4,607 research outputs found
Cryogenic Microwave Imaging of Metal-Insulator Transition in Doped Silicon
We report the instrumentation and experimental results of a cryogenic
scanning microwave impedance microscope. The microwave probe and the scanning
stage are located inside the variable temperature insert of a helium cryostat.
Microwave signals in the distance modulation mode are used for monitoring the
tip-sample distance and adjusting the phase of the two output channels. The
ability to spatially resolve the metal-insulator transition in a doped silicon
sample is demonstrated. The data agree with a semi-quantitative finite-element
simulation. Effects of the thermal energy and electric fields on local charge
carriers can be seen in the images taken at different temperatures and DC
biases.Comment: 10 pages, 5 Figures, Accepted to Review of Scientific Instrumen
Calibration of shielded microwave probes using bulk dielectrics
A stripline-type near-field microwave probe is microfabricated for microwave
impedance microscopy. Unlike the poorly shielded coplanar probe that senses the
sample tens of microns away, the stripline structure removes the stray fields
from the cantilever body and localizes the interaction only around the
focused-ion beam deposited Pt tip. The approaching curve of an oscillating tip
toward bulk dielectrics can be quantitatively simulated and fitted to the
finite-element analysis result. The peak signal of the approaching curve is a
measure of the sample dielectric constant and can be used to study unknown bulk
materials.Comment: 10 pages, 3 figure
Kitchen Counter Narratives
This major research report is the written reflection accompanying Kitchen Counter
Narratives (2017), a 35-minute documentary film on stories shared by immigrants in
the act of cooking and sharing food. Amidst Canada150 celebrations, the intention
of the film resists the project of multiculturalism through the every day kitchen
counter narratives of immigrants. While the film holds the audio-visual
documentation of the stories, this report responds back to the edited footage and
process through reflecting upon the filmmaker’s implication in and relationship to
the project. Guided by critical pedagogy and expanded by critical food pedagogy,
challenges around these frameworks in the process are identified and explored to
offer a reflexive descriptive account of care taken and work put into creating the
project by those involved. These challenges include building relationships within a
research framework, authenticity and performativity, highlighting the “mundane”,
and unpacking stories
Modeling of a Cantilever-Based Near-Field Scanning Microwave Microscope
We present a detailed modeling and characterization of our scalable microwave
nanoprobe, which is a micro-fabricated cantilever-based scanning microwave
probe with separated excitation and sensing electrodes. Using finite-element
analysis, the tip-sample interaction is modeled as small impedance changes
between the tip electrode and the ground at our working frequencies near 1GHz.
The equivalent lumped elements of the cantilever can be determined by
transmission line simulation of the matching network, which routes the
cantilever signals to 50 Ohm feed lines. In the microwave electronics, the
background common-mode signal is cancelled before the amplifier stage so that
high sensitivity (below 1 atto-Farad capacitance changes) is obtained.
Experimental characterization of the microwave probes was performed on
ion-implanted Si wafers and patterned semiconductor samples. Pure electrical or
topographical signals can be realized using different reflection modes of the
probe.Comment: 7 figure
Microwave imaging of mesoscopic percolating network in a manganite thin film
Many unusual behaviors in complex oxides are deeply associated with the
spontaneous emergence of microscopic phase separation. Depending on the
underlying mechanism, the competing phases can form ordered or random patterns
at vastly different length scales. Using a microwave impedance microscope, we
observed an orientation-ordered percolating network in strained Nd0.5Sr0.5MnO3
thin films with a large period of 100 nm. The filamentary metallic domains
align preferentially along certain crystal axes of the substrate, suggesting
the anisotropic elastic strain as the key interaction in this system. The local
impedance maps provide microscopic electrical information of the hysteretic
behavior in strained thin film manganites, suggesting close connection between
the glassy order and the colossal magnetoresistance effects at low
temperatures.Comment: 4 pages,4 figure
Ultra-thin Topological Insulator Bi2Se3 Nanoribbons Exfoliated by Atomic Force Microscopy
Ultra-thin topological insulator nanostructures, in which coupling between
top and bottom surface states takes place, are of great intellectual and
practical importance. Due to the weak Van der Waals interaction between
adjacent quintuple layers (QLs), the layered bismuth selenide (Bi2Se3), a
single Dirac-cone topological insulator with a large bulk gap, can be
exfoliated down to a few QLs. In this paper, we report the first controlled
mechanical exfoliation of Bi2Se3 nanoribbons (> 50 QLs) by an atomic force
microscope (AFM) tip down to a single QL. Microwave impedance microscopy is
employed to map out the local conductivity of such ultra-thin nanoribbons,
showing drastic difference in sheet resistance between 1~2 QLs and 4~5 QLs.
