Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session X20: Understanding Glasses and Disordered Matter Through Computational Models IIFocus Live
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Sponsoring Units: DCOMP DSOFT GSNP DPOLY Chair: Joerg Rottler, University of British Columbia |
Friday, March 19, 2021 8:00AM - 8:36AM Live |
X20.00001: Understanding the boson peak in glasses and glassy polymers Invited Speaker: Alessio Zaccone The boson peak is the ubiquitously observed peak at THz frequencies in the normalized vibrational density of states (DOS) of glasses. Its name is due to the fact that, in early Raman scattering experiments (where the Raman intensity is proportional to the DOS times the Bose-Einstein occupation factor), the temperature dependence of the peak in the Raman intensity displays the same temperature dependence as the Bose function. This observation induced some researchers, since those early days, to think that the boson peak oughts to be temperature-independent. As a result, the current theoretical paradigm holds that the boson peak originates from purely "harmonic" disorder-induced scattering of phonons, in spite of several other studies (including experimental investigations) showing evidence of a strongly "anharmonic" origin of the boson peak. We will show, using numerical simulations on polymer systems and metallic glasses, and theoretical arguments, that the boson peak is instead strongly temperature-dependent provided that one correctly accounts for the role of temperature and instantaneous normal modes in molecular simulations of glasses. I will also show that evidence from different angles and systems points to the anharmonic origin of the boson peak in glasses, while earlier statements about the "harmonic" origin of the boson peak are mislead by the strong temperature dependence of the Bose function in Raman scattering (which effectively obscures all other temperature dependencies) and by simulations of well-relaxed glasses in the inherent structures, where instantaneous normal modes are neglected and where, therefore, the boson peak appears as invariant with temperature. |
Friday, March 19, 2021 8:36AM - 8:48AM Live |
X20.00002: Characterizing the structure of the disordered micellar regime of asymmetric diblock copolymers using simulations Anshul Chawla, Frank Bates, Kevin D Dorfman, David Clark Morse Experiments on highly asymmetric diblock copolymers have revealed that disordered micelles exist above the order-disorder transition temperature. Self-consistent field theory is incapable of describing this disordered micellar state. In this work, we perform molecular dynamics simulations of a coarse-grained model to study the disordered micellar state of asymmetric diblock copolymers with a minority block volume fraction of 0.125. We characterize the structure of these disordered micelles in reciprocal space by measuring the structure factor and in real space by identifying clusters rich in the minority block type. Near the order-disorder transition, the structure factor exhibits a secondary shoulder at high wave numbers similar to that seen in experiments, which has been interpreted as arising from correlations in the positions of disordered micelles. Direct analysis of configurations shows the existence of micelle-like clusters at segregation strengths well below those at which they become apparent from the appearance of this feature in the structure factor. |
Friday, March 19, 2021 8:48AM - 9:00AM Live |
X20.00003: A method for the accurate determination of basins of attraction of jammed packings Praharsh Suryadevara, Stefano Martiniani The efficient computation of the volume of basins of attraction in the energy landscape of jammed solids has recently been enabled by Monte Carlo integration schemes akin to the Frenkel-Ladd method for calculating the free energy of atomic solids. The rate-limiting step for this approach is the energy minimization performed at every Monte Carlo step to determine whether a point lies within a given basin of attraction. Here, we show that directly solving the steepest descent trajectory using a variable-order variable-step implicit scheme based on the Backward Differentiation Formulas, implemented by the CVODE solver, identifies basins with near-perfect accuracy with comparable performance to other efficient optimizers (e.g., FIRE) that, however, do not preserve the basin geometry. We use this new scheme to produce accurate projections of basins of attraction of Hertzian particles in 2 dimensions. We also propose an efficient new method that uses the CVODE solver in non-convex regions and Newton's method otherwise, allowing us to improve performance further while significantly reducing the loss in accuracy characteristic of (quasi-) Newton methods. |
Friday, March 19, 2021 9:00AM - 9:12AM Live |
X20.00004: The effect of different cooling rates on two-level system distributions Alec Mishkin, Jun Jiang, Rui Zhang, Kiran Prasai, Riccardo Bassiri, Martin M Fejer, Hai-Ping Cheng Gravitational wave detectors like LIGO and Virgo use amorphous oxide optical coatings on the mirrors used to reflect LASER beams. The thermal noise from these coatings can be a major limit to the sensitivity of these detectors. It has been shown that annealing can lower the room temperature mechanical loss (which is directly related to thermal noise) of these coatings. In this talk, I will present results from atomic simulations aimed to understand this observation. We generate atomic models of amorphous Ta2O5 and TiO2-doped Ta2O5 using the melt-quench molecular dynamics method. By crudely assuming that fast and slow cooling rates might generate structures similar to those of unannealed and annealed thin films, we generate fast cooled and slow computer models. We use these models to find the two-level system (TLSs) in the underlying potential energy landscape (PEL); and the TLSs are subsequently used to compute the mechanical loss. I will present how changing the cooling rate affects the composition of the two-level systems as well as their barrier heights and asymmetries. Based on these calculations, we make some speculations about how annealing might alter the mechanical loss, if the analogy between slow quenching and annealing is true. |
Friday, March 19, 2021 9:12AM - 9:24AM Live |
