Fig. 4. Theoretical model.

Dissipative response of a spherical rotor χ″ as function of normalised rotation frequency for the the two components corotating (blue line) and counterrotating (cyan line). The red line is the sum of the two terms, showing that the total dissipation can indeed become negative. Here the field frequency is set to ω = 50ω_* (dashed vertical line), where ${\omega }_{*}={\left({\mu }_0{\mu }_r\sigma a^2\right)}^{-1}$ is a scaling frequency at which the penetration depth equals the radius a of a spherical rotor with permeability μ₀μ_r and conductivity σ (see Supplemental Material for additional information on the theoretical model with a sphere and with a cylinder).