849 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
Single Photon Transport through an Atomic Chain Coupled to a One-dimensional Nanophotonic Waveguide
We study the dynamics of a single photon pulse travels through a linear
atomic chain coupled to a one-dimensional (1D) single mode photonic waveguide.
We derive a time-dependent dynamical theory for this collective many-body
system which allows us to study the real time evolution of the photon transport
and the atomic excitations. Our analytical result is consistent with previous
numerical calculations when there is only one atom. For an atomic chain, the
collective interaction between the atoms mediated by the waveguide mode can
significantly change the dynamics of the system. The reflectivity of a photon
can be tuned by changing the ratio of coupling strength and the photon
linewidth or by changing the number of atoms in the chain. The reflectivity of
a single photon pulse with finite bandwidth can even approach . The
spectrum of the reflected and transmitted photon can also be significantly
different from the single atom case. Many interesting physical phenomena can
occur in this system such as the photonic bandgap effects, quantum entanglement
generation, Fano-like interference, and superradiant effects. For engineering,
this system may serve as a single photon frequency filter, single photon
modulation and may find important applications in quantum information
SMEs’ Entrepreneurship from the Perspective of Social Networks
Companies utilize social networks which don\u27t entail any additional resources to promote their products, services as well as brands, build a brand image and handle customer relationships. Therefore, numerous SMEs are more likely to turn to social media when they launch a business. The current research mainly uses questionnaires or case studies to illustrate the benefits resulted from using social media by SMEs to start up a business. A large amount of information flow in social media has brought a lot of opportunities to SMEs. Still, meantime it also puts more pressure on SMEs that lack funds and technology to use such information. In the end, whether social media brings benefits or disadvantages to entrepreneurship still needs empirical data to confirm. From this perspective, this article looks for empirical data to demonstrate the role of social media in entrepreneurship for SMEs. This study obtains relevant data of sample companies from e-commerce and social media websites and applies the data envelopment model to measure the efficiency of these enterprises using social media entrepreneurship
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
Coupled spin and valley physics in monolayers of MoS2 and other group-VI dichalcogenides
We show that inversion symmetry breaking together with spin-orbit coupling
leads to coupled spin and valley physics in monolayers of MoS2 and other
group-VI dichalcogenides, making possible controls of spin and valley in these
2D materials. The spin-valley coupling at the valence band edges suppresses
spin and valley relaxation, as flip of each index alone is forbidden by the
valley contrasting spin splitting. Valley Hall and spin Hall effects coexist in
both electron-doped and hole-doped systems. Optical interband transitions have
frequency-dependent polarization selection rules which allow selective
photoexcitation of carriers with various combination of valley and spin
indices. Photo-induced spin Hall and valley Hall effects can generate long
lived spin and valley accumulations on sample boundaries. The physics discussed
here provides a route towards the integration of valleytronics and spintronics
in multi-valley materials with strong spin-orbit coupling and inversion
symmetry breaking.Comment: published versio
Avoidance Behavior toward Social Network Advertising: Dimensions and Measurement
While social network advertising is pervasive, research focused on avoidance behavior toward it is relatively rare. This study provides the development of a three-dimension scale to measure avoidance behavior toward social network advertising. Based on the survey of 195 social network users, evidence is provided for the reliability, factor structure and validity. Meanwhile, T-tests are used to examine the effects of gender, sample source and purchasing experience on the three-dimension avoidance behavior (i.e., skimming, ignoring and blocking). The results show males on social network are more likely to block social network advertising than females while users without purchasing experience on social network are more likely to skimming through advertisements on social network
Majorana Fermions on Zigzag Edge of Monolayer Transition Metal Dichalcogenides
Majorana fermions, quantum particles with non-Abelian exchange statistics,
are not only of fundamental importance, but also building blocks for
fault-tolerant quantum computation. Although certain experimental breakthroughs
for observing Majorana fermions have been made recently, their conclusive
dection is still challenging due to the lack of proper material properties of
the underlined experimental systems. Here we propose a new platform for
Majorana fermions based on edge states of certain non-topological
two-dimensional semiconductors with strong spin-orbit coupling, such as
monolayer group-VI transition metal dichalcogenides (TMD). Using
first-principles calculations and tight-binding modeling, we show that zigzag
edges of monolayer TMD can host well isolated single edge band with strong
spin-orbit coupling energy. Combining with proximity induced s-wave
superconductivity and in-plane magnetic fields, the zigzag edge supports robust
topological Majorana bound states at the edge ends, although the
two-dimensional bulk itself is non-topological. Our findings points to a
controllable and integrable platform for searching and manipulating Majorana
fermions.Comment: 12 pages, 7 figure
OPTIMAL PREVENTIVE MAINTENANCE POLICIES FOR UNRELIABLE QUEUEING AND PRODUCTION SYSTEMS
Preventive Maintenance (PM) models have traditionally concentrated on utilizing machine ``technical" state information such as the degree of deterioration. However, in real manufacturing systems, additional system operational information such as work-in-process (WIP) inventory levels critically impact actual PM decisions. Surprisingly, the literature on models incorporating this important aspect is relatively sparse. This thesis attempts to fill some of the research gaps in this area by considering problems of optimal preventive maintenance explicitly under the context of unreliable queueing and production-inventory systems. We propose a two-level hierarchical modeling framework for PM planning and scheduling problems. In the higher level, our objective is to characterize structure of optimal PM policies. We start with a simple case in which queueing is not taken into account in the model. We show that a randomized PM policy, like the widely used ``time-window" policy in industry, is in general not optimal. We then consider the problem of optimal PM policies for an M/G/1 queueing system with an unreliable server. The decision problem is formulated as a semi-Markov decision process. We establish some structural properties, e.g., ``control-limit" type structure, that optimal policies will satisfy. We then take the optimal PM problem a step further by considering optimal joint PM and production control policies for unreliable production-inventory systems with time-dependent or operation-dependent failures. We show the optimal joint policies retain the ``control-limit" type structure in terms of the PM portion of the policy. For the production portion of the policy, some properties are also derived, but numerical studies show that in general optimal policies have more complicated structure than the simple control-limit form. The last part of the thesis is devoted to the lower level of the framework where the objective is to optimally schedule multiple PM tasks across a group of tools. We take into account information such as interdependence of PM tasks, WIP data and resource constraints, and formulate the problem as a mixed-integer program. Results of a simulation study comparing the performance of the model-based PM schedule with that of a baseline reference schedule are presented to illustrate the fitness of our solutions
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