X20.00005: Dynamical caging and activation in random Lorentz gas model Giulio Biroli, Patrick Charbonneau, Giampaolo Felona, Yi Hu, Francesco Zamponi Mean-field theory predicts that a dynamical (or mode-coupling) transition leads to particle caging and hence glass formation. In finite-dimensional systems, however, this transition is avoided due to activated processes and other effects. Obtaining a first-principle description of these effects remains one of the most challenging aspects of the glass problem. We here study these effects in the random Lorentz gas (RLG). This minimal model indeed shares a same mean-field description with structural glasses, but only allows certain instantonic corrections to proceed. The vicinity of the dynamical transition can then be carefully examined. The systematic comparison between theoretical descriptions and high-d simulations reveals both qualitative and quantitative corrections to the mean-field behavior. In the caging regime, we have thus identified two types of finite-d corrections: perturbative corrections to the cage size and non-perturbative cage escapes. In the diffusing regime, we further identified a microscopic origin of the Stokes-Einstein relation breakdown. Our results thereby provide strong guidelines for a theory of activation, and offer a first-principle pathway for relating local structure and dynamics in glass-forming liquids. |
Friday, March 19, 2021 9:24AM - 9:36AM Live |
X20.00006: Ab initio study of density-dependent structure of amorphous strontium titanate (a-SrTiO3) Bishal Bhattarai, Ivan Zhuravlev, Julia Medvedeva Crystalline SrTiO3 (c-STO), a common substrate material with singly terminated TiO2 is crucial to grow complex oxide heterostructures. However, thin-films of c-STO suffer many limitations including high sensitivity to deposition process, heat treatment techniques and poor electrical properties compared to bulk STO. In contrast, amorphous materials of ABO3 composition may provide an alternative route to attain structural stability and smooth surfaces while avoiding grain boundaries and internal lattice strain. |
Friday, March 19, 2021 9:36AM - 9:48AM Live |
X20.00007: First Principles Model of Amorphous ALD Deposited Al2O3 Angela Harper, Andrew J Morris, Steffen Emege, Clare Grey Amorphous alumina (a-Al2O3) is widley used for applications such as Si passivation, dielectric layers in field effect transistors, and as a coating material at interfaces in Li-ion batteries. Through the use of ab initio molecular dynamics (AIMD) we develop a model for a-Al2O3 which is verified experimentally by both X-ray Absorption Spectroscopy (XAS) and solid-state Nuclear Magnetic Resonance (NMR). While crystalline models are often verified using X-ray diffraction, it is not the case that amorphous models can be verified in the same way. To create an XAS spectra of a-Al2O3 with density-functional theory (DFT), we calculated core-hole spectra for each Al environment in the model, and the resulting summed spectra matched the experimental XAS. Similarly, NMR from first principles was calculated as the sum of a set of snapshots from the AIMD. We constructed a model from first principles which matches both the electronic and structural properties of ALD deposited a-Al2O3 and therefore can be used to study this phase in conjunction with other materials. |
Friday, March 19, 2021 9:48AM - 10:00AM Live |
X20.00008: Calculated Raman Spectra for LIGO’s Coating Amorphous Oxides Rui Zhang, Jun Jiang, Alec Mishkin, Maher Yazback, Kiran Prasai, Riccardo Bassiri, Martin M Fejer, Hai-Ping Cheng We calculate Raman spectra from a density functional approach for several amorphous oxides that are candidate optical coating materials for gravitational wave detection interferometers. The model amorphous systems are prepared by fitting the experimental radial distribution function using reverse Monte Carlo combined with melt-quench. Each model is about 360 atoms in size. Different approaches to eliminate spurious crystalline phonon modes due to finite supercell size are compared. The calculated Raman spectra are in reasonable agreement with the experimentally determined ones. |
Friday, March 19, 2021 10:00AM - 10:12AM Live |
X20.00009: Hydrogen doping is wide-bandgap amorphous In-Ga-O semiconductors Julia Medvedeva, Bishal Bhattarai Microscopic mechanisms of the formation of H defects and their role in passivation of under-coordinated atoms, short- and long-range structural transformations, and the resulting electronic properties of amorphous In-Ga-O with In:Ga=6:4 are investigated using ab-initio molecular dynamics simulations and hybrid density-functional calculations. The results reveal a stark difference between H-passivation in covalent Si-based and ionic oxide semiconductors. Specifically, it is found that hydrogen doping triggers an extended bond reconfiguration and rearrangement in the network of shared polyhedra in the disordered oxide, resulting in energy gains that outweigh passivation of dangling O-p-orbitals. The H-induced structural changes in the coordination and morphology favor a more uniform charge density distribution in the conduction band, in accord with 70-fold increase in carrier mobility measured in H-doped In-Ga-O [1]. |
Friday, March 19, 2021 10:12AM - 10:24AM Live |
X20.00010: The connection between the density of Two-level systems and density of Quasi-localised modes in glasses WENCHENG JI The density of Two-level systems (TLS) $n_0$, which controls the low-temperature properties of glass, dramatically decreases in ultrastable glasses. Meanwhile, $A_4$, -the prefactor of the density of quasi-localised modes (QLMs)-, also dramatically decreases by several hundred times in recent numerical simulations. However, the relationship between $A_4$ and $n_0$ is unclear. In this work, I theoretically build up the relationship between $n_0$ and the density of QLMs based on the soft-potential model. I find that $n_0$ corresponds to the QLMs with a typical frequency $\omega_0$ that is about 10\% - 20\% of the Debye frequency. The decrease in $n_0$ is influenced not only by the rarefaction of the density of QLMs but also by a factor that is related to the fraction of double-well potentials. Further, I find that the estimations of $n_0$ are consistent with the order of magnitude of the change in $n_0$ found in experiment. |
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