Counterpropagating topological and quantum Hall edge channels

Abstract

The survival of the quantum spin Hall edge channels in presence of an external magnetic field has been a subject of experimental and theoretical research. The inversion of Landau levels that accommodates the quantum spin Hall effect is destroyed at a critical magnetic field, and a trivial insulating gap appears in the spectrum for stronger fields. In this work, we report the absence of this transport gap in disordered two dimensional topological insulators in perpendicular magnetic fields of up to 16 T. Instead, we observe that a topological edge channel (from band inversion) coexists with a counterpropagating quantum Hall edge channel for magnetic fields at which the transition to the insulating regime is expected. For larger fields, we observe only the quantum Hall edge channel with transverse resistance close to h/e2. By tuning the disorder using different fabrication processes, we find evidence that this unexpected ν = 1 plateau originates from extended quantum Hall edge channels along a continuous network of charge puddles at the edges of the device.

2024 roadmap on 2D topological insulators

“…Further, side-contacted Josephson junctions in 2D topological insulators have been realized with this technique to explore concepts of topological superconductivity [57]. Further, wet-etched devices show the theoretically expected trivial insulating state when the inversion of Landau levels is lifted under an external magnetic field, clearly establishing that previous observation of edge conduction in this regime is due to disorder [56].…”

“…However, electrical conductance and microwave impedance spectroscopy measurements revealed an expected edge conductance in the insulating regime [59]. This has now been identified as due to fabrication-induced damage to the material, due to which a quantum Hall edge channel propagates along a network of charge puddles as demonstrated in [56] (figure 4(b)). Finally, one of the challenges for the research on the quantum spin Hall effect is the scarcity of the number of model systems where the conductance quantization has been experimentally demonstrated.…”

“…Another serious consequence of charge puddles is that they lead to the observation of conductance in the regime where one expects a trivial insulating state [56,59]. When a 2D TI is subjected to an external magnetic field, the inversion of Landau levels is lifted and the system transitions to a trivial insulating state where the expected conductance is zero [56]. However, electrical conductance and microwave impedance spectroscopy measurements revealed an expected edge conductance in the insulating regime [59].…”

Two-Dimensional Topological Insulator State in Cadmium Arsenide Thin Films

“…In summary, we have observed the hallmarks of a nearly ideal 2D TI state in thin films of (001)-oriented Cd3As2, including insulating behavior at zero field and an energy gap between two n = 0 Landau levels that closes at Bc. The zeroth Landau level crossing is remarkably well resolved, aided by relatively high Fermi velocities of the electronic states that give rise to the zero-energy Landau levels, low disorder, and a good separation in energy from, e.g., a high density of low mobility valence band states found in other systems [37,40]. These results establish that thin films of Cd3As2 constitute a new member of the very small, highly sought-after family of 2D TIs discovered to date.…”

“…We note that it is the significant dispersion (in B) of both n = 0 levels, and the absence of any other states nearby, that allows for the clear re-entrant ν = 0 quantum Hall plateau. By comparison, for HgTe quantum wells the p-type n = 0 quantum Hall plateau is nearly nondispersive (indicative of a low Fermi velocity) and has a more complex dispersion [37,38], causing the re-entrant quantum Hall effect to be rather complex (e.g., ν = 0 → ν = 1 (-1) → ν = 0 [6]).…”