38 research outputs found
Correlation effects in solids from first principles
この論文は国立情報学研究所の電子図書館事業により電子化されました。サブゼミFirst principles calculations of bandstructures of crystals are usually based on one-particle theories where the electrons are assumed to move in some effective potential. The most commonly used method is based on density functional theory within the local density approximation (LDA). There is, however, no clear justification for interpreting the one-particle eigenvalues as the bandstructure. Indeed, the LDA failure to reproduce the experimental bandstructure is not uncommon. The most famous example is the bandgap problem in semiconductors and insulators where the LDA generally underestimates the gaps. A rigorous approach for calculating bandstructures or quasiparticle energies is provided by the Green function method. The main ingredient is the self-energy operator which acts like an effective potential but unlike in the LDA, it is nonlocal and energy dependent. The selfenergy contains the effects of exchange and correlations. An approximation to the self-energy which has proven fruitful in a wide range of materials is the so-called GW approximation (GWA). This approximation has successfully cured the LDA problems and has produced bandstructures with a rather high accuracy. For example, bandgaps in s-p semiconductors and insulators can be obtained typically to within 0.1-0.2 eV of the experimental values. Despite its success, the GWA has some problems. One of the most serious problems is its inadequacy to describe satellite structures in photoemission spectra. For example, multiple plasmon satellites observed in alkalis cannot be obtained by the GWA. Recently, a theory based on the cumulant expansion was proposed and shown to remedy this problem. Apart from plasmon satellites which are due to long-range correlations, there are also satellite structures arising from short-range correlations. This type of satellite cannot be described by the cumulant expansion. A t-matrix approach was proposed to account for this. Although traditionally the Green function method is used to calculate excitation spectra, groundstate energies can also be obtained from the Green function. Recent works on the electron gas have shown promising results and some approaches for calculating total energies will be discussed
A First Principles Scheme for Calculating the Electronic Structure of Strongly Correlated Materials: GW+DMFT
We give a detailed description of a recently proposed first principles
approach to the electronic structure of strongly correlated materials. The
method combines the GW approximation with dynamical mean field theory. It is
designed to describe Coulomb interactions and screening effects without
adjustable parameters, thus avoiding the conceptual problems inherent to
LDA+DMFT techniques.Comment: 18 pages, 3 figures, proceedings of the conference on "Coincidence
Studies of Surfaces, Thin Films and Nanostructures", Ringberg castle, Sept.
2003. To be published by Wile
Correlation Effects in Orbital Magnetism
Orbital magnetization is known empirically to play an important role in
several magnetic phenomena, such as permanent magnetism and ferromagnetic
superconductivity. Within the recently developed ''modern theory of orbital
magnetization'', theoretical insight has been gained into the nature of this
often neglected contribution to magnetism, but is based on an underlying
mean-field approximation. From this theory, a few treatments have emerged which
also take into account correlations beyond the mean-field approximation. Here,
we apply the scheme developed in a previous work [Phys. Rev. B , 161104(R) (2016)] to the Haldane-Hubbard model to investigate the
effect of charge fluctuations on the orbital magnetization within the
approximation. Qualitatively, we are led to distinguish between two quite
different situations: (i) When the lattice potential is larger than the nearest
neighbor hopping, the correlations are found to boost the orbital
magnetization. (ii) If the nearest neighbor hopping is instead larger than the
lattice potential, the correlations reduce the magnetization.Comment: 8 pages, 9 figure
Self-energy calculation of the hydrogen atom: Importance of the unbound states
We present the calculation of the self-energy of the isolated hydrogen atom within the GW approximation starting from the noninteracting Green's function constructed from the exact wave functions of the hydrogen atom. The error in the electron removal energy of the 1s state is found to be about 0.02 eV, which is much smaller than what one would expect. This small error is explained by the cancellation of the self-screening errors between different l contributions of the self-energy. The unbound continuum states are found to be crucial to get the correct self-energy
Dynamical screening in La2CuO4
We show that the dynamical screening of the Coulomb interaction among Cu-d
electrons in high-Tc cuprates is very strong and that a proper treatment of
this effect is essential for a consistent description of the electronic
structure. In particular, we find that ab-initio calculations for undoped
La2CuO4 yield an insulator only if the frequency dependence of the Coulomb
interaction is taken into account. We also identify a collective excitation in
the screened interaction at 9 eV which is rather localized on the copper site,
and which is responsible for a satellite structure at energy -13 eV, located
below the p bands
Multitier self-consistent +EDMFT
We discuss a parameter-free and computationally efficient ab initio
simulation approach for moderately and strongly correlated materials, the
multitier self-consistent +EDMFT method. This scheme treats different
