112 research outputs found
Quasiparticle bands in cuprates by quantum chemical methods: towards an ab initio description of strong electron correlations
Realistic electronic-structure calculations for correlated Mott insulators
are notoriously hard. Here we present an ab initio multiconfiguration scheme
that adequately describes strong correlation effects involving Cu 3d and O 2p
electrons in layered cuprates. In particular, the O 2p states giving rise to
the Zhang-Rice band are explicitly considered. Renormalization effects due to
nonlocal spin interactions are also treated consistently. We show that the
dispersion of the lowest band observed in photoemission is reproduced with
quantitative accuracy. Additionally, the evolution of the Fermi surface with
doping follows directly from our ab initio data. Our results thus open a new
avenue for the first-principles investigation of the electronic structure of
correlated Mott insulators
Correlated electrons in Fe-As compounds: a quantum chemical perspective
State-of-the-art quantum chemical methods are applied to the study of the
multiorbital correlated electronic structure of a Fe-As compound, the recently
discovered LiFeAs. Our calculations predict a high-spin, S=2, ground-state
configuration for the Fe ions, which shows that the on-site Coulomb
interactions are substantial. Also, orbital degeneracy in the (xz,yz) sector
and a three-quarter filling of these levels suggest the presence of strong
fluctuations and are compatible with a low metallic conductivity in the normal
state. The lowest electron-removal states have As 4p character, in analogy to
the ligand hole states in p-type cuprate superconductors
Near degeneracy and pseudo Jahn-Teller effects in mixed-valence ladders: the phase transition in NaVO
We analyze the electronic structure of a mixed-valence ladder system. We find
that structural anisotropy and complex electron correlations lead to on-rung
charge localization and insulating character. Charge fluctuations within the
rung of the ladder interact strongly to the lattice degrees of freedom, which
gives rise to large pseudo Jahn--Teller effects. The phase transition in
NaVO should be driven by this kind of mechanism.Comment: 5 pages, 2 figures, 1 table, submitted to PR
Ab initio determination of excitation energies and magnetic couplings in correlated, quasi two-dimensional iridates
To determine the strength of essential electronic and magnetic interactions
in the iridates SrIrO and BaIrO - potential platforms for
high-temperature superconductivity - we use many-body techniques from
wavefunction-based electronic-structure theory. Multiplet physics, spin-orbit
interactions, and Ir-O hybridization are all treated on equal footing, fully
{\it ab initio}. Our calculations put the lowest d-d excitations of
SrIrO/BaIrO at 0.69/0.64 eV, substantially lower than in
isostructural cuprates. Charge-transfer excitations start at 3.0/1.9 eV and the
magnetic nearest-neighbor exchange coupling is 51/58 meV. Available
experimental results are fully consistent with these values, which strongly
constrains the parametrization of effective iridate Hamiltonians
Correlation-induced corrections to the band structure of boron nitride: a wave-function-based approach
We present a systematic study of the correlation-induced corrections to the
electronic band structure of zinc-blende BN. Our investigation employs an ab
initio wave-function-based local Hamiltonian formalism which offers a rigorous
approach to the calculation of the polarization and local charge redistribution
effects around an extra electron or hole placed into the conduction or valence
bands of semiconducting and insulating materials. Moreover, electron
correlations beyond relaxation and polarization can be readily incorporated.
The electron correlation treatment is performed on finite clusters. In
conducting our study, we make use of localized Wannier functions and embedding
potentials derived explicitly from prior periodic Hartree-Fock calculations.
The on-site and nearest-neighbor charge relaxation bring corrections of several
eV to the Hartree-Fock band gap. Additional corrections are caused by
long-range polarization effects. In contrast, the dispersion of the
Hartree-Fock bands is marginally affected by electron correlations. Our final
result for the fundamental gap of zinc-blende BN compares well with that
derived from soft x-ray experiments at the B and N K-edges.Comment: 18 pages, 8 figures; the following article has been submitted to J.
