18 research outputs found
Nonlocal electron-phonon interaction as a source of dynamic charge stripes in the cuprates
We calculate for La2CuO4 the phonon-induced redistribution of the electronic
charge density in the insulating, the underdoped pseudogap and the more
conventional metallic state as obtained for optimal and overdoping,
respectively. The investigation is performed for the anomalous
high-frequency-oxygen-bond stretching modes (OBSM) which experimentally are
known to display a strong softening of the frequencies upon doping in the
cuprates. This most likely generic anomalous behaviour of the OBSM has been
shown to be due to a strong nonlocal electron-phonon interaction (EPI) mediated
by charge fluctuations on the ions. The modeling of the competing electronic
states of the cuprates is achieved in terms of consecutive orbital selective
incompressibility-compressibility transitions for the charge response. We
demonstrate that the softening of the OBSM in these states is due to nonlocally
induced dynamic charge inhomogenities in form of charge stripes along the CuO
bonds with different orbital character. Thus, a multi-orbital approach is
essential for the CuO plane. The dynamic charge inhomogeneities may in turn be
considered as precursors of static charge stripe order as recently observed in
LaBaCuO in a broad range of doping around x=1/8. The latter
may trigger a reconstruction of the Fermi surface into small pockets with
reduced doping. We argue that the incompressibility of the Cu3d orbital and
simultaneously the compressibility of the O2p orbital in the pseudogap state
seems to be required to nucleate dynamic stripes.Comment: 10 pages, 4 figures, to be published in "Advances in Condensed Matter
Physics
A microscopic modeling of phonon dynamics and charge response in metallic BaBiO
We use our recently proposed microscopic modeling in the framework of linear
response theory to investigate the complete phonon dispersion, the phonon
density of states, certain phonon-induced electronic charge distributions and
charge fluctuations (CF's) for anomalous soft modes of metallic BaBiO in
its simple cubic phase where superconductivity with up to 32 K appears.
The theoretical approach already has been applied successfully to the cuprate
high-temperature superconductors (HTSC's), simple ionic crystals (NaCl, MgO)
and perovskite oxides (SrTiO, BaTiO). It is well suited for
materials with a strong component of ionic binding and especially for "ionic"
metals. In particular, the giant phonon anomalies related to the breathing
vibration of the oxygen as found experimentally in superconducting doped
BaKBiO, resembling those observed in the high
cuprates, are investigated. The origin of these anomalies is explored and
attributed to a strong nonlocal coupling of the displaced oxygen ions to CF's
of ionic type, essentially of the Bi6s- and Bi6p orbital. This points to the
importance of both of these states at the Fermi energy. Starting from an
ab-initio rigid ion model (RIM) we calculate the effect on the lattice dynamics
and charge response of the most important electronic polarization processes in
the material, i.e. CF's and dipole fluctuations (DF's). Taking into account
these electronic degrees of freedom in linear response theory, we obtain a good
agreement with the measured phonon dispersion and in particular with the strong
phonon anomalies.Comment: Additional comparison with the cuprate HTSC's. A slightly shorter
version has been published in PR
Modeling of the electronic state of the High-Temperature Superconductor LaCuO: Phonon dynamics and charge response
A modeling of the normal state of the p-doped high-temperature
superconductors (HTSC's) is presented. This is achieved starting from a more
conventional metallic phase for optimal- and overdoping and passing via the
underdoped to the insulating state by consecutive orbital selective
compressibility-incompressibility transitions in terms of sum rules for the
charge response. The modeling is substantiated by corresponding phonon
calculations. Extending investigations of the full dispersion and in particular
of the strongly doping dependent anomalous phonon modes in LaCuO, which so far
underpin our treatment of the density response of the electrons in the p-doped
HTSC's, gives additional support for the modeling of the electronic state,
compares well with recent experimental data and predicts the dispersion for the
overdoped regime. Moreover, phonon densities of states have been calculated and
compared for the insulating, underdoped, optimally doped and overdoped state of
LaCuO. From our modeling of the normal state a consistent picture of the
superconducting phase also can be extracted qualitatively pointing in the
underdoped regime to a phase ordering transition. On the other hand, the
modeling of the optimal and overdoped state is consistent with a quasi-particle
picture with a well defined Fermi surface. Thus, in the latter case a Fermi
surface instability with an evolution of pairs of well defined quasiparticles
is possible and can lead to a BCS-type ordering. So, it is tempting to
speculate that optimal in the HTSC's marks a crossover region between
these two forms of ordering.Comment: 18 RevTex pages, 10 figures, revised version, references updated,
accepted for publication in Physical Review
Microscopic calculation of the phonon dynamics of SrRuO compared with LaCuO
The phonon dynamics of the low-temperature superconductor SrRuO
is calculated quantitatively in linear response theory and compared with the
structurally isomorphic high-temperature superconductor LaCuO. Our
calculation corrects for a typical deficit of LDA-based calculations which
always predict a too large electronic -dispersion insufficient to
describe the c-axis response in the real materials. With a more realistic
computation of the electronic band structure the frequency and wavevector
dependent irreducible polarization part of the density response function is
determined and used for adiabatic and nonadiabatic phonon calculations. Our
analysis for SrRuO reveals important differences from the lattice
dynamics of - and -doped cuprates. Consistent with experimental evidence
from inelastic neutron scattering the anomalous doping related softening of the
strongly coupling high-frequency oxygen bond-stretching modes (OBSM) which is
generic for the cuprate superconductors is largely suppressed or completely
absent, respectively, depending on the actual value of the on-site Coulomb
repulsion of the Ru4d orbitals. Also the presence of a characteristic
-mode with a very steep dispersion coupling strongly with the
electrons is missing in SrRuO. Moreover, we evaluate the
possibility of a phonon-plasmon scenario for SrRuO which has been
shown recently to be realistic for LaCuO. In contrast to
LaCuO in SrRuO the very low lying plasmons are
overdamped along the c-axis.Comment: 30 pages, 16 figures, 4 tables, 33 reference
Dynamic charge inhomogenity in cuprate superconductors
The inelastic x-ray scattering spectrum for phonons of -symmetry
including the CuO bond-stretching phonon dispersion is analyzed by a Lorentz
fit in HgBaCuO and BiSrCuO, respectively, using
recently calculated phonon frequencies as input parameters. The resulting mode
frequencies of the fit are almost all in good agreement with the calculated
data. An exception is the second highest -branch compromising the
bond-stretching modes which disagrees in both compounds with the calculations.
This branch unlike the calculations shows an anomalous softening with a minimum
around the wavevector \vc{q}=\frac{2\pi}{a}(0.25, 0, 0). Such a disparity
with the calculated results, that are based on the assumption of an undisturbed
translation- and point group invariant electronic structure of the CuO plane,
indicates some {\it static} charge inhomogenities in the measured probes. Most
likely these will be charge stripes along the CuO bonds which have the
strongest coupling to certain longitudinal bond-stretching modes that in turn
selfconsistently induce corresponding {\it dynamic} charge inhomogenities. The
symmetry breaking by the mix of dynamic and static charge inhomogenities can
lead to a reconstruction of the Fermi surface into small pockets.Comment: 7 pages, 4 figure