Electronic transport in InAs/GaSb composite quantum well: A candidate for 2D topological insulator

Recently, there is a growing interest in the InAs/GaSb composite quantum well sandwiched between two AlSb barriers because of its unique band alignment. In this hybrid system an electron layer in InAs can coexist with a hole layer in the GaSb and a hybridization gap is expected to occur due to quantum tunnelling between the layers. Depending on the QWs’ thicknesses and on the perpendicular electric field, a rich phase diagram is predicted. It should be possible to electrically tune the sample from standard conducting phases to insulating, semimetallic, or topological insulator phases. 

In this talk, I will present transport measurements in a Hall bar device where it was possible to observe both electron and hole transport by tuning a top gate voltage. At high magnetic field we observe well defined quantum Hall plateaus for both electron and holes, indicating the good quality of the sample. Surprisingly, at the charge neutrality point we observe that the longitudinal resistivity increases to much higher than the resistance quantum, together with a strong non-local resistance of similar magnitude. We explain our observation with a model which includes the existence of counter propagating dissipative edge channels coupled with residual bulk scattering.
 
Helical edge states are predicted to dominate transport in the gap at zero magnetic field for devices smaller than the spin-relaxation length. Indeed, by optimizing the etching process and reducing the bulk conductivity, mesoscopic samples in the micrometer range do not show the charge neutrality resistance-peak at negative top gate voltage, but a plateau in the local as well as in the non-local resistances appears in all devices. The non-local resistances scale according to Landauer-Buttiker’s expectations for helical edge modes, which could be a strong signature of the topological edge states in this material. 

References: 
1. 	Fabrizio Nichele, Atindra Nath Pal, Patrick Pietsch, Thomas Ihn, Klaus Ensslin, Christophe Charpentier, Werner Wegscheider PRL, 112, 036802 (2014). 
2.	Lingjie Du, Ivan Knez, Gerard Sullivan, and Rui-Rui Du, PRL 114, 096802 (2015). 
3. 	C. Charpentier, S. Fält, C. Reichl, F. Nichele, A. Nath Pal, P. Pietsch, T. Ihn, K. Ensslin, W. Wegscheider, APL 103, 112102 (2013). 
4. 	A. Nath Pal, S. Mueller, T. Ihn, K. Ensslin, T. Tschirky, C. Charpentier, W. Wegscheider, ArXiv: 1502.06697.