Showing papers by "Federal University of São Carlos published in 1979"

Study of two-dimensional electrons in a magnetic field. II. Intermediate field

Abstract: Taking into consideration the first-order-exchange and ring diagrams, we present a theory of two-dimensional (2-D) electrons in a magnetic field under the de Haas-van Alphen condition The Fermi momentum or the chemical potential of the system oscillates with the magnetic field, its interaction term being characterized by a factor ${(\frac{{e}^{2}}{{p}_{0}})}^{\frac{3}{4}}={({2}^{\frac{1}{2}}{r}_{s})}^{\frac{3}{4}}$ Due to a 2-D peculiarity, the susceptibility oscillates as in the ideal case without a constant phase $\ensuremath{-}\frac{\ensuremath{\pi}}{4}$ characteristic of the three-dimensional case A relation between the amplitude of the oscillating susceptibility and the field and temperature is derived The energy variation is like ${a}^{4}cos(\frac{\ensuremath{\pi}{\ensuremath{\epsilon}}_{0}}{{a}^{2}})$, where ${a}^{2}$ represents the field energy and ${\ensuremath{\epsilon}}_{0}$ is the Fermi energy for the ideal case, ie, $2\ensuremath{\pi}\ensuremath{\hbar}$, in the units $\ensuremath{\hbar}=1$ and $2m=1$, where $n$ is the number density The amplitude of the energy oscillation increases with the field strength squared A new specific-heat formula is also presented

Screening effects in n-type Si inversion layers

Abstract: The screening of a charged impurity center localized inside the oxide and at the interface of a $n$-type Si(001) inversion layer is considered and its effects on the electron-binding energies have been calculated. The results reported here show that the screening of a point-charge impurity by electrons in a silicon inversion layer can account for the observed electron binding energies of Fowler and Hartstein. It is shown that the linear theory of screening underestimates the electron density around the impurity point charge. The Coulomb potential associated with the impurity located at the interface was obtained for several values of both the screening parameter and the external electric field.

Correlations and plasma oscillations of a two-dimensional classical electron system

Abstract: Numerical self-consistent calculations of the static and dynamic properties of a two-dimensional classical electron plasma have been carried out on the basis of the theory derived by Singwi el al. In this theory the short-range correlations are present through a local-field correction depending on the pair correlation function. The static structure factor and the density-density response function are determined in a self-consistent scheme. The pair correlation function and the correlation energy obtained are in very good agreement with the numerical experiments carried out by Totsuji. It is found that the onset of short-range order appears at the plasma parameter in the range 14 < ..cap alpha.. < 15. The plasmon dispersion relation is determined and is much more accurate than previous theories.

Effects of confinement on a magnetized ideal gas II. The boson gas

Abstract: The behavior of a confined magnetized ideal boson gas is examined. The effects of the dimensionalities of the system and the confinement potential as well as the effects of the field strength on the Bose-Einstein condensation are analyzed. It is shown that a phase transition does not occur in strongly confined systems. For the weakly confined gas a connection with the results of the non-confined magnetized gas is made. A correspondence is established between our results and those of a free gas of different dimensionality. The density of states of a particle moving in a three-dimensional space and confined by a cylindrically symmetric potential is calculated and used to explain the correspondence. The thermodynamic properties of the 3D - 2d gas are worked out in detail.

Anomaly of the temperature dependence in an electron gas system

Abstract: A re-evaluation is given of the temperature dependent term in the first order exchange contribution to the grand partition function for an electron gas. The existence and origin of the logarithmic term in the temperature-dependent part are shown in a very simple manner.

Utilização de um leite em pó acidificado num programa de suplementação alimentar a crianças de zero a 2 anos de idade

Abstract: A desnutricao apresenta seus maiores coeficientes de mortalidade e morbidade na infância. A situacao agrava-se quando a desnutricao associa-se as gastro-enterites. Isto reflete uma alimentacao inadequada por parte das criancas, principalmente por aquelas menores de 2 anos onde se encontram taxas mais elevadas de desnutricao. A baixa condicao socio-economica sanitaria e de escolaridade das familias somada e inadequada assistencia prestada pelos servicos de alimentacao e nutricao agravam a situacao. A isto acresce-se o fato de que no domicilio o suplemento alimentar e fornecido as criancas e distribuido para todos os membros da familia. Distribuiu-se, pois, para 89 criancas suplemento alimentar um leite em po acidificado, durante o periodo de 2 anos. Ao termino do trabalho verificou-se que houve aceitacao do leite acidificado pelas criancas; ganho de peso satisfatorio o que parece refletir a ingestao do leite pela crianca e nao por outros membros da familia, e que a educacao sistematica por ocasiao do recebimento das quotas de leite, venceu a resistencia apresentada pelas maes no inicio do programa.