8,124 research outputs found
Effects of thermal motion on electromagnetically induced absorption
We describe the effect of thermal motion and buffer-gas collisions on a
four-level closed N system interacting with strong pump(s) and a weak probe.
This is the simplest system that experiences electromagnetically induced
absorption (EIA) due to transfer of coherence via spontaneous emission from the
excited to ground state. We investigate the influence of Doppler broadening,
velocity-changing collisions (VCC), and phase-changing collisions (PCC) with a
buffer gas on the EIA spectrum of optically active atoms. In addition to exact
expressions, we present an approximate solution for the probe absorption
spectrum, which provides physical insight into the behavior of the EIA peak due
to VCC, PCC, and wave-vector difference between the pump and probe beams. VCC
are shown to produce a wide pedestal at the base of the EIA peak, which is
scarcely affected by the pump-probe angular deviation, whereas the sharp
central EIA peak becomes weaker and broader due to the residual Doppler-Dicke
effect. Using diffusion-like equations for the atomic coherences and
populations, we construct a spatial-frequency filter for a spatially structured
probe beam and show that Ramsey narrowing of the EIA peak is obtained for beams
of finite width
Narrowband Biphoton Generation due to Long-Lived Coherent Population Oscillations
We study the generation of paired photons due to the effect of four-wave
mixing in an ensemble of pumped two-level systems that decay via an
intermediate metastable state. The slow population relaxation of the metastable
state to the ground state is utilized to create long-lived coherent population
oscillation, leading to narrowband nonlinear response of the medium. The
biphotons have a narrow bandwidth, long coherence time and length, which can be
controlled by the pump field. In addition, the biphotons are antibunched, with
antibunching period determined by the dephasing time. During this period,
damped oscillations of the biphoton wavefunction occurs if the pump detuning is
non-zero.Comment: 7 pages, 2 figure
Infrared Spectra and Ab Initio Calculations for the F-−(CH4)n (n = 1−8) Anion Clusters
Infrared spectra of mass-selected F-−(CH4)n (n = 1−8) clusters are recorded in the CH stretching region (2500−3100 cm-1). Spectra for the n = 1−3 clusters are interpreted with the aid of ab initio calculations at the MP2/6-311++G(2df 2p) level, which suggest that the CH4ligands bind to F- by equivalent, linear hydrogen bonds. Anharmonic frequencies for CH4 and F-−CH4 are determined using the vibrational self-consistent field method with second-order perturbation theory correction. The n = 1 complex is predicted to have a C3v structure with a single CH group hydrogen bonded to F-. Its spectrum exhibits a parallel band associated with a stretching vibration of the hydrogen-bonded CH group that is red-shifted by 380 cm-1 from the ν1 band of free CH4 and a perpendicular band associated with the asymmetric stretching motion of the nonbonded CH groups, slightly red-shifted from the ν3 band of free CH4. As nincreases, additional vibrational bands appear as a result of Fermi resonances between the hydrogen-bonded CH stretching vibrational mode and the 2ν4 overtone and ν2 + ν4combination levels of the methane solvent molecules. For clusters with n ≤ 8, it appears that the CH4 molecules are accommodated in the first solvation shell, each being attached to the F- anion by equivalent hydrogen bonds
Orientation-to-alignment conversion and spin squeezing
The relationship between orientation-to-alignment conversion (a form of
atomic polarization evolution induced by an electric field) and the phenomenon
of spin squeezing is demonstrated. A "stretched" state of an atom or molecule
with maximum angular-momentum projection along the quantization axis possesses
orientation and is a quantum-mechanical minimum-uncertainty state, where the
product of the equal uncertainties of the angular-momentum projections on two
orthogonal directions transverse to the quantization axis is the minimum
allowed by the uncertainty relation. Application of an electric field for a
short time induces orientation-to-alignment conversion and produces a
spin-squeezed state, in which the quantum state essentially remains a
minimum-uncertainty state, but the uncertainties of the angular-momentum
