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Next Quantum Material Seminar/LANEF:


Thursday, May 25th 2023 at 2:00 pm

Radu Coldea (Oxford University)

Order-by-disorder in a honeycomb cobaltate



To access this seminar:

Institut Néel CNRS, Building K, Room Rémy Lemaire  (K223)


Abstract: Order-by-disorder occurs when the ground state magnetic structure is selected out of a manifold of many classically-degenerate structures. We explore the manifestation of such physics experimentally in the honeycomb easy-plane magnet CoTiO3 using high-resolution inelastic neutron scattering measurements of the magnetic excitations. We observe a finite energy gap above the magnetic Bragg peaks, which indicates a set of six symmetry-equivalent preferred moment directions in the ab plane. The gap cannot be understood classically considering only the lowest Kramers doublet (effective spin-1/2) of the Co2+ ions, as the mean-field energy is independent of the moment orientation in the ab plane for any two-site spin exchanges allowed by the rhombohedral crystal structure symmetry. As possible mechanisms for the gap generation we discuss i) zero-point quantum spin-wave fluctuations within the lowest Kramers doublet in the presence of bond-dependent exchanges and ii) spin-orbital exchange in the case when magnetic exchange, spin-orbit coupling and crystal field splittings are comparable as is the case here, and we provide a flavour-wave model to capture all key features of the observed spectrum.
M. Elliot, P.A. McClarty, D. Prabhakaran, R.D. Johnson, H.C. Walker, P. Manuel and R. Coldea, Nature Communications 12, 3936 (2021)

The organizers:

Elsa Lhotel (elsa.lhotel@neel.cnrs.fr)
Florence Levy-Bertrand (florence.levy-bertrand@neel.cnrs.fr)