137 research outputs found
Quantum effects with an X-ray free electron laser
A quantum kinetic equation coupled with Maxwell's equation is used to
estimate the laser power required at an XFEL facility to expose intrinsically
quantum effects in the process of QED vacuum decay via spontaneous pair
production. A 9 TW-peak XFEL laser with photon energy 8.3 keV could be
sufficient to initiate particle accumulation and the consequent formation of a
plasma of spontaneously produced pairs. The evolution of the particle number in
the plasma will exhibit non-Markovian aspects of the strong-field pair
production process and the plasma's internal currents will generate an electric
field whose interference with that of the laser leads to plasma oscillations.Comment: 4 pages, LaTeX2
Comment on ``Superconducting PrBa_2Cu_3O_x''
Recently, Zou et al. (Phys. Rev. Lett. 80, 1074, 1998) reported the
observation of bulk superconductivity (SC) for a PrBa_2Cu_3O_x (Pr123) single
crystal grown by the traveling-solvent floating zone (TSFZ) method. The aim of
this Comment is to show the inconsistency of the value of effective magnetic
moment \mu_{eff} reported by Zou et al. (2.92\mu_B) with their magnetic
susceptibility data. The estimation made directly from their data points gives
a considerably smaller value of \mu_{eff}=2.09\mu_B. At the same time the
values of mu_{eff}=2.9\mu_B and 3.1\mu_B were obtained for our Pr123 single
crystals grown by flux method for H||ab-plane and H||c-axis, respectively. This
suggests that Pr occupies only about a half of the RE sites in TSFZ crystal.
The other half of the RE sites is occupied most probably by the nonmagnetic Ba.
Noteworthy, SC with T_c=43 K was observed earlier for
Pr_{0.5}Ca_{0.5}Ba_2Cu_3O_{7-y} thin films. Ba^{2+} has a larger ionic radius
than Pr^{3+} and so the substitution of Ba for Pr could give a natural
explanation not only for the SC in TSFZ Pr123 but also for the elongation of
the distance between the CuO_2 planes observed by Zou et al.Comment: Slightly extended version of Comment accepted to Phys. Rev. Lett.
(v.81, N24, 1998), tentatevely to be publ. 14Dec98. 1 page, REVTex; 1 EPS fi
Effective Action of QED in Electric Field Backgrounds II: Spatially Localized Fields
We find the Bogoliubov coefficient from the tunneling boundary condition on a
charged particle coupled to a static electric field and,
using the regularization scheme in Phys. Rev. D 78, 105013 (2008), obtain the
exact one-loop effective action in scalar and spinor QED. It is shown that the
effective action satisfies the general relation between the vacuum persistence
and the mean number of produced pairs. We advance an approximation method for
general electric fields and show the duality between the space-dependent and
time-dependent electric fields of the same form at the leading order of the
effective actions.Comment: RevTex 7 pages, no figure; extension of arXiv:0807.2696 to
space-dependent electric fields; new section added on approximate effective
actions in general electric fields and conclusion shortened; references
added; replaced by the version to be published in Phys. Rev.
The probability distribution of the number of electron-positron pairs produced in a uniform electric field
The probability-generating function of the number of electron-positron pairs
produced in a uniform electric field is constructed. The mean and variance of
the numbers of pairs are calculated, and analytical expressions for the
probability of low numbers of electron-positron pairs are given. A recursive
formula is derived for evaluating the probability of any number of pairs. In
electric fields of supercritical strength |eE| > \pi m^2/ \ln 2, where e is the
electron charge, E is the electric field, and m is the electron mass, a
branch-point singularity of the probability-generating function penetrates the
unit circle |z| = 1, which leads to the asymptotic divergence of the cumulative
probability. This divergence indicates a failure of the continuum limit
approximation. In the continuum limit and for any field strength, the positive
definiteness of the probability is violated in the tail of the distribution.
Analyticity, convergence, and positive definiteness are restored upon the
summation over discrete levels of electrons in the normalization volume.
