2026, Twist-Controlled Phonons and Symmetry Breaking in Asymmetric Moiré van der Waals Heterostructures, Applied Physics Letters.

2026, Twist-Controlled Phonons and Symmetry Breaking in Asymmetric Moiré van der Waals Heterostructures, Applied Physics Letters.

Abstract:

Twistronics in van der Waals heterostructures enables programmable electronic, vibrational and nonlinear optical responses through moiré superlattices but most platforms are assembled from high-symmetry layers, restricting the accessible symmetry landscape. Here we introduce an intrinsically asymmetric moiré system by fabricating a van der Waals heterobilayer of monolayer WSe₂ (D3h group, C₃ rotational symmetry) and monolayer Mo_0.4W_0.6Se₂ alloy (P1 group. C1 rotational symmetry) with intrinsically broken rotational symmetry. Using polarization-resolved Raman spectroscopy in combination with second-harmonic generation (SHG) microscopy over the twist-angle range (0°–60°), we track how interlayer coupling, lattice reconstruction and global symmetry evolve with twist angle, showing that interlayer phonons act as sensitive probes of the moiré length scale and reveal a continuous crossover from strong coupling near 0°/60° to weak coupling approaching ~30°. Circularly polarized Raman measurements resolve nearly degenerate phonons and uncover a twist-tunable splitting of the in-plane E_2g mode of WSe₂. Polarization-resolved SHG visualizes twist-driven modulation of the effective point-group symmetry, with polar patterns evolving from sixfold, C3-like to twofold, C1-like lobes, quantitatively captured by a bond angular momentum model. Our results establish the combination of native symmetry breaking and twist engineering as a generic strategy for programming phonon and symmetry landscapes in van der Waals materials, opening a route to designer moiré crystals in which interfacial coupling, nonlinear optics and correlated quantum degrees of freedom can be co-engineered within a single, twist-tunable architecture.

Keywords: Two-dimensional materials, transition metal dichalcogenides, moiré superlattice, symmetry breaking, polarization analysis spectroscopy.

Link: https://pubs.aip.org/aip/apl/article/128/15/151907/3387144/Twist-controlled-phonons-and-symmetry-breaking-in?guestAccessKey=

Twist-Controlled Phonons and Symmetry Breaking in Asymmetric Moiré van der Waals Heterostructures.pdf