Density-wave-like gap evolution in La$_3$Ni$_2$O$_7$ under high pressure revealed by ultrafast optical spectroscopy

TL;DR

This study investigates how density wave-like gaps in La$_3$Ni$_2$O$_7$ evolve under high pressure using ultrafast optical spectroscopy, revealing suppression and emergence of different density wave orders related to superconductivity.

Contribution

It provides the first experimental evidence of the pressure-dependent evolution of density wave-like gaps in La$_3$Ni$_2$O$_7$, linking DW order to superconductivity.

Findings

SDW order is suppressed up to 13.3 GPa

A new DW-like order emerges above 29.4 GPa

Energy gap associated with DW is 66 meV

Abstract

Density wave (DW) order is believed to be correlated with superconductivity in the recently discovered high-temperature superconductor La$_3$Ni$_2$O$_7$. However, experimental investigations of its evolution under high pressure are still lacking. Here, we explore the quasiparticle dynamics in bilayer nickelate La$_3$Ni$_2$O$_7$ single crystals using ultrafast optical pump-probe spectroscopy under high pressures up to 34.2 GPa. At ambient pressure, the temperature-dependent relaxation dynamics demonstrate a phonon bottleneck effect due to the opening of an energy gap around 151 K. The energy scale of the DW-like gap is determined to be 66 meV by the Rothwarf-Taylor model. Combined with recent experiential results, we propose that this DW-like transition at ambient pressure and low temperature is spin density wave (SDW). With increasing pressure, this SDW order is significantly suppressed up to 13.3 GPa before it completely disappears around 26 GPa. Remarkably, at pressures above 29.4 GPa, we observe the emergence of another DW-like order with a transition temperature of approximately 135 K, which is probably related to the predicted charge density wave (CDW) order. Our study provides the experimental evidence of the evolution of the DW-like gap under high pressure, offering critical insights into the correlation between DW order and superconductivity in La$_3$Ni$_2$O$_7$.