[Paper Review] Electron correlations and superconductivity in La$_3$Ni$_2$O$_7$ under pressure tuning

Zhiguang Liao, Lei Chen+5 more|arXiv (Cornell University)|Jul 31, 2023

Magnetic and transport properties of perovskites and related materials27 references14 citations

TL;DR

The paper develops a bilayer two-orbital Hubbard model for La3Ni2O7 under pressure, finds a crossover from S=1/2 to S=3/2, identifies an orbital-selective Mott phase, and derives a multiorbital t-J model predicting orbital-selective superconducting pairing with a symmetry transition.

ABSTRACT

Motivated by the recent discovery of superconductivity in La$_3$Ni$_2$O$_7$ under pressure, we discuss the basic ingredients of a model that captures its microscopic physics under pressure tuning. We anchor our description in terms of the spectroscopic evidence of strong correlations in this system. In a bilayer Hubbard model including the Ni $3d$ $x^2-y^2$ and $z^2$ orbitals, we show the ground state of the model crosses over from a low-spin $S=1/2$ state to a high-spin $S=3/2$ state. In the high-spin state, the two $x^2-y^2$ and the bonding $z^2$ orbitals are all close to half-filling, which promotes a strong orbital selectivity in a broad crossover regime of the phase diagram pertinent to the system. Based on these results, we construct an effective multiorbital $t$-$J$ model to describe the superconductivity of the system, and find the leading pairing channel to be an intraorbital spin singlet with a competition between the extended $s$-wave and $d_{x^2-y^2}$ symmetries. Our results highlight the role of strong multiorbital correlation effects in driving the superconductivity of La$_3$Ni$_2$O$_7$.

Motivation & Objective

• Motivate understanding of superconductivity in La3Ni2O7 under high pressure in terms of strong electron correlations and multiple Ni e_g orbitals.
• Anchor the description to spectroscopic evidence for strong correlations in the material.
• Model the system with a bilayer two-orbital Hubbard framework reflecting Ni 3d x^2−y^2 and z^2 orbitals and interlayer bonding.
• Identify the ground-state spin evolution and orbital-selective correlation effects relevant to superconductivity.

Proposed method

• Construct a bilayer two-orbital Hubbard model with Ni x^2−y^2 and z^2 orbitals and fit tight-binding parameters to DFT-derived bands.
• Transform to bonding-antibonding molecular orbital basis to analyze interlayer Ni z^2 bonding, and rewrite H_int in the MO basis.
• Use a U(1) slave-spin theory to compute orbital-resolved quasiparticle weights Z_alpha and electron densities to map the phase diagram.
• Find a low-spin to high-spin crossover and a region of strong orbital selectivity leading to an orbital-selective Mott phase (OSMP).
• Derive an effective multiorbital t-J model in the high-spin/Crossing regime and study superconducting pairing channels via a Bogoliubov mean-field decomposition.

Experimental results

Research questions

• RQ1 What is the ground-state spin configuration and orbital occupancy in La3Ni2O7 under pressure-tuned interactions?
• RQ2 How do orbital-selective correlations influence the electronic structure and potential superconducting pairing in the material?
• RQ3 What is the nature of the superconducting pairing symmetry and its orbital character in the presence of orbital-selective Mott physics?

Key findings

• Electron correlations drive a crossover from a low-spin S=1/2 state to a high-spin S=3/2 state in the bilayer two-orbital Hubbard model.
• In the crossover regime, strong orbital selectivity emerges with Z_x2−y2 ≪ Z_z2 and near half-filling of bonding orbitals.
• Further increasing interactions leads to an orbital-selective Mott phase where x^2−y^2 is localized while z^2 bonding remains itinerant, explaining suppressed Drude weight and two-component conductivity.
• An effective multiorbital t-J model predicts leading superconducting pairing that evolves from extended s-wave (A1g) to d-wave (B1g) as interorbital exchange J^xx is increased, with strong orbital selectivity in pairing amplitudes.
• The results highlight the role of multiorbital correlations and OSMP proximity in driving superconductivity in La3Ni2O7 under pressure.