Magneto-elastic coupling induced vibronic bound state in the spin ice pyrochlore Ho₂Ti₂O₇

The holmium pyrochlore Ho₂Ti₂O₇ is renowned as an exemplary realization of dipolar spin ice physics. While the Ising anisotropy that originates from crystal electric field (CEF) effects is a key ingredient to the spin ice state, it has only been the subject of a select few investigations [1,2]. Here, taking advantage of recent advances in the instrumentation of time-of-flight neutron spectroscopy, we take a closer look at the crystal electric field scheme of Ho₂Ti₂O₇. In doing so, we observe the splitting of a high energy CEF excitation, a feature that could not be observed in previous neutron scattering works due to lower energy resolution. We show that this split excitation cannot be accounted for by either a pure CEF excitation or by a pure phonon excitation. After ruling out several conventional origins for the splitting, we show that its origin is a magneto-elastic coupling induced vibronic bound state, which is a hybridized excitation resulting from the entanglement of a phonon and a crystal field excitation [3]. [1] S. Rosenkranz et al., J. Appl. Phys. 87, 5914 (2000), [2] M. Ruminy et al., Phys. Rev. B 94, 024430 (2016), [3] P.Thalmeier and P.Fulde, Phys. Rev. Lett. 49, 1588(1982)