Research on topological insulators (TIs) has drawn intensive interests. The type-II InAs/GaSb heterostructure, due to its unusual alignment of the conduction and valence band edges of InAs and GaSb, is predicted to be a two-dimensional (2D) TI.

σxx vs σxy at B = 20 T (blue dots), 25 T (black squares), and 30 T (red diamonds). Gray circles are defined by (σxx − N)2 + σxy2 = N2, with N = 1, 2, 3, and 4.

Historically, type-II InAs/GaSb heterostructures have been extensively studied for infrared applications. With the new prediction as a 2D TI, this material system is believed to hold great potential for future quantum computation. In their article, the team presents their recent quantum transport results around the charge neutrality point (CNP) in a type-II InAs/GaSb field-effect transistor.

Charged carrier (electron and hole) transport shows noisy behavior around the CNP at extremely high B fields. When the diagonal conductivity σxx is plotted against the Hall conductivity σxy, an unexpected conductivity circle law is observed (see figure). A better understanding of this new conductivity circle law is expected to lead to better quantum applications.

The article’s authors are Wei Pan and Sungkwun Lyo (both in the Quantum Phenomena Dept.), John Klem and Jin Kim (both in the RF Optoelectronics Dept.), Madhu Thalakulam (formerly in the Quantum Phenomena Dept., now at the India Institute of Science), and Mike Cich (formerly in the RF Optoelectronics Dept.).

This work was supported by the DOE Office of Basic Energy Sciences. DOE’s Office of Science is the single largest supporter of basic research in the physical sciences in the U.S. and is working to address some of the most pressing challenges of our time. For more information, please visit