18,231 research outputs found
Representations of Hecke algebras on quotients of path algebras
Let be a Coxeter system. A -graph encodes a representation of the
Hecke algebra of . We construct universal representations of
multi-parameter Hecke algebras on certain quotients of path algebras, and study
their relationships with -graph representations. We also study the quotients
of path algebras on their own, motivated by one example where the quotient path
algebra is isomorphic to an ideal of Lusztig asymptotic Hecke algebra. Finally,
we describe a method to obtain a generating set for the ideals by which we
quotient the path algebras
Magnetic field induced spin-wave energy focusing
Spin waves can transport both energy and angular momentum over long distances
as they propagate. However, due to damping, their amplitude decreases
exponentially as they move away from the source, leaving little capability for
manipulating how much energy and angular momentum is to be delivered where.
Here we show that a suitable local reduction of the effective field can lead to
a large accumulation of spin wave energy far from the source. Moreover, both
the location and the amount of energy to be delivered can be controlled
accurately with geometry and externalm magnetic fields. Thus, we put forward a
general, robust and flexible approach to convey both heat and spin in
ferromagnets, which can be directly used in spintronic devices
Compensation temperature in spin- Ising trilayers: A Monte Carlo study
We study the magnetic and thermodynamic properties of a spin- Ising
system containing three layers, each of which is composed exclusively of one
out of two possible types of atoms, \textbf{A} or \textbf{B}. The \textbf{A-A}
and \textbf{B-B} bonds are ferromagnetic while the \textbf{A-B} bonds are
antiferromagnetic. The study is performed through Monte Carlo simulations using
the Wolff algorithm and the data are analyzed with the aid of the
multiple-histogram reweighting technique and finite-size scaling tools. We
verify the occurrence of a compensation phenomenon and obtain the compensation
and critical temperatures of the model as functions of the Hamiltonian
parameters. The influence of each parameter on the overall behavior of the
system is discussed in detail and we present our results in the form of phase
diagrams dividing the parameter space in regions where the compensation
phenomenon is present or absent. Our results may provide invaluable information
for experimentalists seeking to build materials with desired characteristics.Comment: 8 pages of text; 20 figures. arXiv admin note: substantial text
overlap with arXiv:1705.10192, arXiv:1710.1029
Pinned domain wall oscillator as tunable direct current spin wave emitter
Spin waves are perturbations in the relative orientation of magnetic moments
in a continuous magnetic system, which have been proposed as a new kind of
information carrier for spin-based low power applications. For this purpose, a
major obstacle to overcome is the energy-efficient excitation of coherent short
wavelength spin waves and alternatives to excitation via the Oersted field of
an alternating current need to be explored. Here we show, by means of
micromagnetic simulations, how, in a perpendicularly magnetized thin strip, a
domain wall pinned at a geometrical constriction emits spin waves when forced
to rotate by the application of a low direct current flowing along the strip.
Spin waves propagate only in the direction of the electron's flow at the first
odd harmonic of the domain wall rotation frequency for which propagation is
allowed. Excitation is due to in-plane dipolar stray field of the rotating
domain wall and that the resulting unidirectionality is a consequence of the
domain wall displacement at the constriction. On the other hand, the
application of an external field opposing domain wall depinning breaks the
symmetry for spin wave propagation in the two domains, allowing emission in
both directions but at different frequencies. The results presented define a
new approach to produce tunable high frequency spin wave emitters of easy
fabrication and low power consumption
Monte Carlo simulations of an Ising bilayer with non-equivalent planes
We study the thermodynamic and magnetic properties of an Ising bilayer
ferrimagnet. The system is composed of two interacting non-equivalent planes in
which the intralayer couplings are ferromagnetic while the interlayer
interactions are antiferromagnetic. Moreover, one of the planes is randomly
diluted. The study is carried out within a Monte Carlo approach employing the
multiple histogram reweighting method and finite-size scaling tools. The
occurrence of a compensation phenomenon is verified and the compensation
temperature, as well as the critical temperature for the model, are obtained as
functions of the Hamiltonian parameters. We present a detailed discussion of
the regions of the parameter space where the compensation effect is present or
absent. Our results are then compared to a mean-field-like approximation
