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Finding the right combination of conditions to synthesize and characterize new semiconducting superatomic crystals requires strategy, perseverance, and a little bit of luck. 

We establish the synthesis, physical properties, and highly-frustrated magnetism of Mn₂In₂Se₅and Mn₂Ga₂S₅van der Waals crystals. 

Abstract Two-dimensional (2D) materials have drawn immense interests in scientific and technological communities, owing to their extraordinary properties and their tunability by gating, proximity, strain and external fields. For electronic applications, an ideal 2D material would have high mobility, air stability, sizable band gap, and be compatible with large scale synthesis. Here we demonstrate air stable field effect transistors using atomically thin few-layer PdSe₂sheets that are sandwiched between hexagonal BN (hBN), with large saturation current > 350 μA/μm, and high field effect mobilities of ~ 700 and 10,000 cm²/Vs at 300 K and 2 K, respectively. At low temperatures, magnetotransport studies reveal unique octets in quantum oscillations that persist at all densities, arising from 2-fold spin and 4-fold valley degeneracies, which can be broken by in-plane and out-of-plane magnetic fields toward quantum Hall spin and orbital ferromagnetism. 

A symmetry-based approach leads to the efficient discovery of magnets hosting topological magnons. 

Optical detection of magnetic resonance using quantum spin sensors (QSSs) provides a spatially local and sensitive technique to probe spin dynamics in magnets. However, its utility as a probe of antiferromagnetic resonance (AFMR) remains an open question. We report the experimental demonstration of optically detected AFMR in layered van der Waals antiferromagnets (AFM) up to frequencies of 24 gigahertz. We leverage QSS spin relaxation due to low-frequency magnetic field fluctuations arising from collective dynamics of magnons excited by the uniform AFMR mode. First, through AFMR spectroscopy, we characterize the intrinsic exchange fields and magnetic anisotropies of the AFM. Second, using the localized sensitivity of the QSS, we demonstrate magnon transport over tens of micrometers. Last, we find that optical detection efficiency increases with increasing frequency. This showcases the dual capabilities of QSS as detectors of high-frequency magnetization dynamics and magnon transport, paving the way for understanding and controlling the magnetism of antiferromagnets.