Electron Confinement and Magnetism of (LaTiO₃)₁/(SrTiO₃)₅ Heterostructure: A Diffusion Quantum Monte Carlo Study

We have applied the diffusion quantum Monte Carlo (DMC) method to study the electron confinement and magnetic structure in the (LaTiO₃)₁/(SrTiO₃)₅ heterostructure. The DMC results were compared with various density functional theory (DFT) methods, including local density approximation (LDA), generalized gradient approximation (GGA), LDA+U, and GGA+U, as well as the recently proposed strongly constrained appropriately normed (SCAN) and van der Waals-Bayesian error estimation functional (vdW-BEEF). We found that many-body correlations are crucial to accurately describe the localization of the two-dimensional (2D) electron gas around the lanthanum planes. DMC predicts 20% more electron density within the first interfacial titanium layer in (LaTiO₃)₁/(SrTiO₃)₅ than LDA+U, suggesting that the degree of confinement of the 2D electron gas in the interfacial region is underestimated with semilocal DFT approximations. DMC yields the ferromagnetic (FM) state as the ground state of (LaTiO₃)₁/(SrTiO₃)₅ and the antiferromagnetic (AFM) and nonmagnetic (NM) states that are higher in energy by 37(15) and 238(15) meV per lanthanum atom at the interface, respectively. Most DFT methods yield the FM and NM states within less than 25 meV in energy and could not find the AFM state.