We report microwave spectroscopy of 167Er3+ doped in CaWO4 which reveals the hyperfine splitting of the erbium electronic ground state (Z1, Jeff.=15/2) induced by the I=7/2 nuclear spin. From spectra measured below∼50 mK in magnetic fields up to 200 mT, we extract spin Hamiltonian parameters including the electron g, hyperfine A, and nuclear electric quadrupolar Q tensors. Crucially, our analysis demonstrate unambiguously, that the previously unobserved nuclear electric quadrupolar moment is essential to reproduce the experimental data. With these refined parameters, we identify zero first-order Zeeman (ZEFOZ) transitions at zero magnetic field. Extending the analysis to finite fields, we uncover that ZEFOZ points lie either along the c axis or within the a-b plane. These results establish CaWO4 as a promising host for long lifetime quantum memories.
@article{arxiv.2601.16362,
title = {Nuclear quadrupole interaction and zero first-order Zeeman transitions of $^{167}$Er$^{3+}$ in CaWO$_4$},
author = {Lewin Marsh and Yikai Yang and Cesare Mattiroli and Mikhael T. Sayat and Đàm Minh Trí and Henrik M. Rønnow and Jevon J. Longdell and Jian-Rui Soh},
journal= {arXiv preprint arXiv:2601.16362},
year = {2026}
}