Less material conducts more

Physicists at the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) in Hamburg have discovered a counterintuitive form of electronic transport. In microscopic devices carved from the semimetal bismuth, removing material does not reduce electrical conductance—as conventional wisdom would suggest—but can instead increase it. The study, published in Nature Physics and selected for the magazines cover, shows that when strong magnetic fields are applied to three-dimensional metals, electric currents can flow preferentially along the surfaces of the material known as chiral surface states. This finding sheds new light on the previously overlooked role of surface conduction in semimetals driven to the quantum limit.

Electrons forced into new paths
In ordinary metals, electrons move throughout the bulk of the material. If a portion of the conductor is removed, the number of available channels for electrons decreases, and the electrical conductance drops accordingly. Strong magnetic fields, however, dramatically reshape how electrons move. The Lorentz force confines them to cyclotron orbits. At the boundaries of a system, these orbits cannot close. Instead, electrons bounce along the boundary in so-called skipping orbits, leading to a directional flow of electrons along the edge. This concept is well known from the quantum Hall effect in 2D systems. But what happens in 3D systems? “Under these conditions, the boundary—now a surface—similarly becomes an important highway for electrons,” explains Junho Seo, lead author of the study. “The advantage of being in 3D is that we have more geometric degrees of freedom to play with”.  

Sculpting current pathways
To explore this idea experimentally, the team sculpted narrow grooves on the surface of a microscopic device cut from high-purity bismuth crystals using focused ion beam (FIB) machining. Intuitively, cutting grooves into a conductor should reduce the amount of material available to carry current. Yet the transport measurements revealed the opposite effect: the conductance increased as more grooves were introduced under strong magnetic fields. Each groove adds additional sidewalls and these surfaces host chiral surface states, effectively creating new highways along which electrons can travel. Instead of suppressing electrical transport, the grooves expand the network of surface pathways that carry current. “The experiment shows that geometry alone can strongly influence how electrons move through a 3D metal under strong magnetic fields” says Philip Moll, Director of the corresponding department at the MPSD.

Geometry as a new playground
The findings reveal a new aspect of electronic transport in the quantum limit. While conventional transport in metals is determined mainly by the material’s electronic structure, in strong magnetic fields the shape of the device itself can become equally important. Because the effect arises from Landau quantization rather than from a specific topological band structure, it may occur in a wide range of 3D metals in the quantum limit. “A much richer design space for geometries of 3D objects provides a versatile platform to tune and design response functions in the quantum limit” says Seo. “Next to the investigations of gapped and quantized topological phases of matter, the physics of gapless quantum-limit metals deserves closer attention as well.”