Chem. Phy
Fermiology of Cuprates from First Principles: From Small Pockets to the Luttinger Fermi surface
Fermiology, the shape and size of the Fermi surface, underpins the
low-temperature physical properties of a metal. Recent investigations of the
Fermi surface of high-Tc superconductors, however, show a most unusual
behavior: upon addition of carriers, ``Fermi'' pockets appear around nodal
(hole doping) and antinodal (electron doping) regions of the Brillouin zone in
the ``pseudogap'' state. With progressive doping, p, these evolve into
well-defined Fermi surfaces around optimal doping (p_opt), with no pseudogap.
Correspondingly, various physical responses, including d-wave
superconductivity, evolve from highly anomalous, up to p_opt, to more
conventional beyond. Describing this evolution holds the key to understanding
high-temperature superconductivity. Here, we present ab initio quantum chemical
results for cuprates, providing a quantitative description of the evolution of
the Fermi surface with doping. Our results constitute an ab initio
justification for several, hitherto proposed semiphenomenological theories,
offering an unified basis for understanding of various, unusual physical
responses of doped cuprates
Ferromagnetic ordering of linearly coordinated Co ions in LiSr[CoN]
LiSr[CoN] single crystals were successfully grown out of Li-rich
flux. Temperature- and field-dependent measurements of the magnetization in the
range of K and up to T as well as
measurements of the heat capacity are presented. Ferromagnetic ordering emerges
below K and comparatively large coercivity fields of
T as well as pronounced anisotropy are observed upon cooling. Polycrystalline
samples of the Ca analog LiCa[CoN] were obtained and investigated in a
similar way. In both compounds Co manifests orbital contributions to the
magnetic moment and large single-ion anisotropy that is caused by second-order
Spin-orbit coupling. Quantum chemistry calculations reveal a magnetic
anisotropy energy of 7 meV, twice as large as the values reported for similar
Co systems.Comment: 21 pages, 6 figures, 5 table
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V4 tetrahedral units in AV4X8 lacunar spinels: Near degeneracy, charge fluctuations, and configurational mixing within a valence space of up to 21 d orbitals
All properties of a given molecule or solid are determined by the way valence electrons are distributed over single-particle energy levels. For multiple, closely spaced single-particle levels, different occupation patterns may provide many-electron quantum states that are close in energy, interact, and admix. We address such near-degeneracy electron correlation effects for V4 vanadium tetrahedral units as encountered in the lacunar spinel GaV4S8, explicitly taking into account up to 21 vanadium valence orbitals, and find effective orbital occupation numbers much different as compared to the picture previously laid out on the basis of mean-field calculations. In light of these results, a modified theoretical frame seems necessary to explain the peculiar magnetic properties of lacunar spinels and of related compounds
Electron correlations and bond-length fluctuations in copper oxides: from Zhang--Rice singlets to correlation bags
We perform first principles, multiconfiguration calculations on clusters
including several CuO octahedra and study the ground-state electron
distribution and electron--lattice couplings when holes are added to the
undoped configuration. We find that the so-called Zhang--Rice state
on a single CuO plaquette is nearly degenerate with a state whose leading
configuration is of the form Cu -- O -- Cu . A strong coupling
between the electronic and nuclear motion gives rise to large inter-site charge
transfer effects for half-breathing displacements of the oxygen ions. Under the
assumption of charge segregation into alternating hole-free and hole-rich
stripes of Goodenough \cite{jbg_02,jbg_03}, our results seem to support the
vibronic mechanism and the traveling charge-density wave model from
Refs.\cite{jbg_02,jbg_03} for the superconductivity in copper oxides.Comment: submitted to Phys. Rev.
Spin-state transition and spin-polaron physics in cobalt oxide perovskites: ab initio approach based on quantum chemical methods
A fully ab initio scheme based on quantum chemical wavefunction methods is
used to investigate the correlated multiorbital electronic structure of a
3d-metal compound, LaCoO3. The strong short-range electron correlations,
involving both Co and O orbitals, are treated by multireference techniques. The
use of effective parameters like the Hubbard U and interorbital U', J terms and
the problems associated with their explicit calculation are avoided with this
approach. We provide new insight into the spin-state transition at about 90 K
and the nature of charge carriers in the doped material. Our results indicate
the formation of a t4e2 high-spin state in LaCoO3 for T>90 K. Additionally, we
explain the paramagnetic phase in the low-temperature lightly doped compound
through the formation of Zhang-Rice-like O hole states and ferromagnetic
clusters
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