5,556 research outputs found
Quantum electron self-interaction in a strong laser field
The quantum state of an electron in a strong laser field is altered if the
interaction of the electron with its own electromagnetic field is taken into
account. Starting from the Schwinger-Dirac equation, we determine the states of
an electron in a plane-wave field with inclusion, at leading order, of its
electromagnetic self-interaction. On the one hand, the electron states show a
pure "quantum" contribution to the electron quasi-momentum, conceptually
different from the conventional "classical" one arising from the quiver motion
of the electron. On the other hand, the electron self-interaction induces a
distinct dynamics of the electron spin, whose effects are shown to be
measurable in principle with available technology.Comment: 5 pages, 2 figure
Cooling nonlinear lattices toward localisation
We describe the energy relaxation process produced by surface damping on
lattices of classical anharmonic oscillators. Spontaneous emergence of
localised vibrations dramatically slows down dissipation and gives rise to
quasi-stationary states where energy is trapped in the form of a gas of weakly
interacting discrete breathers. In one dimension (1D), strong enough on--site
coupling may yield stretched--exponential relaxation which is reminiscent of
glassy dynamics. We illustrate the mechanism generating localised structures
and discuss the crucial role of the boundary conditions. For two--dimensional
(2D) lattices, the existence of a gap in the breather spectrum causes the
localisation process to become activated. A statistical analysis of the
resulting quasi-stationary state through the distribution of breathers'
energies yield information on their effective interactions.Comment: 10 pages, 11 figure
Slow energy relaxation and localization in 1D lattices
We investigate the energy relaxation process produced by thermal baths at
zero temperature acting on the boundary atoms of chains of classical anharmonic
oscillators. Time-dependent perturbation theory allows us to obtain an explicit
solution of the harmonic problem: even in such a simple system nontrivial
features emerge from the interplay of the different decay rates of Fourier
modes. In particular, a crossover from an exponential to an inverse-square-root
law occurs on a time scale proportional to the system size . A further
crossover back to an exponential law is observed only at much longer times (of
the order ). In the nonlinear chain, the relaxation process is initially
equivalent to the harmonic case over a wide time span, as illustrated by
simulations of the Fermi-Pasta-Ulam model. The distinctive feature is
that the second crossover is not observed due to the spontaneous appearance of
breathers, i.e. space-localized time-periodic solutions, that keep a finite
residual energy in the lattice. We discuss the mechanism yielding such
solutions and also explain why it crucially depends on the boundary conditions.Comment: 16 pages, 6 figure
A relativistic non-relativistic Goldstone theorem: gapped Goldstones at finite charge density
We adapt the Goldstone theorem to study spontaneous symmetry breaking in
relativistic theo- ries at finite charge density. It is customary to treat
systems at finite density via non-relativistic Hamiltonians. Here we highlight
the importance of the underlying relativistic dynamics. This leads to seemingly
new results whenever the charge in question is spontaneously broken and does
not commute with other broken charges. We find that that the latter interpolate
gapped excitations. In contrast, all existing versions of the Goldstone theorem
predict the existence of gapless modes. We derive exact non-perturbative
expressions for their gaps, in terms of the chemical potential and of the
symmetry algebra.Comment: 5 pages. v2: minor modifications, matches the PRL versio
Path integral quantization of the relativistic Hopfield model
The path integral quantization method is applied to a relativistically
covariant version of the Hopfield model, which represents a very interesting
mesoscopic framework for the description of the interaction between quantum
light and dielectric quantum matter, with particular reference to the context
of analogue gravity. In order to take into account the constraints occurring in
the model, we adopt the Faddeev-Jackiw approach to constrained quantization in
the path integral formalism. In particular we demonstrate that the propagator
obtained with the Faddeev-Jackiw approach is equivalent to the one which, in
the framework of Dirac canonical quantization for constrained systems, can be
directly computed as the vacuum expectation value of the time ordered product
of the fields. Our analysis also provides an explicit example of quantization
of the electromagnetic field in a covariant gauge and coupled with the
polarization field, which is a novel contribution to the literature on the
Faddeev-Jackiw procedure.Comment: 16 page
Phi-Psi model for Electrodynamics in dielectric media: exact quantisation in the Heisenberg representation
We investigate the quantization in the Heisenberg representation of a model
which represents a simplification of the Hopfield model for dielectric media,
where the electromagnetic field is replaced by a scalar field and the
role of the polarization field is played by a further scalar field . The
model, which is quadratic in the fields, is still characterized by a nontrivial
physical content, as the physical particles correspond to the polaritons of the
standard Hopfield model of condensed matter physics. Causality is also taken
into account and a discussion of the standard interaction representation is
also considered.Comment: 9 page
Improved local-constant-field approximation for strong-field QED codes
The local-constant-field approximation (LCFA) is an essential theoretical
tool for investigating strong-field QED phenomena in background electromagnetic
fields with complex spacetime structure. In our previous work
[Phys.~Rev.~A~\textbf{98}, 012134 (2018)] we have analyzed the shortcomings of
the LCFA in nonlinear Compton scattering at low emitted photon energies for the
case of a background plane-wave field. Here, we generalize that analysis to
background fields, which can feature a virtually arbitrary spacetime structure.
In addition, we provide an explicit and simple implementation of an improved
expression of the nonlinear Compton scattering differential probability that
solves the main shortcomings of the standard LCFA in the infrared region, and
is suitable for background electromagnetic fields with arbitrary spacetime
structure such as those occurring in particle-in-cell simulations. Finally, we
carry out a systematic procedure to calculate the probability of nonlinear
Compton scattering per unit of emitted photon light-cone energy and of
nonlinear Breit-Wheeler pair production per unit of produced positron
light-cone energy beyond the LCFA in a plane-wave background field, which
allows us to identify the limits of validity of this approximation
quantitatively.Comment: 15 pages, 3 figure
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