Investigation of the structure of the ground state of lithium donor centers in silicon enriched in $^{28}$Si isotope and the influence of internal strain in the crystal on this structure

A study of silicon enriched in the $^{28}$Si isotope with a low oxygen content ($N\approx$ 2 $\times$ 10$^{14}$ cm$^{-3}$) at low temperatures ($T$ = 3.8 K) has revealed a set of electron spin resonance (ESR) spectra with anisotropic $g$ factors associated with an isolated Li donor center, whose lines are assigned to triplet and doublet states. The spectra were investigated without and with application of external load to the sample and their $g$ factors were found to be smaller than 2 ($g <$ 2.000), which distinguishes them from the previously obtained spectra. Based on theoretical and experimental estimates made within the spin-Hamiltonian analysis of the spectra, the states of donor electrons of lithium and their $g$ factors are found to depend strongly on internal strains in the crystal and intervalley spin-orbit interactions.