Structural Basis for the Conducting Interface between LaAlO₃ and SrTiO₃

The complete atomic structure of a five-monolayer film of LaAlO₃ on SrTiO₃ has been determined for the first time by surface x-ray diffraction in conjunction with the coherent Bragg rod analysis phase-retrieval method and further structural refinement. Cationic mixing at the interface results in dilatory distortions and the formation of metallic La_1-xSr_xTiO₃. By invoking electrostatic potential minimization, the ratio of Ti⁴⁺/Ti³⁺ across the interface was determined, from which the lattice dilation could be quantitatively explained using ionic radii considerations. The correctness of this model is supported by density functional theory calculations. Thus, the formation of a quasi-two-dimensional electron gas in this system is explained, based on structural considerations.