Manipulating single excess electrons in monolayer transition metal dihalide.

Polarons are entities of excess electrons dressed with local response of lattices, whose atomic-scale characterization is essential for understanding the many body physics arising from the electron-lattice entanglement, yet difficult to achieve. Here, using scanning tunneling microscopy and spectroscopy (STM/STS), we show the visualization and manipulation of single polarons in monolayer CoCl2, that are grown on HOPG substrate via molecular beam epitaxy. Two types of polarons are identified, both inducing upward local band bending, but exhibiting distinct appearances, lattice occupations and polaronic states. First principles calculations unveil origin of polarons that are stabilized by cooperative electron-electron and electron-phonon interactions. Both types of polarons can be created, moved, erased, and moreover interconverted individually by the STM tip, as driven by tip electric field and inelastic electron tunneling effect. This finding identifies the rich category of polarons in CoCl2 and their feasibility of precise control unprecedently, which can be generalized to other transition metal halides.

Planar Heterojunction of Ultrathin CrTe3 and CrTe2 van der Waals Magnet.

The fabrication of planar heterojunctions with magnetic van der Waals ultrathin crystals is essential for constructing miniaturized spintronic devices but is yet to be realized. Here, we report the growth of CrTe3 and CrTe2 ultrathin films with molecular beam epitaxy and characterize their morphological and electronic structure through low-temperature scanning tunneling microscopy/spectroscopy. The former is identified as a Mott insulator, and the latter has shown a robust magnetic order previously. Through vacuum annealing, CrTe3 can be transformed into CrTe2, whose relative ratio is controlled via the annealing time. This renders the feasibility of constructing CrTe3-CrTe2 planar heterojunctions, which express atomically sharp interfaces and smooth band bending. We also identified a superstructure conceivably formed via hybrid units of CrTe3 and CrTe2, whose electronic structure exhibits stunning tunability with the length of the superstructure. Our study sets a foundation for the development of magnetic tunneling junctions for building spintronic circuits and engineering electronic states in artificial superlattice structures.

Spin mapping of intralayer antiferromagnetism and field-induced spin reorientation in monolayer CrTe2.

Intrinsic antiferromagnetism in van der Waals (vdW) monolayer (ML) crystals enriches our understanding of two-dimensional (2D) magnetic orders and presents several advantages over ferromagnetism in spintronic applications. However, studies of 2D intrinsic antiferromagnetism are sparse, owing to the lack of net magnetisation. Here, by combining spin-polarised scanning tunnelling microscopy and first-principles calculations, we investigate the magnetism of vdW ML CrTe2, which has been successfully grown through molecular-beam epitaxy. We observe a stable antiferromagnetic (AFM) order at the atomic scale in the ML crystal, whose bulk is ferromagnetic, and correlate its imaged zigzag spin texture with the atomic lattice structure. The AFM order exhibits an intriguing noncollinear spin reorientation under magnetic fields, consistent with its calculated moderate magnetic anisotropy. The findings of this study demonstrate the intricacy of 2D vdW magnetic materials and pave the way for their in-depth analysis.