45 research outputs found
Electrons in High-Tc Compounds: Ab-Initio Correlation Results
Electronic correlations in the ground state of an idealized infinite-layer
high-Tc compound are computed using the ab-initio method of local ansatz.
Comparisons are made with the local-density approximation (LDA) results, and
the correlation functions are analyzed in detail. These correlation functions
are used to determine the effective atomic-interaction parameters for model
Hamiltonians. On the resulting model, doping dependencies of the relevant
correlations are investigated. Aside from the expected strong atomic
correlations, particular spin correlations arise. The dominating contribution
is a strong nearest neighbor correlation that is Stoner-enhanced due to the
closeness of the ground state to the magnetic phase. This feature depends
moderately on doping, and is absent in a single-band Hubbard model. Our
calculated spin correlation function is in good qualitative agreement with that
determined from the neutron scattering experiments for a metal.Comment: 21pp, 5fig, Phys. Rev. B (Oct. 98
The dimpling in the CuO_2 planes of YBa_2Cu_3O_x (x=6.806-6.984, T=20-300 K) measured by yttrium EXAFS
The dimpling of the CuO_2 planes (spacing between the O2,3 and Cu2 layers) in
YBa_2Cu_3O_x has been measured as a function of oxygen concentration and
temperature by yttrium x-ray extended-fine-structure spectroscopy (EXAFS). The
relative variations of the dimpling with doping (x=6.806-6.984) and temperature
(20-300 K) are weak (within 0.05 AA), and arise mainly from displacements of
the Cu2 atoms off the O2,3 plane towards Ba. The dimpling appears to be
connected with the transition from the underdoped to the overdoped regimes at
x=6.95, and with a characteristic temperature in the normal state, T*=150 K.Comment: 6 pages, 2 ps figs, LaTEX, Elsevier Elsart styl
On the ground state of solids with strong electron correlations
We formulate the calculation of the ground-state wavefunction and energy of a
system of strongly correlated electrons in terms of scattering matrices. A
hierarchy of approximations is introduced which results in an incremental
expansion of the energy. The present approach generalizes previous work
designed for weakly correlated electronic systems.Comment: 17 pages, Latex(revtex
Magnetic phases near the Van Hove singularity in s- and d-band Hubbard model
We investigate the magnetic instabilities of the nondegenerate (s-band) and a
degenerate (d-band) Hubbard model in two dimensions using many-body effects due
to the particle-particle diagrams and Hund's rule local correlations. The
density of states and the position of Van Hove singularity change depending on
the value of next-nearest neighbor hopping t'. The Stoner parameter is strongly
reduced in the s-band case, and ferromagnetism survives only if electron
density is small, and the band is almost flat at small momenta due to
next-nearest neighbor hopping. In contrast, for the d-band case the reduction
of the Stoner parameter which follows from particle-particle correlations is
much smaller and ferromagnetism survives to a large extent. Inclusion of local
spin-spin correlations has a limited destabilizing effect on the magnetic
states.Comment: 8 pages, 7 figure
Magnetic Properties of Undoped
The Heisenberg antiferromagnet, which arises from the large Hubbard
model, is investigated on the molecule and other fullerenes. The
connectivity of leads to an exotic classical ground state with
nontrivial topology. We argue that there is no phase transition in the Hubbard
model as a function of , and thus the large solution is relevant for
the physical case of intermediate coupling. The system undergoes a first order
metamagnetic phase transition. We also consider the S=1/2 case using
perturbation theory. Experimental tests are suggested.Comment: 12 pages, 3 figures (included
Suppression of Mott–Hubbard states and metal–insulator transitions in the two-band Hubbard model
I investigate band and Mott insulating states in a two-band Hubbard model, with the aim of understanding the differences between the idealized one-orbital model and the more realistic multi-band case. Using a projection ansatz I show that additional orbitals suppress the metal–insulator transition, leading to a critical coupling of approximately eight times the bare bandwidth. I also demonstrate the effects of orbital ordering, which hinder Mott–Hubbard states and open a bandgap. Since multi-band correlations are common in real materials, this work suggests that very strongly correlated band insulators may be more common than Mott–Hubbard insulators
Nonperturbative approach to the Hubbard model in C60 cluster
We propose a computational scheme for the Hubbard model in the C60 cluster in
which the interaction with the Fermi sea of charges added to the neutral
molecule is switched on sequentially. This is applied to the calculation of the
balance of charging energies, within a low-energy truncation of the space of
states which produces moderate errors for an intermediate range of the
interaction strength.Comment: 5 pages, Revtex, 2 figure
Electron correlations for ground state properties of group IV semiconductors
Valence energies for crystalline C, Si, Ge, and Sn with diamond structure
have been determined using an ab-initio approach based on information from
cluster calculations. Correlation contributions, in particular, have been
evaluated in the coupled electron pair approximation (CEPA), by means of
increments obtained for localized bond orbitals and for pairs and triples of
such bonds. Combining these results with corresponding Hartree-Fock (HF) data,
we recover about 95 % of the experimental cohesive energies. Lattice constants
are overestimated at the HF level by about 1.5 %; correlation effects reduce
these deviations to values which are within the error bounds of this method. A
similar behavior is found for the bulk modulus: the HF values which are
significantly too high are reduced by correlation effects to about 97 % of the
experimental values.Comment: 22 pages, latex, 2 figure
Cohesive properties of alkali halides
We calculate cohesive properties of LiF, NaF, KF, LiCl, NaCl, and KCl with
ab-initio quantum chemical methods. The coupled-cluster approach is used to
correct the Hartree-Fock crystal results for correlations and to systematically
improve cohesive energies, lattice constants and bulk moduli. After inclusion
of correlations, we recover 95-98 % of the total cohesive energies. The lattice
constants deviate from experiment by at most 1.1 %, bulk moduli by at most 8 %.
We also find good agreement for spectroscopic properties of the corresponding
diatomic molecules.Comment: LaTeX, 10 pages, 1 figure, accepted by Phys. Rev.
Correlation effects in MgO and CaO: Cohesive energies and lattice constants
A recently proposed computational scheme based on local increments has been
applied to the calculation of correlation contributions to the cohesive energy
of the CaO crystal. Using ab-initio quantum chemical methods for evaluating
individual increments, we obtain 80% of the difference between the experimental
and Hartree-Fock cohesive energies. Lattice constants corrected for correlation
effects deviate by less than 1% from experimental values, in the case of MgO
and CaO.Comment: LaTeX, 4 figure