3,428 research outputs found
Friedel oscillations in graphene gapped by breaking \u3ci\u3eƤ\u3c/i\u3e and \u3ci\u3eT\u3c/i\u3e symmetries: Topological and geometrical signatures of electronic structure
The measurement of Friedel oscillations (FOs) is conventionally used to recover the energy dispersion of electronic structure. Besides the energy dispersion, the modern electronic structure also embodies other key ingredients such as the geometrical and topological properties; it is one promising direction to explore the potential of FOs for the relevant measurement. Here, we present a comprehensive study of FOs in substrate-supported graphene under off-resonant circularly polarized light, in which a valley-contrasting feature and topological phase transition occur due to the combined breaking of inversion (Ƥ) and time reversal (T) symmetries. Depending on the position of the Fermi level, FOs may be contributed by electronic backscattering in one single valley or two valleys. In the single-valley regime, the oscillation periods of FOs can be used to determine the topological phase boundary of electronic structure, while the amplitudes of FOs distinguish trivial insulators and topological insulators in a quantitative way. In the two-valley regime, the unequal Fermi surfaces lead to a beating pattern (robust two-wave-front dislocations) of FOs contributed by intravalley (intervalley) scattering. This study implies the great potential of FOs in characterizing topological and geometrical properties of the electronic structure of two-dimensional materials
Robust wavefront dislocations of Friedel oscillations in gapped graphene
Friedel oscillation is a well-known wave phenomenon, which represents the
oscillatory response of electron waves to imperfection. By utilizing the
pseudospin-momentum locking in gapless graphene, two recent experiments
demonstrate the measurement of the topological Berry phase by corresponding to
the unique number of wavefront dislocations in Friedel oscillations. Here, we
study the Friedel oscillations in gapped graphene, in which the
pseudospin-momentum locking is broken. Unusually, the wavefront dislocations do
occur as that in gapless graphene, which expects the immediate verification in
the current experimental condition. The number of wavefront dislocations is
ascribed to the invariant pseudospin winding number in gaped and gapless
graphene. This study deepens the understanding of correspondence between
topological quantity and wavefront dislocations in Friedel oscillations, and
implies the possibility to observe the wavefront dislocations of Friedel
oscillations in intrinsic gapped two-dimensional materials, e.g., transition
metal dichalcogenides.Comment: 5 pages, 3 figure
Make Transformer Great Again for Time Series Forecasting: Channel Aligned Robust Dual Transformer
Recent studies have demonstrated the great power of deep learning methods,
particularly Transformer and MLP, for time series forecasting. Despite its
success in NLP and CV, many studies found that Transformer is less effective
than MLP for time series forecasting. In this work, we design a special
Transformer, i.e., channel-aligned robust dual Transformer (CARD for short),
that addresses key shortcomings of Transformer in time series forecasting.
First, CARD introduces a dual Transformer structure that allows it to capture
both temporal correlations among signals and dynamical dependence among
multiple variables over time. Second, we introduce a robust loss function for
time series forecasting to alleviate the potential overfitting issue. This new
loss function weights the importance of forecasting over a finite horizon based
on prediction uncertainties. Our evaluation of multiple long-term and
short-term forecasting datasets demonstrates that CARD significantly
outperforms state-of-the-art time series forecasting methods, including both
Transformer and MLP-based models
Decentralized Federated Reinforcement Learning for User-Centric Dynamic TFDD Control
The explosive growth of dynamic and heterogeneous data traffic brings great
challenges for 5G and beyond mobile networks. To enhance the network capacity
and reliability, we propose a learning-based dynamic time-frequency division
duplexing (D-TFDD) scheme that adaptively allocates the uplink and downlink
time-frequency resources of base stations (BSs) to meet the asymmetric and
heterogeneous traffic demands while alleviating the inter-cell interference. We
formulate the problem as a decentralized partially observable Markov decision
process (Dec-POMDP) that maximizes the long-term expected sum rate under the
users' packet dropping ratio constraints. In order to jointly optimize the
global resources in a decentralized manner, we propose a federated
reinforcement learning (RL) algorithm named federated Wolpertinger deep
deterministic policy gradient (FWDDPG) algorithm. The BSs decide their local
time-frequency configurations through RL algorithms and achieve global training
via exchanging local RL models with their neighbors under a decentralized
federated learning framework. Specifically, to deal with the large-scale
discrete action space of each BS, we adopt a DDPG-based algorithm to generate
actions in a continuous space, and then utilize Wolpertinger policy to reduce
the mapping errors from continuous action space back to discrete action space.
Simulation results demonstrate the superiority of our proposed algorithm to
benchmark algorithms with respect to system sum rate
Vortex Dynamics in Rotating Rayleigh-B\'enard Convection
We investigate the spatial distribution and dynamics of the vortices in
rotating Rayleigh-B\'enard convection in a reduced Rayleigh-number range
. Under slow rotations (), the
vortices are randomly distributed. The size-distribution of the Voronoi cells
of the vortex centers is well described by the standard distribution.
