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T. H. Lee

Publications and source records attributed to T. H. Lee.

10 recordsLinked to original sources
arXiv2020

Chemical bonding in chalcogenides: the concept of multi-centre hyperbonding

The precise nature of chemical-bonding interactions in amorphous, and crystalline, chalcogenides is still unclear due to the complexity arising from the delocalization of bonding, and non-bonding, electrons. Although an increasing degree of electron delocalization for elements down a column of the periodic table is widely recognized, its influence on chemical-bonding interactions, and on consequent material properties, of chalcogenides has not previously been comprehensively understood from an atomistic point of view. Here, we provide a chemical-bonding framework for understanding the behaviour of chalcogenides (and, in principle, other lone-pair materials) by studying prototypical Telluride non-volatile-memory, 'phase-change' materials (PCMs), and related chalcogenide compounds, via density-functional-theory, molecular-dynamics (DFT-MD) simulations. Identification of the presence of previously unconsidered multi-centre 'hyperbonding' (lone-pair-antibonding-orbital) interactions elucidates not only the origin of various material properties, and their contrast in magnitude between amorphous and crystalline phases, but also the very similar chemical-bonding nature between crystalline PCMs and one of the bonding subgroups (with the same bond length) found in amorphous PCMs, in marked contrast to existing viewpoints. The structure-property relationship established from this new bonding-interaction perspective will help in designing improved chalcogenide materials for diverse applications, based on a fundamental chemical-bonding point of view.

cond-mat.mtrl-sci↗
arXiv2011

Hybrid 3-D Formation Control for Unmanned Helicopters

Teams of Unmanned Aerial Vehicles (UAVs) form typical networked cyber-physical systems that involve the interaction of discrete logic and continuous dynamics. This paper presents a hybrid supervisory control framework for the three-dimensional leader follower formation control of unmanned helicopters. The proposed hybrid control framework captures internal interactions between the decision making unit and the path planner continuous dynamics of the system, and hence improves the system's overall reliability. To design such a hybrid controller, a spherical abstraction of the state space is proposed as a new method of abstraction. Utilizing the properties of multi-affine functions over the partitioned space leads to a finite state Discrete Event System (DES) model, which is shown to be bisimilar to the original continuous-variable dynamical system. Then, in the discrete domain, a logic supervisor is modularly designed for the abstracted model. Due to the bisimilarity between the abstracted DES model and the original UAV dynamics, the designed logic supervisor can be implemented as a hybrid controller through an interface layer. This supervisor drives the UAV dynamics to satisfy the design requirements. In other words, the hybrid controller is able to bring the UAVs to the desired formation starting from any initial state inside the control horizon and then, maintain the formation. Moreover, a collision avoidance mechanism is embedded in the designed supervisor. Finally, the algorithm has been verified by a hardware-in-the-loop simulation platform, which is developed for unmanned helicopters. The presented results show the effectiveness of the algorithm.

eess.SY↗
arXiv2011

An Evolutionary Algorithm with Advanced Goal and Priority Specification for Multi-objective Optimization

This paper presents an evolutionary algorithm with a new goal-sequence domination scheme for better decision support in multi-objective optimization. The approach allows the inclusion of advanced hard/soft priority and constraint information on each objective component, and is capable of incorporating multiple specifications with overlapping or non-overlapping objective functions via logical 'OR' and 'AND' connectives to drive the search towards multiple regions of trade-off. In addition, we propose a dynamic sharing scheme that is simple and adaptively estimated according to the on-line population distribution without needing any a priori parameter setting. Each feature in the proposed algorithm is examined to show its respective contribution, and the performance of the algorithm is compared with other evolutionary optimization methods. It is shown that the proposed algorithm has performed well in the diversity of evolutionary search and uniform distribution of non-dominated individuals along the final trade-offs, without significant computational effort. The algorithm is also applied to the design optimization of a practical servo control system for hard disk drives with a single voice-coil-motor actuator. Results of the evolutionary designed servo control system show a superior closed-loop performance compared to classical PID or RPT approaches.

cs.AI↗
arXiv2008

Higher dimensional cosmological model with a phantom field

We consider a higher dimensional gravity theory with a negative kinetic energy scalar field and a cosmological constant. We find that the theory admits an exact cosmological solution for the scale factor of our universe. It has the feature that the universe undergoes a continuous transition from deceleration to acceleration at some finite time. This transition time can be interpreted as that of recent acceleration of our universe.

gr-qc↗
arXiv2002

Neutrino Clustering in the Galaxy with a Global Monopole

In spherically symmetric, static spacetime, we show that only j=1/2 fermions can satisfy both Einstein's field equation and Dirac's equation. It is also shown that neutrinos are able to have effective masses and cluster in the galactic halo when they are coupled to a global monopole situated at the galactic core. Astronomical implications of the results are discussed.

gr-qc↗
arXiv2001

Scalar Non-Luminous Matter in Galaxies

As a candidate for dark matter in galaxies, we study an SU(3) triplet of complex scalar fields which are non-minimally coupled to gravity. In the spherically symmetric static spacetime where the flat rotational velocity curves of stars in galaxies can be explained, we find simple solutions of scalar fields with SU(3) global symmetry broken to U(1) X U(1), in an exponential scalar potential, which will be useful in a quintessence model of the late-time acceleration of the Universe.

gr-qc↗
arXiv1997

Magnetic properties of neutrinos in high temperature SU(2)XU(1) gauge theory

We calculate the finite temperature self-energy for neutrinos in the presence of a constant magnetic field in a medium in the unbroken $SU(2) \otimes U(1)$ model. We obtain the exact dispersion relation for such neutrinos and find that the thermal effective mass is modified by the magnetic field. We also find a simple analytic expression for the dispersion relation and obtain the index of refraction for large neutrino momentum.

hep-ph↗
arXiv1995

A scale-dependent cosmology for the inhomogeneous Universe

A scale-dependent cosmology is proposed in which the Robertson-Walker metric and the Einstein equation are modified in such a way that $Ω_0$, $H_0$ and the age of the Universe all become scale-dependent. Its implications on the observational cosmology are discussed.

hep-ph↗
arXiv1995

Modifications of the Hubble Law in a Scale-Dependent Cosmology

We study some observational consequences of a recently proposed scale--dependent cosmological model for an inhomogeneous Universe. In this model the Universe is pictured as being inside a highly dense and rapidly expanding shell with the underdense center. For nearby objects ($z \ll 1$), the linear Hubble diagram is shown to remain valid even in this model, which has been demonstrated both analytically and numerically. For large $z$, we present some numerical results of the redshift--luminosity distance relation and the behavior of the mass density as a function of the redshift. It is shown that the Hubble diagram in this model for a locally $open$ Universe ($Ω(t_0, r \sim 0)=0.1$) resembles that of the $flat$ Friedmann cosmology. This implies that study of the Hubble diagram cannot uniquely determine the value of $q_0$ or $Ω_0$ in a model--independent way. The model also accounts for the fact that $Ω_0$ is an increasing function of the redshift.

hep-ph↗