12,272 research outputs found
First order phase transition in the anisotropic quantum orbital compass model
We investigate the anisotropic quantum orbital compass model on an infinite
square lattice by means of the infinite projected entangled-pair state
algorithm. For varying values of the and coupling constants of the
model, we approximate the ground state and evaluate quantities such as its
expected energy and local order parameters. We also compute adiabatic time
evolutions of the ground state, and show that several ground states with
different local properties coexist at . All our calculations are
fully consistent with a first order quantum phase transition at this point,
thus corroborating previous numerical evidence. Our results also suggest that
tensor network algorithms are particularly fitted to characterize first order
quantum phase transitions.Comment: 4 pages, 3 figures, major revision with new result
Gravitational Effects of Quantum Fields in the Interior of a Cylindrical Black Hole
The gravitational back-reaction is calculated for the conformally invariant
scalar field within a black cosmic string interior with cosmological constant.
Using the perturbed metric, the gravitational effects of the quantum field are
calculated. It is found that the perturbations initially strengthen the
singularity. This effect is similar to the case of spherical symmetry (without
cosmological constant). This indicates that the behaviour of quantum effects
may be universal and not dependent on the geometry of the spacetime nor the
presence of a non-zero cosmological constant.Comment: 13 pages, 1 figure, uses AMS package. D.E. solution corrected. Some
qualitative results are change
A review of the potential for competitive cereal cultivars as a tool in integrated weed management
Competitive crop cultivars offer a potentially cheap option to include in integrated weed management strategies (IWM). Although cultivars with high competitive potential have been identified amongst cereal crops, competitiveness has not traditionally been considered a priority for breeding or farmer cultivar choice. The challenge of managing herbicide-resistant weed populations has, however, renewed interest in cultural weed control options, including competitive cultivars. We evaluated the current understanding of the traits that explain variability in competitive ability between cultivars, the relationship between suppression of weed neighbours and tolerance of their presence and the existence of trade-offs between competitive ability and yield in weed-free scenarios. A large number of relationships between competitive ability and plant traits have been reported in the literature, including plant height, speed of development, canopy architecture and partitioning of resources. There is uncertainty over the relationship between suppressive ability and tolerance, although tolerance is a less stable trait over seasons and locations. To realise the potential of competitive crop cultivars as a tool in IWM, a quick and simple-to-use protocol for assessing the competitive potential of new cultivars is required; it is likely that this will not be based on a single trait, but will need to capture the combined effect of multiple traits. A way needs to be found to make this information accessible to farmers, so that competitive cultivars can be better integrated into their weed control programmes
Methanesulfonic Acid in Coastal Antarctic Snow Related to Sea‐ice Extent
Proxy records of biogenic sulfur gas obtained from ice cores suggest that variability in marine biogenic sulfur emissions may reflect changes in climate [Saigne and Legrand, 1987; Legrand et al., 1988, Legrand et al., 1991; Anderson and Charlson, 1991]. Increased sea‐ice extent has previously been proposed as one cause of relatively high methanesulfonic acid (MSA) in glacial‐age ice core samples [Gibson et al., 1990]. We have analyzed MSA, one of the oxidation products of the biogenic sulfur gas dimethylsulfide [Hatakeyama et al., 1985], from snowpit samples recovered from a coastal site in Southern Victoria Land, Antarctica. Time series of MSA correlate significantly with the longest continuous record available of Southern Ocean sea‐ice extent (two decades) [Jacka, 1990]
Sea Level Pressure Variability in the Amundsen Sea Region Inferred from a West Antarctic Glaciochemical Record