projections on the orthogonal directions are unequal. This property can be
visualized using the angular-momentum probability surfaces, where the radius of
the surface is given by the probability of measuring the maximum
angular-momentum projection in that direction. Brief remarks are also given
concerning collective-spin squeezing and quantum nondemolition measurements.Comment: 7 pages, 6 figure
Dissecting the origin of the submillimeter emission in nearby galaxies with Herschel and LABOCA
We model the infrared to submillimeter spectral energy distribution of 11
nearby galaxies of the KINGFISH sample using Spitzer and Herschel data and
compare model extrapolations at 870um (using different fitting techniques) with
LABOCA 870um observations. We investigate how the differences between
predictions and observations vary with model assumptions or environment. At
global scales, we find that modified blackbody models using realistic cold
emissivity indices (beta_c=2 or 1.5) are able to reproduce the 870um observed
emission within the uncertainties for most of the sample. Low values
(beta_c<1.3) would be required in NGC0337, NGC1512 and NGC7793. At local
scales, we observe a systematic 870um excess when using beta_=2.0. The
beta_c=1.5 or the Draine and Li (2007) models can reconcile predictions with
observations in part of the disks. Some of the remaining excesses occur towards
the centres and can be partly or fully accounted for by non-dust contributions
such as CO(3-2) or, to a lesser extent, free-free or synchrotron emission. In
three non-barred galaxies, the remaining excesses rather occur in the disk
outskirts. This could be a sign of a flattening of the submm slope (and
decrease of the effective emissivity index) with radius in these objects.Comment: 31 pages (including appendix), 7 figures, accepted for publication in
MNRA
Pancreatic cysts suspected to be branch duct intraductal papillary mucinous neoplasm without concerning features have low risk for development of pancreatic cancer.
BackgroundThe risk of developing pancreatic cancer is uncertain in patients with clinically suspected branch duct intraductal papillary mucinous neoplasm (BD-IPMN) based on the "high-risk stigmata" or "worrisome features" criteria proposed in the 2012 international consensus guidelines ("Fukuoka criteria").MethodsRetrospective case series involving patients referred for endoscopic ultrasound (EUS) of indeterminate pancreatic cysts with clinical and EUS features consistent with BD-IPMN. Rates of pancreatic cancer occurring at any location in the pancreas were compared between groups of patients with one or more Fukuoka criteria ("Highest-Risk Group", HRG) and those without these criteria ("Lowest-Risk Group", LRG).ResultsAfter exclusions, 661 patients comprised the final cohort (250 HRG and 411 LRG patients), 62% female with an average age of 67 years and 4 years of follow up. Pancreatic cancer, primarily adenocarcinoma, occurred in 60 patients (59 HRG, 1 LRG). Prevalent cancers diagnosed during EUS, immediate surgery, or first year of follow up were found in 48/661 (7.3%) of cohort and exclusively in HRG (33/77, 42.3%). Using Kaplan-Meier method, the cumulative incidence of cancer at 7 years was 28% in HRG and 1.2% in LRG patients (P<0.001).ConclusionsThis study supports using Fukuoka criteria to stratify the immediate and long-term risks of pancreatic cancer in presumptive BD-IPMN. The risk of pancreatic cancer was highest during the first year and occurred exclusively in those with "high-risk stigmata" or "worrisome features" criteria. After the first year all BD-IPMN continued to have a low but persistent cancer risk
Activation of an Endogenous Retrovirus-Associated Long Non-Coding RNA in Human Adenocarcinoma
Background
Long non-coding RNAs (lncRNAs) are emerging as molecules that significantly impact many cellular processes and have been associated with almost every human cancer. Compared to protein-coding genes, lncRNA genes are often associated with transposable elements, particularly with endogenous retroviral elements (ERVs). ERVs can have potentially deleterious effects on genome structure and function, so these elements are typically silenced in normal somatic tissues, albeit with varying efficiency. The aberrant regulation of ERVs associated with lncRNAs (ERV-lncRNAs), coupled with the diverse range of lncRNA functions, creates significant potential for ERV-lncRNAs to impact cancer biology.
