Left: Schematic illustration of an artificial Kondo moiré network made of corner-aligned MoTe2/WSe2 moiré bilayers. The MoTe2 and WSe2 The layer hosts a network of local magnetic moments and a gas of itinerant carriers, respectively. Right: Phase diagram of temperature and density in the Kondo lattice regime. The Kondo destruction transition occurs at the critical density marked by the vertical dotted line, where ferromagnetic correlations and ordering are observed. Credit: Wenjin Zhao, adapted from Natural physics (2024). DOI: 10.1038/s41567-024-02636-4
Moire superlattices are materials consisting of two layers stacked on top of each other with either a small rotational misalignment or a lattice offset between them. The Kondo lattice model, on the other hand, describes systems in which conduction electrons interact with localized magnetic impurities, which changes the electrical and magnetic properties of the systems.
In recent years, physicists have succeeded in designing materials combining the physical properties of these two types of systems. These materials, known as Kondo moiré lattices, are essentially moiré superlattice structures with a periodic arrangement of localized magnetic moments, resembling that described by the Kondo lattice model.
Researchers at Cornell’s Kavli Institute for Nanoscale Science, Cornell University and the National Institute of Materials Science in Japan synthesized and examined Kondo moiré networks, hoping to better understand their underlying physics.
Their most recent article, published in Natural physicsreports the emergence of ferromagnetism at the onset of a density-adapted Kondo rupture in MoTe2/WSe2 moiré bilayers.
“Our work builds on our previous report on realizing an electrically tunable Kondo moiré array system,” Kin Fai Mak, co-author of the paper, told Phys.org. “There we reported the fabrication of an artificial Kondo lattice using moiré semiconductors and the observation of gate-tunable heavy fermions.”
A key goal of Kondo’s lattice physics research efforts is to better understand how heavy fermions in these systems decay under different external parameters, such as doping density, magnetic field, and interaction force. This breakdown, known as the Kondo destruction transition, is often accompanied by the emergence of exotic states of matter (e.g., non-Fermi liquid phase and unconventional superconductivity).
In their previous research, Mak and colleagues designed a highly tunable moiré Kondo array system based on MoTe.2/WSe2 moiré bilayers. This material provides a unique opportunity to examine the Kondo destruction transition in a continuous manner, which has proven very difficult in the case of bulk heavy fermion materials.
“In this context, our Natural physics The paper studied the fate of heavy fermions by continuously adjusting the density of roaming carriers in the system, which adjusts the effective Kondo coupling strength,” Mak said. “Near a critical density, we observed a destruction of heavy fermions and the simultaneous emergence of a ferromagnetic Anderson insulator.
In their new study, the researchers examined the physics of the Kondo lattice emerging in the moiré semiconductor: angularly aligned MoTe2/WSe2 heterobilayer presented in their previous article. Their results highlight the promise of Kondo moiré networks for studying the Kondo destruction transition using a tunable platform, as well as the possibility of realizing other exotic states of matter in the vicinity of such a transition.
“A moire lattice appears in the material due to the 7% mismatch between the MoTe lattice2 and WSe2“, explained Mak. “As a result, no twist angle is required to create the moiré superlattice potential. We have manufactured Double Door Hall Bar Devices; the device allows independent control of the total doping density in the material as well as the relative distribution of doping densities in each transition metal dichalcogenide layer.
Thanks to their manufacturing strategy, Mak and his colleagues were able to prepare their material in the Kondo lattice regime, which allowed them to continuously study the Kondo destruction transition as it occurred. . To probe the emergence of magnetic states near this phase transition, they used a combination of magneto-transport and magneto-optical spectroscopy.
“We measured the anomalous Hall response and spontaneous circular dichroism in the material to demonstrate the emergence of a ferromagnetic Anderson insulator,” Mak said. “We also examined temperature- and magnetic-field-dependent transport properties to show that the ferromagnetic Anderson insulator emerges near the Kondo destruction transition.”
The measurements collected by this research team gave interesting results. As their material approached the density-adapted Kondo destruction transition, Mak and his colleagues observed the almost simultaneous appearance of a metal-insulator transition and a magnetic quantum phase transition.
“Since the two transitions involve different degrees of freedom (one is charge and the other is spin), the appearance of both transitions at almost the same critical density is unexpected,” Mak said. “The observation invites new ideas about how to describe this transition without resorting to fine-tuning parameters in theory.”
As their material approached the Kondo destruction transition, the team also observed ferromagnetic correlations. This observation contrasts with most previous studies of other known Kondo destruction transitions, which instead reported antiferromagnetic correlations and magnetic ordering.
The recent study by Mak and colleagues thus opens exciting new opportunities for the study of the Kondo destruction transition. More precisely, this allows us to study this transition in a different regime, marked by ferromagnetism instead of antiferromagnetism.
“An immediate plan for further research will be to push the Kondo destruction transition to occur at higher critical densities by changing the twist angle of the material,” Mak said. “Much less impact of disorder is expected at higher critical densities, allowing us to study the quantum phase transition more intrinsically.
“Previous theoretical studies have predicted the signatures of quantum spin liquids in the intrinsic regime, where exotic non-Fermi liquids could also emerge.”
In their next studies, the researchers also plan to search for emergent topological states of matter in the Kondo lattice regime. This will be achieved by examining the Kondo moiré network they synthesized while further increasing the density of roaming charge carriers.
“Recent theoretical studies have pointed out that the Kondo interaction in our material system is chiral, thus opening the door to the realization of Kondo topological insulators and Kondo topological semimetals,” Mak added. “We are actively looking for the transport and thermodynamic signatures of these phases of matter.”
More information:
Wenjin Zhao et al, Emergence of ferromagnetism at the start of Kondo moiré breakdown, Natural physics (2024). DOI: 10.1038/s41567-024-02636-4
© 2024 Science X Network
Quote: Physicists report the emergence of ferromagnetism at the start of the Kondo breakup in moiré bilayer networks (October 17, 2024) retrieved October 18, 2024 from
This document is subject to copyright. Apart from fair use for private study or research purposes, no part may be reproduced without written permission. The content is provided for informational purposes only.
