Photoinduced Nonequilibrium Response in Underdoped YBa2Cu3O6+x Probed by Time-Resolved Terahertz Spectroscopy

S J Zhang,H Xiang,Z X Wang,L Y Shi,D Wu,T Dong,N L Wang,X Yao,T Lin,Q M Liu

doi:10.1103/physrevx.10.011056

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 photo-induced nonequilibrium response in underdoped YBa$_2$Cu$_3$O$_{6+x}$. We find that upon photo-excitations, Josephson plasma edge in superconducting state is initially removed accompanied by quasiparticle excitations, and subsequently reappears at frequency lower than the static plasma edge within short time. In normal state, an enhancement or weaker edge-like 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 mid-infrared excitations and exclude phonon pumping as a scenario for the photo-induced effects above. We further elaborate the transient responses in normal state are unlikely to be explained by photo-induced superconductivity.

Highlights

High-Tc superconducting cuprates are highly anisotropic materials
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
When the cuprate system goes into the superconducting state, a very sharp plasma edge suddenly develops in the c-axis reflectivity spectrum

Summary

Introduction

High-Tc superconducting cuprates are highly anisotropic materials. The conducting CuO2 layers are separated by different block layers, leading to less-conducting or insulatorlike c-axis dc and optical responses in the normal state. Plasmalike edge in the reflectance spectra and 1=ω-like dependence in the imaginary part of conductivity along the c axis of cuprates in the normal state, which was taken as an indication of light-induced transient superconductivity.

Results

Conclusion