Phonon Quasiparticle Breakdown and Novel Excitations in Strongly Disordered Solids

A deeper understanding atomic motions in solids is needed to refine microscopic theories of transport and thermodynamics, in order to design improved materials. In particular, the behavior of phonons in crystalline materials is key to rationalize numerous functional properties, ranging from thermoelectrics for cooling or waste-heat harvesting to multiferroics and phase-change materials for information processing, or superionics for solid batteries. Yet, textbook models of lattice dynamics fall short in crystalline systems featuring disorder, which disrupts translational periodicity and conventional phonon band structures. For instance, alloying may result in resonance modes that lack the propagation of conventional phonons [1,2], and spontaneous nanostructures associated with structural instabilities and sublattice disordering can lead to strong phonon scattering and glassy thermal transport [3,4]. In addition, weak bonding and strong anharmonicity can yield large vibrational amplitudes [5] and even thermally driven ionic delocalization that can lead to atomic dynamics intermediate between those of crystalline solids and those of liquids [6,7]. I will describe our combined use of state-of-the-art neutron and x-ray scattering techniques together with first-principles and machine-learning augmented simulations to probe the complex atomic structure and dynamics of disordered materials. Deviations from long-range crystalline order are probed through diffuse scattering measurements while inelastic neutron/x-ray scattering reveals both the spatial and temporal correlations of atomic motions. We rationalize our experimental data using our large-scale simulations based on surrogate neural-net force-fields trained against ab-initio data with machine learning. This integrated approach provides insights into the effect of disorder on phonons and points to descriptors that could enable future materials design.<br/><br/>[1] O. Delaire, I. I. Al-Qasir, A. F. May, C. W. Li, B. C. Sales, J. L. Niedziela, J. Ma, M. Matsuda, D. L. Abernathy, T. Berlijn, “Heavy-impurity resonance, hybridization, and phonon spectral functions in Fe_1-xM_xSi, M=Ir,Os”, Phys. Rev. B 91, 094307 (2015)<i>.</i><br/>[2] O. Delaire, T. Swan-Wood, and B. Fultz, “Negative entropy of mixing for solutions of vanadium-platinum”, Physical Review Letters 93, 185704 (2004).<br/>[3] J. L. Niedziela, D. Bansal, J. Ding, T. Lanigan-Atkins, C. Li, A. F. May, H. Wang, J. Y. Y. Lin, D. L. Abernathy, G. Ehlers, A. Huq, D. Parshall, J. W. Lynn, and O. Delaire, “Controlling phonon lifetimes via sublattice disordering in AgBiSe₂”, Phys. Rev. Materials 4, 105402 (2020).<br/>[4] J. Ma*, O. Delaire*, A. F. May, C. E. Carlton, M. A. McGuire, L. H. VanBebber, D. L. Abernathy, G. Ehlers, Tao Hong, A. Huq, Wei Tian, V. M. Keppens, Y. Shao-Horn, and B. C. Sales, “Glass-like phonon scattering from spontaneous nanostructure in AgSbTe2”, Nature Nanotechnology 8, 445–451 (2013).<br/>[5] T. Lanigan-Atkins*, X. He*, M. J. Krogstad, D. M. Pajerowski, D. L. Abernathy, Guangyong NMN Xu, Zhijun Xu, D.-Y. Chung, M. G. Kanatzidis, S. Rosenkranz, R. Osborn, and O. Delaire, “Two-dimensional overdamped fluctuations of soft perovskite lattice in CsPbBr3”, Nature Materials 20, 977–983 (2021).<br/>[6] J. L. Niedziela, D. Bansal, A. F. May, J. Ding, T. Lanigan-Atkins, G. Ehlers, D. L. Abernathy, A. Said & O. Delaire, “Selective Breakdown of Phonon Quasiparticles across Superionic Transition in CuCrSe2”, Nature Physics, 15, 73–78 (2019).<br/>[7] Jingxuan Ding, Jennifer L. Niedziela, Dipanshu Bansal, Jiuling Wang, Xing He, Andrew F. May, Georg Ehlers, Douglas L. Abernathy, Ayman Said, Ahmet Alatas, Yang Ren, Gaurav Arya, and Olivier Delaire, “Anharmonic lattice dynamics and superionic transition in AgCrSe2”, PNAS 117 (8) 3930-3937 (2020).