110 research outputs found
Incommensurate antiferromagnetic fluctuations in single-crystalline LiFeAs studied by inelastic neutron scattering
We present an inelastic neutron scattering study on single-crystalline LiFeAs
devoted to the characterization of the incommensurate antiferromagnetic
fluctuations at . Time-of-flight
measurements show the presence of these magnetic fluctuations up to an energy
transfer of 60 meV, while polarized neutrons in combination with longitudinal
polarization analysis on a triple-axis spectrometer prove the pure magnetic
origin of this signal. The normalization of the magnetic scattering to an
absolute scale yields that magnetic fluctuations in LiFeAs are by a factor
eight weaker than the resonance signal in nearly optimally Co-doped
BaFeAs, although a factor two is recovered due to the split peaks owing
to the incommensurability. The longitudinal polarization analysis indicates
weak spin space anisotropy with slightly stronger out-of-plane component
between 6 and 12 meV. Furthermore, our data suggest a fine structure of the
magnetic signal most likely arising from superposing nesting vectors.Comment: 9 pages, 8 figure
Spin-orbit excitons in CoO
CoO has an odd number of electrons in its unit cell, and therefore is
expected to be metallic. Yet, CoO is strongly insulating owing to significant
electronic correlations, thus classifying it as a Mott insulator. We
investigate the magnetic fluctuations in CoO using neutron spectroscopy. The
strong and spatially far-reaching exchange constants reported in [Sarte et al.
Phys. Rev. B 98 024415 (2018)], combined with the single-ion spin-orbit
coupling of similar magnitude [Cowley et al. Phys. Rev. B 88, 205117 (2013)]
results in significant mixing between spin-orbit levels in the low
temperature magnetically ordered phase. The high degree of entanglement,
combined with the structural domains originating from the Jahn-Teller
structural distortion at 300 K, make the magnetic excitation spectrum
highly structured in both energy and momentum. We extend previous theoretical
work on PrTl [Buyers et al. Phys. Rev. B 11, 266 (1975)] to construct a
mean-field and multi-level spin exciton model employing the aforementioned spin
exchange and spin-orbit coupling parameters for coupled Co ions on a
rocksalt lattice. This parameterization, based on a tetragonally distorted
type-II antiferromagnetic unit cell, captures both the sharp low energy
excitations at the magnetic zone center, and the energy broadened peaks at the
zone boundary. However, the model fails to describe the momentum dependence of
the excitations at high energy transfers, where the neutron response decays
faster with momentum than the Co form factor. We discuss such a failure
in terms of a possible breakdown of localized spin-orbit excitons at high
energy transfers.Comment: (main text - 21 pages, 12 figures; supplementary information - 15
pages, 3 figures, to be published in Phys. Rev. B
Magnetic Excitations of Undoped Iron Oxypnictides
We study the magnetic excitations of undoped iron oxypnictides using a
three-dimensional Heisenberg model with single-ion anisotropy. Analytic forms
of the spin wave dispersion, velocities, and structure factor are given. Aside
from quantitative comparisons which can be made to inelastic neutron scattering
experiments, we also give qualitative criteria which can distinguish various
regimes of coupling strength. The magnetization reduction due to quantum zero
point fluctuations shows clear dependence on the c-axis coupling.Comment: 4 pages, 5 figures, to appear in Frontiers of Physics in China: a
special issue on Iron-based superconductor
Patient, health service factors and variation in mortality following resuscitated out-of-hospital cardiac arrest in acute coronary syndrome : analysis of the Myocardial Ischaemia National Audit Project
Aims
To determine patient and health service factors associated with variation in hospital mortality among resuscitated cases of out-of-hospital cardiac arrest (OHCA) with acute coronary syndrome (ACS).
Methods
In this cohort study, we used the Myocardial Ischaemia National Audit Project database to study outcomes in patients hospitalised with resuscitated OHCA due to ACS between 2003 and 2015 in the United Kingdom. We analysed variation in inter-hospital mortality and used hierarchical multivariable regression models to examine the association between patient and health service factors with hospital mortality.
Results
We included 17604 patients across 239 hospitals. Overall hospital mortality was 28.7%. In 94 hospitals that contributed at least 60 cases, mortality by hospital ranged from 10.7% to 66.3% (median 28.6%, IQR 23.2% to 39.1%)). Patient and health service factors explained 36.1% of this variation.
After adjustment for covariates, factors associated with higher hospital mortality included increasing serum glucose, ST-Elevation myocardial infarction (STEMI) diagnosis, and initial admission to a primary percutaneous coronary intervention (pPCI) capable hospital. Hospital OHCA volume was not associated with mortality. The key modifiable factor associated with lower mortality was early reperfusion therapy in STEMI patients.
Conclusion
There was wide variation in inter-hospital mortality following resuscitated OHCA due to ACS that was only partially explained by patient and health service factors. Hospital OHCA volume and pPCI capability were not associated with lower mortality. Early reperfusion therapy was associated with lower mortality in STEMI patients
Variation in outcome of hospitalised patients with out-of-hospital cardiac arrest from acute coronary syndrome : a cohort study
Background
Each year, approximately 30,000 people have an out-of-hospital cardiac arrest (OHCA) that is treated by UK ambulance services. Across all cases of OHCA, survival to hospital discharge is less than 10%. Acute coronary syndrome (ACS) is a common cause of OHCA.
Objectives
To explore factors that influence survival in patients who initially survive an OHCA attributable to ACS.
