Transport evidence for chiral boundary states in a microstructured 3D semimetal

Electrons under strong magnetic fields enter the quantum limit and exhibit profound transport phenomena. In 2D systems, the most prominent examples are the quantum Hall effect (QHE) and chiral edge states, which arise from the quantization of electron energy levels into Landau levels. The 3D counterpart of the 2D QHE has also attracted interest, with much focus on the observation of quantized Hall plateaus in 3D semimetals like ZrTe₅ or Cd3As₂. However, it is less appreciated that a 3D analog of chiral edge states also exists, manifesting as surface states that emerge in the quantum limit of 3D systems. Here, we demonstrate the transport signature of chiral boundary states in a microstructured 3D semimetal, bismuth. Using focused ion beam, we have modified the surface of the microstructure by cutting sub-micron-sized grooves and observed a ~1% increase in zero-field resistance, corresponding to a negligible volume change by the surface modification. However, at high magnetic fields in the quantum limit, these grooves decrease the magnetoresistance by ~8%, contrasting the zero-field behavior. Furthermore, rotating the magnetic field orientation clearly shows that the dominant conduction in the device is along the side surfaces and side walls of the grooves, which are parallel to the magnetic field orientation. Our results provide evidence that transport properties of 3D semimetals in the quantum limit are strongly influenced by chiral boundary states, as exemplified by our microstructured bismuth device.