A new topological phase in a two-dimensional system has been discovered by scientists at University of Cambridge. It can be used as a new platform for examining topological physics in nanoscale devices.
Two-dimensional materials such as graphene have acted as a playground for experimental breakthroughs and theoretical knowledge of various phenomena in materials science and physics.
A wide range of functionality can be achieved in devices by utilizing various 2D materials or assembling combinations of layers.
Recently, it was found that in materials such as hexagonal boron nitride (hBN), which is less symmetrical compared to graphene, ferroelectricity occurs when one layer slides over another and further splits the symmetry.
Ferroelectricity is the shift of the electric dipole moment of a material with an electric field, which is a useful property for memory storage and information processing.
They developed an interesting interference pattern called moiré superlattice when 2D materials distort one another. This can radically change the physical properties.
The different stacking regions turn polarized when hBN and identical materials are twisted. This is the result of the polar domain normal network, which has also been shown to generate ferroelectricity.
In this new study, scientists from Cambridge’s Cavendish Laboratory and the University of Liège, Belgium, have discovered that there are more such polar domains: they are topological in nature and form objects called antimerons and merons.
This study has been published in the journal Nature Communications.
The polarization in the bent system points out of plane, meaning it is perpendicular to the seam.
Dr Daniel Bennett, First Study Author, Theory of Condensed Matter Group, Cavendish Laboratory, University of Cambridge
“What we found is that the symmetry breakage caused by shearing or twisting also results in in-plane polarizations that are similar in strength to out-of-plane polarizations. In-plane polarization forms beautiful vector fields, and their shape is determined entirely by the symmetry of the layers.”
Bennett started this project at the Cavendish Laboratory and is currently based at Harvard University, USA.
The breakthrough in-plane polarization revealed the fact that the electrical properties of 2D bent systems are more complicated than previously thought. Most significantly, by integrating the in-plane and out-of-plane polarization sections, the group realized that the polarization in such a twisted bilayer is known to be topologically non-trivial.
“In each domain, the polarizing field rotates about half a revolution, forming a topological object known as a meron (half skyrmion).. Along the twisted layers, a strong network of meron and antimeron is formedsaid Dr Robert-Jan Slager, whose group at the Cavendish Laboratory was involved in the study.
In physics, most things can be understood in terms of energy. Nature is lazy and likes to do things in the most efficient way possible, doing so while minimizing the energy of a system.
Daniel Bennett, First Author Study, Theory of Condensed Matter Group, Cavendish Laboratory, University of Cambridge
Usually, the phase that a material will adopt has the lowest energy. But topological properties and topological phases are not identified by energy but by some system symmetry.
Physical properties of systems, such as their magnetic or electric fields, can develop complex structures that coil or bind themselves together as imposed by symmetry.
The energy cost of untying these knots is very high, so these structures eventually become quite strong. Being able to create, destroy and control these topological objects is of great interest, for example in the field of topological quantum computing.
Robert-Jan SlagerView, Group Theory of Condensed Matter, Cavendish Laboratory, University of Cambridge
For this to be possible, the scientists’ future goal is to develop better knowledge of topological polarization and also to develop a proof-of-concept for a device in which the polar merons or antimerons they discover can be arranged or produce interesting novelties. physical phenomenon.
Bennett started this project during his Ph.D. at Cambridge’s Cavendish Laboratory before moving to the University of Liège, Belgium, where he continued this research with Professor Philippe Ghosez and Dr. Eric Bousquet, where both are experts in the field of ferroelectrics.
Bennett, D., et al. (2023) Polar meron–antimeron networks in a strained and twisted double layer. Nature Communications. https://doi.org/10.1038/s41467-023-37337-8.