Summary Hydrodynamic phonon transport, a collective viscous-like Poiseuille heat flow, is an intriguing regime in solids. Here we show that in quantum-paraelectric strontium titanate (SrTiO3), this transport exhibits an anomalous millimeter-scale size dependence, unlike other materials where pronounced size effects appear only when sample dimensions approach the phonon mean free path. Narrowing high-purity crystals from 1.5 to 0.3 mm suppresses the super-cubic upturn in thermal conductivity and the Poiseuille shoulder in thermal diffusivity in the 6- to 13-K range, alongside a systematic shift of the Knudsen minimum to higher temperatures, signaling a hydrodynamic-to-ballistic crossover. Polarized optical microscopy, first-principles phonon calculations, and thermal transport analysis reveal ferroelastic domain walls—with average spacing following Kittel-Roitburd scaling—acting as dominant diffuse internal boundaries. These findings establish ferroelastic domain wall engineering as a knob for size-tailored hydrodynamic heat transport, enabling cryogenic thermal management and phononic device design.
Guo et al. (Sun,) studied this question.