REVIEW
Variational Quantum Monte Carlo investigations of the superconducting pairing in La$_3$Ni$_2$O$_7$
Superconductivity · Submitted 12 May 2025 · 74 references
Pith's verdict: First VQMC comparison of s± and d-wave pairing in the bilayer two-orbital nickelate model, but the d-wave ansatz is under-flexible and the tiny energy differences come without error bars; the s± conclusion is plausible but not airtight.
T0 · AI review
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Referee report
The paper
The paper claims
The paper claims that strong local correlations in La3Ni2O7 favor s±-wave pairing, led by the 3d_{3z^2-r^2} vertical bond.
The reading
What carries the argument
The central object is the variational wavefunction $|\Psi_G\rangle = \hat{P}|\Psi_0\rangle$, where $|\Psi_0\rangle$ is the ground state of a free BCS-type variational Hamiltonian and $\hat{P}$ is a product of Gutzwiller and Jastrow factors that enforce the relevant local Hilbert space: one or two electrons per site for the t-J model, with doublon-holon processes for the Hubbard model. The argument runs by optimizing all hoppings, pairings, and Jastrow parameters with stochastic reconfiguration and comparing the optimized energies of the s±-wave ansatz (onsite, vertical-bond, and in-plane nearest-neighbor pairings) against the d_{$x^{2}$-$y^{2}$}-wave ansatz (in-plane nearest-neighbor pairings only). The pairing parameters are inspected after optimization, which is how the paper identifies the 3d_{$3z^{2}$-$r^{2}$} vertical bond as the leading pairing channel.
Extended reading notes
Core claim
On the paper's own terms, the discovery is an energy ordering established by unbiased variational optimization: in the two-orbital bilayer model with parameters from band-structure studies, the Jastrow-projected BCS ansatz with s±-wave pairing lowers the ground-state energy more than the analogous d_{$x^{2}$-$y^{2}$}-wave ansatz. For the Hubbard model, s± is favored for Hund's coupling above about 0.2 eV; for the extended t-J model, it wins over the entire parameter range studied. The optimized pairing amplitudes show the dominant component is $\Delta^z_{ij}$ on the vertical bond between 3d_{$3z^{2}$-$r^{2}$} orbitals, with onsite pairings of opposite sign on the two orbitals, consistent with pair-hopping. The result bridges the weak-to-moderate coupling regime, where renormalization-group studies find s±, and the strong-coupling regime, where the t-J model applies. A secondary finding is that intra-orbital double occupancy stays finite as U grows, so the electrons do not become well-defined local moments despite strong correlations.
Load-bearing premise
Everything rests on the variational wavefunction being flexible enough: the calculation only lets pairs form on a fixed set of bonds (onsite, vertical, and in-plane nearest-neighbor), so if a pairing channel outside that set would lower the energy, the s± versus d-wave comparison is incomplete.
Editorial extensions
If this is right
- If s± is the ground state, the superconducting gap must change sign between the α pocket around Γ and the β/γ pockets around M, a signature phase-sensitive experiments can look for.
- The dominance of the vertical 3d_{3z^2-r^2} pairing means interlayer coupling is not a minor correction: materials or pressures that alter the Ni-O-Ni vertical bonding should strongly affect the critical temperature.
- The consistency between weak-coupling renormalization-group and strong-coupling variational results argues that s±-wave pairing survives across the correlation regime, not just at weak or strong coupling.
- Finite double occupancy even at large U implies the carriers retain itinerant character, so treatments that start from a local-moment picture will miss part of the pairing physics.
Reading between the lines
- The variational ansatz restricts pairing to a fixed bond set; adding d_{xy} or longer-range extended-s channels could lower the energy further, in which case the paper's s± versus d-wave comparison would be incomplete rather than wrong.
- The paper's 'not too weak Hund's coupling' threshold (about 0.2 eV in the Hubbard model) makes the pairing-symmetry question partly an experimental one: a reliable determination of the Hund's coupling in La3Ni2O7 would say which side of the competition the real material sits on.
