195 research outputs found
Properties of Hubbard models with degenerate localized single particle eigenstates
We consider the repulsive Hubbard model on a class of lattices or graphs for
which there is a large degeneracy of the single particle ground states and
where the projector onto the space of single particle ground states is highly
reducible. This means that one can find a basis in the space of the single
particle ground states such that the support of each single particle ground
state belongs to some small cluster and these clusters do not overlap. We show
how such lattices can be constructed in arbitrary dimensions. We construct all
multi-particle ground states of these models for electron numbers not larger
than the number of localized single particle eigenstates. We derive some of the
ground state properties, esp. the residual entropy, i.e. the finite entropy
density at zero temperature.Comment: 11 pages, no figures. Complete revision of the paper with many
change
Flow equations for band--matrices
Continuous unitary transformations can be used to diagonalize or
approximately diagonalize a given Hamiltonian. In the last four years, this
method has been applied to a variety of models of condensed matter physics and
field theory. With a new generator for the continuous unitary transformation
proposed in this paper one can avoid some of the problems of former
applications. General properties of the new generator are derived. It turns out
that the new generator is especially useful for Hamiltonians with a banded
structure. Two examples, the Lipkin model, and the spin--boson model are
discussed in detail.Comment: 12 pages, one eps-figure, uses epsfig.sty. Accepted for publication
in European Physical Journa
Hard-core bosons in flat band systems above the critical density
We investigate the behaviour of hard-core bosons in one- and two-dimensional
flat band systems, the chequerboard and the kagom\'e lattice and
one-dimensional analogues thereof. The one dimensional systems have an exact
local reflection symmetry which allows for exact results. We show that above
the critical density an additional particle forms a pair with one of the other
bosons and that the pair is localised. In the two-dimensional systems exact
results are not available but variational results indicate a similar physical
behaviour
Interacting bosons in two-dimensional flat band systems
The Hubbard model of bosons on two dimensional lattices with a lowest flat
band is discussed. In these systems there is a critical density, where the
ground state is known exactly and can be represented as a charge density wave.
Above this critical filling, depending on the lattice structure and the
interaction strength, the additional particles are either delocalised and
condensate in the ground state, or they form pairs. Pairs occur at strong
interactions, e.g., on the chequerboard lattice. The general mechanism behind
this phenomenon is discussed.Comment: small changes, one figure added. Accepted for publication in EPJ
Diagonalization of system plus environment Hamiltonians
A new approach to dissipative quantum systems modelled by a system plus
environment Hamiltonian is presented. Using a continuous sequence of
infinitesimal unitary transformations the small quantum system is decoupled
from its thermodynamically large environment. Dissipation enters through the
observation that system observables generically decay completely into a
different structure when the Hamiltonian is transformed into diagonal form. The
method is particularly suited for studying low-temperature properties. This is
demonstrated explicitly for the super-Ohmic spin-boson model.Comment: 4 pages, Latex, uses Revte
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