Magnetoelastic dynamics of the ``spin Jahn-Teller'' transition in CoTi₂O₅

CoTi₂O₅ has the paradox that the low temperature static and spatially long-range magnetic order is incompatible with the crystal structure owing to a mirror plane that exactly frustrates magnetic interactions. However, despite no observable structural distortion with diffraction, CoTi₂O₅ 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 CoTi₂O₅. 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, with an atypical temperature dependence. Taken together, we propose that the energetics of the spin Jahn-Teller effect in CoTi₂O₅ are related to cooperative magnetoelastic fluctuations as opposed to conventional soft critical dynamics which typically drive large measurable static displacements.