3,113 research outputs found
Particle creation from the vacuum by an exponentially decreasing electric field
We analyze the creation of fermions and bosons from the vacuum by the
exponentially decreasing in time electric field in detail. In our calculations
we use QED and follow in main the consideration of particle creation effect in
a homogeneous electric field. To this end we find complete sets of exact
solutions of the -dimensional Dirac equation in the exponentially decreasing
electric field and use them to calculate all the characteristics of the effect,
in particular, the total number of created particles and the probability of a
vacuum to remain a vacuum. It should be noted that the latter quantities were
derived in the case under consideration for the first time. All possible
asymptotic regimes are discussed in detail. In addition, switching on and
switching off effects are studied.Comment: We add some references and minor comments. Version accepted for
publication in Physica Scripta as a Invited Commen
New insights into electron spin dynamics in the presence of correlated noise
The changes of the spin depolarization length in zinc-blende semiconductors
when an external component of correlated noise is added to a static driving
electric field are analyzed for different values of field strength, noise
amplitude and correlation time. Electron dynamics is simulated by a Monte Carlo
procedure which keeps into account all the possible scattering phenomena of the
hot electrons in the medium and includes the evolution of spin polarization.
Spin depolarization is studied by examinating the decay of the initial spin
polarization of the conduction electrons through the D'yakonov-Perel process,
the only relevant relaxation mechanism in III-V crystals. Our results show
that, for electric field amplitude lower than the Gunn field, the dephasing
length shortens with the increasing of the noise intensity. Moreover, a
nonmonotonic behavior of spin depolarization length with the noise correlation
time is found, characterized by a maximum variation for values of noise
correlation time comparable with the dephasing time. Instead, in high field
conditions, we find that, critically depending on the noise correlation time,
external fluctuations can positively affect the relaxation length. The
influence of the inclusion of the electron-electron scattering mechanism is
also shown and discussed.Comment: Published on "Journal of Physics: Condensed Matter" as "Fast Track
Communications", 11 pages, 9 figure
The utopian function of film music
In this article I apply Ernst Bloch's utopian philosophy to film music
The beautiful and the political
Competing and polarised positions related to the possible political nature of material in contemporary music are exemplified by the work of postmodern composers and of post-war modernist composers. Whilst the former argue for the political nature of their compositions by the inclusion of contemporary issues and imagery, the latter argue for the political nature of their manipulation of otherwise politically neutral musical material. This opposition can be understood as a dialectic between content and form, and is expressed by Adorno as the opposition between representational and ‘committed’ work. This paper examines one example of each type of work—Luigi Nono’s Il Canto Sospeso (1955-56) and Johannes Kreidler’s Audioguide—and their relationship to a conception of the ‘beautiful’ in music. These expressions of the ‘political’ offer a framework through which the musically beautiful can be interrogated in the opposition of committed and autonomous artworks, and understood as an experience of alienation. Eco's exploration of Entfremdung and Kristeva's concept of abjection can both be employed to argue that the ‘political’ dimension of autonomous works offers the potential for a radical experience of beauty as a transcendence derived from present conditions, whilst committed works negate beauty as a condition of re-presenting the present
Noncommutative magnetic moment of charged particles
It has been argued, that in noncommutative field theories sizes of physical
objects cannot be taken smaller than an elementary length related to
noncommutativity parameters. By gauge-covariantly extending field equations of
noncommutative U(1)_*-theory to the presence of external sources, we find
electric and magnetic fields produces by an extended charge. We find that such
a charge, apart from being an ordinary electric monopole, is also a magnetic
dipole. By writing off the existing experimental clearance in the value of the
lepton magnetic moments for the present effect, we get the bound on
noncommutativity at the level of 10^4 TeV.Comment: 9 pages, revtex; v2: replaced to match the published versio
Environmental Noise and Nonlinear Relaxation in Biological Systems
We analyse the effects of environmental noise in three different biological
systems: (i) mating behaviour of individuals of \emph{Nezara viridula} (L.)
(Heteroptera Pentatomidae); (ii) polymer translocation in crowded solution;
(iii) an ecosystem described by a Verhulst model with a multiplicative L\'{e}vy
noise.Comment: 32 pages; In "Ecological Modeling" by Ed. Wen-Jun Zhang. ISBN:
978-1-61324-567-5. - Nova Science Publishers, New York, 201
Violation of vacuum stability by inverse square electric fields
In the framework of QED with a strong background, we study particle creation
(the Schwinger effect) by a time-dependent inverse square electric field. To
this end corresponding exact in- and out-solutions of the Dirac and
Klein-Gordon equations are found. We calculate the vacuum-to-vacuum probability
and differential and total mean numbers of pairs created from the vacuum. For
electric fields varying slowly in time, we present detailed calculations of the
Schwinger effect and discuss possible asymptotic regimes. The obtained results
are consistent with universal estimates of the particle creation effect by
electric fields in the locally constant field approximation. Differential and
total quantities corresponding to asymmetrical configurations are also
discussed in detail. Finally, the inverse square electric field is used to
imitate switching on and off processes. Then the case under consideration is
compared with the one where an exponential electric field is used to imitate
switching on and off processes.Comment: 30 pages, 9 figures. Published versio
Schwinger mechanism of magnon-antimagnon pair production on magnetic field inhomogeneities and the bosonic Klein effect
Effective field theory of low-energy exitations-magnons that describes
antiferromagnets is mapped into scalar electrodynamics of a charged scalar
field interacting with an external electromagnetic potential. In the presence
of a constant inhomogeneous external magnetic field the latter problem is
technically reduced to the problem of charged-particle creation from the vacuum
by an electric potential step (x-step). Magnetic moment plays here the role of
the electric charge, and magnons and antimagnons differ from each other in the
sign of the magnetic moment. In the framework of such a consideration, it is
important to take into account the vacuum instability (the Schwinger effect)
under the magnon-antimagnon production on magnetic field inhomogeneities (an
analog of pair creation from the vacuum by electric-like fields). We
demonstrate how to use the strong field QED with x-steps developed by the
authors (SPG and DMG) to study the magnon-antimagnon pair production on
magnetic field inhomogeneities. Characteristics of the vacuum instability
obtained for some magnetic steps that allows exact solving the Klein-Gordon
equation are presented. In particular, we consider examples of magnetic steps
with very sharp field derivatives that correspond to a regularization of the
Klein step. In the case of smooth-gradient steps, we describe an universal
behavior of the flux density of created magnon pairs. We also note that since
the low-energy magnons are bosons with small effective mass, then for the first
time maybe the opportunity will arise to observe the Schwinger effect in the
case of the Bose statistics, in particular, the bosonic Klein effect in
laboratory conditions. Moreover, it turns out that in the case of the Bose
statistics appears a new mechanism for amplifying the effect of the pair
creation, which we call statistically-assisted Schwinger effect.Comment: 26 page
Adorno on Hope
I argue that Theodor W. Adorno’s philosophy articulates a radical conception of hope. According to Lear, radical hope is ‘directed toward a future goodness that transcends the current ability to understand what it is’ (2006:103). Given Adorno’s claim that the current world is radically evil, and that we cannot know or even imagine what the good is, it is plausible that his conception of hope must be radical in this sense. I develop this argument through an analysis of (i) Adorno’s engagement with Kant’s conception of hope, (ii) Adorno’s references to hope, and (iii) his critical diagnosis of a metaphysical need for hope. Having demonstrated that Adorno must reject both ordinary and Kantian hope, I examine why Adorno thinks that we still may have reasons for hope. I also show that Adorno’s conception of hope differs from Lear’s in one important respect
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