276 research outputs found
Robust half-metallic antiferromagnets LaVOsO and LaMoO ( = Ca, Sr, Ba; = Re, Tc) from first-principles calculations
We have theoretically designed three families of the half-metallic (HM)
antiferromagnets (AFM), namely, LaVOsO, LaMoTcO and
LaMoReO ( = Ca, Sr, Ba), based on a systematic {\it ab initio} study
of the ordered double perovskites LaO with the possible and
pairs from all the 3, 4 and 5 transtion metal elements being
considered. Electronic structure calculations based on first-principles
density-functional theory with generalized gradient approximation (GGA) for
more than sixty double perovskites LaCaO have been performed using the
all-electron full-potential linearized augmented-plane-wave method. The found
HM-AFM state in these materials survives the full {\it ab initio} lattice
constant and atomic position optimizations which were carried out using
frozen-core full potential projector augmented wave method. It is found that
the HM-AFM properties predicted previously in some of the double perovskites
would disappear after the full structural optimizations. The AFM is attributed
to both the superexchange mechanism and the generalized double exchange
mechanism via the () - O (2) - () coupling
and the latter is also believed to be the origin of the HM. Finally, in our
search for the HM-AFMs, we find LaCrTcO and LaCrReO to be AFM
insulators of an unconventional type in the sense that the two
antiferromagnetic coupled ions consist of two different elements and that the
two spin-resolved densities of states are no longer the same.Comment: To appear in Phys. Rev.
Orbital densities functional
Local density approximation (LDA) to the density functional theory (DFT) has
continuous derivative of total energy as a number of electrons function and
continuous exchange-correlation potential, while in exact DFT both should be
discontinuous as number of electrons goes through an integer value. We propose
orbital densities functional (ODF) (with orbitals defined as Wannier functions)
that by construction obeys this discontinuity condition. By its variation
one-electron equations are obtained with potential in the form of projection
operator. The operator increases a separation between occupied and empty bands
thus curing LDA deficiency of energy gap value systematic underestimation.
Orbital densities functional minimization gives ground state orbital and total
electron densities. The ODF expression for the energy of orbital densities
fluctuations around the ground state values defines ODF fluctuation Hamiltonian
that allows to treat correlation effects. Dynamical mean-field theory (DMFT)
was used to solve this Hamiltonian with quantum Monte Carlo (QMC) method for
effective impurity problem. We have applied ODF method to the problem of
metal-insulator transition in lanthanum trihydride LaH_{3-x}. In LDA
calculations ground state of this material is metallic for all values of
hydrogen nonstoichiometry x while experimentally the system is insulating for x
< 0.3. ODF method gave paramagnetic insulator solution for LaH_3 and LaH_{2.75}
but metallic state for LaH_{2.5}.Comment: 35 pages, 5 figure
Self-consistency over the charge-density in dynamical mean-field theory: a linear muffin-tin implementation and some physical implications
We present a simple implementation of the dynamical mean-field theory
approach to the electronic structure of strongly correlated materials. This
implementation achieves full self-consistency over the charge density, taking
into account correlation-induced changes to the total charge density and
effective Kohn-Sham Hamiltonian. A linear muffin-tin orbital basis-set is used,
and the charge density is computed from moments of the many body
momentum-distribution matrix. The calculation of the total energy is also
considered, with a proper treatment of high-frequency tails of the Green's
function and self-energy. The method is illustrated on two materials with
well-localized 4f electrons, insulating cerium sesquioxide Ce2O3 and the
gamma-phase of metallic cerium, using the Hubbard-I approximation to the
dynamical mean-field self-energy. The momentum-integrated spectral function and
momentum-resolved dispersion of the Hubbard bands are calculated, as well as
the volume-dependence of the total energy. We show that full self-consistency
over the charge density, taking into account its modification by strong
correlations, can be important for the computation of both thermodynamical and
spectral properties, particularly in the case of the oxide material.Comment: 20 pages, 6 figures (submitted in The Physical Review B
Adlayer core-level shifts of random metal overlayers on transition-metal substrates
We calculate the difference of the ionization energies of a core-electron of
a surface alloy, i.e., a B-atom in a A_(1-x) B_x overlayer on a
fcc-B(001)-substrate, and a core-electron of the clean fcc-B(001) surface using
