Thermal energy can be conducted by different mechanisms including by single particles or collective excitations. Thermal conductivity is system-specific and shows a richness of behaviors currently explored in different systems, including insulators, strange metals, and cuprate superconductors. Here, we show that despite the seeming complexity of thermal transport, the thermal diffusivity $\alpha$ of liquids and supercritical fluids has a lower bound that is fixed by fundamental physical constants for each system as ${\alpha }_m=\frac{1}{4\pi }\frac{\hslash }{\sqrt{m_{e}m}}$, where $m_e$ and $m$ are electron and molecule masses. The newly introduced elementary thermal diffusivity has an absolute lower bound dependent on $\hslash$ and the proton-to-electron mass ratio only. We back up this result by a wide range of experimental data. We also show that theoretical minima of $\alpha$ coincide with the fundamental lower limit of kinematic viscosity ${\nu }_m$. Consistent with experiments, this points to a universal lower bound for two distinct properties—energy and momentum diffusion—and a surprising correlation between the two transport mechanisms at their minima. We observe that ${\alpha }_m$ gives the minimum on the phase diagram except in the vicinity of the critical point, whereas ${\nu }_m$ gives the minimum on the entire phase diagram.

• Received 12 October 2020
• Revised 3 January 2021
• Accepted 19 January 2021

DOI:https://doi.org/10.1103/PhysRevB.103.014311