Higher-order Mpemba effect in a colloidal system*

The Mpemba effect is observed if an initially hot system cools faster than an initially warm system when coupled to the same thermal bath. Previous experiments on the Mpemba effect were performed in a system where a Brownian colloidal particle is subjected to forces from an asymmetric double-well potential. In this system, the presence of anomalous relaxation traces back to a non-monotonic behavior in temperature of the amplitude ₂ of the second eigenfunction of the Fokker-Planck operator. Here we present observations showing that anomalous relaxation is present in a symmetric potential. By symmetry, the quantity ₂ vanishes in such a potential, and the particle dynamics are governed by the next higher-order term, ₃. The non-monotonic temperature dependence of ₃ leads to this new, higher-order Mpemba effect.