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Semiconductor moiré superlattices provide a versatile platform to engineer quantum solids composed of artificial atoms on moiré sites. Previous studies have mostly focused on the simplest correlated quantum solid—the Fermi-Hubbard model—in which intra-atom interactions are simplified to a single onsite repulsion energy U. Here we report the experimental observation of Wigner molecular crystals emerging from multielectron artificial atoms in twisted bilayer tungsten disulfide moiré superlattices. Using scanning tunneling microscopy, we demonstrate that Wigner molecules appear in multielectron artificial atoms when Coulomb interactions dominate. The array of Wigner molecules observed in a moiré superlattice comprises a crystalline phase of electrons: the Wigner molecular crystal, which is shown to be highly tunable through mechanical strain, moiré period, and carrier charge type. [ABSTRACT FROM AUTHOR]

X-ray absorption spectroscopy (XAS) is an experimental technique used to probe the atomic properties of materials. XAS measurements performed in fluorescence mode are subject to so-called "self-absorption" distortion, especially when the sample under investigation is thick or concentrated with respect to the atom of interest. Here we investigate the behavior of self-absorption and present several suggestions to avoid and manage self-absorption through experimental design. Particularly, we find that it is easier to correct for very thick samples than for samples of intermediate thickness. Self-absorption distortion necessitates a correction of the measured spectrum during data processing. We present a technique for correcting self-absorbed spectra in the XANES region that is effective for all sample thicknesses and experimental geometries. [ABSTRACT FROM AUTHOR]

Semiconductor moiré superlattices have been shown to host a wide array of interaction-driven ground states. However, twisted homobilayers have been difficult to study in the limit of large moiré wavelengths, where interactions are most dominant. In this study, we conducted local electronic compressibility measurements of twisted bilayer WSe2 (tWSe2) at small twist angles. We demonstrated multiple topological bands that host a series of Chern insulators at zero magnetic field near a “magic angle” around 1.23°. Using a locally applied electric field, we induced a topological quantum-phase transition at one hole per moiré unit cell. Our work establishes the topological phase diagram of a generalized Kane-Mele-Hubbard model in tWSe2, demonstrating a tunable platform for strongly correlated topological phases. [ABSTRACT FROM AUTHOR]