applied to the same model by Balcerzak and Sza{\l}owski (2014). Although the
Monte Carlo and mean-field results agree qualitatively, our quantitative
results are significantly different
Monte Carlo study of an anisotropic Ising multilayer with antiferromagnetic interlayer couplings
We present a Monte Carlo study of the magnetic properties of an Ising
multilayer ferrimagnet. The system consists of two kinds of non-equivalent
planes, one of which is site-diluted. All intralayer couplings are
ferromagnetic. The different kinds of planes are stacked alternately and the
interlayer couplings are antiferromagnetic. We perform the simulations using
the Wolff algorithm and employ multiple histogram reweighting and finite-size
scaling methods to analyze the data with special emphasis on the study of
compensation phenomena. Compensation and critical temperatures of the system
are obtained as functions of the Hamiltonian parameters and we present a
detailed discussion about the contribution of each parameter to the presence or
absence of the compensation effect. A comparison is presented between our
results and those reported in the literature for the same model using the pair
approximation. We also compare our results with those obtained through both the
pair approximation and Monte Carlo simulations for the bilayer system.Comment: 13 figures. arXiv admin note: text overlap with arXiv:1607.0522
Computational study of microwave oscillations in absence of external field in nonstandard spin valves in the diffusive transport limit
An anomalous (inverse) spin accumulation in the nonmagnetic spacer may build
up when the spin valve consists of magnetic films having different spin
symmetries. This leads to wavy-like dependence of spin-transfer torque on the
angle between magnetizations, as predicted by spin-dependent diffusive
transport model, and also confirmed experimentally. Making use of these
predictions, we have numerically studied the magnetization dynamics in presence
of such a wavy-torque in Co(8 nm)/Cu(10 nm)/Py(8 nm) nanopillar, considering
geometry with extended and etched Co layer. In both cases we specify conditions
for the out-of-plane precession to appear in absence of external magnetic field
and neglecting thermal fluctuations. We prove the assumption of wavy-like
torque angular dependence to be fully consistent with experimental
observations. We also show that some features reported experimentally, like
nonlinear slope of frequency vs. current behavior, are beyond the applicability
range of macrospin approximation and can be explained only by means of full
micromagnetic analysis.Comment: 9 pages, 11 figure
Sobolev extremal polynomials with respect to mutually singular measures
We consider extremal polynomials with respect to a Sobolev-type -norm,
with and measures supported on compact subsets of the real line.
For a wide class of such extremal polynomials with respect to mutually singular
measures (i.e. supported on disjoint subsets of the real line), it is proved
that their critical points are simple and contained in the interior of the
convex hull of the support of the measures involved and the asymptotic critical
point distribution is studied. We also find the th root asymptotic behavior
of the corresponding sequence of Sobolev extremal polynomials and their
derivatives.Comment: 18 page
Perfect fluid brane-world model
By considering 5--dimensional cosmological models with a bulk filled with a
perfect fluid and a cosmological constant, we have found regular instantonic
solution which is free from any singularity at the origin of the
extra--coordinate and describe 5--dimensional asymptotically anti de Sitter
wormhole, when the bulk has a topology and is filled with dust
and a negative cosmological constant. Compactified brane-world instantons which
are built up from such instantonic solution describe either a single brane or a
string of branes. Their analytical continuation to the pseudo--Riemannian
metric can give rise to either 4-dimensional inflating branes or solutions with
the same dynamical behaviour for extra--dimension and branes, in addition to
multitemporal solutions. Dust brane-world models with other spatial topologies
are also considered.Comment: 14 pages, LateX2e, 4 figure
Lepton pair emission in the top quark decay
The heaviness of the top quark makes its 2-body decay mode to be
dominant, at such level that hardly any other decay mode reaches a detectable
branching ratio (BR) within the SM. Here we study the decay (), which diverges for massless leptons,
and it can reach a BR for reasonable
values of the low energy cut in the lepton-pair invariant mass. This rate
surpasses almost any other rare decays such as (), and thus offers the possibility of being detectable. Furthermore, the
estimate of this channel is relevant because it can mimic the signal arising
from the lepton number violating decay , when the
boson decays into lepton channels.Comment: 9 pages, 4 figures, v2: comments and references added, matches with
version accepted in Phys. Rev.
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