292 research outputs found
Towards two-dimensional metallic behavior at LaAlO3/SrTiO3 interfaces
Using a low-temperature conductive-tip atomic force microscope in
cross-section geometry we have characterized the local transport properties of
the metallic electron gas that forms at the interface between LaAlO3 and
SrTiO3. At low temperature, we find that the carriers do not spread away from
the interface but are confined within ~10 nm, just like at room temperature.
Simulations taking into account both the large temperature and electric-field
dependence of the permittivity of SrTiO3 predict a confinement over a few nm
for sheet carrier densities larger than ~6 10^13 cm-2. We discuss the
experimental and simulations results in terms of a multi-band carrier system.
Remarkably, the Fermi wavelength estimated from Hall measurements is ~16 nm,
indicating that the electron gas in on the verge of two-dimensionality.Comment: Accepted for publication in Physical Review Letter
Towards A Census of Earth-mass Exo-planets with Gravitational Microlensing
Thirteen exo-planets have been discovered using the gravitational
microlensing technique (out of which 7 have been published). These planets
already demonstrate that super-Earths (with mass up to ~10 Earth masses) beyond
the snow line are common and multiple planet systems are not rare. In this
White Paper we introduce the basic concepts of the gravitational microlensing
technique, summarise the current mode of discovery and outline future steps
towards a complete census of planets including Earth-mass planets. In the
near-term (over the next 5 years) we advocate a strategy of automated follow-up
with existing and upgraded telescopes which will significantly increase the
current planet detection efficiency. In the medium 5-10 year term, we envision
an international network of wide-field 2m class telescopes to discover
Earth-mass and free-floating exo-planets. In the long (10-15 year) term, we
strongly advocate a space microlensing telescope which, when combined with
Kepler, will provide a complete census of planets down to Earth mass at almost
all separations. Such a survey could be undertaken as a science programme on
Euclid, a dark energy probe with a wide-field imager which has been proposed to
ESA's Cosmic Vision Programme.Comment: 10 pages. White Paper submission to the ESA Exo-Planet Roadmap
Advisory Team. See also "Inferring statistics of planet populations by means
of automated microlensing searches" by M. Dominik et al. (arXiv:0808.0004
Probing the atmosphere of the bulge G5III star OGLE-2002-BUL-069 by analysis of microlense H alpha line
We discuss high-resolution, time-resolved spectra of the caustic exit of the
binary microlensing event OGLE 2002-BUL-69 obtained with UVES on the VLT. The
source star is a G5III giant in the Galactic Bulge. During such events, the
source star is highly magnified, and a strong differential magnification around
the caustic resolves its surface. Using an appropriate model stellar atmosphere
generated by the NextGEN code we obtained a model light curve for the caustic
exit and compared it with a dense set of photometric observations obtained by
the PLANET microlensing follow up network. We further compared predicted
variations in the H alpha equivalent width with those measured from our
spectra. While the model and observations agree in the gross features, there
are discrepancies suggesting shortcomings in the model, particularly for the H
alpha line core, where we have detected amplified emission from the stellar
chromosphere as the source star's trailing limb exited the caustic. This
achievement became possible by the provision of the OGLE-III Early Warning
System, a network of small telescopes capable of nearly-continuous
round-the-clock photometric monitoring, on-line data reduction, daily
near-real-time modelling in order to predict caustic crossing parameters, and a
fast and efficient response of a 8m-class telescope to a
``Target-Of-Opportunity'' observation request.Comment: 4 pages Latex, 3 figures, accepted for publication to astronomy and
astrophysics letter
Possible Metal/Insulator Transition at B=0 in Two Dimensions
We have studied the zero magnetic field resistivity of unique high- mobility
two-dimensional electron system in silicon. At very low electron density (but
higher than some sample-dependent critical value,
cm), CONVENTIONAL WEAK LOCALIZATION IS OVERPOWERED BY A SHARP DROP OF
RESISTIVITY BY AN ORDER OF MAGNITUDE with decreasing temperature below 1--2 K.
No further evidence for electron localization is seen down to at least 20 mK.
