Interlayer interference in double wells in a tilted magnetic field

Magnetotransport measurements on bilayer electron systems reveal repeated reentrance of the resistance minima at filling factors nu=4N+1 and nu=4N+3, where N is the Landau index number, in the tilted magnetic field. At high filling factors, the Shubnikov-de Haas oscillations exhibit beating effects at certain tilt angles. We attribute such behavior to oscillations of the tunneling gap due to Aharonov-Bohm interference effect between cyclotron orbits in different layers. The interplay between quantum and quasiclassical regimes is established.

Anomalous dephasing scattering rate of two-dimensional electrons in double quantum well structures

The results on the measurement of electrical conductivity and magnetoconductivity of a GaAs double quantum well between 0.5 and 1.1 K are reported. The zero magnetic-field conductivity is well described from the point of view of contributions made by both the weak localization and electron-electron interaction. At low field and low temperature, the magnetoconductivity is dominated by the weak localization effect only. Using the weak localization method, we have determined the electron dephasing times tau(phi) and tunneling times tau(t). Concerning tunneling, we concluded that tau(t) presents a minimum around the balance point; concerning dephasing, we observed an anomalous dependence on temperature and conductivity (or elastic mean free path) of tau(phi). This anomalous behavior cannot be explained in terms of the prevailing concepts for the electron-electron interaction in high-mobility two-dimensional electron systems.

Interference oscillations of microwave photoresistance in double quantum wells

We observe oscillatory magnetoresistance in double quantum wells under microwave irradiation. The results are explained in terms of the influence of subband coupling on the frequency dependent photoinduced part of the electron distribution function. As a consequence, the magnetoresistance demonstrates the interference of magnetointersubband oscillations and conventional microwave induced resistance oscillations.

Resonance oscillations of magnetoresistance in double quantum wells

We present the experimental and theoretical studies of the magnetoresistance oscillations induced by the resonance transitions of electrons between the tunnel-coupled states in double quantum wells. The suppression of these oscillations with increasing temperature is irrelevant to the thermal broadening of the Fermi distribution and reflects the temperature dependence of the quantum lifetime of electrons. The gate control of the period and amplitude of the oscillations is demonstrated.

A new social parasite in the ant genus Ectatomma F. Smith (Hymenoptera, Formicidae, Ectatomminae)

Ectatomma parasiticum Feitosa & Fresneau, a new ant species socially parasitic on Ectatomma tuberculatum (Olivier), is described from gynes discovered in Apazapan, state of Veracruz, Mexico, and reared in the laboratory. Ectatomma parasiticum is the first social parasite described in the Ectatomminae. This species can be distinguished from its host by morphological and behavioral features characteristic of the inquilines known in other ant subfamilies including reduced size, thickened petiole, and agonistic interactions with host species.

Transforma????es na gest??o de compras da administra????o p??blica brasileira

A utiliza????o da tecnologia da informa????o nos procedimentos de compras e contrata????es da administra????o federal no Brasil representa importante experi??ncia de aplica????o de sistemas de compra eletr??nica ao ambiente dos governos. O Sistema Integrado de Administra????o de Servi??os Gerais ??? SIASG ?? a ferramenta de apoio informatizado aos processos de licita????o, que tem percorrido trajet??ria de avan??o em dire????o a sua consolida????o como ferramenta de uso unificado e abrangente. Ao mesmo tempo, o desenvolvimento do SIASG tem contemplado mais recentemente a tentativa de aprofundar a explora????o das potencialidades da tecnologia em dire????o ?? introdu????o de inova????es nos procedimentos de compra, abarcando inclusive a revis??o de leis e normas. Um marco nesse sentido foi a cria????o do portal de compras Comprasnet, na Internet, que permitiu a amplia????o do leque de funcionalidades oferecido bem como a forma de acesso. As se????es de 1 a 4 a seguir, abordam de forma descritiva a estrutura organizacional, o marco legal e normativo, os procedimentos de contrata????o vigentes e o perfil do Governo Federal como comprador. A se????o 5 analisa o desenvolvimento do SIASG/Comprasnet e a sua conforma????o atual sob os aspectos de inser????o institucional, abrang??ncia sobre o ciclo de compra e impactos transformadores que tem ensejado sobre as estruturas e processos da ??rea de compras.3 As conclus??es do artigo apresentam uma aprecia????o do seu grau de maturidade e consolida????o, indicando as limita????es e lacunas a serem equacionadas no seu desenvolvimento futuro.