In the lab, researchers trace beating signal in topological nanowires

A magnetic field threads a topological-insulator nanowire, and electrons moving along its perimeter can take different paths around the material. Researchers at the Korea Research Institute of Standards and Science (KRISS) and the Gwangju Institute of Science and Technology (GIST) have identified the origin of the “beating” signal produced by those paths in antimony-doped bismuth selenide nanowires.

A topological insulator, or TI, blocks electrical conduction through its interior while allowing electrons to move along its edges or outer surface. When a magnetic field is applied, the electron waves interfere and create Aharonov–Bohm, or AB, oscillations: the material’s conductance changes at regular intervals as the field intensity changes. The newly explained pattern appears when two oscillations with slightly different periods overlap.

The two contributors are the topological surface state, which carries electrons at the TI’s surface, and a two-dimensional electron gas (2DEG), a layer where electrons flow just beneath it. In the antimony-doped bismuth selenide nanowires, the two routes cover slightly different cross-sectional areas, producing the shifting rhythm. The team found evidence of the extra oscillation in electrical-conductance data from previous years of TI research.

The distinction was difficult to make with conventional frequency analysis, especially as the pattern changed with gate voltage, the control voltage used to tune the material. With help from Song Taegeun, a professor at Kongju National University, the researchers used machine learning to separate the overlapping oscillation components. Bae Myung-Ho, a principal research scientist at KRISS, said the result shows that quantum interference can involve ordinary electronic states as well as topological ones.

So what changes in practice? For future topological quantum devices, the principle could improve signal interpretation and help attain desired electronic states. Using only the desired topological state requires precise control of doping and the gate so that the ordinary conduction state does not intervene. The result remains a laboratory-stage study of nanowire conductance; applying the principle to device design is described by the researchers as a future possibility. The findings were published in Nano Letters.