364 research outputs found
Room temperature magneto-optic effect in silicon light-emitting diodes
In weakly spin-orbit coupled materials, the spin-selective nature of
recombination can give rise to large magnetic-field effects, for example on
electro-luminescence from molecular semiconductors. While silicon has weak
spin-orbit coupling, observing spin-dependent recombination through
magneto-electroluminescence is challenging due to the inefficiency of emission
due to silicon's indirect band-gap, and to the difficulty in separating
spin-dependent phenomena from classical magneto-resistance effects. Here we
overcome these challenges to measure magneto-electroluminescence in silicon
light-emitting diodes fabricated via gas immersion laser doping. These devices
allow us to achieve efficient emission while retaining a well-defined geometry
thus suppressing classical magnetoresistance effects to a few percent. We find
that electroluminescence can be enhanced by up to 300\% near room temperature
in a seven Tesla magnetic field showing that the control of the spin degree of
freedom can have a strong impact on the efficiency of silicon LEDs
Microwave response of an NS ring coupled to a superconducting resonator
A long phase coherent normal (N) wire between superconductors (S) is
characterized by a dense phase dependent Andreev spectrum . We probe this
spectrum in a high frequency phase biased configuration, by coupling an NS ring
to a multimode superconducting resonator. We detect a dc flux and frequency
dependent response whose dissipative and non dissipative components are related
by a simple Debye relaxation law with a characteristic time of the order of the
diffusion time through the N part of the ring. The flux dependence exhibits
periodic oscillations with a large harmonics content at temperatures
where the Josephson current is purely sinusoidal. This is explained considering
that the populations of the Andreev levels are frozen on the time-scale of the
experiments.Comment: 5 pages,4 figure
Proximity DC squids in the long junction limit
We report the design and measurement of
Superconducting/normal/superconducting (SNS) proximity DC squids in the long
junction limit, i.e. superconducting loops interrupted by two normal metal
wires roughly a micrometer long. Thanks to the clean interface between the
metals, at low temperature a large supercurrent flows through the device. The
dc squid-like geometry leads to an almost complete periodic modulation of the
critical current through the device by a magnetic flux, with a flux periodicity
of a flux quantum h/2e through the SNS loop. In addition, we examine the entire
field dependence, notably the low and high field dependence of the maximum
switching current. In contrast with the well-known Fraunhoffer-type
oscillations typical of short wide junctions, we find a monotonous gaussian
extinction of the critical current at high field. As shown in [15], this
monotonous dependence is typical of long and narrow diffusive junctions. We
also find in some cases a puzzling reentrance at low field. In contrast, the
temperature dependence of the critical current is well described by the
proximity effect theory, as found by Dubos {\it et al.} [16] on SNS wires in
the long junction limit. The switching current distributions and hysteretic IV
curves also suggest interesting dynamics of long SNS junctions with an
important role played by the diffusion time across the junction.Comment: 12 pages, 16 figure
Contactless photoconductivity measurements on (Si) nanowires
Conducting nanowires possess remarkable physical properties unattainable in
bulk materials. However our understanding of their transport properties is
limited by the difficulty of connecting them electrically. In this Letter we
investigate phototransport in both bulk silicon and silicon nanowires using a
superconducting multimode resonator operating at frequencies between 0.3 and 3
GHz. We find that whereas the bulk Si response is mainly dissipative, the
nanowires exhibit a large dielectric polarizability. This technique is
contactless and can be applied to many other semiconducting nanowires and
molecules. Our approach also allows to investigate the coupling of electron
transport to surface acoustic waves in bulk Si and to electro-mechanical
resonances in the nanowires
Geometry-related magnetic interference patterns in long SNS Josephson junctions
We have measured the critical current dependence on the magnetic flux of two
long SNS junctions differing by the normal wire geometry. The samples are made
by a Au wire connected to W contacts, via Focused Ion Beam assisted deposition.
We could tune the magnetic pattern from the monotonic gaussian-like decay of a
quasi 1D normal wire to the Fraunhofer-like pattern of a square normal wire. We
explain the monotonic limit with a semiclassical 1D model, and we fit both
field dependences with numerical simulations of the 2D Usadel equation.
