1,857 research outputs found
Brightened spin-triplet interlayer excitons and optical selection rules in van der Waals heterobilayers
We investigate the optical properties of spin-triplet interlayer excitons in
heterobilayer transition metal dichalcogenides in comparison with the
spin-singlet ones. Surprisingly, the optical transition dipole of the
spin-triplet exciton is found to be in the same order of magnitude to that of
the spin-singlet exciton, in sharp contrast to the monolayer excitons where the
spin triplet species is considered as dark compared to the singlet. Unlike the
monolayer excitons whose spin-conserved (spin-flip) transition dipole can only
couple to light of in-plane (out-of-plane) polarization, such restriction is
removed for the interlayer excitons due to the breaking of the out-of-plane
mirror symmetry. We find that as the interlayer atomic registry changes, the
optical transition dipole of interlayer exciton crosses between in-plane ones
of opposite circular polarization and the out-of-plane one for both the
spin-triplet and spin-singlet species. As a result, excitons of both species
have non-negligible coupling into photon modes of both in-plane and
out-of-plane propagations, another sharp difference from the monolayers where
the exciton couples predominantly into the out-of-plane propagation channel. At
given atomic registry, the spin-triplet and spin-singlet excitons have distinct
valley polarization selection rules, allowing the selective optical addressing
of both the valley configuration and the spin singlet/triplet configuration of
interlayer excitons
Kinematic Evolution of a Slow CME in Corona Viewed by STEREO-B on 8 October 2007
We studied the kinematic evolution of the 8 October 2007 CME in the corona
based on Sun-Earth Connection Coronal and Heliospheric Investigation (SECCHI)
onboard satellite B of Solar TErrestrial RElations Observatory (STEREO). The
observational results show that this CME obviously deflected to a lower
latitude region for about 30 at the beginning. After this, the CME
propagated radially. We also analyze the influence of the background magnetic
field on the deflection of this CME. We find that the deflection of this CME at
an early stage may be caused by the nonuniform distribution of the background
magnetic field energy density and that the CME tended to propagate to the
region with lower magnetic energy density. In addition, we found that the
velocity profile of this gradual CME shows multiphased evolution during its
propagation in COR1-B FOV. The CME velocity first kept at a constant of
23.1km.s-1. Then, it accelerated continuously with a positive acceleration of
7.6m.s-2.Comment: 10 pages, 7 figure
An efficient method for hybrid density functional calculation with spin-orbit coupling
In first-principles calculations, hybrid functional is often used to improve
accuracy from local exchange correlation functionals. A drawback is that
evaluating the hybrid functional needs significantly more computing effort.
When spin-orbit coupling (SOC) is taken into account, the non-collinear spin
structure increases computing effort by at least eight times. As a result,
hybrid functional calculations with SOC are intractable in most cases. In this
paper, we present an approximate solution to this problem by developing an
efficient method based on a mixed linear combination of atomic orbital (LCAO)
scheme. We demonstrate the power of this method using several examples and we
show that the results compare very well with those of direct hybrid functional
calculations with SOC, yet the method only requires a computing effort similar
to that without SOC. The presented technique provides a good balance between
computing efficiency and accuracy, and it can be extended to magnetic
materials.Comment: 10 pages, 3 figure
Intervalley coupling by quantum dot confinement potentials in monolayer transition metal dichalcogenides
Monolayer transition metal dichalcogenides (TMDs) offer new opportunities for
realizing quantum dots (QDs) in the ultimate two-dimensional (2D) limit. Given
the rich control possibilities of electron valley pseudospin discovered in the
monolayers, this quantum degree of freedom can be a promising carrier of
information for potential quantum spintronics exploiting single electrons in
TMD QDs. An outstanding issue is to identify the degree of valley
hybridization, due to the QD confinement, which may significantly change the
valley physics in QDs from its form in the 2D bulk. Here we perform a
systematic study of the intervalley coupling by QD confinement potentials on
extended TMD monolayers. We find that the intervalley coupling in such geometry
is generically weak due to the vanishing amplitude of the electron wavefunction
at the QD boundary, and hence valley hybridization shall be well quenched by
the much stronger spin-valley coupling in monolayer TMDs and the QDs can well
inherit the valley physics of the 2D bulk. We also discover sensitive
dependence of intervalley coupling strength on the central position and the
lateral length scales of the confinement potentials, which may possibly allow
tuning of intervalley coupling by external controlsComment: 17 pages, 14 figure
Statistical Study of Coronal Mass Ejection Source Locations: Understanding CMEs Viewed in Coronagraphs
