Emergence of ferromagnetism at the onset of moiré Kondo breakdown

In the Kondo lattice model, the interaction of a lattice of localized magnetic moments with a sea of conduction electrons induces rich quantum phases of matter, including Fermi liquids with heavily renormalized electronic quasiparticles, quantum critical non-Fermi liquid metals and unconventional superconductors. The recent demonstration of moiré Kondo lattices has presented an opportunity to study the Kondo problem with continuously tunable parameters. Although a heavy Fermi liquid phase has been identified, the magnetic phases remain unexplored in moiré Kondo lattices. Here we report a density-tuned Kondo breakdown in MoTe2/WSe2 moiré bilayers by combining magnetotransport and optical studies. 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 emergence of a ferromagnetic Anderson insulator and suppression of the Kondo screening effect below the critical density. Our results suggest a path for realizing other quantum phase transitions in moiré Kondo lattices. Kondo physics has been observed in moiré bilayers, but the expected magnetic transitions have not been reported. Now, a metal–insulator transition with ferromagnetic order that develops at nearly the same time is reported in a moiré bilayer.