Integer and Fractional Helical States in the Quantum Hall Effect regime

In recent years, efforts to observe helical edge states in materials with large spin-orbit coupling have accelerated. These material, once in proximity to an s-wave superconductor may, under certain conditions, manifest a topological superconductive (TS) phase [1]. Consequently, Majorana fermions, allusive quasiparticles with a non-Abelian exchange statistic, are expected to emerge. Even more interesting are the fractional helical states, previously not observed, which manifest the more exotic para-fermion anyons. Though evidence for the presence of Majorana fermions accumulates, observations of helical edge transport, being a prerequisite for the formation of Majorana quasiparticles, are scarce.  Encouraged by proposals that induce topological superconductivity in 2DEG at the quantum Hall effect (QHE) regime [2, 3], we succeeded to form such, the sought after, robust chiral helical edge modes in GaAs-AlGaAs heterostructures.

In order to have two adjacent counter-propagating edge modes with opposite spin, the 2DEG is embedded in a unique quantum well structure, which hosts two weakly interacting electronic sub-bands. Gating the 2DEG with two half-plain gates, enable a scenario where two different filling factors  are applied to the lower and upper sub-bands. Landau levels of different sub-bands cross at the interface between the two gates; thus forming overlapping, counter-propagating, chiral edge modes. Two counter-propagating edges with opposite spins, both in the integer and fractional regime were observed, propagating for more than 300 microns without mixing. In addition, spin protected tunneling was observed depending on spin orientation.

[1] L. Fu and C. Kane, PRL 100, 096407 (2008).
[2] N.H. Lindner, E. Berg, G. Refael and A. Stern, PRX 2, 041002 (2012).
[3] D.J. Clarke, J. Alicea and K. Shtengel, Nature. Communication 4, 1348 (2013).