153 research outputs found
Dynamical Functions of a 1D Correlated Quantum Liquid
We extend to initial ground states with zero spin density m = 0 the
expressions provided by the pseudofermion dynamical theory (PDT) for the
finite-energy one- and two-electron spectral-weight distributions of a
one-dimensional (1D) correlated metal with on-site particle-particle repulsion.
The spectral-function expressions derived in this paper were used in recent
successful and detailed theoretical studies of the finite-energy singular
features in photoemission of the organic compound
tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) metallic phase. Our
studies take into account spectral contributions from types of microscopic
processes that do not occur for finite values of the spin density. Expressions
for the spectral functions in the vicinity of the singular border lines which
also appear in the TTF- TCNQ spectral-weight distribution are derived. In
addition, the PDT expressions are generalized for electronic densities in the
vicinity of half filling. Further details on the processes involved in the
applications to TTF-TCNQ are reported. Our results are useful for the further
understanding of the unusual spectral properties observed in low-dimensional
organic metals and also provide expressions for the one- and two-atom spectral
functions of a correlated quantum system of ultracold fermionic atoms in a 1D
optical lattice with on-site two-atom repulsion
Scattering mechanisms and spectral properties of the one-dimensional Hubbard model
It is found that the finite-energy spectral properties of the one-dimensional
Hubbard model are controlled by the scattering of charged -spin-zero
-holon composite objects, spin-zero -spinon composite objects, and
charged -spin-less and spin-less objects, rather than by the scattering
of independent -spin 1/2 holons and spin 1/2 spinons. Here . The corresponding matrix is calculated and its relation to the
spectral properties is clarified.Comment: 8 pages, no figure
Phase diagram and magnetic collective excitations of the Hubbard model in graphene sheets and layers
We discuss the magnetic phases of the Hubbard model for the honeycomb lattice
both in two and three spatial dimensions. A ground state phase diagram is
obtained depending on the interaction strength
U and electronic density n. We find a first order phase transition between
ferromagnetic regions where the spin is maximally polarized (Nagaoka
ferromagnetism) and regions with smaller magnetization (weak ferromagnetism).
When taking into account the possibility of spiral states, we find that the
lowest critical U is obtained for an ordering momentum different from zero. The
evolution of the ordering momentum with doping is discussed. The magnetic
excitations (spin waves) in the antiferromagnetic insulating phase are
calculated from the random-phase-approximation for the spin susceptibility. We
also compute the spin fluctuation correction to the mean field magnetization by
virtual emission/absorpion of spin waves. In the large limit, the
renormalized magnetization agrees qualitatively with the Holstein-Primakoff
theory of the Heisenberg antiferromagnet, although the latter approach produces
a larger renormalization
The TTF finite-energy spectral features in photoemission of TTF-TCNQ: The Hubbard-chain description
A dynamical theory which accounts for all microscopic one-electron processes
is used to study the spectral function of the 1D Hubbard model for the whole
-plane, beyond previous studies which focused on the weight
distribution in the vicinity of the singular branch lines only. While our
predictions agree with those of the latter studies concerning the
tetracyanoquinodimethane (TCNQ) related singular features in photoemission of
the organic compound tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ)
metallic phase, the generalized theory also leads to quantitative agreement
concerning the tetrathiafulvalene (TTF) related finite-energy spectral
features, which are found to correspond to a value of the on-site repulsion
larger than for TCNQ. Our study reveals the microscopic mechanisms behind the
unusual spectral features of TTF-TCNQ and provides a good overall description
of those features for the whole -plane.Comment: To appear in Journal of Physics: Condensed Matte
Spectral microscopic mechanisms and quantum phase transitions in a 1D correlated problem
In this paper we study the dominant microscopic processes that generate
nearly the whole one-electron removal and addition spectral weight of the
one-dimensional Hubbard model for all values of the on-site repulsion . We
find that for the doped Mott-Hubbard insulator there is a competition between
the microscopic processes that generate the one-electron upper-Hubbard band
spectral-weight distributions of the Mott-Hubbard insulating phase and
finite-doping-concentration metallic phase, respectively. The spectral-weight
distributions generated by the non-perturbative processes studied here are
shown elsewhere to agree quantitatively for the whole momentum and energy
bandwidth with the peak dispersions observed by angle-resolved photoelectron
spectroscopy in quasi-one-dimensional compounds.Comment: 18 pages, 2 figure
Tracking spin and charge with spectroscopy in spin-polarised 1D systems
We calculate the spectral function of a one-dimensional strongly interacting
chain of fermions, where the response can be well understood in terms of spinon
and holon excitations. Upon increasing the spin imbalance between the spin
species, we observe the single-electron response of the fully polarised system
to emanate from the holon peak while the spinon response vanishes. For
experimental setups that probe one-dimensional properties, we propose this
method as an additional generic tool to aid the identification of spectral
structures, e.g. in ARPES measurements. We show that this applies even to
trapped systems having cold atomic gas experiments in mind.Comment: 5 pages, 4 figure
Electron-electron interaction effects on the photophysics of metallic single-walled carbon nanotubes
Single-walled carbon nanotubes are strongly correlated systems with large
Coulomb repulsion between two electrons occupying the same orbital.
