1,265 research outputs found
An Investigation of the Maximum Specimen Thickness for Differential Phase Contrast Lorentz Microscopy
Examination of magnetic domain structure in the transmission electron microscope is generally confined to very thin foils, where the specimen approximates to a pure phase object, and is achieved by the long established methods of Fresnel or Foucault contrast Lorentz microscopy, or by differential phase contrast (DPC) imaging in a scanning transmission electron microscope (STEM).
If no quantitative interpretation of the image is required then magnetic contrast can be observed from thicker foils, and in this paper we describe an attempt to determine experimentally the range of foil thickness over which this is possible. To this end we have examined electropolished foils of single crystal Incalloy using an extended VG HB501 STEM to produce both DPC and Fresnel contrast images of the same area. The foil thickness at points along the domain walls was measured from the change in the Lorentz deflection angle as the STEM probe was moved across the domain wall, and this led to an estimate of ~ 700nm for the limiting thickness at which domain contrast was still visible in the DPC images.
This value is obviously influenced by a number of factors, including the degree of inelastic scattering and the saturation magnetisation of the material, but it is sufficiently high that there might exist a range of thickness over which both transmission and scanning electron microscopes could be used to study the domain structure in the same areas of specimen
Remarks on the tight-binding model of graphene
We address a simple but fundamental issue arising in the study of graphene,
as well as of other systems that have a crystalline structure with more than
one atom per unit cell. For these systems, the choice of the tight-binding
basis is not unique. For monolayer graphene two bases are widely used in the
literature. While the expectation values of operators describing physical
quantities should be independent of basis, the form of the operators may depend
on the basis, especially in the presence of disorder or of an applied magnetic
field. Using the inappropriate form of certain operators may lead to erroneous
physical predictions. We discuss the two bases used to describe monolayer
graphene, as well as the form of the most commonly used operators in the two
bases. We repeat our analysis for the case of bilayer graphene.Comment: 15 pages, 4 figure
Altered hippocampal function in major depression despite intact structure and resting perfusion
Background: Hippocampal volume reductions in major depression have been frequently reported. However, evidence for functional abnormalities in the same region in depression has been less clear. We investigated hippocampal function in depression using functional magnetic resonance imaging (fMRI) and neuropsychological tasks tapping spatial memory function, with complementing measures of hippocampal volume and resting blood flow to aid interpretation. Method: A total of 20 patients with major depressive disorder (MDD) and a matched group of 20 healthy individuals participated. Participants underwent multimodal magnetic resonance imaging (MRI): fMRI during a spatial memory task, and structural MRI and resting blood flow measurements of the hippocampal region using arterial spin labelling. An offline battery of neuropsychological tests, including several measures of spatial memory, was also completed. Results: The fMRI analysis showed significant group differences in bilateral anterior regions of the hippocampus. While control participants showed task-dependent differences in blood oxygen level-dependent (BOLD) signal, depressed patients did not. No group differences were detected with regard to hippocampal volume or resting blood flow. Patients showed reduced performance in several offline neuropsychological measures. All group differences were independent of differences in hippocampal volume and hippocampal blood flow. Conclusions: Functional abnormalities of the hippocampus can be observed in patients with MDD even when the volume and resting perfusion in the same region appear normal. This suggests that changes in hippocampal function can be observed independently of structural abnormalities of the hippocampus in depression
On the correlation between fragility and stretching in glassforming liquids
We study the pressure and temperature dependences of the dielectric
relaxation of two molecular glassforming liquids, dibutyl phtalate and
m-toluidine. We focus on two characteristics of the slowing down of relaxation,
the fragility associated with the temperature dependence and the stretching
characterizing the relaxation function. We combine our data with data from the
