High-energy electronic excitations in La3Ni2O7 by time-resolved optical spectroscopy

TL;DR

This study uses time-resolved optical spectroscopy to investigate high-energy electronic excitations and phonon dynamics in La3Ni2O7, revealing complex gap structures and electron-phonon interactions relevant to its density-wave and superconducting states.

Contribution

It provides the first detailed analysis of high-energy electronic excitations and phonon coupling in La3Ni2O7, elucidating their roles in density-wave behavior and superconductivity.

Findings

Identified two high-energy electronic excitations with distinct DW gaps.

Observed four Raman-active phonon modes with temperature-dependent coupling.

Revealed deviations at low temperatures indicating electron-phonon interactions.

Abstract

Recently, high-temperature superconductivity has been established in bilayer La3Ni2O7, which exhibits a density-wave (DW) transition at ~ 150 K under ambient pressure. The DW order is believed to be linked to superconductivity, as it is suppressed upon the emergence of superconductivity at high pressures. Here, we explore the ultrafast dynamics of high-energy electronic excitations from 10 K to room temperature under ambient pressure using time-resolved optical spectroscopy. Two high-energy electronic excitations at ~1.8 and ~ 2.4 eV, arising from distinct interband transitions, are identified. They exhibit different DW gaps of approximately 54 and 67 meV, respectively, along with relaxation dynamics that can be well described by the Rothwarf-Taylor model. In addition, we observe four coherent Raman-active phonon modes that exhibit distinct coupling with different electronic excitations. The phonon softening with increasing temperature can be well described between ~100 K and room temperature by a semi-quantitative model, which includes thermal expansion and anharmonic phonon-phonon coupling. At cryogenic temperatures, deviations from the measured temperature-dependent phonon frequencies and the model fits suggest an additional contribution from electron-phonon coupling. Our study provides direct evidence of the complex gap structure and phonon dynamics in this material, offering critical insights into the DW mechanism and many-body effects.