754 research outputs found
Variational Cluster Perturbation Theory for Bose-Hubbard models
We discuss the application of the variational cluster perturbation theory
(VCPT) to the Mott-insulator--to--superfluid transition in the Bose-Hubbard
model. We show how the VCPT can be formulated in such a way that it gives a
translation invariant excitation spectrum -- free of spurious gaps -- despite
the fact that if formally breaks translation invariance. The phase diagram and
the single-particle Green function in the insulating phase are obtained for
one-dimensional systems. When the chemical potential of the cluster is taken as
a variational parameter, the VCPT reproduces the dimension dependence of the
phase diagram even for one-site clusters. We find a good quantitative agreement
with the results of the density-matrix renormalization group when the number of
sites in the cluster becomes of order 10. The extension of the method to the
superfluid phase is discussed.Comment: v1) 10 pages, 6 figures. v2) Final version as publishe
Semanticizing syntactic patterns in NLP processing using SPARQL-DL queries
Some recent works on natural language semantic parsing make use of syntax and semantics together using different combination models. In our work we attempt to use SPARQL-DL as an interface between syntactic information given by the Stanford statistical parser (namely part-of-speech tagged text and typed dependency representation) and semantic information obtained from the FrameNet database. We use SPARQL-DL queries to check the presence of syntactic patterns within a sentence and identify their role as frame elements. The choice of SPARQL-DL is due to its usage as a common reference language for semantic applications and its high expressivity, which let rules to be generalized exploiting the inference capabilities of the underlying reasoner
Characterization of Mott-insulating and superfluid phases in the one-dimensional Bose--Hubbard model
We use strong-coupling perturbation theory, the variational cluster approach
(VCA), and the dynamical density-matrix renormalization group (DDMRG) method to
investigate static and dynamical properties of the one-dimensional
Bose--Hubbard model in both the Mott-insulating and superfluid phases. From the
von Neumann entanglement entropy we determine the central charge and the
transition points for the first two Mott lobes. Our DMRG results for the
ground-state energy, momentum distribution function, boson correlation function
decay, Mott gap, and single particle-spectral function are reproduced very well
by the strong-coupling expansion to fifth order, and by VCA with clusters up to
12 sites as long as the ratio between the hopping amplitude and on-site
repulsion, t/U, is smaller than 0.15 and 0.25, respectively. In addition, in
the superfluid phase VCA captures well the ground-state energy and the sound
velocity of the linear phonon modes. This comparison provides an authoritative
estimate for the range of applicability of these methods. In strong-coupling
theory for the Mott phase, the dynamical structure factor is obtained from the
solution of an effective single-particle problem with an attractive potential.
The resulting resonances show up as double-peak structure close to the
Brillouin zone boundary. These high-energy features also appear in the
superfluid phase which is characterized by a pronounced phonon mode at small
momenta and energies, as predicted by Bogoliubov and field theory. In one
dimension, there are no traces of an amplitude mode in the dynamical
single-particle and two-particle correlation functions.Comment: 15 pages, 12 figure
Phase diagram of a Bose gas near a wide Feshbach resonance
In this paper, we study the phase diagram of a homogeneous Bose gas with a
repulsive interaction near a wide Feshbach resonance at zero temperature. The
Bose-Einstein-condensation (BEC) state of atoms is a metastable state. When the
scattering length exceeds a critical value depending on the atom density
, , the molecular excitation energy is imaginary and the atomic
BEC state is dynamically unstable against molecule formation. The BEC state of
diatomic molecules has lower energy, where the atomic excitation is gapped and
the molecular excitation is gapless. However when the scattering length is
above another critical value, , the molecular BEC state becomes a
unstable coherent mixture of atoms and molecules. In both BEC states, the
binding energy of diatomic molecules is reduced due to the many-body effect.Comment: 5 pages, 4 figure
Prevalence of Mycobacterium avium subsp. paratuberculosis in milk and dairy cattle in Southern Italy; preliminary results
