Abstract
In the Landau levels of a two-dimensional electron system or when flat bands are present, e.g., in twisted van der Waals bilayers, strong electron-electron interaction gives rise to quantum Hall ferromagnetism with spontaneously broken symmetries in the spin and isospin sectors. Quantum Hall ferromagnets support a rich variety of low-energy collective excitations that are instrumental to understand the nature of the magnetic ground states and are also potentially useful as carriers of quantum information. Probing such collective excitations, especially their dispersion , is experimentally challenging due to small sample size and measurement constraints. In this work, we demonstrate an all-electrical approach that integrates a Fabry-Pérot cavity with nonequilibrium transport to achieve the excitation, wave vector selection, and detection of spin waves in graphene heterostructures. Our experiments reveal gapless, linearly dispersed spin wave excitations in the Landau level of bilayer graphene, thus providing direct experimental evidence for a predicted canted antiferromagnetic order. We show that the gapless spin wave mode propagates with a high group velocity of several tens of kilometers per second and maintains phase coherence over a distance of many micrometers. Its dependence on the magnetic field and temperature agree well with the hydrodynamic theory of spin waves. These results lay the foundation for the quest of spin superfluidity in this high-quality material. The resonant cavity technique we develop offers a powerful and timely method to explore the collective excitation of many spin- and isospin-ordered many-body ground states in van der Waals heterostructures and opens the possibility of engineering magnonic devices.
10 More- Received 27 October 2020
- Revised 15 February 2021
- Accepted 24 February 2021
DOI:https://doi.org/10.1103/PhysRevX.11.021012
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
Spin waves, also known as magnons, are traveling disturbances of magnetization that encode information about the nature of the underlying magnetic architecture. Two-dimensional materials such as graphene exhibit a type of magnetism known as quantum Hall magnetism, which hosts a plethora of spin-wave excitations with fascinating fundamental properties and technological potentials. But probing these materials using conventional techniques is experimentally challenging. Here, we report on an all-electrical technique to probe magnons of a quantum Hall antiferromagnet formed in bilayer graphene, shedding light on its ground-state order. This method can potentially be applied to analyze a variety of 2D magnets inaccessible to conventional techniques.
Spin waves are commonly studied using neutrons or light. Measuring the difference between the incoming and outgoing neutron or light beams—a technique known as scattering—allows physicists to obtain the energy-momentum (or dispersion) relation of a spin wave, which carries important information on the nature of the magnetism involved. A recent experiment showed that spin waves can be generated and detected in graphene devices using an all-electrical method, but it could not resolve the dispersion relation.
We have built a Fabry-Pérot cavity into a bilayer graphene transport device and used the discrete resonant modes of the cavity to selectively transmit magnons of certain wave vectors. This method, combined with electrical excitation and detection, allows us to obtain the dispersion relation of spin waves. We show that bilayer graphene supports gapless, linearly dispersed spin-wave excitations—evidence of a type of antiferromagnetic order formed by strong electron correlations in this nonmagnetic material.
Our technique opens new pathways to probe spin transport in quantum Hall and other 2D magnets.