Photoinduced Nonequilibrium Response in Underdoped YBa2Cu3 O6+x Probed by Time-Resolved Terahertz Spectroscopy

Abstract

Intense laser pulses have recently emerged as a tool to tune between different orders in complex quantum materials. Among different light-induced phenomena, transient superconductivity far above the equilibrium transition temperature in cuprates is particularly attractive. Key to those experiments was the resonant pumping of specific phonon modes, which was believed to induce superconducting phase coherence by suppressing the competing orders or modifying the structure slightly. Here, we present a comprehensive study of photoinduced nonequilibrium response in underdoped YBa2Cu3O6+x. We find that upon photoexcitations, the Josephson plasma edge in the superconducting state is initially removed accompanied by quasiparticle excitations, and subsequently reappears at a frequency lower than the static plasma edge within a short time. In the normal state, an enhancement or weaker edgelike shape is indeed induced by pump pulses in the reflectance spectrum accompanied by simultaneous rises in both real and imaginary parts of conductivity. We compare the pump-induced effects between near- and midinfrared excitations and exclude phonon pumping as a scenario for the photoinduced effects above. We further elaborate that the transient responses in the normal state are unlikely to be explained by photoinduced superconductivity.