Seminar: Deconfinement of fractional excitations in ordered quantum magnets

ABSTRACT

Emergent quantum states can host excitations with properties distinct from those of the underlying degrees of freedom. Quantum spin liquids provide a well-known setting for such fractional excitations, but related phenomena can also arise in magnetically ordered systems. In this talk, I will present experimental evidence for fractionalization in quantum magnets, starting from neutron spectroscopy studies of spinon continua in one-dimensional systems, and complementary observations using resonant inelastic X-ray scattering (RIXS).

I will then turn to square-lattice antiferromagnets, where the excitation spectra of undoped cuprates, iridates, and related systems can be described using Hubbard-based models. At the same time, anomalies—particularly near (π,0) — and associated high-energy continua indicate departures from conventional spin-wave theory. In the infinite-layer cuprate SrCuO₂, these effects become especially pronounced, suggesting proximity to a regime where spin-wave descriptions break down and fractionalized excitations may emerge despite long-range order.

These results point to quantum correlations beneath symmetry-broken states as a route to accessing physics akin to quantum spin liquids. I will illustrate this with SeCuO₃, where magnons, triplons, and possible spinon-like excitations coexist. Finally, I will discuss a data-driven approach based on systematic spectroscopic measurements across material families, enabling new directions in materials discovery and AI-assisted modelling.

BIOGRAPHY

Henrik Moodysson Rønnow is a Professor of Physics at École Polytechnique Fédérale de Lausanne, where he heads the Laboratory for Quantum Magnetism and has served as Director of the Institute of Physics. He received his PhD in physics from the University of Copenhagen in collaboration with Risø National Laboratory. His research focuses on quantum magnetism and correlated electron systems, with an emphasis on magnetic excitations and phases studied using neutron and X-ray spectroscopy combined with in-house low temperature methods alongside quantum materials discovery and theoretical modelling. He has contributed to work on quantum spin systems, skyrmion textures, and the development of neutron scattering instrumentation. He holds an honorary professorship at the University of Copenhagen and is an affiliated scientist at RIKEN Center for Emergent Matter Science