4,097 research outputs found
Supercurrent-Induced Magnetization Dynamics
We investigate supercurrent-induced magnetization dynamics in a Josephson
junction with two misaligned ferromagnetic layers, and demonstrate a variety of
effects by solving numerically the Landau-Lifshitz-Gilbert equation. In
particular, we demonstrate the possibility to obtain supercurrent-induced
magnetization switching for an experimentally feasible set of parameters, and
clarify the favorable condition for the realization of magnetization reversal.
These results constitute a superconducting analogue to conventional
current-induced magnetization dynamics and indicate how spin-triplet
supercurrents may be utilized for practical purposes in spintronics.Comment: 4 pages, 3 figures. Submitted to Physical Revie
Cosmic Growth History and Expansion History
The cosmic expansion history tests the dynamics of the global evolution of
the universe and its energy density contents, while the cosmic growth history
tests the evolution of the inhomogeneous part of the energy density. Precision
comparison of the two histories can distinguish the nature of the physics
responsible for the accelerating cosmic expansion: an additional smooth
component - dark energy - or a modification of the gravitational field
equations. With the aid of a new fitting formula for linear perturbation growth
accurate to 0.05-0.2%, we separate out the growth dependence on the expansion
history and introduce a new growth index parameter \gamma that quantifies the
gravitational modification.Comment: 8 pages, 3 figures; minor changes to match version accepted to PR
Spin-triplet Supercurrent through Inhomogeneous Ferromagnetic Trilayers
Motivated by a recent experiment [J. W. A. Robinson, J. D. S. Witt and M. G.
Blamire, Science, \textbf{329}, 5987 (2010)], we here study the possibility of
establishing a long-range spin-triplet supercurrent through an inhomogeneous
ferromagnetic region consisting of a HoCoHo trilayer sandwiched
between two conventional s-wave superconductors. We utilize a full numerical
solution in the diffusive regime of transport and study the behavior of the
supercurrent for various experimentally relevant configurations of the
ferromagnetic trilayer. We obtain qualitatively very good agreement with
experimental data regarding the behavior of the supercurrent as a function of
the width of the Co-layer, . Moreover, we find a synthesis of
0- oscillations with superimposed rapid oscillations when varying the
width of the Ho-layer which pertain specifically to the spiral magnetization
texture in Ho. We are not able to reproduce the anomalous peaks in the
supercurrent observed experimentally in this regime, but note that the results
obtained are quite sensitive to the exact magnetization profile in the
Ho-layers, which could be the reason for the discrepancy between our model and
the experimental reported data for this particular aspect. We also investigate
the supercurrent in a system where the intrinsically inhomogeneous Ho
ferromagnets are replaced with domain-wall ferromagnets, and find similar
behavior as in the HoCoHo case. Furthermore, we propose a novel
type of magnetic Josephson junction including only a domain-wall ferromagnet
and a homogeneous ferromagnetic layer, which in addition to simplicity
regarding the magnetization profile also offers a tunable long-range
spin-triplet supercurrent. Finally, we discuss some experimental aspects of our
findings.Comment: 7 pages, 7 figures. Submitted to Physical Revie
Reducing Zero-point Systematics in Dark Energy Supernova Experiments
We study the effect of filter zero-point uncertainties on future supernova
dark energy missions. Fitting for calibration parameters using simultaneous
analysis of all Type Ia supernova standard candles achieves a significant
improvement over more traditional fit methods. This conclusion is robust under
diverse experimental configurations (number of observed supernovae, maximum
survey redshift, inclusion of additional systematics). This approach to
supernova fitting considerably eases otherwise stringent mission calibration
requirements. As an example we simulate a space-based mission based on the
proposed JDEM satellite; however the method and conclusions are general and
valid for any future supernova dark energy mission, ground or space-based.Comment: 30 pages,8 figures, 5 table, one reference added, submitted to
Astroparticle Physic
Long-range epidemic spreading with immunization
We study the phase transition between survival and extinction in an epidemic
process with long-range interactions and immunization. This model can be viewed
as the well-known general epidemic process (GEP) in which nearest-neighbor
interactions are replaced by Levy flights over distances r which are
distributed as P(r) ~ r^(-d-sigma). By extensive numerical simulations we
confirm previous field-theoretical results obtained by Janssen et al. [Eur.
Phys. J. B7, 137 (1999)].Comment: LaTeX, 14 pages, 4 eps figure
Exploring the Expanding Universe and Dark Energy using the Statefinder Diagnostic
The coming few years are likely to witness a dramatic increase in high
quality Sn data as current surveys add more high redshift supernovae to their
inventory and as newer and deeper supernova experiments become operational.
