1,615 research outputs found
Semiclassical instanton formulation of Marcus-Levich-Jortner theory
Marcus-Levich-Jortner (MLJ) theory is one of the most commonly used methods
for including nuclear quantum effects into the calculation of electron-transfer
rates and for interpreting experimental data. It divides the molecular problem
into a subsystem treated quantum-mechanically by Fermi's golden rule and a
solvent bath treated by classical Marcus theory. As an extension of this idea,
we here present a "reduced" semiclassical instanton theory, which is a
multiscale method for simulating quantum tunnelling of the subsystem in
molecular detail in the presence of a harmonic bath. We demonstrate that
instanton theory is typically significantly more accurate than the cumulant
expansion or the semiclassical Franck-Condon sum, which can give
orders-of-magnitude errors and in general do not obey detailed balance. As
opposed to MLJ theory, which is based on wavefunctions, instanton theory is
based on path integrals and thus does not require solutions of the
Schr\"odinger equation, nor even global knowledge of the ground- and
excited-state potentials within the subsystem. It can thus be efficiently
applied to complex, anharmonic multidimensional subsystems without making
further approximations. In addition to predicting accurate rates, instanton
theory gives a high level of insight into the reaction mechanism by locating
the dominant tunnelling pathway as well as providing information on the
reactant and product vibrational states involved in the reaction and the
activation energy in the bath similarly to what would be found with MLJ theory.Comment: 21 pages, 4 figure
Spectral analysis of 636 white dwarf - M star binaries from the Sloan Digital Sky Survey
We present a catalog of 857 white dwarf (WD)-M binaries from the sixth data
release (DR6) of the Sloan Digital Sky Survey (SDSS), most of which were
previously identified. For 636 of them, we complete a spectral analysis and
derive the basic parameters of their stellar constituents and their distances
from Earth. We attempt to measure fundamental parameters of these systems by
completing spectral analyses. We use a Chi^2 minimization technique to
decompose each combined spectrum and derive independent parameter estimates for
its components. Forty-one of the stellar duets in our spectroscopic sample are
optically resolved in their respective SDSS images. For these systems, we also
derive a minimum true spatial separation and a lower limit to their orbital
periods, typically which are some 10^4 yr. Spectra of 167 stellar duets show
significant hydrogen emission and in most cases no additional He i or He ii
features. We also find that 20 of the 636 WDs are fitted to be DOs, with 16
measured to have T_eff around 40,000 K. Furthermore, we identify 70 very
low-mass objects, which are secondaries of masses smaller than about 0.1 solar
masses, to be candidate substellar companions. Although various selection
effects may play a role, the fraction 6.4 % of WD-M star binaries with orbital
separations of around 500 AU is a criterion for evolutionary models of stellar
binary systems. Active M dwarfs are likely present in 155 Balmer-emitting
systems, corresponding to a fraction of 24.4 %. The excess of cool DOs is most
likely due to additional WDs in the DB-DO T_eff range, for which no detailed
fitting was completed. The trend of the M stars being closer to Earth than the
WD component is probably due to an underestimation of the theoretical M star
radii.Comment: accepted by A&A October 3, 2008, 15 pages, 16 figures, 3 tables; v2,
minor grammatical changes, essential changes in Sect. 5.
