Terahertz spectroscopy of the cuprate superconductor La2-xSrxCuO4 in the photoexcited nonequilibrium state

Ultrafast pump-probe spectroscopy has been a promising tool to study the interplay of complex orders and to reveal hidden states of condensed matter. In particular, the high-Tc cuprate superconductors provide an interesting platform since they exhibit various electronic phases such as pseudogaps, charge/spin density-waves, stripe order, and superconductivity. Here, we focused on an archetypal cuprate superconductor La_2-xSr_xCuO₄ with the doping levels of x = 0.125 to x = 0.15, and investigated the dynamics after intense photoexcitation by utilizing near-infrared (800 nm) optical pump-terahertz probe spectroscopy. In the superconducting state, after photoexcitation we observed an emergence of a peak structure in the real-part conductivity accompanied by the destruction of the superconducting coherence indicated by the redshift of the Josephson plasma resonance. The peak structure sustains for several hundreds of picoseconds after the excitation and its height is strongly dependent on the excitation intensity. The doping dependence of the photoexcited state suggests the correlation between the peak structure and superconductivity.