Emergent Chiral Metal near a Kondo Breakdown Quantum Phase Transition

Abstract: The destruction of the Kondo effect in a local-moment metal can lead to a topological non-Fermi-liquid phase, dubbed fractionalized Fermi liquid, with spinon-type excitations and an emergent gauge field. We demonstrate that, if the latter displays an internal $π$-flux structure, a chiral heavy-fermion metal emerges near the Kondo-breakdown transition. Utilizing a parton mean-field theory describing the transition between a conventional heavy Fermi liquid and a U(1) fractionalized Fermi liquid, we find a novel intermediate phase near the transition whose emergent flux pattern spontaneously breaks both translation and time-reversal symmetries. This phase is an orbital antiferromagnet, and we derive a Landau-type theory which shows that such a phase generically emerges from a $π$-flux spin liquid. We discuss the relevance to pertinent experiments.

• The paper identifies a novel chiral heavy-fermion metal phase that emerges between the conventional heavy Fermi liquid and the fractionalized Fermi liquid states, marked by broken time-reversal symmetry and emergent plaquette fluxes.
• The paper employs a parton mean-field theory combined with the Abrikosov-fermion representation to calculate single-particle spectral functions and reveal complex Fermi surface modifications.
• The findings suggest experimental implications for materials like Pr₂Ir₂O₇, underscoring the importance of low-temperature studies to validate non-Fermi-liquid behavior and Kondo breakdown phenomena.

Emergent Chiral Metal near a Kondo Breakdown Quantum Phase Transition

Introduction

The study investigates the emergence of a chiral heavy-fermion metallic phase near the quantum phase transition from a conventional heavy Fermi liquid (FL) to a fractionalized Fermi liquid (FL$^\ast_\pi$) characterized by $\pi$-flux spin liquid behavior. Through a parton mean-field method, the authors demonstrate that intermediate chiral metal phases emerge naturally, exhibiting broken translation and time-reversal symmetries. This phase is distinct as an orbital antiferromagnet, providing new insights into non-Fermi-liquid behavior and Kondo physics.

Methodology

The research utilizes a parton mean-field theory to model the Kondo-Heisenberg system, conceptualizing transitions between FL and FL$^\ast_\pi$ phases. The model assumes a square lattice with a $\pi$-flux Dirac spin liquid, where the conduction electron system interacts with $f$-electron local moments leading to various phase transitions. The theory involves a U(1) gauge symmetry, which helps understand emergent phases through adjustments in the Kondo coupling and conduction band filling.

A significant component is the use of the Abrikosov-fermion representation for spins, facilitating the mean-field decoupling. This enables the detailed calculation of single-particle spectral functions and their resulting band structures across different phases. The analysis incorporates fluctuations to a limited extent, recognizing the stability of emergent phases against the same.

Results

The key findings include the identification of the chiral heavy-fermion metal phase (FL$_c$) between the FL and FL$^\ast_\pi$ regions in the phase diagram. Predominantly occupying low-temperature zones, the FL$_c$ phase is characterized by non-zero emergent plaquette fluxes and hybridization gaps, demonstrating broken time-reversal symmetry. The transition into this phase involves continuous or first-order transitions marking symmetry breakage.

Elective results illustrate that in FL$_c$, multiple conductive electron pockets emerge, distinguishable from FL$^\ast_\pi$ and conventional heavy Fermi-liquid phases. The heavy-electron bands demonstrate complex Fermi surface modifications, attributing to the chiral nature of the emergent phase. Moreover, the continuous transition from FL$^\ast_\pi$ to FL$_c$ revelaes non-Landau behavior, accentuated by non-analytic gauge-flux dependencies.

Experimental Implications

The research points to materials such as Pr$_2$Ir$_2$O$_7$, a known candidate exhibiting T-breaking states without traditional dipole ordering, as potential real-world representations of the proposed chiral metal phase. Such materials, under suitable experimental conditions exhibiting weak Kondo screening, could validate the theoretical predictions made. This stresses the imperative need for low-temperature electronic structure investigations to substantiate the theoretical models and aid in discovering new materials hosting similar physics.

Conclusion

The paper elucidates a novel chiral metal phase near a Kondo breakdown transition, showcasing the potential for discovering new metallic phases with exotic properties. The approach broadens understanding of Kondo systems and non-Fermi-liquid phases, offering insights for future theoretical and experimental studies. The implications of such work are significant in quantum materials research, particularly in systems with competing magnetic interactions and quantum criticality.