10,418 research outputs found
Effects of polymer additives in the bulk of turbulent thermal convection
We present experimental evidence that a minute amount of polymer additives
can significantly enhance heat transport in the bulk region of turbulent
thermal convection. The effects of polymer additives are found to be the
\textit{suppression} of turbulent background fluctuations that give rise to
incoherent heat fluxes that make no net contribution to heat transport, and at
the same time to \textit{increase} the coherency of temperature and velocity
fields. The suppression of small-scale turbulent fluctuations leads to more
coherent thermal plumes that result in the heat transport enhancement. The fact
that polymer additives can increase the coherency of thermal plumes is
supported by the measurements of a number of local quantities, such as the
extracted plume amplitude and width, the velocity autocorrelation functions and
the velocity-temperature cross-correlation coefficient. The results from local
measurements also suggest the existence of a threshold value for the polymer
concentration, only above which can significant modification of the plume
coherent properties and enhancement of the local heat flux be observed.
Estimation of the plume emission rate suggests that the second effect of
polymer additives is to stabilize the thermal boundary layers.Comment: 8 figures, 11 page
Nonlocal coherence harvesting from quantum vacuum
It is well known that nonlocal coherence reflects nonclassical correlations
better than quantum entan-glement. Here, we analyze nonlocal coherence
harvesting from the quantum vacuum to particle detectors adiabatically
interacting with a quantum scalar field in Minkowski spacetime. We find that
the harvesting-achievable separation range of nonlocal coherence is larger than
that of quantum entanglement. As the energy gap grows sufficiently large, the
detectors harvest less quantum coherence, while the detectors could extract
more quantum entanglement from the vacuum state. Compared with the linear
configuration and the scalene configuration, the equilateral triangle
configuration is the best model to harvest tripartite coherence. Finally, we
find a monogamous relationship, which means that tripartite l1-norm of
coherence is essentially bipartite types.Comment: 18 pages, 5 figure
Does Hawking effect always degrade fidelity of quantum teleportation in Schwarzschild spacetime?
Previous studies have shown that the Hawking effect always destroys quantum
correlations and the fidelity of quantum teleportation in the Schwarzschild
black hole. Here, we investigate the fidelity of quantum teleportation of Dirac
fields between users in Schwarzschild spacetime. We find that, with the
increase of the Hawking temperature, the fidelity of quantum teleportation can
monotonically increase, monotonically decrease, or non-monotonically increase,
depending on the choice of the initial state, which means that the Hawking
effect can create net fidelity of quantum teleportation. This striking result
banishes the extended belief that the Hawking effect of the black hole can only
destroy the fidelity of quantum teleportation. We also find that quantum
steering cannot fully guarantee the fidelity of quantum teleportation in
Schwarzschild spacetime. This new unexpected source may provide a new idea for
the experimental evidence of the Hawking effect.Comment: 21 pages, 3 figures, accepted for publication in JHE
Van der Waals Engineering of Ferromagnetic Semiconductor Heterostructures for Spin and Valleytronics
The integration of magnetic material with semiconductors has been fertile
ground for fundamental science as well as of great practical interest toward
the seamless integration of information processing and storage. Here we create
van der Waals heterostructures formed by an ultrathin ferromagnetic
semiconductor CrI3 and a monolayer of WSe2. We observe unprecedented control of
the spin and valley pseudospin in WSe2, where we detect a large magnetic
exchange field of nearly 13 T and rapid switching of the WSe2 valley splitting
and polarization via flipping of the CrI3 magnetization. The WSe2
photoluminescence intensity strongly depends on the relative alignment between
photo-excited spins in WSe2 and the CrI3 magnetization, due to ultrafast
spin-dependent charge hopping across the heterostructure interface. The
photoluminescence detection of valley pseudospin provides a simple and
sensitive method to probe the intriguing domain dynamics in the ultrathin
magnet, as well as the rich spin interactions within the heterostructure.Comment: Supplementary Materials included. To appear in Science Advance
Electrical Control of 2D Magnetism in Bilayer CrI3
The challenge of controlling magnetism using electric fields raises
fundamental questions and addresses technological needs such as low-dissipation
magnetic memory. The recently reported two-dimensional (2D) magnets provide a
new system for studying this problem owing to their unique magnetic properties.
For instance, bilayer chromium triiodide (CrI3) behaves as a layered
antiferromagnet with a magnetic field-driven metamagnetic transition. Here, we
demonstrate electrostatic gate control of magnetism in CrI3 bilayers, probed by
magneto-optical Kerr effect (MOKE) microscopy. At fixed magnetic fields near
the metamagnetic transition, we realize voltage-controlled switching between
antiferromagnetic and ferromagnetic states. At zero magnetic field, we
demonstrate a time-reversal pair of layered antiferromagnetic states which
exhibit spin-layer locking, leading to a remarkable linear dependence of their
MOKE signals on gate voltage with opposite slopes. Our results pave the way for
exploring new magnetoelectric phenomena and van der Waals spintronics based on
2D materials.Comment: To appear in Nature Nanotechnolog
The Origin of the Prompt Emission for Short GRB 170817A: Photosphere Emission or Synchrotron Emission?
The first gravitational-wave event from the merger of a binary neutron star system (GW170817) was detected recently. The associated short gamma-ray burst (GRB 170817A) has a low isotropic luminosity (~1047 erg s−1) and a peak energy E p ~ 145 keV during the initial main emission between −0.3 and 0.4 s. The origin of this short GRB is still under debate, but a plausible interpretation is that it is due to the off-axis emission from a structured jet. We consider two possibilities. First, since the best-fit spectral model for the main pulse of GRB 170817A is a cutoff power law with a hard low-energy photon index (), we consider an off-axis photosphere model. We develop a theory of photosphere emission in a structured jet and find that such a model can reproduce a low-energy photon index that is softer than a blackbody through enhancing high-latitude emission. The model can naturally account for the observed spectrum. The best-fit Lorentz factor along the line of sight is ~20, which demands that there is a significant delay between the merger and jet launching. Alternatively, we consider that the emission is produced via synchrotron radiation in an optically thin region in an expanding jet with decreasing magnetic fields. This model does not require a delay of jet launching but demands a larger bulk Lorentz factor along the line of sight. We perform Markov Chain Monte Carlo fitting to the data within the framework of both models and obtain good fitting results in both cases
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