6,315 research outputs found
Phonon-mediated electron spin phase diffusion in a quantum dot
An effective spin relaxation mechanism that leads to electron spin
decoherence in a quantum dot is proposed. In contrast to the common
calculations of spin-flip transitions between the Kramers doublets, we take
into account a process of phonon-mediated fluctuation in the electron spin
precession and subsequent spin phase diffusion. Specifically, we consider
modulations in the longitudinal g-factor and hyperfine interaction induced by
the phonon-assisted transitions between the lowest electronic states. Prominent
differences in the temperature and magnetic field dependence between the
proposed mechanisms and the spin-flip transitions are expected to facilitate
its experimental verification. Numerical estimation demonstrates highly
efficient spin relaxation in typical semiconductor quantum dots.Comment: 5 pages, 1 figur
Mixed Quantum/Classical Approach for Description of Molecular Collisions in Astrophysical Environments
An efficient and accurate mixed quantum/classical theory approach for computational treatment of inelastic scattering is extended to describe collision of an atom with a general asymmetric-top rotor polyatomic molecule. Quantum mechanics, employed to describe transitions between the internal states of the molecule, and classical mechanics, employed for description of scattering of the atom, are used in a self-consistent manner. Such calculations for rotational excitation of HCOOCH3 in collisions with He produce accurate results at scattering energies above 15 cm–1, although resonances near threshold, below 5 cm–1, cannot be reproduced. Importantly, the method remains computationally affordable at high scattering energies (here up to 1000 cm–1), which enables calculations for larger molecules and at higher collision energies than was possible previously with the standard full-quantum approach. Theoretical prediction of inelastic cross sections for a number of complex organic molecules observed in space becomes feasible using this new computational tool
First Time-dependent Study of H2 and H3+ Ortho-Para Chemistry in the Diffuse Interstellar Medium: Observations Meet Theoretical Predictions
The chemistry in the diffuse interstellar medium initiates the gradual
increase of molecular complexity during the life cycle of matter. A key
molecule that enables build-up of new molecular bonds and new molecules via
proton-donation is H3+. Its evolution is tightly related to molecular hydrogen
and thought to be well understood. However, recent observations of ortho and
para lines of H2 and H3+ in the diffuse ISM showed a puzzling discrepancy in
nuclear spin excitation temperatures and populations between these two key
species. H3+, unlike H2, seems to be out of thermal equilibrium, contrary to
the predictions of modern astrochemical models. We conduct the first
time-dependent modeling of the para-fractions of H2 and H3+ in the diffuse ISM
and compare our results to a set of line-of-sight observations, including new
measurements presented in this study. We isolate a set of key reactions for H3+
and find that the destruction of the lowest rotational states of H3+ by
dissociative recombination largely control its ortho/para ratio. A plausible
agreement with observations cannot be achieved unless a ratio larger than 1:5
for the destruction of (1,1)- and (1,0)-states of H3+ is assumed. Additionally,
an increased CR ionization rate to 10(-15) 1/s further improves the fit whereas
variations of other individual physical parameters, such as density and
chemical age, have only a minor effect on the predicted ortho/para ratios. Thus
our study calls for new laboratory measurements of the dissociative
recombination rate and branching ratio of the key ion H3+ under interstellar
conditions.Comment: 27 pages, 6 figures, 3 table
Search for solar axions produced by Compton process and bremsstrahlung using the resonant absorption and axioelectric effect
The search for resonant absorption of Compton and bremsstrahlung solar axions
by Tm nuclei have been performed. Such an absorption should lead to the
excitation of low-lying nuclear energy level: Tm Tm Tm (8.41 keV). Additionally the
axio-electric effect in silicon atoms is sought. The axions are detected using
a Si(Li) detectors placed in a low-background setup. As a result, a new model
independent restrictions on the axion-electron and the axion-nucleon coupling:
and the axion-electron
coupling constant: has been obtained. The
limits leads to the bounds 7.9 eV and 1.3 keV for the
mass of the axion in the DFSZ and KSVZ models, respectively ( C.L.).Comment: 6 pages, 3 figures, contributed to the 9th Patras Workshop on Axions,
WIMPs and WISPs, Mainz, June 24-28, 201
High Resolution BPM Upgrade for the ATF Damping Ring at KEK
A beam position monitor (BPM) upgrade at the KEK Accelerator Test Facility
(ATF) damping ring has been accomplished, carried out by a KEK/FNAL/SLAC
collaboration under the umbrella of the global ILC R&D effort. The upgrade
consists of a high resolution, high reproducibility read-out system, based on
analog and processing, and also implements a new automatic gain error
correction schema. The technical concept and realization as well as results of
beam studies are presented.Comment: 3 pp. 10th European Workshop on Beam Diagnostics and Instrumentation
for Particle Accelerators DIPAC 2011, 16-18 May 2011. Hamburg, German
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