2,262 research outputs found
Spin-1 effective Hamiltonian with three degenerate orbitals: An application to the case of V_2O_3
Motivated by recent neutron and x-ray observations in V_2O_3, we derive the
effective Hamiltonian in the strong coupling limit of an Hubbard model with
three degenerate t_{2g} states containing two electrons coupled to spin S = 1,
and use it to re-examine the low-temperature ground-state properties of this
compound. An axial trigonal distortion of the cubic states is also taken into
account. Since there are no assumptions about the symmetry properties of the
hopping integrals involved, the resulting spin-orbital Hamiltonian can be
generally applied to any crystallographic configuration of the transition metal
ion giving rise to degenerate t_{2g} orbitals. Specializing to the case of
V_2O_3 we consider the antiferromagnetic insulating phase. We find two
variational regimes, depending on the relative size of the correlation energy
of the vertical pairs and the in-plane interaction energy. The former favors
the formation of stable molecules throughout the crystal, while the latter
tends to break this correlated state. We determine in both cases the minimizing
orbital solutions for various spin configurations, and draw the corresponding
phase diagrams. We find that none of the symmetry-breaking stable phases with
the real spin structure presents an orbital ordering compatible with the
magnetic space group indicated by very recent observations of non-reciprocal
x-ray gyrotropy in V_2O_3. We do however find a compatible solution with very
small excitation energy in two distinct regions of the phase space, which might
turn into the true ground state of V_2O_3 due to the favorable coupling with
the lattice. We illustrate merits and drawbacks of the various solutions and
discuss them in relation to the present experimental evidence.Comment: 36 pages, 19 figure
Fermipy: An open-source Python package for analysis of Fermi-LAT Data
Fermipy is an open-source python framework that facilitates analysis of data
collected by the Fermi Large Area Telescope (LAT). Fermipy is built on the
Fermi Science Tools, the publicly available software suite provided by NASA for
the LAT mission. Fermipy provides a high-level interface for analyzing LAT data
in a simple and reproducible way. The current feature set includes methods for
extracting spectral energy distributions and lightcurves, generating test
statistic maps, finding new source candidates, and fitting source position and
extension. Fermipy leverages functionality from other scientific python
packages including NumPy, SciPy, Matplotlib, and Astropy and is organized as a
community-developed package following an open-source development model. We
review the current functionality of Fermipy and plans for future development.Comment: Proc. 35th ICRC, Busan, South Korea, PoS(ICRC2017)82
Variational study of the antiferromagnetic insulating phase of V2O3 based on Nth order Muffin-Tin-Orbitals
Motivated by recent results of th order muffin-tin orbital (NMTO)
implementation of the density functional theory (DFT), we re-examine
low-temperature ground-state properties of the anti-ferromagnetic insulating
phase of vanadium sesquioxide VO. The hopping matrix elements obtained
by the NMTO-downfolding procedure differ significantly from those previously
obtained in electronic structure calculations and imply that the in-plane
hopping integrals are as important as the out-of-plane ones. We use the NMTO
hopping matrix elements as input and perform a variational study of the ground
state. We show that the formation of stable molecules throughout the crystal is
not favorable in this case and that the experimentally observed magnetic
structure can still be obtained in the atomic variational regime. However the
resulting ground state (two electrons occupying the degenerate
doublet) is in contrast with many well established experimental observations.
We discuss the implications of this finding in the light of the non-local
electronic correlations certainly present in this compound.Comment: 7 pages, 2 figure
Valence-Bond Crystal, and Lattice Distortions in a Pyrochlore Antiferromagnet with Orbital Degeneracy
We discuss the ground state properties of a spin 1/2 magnetic ion with
threefold orbital degeneracy on a highly frustrated pyrochlore
lattice, like Ti ion in B-spinel MgTiO. We formulate an
effective spin-orbital Hamiltonian and study its low energy sector by
constructing several exact-eigenstates in the limit of vanishing Hund's
coupling. We find that orbital degrees of freedom modulate the spin-exchange
energies, release the infinite spin-degeneracy of pyrochlore structure, and
drive the system to a non-magnetic spin-singlet manifold. The latter is a
collection of spin-singlet dimers and is, however, highly degenerate with
respect of dimer orientations. This ``orientational'' degeneracy is then lifted
by a magneto-elastic interaction that optimizes the previous energy gain by
distorting the bonds in suitable directions and leading to a tetragonal phase.
In this way a valence bond crystal state is formed, through the condensation of
dimers along helical chains running around the tetragonal c-axis, as actually
observed in MgTiO. The orbitally ordered pattern in the dimerized phase
is predicted to be of ferro-type along the helices and of antiferro-type
between them. Finally, through analytical considerations as well as numerical
ab-initio simulations, we predict a possible experimental tool for the
observation of such an orbital ordering, through resonant x-ray scattering.Comment: 15 pages, 8 figure
Statin therapy in critical illness : an international survey of intensive care physicians' opinions, attitudes and practice
Background
Pleotropic effects of statins on inflammation are hypothesised to attenuate the severity of and possibly prevent the occurrence of the host inflammatory response to pathogen and infection-related acute organ failure. We conducted an international survey of intensive care physicians in Australia, New Zealand (ANZ) and United Kingdom (UK). The aims of the survey were to assess the current prescribing practice patterns, attitudes towards prescribing statin therapy in critically ill patients and opinions on the need for an interventional trial of statin therapy in critically ill patients.
