Ever since the discovery of the quantum Hall effect, symmetry has been the guiding principle in the search for topological materials. Now, scientists have introduced an alternative guiding principle named “quasi-symmetry”. It leads to the discovery of a new type of topological material with great potential for applications in spintronics and quantum technologies.
The quasi-symmetry operation acts selectively on different parts of the system.A simplified example can be an incomplete mirror image. Because here some parts of the object are mirrored but others are not. It corresponds to a system that has exact symmetry when taking only the basic approximation into consideration while additional approximative terms break such symmetry.
Scientists demonstrate that quasi-symmetry in the semi-metal CoSi stabilizes tiny energy gaps over a large near-degenerate plane. This is reflected in the way the electrons are bent into circular motion by a magnetic field which is known as quantum oscillations.
The application of in-plane strain breaks the crystal symmetry which gaps only the corresponding degenerate points but the quasi-symmetry-protected points remain intact. These results demonstrate one of the most important features of quasi-symmetry.
Most of the topological materials discovered in recent years require precise engineering of their chemical composition for them to be relevant for future technological applications. Quasi-symmetries delete the need for such fine-tuning as the topological features can be found at any arbitrary chemical potential.
Quasi-symmetry-protected topological materials are robust against any physical deformation which breaks the crystalline symmetry. Ouasi-symmetry-protected topological materials are robust against physical deformations which break the crystalline symmetry which is a key prerequisite for their technological application via thin-film processes.
