Guratinder K, Johnson R D, Prabhakaran D, Taylor R A, Lang F, Blundell S J, Taran L S, Streltsov S V, Williams T J, Giblin S R, Fennell T, Schmalzl K, Stock C
University of Edinburgh, School of Physics and Astronomy, Edinburgh EH9 3JZ, United Kingdom.
University College London, Department of Physics and Astronomy, Gower Street, London WC1E 6BT, United Kingdom.
Phys Rev Lett. 2025 Jun 27;134(25):256702. doi: 10.1103/sr9s-ll1y.
CoTi_{2}O_{5} has the paradox that low temperature static magnetic order is incompatible with the crystal structure owing to a mirror plane that exactly frustrates magnetic interactions. Despite no observable structural distortion with diffraction, CoTi_{2}O_{5} does magnetically order below T_{N}∼25 K with the breaking of spin ground state degeneracy proposed to be a realization of the spin Jahn-Teller effect in analogy to the celebrated orbital Jahn-Teller transition. We apply neutron and Raman spectroscopy to study the dynamics of this transition in CoTi_{2}O_{5}. We find anomalous acoustics associated with a symmetry breaking strain that characterizes the spin Jahn-Teller transition. Crucially, the energy of this phonon coincides with the energy scale of the magnetic excitations, and has the same symmetry of an optic mode, observed with Raman spectroscopy, which atypically softens in energy with decreasing temperature. Taken together, we propose that the energetics of the spin Jahn-Teller effect in CoTi_{2}O_{5} are related to cooperative magnetoelastic fluctuations as opposed to conventional soft critical dynamics which typically drive large measurable static displacements.
