780 research outputs found
Density-matrix formalism with three-body ground-state correlations
A density-matrix formalism which includes the effects of three-body ground-
state correlations is applied to the standard Lipkin model. The reason to
consider the complicated three-body correlations is that the truncation scheme
of reduced density matrices up to the two-body level does not give satisfactory
results to the standard Lipkin model. It is shown that inclusion of the
three-body correlations drastically improves the properties of the ground
states and excited states. It is pointed out that lack of mean-field effects in
the standard Lipkin model enhances the relative importance of the three-body
ground-state correlations. Formal aspects of the density-matrix formalism such
as a relation to the variational principle and the stability condition of the
ground state are also discussed. It is pointed out that the three-body
ground-state correlations are necessary to satisfy the stability condition
Mott Gap Excitations and Resonant Inelastic X-Ray Scattering in Doped Cuprates
Predictions are made for the momentum- and carrier-dependent degradation of
the Mott gap upon doping in high-Tc cuprates as would be observed in Cu K-edge
resonant inelastic x-ray scattering (RIXS). The two-dimensional Hubbard model
with second- and third-nearest-neighbor hopping terms has been studied by
numerical exact diagonalization. Special emphasis is placed on the
particle-hole asymmetry of the Mott gap excitations. We argue that the Mott gap
excitations observed by RIXS are significantly influenced by the interaction
between charge carriers and antiferromagnetic correlations.Comment: 4 pages, 4 figures, revised version; to be published in Phys. Rev.
Let
Exact diagonalization study of optical conductivity in two-dimensional Hubbard model
The optical conductivity \sigma(\omega) in the two-dimensional Hubbard model
is examined by applying the exact diagonalization technique to small square
clusters with periodic boundary conditions up to \sqrt{20} X \sqrt{20} sites.
Spectral-weight distributions at half filling and their doping dependence in
the 20-site cluster are found to be similar to those in a \sqrt{18} X \sqrt{18}
cluster, but different from 4 X 4 results. The results for the 20-site cluster
enable us to perform a systematic study of the doping dependence of the
spectral-weight transfer from the region of the Mott-gap excitation to
lower-energy regions. We discuss the dependence of the Drude weight and the
effective carrier number on the electron density at a large on-site Coulomb
interaction.Comment: 5 pages, 5 figure
Temperature dependence of spinon and holon excitations in one-dimensional Mott insulators
Motivated by the recent angle-resolved photoemission spectroscopy (ARPES)
measurements on one-dimensional Mott insulators, SrCuO and
NaVO, we examine the single-particle spectral weight
of the one-dimensional (1D) Hubbard model at half-filling. We are particularly
interested in the temperature dependence of the spinon and holon excitations.
For this reason, we have performed the dynamical density matrix renormalization
group and determinantal quantum Monte Carlo (QMC) calculations for the
single-particle spectral weight of the 1D Hubbard model. In the QMC data, the
spinon and holon branches become observable at temperatures where the
short-range antiferromagnetic correlations develop. At these temperatures, the
spinon branch grows rapidly. In the light of the numerical results, we discuss
the spinon and holon branches observed by the ARPES experiments on
SrCuO. These numerical results are also in agreement with the
temperature dependence of the ARPES results on NaVO.Comment: 8 pages, 8 figure
Resonant Two-Magnon Raman Scattering and Photoexcited States in Two-Dimensional Mott Insulators
We investigate the resonant two-magnon Raman scattering in two-dimensional
(2D) Mott insulators by using a half-filled 2D Hubbard model in the strong
coupling limit. By performing numerical diagonalization calculations for small
clusters, we find that the Raman intensity is enhanced when the incoming photon
energy is not near the optical absorption edge but well above it, being
consistent with experimental data. The absence of resonance near the gap edge
is associated with the presence of background spins, while photoexcited states
for resonance are found to be characterized by the charge degree of freedom.
The resonance mechanism is different from those proposed previously.Comment: REVTeX4, 4 pages, 3 figures, to be published in Phys. Rev. Let
Theory of RIXS in strongly correlated electron systems: Mott gap excitations in cuprates
We theoretically examine the momentum dependence of resonant inelastic x-ray
scattering (RIXS) spectrum for one-dimensional and two-dimensional cuprates
based on the single-band Hubbard model with realistic parameter values. The
spectrum is calculated by using the numerical diagonalization technique for
finite-size clusters. We focus on excitations across the Mott gap and clarify
spectral features coming from the excitations as well as the physics behind
them. Good agreement between the theoretical and existing experimental results
clearly demonstrates that the RIXS is a potential tool to study the
momentum-dependent charge excitations in strongly correlated electron systems.Comment: 9 pages, 8 figures, Proceedings of 5th International Conference on
Inelastic X-ray Scattering (IXS 2004
Theoretical study of angle-resolved two-photon photoemission in two-dimensional insulating cuprates
We propose angle-resolved two-photon photoemission spectroscopy (AR-2PPES) as
a technique to detect the location of the bottom of the upper Hubbard band
(UHB) in two-dimensional insulating cuprates. The AR-2PPES spectra are
numerically calculated for small Hubbard clusters. When the pump photon excites
an electron from the lower Hubbard band, the bottom of the UHB is less clear,
but when an electron in the nonbonding oxygen band is excited, the bottom of
the UHB can be identified clearly, accompanied with additional spectra
originated from the spin-wave excitation at half filling.Comment: 5 pages, 4 figure
Exciton doublet in the Mott-Hubbard LiCuVO insulator identified by spectral ellipsometry
Spectroscopic ellipsometry was used to study the dielectric function of
LiCuVO, a compound comprised of chains of edge-sharing CuO
plaquettes, in the spectral range (0.75 - 6.5) eV at temperatures (7-300) K.
For photon polarization along the chains, the data reveal a weak but
well-resolved two-peak structure centered at 2.15 and 2.95 eV whose spectral
weight is strongly enhanced upon cooling near the magnetic ordering
temperature. We identify these features as an exciton doublet in the
Mott-Hubbard gap that emerges as a consequence of the Coulomb interaction
between electrons on nearest and next-nearest neighbor sites along the chains.
Our results and methodology can be used to address the role of the long-range
Coulomb repulsion for compounds with doped copper-oxide chains and planes.Comment: 4 pages with 4 figures and EPAPS supplementary online material (3
pages with 4 figures), accepted in Phys. Rev. Let
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