In this flow regime the vortices exhibit Brownian-type horizontal motion. The
probability density functions of the vortex displacements are, however,
non-Gaussian at short time scales. At modest rotating rates
() the centrifugal force leads to radial
vortex motions, i.e., warm cyclones (cold anticyclones) moving towards (outward
from) the rotation axis. The mean-square-displacements of the vortices increase
faster than linearly at large time. This super-diffusive behavior can be
satisfactorily explained by a Langevin model incorporating the centrifugal
force. In the rapidly rotating regime () the
vortices are densely distributed, with the size-distribution of their Voronoi
cells differing significantly from the standard distribution. The
hydrodynamic interaction of neighboring vortices results in formation of vortex
clusters. Inside clusters the correlation of the vortex velocity fluctuations
is scale free, with the correlation length being approximately of the
cluster length. We examine the influence of cluster forming on the dynamics of
individual vortex. Within clusters, cyclones exhibit inverse-centrifugal motion
as they submit to the motion of strong anticyclones, while the velocity for
outward motion of the anticyclones is increased. Our analysis show that the
mobility of isolated vortices, scaled by their vorticity strength, is a simple
power function of the Froude number
Universal critical properties of the Eulerian bond-cubic model
We investigate the Eulerian bond-cubic model on the square lattice by means
of Monte Carlo simulations, using an efficient cluster algorithm and a
finite-size scaling analysis. The critical points and four critical exponents
of the model are determined for several values of . Two of the exponents are
fractal dimensions, which are obtained numerically for the first time. Our
results are consistent with the Coulomb gas predictions for the critical O()
branch for and the results obtained by previous transfer matrix
calculations. For , we find that the thermal exponent, the magnetic
exponent and the fractal dimension of the largest critical Eulerian bond
component are different from those of the critical O(2) loop model. These
results confirm that the cubic anisotropy is marginal at but irrelevant
for
EVE: Environmental Adaptive Neural Network Models for Low-power Energy Harvesting System
IoT devices are increasingly being implemented with neural network models to
enable smart applications. Energy harvesting (EH) technology that harvests
energy from ambient environment is a promising alternative to batteries for
powering those devices due to the low maintenance cost and wide availability of
the energy sources. However, the power provided by the energy harvester is low
and has an intrinsic drawback of instability since it varies with the ambient
environment. This paper proposes EVE, an automated machine learning (autoML)
co-exploration framework to search for desired multi-models with shared weights
for energy harvesting IoT devices. Those shared models incur significantly
reduced memory footprint with different levels of model sparsity, latency, and
accuracy to adapt to the environmental changes. An efficient on-device
implementation architecture is further developed to efficiently execute each
model on device. A run-time model extraction algorithm is proposed that
retrieves individual model with negligible overhead when a specific model mode
is triggered. Experimental results show that the neural networks models
generated by EVE is on average 2.5X times faster than the baseline models
without pruning and shared weights
Geometric density of states of electronic structures for local responses: Phase information from the amplitudes of STM measurement
Electronic band structures underlie the physical properties of crystalline
materials, their geometrical exploration renovates the conventional cognition
and brings about novel applications. Inspired by geometry phases, we introduce
a geometric amplitude named as the geometric density of states (GDOS) dictated
by the differential curvature of the constant-energy contour. The GDOS
determines the amplitude of the real-space Green's function making it attain
the ultimate expression with transparent physics. The local responses of
crystalline materials are usually formulated by the real-space Green's
function, so the relevant physics should be refreshed by GDOS. As an example of
local responses, we suggest using scanning tunneling microscopy (STM) to
characterize the surface states of three-dimensional topological insulator
under an in-plane magnetic field. The GDOS favors the straightforward
simulation of STM measurement without resorting to Fourier transform of the
real-space measurement, and also excavates the unexplored potential of STM
measurement to extract the phase information of wavefunction through its
amplitude, i.e., the spin and curvature textures. Therefore, the proposed GDOS
deepens the understanding of electronic band structures and is indispensable in
local responses, and it should be universal for any periodic systems.Comment: 6 pages, 2 figure
Scale-free download network for publications
The scale-free power-law behavior of the statistics of the download frequency
of publications has been, for the first time, reported. The data of the
download frequency of publications are taken from a well-constructed web page
in the field of economic physics (http://www.unifr.ch/econophysics/). The
Zipf-law analysis and the Tsallis entropy method were used to fit the download
frequency. It was found that the power-law exponent of rank-ordered frequency
distribution is which is consistent with the
power-law exponent for the cumulated frequency
distributions. Preferential attachment model of Barabasi and Albert network has
been used to explain the download network.Comment: 3 pages, 2 figure
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