Jonathan ATTEIA – Skyrmions and magnons in the graphene quantum Hall ferromagnet

Jonathan Atteia, LPS Orsay

As a consequence of the approximate spin-valley symmetry in graphene in a strong magnetic field, the ground state of electrons at charge neutrality is a particular SU(4) quantum Hall ferromagnet. If only the Coulomb interaction is taken into account, this ferromagnet can concern either the spin or the valley pseudo-spin degrees of freedom. This choice is limited by subleading energy scales that explicitly break the SU(4) symmetry, the simplest of which is the Zeeman energy. Additionally, valley symmetry breaking terms can arise from short-range interactions or electron-phonon couplings.
We build upon the phase diagram of the ground state in order to identify the different skyrmions at filling ν=0 that are compatible with these quantum-Hall ferromagnets. Similarly to the ferromagnets, the skyrmions are described by the Gr(2,4) Grassmannian at the center, allowing us to construct the skyrmion spinors. By a variational approach we obtain a phase diagram with 12 different skyrmion types, each having a clear signature in local sublattice-resolved spin magnetization, possibly accessible in scanning-tunneling microscopy and spectroscopy.
We also describe generalized spin waves at ν =±1, finding spin waves, valley-pseudospin waves, as well as more exotic entanglement waves with a mixed spin-valley character. The SU(4) symmetry breaking terms yield spectral gaps, but also in the case of residual ground state symmetries change the spin-wave dispersion from quadratic to linear. Several recent experiments successfully emitted and detected spin waves in the graphene quantum Hall ferromagnet and observed their scattering at junctions between regions with different filling factors.

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(Meeting ID: 861 6817 3007; PASSWORD: 91405)