1,290 research outputs found
Quantum Chaos Versus Classical Chaos: Why is Quantum Chaos Weaker?
We discuss the questions: How to compare quantitatively classical chaos with
quantum chaos? Which one is stronger? What are the underlying physical reasons
VLA 3.5 cm continuum sources in the Serpens cloud core
We present VLA 3.5 cm continuum observations of the Serpens cloud core. 22
radio continuum sources are detected. 16 out of the 22 cm sources are suggested
to be associated with young stellar objects (Class 0, Class I, flat-spectrum,
and Class II) of the young Serpens cluster. The rest of the VLA sources
plausibly are background objects. Most of the Serpens cm sources likely
represent thermal radio jets; on the other hand, the radio continuum emission
of some sources could be due to a gyrosynchroton mechanism arising from
coronally active young stars. The Serpens VLA sources are spatially distributed
into two groups; one of them located towards the NW clump of the Serpens core,
where only Class 0 and Class I protostars are found to present cm emission, and
a second group located towards the SE clump, where radio continuum sources are
associated with objects in evolutionary classes from Class 0 to Class II. This
subgrouping is similar to that found in the near IR, mid-IR and mm wavelength
regimes.Comment: 2 figures, accepted by Astronomical journa
Sensitivity and parameter-estimation precision for alternate LISA configurations
We describe a simple framework to assess the LISA scientific performance
(more specifically, its sensitivity and expected parameter-estimation precision
for prescribed gravitational-wave signals) under the assumption of failure of
one or two inter-spacecraft laser measurements (links) and of one to four
intra-spacecraft laser measurements. We apply the framework to the simple case
of measuring the LISA sensitivity to monochromatic circular binaries, and the
LISA parameter-estimation precision for the gravitational-wave polarization
angle of these systems. Compared to the six-link baseline configuration, the
five-link case is characterized by a small loss in signal-to-noise ratio (SNR)
in the high-frequency section of the LISA band; the four-link case shows a
reduction by a factor of sqrt(2) at low frequencies, and by up to ~2 at high
frequencies. The uncertainty in the estimate of polarization, as computed in
the Fisher-matrix formalism, also worsens when moving from six to five, and
then to four links: this can be explained by the reduced SNR available in those
configurations (except for observations shorter than three months, where five
and six links do better than four even with the same SNR). In addition, we
prove (for generic signals) that the SNR and Fisher matrix are invariant with
respect to the choice of a basis of TDI observables; rather, they depend only
on which inter-spacecraft and intra-spacecraft measurements are available.Comment: 17 pages, 4 EPS figures, IOP style, corrected CQG versio
Quantum interference and Klein tunneling in graphene heterojunctions
The observation of quantum conductance oscillations in mesoscopic systems has
traditionally required the confinement of the carriers to a phase space of
reduced dimensionality. While electron optics such as lensing and focusing have
been demonstrated experimentally, building a collimated electron interferometer
in two unconfined dimensions has remained a challenge due to the difficulty of
creating electrostatic barriers that are sharp on the order of the electron
wavelength. Here, we report the observation of conductance oscillations in
extremely narrow graphene heterostructures where a resonant cavity is formed
between two electrostatically created bipolar junctions. Analysis of the
oscillations confirms that p-n junctions have a collimating effect on
ballistically transmitted carriers. The phase shift observed in the conductance
fringes at low magnetic fields is a signature of the perfect transmission of
carriers normally incident on the junctions and thus constitutes a direct
experimental observation of ``Klein Tunneling.''Comment: 13 pages and 6 figures including supplementary information. The paper
has been modified in light of new theoretical results available at
arXiv:0808.048
Gate-controlled Guiding of Electrons in Graphene
Ballistic semiconductor structures have allowed the realization of
optics-like phenomena in electronics, including magnetic focusing and lensing.
