565 research outputs found
Comment on "Spatial optical solitons in highly nonlocal media" and related papers
In a recent paper [A. Alberucci, C. Jisha, N. Smyth, and G. Assanto, Phys.
Rev. A 91, 013841 (2015)], Alberucci et al. have studied the propagation of
bright spatial solitary waves in highly nonlocal media. We find that the main
results in that and related papers, concerning soliton shape and dynamics,
based on the accessible soliton (AS) approximation, are incorrect; the correct
results have already been published by others. These and other inconsistencies
in the paper follow from the problems in applying the AS approximation in
earlier papers by the group that propagated to the later papers. The accessible
soliton theory cannot describe accurately the features and dynamics of solitons
in highly nonlocal media.Comment: 2 page
Galactic Punctuated Equilibrium: How to Undermine Carter's Anthropic Argument in Astrobiology
We investigate a new strategy which can defeat the (in)famous Carter's
"anthropic" argument against extraterrestrial life and intelligence. In
contrast to those already considered by Wilson, Livio, and others, the present
approach is based on relaxing hidden uniformitarian assumptions, considering
instead a dynamical succession of evolutionary regimes governed by both global
(Galaxy-wide) and local (planet- or planetary system-limited) regulation
mechanisms. This is in accordance with recent developments in both astrophysics
and evolutionary biology. Notably, our increased understanding of the nature of
supernovae and gamma-ray bursts, as well as of strong coupling between the
Solar System and the Galaxy on one hand, and the theories of "punctuated
equilibria" of Eldredge and Gould and "macroevolutionary regimes" of Jablonski,
Valentine, et al. on the other, are in full accordance with the regulation-
mechanism picture. The application of this particular strategy highlights the
limits of application of Carter's argument, and indicates that in the real
universe its applicability conditions are not satisfied. We conclude that
drawing far-reaching conclusions about the scarcity of extraterrestrial
intelligence and the prospects of our efforts to detect it on the basis of this
argument is unwarranted.Comment: 3 figures, 26 page
Spin Hall Current Driven by Quantum Interferences in Mesoscopic Rashba Rings
We propose an all-electrical nanoscopic structure where {\em pure} spin
current is induced in the transverse voltage probes attached to {\em
quantum-coherent} one-dimensional ring when conventional unpolarized charge
current is injected through its longitudinal leads. Tuning of the Rashba
spin-orbit coupling in semiconductor heterostructure hosting the ring generates
quasi-periodic oscillations of the predicted spin Hall current due to {\em
spin-sensitive quantum-interference effects} caused by the difference in
Aharonov-Casher phase acquired by opposite spins states traveling clockwise and
counterclockwise. Its amplitude is comparable to the mesoscopic spin Hall
current predicted for finite-size two-dimensional electron gases, while it gets
reduced in wide two-dimensional or disordered rings.Comment: 5 pages, 4 color figure
Gr\"obner bases in the mod cohomology of oriented Grassmann manifolds
For a power of two, we give a complete description of the cohomology
algebra of the Grassmann manifold
of oriented -planes in . We do this by
finding a reduced Gr\"obner basis for an ideal closely related to this
cohomology algebra. Using this Gr\"obner basis we also present an additive
basis for
Transverse Spin-Orbit Force in the Spin Hall Effect in Ballistic Semiconductor Wires
We introduce the spin and momentum dependent {\em force operator} which is
defined by the Hamiltonian of a {\em clean} semiconductor quantum wire with
homogeneous Rashba spin-orbit (SO) coupling attached to two ideal (i.e., free
of spin and charge interactions) leads. Its expectation value in the
spin-polarized electronic wave packet injected through the leads explains why
the center of the packet gets deflected in the transverse direction. Moreover,
the corresponding {\em spin density} will be dragged along the transverse
direction to generate an out-of-plane spin accumulation of opposite signs on
the lateral edges of the wire, as expected in the phenomenology of the spin
Hall effect, when spin- and spin- polarized packets
(mimicking the injection of conventional unpolarized charge current) propagate
simultaneously through the wire. We also demonstrate that spin coherence of the
injected spin-polarized wave packet will gradually diminish (thereby
diminishing the ``force'') along the SO coupled wire due to the entanglement of
spin and orbital degrees of freedom of a single electron, even in the absence
of any impurity scattering.Comment: 5 pages, 4 color EPS figures; 2 new figures and expanded discussion
on the sign of spin Hall quantities. To appear in Phys. Rev. B 72 (2005
Mesoscopic Spin Hall Effect in Multiprobe Ballistic Spin-Orbit Coupled Semiconductor Bridges
We predict that unpolarized charge current driven through the longitudinal
leads attached to ballistic quantum-coherent two-dimensional electron gas
(2DEG) in semiconductor heterostructure will induce a {\em pure} spin current,
which is not accompanied by any net charge flow, in the transverse voltage
probes. Its magnitude can be tuned by the Rashba spin-orbit (SO) interaction
and, moreover, it is resilient to weak spin-independent scattering off
impurities within the metallic diffusive regime. While the polarization vector
of the spin transported through the transverse leads is not orthogonal to the
plane of 2DEG, we demonstrate that only two components (out-of-plane and
longitudinal) of the transverse spin current are signatures of the spin Hall
effect in four-probe Rashba spin-split semiconductor nanostructures. The linear
response spin Hall current, obtained from the multiprobe Landauer-B\" uttiker
scattering formalism generalized for quantum transport of spin, is the
Fermi-surface determined nonequilibrium quantity whose scaling with the 2DEG
size reveals the importance of processes occurring on the spin precession
{\em mesoscale} (on which spin precesses by an angle )--the
out-of-plane component of the transverse spin current exhibits quasioscillatory
behavior for (attaining the maximum value in 2DEGs of
the size ), while it reaches the asymptotic value
in the macroscopic regime . Furthermore, these values of the
spin Hall current can be manipulated by the measuring geometry defined by the
attached leads.Comment: 12 pages, 6 color EPS figures; expanded discussion to emphasize
crucial role played by processes on the spin precession mesoscal
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