8,041 research outputs found
Phase diagram of an extended Agassi model
Background: The Agassi model is an extension of the Lipkin-Meshkov-Glick
model that incorporates the pairing interaction. It is a schematic model that
describes the interplay between particle-hole and pair correlations. It was
proposed in the 1960's by D. Agassi as a model to simulate the properties of
the quadrupole plus pairing model.
Purpose: The aim of this work is to extend a previous study by Davis and
Heiss generalizing the Agassi model and analyze in detail the phase diagram of
the model as well as the different regions with coexistence of several phases.
Method: We solve the model Hamiltonian through the Hartree-Fock-Bogoliubov
(HFB) approximation, introducing two variational parameters that play the role
of order parameters. We also compare the HFB calculations with the exact ones.
Results: We obtain the phase diagram of the model and classify the order of
the different quantum phase transitions appearing in the diagram. The phase
diagram presents broad regions where several phases, up to three, coexist.
Moreover, there is also a line and a point where four and five phases are
degenerated, respectively.
Conclusions: The phase diagram of the extended Agassi model presents a rich
variety of phases. Phase coexistence is present in extended areas of the
parameter space. The model could be an important tool for benchmarking novel
many-body approximations.Comment: Accepted for publication in PR
An extended Agassi model: algebraic structure, phase diagram, and large size limit
The Agassi model is a schematic two-level model that involves pairing and
monopole-monopole interactions. It is, therefore, an extension of the well
known Lipkin-Meshkov-Glick (LMG) model. In this paper we review the algebraic
formulation of an extension of the Agassi model as well as its bosonic
realization through the Schwinger representation. Moreover, a mean-field
approximation for the model is presented and its phase diagram discussed.
Finally, a analysis, with proportional to the degeneracy of each
level, is worked out to obtain the thermodynamic limit of the ground state
energy and some order parameters from the exact Hamiltonian diagonalization for
finite.Comment: Accepted in Physica Scripta. Focus on SSNET 201
Point defects on graphene on metals
Understanding the coupling of graphene with its local environment is critical
to be able to integrate it in tomorrow's electronic devices. Here we show how
the presence of a metallic substrate affects the properties of an atomically
tailored graphene layer. We have deliberately introduced single carbon
vacancies on a graphene monolayer grown on a Pt(111) surface and investigated
its impact in the electronic, structural and magnetic properties of the
graphene layer. Our low temperature scanning tunneling microscopy studies,
complemented by density functional theory, show the existence of a broad
electronic resonance above the Fermi energy associated with the vacancies.
Vacancy sites become reactive leading to an increase of the coupling between
the graphene layer and the metal substrate at these points; this gives rise to
a rapid decay of the localized state and the quenching of the magnetic moment
associated with carbon vacancies in free-standing graphene layers
Frustration free gapless Hamiltonians for Matrix Product States
For every Matrix Product State (MPS) one can always construct a so-called
parent Hamiltonian. This is a local, frustration free, Hamiltonian which has
the MPS as ground state and is gapped. Whenever that parent Hamiltonian has a
degenerate ground state (the so-called non-injective case), we construct
another 'uncle' Hamiltonian which is local and frustration free but gapless,
and its spectrum is . The construction is obtained by linearly perturbing
the matrices building up the state in a random direction, and then taking the
limit where the perturbation goes to zero. For MPS where the parent Hamiltonian
has a unique ground state (the so-called injective case) we also build such
uncle Hamiltonian with the same properties in the thermodynamic limit.Comment: 36 pages, new version with some contents rearranged, and a correction
in the injective cas
Effectiveness of a program of romana’s Pilates for non-specific low back pain: A pilot study
Objetivo: comprobar la efectividad del método Pilates Romana para conseguir
una mayor flexibilidad de la columna, junto con una mejora en la movilidad de la
misma, así como del dolor que presentan en su vida diaria los pacientes.
Metodología: ensayo clínico con intención de tratar a treinta pacientes con dolor
lumbar inespecífico. Asistieron a 15 sesiones, 2 veces a la semana, del Método
Pilates Romana.. Se evaluaron parámetros tales como dolor, test de Schöber,
SRS-22 y distancia dedos- suelo.
