779 research outputs found
Excitations and Quantum Fluctuations in Site Diluted Two-Dimensional Antiferromagnets
We study the effect of site dilution and quantum fluctuations in an
antiferromagnetic spin system on a square lattice within the linear spin-wave
approximation. By performing numerical diagonalization in real space and
finite-size scaling, we characterize the nature of the low-energy spin
excitations for different dilution fractions up to the classical percolation
threshold. We find nontrivial signatures of fractonlike excitations at high
frequencies. Our simulations also confirm the existence of an upper bound for
the amount of quantum fluctuations in the ground state of the system, leading
to the persistence of long-range order up to the percolation threshold. This
result is in agreement with recent neutron-scattering experimental data and
quantum Monte Carlo numerical calculations. We also show that the absence of a
quantum critical point below the classical percolation threshold holds for a
large class of systems whose Hamiltonians can be mapped onto a system of
coupled noninteracting massless bosons.Comment: RevTex 4, 16 pages, 8 EPS figures, typos corrected, data from Ref. 9
added, few minor changes in the text, to appear in Phys. Rev.
A Survey on Long-Range Wide-Area Network Technology Optimizations
Long-Range Wide-Area Network (LoRaWAN) enables flexible long-range service communications with low power consumption which is suitable for many IoT applications. The densification of LoRaWAN, which is needed to meet a wide range of IoT networking requirements, poses further challenges. For instance, the deployment of gateways and IoT devices are widely deployed in urban areas, which leads to interference caused by concurrent transmissions on the same channel. In this context, it is crucial to understand aspects such as the coexistence of IoT devices and applications, resource allocation, Media Access Control (MAC) layer, network planning, and mobility support, that directly affect LoRaWAN’s performance.We present a systematic review of state-of-the-art works for LoRaWAN optimization solutions for IoT networking operations. We focus on five aspects that directly affect the performance of LoRaWAN. These specific aspects are directly associated with the challenges of densification of LoRaWAN. Based on the literature analysis, we present a taxonomy covering five aspects related to LoRaWAN optimizations for efficient IoT networks. Finally, we identify key research challenges and open issues in LoRaWAN optimizations for IoT networking operations that must be further studied in the future
Ice: a strongly correlated proton system
We discuss the problem of proton motion in Hydrogen bond materials with
special focus on ice. We show that phenomenological models proposed in the past
for the study of ice can be recast in terms of microscopic models in close
relationship to the ones used to study the physics of Mott-Hubbard insulators.
We discuss the physics of the paramagnetic phase of ice at 1/4 filling (neutral
ice) and its mapping to a transverse field Ising model and also to a gauge
theory in two and three dimensions. We show that H3O+ and HO- ions can be
either in a confined or deconfined phase. We obtain the phase diagram of the
problem as a function of temperature T and proton hopping energy t and find
that there are two phases: an ordered insulating phase which results from an
order-by-disorder mechanism induced by quantum fluctuations, and a disordered
incoherent metallic phase (or plasma). We also discuss the problem of
decoherence in the proton motion introduced by the lattice vibrations (phonons)
and its effect on the phase diagram. Finally, we suggest that the transition
from ice-Ih to ice-XI observed experimentally in doped ice is the
confining-deconfining transition of our phase diagram.Comment: 12 pages, 9 figure
Longitudinal study of patients with chronic Chagas cardiomyopathy in Brazil (SaMi-Trop project): a cohort profile.
PurposeWe have established a prospective cohort of 1959 patients with chronic Chagas cardiomyopathy to evaluate if a clinical prediction rule based on ECG, brain natriuretic peptide (BNP) levels, and other biomarkers can be useful in clinical practice. This paper outlines the study and baseline characteristics of the participants.ParticipantsThe study is being conducted in 21 municipalities of the northern part of Minas Gerais State in Brazil, and includes a follow-up of 2 years. The baseline evaluation included collection of sociodemographic information, social determinants of health, health-related behaviours, comorbidities, medicines in use, history of previous treatment for Chagas disease, functional class, quality of life, blood sample collection, and ECG. Patients were mostly female, aged 50-74 years, with low family income and educational level, with known Chagas disease for >10 years; 46% presented with functional class >II. Previous use of benznidazole was reported by 25.2% and permanent use of pacemaker by 6.2%. Almost half of the patients presented with high blood cholesterol and hypertension, and one-third of them had diabetes mellitus. N-terminal of the prohormone BNP (NT-ProBNP) level was >300 pg/mL in 30% of the sample.Findings to dateClinical and laboratory markers predictive of severe and progressive Chagas disease were identified as high NT-ProBNP levels, as well as symptoms of advanced heart failure. These results confirm the important residual morbidity of Chagas disease in the remote areas, thus supporting political decisions that should prioritise in addition to epidemiological surveillance the medical treatment of chronic Chagas cardiomyopathy in the coming years. The São Paulo-Minas Gerais Tropical Medicine Research Center (SaMi-Trop) represents a major challenge for focused research in neglected diseases, with knowledge that can be applied in primary healthcare.Future plansWe will continue following this patients' cohort to provide relevant information about the development and progression of Chagas disease in remotes areas, with social and economic inequalities.Trial registration numberNCT02646943; Pre-results
Strongly correlated fermions with nonlinear energy dispersion and spontaneous generation of anisotropic phases
Using the bosonization approach we study fermionic systems with a nonlinear
dispersion relation in dimension d>2. We explicitly show how the band curvature
gives rise to interaction terms in the bosonic version of the model. Although
these terms are perturbatively irrelevant in relation to the Landau Fermi
liquid fixed point, they become relevant perturbations when instabilities take
place. Using a coherent state path integral technique we built up the effective
action that governs the dynamics of the Fermi surface fluctuations. We consider
the combined effect of fermionic interactions and band curvature on possible
anisotropic phases triggered by negative Landau parameters. In particular we
study in some detail the phase diagram for the isotropic/nematic/hexatic
quantum phase transition.Comment: RevTeX4, 9 pages, 2 eps figures, Final version as appeared in
Phys.Rev.
