One of the main paradigms of quantum magnetism is that collective
excitations in systems with long-range order, such as ferro- or
antiferro-magnets, are well described in terms of bosonic
quasiparticles -- magnons. This approach is extremely successful,
since these magnons very weakly interact. On the other hand, once the
long-range order collapses, for instance due to geometric (or
doping-induced) frustration of spin couplings, magnons interact and
such a description seems to fail. Instead the low-energy magnetic
excitations are typically described in terms of fermionic spinons.
Unfortunately the latter quasiparticles are less intuitive, for they
carry fractional quantum numbers and, often, they also do interact.

In this talk I will first give a not-so-standard but intuitive
understanding of both magnons [1] and spinons [2]. Next, I will
discuss whether the observed with RIXS persistence of collective spin
excitations in the high-Tc cuprates is better described in terms of
spinons [3] or magnons [4].

References:
[1] P. Wrzosek et al.; Physical Review B 102, 02440 (2020).
[2] T. Kulka et al, arXiv:2303.02276 (under review in Phys. Rev. Lett).
[3] YF Kung et al., Physical Review B 96, 195106 (2017); E.M. Paerschke et
al., Physical Review B 99, 205102 (2019).
[4] W. Zhang et al., npj Quantum Mater. 7, 123 (2022).