Cambridge researchers identified a novel topological phase in a two-dimensional system that could serve as a new platform for investigating topological physics in nanoscale devices.
Two-dimensional materials, such as graphene, have acted as a laboratory for the experimental discovery and theoretical understanding of a wide range of physics and materials science phenomena.
Beyond graphene, there are several 2D materials with varying physical properties. This is exciting for possible applications in nanotechnology, as different 2D materials or stacking combinations of multiple layers can create a wide range of functionality in devices.
Ferroelectricity has recently been identified in materials less symmetric than graphene, such as hexagonal boron nitride (hBN), when one layer slides over the other and breaks a symmetry.
The switching of an electric dipole moment of a material with an electric field is known as ferroelectricity, and it is a useful trait for information processing and memory storage.
When two-dimensional materials are twisted with respect to one another, they generate a stunning interference pattern known as a moiré superlattice, which has the potential to drastically alter physical properties. When hBN and related materials are twisted, the distinct stacking regions become polarized, resulting in a regular network of polar domains and ferroelectricity.
In each domain, the polarization field winds around by half a revolution, forming a topological object known as a meron (half a skyrmion). Throughout the twisted layer, a robust network of merons and antimerons forms.
Dr. Robert-Jan Slager
Researchers from Cambridge’s Cavendish Laboratory and the University of Liège, Belgium, discovered that these polar domains are inherently topological and form objects known as merons and antimerons in this new study published in Nature Communications.
“The polarization in twisted systems points in the out-of-plane direction, that is to say perpendicular to the layers,” said first author Dr. Daniel Bennett, who started this project at the Cavendish Laboratory and is now based at Harvard University, U.S..
“What we found is that the symmetry breaking caused by sliding or twisting also results in an in-plane polarization which is similar in strength to the out-of-plane polarization. The in-plane polarization forms a beautiful vector field, and its shape is determined entirely by the symmetry of the layers.”
The discovery of in-plane polarization demonstrates that the electrical properties of 2D twisted systems are far more complex than previously assumed. More importantly, the team discovered that polarization in these twisted bilayers is topologically non-trivial by combining both the in-plane and out-of-plane parts of the polarization.
“In each domain, the polarization field winds around by half a revolution, forming a topological object known as a meron (half a skyrmion),” said Dr. Robert-Jan Slager, whose group at the Cavendish Laboratory was involved in the study. “Throughout the twisted layer, a robust network of merons and antimerons forms.”
“In physics, most things can be understood in terms of energy,” said Bennett. “Nature is lazy and likes to do things in the most efficient way possible, doing so by minimizing the energy of a system.”
The phase that a material will adopt is typically the one that has the lowest energy. Topological phases and topological qualities, on the other hand, are defined by a system’s multiple symmetries, not by energetics.
Because of symmetry, a system’s physical features, such as its electric or magnetic fields, can build intricate structures that wind or tie themselves in knots.
“The energetic cost of untying these knots is very high, so these structures end up being quite robust,” said Slager. “Being able to create, destroy and control these topological objects is very appealing, for example in the field of topological quantum computing.”
The researchers’ future goals are to build a better understanding of topological polarization as well as a proof of concept for a device that can regulate or lead to fascinating new physical events using the polar merons/antimerons they discovered.