- Because the leading pair lives on the vertical 3d_{3z^2-r^2} bond, tuning interlayer hybridization through chemical pressure or rare-earth substitution in the R3Ni2O7 family gives a testable route to confirm the mechanism.
- The finite double occupancy at strong coupling suggests spin-fluctuation theories built purely on local moments may underestimate pairing strength; an orbital-resolved susceptibility measurement could test the itinerant picture.
Original abstract
We investigate the pairing symmetry in the novel superconductor La$_3$Ni$_2$O$_7$ under pressure by the non-perturbative variational quantum Monte Carlo. Within the bilayer Hubbard model and extended $t-J$ model with two orbitals in the $E_g$ doublet, we find the local strong correlation triggers $s_\pm$-wave Cooper pairing, with sign change of the gap function among the various Fermi pockets, while the $d_{x^2-y^2}$-wave pairing is generically disfavored. This is in agreement with the results from functional renormalization group applied in the weak up to moderate correlation limit. We find the 3d$_{3z^2-r^2}$ orbital plays a leading role in the superconducting pairing. We also demonstrate the finite intra-orbital double occupancy even in the strong correlation limit, shedding light on the itinerant versus local moment picture of the electrons in this material.
The compiled record
The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.
The abstract, sentence by sentence
We investigate the pairing symmetry in the novel superconductor La$_3$Ni$_2$O$_7$ under pressure by the non-perturbative variational quantum Monte Carlo. Within the bilayer Hubbard model and extended $t-J$ model with two orbitals in the $E_g$ doublet, we find the local strong correlation triggers $s_\pm$-wave Cooper pairing, with sign change of the gap function among the various Fermi pockets, while the $d_{x^2-y^2}$-wave pairing is generically disfavored. This is in agreement with the results from functional renormalization group applied in the weak up to moderate correlation limit. We find the 3d$_{3z^2-r^2}$ orbital plays a leading role in the superconducting pairing. We also demonstrate the finite intra-orbital double occupancy even in the strong correlation limit, shedding light on the itinerant versus local moment picture of the electrons in this material.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | We investigate the pairing symmetry in the novel superconductor La$_3$Ni$_2$O$_7$ under pressure by the non-perturbative variational quantum Monte Carlo. | contribution 0.98 | 0.95 | 0.06 | asserted 0.88 |
| s1 | Within the bilayer Hubbard model and extended $t-J$ model with two orbitals in the $E_g$ doublet, we find the local strong correlation triggers $s_\pm$-wave Cooper pairing, with sign change of the gap function among the various Fermi pockets, while the $d_{x^2-y^2}$-wave pairing is generically disfavored. | result 1.00 | 0.97 | 0.83 | asserted 1.00 |
| s2 | This is in agreement with the results from functional renormalization group applied in the weak up to moderate correlation limit. | interpretation 0.59 | 0.07 | 0.06 | asserted 0.72 |
| s3 | We find the 3d$_{3z^2-r^2}$ orbital plays a leading role in the superconducting pairing. | result 1.00 | 0.96 | 0.19 | asserted 1.00 |
| s4 | We also demonstrate the finite intra-orbital double occupancy even in the strong correlation limit, shedding light on the itinerant versus local moment picture of the electrons in this material. | result 0.81 | 0.96 | 0.06 | asserted 1.00 |
What was examined, and what was not
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- P abstract
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Figures
Figures reproduced from arXiv:2505.07341 by the authors.
References
How the literature uses it
1 arXiv paper cite this in 1 sentence
Papers whose main result relates to this one's
States a conflicting result 1 paper
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However, we held a different viewpoint that the interlayer pairing $s$-wave SC is induced by the $3d_{x^2-y^2}$ orbital, driven by the strong interlayer superexchange interaction.
Interplay of two $E_g$ orbitals in Superconducting La$_3$Ni$_2$O$_7$ Under Pressure · 0.91
Papers citing it
Cites it as background 1 paper
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A large number of experimental [ref] and theoretical [ref] studies have been conducted to investigate the superconductivity, density wave and normal state electronic properties.
Strain-Engineered Electronic Structure and Superconductivity in La$_3$Ni$_2$O$_7$ Thin Films · Introduction · 1.00
Discussion
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