188 research outputs found
Exact stripe, checkerboard, and droplet ground states in two dimensions
Exact static nondegenerate stripe and checkerboard ground states are obtained
in a two-dimensional generalized periodic Anderson model, for a broad
concentration range below quarter filling. The random droplet states, also
present in the degenerate ground state, are eliminated by extending the
Hamiltonian with terms of different physical origin such as dimerization,
periodic charge displacements, density waves, or distorsion lines.Comment: 12 pages, 8 figure
Ferromagnetism without flat bands in thin armchair nanoribbons
Describing by a Hubbard type of model a thin armchair graphene ribbon in the
armchair hexagon chain limit, one shows in exact terms, that even if the system
does not have flat bands at all, at low concentration a mesoscopic sample can
have ferromagnetic ground state, being metallic in the same time. The mechanism
is connected to a common effect of correlations and confinement.Comment: 37 pages, 12 figures, in press at Eur. Phys. Jour.
Bosonization solution of the Falicov-Kimball model
We use a novel approach to analyze the one dimensional spinless
Falicov-Kimball model. We derive a simple effective model for the occupation of
the localized orbitals which clearly reveals the origin of the known ordering.
Our study is extended to a quantum model with hybridization between the
localized and itinerant states; we find a crossover between the well-known
weak- and strong-coupling behaviour. The existence of electronic polarons at
intermediate coupling is confirmed. A phase diagram is presented and discussed
in detail.Comment: RevTex, 10 pages, 1 figur
Exact Insulating and Conducting Ground States of a Periodic Anderson Model in Three Dimensions
We present a class of exact ground states of a three-dimensional periodic
Anderson model at 3/4 filling. Hopping and hybridization of d and f electrons
extend over the unit cell of a general Bravais lattice. Employing novel
composite operators combined with 55 matching conditions the Hamiltonian is
cast into positive semidefinite form. A product wave function in position space
allows one to identify stability regions of an insulating and a conducting
ground state. The metallic phase is a non-Fermi liquid with one dispersing and
one flat band.Comment: 4 pages, 3 figure
Competing Orderings in an Extended Falicov-Kimball Model
We present a Hartree-Fock study of the Falicov-Kimball model extended by both
on-site and non-local hybridization. We examine the interplay between excitonic
effects and the charge-density wave (CDW) instability known to exist at zero
hybridization. It is found that the CDW state remains stable in the presence of
finite hybridization; for on-site hybridization the Coulomb interaction
nevertheless strongly enhances the excitonic average above its value in the
noninteracting system. In contrast, for non-local hybridization, we observe no
such enhancement of the excitonic average or a spontaneous on-site
hybridization potential. Instead, we find only a significant suppression of the
excitonic correlations in the CDW state. A phenomenological Ginzburg-Landau
analysis is also provided to understand the interplay.Comment: RevTex, 5 pages, 4 figures; expanded and corrected, typos added,
references adde
Spin gap and Luttinger liquid description of the NMR relaxation in carbon nanotubes
Recent NMR experiments by Singer et al. [Singer et al. Phys. Rev. Lett. 95,
236403 (2005).] showed a deviation from Fermi-liquid behavior in carbon
nanotubes with an energy gap evident at low temperatures. Here, a comprehensive
theory for the magnetic field and temperature dependent NMR 13C spin-lattice
relaxation is given in the framework of the Tomonaga-Luttinger liquid. The low
temperature properties are governed by a gapped relaxation due to a spin gap (~
30K), which crosses over smoothly to the Luttinger liquid behaviour with
increasing temperature.Comment: 5 pages, 1 figure, 1 tabl
Exact Ground States of the Periodic Anderson Model in D=3 Dimensions
We construct a class of exact ground states of three-dimensional periodic
Anderson models (PAMs) -- including the conventional PAM -- on regular Bravais
lattices at and above 3/4 filling, and discuss their physical properties. In
general, the f electrons can have a (weak) dispersion, and the hopping and the
non-local hybridization of the d and f electrons extend over the unit cell. The
construction is performed in two steps. First the Hamiltonian is cast into
positive semi-definite form using composite operators in combination with
coupled non-linear matching conditions. This may be achieved in several ways,
thus leading to solutions in different regions of the phase diagram. In a
second step, a non-local product wave function in position space is constructed
which allows one to identify various stability regions corresponding to
insulating and conducting states. The compressibility of the insulating state
is shown to diverge at the boundary of its stability regime. The metallic phase
is a non-Fermi liquid with one dispersing and one flat band. This state is also
an exact ground state of the conventional PAM and has the following properties:
(i) it is non-magnetic with spin-spin correlations disappearing in the
thermodynamic limit, (ii) density-density correlations are short-ranged, and
(iii) the momentum distributions of the interacting electrons are analytic
functions, i.e., have no discontinuities even in their derivatives. The
stability regions of the ground states extend through a large region of
parameter space, e.g., from weak to strong on-site interaction U. Exact
itinerant, ferromagnetic ground states are found at and below 1/4 filling.Comment: 47 pages, 10 eps figure
Charge Order in the Falicov-Kimball Model
We examine the spinless one-dimensional Falicov-Kimball model (FKM) below
half-filling, addressing both the binary alloy and valence transition
interpretations of the model. Using a non-perturbative technique, we derive an
effective Hamiltonian for the occupation of the localized orbitals, providing a
comprehensive description of charge order in the FKM. In particular, we uncover
the contradictory ordering roles of the forward-scattering and backscattering
itinerant electrons: the latter are responsible for the crystalline phases,
while the former produces the phase separation. We find an Ising model
describes the transition between the phase separated state and the crystalline
phases; for weak-coupling we present the critical line equation, finding
excellent agreement with numerical results. We consider several extensions of
the FKM that preserve the classical nature of the localized states. We also
investigate a parallel between the FKM and the Kondo lattice model, suggesting
a close relationship based upon the similar orthogonality catastrophe physics
of the associated single-impurity models.Comment: 39 pages, 6 figure
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