How to properly understand coronal mass ejections (CMEs) viewed in
white-light coronagraphs is crucial to many relative researches in solar and
space physics. The issue is now particularly addressed in this paper through
studying the source locations of all the 1078 LASCO CMEs listed in CDAW CME
catalog during 1997 -- 1998 and their correlation with CMEs' apparent
parameters. By manually checking LASCO and EIT movies of these CMEs, we find
that, except 231 CMEs whose source locations can not be identified due to poor
data, there are 288 CMEs with location identified on the front-side solar disk,
234 CMEs appearing above solar limb, and 325 CMEs without evident eruptive
signatures in the field of view of EIT. Based on the statistical results of
CMEs' source locations, four physical issues, including (1) the missing rate of
CMEs by SOHO LASCO and EIT, (2) the mass of CMEs, (3) the causes of halo CMEs
and (4) the deflections of CMEs in the corona, are exhaustively analyzed. It is
found that (1) about 32% of front-side CMEs can not be recognized by SOHO, (2)
the brightness of a CME at any heliocentric distance is roughly positively
correlated with its speed, and the CME mass derived from the brightness is
probably overestimated, (3) both projection effect and violent eruption are the
major causes of halo CMEs, and especially for limb halo CMEs, the latter is the
primary one, (4) most CMEs deflected towards equator near the solar minimum,
and these deflections can be classified into three types, the asymmetrical
expansion, non-radial ejection, and the deflected propagation.Comment: 15 pages, 14 figure
Lower bound of local quantum uncertainty for high-dimensional bipartite quantum systems
Quantum correlations are of fundamental importance in quantum phenomena and
quantum information processing studies. The measure of quantum correlations is
one central issue. The recently proposed measure of quantum correlations, the
local quantum uncertainty (LQU), satisfies the full physical requirements of a
measure of quantum correlations. In this work, by using operator relaxation, a
closed form lower bound of the LQU for arbitrary-dimensional bipartite quantum
states is derived. We have compared the lower bound and the optimized LQU for
several typical quantum states.Comment: We have revised the manuscript. Comments are welcom
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
Spin-valley qubit in nanostructures of monolayer semiconductors: Optical control and hyperfine interaction
We investigate the optical control possibilities of spin-valley qubit carried
by single electrons localized in nanostructures of monolayer TMDs, including
small quantum dots formed by lateral heterojunction and charged impurities. The
quantum controls are discussed when the confinement induces valley
hybridization and when the valley hybridization is absent. We show that the
bulk valley and spin optical selection rules can be inherited in different
forms in the two scenarios, both of which allow the definition of spin-valley
qubit with desired optical controllability. We also investigate nuclear spin
induced decoherence and quantum control of electron-nuclear spin entanglement
via intervalley terms of the hyperfine interaction. Optically controlled
two-qubit operations in a single quantum dot are discussed.Comment: 17pages, 10 figure
Quantitative Analysis of CME Deflections in the Corona
In this paper, ten CME events viewed by the STEREO twin spacecraft are
analyzed to study the deflections of CMEs during their propagation in the
corona. Based on the three-dimensional information of the CMEs derived by the
graduated cylindrical shell (GCS) model [Thernisien et al., 2006], it is found
that the propagation directions of eight CMEs had changed. By applying the
theoretical method proposed by Shen et al. [2011] to all the CMEs, we found
that the deflections are consistent, in strength and direction, with the
gradient of the magnetic energy density. There is a positive correlation
between the deflection rate and the strength of the magnetic energy density
gradient and a weak anti-correlation between the deflection rate and the CME
speed. Our results suggest that the deflections of CMEs are mainly controlled
by the background magnetic field and can be quantitatively described by the
magnetic energy density gradient (MEDG) model.Comment: 19 pages, 20 figure
Triply degenerate nodal line and tunable contracted-drumhead surface state in a tight-binding model
The study of topological semimetals has been extended to more general
topological nodal systems such as metamaterials and artificial periodic
structures. Among various nodal structures, triply degenerate nodal line (TNL)
is rare and hence lack of attention. In this work, we have proposed a simple
tight-binding model which hosts a topological non-trivial TNL. This TNL not
only has the drumhead surface states as usual nodal line systems, but also has
surface states which form a contracted-drumhead shape. And the shape and area
of this contracted-drumhead can be tuned by the hopping parameters of the
model. This provides an effective way to modulate surface states as well as
their density of states, which can be important in future applications of
topological nodal systems.Comment: 6 pages, 5 figure
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