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Polarons in dielectric crystals play a crucial role for applications in integrated electronics and optoelectronics. In this work, we use density-functional theory and Green's function methods to explore the microscopic structure and spectroscopic signatures of electron polarons in lithium niobate (LiNbO_{3}). Total-energy calculations and the comparison of calculated electron paramagnetic resonance data with available measurements reveal the formation of bound polarons at Nb_{Li} antisite defects with a quasi-Jahn-Teller distorted, tilted configuration. The defect-formation energies further indicate that (bi)polarons may form not only at Nb_{Li} antisites but also at structures where the antisite Nb atom moves into a neighboring empty oxygen octahedron. Based on these structure models, and on the calculated charge-transition levels and potential-energy barriers, we propose two mechanisms for the optical and thermal splitting of bipolarons, which provide a natural explanation for the reported two-path recombination of bipolarons. Optical-response calculations based on the Bethe-Salpeter equation, in combination with available experimental data and new measurements of the optical absorption spectrum, further corroborate the geometries proposed here for free and defect-bound (bi)polarons.

In recent years, spin-orbit coupling has attracted significant attention due to its promising applications in spintronic devices. In solid-state spin qubits, the spin-orbit coupling allows for the lifting of spin degeneracy in the absence of an external magnetic field. Such spin-orbit driven zero-field splitting can be directly tuned by external electric fields. Here we present a reliable theoretical framework to address this phenomenon in extended periodic systems. We unravel the microscopic origin of the zero-field splitting in light-element semiconductors and propose its implications for coherent electrical control. The reported theoretical results open up promising possibilities for a rational design and tuning of high-spin centers suitable for quantum information processing.

An accurate description of the two-electron density, crucial for magnetic coupling in spin systems, provides in general a major challenge for density functional theory calculations. It affects, e.g., the calculated zero-field splitting (ZFS) energies of spin qubits in semiconductors that frequently deviate significantly from experiment. In the present work, (i) we propose an efficient and robust strategy to correct for spin contamination in both extended periodic and finite-size systems, (ii) verify its accuracy using model high-spin molecules, and finally, (iii) apply the methodology to calculate accurate ZFS of spin qubits (NV^{−} centers, divacancies) in diamond and silicon carbide. The approach is shown to reduce the dependence on the used exchange-correlation functional to a minimum.