Furthermore, we observe both integer and fractional Shapiro steps. The magnetic
flux dependence of the integer steps reproduces as expected that of the
critical current Ic, while fractional steps decay slower with the flux than Ic.Comment: 5 pages, 4 figure
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Migration, Remittances and Capital Accumulation: Evidence from Rural Mexico
This article studies the link between migration, remittances and asset accumulation for a panel of poor rural households in Mexico over the period 1997--2006. In a context of financial markets' imperfections, migration may act as a substitute for imperfect credit and insurance provision (through remittances from migrants) and, thus, exert a positive effect on investment. However, it may well be the case that remittances are channelled towards increasing consumption and leisure goods instead. Exploiting within family variation and an instrumental variable strategy, we show that migration indeed accelerates productive assets' accumulation. However, when we look at the effect of migration on non-productive assets (durable goods), we find a negative effect. Our results then suggest that poor rural families resort to migration as a way to mitigate constraints that prevent them from investing in productive assets
Comment on Qian et al. 2008: La Niña and El Niño composites of atmospheric CO2 change
It is well known that interannual extremes in the rate of change of atmospheric CO2 are strongly influenced by the occurrence of El Niño-Southern Oscillation (ENSO) events. Qian et al. presented ENSO composites of atmospheric CO2 changes. We show that their composites do not reflect the atmospheric changes that are most relevant to understanding the role of ENSO on atmospheric CO2 variability. We present here composites of atmospheric CO2 change that differ markedly from those of Qian et al., and reveal previously unreported asymmetries between the effects on the global carbon system of El Niño and La Niña events. The calendar-year timing differs; La Niña changes in atmospheric CO2 typically occur primarily over September–May, while El Niño changes occur primarily over December–August. And the net concentration change is quite different; La Niña changes are about half the size of El Niño changes. These results illustrate new aspects of the ENSO/global carbon budget interaction and provide useful global-scale benchmarks for the evaluation of Earth System Model studies of the carbon system
Portfolio selection problems in practice: a comparison between linear and quadratic optimization models
Several portfolio selection models take into account practical limitations on
the number of assets to include and on their weights in the portfolio. We
present here a study of the Limited Asset Markowitz (LAM), of the Limited Asset
Mean Absolute Deviation (LAMAD) and of the Limited Asset Conditional
Value-at-Risk (LACVaR) models, where the assets are limited with the
introduction of quantity and cardinality constraints. We propose a completely
new approach for solving the LAM model, based on reformulation as a Standard
Quadratic Program and on some recent theoretical results. With this approach we
obtain optimal solutions both for some well-known financial data sets used by
several other authors, and for some unsolved large size portfolio problems. We
also test our method on five new data sets involving real-world capital market
indices from major stock markets. Our computational experience shows that,
rather unexpectedly, it is easier to solve the quadratic LAM model with our
algorithm, than to solve the linear LACVaR and LAMAD models with CPLEX, one of
the best commercial codes for mixed integer linear programming (MILP) problems.
Finally, on the new data sets we have also compared, using out-of-sample
analysis, the performance of the portfolios obtained by the Limited Asset
models with the performance provided by the unconstrained models and with that
of the official capital market indices
Subsurface life can modify volatile cycling on a planetary scale
The past decade of environmental microbiology has revealed that subsurface environments, both marine and continental, harbor one of the largest ecosystems of our planet, with diversity and biomass rivaling those of the surface. In addition, subsurface life has been recently shown to contribute significantly to the planet’s biogeochemistry, with microbial activity potentially playing an important role in controlling the flux and composition of volatiles recycled between the Earth’s surface and interior, which has broad implications for the search for life beyond our planet. Current efforts to discover extraterrestrial life are focused on planetary bodies with largely inhospitable surfaces, such as Mars, Venus, Europa, Titan, and Enceladus. In these locations, subsurface environments might provide niches of habitability, making the study of deep microbial life a priority for future astrobiological missions. Understanding how volatile elements are exchanged between planetary surfaces and interiors and the role of a subsurface biosphere in altering their composition and flux might provide a tractable target for defining planetary habitability and the detection of subsurface life forms.Fil: Giovanelli, D.. UniversitĂ degli Studi di Napoli Federico II; Italia. Tokyo Institute of Technology; JapĂłn. Rutgers University; Estados Unidos. Consiglio Nazionale delle Ricerche; Italia. Woods Hole Oceanographic Institution; Estados UnidosFil: Barry, P. H.. Woods Hole Oceanographic Institution; Estados UnidosFil: Bekaert, D. V.. Woods Hole Oceanographic Institution; Estados UnidosFil: Chiodi, Agostina Laura. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico Conicet - Salta. Instituto de Bio y Geociencias del NOA. Universidad Nacional de Salta. Facultad de Ciencias Naturales. Museo de Ciencias Naturales. Instituto de Bio y Geociencias del NOA; ArgentinaFil: Cordone, A.. UniversitĂ degli Studi di Napoli Federico II; ItaliaFil: Covone, G.. UniversitĂ degli Studi di Napoli Federico II; Italia. Istituto Nazionale di Astrofisica; Italia. Istituto Nazionale di Fisica Nucleare; ItaliaFil: Jessen, G.. Universidad Austral de Chile; ChileFil: Lloyd, K.. University of Tennessee; Estados UnidosFil: de Moor, J. M.. Universidad Nacional; Costa RicaFil: Morrison, S. M.. Carnegie Institution For Science; Estados UnidosFil: Schrenk, M. O.. Michigan State University; Estados UnidosFil: Vitale Brovarone, A.. Alma Mater Studiorum Universit`a Di Bologna; Italia. Sorbonne University; Francia. Museum National d’Histoire Naturelle; Franci
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