543 research outputs found
Kinetic and finite ion mass effects on the transition to relativistic self-induced transparency in laser-driven ion acceleration
We study kinetic effects responsible for the transition to relativistic
self-induced transparency in the interaction of a circularly-polarized
laser-pulse with an overdense plasma and their relation to hole-boring and ion
acceleration. It is demonstrated using particle-in-cell simulations and an
analysis of separatrices in single-electron phase-space, that ion motion can
suppress fast electron escape to the vacuum, which would otherwise lead to
transition to the relativistic transparency regime. A simple analytical
estimate shows that for large laser pulse amplitude the time scale over
which ion motion becomes important is much shorter than usually anticipated. As
a result, the threshold density above which hole-boring occurs decreases with
the charge-to-mass ratio. Moreover, the transition threshold is seen to depend
on the laser temporal profile, due to the effect that the latter has on
electron heating. Finally, we report a new regime in which a transition from
relativistic transparency to hole-boring occurs dynamically during the course
of the interaction. It is shown that, for a fixed laser intensity, this dynamic
transition regime allows optimal ion acceleration in terms of both energy and
energy spread.Comment: Added new material. 15 pages, 12 figure
Deviation from the Fourier law in room-temperature heat pulse experiments
We report heat pulse experiments at room temperature that cannot be described
by Fourier's law. The experimental data is modelled properly by the
Guyer--Krumhansl equation, in its over-diffusion regime. The phenomenon is due
to conduction channels with differing conductivities, and parallel to the
direction of the heat flux.Comment: 9 pages, 4 figure
Electronic States of Graphene Grain Boundaries
We introduce a model for amorphous grain boundaries in graphene, and find
that stable structures can exist along the boundary that are responsible for
local density of states enhancements both at zero and finite (~0.5 eV)
energies. Such zero energy peaks in particular were identified in STS
measurements [J. \v{C}ervenka, M. I. Katsnelson, and C. F. J. Flipse, Nature
Physics 5, 840 (2009)], but are not present in the simplest pentagon-heptagon
dislocation array model [O. V. Yazyev and S. G. Louie, Physical Review B 81,
195420 (2010)]. We consider the low energy continuum theory of arrays of
dislocations in graphene and show that it predicts localized zero energy
states. Since the continuum theory is based on an idealized lattice scale
physics it is a priori not literally applicable. However, we identify stable
dislocation cores, different from the pentagon-heptagon pairs, that do carry
zero energy states. These might be responsible for the enhanced magnetism seen
experimentally at graphite grain boundaries.Comment: 10 pages, 4 figures, submitted to Physical Review
One dimensional drift-diffusion between two absorbing boundaries: application to granular segregation
Motivated by a novel method for granular segregation, we analyze the one
dimensional drift-diffusion between two absorbing boundaries. The time
evolution of the probability distribution and the rate of absorption are given
by explicit formulae, the splitting probability and the mean first passage time
are also calculated. Applying the results we find optimal parameters for
segregating binary granular mixtures.Comment: RevTeX, 5 pages, 6 figure
Twisted N=8, D=2 super Yang-Mills theory as example of a Hodge-type cohomological theory
It is shown that the dimensional reduction of the N_T=2, D=3 Blau-Thompson
model to D=2, i.e., the novel topological twist of N=8, D=2 super Yang-Mills
theory, provides an example of a Hodge-type cohomological theory. In that
theory the generators of the topological shift, co-shift and gauge symmetry,
together with a discrete duality operation, are completely analogous to the de
Rham cohomology operators and the Hodge *-operation.Comment: 8 pages, Late
Inter-layer spin diffusion and electric conductivity in the organic conductors {\kappa}-ET2-Cl and {\kappa}-ET2-Br
A high frequency (111.2-420 GHz) electron spin resonance study of the
inter-layer (perpendicular) spin diffusion as a function of pressure and
temperature is presented in the conducting phases of the layered organic
compounds, {\kappa}-(BEDT-TTF)2-Cu[N(CN)2]X ({\kappa}-ET2-X), X=Cl or Br. The
resolved ESR lines of adjacent layers at high temperatures and high frequencies
allows for the determination of the inter-layer cross spin relaxation time, Tx
and the intrinsic spin relaxation time, T2 of single layers. In the bad metal
phase spin diffusion is two-dimensional, i.e. spins are not hopping to adjacent
layers within T2. Tx is proportional to the perpendicular resistivity at least
approximately, as predicted in models where spin and charge excitations are
tied together. In {\kappa}-ET2-Cl, at zero pressure Tx increases as the bad
metal-insulator transition is approached. On the other hand, Tx decreases as
the normal metal and superconducting phases are approached with increasing
pressure and/or decreasing temperature.Comment: 18 pages, 11 figure
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