6,310 research outputs found
Electron-beam dynamics in a strong laser field including quantum radiation reaction
The evolution of an electron beam colliding head-on with a strong plane-wave
field is investigated in the framework of strong-field QED including
radiation-reaction effects due to photon emission. Employing a kinetic approach
to describe the electron and the photon distribution it is shown that at a
given total laser fluence the final electron distribution depends on the shape
of the laser envelope and on the pulse duration, in contrast to the classical
predictions of radiation reaction based on the Landau-Lifshitz equation.
Finally, it is investigated how the pair-creation process leads to a nonlinear
coupled evolution of the electrons in the beam, of the produced charged
particles, and of the emitted photons.Comment: 25 pages, 11 figure
Orbital currents, anapoles, and magnetic quadrupoles in CuO
We show that orbital currents in a CuO2 plane, if present, should be
described by two independent parity and time-reversal odd order parameters, a
toroidal dipole (anapole) and a magnetic quadrupole. Based on this, we derive
the resonant X-ray diffraction cross-section for monoclinic CuO at the
antiferromagnetic wavevector and show that the two order parameters can be
disentangled. From our analysis, we examine a recent claim of detecting
anapoles in CuO.Comment: 7 pages, 5 figure
The magnetic ground state of Sr2IrO4 and implications for second-harmonic generation
The currently accepted magnetic ground state of Sr2IrO4 (the -++- state)
preserves inversion symmetry. This is at odds, though, with recent experiments
that indicate a magnetoelectric ground state, leading to the speculation that
orbital currents or more exotic magnetic multipoles might exist in this
material. Here, we analyze various magnetic configurations and demonstrate that
two of them, the magnetoelectric -+-+ state and the non-magnetoelectric ++++
state, can explain these recent second-harmonic generation (SHG) experiments,
obviating the need to invoke orbital currents. The SHG-probed magnetic order
parameter has the symmetry of a parity-breaking multipole in the -+-+ state and
of a parity-preserving multipole in the ++++ state. We speculate that either
might have been created by the laser pump used in the experiments. An
alternative is that the observed magnetic SHG signal is a surface effect. We
suggest experiments that could be performed to test these various
possibilities, and also address the important issue of the suppression of the
RXS intensity at the L2 edge.Comment: 28 pages, 8 figures, v3 - an expanded discussion of the origin of the
SHG signa
The nature of the tensor order in Cd2Re2O7
The pyrochlore metal Cd2Re2O7 has been recently investigated by
second-harmonic generation (SHG) reflectivity. In this paper, we develop a
general formalism that allows for the identification of the relevant tensor
components of the SHG from azimuthal scans. We demonstrate that the secondary
order parameter identified by SHG at the structural phase transition is the
x2-y2 component of the axial toroidal quadrupole. This differs from the 3z2-r2
symmetry of the atomic displacements associated with the I-4m2 crystal
structure that was previously thought to be its origin. Within the same
formalism, we suggest that the primary order parameter detected in the SHG
experiment is the 3z2-r2 component of the magnetic quadrupole. We discuss the
general mechanism driving the phase transition in our proposed framework, and
suggest experiments, particularly resonant X-ray scattering ones, that could
clarify this issue.Comment: some additions and clarifications adde
A new tool for the performance analysis of massively parallel computer systems
We present a new tool, GPA, that can generate key performance measures for
very large systems. Based on solving systems of ordinary differential equations
(ODEs), this method of performance analysis is far more scalable than
stochastic simulation. The GPA tool is the first to produce higher moment
analysis from differential equation approximation, which is essential, in many
cases, to obtain an accurate performance prediction. We identify so-called
switch points as the source of error in the ODE approximation. We investigate
the switch point behaviour in several large models and observe that as the
scale of the model is increased, in general the ODE performance prediction
improves in accuracy. In the case of the variance measure, we are able to
justify theoretically that in the limit of model scale, the ODE approximation
can be expected to tend to the actual variance of the model
X-ray Dichroism and the Pseudogap Phase of Cuprates
A recent polarized x-ray absorption experiment on the high temperature
cuprate superconductor Bi2Sr2CaCu2O8 indicates the presence of broken parity
symmetry below the temperature, T*, where a pseudogap appears in photoemission.
