Current-Driven Nonlinear Skyrmion Dynamics in d-Wave Altermagnets.

The center of mass and helicity are two dynamic degrees of freedom of skyrmions. In this Letter, we study the current-driven skyrmion motion in frustrated d-wave altermagnets. Contrary to conventional wisdom, we find that the skyrmion helicity is not locked with the skyrmion Hall angle, but unidirectionally rotates with a global angular velocity proportional to the square of the current density. In addition, we find that the helicity rotation velocity is highly anisotropic, depending on the direction of current flows. We also observe helicity oscillation in the terahertz regime, where the nonlinear mixing between the fast and slow modes generates a comblike spectrum. Full atomistic spin dynamics simulations verify our theoretical predictions. Our results establish frustrated altermagnets as a promising platform for skyrmionics, terahertz technology, and magnon frequency comb.