First-Principles Study of Frequency-Dependent Resonant Raman Scattering

Gillet, Yannick [UCL] Draxl, Claudia [Humboldt Universitat zu Berlin] Gonze, Xavier [UCL]

A resonance phenomenon appears in the Raman response when the exciting light has frequency close to electronic transitions. Unlike for molecules and for graphene, the theoretical prediction of such frequency-dependent Raman response of crystalline systems has remained a challenge. Indeed, many Raman intensity first-principle calculations are nowadays done at vanishing light frequency, using static Density-Functional Perturbation Theory, thus neglecting the frequency dependence and excitonic effects. During this presentation, I will describe the finite-difference method we propose to compute frequency-dependent Raman intensities. Recently, we used this methodology for the computation of the first-order frequency-dependent Raman intensity of silicon [1], with excitonic effects described by the Bethe-Salpeter equation. We found these to be crucial for the accurate description of the experimental enhancement for laser photon energies around the gap. I will also present a recent study on transition-metal dichalcogenides that reveals an atypical exciton-phonon interaction for WS2 and WSe2 [2]. This approach can be generalized to the more complex second-order Raman intensity, with phonon losses coming from the entire Brillouin zone. Interestingly, even without excitonic effects, one is able to capture the main relative changes in the frequency-dependent Raman spectrum of silicon at fixed laser frequencies. However, excitonic effects might affect significantly the intensity of specific modes and also lead to a global tenfold increase of absolute intensities. [1] Y. Gillet, M. Giantomassi, X. Gonze, Phys. Rev. B 88, 094305 (2013). [2] E. del Corro, A. Botello-Méndez, Y. Gillet et al., Nano Letters 16, 2363 (2016).