68 research outputs found
Hierarchical Theory of Quantum Adiabatic Evolution
Quantum adiabatic evolution is a dynamical evolution of a quantum system
under slow external driving. According to the quantum adiabatic theorem, no
transitions occur between non-degenerate instantaneous eigen-energy levels in
such a dynamical evolution. However, this is true only when the driving rate is
infinitesimally small. For a small nonzero driving rate, there are generally
small transition probabilities between the energy levels. We develop a
classical mechanics framework to address the small deviations from the quantum
adiabatic theorem order by order. A hierarchy of Hamiltonians are constructed
iteratively with the zeroth-order Hamiltonian being determined by the original
system Hamiltonian. The th-order deviations are governed by a th-order
Hamiltonian, which depends on the time derivatives of the adiabatic parameters
up to the th-order. Two simple examples, the Landau-Zener model and a
spin-1/2 particle in a rotating magnetic field, are used to illustrate our
hierarchical theory. Our analysis also exposes a deep, previously unknown
connection between classical adiabatic theory and quantum adiabatic theory.Comment: 10 pages, 6 figures, 29 reference
Sensitive frequency-dependence of the carrier-envelope phase effect on bound-bound transition: an interference perspective
We investigate numerically with Hylleraas coordinates the frequency
dependence of the carrier-envelope phase (CEP) effect on bound-bound
transitions of helium induced by an ultrashort laser pulse of few cycles. We
find that the CEP effect is very sensitive to the carrier frequency of the
laser pulse, occurring regularly even at far-off resonance frequencies. By
analyzing a two-level model, we find that the CEP effect can be attributed to
the quantum interference between neighboring multi-photon transition pathways,
which is made possible by the broadened spectrum of the ultrashort laser pulse.
A general picture is developed along this line to understand the sensitivity of
the CEP effect to laser's carrier frequency. Multi-level influence on the CEP
effect is also discussed
Crystal structure of [5,5′-((propane-1,3-diylbis(azanylylidene))bis(ethan-1-yl-2-ylidene))bis(3-(ethoxycarbonyl)-2,4-dimethylpyrrol-1-ido)-κ4N,N′,N′′,N′′′]nickel(II), C23H30N4O4Ni
Abstract
C23H30N4O4Ni, triclinic, P1̄ (no. 2), a = 7.5883(9) Å, b = 12.3110(15) Å, c = 12.7718(15) Å, α = 95.621(2)°, β = 99.908(2)°, γ = 101.30(2)°, V = 1141.8(2) Å3, Z = 2, R
gt(F) = 0.0433, wR
ref(F
2) = 0.1239, T = 296 K
Reaction kinetics of CN + toluene and its implication on the productions of aromatic nitriles in the Taurus molecular cloud and Titan's atmosphere
Reactions between cyano radical and aromatic hydrocarbons are believed to be
important pathways for the formation of aromatic nitriles in the interstellar
medium (ISM) including those identified in the Taurus molecular cloud (TMC-1).
Aromatic nitriles might participate in the formation of polycyclic aromatic
nitrogen containing hydrocarbons (PANHs) in Titan's atmosphere. Here, ab initio
kinetics simulations reveal a high efficiency of and the competition of the different products of
30-1800 K and -100 atm of the CN + toluene reaction. In the
star-forming region of TMC-1 environment, the product yields of benzonitrile
and tolunitriles for CN reacting with toluene may be approximately 17 and
83, respectively. The detection of main products, tolunitriles, can serve
as proxies for the undetected toluene in the ISM due to their much larger
dipole moments. The competition between bimolecular and unimolecular products
is extremely intense under the warmer and denser PANH forming region of Titan's
stratosphere. The computational results show that the fractions of
tolunitriles, adducts, and benzonitrile are 19-68, 15-64 and
17, respectively, at 150-200 K and 0.0001-0.001 atm (Titan's stratosphere).
Then, benzonitrile and tolunitriles may contribute to the formation of PANHs by
consecutive additions. Kinetic information of aromatic nitriles
for the CN + toluene reaction calculated here helps to explain the formation
mechanism of polycyclic aromatic hydrocarbons (PAHs) or PANHs under different
interstellar environments and constrains corresponding astrochemical models
Interlayer Interactions in Anisotropic Atomically-thin Rhenium Diselenide
Recently, two-dimensional (2D) materials with strong in-plane anisotropic
properties such as black phosphorus have demonstrated great potential for
developing new devices that can take advantage of its reduced lattice symmetry
with potential applications in electronics, optoelectronics and
thermoelectrics. However, the selection of 2D material with strong in-plane
anisotropy has so far been very limited and only sporadic studies have been
devoted to transition metal dichalcogenides (TMDC) materials with reduced
lattice symmetry, which is yet to convey the full picture of their optical and
phonon properties, and the anisotropy in their interlayer interactions. Here,
we study the anisotropic interlayer interactions in an important TMDC 2D
material with reduced in-plane symmetry - atomically thin rhenium diselenide
(ReSe2) - by investigating its ultralow frequency interlayer phonon vibration
modes, the layer dependent optical bandgap, and the anisotropic
photoluminescence (PL) spectra for the first time. The ultralow frequency
interlayer Raman spectra combined with the first study of polarization-resolved
high frequency Raman spectra in mono- and bi-layer ReSe2 allows deterministic
identification of its layer number and crystal orientation. PL measurements
show anisotropic optical emission intensity with bandgap increasing from 1.26
eV in the bulk to 1.32 eV in monolayer, consistent with the theoretical results
based on first-principle calculations. The study of the layer-number dependence
of the Raman modes and the PL spectra reveals the relatively weak van der Waals
interaction and 2D quantum confinement in atomically-thin ReSe2.Comment: 17 pages, 5 figures, supplementary informatio
Leveraging Graph-based Cross-modal Information Fusion for Neural Sign Language Translation
Sign Language (SL), as the mother tongue of the deaf community, is a special
visual language that most hearing people cannot understand. In recent years,
neural Sign Language Translation (SLT), as a possible way for bridging
communication gap between the deaf and the hearing people, has attracted
widespread academic attention. We found that the current mainstream end-to-end
neural SLT models, which tries to learning language knowledge in a weakly
supervised manner, could not mine enough semantic information under the
condition of low data resources. Therefore, we propose to introduce additional
word-level semantic knowledge of sign language linguistics to assist in
improving current end-to-end neural SLT models. Concretely, we propose a novel
neural SLT model with multi-modal feature fusion based on the dynamic graph, in
which the cross-modal information, i.e. text and video, is first assembled as a
dynamic graph according to their correlation, and then the graph is processed
by a multi-modal graph encoder to generate the multi-modal embeddings for
further usage in the subsequent neural translation models. To the best of our
knowledge, we are the first to introduce graph neural networks, for fusing
multi-modal information, into neural sign language translation models.
Moreover, we conducted experiments on a publicly available popular SLT dataset
RWTH-PHOENIX-Weather-2014T. and the quantitative experiments show that our
method can improve the model
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