4,398 research outputs found
Crossover from Fermi liquid to Wigner molecule behavior in quantum dots
The crossover from weak to strong correlations in parabolic quantum dots at
zero magnetic field is studied by numerically exact path-integral Monte Carlo
simulations for up to eight electrons. By the use of a multilevel blocking
algorithm, the simulations are carried out free of the fermion sign problem. We
obtain a universal crossover only governed by the density parameter . For
, the data are consistent with a Wigner molecule description, while
for , Fermi liquid behavior is recovered. The crossover value is surprisingly small.Comment: 4 pages RevTeX, 3 figures, corrected Tabl
Quantum switches and quantum memories for matter-wave lattice solitons
We study the possibility of implementing a quantum switch and a quantum
memory for matter wave lattice solitons by making them interact with
"effective" potentials (barrier/well) corresponding to defects of the optical
lattice. In the case of interaction with an "effective" potential barrier, the
bright lattice soliton experiences an abrupt transition from complete
transmission to complete reflection (quantum switch) for a critical height of
the barrier. The trapping of the soliton in an "effective" potential well and
its release on demand, without loses, shows the feasibility of using the system
as a quantum memory. The inclusion of defects as a way of controlling the
interactions between two solitons is also reported
The Ultraluminous X-ray Sources near the Center of M82
We report the identification of a recurrent ultraluminous X-ray source (ULX),
a highly absorbed X-ray source (possibly a background AGN), and a young
supernova remnant near the center of the starburst galaxy M82. From a series of
Chandra observations taken from 1999 to 2005, we found that the transient ULX
first appeared in 1999 October. The source turned off in 2000 January, but
later reappeared and has been active since then. The X-ray luminosity of this
source varies from below the detection level (~2.5e38 erg/s) to its active
state in between ~7e39 erg/s and 1.3e40 erg/s (in the 0.5-10 keV energy band)
and shows unusual spectral changes. The X-ray spectra of some Chandra
observations are best fitted with an absorbed power-law model with photon index
ranging from 1.3 to 1.7. These spectra are similar to those of Galactic black
hole binary candidates seen in the low/hard state except that a very hard
spectrum was seen in one of the observations. By comparing with near infrared
images taken with the Hubble Space Telescope, the ULX is found to be located
within a young star cluster. Radio imaging indicates that it is associated with
a H II region. We suggest that the ULX is likely to be a > 100 solar mass
intermediate-mass black hole in the low/hard state. In addition to the
transient ULX, we also found a highly absorbed hard X-ray source which is
likely to be an AGN and an ultraluminous X-ray emitting young supernova remnant
which may be related to a 100-year old gamma-ray burst event, within 2 arcsec
of the transient ULX.Comment: 9 pages, 8 figures. Accepted for publication in Ap
General Relativistic Radiant Shock Waves in the Post-Quasistatic Approximation
An evolution of radiant shock wave front is considered in the framework of a
recently presented method to study self-gravitating relativistic spheres, whose
rationale becomes intelligible and finds full justification within the context
of a suitable definition of the post-quasistatic approximation. The spherical
matter configuration is divided into two regions by the shock and each side of
the interface having a different equation of state and anisotropic phase. In
order to simulate dissipation effects due to the transfer of photons and/or
neutrinos within the matter configuration, we introduce the flux factor, the
variable Eddington factor and a closure relation between them. As we expected
the strength of the shock increases the speed of the fluid to relativistic
values and for some critical ones is larger than light speed. In addition, we
find that energy conditions are very sensible to the anisotropy, specially the
strong one. As a special feature of the model, we find that the contribution of
the matter and radiation to the radial pressure are the same order of magnitude
as in the mant as in the core, moreover, in the core radiation pressure is
larger than matter pressure.Comment: To appear in Journal of Physics:Conference Series:"XXIX Spanish
Relativity Meeting (ERE 2006): Einstein's Legacy: From the Theoretical
Paradise to Astrophysical Observations
Cachexia and protein-energy wasting in children with chronic kidney disease
Children with chronic kidney disease (CKD) are at risk for “cachexia” or “protein-energy wasting” (PEW). These terms describe a pathophysiologic process resulting in the loss of muscle, with or without loss of fat, and involving maladaptive responses, including anorexia and elevated metabolic rate. PEW has been defined specifically in relation to CKD. We review the diagnostic criteria for cachexia and PEW in CKD and consider the limitations and applicability of these criteria to children with CKD. In addition, we present an overview of the manifestations and mechanisms of cachexia and PEW. A host of pathogenetic factors are considered, including systemic inflammation, endocrine perturbations, and abnormal neuropeptide signaling, as well as poor nutritional intake. Mortality risk, which is 100- to 200-fold higher in patients with end-stage renal disease than in the general population, is strongly correlated with the components of cachexia/PEW. Further research into the causes and consequences of wasting and growth retardation is needed in order to improve the survival and quality of life for children with CKD
Effect of monolingualism and bilingualism in the anterior cingulate cortex: a proton magnetic resonance spectroscopy study in two centers
Reports of an advantage of bilingualism on brain structure in young adult participants are inconsistent. Abutalebi et al. (2012) reported more efficient monitoring of conflict during the Flanker task in young bilinguals compared to young monolingual speakers. The present study compared young adult (mean age = 24) Cantonese-English bilinguals in Hong Kong and young adult monolingual speakers. We expected (a) differences in metabolites in neural tissue to result from bilingual experience, as measured by 1H-MRS at 3T, (b) correlations between metabolic levels and Flanker conflict and interference effects (c) different associations in bilingual and monolingual speakers. We found evidence of metabolic differences in the ACC due to bilingualism, specifically in metabolites Cho, Cr, Glx and NAA. However, we found no significant correlations between metabolic levels and conflict and interference effects and no significant evidence of differential relationships between bilingual and monolingual speakers. Furthermore, we found no evidence of significant differences in the mean size of conflict and interference effects between groups i.e. no bilingual advantage. Lower levels of Cho, Cr, Glx and NAA in bilingual adults compared to monolingual adults suggest that the brains of bilinguals develop greater adaptive control during conflict monitoring because of their extensive bilingual experience
Optoelectronics with electrically tunable PN diodes in a monolayer dichalcogenide
One of the most fundamental devices for electronics and optoelectronics is
the PN junction, which provides the functional element of diodes, bipolar
transistors, photodetectors, LEDs, and solar cells, among many other devices.
