Twist-controlled phonons and symmetry breaking in asymmetric moiré van der Waals heterostructures

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 Mo0.4W0.6Se2 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 E2g 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.