revealed by time-resolved Raman scattering*

Quantum matter with striking properties has recently been generated by driving materials with ultrafast laser pulses. However, observing these phases by following a single property as a function of time after photoexcitation provides a limited picture. We directly tracked hot phonons and transient heating in a high temperature superconductor YBa₂Cu₃O_6.9 by time-resolved Raman scattering with time-resolution of 220 fs. In addition, we used the hardening of the Raman-active apical oxygen phonon right after photoexcitation as an indirect probe of hot electrons, which equilibrated with hot phonons around 0.5 ps. Analysis based on the two-temperature model revealed temperature-independent electron-phonon coupling with λ ≈ 0.8 as well as significant anharmonicity that increased with temperature. It is essential to reduce this anharmonicity to generate longer lived nonequilibrium states in copper oxides. We will also discuss new insights into photoinduced superconductivity recently reported at lower doping that follow from these results.