Observation of Oriented Landau Levels and Helical Zero Modes in Berry Dipole Acoustic Crystals

Abstract

<p>Landau levels play a fundamental role in condensed matter physics, as they offer critical insights into charged particles' behavior in magnetic fields. These unique characteristics have significant implications for understanding various quantum phenomena and materials' electronic properties, such as the renowned quantum Hall effect. Over the past two decades, substantial advancements have been achieved in Landau levels in systems with linear band crossing points, like Weyl points, which exhibit fascinating features like chiral zero modes. The Landau levels observed thus far are independent of the magnetic field's orientation. In this Letter, we experimentally demonstrate unconventional oriented Landau levels in acoustic Berry dipole systems, wherein by reversing the orientation of the pseudomagnetic field, the system exhibits distinct Landau spectra. Remarkably, we observe a new type of helical zero mode, whose existence critically depends on the magnetic field's orientation. This is in sharp contrast to the well-studied chiral zero modes in Weyl systems, whose propagation direction, but not existence, depends on the applied magnetic field's orientation. Our Letter opens up avenues for understanding the interaction of gauge fields with band singularities that go beyond Chern class, including Berry multipoles and even-dimensional monopoles, and offers new insights for exploring topological devices.</p>