Direct observation of a nuclear spin excitation in Ho2Ti2O7

A single nondispersive excitation is observed by means of neutron backscattering, at E0=26.3μeV in the spin ice Ho2Ti2O7 but not in the isotopically enriched Dy2162Ti2O7 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 ∼65K. The application of a magnetic field up to μ0H=4.5T, 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.