Imaging twisty magnets Twisting monolayers of graphene with respect to each other has led to a number of unusual correlated states. This approach has inspired researchers to try their hand at twisting two-dimensional (2D) magnets, but such experiments have proven a difficult challenge. Song et al . made structures out of layers of the 2D magnet chromium triiodide with a small twist angle (see the Perspective by Lado). Using nitrogen vacancy centers in diamond as a magnetometer, the authors imaged the magnetic domains in both twisted monolayer and twisted trilayer structures. For twisted trilayers, a periodic pattern of ferromagnetic and antiferromagnetic domains was revealed. —JS

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For example, in transition metal dichalcogenides (TMDCs) such as MoS 2 , the stacking order controls the bandgap energy and determines the spin-valley coupling. [1,2] For trilayer graphene, the electronic structures of ABA-and ABCstacking are totally different: the ABAstacking is a semimetal and ABC-stacking is a semiconductor upon applying an electric field. [3][4][5] The unambiguous determination of the interlayer stacking order is therefore crucial for understanding and utilizing van der Waals materials. Recently, indium selenide (InSe) has attracted much attention as a highly promising candidate for bendable electronics and ultrafast optoelectronics. [6][7][8] It belongs to the family of III-VI metal monochalcogenide semiconductors including GaS and GaSe, exhibiting high carrier mobility and direct bandgap from bulk to thin flakes. Prototype InSe-based devices such as bendable photodetectors, [7] p-n junctions, [9] and field-effect-transistors [10] showed better performance than those based on graphene and TMDCs.Nonetheless, stacking order, the basic structural property of InSe, remains largely controversial. The monolayer InSe Indium selenide (InSe), a layered semiconductor with direct band gap and high carrier mobility, holds promising applications in bendable electronics and ultrafast optoelectronics. Yet its crystal structure exhibits polytypism with four different stacking orders (γ-, ε-, βand δ-phases), arising from the weak van der Waals interlayer coupling. These phases are nearly-degenerate in energy but are predicted to have contrasting electronic structures for versatile applications. It remains highly challenging to distinguish between these polytypes due to the lack of noninvasive tools that are sensitive enough to the interlayer structural variations. Here, the unambiguous discrimination of different InSe polytypes using symmetry-sensitive oblique incident optical second harmonic generation (SHG) is demonstrated. Surprisingly, the ε-phase is found to be dominant, although the samples from two popular commercial vendors (2D Semiconductors and Six Carbon Technology) are claimed to be the γ-phase. These results would help promoting the precise application of InSe crystal for piezoelectric transducer and strain sensing, as well as showing the oblique incident SHG to be a powerful structural analytical tool for van der Waals layered materials.

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