241 research outputs found
Topological field-effect transistor with quantized ON/OFF conductance of helical/chiral dislocation states
Topology is a key ingredient driving the emergence of quantum devices.
Topological field-effect transistor (TFET) has been proposed to outperform the
conventional FET by replacing the ON state with topology-protected quantized
conductance, while the OFF state remains the same normal insulating
characteristics and hence bears similar drawbacks. Here, we demonstrate a
proof-of-concept TFET having both ON and OFF quantized conductance, by
switching between helical and chiral topological screw dislocation (SD) states
in three-dimensional topological insulators. A pair of SDs are configured with
one acting as channel and the other as gate controlled by local magnetic field.
A reversible field-switching is achieved with the ON and OFF conductance of
and , respectively, as shown by tight-binding quantum transport
calculations. Furthermore, BaBiO is shown as a candidate material having
the desired topological SD states, based on first-principles calculations. Our
findings open a new route to high-fidelity topological quantum devices
Higher-order Topological Point State
Higher-order topological insulators (HOTIs) have attracted increasing
interest as a unique class of topological quantum materials. One distinct
property of HOTIs is the crystalline symmetry-imposed topological state at the
lower-dimensional outer boundary, e.g. the zero-dimensional (0D) corner state
of a 2D HOTI, used exclusively as a universal signature to identify
higher-order topology but yet with uncertainty. Strikingly, we discover the
existence of inner topological point states (TPS) in a 2D HOTI, as the embedded
"end" states of 1D first-order TI, as exemplified by those located at the
vacancies in a Kekule lattice. Significantly, we demonstrate that such inner
TPS can be unambiguously distinguished from the trivial point-defect states, by
their unique topology-endowed inter-TPS interaction and correlated magnetic
response in spectroscopy measurements, overcoming an outstanding experimental
challenge. Furthermore, based on first-principles calculations, we propose
{\gamma}-graphyne as a promising material to observe the higher-order TPS. Our
findings shed new light on our fundamental understanding of HOTIs, and also
open an avenue to experimentally distinguishing and tuning TPS in the interior
of a 2D sample for potential applications
The improvement and development of Shanghaiβs urban economic competitiveness
As one of the greatest metropolises in China, Shanghai has been playing a critical role in Chinaβs economic development. With the launching of its construction of Β«Big Four CentersΒ», namely the Shanghai International Economic Center, the International Financial Center, the International Trade Center and the International Shipping Center, Shanghai is now having its industrial structures being adjusted and bettering the proportion between its secondary and tertiary industries. Meanwhile, Shanghai is also making full use of the Β«post-ExpoΒ» economic effect, focusing on constructing the Free Trade Zone, attracting high quality foreign investment, building its brand new international image, and enhancing its urban economic competitiveness
Design, simulation and experiment of particle dampers attached to a precision instrument in spacecraft
Aiming at attenuating the vibration of a precision instrument in spacecraft, multiple particle dampers are designed and their damping performances are evaluated. Firstly, the vibrating table test for the primary system under sin-swept excitation is conducted to acquire the vibration characteristic. Then enclosures attached to the installing bracket are designed and fabricated elaborately. Using discrete element-finite element (DE-FE) coupling algorithm, the effects of some system parameters (such as: mass ratio, particle material, numbers of dampers and cavity depth) are investigated to optimize the damping capacity of particle dampers. Furthermore, a series of experiments are conducted to verify the performance of particle dampers under dynamic load. The results indicate that the transfer functions of acceleration in Y and Z direction decrease at 22.58Β % and 77.38Β % respectively, while only 3.1Β % mass of the primary system is attached
CDOpt: A Python Package for a Class of Riemannian Optimization
Optimization over the embedded submanifold defined by constraints
has attracted much interest over the past few decades due to its wide
applications in various areas. Plenty of related optimization packages have
been developed based on Riemannian optimization approaches, which rely on some
basic geometrical materials of Riemannian manifolds, including retractions,
vector transports, etc. These geometrical materials can be challenging to
determine in general. Existing packages only accommodate a few well-known
manifolds whose geometrical materials are easily accessible. For other
manifolds which are not contained in these packages, the users have to develop
the geometric materials by themselves. In addition, it is not always tractable
to adopt advanced features from various state-of-the-art unconstrained
optimization solvers to Riemannian optimization approaches.
