Mechanical and Industrial Engineering Faculty Publication SeriesCopyright (c) 2016 University of Massachusetts - Amherst All rights reserved.
http://scholarworks.umass.edu/mie_faculty_pubs
Recent documents in Mechanical and Industrial Engineering Faculty Publication Seriesen-usThu, 22 Sep 2016 02:15:56 PDT3600Robust Identification of Dynamically Distinct Regions in Stratified Turbulence
http://scholarworks.umass.edu/mie_faculty_pubs/615
http://scholarworks.umass.edu/mie_faculty_pubs/615Tue, 20 Sep 2016 13:19:50 PDT
we present a new robust method for identifying three dynamically distinct regions in a stratified turbulent flow, which we characterise as quiescent flow, intermittent layers, and turbulent patches. The method uses the cumulative filtered distribution function of the local density gradient to identify each region. We apply it to data from direct numerical simulations of homogeneous stratified turbulence, with unity Prandtl number, resolved on up to 8192x8192x4092 grid points. In addition to classifying regions consistently with contour plots of potential enstropy, our method identifies quiescent regions as regions where ∊ ⁄ νΝ^{2} ~ Ο(1), layers as regions where ∊ ⁄ νΝ^{2} ~ Ο(10), and patches as regions where ∊ ⁄ νΝ^{2} ~ Ο(100). Here ∊ is the dissipation rate of turbulence kinetic energy, ν is the kinematic viscosity, and Ν is the (overall) buoyancy frequency. By far the highest local dissipation and mixing rates, and the majority of dissipation and mixing, occur in patch regions, even when patch regions occupy only 5% of the flow volume. we conjecture that treating stratified turbulence as an instantaneous assemblage of these different regions in varying proportions may explain some of the apparently high scattered flow dynamics and statistics previously reported in the literature.
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Portwood, Gavin D. et al.Large excitonic effects in monolayers of molybdenum and tungsten dichalcogenides
http://scholarworks.umass.edu/mie_faculty_pubs/614
http://scholarworks.umass.edu/mie_faculty_pubs/614Thu, 15 Sep 2016 09:49:16 PDT
Quasiparticle band structures and optical properties of MoS2, MoSe2, MoTe2, WS2, and WSe2 monolayers are studied using the GW approximation in conjunction with the Bethe-Salpeter equation (BSE). The inclusion of two-particle excitations in the BSE approach reveals the presence of two strongly bound excitons (A and B) below the quasiparticle absorption onset arising from vertical transitions between a spin-orbit-split valence band and the conduction band at the K point of the Brillouin zone. The transition energies for monolayer MoS2, in particular, are shown to be in excellent agreement with available absorption and photoluminescence measurements. Excitation energies for the remaining monolayers are predicted to lie in the range of 1–2 eV. Systematic trends are identified for quasiparticle band gaps, transition energies, and exciton binding energies within as well as across the Mo and W families of dichalcogenides. Overall, the results suggest that quantum confinement of carriers within monolayers can be exploited in conjunction with chemical composition to tune the optoelectronic properties of layered transition-metal dichalcogenides at the nanoscale.
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Ramasubramaniam, AshwinThree-dimensional simulations of self-assembly of hut shaped Si-Ge quantum dots
http://scholarworks.umass.edu/mie_faculty_pubs/613
http://scholarworks.umass.edu/mie_faculty_pubs/613Thu, 15 Sep 2016 09:49:13 PDT
This article presents the results of three-dimensional modeling of heteroepitaxial thin film growth with the objective of understanding recent experiments on the early stages of quantum dot formation in SiGe/Si systems. We use a continuum model, based on the underlying physics of crystallographic surface steps, to study the growth of quantum dots, their spatial ordering and coarsening behavior. Using appropriate parameters, obtained from atomistic calculations, the (100) orientation is found to be unstable under compressive strains. The surface energy now develops a minimum at an orientation that may be interpreted as the (105) facet observed in SiGe/Si systems. This form of the surface energy allows for the growth of quantum dots without any barrier to nucleation—dots are seen to start off via a surface instability as shallow stepped mounds, which steepen continuously to reach their low energy orientations. During the very initial stages of growth, mounds are seen to grow in a dense array with several of them impinging on each other and subsequently coalescing to form larger mounds. This behavior occurs due to the competition between surface energy which seeks to minimize the free-energy by the formation of islands with side-walls at the strain stabilized orientations and repulsive elastic interactions between such closely spaced islands. Using simple analytical calculations, we show the existence of a critical island size for this coalescence behavior. A key result of our analysis is the inverse scaling of this critical size with the misfit strain in the film. While energetic analyses may be used to obtain useful insights, the growth of quantum dots is essentially a nonequilibrium process and requires a fundamental understanding of the kinetics. Numerical studies show that the growth kinetics has a profound effect on surface morphology: arrays of well-separated islands or, alternatively, intersecting ridges are obtained in different kinetic regimes. We also study an alternative model of a stable but nonfacet (100) orientation and point out the inconsistencies of this assumption.
