Experimental Signatures of Higher Order Topology

In second-order topological insulators (SOTIs), the bulk and surfaces are insulating, while the edges or hinges conduct current in a quasi-ideal (ballistic) manner, insensitive to disorder. As in the case of quantum spin Hall edges of 2D topological insulators, the current should be transported without dissipation by counter-propagating ballistic helical states with spin orientation locked to momentum. These edge or hinge states open up many possibilities, ranging from dissipation-free charge and spin transport to new avenues for quantum computing. Bismuth, although a semi-metal, has been shown to belong to this class of materials. In our group, we have studied Josephson junctions based on crystalline Bi nanowires and found that they exhibit robust sawtooth current phase relations in a high magnetic field, which is the signature of one-dimensional ballistic edge states. We also demonstrated the topological nature of Andreev states through the dissipative microwave response in a phase-biased configuration. More recently, in a SQUID constructed from a bismuth ring, we have identified the parity relaxation rate by exploring the statistics of the switching current. In order to find different topological insulators with reduced contribution from bulk non-topological states, we are now exploring the quantum transport properties of WTe2, which has been shown to exhibit a quantum spin Hall effect in the few-layer limit, and Bi4Br4. The latter material is a SOTI with a high bulk band gap.