Low-temperature resonant Raman asymmetry in 2H-MoS₂ under high pressure

We report on the combined effect of temperature (6 K–300 K) and high pressure (up to 6 GPa) on the resonant Raman scattering by A_1g phonons in bulk 2H-MoS₂, as the energy of the A exciton is tuned into resonance with an exciting laser at E_L  =  1.96 eV. As expected, the pressure to be applied for attaining resonant conditions decreases with decreasing temperature. A striking result concerns the combined effect of temperature and pressure on the strength of the incoming relative to the outgoing resonance of the A_1g phonon. When its Raman intensity is normalized by that of the 'non-resonant' $E_{2g}^1$ phonon (IA_1g/I$E_{2g}^1$), we find that the contribution of the pressure-tuned outgoing resonance relative to that of the incoming channel changes with temperature. At room temperature both contributions are about equal, as expected. Interestingly, with decreasing temperature an asymmetry in the relative magnitude of the resonances develops, becoming the outgoing contribution about half of the incoming resonance below ∼50 K. We discuss the different possibilities for the origin of this effect. (paper)