46 research outputs found
Surface Conductivity of Si(100) and Ge(100) Surfaces Determined from Four-Point Transport Measurements Using an Analytical N-Layer Conductance Model
An analytical N-layer model for charge transport close to a surface is
derived from the solution of Poisson's equation and used to describe
distance-dependent electrical four-point measurements on the microscale. As the
N-layer model comprises a surface channel, multiple intermediate layers and a
semi-infinite bulk, it can be applied to semiconductors in combination with a
calculation of the near-surface band-bending to model very precisely the
measured four-point resistance on the surface of a specific sample and to
extract a value for the surface conductivity. For describing four-point
measurements on sample geometries with mixed 2D-3D conduction channels often a
very simple parallel-circuit model has so far been used in the literature, but
the application of this model is limited, as there are already significant
deviations, when it is compared to the lowest possible case of the N-layer
model, i.e. the 3-layer model. Furthermore, the N-layer model is applied to
published distance-dependent four-point resistance measurements obtained with a
multi-tip scanning tunneling microscope (STM) on Germanium(100) and
Silicon(100) with different bulk doping concentrations resulting in the
determination of values for the surface conductivities of these materials.Comment: 11 pages, 6 color figure
Surface and Step Conductivities on Si(111) Surfaces
Four-point measurements using a multi-tip scanning tunneling microscope (STM)
are carried out in order to determine surface and step conductivities on
Si(111) surfaces. In a first step, distance-dependent four-point measurements
in the linear configuration are used in combination with an analytical
three-layer model for charge transport to disentangle the 2D surface
conductivity from non-surface contributions. A termination of the Si(111)
surface with either Bi or H results in the two limiting cases of a pure 2D or
3D conductance, respectively. In order to further disentangle the surface
conductivity of the step-free surface from the contribution due to atomic
steps, a square four-probe configuration is applied as function of the rotation
angle. In total this combined approach leads to an atomic step conductivity of
and
to a step-free surface conductivity of for the Si(111)-(77) surface.Comment: Main paper: 5 pages, 4 figures, Supplemental material: 6 pages, 3
figures. The Supplemental Material contains details on the sample preparation
and measurement procedure, additional experimental results for Si(111)
samples with different doping levels, and the description of the three-layer
conductance mode
Lifting paddle wheel : full-size protype craft development.
This report describes the construction, testing and analysis of a large scale lifting paddlewheel craft.
No large scale vehicle of this style exists to date; a small scale version has previously been successful. A 4wd farm bike provided the basis for the prototype with modifications for use in an aquatic environment. Following open water tests, ongoing developmental work was carried out. This included the analysis and prediction of the crafts performance, operation of the lifting paddlewheels and comparison of the successful small scale craft to that of the prototype constructed.
The prototype did not operate as the lifting paddlewheels were intended. Predictions initially showed a deficit in power comparable with the craft comparison which showed a large difference in the power to weight of the crafts. Modifications to increase the power and following tests proved to also be unproductive. Analysis of the farm bike prototype dynamics showed a possibility of successful operation should certain criteria be met
From Vulnerable Plaque to Vulnerable Patient
Atherosclerotic cardiovascular disease results in >19 million deaths annually, and coronary heart disease accounts for the majority of this toll. Despite major advances in treatment of coronary heart disease patients, a large number of victims of the disease who are apparently healthy die suddenly without prior symptoms. Available screening and diagnostic methods are insufficient to identify the victims before the event occurs. The recognition of the role of the vulnerable plaque has opened new avenues of opportunity in the field of cardiovascular medicine. This consensus document concludes the following. (1) Rupture-prone plaques are not the only vulnerable plaques. All types of atherosclerotic plaques with high likelihood of thrombotic complications and rapid progression should be considered as vulnerable plaques. We propose a classification for clinical as well as pathological evaluation of vulnerable plaques. (2) Vulnerable plaques are not the only culprit factors for the development of acute coronary syndromes, myocardial infarction, and sudden cardiac death. Vulnerable blood (prone to thrombosis) and vulnerable myocardium (prone to fatal arrhythmia) play an important role in the outcome. Therefore, the term "vulnerable patient" may be more appropriate and is proposed now for the identification of subjects with high likelihood of developing cardiac events in the near future. (3) A quantitative method for cumulative risk assessment of vulnerable patients needs to be developed that may include variables based on plaque, blood, and myocardial vulnerability. In Part I of this consensus document, we cover the new definition of vulnerable plaque and its relationship with vulnerable patients. Part II of this consensus document will focus on vulnerable blood and vulnerable myocardium and provide an outline of overall risk assessment of vulnerable patients. Parts I and II are meant to provide a general consensus and overviews the new field of vulnerable patient. Recently developed assays (eg, C-reactive protein), imaging techniques (eg, CT and MRI), noninvasive electrophysiological tests (for vulnerable myocardium), and emerging catheters (to localize and characterize vulnerable plaque) in combination with future genomic and proteomic techniques will guide us in the search for vulnerable patients. It will also lead to the development and deployment of new therapies and ultimately to reduce the incidence of acute coronary syndromes and sudden cardiac death. We encourage healthcare policy makers to promote translational research for screening and treatment of vulnerable patients