Phonons in twisted transition-metal dichalcogenide bilayers: Ultrasoft phasons and a transition from a superlubric to a pinned phase

Abstract

The tunability of the interlayer coupling by twisting one layer with respect to another layer of two-dimensional materials provides a unique way to manipulate the phonons and related properties. We refer to this engineering of phononic properties as twistnonics. We study the effects of twisting on low-frequency shear modes (SMs) and layer breathing modes in a transition-metal dichalcogenide (TMD) bilayer using atomistic classical simulations. We show that these low-frequency modes are extremely sensitive to twisting and can be used to infer the twist angle. We find ultrasoft phason modes (frequency 1 cm −1 , comparable to acoustic modes) for any nonzero twist, corresponding to an effective translation of the moiré lattice by relative displacement of the constituent layers in a nontrivial way. Unlike the acoustic modes, the velocity of the phason modes are quite sensitive to the twist angle. Also, high-frequency SMs appear for small twist angles, identical to those in stable bilayer TMD (θ = 0 • or 60 •), due to the overwhelming growth of stable stacking regions in relaxed twisted structures. Our study reveals the possibility of an intriguing θ-dependent superlubric to pinning behavior and of the existence of ultrasoft modes in all two-dimensional materials.

Moiré phonons in twisted MoSe2–WSe2 heterobilayers and their correlation with interlayer excitons

“…The range here is from several tens to one hundred nanometers [33]. The domain formation was first predicted in [36], and confirmed by density functional theory calculations [33,34,37,38], and by a classical modelling [31]. Interestingly, in atomically-reconstructed structures the domain periodicity depends in the same way on the twist-angle deviation, as the moiré periodicity in structures, where the twist angle is larger than the critical value for atomic reconstruction [37].…”

Low-frequency Raman scattering in WSe2−MoSe2 heterobilayers: Evidence for atomic reconstruction

“…27 via conformational considerations, and indeed, density functional theory calculations confirmed that in the above cases, the two stacking configurations are the energetically favorable lattice arrangements 24,25,28,29 (also see Ref. 30 for a classical modeling). For R-type stacking, two energetically degenerate commensurate configurations are possible for a heterobilayer [see Fig.…”

TMDs as a platform for spin liquid physics: A strong coupling study of twisted bilayer WSe$_2$

“…If it's possible to tune to such a large J 2 /J 1 ratio, it would allow to explore the interplay between the moiré scale incommensurate magnetic structure, and its gapless phason modes, with the atomic scale incommensurate lattice structure, and its gapless phonon modes. 67,68 For smaller ratios of J 2 /J 1 0.3, the 120 • order is stabilized. For φ = π/6 it is particularly stable, and has a fixed chirality, which leaves only a single BKT transition at finite temperature, with an expected T BKT J 1 .…”