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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. 

Defects in two-dimensional (2D) transition-metal dichalcogenides play a crucial role in controlling the spatiotemporal dynamics of photogenerated charge carriers, which remain poorly understood to date. In this paper, the defect-mediated carrier diffusion and recombination in WS₂monolayers are quantitatively investigated by laser-illuminated microwave impedance microscopy. Surprisingly, the photoresponse is in general stronger in the more disordered regions and samples. Such counterintuitive observations are reconciled by spatiotemporally resolved experiments, which indicate that the electron lifetime is prolonged due to the slow release of holes from the trap states. The results reveal the intrinsic time and length scales of photocarriers in van der Waals materials, providing the guidance for implementing nanooptoelectronic devices based on 2D semiconductors.

 

Interlayer exciton diffusion in transition metal dichalcogenide heterostructures is controlled by the moiré potential.

 

Optical dielectric constants are critical to modeling the electronic and optical properties of materials. Silver, as a noble metal with low loss, has been extensively investigated. The recently developed epitaxial growths of single crystalline Ag on dielectric substrates have prompted efforts to characterize their intrinsic optical dielectric function. In this paper, we report spectral ellipsometry measurements and analysis of a thick, epitaxially-grown, single-crystalline Ag film. We focus on the range of 0.18 – 1.0 eV or 1.24 – 7 µm, an energy and wavelength range that has not been examined previously using epitaxial films. We compare the extracted dielectric constants and the predicted optical performances with previous measurements. The loss is appreciably lower than the widely quoted Palik’s optical constants (i.e., up to a factor of 2) in the infrared frequency range. The improved knowledge of fundamental optical properties of the high-quality epitaxial Ag film will have a broad impact on simulations and practical applications based on Ag in the long wavelength range.