297 research outputs found
Position dependent energy level shifts of an accelerated atom in the presence of a boundary
We consider a uniformly accelerated atom interacting with a vacuum
electromagnetic field in the presence of an infinite conducting plane boundary
and calculate separately the contributions of vacuum fluctuations and radiation
reaction to the atomic energy level shift. We analyze in detail the behavior of
the total energy shift in three different regimes of the distance in both the
low acceleration and high acceleration limits. Our results show that, in
general, an accelerated atom does not behave as if immersed in a thermal bath
at the Unruh temperature in terms of the atomic energy level shifts, and the
effect of the acceleration on the atomic energy level shifts may in principle
become appreciable in certain circumstances, although it may not be realistic
for actual experimental measurements. We also examine the effects of the
acceleration on the level shifts when the acceleration is of the order of the
transition frequency of the atom and we find some features differ from what was
obtained in the existing literature.Comment: 26 pages, 6 figures, version published in PR
Thermal nature of de Sitter spacetime and spontaneous excitation of atoms
We consider, in de Sitter spacetime, both freely falling and static two-level
atoms in interaction with a conformally coupled massless scalar field in the de
Sitter-invariant vacuum, and separately calculate the contributions of vacuum
fluctuations and radiation reaction to the atom's spontaneous excitation rate.
We find that spontaneous excitations occur even for the freely falling atom as
if there is a thermal bath of radiation at the Gibbons-Hawking temperature and
we thus recover, in a different physical context, the results of Gibbons and
Hawking that reveals the thermal nature of de Sitter spacetime. Similarly, for
the case of the static atom, our results show that the atom also perceives a
thermal bath which now arises as a result of the intrinsic thermal nature of de
Sitter spacetime and the Unruh effect associated with the inherent acceleration
of the atom.Comment: 11 page
Spontaneous excitation of an accelerated hydrogen atom coupled with electromagnetic vacuum fluctuations
We consider a multilevel hydrogen atom in interaction with the quantum
electromagnetic field and separately calculate the contributions of the vacuum
fluctuation and radiation reaction to the rate of change of the mean atomic
energy of the atom for uniform acceleration. It is found that the acceleration
disturbs the vacuum fluctuations in such a way that the delicate balance
between the contributions of vacuum fluctuation and radiation reaction that
exists for inertial atoms is broken, so that the transitions to higher-lying
states from ground state are possible even in vacuum. In contrast to the case
of an atom interacting with a scalar field, the contributions of both
electromagnetic vacuum fluctuations and radiation reaction to the spontaneous
emission rate are affected by the acceleration, and furthermore the
contribution of the vacuum fluctuations contains a non-thermal
acceleration-dependent correction, which is possibly observable.Comment: 8 pages, Revtex4, accepted for publication in PR
Modification of energy shifts of atoms by the presence of a boundary in a thermal bath and the Casimir-Polder force
We study the modification by the presence of a plane wall of energy level
shifts of two-level atoms which are in multipolar coupling with quantized
electromagnetic fields in a thermal bath in a formalism which separates the
contributions of thermal fluctuations and radiation reaction and allows a
distinct treatment to atoms in the ground and excited states. The position
dependent energy shifts give rise to an induced force acting on the atoms. We
are able to identify three different regimes where the force shows distinct
features and examine, in all regimes, the behaviors of this force in both the
low temperature limit and the high temperature limit for both the ground state
and excited state atoms, thus providing some physical insights into the
atom-wall interaction at finite temperature. In particular, we show that both
the magnitude and the direction of the force acting on an atom may have a clear
dependence on atomic the polarization directions. In certain cases, a change of
relative ratio of polarizations in different directions may result in a change
of direction of the force.Comment: 29 pages, 3 figure
Tailoring the mechanical properties of 3D microstructures: a deep learning and genetic algorithm inverse optimization framework
Materials-by-design has been historically challenging due to complex
process-microstructure-property relations. Conventional analytical or
simulation-based approaches suffer from low accuracy or long computational time
and poor transferability, further limiting their applications in solving the
inverse material design problem. Here, we establish a deep learning and genetic
algorithm framework that integrates forward prediction and inverse exploration.
This framework provides an end-to-end solution to achieve application-specific
mechanical properties by microstructure optimization. In this study, we select
the widely used Ti-6Al-4V to demonstrate the effectiveness of this framework by
tailoring its microstructure and achieving various yield strength and elastic
modulus across a large design space, while minimizing the stress concentration
factor. Compared with conventional methods, our framework is efficient,
versatile, and readily transferrable to other materials and properties. Paired
with additive manufacturing's potential in controlling local microstructural
features, our method has far-reaching potential for accelerating the
development of application-specific, high-performing materials.Comment: 19 pages, 5 figure
Spontaneous absorption of an accelerated hydrogen atom near a conducting plane in vacuum
We study, in the multipolar coupling scheme, a uniformly accelerated
multilevel hydrogen atom in interaction with the quantum electromagnetic field
near a conducting boundary and separately calculate the contributions of the
vacuum fluctuation and radiation reaction to the rate of change of the mean
atomic energy. It is found that the perfect balance between the contributions
of vacuum fluctuations and radiation reaction that ensures the stability of
ground-state atoms is disturbed, making spontaneous transition of ground-state
atoms to excited states possible in vacuum with a conducting boundary. The
boundary-induced contribution is effectively a nonthermal correction, which
enhances or weakens the nonthermal effect already present in the unbounded
case, thus possibly making the effect easier to observe. An interesting feature
worth being noted is that the nonthermal corrections may vanish for atoms on
some particular trajectories.Comment: 19 pages, no figures, Revtex
- …