541 research outputs found
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
Realistic many-body models for Manganese Monoxide under pressure
In materials like transition metals oxides where electronic Coulomb
correlations impede a description in terms of standard band-theories, the
application of genuine many-body techniques is inevitable. Interfacing the
realism of density-functional based methods with the virtues of Hubbard-like
Hamiltonians, requires the joint ab initio construction of transfer integrals
and interaction matrix elements (like the Hubbard U) in a localized basis set.
In this work, we employ the scheme of maximally localized Wannier functions and
the constrained random phase approximation to create effective low-energy
models for Manganese monoxide, and track their evolution under external
pressure. We find that in the low pressure antiferromagnetic phase, the
compression results in an increase of the bare Coulomb interaction for specific
orbitals. As we rationalized in recent model considerations [Phys. Rev. B 79,
235133 (2009)], this seemingly counter-intuitive behavior is a consequence of
the delocalization of the respective Wannier functions. The change of screening
processes does not alter this tendency, and thus, the screened on-site
component of the interaction - the Hubbard U of the effective low-energy system
- increases with pressure as well. The orbital anisotropy of the effects
originates from the orientation of the orbitals vis-a-vis the deformation of
the unit-cell. Within the high pressure paramagnetic phase, on the other hand,
we find the significant increase of the Hubbard U is insensitive to the orbital
orientation and almost exclusively owing to a substantial weakening of
screening channels upon compression.Comment: 13 pages, 6 figure
Charge redistribution at Pd surfaces: ab initio grounds for tight-binding interatomic potentials
A simplified tight-binding description of the electronic structure is often
necessary for complex studies of surfaces of transition metal compounds. This
requires a self-consistent parametrization of the charge redistribution, which
is not obvious for late transition series elements (such as Pd, Cu, Au), for
which not only d but also s-p electrons have to be taken into account. We show
here, with the help of an ab initio FP-LMTO approach, that for these elements
the electronic charge is unchanged from bulk to the surface, not only per site
but also per orbital. This implies different level shifts for each orbital in
order to achieve this orbital neutrality rule. Our results invalidate any
neutrality rule which would allow charge redistribution between orbitals to
ensure a common rigid shift for all of them. Moreover, in the case of Pd, the
power law which governs the variation of band energy with respect to
coordination number, is found to differ significantly from the usual
tight-binding square root.Comment: 6 pages, 2 figures, Latex; Phys.Rev. B 56 (1997
Electronic Structure and Lattice Relaxation Related to Fe in Mgo
The electronic structure of Fe impurity in MgO was calculated by the linear
muffin-tin orbital--full-potential method within the conventional local-density
approximation (LDA) and making use of the LDA+ formalism. The importance of
introducing different potentials, depending on the screened Coulomb integral
, is emphasized for obtaining a physically reasonable ground state of the
Fe ion configuration. The symmetry lowering of the ion electrostatic
field leads to the observed Jahn--Teller effect; related ligand relaxation
confined to tetragonal symmetry has been optimized based on the full-potential
total energy results. The electronic structure of the Fe ion is also
calculated and compared with that of Fe.Comment: 13 pages + 4 PostScript figures, Revtex 3.0, SISSA-CM-94-00
Extended moment formation and magnetic ordering in the trigonal chain compound Ca3Co2O6
The results of electronic structure calculations for the one-dimensional
magnetic chain compound Ca3Co2O6 are presented. The calculations are based on
density functional theory and the local density approximation and used the
augmented spherical wave (ASW) method. Our results allow for deeper
understanding of recent experimental findings. In particular, alternation of Co
3d low- and high-spin states along the characteristic chains is related to
differences in the oxygen coordination at the inequivalent cobalt sites. Strong
hybridization of the d states with the O 2p states lays ground for polarization
of the latter and the formation of extended localized magnetic moments centered
at the high-spin sites. In contrast, strong metal-metal overlap along the
