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Correlated states have been shown to emerge in bilayer and trilayer graphene with the two-dimensional layers at just the right angle with respect to each other. Key to the enhanced importance of interactions are the so-called moiré electronic bands that form in such systems. Rickhauset al. explored a related system of two graphene bilayers twisted with respect to each other. The twist angle was set so that the layer coupling was strong enough to form moiré bands but weak enough for the carrier concentration in the top and bottom bilayer to be controlled separately. Doping the top bilayer with electrons and the bottom bilayer with holes, the researchers created a correlated state with nested Fermi surfaces. —JS

 

In moiré crystals formed by stacking van der Waals materials, surprisingly diverse correlated electronic phases and optical properties can be realized by a subtle change in the twist angle. Here, we discover that phonon spectra are also renormalized in MoS2 twisted bilayers, adding an insight to moiré physics. Over a range of small twist angles, the phonon spectra evolve rapidly owing to ultra-strong coupling between different phonon modes and atomic reconstructions of the moiré pattern. We develop a low-energy continuum model for phonons that overcomes the outstanding challenge of calculating the properties of large moiré supercells and successfully captures the essential experimental observations. Remarkably, simple optical spectroscopy experiments can provide information on strain and lattice distortions in moiré crystals with nanometre-size supercells. The model promotes a comprehensive and unified understanding of the structural, optical and electronic properties of moiré superlattices.