1,565 research outputs found
Temperature and Dimensionality Dependences of Optical Absorption Spectra in Mott Insulators
We investigate the temperature dependence of optical absorption spectra of
one-dimensional (1D) and two-dimensional (2D) Mott insulators by using an
effective model in the strong-coupling limit of a half-filed Hubbard model. In
the numerically exact diagonalization calculations on finite-size clusters, we
find that in 1D the energy position of the absorption edge is almost
independent of temperature, while in 2D the edge position shifts to lower
energy with increasing temperature. The different temperature dependence
between 1D and 2D is attributed to the difference of the coupling of the charge
and spin degrees of freedom. The implications of the results on experiments are
discussed in terms of the dimensionality dependence.Comment: 5 pages, 4 figure
Random dispersion approximation for the Hubbard model
We use the Random Dispersion Approximation (RDA) to study the Mott-Hubbard
transition in the Hubbard model at half band filling. The RDA becomes exact for
the Hubbard model in infinite dimensions. We implement the RDA on finite chains
and employ the Lanczos exact diagonalization method in real space to calculate
the ground-state energy, the average double occupancy, the charge gap, the
momentum distribution, and the quasi-particle weight. We find a satisfactory
agreement with perturbative results in the weak- and strong-coupling limits. A
straightforward extrapolation of the RDA data for lattice results in
a continuous Mott-Hubbard transition at . We discuss the
significance of a possible signature of a coexistence region between insulating
and metallic ground states in the RDA that would correspond to the scenario of
a discontinuous Mott-Hubbard transition as found in numerical investigations of
the Dynamical Mean-Field Theory for the Hubbard model.Comment: 10 pages, 11 figure
Thermodynamics of the one-dimensional half-filled Hubbard model in the spin-disordered regime
We analyze the Thermodynamic Bethe Ansatz equations of the one-dimensional
half-filled Hubbard model in the "spin-disordered regime", which is
characterized by the temperature being much larger than the magnetic energy
scale but small compared to the Mott-Hubbard gap. In this regime the
thermodynamics of the Hubbard model can be thought of in terms of gapped
charged excitations with an effective dispersion and spin degrees of freedom
that only contribute entropically. In particular, the internal energy and the
effective dispersion become essentially independent of temperature. An
interpretation of this regime in terms of a putative interacting-electron
system at zero temperature leads to a metal-insulator transition at a finite
interaction strength above which the gap opens linearly. We relate these
observations to studies of the Mott-Hubbard transition in the limit of infinite
dimensions.Comment: 15 pages, 3 figure
Cluster dynamical mean field theory of quantum phases on a honeycomb lattice
We have studied the ground state of the half-filled Hubbard model on a
honeycomb lattice by performing the cluster dynamical mean field theory
calculations with exact diagonalization on the cluster-impurity solver. Through
using elaborate numerical analytic continuation, we identify the existence of a
`spin liquid' from the on-site interaction U=0 to (between and
) with a smooth crossover correspondingly from the charge fluctuation
dominating phase into the charge correlation dominating phase. The
semi-metallic state exits only at U=0. We further find that the magnetic phase
transition at from the `spin liquid' to the N\'{e}el antiferromagnetic
Mott insulating phase is a first-order quantum phase transition. We also show
that the charge fluctuation plays a substantial role on keeping the `spin
liquid' phase against the emergence of a magnetic order.Comment: 5 pages and 8 figure
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
Antiferromagnetic order in multi-band Hubbard models for iron-pnictides
We investigate multi-band Hubbard models for the three iron 3-
bands and the two iron 3- bands in by means of the
Gutzwiller variational theory. Our analysis of the paramagnetic ground state
shows that neither Hartree--Fock mean-field theories nor effective spin models
describe these systems adequately. In contrast to Hartree--Fock-type
approaches, the Gutzwiller theory predicts that antiferromagnetic order
requires substantial values of the local Hund's-rule exchange interaction. For
the three-band model, the antiferromagnetic moment fits experimental data for a
broad range of interaction parameters. However, for the more appropriate
five-band model, the iron electrons polarize the electrons and
they substantially contribute to the ordered moment.Comment: 4 pages, 4 figure
Effective mass in quasi two-dimensional systems
The effective mass of the quasiparticle excitations in quasi two-dimensional
systems is calculated analytically. It is shown that the effective mass
increases sharply when the density approaches the critical one of
metal-insulator transition. This suggests a Mott type of transition rather than
an Anderson like transition.Comment: 3 pages 3 figure
Brueckner-Goldstone perturbation theory for the half-filled Hubbard model in infinite dimensions
We use Brueckner-Goldstone perturbation theory to calculate the ground-state
energy of the half-filled Hubbard model in infinite dimensions up to fourth
order in the Hubbard interaction. We obtain the momentum distribution as a
functional derivative of the ground-state energy with respect to the bare
dispersion relation. The resulting expressions agree with those from
Rayleigh-Schroedinger perturbation theory. Our results for the momentum
distribution and the quasi-particle weight agree very well with those obtained
earlier from Feynman-Dyson perturbation theory for the single-particle
self-energy. We give the correct fourth-order coefficient in the ground-state
energy which was not calculated accurately enough from Feynman-Dyson theory due
to the insufficient accuracy of the data for the self-energy, and find a good
agreement with recent estimates from Quantum Monte-Carlo calculations.Comment: 15 pages, 8 fugures, submitted to JSTA
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