[Paper Review] Emergence of ferromagnetism at the onset of moiré Kondo breakdown

Wenjin Zhao, Bowen Shen+10 more|arXiv (Cornell University)|Oct 9, 2023

Rare-earth and actinide compoundsPhysics and Astronomy3 citations

TL;DR

This study demonstrates the emergence of ferromagnetism at the onset of moiré Kondo breakdown in AB-stacked MoTe2/WSe2 bilayers via density-tuned transport and optical measurements. As carrier density decreases, the Kondo temperature drops, leading to a heavy Fermi liquid to insulator transition accompanied by the onset of ferromagnetic order, consistent with a ferromagnetic Anderson insulator and suppressed Kondo screening.

ABSTRACT

The interaction of a lattice of localized magnetic moments with a sea of conduction electrons in Kondo lattice models induces rich quantum phases of matter, such as Fermi liquids with heavily renormalized electronic quasiparticles, quantum critical non-Fermi liquid metals and unconventional superconductors, among others. The recent demonstration of moiré Kondo lattices has opened the door to investigate the Kondo problem with continuously tunable parameters. Although a heavy Fermi liquid phase has been identified in moiré Kondo lattices, the magnetic phases and Kondo breakdown transitions remain unexplored. Here we report a density-tuned Kondo destruction in AB-stacked MoTe2/WSe2 moiré bilayers by combining magneto transport and optical studies. As the itinerant carrier density decreases, the Kondo temperature decreases. At a critical density, we observe a heavy Fermi liquid to insulator transition, and a nearly concomitant emergence of ferromagnetic order. The observation is consistent with the scenario of a ferromagnetic Anderson insulator and suppression of the Kondo screening effect. Our results pave the path for inducing other exotic quantum phase transitions in moiré Kondo lattices.

Motivation & Objective

• To investigate magnetic phases and Kondo breakdown transitions in moiré Kondo lattices, which remain unexplored despite the observation of heavy Fermi liquids.
• To explore the interplay between Kondo screening, electron correlation, and emergent magnetism in two-dimensional moiré superlattices.
• To determine the role of carrier density in driving quantum phase transitions, particularly the transition from heavy Fermi liquid to insulator with concomitant ferromagnetism.
• To provide experimental evidence for the formation of a ferromagnetic Anderson insulator in a moiré Kondo lattice system.
• To establish a platform for inducing exotic quantum phase transitions in moiré heterostructures through tunable parameters.

Proposed method

• Employed density-tuned magneto-transport measurements in AB-stacked MoTe2/WSe2 moiré bilayers to probe electronic correlations.
• Conducted optical spectroscopy to probe the electronic response and identify signatures of Kondo screening and magnetic order.
• Monitored the evolution of the Kondo temperature as a function of carrier density to identify the onset of Kondo breakdown.
• Analyzed the transport behavior near the Mott transition to detect the emergence of insulating and magnetic phases.
• Used the observed suppression of Kondo screening and onset of ferromagnetism to infer the formation of a ferromagnetic Anderson insulator.

Experimental results

Research questions

• RQ1 How does carrier density tuning affect the Kondo screening and electronic phases in moiré Kondo lattices?
• RQ2 What is the nature of the quantum phase transition from a heavy Fermi liquid to an insulator in MoTe2/WSe2 moiré heterostructures?
• RQ3 Does the onset of insulating behavior coincide with the emergence of ferromagnetic order, and if so, what is the underlying mechanism?
• RQ4 Can the observed magnetic phase be explained by a ferromagnetic Anderson insulator scenario with suppressed Kondo screening?
• RQ5 What role does the moiré superlattice potential play in stabilizing exotic quantum phases such as Kondo breakdown and ferromagnetism?

Key findings

• A heavy Fermi liquid to insulator transition is observed at a critical carrier density, accompanied by a sharp drop in the Kondo temperature.
• Ferromagnetic order emerges nearly simultaneously with the insulating transition, indicating a close correlation between Kondo breakdown and magnetic ordering.
• The observed behavior is consistent with a ferromagnetic Anderson insulator, where localized magnetic moments are not screened by conduction electrons due to Kondo breakdown.
• Suppression of Kondo screening is directly linked to the onset of insulating and ferromagnetic phases, supporting a scenario of competing Kondo and magnetic correlations.
• The results demonstrate that moiré Kondo lattices provide a tunable platform for exploring exotic quantum phase transitions, including Kondo breakdown and emergent magnetism.
• The combination of transport and optical measurements confirms the coexistence of insulating behavior and ferromagnetism at low carrier densities.