Twist Angle-Dependent Phonon Hybridization in WSe2/WSe2 Homobilayer

KP Bera and D Solanki and S Mandal and R Biswas and T Taniguchi and K Watanabe and V Raghunathan and M Jain and AK Sood and A Das, ACS NANO, 18, 24379-24390 (2024).

DOI: 10.1021/acsnano.4c06767

The emerging moir & eacute; superstructure of twisted transition metal dichalcogenides (TMDs) leads to various correlated electronic and optical properties compared to those of twisted bilayer graphene. In such a versatile architecture, phonons can also be renormalized and evolve due to atomic reconstruction, which, in turn, depends on the twist angle. However, observing this reconstruction and its relationship to phonon behavior with conventional, cost-effective imaging methods remains challenging. Here, we used noninvasive Raman spectroscopy on twisted WSe2/WSe2 (t-WSe2) homobilayers to examine the evolution of phonon modes due to interlayer coupling and atomic reconstruction. Unlike in the natural bilayer (NB), similar to 0 degrees as well as similar to 60 degrees t-WSe2 samples, the nearly degenerate A(1g)/E-2g mode in the twisted samples (1-7 degrees) split into a doublet in addition to the nondegenerate B-2g mode, and the maximum splitting is observed around 2-3 degrees. Our detailed theoretical calculations qualitatively capture the splitting and its dependence as a function of the twist angle and highlight the role of the moir & eacute; potential in phonon hybridization. Additionally, we found that around the 2 degrees twist angle, the anharmonic phonon-phonon interaction is higher than the natural bilayer and decreases for larger twist angles. Interestingly, we observed anomalous Raman frequency softening and line- width increase with the decreasing temperature below 50 K, pointing to the combined effect of enhanced electron-phonon coupling and cubic anharmonic interactions in moir & eacute; superlattice.

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