12,100 research outputs found
Multiple colliding electromagnetic pulses: a way to lower the threshold of pair production from vacuum
The scheme of simultaneous multiple pulse focusing on one spot naturally
arises from the structural features of projected new laser systems, such as ELI
and HiPER. It is shown that the multiple pulse configuration is beneficial for
observing pair production from vacuum under the action of sufficiently
strong electromagnetic fields. The field of the focused pulses is described
using a realistic three-dimensional model based on an exact solution of the
Maxwell equations. The pair production threshold in terms of
electromagnetic field energy can be substantially lowered if, instead of one or
even two colliding pulses, multiple pulses focused on one spot are used. The
multiple pulse interaction geometry gives rise to subwavelength field features
in the focal region. These features result in the production of extremely short
bunches.Comment: 10 pages, 4 figure
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
Antiferromagnetic spin phase transition in nuclear matter with effective Gogny interaction
The possibility of ferromagnetic and antiferromagnetic phase transitions in
symmetric nuclear matter is analyzed within the framework of a Fermi liquid
theory with the effective Gogny interaction. It is shown that at some critical
density nuclear matter with D1S effective force undergoes a phase transition to
the antiferromagnetic spin state (the opposite direction of neutron and proton
spins). The self--consistent equations of spin polarized nuclear matter with
D1S force have no solutions, corresponding to the ferromagnetic spin ordering
(the same direction of neutron and proton spins) and, hence, the ferromagnetic
transition does not appear. The dependence of antiferromagnetic spin
polarization parameter as a function of density is found at zero temperature.Comment: Report at the workshop "Hot points in astrophysics and cosmology",
Dubna, August, 2-13, 2004. REVTeX4, 9 pages, 3 figure
Spin dynamics across the superfluid-insulator transition of spinful bosons
Bosons with non-zero spin exhibit a rich variety of superfluid and insulating
phases. Most phases support coherent spin oscillations, which have been the
focus of numerous recent experiments. These spin oscillations are Rabi
oscillations between discrete levels deep in the insulator, while deep in the
superfluid they can be oscillations in the orientation of a spinful condensate.
We describe the evolution of spin oscillations across the superfluid-insulator
quantum phase transition. For transitions with an order parameter carrying
spin, the damping of such oscillations is determined by the scaling dimension
of the composite spin operator. For transitions with a spinless order parameter
and gapped spin excitations, we demonstrate that the damping is determined by
an associated quantum impurity problem of a localized spin excitation
interacting with the bulk critical modes. We present a renormalization group
analysis of the quantum impurity problem, and discuss the relationship of our
results to experiments on ultracold atoms in optical lattices.Comment: 43 pages (single-column format), 8 figures; v2: corrected discussion
of fixed points in Section V
BCS-BEC crossover in a system of microcavity polaritons
We investigate the thermodynamics and signatures of a polariton condensate
over a range of densities, using a model of microcavity polaritons with
internal structure. We determine a phase diagram for this system including
fluctuation corrections to the mean-field theory. At low densities the
condensation temperature, T_c, behaves like that for point bosons. At higher
densities, when T_c approaches the Rabi splitting, T_c deviates from the form
for point bosons, and instead approaches the result of a BCS-like mean-field
theory. This crossover occurs at densities much less than the Mott density. We
show that current experiments are in a density range where the phase boundary
is described by the BCS-like mean-field boundary. We investigate the influence
of inhomogeneous broadening and detuning of excitons on the phase diagram.Comment: 20 pages, 6 figure
The Stokes Phenomenon and Schwinger Vacuum Pair Production in Time-Dependent Laser Pulses
Particle production due to external fields (electric, chromo-electric or
gravitational) requires evolving an initial state through an interaction with a
time-dependent background, with the rate being computed from a Bogoliubov
transformation between the in and out vacua. When the background fields have
temporal profiles with sub-structure, a semiclassical analysis of this problem
confronts the full subtlety of the Stokes phenomenon: WKB solutions are only
local, while the production rate requires global information. Incorporating the
Stokes phenomenon, we give a simple quantitative explanation of the recently
computed [Phys. Rev. Lett. 102, 150404 (2009)] oscillatory momentum spectrum of
e+e- pairs produced from vacuum subjected to a time-dependent electric field
with sub-cycle laser pulse structure. This approach also explains naturally why
for spinor and scalar QED these oscillations are out of phase.Comment: 5 pages, 4 figs.; v2 sign typo corrected, version to appear in PR
- …