An extension that appears unique to graphene is to use both n and p carrier
types to create electronic analogs of optical devices having both positive and
negative indices of refraction. Here, we use gate-controlled density with both
p and n carrier types to demonstrate the analog of the fiber-optic guiding in
graphene. Two basic effects are investigated: (1) bipolar p-n junction guiding,
based on the principle of angle-selective transmission though the graphene p-n
interface, and (2) unipolar fiber-optic guiding, using total internal
reflection controlled by carrier density. Modulation of guiding efficiency
through gating is demonstrated and compared to numerical simulations, which
indicates that interface roughness limits guiding performance, with
few-nanometer effective roughness extracted. The development of p-n and
fiber-optic guiding in graphene may lead to electrically reconfigurable wiring
in high-mobility devices.Comment: supplementary materal at
http://marcuslab.harvard.edu/papers/OG_SI.pd
Binding Energy of Charged Excitons in ZnSe-based Quantum Wells
Excitons and charged excitons (trions) are investigated in ZnSe-based quantum
well structures with (Zn,Be,Mg)Se and (Zn,Mg)(S,Se) barriers by means of
magneto-optical spectroscopy. Binding energies of negatively () and positively
(X+) charged excitons are measured as functions of quantum well width, free
carrier density and in external magnetic fields up to 47 T. The binding energy
of shows a strong increase from 1.4 to 8.9 meV with decreasing quantum well
width from 190 to 29 A. The binding energies of X+ are about 25% smaller than
the binding energy in the same structures. The magnetic field behavior of and
X+ binding energies differ qualitatively. With growing magnetic field strength,
increases its binding energy by 35-150%, while for X+ it decreases by 25%.
Zeeman spin splittings and oscillator strengths of excitons and trions are
measured and discussed
A Study of the Luminosity and Mass Functions of the Young IC 348 Cluster using FLAMINGOS Wide-Field Near-Infrared Images
We present wide-field near-infrared (JHK) images of the young, 2 Myr IC 348
cluster taken with FLAMINGOS. We use these new data to construct an infrared
census of sources, which is sensitive enough to detect a 10 Mjup brown dwarf
seen through an extinction of Av=7mag. We examine the cluster's structure and
relationship to the molecular cloud and construct the cluster's K band
luminosity function. Using our model luminosity function algorithm, we derive
the cluster's initial mass function throughout the stellar and substellar
regimes and find that the IC 348 IMF is very similar to that found for the
Trapezium Cluster with both cluster IMFs having a mode between 0.2 - 0.08 Msun.
In particular we find that, similar to our results for the Trapezium, brown
dwarfs constitute only 1 in 4 of the sources in the IC 348 cluster. We show
that a modest secondary peak forms in the substellar IC 348 KLF, corresponding
to the same mass range responsible for a similar KLF peak found in the
Trapezium. We interpret this KLF peak as either evidence for a corresponding
secondary IMF peak at the deuterium burning limit, or as arising from a feature
in the substellar mass-luminosity relation that is not predicted by current
theoretical models. Lastly, we find that IC 348 displays radial variations of
its sub-solar (0.5 - 0.08 Msun) IMF on a parsec scale. Whatever mechanism that
is breaking the universality of the IMF on small spatial scales in IC 348 does
not appear to be acting upon the brown dwarf population, whose relative size
does not vary with distance from the cluster center.Comment: 53 pages, 20 figures, AASTeX5.0. Color version of Figure 1 made
available in jpg format. Figure(s) 2,3,5 are reduced in resolution. Accepted
14 Jan 2003 to the Astronomical Journa
The luminosities of protostars in the spitzer c2d and gould belt legacy clouds
