831 research outputs found
Uncovering the effects of interface-induced ordering of liquid on crystal growth using machine learning
The process of crystallization is often understood in terms of the
fundamental microstructural elements of the crystallite being formed, such as
surface orientation or the presence of defects. Considerably less is known
about the role of the liquid structure on the kinetics of crystal growth. Here
atomistic simulations and machine learning methods are employed together to
demonstrate that the liquid adjacent to solid-liquid interfaces presents
significant structural ordering, which effectively reduces the mobility of
atoms and slows down the crystallization kinetics. Through detailed studies of
silicon and copper we discover that the extent to which liquid mobility is
affected by interface-induced ordering (IIO) varies greatly with the degree of
ordering and nature of the adjacent interface. Physical mechanisms behind the
IIO anisotropy are explained and it is demonstrated that incorporation of this
effect on a physically-motivated crystal growth model enables the quantitative
prediction of the growth rate temperature dependence
Supply Shock Versus Demand Shock: The Local Effects of New Housing in Low-Income Areas
We study the local effects of new market-rate housing in low-income areas using microdata on large apartment buildings, rents, and migration. New buildings decrease nearby rents by 5 to 7 percent relative to locations slightly farther away or developed later, and they increase in-migration from low-income areas. Results are driven by a large supply effectâwe show that new buildings absorb many high-income householdsâthat overwhelms any offsetting endogenous amenity effect. The latter may be small because most new buildings go into already-changing areas. Contrary to common concerns, new buildings slow local rent increases rather than initiate or accelerate them
Optical, electronic, and dynamical phenomena in the shock compression of condensed matter
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2003.Includes bibliographical references (leaves 109-113).This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Despite the study of shock wave compression of condensed matter for over 100 years, scant progress has been made in understanding the microscopic details. This thesis explores microscopic phenomena in shock compression of condensed matter including electronic excitations at the shock front, a new dynamical formulation of shock waves that links the microscopic scale to the macroscopic scale, and basic questions regarding the role of crystallinity in the propagation of electromagnetic radiation in a shocked material. In Chapter 2, the nature of electronic excitations in crystalline solid nitromethane are examined under conditions of shock compression. Density functional theory calculations are used to determine the crystal bandgap under hydrostatic stress, uniaxial strain, and shear strain for pure and defective materials. In all cases, the bandgap is not lowered enough to produce a significant population of excited states. In Chapter 3, a new multi-scale simulation method is formulated for the study of shocked materials. The method allows the molecular dynamics simulation of the system under dynamical shock conditions for orders of magnitude longer time periods than is possible using the popular non-equilibrium molecular dynamics (NEMD) approach. An example calculation is given for a model potential for silicon in which a computational speedup of 10â” is demonstrated. Results of these simulations are consistent with some recent experimental observations. Chapters 4 and 5 present unexpected new physical phenomena that result when light interacts with a shock wave propagating through a photonic crystal.(cont.) These new phenomena include the capture of light at the shock wave front and re-emission at a tunable pulse rate and carrier frequency across the bandgap, and bandwidth narrowing of an arbitrary signal as opposed to the ubiquitous bandwidth broadening. Reversed and anomalous Doppler shifts are also predicted in light reflected from the shock front.by Evan J. Reed.Ph.D
A data-centric framework for crystal structure identification in atomistic simulations using machine learning
Atomic-level modeling performed at large scales enables the investigation of
mesoscale materials properties with atom-by-atom resolution. The spatial
complexity of such cross-scale simulations renders them unsuitable for simple
human visual inspection. Instead, specialized structure characterization
techniques are required to aid interpretation. These have historically been
challenging to construct, requiring significant intuition and effort. Here we
propose an alternative framework for a fundamental structural characterization
task: classifying atoms according to the crystal structure to which they
belong. Our approach is data-centric and favors the employment of Machine
Learning over heuristic rules of classification. A group of data-science tools
and simple local descriptors of atomic structure are employed together with an
efficient synthetic training set. We also introduce the first standard and
publicly available benchmark data set for evaluation of algorithms for
crystal-structure classification. It is demonstrated that our data-centric
framework outperforms all of the most popular heuristic methods -- especially
at high temperatures when lattices are the most distorted -- while introducing
a systematic route for generalization to new crystal structures. Moreover,
through the use of outlier detection algorithms our approach is capable of
discerning between amorphous atomic motifs (i.e., noncrystalline phases) and
unknown crystal structures, making it uniquely suited for exploratory materials
synthesis simulations.Comment: 16 pages, 7 figure
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