Imaging currents in HgTe quantum wells in the quantum spin Hall regime

Quantum wells based on mercury telluride are an experimental realization of a two-dimensional topological insulator. By using a scanning superconducting quantum interference device (SQUID) technique, the magnetic fields flowing through HgTe/CdTe heterostructures are imaged both in the quantum spin Hall and the trivial regimes, revealing the edge states associated with the quantum spin Hall state. The quantum spin Hall (QSH) state is a state of matter characterized by a non-trivial topology of its band structure, and associated conducting edge channels 1 , 2 , 3 , 4 , 5 . The QSH state was predicted 6 and experimentally demonstrated 7 to be realized in HgTe quantum wells. The existence of the edge channels has been inferred from local and non-local transport measurements in sufficiently small devices 7 , 8 , 9 . Here we directly confirm the existence of the edge channels by imaging the magnetic fields produced by current flowing in large Hall bars made from HgTe quantum wells. These images distinguish between current that passes through each edge and the bulk. On tuning the bulk conductivity by gating or raising the temperature, we observe a regime in which the edge channels clearly coexist with the conducting bulk, providing input to the question of how ballistic transport may be limited in the edge channels. Our results represent a versatile method for characterization of new QSH materials systems 10 , 11 , 12 , 13 .