Surface symmetry-breaking and strain effects on orbital occupancy in transition metal perovskite epitaxial films

Abstract

The electron occupancy of 3d-orbitals determines the properties of transition metal oxides. This can be achieved, for example, through thin-film heterostructure engineering of ABO 3 oxides, enabling emerging properties at interfaces. Interestingly, epitaxial strain may break the degeneracy of 3d-e g and t 2g orbitals, thus favoring a particular orbital filling with consequences for functional properties. Here we disclose the effects of symmetry breaking at free surfaces of ABO 3 perovskite epitaxial films and show that it can be combined with substrate-induced epitaxial strain to tailor at will the electron occupancy of in-plane and out-of-plane surface electronic orbitals. We use X-ray linear dichroism to monitor the relative contributions of surface, strain and atomic terminations to the occupancy of 3z 2 -r 2 and x 2 -y 2 orbitals in La 2/3 Sr 1/3 MnO 3 films. These findings open the possibility of an active tuning of surface electronic and magnetic properties as well as chemical properties (catalytic reactivity, wettability and so on).

Thickness dependence of La0.7Sr0.3MnO3/PbZr0.2Ti0.8O3 magnetoelectric interfaces

“…The Mn valence near the interface (small i's) is suppressed due to the large PZT surface charge and gradually returns to nominal valence þ3.3 near the 7th MnO 2 plane. While the simulated surface Mn valence (i ¼ 10) is around 3.3 or larger, the surface is affected by the absence of apical oxygen coordination 25 and should be smaller due to higher 3z 2 -r 2 orbital occupancy than in the bulk.…”

Chemically induced Jahn–Teller ordering on manganite surfaces

“…Many of the same classes of correlated oxides are broadly explored for their electrochemical properties, enabling applications from solid oxide fuel cells to oxygen sensors [5][6][7][8] , and suggesting intrinsic links between physical and electrochemical functionalities. While the bulk properties of these materials are now amenable to a broad range of scattering and electron microscopy techniques, detailed studies of the structure and properties of their surfaces have remained challenging [9][10][11][12][13][14] . Understanding the emergent physical phenomena at surfaces requires the capability to probe local composition, recognize minute deviations from ideal structures and explore atomic coupling to electronic properties.…”

Interfaces and Nanostructures of Functional Oxide Octahedral Framework Structures

“…In the La 1-x Sr x MnO 3 solid solution, the electron occupancy of the e g doublet depends on the strength of the JahnTeller distortion, which in turn is determined by the hole doping level, x (Tokura and Nagaosa, 2000). In epitaxial thin films, however, similarly to the Jahn-Teller distortion, misfit strains have the ability to break the x 2 − y 2 /3z 2 − r 2 orbital degeneracy (Fang et al, 2000) as recently confirmed by X-ray linear dichroism investigations (Huijben et al, 2008;Tebano et al, 2008;Pesquera et al, 2012). A thorough investigation taking into account the attenuation of the signal with the depth in the sample, suggests that the observed X-ray linear dichroism is rather a consequence of the superposition of interfacial and free surface effects (Pesquera et al, 2012).…”

“…In epitaxial thin films, however, similarly to the Jahn-Teller distortion, misfit strains have the ability to break the x 2 − y 2 /3z 2 − r 2 orbital degeneracy (Fang et al, 2000) as recently confirmed by X-ray linear dichroism investigations (Huijben et al, 2008;Tebano et al, 2008;Pesquera et al, 2012). A thorough investigation taking into account the attenuation of the signal with the depth in the sample, suggests that the observed X-ray linear dichroism is rather a consequence of the superposition of interfacial and free surface effects (Pesquera et al, 2012). As shown in Figure 1C, those results suggest that a tensile strain would favor the occupancy of the in-plane x 2 − y 2 orbitals, while a compressive strain would induce the occupancy of the out-of-plane 3z 2 − r 2 ones, as expected from electrostatic arguments.…”

“…Orbitals pointing to the oxygen ions, e g , 3z 2 − r 2 and x 2 − y 2 , are higher in energy than those pointing between them, t 2g : xy, yz, zx (not shown). According to recent findings (Pesquera et al, 2012), a tensile misfit strain would favor the preferential occupation of in-plane x 2 − y 2 orbitals, while a compressive misfit strain would favor the occupation of the out-of-plane 3z 2 − r 2 ones. Near the polar La 0.7 Sr 0.3 MnO 3 /SrTiO 3 interface, the Mn 3+ /Mn 4+ ratio is high and the strong localization of the e g electron gives rise to a measurable deformation of the MnO 6 octahedra that contributes to the relaxation of the misfit strain.…”