24 research outputs found
Phenomenological Modeling of Photoemission Spectra in Strongly Correlated Electron Systems
A phenomenological approach is presented that allows one to model, and
thereby interpret, photoemission spectra of strongly correlated electron
systems. A simple analytical formula for the self-energy is proposed. This
self-energy describes both coherent and incoherent parts of the spectrum
(quasiparticle and Hubbard peaks, respectively). Free parameters in the
expression are determined by fitting the density of states to experimental
photoemission data. An explicit fitting is presented for the
LaSrTiO system with . In general, our
phenomenological approach provides information on the effective mass, the
Hubbard interaction, and the spectral weight distribution in different parts of
the spectrum. Limitations of this approach are also discussed.Comment: 13 pages, 4 figures, IJMPB style (included
Correlation Strength, Gaps and Particle-Hole Asymmetry in High-Tc Cuprates: a Dynamical Mean Field Study of the Three-Band Copper-Oxide Model
The three-band model relevant to high temperature copper-oxide
superconductors is solved using single-site dynamical mean field theory and a
tight-binding parametrization of the copper and oxygen bands. For a band
filling of one hole per unit cell the metal/charge-transfer-insulator phase
diagram is determined. The electron spectral function, optical conductivity and
quasiparticle mass enhancement are computed as functions of electron and hole
doping for parameters such that the corresponding to the paramagnetic metal and
charge-transfer insulator sides of the one hole per cell phase diagram. The
optical conductivity is computed using the Peierls phase approximation for the
optical matrix elements. The calculation includes the physics of "Zhang-Rice
singlets". The effects of antiferromagnetism on the magnitude of the gap and
the relation between correlation strength and doping-induced changes in state
density are determined. Three band and one band models are compared. The two
models are found to yield quantitatively consistent results for all energies
less than about 4eV, including energies in the vicinity of the charge-transfer
gap. Parameters on the insulating side of the metal/charge-transfer insulator
phase boundary lead to gaps which are too large and near-gap conductivities
which are too small relative to data. The results place the cuprates clearly in
the intermediate correlation regime, on the paramagnetic metal side of the
metal/charge-transfer insulator phase boundary.Comment: 9 pages, 6 figures. accepted by Phys. Rev.
Electronic Raman scattering in a multiband model for cuprate superconductors
Charge-charge, current-current and Raman correlation functions are derived in
a consistent way using the unified response theory. The theory is based on the
improved description of the conduction electron coupling to the external
electromagnetic fields, distinguishing further the direct and indirect
(assisted) scattering on the quasi-static disorder. The two scattering channels
are distinguished in terms of the energy and momentum conservation laws. The
theory is illustrated on the Emery three-band model for the normal state of the
underdoped high- cuprates which includes the incoherent electron
scattering on the disorder associated with the quasi-static fluctuations around
the static antiferromagnetic (AF) ordering. It is shown, for the first time
consistently, that the incoherent indirect processes dominate the low-frequency
part of the Raman spectra, while the long-range screening which is dynamic
removes the long-range forces in the channel. In the mid-infrared
frequency range the coherent AF processes are dominant. In contrast to the
nonresonant response, which is large by itself, the resonant interband
transitions enhance both the and Raman spectra to comparable
values, in good agreement with experimental observation. It is further argued
that the AF correlations give rise to the mid-infrared peak in the
Raman spectrum, accompanied by a similar peak in the optical conductivity. The
doping behavior of these peaks is shown to be correlated with the linear doping
dependence of the Hall number, as observed in all underdoped high-
compounds.Comment: 18 pages, 14 figures; to appear in Phys. Rev.
The Cerium volume collapse: Results from the LDA+DMFT approach
The merger of density-functional theory in the local density approximation
(LDA) and many-body dynamical mean field theory (DMFT) allows for an ab initio
calculation of Ce including the inherent 4f electronic correlations. We solve
the DMFT equations by the quantum Monte Carlo (QMC) technique and calculate the
Ce energy, spectrum, and double occupancy as a function of volume. At low
temperatures, the correlation energy exhibits an anomalous region of negative
curvature which drives the system towards a thermodynamic instability, i.e.,
the -to- volume collapse, consistent with experiment. The
connection of the energetic with the spectral evolution shows that the physical
origin of the energy anomaly and, thus, the volume collapse is the appearance
of a quasiparticle resonance in the 4f-spectrum which is accompanied by a rapid
growth in the double occupancy.Comment: 4 pages, 3 figure
Dynamical Mean-Field Theory and Its Applications to Real Materials
Dynamical mean-field theory (DMFT) is a non-perturbative technique for the
investigation of correlated electron systems. Its combination with the local
density approximation (LDA) has recently led to a material-specific
computational scheme for the ab initio investigation of correlated electron
materials. The set-up of this approach and its application to materials such as
(Sr,Ca)VO_3, V_2O_3, and Cerium is discussed. The calculated spectra are
compared with the spectroscopically measured electronic excitation spectra. The
surprising similarity between the spectra of the single-impurity Anderson model
and of correlated bulk materials is also addressed.Comment: 20 pages, 9 figures, invited paper for the JPSJ Special Issue "Kondo
