2D Magnetic, Ferroelectric and Superconducting van der Waal’s Structures

Van der waal’s structures provide for many possibilities for tailoring magnetic, ferroelectric and superconducting properties. Firstly, we discuss the observation of topological chiral spin textures in thin lamellae prepared from single crystals of two vdw’s compounds, Fe₃GeTe₂¹ and and Cr_1+δTe₂(δ ≈ 0.3)². We find direct evidence for Néel skyrmions using Lorentz transmission electron microscopy. We show that these topological magnetic structures are made possible by self-intercalation of Cr in the latter and by an asymmetric distribution of Fe vacancies in the former. These significant structural changes lower the symmetry of the respective crystals and, in particular, break the structural inversion symmetry, thereby, making possible the chiral spin textures that we observe. In both cases the skymion size increases significantly as the thickness of the lamella in which the skyrmions are hosted is increased. In the extreme monolayer limit, using molecular beam epitaxy, we have prepared single monolayers of CrCl₃ on graphene/6H-SiC(0001) substrates and observed robust nearly ideal easy-plane ferromagnetic ordering with critical scaling characteristic of a 2D-XY system³. Secondly, we discuss ferroelectric ordering of SnTe⁴ and SnSe⁵ in the monolayer limit and demonstrate greatly enhanced transition temperatures compared to the bulk materials related to significant structural changes. Finally, we discuss our recent work on proximity induced superconductivity in a Dirac semi-metal vdw’s layer and our observation of a very large Josephson diode effect.<br/>1 Chakraborty, A.<i> et al.</i> Magnetic skyrmions in a thickness tunable 2D ferromagnet from a defect driven Dzyaloshinskii-Moriya interaction. <i>under review</i> (2021).<br/>2 Saha, R.<i> et al.</i> Observation of Néel-type skyrmions in acentric self-intercalated Cr_1+δTe₂. <i>submitted</i> (2021).<br/>3 Bedoya-Pinto, A.<i> et al.</i> Intrinsic 2D-XY ferromagnetism in a van der Waals monolayer. <i>Science</i> <b>374</b>, 616-620 (2021).<br/>4 Chang, K.<i> et al.</i> Enhanced Spontaneous Polarization in Ultrathin SnTe Films with Layered Antipolar Structure. <i>Adv. Mater.</i> <b>31</b>, 1804428, doi:10.1002/adma.201804428 (2019).<br/>5 Chang, K.<i> et al.</i> Microscopic manipulation of ferroelectric domains in SnSe monolayers at room temperature. <i>Nano Lett.</i> <b>20</b>, 6590-6597 (2020).