Giant THz photoconductivity and possible non-equilibrium superconductivity in metallic K(3)C(60)

The non-equilibrium control of emergent phenomena in solids is an important research frontier, encompassing effects like the optical enhancement of superconductivity (1) . Recently, nonlinear excitation (2 , 3) of certain phonons in bilayer cuprates was shown to induce superconducting-like optical properties at temperatures far above T(c) (4,5,6). This effect was accompanied by the disruption of competing charge-density-wave correlations(7,8), which explained some but not all of the experimental results. Here, we report a similar phenomenon in a very different compound. By exciting metallic K(3)C(60) with mid-infrared optical pulses, we induce a large increase in carrier mobility, accompanied by the opening of a gap in the optical conductivity. Strikingly, these same signatures are observed at equilibrium when cooling metallic K(3)C(60) below the superconducting transition temperature (T(c) = 20 K). Although optical techniques alone cannot unequivocally identify non-equilibrium high-temperature superconductivity, we propose this scenario as a possible explanation of our results.