Methods
Survey questions were developed through an iterative process. An expert group reviewed the resulting 26 items for face and content validity and clarity. The questions were further refined following pilot testing by ICU physicians from Australia, Canada and the UK. We used the online Smart SurveyTM software to administer the survey.
Results
Of 239 respondents (62 from ANZ and 177 from UK) 58% worked in teaching hospitals; most (78.2%) practised in ‘closed’ units with a mixed medical and surgical case mix (71.0%). The most frequently prescribed statins were simvastatin (77.6%) in the UK and atorvastatin (66.1%) in ANZ. The main reasons cited to explain the choice of statin were preadmission prescription and pharmacy availability. Most respondents reported never starting statins to prevent (65.3%) or treat (89.1%) organ dysfunction. Only a minority (10%) disagreed with a statement that the risks of major side effects of statins when prescribed in critically ill patients were low. The majority (84.5%) of respondents strongly agreed that a clinical trial of statins for prevention is needed. More than half (56.5%) favoured rates of organ failure as the primary outcome for such a trial, while a minority (40.6%) favoured mortality.
Conclusions
Despite differences in type of statins prescribed, critical care physicians in the UK and ANZ reported similar prescription practices. Respondents from both communities agreed that a trial is needed to test whether statins can prevent the onset of new organ failure in patients with sepsis
Knowledge Translation Approaches in Occupational Therapy: A Scoping Review
A gap exists between what is known in occupational therapy and how occupational therapists practice. Knowledge translation approaches have been designed to bridge the gap between research and practice. Currently there is limited literature exploring the knowledge translation approaches being implemented specifically within occupational therapy. Therefore, a scoping review was completed to provide an overview of the existing literature on knowledge translation approaches in occupational therapy. Three electronic databases were searched. All peer-reviewed quantitative and qualitative articles which met the inclusion criteria were reviewed. A data extraction table aided the analysis and synthesis of the literature. The initial search returned 565 articles, of which 59 were selected based on the inclusion criteria. Comprehensive screening of the 59 articles resulted in 16 peer-reviewed articles being included in the review. A range of knowledge translation methods have been used in occupational therapy including face-to-face education sessions, online resources, and clinical audits of documentation with feedback to the therapists. A small number of studies used a knowledge translation framework to guide the knowledge translation approach. Findings from this study highlighted that knowledge translation approaches are useful for overcoming challenges and changing practice. In particular, a knowledge translation framework may be useful to guide the design and implementation of a knowledge translation initiative. Although there were a number of knowledge transfer strategies used in the studies, face to face education was used most often. However, a combination of transfer strategies had the most lasting impact on practice change. Including participants’ perspectives in the planning, delivery, and evaluation was beneficial. More research is needed to identify how the use of a theoretical framework might support positive outcomes for knowledge translation
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Urban form, infrastructure, and spatial organization in the Roman Empire
Although there has been considerable interest in the nature of ancient cities, it has been difficult to identify and explore quantitative patterns in their design and amenities. In this article, we offer a model for the relationship between the size and infrastructure of settlements, before testing it against measures of the urban form of cities in the Roman
Empire. This allows us to advocate a more consistent approach to investigating settlements that is capable of not only incorporating sites with divergent experiences into the same model, but also exposing their similarities and differences
An effective spin-orbital Hamiltonian for the double perovskite SrFeW O: Derivation of the phase diagram
We formulate a superexchange theory of insulating double-perovskite compounds
such as SrFeWO. An effective spin-orbital Hamiltonian is derived in the
strong coupling limit of Hubbard model for d-electrons on Fe and W ions. The
relevant degrees of freedom are the spins S=2 and the three-fold orbital
degeneracy of Fe-ions. W-sites are integrated out by means of a
fourth-order perturbative expansion. The magnetically and orbitally ordered
ground states of the effective Hamiltonia n are discussed as a function of the
model parameters. We show that for realistic values of such parameters the
ground state is antiferromagnetic, as experimentally observed. The order found
is of type-II, consisting of \{111\} ferromagnetic planes stac ked
antiferromagnetically. The orbital order energy scale found is one order of
magnitude less than the spi n one.Comment: 12 pages, 4 figure
Designing Engaging Learning Experiences in Programming
In this paper we describe work to investigate the creation of engaging programming learning experiences. Background research informed the design of four fieldwork studies to explore how programming tasks could be framed to motivate learners. Our empirical findings from these four field studies are summarized here, with a particular focus upon one – Whack a Mole – which compared the use of a physical interface with the use of a screen-based equivalent interface to obtain insights into what made for an engaging learning experience. Emotions reported by two sets of participant undergraduate students were analyzed, identifying the links between the emotions experienced during programming and their origin. Evidence was collected of the very positive emotions experienced by learners programming with a physical interface (Arduino) in comparison with a similar program developed using a screen-based equivalent interface. A follow-up study provided further evidence of the motivation of personalized design of programming tangible physical artefacts. Collating all the evidence led to the design of a set of ‘Learning Dimensions’ which may provide educators with insights to support key design decisions for the creation of engaging programming learning experiences
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