[Paper Review] Superconductivity and Antiferromagnetism in NdNiO$_2$ and CaCuO$_2$: A Cluster DMFT Study

Jonathan Karp, Alexander Hampel+1 more|arXiv (Cornell University)|Jan 25, 2022

Physics of Superconductivity and Magnetism4 citations

TL;DR

This study uses a 2×2 real-space cluster dynamical mean-field theory (DMFT) with Nambu spinors to simultaneously model superconducting and antiferromagnetic order in NdNiO₂ and CaCuO₂. It finds that both materials exhibit cuprate-like superconductivity emerging at similar hole doping, with a superconducting dome bounded by antiferromagnetism at low doping and suppressed Tc at high doping, supporting the idea that nickelate superconductivity is fundamentally similar to cuprate physics.

ABSTRACT

We perform a comparative $2 imes 2$ real space cluster DMFT study on minimal models for NdNiO$_2$ and CaCuO$_2$ obtained from downfolding DFT states, using a Nambu formalism that allows for both superconducting and antiferromagnetic order. We produce a phase diagram in temperature and doping. We find that for the nickelate, like the cuprate, the stoichiometric compound is antiferromagnetic. We find superconductivity in a doping range bounded, with a small coexistence region, by the onset of antiferromagnetism at low doping and with transition temperature becoming immeasurably small at high doping. Superconductivity emerges at around the same hole doping for both compounds, but requires a larger deviation from half filling for the nickelate. Both antiferromagnetic and superconducting order lead to a partial gapping of the $d_{x^2-y^2}$ Fermi surface sheet. Our similar results for the cuprate and nickelate suggest that nickelate superconductivity is cupratelike. We compare our results to the experimental phase diagram.

Motivation & Objective

• To investigate whether superconductivity in the infinite-layer nickelate NdNiO₂ is fundamentally similar to that in cuprates like CaCuO₂.
• To determine the role of Nd-derived bands and orbital hybridization in modifying the electronic structure and superconducting pairing.
• To resolve conflicting theoretical interpretations about the importance of multiple orbitals and correlation effects in nickelates.
• To compare theoretical predictions with experimental phase diagrams for both nickelates and cuprates.
• To assess the validity of the double-counting correction in DFT+DMFT for systems with strong orbital hybridization and competing orders.

Proposed method

• Performs a 2×2 real-space cluster DMFT calculation on minimal models derived from DFT downfolding for NdNiO₂ and CaCuO₂.
• Uses a Nambu formalism to simultaneously treat superconducting and antiferromagnetic order parameters in the same calculation.
• Applies a double-counting correction using either DFT or DMFT densities to account for self-interaction effects.
• Employs a three-orbital model (Ni-dx²−y², dxy, dz²) and a seven-orbital model including Nd-dz² and dxy orbitals to test robustness.
• Calculates the anomalous self-energy and order parameters to identify d-wave superconducting and antiferromagnetic phases.
• Compares results across different double-counting schemes and orbital model sizes to assess numerical stability and physical consistency.

Experimental results

Research questions

• RQ1 Does superconductivity in NdNiO₂ emerge at a doping level comparable to that in CaCuO₂, suggesting cuprate-like physics?
• RQ2 How does the presence of Nd-derived bands and hybridization with Ni-d orbitals affect the stability of superconducting and antiferromagnetic phases?
• RQ3 Is the superconducting dome in NdNiO₂ bounded by antiferromagnetic order at low doping, as seen in cuprates?
• RQ4 To what extent do orbital degrees of freedom beyond dx²−y² contribute to the correlation physics in nickelates?
• RQ5 How sensitive are the results to the choice of double-counting correction in DFT+DMFT for strongly correlated, multi-orbital systems?

Key findings

• The stoichiometric NdNiO₂ compound is antiferromagnetic, like its cuprate counterpart CaCuO₂, despite experimental absence of long-range AFM order down to 1.7 K.
• Superconductivity emerges at a hole doping of approximately x ≈ 0.12 in both NdNiO₂ and CaCuO₂, with a superconducting dome bounded by antiferromagnetism at low doping.
• The transition temperature Tc decreases with increasing doping, becoming immeasurably small at high doping, consistent with experimental observations.
• Both antiferromagnetic and superconducting orderings lead to partial gapping of the dx²−y² Fermi surface sheet, indicating a common electronic response.
• The inclusion of additional orbitals (e.g., Nd-dz² and dxy) in the seven-band model does not qualitatively alter the anomalous self-energy, supporting the robustness of the d-wave pairing scenario.
• The choice of double-counting correction has a small quantitative effect on the anomalous self-energy but does not change the qualitative phase boundaries, validating the methodological approach.