23,377 research outputs found
SU(4) Theory for Spin Systems with Orbital Degeneracy
The isotropic limit of spin systems with orbital degeneracy is shown to have
global SU(4) symmetry. On many 2D lattices, the ground state does not posses
long range order, which may explain the observed spin liquid properties of
. In the SU(4) Neel ordered state, spin-spin correlations can be
antiferromagneitc between two neighboring sites with parallel magnetic moments.Comment: 11 pages, 2 figures. submitted to PR
Dynamic Characteristic Analysis of Doubly-fed Induction Generator Low Voltage Ride-through
AbstractFor studying the impacts of wind turbines integrated into grid, the relation between the protection of doubly-fed induction generator (DFIG) during low voltage fault and the dynamic characteristic of grid is established from the points of generator operation constrains and district grid voltage stability. Then the resistance value and switching strategy of crowbar are discussed. Based on analyzing the electric characteristic of the voltage or current during the short-circuit fault in wind turbines with crowbar switching, the equation to estimate peak current of stator and rotor of DFIG with crowbar switching and the value range of crowbar resistance are derived. The numeric test analyzes the impacts of crowbar switching on district grid voltage stability with different fault types, crowbar switching time and crowbar resistance values. Also the interaction impact of crowbar switching on multi-wind farms is analyzed. The results show that reasonable crowbar resistance value and switching strategy can improve low voltage ride through (LVRT) ability of wind turbines and reduce bad impacts on district grid voltage stability with large-scale crowbar switching of wind farms
Fair signature exchange via delegation on ubiquitous networks
This paper addresses the issue of autonomous fair signature exchange in emerging ubiquitous (u-) commerce systems, which require that the exchange task be delegated to authorised devices for its autonomous and secure execution. Relevant existing work is either inefficient or ineffective in dealing with such delegated exchange. To rectify this situation, this paper aims to propose an effective, efficient and secure solution to the delegated exchange to support the important autonomy feature offered by u-commerce systems. The proposed work includes a novel approach to symmetric-key based verifiable proxy encryption to make the exchange delegation flexible, efficient and simple to implement on resource-limited devices commonly used in u-commerce systems. This approach is then applied to design a new exchange protocol. An analysis of the protocol is also provided to confirm its security and fairness. Moreover, a comparison with related work is presented to demonstrate its much better efficiency and simplicity
A GPS-Less Localization and Mobility Modelling (LMM) System for Wildlife Tracking
Existing wildlife tracking solutions typically use sensor nodes with specialised facilities, such as long-range radio, solar array of cells and Global Positioning System (GPS). This introduces additional manufacturing cost, increased energy and memory consumptions and increased sensor node weight. This paper proposes a novel Localization and Mobility Modelling (LMM) system, that can carry out wildlife tracking by merely using low-cost, lightweight sensor nodes and using short-range peer-to-peer communication facilities only, i.e. without the need for any specialised facilities. This is done by using two computationally simple operations, which are: (i) aggregated data collections from sensor nodes via peer-to-peer communications in a distributed manner, and (ii) estimation of sensor nodes' movement traces using trilateration. The computational load placed on each sensor node is just that of data collection and aggregation, whereas movement traces estimation is carried out on a backend server, separated from the sensor nodes. In the design of the LMM system, we have: (i) carried out an empirical evaluation of different parameter value settings for data collection to develop a Multi-Zone Multi-Hierarchy (MZMH) communication structure, (ii) demonstrated a novel use of an Aggregation based Topology Learning (ATL) protocol for collecting sensor nodes' topology data using peer-to-peer multi-hop communications, and (iii) used a novel Location Estimation (LE) method for estimating sensor nodes' movement traces from the collected topology data. The evaluation results show that the LMM system can accurately estimate sensor nodes' movement traces but with significantly less energy and memory costs, demonstrating its cost-efficiency as compared to the related wildlife tracking solutions. © 2020 IEEE
Observation of Weyl nodes in robust type-II Weyl semimetal WP2
Distinct to type-I Weyl semimetals (WSMs) that host quasiparticles described
by the Weyl equation, the energy dispersion of quasiparticles in type-II WSMs
violates Lorentz invariance and the Weyl cones in the momentum space are
tilted. Since it was proposed that type-II Weyl fermions could emerge from
(W,Mo)Te2 and (W,Mo)P2 families of materials, a large numbers of experiments
have been dedicated to unveil the possible manifestation of type-II WSM, e.g.
the surface-state Fermi arcs. However, the interpretations of the experimental
results are very controversial. Here, using angle-resolved photoemission
spectroscopy supported by the first-principles calculations, we probe the
tilted Weyl cone bands in the bulk electronic structure of WP2 directly, which
are at the origin of Fermi arcs at the surfaces and transport properties
related to the chiral anomaly in type-II WSMs. Our results ascertain that due
to the spin-orbit coupling the Weyl nodes originate from the splitting of
4-fold degenerate band-crossing points with Chern numbers C = 2 induced by
the crystal symmetries of WP2, which is unique among all the discovered WSMs.
Our finding also provides a guiding line to observe the chiral anomaly which
could manifest in novel transport properties.Comment: 13 pages, 3 figure
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