[Paper Review] Correlations turn electronic structure of finite-layer nickelates upside down

Paul Worm, Liang Si+4 more|arXiv (Cornell University)|Nov 24, 2021

Magnetic and transport properties of perovskites and related materials1 references4 citations

TL;DR

This paper demonstrates that electronic correlations in finite-layer nickelates, particularly in the pentalayer Nd6Ni5O12, shift Nd-derived electron pockets above the Fermi level, leaving only a single d_{x²−y²} orbital per Ni layer to form the Fermi surface—reminiscent of cuprate superconductors. Using DFT+DMFT and dynamical vertex approximation, the authors show that this correlation-driven reconstruction enables a minimal one-orbital Hubbard model with superconducting critical temperatures comparable to experiment, and propose Zr-doping of bilayer nickelates to restore similar single-orbital physics.

ABSTRACT

Motivated by the recent discovery of superconductivity in the pentalayer nickelate Nd$_6$Ni$_5$O$_{12}$ [Nature Materials 10.1038], we calculate its electronic structure and superconducting critical temperature. We find that electronic correlations are essential for pushing Nd$_6$Ni$_5$O$_{12}$ into the superconducting doping range as they shift the electron pockets above the Fermi energy. As a consequence, Nd$_6$Ni$_5$O$_{12}$ can be described with a single $d_{x^2-y^2}$ orbital per Ni. Instead, for the bilayer nickelate Nd$_3$Ni$_2$O$_6$ we find correlations to drive the system into a three-orbital regime also involving the Ni $d_{xz,yz}$ states. We suggest, however, that single-orbital physics with optimal doping can be restored by substituting 60% of the trivalent Nd or La by tetravalent Zr.

Motivation & Objective

• To understand the electronic structure of the superconducting pentalayer nickelate Nd6Ni5O12 and its relation to cuprates.
• To determine the role of electronic correlations in shifting Nd-derived electron pockets above the Fermi level.
• To identify minimal model parameters for superconductivity in finite-layer nickelates.
• To propose a doping strategy—specifically Zr substitution—to restore single-orbital physics in bilayer nickelates.

Proposed method

• Employed state-of-the-art DFT+DMFT to account for local electronic correlations in Nd6Ni5O12 and Nd3Ni2O6.
• Used Wannier function projections to isolate orbital contributions to the Fermi surface and spectral functions.
• Applied the dynamical vertex approximation (DΓA) to include non-local correlations and study superconducting pairing.
• Performed structural relaxations using VASP and DFT band structure calculations with wien2k and PBE/PBESol functionals.
• Used analytic continuation with adjustable parameters to improve spectral function reconstruction from imaginary-time DMFT data.
• Proposed Zr-doping in La2+δZr1−δNi2O6 with δ=0.4 to tune electron count and restore d_{x²−y²}-only Fermi surface.

Experimental results

Research questions

• RQ1 How do electronic correlations affect the Fermi surface topology in the pentalayer nickelate Nd6Ni5O12?
• RQ2 Why does Nd6Ni5O12 exhibit superconductivity while the trilayer Nd4Ni3O8 does not, despite similar structures?
• RQ3 Can a minimal one-orbital Hubbard model describe the superconducting state in finite-layer nickelates?
• RQ4 What role do Nd-derived electron pockets play in the electronic structure, and how are they modified by correlations?
• RQ5 Can electron doping via Zr substitution restore single-orbital d_{x²−y²} physics in the bilayer Nd3Ni2O6 system?

Key findings

• In Nd6Ni5O12, DFT+DMFT calculations show that local electronic correlations push Nd-derived electron pockets above the Fermi level, leaving only a single d_{x²−y²} orbital per Ni layer to form the Fermi surface.
• The effective tight-binding parameters and effective masses in the pentalayer system are remarkably similar to those in infinite-layer nickelates, supporting a minimal one-orbital Hubbard model.
• Superconductivity in the pentalayer system arises from dominant spin fluctuations in the DΓA framework, yielding a Tc comparable to experimental observations.
• In the undoped bilayer Nd3Ni2O6, correlations drive charge transfer from d_{x²−y²} to d_{xz/yz} orbitals, resulting in a three-orbital, three-dimensional electronic structure.
• Electron doping to a nominal Ni valence of d^8.8 via 40% Zr substitution in La2+δZr1−δNi2O6 restores a d_{x²−y²}-only Fermi surface, recovering single-orbital physics.
• The DFT+DMFT Fermi surface of the doped bilayer system closely resembles that of the pentalayer and infinite-layer superconducting nickelates, indicating a common low-energy physics.