Methods
We used RNA-seq analysis to identify and profile the expression of a novel lncRNA in six large cohorts, including over 7,500 samples from The Cancer Genome Atlas (TCGA).
Results
We identified the tumor-specific expression of a novel lncRNA that we have named Endogenous retroViral-associated ADenocarcinoma RNA or ‘EVADR’, by analyzing RNA-seq data derived from colorectal tumors and matched normal control tissues. Subsequent analysis of TCGA RNA-seq data revealed the striking association of EVADR with adenocarcinomas, which are tumors of glandular origin. Moderate to high levels of EVADR were detected in 25 to 53% of colon, rectal, lung, pancreas and stomach adenocarcinomas (mean = 30 to 144 FPKM), and EVADR expression correlated with decreased patient survival (Cox regression; hazard ratio = 1.47, 95% confidence interval = 1.06 to 2.04, P = 0.02). In tumor sites of non-glandular origin, EVADR expression was detectable at only very low levels and in less than 10% of patients. For EVADR, a MER48 ERV element provides an active promoter to drive its transcription. Genome-wide, MER48 insertions are associated with nine lncRNAs, but none of the MER48-associated lncRNAs other than EVADR were consistently expressed in adenocarcinomas, demonstrating the specific activation of EVADR. The sequence and structure of the EVADR locus is highly conserved among Old World monkeys and apes but not New World monkeys or prosimians, where the MER48 insertion is absent. Conservation of the EVADR locus suggests a functional role for this novel lncRNA in humans and our closest primate relatives.
Conclusions
Our results describe the specific activation of a highly conserved ERV-lncRNA in numerous cancers of glandular origin, a finding with diagnostic, prognostic and therapeutic implications
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Cool dust heating and temperature mixing in nearby star-forming galaxies
Physical conditions of the interstellar medium in galaxies are closely linked
to the ambient radiation field and the heating of dust grains. In order to
characterize dust properties in galaxies over a wide range of physical
conditions, we present here the radial surface brightness profiles of the
entire sample of 61 galaxies from Key Insights into Nearby Galaxies:
Far-Infrared Survey with Herschel (KINGFISH). The main goal of our work is the
characterization of the grain emissivities, dust temperatures, and interstellar
radiation fields responsible for heating the dust. After fitting the dust and
stellar radial profiles with exponential functions, we fit the far-infrared
spectral energy distribution (SED) in each annular region with
single-temperature modified black bodies using both variable (MBBV) and fixed
(MBBF) emissivity indices beta, as well as with physically motivated dust
models. Results show that while most SED parameters decrease with radius, the
emissivity index beta also decreases with radius in some galaxies, but in
others is increasing, or rising in the inner regions and falling in the outer
ones. Despite the fixed grain emissivity (average beta~ 2.1) of the
physically-motivated models, they are well able to accommodate flat spectral
slopes with beta<= 1. We find that flatter slopes (beta<= 1.5) are associated
with cooler temperatures, contrary to what would be expected from the usual
Tdust-beta degeneracy. This trend is related to variations in Umin since beta
and Umin are very closely linked over the entire range in Umin sampled by the
KINGFISH galaxies: low Umin is associated with flat beta<=1. Both these results
strongly suggest that the low apparent \beta values (flat slopes) in MBBV fits
are caused by temperature mixing along the line-of-sight, rather than by
intrinsic variations in grain properties. Abstract truncated for arXiv.Comment: 28 pages, 20 figures, accepted for publication in A&
Generalized Numerical Renormalization Group for Dynamical Quantities
In this paper we introduce a new approach for calculating dynamical
properties within the numerical renormalization group. It is demonstrated that
the method previously used fails for the Anderson impurity in a magnetic field
due to the absence of energy scale separation. The problem is solved by
evaluating the Green function with respect to the reduced density matrix of the
full system, leading to accurate spectra in agreement with the static
magnetization. The new procedure (denoted as DM-NRG) provides a unifying
framework for calculating dynamics at any temperature and represents the
correct extension of Wilson's original thermodynamic calculation.Comment: 4 pages RevTeX, 6 eps figures include
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