Direct Experimental Evidence of Longitudinal and Transverse Mode Hybridization and Anticrossing in Simple Model Fluids

A significant number of key properties of condensed matter are determined by the spectra of elementary excitations and, in particular, collective vibrations. However, the behavior and description of collective modes in disordered media (e.g., liquids and glasses) remains a challenging area of modern condensed matter science. Recently, anticrossing between longitudinal and transverse modes was predicted theoretically and observed in molecular dynamics simulations, but this fundamental phenomenon has never been observed experimentally. Here we demonstrate the mode anticrossing in a simple Yukawa fluid constructed from charged microparticles in weakly ionized gas. Theory, simulations, and experiments show clear evidence of mode anticrossing that is accompanied by mode hybridization and strong redistribution of the excitation spectra. Our results provide a significant advance in understanding excitations of fluids, opening new perspectives for studies of dynamics, thermodynamics, and transport phenomena in a wide variety of systems from noble-gas fluids and metallic melts to strongly coupled plasmas and molecular and complex fluids.