Coupling between CaWO$_4$ phonons and Er$^{3+}$ dopants

TL;DR

This study combines experimental and theoretical methods to analyze phonon modes in CaWO$_4$, revealing specific vibrational modes that interact with Er$^{3+}$ ions, crucial for quantum memory applications.

Contribution

The paper provides a detailed microscopic understanding of phonon modes in CaWO$_4$ and their coupling to Er$^{3+}$ dopants, aiding quantum memory development.

Findings

Phonon dispersion extends up to 130 meV with a gap between 60-80 meV.

Identified eight Raman-active modes coupling to Er$^{3+}$.

A low-energy $B_g$ mode at 9.1 meV influences spin-lattice relaxation.

Abstract

We investigate the lattice dynamics of CaWO$_4$, a promising host crystal for erbium-based quantum memories, using inelastic neutron scattering together with density-functional perturbation theory. The measured phonon dispersion along the (100), (001), and (101) reciprocal space direction reveals phonon bands extending up to 130 meV, with a gap between 60 and 80 meV, in good agreement with our calculations. From a symmetry analysis of the phonon eigenmodes, we identify eight Raman-active modes that can couple directly to the Er$^{3+}$ crystal-field operators, including a low-energy $B_g$ mode at 9.1 meV that is expected to play a dominant role in phonon-assisted spin-lattice relaxation. These results provide a microscopic description of the phonon bath in CaWO$_4$ and establish a basis for engineering phononic environments to mitigate the loss of stored quantum states and optimize Er-doped CaWO$_4$ for quantum-memory applications.