2,644 research outputs found
Electron Spin Dynamics in Impure Quantum Wells for Arbitrary Spin-Orbit Coupling
Strong interest has arisen recently on low-dimensional systems with strong
spin-orbit interaction due to their peculiar properties of interest for some
spintronic applications. Here, the time evolution of the electron spin
polarization of a disordered two-dimensional electron gas is calculated exactly
within the Boltzmann formalism for arbitrary couplings to a Rashba spin-orbit
field. The classical Dyakonov-Perel mechanism of spin relaxation is shown to
fail for sufficiently strong Rashba fields, in which case new regimes of spin
decay are identified. These results suggest that spin manipulation can be
greatly improved in strong spin-orbit interaction materials.Comment: 5 pages, 2 figures -revised versio
Polaronic and nonadiabatic phase diagram from anomalous isotope effects
Isotope effects (IEs) are powerful tool to probe directly the dependence of
many physical properties on the lattice dynamics. In this paper we invenstigate
the onset of anomalous IEs in the spinless Holstein model by employing the
dynamical mean field theory. We show that the isotope coefficients of the
electron effective mass and of the dressed phonon frequency are sizeable also
far away from the strong coupling polaronic crossover and mark the importance
of nonadiabatic lattice fluctuations in the weak to moderate coupling region.
We characterize the polaronic regime by the appearence of huge IEs. We draw a
nonadiabatic phase diagram in which we identify a novel crossover, not related
to polaronic features, where the IEs attain their largest anomalies.Comment: 5 pages, 4 figure
Nonadiabatic Superconductivity and Vertex Corrections in Uncorrelated Systems
We investigate the issue of the nonadiabatic superconductivity in
uncorrelated systems. A local approximation is employed coherently with the
weak dependence on the involved momenta. Our results show that nonadiabatic
vertex corrections are never negligible, but lead to a strong suppression of
with respect to the conventional theory. This feature is understood in
terms of the momentum-frequency dependence of the vertex function. In contrast
to strongly correlated systems, where the small -selection probes the
positive part of vertex function, vertex corrections in uncorrelated systems
are essentially negative resulting in an effective reduction of the
superconducting pairing. Our analysis shows that vertex corrections in
nonadiabatic regime can be never disregarded independently of the degree of
electronic correlation in the system.Comment: 4 pages, 3 eps fig
Nonadiabatic Pauli susceptibility in fullerene compounds
Pauli paramagnetic susceptibility is unaffected by the electron-phonon
interaction in the Migdal-Eliashberg context. Fullerene compounds however do
not fulfill the adiabatic assumption of Migdal's theorem and nonadiabatic
effects are expected to be relevant in these materials. In this paper we
investigate the Pauli spin susceptibility in nonadiabatic regime by following a
conserving approach based on Ward's identity. We find that a sizable
renormalization of due to electron-phonon coupling appears when
nonadiabatic effects are taken into account. The intrinsic dependence of
on the electron-phonon interaction gives rise to a finite and negative isotope
effect which could be experimentally detected in fullerides. In addition, we
find an enhancement of the spin susceptibility with temperature increasing, in
agreement with the temperature dependence of observed in fullerene
compounds. The role of electronic correlation is also discussed.Comment: Revtex, 10 pages, 8 figures include
Electron-phonon renormalization in small Fermi energy systems
The puzzling features of recent photoemission data in cuprates have been
object of several analysis in order to identity the nature of the underlying
electron-boson interaction. In this paper we point out that many basilar
assumptions of the conventional analysis as expected to fail in small Fermi
energy systems when, as the cuprates, the Fermi energy is
comparable with the boson energy scale. We discuss in details the novel
features appearing in the self-energy of small Fermi energy systems and the
possible implications on the ARPES data in cuprates.Comment: 4 pages, 5 eps figures include
Longitudinal and transversal piezoresistive response of granular metals
In this paper, we study the piezoresistive response and its anisotropy for a
bond percolation model of granular metals. Both effective medium results and
numerical Monte Carlo calculations of finite simple cubic networks show that
the piezoresistive anisotropy is a strongly dependent function of bond
probability p and of bond conductance distribution width \Delta g. We find that
piezoresistive anisotropy is strongly suppressed as p is reduced and/or \Delta
g is enhanced and that it vanishes at the percolation thresold p=p_c. We argue
that a measurement of the piezoresistive anisotropy could be a sensitive tool
to estimate critical metallic concentrations in real granular metals.Comment: 14 pages, 7 eps figure
