Phonon-pump XUV-photoemission-probe in graphene: evidence for non-adiabatic heating of Dirac carriers by lattice deformation

TL;DR

This study uses time-resolved photoemission spectroscopy to investigate how resonant lattice vibrations in bilayer graphene influence Dirac electron dynamics, revealing non-adiabatic heating effects and band structure changes.

Contribution

It demonstrates the impact of resonant phonon excitation on Dirac carriers and provides experimental evidence for non-adiabatic effects in graphene.

Findings

Lattice modulation causes Dirac electrons to reach lower peak temperatures.

Resonant phonon driving leads to faster electron relaxation.

Band structure changes are predicted and observed during phonon excitation.

Abstract

We modulate the atomic structure of bilayer graphene by driving its lattice at resonance with the in-plane E1u lattice vibration at 6.3um. Using time- and angle-resolved photoemission spectroscopy (tr-ARPES) with extreme ultra-violet (XUV) pulses, we measure the response of the Dirac electrons near the K-point. We observe that lattice modulation causes anomalous carrier dynamics, with the Dirac electrons reaching lower peak temperatures and relaxing at faster rate compared to when the excitation is applied away from the phonon resonance or in monolayer samples. Frozen phonon calculations predict dramatic band structure changes when the E1u vibration is driven, which we use to explain the anomalous dynamics observed in the experiment.