Superparamagnetism-induced Mesoscopic Electron Focusing in Topological Insulators*

Breaking time-reversal symmetry by the presence of magnetic dopants re-opens the backscattering channel in the surface state of strong topological insulators. Recent quasiparticle interference measurements at the surface of Mn-doped Bi₂Te₃ at 4K reveal directionally focused and undamped interference patterns [1].
By means of ab-initio calculations we reveal that the measured mesoscopic coherence relies on two conditions: (i) a hexagonal Fermi surface with large parallel segments (nesting) and (ii) magnetic dopants, which couple to a high-spin superparamagnetic state.
Our theory is supported by x-ray magnetic circular dichroism experiments showing superparamagnetism even at very dilute Mn concentrations, which is a result of Dirac-fermion-mediated RKKY interactions [1,2]. Thus, our findings demonstrate how spin information can be transmitted over long distances, allowing the design of experiments based on coherent quantum effects in topological insulators.

[1] P. Sessi, P. Rüßmann et al., PRB 94, 075137 (2016)
[2] A. Barla, P. Rüßmann et al., in preparation