Transport measurement carried out on an exfoliated (\leq 5 QLs) Bi2Se3 device
shows non-metallic temperature dependence of resistance, in sharp contrast to
the metallic behavior seen in thick (> 50 QLs) ribbons. These AFM-exfoliated
thin nanoribbons afford interesting candidates for studying the transition from
quantum spin Hall surface to edge states
Depression, anxiety, substance misuse and self-harm in children and young people with rare chronic liver disease
The burden of mental illness in young people with chronic liver disease is not known. In this population cohort study in England, we identified 358 individuals (aged ≤25 years) diagnosed with autoimmune hepatitis or liver disease related to cystic fibrosis and 1541 propensity-score-matched controls. By the first year of follow-up, the cumulative burden of psychiatric events in participants with liver disease was high compared with controls: anxiety disorder (6.87 per 100 individuals [95% CI 4.00-9.73] v. 2.22 [95% CI 1.37-3.07]), depression (5.10 [95% CI 2.83-7.37] v. 0.86 [95% CI 0.53-1.19]), substance misuse (10.61 [95% CI 9.50-11.73] v. 1.23 [95% CI 0.71-1.75]) and self-harm (3.09 [95% CI 1.12-5.05] v. 0.20 [95% CI 0.07-0.33]). Participants with liver disease had a 2-fold increase (OR = 1.94, 95% CI 1.45-2.58), a 2.5-fold increase (OR = 2.59, 95% CI 1.91-3.50) and 4.4-fold increase (OR = 4.44; 95% CI 3.46-5.71) in the risk of anxiety, depression and substance misuse, respectively. These findings highlight the need for effective intervention in psychiatric disorders in young people with rare liver disease
AFM-compatible near-field scanning microwave microscope with separated excitation and sensing probes
We present the design and experimental results of a near-field scanning
microwave microscope (NSMM) working at a frequency of 1GHz. Our microscope is
unique in that the sensing probe is separated from the excitation electrode to
significantly suppress the common-mode signal. Coplanar waveguides were
patterned onto a silicon nitride cantilever interchangeable with atomic force
microscope (AFM) tips, which are robust for high speed scanning. In the contact
mode that we are currently using, the numerical analysis shows that contrast
comes from both the variation in local dielectric properties and the sample
topography. Our microscope demonstrates the ability to achieve high resolution
microwave images on buried structures, as well as nano-particles, nano-wires,
and biological samples.Comment: 14 pages, 7 figures, submitted to Review of Scientific Instrument
Sex and Diffusion Tensor Imaging of White Matter in Schizophrenia: A Systematic Review Plus Meta-analysis of the Corpus Callosum
Sex is considered an understudied variable in health research. Schizophrenia is a brain disorder with known sex differences in epidemiology and clinical presentation. We systematically reviewed the literature for sex-based differences of diffusion properties of white matter tracts in schizophrenia. We then conducted a meta-analysis examining sex-based differences in the genu and splenium of the corpus callosum in schizophrenia. Medline and Embase were searched to identify relevant papers. Studies fulfilling the following criteria were included: (1) included individuals with a diagnosis of schizophrenia, (2) included a control group of healthy individuals, (3) included both sexes in the patient and the control groups, (4) used diffusion tensor imaging, and (5) involved analyzing metrics of white matter microstructural integrity. Fractional anisotropy (FA) was used as the measure of interest in the meta-analysis. Of 730 studies reviewed, 75 met the inclusion criteria. Most showed no effect of sex, however, those that did found either that females have lower FA than males, or that the effect of disease in females is larger than that in males. The findings of the meta-analysis in the corpus callosum supported this result. There is a recognized need for studies on schizophrenia with a sufficient sample of female patients. Lack of power undermines the ability to detect sex-based differences. Understanding the sex-specific impact of illness on neural circuits may help inform development of new treatments, and improvement of existing interventions
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