Density-wave-like gap evolution in La3Ni2O7 under high pressure revealed by ultrafast optical spectroscopy

Abstract Density-wave-like (DW) order is believed to be correlated with superconductivity in the recently discovered high-temperature superconductor La3Ni2O7. However, experimental investigations of its evolution under high pressure are still lacking. Here, we investigate the quasiparticle dynamics in bilayer nickelate La3Ni2O7 single crystals using ultrafast optical pump-probe spectroscopy under high pressures up to 34.2 GPa. Near ambient pressure, the temperature-dependent relaxation dynamics demonstrate a phonon bottleneck effect due to the opening of a DW gap at 151 K, with an energy scale of 66 meV as determined by the Rothwarf-Taylor model. With increasing pressure, this phonon bottleneck effect is gradually suppressed and completely disappears around 26 GPa. Remarkably, at pressures above 29.4 GPa, we observe the emergence of a new DW order with a transition temperature of approximately 130 K. Our study provides the first experimental evidence of the evolution of the DW gap under high pressure, offering critical insights into the correlation between DW order and superconductivity in La3Ni2O7. These findings highlight the complex electronic phase transitions in this material and underscore the role of high pressure in tuning its superconducting and DW properties.