1,932 research outputs found
On the correct continuum limit of the functional-integral representation for the four-slave-boson approach to the Hubbard model: Paramagnetic phase
The Hubbard model with finite on-site repulsion U is studied via the
functional-integral formulation of the four-slave-boson approach by Kotliar and
Ruckenstein. It is shown that a correct treatment of the continuum imaginary
time limit (which is required by the very definition of the functional
integral) modifies the free energy when fluctuation (1/N) corrections beyond
mean-field are considered. Our analysis requires us to suitably interpret the
Kotliar and Ruckenstein choice for the bosonic hopping operator and to abandon
the commonly used normal-ordering prescription, in order to obtain meaningful
fluctuation corrections. In this way we recover the exact solution at U=0 not
only at the mean-field level but also at the next order in 1/N. In addition, we
consider alternative choices for the bosonic hopping operator and test them
numerically for a simple two-site model for which the exact solution is readily
available for any U. We also discuss how the 1/N expansion can be formally
generalized to the four-slave-boson approach, and provide a simplified
prescription to obtain the additional terms in the free energy which result at
the order 1/N from the correct continuum limit.Comment: Changes: Printing problems (due to non-standard macros) have been
removed, 44 page
Phase separation and competition of superconductivity and magnetism in the two-dimensional Hubbard model: From strong to weak coupling
Cooperation and competition between the antiferromagnetic, d-wave
superconducting and Mott-insulating states are explored for the two-dimensional
Hubbard model including nearest and next-nearest-neighbor hoppings at zero
temperature. Using the variational cluster approach with clusters of different
shapes and sizes up to 10 sites, it is found that the doping-driven transition
from a phase with microscopic coexistence of antiferromagnetism and
superconductivity to a purely superconducting phase is discontinuous for strong
interaction and accompanied by phase separation. At half-filling the system is
in an antiferromagnetic Mott-insulating state with vanishing charge
compressibility. Upon decreasing the interaction strength U below a certain
critical value of roughly U=4 (in units of the nearest-neighbor hopping),
however, the filling-dependent magnetic transition changes its character and
becomes continuous. Phase separation or, more carefully, the tendency towards
the formation of inhomogeneous states disappears. This critical value is in
contrast to previous studies, where a much larger value was obtained. Moreover,
we find that the system at half-filling undergoes the Mott transition from an
insulator to a state with a finite charge compressibility at essentially the
same value. The weakly correlated state at half-filling exhibits
superconductivity microscopically admixed to the antiferromagnetic order. This
scenario suggests a close relation between phase separation and the
Mott-insulator physics.Comment: 7 pages, 8 figures, revised version to be published in Phys. Rev.
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 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
Phase diagram and single-particle spectrum of CuO layers within a variational cluster approach to the 3-band Hubbard model
We carry out a detailed numerical study of the three-band Hubbard model in
the underdoped region both in the hole- as well as in the electron-doped case
by means of the variational cluster approach. Both the phase diagram and the
low-energy single-particle spectrum are very similar to recent results for the
single-band Hubbard model with next-nearest-neighbor hoppings. In particular,
we obtain a mixed antiferromagnetic+superconducting phase at low doping with a
first-order transition to a pure superconducting phase accompanied by phase
separation. In the single-particle spectrum a clear Zhang-Rice singlet band
with an incoherent and a coherent part can be seen, in which holes enter upon
doping around . The latter is very similar to the coherent
quasi-particle band crossing the Fermi surface in the single-band model. Doped
electrons go instead into the upper Hubbard band, first filling the regions of
the Brillouin zone around . This fact can be related to the enhanced
robustness of the antiferromagnetic phase as a function of electron doping
compared to hole doping.Comment: 14 pages, 15 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
Extended self-energy functional approach for strongly-correlated lattice bosons in the superfluid phase
Among the various numerical techniques to study the physics of strongly
correlated quantum many-body systems, the self-energy functional approach (SFA)
has become increasingly important. In its previous form, however, SFA is not
applicable to Bose-Einstein condensation or superfluidity. In this paper we
show how to overcome this shortcoming. To this end we identify an appropriate
quantity, which we term , that represents the correlation correction of the
condensate order parameter, as it does the self-energy for the Green's
function. An appropriate functional is derived, which is stationary at the
exact physical realizations of and of the self-energy. Its derivation is
based on a functional-integral representation of the grand potential followed
by an appropriate sequence of Legendre transformations. The approach is not
perturbative and therefore applicable to a wide range of models with local
interactions. We show that the variational cluster approach based on the
extended self-energy functional is equivalent to the "pseudoparticle" approach
introduced in Phys. Rev. B, 83, 134507 (2011). We present results for the
superfluid density in the two-dimensional Bose-Hubbard model, which show a
remarkable agreement with those of Quantum-Monte-Carlo calculations.Comment: 1 additional figure showing the region close to the tip of the Mott
lobe, minor changes in the tex
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