Session X19: Invited Session: Two Dimensional Electron Systems at Oxide Interfaces

Electronic structure of the interfacial LaAlO$_3$/SrTiO$_3$ 2D electron gas

The interface between LaAlO$_3$ and SrTiO$_3$, two good band insulators, was found in 2004 to be conducting with a high mobility [1] and, in some doping range, superconducting with a maximum critical temperature of about 200~mK [2,3]. I will describe recent experiments aiming at determining the origin of the electron gas. I will then discuss the transport properties of high mobility samples that display Shubnikov de Haas (SdH) oscillations [4]. In such high mobility samples, electric field tuning of the carrier density allows the electronic structure to be followed through analysis of the evolution of the SdH oscillations. \\[4pt] [1] A. Ohtomo, H. Y. Hwang, Nature 427, 423 (2004).\\[0pt] [2] N. Reyren, S. Thiel, A. D. Caviglia, L. Fitting Kourkoutis, G. Hammerl, C. Richter, C. W. Schneider, T. Kopp, A.-S. Ruetschi, D. Jaccard, M. Gabay, D. A. Muller, J.-M. Triscone and J. Mannhart, Science 317, 1196 (2007).\\[0pt] [3] A. Caviglia, S. Gariglio, N. Reyren, D. Jaccard, T. Schneider, M. Gabay, S. Thiel, G. Hammerl, J. Mannhart, and J.-M. Triscone, Nature 456, 624 (2008).\\[0pt] [4] A.D. Caviglia, S. Gariglio, C. Cancellieri, B. Sac\'ep\'e, A. F\^ete, N. Reyren, M. Gabay, A.F. Morpurgo, J.-M. Triscone, Physical Review Letters 105, 236802 (2010).   

Fundamental Properties of the 2-D Electron Liquid Generated by LaAlO$_{3}$-SrTiO$_{3}$ Interfaces

Extraordinary electron systems can be generated at well-defined interfaces between complex oxides. Much more so than the 2-D electron gases formed at interfaces between conventional semiconductors, the electron systems at oxide interfaces may be shaped by the character of the underlying ionic lattice and be characterized by substantial correlations. Focusing on the electron liquid produced by n-type LaAlO$_{3}$-SrTiO$_{3}$ interfaces, I will present our studies of such structures and discuss in particular our results pertaining to the magnetism, superconductivity, and negative electronic compressibility of these systems.   

Gate control of the mobility, carrier density and superconductivity at the LaAlO$_3$/SrTiO$_3$ interface

The conductivity at the LaAlO$_3$/SrTiO$_3$ interface [1], and in particular its control with a back-gate electrode in the superconducting regime [2], offers a powerful route to explore the 2D superconductor-insulator transition. An essential requirement is to fully understand how the gate voltage changes the various characteristic properties of the system, such as the mobility, carrier density, critical temperature, critical fields and superfluid density. One important point, for example, is that with back-gating the Hall mobility variation is significantly larger than the change in sheet carrier density [3]. These results indicate that the relative disorder strength increases across the superconductor-insulator transition, and that disorder is the primary control parameter associated with back-gating. I will discuss how this 2D superconductor-insulator transition can be understood, in analogy to thickness variations in other more conventional systems, and from studies of symmetrically confined 2D superconductivity in STO [4,5]. The importance of the strong dielectric nonlinearity in STO at low temperatures will be stressed, leading to the result that the gating phase diagram is a nonlinear function of the starting free carrier density. Our collaborative efforts using real space imaging of the superfluid density by scanning SQUID microscopy, offering an extremely powerful complementary tool to transport studies, will also be discussed, as well as the relationship between the superconductivity and the ferromagnetism in this fascinating system [6]. \\[4pt] [1] A. Ohtomo and H. Y. Hwang, Nature {\bf 427}, 423 (2004).\\[0pt] [2] A. D. Caviglia $et$ $al.$, Nature {\bf 456}, 624 (2008).\\[0pt] [3] C. Bell $et$ $al.$, Phys. Rev. Lett. {\bf 103}, 226802 (2009).\\[0pt] [4] Y. Kozuka $et$ $al.$, Nature {\bf 462}, 487 (2009).\\[0pt] [5] M. Kim $et$ $al.$, arxiv/1106.5193 (2011). \\[0pt] [6] J. Bert $et$ $al.$, Nature Phys. {\bf 7}, 767 (2011).   