degrees of freedom, such as high-energy and low-energy bands, or local and
nonlocal interactions, within appropriate levels of approximation, and provides
a fully self-consistent description of correlation and screening effects in the
solid. The ab initio input is provided by a one-shot calculation,
while the strong-correlation effects originating from narrow bands near the
Fermi level are captured by a combined plus extended dynamical mean-field
(EDMFT) treatment. We present the formalism and technical details of our
implementation and discuss some general properties of the effective EDMFT
impurity action. In particular, we show that the retarded impurity interactions
can have non-causal features, while the physical observables, such as the
screened interactions of the lattice system, remain causal. We then turn to
stretched sodium as a model system to explore the performance of the multitier
self-consistent +EDMFT method in situations with different degrees of
correlation. While the results for the physical lattice spacing show that
the scheme is not very accurate for electron-gas like systems, because nonlocal
corrections beyond are important, it does provide physically correct
results in the intermediate correlation regime, and a Mott transition around a
lattice spacing of . Remarkably, even though the Wannier functions in
the stretched compound are less localized, and hence the bare interaction
parameters are reduced, the self-consistently computed impurity interactions
show the physically expected trend of an increasing interaction strength with
increasing lattice spacing.Comment: 22 pages, 19 figure
Position Representation of Effective Electron-Electron Interactions in Solids
An essential ingredient in many model Hamiltonians, such as the Hubbard
model, is the effective electron-electron interaction , which enters as
matrix elements in some localized basis. These matrix elements provide the
necessary information in the model, but the localized basis is incomplete for
describing . We present a systematic scheme for computing the manifestly
basis-independent dynamical interaction in position representation, , and its Fourier transform to time domain, . These functions can serve as an unbiased tool for the construction
of model Hamiltonians. For illustration we apply the scheme within the
constrained random-phase approximation to the cuprate parent compounds
LaCuO and HgBaCuO within the commonly used 1- and 3-band
models, and to non-superconducting SrVO within the model. Our
method is used to investigate the shape and strength of screening channels in
the compounds. We show that the O 2Cu 3 screening gives
rise to regions with strong attractive static interaction in the minimal
(1-band) model in both cuprates. On the other hand, in the minimal ()
model of SrVO only regions with a minute attractive interaction are found.
The temporal interaction exhibits generic damped oscillations in all compounds,
and its time-integral is shown to be the potential caused by inserting a frozen
point charge at . When studying the latter within the three-band model
for the cuprates, short time intervals are found to produce a negative
potential.Comment: 15 pages, 13 figure
Ab-initio procedure for effective models of correlated materials with entangled band structure
We present a first-principles method for deriving effective low-energy models
of electrons in solids having entangled band structure. The procedure starts
with dividing the Hilbert space into two subspaces, the low-energy part ("
space'') and the rest of the space (" space''). The low-energy model is
constructed for the space by eliminating the degrees of freedom of the
space. The thus derived model contains the strength of electron correlation
expressed by a partially screened Coulomb interaction, calculated in the
constrained random-phase-approximation (cRPA) where screening channels within
the space, , are subtracted. One conceptual problem of this
established downfolding method is that for entangled bands it is not clear how
to cut out the space and how to distinguish from the total
polarization. Here, we propose a simple procedure to overcome this difficulty.
In our scheme, the subspace is cut out from the Hilbert space of the Kohn
Sham eigenfunctions with the help of a procedure to construct a localized
Wannier basis. The subspace is constructed as the complementary space
orthogonal to the subspace. After this disentanglement, becomes well
defined. Using the disentangled bands, the effective parameters are uniquely
determined in the cRPA. The method is successfully applied to 3 transition
metals.Comment: 14 pages, 4 figure
Combined GW and dynamical mean field theory: Dynamical screening effects in transition metal oxides
We present the first dynamical implementation of the combined GW and
dynamical mean field scheme ("GW+DMFT") for first principles calculations of
the electronic properties of correlated materials. The application to the
ternary transition metal oxide SrVO3 demonstrates that this schemes inherits
the virtues of its two parent theories: a good description of the local low
energy correlation physics encoded in a renormalized quasi-particle band
structure, spectral weight transfer to Hubbard bands, and the physics of
screening driven by long-range Coulomb interactions. Our data is in good
agreement with available photoemission and inverse photoemission spectra; our
analysis leads to a reinterpretation of the commonly accepted "three-peak
structure" as originating from orbital effects rather than from the electron
addition peak within the t2g manifold.Comment: replaced with published version (6 pages, 3 figures); first version
was submitted to PRL on June 19, 201