Electronic reconstruction in complex oxide multilayers explored with resonant soft x-ray reflectivity
In the recent years, progress in pulsed-laser deposition and molecular-beam epitaxy has enabled the growth of transition-metal oxide (TMO) heterostructures with well-controlled, atomically flat interfaces. Their orbital properties such as the occupation of the transition metal d-orbitals exert a key influence on the electronic phase behavior of TMOs ; in particular, orbital degrees of freedom are directly coupled to magnetic and transport properties. However, whereas spectroscopic probes of spin and charge degrees of freedom at metal-oxide interfaces are already available [2-4], it remains difficult to obtain information about orbital occupations directly at interfaces. A precise determination of the bulk-averaged orbital occupation is possible by x-ray linear dichroism (XLD) , that is, X-ray absorption resonantly tuned to the energy of a transition from a core level to the orbitals in question, supported by powerful sum rules for differently polarized X-rays. Orbital "reconstruction" (reoccupation compared to the bulk) is believed to occur at interfaces but could hitherto only be studied indirectly [5-7].
Resonant x-ray reflectometry (RXR) is an emerging technique which is ideally suited to address interfacial reconstruction phenomena. By studying the energy and angular dependence of reflected x-rays, one can extract depth resolved orbital occupations in oxide heterostructures (Fig. 1). Recently, using RXR we have derived quantitative, spatially resolved orbital occupation depth profiles in LaNiO3/LaAlO3 superlattices , demonstrated atomic-layer-resolved elemental concentration sensitivity [9,10], and probed polarity-induced electronic reconstruction effects in LaCoO3-based heterostructures . Very recently, we have used RXR to extract depth dependent charge density and orbital symmetry profiles at the LaAlO3/SrTiO3 interface. In this case, RXR measured at the Ti L2,3 resonance is extremely sensitive to the presence or absence of the highly studied 2D electron liquid at the interface (Fig. 2).
Within the MPG-UBC collaboration, we are in the ideal situation to be able to further develop RXR to probe orbital population profiles and orbital reconstruction at surfaces and buried interfaces: We operate a dedicated RXR instrument at the Canadian Light Source, which provides us with continuous access to beam time and expertise in resonant x-ray measurements from first hand. At the same time, the growth of high-quality TMO multilayers, e.g. comprising YBa2Cu3O6+x, La1-xCaxMnO3, LaNiO3 and SrRuO3 has been perfected at the MPI-FKF and MPI-Halle in the recent years. We also collaborate with the groups of Harold Hwang (Stanford), Suzanne Stemmer (UCSB), Ho-Nyung Lee (ORNL), and Guus Rijnders (U. Twente) to investigate their high-quality transition metal oxide heterostructures.
Finally, have developed multiple resonant x-ray simulation tools, which can fully model the resonant reflection and diffraction of x-rays. The programs ReMagX  and QUAD  can simulate resonant reflectivity and diffraction, respectively, for materials with arbitrary scattering tensors and including full dynamical effects. Recently we used QUAD to demonstrate the importance of dynamical effects in resonant diffraction, using a LaNiO3 superlattice as an example .
The work in this project is performed in the group of George Sawatzky at the Stewart Blusson Quantum Matter Institute of the University of British Columbia, in close collaboration with the department of Bernhard Keimer (MPI-FKF Stuttgart), Hao Tjeng and Maurits Haverkort (MPI-CPfS Dresden), and with theoreticians at UBC (Ilya Elfimov).
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