Three-Dimensional Acoustic Quantum Hall Effect Demonstrated in New Study

The quantum Hall effect (QHE) stands as a significant milestone in condensed matter physics, paving the way for advancements in topological physics. Extending QHE into three dimensions, however, presents substantial challenges. The primary difficulty lies in the extension of Landau levels into bands along the magnetic field direction, preventing the formation of bulk gaps.

A recent approach has shown promise using Weyl semimetals, where Fermi arc states on opposite surfaces connect through bulk Weyl points to form a complete Fermi loop. Under a magnetic field, one-dimensional edge states appear on the boundary of opposite surfaces. Despite this theoretical foundation, these unique edge states had not been experimentally observed-until now.

A new study published in Science Bulletin by researchers from Shanxi University and Wuhan University in China has both theoretically proposed and experimentally demonstrated the three-dimensional QHE for acoustic waves in a Weyl acoustic crystal. Notably, they have directly observed the intriguing one-dimensional edge states on opposite surfaces.

Since magnetic fields do not affect acoustic waves, a pseudomagnetic field (PMF) was constructed to mimic the effects of magnetic fields on electrons. Typically, an acoustic wave PMF is achieved by introducing a structural gradient.

In this study, the researchers introduced a gradient structure by varying the acoustic cavities corresponding to the on-site energy. This caused the Fermi arcs connected to the Weyl points to shift in the same direction, allowing both bulk and surface states to experience the same pseudomagnetic field. Consequently, the surface states formed Landau levels, and the one-dimensional edge states emerged and localized near the diagonal hinges.

Experimentally, an acoustic crystal sample was fabricated using 3D printing technology. The one-dimensional edge states were directly observed by measuring the acoustic pressure field within the sample.

"This study may open new manners for manipulating acoustic waves, which serves as the basis for acoustic devices with unconventional functions. It provides an ideal and tunable platform to explore the Hall physics, and can extend to other artificial structures, such as the optical and cold atomic systems." The researchers forecast.

Research Report:Observation of 3D acoustic quantum Hall states