(x = 0.115) Following Near-infrared Excitation*

Numerous cuprate-based superconductors exhibit a complex interplay between spin order, charge order and superconductivity. We utilize THz time domain spectroscopy to monitor the c-axis Josephson Plasma Resonance (JPR) as a direct probe of the superconducting condensate following excitation with 1.55 eV pulses. La_2-xBa_xCuO₄ (x=0.115) exhibits charge order below ~50K, with an onset of spin order at ~40K and a superconducting transition temperature of T_C = 13K [1]. Recent work on this compound has suggested a brief enhancement of superconductivity following photoexcitation [2] which has motivated further investigation. We show that starting from the superconducting state, photoexcitation with 1.5eV near infrared pulses results in a novel electrodynamic response which persists for >300ps. In particular, c-axis polarized excitation with fluences <1mJ/cm² induces a new plasma edge around ~0.8THz along with a commensurate increase in the spectral weight below 0.5THz. For photoexcitation at temperatures above TC, a qualitatively similar state emerges that decays on the order of several picoseconds. [1] C. C. Homes, et al, Phys. Rev. B 85, 134510 (2012) [2] D. Nicoletti, et al, Phys. Rev. B 90, 100503(R) (2014)