Direct Observation of a Nuclear Spin Excitation in Ho₂Ti₂O₇

A single nondispersive excitation is observed by means of neutron backscattering, at E₀=26.3  μeV in the spin ice Ho₂Ti₂O₇ but not in the isotopically enriched ¹⁶²Dy₂Ti₂O₇ analogue. The intensity of this excitation is rather small, ≲0.2% of the elastic intensity. It is clearly observed below 80 K but resolution limited only below ∼65  K. The application of a magnetic field up to μ₀H=4.5  T, at 1.6 K, has no measurable effect on the energy or intensity. This nuclear excitation is believed to perturb the electronic, Ising spin system resulting in the persistent spin dynamics observed in spin ice compounds.