900 research outputs found
Metallic ferromagnetism without exchange splitting
In the band theory of ferromagnetism there is a relative shift in the
position of majority and minority spin bands due to the self-consistent field
due to opposite spin electrons. In the simplest realization, the Stoner model,
the majority and minority spin bands are rigidly shifted with respect to each
other. Here we consider models at the opposite extreme, where there is no
overall shift of the energy bands. Instead, upon spin polarization one of the
bands broadens relative to the other. Ferromagnetism is driven by the resulting
gain in kinetic energy. A signature of this class of mechanisms is that a
transfer of spectral weight in optical absorption from high to low frequencies
occurs upon spin polarization. We show that such models arise from generalized
tight binding models that include off-diagonal matrix elements of the Coulomb
interaction. For certain parameter ranges it is also found that reentrant
ferromagnetism occurs. We examine properties of these models at zero and finite
temperatures, and discuss their possible relevance to real materials
Synthesizing attractors of Hindmarsh-Rose neuronal systems
In this paper a periodic parameter switching scheme is applied to the
Hindmarsh-Rose neuronal system to synthesize certain attractors. Results show
numerically, via computer graphic simulations, that the obtained synthesized
attractor belongs to the class of all admissible attractors for the
Hindmarsh-Rose neuronal system and matches the averaged attractor obtained with
the control parameter replaced with the averaged switched parameter values.
This feature allows us to imagine that living beings are able to maintain vital
behavior while the control parameter switches so that their dynamical behavior
is suitable for the given environment.Comment: published in Nonlinear Dynamic
Evaluation of the CABLEv2.3.4 land surface model coupled to NUâWRFv3.9.1.1 in simulating temperature and precipitation means and extremes over CORDEX AustralAsia within a WRF physics ensemble
The Community Atmosphere Biosphere Land Exchange (CABLE) model is a thirdâgeneration land surface model (LSM). CABLE is commonly used as a standâalone LSM, coupled to the Australian Community Climate and Earth Systems Simulator global climate model and coupled to the Weather Research and Forecasting (WRF) model for regional applications. Here, we evaluate an updated version of CABLE within a WRF physics ensemble over the COordinated Regional Downscaling EXperiment (CORDEX) AustralAsia domain. The ensemble consists of different cumulus, radiation and planetary boundary layer (PBL) schemes. Simulations are carried out within the NASA Unified WRF modeling framework, NUâWRF. Our analysis did not identify one configuration that consistently performed the best for all diagnostics and regions. Of the cumulus parameterizations the GrellâFreitas cumulus scheme consistently overpredicted precipitation, while the new Tiedtke scheme was the best in simulating the timing of precipitation events. For the radiation schemes, the RRTMG radiation scheme had a general warm bias. For the PBL schemes, the YSU scheme had a warm bias, and the MYJ PBL scheme a cool bias. Results are strongly dependent on the region of interest, with the northern tropics and southwest Western Australia being more sensitive to the choice of physics options compared to southeastern Australia which showed less overall variation and overall better performance across the ensemble. Comparisons with simulations using the Unified Noah LSM showed that CABLE in NUâWRF has a more realistic simulation of evapotranspiration when compared to GLEAM estimates
Understanding and engineering beneficial plantâmicrobe interactions:Plant growth promotion in energy crops
Plant production systems globally must be optimized to produce stable high yields from limited land under changing and variable climates. Demands for food, animal feed, and feedstocks for bioenergy and biorefining applications, are increasing with population growth, urbanization and affluence. Low-input, sustainable, alternatives to petrochemical-derived fertilizers and pesticides are required to reduce input costs and maintain or increase yields, with potential biological solutions having an important role to play. In contrast to crops that have been bred for food, many bioenergy crops are largely undomesticated, and so there is an opportunity to harness beneficial plantâmicrobe relationships which may have been inadvertently lost through intensive crop breeding. Plantâmicrobe interactions span a wide range of relationships in which one or both of the organisms may have a beneficial, neutral or negative effect on the other partner. A relatively small number of beneficial plantâmicrobe interactions are well understood and already exploited; however, others remain understudied and represent an untapped reservoir for optimizing plant production. There may be near-term applications for bacterial strains as microbial biopesticides and biofertilizers to increase biomass yield from energy crops grown on land unsuitable for food production. Longer term aims involve the design of synthetic genetic circuits within and between the host and microbes to optimize plant production. A highly exciting prospect is that endosymbionts comprise a unique resource of reduced complexity microbial genomes with adaptive traits of great interest for a wide variety of applications
Dynamics of the Hubbard model: a general approach by time dependent variational principle
We describe the quantum dynamics of the Hubbard model at semi-classical
level, by implementing the Time-Dependent Variational Principle (TDVP)
procedure on appropriate macroscopic wavefunctions constructed in terms of
su(2)-coherent states. Within the TDVP procedure, such states turn out to
include a time-dependent quantum phase, part of which can be recognized as
Berry's phase. We derive two new semi-classical model Hamiltonians for
describing the dynamics in the paramagnetic, superconducting, antiferromagnetic
and charge density wave phases and solve the corresponding canonical equations
of motion in various cases. Noticeably, a vortex-like ground state phase
dynamics is found to take place for U>0 away from half filling. Moreover, it
appears that an oscillatory-like ground state dynamics survives at the Fermi
surface at half-filling for any U. The low-energy dynamics is also exactly
solved by separating fast and slow variables. The role of the time-dependent
phase is shown to be particularly interesting in the ordered phases.Comment: ReVTeX file, 38 pages, to appear on Phys. Rev.
