The single ion physics of Ho2Ti2O7 is well-understood to produce strong Ising anisotropy, which is an essential ingredient to its low-temperature spin ice state. We present inelastic neutron scattering measurements on Ho2Ti2O7 that reveal a clear inconsistency with its established single ion Hamiltonian. Specifically, we show that a crystal field doublet near 60~meV is split by approximately 3~meV. Furthermore, this crystal field splitting is not isolated to Ho2Ti2O7 but can also be found in its chemical pressure analogs, Ho2Ge2O7 and Ho2Sn2O7. We demonstrate that the origin of this effect is a vibronic bound state, resulting from the entanglement of a phonon and crystal field excitation. We derive the microscopic Hamiltonian that describes the magneto-elastic coupling and provides a quantitative description of the inelastic neutron spectra.
@article{arxiv.1805.08909,
title = {Magneto-elastic induced vibronic bound state in the spin ice pyrochlore Ho$_2$Ti$_2$O$_7$},
author = {J. Gaudet and A. M. Hallas and C. R. C. Buhariwalla and G. Sala and M. B. Stone and M. Tachibana and K. Baroudi and R. J. Cava and B. D. Gaulin},
journal= {arXiv preprint arXiv:1805.08909},
year = {2018}
}