Using European Center for Medium-Range Weather Forecasts (ECMWF) numerical operational analyses, sea ice extent records, and station pressure data, we investigate the influence of sea level pressure variability in the Amundsen Sea region on a West Antarctic (Siple Dome) glaciochemical record. Empirical orthogonal function analysis of the high-resolution Siple Dome multivariate ice core chemical time series record (SDEOF1) documents lower tropospheric transport of sea-salt aerosols to the site. During 1985–1994 the SDEOF1 record of high (low) aerosol transport corresponds to anomalously low (high) sea level pressure (SLP) in the Amundsen Sea region. Spatial correlation patterns between ECMWF monthly SLP fields and the annual SDEOF1 record suggest that a majority of sea-salt aerosol is transported to Siple Dome during spring (September, October, and November). Analysis of zonal and meridional wind fields supports the SLP/SDEOF1 correlation and suggests the SDEOF1 record is sensitive to changes in regional circulation strength. No relationship is found between sea ice extent and the SDEOF1 record for the period 1973–1994. To investigate the SDEOF1 record prior to ECMWF coverage, a spring transpolar index (STPI) is created, using normalized SLP records from the New Zealand and South America/Antarctic Peninsula sectors, and is significantly correlated (at least 95% c.l.) with the SDEOF1 record on an annual (r = 0.32, p\u3c 0.001) and interannual (3 years; r = 0.51, p \u3c 0.001) basis. Dominant periodicities (3.3 and 7.1 years) in the annual SDEOF1 record (1890–1994 A.D.) suggest that a portion of the recorded interannual variability may be related tropical/extratropical ENSO teleconnections. Changes in the periodic structure of the full (850–1994 A.D.) Siple Dome record suggests a shift in SLP forcing during the Little Ice Age (∼1400–1900 A.D.) interval
The Murmur of the Sleeping Black Hole: Detection of Nuclear Ultraviolet Variability in LINER Galaxies
LINER nuclei, which are present in many nearby galactic bulges, may be the
manifestation of low-rate or low-radiative-efficiency accretion onto
supermassive central black holes. However, it has been unclear whether the
compact UV nuclear sources present in many LINERs are clusters of massive
stars, rather than being directly related to the accretion process. We have
used HST to monitor the UV variability of a sample of 17 galaxies with LINER
nuclei and compact nuclear UV sources. Fifteen of the 17 galaxies were observed
more than once, with two to five epochs per galaxy, spanning up to a year. We
detect significant variability in most of the sample, with peak-to-peak
amplitudes from a few percent to 50%. In most cases, correlated variations are
seen in two independent bands (F250W and F330W). Comparison to previous UV
measurements indicates, for many objects, long-term variations by factors of a
few over decade timescales. Variability is detected in LINERs with and without
detected compact radio cores, in LINERs that have broad H-alpha wings detected
in their optical spectra (``LINER 1's''), and in those that do not (``LINER
2s''). This variability demonstrates the existence of a non-stellar component
in the UV continuum of all types, and sets a lower limit to the luminosity of
this component. We note a trend in the UV color (F250W/F330W) with spectral
type - LINER 1s tend to be bluer than LINER 2s. This trend may indicate a link
between the shape of the nonstellar continuum and the presence or the
visibility of a broad-line region. In one target, the post-starburst galaxy NGC
4736, we detect variability in a previously noted UV source that is offset by
2.5" (60 pc in projection) from the nucleus. This may be the nearest example of
a binary active nucleus, and of the process leading to black hole merging.Comment: accepted to Ap
Primordial perturbations from slow-roll inflation on a brane
In this paper we quantise scalar perturbations in a Randall-Sundrum-type
model of inflation where the inflaton field is confined to a single brane
embedded in five-dimensional anti-de Sitter space-time. In the high energy
regime, small-scale inflaton fluctuations are strongly coupled to metric
perturbations in the bulk and gravitational back-reaction has a dramatic effect
on the behaviour of inflaton perturbations on sub-horizon scales. This is in
contrast to the standard four-dimensional result where gravitational
back-reaction can be neglected on small scales. Nevertheless, this does not
give rise to significant particle production, and the correction to the power
spectrum of the curvature perturbations on super-horizon scales is shown to be
suppressed by a slow-roll parameter. We calculate the complete first order
slow-roll corrections to the spectrum of primordial curvature perturbations.Comment: 23 pages, 10 figure
