High-Field Skyrmions in Graphene
Theoretical Condensed Matter Physics
Final Report Abstract
The project combined a theory group from France (Paris-Orsay) and an experimental group from Germany (Aachen). They tackled one of the central paradigmatic electron systems prone to quantum mechanical behaviour, namely electron systems exhibiting the quantum Hall effect. The aim was to probe the excitations of particular quantum Hall phases in real space. These excitations can be collective in nature with a texture of the quantum mechanical degrees of freedom that looks like the spikes of a hedgehog, i.e. the texture rotates in real space from pointing forward to pointing backward and back to forward, if one moves along a circle on the surface of the hedgehog. These particular structures are called skyrmions and are robust due to a topological protection mechanism. They can be regarded as a novel type of quasi-particle emerging from the interactions between the electrons of the system. The skyrmions in the quantum Hall regime at charge neutrality, moreover, exhbit a complex interplay between the magnetic spin and a so-called pseudospin that is related to the distribution of the electronic wave function to the two atomic sublattices of graphene. The aim of the project was to disentangle this complex interplay by direct imaging of the skyrmion texture. During the project, the theory group has calculated the resulting interplay of the two degrees of freedom in detail as a function of the parameters that guide the interaction between the electrons. This served as the background for the first real space imaging of such skyrmions. The real space imaging was realized by a competitor group, but described in detail by the applicants. The experimental group has optimized the base for imaging of such skyrmions by developing new sample structures suited to combine quantum transport measurements and imaging of the resulting textures. Moreover, they have tackled another crucial issue in quantum Hall physics, namely the imaging of edge states that are responsible for the quantized transport at the relevant length scale dubbed the magnetic length and being about 10 nm. They used a scanning tunneling microscope for that purpose and established conditions where the probe that is electrically charged does not perturb the mapping of the edge states. For this purpose, they analyzed the perturbing influence of the probe in quantitative detail such they could tune the parameters into the non-disturbing regime. This breakthrough result has so far only been used to show such edge states as proof of principle, but will soon enable a detailed investigation of the edge states for varying interfaces between different quantum Hall phases using the optimized sample structures. Concerning the skyrmions, only the proof of principle has been shown by mapping the sublattice degree of freedom of the skyrmion. This leaves the mapping of the competing spin degree of freedom for future studies.
Publications
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Skyrmion zoo in graphene at charge neutrality in a strong magnetic field. Physical Review B, 103(3).
Atteia, Jonathan; Lian, Yunlong & Goerbig, Mark Oliver
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SU(4) spin waves in the ν=±1 quantum Hall ferromagnet in graphene. Physical Review B, 103(19).
Atteia, Jonathan & Goerbig, Mark Oliver
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Many-particle electron states in graphene. Science, 375(6578), 263-264.
Morgenstern, Markus & Goerbig, Mark
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Ph.D. thesis, RWTH Aachen University 2022: Interplay of quantum Hall edge states in graphene with the tip-induced quantum dot and graphene sample fabrication techniques for advanced scanning tunneling microscop
Tjorven Johnsen
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Mapping quantum Hall edge states in graphene by scanning tunneling microscopy. Physical Review B, 107(11).
Johnsen, T.; Schattauer, C.; Samaddar, S.; Weston, A.; Hamer, M. J.; Watanabe, K.; Taniguchi, T.; Gorbachev, R.; Libisch, F. & Morgenstern, M.