We introduce CDOpt (available at https://cdopt.github.io/), a user-friendly
Python package for a class Riemannian optimization. Based on constraint
dissolving approaches, Riemannian optimization problems are transformed into
their equivalent unconstrained counterparts in CDOpt. Therefore, solving
Riemannian optimization problems through CDOpt directly benefits from various
existing solvers and the rich expertise gained over decades for unconstrained
optimization. Moreover, all the computations in CDOpt related to any manifold
in question are conducted on its constraints expression, hence users can easily
define new manifolds in CDOpt without any background on differential geometry.
Furthermore, CDOpt extends the neural layers from PyTorch and Flax, thus allows
users to train manifold constrained neural networks directly by the solvers for
unconstrained optimization. Extensive numerical experiments demonstrate that
CDOpt is highly efficient and robust in solving various classes of Riemannian
optimization problems.Comment: 31 page
Graph Theorem for Chiral Exact Flat Bands at Charge Neutrality
Chiral exact flat bands (FBs) at charge neutrality have attracted much recent
interest, presenting an intriguing condensed-matter system to realize exact
many-body phenomena, as specifically shown in "magic angle" twisted bilayer
graphene for superconductivity and triangulene-based superatomic graphene for
excitonic condensation. Yet, no generic physical model to realize such FBs has
been developed. Here we present a new mathematical theorem, called bipartite
double cover (BDC) theorem, and prove that the BDC of line-graph (LG) lattices
hosts at least two chiral exact FBs of opposite chirality, i.e., yin-yang FBs,
centered-around/at charge neutrality (E = 0) akin to the "chiral limit" of
twisted bilayer graphene. We illustrate this theorem by mapping it exactly onto
tight-binding lattice models of the BDC of LGs of hexagonal lattice for strong
topological and of triangular lattice for fragile topological FBs,
respectively. Moreover, we use orbital design principle to realize such exotic
yin-yang FBs in non-BDC lattices to instigate their real material discovery.
This work not only enables the search for exact chiral FBs at zero energy
beyond moir\'e heterostructures, but also opens the door to discovering quantum
semiconductor features with FB-enabled strongly correlated carriers
Performance Regulation of Thieno[3,2-b]benzothiophene Ο-Spacer-Based D-Ο-A Organic Dyes for Dye-Sensitized Solar Cell Applications: Insights From Computational Study
Dye-sensitized solar cells (DSSCs) have been widely investigated; however, the development of promising dye sensitizers is still appealing. In this work, we perform a detailed theoretical search for high-efficiency D-Ο-A organic dyes using density functional theory and time-dependent density functional theory calculations. Specifically, we perform geometric optimization, and electronic structure and absorption spectra calculations for isolated dyes for two thieno[3,2-b]benzothiophene Ο-spacer-based D-Ο-A organic dyes SGT129 and SGT130, which show significant efficiency difference, before and after binding to a TiO2 semiconductor. The calculation results reveal that the coplanar configuration between the electron donor and the Ο-spacer can enhance electronic communication efficiently, thus facilitating intra-molecular charge transfer from the electron donor to the acceptor groups in SGT130. The absorption spectrum of SGT130 broadens and is red-shifted owing to the decreased bandgap. The higher light-harvesting efficiency, favorable intra-molecular charge transfer, larger shift of the conduction band edge in the TiO2 semiconductor, and slower charge recombination between the injected electrons in the TiO2 conduction band and the electrolyte explain the superior efficiency of SGT130 over that of SGT129. Using SGT130 as the reference dye, we further design four novel dyes 1β4 by modifying the Ο-spacer with electron-rich and electron-withdrawing moieties. Judging from the theoretical parameters influencing the short-circuit current and open-circuit voltage, we found that all dyes would perform better than SGT130 in terms of the favorable interfacial charge transfer (ICT) and light-harvesting efficiency, as well as the larger shift of the TiO2 conduction band edge. Our theoretical research is expected to provide valuable insights into the molecular modification of TBT-based D-Ο-A organic dyes for DSSC applications
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