Direct observation of the nonaxisymmetric vortex in superfluidB3

We present experimental proof that in rotating ${}_{}{}^3{}_{}{}^{}\mathit{B}$ the vortex-core transition temperature ${\mathit{T}}_{\mathit{V}}$ separates axisymmetric vortices above ${\mathit{T}}_{\mathit{V}}$ from vortices with spontaneously broken axial symmetry below ${\mathit{T}}_{\mathit{V}}$. The nonaxisymmetry is observed in the presence of coherent spin precession as a new soft Goldstone mode, manifested as oscillations and spiral twisting of the core anisotropy axis. These are driven by the precessing spin via spin-orbit coupling and lead to magnetic relaxation from viscous losses, which depend on vortex pinning.