Photo-induced nonequilibrium response in underdoped cuprates probed by time-resolved terahertz spectroscopy*

Utilizing ultrashort laser pulses to explore and manipulate quantum materials has led to many remarkable discoveries, one of which is the light-induced transient superconductivity. In cuprates, the transient superconductivity was first claimed in LESCO, then YBCO and LBCO through time-resolved terahertz measurements along the c-axis, though the observations varied in terms of pump fluence, polarization, wavelength, or time delay after excitation. To gain further insight into this issue, we conducted non-equilibrium terahertz spectroscopy measurements on underdoped YBCO and LBCO with tunable pump wavelengths from near-infrared to mid-infrared, in both the ab-plane and c-axis. Our results raise questions about the notion of photoinduced superconductivity and whether the observed phenomena can be exclusively interpreted as "transient superconductivity," as well as whether phonon resonant pumping is necessary to observe the phenomenon. Notably, we have found that the pump-induced superconducting condensate is absent in the ab-plane response, suggesting that the c-axis transient responses cannot be explained by an equivalent of Josephson tunneling, but instead are likely linked to the generation of quasi-particles.
Work done with S. J. Zhang, T. Dong, X. Y. Zhou, Z. X. Wang, S. X. Xu, Q. Wu, L. Yue, Q. M. Liu, T. C. Hu, R. S. Li, J. Y. Yuan, C. C. Homes, G. D. Gu.