Self-organization in the avalanche from molecular Rydberg gas to ultracold plasma

Long-range correlations can steer dissipative many-body systems to transient states with emergent dynamics far different from the equipartition of energy, where small driving forces produce local fluctuations that trigger avalanche-like energy dissipation events. Driven dissipation sometimes leads to size distributions described by power laws signifying self-organization to a scale-invariant critical state. Recent experiments in Strasbourg and Durham have found such signs of self-organized criticality in the off-resonant excitation of a Rydberg gas in the anti-blockade regime, where interaction-induced line shifts and broadening determine size distributions of transition avalanches that underlie a physics of self-organization. A similar mosaic of local kinetic processes and long-range interactions drives the relaxation of a dense molecular Rydberg gas of nitric oxide to form an ultracold plasma exhibiting a scale-invariant avalanche-size distribution described by a power-law: P(N) = N^awith a = -1.37. An evident balance in the optimum density, ρ₀, to form a long-lived plasma for spectroscopically selected intervals of initial principal quantum number, n₀, reflects a sensitivity to the Rydberg orbital radius compared with the average distance between nearest neighbors. Non-linearity enters in the variation of dipole-dipole coupling and unimolecular dissociation with the evolving electron binding energy. Owing to these factors, n₀ and ρ₀ constitute important control parameters to consider in models that seek to explain the apparent self-organized criticality of this system in terms of the dynamics of microscopic interactions.