Extremely Weakly Interacting $\mathrm{\Delta }{S}_z=0$ and $\mathrm{\Delta }{S}_z=1$ Excitations and Evidence for Fractional Quantization in a Magnetization Plateau: CeSb

The plateau at 1/3 of the saturation magnetization, $M_s$, in the metamagnet CeSb is accompanied by a state of ferromagnetic layers of spins in an up-up-down sequence. We measured $M$ and the specific heat, $C$, in the plateau, spin wave analyses of which reveal two distinct branches of excitations. Those with $\mathrm{\Delta }{S}_z=1$ as measured by $M$, coexist with a much larger population of $\mathrm{\Delta }{S}_z=0$ excitations measured by $C$ but invisible to $M$. The large density of $\mathrm{\Delta }{S}_z=0$ excitations, their energy gap, and their seeming lack of interaction with $\mathrm{\Delta }{S}_z=1$ excitations suggest an analogy with astrophysical dark matter. Additionally, in the middle of the plateau three sharp jumps in $M(H)$ are seen, the size of which, $0.15$%$M_s$, is consistent with fractional quantization of magnetization-per-site in the down-spin layers.