Journal ArticlePublished version available online at the Astronomical Journal, Volume 145, Number 4, Article 94; doi: doi: 10.1088/0004-6256/145/4/94Motivated by the long-standing "luminosity problem" in low-mass star formation whereby protostars are underluminous compared to theoretical expectations, we identify 230 protostars in 18 molecular clouds observed by two Spitzer Space Telescope Legacy surveys of nearby star-forming regions. We compile complete spectral energy distributions, calculate L bol for each source, and study the protostellar luminosity distribution. This distribution extends over three orders of magnitude, from 0.01 L ÈŻ to 69 L ÈŻ, and has a mean and median of 4.3 L ÈŻ and 1.3 L ÈŻ, respectively. The distributions are very similar for Class 0 and Class I sources except for an excess of low luminosity (L bol âČ 0.5 L) Class I sources compared to Class 0. 100 out of the 230 protostars (43%) lack any available data in the far-infrared and submillimeter (70 ÎŒm <λ < 850 ÎŒm) and have L bol underestimated by factors of 2.5 on average, and up to factors of 8-10 in extreme cases. Correcting these underestimates for each source individually once additional data becomes available will likely increase both the mean and median of the sample by 35%-40%. We discuss and compare our results to several recent theoretical studies of protostellar luminosities and show that our new results do not invalidate the conclusions of any of these studies. As these studies demonstrate that there is more than one plausible accretion scenario that can match observations, future attention is clearly needed. The better statistics provided by our increased data set should aid such future work. © 2013. The American Astronomical Society. All rights reserved..National Science FoundationNational Aeronautics and Space AdministrationJet Propulsion Laboratory, California Institute of Technolog
RNA isolation for transcriptomics of human and mouse small skin biopsies
<p>Abstract</p> <p>Background</p> <p>Isolation of RNA from skin biopsies presents a challenge, due to the tough nature of skin tissue and a high presence of RNases. As we lacked the dedicated equipment, i.e. homogenizer or bead-beater, needed for the available RNA from skin isolation methods, we adapted and tested our zebrafish single-embryo RNA-isolation protocol for RNA isolation from skin punch biopsies.</p> <p>Findings</p> <p>We tested our new RNA-isolation protocol in two experiments: a large-scale study with 97 human skin samples, and a small study with 16 mouse skin samples. Human skin was sampled with 4.0 mm biopsy punches and for the mouse skin different punch diameter sizes were tested; 1.0, 1.5, 2.0, and 2.5 mm. The average RNA yield in human samples was 1.5 ÎŒg with an average RNA quality RIN value of 8.1. For the mouse biopsies, the average RNA yield was 2.4 ÎŒg with an average RIN value of 7.5. For 96% of the human biopsies and 100% of the mouse biopsies we obtained enough high-quality RNA. The RNA samples were successfully tested in a transcriptomics analysis using the Affymetrix and Roche NimbleGen platforms.</p> <p>Conclusions</p> <p>Using our new RNA-isolation protocol, we were able to consistently isolate high-quality RNA, which is apt for further transcriptomics analysis. Furthermore, this method is already useable on biopsy material obtained with a punch diameter as small as 1.5 mm.</p
A sensitive survey for water maser emission towards Bok globules using the Robledo 70m antenna
We report the most sensitive water maser survey towards Bok globules to date,
using NASA's 70m antenna in Robledo de Chavela (Spain). We observed 207
positions within the CB catalog with a higher probability of harboring a young
star, using as selection criteria the presence of radio continuum emission
(from submillimeter to centimeter wavelengths), geometrical centers of
molecular outflows, peaks in maps of high-density gas tracers (NH3 or CS), and
IRAS point sources. We have obtained 7 maser detections, 6 of which (in CB 34,
CB 54, CB 65, CB 101, CB 199, and CB 232) are reported for the first time here.
Most of the water masers we detected are likely to be associated with young
stellar objects (YSOs), except for CB 101 (probably an evolved object) and CB
65 (uncertain nature). The water maser in CB 199 shows a relatively high shift
(~30 km/s) of its velocity centroid with respect to the cloud velocity, which
is unusual for low-mass YSOs. We speculate that high-velocity masers in this
kind of object could be related with episodes of energetic mass-loss in close
binaries. Alternatively, the maser in CB 199 could be pumped by a
protoplanetary or a young planetary nebula. CB 232 is the smallest Bok globule
(~0.6 pc) known to be associated with water maser emission, although it would
be superseded by the cases of CB 65 (~0.3 pc) and CB 199 (~0.5 pc) if their
association with YSOs is confirmed. All our selection criteria have
statistically compatible detection rates, except for IRAS sources, which tend
to be a somewhat worse predictor for the presence of maser emission.Comment: 42 pages, 7 figures. Accepted by The Astronomical Journal. Corrected
typos in Tables 1 &
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