31 research outputs found
Dispersion of the dielectric function of a charge-transfer insulator
We study the problem of dielectric response in the strong coupling regime of
a charge transfer insulator. The frequency and wave number dependence of the
dielectric function and its inverse is the main object of consideration. We show that the
problem, in general, cannot be reduced to a calculation within the Hubbard
model, which takes into account only a restricted number of electronic states
near the Fermi energy. The contribution of the rest of the system to the
longitudinal response (i.e. to ) is essential
for the whole frequency range. With the use of the spectral representation of
the two-particle Green's function we show that the problem may be divided into
two parts: into the contributions of the weakly correlated and the Hubbard
subsystems. For the latter we propose an approach that starts from the
correlated paramagnetic ground state with strong antiferromagnetic
fluctuations. We obtain a set of coupled equations of motion for the
two-particle Green's function that may be solved by means of the projection
technique. The solution is expressed by a two particle basis that includes the
excitonic states with electron and hole separated at various distances. We
apply our method to the multiband Hubbard (Emery) model that describes layered
cuprates. We show that strongly dispersive branches exist in the excitonic
spectrum of the 'minimal' Emery model () and consider the
dependence of the spectrum on finite oxygen hopping and on-site
repulsion . The relationship of our calculations to electron energy loss
spectroscopy is discussed.Comment: 22 pages, 5 figure
The dynamics of a hole in a CuO_4 plaquette: electron energy-loss spectroscopy of Li_2CuO_2
We have measured the energy and momentum dependent loss function of Li_2CuO_2
single crystals by means of electron energy-loss spectroscopy in transmission.
Using the same values for the model parameters, the low-energy features of the
spectrum as well as published Cu 2p_(3/2) x-ray photoemission data of Li_2CuO_2
are well described by a cluster model that consists of a single CuO_4 plaquette
only. This demonstrates that charge excitations in Li_2CuO_2 are strongly
localized.Comment: 5 pages, 5 figure
One-dimensional dynamics of the d-electrons in -NaVO
We have studied the electronic properties of the ladder compound
-NaVO, adopting a joint experimental and theoretical
approach. The momentum-dependent loss function was measured using electron
energy-loss spectroscopy in transmission. The optical conductivity derived from
the loss function by a Kramers-Kronig analysis agrees well with our results
from LSDA+U band-structure calculations upon application of an
antiferromagnetic alignment of the V~3 spins along the legs and an
on-site Coulomb interaction U of between 2 and 3 eV. The decomposition of the
calculated optical conductivity into contributions from transitions between
selected energy regions of the DOS reveals the origin of the observed
anisotropy of the optical conductivity. In addition, we have investigated the
plasmon excitations related to transitions between the vanadium states within
an effective 16 site vanadium cluster model. Good agreement between the
theoretical and experimental loss function was obtained using the hopping
parameters derived from the tight binding fit to the band-structure and
moderate Coulomb interactions between the electrons within the ab plane.Comment: 23 pages, 8 figures; submitted to PR
Valence band excitations in V_2O_5
We present a joint theoretical and experimental investigation of the
electronic and optical properties of vanadium pentoxide. Electron energy-loss
spectroscopy in transmission was employed to measure the momentum-dependent
loss function. This in turn was used to derive the optical conductivity, which
is compared to the results of band structure calculations. A good qualitative
and quantitative agreement between the theoretical and the experimental optical
conductivity was observed. The experimentally observed anisotropy of the
optical properties of V_2O_5 could be understood in the light of an analysis of
the theoretical data involving the decomposition of the calculated optical
conductivity into contributions from transitions into selected energy regions
of the conduction band. In addition, based upon a tight binding fit to the band
structure, values are given for the effective V3d_xy-O2p hopping terms and are
compared to the corresponding values for alpha'-NaV_2O_5.Comment: 6 pages (revtex),6 figures (jpg
Temperature dependence of optical spectral weights in quarter-filled ladder systems
The temperature dependence of the integrated optical conductivity I(T)
reflects the changes of the kinetic energy as spin and charge correlations
develop. It provides a unique way to explore experimentally the kinetic
properties of strongly correlated systems. We calculated I(T) in the frame of a
t-J-V model at quarter-filling for ladder systems, like NaV_2O_5, and show that
the measured strong T dependence of I(T) for NaV_2O_5 can be explained by the
destruction of short range antiferromagnetic correlations. Thus I(T) provides
detailed information about super-exchange and magnetic energy scales.Comment: 4 pages, 5 figure
Electron Correlation Effects in Resonant Inelastic X-ray Scattering of NaV2O5
Element- and site-specific resonant inelastic x-ray scattering spectroscopy
(RIXS) is employed to investigate electron correlation effects in {}. In contrast to single photon techniques, RIXS at the vanadium
edge is able to probe transitions between V d-bands. A sharp energy
loss feature is observed at -1.56 eV, which is well reproduced by a model
calculation including correlation effects. The calculation identifies the loss
feature as excitation between the lower and upper Hubbard bands and permits an
accurate determination of the Hubbard interaction term eV.Comment: 15 pages, four figures, accepted to Phys. Rev. Let
Polarized x-ray absorption spectra of CuGeO3 at the Cu and Ge K edges
Polarized x-ray absorption near edge structure (XANES) spectra at both the Cu
and the Ge K-edges of CuGeO3 are measured and calculated relying on the
real-space multiple-scattering formalism within a one-electron approach. The
polarization components are resolved not only in the unit cell coordinate
system but also in a local frame attached to the nearest neighborhood of the
photoabsorbing Cu atom. In that way, features which resist a particular
theoretical description can be identified. We have found that it is the
out-of-CuO4-plane p_{z'} component which defies the one-electron calculation
based on the muffin-tin potential. For the Ge K-edge XANES, the agreement
between the theory and the experiment appears to be better for those
polarization components which probe more compact local surroundings than for
those which probe regions with lower atomic density. Paper published in Phys.
Rev. B 66, 155119 (2002) and available on-line at
http://link.aps.org/abstract/PRB/v66/e155119.Comment: 15 pages, 6 figures. Published in Physical Review B, abstract
available on-line at http://link.aps.org/abstract/PRB/e15511
Elektronen-Energieverlustspektroskopie von quasi-eindimensionalen Kupraten und Vanadaten
In a combination of experimental and theoretical methods in this thesis the electronic structures of quasi-one-dimensional cuprates and vanadates were studied. For this the momentum-dependent loss function was measured by means of the electron energy-loss spectroscopy in transmission on monocrystals of Li_2CuO_2, CuGeO_3, V_2O_5 and #alpha#'-NaVO_5. The comparison of the experimental data with results from band-structure and cluster calculations allowed conclusions on the mobility and correlations of the electrons in these systems.Available from TIB Hannover / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman