Novel Electron-Phonon Relaxation Pathway in Graphite Revealed by Time-Resolved Raman Scattering and Angle-Resolved Photoemission Spectroscopy

TL;DR

This study reveals a new electron-phonon relaxation pathway in graphite, showing that energy transfer to G-phonons occurs indirectly via optical phonons near zone boundary K-points, challenging previous models.

Contribution

The paper uncovers a previously unknown relaxation pathway involving phonon-phonon interactions, which must be considered in hot carrier relaxation models for graphite.

Findings

G-phonon population increase is delayed by ~65 fs after excitation

The relaxation rate depends on pump fluence, contradicting two-temperature model predictions

Energy transfer occurs via optical phonons near K-points, not directly to G-phonons

Abstract

Time dynamics of photoexcited electron-hole pairs is important for a number of technologies, in particular solar cells. We combined ultrafast pump-probe Raman scattering and photoemission to directly follow electron-hole excitations as well as the G-phonon in graphite after an excitation by an intense laser pulse. This phonon is known to couple relatively strongly to electrons. Cross-correlating effective electronic and phonon temperatures places new constraints on model-based fits. The accepted two-temperature model predicts that G-phonon population should start to increase as soon as excited electron-hole pairs are created and that the rate of increase should not depend strongly on the pump fluence. Instead we found that the increase of the G-phonon population occurs with a delay of $\sim$65 fs. This time-delay is also evidenced by the absence of the so-called self-pumping for G phonons. It decreases with increased pump fluence. We show that these observations imply a new relaxation pathway: Instead of hot carriers transferring energy to G-phonons directly, the energy is first transferred to optical phonons near the zone boundary K-points, which then decay into G-phonons via phonon-phonon scattering. Our work demonstrates that phonon-phonon interactions must be included in any calculations of hot carrier relaxation in optical absorbers even when only short timescales are considered.