Band-monitoring Payload for a CubeSat Satellite

During changing sun activity, the ionosphere is responding accordingly and therefore it is interesting to observe the propagation behavior of shortwave bands. For the above mentioned purpose we have designed a band-monitoring payload for an experimental CubeSat satellite. The payload consists of a receiver, which is able to receive SSB modulated narrowband signals in 28 MHz uplink band, and a transmitter with FM modulation in UHF downlink band. The receiver frequency is selected to be at the center of radio amateur activity with low data rate digital modulations

Coherent resistance of a disordered 1D wire: Expressions for all moments and evidence for non-Gaussian distribution

We study coherent electron transport in a one-dimensional wire with disorder modeled as a chain of randomly positioned scatterers. We derive analytical expressions for all statistical moments of the wire resistance $\rho$. By means of these expressions we show analytically that the distribution $P(f)$ of the variable $f=\ln(1+\rho)$ is not exactly Gaussian even in the limit of weak disorder. In a strict mathematical sense, this conclusion is found to hold not only for the distribution tails but also for the bulk of the distribution $P(f)$.Comment: Revised version, 8 pages, 4 figures, RevTeX

Spin-engineered quantum dots

Spatially nonhomogeneously spin polarized nuclei are proposed as a new mechanism to monitor electron states in a nanostructure, or as a means to createn and, if necessary, reshape such nanostructures in the course of the experiment. We found that a polarization of nulear spins may lift the spin polarization of the electron states in a nanostructure and, if sufficiently strong, leads to a polarization of the electron spins. Polarized nuclear spins may form an energy landscape capable of binding electrons with energy up to several meV and the localization radius $>$ 100\AA.Comment: 9 pages, 1 figure, submitted to Physica E, Augist 31, 200

Massive Spin Collective Mode in Quantum Hall Ferromagnet

It is shown that the collective spin rotation of a single Skyrmion in quantum Hall ferromagnet can be regarded as precession of the entire spin texture in the external magnetic field, with an effective moment of inertia which becomes infinite in the zero g-factor limit. This low-lying spin excitation may dramatically enhance the nuclear spin relaxation rate via the hyperfine interaction in the quantum well slightly away from filling factor equal one.Comment: 4 page

Nucleus-mediated spin-flip transitions in GaAs quantum dots

Spin-flip rates in GaAs quantum dots can be quite slow, thus opening up the possibilities to manipulate spin states in the dots. We present here estimations of inelastic spin-flip rates mediated by hyperfine interaction with nuclei. Under general assumptions the nucleus mediated rate is proportional to the phonon relaxation rate for the corresponding non-spin-flip transitions. The rate can be accelerated in the vicinity of a singlet-triplet excited states crossing. The small proportionality coefficient depends inversely on the number of nuclei in the quantum dot. We compare our results with known mechanisms of spin-flip in $GaAs$ quantum dot.Comment: RevTex 4 pages, 1 figure, submitted to Phys. Rev.

Nuclear-spin qubits interaction in mesoscopic wires and rings

Theoretical study of the indirect coupling of nuclear spins (qubits) embedded into a mesoscopic ring and in a finite length quantum wire in a magnetic field is presented. It is found that the hyperfine interaction, via the conduction electrons, between nuclear spins exhibits sharp maxima as function of the magnetic field and nuclear spin positions. This phenomenon can be used for manipulation of qubits with almost atomic precision. Experimental feasibility and implications for quantum logics devices is discussed.Comment: 3 figures, 12 page

Metilfenidato: influência da notificação de receita A (cor amarela) sobre a prática de prescrição por médicos brasileiros

Four hundred and sixty three brazilian neurologists and 411 brazilian psychiatrists answered a questionnaire aimed at to obtain information on the prescription of methylphenidate for the treatment of the Attention Deficit and Hyperactivity Disorder. The majority of the doctors reported the erratic occurrence of a few non serious adverse reactions such as headache, weight loss, inapetence, hyperexcitability and tachycardia. On the other hand, only six doctors reported cases that cold raise a suspicion of drug dependence, although none of these six cases satisfied the ICD-10 criteria for dependence. The vast majority of the medical doctors manifested the opinion that the especial prescriptions the Notifications A of yellow color, the same used for opiates also, as demanded by the Brazilian Health Authorities, is inadequate as it raises misconceptions in relation to the real patient disease and frightens the parents and relatives. The doctors also claimed the such especial prescription form encumbers the prescription of the medicament making methylphenidate the second choice for the treatment and, for this reason, less available in pharmacies and drugstores. As a consequence the adherence to the treatment is more difficult to attain. The neurologists and psychiatrists agree that a certain control over methylphenidate is necessary, but as it is prescribed mostly by specialists and does not possess an evident abuse potential, the Prescription Notification B, of blue color, would be enough for efficient control. In fact, according to the doctors, the scheduling in Brazil of methylphenidate in the same list as the amphetamines and dronabinol (the active principle of Cannabis sativa) should be reviewed.Oitocentos e noventa e dois médicos, entre eles neurologistas (463) e psiquiatras (411), responderam a um questionário sobre a prescrição do metilfenidato para o transtorno do déficit de atenção e hiperatividade (TDAH). A maioria dos médicos relatou a ocorrência esporádica de reações adversas não-sérias, tais como dor de cabeça, emagrecimento, inapetência, hiperexcitabilidade, taquicardia, etc; apenas seis médicos, entretanto, relataram casos que poderiam levar à suspeita de dependência, embora nenhum deles satisfizesse os critérios da CID-10 para tal. A grande maioria opinou que a exigência da notificação de receita A (amarela) é inadequada, criando preconceitos em relação à condição dos pacientes e amedrontando seus parentes. Relatam ainda que tal classificação dificulta a prescrição do medicamento, fazendo muitas vezes com que o metilfenidato se torne a segunda opção para o tratamento de TDAH, além de tornar o produto pouco disponível nas farmácias e dificultar o trabalho dos profissionais. Isso faz com que a adesão ao tratamento se torne mais difícil, pois, além dos vários entraves burocráticos para se adquirir o medicamento, há também o medo por parte dos familiares e do próprio paciente de estar tomando tal medicação. Neurologistas e psiquiatras argumentam que o controle do metilfenidato deve existir, mas, em se tratando de um medicamento prescrito por especialistas e de não possuir grande potencial de dependência, o receituário azul já seria suficiente para o controle adequado. A classificação do metilfenidato, juntamente com as anfetaminas e com o princípio ativo da maconha (dronabinol), é uma outra questão levantada, sendo tal fato para grande parte dos profissionais uma total incoerência.UNIFESP-EPM Departamento de Psicobiologia Centro Brasileiro de Informações sobre Drogas PsicotrópicasNovartis BiociênciasUNIFESP, EPM, Depto. de Psicobiologia Centro Brasileiro de Informações sobre Drogas PsicotrópicasSciEL

Quantum dots based on spin properties of semiconductor heterostructures

The possibility of a novel type of semiconductor quantum dots obtained by spatially modulating the spin-orbit coupling intensity in III-V heterostructures is discussed. Using the effective mass model we predict confined one-electron states having peculiar spin properties. Furthermore, from mean field calculations (local-spin-density and Hartree-Fock) we find that even two electrons could form a bound state in these dots.Comment: 9 pages, 3 figures. Accepted in PRB (Brief Report) (2004