The Longitudinal Interplay between Sleep, Anthropometric Indices, Eating Behaviors, and Nutritional Aspects: A Systematic Review and Meta-Analysis
Sleep is fundamental for adolescents’ healthy development but undergoes dramatic changes in quantity and quality due to the conflict between biological and social rhythms. Insufficient sleep has been associated with worse physical health status and irregular eating behaviors in adolescents. This review aims to systematically synthesize the longitudinal associations between adolescents’ sleep dimensions (i.e., duration, timing, quality, and insomnia symptoms) and physical health indicators (i.e., anthropometric indices, fat percentage, and risk of obesity), eating behaviors, and nutritional aspects (i.e., type of diet related to the intake of specific foods and nutrients, amount and timing of food consumption, energy expenditure). A total of 28 longitudinal studies were included. The meta-analytic results showed that longer sleep duration, better sleep quality, and lower insomnia symptoms were associated with lower BMI and fat percentage and that shorter sleep duration (<7 h) and lower sleep quality were associated with a higher risk of obesity. Conversely, anthropometric indices were not related to sleep over time. Limited literature examined the bidirectional association between adolescents’ sleep and their eating behaviors and nutritional aspects. Such knowledge sheds new light on the role of sleep for adolescents’ health, highlighting the need to examine further the interplay between these variables
Band-filling effects on electron-phonon properties of normal and superconducting state
We address the effect of band filling on the effective electron mass
and the superconducting critical temperature in a electron-phonon system.
We compare the vertex corrected theory with the non-crossing approximation of
the Holstein model within a local approximation. We identify two regions of the
electron density where and are enhanced or decreased by the
inclusion of the vertex diagrams. We show that the crossover between the
enhancement at low density and the decrease towards half filling is almost
independent of the microscopic electron-phonon parameters. These different
behaviors are explained in terms of the net sign of the vertex diagrams which
is positive at low densities and negative close to half filling. Predictions of
the present theory for doped MgB, which is argued to be in the low density
regime, are discussed.Comment: 13 revtex pages, figures eps include
Características químicas de um gleissolo sob diferentes sistemas de uso, nas margens do rio Guamá, Belém, Pará.
Foram estudados os efeitos do uso do solo sobre a fertilidade, em Gleissolo sob três diferentes sistemas de manejo. As coletas foram feitas antes do período chuvoso nos seguintes sistemas de manejo: área cultivada com arroz (Oriza sativa L.) há aproximadamente 40 anos, com algumas interrupções ao longo deste período, com o cultivo algumas vezes mecanizado e uso de aração e gradagem; área sob pastagem de canarana erecta lisa (Echinochloa pyramidalis Hitch.), formada há cerca de 20 anos sem uso de adubações ou calagens; e área sob vegetação natural de floresta típica das áreas de várzea alta. As amostras de solo foram coletadas em dez pontos a partir de um transecto nas áreas, em quatro profundidades 0-10, 10-20, 20-30 e 30-40 cm. Os resultados demonstraram que o solo em condições naturais, de modo geral, apresentou atributos químicos que variaram de bons a muito bons, o que demonstra um grande potencial para o uso agrícola. O cultivo agrícola dos solos das várzeas do rio Guamá provocou uma redução da concentração de P e de K. O sistema de uso com pastagem apresentou maior sustentabilidade da fertilidade do que o sistema sob cultivo com arroz, visto que, além da melhoria das características químicas, a matéria orgânica elevou-se, também, em relação ao sistema natural
50 Hz Extremely Low Frequency Electromagnetic Fields Enhance Protein Carbonyl Groups Content in Cancer Cells: Effects on Proteasomal Systems
Electromagnetic fields are an assessed cause of prolonging free radicals lifespan. This study was carried out to investigate the influence of extremely low frequency electromagnetic fields on protein oxidation and on the 20S proteasome functionality, the complex responsible for the degradation of oxidized proteins. Caco 2 cells were exposed, for 24–72 hours, to 1 mT, 50 Hz electromagnetic fields. The treatment induced a time-dependent increase both in cell growth and in protein oxidation, more evident in the presence of TPA, while no changes in cell viability were detected. Exposing the cells to 50 Hz electromagnetic fields caused a global activation of the 20S proteasome catalytic components, particularly evident at 72 hours exposure and in the presence of TPA. The finding that EGCG, a natural antioxidant compound, counteracted the field-related pro-oxidant effects demonstrates that the increased proteasome activity was due to an enhancement in intracellular free radicals
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