Optically induced coherent transport far above Tc in underdoped YBa2Cu3 O6+Î

Abstract

We report on a photoinduced transient state of YBa2Cu2O6+Î in which transport perpendicular to the Cu-O planes 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 Tc, 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 Ï2(ω). Above Tc, the incoherent equilibrium conductivity becomes highly coherent, with the appearance of a reflectivity edge and a positive Ï2(ω) that increases with decreasing frequency. These features are observed up to room temperature in YBa2Cu2O6.45 and YBa2Cu2O6.5. The data above Tc 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 104 cm2/V sec at 100 K, with a density of charge carriers similar to the below-Tc superfluid density. Our results are indicative of highly unconventional nonequilibrium physics and open new prospects for optical control of complex solids. © 2014 American Physical Society.