chains gives rise to intrachain ferromagnetic exchange coupling of the extended
moments via the d_{3z^2-r^2} orbitals of the low-spin cobalt atoms.Comment: 10 pages, 4 figures more information at
http://www.physik.uni-augsburg.de/~eyert
Magnetic polarons and magnetoresistance in EuB6
EuB6 is a low carrier density ferromagnet which exhibits large
magnetoresistance, positive or negative depending on temperature. The formation
of magnetic polarons just above the magnetic critical temperature has been
suggested by spin-flip Raman scattering experiments. We find that the fact that
EuB6 is a semimetal has to be taken into account to explain its electronic
properties, including magnetic polarons and magnetoresistance.Comment: 6 pages, 1 figur
Phonon Properties of Knbo3 and Ktao3 from First-Principles Calculations
The frequencies of transverse-optical phonons in KNbO and
KTaO are calculated in the frozen-phonon scheme making use of the
full-potential linearized muffin-tin orbital method. The calculated frequencies
in the cubic phase of KNbO and in the tetragonal ferroelectric phase are in
good agreement with experimental data. For KTaO, the effect of lattice
volume was found to be substantial on the frequency of the soft mode, but
rather small on the relative displacement patterns of atoms in all three modes
of the symmetry. The TO frequencies in KTaO are found to be of the
order of, but somehow higher than, the corresponding frequencies in cubic
KNbO.Comment: 8 pages + 1 LaTeX figure, Revtex 3.0, SISSA-CM-94-00
An analysis of photoemission and inverse photoemission spectra of Si(111) and sulphur-passivated InP(001) surfaces
Photoemission (PES) and inverse-photoemission spectra (IPES) for the
sulphur-passivated InP(001) surface are compared with theoretical predictions
based on density-functional calculations. As a test case for our methods, we
also present a corresponding study of the better known Si(111) surface. The
reported spectra for InP(001)-S agree well with the calculated ones if the
surface is assumed to consist of a mixture of two phases, namely, the fully
S-covered -reconstructed structure, which contains four S atoms in
the surface unit-cell, and a structure containing two S and two P
atoms per unit cell. The latter has recently been identified in total-energy
calculations as well as in core-level spectra of S-passivated
Si(111)- is in excellent agreement with the calculations. The
comparison of the experimental-PES with our calculations provides additional
considerations regarding the nature of the sample surface. It is also found
that the commonly-used density-of-states approximation to the photo- and
inverse- photoemission spectra is not valid for these systems.Comment: Submitted to Phys. Rev. B; 6 postscript formatted pages; 7 figures in
gif format; postscript figures available upon reques
A new charge-transfer complex in UHV co-deposited tetramethoxypyrene and tetracyanoquinodimethane
UHV-deposited films of the mixed phase of tetramethoxypyrene and
tetracyanoquinodimethane (TMP1-TCNQ1) on gold have been studied using
ultraviolet photoelectron spectroscopy (UPS), X-ray-diffraction (XRD), infrared
(IR) spectroscopy and scanning tunnelling spectroscopy (STS). The formation of
an intermolecular charge-transfer (CT) compound is evident from the appearance
of new reflexes in XRD (d1= 0.894 nm, d2= 0.677 nm). A softening of the CN
stretching vibration (red-shift by 7 cm-1) of TCNQ is visible in the IR
spectra, being indicative of a CT of the order of 0.3e from TMP to TCNQ in the
complex. Characteristic shifts of the electronic level positions occur in UPS
and STS that are in reasonable agreement with the prediction of from DFT
calculations (Gaussian03 with hybrid functional B3LYP). STS reveals a HOMO-LUMO
gap of the CT complex of about 1.25 eV being much smaller than the gaps (>3.0
eV) of the pure moieties. The electron-injection and hole-injection barriers
are 0.3 eV and 0.5 eV, respectively. Systematic differences in the positions of
the HOMOs determined by UPS and STS are discussed in terms of the different
information content of the two methods.Comment: 20 pages, 6 figure
Systematic Control of Carrier Doping without Disorder at Interface of Oxide Heterostructures
We propose a method to systematically control carrier densities at the
interface of transition-metal oxide heterostructures without introducing
disorders. By inserting non-polar layers sandwiched by polar layers, continuous
carrier doping into the interface can be realized. This method enables us to
control the total carrier densities per unit cell systematically up to high
values of the order unity.Comment: 8 pages, 9 figure
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