502 research outputs found
Spectral functions and optical conductivity of spinless fermions on a checkerboard lattice
We study the dynamical properties of spinless fermions on the checkerboard
lattice. Our main interest is the limit of large nearest-neighbor repulsion
as compared with hopping . The spectral functions show broad low-energy
excitation which are due to the dynamics of fractionally charged excitations.
Furthermore, it is shown that the fractional charges contribute to the
electrical current density.Comment: 9 Pages, 9 Figure
Kinetic ferromagnetism on a kagome lattice
We study strongly correlated electrons on a kagome lattice at 1/6 and 1/3
filling. They are described by an extended Hubbard Hamiltonian. We are
concerned with the limit |t|<<V<<U with hopping amplitude t, nearest-neighbor
repulsion V and on-site repulsion U. We derive an effective Hamiltonian and
show, with the help of the Perron-Frobenius theorem, that the system is
ferromagnetic at low temperatures. The robustness of ferromagnetism is
discussed and extensions to other lattices are indicated.Comment: 4 pages, 2 color eps figures; updated version published in Phys. Rev.
Lett.; one reference adde
Charge degrees in the quarter-filled checkerboard lattice
For a systematic study of charge degrees of freedom in lattices with
geometric frustration, we consider spinless fermions on the checkerboard
lattice with nearest-neighbor hopping and nearest-neighbor repulsion at
quarter-filling. An effective Hamiltonian for the limit is given to
lowest non-vanishing order by the ring exchange (). We show
that the system can equivalently be described by hard-core bosons and map the
model to a confining U(1) lattice gauge theory.Comment: Proceedings of ICM200
Strongly correlated fermions on a kagome lattice
We study a model of strongly correlated spinless fermions on a kagome lattice
at 1/3 filling, with interactions described by an extended Hubbard Hamiltonian.
An effective Hamiltonian in the desired strong correlation regime is derived,
from which the spectral functions are calculated by means of exact
diagonalization techniques. We present our numerical results with a view to
discussion of possible signatures of confinement/deconfinement of fractional
charges.Comment: 10 pages, 10 figure
A Generic Model for Current Collapse in Spin Blockaded Transport
A decrease in current with increasing voltage, often referred to as negative
differential resistance (NDR), has been observed in many electronic devices and
can usually be understood within a one-electron picture. However, NDR has
recently been reported in nanoscale devices with large single-electron charging
energies which require a many-electron picture in Fock space. This paper
presents a generic model in this transport regime leading to a simple criterion
for the conditions required to observe NDR and shows that this model describes
the recent observation of multiple NDR's in Spin Blockaded transport through
weakly coupled-double quantum dots quite well. This model shows clearly how a
delicate interplay of orbital energy offset, delocalization and Coulomb
interaction lead to the observed NDR under the right conditions, and also aids
in obtaining a good match with experimentally observed features. We believe the
basic model could be useful in understanding other experiments in this
transport regime as well.Comment: 10 pages, 10 figures. to appear in Phys Rev
Cluster approach study of intersite electron correlations in pyrochlore and checkerboard lattices
To treat effects of electron correlations in geometrically frustrated
pyrochlore and checkerboard lattices, an extended single-orbital Hubbard model
with nearest neighbor hopping and Coulomb repulsion is
applied. Infinite on-site repulsion, , is assumed, thus double
occupancies of sites are forbidden completely in the present study. A
variational Gutzwiller type approach is extended to examine correlations due to
short-range interaction and a cluster approximation is developed to
evaluate a variational ground state energy of the system. Obtained analytically
in a special case of quarter band filling appropriate to LiVO, the
resulting simple expression describes the ground state energy in the regime of
intermediate and strong coupling . Like in the Brinkman-Rice theory based on
the standard Gutzwiller approach to the Hubbard model, the mean value of the
kinetic energy is shown to be reduced strongly as the coupling approaches a
critical value . This finding may contribute to explaining the observed
heavy fermion behavior in LiVO
Kondo lattice model: Unitary transformations, spin dynamics, strongly correlated charged modes, and vacuum instability
Using unitary transformations, we express the Kondo lattice Hamiltonian in
terms of fermionic operators that annihilate the ground state of the
interacting system and that represent the best possible approximations to the
actual charged excitations. In this way, we obtain an effective Hamiltonian
which, for small couplings, consists in a kinetic term for conduction electrons
and holes, an RKKY-like term, and a renormalized Kondo interaction. The
physical picture of the system implied by this formalism is that of a vacuum
state consisting in a background of RKKY-induced spin correlations, where two
kinds of elementary modes can be excited: Soft neutral modes associated with
deformations of the spin liquid, which lead to very large low-temperature
values of the heat capacity and magnetic susceptibility, and charged modes
corresponding to the excitation of electrons and holes in the system. Using the
translational and spin rotational symmetries, we construct a simple ansatz to
determine the charged excitations neglecting the effects of the spin
correlations. Apart from the `normal', uncorrelated states, we find strongly
correlated charged modes involving soft electrons (or holes) and spin
fluctuations, which strongly renormalize the low-energy charged spectrum, and
whose energy becomes negative beyond a critical coupling, signaling a vacuum
instability and a transition to a new phase.Comment: 35 pages, revtex 3.
Fast computation of the Kohn-Sham susceptibility of large systems
For hybrid systems, such as molecules grafted onto solid surfaces, the
calculation of linear response in time dependent density functional theory is
slowed down by the need to calculate, in N^4 operations, the susceptibility of
N non interacting Kohn-Sham reference electrons. We show how this
susceptibility can be calculated N times faster within finite precision. By
itself or in combination with previous methods, this should facilitate the
calculation of TDDFT response and optical spectra of hybrid systems.Comment: submitted 25/1/200
Spectral properties of the three-dimensional Hubbard model
We present momentum resolved single-particle spectra for the
three-dimensional Hubbard model for the paramagnetic and antiferromagnetically
ordered phase obtained within the dynamical cluster approximation. The
effective cluster problem is solved by continuous-time Quantum Monte Carlo
simulations. The absence of a time discretization error and the ability to
perform Monte Carlo measurements directly in Matsubara frequencies enable us to
analytically continue the self-energies by maximum entropy, which is essential
to obtain momentum resolved spectral functions for the N'eel state. We
investigate the dependence on temperature and interaction strength and the
effect of magnetic frustration introduced by a next-nearest neighbor hopping.
One particular question we address here is the influence of the frustrating
interaction on the metal insulator transition of the three-dimensional Hubbard
model.Comment: 16 pages, 14 figure
Renormalization of the periodic Anderson model: an alternative analytical approach to heavy Fermion behavior
In this paper a recently developed projector-based renormalization method
(PRM) for many-particle Hamiltonians is applied to the periodic Anderson model
(PAM) with the aim to describe heavy Fermion behavior. In this method
high-energetic excitation operators instead of high energetic states are
eliminated. We arrive at an effective Hamiltonian for a quasi-free system which
consists of two non-interacting heavy-quasiparticle bands. The resulting
renormalization equations for the parameters of the Hamiltonian are valid for
large as well as small degeneracy of the angular momentum. An expansion
in is avoided. Within an additional approximation which adapts the
idea of a fixed renormalized \textit{f} level , we obtain
coupled equations for and the averaged \textit{f}
occupation . These equations resemble to a certain extent those of the
usual slave boson mean-field (SB) treatment. In particular, for large
the results for the PRM and the SB approach agree perfectly whereas
considerable differences are found for small .Comment: 26 pages, 5 figures included, discussion of the DOS added in v2,
accepted for publication in Phys. Rev.
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