21,358 research outputs found
Electronic structure and magnetic properties of few-layer CrGeTe: the key role of nonlocal electron-electron interaction effects
Atomically-thin magnetic crystals have been recently isolated experimentally,
greatly expanding the family of two-dimensional materials. In this Article we
present an extensive comparative analysis of the electronic and magnetic
properties of , based on density functional
theory (DFT). We first show that the often-used approaches fail
in predicting the ground-state properties of this material in both its
monolayer and bilayer forms, and even more spectacularly in its bulk form. In
the latter case, the fundamental gap {\it decreases} by increasing the
Hubbard- parameter, eventually leading to a metallic ground state for
physically relevant values of , in stark contrast with experimental data. On
the contrary, the use of hybrid functionals, which naturally take into account
nonlocal exchange interactions between all orbitals, yields good account of the
available ARPES experimental data. We then calculate all the relevant exchange
couplings (and the magneto-crystalline anisotropy energy) for monolayer,
bilayer, and bulk with a hybrid functional,
with super-cells containing up to atoms, commenting on existing
calculations with much smaller super-cell sizes. In the case of bilayer , we show that two distinct intra-layer
second-neighbor exchange couplings emerge, a result which, to the best of our
knowledge, has not been noticed in the literature.Comment: 13 pages, 6 figures, 3 table
To develop an efficient variable speed compressor motor system
This research presents a proposed new method of improving the energy efficiency of a Variable Speed Drive (VSD) for induction motors. The principles of VSD are reviewed with emphasis on the efficiency and power losses associated with the operation of the variable speed compressor motor drive, particularly at low speed operation.The efficiency of induction motor when operated at rated speed and load torque
is high. However at low load operation, application of the induction motor at rated flux will cause the iron losses to increase excessively, hence its efficiency will reduce
dramatically. To improve this efficiency, it is essential to obtain the flux level that minimizes the total motor losses. This technique is known as an efficiency or energy
optimization control method. In practice, typical of the compressor load does not require high dynamic response, therefore improvement of the efficiency optimization
control that is proposed in this research is based on scalar control model.In this research, development of a new neural network controller for efficiency optimization control is proposed. The controller is designed to generate both voltage and frequency reference signals imultaneously. To achieve a robust controller from variation of motor parameters, a real-time or on-line learning algorithm based on a second order optimization Levenberg-Marquardt is employed. The simulation of the proposed controller for variable speed compressor is presented. The results obtained
clearly show that the efficiency at low speed is significant increased. Besides that the speed of the motor can be maintained. Furthermore, the controller is also robust to the motor parameters variation. The simulation results are also verified by experiment
A highly optimized vectorized code for Monte Carlo simulations of SU(3) lattice gauge theories
New methods are introduced for improving the performance of the vectorized Monte Carlo SU(3) lattice gauge theory algorithm using the CDC CYBER 205. Structure, algorithm and programming considerations are discussed. The performance achieved for a 16(4) lattice on a 2-pipe system may be phrased in terms of the link update time or overall MFLOPS rates. For 32-bit arithmetic, it is 36.3 microsecond/link for 8 hits per iteration (40.9 microsecond for 10 hits) or 101.5 MFLOPS
On the consistency of the N=1 SYM spectra from wrapped five-branes
We discuss the existence of glueball states for N=1 SYM within the
Maldacena-Nunez model. We find that for this model the existence of an area law
in the Wilson loop operator does not imply the existence of a discrete glueball
spectrum. We suggest that implementing the model with an upper hard cut-off can
amend the lack of spectrum. As a result the model can be only interpreted in
the infra-red region. A direct comparison with the lattice data allows us to
fix the scale up to where the model is sensible to describe low-energy
observables. Nevertheless, taking for granted the lattice results, the
resulting spectrum does not follow the general trends found in other
supergravity backgrounds. We further discuss the decoupling of the non-singlet
Kaluza-Klein states by analysing the associated supergravity equation of
motion. The inclusion of non-commutative effects is also analysed and we find
that leads to an enhancement on the value of the masses.Comment: 30 pages, late
Transient behavior of wind towers grounding systems under lightning strikes
Wind turbines, because of their height and localization, are frequently subject to direct lightning strikes. Thus, the investigation on the performance of their grounding system is of paramount interest for the prediction of the potential threats either for people working in (or animals passing through) the wind farm area and for the power and control units installed in close proximity of the turbine towers. In this paper,we perform a comprensive study of the transient behavior of the earthing systems of wind turbines. The analysis is conducted in the frequency domain and an hybrid approach, based on circuit theory and Method of Moments, is adopted to fully account for resistive, inductive and capacitive couplings between elements of the ground system. The actual transient behavior is obtained by means of an Inverse Fourier Transform. The results, computed considering a typical wind turbine grounding system configuration, provide more insight on the nature of the early-time transient response of grounding systems and allow to draw up useful design guidelines
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