The self-energy corrections to phonons have mostly been observed for modes with zone-center ($q=0$) wave vectors. Here, we investigated the self-energy corrections to phonons in ionic-liquid-gated semiconductors, i.e., monolayer and bilayer ${\mathrm{WS}}_2$, experimentally. Apart from the previously observed $A_{1g}\left(\mathrm{\Gamma }\right)$ mode renormalization by Raman spectroscopy, we additionally discovered that the zone-edge longitudinal acoustic (LA($M$)) phonon also softened under finite electron doping. This observed renormalization effect arises from the LA($M$) phonon-assisted electron scattering between the $\mathbf{K}$ and $\mathbf{Q}$ valleys in the conduction bands, indicating the nonadiabatic self-energy corrections dominate, different from the $A_{1g}\left(\mathrm{\Gamma }\right)$ mode. The self-energy corrections of the two modes both depend on layer configurations. Our findings suggest that ${\mathrm{WS}}_2$ and other similar two-dimensional materials are intriguing playgrounds to study the electron-phonon interactions and valley-related physics.

Close

PrevNext

View figure in article

• Acoustic phonons
• Electron-phonon coupling
• Layered crystals
• Semiconductors
• Wide band gap systems
• Density functional theory
• Raman spectroscopy