Reliability Testing of AlGaN/GaN HEMTs Under Multiple Stressors
We performed an experiment on AlGaN/GaN HEMTs with high voltage and high power as stressors. We found that devices tested under high power generally degraded more than those tested under high voltage. In particular, the high-voltage-tested devices did not degrade significantly as suggested by some papers in the literature. The same papers in the literature also suggest that high voltages cause cracks and pits. However, the high-voltage-tested devices in this study do not exhibit cracks or pits in TEM images, while the high-power-tested devices exhibit pits
Statistical Properties of Many Particle Eigenfunctions
Wavefunction correlations and density matrices for few or many particles are
derived from the properties of semiclassical energy Green functions. Universal
features of fixed energy (microcanonical) random wavefunction correlation
functions appear which reflect the emergence of the canonical ensemble as the
number of particles approaches infinity. This arises through a little known
asymptotic limit of Bessel functions. Constraints due to symmetries,
boundaries, and collisions between particles can be included.Comment: 13 pages, 4 figure
Uniform semiclassical wave function for coherent 2D electron flow
We find a uniform semiclassical (SC) wave function describing coherent
branched flow through a two-dimensional electron gas (2DEG), a phenomenon
recently discovered by direct imaging of the current using scanned probed
microscopy. The formation of branches has been explained by classical
arguments, but the SC simulations necessary to account for the coherence are
made difficult by the proliferation of catastrophes in the phase space. In this
paper, expansion in terms of "replacement manifolds" is used to find a uniform
SC wave function for a cusp singularity. The method is then generalized and
applied to calculate uniform wave functions for a quantum-map model of coherent
flow through a 2DEG. Finally, the quantum-map approximation is dropped and the
method is shown to work for a continuous-time model as well.Comment: 9 pages, 7 figure
Hamiltonian approach for the wave packet dynamics: Beyond Gaussian wave functions
It is well known that the Gaussian wave packet dynamics can be written in
terms of Hamilton equations in the extended phase space that is twice as large
as in the corresponding classical system. We construct several generalizations
of this approach that include non-Gausssian wave packets. These generalizations
lead to the further extension of the phase space while retaining the Hamilton
structure of the equations of motion. We compare the Gaussian dynamics with
these non-Gaussian extensions for a particle with the quartic potential.Comment: 5 pages, 3 figure
A new class of semiclassical wave function uniformizations
We present a new semiclassical technique which relies on replacing
complicated classical manifold structure with simpler manifolds, which are then
evaluated by the usual semiclassical rules. Under circumstances where the
original manifold structure gives poor or useless results semiclassically the
replacement manifolds can yield remarkable accuracy. We give several working
examples to illustrate the theory presented here.Comment: 12 pages (incl. 12 figures
Hyperglycaemia but not hyperlipidaemia causes beta cell dysfunction and beta cell loss in the domestic cat
AIMS/HYPOTHESIS: In vitro studies point to a toxic effect of high glucose and non-esterified fatty acids on beta cells. Whether elevated levels of glucose and lipids induce beta cell loss in vivo is less clear. The domestic cat has recently been proposed as a valuable animal model for human type 2 diabetes because feline diabetes shows several similarities with diabetes in humans, including obesity-induced insulin resistance, impaired beta cell function, decreased number of beta cells and pancreatic amyloid deposition. METHODS: We infused healthy cats with glucose or lipids for 10 days to clamp their blood concentrations at the approximate level found in untreated feline diabetes (glucose: 25-30 mmol/l; triacylglycerols: 3-7 mmol/l). RESULTS: Glucose and lipid levels were adequately targeted. Plasma non-esterified fatty acids were increased by lipid infusion 1.7-fold. A dramatic and progressive decline of plasma insulin levels was observed in glucose-infused cats beginning after 2 days of hyperglycaemic clamp. In contrast, plasma insulin concentration and glucose tolerance test were not affected by hyperlipidaemia. Compared with controls, glucose-infused cats had a 50% decrease in beta cells per pancreatic area. Apoptotic islet cells and cleaved caspase-3-positive beta cells were observed in glucose-infused cats only. CONCLUSIONS/INTERPRETATION: Sustained hyperglycaemia but not hyperlipidaemia induces early and severe beta cell dysfunction in cats, and excess glucose causes beta cell loss via apoptosis in vivo. Hyperglycaemic clamps in cats may provide a good model to study the pathogenesis of glucose toxicity in beta cells
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Thin channel β-Ga2O3 MOSFETs with self-aligned refractory metal gates
We report the first demonstration of self-aligned gate (SAG) β-Ga2O3 metal-oxide-semiconductor field-effect transistors (MOSFETs) as a path toward eliminating source access resistance for low-loss power applications. The SAG process is implemented with a subtractively defined and etched refractory metal, such as Tungsten, combined with ion-implantation. We report experimental and modeled DC performance of a representative SAG device that achieved a maximum transconductance of 35 mS mm-1 and an on-resistance of ∼30 Ω mm with a 2.5 μm gate length. These results highlight the advantage of implant technology for SAG β-Ga2O3 MOSFETs enabling future power switching and RF devices with low parasitic resistance. © Not subject to copyright in the USA. Contribution of Wright-Patterson AFB
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