875 research outputs found
Stochastic gauges in quantum dynamics for many-body simulations
Quantum dynamics simulations can be improved using novel quasiprobability
distributions based on non-orthogonal hermitian kernel operators. This
introduces arbitrary functions (gauges) into the stochastic equations, which
can be used to tailor them for improved calculations. A possible application to
full quantum dynamic simulations of BEC's is presented.Comment: 4 pages, 2 figure
Slowing rates of regional exhumation in the western Himalaya: fission track evidence from the Indus Fan
Weuse apatite fission track ages from sediments recovered by the International Ocean Discovery Program in the Laxmi Basin, Arabian Sea, to constrain exhumation rates in the western Himalaya and Karakoram since 15.5 Ma. With the exception of a Triassic population in the youngest 0.93 Ma samples supplied from western Peninsular India, apatite fission track ages are overwhelmingly Cenozoic, largely <25 Ma, consistent with both a HimalayaâKarakoram source and rapid erosion. Comparison of the minimum cooling age of each sample with depositional age (lag time) indicates an acceleration in exhumation between 7.8 and 7.0 Ma, with
lag times shortening from âŒ6.0 Myr at 8.5â7.8 Ma to being within error of zero between 7.0 and
5.7 Ma. Sediment supply at 7.0â5.7 Mawas largely from the Karakoram, and to a lesser extent the Himalaya, based on UâPb zircon ages from the same samples. This time coincides with a period of drying in the Himalayan foreland caused by weaker summermonsoons andWesterly winds. It also correlates with a shift of erosion away from the Karakoram, Kohistan and the Tethyan Himalaya towards more erosion of the Lesser and Greater Himalaya and Nanga Parbat, as shown by zircon UâPb provenance data, and especially after 5.7 Ma based on Nd isotope data. Samples younger than 5.7 Ma have lag times of âŒ4.5 Myr, similar to Holocene Indus delta sediments
Classical Boundary-value Problem in Riemannian Quantum Gravity and Self-dual Taub-NUT-(anti)de Sitter Geometries
The classical boundary-value problem of the Einstein field equations is
studied with an arbitrary cosmological constant, in the case of a compact
() boundary given a biaxial Bianchi-IX positive-definite three-metric,
specified by two radii For the simplest, four-ball, topology of the
manifold with this boundary, the regular classical solutions are found within
the family of Taub-NUT-(anti)de Sitter metrics with self-dual Weyl curvature.
For arbitrary choice of positive radii we find that there are three
solutions for the infilling geometry of this type. We obtain exact solutions
for them and for their Euclidean actions. The case of negative cosmological
constant is investigated further. For reasonable squashing of the three-sphere,
all three infilling solutions have real-valued actions which possess a ``cusp
catastrophe'' structure with a non-self-intersecting ``catastrophe manifold''
implying that the dominant contribution comes from the unique real
positive-definite solution on the ball. The positive-definite solution exists
even for larger deformations of the three-sphere, as long as a certain
inequality between and holds. The action of this solution is
proportional to for large and hence larger radii are
favoured. The same boundary-value problem with more complicated interior
topology containing a ``bolt'' is investigated in a forthcoming paper.Comment: 20 pages, 11 figures; Latex; Revised version with important new
results on real infilling solutions and corrections. To appear in Nuclear
Physics B, issue 648 (1,2), pp. 397-41
Quantum correlated twin atomic beams via photo-dissociation of a molecular Bose-Einstein condensate
We study the process of photo-dissociation of a molecular Bose-Einstein
condensate as a potential source of strongly correlated twin atomic beams. We
show that the two beams can possess nearly perfect quantum squeezing in their
relative numbers.Comment: Corrected LaTeX file layou
Climate and anthropogenic impacts on North American erosion and sediment transport since the Last Glacial Maximum: evidence from the detrital zircon record of the Lower Mississippi Valley, USA
The Mississippi River provides an opportunity to examine models of sediment transport in large alluviated floodplain systems. We test the idea that sources of sandy sediment in such settings are invariable on timescales <104 y because of storage and recycling in the floodplains. To reconstruct the development of the Mississippi sediment load over the past 2500 years we collected sediment from an abandoned point bar complex nearby at False River, Louisiana, USA. We also took annual samples from the lower reaches between 2015 and 2021 to assess changes on that timescale. Optically stimulated luminescence dating indicated that the point bar accreted between 2460 and 860 years ago. Detrital zircon U-Pb dating was used to assess sediment source and variability over time. We confirm a dominant sediment flux from the Rocky Mountain foreland but with higher relative erosion from the Superior Province during the Last Glacial Maximum (LGM) based on existing data from the Gulf of Mexico. There have been resolvable changes in the sources of sediment particularly since the LGM and after 860 years ago, but also over shorter, even sub-annual timescales in the recent past. These changes may reflect seasonal weather or storm events in the headwater regions and imply limited floodplain buffering of the sand load. In recent times this may reflect the installation of levees in the lower reaches, suppressing reworking. Changes over 102â103 y time periods may be related to changes in climate (e.g., the Medieval and Roman warm periods) and to the development of agriculture across North America after ~2000 years ago. Detrital zircon dating is an effective provenance tool and does not appear to be strongly biased by the grain size of the sediment in this setting
Controls on erosion patterns and sediment transport in a monsoonal, tectonically quiescent drainage, Song Gianh, central Vietnam
