Longitudinal and Transverse Dispersion in Liquids from Elementary Excitations

Phonons are quantized excitations of lattice vibrations. Although they are quasiparticles in the quantum framework, their properties and interactions in a crystal can be studied using the normal modes of vibrations in a periodic lattice. Extending a similar approach to model collective excitations in disordered systems is gaining traction in recent years. However, the absence of well-defined lattice sites hinders the definition of normal modes of atomic displacements in such systems. In this work, we extend our recent methodology of using elementary waves that can unambiguously identify phonons along a specified wavevector in an atomistic system to an aperiodic liquid system. Relaxing the periodic requirement, we employ our recent approach of using the projections of zero-time correlations of accelerations and velocities (ZTR-2) to circumvent the inherent inability to define atomic displacements about mean positions in a liquid. We show that our approach can successfully compute the normal mode dispersion in liquids from elementary atomic excitations using atomistic simulations. We conclude our presentation by discussing the possible applications for probing quasi-phonon modes in a wider class of materials, including supercooled liquids and glasses.