Neutron and X-ray Scattering in Materials Science: Atomic Dynamics in Crystalline Materials
Sponsored by: TMS Functional Materials Division, TMS: Chemistry and Physics of Materials Committee
Program Organizers: Michael Manley, Oak Ridge National Laboratory; Chen Li, University of California-Riverside; Jennifer Niedziela, Oak Ridge National Lab; Hillary Smith, Swarthmore College
Monday 8:30 AM
March 20, 2023
Room: Aqua 311B
Location: Hilton
Session Chair: Hillary Smith, Swarthmore College
8:30 AM Invited
Inelastic Neutron Scattering Studies of New Spectral Features from Nonlinear Phonon Interactions: Brent Fultz1; Vladimir Ladygin1; Camille Bernal-Choban1; Claire Saunders1; Yang Shen1; 1California Institute of Technology
Especially at high temperatures, "anharmonicity'' alters the phonon entropy, free energy, and thermophysical properties such as thermal expansion[1]. Large anharmonicity with nonlinear phonon interactions can bring new features to phonon spectra. Intermodulation phonon sidebands were recently observed by inelastic neutron scattering in NaBr at 300 K[2]. In a crystal of NaBr, the lower sideband is an intrinsic localized mode, previously observed by Manley. The newly-discovered upper sideband completes the picture for flat phonon branches. Higher temperatures may show intermodulations between dispersive phonon branches. Another newly-observed effect of anharmonicity is diffuse inelastic intensity (DII) in the INS phonon spectra of cuprite, Cu2</SIB>O. Today we can say that the DII reflects changes in the dynamics of both Cu and O atoms on a time scale on the order of their vibrational periods. [1] DOI: 10.1103/PhysRevLett.125.085504. [2] DOI: 10.1103/PhysRevB.103.134302.This work is supported by DOE BES award No. DE-FG02-03ER46055.
9:00 AM Invited
Atomic Tunneling in Crystalline Materials: Raphael Hermann1; 1Oak Ridge National Laboratory
Tunneling is a hallmark quantum mechanical behavior intrinsically related to superposition. Whereas tunneling has been proposed as the origin of glass-like thermal conductivity in amorphous materials, a clear identification of these tunneling states is lacking. In contrast, tunneling in crystalline materials is often restricted to a hydrogen or a small fraction of impurity atoms. Here we will report on the tunneling of a heavy atom in the framework of BaTiS3[1] and Eu8Ga16Ge30[2], two crystalline materials with ultra-low glass-like thermal conductivity at low temperature. In both materials, clear tunneling lines observed by spectroscopy or scattering enable insight in the ground state and transport of the material. All collaborators, support from DOE Office of Basic Energy Science, and the neutron sources at ORNL are gratefully acknowledged. [1] Sun et al., Nature Comm. 11, 6039 (2020).[2] Hermann et al., Phys. Rev. Lett. 97, 017401 (2006).
9:30 AM Invited
Understanding the Origin of Kohn Anomalies in Alpha-Uranium: Dipanshu Bansal1; Aditya Roy1; Naini Bajaj1; Ranjan Mittal2; P D Babu2; 1IIT Bombay; 2BARC
The topology of the Fermi surface controls electronic response of metal, including charge density wave (CDW). A topology conducive to Fermi surface nesting (FSN) allows the electronic susceptibility to diverge and induce a CDW at wave vector qCDW. Kohn extended the implications ofFSN to show that the imaginary part of lattice dynamical susceptibility also responds anomalously for all phonon branches at qCDW — a phenomenon referred to as the Kohn anomaly. However, materials exhibiting multiple Kohn anomalies remain rare. Using first-principles simulations of lattice and electron susceptibility, combined with inelastic neutron scattering measurements we show that α-uranium harbors multiple Kohn anomalies enabled by the combined effect of FSN and “hidden” nesting, i.e., nesting of electronic states above and below the Fermi surface. FSN and hidden nesting lead to a ridgelike feature in real part of electronic susceptibility, allowing interatomic forces to modulate strongly and multiple Kohn anomalies to emerge.