density-functional-theory. We analyze the initial-state contributions and the
screening effects induced by the core hole, and study the influence of the
alloy composition for a number of noble metal-transition metal systems. Data
are presented for Cu_(1-x)Pd_x/Pd(001), Ag_(1-x) Pd_x/Pd(001), Pd_(1-x)
Cu_x/Cu(001), and Pd_(1-x) Ag_x/Ag(001), changing x from 0 to 100 %. Our
analysis clearly indicates the importance of final-state screening effects for
the interpretation of measured core-level shifts. Calculated deviations from
the initial-state trends are explained in terms of the change of inter- and
intra-atomic screening upon alloying. A possible role of alloying on the
chemical reactivity of metal surfaces is discussed.Comment: 4 pages, 2 figures, Phys. Rev. Letters, to appear in Feb. 199
Potential, core-level and d band shifts at transition metal surfaces
We have extended the validity of the correlation between the surface
3d-core-level shift (SCLS) and the surface d band shift (SDBS) to the entire 4d
transition metal series and to the neighboring elements Sr and Ag via accurate
first-principles calculations. We find that the correlation is quasilinear and
robust with respect to the differencies both between initial and final-state
calculations of the SCLS's and two distinct measures of the SDBS's. We show
that despite the complex spatial dependence of the surface potential shift
(SPS) and the location of the 3d and 4d orbitals in different regions of space,
the correlation exists because the sampling of the SPS by the 3d and 4d
orbitals remains similar. We show further that the sign change of the SCLS's
across the transition series does indeed arise from the d band-narrowing
mechanism previously proposed. However, while in the heavier transition metals
the predicted increase of d electrons in the surface layer relative to the bulk
arises primarily from transfers from s and p states to d states within the
surface layer, in the lighter transition metals the predicted decrease of
surface d electrons arises primarily from flow out into the vacuum.Comment: RevTex, 22 pages, 5 figures in uufiles form, to appear in Phys.Rev.
Giant magnetic enhancement in Fe/Pd films and its influence on the magnetic interlayer coupling
The magnetic properties of thin Pd fcc(001) films with embedded monolayers of
Fe are investigated by means of first principles density functional theory. The
induced spin polarization in Pd is calculated and analyzed in terms of quantum
interference within the Fe/Pd/Fe bilayer system. An investigation of the
magnetic enhancement effects on the spin polarization is carried out and its
consequences for the magnetic interlayer coupling are discussed. In contrast to
{\it e.g.} the Co/Cu fcc(001) system we find a large effect on the magnetic
interlayer coupling due to magnetic enhancement in the spacer material. In the
case of a single embedded Fe monolayer we find aninduced Pd magnetization
decaying with distance from the magnetic layer as ~ with
. For the bilayer system we find a giant magnetic
enhancement (GME) that oscillates strongly due to interference effects. This
results in a strongly modified magnetic interlayer coupling, both in phase and
magnitude, which may not be described in the pure
Ruderman-Kittel-Kasuya-Yoshida (RKKY) picture. No anti-ferromagnetic coupling
was found and by comparison with magnetically constrained calculations we show
that the overall ferromagnetic coupling can be understood from the strong
polarization of the Pd spacer
Full orbital calculation scheme for materials with strongly correlated electrons
We propose a computational scheme for the ab initio calculation of Wannier
functions (WFs) for correlated electronic materials. The full-orbital
Hamiltonian H is projected into the WF subspace defined by the physically most
relevant partially filled bands. The Hamiltonian H^{WF} obtained in this way,
with interaction parameters calculated by constrained LDA for the Wannier
orbitals, is used as an ab initio setup of the correlation problem, which can
then be solved by many-body techniques, e.g., dynamical mean-field theory
(DMFT). In such calculations the self-energy operator \Sigma(e) is defined in
WF basis which then can be converted back into the full-orbital Hilbert space
to compute the full-orbital interacting Green function G(r,r',e). Using
G(r,r',e) one can evaluate the charge density, modified by correlations,
together with a new set of WFs, thus defining a fully self-consistent scheme.
The Green function can also be used for the calculation of spectral, magnetic
and electronic properties of the system. Here we report the results obtained
with this method for SrVO3 and V2O3. Comparisons are made with previous results
obtained by the LDA+DMFT approach where the LDA DOS was used as input, and with
new bulk-sensitive experimental spectra.Comment: 36 pages, 14 figure
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