M. Reyes Calvo: Edge states interplay in a Quantum Spin Hall lateral heterojunction

Above a certain critical thickness, HgTe quantum wells present an inverted band structure that results in the existence of counter-propagating Quantum Spin Hall (QSH) edge states. This behavior is often referred as the material being a 2D Topological Insulator (2D-TI). In theory, while at zero applied magnetic field, the QSH edge states are protected against backscattering, the application of a magnetic field is expected to lift this protection, and above a critical field the material would enter a trivial insulator regime, where the edge conduction fully disappears [1].

In real HgTe quantum wells, the picture seems to be more complex and scattering occurs over long enough distances (a few microns). Furthermore, although the resistance of the devices increases in the presence of a magnetic field, the removal of the edge conduction has not been yet directly probed. Novel probe techniques are a suitable tool to tackle these challenges [2-4].

After a summary of Scanning Probe results, I will focus on the study of the electronic transport across an electrostatically gated lateral junction in HgTe quantum wells with and without an applied magnetic field [5]. We control the carrier density inside and outside a junction region independently and hence tune the number and nature of 1D edge modes propagating in each of those regions. Outside the bulk gap, the magnetic field drives the system to the quantum Hall regime, and chiral states propagate at the edge. In this regime, we observe fractional plateaus that reflect the equilibration between 1D chiral modes across the junction [6-8]. As the carrier density approaches zero in the central region and at moderate fields, we observe oscillations in the resistance that we attribute to Fabry-Perot interference in the helical states, enabled by the broken time reversal symmetry. At higher fields, those oscillations disappear, in agreement with the expected absence of helical states when band inversion is lifted.

References

[1] M. Konig et al. Science 318, 2007

[2] E.Y. Ma, M.R. Calvo et al. Nat. Comms 6, 2015

[3] M. Konig et al. Phys. Rev. X 3, 021003 (2013)

[4] M. R. Calvo et al. In preparation.

[5] M. R. Calvo et al. Phys. Rev. Lett. 119, 226401 (2017)

[6] B. Ozyilmaz et al. Phys. Rev. Lett. 99, 166804 (2007)

[7] R. J. Haug, Semicond. Sci. Technol. 8 131 (1993)

[8] G. M. Gusev et al. Phys. Rev. Lett . 110, 076805 (2013)