Numerical examples illustrating the field strength dependence of the asymptotic
behavior of the probability distribution are presented.Comment: 7 pages, REVTeX, 4 figures; new references added; a short version of
this e-print has appeared in PR
Schwinger Pair Production in dS_2 and AdS_2
We study Schwinger pair production in scalar QED from a uniform electric
field in dS_2 with scalar curvature R_{dS} = 2 H^2 and in AdS_2 with R_{AdS} =
- 2 K^2. With suitable boundary conditions, we find that the pair-production
rate is the same analytic function of the scalar curvature in both cases.Comment: RevTex 6 pages, no figure; replaced by the version published in PR
Multidimensional Worldline Instantons
We extend the worldline instanton technique to compute the vacuum pair
production rate for spatially inhomogeneous electric background fields, with
the spatial inhomogeneity being genuinely two or three dimensional, both for
the magnitude and direction of the electric field. Other techniques, such as
WKB, have not been applied to such higher dimensional problems. Our method
exploits the instanton dominance of the worldline path integral expression for
the effective action.Comment: 22 pages, 13 figure
Renormalized Effective Actions in Radially Symmetric Backgrounds I: Partial Wave Cutoff Method
The computation of the one-loop effective action in a radially symmetric
background can be reduced to a sum over partial-wave contributions, each of
which is the logarithm of an appropriate one-dimensional radial determinant.
While these individual radial determinants can be evaluated simply and
efficiently using the Gel'fand-Yaglom method, the sum over all partial-wave
contributions diverges. A renormalization procedure is needed to unambiguously
define the finite renormalized effective action. Here we use a combination of
the Schwinger proper-time method, and a resummed uniform DeWitt expansion. This
provides a more elegant technique for extracting the large partial-wave
contribution, compared to the higher order radial WKB approach which had been
used in previous work. We illustrate the general method with a complete
analysis of the scalar one-loop effective action in a class of radially
separable SU(2) Yang-Mills background fields. We also show that this method can
be applied to the case where the background gauge fields have asymptotic limits
appropriate to uniform field strengths, such as for example in the Minkowski
solution, which describes an instanton immersed in a constant background.
Detailed numerical results will be presented in a sequel.Comment: 35 page
Fermion Pair Production From an Electric Field Varying in Two Dimensions
The Hamiltonian describing fermion pair production from an arbitrarily
time-varying electric field in two dimensions is studied using a
group-theoretic approach. We show that this Hamiltonian can be encompassed by
two, commuting SU(2) algebras, and that the two-dimensional problem can
therefore be reduced to two one-dimensional problems. We compare the group
structure for the two-dimensional problem with that previously derived for the
one-dimensional problem, and verify that the Schwinger result is obtained under
the appropriate conditions.Comment: Latex, 14 pages of text. Full postscript version available via the
worldwide web at http://nucth.physics.wisc.edu/ or by anonymous ftp from
ftp://nucth.physics.wisc.edu:/pub/preprints
Momentum dependence in the dynamically assisted Sauter-Schwinger effect
Recently it has been found that the superposition of a strong and slow
electric field with a weaker and faster pulse can significantly enhance the
probability for non-perturbative electron-positron pair creation out of the
vacuum -- the dynamically assisted Sauter-Schwinger effect. Via the WKB method,
we estimate the momentum dependence of the pair creation probability and
compare it to existing numerical results. Besides the theoretical interest, a
better understanding of this pair creation mechanism should be helpful for the
planned experiments aiming at its detection.Comment: 4 pages RevTeX, 1 figur
Infra-red divergences in plane wave backgrounds
We show that the emission of soft photons via nonlinear Compton scattering in
a pulsed plane wave (laser field) is in general infra-red divergent. We give
examples of both soft and soft-collinear divergences, and we pay particular
attention to the case of crossed fields in both classical and quantum theories.Comment: 15 pages, 7 figure
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