Browsing by Subject "Phonon"

Now showing 1 - 4 of 4
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    Bridging scales in modeling and simulation of thermal transport processes
    (2014-08) Wheeler, Vincent Michael
    The vastly disparate length and time scales existing in new devices and materials born out of nanotechnology have made thermal modeling and simulation more important and more difficult. The experimental thermal characterization of such systems, e.g. modern computer processors, can be prohibitively difficult or expensive making numerical simulation the only route to effective technology design. However, obtaining solutions that account for small scales, but are still computationally feasible, requires innovative modeling approaches. The research contained herein represents three independent contributions to the understanding of the modeling of thermal transport processes in systems with nano-sized features. At their common core, all contributions in this thesis are rooted in transport theory--the solution or approximation of the Boltzmann equation (BE)--to statistically describe a system made up of a great many energy-carrying particles. The work roughly divides into the three modes of heat transfer--convection, conduction, and radiation. First, a framework for the discretization of the BE (in its many forms) based on lattices is presented. The widely-used lattice Boltzmann method for the simulation of fluid flow is shown to be a sub-case. The framework gives a new rigorous foundation to the use of lattice methods which have emerged in recent years with applications ranging from Brownian motion to astrophysical radiation. Second, we give a thorough presentation of recently proposed models of heat conduction derived from the phonon BE which provides rigor and insight into the different approaches. Most notably, the "new heat equation" is derived directly from the phonon BE for the first time along with a novel boundary condition. The result is shown to give excellent agreement with the more detailed description provided by the equation of phonon radiative transport. Last, we provide the radiative characterization of a nano-porous material using Maxwell's equations in order to recover coefficients to the linear BE governing thermal radiative transfer.
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    Development of CDMS-II surface event rejection techniques and their extensions to lower energy thresholds
    (2014-12) Hofer, Thomas James
    The CDMS-II phase of the Cryogenic Dark Matter Search, a dark matter direct-detection experiment, was operated at the Soudan Underground Laboratory from 2003 to 2008. The full payload consisted of 30 ZIP detectors, totaling approximately 1.1 kg of Si and 4.8 kg of Ge, operated at temperatures of 50 mK. The ZIP detectors read out both ionization and phonon pulses from scatters within the crystals; channel segmentation and analysis of pulse timing parameters allowed effective fiducialization of the crystal volumes and background rejection sufficient to set world-leading limits at the times of their publications. A full re-analysis of the CDMS-II data was motivated by an improvement in the event reconstruction algorithms which improved the resolution of ionization energy and timing information. The Ge data were re-analyzed using three distinct background-rejection techniques; the Si data from runs 125 - 128 were analyzed for the first time using the most successful of the techniques from the Ge re-analysis. The results of these analyses prompted a novel "mid-threshold" analysis, wherein energy thresholds were lowered but background rejection using phonon timing information was still maintained. This technique proved to have significant discrimination power, maintaining adequate signal acceptance and minimizing background leakage. The primary background for CDMS-II analyses comes from surface events, whose poor ionization collection make them difficult to distinguish from true nuclear recoil events. The novel detector technology of SuperCDMS, the successor to CDMS-II, uses interleaved electrodes to achieve full ionization collection for events occurring at the top and bottom detector surfaces. This, along with dual-sided ionization and phonon instrumentation, allows for excellent fiducialization and relegates the surface-event rejection techniques of CDMS-II to a secondary level of background discrimination. Current and future SuperCDMS results hold great promise for mid- to low-mass WIMP-search results.
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    Nanoscale Coherent-Acoustic-Phonon Dynamics in Molybdenum Disulfide Using Ultrafast Electron Microscopy
    (2021-06) Zhang, Yichao
    In this dissertation, photoexcited, defect-mediated anisotropic acoustic-phonon dynamics in molybdenum disulfide (MoS2) have been directly imaged on the nanometer-picosecond spatiotemporal scales in real space. MoS2 is a prototypical material system of transition metal dichalcogenides extensively studied due to the exceptional tunability of many properties (e.g., electronic band structure) via layer number and strain, and thus attracts interests in a broad range of device applications. Defects have been demonstrated to impact local properties and dynamics in a spatial range from a single atom to hundreds of nanometers. The combined nanometer spatial resolution and femtosecond temporal resolution of ultrafast electron microscopy (UEM) enables direct visualization of photoinduced anisotropic acoustic-phonon dynamics localized at nanoscale defects in freestanding, multilayer MoS2.Excitation of the phonon dynamics is achieved via uniformly illuminating the specimens with 515-nm, 300-fs laser pulses. Propagation of the waves locally deforms the crystal lattice, leading to a modulation of the local Bragg conditions. Visualization of the waves is thus achieved by monitoring modulation of diffraction contrast features (e.g., bend contours). When viewed along the [0001] direction, photoexcited individual phonon wave trains were observed to be emitted from crystal-crystal interfaces (e.g., step edge and terrace) and propagate along a single wave vector perpendicular to the interfaces at frequencies in the tens of gigahertz (GHz) range and at approximately the in-plane speed of sound (7 nm/ps). When viewed along a high-index zone axis (i.e., large angle between specimen normal and the incident electron wave vector), the observed diffraction-contrast dynamics exhibits no propagations but only oscillations. Such specimen configuration allows for projection of c-axis wave dynamics onto the image plane. The c-axis phonon velocity was extracted from the oscillation frequency and specimen thickness, consistent with c-axis speed of sound (2.9 nm/ps). The onset of the c-axis dynamics occurs a few picoseconds earlier than that of the in-plane dynamics, suggesting photoinduced modulation of interlayer spacing leading to launching of in-plane compression waves. In addition to serving as launching sites of phonon wave trains, step edges can mediate localized dynamics that are distinct from that observed in pristine regions. The c-axis phonon modes exhibit dephased oscillations at an individual step edge owing to different specimen thicknesses, and even one-unit-cell of thickness difference manifests a few GHz of frequency shift. Step edges have also been demonstrated to induce new relaxation states extending to several hundred nanometers. Subsequently, a frequency reduction in the c-axis phonon oscillation (i.e., phonon softening) at an individual, nanoscale step edge was observed, indicative of associated softening of the elastic constant. This is consistent with results obtained with finite element modeling. In the process of preparing ultrathin, pristine specimens for studying photoinduced structural dynamics in mono- and bilayer MoS2, substrate-directed spontaneous strain patterning was observed in the term of twelve-fold zone-axis patterns and six-fold centroidal Voronoi tessellation patterns. Vertical deformation of up to 35 nm for several bilayer MoS2 crystals were measured with atomic force microscopy. The formation mechanism of such pattern was elucidated with atomistic simulations.
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    Raman spectroscopy data and phonon calculations for ScV6Sn6 in P6/mmm and R-3m structures, 2023
    (2023-06-27) Ritz, Ethan T; Birol, Turan; Gu, Yanhong; Musfeldt, Janice L; eritz@umn.edu; Ritz, Ethan T; Birol Research Group, University of Minnesota; Musfeldt Group, University of Tennessee Knoxville
    We use density functional theory (DFT) to calculate the phonon frequencies and the distortions associated with them in the compound ScV6Sn6 in the P6/mmm and R-3m space groups, then compute the overlap between the Raman-active phonons in each structure. This data includes scripts to generate the DFT submission files, the results of those simulations, as well as MATLAB scripts to plot the results. We also include experimental Raman spectroscopy data at temperatures from 5.5 K to 300 K.