139 research outputs found
Flat-band ferromagnetism in a correlated topological insulator on a honeycomb lattice
We study the flat-band ferromagnetic phase of a spinfull and time-reversal
symmetric Haldane-Hubbard model on a honeycomb lattice within a bosonization
formalism for flat-band Z topological insulators. Such a study extend our
previous one [L. S. G. Leite and R. L. Doretto, Phys. Rev. B {\bf 104}, 155129
(2021)] concerning the flat-band ferromagnetic phase of a correlated Chern
insulator described by a Haldane-Hubbard model. We consider the topological
Hubbard model at filling of its corresponding noninteracting limit and in
the nearly flat band limit of its lower free-electronic bands. We show that it
is possible to define boson operators associated with two distinct spin-flip
excitations, one that changes (mixed-lattice excitations) and a second one that
preserves (same-lattice excitations) the index related with the two triangular
sublattices. Within the bosonization scheme, the fermionic model is mapped into
an effective interacting boson model, whose quadratic term is considered at the
harmonic approximation in order to determine the spin-wave excitation spectrum.
For both mixed and same-lattice excitations, we find that the spin-wave
spectrum is gapped and has two branches, with an energy gap between the lower
and the upper bands at the and points of the first Brillouin zone.
Such a behavior is distinct from the one of the corresponding correlated Chern
insulator, whose spin-wave spectrum has a Goldstone mode at the center of the
first Brillouin zone and Dirac points at and points. We also find some
evidences that the spin-wave bands for the same-lattice excitations might be
topologically nontrivial even in the completely flat band limit.Comment: 16 pages, 8 figures, companion paper to our previous
arXiv:2106.00468, final versio
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
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.
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
Considering mesohabitat scale in ecological impact assessment of sediment flushing
Benthic macroinvertebrates respond to several factors characterizing the physical habitats, as water depth, current and streambed substrate. Thus, anthropogenic disturbances altering these factors may have different effects on benthos, also depending on mesohabitats. These disturbances include sediment flushing operations, commonly carried out to recover reservoir capacity, and investigating their effects at mesohabitat scale could be relevant for an adequate ecological impact assessment of these operations. Here, we compared benthic macroinvertebrate communities sampled before and after a controlled sediment flushing operation in three different mesohabitats (a pool, a riffle and a step-pool) of an Alpine stream. Contrary from expectations, the composition of macroinvertebrate assemblages was not significantly different among mesohabitats. Moreover, the impact of sediment flushing was more significant in terms of density rather than in richness. Two stressor-specific indices were tested, but only one (the Siltation Index for LoTic EcoSystems - SILTES) clearly detected the impact of sediment flushing on the macroinvertebrate community structure. Finally, some differences in the temporal trajectories and recovery times to pre-flushing conditions were observed among mesohabitats, both if the three mesohabitats were considered separately and if all their possible combinations were accounted for. Particularly, riffle was the most sensitive mesohabitat, not fully recovering one year after the sediment disturbance
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
Beta-diversity and stressor specific index reveal patterns of macroinvertebrate community response to sediment flushing
Anthropogenic increase of fine sediment loading is one of the main pressures for rivers worldwide. Particularly, Alpine streams are increasingly facing this issue due to sediment flushing operations from hydropower reservoirs, aimed at recovering storage for preserving electricity generation. Although Controlled Sediment Flushing Operations (CSFOs) are becoming increasingly frequent, ecological indicators to adequately assess and monitor their impact on the stream ecosystem have been poorly developed. In this work, we aimed to perform a screening of currently available biomonitoring tools to evaluate the CSFO effects on the riverine biota and adequately assess its recovery, starting from the recognition of the main ecological mechanisms triggered by the mentioned activities on benthic macroinvertebrate communities. We used two independent datasets concerning two reservoirs in the central Italian Alps