As a generic property, all substances transfer heat through microscopic collisions of constituent particles¹. A solid conducts heat through both transverse and longitudinal acoustic phonons, but a liquid employs only longitudinal vibrations^2,3. As a result, a solid is usually thermally more conductive than a liquid. In canonical viewpoints, such a difference also serves as the dynamic signature distinguishing a solid from a liquid. Here, we report liquid-like thermal conduction observed in the crystalline AgCrSe₂. The transverse acoustic phonons are completely suppressed by the ultrafast dynamic disorder while the longitudinal acoustic phonons are strongly scattered but survive, and are thus responsible for the intrinsically ultralow thermal conductivity. This scenario is applicable to a wide variety of layered compounds with heavy intercalants in the van der Waals gaps, manifesting a broad implication on suppressing thermal conduction. These microscopic insights might reshape the fundamental understanding on thermal transport properties of matter and open up a general opportunity to optimize performances of thermoelectrics.

作为一般特性，所有物质都通过组成粒子^{1 的}微观碰撞来传递热量。固体通过横向和纵向声子传导热量，但液体仅采用纵向振动^2,3。因此，固体通常比液体更具有导热性。在规范的观点中，这种差异也可以作为区分固体和液体的动态特征。在这里，我们报告了在结晶 AgCrSe _{2 中}观察到的液体状热传导. 横向声子被超快动态无序完全抑制，而纵向声子被强烈散射但仍然存在，因此是本质上超低热导率的原因。这种情况适用于范德华间隙中具有重插入剂的各种层状化合物，对抑制热传导具有广泛的意义。这些微观见解可能会重塑对物质热传输特性的基本认识，并为优化热电性能提供一般机会。