Abstract
Based on first-principles calculations including a Coulomb repulsion term, we identify trends in the electronic reconstruction of $A$NiO$_2$/SrTiO$_3$(001) ($A=$ Pr, La) and $A$CuO$_2$/SrTiO$_3$(001) ($A=$ Ca, Sr). Common to all cases is the emergence of a quasi-two-dimensional electron gas (q2DEG) in SrTiO$_3$(001), albeit the higher polarity mismatch at the interface of nickelates vs. cuprates to the nonpolar SrTiO$_3(001)$ substrate (${3+}/0$ vs. ${2+}/0$) results in an enhanced q2DEG carrier density. The simulations reveal a significant dependence of the interfacial Ti $3d_{xy}$ band bending on the rare-earth ion in the nickelate films, being $20$-$30\%$ larger for PrNiO$_2$ and NdNiO$_2$ than for LaNiO$_2$. Contrary to expectations from the formal polarity mismatch, the electrostatic doping in the films is twice as strong in cuprates as in nickelates. We demonstrate that the depletion of the self-doping rare-earth $5d$ states enhances the similarity of nickelate and cuprate Fermi surfaces in film geometry, reflecting a single hole in the Ni and Cu $3d_{x^2-y^2}$ orbitals. Finally, we show that NdNiO$_2$ films grown on a polar NdGaO$_3(001)$ substrate feature a simultaneous suppression of q2DEG formation as well as Nd~$5d$ self-doping.
Highlights
The very recent observation of superconductivity in Srdoped NdNiO2 and PrNiO2 films grown on SrTiO3(001) (STO) [1,2,3] has sparked considerable interest in infinite-layer nickelates, since their formal Ni1+ (3d9) valence state renders them close to cuprates [4,5,6,7,8,9,10,11,12,13,14,15,16]
Similar to NdNiO2/SrTiO3(001), polar discontinuity drives the emergence of a quasi-twodimensional electron gas (q2DEG) at the interface in all cases due to the occupation of the Ti 3d conduction band that is
Accompanied by substantial ferroelectric-like displacements of the Ti ions. Despite their comparable electronic structure in the bulk, the higher polarity mismatch at the interface of infinite-layer nickelates versus cuprates to the nonpolar SrTiO3(001) substrate enhances the q2DEG carrier density for the nickelate films
Summary
The very recent observation of superconductivity in Srdoped NdNiO2 and PrNiO2 films grown on SrTiO3(001) (STO) [1,2,3] has sparked considerable interest in infinite-layer nickelates, since their formal Ni1+ (3d9) valence state renders them close to cuprates [4,5,6,7,8,9,10,11,12,13,14,15,16]. A considerable electronic reconstruction emerges in NdNiO2/SrTiO3(001) due to the polar discontinuities at the interface and the surface [19], which comprises (i) the formation of a quasi-two-dimensional electron gas (q2DEG) at the interface by occupation of Ti 3d states despite the metallic screening of the nickelate film; (ii) the depletion of the self-doping Nd 5d states, resulting in a cuprate-like Fermi surface; and (iii) an enhanced and modulated Ni eg orbital polarization throughout the film due to electrostatic doping. A fundamental difference between these two materials classes remains, namely, the formal polarity of the consecutive (001) layers, being, for instance, Ca2+(CuO2)2− and Sr2+(CuO2)2− in the superconducting infinite-layer cuprates and Nd3+(NiO2 )3−, Pr3+(NiO2 )3−, and La3+(NiO2 )3− in the infinite-layer nickelates This implies distinct behavior between cuprates and nickelates in film geometry on a nonpolar substrate such as STO(001) despite the same formal 3d9 configuration. We show that NdNiO2 films grown on a polar NdGaO3(001) substrate exhibit depleted Nd 5d states in the film and a simultaneously quenched q2DEG in the substrate, which offers a route to disentangle their contributions to superconductivity in infinite-layer nickelates
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