching LO phonon mode on doped charge density. The measurements were made in the time-of-flight mode with the multiangle position sensitive spectrometer of the ISIS facility on single crystals of YBa 2 Cu 3 O 6+x ͑x = 0.15, 0.35, 0.6, 0.7, and 0.95͒. The focus of the study is the in-plane Cu-O bond-stretching LO phonon mode, which is known for strong electron-phonon coupling and unusual dependence on composition and temperature. It is shown that the dispersions for the samples with x = 0.35, 0.6, and 0.7 are similar to the superposition of those for x = 0.15 and 0.95 samples, and cannot be explained in terms of the structural anisotropy. It is suggested that the results are consistent with the model of nanoscale electronic phase separation, with the fraction of the phases being dependent on the doped charge density.
Physical Review B, 2006
The phonon spectra of YBa 2 Cu 4 O 8 polycrystalline samples with different isotope substitution for Ba, Cu, and O have been studied using far-infrared ellipsometry and Raman scattering. The observed isotope shifts of Raman-and infrared-active phonons provide insight into their relative Ba, Cu, and O normal mode content. This allows us to examine mode eigenvectors obtained from lattice dynamical calculations using ab initio electronic structures based on the full potential linear muffin-tin-orbital within the local density approximation. Our Raman results demonstrate that the 104 cm −1 A g phonon is only very weakly mixed, representing almost pure Ba vibrations. Correspondingly the 151 cm −1 phonon represents pure Cu vibrations. The 339, 436, and 498 cm −1 phonon modes involve mainly O vibrations. Considerable Cu-O mixing takes place in the case of the A g phonons at 257 and 605 cm −1 and for the B 3g phonon at 314 cm −1 . These results are in good agreement with the ab initio calculations. The far-infrared experiments reveal that the modes with energies of 115, 297, 502, and 592 cm −1 are mixed vibrations of O and Cu atoms. The phonon mode at 130 cm −1 corresponds to a Ba-Cu-O complex whereas the modes at 191 and 357 cm −1 reflect almost pure Cu and O vibrations, respectively.
Physical Review B, 2002
All Raman-active phonon modes of YBa 2 Cu 3 O 7 are investigated by first-principles linearized augmented plane-wave calculations based on density-functional theory for a fully optimized crystal structure. The calculated frequencies as well as the Raman scattering intensities are in excellent agreement with measured Raman spectra. The effect of site-selective isotope substitution on the Raman spectra is investigated. The substitution not only shifts the phonon frequencies, but also leads to dramatic changes in the scattering intensities.
Non-equilibrium electrons, holes and phonons in YBa2Cu3O7−x
Journal of Physics and Chemistry of Solids, 1993
Abatrnct-We report an observation by picosecond Stokes/anti-Stokes Raman spectroscopy of high non-equilibrium O(4) apical oxygen and Cu-0 plane buckling phonon populations in insulating and metallic YBa2Cu307_x induced by high peak power pulsed laser excitation. Different excitation and probe geometries were used to selectively meaaun the non-equilibrium phonon populations of the two phonons. We attribute the effects to strong coupling of photoexcited carriers to the lattice. A photoinduced metal-insulator transition is observed in insulating YBa2Cu306.3 accompanied with a qualitative change in nonequilibtium behaviour for the two phonona and an increase in carrier lifetime upon increasing doping. A polaronic model is used to discuss the observations.
Oxygen phonon branches in YBa2Cu3O7
Physical Review B, 2004
We report results of inelastic neutron scattering measurements of phonon dispersions in optimally doped YBa2Cu3O6.95 and compare them with model calculations. The focus is on the in-plane oxygen bond-stretching phonon branches. The study of these modes is complicated by anticrossings with c-axis-polarized branches; such effects are interpreted through lattice-dynamical shell-model calculations. The in-plane anisotropy of the bond-stretching phonons was firmly ascertained from measurements on a detwinned sample. Studying not only the in-plane modes involving in-phase motion for the two Cu-O layers within a unit cell but also those with opposite-phase motion was of great help for establishing a clear experimental picture. The measurements confirm that the in-plane oxygen bond-stretching phonon branches disperse steeply downwards from the zone center in both the a and the b directions indicating a strong electron-phonon coupling. For the b-axispolarized bond-stretching phonons, there is an additional feature of considerable interest: a sharp local frequency minimum was found to develop on cooling from room temperature to T = 10 K at wave vector q≈0.27 r.l.u..
In-plane anisotropy and temperature dependence of oxygen phonon modes in YBa2Cu3O6.6
Physica C: Superconductivity, 2003
Inelastic pulsed neutron scattering measurements on YBa 2 Cu 3 O 6.95 single crystals indicate that the sample has a distinct a-b plane anisotropy in the oxygen vibrations. The Cu-O bond-stretching-type phonons, which are suspected to interact strongly with charge, are simultaneously observed along the a and b directions due to a 7-meV splitting arising from the orthorhombicity, even though the sample is twinned. The bond-stretching LO branch with the polarization along a ͑perpendicular to the chain͒ loses intensity beyond the middle of the zone, indicating branch splitting as seen in doped nickelates, with the second branch being located at 10 meV below. The mode along b has a continuous dispersion. These modes show temperature dependence, which parallels that of superconductive order parameter, suggesting significant involvement of phonons in the superconductivity of this compound.
Electron and phonon characteristics of YBa2Cu3O7−δ
Physica C: Superconductivity, 1995
In this work the experimental results of heat capacity are presented for superconducting yttrium ceramics YBa2Cu30 7_ 8 in the temperature interval 5-300 K. The electron component yT was separated in the normal state (above To). For this purpose, the special technique of data processing had been used which allowed one to extract both the electron component of heat capacity yT and the moments of phonon density of states 0 2, O 4 and some effective moment O, attributed to the phonon spectrum boundary. 0921-4534/95/$09.50
Optical Society of America, Journal, B: Optical Physics, 1989
Phonon-mode characterization of orthorhombic and tetragonal YBa 2 Cu 3 O (7- delta) single crystals by Raman spectroscopy. LV GASPAROV, GA EMEL'CHENKO, VD KULAKOVSKII, OV MISOCHKO, VB TIMOFEEV Optical Society of America, Journal, B: Optical Physics 6, 440-447, 3/1989. 76 SOLID-STATE PHYSICS(AH).