Tracking primary thermalization events in graphene with photoemission at extreme timescales

TL;DR

This study uses ultrafast photoemission spectroscopy to observe inverse Auger scattering as the dominant thermalization process in graphene within the first 25 femtoseconds after excitation, revealing new insights into ultrafast carrier dynamics.

Contribution

First direct measurement of inverse Auger scattering dominance in graphene at extreme timescales using femtosecond photoemission spectroscopy.

Findings

Inverse Auger scattering dominates initial thermalization in graphene.

Carrier density increases while average kinetic energy decreases within 25 fs.

Ultrafast photoemission provides insights into electronic relaxation processes.

Abstract

Direct and inverse Auger scattering are amongst the primary processes that mediate the thermalization of hot carriers in semiconductors. These two processes involve the annihilation or generation of an electron-hole pair by exchanging energy with a third carrier, which is either accelerated or decelerated. Inverse Auger scattering is generally suppressed, as the decelerated carriers must have excess energies higher than the band gap itself. In graphene, which is gapless, inverse Auger scattering is instead predicted to be dominant at the earliest time delays. Here, $<8$ femtosecond extreme-ultraviolet pulses are used to detect this imbalance, tracking both the number of excited electrons and their kinetic energy with time- and angle-resolved photoemission spectroscopy. Over a time window of approximately 25 fs after absorption of the pump pulse, we observe an increase in conduction band carrier density and a simultaneous decrease of the average carrier kinetic energy, revealing that relaxation is in fact dominated by inverse Auger scattering. Measurements of carrier scattering at extreme timescales by photoemission will serve as a guide to ultrafast control of electronic properties in solids for PetaHertz electronics.