Kinetic energy oscillations in annular regions of ultracold neutral plasmas

A study of ion equilibration in annular regions of ultracold strontium plasmas is reported. Plasmas are formed by photoionizing laser-cooled atoms with a pulsed dye laser. The experimental probe is spatially-resolved absorption spectroscopy using the ²S_1/2-²P_1/2 transition of the Sr⁺ ion. The kinetic energy of the ions is calculated from the Doppler broadening of the spectrum, and it displays clear oscillations during the first microsecond after plasma formation. The oscillations, which are a characteristic of strong coulomb coupling, are fit with a simple phenomenological model incorporating damping and density variation in the plasma.     

Defects in a TGBA phase: A theoretical approach

Following our experimental observations of disclination lines in freely suspended droplets and free-standing films (Yu.A. Nastishin et al., Eur. Phys. J. E 5 353 (2001)), topological defects of the twist grain boundary (TGBA) phase are considered according to two aspects: topological and energetical. There are two classes of line defects, disclinations (as in the cholesteric (N*) phase and the liquid vortices phase (NL*), relating to the directors tripod symmetries) and dispirations (relating to the translation-rotation symmetries); there are no topological point defects. Differences between N*, NL* and TGBA disclinations are physical, not topological. The absence of focal conic domains in the TGBA phase is an immediate consequence of the materialization of the helical axis (along the χ-director); the same feature, coupled to the trend to parallelism of the smectic layers, accounts for the predominance of λ-lines. Finally, the presence of defects akin to developable domains is explained in the frame of the leastcurvaturemodel, that requires the introduction of a third type of defects: the densitiesofedgedislocations of the smectic layers. Received 20 February 2002     

Towards a spin-ensemble quantum memory for superconducting qubits

This article reviews efforts to build a new type of quantum device, which combines an ensemble of electronic spins with long coherence times, and a small-scale superconducting quantum processor. The goal is to store over long times arbitrary qubit states in orthogonal collective modes of the spin-ensemble, and to retrieve them on-demand. We first present the protocol devised for such a multi-mode quantum memory. We then describe a series of experimental results using NV (as in nitrogen vacancy) center spins in diamond, which demonstrate its main building blocks: the transfer of arbitrary quantum states from a qubit into the spin ensemble, and the multi-mode retrieval of classical microwave pulses down to the single-photon level with a Hahn-echo like sequence. A reset of the spin memory is implemented in-between two successive sequences using optical repumping of the spins.     

Synaptic inhibition and γ-aminobutyric acid in the mammalian central nervous system

Signal transmission through synapses connecting two neurons is mediated by release of neurotransmitter from the presynaptic axon terminals and activation of its receptor at the postsynaptic neurons. γ-Aminobutyric acid (GABA), non-protein amino acid formed by decarboxylation of glutamic acid, is a principal neurotransmitter at inhibitory synapses of vertebrate and invertebrate nervous system. On one hand glutamic acid serves as a principal excitatory neurotransmitter. This article reviews GABA researches on; (1) synaptic inhibition by membrane hyperpolarization, (2) exclusive localization in inhibitory neurons, (3) release from inhibitory neurons, (4) excitatory action at developmental stage, (5) phenotype of GABA-deficient mouse produced by gene-targeting, (6) developmental adjustment of neural network and (7) neurological/psychiatric disorder. In the end, GABA functions in simple nervous system and plants, and non-amino acid neurotransmitters were supplemented.     

The role of duct thickness on the quenching process of premixed flame propagation

The propagation of premixed laminar flame in ducts of circular cross-section considering a thermal-diffusive model is investigated numerically. Heat losses by conduction to the channels walls are taken into account using the thermally thin wall regime. The effects and the relationship between thickness and diameter of the tube with the flame speed propagation are studied and the quenching condition is obtained as a function of the heat-loss parameter. The mathematical model employs the axisymmetric energy and species equations. The calculations are based on a two-step chemistry, with an Arrhenius, energetically neutral, radical production reaction followed by an exothermic radical recombination reaction. For large values of the heat-loss parameter, the wall temperature is close to the free stream temperature and all the heat losses through the wall are convected away. No heat feedback occurs. On the other hand, for small values of the heat-loss parameter, a feedback mechanism occurs by transferring heat from the burned gas to the fresh mixture along the tube wall. For values of the heat-loss parameter of order unity, the heat feedback mechanism is able to sustain the flame propagation and the quenching condition disappears, producing an almost planar flame front as the propagation velocity reduces. For this two-step reaction mechanism, the radical species behaviour at the duct walls seems to have negligible effect on the quenching process.     

