117 research outputs found
Electrostatics of Vortices in Type II Superconductors
In a type II superconductor the gap variation in the core of a vortex line
induces a local charge modulation. Accounting for metallic screening, we
determine the line charge of individual vortices and calculate the electric
field distribution in the half space above a field penetrated superconductor.
The resulting field is that of an atomic size dipole , is the Bohr radius, acting
on a force microscope in the pico to femto Newton range.Comment: 9 pages, late
On the action potential as a propagating density pulse and the role of anesthetics
The Hodgkin-Huxley model of nerve pulse propagation relies on ion currents
through specific resistors called ion channels. We discuss a number of
classical thermodynamic findings on nerves that are not contained in this
classical theory. Particularly striking is the finding of reversible heat
changes, thickness and phase changes of the membrane during the action
potential. Data on various nerves rather suggest that a reversible density
pulse accompanies the action potential of nerves. Here, we attempted to explain
these phenomena by propagating solitons that depend on the presence of
cooperative phase transitions in the nerve membrane. These transitions are,
however, strongly influenced by the presence of anesthetics. Therefore, the
thermodynamic theory of nerve pulses suggests a explanation for the famous
Meyer-Overton rule that states that the critical anesthetic dose is linearly
related to the solubility of the drug in the membranes.Comment: 13 pages, 8 figure
Exchange Effects in the Invar Hardening: as a test case
An increase of the critical resolved shear stress of Invar alloys (Invar
hardening) with a lowering temperature is explained. The effect is caused by a
growth of the exchange interaction between dangling -electron states of
dislocation cores and paramagnetic obstacles (e.g., Ni atoms in FeNi alloys)
which occurs below the Curie temperature. The spins of the two electrons align
along the magnetization due to the exchange interaction with the surrounding
atoms of the ferromagnetic. The exchange interaction between the dislocations
and obstacles is enhanced in Invars due to a strong growth of the magnetic
moments of atoms under the action of elastic strains near the dislocation
cores. Parameters characterizing the exchange interaction are determined for
the case of the FeNi Invar. The influence of the internal
magnetic field on the dislocation detachment from the obstacles is taken into
account. The obtained temperature dependence of the critical resolved shear
stress in the FeNi Invar agrees well with the available
experimental data. Experiments facilitating a further check of the theoretical
model are suggested.Comment: 8 pages, 2 figure
Soft Stylus Probes for Scanning Electrochemical Microscopy
A soft stylus microelectrode probe has been developed to carry out scanning electrochemical microscopy (SECM) of rough, tilted, and large substrates in contact mode. It is fabricated by first ablating a microchannel in a polyethylene terephthalate thin film and filling it with a conductive carbon ink. After curing the carbon track and lamination with a polymer film, the V- shaped stylus was cut thereby forming a probe, with the cross section of the carbon track at the tip being exposed either by UVphotoablation machining or by blade cutting followed by polishing to produce a crescent moon- shaped carbon microelectrode. The probe properties have been assessed by cyclic voltammetry, approach curves, and line scans over electrochemically active and inactive substrates of different roughness. The influence of probe bending on contact mode imaging was then characterized using simple patterns. Boundary element method simulations were employed to rationalize the distance-dependent electrochemical response of the soft stylus probes
Flux-Induced Vortex in Mesoscopic Superconducting Loops
We predict the existence of a quantum vortex for an unusual situation. We
study the order parameter in doubly connected superconducting samples embedded
in a uniform magnetic field. For samples with perfect cylindrical symmetry, the
order parameter has been known for long and no vortices are present in the
linear regime. However, if the sample is not symmetric, there exist ranges of
the field for which the order parameter vanishes along a line, parallel to the
field. In many respects, the behavior of this line is qualitatively different
from that of the vortices encountered in type II superconductivity. For samples
with mirror symmetry, this flux-induced vortex appears at the thin side for
small fluxes and at the opposite side for large fluxes. We propose direct and
indirect experimental methods which could test our predictions.Comment: 6 pages, Latex, 4 figs., uses RevTex, extended to situations far from
cylindrical symmetr
The influence of anesthetics, neurotransmitters and antibiotics on the relaxation processes in lipid membranes
In the proximity of melting transitions of artificial and biological
membranes fluctuations in enthalpy, area, volume and concentration are
enhanced. This results in domain formation, changes of the elastic constants,
changes in permeability and slowing down of relaxation processes. In this study
we used pressure perturbation calorimetry to investigate the relaxation time
scale after a jump into the melting transition regime of artificial lipid
membranes. This time corresponds to the characteristic rate of domain growth.
