Optically induced coherent transport far above Tc in underdoped YBa2Cu3O6+δ

We report on a photoinduced transient state of YBa2Cu2O6+delta in which transport perpendicular to the Cu-Oplanes becomes highly coherent. This effect is achieved by excitation with mid-infrared optical pulses, tuned to the resonant frequency of apical oxygen vibrations, which modulate both lattice and electronic properties. Below the superconducting transition temperature T-c, the equilibrium signatures of superconducting interlayer coupling are enhanced. Most strikingly, the optical excitation induces a new reflectivity edge at higher frequency than the equilibrium Josephson plasma resonance, with a concomitant enhancement of the low-frequency imaginary conductivity sigma(2)(omega). Above T-c, the incoherent equilibrium conductivity becomes highly coherent, with the appearance of a reflectivity edge and a positive sigma(2)(omega) that increases with decreasing frequency. These features are observed up to room temperature in YBa2Cu2O6.45 and YBa2Cu2O6.5. The data above T-c can be fitted by hypothesizing that the light establishes a transient superconducting state over only a fraction of the solid, with a lifetime of a few picoseconds. Non-superconducting transport could also explain these observations, although one would have to assume transient carrier mobilities near 10(4) cm(2)/V sec at 100 K, with a density of charge carriers similar to the below-T-c superfluid density. Our results are indicative of highly unconventional nonequilibrium physics and open new prospects for optical control of complex solids.