29,857 research outputs found
Electronic Transport in Fullerene C20 Bridge Assisted by Molecular Vibrations
The effect of molecular vibrations on electronic transport is investigated
with the smallest fullerene C20 bridge, utilizing the Keldysh nonequilibrium
Green's function techniques combined with the tight-binding molecular-dynamics
method. Large discontinuous steps appear in the differential conductance when
the applied bias-voltage matches particular vibrational energies. The magnitude
of the step is found to vary considerably with the vibrational mode and to
depend on the local electronic states besides the strength of
electron-vibration coupling. On the basis of this finding, a novel way to
control the molecular motion by adjusting the gate voltage is proposed.Comment: 9 pages, 4 figures, accepted for publication in Phys. Rev. Let
Constraining the Cosmic Star Formation Rate with the MeV Background
The Cosmic Gamma-ray Background (CGB) in the MeV regime has been measured
with COMPTEL and SMM. The origin of the CGB in this energy regime is believed
to be dominated by gamma-rays from Type Ia supernovae. We calculate the CGB
spectrum within the framework of FRW cosmology as a function of the cosmic star
formation rate, SFR(z). Several estimates of the SFR(z) have been reported
since the pioneering work of Madau et al. Here we discuss observational
constraints on SFR(z) derived from models of the CGB. In particular, we
consider the SFR obtained from Gamma-Ray Burst observations, which increases
dramatically with redshift beyond z ~ 1 in contrast to most estimates which
saturate or show a mild increase with redshift. Gamma-ray bursts may be the
most powerful tracers of star formation in the early universe and thus provide
signposts of the initial epoch of element synthesis. The star formation rate
implied by GRB statistics results in a gamma-ray background that matches the
observations more closely than that inferred from other tracers of star
formation. This may provide some support for the GRB/SFR-paradigm, which in
turn promises a powerful diagnostic of star formation, and thus cosmic chemical
evolution, from the era of Population III stars to the present
A study of electronic packages environmental control systems and vehicle thermal systems integration Quarterly report, Nov. 1966 - Jan. 1967
Heat balances of combined astrionic equipment and thermal conditioning subsystem of environmental control system, and vehicle configuration
Microfluidic-SANS: flow processing of complex fluids
Understanding and engineering the flow-response of complex and non-Newtonian fluids at a molecular level is a key challenge for their practical utilisation. Here we demonstrate the coupling of microfluidics with small angle neutron scattering (SANS). Microdevices with high neutron transmission (up to 98%), low scattering background ([Image: see text]), broad solvent compatibility and high pressure tolerance (â3â15 bar) are rapidly prototyped via frontal photo polymerisation. Scattering from single microchannels of widths down to 60â
ÎŒm, with beam footprint of 500â
ÎŒm diameter, was successfully obtained in the scattering vector range 0.01â0.3â
Ă
(â1), corresponding to real space dimensions of [Image: see text]. We demonstrate our approach by investigating the molecular re-orientation and alignment underpinning the flow response of two model complex fluids, namely cetyl trimethylammonium chloride/pentanol/D(2)O and sodium lauryl sulfate/octanol/brine lamellar systems. Finally, we assess the applicability and outlook of microfluidic-SANS for high-throughput and flow processing studies, with emphasis of soft matter
Comparing Organic Urban Consumers in Developing and Developed Countries: First Results in Brazil and France
Despite numerous studies reporting on sustainable consumption or organic consumer profiles, there is a gap in thorough understanding of organic consumers in different places, since most of studies only investigate organic consumption in most developed countries. The goal of this paper is thus to compare French and Brazilian organic consumers, so as to know if people think and behave differently or similarly in different places. Individual interviews were conducted in each country, with consumers in organic producers market in Brazil, and consumers who buy organic products from farmers markets or local organic food network in France. Products were selected to cover examples of different choice situations such as imported organic products that compete with comparable products of local origin, or organic local products in supermarkets that compete with similar products from other distribution outlets. Results show common consumer concerns such as quality or personal and family health, and common preference for local and organic products but for different reasons. However, results also shed light on different patterns related to environmental concerns