Pressure tuning of minibands in MoS2/WSe2 heterostructures revealed by moiré phonons.

Moiré superlattices of two-dimensional heterostructures arose as a new platform to investigate emergent behaviour in quantum solids with unprecedented tunability. To glean insights into the physics of these systems, it is paramount to discover new probes of the moiré potential and moiré minibands, as well as their dependence on external tuning parameters. Hydrostatic pressure is a powerful control parameter, since it allows to continuously and reversibly enhance the moiré potential. Here we use high pressure to tune the minibands in a rotationally aligned MoS2/WSe2 moiré heterostructure, and show that their evolution can be probed via moiré phonons. The latter are Raman-inactive phonons from the individual layers that are activated by the moiré potential. Moiré phonons manifest themselves as satellite Raman peaks arising exclusively from the heterostructure region, increasing in intensity and frequency under applied pressure. Further theoretical analysis reveals that their scattering rate is directly connected to the moiré potential strength. By comparing the experimental and calculated pressure-induced enhancement, we obtain numerical estimates for the moiré potential amplitude and its pressure dependence. The present work establishes moiré phonons as a sensitive probe of the moiré potential as well as the electronic structures of moiré systems.

Electronic Band Tuning and Multivalley Raman Scattering in Monolayer Transition Metal Dichalcogenides at High Pressures.

Transition metal dichalcogenides (TMDs) possess spin-valley locking and spin-split K/K' valleys, which have led to many fascinating physical phenomena. However, the electronic structure of TMDs also exhibits other conduction band minima with similar properties, the Q/Q' valleys. The intervalley K-Q scattering enables interesting physical phenomena, including multivalley superconductivity, but those effects are typically hindered in monolayer TMDs due to the large K-Q energy difference (ΔEKQ). To unlock elusive multivalley phenomena in monolayer TMDs, it is desirable to reduce ΔEKQ, while being able to sensitively probe the valley shifts and the multivalley scattering processes. Here, we use high pressure to tune the electronic properties of monolayer MoS2 and WSe2 and probe K-Q crossing and multivalley scattering via double-resonance Raman (DRR) scattering. In both systems, we observed a pressure-induced enhancement of the double-resonance LA and 2LA Raman bands, which can be attributed to a band gap opening and ΔEKQ decrease. First-principles calculations and photoluminescence measurements corroborate this scenario. In our analysis, we also addressed the multivalley nature of the DRR bands for WSe2. Our work establishes the DRR 2LA and LA bands as sensitive probes of strain-induced modifications to the electronic structure of TMDs. Conversely, their intensity could potentially be used to monitor the presence of compressive or tensile strain in TMDs. Furthermore, the ability to probe K-K' and K-Q scattering as a function of strain shall advance our understanding of different multivalley phenomena in TMDs such as superconductivity, valley coherence, and valley transport.