Chiral effect in plane isotropic micropolar elasticity and its application to chiral lattices

In continuum mechanics, the non-centrosymmetric micropolar theory is usually used to capture the chirality inherent in materials. However when reduced to a two dimensional (2D) isotropic problem, the resulting model becomes non-chiral. Therefore, influence of the chiral effect cannot be properly characterized by existing theories for 2D chiral solids. To circumvent this difficulty, based on reinterpretation of isotropic tensors in a 2D case, we propose a continuum theory to model the chiral effect for 2D isotropic chiral solids. A single material parameter related to chirality is introduced to characterize the coupling between the bulk deformation and the internal rotation which is a fundamental feature of 2D chiral solids. Coherently, the proposed continuum theory is also derived for a triangular chiral lattice from a homogenization procedure, from which the effective material constants of the lattice are analytically determined. The unique behavior in the chiral lattice is demonstrated through the analyses of a static tension problem and a plane wave propagation problem. The results, which cannot be predicted by the non-chiral model, are validated by the exact solution of the discrete model.Comment: 33 pages, 7 figure

The growth and assembly of a massive galaxy at z ~ 2

We study the stellar mass assembly of the Spiderweb Galaxy (MRC 1138-262), a massive z = 2.2 radio galaxy in a protocluster and the probable progenitor of a brightest cluster galaxy. Nearby protocluster galaxies are identified and their properties are determined by fitting stellar population models to their rest-frame ultraviolet to optical spectral energy distributions. We find that within 150 kpc of the radio galaxy the stellar mass is centrally concentrated in the radio galaxy, yet most of the dust-uncorrected, instantaneous star formation occurs in the surrounding low-mass satellite galaxies. We predict that most of the galaxies within 150 kpc of the radio galaxy will merge with the central radio galaxy by z = 0, increasing its stellar mass by up to a factor of ~ 2. However, it will take several hundred Myr for the first mergers to occur, by which time the large star formation rates are likely to have exhausted the gas reservoirs in the satellite galaxies. The tidal radii of the satellite galaxies are small, suggesting that stars and gas are being stripped and deposited at distances of tens of kpc from the central radio galaxy. These stripped stars may become intracluster stars or form an extended stellar halo around the radio galaxy, such as those observed around cD galaxies in cluster cores.Comment: 12 pages, accepted for publication in MNRA

Atomistic-continuum mechanical models for deformations of single-walled carbon nanotubes

Carbon nanotubes (CNTs) are nanometer sized cylinders made of carbon atoms which possess extraordinary electrical, thermal, and mechanical properties. Their potential applications include such diverse areas as conductive and high strength composites, energy storage and conversion devices, sensors, field emission displays and radiation sources, hydrogen storage media, and nanometer sized semiconductor devices, probes and interconnects. A single-walled carbon nanotube (SWNT) is a CNT formed from a single atomic layer comprised of a hexagonal network of carbon atoms that has been rolled up to form a seamless, hollow cylinder, and it is of interest to understand how the underlying atomic structure determines its macroscopic properties. The present dissertation deals with models to study the influence of atomic structure on the macroscopic mechanical properties of SWNTs. In describing such atomic systems, all-atom simulations using appropriate energetic descriptions are accurate, and often employed. However, these are limited by computational expense to a small number of atoms and time steps. Alternatively, continuum models capture a collective behavior of atoms and are computational efficient. However, the accuracy of traditional continuum models suffers from surface, interface and size effects, and ambiguities in model parameters. Hence, there is a need to develop atomistically enriched continuum models which combine the accuracy of all-atom simulations and the efficiency of continuum analyses. The present dissertation focuses on two zero-temperature, atomistically enriched, large-strain, elastic continuum models to study mechanical deformations of SWNTs - (i) a two-dimensional, quasicontinuum membrane model, and (ii) a one-dimensional rod model. The membrane SWNT model has been employed in prior, published work to predict localized effects such as buckled mode shapes of the effective continuum in severe twist and bending deformations. In the present dissertation, modifications to the existing membrane model are proposed, and implemented in studying coupled extension and twist deformations of SWNTs. The rod model is motivated by the need to model global behavior of long SWNTs in which the aforementioned localized effects are not of significant interest. It is a unified, large-strain SWNT model capable of simultaneously accounting for (a) bending, (b) twist, (c) shear, (d) extension, (e) coupled extension and twist, and (f) coupled bending and shear deformation modes. Both the atomistic-continuum SWNT models in the present dissertation demonstrate the benefits of accounting for important anisotropic and large-strain effects as improvements over employing traditional, linearly elastic, isotropic, small-strain, continuum models. It is envisioned that the ideas presented in this dissertation can be extended to other atomic systems such as silicon or boron nitride nanotubes by use of appropriate lattices and energetic descriptions.Grant # EEC-0303674 of the National Science Foundation to Cornell University

Novel two-four semiconductor nanocrystal gain media

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2002.Includes bibliographical references.This thesis details efforts in using cadmium selenide (CdSe) nanocrystallites (NCs) as a novel lasing media. We begin with the synthesis of polymer/NC composites, in which the NCs are stabilized within a long-chain methacrylate polymer. This chapter serves to highlight the processing flexibility afforded by the NCs as well as some of the salient linear optical properties of NCs. The wide range of colors that are accessible using these NCs and the ability to excite them simultaneously are emphasized and provide an incentive to use them as a gain medium. Chapter 3 summarizes work done in developing a NC-based gain media and lists the stringent requirements for observing amplified spontaneous emission (ASE), a signature manifestation of gain. Then the synthesis of a robust NCs-titania, sol-gel matrix is described which satisfies these requirements. We exploit the stability and processability of these matrices to study the optical properties of the NC gain media.(cont.) True temperature independent gain and ASE thresholds are shown to present, thus confirming early theoretical predictions of strongly-confined, zero dimensional gain media. Chapter 4 considers the incorporation of such structures with a suitable feedback structure and presents evidence for the first NC based distributed feedback laser. Room-temperature operation of such devices is shown to follow naturally from the unique gain features of the constituent NCs. Chapter 5 emphasizes the flexibility inherent in using these NCs as a gain media. We combine the processability of NC-titania films with soft-lithographic techniques to construct more complicated lasing structures. Simultaneous, mixed-colored lasing is shown to be possible, which might allow for new devices that operate within a wide gain window.by Vikram C. Sundar.Ph.D