Abstract
At high magnetic fields, monolayer graphene hosts competing phases distinguished by their breaking of the approximate SU(4) isospin symmetry. Recent experiments have observed an even denominator fractional quantum Hall state thought to be associated with a transition in the underlying isospin order from a spin-singlet charge density wave at low magnetic fields to an antiferromagnet at high magnetic fields, implying that a similar transition must occur at charge neutrality. However, this transition does not generate contrast in typical electrical transport or thermodynamic measurements and no direct evidence for it has been reported, despite theoretical interest arising from its potentially unconventional nature. Here, we measure the transmission of ferromagnetic magnons through the two-dimensional bulk of clean monolayer graphene. Using spin polarized fractional quantum Hall states as a benchmark, we find that magnon transmission is controlled by the detailed properties of the low-momentum spin waves in the intervening Hall fluid, which is highly density dependent. Remarkably, as the system is driven into the antiferromagnetic regime, robust magnon transmission is restored across a wide range of filling factors consistent with Pauli blocking of fractional quantum Hall spin-wave excitations and their replacement by conventional ferromagnetic magnons confined to the minority graphene sublattice. Finally, using devices in which spin waves are launched directly into the insulating charge-neutral bulk, we directly detect the hidden phase transition between bulk insulating charge density wave and a canted antiferromagnetic phase at charge neutrality, completing the experimental map of broken-symmetry phases in monolayer graphene.
- Received 6 February 2021
- Revised 22 February 2022
- Accepted 28 April 2022
DOI:https://doi.org/10.1103/PhysRevX.12.021060
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
A magnon is a collective excitation of electron spins in a solid. As with charge transport, different materials have different magnon-transport properties, and measuring magnon transport can help researchers understand the magnetic order of an electronic system. This approach is especially useful in insulators, where charge transport measurements are not possible. In this work, we study magnon transmission in graphene under a strong magnetic field and detect a long-predicted (but never observed) transition from an antiferromagnet state to one with a charge density wave.
In our experiment, we generate magnons by inducing electron scattering between states with opposite spins, and we subsequently detect the magnons via an inverse process. We find that measuring magnon transmission is an effective probe of the electronic states, finding direct evidence for the phase transition—magnons are transmitted through the antiferromagnet but not through the charge density wave, despite both being equally insulating electrically.
Besides completing the predicted phase diagram of charge-neutral monolayer graphene, our analysis reveals that magnon transmission measurement is a sensitive probe of correlated physics in the fractional quantum Hall regime, where theoretical predictions are not available. Similar measurements may also be useful in a wide variety of systems where strong correlations drive spin ordering.
