Diffuse intensity from phonon noise in inelastic neutron scattering

Creators: Saunders, Claire N.¹; Ladygin, Vladimir V.¹; Kim, Dennis S.²; Bernal-Choban, Camille M.¹; Lohaus, Stefan H.¹; Granroth, Garrett E.³; Abernathy, Douglas L.³; Fultz, Brent¹

1. California Institute of Technology
2. University of California, Los Angeles
3. Oak Ridge National Laboratory

Abstract

Atomic vibrational dynamics in cuprite Cu_(2)O was studied by inelastic neutron scattering and molecular-dynamics (MD) simulations from 10 K to 900 K. At 300 K, a diffuse inelastic intensity (DII) appeared in the phonon dispersions, and dominated the spectral intensity at higher temperatures. Classical MD simulations with a machine-learning interatomic potential reproduced general features of the DII. Better agreement with experiment was obtained with the addition of a stiffer potential at close approaches of the Cu and O-atoms. The DII originates from random phase shifts of vibrating O atoms that have brief (∼10 fs) anharmonic interactions with neighboring Cu atoms. The spectrum of DII gives information about the interaction time of anharmonic interactions between atoms, and its intensity gives a strength of coupling between vibrating atoms and a thermal bath.

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Acknowledgement

This research used resources at the Spallation Neutron Source, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory. In addition, this work used resources from the National Energy Research Scientific Computing Center (NERSC), a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. Finally, this work was supported by the DOE Office of Science, BES, under Contract No. DE-FG02-03ER46055.

Supplemental Material

The section "Concept of DII from Phase Shifts" presents DII as a commutative phase error in atomic vibrations. Section "Extended model of semi-periodic phase shifts" extends the model of semi-periodic phase shifts presented in the main text to include a transition of vibrational dynamics from that of a damped harmonic oscillator to the dynamics of a Gaussian distribution of phase shifts.
Sections 3 give details of multiphonon correction computations.
Sections 4-8 provide additional details on computations presented in the main text.
In the Section "Analyses of characteristic damping time" we analyze the characteristic time of decoherence of atomic vibrations from averaged spectral intensity.
In the last Section, we quantify anharmonicity in the Cu2O system.

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