Phonon-assisted processes in the ultraviolet-transient optical response of graphene

Dino Novko,Marko Kralj

doi:10.1038/s41699-019-0131-5

Abstract

Many recent experiments investigated potential and attractive means of modifying many-body interactions in two-dimensional materials through time-resolved spectroscopy techniques. However, the role of ultrafast phonon-assisted processes in two-dimensional systems is rarely discussed in depth. Here, we investigate the role of electron–phonon interaction in the transient optical absorption of graphene by means of first-principles methods. It is shown at equilibrium that the phonon-assisted transitions renormalize significantly the electronic structure. As a result, absorption peak around the Van-Hove singularity broadens and redshifts by ~100 meV. In addition, temperature increase and chemical doping are shown to notably enhance these phonon-assisted features. In the photoinduced transient response, we obtain spectral changes in close agreement with the experiments, and we associate them to the strong renormalization of occupied and unoccupied pi bands, which predominantly comes from the coupling with the zone-center {E}_{2g} optical phonon. Our estimation of the Coulomb interaction effects shows that the phonon-assisted processes can have a dominant role even in the subpicosecond regime.

Highlights

The ability of altering the electronic structure as well as the corresponding excitation spectrum is one of the most impressive feature of two-dimensional and layered materials.[1,2] Electronic properties of such systems are shown to be highly sensitive to external perturbation that introduces excess electron or hole concentrations
For noninteract- cies are in line with the changes that we get by including the electron–phonon coupling (EPC), ing electrons, i.e., when the EPC is omitted, as it is the case in the density-functional theory, σðωÞ includes only momentum- and i.e., Δπ 1⁄4 77 meV and ΔIπ % πe2=2h
The results have shown that the inclusion of the electron–phonon coupling reduces the energy gap between occupied and unoccupied π band around the Van-Hove singularity point of the Brillouin zone

Summary

Introduction

The ability of altering the electronic structure as well as the corresponding excitation spectrum is one of the most impressive feature of two-dimensional and layered materials.[1,2] Electronic properties of such systems are shown to be highly sensitive to external perturbation that introduces excess electron or hole concentrations. Electrostatic and chemical doping techniques were successfully utilized in order to tune optical absorption as well as plasmon and exciton energies in graphenebased materials,[3,4,5,6,7,8,9] transitions metal dichalcogenides,[10,11,12,13,14,15,16] and corresponding van der Waals heterostructures.[17,18] In addition to these static modifications, it is of utter importance to comprehend transient electronic structure changes in time domain. The sequence of fast and slow relaxation events at two characteristic time scales of transient response associated with electron- and phononassisted dynamics, respectively (the so-called biexponential decay), appears to be universal, and was observed in various photoemission and optical absorption experiments (e.g., in photemission intensity of MoS2 valence bands[25] and in transient reflectivity spectra of graphite[21])

Methods

Results

Conclusion