A New Liquid Phase and Metal-Insulator Transition in Si MOSFETs

We argue that there is a new liquid phase in the two-dimensional electron system in Si MOSFETs at low enough electron densities. The recently observed metal-insulator transition results as a crossover from the percolation transition of the liquid phase through the disorder landscape in the system below the liquid-gas critical temperature. The consequences of our theory are discussed for variety of physical properties relevant to the recent experiments.Comment: 12 pages of RevTeX with 3 postscript figure

A Missão numa Perspectiva Antropológica

This article is a study about religious missions it discusses anthropological publications related to this theme, found within the analyses of various authors of the anthropology of religion. The author analyses academic publications from the middle of the last century to understand the relations between missionaries and anthropologists. The article relates how anthropologists view missions and gives a large theoretical revision about the anthropological interpretations of the missions.Este artigo apresenta um estudo sobre as missões religiosas discutindo as produções antropológicas referentes a este tema, presentes na análise de diversos autores da antropologia da religião. A autora faz uma análise das produções acadêmicas desde meados do século passado para compreender as relações travadas entre missionários e antropólogos. Relata como os antropólogos pensam as missões e faz uma farta revisão teórica sobre as interpretações antropológicas em torno das missões

Mujeres y conocimientos ancestrales en la Amazonia, Brasil.

En este artículo se discute la importancia de la mujer en la actividad productiva y en la producción de los respectivos conocimientos, en esta medida el material utilizado en este documento proviene de la experiencia a lo largo de años de investigación en la Amazonia Brasileña tanto entre indígenas como entre poblaciones caboclas, campesinas, cimarronas entre otros grupos sociales que viven en el medio rural. En este sentido se presenta una reflexión conceptual y una caracterización de la importancia de las mujeres amazónicas. Finalmente, se revela la centralidad de las mujeres en la producción del conocimiento ancestral.This article discusses the importance of women in the productive activity and the production of the respective knowledge, to this extent the material used in this document comes through years of research experience in the Brazilian Amazon among indigenous as populations caboclas, peasant, maroon among other social groups living in rural areas. In this sense is presented a conceptual reflection and characterization the importance of the amazon women. Finally, it is revealed the centrality of women in the production of the ancestral knowledge

Calculations of exchange interaction in impurity band of two-dimensional semiconductors with out of plane impurities

We calculate the singlet-triplet splitting for a couple of two-dimensional electrons in the potential of two positively charged impurities which are located out of plane. We consider different relations between vertical distances of impurities $h_1$ and $h_2$ and their lateral distance $R$. Such a system has never been studied in atomic physics but the methods, worked out for regular two-atomic molecules and helium atom, have been found to be useful. Analytical expressions for several different limiting configurations of impurities are obtained an interpolated formula for intermediate range of parameters is proposed. The $R$-dependence of the splitting is shown to become weaker with increasing $h_1,h_2$.Comment: 14 pages, RevTeX, 5 figures. Submitted to Phys Rev.

On the Theory of Metal-Insulator Transitions in Gated Semiconductors

It is shown that recent experiments indicating a metal-insulator transition in 2D electron systems can be interpreted in terms of a simple model, in which the resistivity is controlled by scattering at charged hole traps located in the oxide layer. The gate voltage changes the number of charged traps which results in a sharp change in the resistivity. The observed exponential temperature dependence of the resistivity in the metallic phase of the transition follows from the temperature dependence of the trap occupation number. The model naturally describes the experimentally observed scaling properties of the transition and effects of magnetic and electric fields.Comment: 4 two-column pages, 4 figures (included in the text

Comment on "Theory of metal-insulator transitions in gated semiconductors" (B. L. Altshuler and D. L. Maslov, Phys. Rev. Lett. 82, 145 (1999))

In a recent Letter, Altshuler and Maslov propose a model which attributes the anomalous temperature and field dependence of the resistivity of two-dimensional electron (or hole) systems to the charging and discharging of traps in the oxide (spacer), rather than to intrinsic behavior of interacting particles associated with a conductor-insulator transition in two dimensions. We argue against this model based on existing experimental evidence.Comment: 1 page; submitted to PR

Indication of the ferromagnetic instability in a dilute two-dimensional electron system

The magnetic field B_c, in which the electrons become fully spin-polarized, is found to be proportional to the deviation of the electron density from the zero-field metal-insulator transition in a two-dimensional electron system in silicon. The tendency of B_c to vanish at a finite electron density suggests a ferromagnetic instability in this strongly correlated electron system.Comment: 4 pages, postscript figures included. Revised versio