127 research outputs found
Coupling of spin and lattice modes in the S=1/2 two-dimensional antiferromagnet KVO with magneto-dielectric couplings
Lattice dynamics and magnetic excitations are investigated to elucidate the
origin of magneto-dielectric effects in the S=1/2 two-dimensional quantum spin
compound KVO. We find evidence for lattice instabilities at 110 K
and 60 K as optical phonon anomalies and a soft mode at 26 cm in A
symmetry. Two-magnon excitations in B symmetry show an unconventional
double-peak structure and temperature dependence. This suggests the existence
of a split mode near the zone boundary caused by a mixing of spin and lattice
modes.Comment: 7 pages, 7 figures, accepted for publication in PR
Magnetic field dependence of the exciton energy in a quantum disk
The groundstate energy and binding energy of an exciton, confined in a^M
quantum disk, are calculated as a function of an external magnetic field. The
confinement potential is a hard wall of finite height. The diamagnetic shift is
investigated for magnetic fields up to 40. Our results are applied to
self-assembled quantum dots and very good
agreement with experiments is obtained. Furthermore, we investigated the
influence of the dot size on the diamagnetic shift by changing the disk radius.
The exciton excited states are found as a function of the magnetic field. The
relative angular momentum is not a quantum number and changes with the magnetic
field strength.Comment: 10 pages, 17 figure
Lattice dynamical analogies and differences between SrTiO3 and EuTiO3 revealed by phonon-dispersion relations and double-well potentials
A comparative analysis of the structural phase transitions of EuTiO3 and
SrTiO3 (at TS = 282 and 105 K, respectively) is made on the basis of
phonon-dispersion and density functional calculations. The phase transition of
EuTiO3 is predicted to arise from the softening of a transverse acoustic
zone-boundary mode caused by the rotations of the TiO6 octahedra, as also found
for the phase transition of SrTiO3. While the temperature dependence of the
soft mode is similar in both compounds, their elastic properties differ
drastically due to a large difference in the double-well potentials associated
with the soft zone boundary-acoustic mode.Comment: 16 pages, 6 figure
Inelastic Light Scattering From Correlated Electrons
Inelastic light scattering is an intensively used tool in the study of
electronic properties of solids. Triggered by the discovery of high temperature
superconductivity in the cuprates and by new developments in instrumentation,
light scattering both in the visible (Raman effect) and the X-ray part of the
electromagnetic spectrum has become a method complementary to optical
(infrared) spectroscopy while providing additional and relevant information.
The main purpose of the review is to position Raman scattering with regard to
single-particle methods like angle-resolved photoemission spectroscopy (ARPES),
and other transport and thermodynamic measurements in correlated materials.
Particular focus will be placed on photon polarizations and the role of
symmetry to elucidate the dynamics of electrons in different regions of the
Brillouin zone. This advantage over conventional transport (usually measuring
averaged properties) indeed provides new insights into anisotropic and complex
many-body behavior of electrons in various systems. We review recent
developments in the theory of electronic Raman scattering in correlated systems
and experimental results in paradigmatic materials such as the A15
superconductors, magnetic and paramagnetic insulators, compounds with competing
orders, as well as the cuprates with high superconducting transition
temperatures. We present an overview of the manifestations of complexity in the
Raman response due to the impact of correlations and developing competing
orders. In a variety of materials we discuss which observations may be
understood and summarize important open questions that pave the way to a
detailed understanding of correlated electron systems.Comment: 62 pages, 48 figures, to appear in Rev. Mod. Phys. High-resolution
pdf file available at http://onceler.uwaterloo.ca/~tpd/RMP.pd
Axial forces and bending moments in the loaded rabbit tibia in vivo
<p>Abstract</p> <p>Background</p> <p>Different animal models are used as fracture models in orthopaedic research prior to implant use in humans, although biomechanical forces can differ to a great extend between species due to variable anatomic conditions, particularly with regard to the gait. The rabbit is an often used fracture model, but biomechanical data are very rare. The objective of the present study was to measure axial forces, bending moments, and bending axis directly in the rabbit tibia <it>in vivo</it>. The following hypothesis was tested: Axial forces and bending moments in the mid-diaphysis of rabbit tibia differ from other experimental animals or indirectly calculated data.</p> <p>Methods</p> <p>A minifixateur system with 4 force sensors was developed and attached to rabbit tibia (<it>n </it>= 4), which were subsequently ostectomised. Axial forces, bending moments and bending angles were calculated telemetrically during weight bearing in motion between 6 and 42 days post operation.</p> <p>Results</p> <p>Highest single values were 201% body weight [% bw] for axial forces and 409% bw cm for bending moments. Whereas there was a continous decrease in axial forces over time after day 10 (<it>P </it>= 0.03 on day 15), a decrease in bending moments was inconsistent (<it>P </it>= 0.03 on day 27). High values for bending moments were frequently, but not consistently, associated with high values for axial forces.</p> <p>Conclusion</p> <p>Axial forces in rabbit tibia exceeded axial forces in sheep, and differed from indirectly calculated data. The rabbit is an appropriate fracture model because axial loads and bending moments in rabbit tibia were more closely to human conditions than in sheep tibia as an animal model.</p
