Rashba spin-orbit coupling in infinite-layer nickelate films on SrTiO3 (001) and KTaO3 (001)

The impact of spin-orbit interactions in ${\mathrm{NdNiO}}_{2}/{\mathrm{SrTiO}}_{3}$(001) and ${\mathrm{NdNiO}}_{2}/{\mathrm{KTaO}}_{3}$(001) is explored by performing density functional theory simulations including a Coulomb repulsion term. Polarity mismatch drives the emergence of an interfacial two-dimensional electron gas in ${\mathrm{NdNiO}}_{2}/{\mathrm{KTaO}}_{3}$(001) involving the occupation of Ta $5d$ conduction-band states, which is twice as pronounced as in ${\mathrm{NdNiO}}_{2}/{\mathrm{SrTiO}}_{3}$(001). We identify a significant anisotropic ${k}^{3}$ Rashba spin splitting of the respective ${d}_{xy}$ states in both systems that results from the broken inversion symmetry at the nickelate-substrate interface and exceeds the width of the superconducting gap. In ${\mathrm{NdNiO}}_{2}/{\mathrm{KTaO}}_{3}$(001), the splitting reaches 210 meV, which is comparable to Bi(111) surface states. At the surface, the Ni $3{d}_{{x}^{2}\ensuremath{-}{y}^{2}}$-derived states exhibit an unexpectedly strong linear Rashba effect with ${\ensuremath{\alpha}}_{\text{R}}\ensuremath{\sim}125$ meV $\text{\AA{}}$, exemplifying its orbital selectivity. The corresponding Fermi sheets present a reconstructed circular shape due to the electrostatic doping, but undergo a Lifshitz transition towards a cupratelike topology deeper in the film that coincides with a realignment of their spin texture. These results promote surface and interface polarity as interesting design parameters to control spin-orbit physics as well as the superconducting pairing in infinite-layer nickelate heterostructures.