8,616 research outputs found
Kinematic dynamo action in a sphere: Effects of periodic time-dependent flows on solutions with axial dipole symmetry
Choosing a simple class of flows, with characteristics that may be present in
the Earth's core, we study the ability to generate a magnetic field when the
flow is permitted to oscillate periodically in time. The flow characteristics
are parameterised by D, representing a differential rotation, M, a meridional
circulation, and C, a component characterising convective rolls. Dynamo action
is sensitive to these flow parameters and fails spectacularly for much of the
parameter space where magnetic flux is concentrated into small regions.
Oscillations of the flow are introduced by varying the flow parameters in
time, defining a closed orbit in the space (D,M). Time-dependence appears to
smooth out flux concentrations, often enhancing dynamo action. Dynamo action
can be impaired, however, when flux concentrations of opposite signs occur
close together as smoothing destroys the flux by cancellation.
It is possible to produce geomagnetic-type reversals by making the orbit
stray into a region where the steady flows generate oscillatory fields. In this
case, however, dynamo action was not found to be enhanced by the
time-dependence.
A novel approach is taken to solving the time-dependent eigenvalue problem,
where by combining Floquet theory with a matrix-free Krylov-subspace method we
avoid large memory requirements for storing the matrix required by the standard
approach.Comment: 22 pages, 12 figures. Geophys. Astrophys. Fluid Dynam., as accepted
(2004
Preserving In-Context Learning ability in Large Language Model Fine-tuning
Pretrained large language models (LLMs) are strong in-context learners that
are able to perform few-shot learning without changing model parameters.
However, as we show, fine-tuning an LLM on any specific task generally destroys
its in-context ability. We discover an important cause of this loss, format
specialization, where the model overfits to the format of the fine-tuned task
and is unable to output anything beyond this format. We further show that
format specialization happens at the beginning of fine-tuning. To solve this
problem, we propose Prompt Tuning with MOdel Tuning (ProMoT), a simple yet
effective two-stage fine-tuning framework that preserves in-context abilities
of the pretrained model. ProMoT first trains a soft prompt for the fine-tuning
target task, and then fine-tunes the model itself with this soft prompt
attached. ProMoT offloads task-specific formats into the soft prompt that can
be removed when doing other in-context tasks. We fine-tune mT5 XXL with ProMoT
on natural language inference (NLI) and English-French translation and evaluate
the in-context abilities of the resulting models on 8 different NLP tasks.
ProMoT achieves similar performance on the fine-tuned tasks compared with
vanilla fine-tuning, but with much less reduction of in-context learning
performances across the board. More importantly, ProMoT shows remarkable
generalization ability on tasks that have different formats, e.g. fine-tuning
on a NLI binary classification task improves the model's in-context ability to
do summarization (+0.53 Rouge-2 score compared to the pretrained model), making
ProMoT a promising method to build general purpose capabilities such as
grounding and reasoning into LLMs with small but high quality datasets. When
extended to sequential or multi-task training, ProMoT can achieve even better
out-of-domain generalization performance
Brominated B1-polycyclic aromatic hydrocarbons for the synthesis of deep-red to near-infrared delayed fluorescence emitters
This project has received funding from the Leverhulme Trust (Grant RPG-2022-032) and the European Research Council (ERC) under the European Unionâs Horizon 2020 research and innovation program (Grant Agreement 769599). C.S. thanks the China Scholarship Council (201806890001), and M.I. and E.Z.-C. thank the EPSRC Programme Grant âBoron: Beyond the Reagentâ (EP/W007517) for support.Bromo-functionalized B1-polycyclic aromatic hydrocarbons (PAHs) with LUMOs of less than â3.0 eV were synthesized and used in cross-couplings to form donorâacceptor materials. These materials spanned a range of S1 energies, with a number showing thermally activated delayed fluorescence and significant emission in the near-infrared region of the spectrum. These B1-PAHs represent a useful family of acceptors that can be readily synthesized and functionalized.Publisher PDFPeer reviewe
Measuring the beaming angle of GRB 030329 by fitting the rebrightenings in its multiband afterglow
Multiple rebrightenings have been observed in the multiband afterglow of GRB
030329. Especially, a marked and quick rebrightening occurred at about t ~ 1.2
* 10^5 s. Energy injection from late and slow shells seems to be the best
interpretation for these rebrightenings. Usually it is assumed that the energy
is injected into the whole external shock. However, in the case of GRB 030329,
the rebrightenings are so quick that the usual consideration fails to give a
satisfactory fit to the observed light curves. Actually, since these late/slow
shells coast freely in the wake of the external shock, they should be cold and
may not expand laterally. The energy injection then should only occur at the
central region of the external shock. Considering this effect, we numerically
re-fit the quick rebrightenings observed in GRB 030329. By doing this, we were
able to derive the beaming angle of the energy injection process. Our result,
with a relative residual of only 5% - 10% during the major rebrightening, is
better than any previous modeling. The derived energy injection angle is about
0.035. We assume that these late shells are ejected by the central engine via
the same mechanism as those early shells that produce the prompt gamma-ray
burst. The main difference is that their velocities are much slower, so that
they catch up with the external shock very lately and manifest as the observed
quick rebrightenings. If this were true, then the derived energy injection
angle can give a good measure of the beaming angle of the prompt gamma-ray
emission. Our study may hopefully provide a novel method to measure the beaming
angle of gamma-ray bursts.Comment: 8 pages, 6 figures, Has been accepted by RAA (Research in Astronomy
and Astrophysics
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