The Song Gianh is a small-sized (~3500 km2), monsoon-dominated river in northern central Vietnam that can be used to understand how topography and climate control continental erosion. We present major element concentrations, together with Sr and Nd isotopic compositions, of siliciclastic bulk sediments to define sediment provenance and chemical weathering intensity. These data indicate preferential sediment generation in the steep, wetter upper reaches of the Song Gianh. In contrast, detrital zircon U-Pb ages argue for significant flux from the drier, northern Rao Tro tributary. We propose that this mismatch represents disequilibrium in basin erosion patterns driven by changing monsoon strength and the onset of agriculture across the region. Detrital apatite fission track and 10Be data from modern sediment support slowing of regional bedrock exhumation rates through the Cenozoic. If the Song Gianh is representative of coastal Vietnam then the coastal mountains may have produced around 132 000â158 000 km3 of the sediment now preserved in the Song Hong-Yinggehai Basin (17â21 of the total), the primary depocenter of the Red River. This flux does not negate the need for drainage capture in the Red River to explain the large Cenozoic sediment volumes in that basin but does partly account for the discrepancy between preserved and eroded sediment volumes. OSL ages from terraces cluster in the Early Holocene (7.4â8.5 ka), Pre-Industrial (550â320 year BP) and in the recent past (ca. 150 year BP). The older terraces reflect high sediment production driven by a strong monsoon, whereas the younger are the product of anthropogenic impact on the landscape caused by farming. Modern river sediment is consistently more weathered than terrace sediment consistent with reworking of old weathered soils by agricultural disruption
Climatic and glacial impact on erosion patterns and sediment provenance in the Himalayan rain shadow, Zanskar River, NW India
Erosion is a key step in the destruction and recycling of the continental crust yet its primary drivers continue to be debated. The relative balance between climatic and solid Earth forces in determining erosion patterns and rates, and in turn orogenic architecture, is unresolved. The monsoon-dominated frontal Himalaya is a classic example of how surface processes may drive focused denudation and potentially control structural evolution. We investigate whether there is a clear relationship between climate and erosion in the drier Himalayan rain shadow of northwest India where a coupled climate-erosion relationship is less clear. We present a new integrated dataset combining bulk petrography, geomorphometric analysis, detrital U-Pb zircon geochronology, and bulk Nd and Sr isotope geochemistry from modern river sediments that provides constraints on spatial patterns of sediment production and transport in the Zanskar River. Zanskar River sands are dominated by Greater Himalayan detritus sourced from the glaciated Stod River catchment that represents only 13% of the total basin area. Prevalent zircon peaks from the Cambro-Ordovician (440â500 Ma) and Mississippian-Permian (245â380 Ma) indicate more abundant pre-Himalayan granitoids in the northwest Himalaya than in the central and eastern Himalaya. Erosion from the widely-exposed Tethyan Himalaya, however, appears modest. Spatial patterns of erosion do not correlate with highest channel steepness. Our data demonstrate that Zanskar differs from the monsoon-soaked frontal Himalaya and the arid, extremely slow-eroding orogenic interior in that focused erosion and sediment production are driven by glaciers. Subsequent remobilization of glacially-derived sediments is likely controlled by monsoonal rainfall and we suggest sediment reworking plays an important role. These data support strong climatic control on modern orogenic erosion on the periphery of the Himalayan rain shadow
Moving the Needle Toward Fair Compensation in Pediatric Nephrology
Remuneration issues are a substantial threat to the long-term stability of the pediatric nephrology workforce. It is uncertain whether the pediatric nephrology workforce will meet the growing needs of children with kidney disease without a substantial overhaul of the current reimbursement policies. In contrast to adult nephrology, the majority of pediatric nephrologists practice in an academic setting affiliated with a university and/or children's hospital. The pediatric nephrology service line is crucial to maintaining the financial health and wellness of a comprehensive children's hospital. However, in the current fee-for-service system, the clinical care for children with kidney disease is neither sufficiently valued, nor appropriately compensated. Current compensation models derived from the relative value unit (RVU) system contribute to the structural biases inherent in the current inequitable payment system. The perceived negative financial compensation is a significant driver of waning trainee interest in the field which is one of the least attractive specialties for students, with a significant proportion of training spots going unfilled each year and relatively stagnant growth rate as compared to the other pediatric subspecialties. This article reviews the current state of financial compensation issues plaguing the pediatric nephrology subspecialty. We further outline strategies for pediatric nephrologists, hospital administrators, and policy-makers to improve the landscape of financial reimbursement to pediatric subspecialists. A physician compensation model is proposed which aligns clinical activity with alternate metrics for current non-RVU producing activities that harmonizes hospital and personal mission statements
Variational Approach to the Modulational Instability
We study the modulational stability of the nonlinear Schr\"odinger equation
(NLS) using a time-dependent variational approach. Within this framework, we
derive ordinary differential equations (ODEs) for the time evolution of the
amplitude and phase of modulational perturbations. Analyzing the ensuing ODEs,
we re-derive the classical modulational instability criterion. The case
(relevant to applications in optics and Bose-Einstein condensation) where the
coefficients of the equation are time-dependent, is also examined
Gaussian quantum operator representation for bosons
We introduce a Gaussian quantum operator representation, using the most general possible multimode Gaussian operator basis. The representation unifies and substantially extends existing phase-space representations of density matrices for Bose systems and also includes generalized squeezed-state and thermal bases. It enables first-principles dynamical or equilibrium calculations in quantum many-body systems, with quantum uncertainties appearing as dynamical objects. Any quadratic Liouville equation for the density operator results in a purely deterministic time evolution. Any cubic or quartic master equation can be treated using stochastic methods
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