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

Benjamin Geisler|arXiv (Cornell University)|Mar 1, 2023

Magnetic and transport properties of perovskites and related materialsMaterials Science81 references3 citations

TL;DR

This study investigates Rashba spin-orbit coupling in infinite-layer NdNiO2 films on SrTiO3 and KTaO3(001) substrates using DFT+U simulations. It reveals a 210 meV anisotropic Rashba splitting in the Ni 3d_{x²−y²} states at the KTaO3 interface—exceeding the superconducting gap and comparable to Bi(111) surface states—driven by interfacial polarity and orbital-selective spin splitting, with a Lifshitz transition in Fermi surface topology and spin texture reorientation across the film.

ABSTRACT

The impact of spin-orbit interactions in NdNiO2/SrTiO3(001) and NdNiO2/KTaO3(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 NdNiO2/KTaO3(001) involving the occupation of Ta $5d$ conduction-band states, which is twice as pronounced as in NdNiO2/SrTiO3(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 NdNiO2/KTaO3(001), the splitting reaches 210 meV, which is comparable to Bi(111) surface states. At the surface, the Ni $3d_{x^2-y^2}$-derived states exhibit a linear Rashba effect with $α_ ext{R} \sim$ 125 meV Å, 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 cuprate-like 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 in infinite-layer nickelate heterostructures.

Motivation & Objective

• To investigate the role of spin-orbit coupling in infinite-layer nickelate heterostructures on SrTiO3 and KTaO3(001) substrates.
• To understand how interfacial polarity and broken inversion symmetry influence Rashba spin splitting in 2DEG states.
• To explore the impact of electrostatic doping and orbital character on Fermi surface reconstruction and spin texture.
• To compare the strength and nature of Rashba splitting in NdNiO2/SrTiO3 versus NdNiO2/KTaO3 systems.
• To assess the implications of these spin-orbit effects for superconductivity and spintronic applications in oxide heterostructures.

Proposed method

• Performed first-principles density functional theory (DFT) simulations using the PBE exchange-correlation functional.
• Applied the DFT+U method with U-J = 3 eV at Ni and Ti sites and U-J = 1 eV at Ta sites to account for static correlation effects.
• Used VASP software with a 520 eV plane-wave cutoff and spin-orbit coupling included in the calculations.
• Analyzed layer-resolved band structures and Fermi surface topology across the film thickness to track electronic reconstruction.
• Tracked orbital contributions (d_{x²−y²}, d_{xy}, d_{xz,yz}, d_{z²}) to identify spin texture evolution and Rashba splitting.
• Compared results between NdNiO2/SrTiO3(001) and NdNiO2/KTaO3(001) to isolate the role of substrate polarity and 5d vs. 3d states.

Experimental results

Research questions

• RQ1 How does interfacial polarity in NdNiO2/SrTiO3(001) and NdNiO2/KTaO3(001) heterostructures influence the formation of a two-dimensional electron gas (2DEG)?
• RQ2 What is the magnitude and orbital dependence of Rashba spin-orbit coupling in the Ni 3d_{x²−y²} and substrate d_{xy} states?
• RQ3 How does electrostatic doping from the 2DEG affect the Fermi surface topology and spin texture of the Ni 3d_{x²−y²} states?
• RQ4 Does a Lifshitz transition occur in the Fermi surface as a function of film layer depth, and how is it linked to spin texture reorientation?
• RQ5 How does the strength of Rashba splitting in NdNiO2/KTaO3(001) compare to that in NdNiO2/SrTiO3(001), and what explains the difference?

Key findings

• The 2DEG in NdNiO2/KTaO3(001) is twice as pronounced as in NdNiO2/SrTiO3(001), primarily due to occupation of Ta 5d_{xy} conduction-band states.
• A significant anisotropic k³ Rashba spin splitting of 210 meV is observed in the Ni 3d_{x²−y²} states at the KTaO3 interface, exceeding the superconducting gap.
• The Rashba splitting in NdNiO2/KTaO3(001) is comparable in magnitude to that of Bi(111) surface states, indicating strong spin-orbit coupling.
• The Ni 3d_{x²−y²} orbital exhibits a linear momentum dependence in Rashba splitting (α_R ≈ 125 meV·Å), demonstrating orbital selectivity.
• The Fermi sheet of Ni 3d_{x²−y²} states at the surface is reconstructed into a quasi-circular shape due to electrostatic doping, suggesting possible s-wave pairing character.
• A Lifshitz transition occurs from the surface to deeper layers, transforming the Fermi surface from circular to cuprate-like, accompanied by a reorientation of the spin texture around the M point.