474 research outputs found
Valley excitons in two-dimensional semiconductors
Monolayer group-VIB transition metal dichalcogenides have recently emerged as
a new class of semiconductors in the two-dimensional limit. The attractive
properties include: the visible range direct band gap ideal for exploring
optoelectronic applications; the intriguing physics associated with spin and
valley pseudospin of carriers which implies potentials for novel electronics
based on these internal degrees of freedom; the exceptionally strong Coulomb
interaction due to the two-dimensional geometry and the large effective masses.
The physics of excitons, the bound states of electrons and holes, has been one
of the most actively studied topics on these two-dimensional semiconductors,
where the excitons exhibit remarkably new features due to the strong Coulomb
binding, the valley degeneracy of the band edges, and the valley dependent
optical selection rules for interband transitions. Here we give a brief
overview of the experimental and theoretical findings on excitons in
two-dimensional transition metal dichalcogenides, with focus on the novel
properties associated with their valley degrees of freedom.Comment: Topical review, published online on National Science Review in Jan
201
Anomalous light cones and valley optical selection rules of interlayer excitons in twisted heterobilayers
We show that, because of the inevitable twist and lattice mismatch in
heterobilayers of transition metal dichalcogenides, interlayer excitons have
six-fold degenerate light cones anomalously located at finite velocities on the
parabolic energy dispersion. The photon emissions at each light cone are
elliptically polarized, with major axis locked to the direction of exciton
velocity, and helicity specified by the valley indices of the electron and the
hole. These finite-velocity light cones allow unprecedented possibilities to
optically inject valley polarization and valley current, and the observation of
both direct and inverse valley Hall effects, by exciting interlayer excitons.
Our findings suggest potential excitonic circuits with valley functionalities,
and unique opportunities to study exciton dynamics and condensation phenomena
in semiconducting 2D heterostructures.Comment: Including the Supplemental Material
Moir\'e excitons: from programmable quantum emitter arrays to spin-orbit coupled artificial lattices
Highly uniform and ordered nanodot arrays are crucial for high performance
quantum optoelectronics including new semiconductor lasers and single photon
emitters, and for synthesizing artificial lattices of interacting
quasiparticles towards quantum information processing and simulation of
many-body physics. Van der Waals heterostructures of 2D semiconductors are
naturally endowed with an ordered nanoscale landscape, i.e. the moir\'e pattern
that laterally modulates electronic and topographic structures. Here we find
these moir\'e effects realize superstructures of nanodot confinements for
long-lived interlayer excitons, which can be either electrically or strain
tuned from perfect arrays of quantum emitters to excitonic superlattices with
giant spin-orbit coupling (SOC). Besides the wide range tuning of emission
wavelength, the electric field can also invert the spin optical selection rule
of the emitter arrays. This unprecedented control arises from the gauge
structure imprinted on exciton wavefunctions by the moir\'e, which underlies
the SOC when hopping couples nanodots into superlattices. We show that the
moir\'e hosts complex-hopping honeycomb superlattices, where exciton bands
feature a Dirac node and two Weyl nodes, connected by spin-momentum locked
topological edge modes.Comment: To appear in Science Advance
Caustic graphene plasmons with Kelvin angle
A century-long argument made by Lord Kelvin that all swimming objects have an
effective Mach number of 3, corresponding to the Kelvin angle of 19.5 degree
for ship waves, has been recently challenged with the conclusion that the
Kelvin angle should gradually transit to the Mach angle as the ship velocity
increases. Here we show that a similar phenomenon can happen for graphene
plasmons. By analyzing the caustic wave pattern of graphene plasmons stimulated
by a swift charged particle moving uniformly above graphene, we show that at
low velocities of the charged particle, the caustics of graphene plasmons form
the Kelvin angle. At large velocities of the particle, the caustics disappear
and the effective semi-angle of the wave pattern approaches the Mach angle. Our
study introduces caustic wave theory to the field of graphene plasmonics, and
reveals a novel physical picture of graphene plasmon excitation during electron
energy-loss spectroscopy measurement.Comment: 15 pages, 4 figure
Improving data quality for 3D electron diffraction (3D ED) by Gatan Image Filter and a new crystal tracking method
3D ED is an effective technique to determine the structures of submicron- or
nano-sized crystals. In this paper, we implemented energy-filtered 3D ED using
a Gatan Energy Filter (GIF) in both selected area electron diffraction mode and
micro/nanoprobe mode. We explained the setup in detail, which improves the
accessibility of energy-filtered 3D ED experiments as more electron microscopes
are equipped with a GIF than an in-column filter. We also proposed a crystal
tracking method in STEM mode using live HAADF image stream. This method enables
us to collect energy-filtered 3D ED datasets in STEM mode with a larger tilt
range without foregoing any frames. In order to compare the differences between
energy-filtered 3D ED and normal 3D ED data, three crystalline samples have
been studied in detail. We observed that the final R1 will improve 20% to 30%
for energy-filtered datasets compared with unfiltered datasets and the
structure became more reasonable. We also discussed the possible reasons that
lead to the improvement
Highly efficient schemes for time fractional Allen-Cahn equation using extended SAV approach
In this paper, we propose and analyze high order efficient schemes for the
time fractional Allen-Cahn equation. The proposed schemes are based on the L1
discretization for the time fractional derivative and the extended scalar
auxiliary variable (SAV) approach developed very recently to deal with the
nonlinear terms in the equation. The main contributions of the paper consist
in: 1) constructing first and higher order unconditionally stable schemes for
different mesh types, and proving the unconditional stability of the
constructed schemes for the uniform mesh; 2) carrying out numerical experiments
to verify the efficiency of the schemes and to investigate the coarsening
dynamics governed by the time fractional Allen-Cahn equation. Particularly, the
influence of the fractional order on the coarsening behavior is carefully
examined. Our numerical evidence shows that the proposed schemes are more
robust than the existing methods, and their efficiency is less restricted to
particular forms of the nonlinear potentials
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