Light-hole states and hyperfine interaction in electrically-defined Ge/GeSn quantum dots

We theoretically investigate hole spins confined in a gate-defined quantum dot (QD) embedded in GeSn/Ge/GeSn quantum well (QW) structure. Owing to the tensile strain in the Ge layer, the system effectively realizes a light-hole qubit. We systematically study how various morphological parameters influence the energy spectrum and the hyperfine coupling to the nuclear spin bath. The simulations are carried out using a realistic, fully atomic sp3d5s∗ tight-binding model. We also perform complementary DFT calculations of wave functions near the atomic cores and use them to parameterize the hyperfine-interaction Hamiltonian. We evaluate the Overhauser field fluctuations and demonstrate that the strength of the hyperfine coupling for the lowest hole doublet crucially depends on the Sn content in the barrier. We highlight the conduction-valence band mixing, which leads to considerable 𝑠-type admixtures to the hole states, providing the dominant channel of hyperfine coupling due to the Fermi contact interaction.