Probing Ultrathin Functional Layers and Buried Interfaces with Advanced X-ray Spectroscopic Techniques

Rational design of low-dimensional electronic phenomena at oxide interfaces is considered to be one of the most promising schemes for realizing new energy-efficient logic and memory devices. An atomically-abrupt interface between paramagnetic LaNiO₃ and antiferromagnetic CaMnO₃ exhibits interfacial ferromagnetism, which can be tuned via a thickness-dependent metal-insulator transition in LaNiO₃. A rich cation picture, emerging from the polarity mismatch and electronic reconstruction at the interface, is considered to be the driving factor for this phenomenon. Once fully understood, such emergent functionality could turn this Mott-interface system into a key building block for the above-mentioned future devices. In this talk, I will discuss three recent studies, in which we utilized a combination of x-ray spectroscopic and electron imaging techniques to investigate the electronic-structural origins of this emergent phenomenon. Starting with the building blocks of this heterojunction (CaMnO₃ and LaNiO₃), we used a combination of hard x-ray photoemission (HAXPES) and x-ray absorption spectroscopy (XAS) to establish a direct link between the in-plane strain and the oxygen-vacancy content in CaMnO₃ [1]. Then, by using a combination of XAS and scanning transmission electron microscopy (STEM), we examined the nature of the metal-insulator transition in LaNiO₃ in the ultrathin limit (<2 u.c.) [2]. Finally, we utilized a combination of HAXPES and standing-wave photoemission spectroscopy (SW-XPS) to demonstrate a depth-dependent charge reconstruction at the LaNiO₃/CaMnO₃ interface [3]. Our findings suggest a new strategy for designing functional Mott oxide heterostructures by tuning the interfacial cation characteristics via controlled manipulation of thickness, strain, and ionic defect states.

[1] R. U. Chandrasena et al., Nano Lett. 17, 794 (2017)
[2] M. Golalikhani et al., Nature Comm. 9, 2206 (2018)
[5] R. U. Chandrasena et al., Phys. Rev. B 98, 155103 (2018)