Fig. 3

Detailed field dependence of the response: The field is applied in the direction of ζ = (−1, 2, 0), and field values are defined in the legend of panel a. a, b Constant Q cuts showing the energy dependence of the scattering. The data are integrated over the wave vector ranges ΔH = [−0.1, 0.1], Δζ = [−0.05, 0.05] and ΔL = [−2, 2] (See Supplementary Fig. S1 for units). Lines are guides to the eye. a Scattering at the M point (1/2, 0, L) shows that the energy of the spin-wave peak at 2.2 ± 0.2 meV does not change with fields up to 6 T. At 8 T the spin waves are suppressed. Defining the energy gap as the point where inelastic scattering exceeds the background leads to value of at least 1.6 ± 0.2 meV. The gap at the M point is also independent of fields up to 6 T (See supplementary fig. S7). b Scattering at the Γ point showing the evolution of the spectrum. The lowest energy zero-field spin-wave peak occurs at 2.69 ± 0.11 meV. The energy gap for the 0–4 T spectra is 1.8 ± 0.2 meV. The gap appears closed at 6 T, but has reopened at 8 T. At 8 T the spectrum shows higher intensity for all energies above the gap. c The 2 T (blue) and 6 T (orange) constant Q cuts at the Γ-point (from panel b) is shown with the corresponding 0 T data subtracted. d The 8 T constant Q cuts at the Γ-point (from panel b) is shown with the corresponding 6 T data subtracted. (See supplementary fig. S5 for 4 T data). e T = 2 K constant energy cuts integrated over Δζ and ΔL ranges as described above, and energy range E = ^5,7 meV, showing the evolution of the peak height and width of the continuum at 0, 2, 6 and 8 T. (See supplementary fig. S6 for 4 T data). f The same cuts comparing the data at T = 2 K, 8 T (grey triangles) and T = 15 K, 0 T (green open squares) demonstrating an overall quantitative agreement of the intensity and width of the Γ point continuum. The 15 K data is scaled by a factor of 2 to account for using a different chopper frequency (see Methods). In (c) and (d) the solid lines are least-square fits to a Gaussian peak plus a constant background. g The peak height (circles) and FWHM (triangles) of the continuum as a function of field at T = 2 K (solid symbols) and T = 15 K (open symbols). The field dependence hints at a discontinuous jump close to 6 T (crossover region, shaded.) Lines are guides to the eye. h AC susceptibility (Re(χ_AC))measured at T = 2 K with a frequency of 1 kHz shows two anomalies at B_C1 = 6.1 ± 0.5 T and B_C2 = 7.3 ± 0.3 T. In all panels (a–g) the error bars represent one standard deviation assuming Poisson counting statistics