The studies were performed on single-component large unilamellar and
multilamellar vesicle systems with and without the addition of small molecules
such as general anesthetics, neurotransmitters and antibiotics. These drugs
interact with membranes and affect melting points and profiles. In all systems
we found that heat capacity and relaxation times are related to each other in a
simple manner. The maximum relaxation time depends on the cooperativity of the
heat capacity profile and decreases with a broadening of the transition. For
this reason the influence of a drug on the time scale of domain formation
processes can be understood on the basis of their influence on the heat
capacity profile. This allows estimations of the time scale of domain formation
processes in biological membranes.Comment: 12 pages, 6 figure
Phase transitions in biological membranes
Native membranes of biological cells display melting transitions of their
lipids at a temperature of 10-20 degrees below body temperature. Such
transitions can be observed in various bacterial cells, in nerves, in cancer
cells, but also in lung surfactant. It seems as if the presence of transitions
slightly below physiological temperature is a generic property of most cells.
They are important because they influence many physical properties of the
membranes. At the transition temperature, membranes display a larger
permeability that is accompanied by ion-channel-like phenomena even in the
complete absence of proteins. Membranes are softer, which implies that
phenomena such as endocytosis and exocytosis are facilitated. Mechanical signal
propagation phenomena related to nerve pulses are strongly enhanced. The
position of transitions can be affected by changes in temperature, pressure, pH
and salt concentration or by the presence of anesthetics. Thus, even at
physiological temperature, these transitions are of relevance. There position
and thereby the physical properties of the membrane can be controlled by
changes in the intensive thermodynamic variables. Here, we review some of the
experimental findings and the thermodynamics that describes the control of the
membrane function.Comment: 23 pages, 15 figure
Action anticipation based on an agent's epistemic state in toddlers and adults
Do toddlers and adults engage in spontaneous Theory of Mind (ToM)? Evidence from anticipatory looking (AL) studies suggests that they do. But a growing body of failed replication studies raised questions about the paradigm’s suitability. In this multi-lab collaboration, we test the robustness of spontaneous ToM measures. We examine whether 18- to 27-month-olds’ and adults’ anticipatory looks distinguish between two basic forms of an agent’s epistemic states: knowledge and ignorance. In toddlers [ANTICIPATED n = 520 50% FEMALE] and adults [ANTICIPATED n = 408, 50% FEMALE] from diverse ethnic backgrounds, we found [SUPPORT/NO SUPPORT] for epistemic state-based action anticipation. Future research can probe whether this conclusion extends to more complex kinds of epistemic states, such as true and false beliefs
The spin label amino acid TOAC and its uses in studies of peptides: chemical, physicochemical, spectroscopic, and conformational aspects
We review work on the paramagnetic amino acid 2,2,6,6-tetramethyl-N-oxyl-4-amino-4-carboxylic acid, TOAC, and its applications in studies of peptides and peptide synthesis. TOAC was the first spin label probe incorporated in peptides by means of a peptide bond. In view of the rigid character of this cyclic molecule and its attachment to the peptide backbone via a peptide bond, TOAC incorporation has been very useful to analyze backbone dynamics and peptide secondary structure. Many of these studies were performed making use of EPR spectroscopy, but other physical techniques, such as X-ray crystallography, CD, fluorescence, NMR, and FT-IR, have been employed. The use of double-labeled synthetic peptides has allowed the investigation of their secondary structure. A large number of studies have focused on the interaction of peptides, both synthetic and biologically active, with membranes. In the latter case, work has been reported on ligands and fragments of GPCR, host defense peptides, phospholamban, and β-amyloid. EPR studies of macroscopically aligned samples have provided information on the orientation of peptides in membranes. More recent studies have focused on peptide–protein and peptide–nucleic acid interactions. Moreover, TOAC has been shown to be a valuable probe for paramagnetic relaxation enhancement NMR studies of the interaction of labeled peptides with proteins. The growth of the number of TOAC-related publications suggests that this unnatural amino acid will find increasing applications in the future
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