Probing Quantum Confinement and Electronic Structure at Polar Oxide Interfaces

A Two-Dimensional Superconducting Electron Gas in Freestanding LaAlO3/SrTiO3 Micromembranes

Freestanding oxide membranes constitute an intriguing material platform for new functionalities and allow integration of oxide electronics with technologically important platforms such as silicon. Sambri et al. recently reported a method to fabricate freestanding LaAlO₃/SrTiO₃ (LAO/STO) membranes by spalling of strained heterostructures. Here, we first develop a scheme for the high-yield fabrication of membrane devices on silicon. Second, we show that the membranes exhibit metallic conductivity and a superconducting phase below ∼200 mK. Using anisotropic magnetotransport we extract the superconducting phase coherence length ξ ≈ 36-80 nm and establish an upper bound on the thickness of the superconducting electron gas d ≈ 17-33 nm, thus confirming its two-dimensional character. Finally, we show that the critical current can be modulated using a silicon-based backgate. The ability to form superconducting nanostructures of LAO/STO membranes, with electronic properties similar to those of the bulk counterpart, opens opportunities for integrating oxide nanoelectronics with silicon-based architectures.

A spin-orbit coupling-induced two-dimensional electron gas in BiAlO3/SrTiO3 heterostructures

Both LaAlO₃ and BiAlO₃ are isostructural, isoelectronic and band insulators. Therefore, in analogy to the LaAlO₃/SrTiO₃ heterostructure, a quasi two dimensional electron gas (q-2DEG) could be anticipated in BiAlO₃/SrTiO₃ heterostructures. Our density functional theory based scalar relativistic calculations show that BiAlO₃/SrTiO₃ heterostructures remain insulating for a BiAlO₃ film thickness up to 5 unit cells. However, with spin orbit coupling included in the crystal Hamiltonian, we find a thickness dependent insulator to metal transition for BiAlO₃/SrTiO₃ heterostructures. However, unlike the Ti³⁺/Ti⁴⁺ electronic reconstruction in LaAlO₃/SrTiO₃, the conductivity in BiAlO₃/SrTiO₃ is found to originate from the subsurface Bi 6p states. The results suggest that the properties of q-2DEG in BiAlO₃/SrTiO₃ can be controlled using an external electric field, leading to a wide range of solid state applications.

Electron Trapping Mechanism in LaAlO_{3}/SrTiO_{3} Heterostructures

In LaAlO_{3}/SrTiO_{3} heterostructures, a still poorly understood phenomenon is that of electron trapping in back-gating experiments. Here, by combining magnetotransport measurements and self-consistent Schrödinger-Poisson calculations, we obtain an empirical relation between the amount of trapped electrons and the gate voltage. The amount of trapped electrons decays exponentially away from the interface. However, contrary to earlier observations, we find that the Fermi level remains well within the quantum well. The enhanced trapping of electrons induced by the gate voltage can therefore not be explained by a thermal escape mechanism. Further gate sweeping experiments strengthen that conclusion. We propose a new mechanism which involves the electromigration and clustering of oxygen vacancies in SrTiO_{3} and argue that such electron trapping is a universal phenomenon in SrTiO_{3}-based two-dimensional electron systems.