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Novel two-dimensional electron gases at the surface of transition-metal oxides : from orbital ordering to orbital symmetry reconstruction.
Electronic states at surfaces or interfaces can lead to groundbreaking phenomena and applications. Paradigmatic examples are the field-effect semiconductor transistors, the quantum Hall effect, the topological insulators, and the two-dimensional electron gases (2DEGs) at interfaces between SrTiO3 (STO) and other insulating transition-metal oxides [1]. These interfaces display stunning properties, such as superconductivity or magnetoresistance. However, they are difficult to produce, and their electric properties depend strongly on the fabrication details. This has hampered the understanding of the physical origin of such 2DEGs and their generalization to other multifunctional oxides.
In this seminar, I will first present our recent discovery that a 2DEG can be simply realized at the vacuum-cleaved surface of STO (STO) [2]. We probed this state using light-polarized angle-resolved photoemission spectroscopy (LP-ARPES), a powerful technique that gives the band structure and symmetries at the surface of materials. Our data unveil an electronic structure consisting on multiple subbands of heavy and light electrons, confined within a region of 5 unit cells beneath the surface, and ordered by their bulk orbital symmetries. The similarity of this 2DEG with those reported in other STO-based interfaces suggests that different forms of electron confinement at the surface of SrTiO3 lead to essentially the same 2DEG. This provides a model system for the study of the electronic structure of 2DEGs in STO-based devices and a novel route to generate 2DEGs at surfaces of functional oxides.
In fact, the generalization of the 2DEG at the surface of STO to other perovskites would open the exciting possibility of using correlated and multifunctional oxides to realize new states of matter and oxide-based devices. Key to this program is to demonstrate that the interactions present in the bulk change the nature of the 2DEG.
In the second part of this seminar, I will show that a 2DEG can also be created at the surface of KTaO3, a wide-gap insulator with a strong spin-orbit coupling (SOC), more than one order of magnitude larger than in STO. Our data and analysis establish that the 2DEG at the surface of KTO is, in contrast to the previous cases of STO-based 2DEGs, a genuinely new physical system with respect to the bulk : the orbital symmetries of its subbands are entirely reconstructed and their masses are significantly renormalized. This stems from the strong spin-orbit interaction inherited from the bulk, which is comparable to the Fermi energy and subband splittings of the 2DEG. These results demonstrate that in transition-metal oxides the strong couplings between the active electronic degrees of freedom, combined with the electron confinement, can lead to novel electronic states at the surface that are not simple extensions of the bulk bands.
[1] A. Ohtomo and H. Y. Hwang, Nature 427, 423 (2004) ; C. H. Ahn, J.-M. Triscone and J. Mannhart, .Nature 424, 1015 (2003) ; C. Cen et al., Nature Mater. 7, 298-302 (2008) ; N. Reyren et al., Science 317, 1196-1199 (2006) ; K. Ueno et al., Nature Mater. 7, 855-858 (2008) ; A. Brinkman et al., Nature Mater. 6, 493-496 (2007).
[2] A. F. Santander-Syro et al., Nature 469, 189 (2011)