Fundamental difference in the electronic reconstruction of infinite-layer versus perovskite neodymium nickelate films on ${\mathrm{SrTiO}}_3$(001)

Motivated by recent reports of superconductivity in Sr-doped ${\mathrm{NdNiO}}_2$ films on ${\mathrm{SrTiO}}_3$(001) [Nature (London) 572, 624 (2019)], we explore the role of the polar interface on the structural and electronic properties of ${\mathrm{NdNiO}}_n/{\mathrm{SrTiO}}_3$(001) ($n=2,3$) by performing first-principles calculations including a Coulomb repulsion term. For infinite-layer nickelate films ($n=2$), electronic reconstruction drives the surprising emergence of a two-dimensional electron gas (2DEG) at the interface involving a strong occupation of the Ti $3d$ states. This effect is more pronounced than in ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$(001) and accompanied by a substantial reconstruction of the Fermi surface: a depletion of the self-doping Nd $5d$ states and an enhanced Ni $e_g$ orbital polarization reaching up to $35\%$ at the surface, reflecting a single hole in the $3d_{x^2-y^2}$ states, i.e., cupratelike behavior. In contrast, no 2DEG forms for perovskite films ($n=3$) or if a single perovskite layer persists at the interface. We show that the topotactic reaction from the perovskite to the infinite-layer phase is confined to the nickelate film, whereas the ${\mathrm{SrTiO}}_3$ substrate remains intact.

Figure 1

(a), (b) Optimized geometry of ${\left({\mathrm{NdNiO}}_n\right)}_4/\mathrm{STO}\left(001\right)$ ($n=2$: infinite layer; $n=3$: perovskite). (c) The vertical displacements $\mathrm{\Delta }z$ of the $B$ site ions from their respective oxygen layers are substantial for $n=2$; for $n=3$, they are negligible (not shown). The apical $B$-O bond lengths reflect the highly distinct ionic relaxation in the two systems. (d), (e) Corresponding layer-resolved density of states and distribution of electron density (integrated between $-0.7\mathrm{eV}$ and $E_{\text{F}}$), visualizing the fundamental differences in electronic reconstruction. The strong oxygen contributions to the electron density in the nickelate region (absent in STO) highlight its covalent nature. (f), (g) Layer-resolved charge difference at Ni and Ti sites relative to the respective bulk compounds and Ni and Ti $e_g$ orbital polarization of ${\left({\mathrm{NdNiO}}_n\right)}_4/\mathrm{STO}\left(001\right)$. The former show the valence modulations arising particularly for $n=2$. A positive orbital polarization indicates a preferential $3d_{z^2}$ occupation. For reference, the Ti charge differences are provided for the paradigmatic ${\left(\mathrm{LAO}\right)}_4/\mathrm{STO}\left(001\right)$ and ${\left(\mathrm{LAO}\right)}_5/\mathrm{STO}\left(001\right)$ 2DEG systems.

Figure 2

(a) Band structures ($\vec{k}$-resolved densities of states, projected on Ni $3d$ and Ti $3d$ orbitals in different layers) highlight the interfacial 2DEG formation in ${\left({\mathrm{NdNiO}}_2\right)}_4/\mathrm{STO}\left(001\right)$. For reference, a panel for bulk ${\mathrm{NdNiO}}_2$ is provided. The orbital characters are denoted. (b) The film Fermi surface is composed predominantly of Ni $3d_{x^2-y^2}$ and $\mathrm{\Gamma }$-centered Ti $3d$ (2DEG) sheets and thus considerably reconstructed with respect to the Fermi surface of bulk ${\mathrm{NdNiO}}_2$, which is shown here for a comparable $\sqrt{2}\times \sqrt{2}$ cell and closely resembles that of ${\mathrm{LaNiO}}_2$ [14, 39, 40].

Figure 3

The self-doping Nd $5d$ states in bulk ${\mathrm{NdNiO}}_2$ (a) are shifted to higher energies and largely depleted in ${\mathrm{NdNiO}}_2/\mathrm{STO}\left(001\right)$ (b).

Figure 4

(a) Energies $E_f^{i,0}$ and $E_f^{i,i-1}$ (per formula unit; see text) required to reduce different $A\mathrm{O}$ layers in ${\left({\mathrm{NdNiO}}_3\right)}_4/\mathrm{STO}\left(001\right)$ heterostructures ($A=\text{Nd}$, Sr). (b) Electronic structure and optimized geometry of ${\left({\mathrm{NdNiO}}_2\right)}_3/{\left({\mathrm{NdNiO}}_3\right)}_1/\mathrm{STO}\left(001\right)$. The oxidized layer at the interface (marked in red) inhibits the formation of a 2DEG. The Nd $5d$ self-doping is simultaneously suppressed.