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

TL;DR

This study demonstrates that mid-infrared optical pulses can induce a transient, highly coherent transport state in underdoped YBa2Cu3O6+x, both below and above the superconducting transition temperature, revealing unconventional non-equilibrium physics.

Contribution

It shows that optical excitation can transiently enhance or induce superconducting-like coherence in underdoped cuprates at temperatures well above Tc, a novel control of electronic states.

Findings

Enhanced interlayer coherence below Tc

Induction of a high-frequency reflectivity edge above Tc

Transient superconducting state with picosecond lifetime

Abstract

We report on a photo-induced transient state of YBa2Cu2O6+x 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. Above Tc, the incoherent equilibrium conductivity becomes highly coherent, with the appearance of a reflectivity edge and a positive imaginary conductivity 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 re-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 Tc superfluid density. Our results are indicative of highly unconventional non-equilibrium physics and open new prospects for optical control of complex solids.