576 research outputs found
Theoretical Study of Sodium and Potassium Resonance Lines Pressure Broadened by Helium Atoms
We perform fully quantum mechanical calculations in the binary approximation
of the emission and absorption profiles of the sodium - and potassium
- resonance lines under the influence of a helium perturbing gas. We
use carefully constructed potential energy surfaces and transition dipole
moments to compute the emission and absorption coefficients at temperatures
from 158 to 3000 K. Contributions from quasi-bound states are included. The
resulting red and blue wing profiles agree well with previous theoretical
calculations and with experimental measurements.Comment: 16 figure
Non-Fourier heat transport in metal-dielectric core-shell nanoparticles under ultrafast laser pulse excitation
Relaxation dynamics of embedded metal nanoparticles after ultrafast laser
pulse excitation is driven by thermal phenomena of different origins the
accurate description of which is crucial for interpreting experimental results:
hot electron gas generation, electron-phonon coupling, heat transfer to the
particle environment and heat propagation in the latter. Regardingthis last
mechanism, it is well known that heat transport in nanoscale structures and/or
at ultrashort timescales may deviate from the predictions of the Fourier law.
In these cases heat transport may rather be described by the Boltzmann
transport equation. We present a numerical model allowing us to determine the
electron and lattice temperature dynamics in a spherical gold nanoparticle core
under subpicosecond pulsed excitation, as well as that of the surrounding shell
dielectric medium. For this, we have used the electron-phonon coupling equation
in the particle with a source term linked with the laser pulse absorption, and
the ballistic-diffusive equations for heat conduction in the host medium.
Either thermalizing or adiabatic boundary conditions have been considered at
the shell external surface. Our results show that the heat transfer rate from
the particle to the matrix can be significantly smaller than the prediction of
Fourier's law. Consequently, the particle temperature rise is larger and its
cooling dynamics might be slower than that obtained by using Fourier's law.
This difference is attributed to the nonlocal and nonequilibrium heat
conduction in the vicinity of the core nanoparticle. These results are expected
to be of great importance for analyzing pump-probe experiments performed on
single nanoparticles or nanocomposite media
Dissociation of Feshbach Molecules into Different Partial Waves
Ultracold molecules can be associated from ultracold atoms by ramping the
magnetic field through a Feshbach resonance. A reverse ramp dissociates the
molecules. Under suitable conditions, more than one outgoing partial wave can
be populated. A theoretical model for this process is discussed here in detail.
The model reveals the connection between the dissociation and the theory of
multichannel scattering resonances. In particular, the decay rate, the
branching ratio, and the relative phase between the partial waves can be
predicted from theory or extracted from experiment. The results are applicable
to our recent experiment in 87Rb, which has a d-wave shape resonance.Comment: Added Refs.[32-38
Climate Change and Fiscal Sustainability: Risks and Opportunities
Both the physical and transition-related impacts of climate change pose substantial macroeconomic risks. Yet, markets still lack credible estimates of how climate change will affect debt sustainability, sovereign creditworthiness, and the public finances of major economies. We present a taxonomy for tracing the physical and transition impacts of climate change through to impacts on sovereign risk. We then apply the taxonomy to the UK's potential transition to net zero. Meeting internationally agreed climate targets will require an unprecedented structural transformation of the global economy over the next two or three decades. The changing landscape of risks warrants new risk management and hedging strategies to contain climate risk and minimise the impact of asset stranding and asset devaluation. Yet, conditional on action being taken early, the opportunities from managing a net zero transition would substantially outweigh the costs
Relativistic coupled-cluster single-double method applied to alkali-metal atoms
A relativistic version of the coupled-cluster single-double (CCSD) method is
developed for atoms with a single valence electron. In earlier work, a
linearized version of the CCSD method (with extensions to include a dominant
class of triple excitations) led to accurate predictions for energies,
transition amplitudes, hyperfine constants, and other properties of monovalent
atoms. Further progress in high-precision atomic structure calculations for
heavy atoms calls for improvement of the linearized coupled-cluster
methodology. In the present work, equations for the single and double
excitation coefficients of the Dirac-Fock wave function, including all
non-linear coupled-cluster terms that contribute at the single-double level are
worked out. Contributions of the non-linear terms to energies, electric-dipole
matrix elements, and hyperfine constants of low-lying states in alkali-metal
atoms from Li to Cs are evaluated and the results are compared with other
calculations and with precise experiments.Comment: 12 page
Lifetime Measurement of the 6s Level of Rubidium
We present a lifetime measurements of the 6s level of rubidium. We use a
time-correlated single-photon counting technique on two different samples of
rubidium atoms. A vapor cell with variable rubidium density and a sample of
atoms confined and cooled in a magneto-optical trap. The 5P_{1/2} level serves
as the resonant intermediate step for the two step excitation to the 6s level.
We detect the decay of the 6s level through the cascade fluorescence of the
5P_{3/2} level at 780 nm. The two samples have different systematic effects,
but we obtain consistent results that averaged give a lifetime of 45.57 +- 0.17
ns.Comment: 10 pages, 9 figure
Two-phonon 1- state in 112Sn observed in resonant photon scattering
Results of a photon scattering experiment on 112Sn using bremsstrahlung with
an endpoint energy of E_0 = 3.8 MeV are reported. A J = 1 state at E_x =
3434(1) keV has been excited. Its decay width into the ground state amounts to
Gamma_0 = 151(17) meV, making it a candidate for a [2+ x 3-]1- two-phonon
state. The results for 112Sn are compared with quasiparticle-phonon model
calculations as well as the systematics of the lowest-lying 1- states
established in other even-mass tin isotopes. Contrary to findings in the
heavier stable even-mass Sn isotopes, no 2+ states between 2 and 3.5 MeV
excitation energy have been detected in the present experiment.Comment: 10 pages, including 2 figures, Phys. Rev. C, in pres
One-way quantum computing in a decoherence-free subspace
We introduce a novel scheme for one-way quantum computing (QC) based on the
use of information encoded qubits in an effective cluster state resource. With
the correct encoding structure, we show that it is possible to protect the
entangled resource from phase damping decoherence, where the effective cluster
state can be described as residing in a Decoherence-Free Subspace (DFS) of its
supporting quantum system. One-way QC then requires either single or two-qubit
adaptive measurements. As an example where this proposal can be realized, we
describe an optical lattice setup where the scheme provides robust quantum
information processing. We also outline how one can adapt the model to provide
protection from other types of decoherence.Comment: 9 pages, 4 figures, RevTeX
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