Broken adiabaticity induced by Lifshitz transition in MoS2 and WS2 single layers

Dino Novko

doi:10.1038/s42005-020-0299-1

Dino Novko

Open Access

https://doi.org/10.1038/s42005-020-0299-1

Copy DOI

Abstract

The breakdown of the adiabatic Born-Oppenheimer approximation is striking dynamical phenomenon, however, it occurs only in a handful of layered materials. Here, I show that adiabaticity breaks down in doped single-layer transition metal dichalcogenides in a quite intriguing manner. Namely, significant nonadiabatic coupling, which acts on frequencies of the Raman-active modes, is prompted by a Lifshitz transition due to depopulation and population of multiple valence and conduction valleys, respectively. The outset of the latter event is shown to be dictated by the interplay of highly non-local electron-electron interaction and spin-orbit coupling. In addition, intense electron-hole pair scatterings due to electron-phonon coupling are inducing phonon linewidth modifications as a function of doping. Comprehending these intricate dynamical effects turns out to be a key for mastering characterization of electron doping in two-dimensional nano-devices by means of Raman spectroscopy.

Highlights

The breakdown of the adiabatic Born-Oppenheimer approximation is striking dynamical phenomenon, it occurs only in a handful of layered materials
Recent study on carrierinduced phonon modifications in atomically-thin transition metal dichalcogenides (TMDs) had shown that the adiabatic density functional theory (DFT) results are largely overestimating the corresponding frequency shifts of the Raman-active modes, but NA corrections were not taken into account[12]
For the former, the global minimum is located in the K point of the Brillouin zone, while the remnant minimum is at the Σ point, i.e., halfway along the Γ − K path [see Fig 1a]

Summary

Introduction

The breakdown of the adiabatic Born-Oppenheimer approximation is striking dynamical phenomenon, it occurs only in a handful of layered materials. Shown that striking nonadiabatic (NA) effects visible in vibrational Raman spectra can be found in metallic layered materials, such as graphene[1,2,3], graphite intercalation compounds[4], and magnesium diboride[4,5], as well as in doped semiconductors, such as boron-doped diamond[6] The strength of this dynamical electron–phonon coupling (EPC) can, be modified as a function of carrier concentration, which makes the Raman spectroscopy a quite powerful tool for characterization of doped two-dimensional and layered materials[7]. Born-Oppenheimer approximation in doped TMDs turns out to be even more intricate than in the well-know examples[1,4,6]

Methods

Results

Conclusion