16 research outputs found
Correlated band structure of electron-doped cuprate materials
We present a numerical study of the doping dependence of the spectral
function of the n-type cuprates. Using a variational cluster-perturbation
theory approach based upon the self-energy-functional theory, the spectral
function of the electron-doped two-dimensional Hubbard model is calculated. The
model includes the next-nearest neighbor electronic hopping amplitude and
a fixed on-site interaction at half filling and doping levels ranging
from to . Our results support the fact that a comprehensive
description of the single-particle spectrum of electron-doped cuprates requires
a proper treatment of strong electronic correlations. In contrast to previous
weak-coupling approaches, we obtain a consistent description of the ARPES
experiments without the need to introduce a doping-dependent on-site
interaction .Comment: 7 pages 4 eps figure
Variational cluster approach to the Hubbard model: Phase-separation tendency and finite-size effects
Using the variational cluster approach (VCA), we study the transition from
the antiferromagnetic to the superconducting phase of the two-dimensional
Hubbard model at zero temperature. Our calculations are based on a new method
to evaluate the VCA grand potential which employs a modified Lanczos algorithm
and avoids integrations over the real or imaginary frequency axis. Thereby,
very accurate results are possible for cluster sizes not accessible to full
diagonalization. This is important for an improved treatment of short-range
correlations, including correlations between Cooper pairs in particular. We
investigate the cluster-size dependence of the phase-separation tendency that
has been proposed recently on the basis of calculations for smaller clusters.
It is shown that the energy barrier driving the phase separation decreases with
increasing cluster size. This supports the conjecture that the ground state
exhibits microscopic inhomogeneities rather than macroscopic phase separation.
The evolution of the single-particle spectum as a function of doping is studied
in addtion and the relevance of our results for experimental findings is
pointed out.Comment: 7 pages, 6 figures, published versio
Variational cluster approach to correlated electron systems in low dimensions
A self-energy-functional approach is applied to construct cluster
approximations for correlated lattice models. It turns out that the
cluster-perturbation theory (Senechal et al, PRL 84, 522 (2000)) and the
cellular dynamical mean-field theory (Kotliar et al, PRL 87, 186401 (2001)) are
limiting cases of a more general cluster method. Results for the
one-dimensional Hubbard model are discussed with regard to boundary conditions,
bath degrees of freedom and cluster size.Comment: 4 pages, final version with minor change
Correlated band structure of electron-doped cuprate materials
We present a numerical study of the doping dependence of the spectral function of the n-type
cuprates. Using a variational cluster-perturbation theory approach based upon the self-energyfunctional
theory, the spectral function of the electron-doped two-dimensional Hubbard model is
calculated. The model includes the next-nearest neighbor electronic hopping amplitude t' and a
fixed on-site interaction U - 8t at half-filling and doping levels ranging from x - 0.077 to x - 0.20 .
Our results support the fact that a comprehensive description of the single-particle spectrum of
electron-doped cuprates requires a proper treatment of strong electronic correlations. In contrast
to previous weak-coupling approaches, we obtain a consistent description of the ARPES
experiments without the need to (artificially) introduce a doping-dependent on-site interaction U
Variational Cluster Perturbation Theory for Bose-Hubbard models
We discuss the application of the variational cluster perturbation theory
(VCPT) to the Mott-insulator--to--superfluid transition in the Bose-Hubbard
model. We show how the VCPT can be formulated in such a way that it gives a
translation invariant excitation spectrum -- free of spurious gaps -- despite
the fact that if formally breaks translation invariance. The phase diagram and
the single-particle Green function in the insulating phase are obtained for
one-dimensional systems. When the chemical potential of the cluster is taken as
a variational parameter, the VCPT reproduces the dimension dependence of the
phase diagram even for one-site clusters. We find a good quantitative agreement
with the results of the density-matrix renormalization group when the number of
sites in the cluster becomes of order 10. The extension of the method to the
superfluid phase is discussed.Comment: v1) 10 pages, 6 figures. v2) Final version as publishe
Cluster Perturbation Theory for Hubbard models
Cluster perturbation theory is a technique for calculating the spectral
weight of Hubbard models of strongly correlated electrons, which combines exact
diagonalizations on small clusters with strong-coupling perturbation theory at
leading order. It is exact in both the strong- and weak-coupling limits and
provides a good approximation to the spectral function at any wavevector.
Following the paper by S\'en\'echal et al. (Phys. Rev. Lett. {\bf 84}, 522
(2000)), we provide a more complete description and derivation of the method.
We illustrate some of its capabilities, in particular regarding the effect of
doping, the calculation of ground state energy and double occupancy, the
disappearance of the Fermi surface in the Hubbard model, and so on. The
method is applicable to any model with on-site repulsion only.Comment: 11 pages, 10 figures (RevTeX 4
The 3-Band Hubbard-Model versus the 1-Band Model for the high-Tc Cuprates: Pairing Dynamics, Superconductivity and the Ground-State Phase Diagram
One central challenge in high- superconductivity (SC) is to derive a
detailed understanding for the specific role of the - and
- orbital degrees of freedom. In most theoretical studies an
effective one-band Hubbard (1BH) or t-J model has been used. Here, the physics
is that of doping into a Mott-insulator, whereas the actual high- cuprates
are doped charge-transfer insulators. To shed light on the related question,
where the material-dependent physics enters, we compare the competing magnetic
and superconducting phases in the ground state, the single- and two-particle
excitations and, in particular, the pairing interaction and its dynamics in the
three-band Hubbard (3BH) and 1BH-models. Using a cluster embedding scheme, i.e.
the variational cluster approach (VCA), we find which frequencies are relevant
for pairing in the two models as a function of interaction strength and doping:
in the 3BH-models the interaction in the low- to optimal-doping regime is
dominated by retarded pairing due to low-energy spin fluctuations with
surprisingly little influence of inter-band (p-d charge) fluctuations. On the
other hand, in the 1BH-model, in addition a part comes from "high-energy"
excited states (Hubbard band), which may be identified with a non-retarded
contribution. We find these differences between a charge-transfer and a Mott
insulator to be renormalized away for the ground-state phase diagram of the
3BH- and 1BH-models, which are in close overall agreement, i.e. are
"universal". On the other hand, we expect the differences - and thus, the
material dependence to show up in the "non-universal" finite-T phase diagram
(-values).Comment: 17 pages, 9 figure