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Ergodicity, the central tenet of statistical mechanics, requires an isolated system to explore all available phase space constrained by energy and symmetry. Mechanisms for violating ergodicity are of interest for probing nonequilibrium matter and protecting quantum coherence in complex systems. Polyatomic molecules have long served as a platform for probing ergodicity breaking in vibrational energy transport. Here, we report the observation of rotational ergodicity breaking in an unprecedentedly large molecule,¹²C₆₀, determined from its icosahedral rovibrational fine structure. The ergodicity breaking occurs well below the vibrational ergodicity threshold and exhibits multiple transitions between ergodic and nonergodic regimes with increasing angular momentum. These peculiar dynamics result from the molecule’s distinctive combination of symmetry, size, and rigidity, highlighting its relevance to emergent phenomena in mesoscopic quantum systems. 

The eigenstate thermalisation hypothesis (ETH) is a statisticalcharacterisation of eigen-energies, eigenstates and matrix elements oflocal operators in thermalising quantum systems. We develop an ETH-likeansatz of a partially thermalising system composed of aspin- \tfrac{1}{2} 1 2 coupled to a finite quantum bath. The spin-bath coupling is sufficientlyweak that ETH does not apply, but sufficiently strong that perturbationtheory fails. We calculate (i) the distribution of fidelitysusceptibilities, which takes a broadly distributed form, (ii) thedistribution of spin eigenstate entropies, which takes a bi-modal form,(iii) infinite time memory of spin observables, (iv) the distribution ofmatrix elements of local operators on the bath, which is non-Gaussian,and (v) the intermediate entropic enhancement of the bath, whichinterpolates smoothly between S = 0 S = 0 and the ETH value of S = \log 2 S = log 2 .The enhancement is a consequence of rare many-body resonances, and isasymptotically larger than the typical eigenstate entanglement entropy.We verify these results numerically and discuss their connections to themany-body localisation transition. 

The many-body localised (MBL) to thermal crossover observed in exactdiagonalisation studies remains poorly understood as the accessiblesystem sizes are too small to be in an asymptotic scaling regime.We develop a model of the crossover in short 1D chains in which theMBL phase is destabilised by the formation of many-body resonances.The model reproduces several properties of the numerically observedcrossover, including an apparent correlation length exponent \nu=1 ν = 1 ,exponential growth of the Thouless time with disorder strength, lineardrift of the critical disorder strength with system size, scale-freeresonances, apparent 1/\omega 1 / ω dependence of disorder-averaged spectral functions, and sub-thermalentanglement entropy of small subsystems.In the crossover, resonances induced by a local perturbation are rareat numerically accessible system sizes L L which are smaller than a \lambda λ .For L \gg \sqrt{\lambda} L ≫ λ (in lattice units), resonances typically overlap, and this model doesnot describe the asymptotic transition.The model further reproduces controversial numerical observationswhich Refs. claimed to be inconsistent with MBL. We thus argue that thenumerics to date is consistent with a MBL phase in the thermodynamiclimit.