Abstract: Long-wavelength longitudinal phonons can propagate in liquids, but whether transverse phonons exist in liquids has been debated since the 1970s. The classic hydrodynamic theory refutes the existence of the latter because the transverse current fluctuation does not directly couple with the density fluctuation and the Brillouin zone is not well-defined. However, such arguments fail to describe the viscoelastic response of liquids and glasses.

Recently, we have developed a viscoelastic hydrodynamic theory by incorporating both viscoelasticity and anisotropy. As a result of the breaking of time-reversal and isotropic symmetries, transverse density fluctuations emerge in liquids and manifest in glasses, and thus we can interpret phonon delocalization as the microscopic driving force of the strongly-correlated behavior of supercooled liquids and glasses. This approach provides a unified framework to examine closely related phenomena such as Boson peak, beta-relaxation, fragility, Arrhenius crossover, Ioffe-Regel localization, dynamic heterogeneity, and the breakdown of Stokes-Einstein relation. We will also show evidence from coherent inelastic neutron scattering measurements and MD simulations.

Bio: YZ is an associate professor and a Donald Biggar Willett Faculty Scholar in the Department of Nuclear, Plasma, and Radiological Engineering; Department of Electrical and Computer Engineering; Program of Computational Science and Engineering; Center for Biophysics and Quantitative Biology; and Beckman Institute of Advanced Science and Technology at the University of Illinois at Urbana-Champaign. He is also the associate head of the Department of Nuclear, Plasma, and Radiological Engineering. He received his Ph.D. in nuclear science and engineering from the Massachusetts Institute of Technology.