Braiding reflectionless states in non-Hermitian magnonics

A thorough understanding of the topological classifications of non-Hermitian energy bands is essential for advancing non-Hermitian band theory and its applications. As evidenced in various disciplines of physics, including optics, electronics and acoustics, the process of braiding plays a crucial role in the classification of non-Hermitian bands that manifest topological characteristics. Here we demonstrate topological braiding of both reflectionless states and resonant states in non-Hermitian magnons, unveiling a reversal in their braiding handedness. Furthermore, we constitute parity-time symmetric reflectionless scattering modes, along with their degenerate exceptional points. Our results not only underscore the importance of braided scattering states, but also establish magnonics as a versatile platform for exploring non-Hermitian band theory and developing magnon-based applications, including topological energy transfer, tunable absorbers and logic circuits. Extending topological braids of complex energy bands to non-Hermitian systems of magnons-the quanta of spin waves-is a crucial step in the development of spin-based topological devices. This has now been experimentally demonstrated.