[Paper Review] Similarities and differences between nickelate and cuprate films grown on a SrTiO$_3$ substrate

Yang Zhang, Ling-Fang Lin+4 more|arXiv (Cornell University)|Aug 10, 2020

Physics of Superconductivity and Magnetism4 citations

TL;DR

This study uses density functional theory (DFT) to compare NdNiO₂/SrTiO₃ and CaCuO₂/SrTiO₃ films, revealing that NiO₂ layers undergo strong polar lattice reconstruction with opposite displacements at the surface and interface, triggering a 2DEG and charge transfer to Ti 3d_{xy} orbitals. In contrast, CuO₂ layers distort uniformly, and the 2DEG is more localized. The Ni d_{3z²−r²} orbital becomes itinerant, while Cu remains doubly occupied, highlighting a key electronic difference despite isoelectronic d⁹ configurations.

ABSTRACT

The recent discovery of superconductivity in Sr-doped NdNiO$_2$ films grown on SrTiO$_3$ started a novel field within unconventional superconductivity. To understand the similarities and differences between nickelate and cuprate layers on the same SrTiO$_3$ substrate, here based on the density functional theory we have systematically investigated the structural, electronic, and magnetic properties of NdNiO$_2$/SrTiO$_3$ and CaCuO$_2$/SrTiO$_3$ systems. Our results revealed a strong lattice reconstruction in the case of NdNiO$_2$/SrTiO$_3$, resulting in a polar film, with the surface and interfacial NiO$_2$ layers presenting opposite displacements. However, for CaCuO$_2$/SrTiO$_3$, the distortions of those same two CuO$_2$ layers were in the same direction. In addition, we found this distortion to be approximately independent of the studied range of film thickness for both the nickelate and cuprates films. Furthermore, we also observed a two-dimensional electron gas at the interface between NdNiO$_2$ and SrTiO$_3$, caused by the polar discontinuity, in agreement with recent literature. For NdNiO$_2$/SrTiO$_3$ the two-dimensional electron gas extends over several layers, while for CaCuO$_2$/SrTiO$_3$ this electronic rearrangement is very localized at the interface between CaCuO$_2$ and SrTiO$_3$. The electronic reconstruction found at the interface involves a strong occupation of the Ti $3d_{xy}$ state. In both cases, there is a significant electronic charge transfer from the surface Ni or Cu layers to the Ti interface layer. The interfacial Ni and Cu layer is hole and electron doped, respectively. By introducing magnetism and electronic correlation, we observed that the $d_{3z^2-r^2}$ orbital of Ni becomes itinerant while the same orbital for Cu remains doubly occupied, establishing a clear two- vs one-orbital active framework for the description of these systems.

Motivation & Objective

• To understand the structural, electronic, and magnetic differences between nickelate and cuprate films on SrTiO₃ substrates.
• To investigate the role of polar discontinuity and lattice reconstruction in inducing interfacial 2DEG and charge transfer.
• To compare the electronic behavior of Ni and Cu 3d orbitals under similar epitaxial strain and interface conditions.
• To determine whether the superconductivity in NNO films arises from interface-driven electronic reconstruction, as in LAO/STO or CCO/STO systems.

Proposed method

• Density functional theory (DFT) with GGA and DFT+Ueff (Ueff = 4 eV) to account for electronic correlations.
• Slab models of (NdNiO₂)n/(SrTiO₃)4 and (CaCuO₂)n/(SrTiO₃)4 with fixed in-plane lattice constants (a = b = 3.905 Å or 3.943 Å) to simulate epitaxial strain.
• Structural relaxation to determine atomic displacements and polar distortions in NiO₂ and CuO₂ layers.
• Charge population analysis using Bader charges to quantify electron transfer from Ni/Cu to Ti ions.
• Projected band structures and Fermi surface analysis to examine orbital character and electronic states near the Fermi level.
• Comparison of non-magnetic and G-AFM (G-type antiferromagnetic) states to assess magnetic reconstruction effects.

Experimental results

Research questions

• RQ1 How do the lattice distortions in NdNiO₂ and CaCuO₂ films differ when grown on SrTiO₃ substrates?
• RQ2 What is the nature and extent of the interfacial two-dimensional electron gas (2DEG) in NdNiO₂/SrTiO₃ versus CaCuO₂/SrTiO₃?
• RQ3 How does charge transfer from Ni or Cu to Ti 3d orbitals differ between the two systems, and what role does the Ti 3d_{xy} orbital play?
• RQ4 Why does the Ni d_{3z²−r²} orbital become itinerant while the Cu d_{3z²−r²} remains doubly occupied in the presence of correlations?
• RQ5 To what extent does magnetic reconstruction at the NdNiO₂ surface suppress magnetism compared to the CaCuO₂ system?

Key findings

• NdNiO₂/SrTiO₃ exhibits strong polar lattice reconstruction with NiO₂ surface and interface layers displacing in opposite directions (Δ = +0.279 Å and −0.169 Å), while CaCuO₂/SrTiO₃ shows same-direction distortions.
• The 2DEG in NdNiO₂/SrTiO₃ extends over several layers due to polar discontinuity, whereas in CaCuO₂/SrTiO₃ it is highly localized at the interface.
• Ti 3d_{xy} orbitals at the interface are strongly occupied (gaining ~0.129 e/Ti), and charge transfer from Ni to Ti is enhanced by DFT+Ueff (from −0.101 e/Ni to −0.141 e/Ni at interface).
• The Ni d_{3z²−r²} orbital becomes itinerant under electronic correlations, while the Cu d_{3z²−r²} remains doubly occupied, establishing a two-orbital vs. one-orbital active framework.
• Magnetic reconstruction suppresses magnetism at the NdNiO₂ surface, while the interface remains metallic and doped.
• The structural and electronic differences are robust across film thicknesses and are consistent under both non-magnetic and G-AFM DFT+Ueff calculations.