Hotter Cools Faster: The Mpemba Effect for Thermal Radiation express
In a hot mug, water sometimes freezes faster than in a warm one. This paradox is over half a century old and is called the Mpemba effect. It is usually attributed to fluid motion or evaporation. Now physicists have shown that a similar trick is possible without water, just through radiation of heat.
The hero of this story is a tiny VO₂ particle suspended a few tens of nanometers from a cold silicon carbide (SiC) plate. When the particle is hotter, it reaches the transition temperature faster and 'switches on' the metallic state. Latent heat temporarily locks away some energy, preventing the temperature from dropping. When the particle cools slightly, the lattice returns to the insulator, and the stored heat is released, like a spring. This thermal buffer creates conditions where a hotter body can zip through the dangerous temperature range faster than one that started with less heat.
A key role is played by the near field — radiative heat exchange at distances smaller than the wavelength. It dramatically accelerates cooling and allows the substrate to act as an external memory: by reflecting radiation, the plate retains information about the state the particle was in. This gives rise to a passive Mpemba effect — without pumps, fans, or complex electronics.
Original abstract
as written by the authors arXiv:2606.22552
The Mpemba effect states that initially hotter systems cool faster than colder ones. While known in convective, conductive, and quantum systems, its radiative analogue is unexplored. Here, this anomaly is realized via phase-change hysteresis of a VO$$_2$$ nanoparticle near a SiC substrate. After analytically deriving an onset condition, the phase space is mapped. Crucially, latent heat acts as a thermal buffer enabling both ordinary and inverse effects. Near-field coupling governs the relaxation time and enables a passive effect where memory is stored externally via substrate reflection.
The Mpemba effect is named after Tanzanian schoolboy Erasto Mpemba, who in 1963 noticed that hot ice cream freezes faster than cold and persuaded a professor to test it in the lab.