Site-Selective Polar Compensation of Mott Electrons in a Double Perovskite Heterointerface

Double perovskite oxides (DPOs) of the general formula A₂BB’O₆ (where B, B’ are the transition metal elements) exhibit diverse phenomena, including room temperature magnetoresistance, multiglass behavior, insulating ferromagnetism, etc [1-2]. Studying these DPOs as ultrathin epitaxial thin films on single crystalline substrates can add another dimension to engineering electronic, magnetic, and topological phenomena [3]. Understanding the consequence of polar symmetry mismatch between the substrate and DPO would be a first step towards this broad goal. This question is further connected with a more fundamental question about the role of Mott physics for polar compensation in oxide heterostructures as transition metal oxides (TMOs), specifically 3d TMOs, are strongly correlated electron systems. Typically, polar compensation mechanisms in TMOs have been addressed in a semiconductor-like band bending framework. We hypothesize that if strong correlation physics is crucial in contrast to the conventional band bending framework, B and B′ in a DPO would respond differently to the polar catastrophe owing to their different characteristic correlated energy scales of on-site coulomb repulsion energy (U), charge transfer energy (Δ) and the hopping parameter (t).
In this work [4], we have investigated ultra-thin films of various thicknesses of a prototypical DPO Nd₂NiMnO₆ (NNMO), grown on single crystalline NdGaO₃ [110]_or (NGO) and SrTiO₃ (STO) [001] substrates by pulsed laser deposition technique. We comparatively studied the above two sets since, contrary to NNMO/NGO where there is no interfacial polarity mismatch, the NNMO/STO heterostructure encompasses interfacial polarity mismatch, similar to the well-known LaAlO₃/SrTiO₃ system [5]. We have explored the effect of polar catastrophe using element-sensitive X-ray absorption spectroscopy, scanning transmission electron microscopy, electron energy loss spectroscopy measurements along with first-principles calculations. Our findings establish that polar compensation happens through the formation of interfacial Mn³⁺ within the film side near the interface while the Ni site remains unperturbed. The additional electron comes due to surface oxygen vacancies. This surprising site-selective charge compensation, which cannot be explained by existing mechanisms of polar compensation, emanates from the relative difference between U and Δ of Ni and Mn. This demonstrates the direct correlation between polar compensation and electronic structure parameters, emphasizing the crucial role of Mott physics in polar compensation and paving the way for designer doping strategies in complex oxides.

[1] Kobayashi et al., Nature 395, 677 (1998)
[2] Vasala et al., Progress in Solid State Chemistry 43, 1 (2015).
[3] Cook et al., Phys. Rev. Lett. 113, 077203 (2014).
[4] Bhattacharya et al., arXiv:2311.15726 [cond-mat.mtrl-sci].
[5] Ohtomo et al., Nature 427, 423 (2004).