Given the current variety in dark energy models and the expected improvement in
observational data, an accurate and versatile diagnostic of dark energy is the
need of the hour. This paper examines the Statefinder diagnostic in the light
of the proposed SNAP satellite which is expected to observe about 2000
supernovae per year. We show that the Statefinder is versatile enough to
differentiate between dark energy models as varied as the cosmological constant
on the one hand, and quintessence, the Chaplygin gas and braneworld models, on
the other. Using SNAP data, the Statefinder can distinguish a cosmological
constant () from quintessence models with and Chaplygin gas
models with at the level if the value of \om is
known exactly. The Statefinder gives reasonable results even when the value of
\om is known to only accuracy. In this case, marginalizing over
\om and assuming a fiducial LCDM model allows us to rule out quintessence
with and the Chaplygin gas with (both at
). These constraints can be made even tighter if we use the
Statefinders in conjunction with the deceleration parameter. The Statefinder is
very sensitive to the total pressure exerted by all forms of matter and
radiation in the universe. It can therefore differentiate between dark energy
models at moderately high redshifts of z \lleq 10.Comment: 21 pages, 17 figures. Minor typos corrected to agree with version
published in MNRAS. Results unchange
Magnetization dynamics and Majorana fermions in ferromagnetic Josephson junctions along the quantum spin Hall edge
We investigate the interplay between ferromagnetic and superconducting order
at the edge of a quantum spin Hall insulator. Using complementary analytical
and self-consistent numerical approaches, we study a ferromagnetic Josephson
junction and show how the direct coupling between magnetism and the
superconducting U(1) phase gives rise to several unusual phenomena which
distinguishes the present system from its non-topological equivalent. In
particular, we demonstrate how the anomalous current-phase relation triggers
supercurrent-induced magnetization dynamics and also study the spatial
localization of the Majorana fermions appearing in the junction.Comment: 4+ pages, 4 figures. Version 2 contains only minor change
Probing Gravitation, Dark Energy, and Acceleration
The acceleration of the expansion of the universe arises from unknown
physical processes involving either new fields in high energy physics or
modifications of gravitation theory. It is crucial for our understanding to
characterize the properties of the dark energy or gravity through cosmological
observations and compare and distinguish between them. In fact, close
consistencies exist between a dark energy equation of state function w(z) and
changes to the framework of the Friedmann cosmological equations as well as
direct spacetime geometry quantities involving the acceleration, such as
``geometric dark energy'' from the Ricci scalar. We investigate these
interrelationships, including for the case of superacceleration or phantom
energy where the fate of the universe may be more gentle than the Big Rip.Comment: 12 pages, 8 figure
Cosmology with X-ray Cluster Baryons
X-ray cluster measurements interpreted with a universal baryon/gas mass
fraction can theoretically serve as a cosmological distance probe. We examine
issues of cosmological sensitivity for current (e.g. Chandra X-ray Observatory,
XMM-Newton) and next generation (e.g. Con-X, XEUS) observations, along with
systematic uncertainties and biases. To give competitive next generation
constraints on dark energy, we find that systematics will need to be controlled
to better than 1% and any evolution in f_gas (and other cluster gas properties)
must be calibrated so the residual uncertainty is weaker than (1+z)^{0.03}.Comment: 6 pages, 5 figures; v2: 13 pages, substantial elaboration and
reordering, matches JCAP versio
Josephson effect in thin-film superconductor/insulator/superconductor junctions with misaligned in-plane magnetic fields
We study a tunnel junction consisting of two thin-film s-wave superconductors
separated by a thin, insulating barrier in the presence of misaligned in-plane
exchange fields. We find an interesting interplay between the superconducting
phase difference and the relative orientation of the exchange fields,
manifested in the Josephson current across the junction. Specifically, this may
be written , where
I_0 and I_m are constants, and is the relative orientation of the
exchange fields while is the superconducting phase difference.
Similar results have recently been obtained in other S/I/S junctions coexisting
with helimagnetic or ferromagnetic order. We calculate the superconducting
order parameter self-consistently, and investigate quantitatively the effect
which the misaligned exchange fields constitute on the Josephson current, to
see if I_m may have an appreciable effect on the Josephson current. It is found
that I_0 and I_m become comparable in magnitude at sufficiently low
temperatures and fields close to the critical value, in agreement with previous
work. From our analytical results, it then follows that the Josephson current
in the present system may be controlled in a well-defined manner by a rotation
of the exchange fields on both sides of the junction. We discuss a possible
experimental realization of this proposition.Comment: 8 pages, 8 figures. Accepted for publication in Phys. Rev.
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