Fig. 4: Spin transmission visibility and elementary spin excitations.

a Transmission visibility of the spin resonance obtained from the experimental results shown in Fig. 3a (red points) and from theoretical simulations based on describing the spin excitations with either a classical mean-field model (dashed light-red line) or as bosonic magnons (solid red line). b Experimental (open blue symbols) and theoretical (lines) spin resonance width. The different lines progressively add different contributions to the line width: the dashed-pointed gray line represents the paramagnetic inhomogeneous broadening, the dashed light-blue line includes the additional damping arising from the emergence of antiferromagnetic spin-spin correlations and, finally, the solid darker blue line, obtained from the results in Fig. 3b, adds also the broadening caused by the spin-spin interaction anisotropy that makes chains with different orientations interact differently with the microwave field. The inset illustrates the nature of the excitations that couple to propagating photons: single spin excitations in the paramagnetic regime and bosonic spin waves in the low-T region.