Figure 1: Temperature dependence of direct current (DC) RDNMR spectra of ¹¹⁵In in a Corbino disk.

(a) versus f as a function of temperature at B=12.3 T with current I=0.6 μA (black curve) and I=−0.6 μA (pink cruve). The dashed–dotted line represents the zero level. Quadrupole resonances are indicated by vertical solid lines with numbers 1–9. (b) Schematic domain structures of quantum Hall ferromagnet (QHF). The grey and green areas denote the spin-unpolarized (spin polarization P=0, spin-up (black solid arrow) and spin-down (red solid arrow) electrons in the two Zeeman levels of the n=0 Landau level (LL), see inset of Supplementary Fig. 4) and spin-polarized (P=1, spin-up electrons in both n=0 and n=1 LLs) domains, respectively, and a domain wall (DW) occurs in between. For clarity, spin-up electrons in the n=0 LL are not shown in the graph. The electron-spin flip (say from spin-down to spin-up, red dashed arrows) flops one nuclear spin from spin-up (black hollow arrow) to spin-down (red hollow arrow) at DW boundaries. Note that the arrow length is not scaled with the magnetic moment of each particle. (c) A possible population distribution (energy E versus population N) of ¹¹⁵In with 10 nuclear spin states |m> near the P=0 and P=1 domains and the total population distribution by assigning the same weight from these two domains to the RDNMR response. The presence of electric quadrupole coupling accounts for a difference in the splitting between these levels (where f₀ and Δf are the Zeeman and quadrupole frequencies, respectively, and h is Planck’s constant). (d) The corresponding population difference between adjacent levels (denoted by numbers 1–9), ΔN=N_|m>−N_|m-1>, as a function of f, where the frequency interval is equally spaced by Δf and the largest ΔN is taken as unity. The total ΔN−f dependence is proposed to be responsible for quadrupole resonances in data that are equally spaced by Δf∼85 kHz.