Observation of oriented Landau levels in Berry dipole semimetals

TL;DR

This study experimentally demonstrates oriented Landau levels in Berry dipole semimetals using engineered acoustic lattices, revealing orientation-dependent spectra and novel helical zero modes, advancing topological material research.

Contribution

It provides the first experimental observation of oriented Landau levels in Berry dipole semimetals, highlighting the effects of magnetic field orientation on quantum states.

Findings

Observation of distinct Landau level spectra with reversed magnetic field orientation

Discovery of helical zero modes dependent on magnetic field direction

Demonstration of engineered acoustic lattice to realize Berry dipole phenomena

Abstract

Band crossing points, such as Weyl and Dirac points, play a crucial role in the topological classification of materials and guide the exploration of exotic topological phases. The Berry dipole, a three-dimensional band crossing point beyond the Chern class, hosts a dipolar Berry curvature field and gives rise to numerous nontrivial quantum geometric effects. It has been proposed that the Berry dipole exhibits oriented Landau levels, whose spectrum critically relies on the orientation of the applied magnetic field. However, experimental demonstration of this phenomenon has remained elusive. Here we experimentally demonstrate oriented Landau levels by carefully engineering an inhomogeneous acoustic lattice. We observe distinct Landau level spectra and different propagation properties when the orientation of the pseudomagnetic field is reversed. Notably, we discover a new type of helical zero modes whose existence critically depends on the magnetic field's orientation. Our work paves the way for studying band crossings beyond Chern-class crossing points, including Berry multipoles and even-dimensional monopoles. Furthermore, it offers new insight for exploring topological devices.