Observation and Manipulation of Chiral Phasons in a Magnetic System

Abstract Phasons, quasiparticle excitations corresponding to phase shifts in incommensurate orders, represent the lowest-energy Goldstone modes of such systems, yet have remained unobserved in magnetic materials. Here, we activate the Goldstone mode of a one-dimensional chiral spin helix in a helimagnet by exploiting nonlinear spin dynamics, revealing an Archimedean screw-like phason mode at megahertz frequencies. This unique phase-sliding behavior is directly evidenced through a Doppler-shift signature detected via stroboscopic magneto-optical Kerr effect measurements. Remarkably, owing to the intrinsic screw symmetry of the system, forward- and backward-propagating phasons can be selectively excited, demonstrating well-defined mode chirality. Our results unveil a novel pumping mechanism in magnetic materials and establish a tunable experimental platform for the study of Floquet magnetism.