19,451 research outputs found
Momentum alignment and the optical valley Hall effect in low-dimensional Dirac materials
We study the momentum alignment phenomenon and the optical control of valley
population in gapless and gapped graphene-like materials. We show that the
trigonal warping effect allows for the spatial separation of carriers belonging
to different valleys via the application of linearly polarized light. Valley
separation in gapped materials can be detected by measuring the degree of
circular polarization of band-edge photoluminescence at different sides of the
sample or light spot (optical valley Hall effect). We also show that the
momentum alignment phenomenon leads to the giant enhancement of near-band-edge
interband optical transitions in narrow-gap carbon nanotubes and graphene
nanoribbons independent of the mechanism of the gap formation. A detection
scheme to observe these giant interband transitions is proposed which opens a
route for creating novel terahertz radiation emitters.Comment: 28 pages, 9 figure
Interpolation and harmonic majorants in big Hardy-Orlicz spaces
Free interpolation in Hardy spaces is caracterized by the well-known Carleson
condition. The result extends to Hardy-Orlicz spaces contained in the scale of
classical Hardy spaces , . For the Smirnov and the Nevanlinna
classes, interpolating sequences have been characterized in a recent paper in
terms of the existence of harmonic majorants (quasi-bounded in the case of the
Smirnov class). Since the Smirnov class can be regarded as the union over all
Hardy-Orlicz spaces associated with a so-called strongly convex function, it is
natural to ask how the condition changes from the Carleson condition in
classical Hardy spaces to harmonic majorants in the Smirnov class. The aim of
this paper is to narrow down this gap from the Smirnov class to ``big''
Hardy-Orlicz spaces. More precisely, we characterize interpolating sequences
for a class of Hardy-Orlicz spaces that carry an algebraic structure and that
are strictly bigger than . It turns out that the
interpolating sequences are again characterized by the existence of
quasi-bounded majorants, but now the weights of the majorants have to be in
suitable Orlicz spaces. The existence of harmonic majorants in such Orlicz
spaces will also be discussed in the general situation. We finish the paper
with an example of a separated Blaschke sequence that is interpolating for
certain Hardy-Orlicz spaces without being interpolating for slightly smaller
ones.Comment: 19 pages, 2 figure
Evidence for from photoproduction and consequence for chiral-symmetry restoration at high mass
We report a partial-wave analysis of new data on the double-polarization
variable for the reactions and
and of further data published earlier. The analysis within the Bonn-Gatchina
(BnGa) formalism reveals evidence for a poorly known baryon resonance, the
one-star . This is the lowest-mass resonance with
spin-parity . Its mass is significantly higher than the mass of its
parity partner which is the lowest-mass
resonance with spin-parity . It has been suggested that chiral
symmetry might be restored in the high-mass region of hadron excitations, and
that these two resonances should be degenerate in mass. Our findings are in
conflict with this prediction.Comment: 5 pages, 3 figures; Physics Letters B in pres
Self-tuning of threshold for a two-state system
A two-state system (TSS) under time-periodic perturbations (to be regarded as
input signals) is studied in connection with self-tuning (ST) of threshold and
stochastic resonance (SR). By ST, we observe the improvement of signal-to-noise
ratio (SNR) in a weak noise region. Analytic approach to a tuning equation
reveals that SNR improvement is possible also for a large noise region and this
is demonstrated by Monte Carlo simulations of hopping processes in a TSS. ST
and SR are discussed from a little more physical point of energy transfer
(dissipation) rate, which behaves in a similar way as SNR. Finally ST is
considered briefly for a double-well potential system (DWPS), which is closely
related to the TSS
Bound States for a Magnetic Impurity in a Superconductor
We discuss a solvable model describing an Anderson like impurity in a BCS
superconductor. The model can be mapped onto an Ising field theory in a
boundary magnetic field, with the Ising fermions being the quasi-particles of
the Bogoliubov transformation in BCS theory. The reflection S-matrix exhibits
Andreev scattering, and the existence of bound states of the quasi-particles
with the impurity lying inside the superconducting gap.Comment: 7 pages, Plain Te
Computation of the asymptotic states of modulated open quantum systems with a numerically exact realization of the quantum trajectory method
Quantum systems out of equilibrium are presently a subject of active
research, both in theoretical and experimental domains. In this work we
consider time-periodically modulated quantum systems which are in contact with
a stationary environment. Within the framework of a quantum master equation,
the asymptotic states of such systems are described by time-periodic density
operators. Resolution of these operators constitutes a non-trivial
computational task. To go beyond the current size limits, we use the quantum
trajectory method which unravels master equation for the density operator into
a set of stochastic processes for wave functions. The asymptotic density matrix
is calculated by performing a statistical sampling over the ensemble of quantum
trajectories, preceded by a long transient propagation. We follow the ideology
of event-driven programming and construct a new algorithmic realization of the
method. The algorithm is computationally efficient, allowing for long 'leaps'
forward in time, and is numerically exact in the sense that, being given the
list of uniformly distributed (on the unit interval) random numbers, , one could propagate a quantum trajectory (with 's
as norm thresholds) in a numerically exact way. %Since the quantum trajectory
method falls into the class of standard sampling problems, performance of the
algorithm %can be substantially improved by implementing it on a computer
cluster. By using a scalable -particle quantum model, we demonstrate that
the algorithm allows us to resolve the asymptotic density operator of the model
system with states on a regular-size computer cluster, thus reaching
the scale on which numerical studies of modulated Hamiltonian systems are
currently performed
On "the authentic damping mechanism" of the phonon damping model
Some general features of the phonon damping model are presented. It is
concluded that the fits performed within this model have no physical content
First excitations in two- and three-dimensional random-field Ising systems
We present results on the first excited states for the random-field Ising
model. These are based on an exact algorithm, with which we study the
excitation energies and the excitation sizes for two- and three-dimensional
random-field Ising systems with a Gaussian distribution of the random fields.
Our algorithm is based on an approach of Frontera and Vives which, in some
cases, does not yield the true first excited states. Using the corrected
algorithm, we find that the order-disorder phase transition for three
dimensions is visible via crossings of the excitations-energy curves for
different system sizes, while in two-dimensions these crossings converge to
zero disorder. Furthermore, we obtain in three dimensions a fractal dimension
of the excitations cluster of d_s=2.42(2). We also provide analytical droplet
arguments to understand the behavior of the excitation energies for small and
large disorder as well as close to the critical point.Comment: 17 pages, 12 figure
Impurity relaxation mechanism for dynamic magnetization reversal in a single domain grain
The interaction of coherent magnetization rotation with a system of two-level
impurities is studied. Two different, but not contradictory mechanisms, the
`slow-relaxing ion' and the `fast-relaxing ion' are utilized to derive a system
of integro-differential equations for the magnetization. In the case that the
impurity relaxation rate is much greater than the magnetization precession
frequency, these equations can be written in the form of the Landau-Lifshitz
equation with damping. Thus the damping parameter can be directly calculated
from these microscopic impurity relaxation processes
- âŠ