Paratuberculosis affects all ruminants worldwide. Mycobacterium avium subsp. paratuberculosis could have a role in human diseases like Crohn\u2019s. Some extra EU countries request importation of MAP-free products. Italy has not yet actualized a control program and the diffusion of the infection is still unknown in Southern Italy. The aim of this study was to evaluate the prevalence of the infection in five regions of Southern Italy. Bulk tank milk and in-line milk filters were sampled in 780 dairy cattle herds and respectively analyzed by ELISA and real time PCR. One hundred and fifty-five out of 780 herds (19.9%) were found positive by ELISA and/or real time PCR. Individual milk samples were then collected from all the producing animals of positive herds and from a selection of negative herds. The estimated prevalence varies from region to region between 2.8% and 5.5%. Our results indicate that the disease is widespread in the five regions. The observed prevalence could be underestimated
Theory of the density fluctuation spectrum of strongly correlated electrons
The density response function of the two-dimensional
model is studied starting from a mixed gauge formulation of the slave boson
approach. Our results for are in remarkable agreement with exact
diagonalization studies, and provide a natural explanation of the anomalous
features in the density response in terms of the spin polaron nature of the
charge carriers. In particular we have identified unexplained low energy
structures in the diagonalization data as arising from the coherent polaron
motion of holes in a spin liquid.Comment: 4 pages with 4 figures, to be published in Physical Review B (RC
Electron transport in coupled chains of interacting fermions with impurities
We study the low-temperature transport of a doped two-chain ladder system of
interacting fermions in the presence of a barrier or of a low concentration of
impurities. Above a certain value of the interaction, the conductance is
suppressed, like for a single chain, despite the presence of dominant
superconducting correlations. There is, however, a region of repulsive
interaction where perfect transmission across the barrier occurs unlike the
single-chain case. We provide a possible explanation for the temperature
maximum of the resistivity in the normal state of \srca.Comment: 4 pages, 2 figures, to be published in Phys. Rev. Let
Time-dependent Gutzwiller approximation for the Hubbard model
We develop a time-dependent Gutzwiller approximation (GA) for the Hubbard
model analogous to the time-dependent Hartree-Fock (HF) method. The formalism
incorporates ground state correlations of the random phase approximation (RPA)
type beyond the GA. Static quantities like ground state energy and double
occupancy are in excellent agreement with exact results in one dimension up to
moderate coupling and in two dimensions for all couplings. We find a
substantial improvement over traditional GA and HF+RPA treatments. Dynamical
correlation functions can be easily computed and are also substantially better
than HF+RPA ones and obey well behaved sum rules.Comment: 4 pages, 2 figure
Bovine paratuberculosis: results of a control plan in 64 dairy farms in a 4-year period
Paratuberculosis is considered one of the most economically devastating infectious diseases of domestic livestock, and the most effective control strategy is a combination of 'test-and-cull' and on-farm biosecurity measures. In Italy, a Voluntary National Control Plan (VNCP) and guidelines have been introduced to reduce the impact of the disease, and farmers can voluntarily enroll in the control plan. The main aims of this study were: i) the description of the trend over a 4-year period on total, within-herd (WH) and between herd (BH) apparent seroprevalences observed in 64 dairy herds members of a mutual company located in Italy after the introduction of a proposed "Customized Control Plan" (CCP); ii) the evaluation of its effectiveness in terms of percentage of participating farms that decided to join the VNCP. Analyses on serum samples were performed with Enzyme-Linked Immuno Sorbent Assay (ELISA) method and revealed a general decrease in both total, WH and BH apparent seroprevalence. Total average apparent seroprevalence decreased from 2.39% in 2017 to 1% in 2020. Negative herds raised from 51.9% in 2017 to 71.1% in 2020, while farms with WH apparent seroprevalence > 5% decreased from 17.3% in 2017 to 4.4% in 2020. BH apparent seroprevalence decreased from 51.2% in 2017 to 29.2% in 2020. Among the 52 out of 64 herds that accepted to continue the proposed CCP after the first year, 41 (78.8%) joined in 2020 the VNCP, that assessed the health ranking of the herds. The results provide evidence that a control plan based on a farm-specific strategy and a subsidized testing process can effectively reduce the impact of paratuberculosis in dairy herds, especially in convincing farmers to continue in paratuberculosis control by joining the VNCP, including them in a National context and increasing their awareness of the disease
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