69 research outputs found
Flat-band ferromagnetism and spin waves in the Haldane-Hubbard model
We study the flat-band ferromagnetic phase of the Haldane-Hubbard model on a
honeycomb lattice within a bosonization scheme for flat-band Chern insulators,
focusing on the calculation of the spin-wave excitation spectrum. We consider
the Haldane-Hubbard model with the noninteracting lower bands in a nearly-flat
band limit, previously determined for the spinless model, and at 1/4-filling of
its corresponding noninteracting limit. Within the bosonization scheme, the
Haldane-Hubbard model is mapped into an effective interacting boson model,
whose quadratic term allows us to determine the spin-wave spectrum at the
harmonic approximation. We show that the excitation spectrum has two branches
with a Goldstone mode and Dirac points at center and at the K and K' points of
the first Brillouin zone, respectively. We also consider the effects on the
spin-wave spectrum due to an energy offset in the on-site Hubbard repulsion
energies and due to the presence of an staggered on-site energy term, both
quantities associated with the two triangular sublattices. In both cases, we
find that an energy gap opens at the K and K' points. Moreover, we also find
some evidences for an instability of the flat-band ferromagnetic phase in the
presence of the staggered on-site energy term. We provide some additional
results for the square lattice topological Hubbard model previous studied
within the bosonization formalism and comment on the differences between the
bosonization scheme implementation for the correlated Chern insulators on both
square and honeycomb lattices.Comment: 17 pages, 11 figure
Entanglement entropy for the valence bond solid phases of two-dimensional dimerized Heisenberg antiferromagnets
We calculate the bipartite von Neumann and second R\'enyi entanglement
entropies of the ground states of spin-1/2 dimerized Heisenberg
antiferromagnets on a square lattice. Two distinct dimerization patterns are
considered: columnar and staggered. In both cases, we concentrate on the
valence bond solid (VBS) phase and describe such a phase with the bond-operator
representation. Within this formalism, the original spin Hamiltonian is mapped
into an effective interacting boson model for the triplet excitations. We study
the effective Hamiltonian at the harmonic approximation and determine the
spectrum of the elementary triplet excitations. We then follow an analytical
procedure, which is based on a modified spin-wave theory for finite systems and
was originally employed to calculate the entanglement entropies of magnetic
ordered phases, and calculate the entanglement entropies of the VBS ground
states. In particular, we consider one-dimensional (line) subsystems within the
square lattice, a choice that allows us to consider line subsystems with sizes
up to . We combine such a procedure with the results of the
bond-operator formalism at the harmonic level and show that, for both dimerized
Heisenberg models, the entanglement entropies of the corresponding VBS ground
states obey an area law as expected for gapped phases. For both columnar-dimer
and staggered-dimer models, we also show that the entanglement entropies
increase but do not diverge as the dimerization decreases and the system
approaches the N\'eel--VBS quantum phase transition. Finally, the entanglement
spectra associated with the VBS ground states are presented.Comment: 14 pages, 9 figure
Photoluminescence spectrum of an interacting two-dimensional electron gas at \nu=1
We report on the theoretical photoluminescence spectrum of the interacting
two-dimensional electron gas at filling factor one (\nu=1). We considered a
model similar to the one adopted to study the X-ray spectra of metals and
solved it analytically using the bosonization method previously developed for
the two-dimensional electron gas at \nu=1. We calculated the emission spectra
of the right and the left circularly polarized radiations for the situations
where the distance between the two-dimensional electron gas and the valence
band hole are smaller and greater than the magnetic length. For the former, we
showed that the polarized photoluminescence spectra can be understood as the
recombination of the so-called excitonic state with the valence band hole
whereas, for the latter, the observed emission spectra can be related to the
recombination of a state formed by a spin down electron bound to n spin waves.
This state seems to be a good description for the quantum Hall skyrmion.Comment: Revised version, 10 pages, 5 figures, accepted to Phys. Rev.
NMR linewidth and Skyrmion localization in quantum Hall ferromagnets
The non-monotonic behavior of the NMR signal linewidth in the 2D quantum Hall
system is explained in terms of the interplay between skyrmions localization,
due to the influence of disorder, and the non-trivial temperature dependent
skyrmion dynamics.Comment: 5 pages, 2 figure
Finite-momentum condensate of magnetic excitons in a bilayer quantum Hall system
We study the bilayer quantum Hall system at total filling factor \nu_T = 1
within a bosonization formalism which allows us to approximately treat the
magnetic exciton as a boson. We show that in the region where the distance
between the two layers is comparable to the magnetic length, the ground state
of the system can be seen as a finite-momentum condensate of magnetic excitons
provided that the excitation spectrum is gapped. We analyze the stability of
such a phase within the Bogoliubov approximation firstly assuming that only one
momentum Q0 is macroscopically occupied and later we consider the same
situation for two modes \pm Q0. We find strong evidences that a first-order
quantum phase transition at small interlayer separation takes place from a
zero-momentum condensate phase, which corresponds to Halperin 111 state, to a
finite-momentum condensate of magnetic excitons.Comment: 18 pages, 11 figures, final versio
Tracking System with Re-identification Using a RGB String Kernel
International audiencePeople re-identification consists to identify a person which comes back in a scene where it has been previously detected. This key problem in visual surveillance applications may concern single or multi camera systems. Features encoding each person should be rich enough to provide an efficient re-identification while being sufficiently robust to remain significant through the different phenomena which may alter the appearance of a person in a video. We propose in this paper a method which encodes people's appearance through a string of salient points. The similarity between two such strings is encoded by a kernel. This last kernel is combined with a tracking algorithm in order to associate a set of strings to each person and to measure similarities between persons entering into the scene and persons who left it
Two-particle decay and quantum criticality in dimerized antiferromagnets
In certain Mott-insulating dimerized antiferromagnets, triplet excitations of
the paramagnetic phase can decay into the two-particle continuum. When such a
magnet undergoes a quantum phase transition into a magnetically ordered state,
this coupling becomes part of the critical theory provided that the lattice
ordering wavevector is zero. One microscopic example is the staggered-dimer
antiferromagnet on the square lattice, for which deviations from O(3)
universality have been reported in numerical studies. Using both symmetry
arguments and microscopic calculations, we show that a non-trivial cubic term
arises in the relevant order-parameter quantum field theory, and assess its
consequences using a combination of analytical and numerical methods. We also
present finite-temperature quantum Monte Carlo data for the staggered-dimer
antiferromagnet which complement recently published results. The data can be
consistently interpreted in terms of critical exponents identical to that of
the standard O(3) universality class, but with anomalously large corrections to
scaling. We argue that the two-particle decay of critical triplons, although
irrelevant in two spatial dimensions, is responsible for the leading
corrections to scaling due to its small scaling dimension.Comment: 14 pages, 7 fig
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