Two-dimensionale ferroelectrics

• Applicant: Professor Dr. Carsten Busse
• Subject Area: Experimental Condensed Matter Physics
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 495846840

Ferroelectric materials are a recent addition to the two-dimensional world with fascinating fun-damental properties as well as a high potential for applications. In analogy to ferromagnets, ferroelectrics show a permanent electrical polarization that can be switched by an external electric field. The reduced dimensionality in ultrathin layers as well as competing modes of coupling in van der Waals stacks induce novel effects in 2D-ferroelectrics.In thin films of conventional ferroelectrics, the critical temperature TC decreases with decreas-ing thickness and quickly drops below feasible values. In contrast, in two-dimensional group-IV monochalcogenides, very stable (in-plane) ferroelectricity is found. These materials of the form MX (M=Ge, Sn; X=S, Se, Te) are in the focus of this project. Here, ferroelectricity arises from the loss of centrosymmetry in the puckered crystal structure in combination with the ionic character of the bonds. The samples in this project will be prepared by epitaxial growth under highly-controlled condi-tions (well-defined substrate, UHV environment) which is a versatile method to prepare ad-vanced 2D materials, in this case quasi-freestanding mono- as well as few-layered MX. Our main methods for sample characterization are scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS).We will investigate the switching of the polarization by applying a point-like electric field using the STM tip and by exploiting the auxetic behavior of MX and apply a vertical strain with the AFM which induces a compression in-plane, moving the system towards the saddle point of the switching. A switchable polarization of this kind would also be interesting from the view-point of applications. On a more fundamental scale, in the MX also a ferroelastic switching is possible, which corresponds to a rotation by 90°.The interaction between neighboring layers in van der Waals-stacks is decisive for the ferroe-lectricity: The in-plane dipoles prefer an antiferroelectric coupling (AB-stacking), while the local ionic and covalent coupling prefers an AA stacking with ferroelectric coupling. The resulting interaction energies have different dependencies on the number of layers, and hence the pre-ferred stacking can change with film thickness. For the specific case of GeSe, AA stacking is the energetic minimum for the bilayer, but AB stacking is preferred in the bulk. We will investi-gate this interplay of local and non-local coupling in different MX (GeSe, GeS, SnSe, SnS) to pin down the influence of the individual constituents. In such a system, switching of one layer only would lead to a moiré-structure between two ferroelectric layers, which is a completely unexplored system.

• DFG Programme: Research Grants