1,842 research outputs found
Fermi Liquid Properties of a Two Dimensional Electron System With the Fermi Level Near a van Hove Singularity
We use a diagrammatic approach to study low energy physics of a two
dimensional electron system where the Fermi level is near van-Hove singularies
in the energy spectrum. We find that in most regions of the
phase diagram the system behaves as a normal Fermi liquid rather than a
marginal Fermi liquid. Particularly, the imaginary part of the self energy is
much smaller than the excitation energy, which implies well defined
quasiparticle excitations, and single particle properties are only weakly
affected by the presence of the van-Hove singularities. The relevance to high
temperature superconductivity is also discussed.Comment: 10 pages, 4 postscript figure
Fermi surface renormalization in Hubbard ladders
We derive the one-loop renormalization equations for the shift in the
Fermi-wavevectors for one-dimensional interacting models with four Fermi-points
(two left and two right movers) and two Fermi velocities v_1 and v_2. We find
the shift to be proportional to (v_1-v_2)U^2, where U is the Hubbard-U. Our
results apply to the Hubbard ladder and to the t_1-t_2 Hubbard model. The
Fermi-sea with fewer particles tends to empty. The stability of a saddle point
due to shifts of the Fermi-energy and the shift of the Fermi-wavevector at the
Mott-Hubbard transition are discussed.Comment: 5 pages, 4 Postscript figure
Daily physical activity and macronutrient distribution of low-calorie diets jointly affect body fat reduction in obese women.
Inadequate dietary patterns and sedentary lifestyles are believed to be important factors in predisposing people to obesity. This study analyzed the potential interaction between habitual physical activity and the carbohydrate (CHO)-fat
distribution in 2 hypocaloric diets and the impact of such interplay on body composition changes. Forty healthy obese women, 20â50 years old, were randomly assigned to a high- or low-CHO energy-restricted diet, which was low or high in fat, respectively, during 10 weeks. Baseline and final measurements were performed to assess dietary habits, resting metabolic rate, and body composition changes. Physical activity was measured with a triaxial accelerometer and with a questionnaire. There were no significant differences in anthropometric and metabolic variables between both dietary groups at
baseline. However, there was a positive correlation between total free-living physical activity and arm muscle preservation
after 10 weeks (r = 0.371; p = 0.024). Interestingly, an interaction between macronutrient (CHOâfat distribution) intake
and physical activity was found, since less-active subjects with a high-CHOâlow-fat diet showed a greater fat loss than those more active with a lower-CHOâhigh-fat diet, whereas more-active subjects with a high-CHOâlow-fat diet showed a smaller fat loss than those receiving a low-CHOâhigh-fat diet. Physical activity and the macronutrient content of energyrestricted diets, when designed to promote body fat mass reduction, should be considered together to better predict the outcome
Semiquantitative theory of electronic Raman scattering from medium-size quantum dots
A consistent semiquantitative theoretical analysis of electronic Raman
scattering from many-electron quantum dots under resonance excitation
conditions has been performed. The theory is based on
random-phase-approximation-like wave functions, with the Coulomb interactions
treated exactly, and hole valence-band mixing accounted for within the
Kohn-Luttinger Hamiltonian framework. The widths of intermediate and final
states in the scattering process, although treated phenomenologically, play a
significant role in the calculations, particularly for well above band gap
excitation. The calculated polarized and unpolarized Raman spectra reveal a
great complexity of features and details when the incident light energy is
swept from below, through, and above the quantum dot band gap. Incoming and
outgoing resonances dramatically modify the Raman intensities of the single
particle, charge density, and spin density excitations. The theoretical results
are presented in detail and discussed with regard to experimental observations.Comment: Submitted to Phys. Rev.
