94 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
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
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.
Renormalization of the quasiparticle hopping integrals by spin interactions in layered copper oxides
Holes doped within the square CuO2 network specific to the cuprate
superconducting materials have oxygen 2p character. We investigate the basic
properties of such oxygen holes by wavefunction-based quantum chemical
calculations on large embedded clusters. We find that a 2p hole induces
ferromagnetic correlations among the nearest-neighbor Cu 3d spins. When moving
through the antiferromagnetic background the hole must bring along this spin
polarization cloud at nearby Cu sites, which gives rise to a substantial
reduction of the effective hopping parameters. Such interactions can explain
the relatively low values inferred for the effective hoppings by fitting the
angle-resolved photoemission data. The effect of the background
antiferromagnetic couplings of renormalizing the effective nearest-neighbor
hopping is also confirmed by density-matrix renormalization-group model
Hamiltonian calculations for chains and ladders of CuO4 plaquettes
Dispersion of orbital excitations in 2D quantum antiferromagnets
We map the problem of the orbital excitation (orbiton) in a 2D
antiferromagnetic and ferroorbital ground state onto a problem of a hole in 2D
antiferromagnet. The orbiton turns out to be coupled to magnons and can only be
mobile on a strongly renormalized scale by dressing with magnetic excitations.
We show that this leads to a dispersion relation reflecting the two-site unit
cell of the antiferromagnetic background, in contrast to the predictions based
on a mean-field approximation and linear orbital-wave theory.Comment: 4 pages, 2 figures, submitted to SCES 2011 conference proceeding
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