29,032 research outputs found
Nonlinear chiral refrigerators
We investigate a mesoscopic refrigerator based on chiral quantum Hall edge
channels. We discuss a three-terminal cooling device in which charge transport
occurs between a pair of voltage-biased terminals only. The third terminal,
which is to be cooled, is set as a voltage probe with vanishing particle flux.
This largely prevents the generation of direct Joule heating which ensures a
high coefficient of performance. Cooling operation is based on energy-dependent
quantum transmissions. The latter are implemented with the aid of two tunable
scattering resonances (quantum dots). To find the optimal performance point and
the largest temperature difference created with our refrigerator, it is crucial
to address the nonlinear regime of transport, accounting for electron-electron
interaction effects. Our numerical simulations show that the maximal cooling
power can be tuned with the quantum dot couplings and energy levels. Further,
we provide analytical expressions within a weakly nonlinear scattering-matrix
formalism which allow us to discuss the conditions for optimal cooling in terms
of generalized thermopowers. Our results are important for the assessment of
chiral conductors as promising candidates for efficient quantum refrigerators
with low dissipation.Comment: 9 pages, 4 figures. v2: minor changes. Published versio
Massive star formation in Wolf-Rayet galaxies. IV: Colours, chemical composition analysis and metallicity-luminosity relations
(Abridged) We performed a multiwavelength analysis of a sample of starburst
galaxies that show the presence of a substantial population of very young
massive (WR) stars. Here we present the global analysis of the derived
photometric and chemical properties. We compare optical/NIR colours and the
physical properties (reddening coefficient, equivalent widths of the emission
and underlying absorption lines, ionization degree, electron density, and
electron temperature) and chemical properties with previous observations and
galaxy evolution models. Attending to their absolute B-magnitude many of them
are not dwarf galaxies, but they should be during their quiescent phase. We
found that both C(Hb) and Wabs increase with increasing metallicity. We
detected a high N/O ratio in objects showing strong WR features. The ejecta of
the WR stars may be the origin of the N enrichment in these galaxies. We
compared the abundances provided by the direct method with those obtained using
empirical calibrations, finding that (i) the Pilyugin method is the best
suitable empirical calibration, (ii) the relations between the oxygen abundance
and the N2 or the O3N2 parameters provided by Pettini & Pagel (2004) give
acceptable results for objects with 12+log(O/H)>8.0, and (iii) the results
provided by empirical calibrations based on photoionization models are
systematically 0.2-0.3 dex higher than the values derived from the direct
method. The O and N abundances and the N/O ratios are related to the
optical/NIR luminosity; the dispersion is consequence of the differences in the
star-formation histories. Galaxies with redder colours tend to have higher
oxygen and nitrogen abundances. Our detailed analysis is fundamental to
understand the nature of galaxies showing strong starbursts, as well as to know
their star formation history and the relationships with the environment.Comment: 30 pages, 22 figures, accepted to A&A. Updated with the final
version
Quantum point contacts as heat engines
The efficiency of macroscopic heat engines is restricted by the second law of
thermodynamics. They can reach at most the efficiency of a Carnot engine. In
contrast, heat currents in mesoscopic heat engines show fluctuations. Thus,
there is a small probability that a mesoscopic heat engine exceeds Carnot's
maximum value during a short measurement time. We illustrate this effect using
a quantum point contact as a heat engine. When a temperature difference is
applied to a quantum point contact, the system may be utilized as a source of
electrical power under steady state conditions. We first discuss the optimal
working point of such a heat engine that maximizes the generated electrical
power and subsequently calculate the statistics for deviations of the
efficiency from its most likely value. We find that deviations surpassing the
Carnot limit are possible, but unlikely.Comment: 9 pages, 2 figures. Contribution to the Physica E special issue on
"Frontiers in quantum electronic transport" in memory of Markus Buttiker.
Published versio
Representaciones y expresiones de lo mexicano en los muralistas de la primera generación
El movimiento muralista mexicano, que tuvo sus orÃgenes en la etapa posrevolucionaria y fue impulsado por José Vasconcelos, se convirtió en una expresión artÃstica cuyas obras expusieron contenidos que enfatizaban lo nacional y lo mexicano. En ellas se puede encontrar un amplio repertorio iconográfico del que destacan escenas alusivas a la historia nacional y a la vida popular. Entre los principales exponentes de la primera generación de muralistas se encuentran: Diego Rivera, David Alfaro Siqueiros, FermÃn Revueltas, Fernando Leal, Jean Charlot, Amado de la Cueva y Roberto Montenegro, artistas que en sus murales plasmaron paisajes, personajes, acontecimientos y demás escenarios, cuyos componentes pretenden ser un ref lejo de lo nacional, para lo cual rescataron, en sus representaciones, sÃmbolos y formas relacionadas con la identidad mexicana
- …