12,119 research outputs found
Metal shearing energy absorber
A metal shearing energy absorber is described. The absorber is composed of a flat thin strip of metal which is pulled through a slot in a cutter member of a metal, harder than the metal of the strip. The slot's length, in the direction perpendicular to the pull direction, is less than the strip's width so that as the strip is pulled through the slot, its edges are sheared off, thereby absorbing some of the pulling energy. In one embodiment the cutter member is a flat plate of steel, while in another embodiment the cutter member is U-shaped with the slot at its base
Radical pair intersystem crossing: Quantum dynamics or incoherent kinetics?
Magnetic field effects on radical pair reactions arise due to the interplay
of coherent electron spin dynamics and spin relaxation effects, a rigorous
treatment of which requires the solution of the Liouville-von Neumann equation.
However, it is often found that simple incoherent kinetic models of the radical
pair singlet-triplet intersystem crossing provide an acceptable description of
experimental measurements. In this paper we outline the theoretical basis for
this incoherent kinetic description, elucidating its connection to exact
quantum mechanics. We show in particular how the finite lifetime of the radical
pair spin states, as well as any additional spin-state dephasing, leads to
incoherent intersystem crossing. We arrive at simple expressions for the
radical pair spin state interconversion rates to which the functional form
proposed recently by Steiner et al. [J. Phys. Chem. C 122, 11701 (2018)] can be
regarded as an approximation. We also test the kinetic master equation against
exact quantum dynamical simulations for a model radical pair and for a series
of molecular
wires
Spin-selective electron transfer reactions of radical pairs: beyond the Haberkorn master equation
Radical pair recombination reactions are normally described using a quantum
mechanical master equation for the electronic and nuclear spin density
operator. The electron spin state selective (singlet and triplet) recombination
processes are described with a Haberkorn reaction term in this master equation.
Here we consider a general spin state selective electron transfer reaction of a
radical pair and use Nakajima-Zwanzig theory to derive the master equation for
the spin density operator, thereby elucidating the relationship between
non-adiabatic reaction rate theory and the Haberkorn reaction term. A second
order perturbation theory treatment of the diabatic coupling naturally results
in the Haberkorn master equation with an additional reactive scalar electron
spin coupling term. This term has been neglected in previous spin chemistry
calculations, but we show that it will often be quite significant. We also show
that beyond second order in perturbation theory, i.e., beyond the Fermi golden
rule limit, an additional reactive singlet-triplet dephasing term appears in
the master equation. A closed form expression for the reactive scalar electron
spin coupling in terms of the Marcus theory parameters that determine the
singlet and triplet recombination rates is presented. By performing simulations
of radical pair reactions with the exact Hierarchical Equations of Motion
(HEOM) method, we demonstrate that our master equations provide a very accurate
description of radical pairs undergoing spin-selective non-adiabatic electron
transfer reactions. The existence of a reactive electron spin coupling may well
have implications for biologically relevant radical pair reactions such as
those which have been suggested to play a role in avian magnetoreception
Exact and Asymptotic Weighted Logrank Tests for Interval Censored Data: The interval R Package
For right-censored data perhaps the most commonly used tests are weighted logrank tests, such as the logrank and Wilcoxon-type tests. In this paper we review several generalizations of those weighted logrank tests to interval-censored data and present an R package, interval, to implement many of them. The interval package depends on the perm package, also presented here, which performs exact and asymptotic linear permutation tests. The perm package performs many of the tests included in the already available coin package, and provides an independent validation of coin. We review analysis methods for interval-censored data, and we describe and show how to use the interval and perm packages.
Spin-dependent charge recombination along para-phenylene molecular wires
We have used an efficient new quantum mechanical method for radical pair
recombination reactions to study the spin-dependent charge recombination along
PTZ--Ph--PDI molecular wires. By comparing our
results to the experimental data of E. Weiss {\em et al.} [J. Am. Chem. Soc.
{\bf 126}, 5577 (2004)], we are able to extract the spin-dependent (singlet and
triplet) charge recombination rate constants for wires with . These
spin-dependent rate constants have not been extracted previously from the
experimental data because they require fitting its magnetic field-dependence to
the results of quantum spin dynamics simulations. We find that the triplet
recombination rate constant decreases exponentially with the length of the
wire, consistent with the superexchange mechanism of charge recombination.
However, the singlet recombination rate constant is nearly independent of the
length of the wire, suggesting that the singlet pathway is dominated by an
incoherent hopping mechanism. A simple qualitative explanation for the
different behaviours of the two spin-selective charge recombination pathways is
provided in terms of Marcus theory. We also find evidence for a magnetic
field-independent background contribution to the triplet yield of the charge
recombination reaction, and suggest several possible explanations for it. Since
none of these explanations is especially compelling given the available
experimental evidence, and since the result appears to apply more generally to
other molecular wires, we hope that this aspect of our study will stimulate
further experimental work.Comment: 12 pages, 10 figure
Bianchi type IX asymptotical behaviours with a massive scalar field: chaos strikes back
We use numerical integrations to study the asymptotical behaviour of a
homogeneous but anisotropic Bianchi type IX model in General Relativity with a
massive scalar field. As it is well known, for a Brans-Dicke theory, the
asymptotical behaviour of the metric functions is ruled only by the Brans-Dicke
coupling constant with respect to the value -3/2. In this paper we examine if
such a condition still exists with a massive scalar field. We also show that,
contrary to what occurs for a massless scalar field, the singularity
oscillatory approach may exist in presence of a massive scalar field having a
positive energy density.Comment: 31 pages, 7 figures (low resolution
Self heating and nonlinear current-voltage characteristics in bilayer graphene
We demonstrate by experiments and numerical simulations that the
low-temperature current-voltage characteristics in diffusive bilayer graphene
(BLG) exhibit a strong superlinearity at finite bias voltages. The
superlinearity is weakly dependent on doping and on the length of the graphene
sample. This effect can be understood as a result of Joule heating. It is
stronger in BLG than in monolayer graphene (MLG), since the conductivity of BLG
is more sensitive to temperature due to the higher density of electronic states
at the Dirac point.Comment: 9 pages, 7 figures, REVTeX 4.
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