Probing Quantum Confinement and Electronic Structure at Polar Oxide Interfaces

Polar discontinuities occurring at interfaces between two materials constitute both a challenge and an opportunity in the study and application of a variety of devices. In order to cure the large electric field occurring in such structures, a reconfiguration of the charge landscape sets in at the interface via chemical modifications, adsorbates, or charge transfer. In the latter case, one may expect a local electronic doping of one material: one example is the two-dimensional electron liquid (2DEL) appearing in SrTiO3 once covered by a polar LaAlO3 layer. Here, it is shown that tuning the formal polarization of a (La,Al)1−x(Sr,Ti)xO3 (LASTO:x) overlayer modifies the quantum confinement of the 2DEL in SrTiO3 and its electronic band structure. The analysis of the behavior in magnetic field of superconducting field-effect devices reveals, in agreement with ab initio calculations and self-consistent Poisson–Schrödinger modeling, that quantum confinement and energy splitting between electronic bands of different symmetries strongly depend on the interface total charge densities. These results strongly support the polar discontinuity mechanisms with a full charge transfer to explain the origin of the 2DEL at the celebrated LaAlO3/SrTiO3 interface and demonstrate an effective tool for tailoring the electronic structure at oxide interfaces.

两种材料界面处存在的极不连续性（polar discontinuities），在各类器件的研究与应用中既是挑战亦是机遇。为抑制此类结构中出现的强电场，可通过化学修饰、吸附物或电荷转移在界面处实现电荷分布的重构。在后述情形中，可预期实现单一材料的局域电子掺杂：典型案例为极性铝酸镧（LaAlO3）层覆盖的钛酸锶（SrTiO3）中出现的二维电子液体（two-dimensional electron liquid, 2DEL）。本文研究表明，调控(La,Al)1-x(Sr,Ti)xO3（LASTO:x）覆盖层的形式极化，可改变钛酸锶中二维电子液体的量子限制效应及其电子能带结构。通过对超导场效应器件的磁场响应特性分析，并结合从头算（ab initio）与自洽泊松-薛定谔（Poisson–Schrödinger）模拟验证，研究发现量子限制效应与不同对称性电子能带间的能级劈裂，强烈依赖于界面的总电荷密度。上述结果有力支持了通过完全电荷转移来解释经典LaAlO3/SrTiO3界面二维电子液体起源的极不连续性机制，并证明了一种可用于调控氧化物界面电子结构的有效手段。