Fatty Acid Methyl Esters as Biosolvents of Epoxy Resins: A Physicochemical Study
The C8 to C18 fatty acid methyl esters (FAME) have been compared as solvents for two epoxy resin pre-polymers, bisphenol A diglycidyl ether (DGEBA) and triglycidyl paminophenol ether (TGPA). It was found that the solubilization limits vary according to the ester and that methyl caprylate is the best solvent of both resins. To explain these solubility performances, physical and chemical properties of FAME were studied, such as the Hansen parameters, viscosity, binary diffusion coefficient and vaporization enthalpy. Determination of the physicochemical parameters of FAME was carried out by laboratory experimentations and by calculation from bibliographic data. The Hansen parameters of FAME and epoxy resins pre-polymers were theoretically and experimentally determined. The FAME chain length showed a long dependence on the binary diffusion parameters and kinematic viscosity, which are mass and momentum transport properties. Moreover, the vaporization enthalpy of these compounds was directly correlated with the solubilization limits
Krein-Space Formulation of PT-Symmetry, CPT-Inner Products, and Pseudo-Hermiticity
Emphasizing the physical constraints on the formulation of a quantum theory
based on the standard measurement axiom and the Schroedinger equation, we
comment on some conceptual issues arising in the formulation of PT-symmetric
quantum mechanics. In particular, we elaborate on the requirements of the
boundedness of the metric operator and the diagonalizability of the
Hamiltonian. We also provide an accessible account of a Krein-space derivation
of the CPT-inner product that was widely known to mathematicians since 1950's.
We show how this derivation is linked with the pseudo-Hermitian formulation of
PT-symmetric quantum mechanics.Comment: published version, 17 page
Instrumentation mĂ©canique des conduits radiculaires. Une Ă©tude Ă lâaide du M.E.B. et analyse informatisĂ©e dâimage
The effectiveness of ultrasonic and sonic instrumentation in eliminating the smear layer from instrumented root channels. The results were examined with S.E.M. and the effective areas of dental diffusion were calculated using the computerized image analyzer. The agents used: citric acid at concentrations of 10, 25 and 50% as well as 15% EDTA, proved their efficiency with both types of mechanical instrumentation. However, the area of diffusion found was always greater using the ultrasonic instrumentation technique as opposed to the sonic instrumentation technique. 1, 2.5 and 5.25% sodium hypochlorite, as well as 10 volume hydrogen peroxide were not effective in eliminating the smear layer using both types of instrumentation.Nous Ă©tudions lâefficacitĂ© de lâinstrumentation ultrasonique et sonique pour Ă©liminer la boue dentinaire des conduits radiculaires instrumentĂ©s. Les rĂ©sultats ont Ă©tĂ© objectivĂ©s au moyen du M.E.B. et nous avons calculĂ© les aires effectives de diffusion dentinaire avec lâanalyseur dâimages. Les agents employĂ©s: acide citrique Ă 10, 25 et 50% de concentration, ainsi que lâEDTA Ă 15% ont confirmĂ© leur efficacitĂ© avec les deux types dâinstrumentation mĂ©canique. Cependant lâaire de diffusion vĂ©rifiĂ©e, est toujours supĂ©rieure avec la technique dâinstrumentation ultrasonique par rapport Ă la technique dâinstrumentation sonique. Lâhypochlorite sodique Ă 1, 2,5 et 5,25%; ainsi que lâeau oxygĂ©nĂ©e Ă 10 volumes, se sont avĂ©rĂ©s inefficaces dans lâĂ©limination de la boue dentinaire avec les deux types dâinstrumentation
A comparative study of the electrochemical properties of vitamin B-6 related compounds at physiological pH
A comparative study of vitamin B6 group and related compounds in buffered solutions using electrochemical techniques has been performed at neutral pH. Irreversible bi- or tetra-electronic processes are observed for these substances, and the electron transfer coefficient (αn) calculated. It was concluded that either the first or second electron transfer were the rate determining step of the electrode process. The diffusion coefficient of these substances was calculated and the values given follow an inverse tendency to the molecular size. For aldehydes the values obtained were corrected of the hydration reaction.
It is important to remark that catalytic waves were reported for the first time for these compounds. Using a model involving the nitrogen of the basic structure the kinetic constants were calculated for most of them
- âŠ