Spin susceptibility and effective mass of two-dimensional electrons in Mg_xZn_1−xO/ZnO heterostructures

We report measurements of the spin susceptibility and the electron effective mass for two-dimensional electrons confined at the interfaces of Mg_xZn_1−xO/ZnO single heterostructures (x=0.05, 0.08, and 0.11), grown by molecular-beam epitaxy on (0001) ZnO substrates. By tuning the built-in polarization through control of the barrier composition, the electron density was systematically varied in the range of 5.6×10¹¹–1.6×10¹² cm⁻², corresponding to a range of 3.1≤r_s≤5.2, where r_s is the average electron spacing measured in units of the effective Bohr radius. We used the coincidence technique, where crossings of the spin-split Landau levels occur at critical tilt angles of magnetic field, to evaluate the spin susceptibility. In addition, we determined the effective mass from the temperature dependence of the Shubnikov–de Haas oscillations measured at the coincidence conditions. The susceptibility and the effective mass both gradually increase with decreasing electron density, reflecting the role of electron-electron interaction.