Magneto-elastic induced vibronic bound state in the spin ice pyrochlore Ho$_2$Ti$_2$O$_7$

TL;DR

This study uncovers a vibronic bound state in Ho$_2$Ti$_2$O$_7$ caused by magneto-elastic coupling, revealing a new interaction affecting its crystal field excitations and extending to related compounds.

Contribution

It introduces the discovery of a vibronic bound state in Ho$_2$Ti$_2$O$_7$ due to magneto-elastic coupling, with a detailed microscopic Hamiltonian modeling this effect.

Findings

Identification of a 3 meV splitting in the crystal field doublet

Observation of similar effects in Ho$_2$Ge$_2$O$_7$ and Ho$_2$Sn$_2$O$_7$

Development of a quantitative model for magneto-elastic coupling

Abstract

The single ion physics of Ho$_2$Ti$_2$O$_7$ 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 Ho$_2$Ti$_2$O$_7$ 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 Ho$_2$Ti$_2$O$_7$ but can also be found in its chemical pressure analogs, Ho$_2$Ge$_2$O$_7$ and Ho$_2$Sn$_2$O$_7$. 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.