Vorticity-induced short-range ordering for heat insulation in anisotropically bonded CsAg2I3.

Vortical motion naturally occurs on various scales, from galaxies to skyrmions, influencing the dynamics of mass and information transport. Yet its impact on lattice dynamics in solids is rarely explored so far, primarily owing to the strong interatomic bonds in real materials that inhibit the actual transport of their constituent species. Here we investigate the effect of vortical modes of polarized phonons on the heat insulation of solid materials using synchrotron X-ray diffuse scattering and inelastic neutron scattering along with ab initio molecular dynamics simulations. We demonstrate this vortical vibration as well as its effect on thermal insulation in CsAg2I3, where the anisotropically bonded iodine atoms form the low-lying transverse optical phonons for enhancing scattering phase space. Moreover, the vortical modes, mainly arising from the chiral symmetric operations acting on the iodine sites, induce two-dimensional short-range-ordering of atomic clusters, lead to a substantially increased phonon scattering centers with a diameter on the order of ~2 primitive cells. This eventually results in extraordinary heat insulation in the single-crystalline CsAg2I3, exhibiting a thermal conductivity as low as 0.08 W/m-K at room temperature. This strategy establishes a material design paradigm for heat insulation and unveils an unexplored avenue for phonon transport manipulation.