We critically analyze the x-ray data, and conclude that a parity-breaking
signal of the kind suggested is unlikely based on the crystal structures
reported in the literature. Possible other origins of the observed dichroism
signal are discussed. We propose x-ray scattering experiments that can be done
in order to determine whether such alternative interpretations are valid or
not.Comment: final version to be published in Phys Rev B: some calculational
details added, clarification of XNLD contamination and biaxiality, more
discussion on possible space groups and previous optics result
On the Effectiveness of Retrieval, Alignment, and Replay in Manipulation
Imitation learning with visual observations is notoriously inefficient when
addressed with end-to-end behavioural cloning methods. In this paper, we
explore an alternative paradigm which decomposes reasoning into three phases.
First, a retrieval phase, which informs the robot what it can do with an
object. Second, an alignment phase, which informs the robot where to interact
with the object. And third, a replay phase, which informs the robot how to
interact with the object. Through a series of real-world experiments on
everyday tasks, such as grasping, pouring, and inserting objects, we show that
this decomposition brings unprecedented learning efficiency, and effective
inter- and intra-class generalisation. Videos are available at
https://www.robot-learning.uk/retrieval-alignment-replay.Comment: Published in IEEE Robotics and Automation Letters (RA-L). (Accepted
December 2023
DINOBot: Robot Manipulation via Retrieval and Alignment with Vision Foundation Models
We propose DINOBot, a novel imitation learning framework for robot
manipulation, which leverages the image-level and pixel-level capabilities of
features extracted from Vision Transformers trained with DINO. When interacting
with a novel object, DINOBot first uses these features to retrieve the most
visually similar object experienced during human demonstrations, and then uses
this object to align its end-effector with the novel object to enable effective
interaction. Through a series of real-world experiments on everyday tasks, we
show that exploiting both the image-level and pixel-level properties of vision
foundation models enables unprecedented learning efficiency and generalisation.
Videos and code are available at https://www.robot-learning.uk/dinobot.Comment: To appear at 2024 IEEE International Conference on Robotics and
Automation (ICRA
Stochastic Calculus of Wrapped Compartments
The Calculus of Wrapped Compartments (CWC) is a variant of the Calculus of
Looping Sequences (CLS). While keeping the same expressiveness, CWC strongly
simplifies the development of automatic tools for the analysis of biological
systems. The main simplification consists in the removal of the sequencing
operator, thus lightening the formal treatment of the patterns to be matched in
a term (whose complexity in CLS is strongly affected by the variables matching
in the sequences).
We define a stochastic semantics for this new calculus. As an application we
model the interaction between macrophages and apoptotic neutrophils and a
mechanism of gene regulation in E.Coli
Novel aspects of radiation reaction in the classical and the quantum regime
This work is dedicated to the study of radiation reaction signatures in the
framework of classical and quantum electrodynamics. Since there has been no
distinct experimental validation of radiation reaction and its underlying
equations so far and its impact is expected to be substantial for the
construction of new experimental devices, e.g., quantum x-free electron lasers,
a profound understanding of radiation reaction effects is of special interest.
Here, we describe how the inclusion of quantum radiation reaction effects
changes the dynamics of ultra-relativistic electron beams colliding with
intense laser pulses significantly. Thereafter, the angular distribution of
emitted radiation is demonstrated to be strongly altered in the quantum
framework, if in addition to single photon emission also higher order photon
emissions are considered. Furthermore, stimulated Raman scattering of an
ultra-intense laser pulse in plasmas is examined and forward Raman scattering
is found to be significantly increased by the inclusion of radiation reaction
effects in the classical regime. The numerical simulations in this work show
the feasibility of an experimental verification of the predicted effects with
presently available lasers and electron accelerators.Comment: 13 pages, 3 figures, Proceedings of the LPHYS'13 Worksho
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