Novel two-dimensional electron systems at the surface of functional oxides

Transition-metal oxides (TMOs) show remarkable properties, not found in standard semiconductors, such as high-temperature superconductivity, photo-catalytic capacity, or metal-to-insulator transitions. The realization of two-dimensional electron gases (2DEGs) in TMOs is crucial for harnessing the functionalities of these materials for future applications. Additionally, such 2DEGs offer the possibility to explore new physics emerging from the combined effects of electron correlations and low-dimensional confinement.

We found that 2DEGs can be simply realized at the surface of various insulating TMOs, such as the quantum paraelectric SrTiO3 [1], the strong spin-orbit coupled KTaO3 [2], or the photo-catalyst TiO2 [3]. I will show how the choice of the surface termination allows tailoring the electronic structure and symmetries of these 2DEGs [4-5], paving the way for the quest of topological states in correlated oxides. Furthermore, I will discuss our observation of magnetism in the 2DEG at the oxygen-deficient surface of SrTiO3 [6]. I will then describe our recent development of a simple method to fabricate these 2DEGs in several other oxides, such as the ferroelectric BaTiO3 [7] or the semiconductor ZnO, which turns out to be a textbook case of a 2D Fermi liquid coupled to a Debye distribution of phonons [8]. This novel fabrication technique allows engineering functional interfaces between the 2DEG and, e.g., tunable magnetic layers [9], and measuring their transport characteristics [10] –and is thus promising for applications.

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• [8] T. C. Rödel et al., Phys. Rev. Mater. 2, 051601(R) (2018).
• [9] P. Lömker et al., Phys. Rev. Mater. 1, 062001(R) (2017).
• [10] S. Sengupta et al. J. Appl. Phys. 124, 213902 (2018).