Pseudogap and Non-Fermi-liquid criticality in double Kondo model for bilayer nickelates

TL;DR

This paper investigates the phase diagram of a bilayer Kondo lattice model relevant to nickelates, revealing a pseudogap phase with non-Fermi-liquid criticality, characterized by a heavy Fermi liquid-like behavior without symmetry breaking.

Contribution

It introduces a novel pseudogap phase in the bilayer Kondo model, explains its properties with an analytical framework, and connects the findings to experimental nickelate systems.

Findings

Identification of a non-Fermi-liquid critical point separating Fermi liquid and pseudogap phases.

Discovery of a 'second Fermi liquid' pseudogap phase with small hole pockets and heavy quasiparticles.

Proposal that current nickelate samples are in the overdoped Fermi liquid regime, with pseudogap accessible via doping.

Abstract

Motivated by recent experimental progress on high-temperature superconductivity in bilayer nickelates, we investigate the phase diagram of the normal state in a bilayer Kondo lattice model using single-site dynamical mean-field theory (DMFT). When the interlayer tunneling $t_\perp$ is absent, we identify a non-Fermi-liquid (NFL) critical point tuned by the interlayer spin coupling $J_\perp$ or hole doping $x$, which separates a standard Fermi liquid in the overdoped region from a distinct pseudogap (PG) metal in the underdoped regime. This PG phase, which we term the `second Fermi liquid' (sFL), exhibits small hole pockets and violates the perturbative Luttinger theorem despite the absence of symmetry breaking or fractionalization. The PG metal behaves like a heavy Fermi liquid, with small quasi-particle residue and large effective mass. We also provide an intuitive analytical description of the pseudogap and the ground-state wave function based on an ancilla-fermion framework. Inside the PG phase, we interpret the ancilla fermion as a spin-polaron and demonstrate a Kondo-like resonance peak in the spectral function of this composite fermion directly in DMFT calculation. Extending the analysis to finite $t_\perp$, we apply this framework to the bilayer nickelate $\mathrm{La}_3\mathrm{Ni}_2\mathrm{O}_7$. We propose that current experimental samples ($x \approx 0.5$) reside in the overdoped FL regime, suggesting that the pseudogap phase and the NFL criticality may be accessed via electron doping.