In conventional PN junctions, the adjacent p- and n-type regions of a
semiconductor are formed by chemical doping. Materials with ambipolar
conductance, however, allow for PN junctions to be configured and modified by
electrostatic gating. This electrical control enables a single device to have
multiple functionalities. Here we report ambipolar monolayer WSe2 devices in
which two local gates are used to define a PN junction exclusively within the
sheet of WSe2. With these electrically tunable PN junctions, we demonstrate
both PN and NP diodes with ideality factors better than 2. Under excitation
with light, the diodes show photodetection responsivity of 210 mA/W and
photovoltaic power generation with a peak external quantum efficiency of 0.2%,
promising numbers for a nearly transparent monolayer sheet in a lateral device
geometry. Finally, we demonstrate a light-emitting diode based on monolayer
WSe2. These devices provide a fundamental building block for ubiquitous,
ultra-thin, flexible, and nearly transparent optoelectronic and electronic
applications based on ambipolar dichalcogenide materials.Comment: 14 pages, 4 figure
Surface superconductivity in multilayered rhombohedral graphene: Supercurrent
The supercurrent for the surface superconductivity of a flat-band
multilayered rhombohedral graphene is calculated. Despite the absence of
dispersion of the excitation spectrum, the supercurrent is finite. The critical
current is proportional to the zero-temperature superconducting gap, i.e., to
the superconducting critical temperature and to the size of the flat band in
the momentum space
Optical signature of symmetry variations and spin-valley coupling in atomically thin tungsten dichalcogenides
Motivated by the triumph and limitation of graphene for electronic
applications, atomically thin layers of group VI transition metal
dichalcogenides are attracting extensive interest as a class of graphene-like
semiconductors with a desired band-gap in the visible frequency range. The
monolayers feature a valence band spin splitting with opposite sign in the two
valleys located at corners of 1st Brillouin zone. This spin-valley coupling,
particularly pronounced in tungsten dichalcogenides, can benefit potential
spintronics and valleytronics with the important consequences of spin-valley
interplay and the suppression of spin and valley relaxations. Here we report
the first optical studies of WS2 and WSe2 monolayers and multilayers. The
efficiency of second harmonic generation shows a dramatic even-odd oscillation
with the number of layers, consistent with the presence (absence) of inversion
symmetry in even-layer (odd-layer). Photoluminescence (PL) measurements show
the crossover from an indirect band gap semiconductor at mutilayers to a
direct-gap one at monolayers. The PL spectra and first-principle calculations
consistently reveal a spin-valley coupling of 0.4 eV which suppresses
interlayer hopping and manifests as a thickness independent splitting pattern
at valence band edge near K points. This giant spin-valley coupling, together
with the valley dependent physical properties, may lead to rich possibilities
for manipulating spin and valley degrees of freedom in these atomically thin 2D
materials
Switching Mechanism in Single-Layer Molybdenum Disulfide Transistors: an Insight into Current Flow across Schottky Barriers
In this article, we study the properties of metal contacts to single-layer
molybdenum disulfide (MoS2) crystals, revealing the nature of switching
mechanism in MoS2 transistors. On investigating transistor behavior as contact
length changes, we find that the contact resistivity for metal/MoS2 junctions
is defined by contact area instead of contact width. The minimum gate dependent
transfer length is ~0.63 {\mu}m in the on-state for metal (Ti) contacted
single-layer MoS2. These results reveal that MoS2 transistors are Schottky
barrier transistors, where the on/off states are switched by the tuning the
Schottky barriers at contacts. The effective barrier heights for source and
drain barriers are primarily controlled by gate and drain biases, respectively.
We discuss the drain induced barrier narrowing effect for short channel
devices, which may reduce the influence of large contact resistance for MoS2
Schottky barrier transistors at the channel length scaling limit.Comment: ACS Nano, ASAP (2013
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