The Modified Weighted Slab Technique: Models and Results
In an attempt to understand the source and propagation of galactic cosmic
rays we have employed the Modified Weighted Slab technique along with recent
values of the relevant cross sections to compute primary to secondary ratios
including B/C and Sub-Fe/Fe for different galactic propagation models. The
models that we have considered are the disk-halo diffusion model, the dynamical
halo wind model, the turbulent diffusion model and a model with minimal
reacceleration. The modified weighted slab technique will be briefly discussed
and a more detailed description of the models will be given. We will also
discuss the impact that the various models have on the problem of anisotropy at
high energy and discuss what properties of a particular model bear on this
issue.Comment: LaTeX - AASTEX format, Submitted to ApJ, 8 figures, 20 page
Recommended from our members
Black carbon physical and optical properties across northern India during pre-monsoon and monsoon seasons
Black carbon (BC) is known to have major impacts on both climate and human health and is therefore of global importance, particularly in regions close to large populations that have strong sources. The size-resolved mixing state of BC-containing particles was characterised using a single-particle soot photometer (SP2). The study focusses on the Indo-Gangetic Plain (IGP) during the pre-monsoon and monsoon seasons. Data presented are from the UK Facility for Airborne Atmospheric Measurements BAe-146 research aircraft that performed flights during the pre-monsoon (11 and 12 June) and monsoon (30 June to 11 July) seasons of 2016.
Over the IGP, BC mass concentrations were greater (1.95 µg m−3) compared to north-west India (1.50 µg m−3) and north-east India (0.70 µg m−3) during the pre-monsoon season. Across northern India, two distinct BC modes were recorded; a mode of small BC particles (core diameter <0.16 µm and coating thickness <50 nm) and a mode of moderately coated BC (core diameter <0.22 µm and coating thickness of 50–200 nm). The IGP and north-east India locations exhibited moderately coated black carbon particles with enhanced coating thicknesses, core sizes, mass absorption cross sections, and scattering enhancement values compared to much lower values present in the north-west. The coating thickness and mass absorption cross section increased with altitude (13 %) compared to those in the boundary layer. As the monsoon arrived across the region, mass concentration of BC decreased over the central IGP and north-east locations (38 % and 28 % respectively), whereas for the north-west location BC properties remained relatively consistent. Post-monsoon onset, the coating thickness, core size, mass absorption cross section, and scattering enhancement values were all greatest over the central IGP much like the pre-monsoon season but were considerably reduced over both north-east and north-west India. Increases in mass absorption cross section through the atmospheric column were still present during the monsoon for the north-west and central IGP locations, but less so over the north-east due to lack of long-range transport aerosol aloft. Across the Indo-Gangetic Plain and north-east India during the pre-monsoon and monsoon seasons, solid-fuel (wood burning) emissions form the greatest proportion of BC with moderately coated particles. However, as the monsoon develops in the north-east there was a switch to small uncoated BC particles indicative of traffic emissions, but the solid-fuel emissions remained in the IGP into the monsoon. For both seasons in the north-west, traffic emissions form the greatest proportion of BC particles.
Our findings will prove important for greater understanding of the BC physical and optical properties, with important consequences for the atmospheric radiative forcing of BC-containing particles. The findings will also help constrain the regional aerosol models for a variety of applications such as space-based remote sensing, chemistry transport modelling, air quality, and BC source and emission inventories
Entropy in Black Hole Pair Production
Pair production of Reissner-Nordstrom black holes in a magnetic field can be
described by a euclidean instanton. It is shown that the instanton amplitude
contains an explicit factor of , where is the area of the event
horizon. This is consistent with the hypothesis that measures the
number of black hole states.Comment: 24 pages (harvmac l mode
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