The local and systemic response to SARS-CoV-2 infection in children and adults
While a substantial proportion of adults infected with SARS-CoV-2 progress to develop severe disease, children rarely manifest respiratory complications. Therefore, understanding differences in the local and systemic response to SARS-CoV-2 infection between children and adults may provide important clues about the pathogenesis of SARS-CoV-2 infection. To address this, we first generated a healthy reference multi-omics single cell data set from children (n=30) in whom we have profiled triple matched samples: nasal and tracheal brushings and PBMCs, where we track the developmental changes for 42 airway and 31 blood cell populations from infancy, through childhood to adolescence. This has revealed the presence of naive B and T lymphocytes in neonates and infants with a unique gene expression signature bearing hallmarks of innate immunity. We then contrast the healthy reference with equivalent data from severe paediatric and adult COVID-19 patients (total n=27), from the same three types of samples: upper and lower airways and blood. We found striking differences: children with COVID-19 as opposed to adults had a higher proportion of innate lymphoid and non-clonally expanded naive T cells in peripheral blood, and a limited interferon-response signature. In the airway epithelium, we found the highest viral load in goblet and ciliated cells and describe a novel inflammatory epithelial cell population. These cells represent a transitional regenerative state between secretory and ciliated cells; they were found in healthy children and were enriched in paediatric and adult COVID-19 patients. Epithelial cells display an antiviral and neutrophil-recruiting gene signature that is weaker in severe paediatric versus adult COVID-19. Our matched blood and airway samples allowed us to study the spatial dynamics of infection. Lastly, we provide a user-friendly interface for this data1 as a highly granular reference for the study of immune responses in airways and blood in children
Local and systemic responses to SARS-CoV-2 infection in children and adults
It is not fully understood why COVID-19 is typically milder in children1–3. To examine differences in response to SARS-CoV-2 infection in children and adults, we analysed paediatric and adult COVID-19 patients and healthy controls (total n=93) using single-cell multi-omic profiling of matched nasal, tracheal, bronchial and blood samples. In healthy paediatric airways, we observed cells already in an interferon-activated state, that upon SARS-CoV-2 infection was further induced especially in airway immune cells. We postulate that higher paediatric innate interferon-responses restrict viral replication and disease progression. The systemic response in children was characterised by increases in naive lymphocytes and a depletion of natural killer cells, while in adults cytotoxic T cells and interferon-stimulated subpopulations were significantly increased. We provide evidence that dendritic cells initiate interferon signaling in early infection, and identify novel epithelial cell states that associate with COVID-19 and age. Our matching nasal and blood data showed a strong interferon response in the airways with the induction of systemic interferon-stimulated populations, which were massively reduced in paediatric patients. Together, we provide several mechanisms that explain the milder clinical syndrome observed in children
Single-cell multi-omics analysis of the immune response in COVID-19
Analysis of human blood immune cells provides insights into the coordinated response to viral infections such as severe acute respiratory syndrome coronavirus 2, which causes coronavirus disease 2019 (COVID-19). We performed single-cell transcriptome, surface proteome and T and B lymphocyte antigen receptor analyses of over 780,000 peripheral blood mononuclear cells from a cross-sectional cohort of 130 patients with varying severities of COVID-19. We identified expansion of nonclassical monocytes expressing complement transcripts (CD16+C1QA/B/C+) that sequester platelets and were predicted to replenish the alveolar macrophage pool in COVID-19. Early, uncommitted CD34+ hematopoietic stem/progenitor cells were primed toward megakaryopoiesis, accompanied by expanded megakaryocyte-committed progenitors and increased platelet activation. Clonally expanded CD8+ T cells and an increased ratio of CD8+ effector T cells to effector memory T cells characterized severe disease, while circulating follicular helper T cells accompanied mild disease. We observed a relative loss of IgA2 in symptomatic disease despite an overall expansion of plasmablasts and plasma cells. Our study highlights the coordinated immune response that contributes to COVID-19 pathogenesis and reveals discrete cellular components that can be targeted for therapy
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