Supersymmetry without R-parity : Constraints from Leptonic Phenomenology
R-parity conservation is an {\it ad hoc} assumption in the most popular
version of the supersymmetric standard model. Most studies of models which do
allow for R-parity violation have been restricted to various limiting
scenarios. The single-VEV parametrization used in this paper provides a
workable framework to analyze phenomenology of the most general theory of SUSY
without R-parity. We perform a comprehensive study of leptonic phenomenology at
tree-level. Experimental constraints on various processes are studied
individually and then combined to yield regions of admissible parameter space.
In particular, we show that large R-parity violating bilinear couplings are not
ruled out, especially for large .Comment: 56 pages Revtex with figures incorporated; typos (including
transcription typo in Table II) and minor corrections; proof-read version, to
appear in Phys. Rev.
A Model for the Development of the Rhizobial and Arbuscular Mycorrhizal Symbioses in Legumes and Its Use to Understand the Roles of Ethylene in the Establishment of these two Symbioses
We propose a model depicting the development of nodulation and arbuscular mycorrhizae. Both processes are dissected into many steps, using Pisum sativum L. nodulation mutants as a guideline. For nodulation, we distinguish two main developmental programs, one epidermal and one cortical. Whereas Nod factors alone affect the cortical program, bacteria are required to trigger the epidermal events. We propose that the two programs of the rhizobial symbiosis evolved separately and that, over time, they came to function together. The distinction between these two programs does not exist for arbuscular mycorrhizae development despite events occurring in both root tissues. Mutations that affect both symbioses are restricted to the epidermal program. We propose here sites of action and potential roles for ethylene during the formation of the two symbioses with a specific hypothesis for nodule organogenesis. Assuming the epidermis does not make ethylene, the microsymbionts probably first encounter a regulatory level of ethylene at the epidermisâoutermost cortical cell layer interface. Depending on the hormone concentrations there, infection will either progress or be blocked. In the former case, ethylene affects the cortex cytoskeleton, allowing reorganization that facilitates infection; in the latter case, ethylene acts on several enzymes that interfere with infection thread growth, causing it to abort. Throughout this review, the difficulty of generalizing the roles of ethylene is emphasized and numerous examples are given to demonstrate the diversity that exists in plants
Status of a Supersymmetric Flavour Violating Solution to the Solar Neutrino Puzzle with Three Generations
We present a general study of a three neutrino flavour transition model based
on the supersymmetric interactions which violate R-parity. These interactions
induce flavour violating scattering reactions between solar matter and
neutrinos. The model does not contain any vacuum mass or mixing angle for the
first generation neutrino. Instead, the effective mixing in the first
generation is induced via the new interactions. The model provides a natural
interpretation of the atmospheric neutrino anomaly, and is consistent with
reactor experiments. We determine all R-parity violating couplings which can
contribute to the effective neutrino oscillations, and summarize the present
laboratory bounds. Independent of the specific nature of the (supersymmetric)
flavour violating model, the experimental data on the solar neutrino rates and
the recoil electron energy spectrum are inconsistent with the theoretical
predictions. The confidence level of the -analysis ranges between and . The incompatibility, is due to the new SNO
results, and excludes the present model. We conclude that a non-vanishing
vacuum mixing angle for the first generation neutrino is necessary in our
model. We expect this also to apply to the solutions based on other flavour
violating interactions having constraints of the same order of magnitude.Comment: 17 pages, Latex fil
Parenting through grief: A cross-sectional study of recently bereaved adults with minor children
Background: Grieving adults raising parentally-bereaved minor children experience persistently elevated symptoms of depression and grief. However, the factors associated with their mental health outcomes are not well understood. Aim: To investigate the psychosocial and demographic characteristics associated with grief distress and depressive symptom severity in bereaved adults with minor children. Design: Cross-sectional, web-based survey. Setting/participants: Eight hundred forty-five bereaved adults raising minor (age <18 years) children who had experienced the death of a co-parent. Primary outcomes were grief distress (Prolonged Grief Disorder-13), depressive symptoms (Patient-Reported Outcomes Measurement Information System-Depression), and widowed parenting self-efficacy (WPSES). Results: Mean grief scores were 33.5; mean depression scores were 58.3. Among the 690 individuals more than 6 months bereaved, 132 (19.3%) met criteria for prolonged grief disorder. In adjusted models, participants reporting higher grief scores were more recently bereaved, identified as mothers, non-Caucasian, had lower education and income, and had not anticipated their co-parentâs death. The statistical modeling results for depression scores were similar to grief scores except that depression was not associated with anticipation of co-parent death. Parents reporting lower WPSES scores had higher grief and depression scores. Retrospective assessments of more intense parenting worries at the time of co-parent death were also associated with higher grief and depression scores. Conclusions: For bereaved adults with minor children, unanticipated co-parent death was linked with higher grief distress but not symptoms of depression. Addressing parenting concerns may represent a common pathway for improving the mental health of parentally-bereaved families
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