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Ramasubramaniam, Ashwin et al.Edge-stress induced warping of graphene sheets and nanoribbons
http://scholarworks.umass.edu/mie_faculty_pubs/612
http://scholarworks.umass.edu/mie_faculty_pubs/612Thu, 15 Sep 2016 09:49:10 PDT
We show that edge stresses introduce intrinsic ripples in freestanding graphene sheets even in the absence of any thermal effects. Compressive edge stresses along zigzag and armchair edges of the sheet cause out-of-plane warping to attain several degenerate mode shapes. Based on elastic plate theory, we identify scaling laws for the amplitude and penetration depth of edge ripples as a function of wavelength. We also demonstrate that edge stresses can lead to twisting and scrolling of nanoribbons as seen in experiments. Our results underscore the importance of accounting for edge stresses in thermal theories and electronic structure calculations for freestanding graphene sheets.
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Ramasubramaniam, Ashwin et al.Tunable band gaps in bilayer graphene-BN heterostructures
http://scholarworks.umass.edu/mie_faculty_pubs/611
http://scholarworks.umass.edu/mie_faculty_pubs/611Thu, 15 Sep 2016 09:49:08 PDT
We investigate band-gap tuning of bilayer graphene between hexagonal boron nitride sheets, by external electric fields. Using density functional theory, we show that the gap is continuously tunable from 0 to 0.2 eV, and is robust to stacking disorder. Moreover, boron nitride sheets do not alter the fundamental response from that of free-standing bilayer graphene, apart from additional screening. The calculations suggest that the graphene-boron nitride heterostructures could provide a viable route to graphene-based electronic devices.
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Ramasubramaniam, Ashwin et al.Interatomic potentials for hydrogen in a-iron based on density functional theory
http://scholarworks.umass.edu/mie_faculty_pubs/610
http://scholarworks.umass.edu/mie_faculty_pubs/610Thu, 15 Sep 2016 09:49:05 PDT
We present two interatomic potentials for hydrogen in α–iron based on the embedded atom method potentials for iron developed by Mendelev et al. Philos. Mag. 83 3977 (2003) and Ackland et al. J. Phys.: Condens. Matter 16 S2629 (2004). Since these latter potentials are unique among existing iron potentials in their ability to produce the same core structure for screw dislocations as density functional theory (DFT) calculations, our interatomic potentials for hydrogen in iron also inherit this important feature. We use an extensive database of energies and atomic configurations from DFT calculations to fit the cross interaction of hydrogen with iron. Detailed tests on the dissolution and diffusion of hydrogen in bulk α–iron, as well as the binding of H to vacancies, free surfaces, and dislocations, indicate that our potentials are in excellent overall agreement with DFT calculations.
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Ramasubramaniam, Ashwin et al.A Comparison of the Elastic Properties of Graphene- and Fullerene-Reinforced Polymer Composites: The Role of Filler Morphology and Size
http://scholarworks.umass.edu/mie_faculty_pubs/609
http://scholarworks.umass.edu/mie_faculty_pubs/609Thu, 15 Sep 2016 09:49:03 PDTLu, Chang-Tsan et al.