Cooling quasiparticles in A3C60 fullerides by excitonic mid-infrared absorption

Long after its discovery, superconductivity in alkali fullerides A3C60 still challenges conventional wisdom. The freshest inroad in such ever-surprising physics is the behaviour under intense infrared excitation. Signatures attributable to a transient superconducting state extending up to temperatures ten times higher than the equilibrium Tc=20 K have been discovered in K3C60 after ultra-short pulsed infrared irradiation-an effect which still appears as remarkable as mysterious. Motivated by the observation that the phenomenon is observed in a broad pumping frequency range that coincides with the mid-infrared electronic absorption peak still of unclear origin, rather than to transverse optical phonons as has been proposed, we advance here a radically new mechanism. First, we argue that this broad absorption peak represents a "super-exciton" involving the promotion of one electron from the t_1u half-filled state to a higher-energy empty t_1g state, dramatically lowered in energy by the large dipole-dipole interaction acting in conjunction with the Jahn-Teller effect within the enormously degenerate manifold of (t_1u)^2 x (t_1g)^1 states. Both long-lived and entropy-rich because they are triplets, the infrared-induced excitons act as a sort of cooling mechanism that permits transient superconductive signals to persist up to much higher temperatures.