Special CPM Seminar

Quantum Hall Ferromagnets and Skyrmions in Graphene

Mark-Oliver Goerbig

Université de Paris-Sud

Skyrmions are particular spin-texture states that arise in low-dimensional ferromagnets. After a short general introduction to such systems, I will argue that Landau levels, formed due to the energy quantization of 2D electrons in a strong perpendicular magnetic field, can be viewed as paradigmatic systems hosting ferromagnets and skyrmions. While quantum Hall systems in the conventional semiconductor heterostructures are rather difficult to access by spectroscopic means, graphene is an ideal 2D system with electrons residing at the surface. Furthermore, graphene electrons display a much richer behavior than semiconductor heterostructures due to a twofold valley degeneracy that conspires with the physical spin of the electrons. This provides a four-component structure that leads to particular spin-valley ferromagnetic states and skyrmions with a generic entanglement between the spin and valley degrees of freedom [1]. In addition to a theoretical classification of these states, I will present calculated spin- and lattice-resolved patterns in the central (n=0) Landau level that one may expect to observe spin-resolved scanning tunnelling microscopy [2,3].

[1] B. Doucot et al., Phys. Rev. B 78, 195327 (2008). [2] Y. Lian, A. Rosch, and M. O. Goerbig, Phys. Rev. Lett. 117, 056806 (2016). [3] Y. Lian and M. O. Goerbig, Phys. Rev. B 95, 245428 (2017).