Electronic and structural correlations at 2DEG oxide heterointerfaces

The formation of a two-dimensional electron gas (2DEG) at complex oxide interfaces is directly influenced by the rich electronic and structural characteristics of the bulk oxides, as well as new phenomena arising at the interface. We investigated how local correlations control oxide 2DEGs by inserting a single atomic layer of a rare-earth oxide (RO) [(R is lanthanum (La), praseodymium (Pr), neodymium (Nd), samarium (Sm), or yttrium (Y)] into an epitaxial SrTiO$_3$ matrix using pulsed-laser deposition with atomic layer control [1]. We find that structures with La, Pr, and Nd ions result in conducting 2DEGs at the inserted layer, whereas the structures with Sm or Y ions are insulating. Our local spectroscopic results, and our theoretical results, indicate that the interfacial conductivity is dependent on electronic and structural correlations that decay spatially into the SrTiO$_3$ matrix and are determined by the local rare-earth ion. We also find that at least the structural correlations play a role in the properties of the 2DEG at LaAlO$_3$/SrTiO$_3$ interfaces. We used different lattice constant single-crystal substrates to produce LaAlO$_3$/SrTiO$_3$ interfaces with controlled levels of biaxial epitaxial strain [2], finding that tensile-strained SrTiO$_3$ destroys the conducting 2DEG, while compressively strained SrTiO$_3$ retains the 2DEG, but with reduced carrier concentration. In addition, the critical LaAlO$_3$ overlayer thickness for 2DEG formation increases with SrTiO$_3$ compressive strain. Our first-principles calculations suggest that a strain-induced electric polarization in the SrTiO$_3$ layer, stabilized by the LaAlO$_3$ overlayer, is responsible for this behavior. This work was done in collaboration with H.W. Jang, C.W. Bark, D.A. Felker, T. Hernandez, Y. Wang, M.K. Niranjan, C.T. Nelson, Y. Zhang, D. Su, C.M. Folkman, S.H. Baek, S. Lee, K. Janicka, H. Zhou, Y. Zhu, X.Q. Pan, D.D. Fong, E.Y. Tsymbal, C. B. Eom. This work was supported by the National Science Foundation through grant DMR-0906443.\\[4pt] [1] H.W. Wang et al., Science {\bf 331}, 886 (2011). \newline [2] C.W. Bark et al., P. Natl Acad Sci USA {\bf 108}, 4720 (2011).   

Electronic structure and two-dimensional electron gas at the surface of SrTiO$_{3}$

Similar to silicon that is the basis of conventional electronics, strontium titanate (SrTiO$_{3})$ is the bedrock of the emerging field of oxide electronics. SrTiO$_{3}$ is the preferred template to create exotic two-dimensional (2D) phases of electron matter at oxide interfaces, exhibiting metal-insulator transitions, superconductivity, large magnetoresistance, or coexistence of superconductivity and ferromagnetism. However, the physical nature of the electronic structure underlying these 2D electron gases (2DEGs) remains elusive, although its determination is crucial to understand their remarkable properties. In this talk, we present our angle-resolved photoemission spectroscopy (ARPES) results showing that there is a highly metallic universal 2DEG at the vacuum-cleaved surface of SrTiO$_{3}$, independent of bulk carrier densities over more than seven decades, including the undoped insulating material [A. F. Santander-Syro \textit{et al}. Nature \textbf{469}, 189-193 (2011)]. Our data unveil a remarkable electronic structure consisting on multiple subbands of heavy and light electrons. We find that the 2DEG is confined within a region of \textit{$\sim $}5 unit cells and has a sheet carrier density of \textit{$\sim $}0$.$33 electrons per $a^{2}$ ($a $ is the cubic lattice parameter). The similarity of this 2DEG with those reported in SrTiO$_{3}$-based heterostructures and field-effect transistors suggests that different forms of electron confinement at the surface of SrTiO$_{3}$ lead to essentially the same 2DEG. Our discovery provides thus a model system for the study of the electronic structure of 2DEGs in SrTiO$_{3}$-based devices and a novel route to generate 2DEGs at surfaces of other functional oxides.