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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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arXiv ID2505.07341 (version 1)
Pith IDT3CWQVYY, Pith's permanent record of this paper and its review
CitationDa Wang, Qiang-Hua Wang, Yi-Qun Liu. "Variational Quantum Monte Carlo investigations of the superconducting pairing in La$_3$Ni$_2$O$_7$." arXiv:2505.07341 (2025). https://pith.science/paper/T3CWQVYY
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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

The paper asks which superconducting pairing symmetry wins in the pressurized nickelate La3Ni2O7, a bilayer material with two active orbitals per nickel. Using variational quantum Monte Carlo on the bilayer two-orbital Hubbard model and its strong-coupling extended t-J version, it claims that the s±-wave state—a gap that changes sign between different Fermi pockets—is lower in energy than the d_{$x^{2}$-$y^{2}$}-wave state whenever Hund's coupling is not too weak, and in the t-J model always. The leading pairing component sits on the vertical bond between 3d_{$3z^{2}$-$r^{2}$} orbitals, identifying that orbital as the main actor. If the claim is right, theory and experiment should focus on sign-changing gap states and on interlayer coupling, not on d-wave pairing.

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

Editorial extensions of the paper, not claims the author makes directly.

  • 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.

Compiled by classifier-1.13.0 via TypeSafe API, question bank abstract-p1-20260923, 2026-09-23: 1 request, 4,455 input tokens.

What the abstract itself states, as probabilities: claims first 0.04 · states limitation 0.11 · human participants 0.01 · controlled comparison 0.04 · randomized assignment 0.01 · states quantitative result 0.06 · compares with prior results 0.97 · mentions code or data availability 0.02.

The abstract, sentence by sentence

arXiv listing abstract v1. Results and the main claim are underlined; .

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

What the classifier reports for each sentence. The number is the probability it gave the label.
#sentencerolethe authors' own workstates the main conclusionstrength
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 kind of paper the classifier reports this is: simulation study 1.00 · evidence from simulation or computation 1.00.

What was examined, and what was not

sources read
  • arXiv listing abstract v1
not examined
  • full text
  • figures and tables
  • references and citing sentences
  • relations to other papers
levels run
  • P abstract
not run
  • S sections
  • T body sentences
  • C citations
  • R cross-paper relations

Every quoted sentence on this record is the authors'. Every label is from Pith's closed vocabulary, with the probability shown beside it. Every number in the measurements table is the authors', checked by code. Record as JSON

Figures
Figure 1. (a) Energy band dispersions along the high-symmet
Figure 2. (a) Schematic plot of the pairing potentials (or va
Figure 3. VQMC results of the t − J model. The energies of the s±-wave and dx 2−y 2 -wave pairings relative to the normal state are plotted versus U by fixing JH = 0.5 eV in (a), and versus JH by fixing U = 7 eV in (b). The corresponding optimized variational pairing parameters for the s±-wave are plotted versus U in (c) and versus JH in (d), respectively. the projection operator for n-electrons on orbital a and site i, Q0e,ia = (1−nia↑)(1−nia↓), Q2e,ia = nia↑nia↓, Q1e,ia = nia↑(1−nia↓) + (1−nia↑)nia↓, (4) and Qne,i is the projection operator for total charge n-electrons on site i, Qne,i = n…
Figure 4. VQMC results of the Hubbard model. The energies of t
Figure 5. Finite size effect with lattice size L = 6 to 12 for the energies of the s±-wave and dx 2−y 2 -wave pairings at U = 7 eV and JH = 0.5 eV
Figure 6. The optimized variational hopping and onsite ener
Figure 7. The optimized Jastrow factors for the Hubbard mode

Figures reproduced from arXiv:2505.07341 by the authors.

References
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Each citing sentence is taken from the citing paper's LaTeX source and typed by Pith's classifier. A label appears at probability 0.8 or more (0.95 for support), a contest at 0.7 or more; the quoted words are the citing authors'. How each label was checked · JSON

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Discussion

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