287 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
Halbach Magnets for Magnetic Resonance
This review is a compilation of relevant concepts in designing Halbach
multipoles for magnetic resonance applications. The main focus is on providing
practical guidelines to plan, design and build such magnets. Therefore,
analytical equations are presented for estimating the magnetic field from ideal
to realistic systems. Various strategies of homogenizing magnetic fields are
discussed together with concepts of opening such magnets without force, or
combining them for variable fields. Temperature compensation and other
practical aspects are also reviewed. For magnetic resonance two polarities (di-
and quadrupole) are of main interest, but higher polarities are also included.Comment: 37 pages, 17 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
Polynomial Chaos Expansion method as a tool to evaluate and quantify field homogeneities of a novel waveguide RF Wien Filter
For the measurement of the electric dipole moment of protons and deuterons, a
novel waveguide RF Wien filter has been designed and will soon be integrated at
the COoler SYnchrotron at J\"ulich. The device operates at the harmonic
frequencies of the spin motion. It is based on a waveguide structure that is
capable of fulfilling the Wien filter condition ()
\textit{by design}. The full-wave calculations demonstrated that the waveguide
RF Wien filter is able to generate high-quality RF electric and magnetic
fields. In reality, mechanical tolerances and misalignments decrease the
simulated field quality, and it is therefore important to consider them in the
simulations. In particular, for the electric dipole moment measurement, it is
important to quantify the field errors systematically. Since Monte-Carlo
simulations are computationally very expensive, we discuss here an efficient
surrogate modeling scheme based on the Polynomial Chaos Expansion method to
compute the field quality in the presence of tolerances and misalignments and
subsequently to perform the sensitivity analysis at zero additional
computational cost.Comment: 12 pages, 19 figure
Neue Aspekte der Chemie des supersauren Mediums HF/GeF4 und BeitrÀge zur Protonierung sehr starker organischer SÀuren
Ischémie mésentérique étendue associée à la prise excessive de naratriptan et de jus de pamplemousse
We reported the case of a 61-year-old woman, who has been hospitalized in ICU because of an extensive mesenteric ischaemia, involving the small bowel, secondary to a naratriptan overuse. This mesenteric ischaemia was complicated by multiple organ failure and was responsible for extensive small bowel resection and left colectomy. A concomitant abundant absorption of grapefruit juice, a well-known P450 inhibitor, may have enhanced this naratriptan toxicity. This case underscore that an abdominal pain occurring in the context of headache treatment may be related to a mesenteric ischaemia
Electromagnetic Simulation and Design of a Novel Waveguide RF Wien Filter for Electric Dipole Moment Measurements of Protons and Deuterons
The conventional Wien filter is a device with orthogonal static magnetic and
electric fields, often used for velocity separation of charged particles. Here
we describe the electromagnetic design calculations for a novel waveguide RF
Wien filter that will be employed to solely manipulate the spins of protons or
deuterons at frequencies of about 0.1 to 2 MHz at the COoler SYnchrotron COSY
at J\"ulich. The device will be used in a future experiment that aims at
measuring the proton and deuteron electric dipole moments, which are expected
to be very small. Their determination, however, would have a huge impact on our
understanding of the universe.Comment: 10 pages, 10 figures, 4 table
How cold is the junction of a millikelvin scanning tunnelling microscope?
We employ a scanning tunnelling microscope (STM) cooled to millikelvin
temperatures by an adiabatic demagnetization refrigerator (ADR) to perform
scanning tunnelling spectroscopy (STS) on an atomically clean surface of
Al(100) in a superconducting state using normal-metal and superconducting STM
tips. Varying the ADR temperatures between 30 mK and 1.2 K, we show that the
temperature of the STM junction is decoupled from the temperature of the
surrounding environment . Simulating the STS data with the
theory, we determine that K, while the
fitting of the superconducting gap spectrum yields the lowest mK.Comment: 12 pages, 10 figure
Preparation and Characterization of Protonated Fumaric Acid
Fumaric acid was reacted with the binary superacidic systems HF/SbF5 and HF/AsF5. The O,O'âdiprotonated [C4H6O4]2+([MF6]â)2 (M = As, Sb) and the Oâmonoprotonated [C4H5O4]+[MF6]â (M = As, Sb) species are formed depending on the stoichiometric ratio of the Lewis acid to fumaric acid. The colorless salts were characterized by lowâtemperature vibrational spectroscopy. In case of the hexafluoridoantimonates singleâcrystal Xâray structure analyses were carried out. The [C4H6O4]2+([SbF6]â)2 crystallizes in the monoclinic space group C2/c with four formula units per unit cell and [C4H5O4]+[SbF6]â crystallizes in the triclinic space group P1 with one formula unit per unit cell. The protonation of fumaric acid does not cause a notable change of the C=C bond length. The experimental data are discussed together with quantum chemical calculations of the cations [C4H6O4 · 4 HF]2+ and [C4H6O4 · 2 H2CO · 2 HF]2+
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