Surfaces and interfaces of infinite-layer nickelates studied by dynamical mean-field theory

TL;DR

The paper addresses how electronic correlations shape the surface and interface electronic structure of infinite-layer nickelates NdNiO$_2$ grown on SrTiO$_3$. It combines DFT with dynamical mean-field theory to model two slab geometries, revealing a polar-field-driven, layer-dependent reconstruction that drives a Mott-insulating NiO$_2$ surface (Ni $3d$ orbitals) in one geometry and an orbital-selective Mott state at the Nd-terminated interface in the other. The work shows that Ti $3d$ states at the interface can become electron-doped in the $n$-type case and that inner layers retain metallic, predominantly $3d^9$ character, highlighting strong orbital-selective correlation effects near surfaces and interfaces. These findings underscore the necessity of including dynamical correlations, realistic lattice relaxations, and spatial inhomogeneity to understand ultrathin nickelate films and have potential implications for interface-engineered superconductivity in nickelate heterostructures.

Abstract

Infinite-layer nickelate superconductors are typically synthesized as thin films and thus include, besides the more bulk-like inner layers, distinct surface and interface layers in contact with the vacuum and substrate, respectively. Here, we employ density-functional theory and dynamical mean-field theory to investigate how electronic correlations influence these surface and interface regions. Our results show that electronic correlations can significantly modify the electronic structure, even driving surface layers into a Mott-insulating state with a 3$d^8$ electronic configuration. Moreover, surface termination effects induce a polar field that can shift the $Γ$ and $A$ pocket above the Fermi level, even for the undoped parent compound NdNiO$_2$. Finally, for an $n$-type interface, often synthesized experimentally, we find the Ti 3$d$ orbitals to become electron doped.

Figures (36)

• Figure 1: DFT relaxed configurations of a NdNiO$_2$ (001) thin film, consisting of 5 NdNiO$_2$ layers on top of 2 SrTiO$_3$ (STO) layers. A stoichiometric ratio of Nd, Ni, and O gives either a NiO$_2$-terminated surface (left) or Nd-terminated surface (right).
• Figure 2: DFT band structures for the NiO$_2$-terminated surface (left) and the Nd-terminated one (right). The band projections are colored according to their atomic character, as in Fig. \ref{['fig:structure']}.
• Figure 3: DFT band structures of the NiO$_2$-terminated surface (top) and the Nd-terminated one (bottom) projected onto the interface, center and surface layer. Nd and Ni contributions are colored in green and blue, respectively.
• Figure 4: Layer-resolved, $\mathbf{k}$-integrated spectral function of Nd 5$d$ orbitals (left column) and Ni 3$d$ orbitals (right column) for the NiO$_2$-terminated slab. The gray line is the additional interstitial $s$ orbital, which is located within the Nd planes. The bottom row is at the interface, the top row is at the surface. The black dotted line visualizes the polar field shift; the vertical dashed line indicates the Fermi energy.
• Figure 5: Comparison of filling in DMFT (solid lines, squares) and DFT (dotted lines, circles) for different orbitals in case of the NiO$_2$-terminated slab. Layer 1 is at the interface, layer 5 is at the surface. Dashed, black horizontals are the fillings for bulk NdNiO$_2$ in DMFT. Black, solid lines in the central row indicate half-filling of Ni $3d_{x^2-y^2}$ and three-quarter filling of the degenerate Ni $3d_{xy}/3d_{yz}$ orbitals, respectively. The Mott-insulating surface Ni $3d_{x^2-y^2}$ and $3d_{xz}$ Wannier functions are depicted in the two central plots.