Data source
Data collected by the Myocardial Ischaemia National Audit Project (MINAP) between 2003 and 2015.
Participants
Adult patients who had a first OHCA attributable to ACS and who were successfully resuscitated and admitted to hospital.
Main outcome measures
Hospital mortality, neurological outcome at hospital discharge, and time to all-cause mortality.
Methods
We undertook a cohort study using data from the MINAP registry. MINAP is a national audit that collects data on patients admitted to English, Welsh and Northern Irish hospitals with myocardial ischaemia. From the data set, we identified patients who had an OHCA. We used imputation to address data missingness across the data set. We analysed data using multilevel logistic regression to identify modifiable and non-modifiable factors that affect outcome.
Results
Between 2003 and 2015, 1,127,140 patient cases were included in the MINAP data set. Of these, 17,604 OHCA cases met the study inclusion criteria. Overall hospital survival was 71.3%. Across hospitals with at least 60 cases, hospital survival ranged from 34% to 89% (median 71.4%, interquartile range 60.7–76.9%). Modelling, which adjusted for patient and treatment characteristics, could account for only 36.1% of this variability. For the primary outcome, the key modifiable factors associated with reduced mortality were reperfusion treatment [primary percutaneous coronary intervention (pPCI) or thrombolysis] and admission under a cardiologist. Admission to a high-volume cardiac arrest hospital did not influence survival. Sensitivity analyses showed that reperfusion was associated with reduced mortality among patients with a ST elevation myocardial infarction (STEMI), but there was no evidence of a reduction in mortality in patients who did not present with a STEMI.
Limitations
This was an observational study, such that unmeasured confounders may have influenced study findings. Differences in case identification processes at hospitals may contribute to an ascertainment bias.
Conclusions
In OHCA patients who have had a cardiac arrest attributable to ACS, there is evidence of variability in survival between hospitals, which cannot be fully explained by variables captured in the MINAP data set. Our findings provide some support for the current practice of transferring resuscitated patients with a STEMI to a hospital that can deliver pPCI. In contrast, it may be reasonable to transfer patients without a STEMI to the nearest appropriate hospital.
Future work
There is a need for clinical trials to examine the clinical effectiveness and cost-effectiveness of invasive reperfusion strategies in resuscitated OHCA patients of cardiac cause who have not had a STEMI.
Funding
The National Institute for Health Research Health Services and Delivery Research programme
Spin-orbital correlations from complex orbital order in MgVO
MgVO is a spinel based on magnetic V ions which host both
spin () and orbital () moments. Owing to the underlying
pyrochlore coordination of the magnetic sites, the spins in MgVO
only antiferromagnetically order once the frustrating interactions imposed by
the lattice are broken through an orbitally-driven structural
distortion at T 60 K. Consequently, a N\'eel transition occurs
at T 40 K. Low temperature spatial ordering of the electronic
orbitals is fundamental to both the structural and magnetic properties, however
considerable discussion on whether it can be described by complex or real
orbital ordering is ambiguous. We apply neutron spectroscopy to resolve the
nature of the orbital ground state and characterize hysteretic spin-orbital
correlations using x-ray and neutron diffraction. Neutron spectroscopy finds
multiple excitation bands and we parameterize these in terms of a multi-level
(or excitonic) theory based on the orbitally degenerate ground state.
Meaningful for the orbital ground state, we report an "optical-like" mode at
high energies that we attribute to a crystal-field-like excitation from the
spin-orbital =2 ground state manifold to an excited =1 energy
level. We parameterize the magnetic excitations in terms of a Hamiltonian with
spin-orbit coupling and local crystalline electric field distortions resulting
from deviations from perfect octahedra surrounding the V ions. We
suggest that this provides compelling evidence for complex orbital order in
MgVO. We then apply the consequences of this model to understand
hysteretic effects in the magnetic diffuse scattering where we propose that
MgVO displays a high temperature orbital memory of the low
temperature spin order.Comment: 21 pages and 13 figure
Antiferromagnetic fluctuations in Fe(Se1-xTex)0.92 (x = 0.75, 1) observed by inelastic neutron scattering
Motivated by the discovery of superconductivity in F-doped LaFeAsO, we
investigated the magnetic fluctuations in a related compound Fe(Se1-xTex)0.92
(x = 0.75, 1) using neutron scattering techniques. Non-superconducting FeTe0.92
shows antiferromagnetic (AF) ordering with the ordering vector |Q| = 0.97 A-1
(Q = (0.5,0,0.5)) at TN ~ 70 K and a structural transition between tetragonal
and monoclinic phases. In the AF monoclinic phase, the low-energy spectral
weight is suppressed, indicating a possible gap formation. On the other hand,
in the paramagnetic tetragonal phase, we observed a pronounced magnetic
fluctuation around at |Q| ~ 0.92 A-1, which is slightly smaller than the
commensurate value. In Fe(Se0.25Te0.75)0.92, which does not show magnetic
ordering and shows superconductivity at Tc ~ 8 K, we observed that a magnetic
fluctuation is located at |Q| ~ 0.9 A-1 at low energies and shifts to a higher
value of |Q| ~ 1.2 A-1 at higher energies. The latter value is close to a
reciprocal lattice vector Q = (0.5,0.5,0.5) or (0.5,0.5,0), where the AF
fluctuations are observed in other FeAs-based materials. The existence of this
common characteristic in different Fe-based superconductors suggests that the
AF fluctuations may play a important role in superconductivity.Comment: 4 pages, 4 figure
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