A escolarização das pessoas com deficiência no Brasil: atendimento, atraso e progressão no ensino fundamental segundo o Censo 2000
O presente artigo tem por objetivo evidenciar a implicação das deficiências
sobre o atendimento, o atraso e a progressão escolar no ensino fundamental.
Para tal, é utilizada a modelagem estatÃstica logÃstica binária. Após apresentação
dos diferentes conceitos de deficiência, por meio do Censo 2000, transcorre-
-se a análise dos quesitos pretendidos. A progressão escolar é pesquisada por
meio da medida nomeada probabilidade de progressão por série (PPS), dos
três pontos de maior relevância no desenvolvimento do ensino fundamental. Os
resultados indicam grande disparidade entre as deficiências na determinação dos
elementos investigados. Em geral, todas as condições se mostram como fatores
que desfavorecem o desenvolvimento na carreira escolar. Porém, a implicação das
deficiências sobre as PPS se reduz com o avanço no ensino fundamental, sendo
também menos significativa a cada etapa sequente. Entre os quesitos analisados,
o atraso escolar demonstrou sofrer relevante variação, devido ao controle por
fatores de natureza socioeconômica. No mais, este trabalho contém resultados
e aponta contributos para o desenvolvimento cientÃfico dessa área de estudo
Double Exchange Model for Magnetic Hexaborides
A microscopic theory for rare-earth ferromagnetic hexaborides, such as
Eu(1-x)Ca(x)B6, is proposed on the basis of the double-exchange Hamiltonian. In
these systems, the reduced carrier concentrations place the Fermi level near
the mobility edge, introduced in the spectral density by the disordered spin
background. We show that the transport properties such as Hall effect,
magnetoresitance, frequency dependent conductivity, and DC resistivity can be
quantitatively described within the model. We also make specific predictions
for the behavior of the Curie temperature, Tc, as a function of the plasma
frequency, omega_p.Comment: 4 pages, 3 figure
Bosonization of Fermi liquids
We bosonize a Fermi liquid in any number of dimensions in the limit of long
wavelengths. From the bosons we construct a set of coherent states which are
related with the displacement of the Fermi surface due to particle-hole
excitations. We show that an interacting hamiltonian in terms of the original
fermions is quadratic in the bosons. We obtain a path integral representation
for the generating functional which in real time, in the semiclassical limit,
gives the Landau equation for sound waves and in the imaginary time gives us
the correct form of the specific heat for a Fermi liquid even with the
corrections due to the interactions between the fermions. We also discuss the
similarities between our results and the physics of quantum crystals.Comment: 42 pages, RevteX, preprint UIUC (1993
Quantum skyrmions and the destruction of long-range antiferromagnetic order in the high-Tc superconductors La(2-x)Sr(x)CuO(4) and YBa(2)Cu(3)O(6+x)
We study the destruction of the antiferromagnetic order in the high-Tc
superconductors La(2-x)Sr(x)CuO(4) and YBa(2)Cu(3)O(6+x) in the framework of
the CP1-nonlinear sigma model formulation of the 2D quantum Heisenberg
antiferromagnet. The dopants are introduced as independent fermions with
appropriate dispersion relations determined by the shape of the Fermi surface.
The energy of skyrmion topological defects, which are shown to be introduced by
doping, is used as an order parameter for antiferromagnetic order. We obtain
analytic expressions for this as a function of doping which allow us to plot
the curves T_N(x_c)\times x_c and M(x)\times x, for both YBCO and LSCO, in good
quantitative agreement with the experimental data.Comment: 4 pages, revtex, 5 embeeded figure
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