Coexisting Liquid-, Glassy-, and Crystal-like Phonons in a Crystalline Solid

Abstract As main states of matter, crystal, glass, and liquid manifest themselves in distinct transverse collective atomic vibrations. In contrast to crystals where transverse phonons are well-defined throughout Brillouin zones, glasses and liquids process such motions in long and/or short wavelength limits, respectively. Considerable efforts have been made to experimentally probe transverse phonon behavior in these highly anharmonic and disordered materials in pursuit of a universal framework for thermal transport. In this work, inelastic neutron scattering on AgCrSe2 single crystals reveals liquid-, glassy-, and crystal-like phonon features along different crystallographic directions. In particular, the liquid-like behavior is attributed to the significant softening and scattering arising from the nesting effect and dynamical atomic disorder while the glassy-like behavior is due to the selective disorder effect. Our findings challenge conventional boundaries between states of matter at the level of atomic dynamics and establish a materials platform for unifying the thermal transport theories across different states.