[Paper Review] Spin-singlet and spin-triplet pairing correlations in antiferromagnetically coupled Kondo systems

Haoyu Hu, Ang Cai+2 more|arXiv (Cornell University)|Sep 27, 2021

Rare-earth and actinide compounds40 references4 citations

TL;DR

This study investigates spin-singlet and spin-triplet pairing correlations in antiferromagnetically coupled Kondo systems using non-perturbative continuous-time quantum Monte Carlo (CT-QMC) simulations in cluster Bose-Fermi Anderson models. It identifies Kondo-destruction quantum critical points where spin-singlet pairing is strongly enhanced in SU(2) symmetric systems, while realistic Ising anisotropy makes spin-triplet (S^z=0) pairing competitive, offering a microscopic explanation for field-induced spin-triplet superconductivity in YbRh2Si2.

ABSTRACT

Recent experiments in quantum critical heavy fermion metals have pushed to the fore the question about whether antiferromagnetic fluctuations can promote both spin-singlet and spin-triplet superconductivity. Here we address the issue through non-perturbative calculations in antiferromagnetically correlated Kondo systems. We identify Kondo-destruction quantum critical points in both the SU(2) symmetric and Ising-anisotropic cluster Bose-Fermi Anderson models. The spin-singlet pairing correlations are significantly enhanced near the quantum critical point in the SU(2) case; however, with adequate but still realistic degree of Ising anisotropy, the spin-triplet pairing correlations are competitive. Our results demonstrate that spin-flip processes strengthen the spin-singlet pairing at the Kondo-destruction quantum critical points, and point towards a way for antiferromagnetic correlations to drive spin-triplet pairing. Further implications of our findings in the broader context of strongly correlated superconductivity are discussed.

Motivation & Objective

• To understand whether antiferromagnetic fluctuations can simultaneously promote both spin-singlet and spin-triplet superconductivity in quantum critical Kondo systems.
• To investigate the role of spin anisotropy in determining the dominance of singlet versus triplet pairing near a Kondo-destruction quantum critical point.
• To provide a microscopic explanation for the recently observed field-induced spin-triplet superconducting phase in YbRh2Si2 near its quantum critical point.
• To extend beyond-Landau quantum criticality models by incorporating spin-flip processes and finite Ising anisotropy in cluster BFAMs.
• To determine the conditions under which spin-triplet pairing becomes competitive or dominant in strongly correlated Kondo systems.

Proposed method

• Non-perturbative continuous-time quantum Monte Carlo (CT-QMC) simulations with a triple expansion in hybridization, transverse exchange, and bosonic coupling.
• Use of cluster Bose-Fermi Anderson models (BFAM) to model antiferromagnetic Kondo systems with RKKY interactions and dynamical magnetic fluctuations.
• Implementation of an SU(2) symmetric CT-QMC method for multi-impurity systems to access extended quantum critical regimes.
• Analysis of pairing susceptibilities in both spin-singlet and spin-triplet channels (S^z = 0, ±1) to identify dominant pairing channels.
• Systematic variation of Ising anisotropy (Jp/Jz) and coupling strength (λ) to probe the transition between SU(2) and Ising limits.
• Identification of Kondo-destruction quantum critical points via diverging spin and pairing susceptibilities in the cluster BFAM framework.

Experimental results

Research questions

• RQ1 Can antiferromagnetic fluctuations in Kondo systems simultaneously promote both spin-singlet and spin-triplet superconductivity?
• RQ2 How does spin anisotropy influence the relative strength of spin-singlet versus spin-triplet pairing near a Kondo-destruction quantum critical point?
• RQ3 What is the role of spin-flip processes in enhancing spin-singlet pairing correlations in quantum critical Kondo systems?
• RQ4 Can the observed field-induced spin-triplet superconductivity in YbRh2Si2 be explained by antiferromagnetic quantum criticality in a Kondo-destruction framework?
• RQ5 Under what conditions does the S^z=0 spin-triplet channel become competitive with the spin-singlet channel in anisotropic Kondo systems?

Key findings

• In the SU(2) symmetric cluster BFAM, spin-singlet pairing correlations are significantly enhanced near the Kondo-destruction quantum critical point (QCP), with a peak in the pairing susceptibility χ^(s,0) at λc ≈ 0.2.
• In the Ising-anisotropic model with Jp/Jz = 0.2, the spin-triplet pairing susceptibility χ^(t,0) in the S^z=0 channel is similarly enhanced near the QCP, with λc ≈ 0.45, indicating competitive pairing.
• Spin-flip processes, mediated by transverse RKKY interactions, strengthen spin-singlet pairing, explaining why singlet pairing dominates in the SU(2) symmetric limit.
• The S^z=0 spin-triplet channel becomes competitive with spin-singlet pairing under realistic Ising anisotropy, suggesting a mechanism for triplet pairing in quantum critical Kondo systems.
• The results provide a natural explanation for the field-induced spin-triplet superconducting phase in YbRh2Si2, where the applied magnetic field enhances Ising anisotropy and stabilizes the S^z=0 triplet channel.
• The findings suggest that antiferromagnetic fluctuations can drive spin-triplet pairing via Kondo-destruction quantum criticality, extending the scope of quantum criticality to unconventional superconductivity in strongly correlated metals.