Effect - 40 Years after the Discovery"; final version, references adde
Plaquette operators used in the rigorous study of ground-states of the Periodic Anderson Model in dimensions
The derivation procedure of exact ground-states for the periodic Anderson
model (PAM) in restricted regions of the parameter space and D=2 dimensions
using plaquette operators is presented in detail. Using this procedure, we are
reporting for the first time exact ground-states for PAM in 2D and finite value
of the interaction, whose presence do not require the next to nearest neighbor
extension terms in the Hamiltonian. In order to do this, a completely new type
of plaquette operator is introduced for PAM, based on which a new localized
phase is deduced whose physical properties are analyzed in detail. The obtained
results provide exact theoretical data which can be used for the understanding
of system properties leading to metal-insulator transitions, strongly debated
in recent publications in the frame of PAM. In the described case, the lost of
the localization character is connected to the break-down of the long-range
density-density correlations rather than Kondo physics.Comment: 34 pages, 5 figure
Heavy Quasi-Particle in the Two-Orbital Hubbard Model
The two-orbital Hubbard model with the Hund coupling is investigated in a
metallic phase close to the Mott insulator. We calculate the one-particle
spectral function and the optical conductivity within dynamical mean field
theory, for which the effective impurity problem is solved by using the
non-crossing approximation. For a metallic system close to quarter filling, a
heavy quasi-particle band is formed by the Hubbard interaction, the effective
mass of which is not so sensitive to the orbital splitting and the Hund
coupling. In contrast, a heavy quasi-particle band near half filling disappears
in the presence of the orbital splitting, but is induced again by the
introduction of the Hund coupling, resulting in a different type of heavy
quasi-particles.Comment: 6page, 7eps figures, to appear in J. Phys. Soc. Jp
The electronic structure of the heavy fermion metal
The electronic structure of the first reported heavy fermion compound without
f-electrons LiV_2O_4 was studied by an ab-initio calculation method. In the
result of the trigonal splitting and d-d Coulomb interaction one electron of
the configuration of V ion is localized and the rest partially fills
a relatively broad conduction band. The effective Anderson impurity model was
solved by Non-Crossing-Approximation method, leading to an estimation for the
single-site Kondo energy scale T_K. Then, we show how the so-called exhaustion
phenomenon of Nozi\`eres for the Kondo lattice leads to a remarkable decrease
of the heavy-fermion (or coherence) energy scale (D
is the typical bandwidth), comparable to the experimental result.Comment: 4 pages, RevTeX; 3 figures in format .eps. submitted to PR
The spectral and magnetic properties of - and -Ce from the Dynamical Mean-Field Theory and Local Density Approximation
We have calculated ground state properties and excitation spectra for Ce
metal with the {\it ab initio} computational scheme combining local density
approximation and dynamical mean-field theory (LDA+DMFT). We considered all
electronic states, i.e. correlated f-states and non-correlated s-, p- and
d-states. The strong local correlations (Coulomb interaction) among the
f-states lead to typical many-body resonances in the partial f-density, such as
lower and upper Hubbard band. Additionally the well known Kondo resonance is
observed. The s-, p- and d-densities show small to mediate renormalization
effects due to hybridization. We observe different Kondo temperatures for
- and -Ce ( and
), due to strong volume dependence of the effective
hybridization strength for the localized f-electrons. Finally we compare our
results with a variety of experimental data, i.e. from photoemission
spectroscopy (PES), inverse photoemission spectroscopy (BIS), resonant inverse
photoemission spectroscopy (RIPES) and magnetic susceptibility measurements.Comment: 7 pages, 4 figure
Consistent LDA'+DMFT approach to electronic structure of transition metal oxides: charge transfer insulators and correlated metals
We discuss the recently proposed LDA'+DMFT approach providing consistent
parameter free treatment of the so called double counting problem arising
within the LDA+DMFT hybrid computational method for realistic strongly
correlated materials. In this approach the local exchange-correlation portion
of electron-electron interaction is excluded from self consistent LDA
calculations for strongly correlated electronic shells, e.g. d-states of
transition metal compounds. Then the corresponding double counting term in
LDA+DMFT Hamiltonian is consistently set in the local Hartree (fully localized
limit - FLL) form of the Hubbard model interaction term. We present the results
of extensive LDA'+DMFT calculations of densities of states, spectral densities
and optical conductivity for most typical representatives of two wide classes
of strongly correlated systems in paramagnetic phase: charge transfer
insulators (MnO, CoO and NiO) and strongly correlated metals (SrVO3 and
Sr2RuO4). It is shown that for NiO and CoO systems LDA'+DMFT qualitatively
improves the conventional LDA+DMFT results with FLL type of double counting,
where CoO and NiO were obtained to be metals. We also include in our
calculations transition metal 4s-states located near the Fermi level missed in
previous LDA+DMFT studies of these monooxides. General agreement with optical
and X-ray experiments is obtained. For strongly correlated metals
LDA+DMFT results agree well with earlier LDA+DMFT calculations and
existing experiments. However, in general LDA'+DMFT results give better
quantitative agreement with experimental data for band gap sizes and oxygen
states positions, as compared to the conventional LDA+DMFT.Comment: 13 pages, 11 figures, 1 table. In v2 there some additional
clarifications are include