To the fundamental questions arising in the study of topological insulators (TI) belongs their behavior near localized extrinsic perturbances such as magnetic impurities and impurity layers, as well as their competition with other phases involving magnetic or orbital ordering. Here we propose to study the properties of TIs using a combination of resonant x-ray absorption spectroscopy (XAS) and reflectivity (RXR), which together allow element specific sensitivity significantly below the 0.1%-range, a depth resolution below 1 nm, resulting in interface sensitivity, and a probing depth of hundreds of nanometers. The technique will be used to obtain both chemical and electronic (density, orbital occupation, magnetism) depth profiles. RXR is well suited to both study strongly correlated systems and the behavior of a weakly correlated system at the localized perturbances mentioned above. It is thus complementary to other powerful techniques employed within the proposed CRC such as photoemission (projects A01, A08) and scanning tunneling spectroscopy (A02), which can directly map the topological state and address itinerant properties, but are strictly limited to the surface.
We plan to focus on two main topics: First, we will contribute to the realization of a TI in transition-metal-oxide (TMO) heterostructures based on LaAlO3 / SrTiO3 and LaNiO3, where correlations are predicted to result in the formation of a topological state in films with a surface normal along the (111) crystallographic direction. Corresponding structures are being grown within project C08. Of key importance is the orbital occupation and polarization within the honeycomb lattice formed by the transition-metal ions in the (111) planes. We will use our expertise in RXR to probe exactly this aspect. The orbital occupation is also at the heart of published and ongoing theoretical work by two theoretical groups participating in the CRC (C07, and Sangiovanni and Trauzettel), close collaboration is therefore warranted. RXR is particularly well suited, because it is also sensitive to predicted competing correlated phases in (111) systems such as magnetic and nematic phases in LaNiO3 heterostructures. The identification of the latter is interesting in its own regard, but also since it will set boundary conditions for the realization of topological phases.
Second, taking advantage of the probing depth and the nm depth resolution of our method we plan to study the spatial distribution and the interaction of incorporated Mn, Cr and Fe impurities and buried layers with the gapless near-surface states in HgTe, α-Sn and (Bi,Sb)2(Se,Te)3 (projects A04, A08, B01). This interaction is predicted to manifest itself in correlation effects such as RKKY-mediated ferromagnetism, and is related to the reported occurrence of a quantum anomalous Hall effect. The latter interpretation, however, crucially depends on the homogeneous distribution of the magnetic impurities and would not be tenable if impurities were clustered. To obtain a full understanding, we will closely collaborate with theoretical colleagues (project A06).
Finally, our method will allow us to provide valuable feedback on the interface quality, purity and homogeneity to the projects dealing with growth.
[C04.9] E. Benckiser, M. W. Haverkort, S. BrÃĳck, E. Goering, S. Macke, A. Frano, X. Yang, O. K. Andersen, G. Cristiani, H. U. Habermeier, A. V. Boris, I. Zegkinoglou, P. Wochner, H. J. Kim, V. Hinkov, and B. Keimer, Orbital reflectometry of oxide heterostructures, Nature Materials 10, 189 (2011).