For , the sample is insulating. The resistivity is empirically
found to SCALE WITH TEMPERATURE BOTH BELOW AND ABOVE WITH A SINGLE
PARAMETER which approaches zero at suggesting a metal/ insulator
phase transition.Comment: 10 pages; REVTeX v3.0; 3 POSTSCRIPT figures available upon request;
to be published in PRB, Rapid Commu
Inelastic Coulomb scattering rates due to acoustic and optical plasmon modes in coupled quantum wires
We report a theoretical study on the inelastic Coulomb scattering rate of an
injected electron in two coupled quantum wires in quasi-one-dimensional doped
semiconductors. Two peaks appear in the scattering spectrum due to the optical
and the acoustic plasmon scattering in the system. We find that the scattering
rate due to the optical plasmon mode is similar to that in a single wire but
the acoustic plasmon scattering depends crucially on its dispersion relation at
small . Furthermore, the effects of tunneling between the two wires are
studied on the inelastic Coulomb scattering rate. We show that a weak tunneling
can strongly affect the acoustic plasmon scattering.Comment: 6 Postscript figure
Limits on additional planetary companions to OGLE-2005-BLG-390L
We investigate constraints on additional planets orbiting the distant M-dwarf
star OGLE-2005-BLG-390L, around which photometric microlensing data has
revealed the existence of the sub-Neptune-mass planet OGLE-2005-BLG-390Lb. We
specifically aim to study potential Jovian companions and compare our findings
with predictions from core-accretion and disc-instability models of planet
formation. We also obtain an estimate of the detection probability for
sub-Neptune mass planets similar to OGLE-2005-BLG-390Lb using a simplified
simulation of a microlensing experiment. We compute the efficiency of our
photometric data for detecting additional planets around OGLE-2005-BLG-390L, as
a function of the microlensing model parameters and convert it into a function
of the orbital axis and planet mass by means of an adopted model of the Milky
Way. We find that more than 50 % of potential planets with a mass in excess of
1 M_J between 1.1 and 2.3 AU around OGLE-2005-BLG-390L would have revealed
their existence, whereas for gas giants above 3 M_J in orbits between 1.5 and
2.2 AU, the detection efficiency reaches 70 %; however, no such companion was
observed. Our photometric microlensing data therefore do not contradict the
existence of gas giant planets at any separation orbiting OGLE-2005-BLG-390L.
Furthermore we find a detection probability for an OGLE-2005-BLG-390Lb-like
planet of around 2-5 %. In agreement with current planet formation theories,
this quantitatively supports the prediction that sub-Neptune mass planets are
common around low-mass stars.Comment: 10 pages, 4 figures, accepted by A&
OGLE-2005-BLG-153: Microlensing Discovery and Characterization of A Very Low Mass Binary
The mass function and statistics of binaries provide important diagnostics of
the star formation process. Despite this importance, the mass function at low
masses remains poorly known due to observational difficulties caused by the
faintness of the objects. Here we report the microlensing discovery and
characterization of a binary lens composed of very low-mass stars just above
the hydrogen-burning limit. From the combined measurements of the Einstein
radius and microlens parallax, we measure the masses of the binary components
of and . This discovery
demonstrates that microlensing will provide a method to measure the mass
function of all Galactic populations of very low mass binaries that is
independent of the biases caused by the luminosity of the population.Comment: 6 pages, 3 figures, 1 tabl
A systematic fitting scheme for caustic-crossing microlensing events
We outline a method for fitting binary-lens caustic-crossing microlensing
events based on the alternative model parameterisation proposed and detailed in
Cassan (2008). As an illustration of our methodology, we present an analysis of
OGLE-2007-BLG-472, a double-peaked Galactic microlensing event with a source
crossing the whole caustic structure in less than three days. In order to
identify all possible models we conduct an extensive search of the parameter
space, followed by a refinement of the parameters with a Markov Chain-Monte
Carlo algorithm. We find a number of low-chi2 regions in the parameter space,
which lead to several distinct competitive best models. We examine the
parameters for each of them, and estimate their physical properties. We find
that our fitting strategy locates several minima that are difficult to find
with other modelling strategies and is therefore a more appropriate method to
fit this type of events.Comment: 12 pages, 11 figure
OGLE-2005-BLG-018: Characterization of Full Physical and Orbital Parameters of a Gravitational Binary Lens
We present the analysis result of a gravitational binary-lensing event
OGLE-2005-BLG-018. The light curve of the event is characterized by 2 adjacent
strong features and a single weak feature separated from the strong features.
The light curve exhibits noticeable deviations from the best-fit model based on
standard binary parameters. To explain the deviation, we test models including
various higher-order effects of the motions of the observer, source, and lens.
From this, we find that it is necessary to account for the orbital motion of
the lens in describing the light curve. From modeling of the light curve
considering the parallax effect and Keplerian orbital motion, we are able to
measure not only the physical parameters but also a complete orbital solution
of the lens system. It is found that the event was produced by a binary lens
located in the Galactic bulge with a distance kpc from the Earth.
The individual lens components with masses and are separated with a semi-major axis of AU and
orbiting each other with a period yr. The event demonstrates
that it is possible to extract detailed information about binary lens systems
from well-resolved lensing light curves.Comment: 19 pages, 6 figure
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