Chemical Origins of Optically Addressable Spin States in Eu(PS) and Eu(PSe).

Lanthanide materials with a 4f electron configuration (S) offer an exciting system for realizing multiple addressable spin states for qubit design. While the S ground state of 4f free ions displays an isotropic character, breaking degeneracy of this ground state and excited states can be achieved through local symmetry of the lanthanide and the choice of ligands. This makes Eu attractive as it mirrors Gd in exhibiting the S ground state, capable of seven spin-allowed transitions. In this work, we identify Eu(PS) and Eu(PSe) as viable candidates for optically addressable spin states. The materials feature paramagnetic behavior at 2.0 ≤ ≤ 400 K and μ = 0.01 and 7 T. The field-dependent magnetization () curve reveals a single-ion spin with effective magnetic moments comparable to the expected magnetic moment of Eu. Seven well-defined narrow peaks in the excitation and emission spectra of Eu are resolved. Phonon contributions to the Eu spin environment are evaluated through heat capacity measurements. Insights into how the spin-polarized band structure and density of states of the materials influence the physical properties are described by using density functional theory calculations. These results present a foundational study of Eu(PS) and Eu(PSe) as a feasible platform for harnessing the spin, charge, orbital, and lattice degrees of freedom of Eu for qubit design.