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flip character (Figure 101a), which, together with their incident-energy behaviour, rules out a more trivial multi- magnon origin. As a comparison, Figure 101b shows the corresponding spectrum in Sr2CuO2Cl2 (SCOC), which has a much smaller ring exchange Jc than CaCuO2. In SCOC, the magnon peak is the dominant excitation even at the X point.

Therefore, sound evidence links such an anomaly in CCO to a two-spinon continuum, and correlates its presence with an exceptionally high Jc~J. This work confirms that in the CuO2 planes of cuprates, long-range antiferromagnetism can coexist with spin-glass phenomena, and paves the way for future RIXS studies on quantum magnetism.

PRINCIPAL PUBLICATION AND AUTHORS

Fractional Spin Excitations in the Infinite-Layer Cuprate CaCuO2, L. Martinelli (a), D. Betto (b), K. Kummer (b), R. Arpaia (c), L. Braicovich (a,b), D. Di Castro (d,e), N.B. Brookes (b), M. Moretti Sala (a), G. Ghiringhelli (a,f), Phys. Rev. X 12, 021041 (2022); https:/doi.org/10.1103/PhysRevX.12.021041 (a) Politecnico di Milano (Italy) (b) ESRF (France) (c) Chalmers University of Technology (Sweden) (d) Università di Roma Tor Vergata (Italy) (e) CNR-SPIN, Roma (Italy) (f) CNR-SPIN, Milano (Italy)

REFERENCES

[1] H. Shao et al., Phys. Rev. X 7, 041072 (2017). [2] N.S. Headings et al., Phys. Rev. Lett. 105, 247001 (2010). [3] Y.Y. Peng et al., Nat. Phys. 13, 1201-1206 (2017).

New perspectives on the dynamics of a Kondo lattice

What would happen if Schrödinger s cats started talking to each other? As in the famous Gedankenexperiment, the electrons of a Kondo system are in a quantum mechanical superposition: Not dead or alive but localised and itinerant. Resonant inelastic X-ray scattering measurements can resolve what happens as such superposition turns cooperative.

The unlikely behaviour of quantum matter, from superconductivity to topological quasiparticles, emerges out of complexity at the atomic scale. One universal source of this complexity, recurrent across quite disparate materials, is the entanglement of local and itinerant electronic degrees of freedom. The prototype of this local- itinerant ambiguity is the Kondo effect, which describes a single electron with counteracting tendencies: either to settle at its lattice site and form a magnetic and insulating state, or to melt into the surrounding metallic environment as part of a non-magnetic conductor.

In Kondo lattices, like CePd3, the entanglement of individual Kondo sites is so strong that their fluctuations become cooperative (dispersive) at relatively high temperatures (Figure 102). Conceptually, this is a pivotal point of quantum matter: as the local-itinerant correlation itself (the self-energy of the electronic state) starts being shaped by the system s lattice character, its consequences can no longer be modelled as a sum of its parts [1]. Progress in theory and neutron and synchrotron techniques [2,3] has made it possible to study this situation on the relevant scales of momentum and energy.

Crucially for the study of CePd3, the absence of long-range order provides a chance to directly glimpse the cooperative character of a Kondo lattice. But for the same reasons because the alterations of magnetic, charge and orbital behaviour are so subtle and span a wide range of energies its experimental characterisation is difficult. In this work, the strategy was therefore first to build a state-of-the-art computational model of the overall electronic structure, to corroborate this calculation with spectroscopies from the meV to eV scale (photoemission and X-ray absorption), and

Fig. 102: a) Impression of a Kondo lattice, where localised spins are

coherently screened by conduction electrons. b) State-of-the-art electronic structure calculations provide a glimpse

of this scenario in momentum space.