10:00 AM Break
10:15 AM Invited
Modification of Phonon Group Velocity in Doped Sapphire: Shuonan Chen1; Javier Garay2; Fariborz Kargar1; Tao Hong3; Alexander Balandin1; Chen Li1; 1University of California-Riverside; 2University of California, San Diego; 3Oak Ridge National Laboratory
Phonon plays an important role in thermal transport of materials. A more than 10% decrease in transverse acoustic phonon group velocity was observed in lightly Ti-doped sapphire using inelastic neutron scattering. Such modification of acoustic phonon group velocity was not shown in Cr-doped samples. We attribute the difference to the large local strain induced by Ti defects. Our results shed light on phonon engineering of bulk crystals and benefit the design of new materials with tailored thermal conductivity and other thermodynamic properties.
10:45 AM Invited
Lattice Dynamics of Incommensurate Crystals: Michael Manley1; Andrew May1; Barry Winn1; Douglas Abernathy1; Raffi Sahul2; Raphael Hermann1; 1Oak Ridge National Laboratory; 2Amphenol Corporation
Crystals or structures with incompatible translational periodicities are described as incommensurate. Such structures exhibit unique dynamical properties associated with the fact that the phase relationship between incommensurate elements can be shifted without changing the energy of the overall system. This allows for highly supersonic acoustic-like waves called phasons, or super lubricity (near zero friction) in the sliding of incommensurate surfaces. Using inelastic neutron scattering and thermal conductivity measurements, we establish that phasons in the piezoelectric mineral fresnoite make a major contribution to thermal conductivity by propagating many times further and faster than acoustic phonons. The phason contribution to thermal conductivity is maximum near room temperature, where it is the single largest contributing degree of freedom, while phonons still dominate at cryogenic temperatures. This work shows how excitations unique to incommensurate lattices can dominate transport properties and must therefore be considered when attempting to model thermal transport behavior in such systems.
11:15 AM
Temperature Dependence of Anharmonic Effects in NaBr by Inelastic Neutron Scattering and Interatomic Potentials from Machine Learning: Vladimir Ladygin1; Claire Sounders1; Camille Bernal-Choban1; Douglas Abernathy2; Michael Manley2; Brent Fultz1; 1California Institute of Technology; 2Oak Ridge National Laboratory
The rocksalt phase of NaBr has highly anharmonic phonons. Its thermal expansion is well explained by anharmonic perturbation theory [1] DOI: 10.1103/PhysRevLett.125.085504. It also displays a new phenomenon from strong anharmonicity: intrinsic localized modes and intermodulation phonon sidebands. These features occur in pairs, resulting from anharmonic interactions of TA and TO phonons [2] DOI: 10.1103/PhysRevB.103.134302. Here we describe another effect of high temperatures on the phonon dispersions of NaBr measured by inelastic neutron scattering (INS). Also, we have found new spectral features in the optical phonon branches, including patches of diffuse intensity and distinct changes in the shapes of the dispersions. These features have now been measured by INS at multiple temperatures to confirm the original results, and monitor their evolution with temperature. These anharmonic phonon spectra are being simulated with ab initio and machine learning driven molecular dynamics methods. This work is supported by DOE BES award No. DE-FG02-03ER46055.
11:35 AM
Validating First-principles Phonon Lifetimes via Inelastic Neutron Scattering: Hao Ma1; Enda Xiao2; Chris Marianetti2; Michael Manley1; 1ORNL; 2Columbia University
Phonon lifetimes are a key component of quasiparticle theories of transport, yet first-principles lifetimes are rarely directly compared to inelastic neutron scattering (INS) results. Existing comparisons show discrepancies even at temperatures where perturbation theory is expected to be reliable. In this work, we demonstrate that the reciprocal space voxel (q-voxel), the finite region in reciprocal space required in INS data analysis, must be explicitly accounted for within theory to draw a meaningful comparison. We demonstrate accurate predictions of peak widths of the scattering function when accounting for the q-voxel in UN. Passing this test implies high fidelity of the phonon interactions and the approximations used to compute the Green's function, serving as a critical benchmark of theory, and indicating that other material properties should be accurately predicted; which we demonstrate for thermal conductivity.