Orbital reflectometry of nickel oxide heterostructures

by Meng Wu

Institution: University of Stuttgart
Department: Fakult├Ąt Mathematik und Physik
Degree: PhD
Year: 2015
Record ID: 1113172
Full text PDF: http://elib.uni-stuttgart.de/opus/volltexte/2015/9903/


Rare-earth nickelate heterostructures, e.g. superlattices of LaNiO3 (LNO) with a large band-gap insulator LaAlO3, were proposed as candidates to imitate the electronic structure of the cuprates (Chaloupka et al., 2008). One important aspect of the electronic structure is the orbital occupation of the Ni d-orbitals. The manipulation of orbital occupation depends on strain, the chemical counter ions and quantum confinement based on theoretical calculations (Hansmann et al., 2009, Han et al., 2010, Han et al., 2011, Han et al., 2012, Pentcheva et al., 2011, Hansmann et al., 2010). In this thesis, orbital reflectometry, which is a combination of x-ray absorption and x-ray reflectivity, was used to study layer-resolved orbital occupations. Two systems were investigated: First, we focused on LNO-based heterostructures, with a structural composition of 4 unit cell (u.c.) LNO and equally thick layer stacks of the band insulators RXO (R =La, Gd, Dy and X =Al, Ga, Sc). Compared to the x-ray absorption spectra which provide averaged information of all LNO layers, orbital reflectometry allows us to partly disentangle the orbital polarization originating from strain (which affects all four LNO layers in the stack) from the change in chemical composition across the LNO-RXO interface (which largely affects the interfacial layers). Our results indicate a linear orbital-lattice coupling and confirm the stabilization of the planar dx2-y2 orbital under tensile strain. We further show that strain is the most effective control parameter, whereas the influence of the chemical composition of the blocking layers is comparatively small at least for superlattices investigated here. We specify the layer-resolved orbital polarization quantitatively and directly compare the experimental data with the results of DFT calculations. In the second part of this thesis, we report an investigation of the orbital properties on PrNiO3-PrAlO3 (PNO-PAO) heterostructures. The PNO-PAO superlattices of interest are grown on different substrates, which show strain-dependent tunable spin and charge order, i.e. a robust insulating phase with both spin and charge order under tensile strain and a weakly metallic phase with no spin but no charge order. We identified the latter as a possible realization of a pure spin density wave for the compressive strain case. The temperature dependent x-ray linear dichroism shows a decrease of the orbital polarization for the insulating phase which is in agreement with the existence of charge disproportionation into two orbitally non-degenerate Ni species, whereas there is almost no change of the orbital polarization at the magnetic transition for the superlattice with a metallic spin density wave state. A similar linear orbital-lattice interaction is observed in agreement with the LNO-based heterostructures. This observation is important for the design of "orbitally engineered" in oxide heterostructures. Die Selten-Erd Nickelate wurden als einer der Kandidaten vorgeschlagen, bei denen mittels Heterostrukturierung diese…