6,689 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
Gauss-Codazzi thermodynamics on the timelike screen
It is a known result by Jacobson that the flux of energy-matter through a
local Rindler horizon is related with the expansion of the null generators in a
way that mirrors the first law of thermodynamics. We extend such a result to a
timelike screen of observers with finite acceleration. Since timelike curves
have more freedom than null geodesics, the construction is more involved than
Jacobson's and few geometrical constraints need to be imposed: the observers'
acceleration has to be constant in time and everywhere orthogonal to the
screen. Moreover, at any given time, the extrinsic curvature of the screen has
to be flat. The latter requirement can be weakened by asking that the extrinsic
curvature, if present at the beginning, evolves in time like on a cone and just
rescales proportionally to the expansion.Comment: 8+1 pages, final versio
Building Consensus. An introduction to a rhetorical approach
The contemporary debate on democracy shows two points of view: the \u201cdeliberative\u201d one and the \u201cagonistic\u201d one. The first one is related to the classical tradition that considers Habermas as its reference point. It emphasizes the role of rational deliberation as a means to produce a legitimate and binding consensus. In contrast, the second one draws its inspiration from C. Schmitt, and considers conflict and disagreement as unavoidable conditions of democratic life. Despite their obvious differences, these two theoretical models have a conception of rhetoric in common that is subjected to, or at least excluded from, the full exercise of argumentative rationality. We propose an interpretation of rhetoric that includes the logical-argumentative dimension in the rhetorical domain is possible. In this way, the recovery of rhetoric, considered both as a practice and as a theory of persuasive speech, may shed light on the role of discursive processes in building consensus, and thus may allow a revision of the dialectical tension between the pairs of concepts that the debate tends to focus on: normative/descriptive, rational/irrational, agreement/conflict
Pair-production of charged Dirac particles on charged Nariai and ultracold black hole manifolds
Spontaneous loss of charge by charged black holes by means of pair-creation
of charged Dirac particles is considered. We provide three examples of exact
calculations for the spontaneous discharge process for 4D charged black holes
by considering the process on three special non-rotating de Sitter black hole
backgrounds, which allow to bring back the problem to a Kaluza-Klein reduction.
Both the zeta-function approach and the transmission coefficient approach are
taken into account. A comparison between the two methods is also provided, as
well as a comparison with WKB results. In the case of non-zero temperature of
the geometric background, we also discuss thermal effects on the discharge
process.Comment: 27 page
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
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