Electron-ion Coulomb Bremsstrahlung in nonideal plasmas

The classical electron-ion Coulomb Bremsstrahlung process is investigated in nonideal plasmas. An effective pseudopotential model taking into account the plasma screening and collective effects is applied to describe the electron-ion interaction potential in a classical nonideal plasma. The classical straight-line trajectory method is applied to the motion of the projectile electron in order to visualize the variation of the differential Bremsstrahlung radiation cross-section (DBRCS) as a function of the scaled impact parameter, nonideal plasma parameter, projectile energy, photon energy, and Debye length. The results show that the DBRCS in ideal plasmas described by the Debye-Hückel potential is always greater than that in nonideal plasmas, i.e., the collective effects reduce the DBRCS for both the soft and hard photon cases. For large impact parameters, the DBRCS for the soft photon case is found to be always greater than that for the hard photon case. Received 1st December 1999     

Tissue-specific δ13C in ancient and modern tropical seabirds and flying fish in the Xisha Islands,South China Sea

We analyzed ¹³C characteristics in samples of bird bones, feathers, eggshell carbonate and membrane from modern specimens of red-footed booby (Sula sula) as well as fish muscle, scales, and bones from its predominant food source, flying fish (Exocoetus volitans), and muscle from its secondary food source squid (Loligo chinensis), as well as in ancient sub-fossil samples of seabird and flying fish at the Xisha Islands, South China Sea. δ¹³C is tissue-specific in both seabirds and flying fish due to the variance in turn-over among the tissues and differences in the type and content of amino acids across a diverse range of tissues. The δ¹³C discrimination factors also differed significantly among the various tissues between tropical seabirds and their prey. A Suess effect, caused by fossil fuel combustion and the emission of carbon with fewer ¹³C isotopes, was observed in the bird and fish tissue from ancient to modern time. Our study provides a multiple variability index for δ¹³C in organisms along a food chain, and verifies that tissue-specific ¹³C analysis is essential to identify diet and species and thus is a valuable tool for research on tropical seabird ecology.     

Quantum hall effect observations in commercial MOSFETs and effects of thermoelectric power

Summary We have performed quantum Hall effect measurements on commercial low-mobilityn-channel MOSFETs. The channel resistancevs. electron concentration has the expected oscillating behaviour and gives the quantized values for the Hall resistance, at least in the region of higher mobility. One class of the tested samples gave a surprising behaviour, both in QHE and in field effect mobility measurements: this is thought to be due to uncontrolled differences in the diffusion zones at the interface between the channel and the contacts. In particular, we attribute the deformation of the QHE curves to the onset of a thermoelectromotive force which is present in the electron gas even if the sample is isothermal. A clear evidence for thermoelectric effects is given by measurements made on a high-quality Hall geometry MOSFET.

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Riassunto Abbiamo eseguito misure di effetto Hall quantistico su MOSFET commerciali di bassa mobilità a canalen. Le curve di resistenza di canale in funzione della tensione di gate presentano l'atteso andamento oscillante e danno i valori quantizzati, almeno nelle regioni di maggiore mobilità. Una categoria di campioni ha mostrato andamenti sorprendenti, sia in misure di QHE che in misure di mobilità per effetto di campo: riteniamo che ciò sia dovuto a differenze incontrollabili nelle regioni di diffusione alle interfacce tra i contatti e il canale. In particolare, attribuiamo la deformazione delle curve di QHE all'instaurarsi di forze termoelettromotrici, che sono presenti nel gas elettronico nonostante i campioni siano isotermi. Misure eseguite su un MOSFET a geometria Hall di alta qualità forniscono una chiara evidenza dell'esistenza di effetti termoelettrici.

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Резюме Проводятся измерения квантового эффекта Холла на коммерческих MOSFET с малой подвижностью вn-канале. Зависмость сопротивления канала от концентрации электронов обнаруживает осцилляторное поведение и дает квантованные значения сопротивления Холла, по крайней мере, в области высокой подвижности. Один класс исследованных образцов обнаруживает удивительное поведение при измерении квантового эффекта Холла и при измерении зависимости подвижности от поля. Такое поведение, по-видимому, обусловлено неконтролируемыми различиями в зонах диффузии между каналом и контактами. В частности, мы приписываем деформацию кривых квантового эффекта Холла возникновению термоэлектродвижущей силы, которая присутствует в электронном газе, даже в изотермических образцах. Приводятся подтверждения для термоэлектрических эффектов, проводя измерения эффекта Холла в MOSFET с высокой подвижностью.

     

Radiation damping in closed expanding universes

The dynamics of a coupled model (harmonic oscillator-relativistic scalar field) in Conformal Robertson-Walker (k = +1) spacetimes is investigated. The exact radiation-reaction equation of the source-including the retarded radiation terms due to the closed space geometry – is obtained and analyzed. A suitable family of Lyapunov functions is constructed to show that, if the spacetime expands monotonely, then the source's energy damps. A numerical simulation of this equation for expanding Universes, with and without Future Event Horizon, is performed.