SPM images of cleaved h-HoMnO3 (110): (top) PFM image of in-plane ferroelectric domains (oriented vertical) and (bottom) conductive AFM image showing enhanced conduction along tail-to-tail domain walls; images span 4 µm on a side.
These 2D sheets of charge are not pinned by unstable defects, chemical dopants, or structural interface, but are formed naturally as the inevitable by-products of topological vortices in this multiferroic material. The team focused on hexagonal HoMnO3, which is a multiferroic material in which antiferromagnetism and ferroelectricity coexist and magnetic, electric, and mechanical forces can be coupled to one another.
In situ conductive atomic force microscopy, piezo-response force microscopy, and Kelvin-probe force microscopy at low temperatures measured the material properties. The discovery is an important step in understanding the semiconducting properties of the domains and domain walls in small-gap ferroelectrics.
It also suggests a new and natural platform for exploring transport of charge carriers confined at interfaces or surfaces, which is one of the major playgrounds in condensed matter physics for emergent phenomena.