Exploring physics at highly correlated oxide interfaces : recent results and prospects in electron microscopy

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Scanning transmission electron microscopy (STEM), with its various structural and spectroscopic imaging techniques, is a highly versatile technique for providing structural, chemical and electronic information on materials at very high spatial resolution. Over the past decade, the development of lens aberration correctors has enabled a real breakthrough towards sub-angstromic resolution. Various types of atomic resolution imaging are now available, providing access to real-space crystallography of materials. These new imaging modes, coupled with new possibilities in spectromicroscopy at the scale of the individual atomic column, provide information channels particularly relevant to the exploration of physics at highly correlated oxide interfaces.

In this lecture, we will show, for example, how it is possible to probe the role of structural distortions at grain boundaries in metal-insulator transitions in vanadium oxide [1], or to quantify in real space atomic displacements to within a few picometers in order to reveal stress effects in ultrathin layers or perovskite superlattices. We will present the case of manganite thin films and show, for example, how the crystallographic lattices and rotations of the oxygen octahedra differ strongly from the bulk material near the interfaces, resulting in "dead layers" where the properties of the bulk material are strongly altered [2].
We will also show how electron energy loss spectroscopy (EELS), spatially resolved at the atomic column scale, can be used to study structural reconstructions at interfaces, such as the nature of termination planes or cation interdiffusion. In addition, we'll show how this technique can be used to probe electronic structure at the local level, for example, quantifying the number of electrons in the 3d bands of transition metals. It then becomes possible, for example, to image charge orders in oxides in order to understand the origin of magnetism in hematite-ilmenite-type oxides [3], or to map the charge distribution associated with interface reconstructions (manganitenickelate or manganite-ferroelectric [4]).

We will conclude this presentation by outlining the prospects of transmission electron microscopy for the study of oxide heterostructures.

References

[1] Li et al, Role of epitaxial microstructure, stress and twin boundaries in the metal-insulator transition mechanism in VO2/Al2O3 heterostructures, Acta Materiala 61, 6443 (2013).

[2] Li et al, Impact of interfacial coupling of oxygen octahedra on ferromagnetic order in LSMO/STO heterostructures, Scientific Report DOI: 10.1038/srep40068 (2017).

[3] Bocher et al, Direct Evidence of Fe2+-Fe3+ Charge Ordering in the Ferrimagnetic Hematite-Ilmenite Fe1.35Ti0.65O3-delta Thin Films, Physical Review Letters 111, 16 (2013).

[4] Marinova et al, Depth profiling charge accumulation from a ferroelectric into a doped Mott insulator, Nano letters 15, 2533 (2015) and Gibert et al, Interfacial Control of Magnetic Properties at LMO/LNO Interfaces, Nano letters 15, 7355 (2015).