to investigate the temporal effects of CSFOs repeated for four consecutive years (case-study I), and the impact of a single CSFO at a seasonal scale through a before/after-control/impact approach (case-study II). Initially, we quantified the CSFO impact on the richness and beta-diversity of macroinvertebrate communities by combining multivariate and univariate statistical techniques. Then, we compared the performance of the Siltation Index for LoTic EcoSystems (SILTES), recently developed for detecting siltation impact in Alpine streams, with that of the generic index currently adopted to assess the ecological status (sensu Water Framework Directive) of the Italian rivers, and of another sediment-specific index, but developed for a different bio-geographical area. The analysis of the two case-studies demonstrated that the nestedness (i.e. taxa loss) is the primary source of biological impairment caused by CSFOs. Moreover, we found that SILTES was more effective than the other indices because of its strong correlation with the nestedness, and since it properly discriminated impaired and pristine conditions, at both multi-annual and seasonal scale. In the first case-study, a threshold in the temporal trend of this index was detected, indicating a recovery within three months. In the second one, SILTES showed a recovery to pre-event seasonal values after nine months from the CSFO, due to larger and more persistent sediment deposition. This study demonstrates that SILTES could be adopted as a benchmark to improve the management of CSFOs from an ecological viewpoint. Our findings can be extended to the management of other sediment-related activities affecting mountainous streams worldwide, and, more generally, the adopted approach can be replicated for developing new ecological tools to manage other disturbances to river environments
Spin-excitations of the quantum Hall ferromagnet of composite fermions
The spin-excitations of a fractional quantum Hall system are evaluated within
a bosonization approach. In a first step, we generalize Murthy and Shankar's
Hamiltonian theory of the fractional quantum Hall effect to the case of
composite fermions with an extra discrete degree of freedom. Here, we mainly
investigate the spin degrees of freedom, but the proposed formalism may be
useful also in the study of bilayer quantum-Hall systems, where the layer index
may formally be treated as an isospin. In a second step, we apply a
bosonization scheme, recently developed for the study of the two-dimensional
electron gas, to the interacting composite-fermion Hamiltonian. The dispersion
of the bosons, which represent quasiparticle-quasihole excitations, is
analytically evaluated for fractional quantum Hall systems at \nu = 1/3 and \nu
= 1/5. The finite width of the two-dimensional electron gas is also taken into
account explicitly. In addition, we consider the interacting bosonic model and
calculate the lowest-energy state for two bosons. Besides a continuum
describing scattering states, we find a bound-state of two bosons. This state
is interpreted as a pair excitation, which consists of a skyrmion of composite
fermions and an antiskyrmion of composite fermions. The dispersion relation of
the two-boson state is evaluated for \nu = 1/3 and \nu = 1/5. Finally, we show
that our theory provides the microscopic basis for a phenomenological
non-linear sigma-model for studying the skyrmion of composite fermions.Comment: Revised version, 14 pages, 4 figures, accepted to Phys. Rev.
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
Prevalence Of Newcastle Disease Virus In Broiler Chickens (gallus Gallus) In Brazil.
This study was carried out during 2002/2003, aiming to determine the prevalence of virulent Newcastle disease virus strains (NDV) in Brazilian commercial poultry farms. Clinical samples were obtained from the Southeastern, Southern and Central-Western regions, which comprise the main area of the Brazilian poultry production. Serum samples and tracheal and cloacal swabs of 23,745 broiler chickens from 1,583 flocks, including both vaccinated chickens and those with no vaccination information, were tested for NDV using a diagnostic ELISA kit. The seropositivity was 39.1%, and the isolation percentage by flock varied from 1.0 to 7.6%, and by region from 6.5 to 58.4%. Higher isolation rates (74.3-83.3%) were obtained after three passages in embryonated chicken eggs. All isolates preliminarily identified as NDV were characterized as nonpathogenic strains, as their Intracerebral Pathogenicity Index (ICPI) was below 0.7. Based on results of this study, Brazil can claim a virulent NDV-free status for commercial flocks.41349-5
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