Magnetoelastically induced vibronic bound state in the spin-ice pyrochlore Ho2Ti2O7

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 magnetoelastic coupling and provides a quantitative description of the inelastic neutron spectra.