Within a molecular Hamiltonian appropriate for correlated -electron
systems, we show that optical excitations polarized parallel to the nanotube
axes in the so-called metallic single-walled carbon nanotubes are to excitons.
Our calculated absolute exciton energies in twelve different metallic
single-walled carbon nanotubes, with diameters in the range 0.8 - 1.4 nm, are
in nearly quantitative agreement with experimental results. We have also
calculated the absorption spectrum for the (21,21) single-walled carbon
nanotube in the E region. Our calculated spectrum gives an excellent fit
to the experimental absorption spectrum. In all cases our calculated exciton
binding energies are only slightly smaller than those of semiconducting
nanotubes with comparable diameters, in contradiction to results obtained
within the {\it ab initio} approach, which predicts much smaller binding
energies. We ascribe this difference to the difficulty of determining the
behavior of systems with strong on-site Coulomb interactions within theories
based on the density functional approach. As in the semiconducting nanotubes we
predict in the metallic nanotubes a two-photon exciton above the lowest
longitudinally polarized exciton that can be detected by ultrafast pump-probe
spectroscopy. We also predict a subgap absorption polarized perpendicular to
the nanotube axes below the lowest longitudinal exciton, blueshifted from the
exact midgap by electron-electron interactions
Combined effect of AMPK/PPAR agonists and exercise training in mdx mice functional performance
The present investigation was undertaken to test whether exercise training (ET) associated with AMPK/PPAR agonists (EM) would improve skeletal muscle function in mdx mice. These drugs have the potential to improve oxidative metabolism. This is of particular interest because oxidative muscle fibers are less affected in the course of the disease than glycolitic counterparts. Therefore, a cohort of 34 male congenic C57Bl/10J mdx mice included in this study was randomly assigned into four groups: vehicle solution (V), EM [AICAR (AMPK agonist, 50 mg/Kg-1.day-1, ip) and GW 1516 (PPAR delta agonist, 2.5 mg/Kg-1.day-1, gavage)], ET (voluntary running on activity wheel) and EM+ET. Functional performance (grip meter and rotarod), aerobic capacity (running test), muscle histopathology, serum creatine kinase (CK), levels of ubiquitined proteins, oxidative metabolism protein expression (AMPK, PPAR, myoglobin and SCD) and intracellular calcium handling (DHPR, SERCA and NCX) protein expression were analyzed. Treatments started when the animals were two months old and were maintained for one month. A significant functional improvement (p<0.05) was observed in animals submitted to the combination of ET and EM. CK levels were decreased and the expression of proteins related to oxidative metabolism was increased in this group. There were no differences among the groups in the intracellular calcium handling protein expression. To our knowledge, this is the first study that tested the association of ET with EM in an experimental model of muscular dystrophy. Our results suggest that the association of ET and EM should be further tested as a potential therapeutic approach in muscular dystrophies.Fundacao de Amparo a Pesquisa do Estado de Sao PauloFundacao de Amparo a Pesquisa do Estado de Sao Paulo [FAPESP - CEPID 98/14254-2]Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (CNPq)Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (CNPq)Instituto Nacional de Ciencia e Tecnologia (INCT)Instituto Nacional de Ciencia e Tecnologia (INCT
Autophagy signaling in skeletal muscle of infarcted rats
Background: Heart failure (HF)-induced skeletal muscle atrophy is often associated to exercise intolerance and poor prognosis. Better understanding of the molecular mechanisms underlying HF-induced muscle atrophy may contribute to the development of pharmacological strategies to prevent or treat such condition. It has been shown that autophagylysosome system is an important mechanism for maintenance of muscle mass. However, its role in HF-induced myopathy has not been addressed yet. Therefore, the aim of the present study was to evaluate autophagy signaling in myocardial infarction (MI)-induced muscle atrophy in rats.\ud
Methods/Principal Findings: Wistar rats underwent MI or Sham surgeries, and after 12 weeks were submitted toechocardiography, exercise tolerance and histology evaluations. Cathepsin L activity and expression of autophagy-related\ud
genes and proteins were assessed in soleus and plantaris muscles by fluorimetric assay, qRT-PCR and immunoblotting, respectively. MI rats displayed exercise intolerance, left ventricular dysfunction and dilation, thereby suggesting the presence of HF. The key findings of the present study were: a) upregulation of autophagy-related genes (GABARAPL1, ATG7, BNIP3, CTSL1 and LAMP2) was observed only in plantaris while muscle atrophy was observed in both soleus and plantaris muscles, and b) Cathepsin L activity, Bnip3 and Fis1 protein levels, and levels of lipid hydroperoxides were increased\ud
specifically in plantaris muscle of MI rats.\ud
Conclusions: Altogether our results provide evidence for autophagy signaling regulation in HF-induced plantaris atrophy but not soleus atrophy. Therefore, autophagy-lysosome system is differentially regulated in atrophic muscles comprising different fiber-types and metabolic characteristics.Fundação de Amparo à Pesquisa do Estado de São Paulo, Brazil (FAPESP #2010/14567-4).FAPESP (#2010/50048-1)Conselho Nacional de Pesquisa e Desenvolvimento (CNPq #302201/2011-4
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