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The authors characterize the spin Hamiltonian, crystal-field levels, and electric field effects of $\text{Dy}^{3+}$ dopants in $\text{CaWO}_4$ single crystals across four orders of magnitude in concentration using high-precision electron paramagnetic resonance (EPR). By jointly constraining crystal-field parameters with both optical data and measured $g$-tensors, they identify the ground doublet as an anticrossing between $|\pm11/2\rangle$ and $|\mp13/2\rangle$ states and resolve longstanding ambiguities in the optical multiplet structure. The measurements uncover an exceptionally large electric-field coupling alongside quadratic Stark broadening, both driven by a dense crystal-field structure and a narrow $\sim 20\text{ cm}^{-1}$ gap to the nearest opposite-symmetry doublet.
$\text{Dy}^{3+}$ in $\text{CaWO}_4$ exhibits an electric-field coupling three times larger than $\text{Er}^{3+}$—the highest ever measured for a rare-earth ion in this host—revealing how dense $J$-multiplet structures can dramatically amplify Stark susceptibility in solid-state spin systems.
We report an electron paramagnetic resonance (EPR) study of Dy${^3+}$ in CaWO$_4$ single crystals spanning dopant concentrations from 80~ppb to 247~ppm. The $g$ factors of the ground Kramers doublet at the $S_4$ point-group site, $g_\parallel = 7.22 \pm 0.01$ and $g_\perp = 5.45 \pm 0.01$, and the hyperfine constants of \ce{^{161}Dy} and $^{163}$Dy are determined with an order-of-magnitude improvement in precision over the only previous report. A crystal-field analysis constrained jointly by published optical levels and by the measured $g$ tensor identifies the ground doublet as the lower branch of an anticrossing between the near-degenerate $\|{\pm}11/2>$ and $\|{\mp}13/2>$ states, and explains why optical data alone left the $g$ values unconstrained. \add{The same parameter set, tested on the excited multiplets of the $^6H$ term, reproduces the ${}^6H_{13/2}$ crystal-field levels better than the optical parameters themselves.} The angular dependence of the linewidth in the $ab$ plane reveals broadening by random internal electric fields through the linear electric field effect. Calibrating these fields with the residual Er$^{3+}$ present in the same crystals yields the first electric-field coupling parameters of Dy$^3+$, $(B_{31}^2+B_{36}^2)^{1/2} = (34\pm5)\times10^{-6}$~cm/V, three times the Er$^{3+}$ value and the largest reported for a rare-earth ion in this host.The residual linewidth and the concentration-dependent lineshape asymmetry follow the quadratic field scaling expected for a second-order Stark shift, which escapes the inversion-site cancellation constraining the linear effect. Both couplings trace back to the $\sim 20$~cm$^{-1}$ gap to the first opposite-symmetry doublet, a consequence of the dense crystal-field structure of the $J=15/2$ manifold.