Spintronics of helical edge states interacting with a quantum magnet
Final Report Abstract
In this project, we investigated several setups where magnetic elements interact with Dirac surface states. We have investigated an interference setup with helical edge states coupled by quantum point contacts and exchange coupled to a magnet with a magnetic anisotropy axis parallel to the spin quantization axis of the helical edge state electrons. Although the current transmission through the magnet is perfect despite the magnetic gap opened in the helical edge, the interference properties are shown to be influenced by the magnet. The Aharonov-Bohm interference in the dc-current is suppressed for energy scales below the magnetic gap induced in the helical edge state. However, an ac-current becomes possible in the interferometer when dc-bias voltages are applied - a phenomenology somewhat reminiscent of the ac-Josephson effect. This is possible since the magnet builds up a rotating magnetization propelled by the dc-voltages and electrons reflected by the magnet change their energy by an amount given by the rotation frequency of the magnet. Interference between a reflected and a non-reflected electron leads to the ac-currents. We also have analyzed anomalous Hall currents of massive Dirac electrons induced by a potential step showing universal results and surprisingly inhomogeneous current densities. Besides the transport current part, the calculated currents exhibit also a magnetization contribution. In twisted bilayer graphene, the resulting moire lattice allows for valleycontrasting physics that is inaccessible in single-layer graphene. We have investigated transport through a potential step and showed that the setup acts as a valley-splitter (valley-layer locked currents) and produces transverse focusing currents in a single valley in the absence of a magnetic field (i.e. without time-reversal symmetry breaking).
Publications
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Anomalous equilibrium currents for massive Dirac electrons, Phys. Rev. B 100, 1154404 (2019)
P.G. Silvestrov, P. Recher
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Valley Splitter and transverse valley focusing in twisted bilayer graphene, Phys. Rev. Research 2, 043151 (2020)
C. De Beule, P.G. Silvestrov, M.-H. Liu, P. Recher
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Interference effects by a precessing easy-plane magnet coupled to a helical edge state, Physical Review B 103, 115142 (2021)
K. A. Madsen, P. W. Brouwer, P. Recher, and P.G. Silvestrov