Resultados. Se encontraron diferencias estadísticamente significativas con
respecto al dolor (escala EVA), distancia dedos- suelo, test de Schöber
(flexibilidad en plano sagital), flexión lateral (flexibilidad en plano frontal) y en
varios ítems de la escala SRS-22, con valores de p<0,001. Por ello, este método
puede ser usado para mejorar el dolor, la flexibilidad axial, la función y los
aspectos relacionados con la calidad de vidaAim: to test the effectiveness of the Romana’s Pilates method to obtain
increased flexibility, improvements in mobility, and reduced pain in daily life.
Methodology: a clinical trial with intention-to-treat thirty patients with nonspecific
low back pain. Participants attended 15 sessions, twice a week. The
Romana’s Pilates method was taught by an external physiotherapist.
Parameters such as pain, the Schober test, and the SRS-22 were evaluated.
Results: Statistically significant differences in pain (VAS), the Schober test
(flexibility in sagittal plane), lateral flexion (flexibility in frontal plane) and several
items of the SRS-22 scale were found, with p <0.001.
This method may be used to improve pain, axial flexibility, function and aspects
related to the quality of life of patient
Gapless Hamiltonians for the toric code using the PEPS formalism
We study Hamiltonians which have Kitaev's toric code as a ground state, and
show how to construct a Hamiltonian which shares the ground space of the toric
code, but which has gapless excitations with a continuous spectrum in the
thermodynamic limit. Our construction is based on the framework of Projected
Entangled Pair States (PEPS), and can be applied to a large class of
two-dimensional systems to obtain gapless "uncle Hamiltonians".Comment: 8 pages, 2 figure
An unbiased genetic screen reveals the polygenic nature of the influenza virus anti-interferon response.
Influenza A viruses counteract the cellular innate immune response at several steps, including blocking RIG I-dependent activation of interferon (IFN) transcription, interferon (IFN)-dependent upregulation of IFN-stimulated genes (ISGs), and the activity of various ISG products; the multifunctional NS1 protein is responsible for most of these activities. To determine the importance of other viral genes in the interplay between the virus and the host IFN response, we characterized populations and selected mutants of wild-type viruses selected by passage through non-IFN-responsive cells. We reasoned that, by allowing replication to occur in the absence of the selection pressure exerted by IFN, the virus could mutate at positions that would normally be restricted and could thus find new optimal sequence solutions. Deep sequencing of selected virus populations and individual virus mutants indicated that nonsynonymous mutations occurred at many phylogenetically conserved positions in nearly all virus genes. Most individual mutants selected for further characterization induced IFN and ISGs and were unable to counteract the effects of exogenous IFN, yet only one contained a mutation in NS1. The relevance of these mutations for the virus phenotype was verified by reverse genetics. Of note, several virus mutants expressing intact NS1 proteins exhibited alterations in the M1/M2 proteins and accumulated large amounts of deleted genomic RNAs but nonetheless replicated to high titers. This suggests that the overproduction of IFN inducers by these viruses can override NS1-mediated IFN modulation. Altogether, the results suggest that influenza viruses replicating in IFN-competent cells have tuned their complete genomes to evade the cellular innate immune system and that serial replication in non-IFN-responsive cells allows the virus to relax from these constraints and find a new genome consensus within its sequence space.
IMPORTANCE In natural virus infections, the production of interferons leads to an antiviral state in cells that effectively limits virus replication. The interferon response places considerable selection pressure on viruses, and they have evolved a variety of ways to evade it. Although the influenza virus NS1 protein is a powerful interferon antagonist, the contributions of other viral genes to interferon evasion have not been well characterized. Here, we examined the effects of alleviating the selection pressure exerted by interferon by serially passaging influenza viruses in cells unable to respond to interferon. Viruses that grew to high titers had mutations at many normally conserved positions in nearly all genes and were not restricted to the NS1 gene. Our results demonstrate that influenza viruses have fine-tuned their entire genomes to evade the interferon response, and by removing interferon-mediated constraints, viruses can mutate at genome positions normally restricted by the interferon response
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