LaAlO3 stoichiometry found key to electron liquid formation at LaAlO3/SrTiO3 interfaces (1303.5352v1)

Abstract: Emergent phenomena, including superconductivity and magnetism, found in the two-dimensional electron liquid (2-DEL) at the interface between the insulators LaAlO3 and SrTiO3 distinguish this rich system from conventional two-dimensional electron gases at compound semiconductor interfaces. The origin of this 2-DEL, however, is highly debated with focus on the role of defects in the SrTiO3 while the LaAlO3 has been assumed perfect. Our experiments and first principles calculations show that the cation stoichiometry of the nominal LaAlO3 layer is key to 2-DEL formation: only Al-rich LaAlO3 results in a 2-DEL. While extrinsic defects including oxygen deficiency are known to render LaAlO3/SrTiO3 samples conducting, our results show that in the absence of such extrinsic defects, an interface 2-DEL can form. Its origin is consistent with an intrinsic electronic reconstruction occurring to counteract a polarization catastrophe. This work provides a roadmap for identifying other interfaces where emergent behaviors await discovery.

Stoichiometry in LaAlO/SrTiO Interfaces: Emergent Electron Liquid Phenomena

The paper presented in the paper focuses on the emergent phenomena associated with the two-dimensional electron liquid (2-DEL) found at the LaAlO/SrTiO interfaces. Conventionally, these interfaces have been sites of interest due to their unique electronic properties, such as magnetism and superconductivity, distinct from those observed in compound semiconductor interfaces. The focus lies in deciphering the fundamental mechanisms leading to the formation of a 2-DEL, emphasizing the significance of LaAlO stoichiometry, specifically its aluminum-rich composition as a pivotal factor.

Experimental and Computational Insights

The experimental approach was crucial in establishing a clear relationship between the La/Al ratio and the conduction properties at the interface. Utilizing molecular-beam epitaxy (MBE) rather than pulsed-laser deposition (PLD) allowed for fine control over stoichiometry, minimizing extrinsic defect formations such as oxygen vacancies. The paper reveals that only when the LaAlO layer is Al-rich does the interface exhibit conductive properties, including a sharp insulator-to-metal transition observed at a critical La/Al ratio of 0.97±0.03.

Density functional theory (DFT) calculations provide further understanding of defect formation within the LaAlO layer, demonstrating that for stoichiometries with La/Al less than unity, aluminum substitutes for lanthanum without charge transfer, facilitating 2-DEL formation. These intrinsic defects are essential for understanding conductivity apart from the extrinsic defects previously considered.

Elimination of Extrinsic Mechanisms

The research effectively eliminates four major proposed pathways of extrinsic conduction: oxygen vacancies due to insufficient oxidizing environment, surface region vacancies from substrate preparation, high-energy species-induced defects during growth, and chemical intermixing leading to La-doped SrTiO which is inherently conductive. The coherent interfaces and homogeneity of the substrate preparation were instrumental in ruling out these defect-driven conduction mechanisms, underscoring the intrinsic electronic reconstruction prompted by the stoichiometry as the source of 2-DEL.

Implications and Future Directions

The findings hold significant implications for the design and development of novel electronic materials. The intrinsic nature of the electronic reconstruction, dictated by LaAlO stoichiometry, suggests potential application in other oxide interfaces, broadening the scope of research in functionalities such as superconductivity and magnetism at oxide heterointerfaces, independent of SrTiO.

By asserting cation stoichiometry as the key driver for emergent electron liquid formation, the research deepens understanding within this domain, establishing a pathway for strategic manipulation of interface properties for technological advances in electronic devices. Future work may involve exploring similar stoichiometric dependencies in other material systems and interfaces to expand the catalog of materials exhibiting 2-DEL characteristics, with potential applications in quantum computing and advanced sensor technologies.

Overall, the paper contributes a crucial piece to the puzzle of understanding emergent electronic states in oxide interfaces, highlighting the pivotal role of intrinsic stoichiometric alterations and laying groundwork for advancements in functional oxide electronics.