literature to revisit the proposed correlation between these two quantities. We
do this in light of constraints that we suggest to put on the search for
empirical correlations among properties of glassformers. In particular, argue
that a meaningful correlation is to be looked for between stretching and
isochoric fragility, as both seem to be constant under isochronic conditions
and thereby reflect the intrinsic effect of temperature
Dynamic heterogeneities in attractive colloids
We study the formation of a colloidal gel by means of Molecular Dynamics
simulations of a model for colloidal suspensions. A slowing down with gel-like
features is observed at low temperatures and low volume fractions, due to the
formation of persistent structures. We show that at low volume fraction the
dynamic susceptibility, which describes dynamic heterogeneities, exhibits a
large plateau, dominated by clusters of long living bonds. At higher volume
fraction, where the effect of the crowding of the particles starts to be
present, it crosses over towards a regime characterized by a peak. We introduce
a suitable mean cluster size of clusters of monomers connected by "persistent"
bonds which well describes the dynamic susceptibility.Comment: 4 pages, 4 figure
Comparison of oxygen-15 PET and transcranial Doppler CO2-reactivity measurements in identifying haemodynamic compromise in patients with symptomatic occlusion of the internal carotid artery
BACKGROUND: Transcranial Doppler (TCD) CO(2)-reactivity and oxygen-15 positron emission tomography (PET) have both been used to measure the cerebral haemodynamic state in patients who may have a compromised blood flow. Our purpose was to investigate whether PET and TCD identify the same patients with an impaired flow state of the brain in patients with internal carotid artery (ICA) occlusion. METHODS: Patients with recent transient ischaemic attack or minor ischaemic stroke associated with ICA occlusion underwent TCD with measurement of CO(2)-reactivity and oxygen-15 PET within a median time interval of 6 days. RESULTS: We included 24 patients (mean age 64 ± 10 years). Seventeen (71%) patients had impaired CO(2)-reactivity (≤20%), of whom six had absent reactivity (0%) or steal (<0%) in the hemisphere ipsilateral to the ICA occlusion. PET of the perfusion state of the hemisphere ipsilateral to the ICA occlusion demonstrated stage 1 haemodynamic compromise (decreased cerebral blood flow (CBF) or increased cerebral blood volume (CBV) without increased oxygen extraction fraction (OEF)) in 13 patients and stage 2 (increased OEF) in 2 patients. In 12 patients (50%), there was agreement between TCD and PET, indicating haemodynamic compromise in 10 and a normal flow state of the brain in 2 patients. There was no significant correlation between CO(2)-reactivity and CBF ipsilateral/contralateral hemispheric ratio (r = 0.168, p value = 0.432), OEF ratio (r = −0.242, p value = 0.255), or CBV/CBF ratio (r = −0.368, p value = 0.077). CONCLUSIONS: In patients with symptomatic ICA occlusion, identification of an impaired flow state of the brain by PET and TCD CO(2)-reactivity shows concordance in only half of the patients
Tunable Nb superconducting resonators based upon a Ne-FIB-fabricated constriction nanoSQUID
Hybrid superconducting--spin systems offer the potential to combine highly
coherent atomic quantum systems with the scalability of superconducting
circuits. To fully exploit this potential requires a high quality-factor
microwave resonator, tunable in frequency and able to operate at magnetic
fields optimal for the spin system. Such magnetic fields typically rule out
conventional Al-based Josephson junction devices that have previously been used
for tunable high- microwave resonators. The larger critical field of niobium
(Nb) allows microwave resonators with large field resilience to be fabricated.
Here, we demonstrate how constriction-type weak links, patterned in parallel
into the central conductor of a Nb coplanar resonator using a neon focused ion
beam (FIB), can be used to implement a frequency-tunable resonator. We study
transmission through two such devices and show how they realise high quality
factor, tunable, field resilient devices which hold promise for future
applications coupling to spin systems
Pinning and switching of magnetic moments in bilayer graphene
We examine the magnetic properties of the localized states induced by lattice
vacancies in bilayer graphene with an unrestricted Hartree-Fock calculation. We
show that with realistic values of the parameters and for experimentally
accessible gate voltages we can have a magnetic switching between an
unpolarized and a fully polarized system.Comment: 9 pages, 4 figure
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