or commitment to supporting small or local farmers. The impacts of the findings of this study relate to a diversity of topics such as social mobilization for sustainable agriculture, local organic food networks and environmental concerns. ...French Abstract : De nombreuses recherches portent sur la consommation durable et le profil des consommateurs de produits biologiques, mais ces recherches portent sur la consommation dans les pays dĂ©veloppĂ©s, et ne permettent pas de savoir s'il existe des diffĂ©rences de consommation entre pays dĂ©veloppĂ©s et pays en dĂ©veloppement. Le but de cet article est de comparer le comportement de consommateurs de produits biologiques en France et au BrĂ©sil, sur la base d'entretiens individuels permettant de comparer plusieurs situations de choix : produits biologiques importĂ©s en concurrence avec des produits locaux comparables, produits biologiques locaux disponibles en supermarchĂ© ou d'en d'autres circuits de distribution,... Les rĂ©sultats mettent en Ă©vidence des attentes communes, comme la qualitĂ© ou la santĂ©, et des prĂ©fĂ©rences partagĂ©es pour les produits biologiques et locaux, mais pour des raisons diffĂ©rentes dans les deux Ă©chantillons. Les rĂ©sultats montrent Ă©galement que l'importance accordĂ©e aux prĂ©occupations environnementales et au soutien aux petits producteurs locaux n'est pas la mĂȘme dans les deux Ă©chantillons. Cette recherche conduit Ă des pistes de rĂ©flexion portant sur la mobilisation des consommateurs pour la consommation durable, les rĂ©seaux de production et consommation de produits biologiques et les prĂ©occupations environnementales des consommateurs.ORGANIC FOOD; LOCAL FOOD; SUSTAINABLE CONSUMPTION; QUALITATIVE STUDY
Structure of vortices in rotating Bose-Einstein condensates
We calculate the structure of individual vortices in rotating Bose-Einstein
condensates in a transverse harmonic trap. Making a Wigner-Seitz approximation
for the unit cell of the vortex lattice, we derive the Gross-Pitaevskii
equation for the condensate wave function in each cell of the lattice,
including effects of varying coarse grained density. We calculate the Abrikosov
parameter, the fractional core area, and the energy of individual cells.Comment: 10 pages, 9 figures, published versio
Quantum Effects in Small-Capacitance Single Josephson Junctions
We have measured the current-voltage (I-V) characteristics of
small-capacitance single Josephson junctions at low temperatures (T=0.02-0.6
K), where the strength of the coupling between the single junction and the
electromagnetic environment was controlled with one-dimensional arrays of dc
SQUIDs. The single-junction I-V curve is sensitive to the impedance of the
environment, which can be tuned IN SITU. We have observed Coulomb blockade of
Cooper-pair tunneling and even a region of negative differential resistance,
when the zero-bias resistance R_0' of the SQUID arrays is much higher than the
quantum resistance R_K = h/e^2 = 26 kohm. The negative differential resistance
is evidence of coherent single-Cooper-pair tunneling within the theory of
current-biased single Josephson junctions. Based on the theory, we have
calculated the I-V curves numerically in order to compare with the experimental
ones at R_0' >> R_K. The numerical calculation agrees with the experiments
qualitatively. We also discuss the R_0' dependence of the
single-Josephson-junction I-V curve in terms of the superconductor-insulator
transition driven by changing the coupling to the environment.Comment: 11 pages with 14 embedded figures, RevTeX4, final versio
Floquet analysis of the modulated two-mode Bose-Hubbard model
We study the tunneling dynamics in a time-periodically modulated two-mode
Bose-Hubbard model using Floquet theory. We consider situations where the
system is in the self-trapping regime and either the tunneling amplitude, the
interaction strength, or the energy difference between the modes is modulated.
In the former two cases, the tunneling is enhanced in a wide range of
modulation frequencies, while in the latter case the resonance is narrow. We
explain this difference with the help of Floquet analysis. If the modulation
amplitude is weak, the locations of the resonances can be found using the
spectrum of the non-modulated Hamiltonian. Furthermore, we use Floquet analysis
to explain the coherent destruction of tunneling (CDT) occurring in a
large-amplitude modulated system. Finally, we present two ways to create a NOON
state (a superposition of particles in mode 1 with zero particles in mode 2
and vice versa). One is based on a coherent oscillation caused by detuning from
a partial CDT. The other makes use of an adiabatic variation of the modulation
frequency. This results in a Landau-Zener type of transition between the ground
state and a NOON-like state.Comment: 16 pages, 11 figures; published in Phys. Rev.
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