  Ergodicity, the central tenet of statistical mechanics, requires that an
isolated system will explore all of its available phase space permitted by
energetic and symmetry constraints. Mechanisms for violating ergodicity are of
great interest for probing non-equilibrium matter and for protecting quantum
coherence in complex systems. For decades, polyatomic molecules have served as
an intriguing and challenging platform for probing ergodicity breaking in
vibrational energy transport, particularly in the context of controlling
chemical reactions. Here, we report the observation of rotational ergodicity
breaking in an unprecedentedly large and symmetric molecule, 12C60. This is
facilitated by the first ever observation of icosahedral ro-vibrational fine
structure in any physical system, first predicted for 12C60 in 1986. The
ergodicity breaking exhibits several surprising features: first, there are
multiple transitions between ergodic and non-ergodic regimes as the total
angular momentum is increased, and second, they occur well below the
traditional vibrational ergodicity threshold. These peculiar dynamics result
from the molecules' unique combination of symmetry, size, and rigidity,
highlighting the potential of fullerenes to uncover emergent phenomena in
mesoscopic quantum systems.
