Non-coplanar swirling field textures, or skyrmions, are now widely recognized as objects of both fundamental interest and technological relevance. So far, skyrmions were amply investigated in magnets, where due to the presence of chiral interactions, these topological objects were found to be intrinsically stabilized. Ferroelectrics on the other hand, lacking such chiral interactions, were somewhat left aside in this quest. Here we demonstrate, via the use of a first-principles-based framework, that skyrmionic configuration of polarization can be extrinsically stabilized in ferroelectric nanocomposites. The interplay between the considered confined geometry and the dipolar interaction underlying the ferroelectric phase instability induces skyrmionic configurations. The topological structure of the obtained electrical skyrmion can be mapped onto the topology of domain-wall junctions. Furthermore, the stabilized electrical skyrmion can be as small as a few nanometers, thus revealing prospective skyrmion-based applications of ferroelectric nanocomposites.

Observation of a new type of nanoscale ferroelectric domains, termed as "bubble domains"-laterally confined spheroids of sub-10 nm size with local dipoles self-aligned in a direction opposite to the macroscopic polarization of a surrounding ferroelectric matrix-is reported. The bubble domains appear in ultrathin epitaxial PbZr Ti O /SrTiO /PbZr Ti O ferroelectric sandwich structures due to the interplay between charge and lattice degrees of freedom. The existence of the bubble domains is revealed by high-resolution piezoresponse force microscopy (PFM), and is corroborated by aberration-corrected atomic-resolution scanning transmission electron microscopy mapping of the polarization displacements. An incommensurate phase and symmetry breaking is found within these domains resulting in local polarization rotation and hence impart a mixed Néel-Bloch-like character to the bubble domain walls. PFM hysteresis loops for the bubble domains reveal that they undergo an irreversible phase transition to cylindrical domains under the electric field, accompanied by a transient rise in the electromechanical response. The observations are in agreement with ab-initio-based calculations, which reveal a very narrow window of electrical and elastic parameters that allow the existence of bubble domains. The findings highlight the richness of polar topologies possible in ultrathin ferroelectric structures and bring forward the prospect of emergent functionalities due to topological transitions.

Magnetic skyrmions are nano-scale spin structures that are promising for ultra-dense memory and logic devices. Recent progresses in two-dimensional magnets encourage the idea to realize skyrmionic states in freestanding monolayers. However, monolayers such as CrI 3 lack Dzyaloshinskii-Moriya interactions (DMI) and thus do not naturally exhibit skyrmions but rather a ferromagnetic state. Here we propose the fabrication of Cr(I,X) 3 Janus monolayers, in which the Cr atoms are covalently bonded to the underlying I ions and top-layer Br or Cl atoms. By performing first-principles calculations and Monte-Carlo simulations, we identify strong enough DMI, which leads to not only helical cycloid phases, but also to intrinsic skyrmionic states in Cr(I,Br) 3 and magnetic-field-induced skyrmions in Cr(I,Cl) 3 . 1 arXiv:1906.04336v2 [cond-mat.mtrl-sci] 28 Jun 2019Magnetic skyrmions are nano-scale spin clusters with topological stability, and are promising for advanced spintronics 1, 2 . One requirement toward such applications is that the hosting materials should be thin films, so that the nano size of skyrmions can be taken full advantage of.Besides previous studies on bulk MnSi 3-6 , recent works focused on ultrathin films, such as FeGe 7, 8 and rare-earth ion garnet 9, 10 , which both take advantage of the Dzyaloshinskii-Moriya interaction (DMI) arising from the heavy metal substrate. However, no skyrmionic state has ever been reported to intrinsically exist in free-standing monolayers, to the best of our knowledge, while two-dimensional (2D) semiconducting magnets, such as monolayer CrI 3 11 , are recently attracting much attention due to their novel physics and rich applications 12 . The ferromagnetic monolayer CrI 3 crystalizes in honeycomb lattice made of edge-sharing octahedra. Its ferromagnetic order is stabilized by an out-of-plane anisotropy 11 , which arises from single ion anisotropy (SIA) andKitaev-type exchange coupling that both result from the SOC of its heavy ligands 13,14 . However, the ingredient DMI is absent between the most strongly coupled first nearest neighbor (1st NN) Cr-Cr pairs, because the inversion center between the two Cr atoms prevents its existence 15 . Interestingly, very recent theoretical study proposed the application of electric field to break the inversion center and induce DMI in monolayer CrI 3 16 . Although this clever method leads to CrI 3 monolayers becoming closer to adopt a skyrmion phase, the weak effects of electric field in generating DMI, as well as the rather strong out-of-plane anisotropy, hinders the actual creation of skyrmions in this system.Here we propose a more effective approach that consists in fabricating Janus monolayers of chromium trihalides Cr(I,X) 3 (X = Br, Cl). One example of Janus monolayers is the transition

Quantum spin liquids (QSLs) form an extremely unusual magnetic state in which the spins are highly correlated and fluctuate coherently down to the lowest temperatures, but without symmetry breaking and without the formation of any static long-range-ordered magnetism. Such intriguing phenomena are not only of great fundamental relevance in themselves, but also hold the promise for quantum computing and quantum information. Among different types of QSLs, the exactly solvable Kitaev model is attracting much attention, with most proposed candidate materials, e.g., RuCl3 and Na2IrO3, having an effective S=1/2 spin value. Here, via extensive first-principlebased simulations, we report the investigation of the Kitaev physics and possible Kitaev QSL state in epitaxially strained Cr-based monolayers, such as CrSiTe3, that rather possess a S=3/2 spin value. Our study thus extends the playground of Kitaev physics and QSLs to 3d transition metal compounds. :2002.12184v1 [cond-mat.mtrl-sci] arXiv

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