Researchers report the observation of nearly motionless electronic states, called flat bands, lying close to the electronic chemical potential (the Fermi level) in the bulk material TaRhTe4. Flat bands are energy ranges where electrons have very little kinetic energy. When they sit near the Fermi level, they can strongly affect how a material conducts electricity and responds to temperature changes.

The team measured the electronic structure of TaRhTe4 using angle-resolved photoemission spectroscopy (ARPES). ARPES is an experimental method that shines light on a material and measures the energies and angles of emitted electrons to map the band structure. Their measurements show flat bands close to the Fermi level in this noncentrosymmetric (that is, lacking inversion symmetry), van-der-Waals layered crystal. These flat bands were not anticipated by standard density functional theory (DFT) calculations, so the finding differs from prior theoretical expectations.

TaRhTe4 is described as a type-II Weyl semimetal. A Weyl semimetal hosts special points where electronic bands cross in ways that give the bands a nontrivial topology, and “type-II” means these crossings occur in a strongly tilted energy landscape. Flat bands are uncommon in Weyl semimetals, and they are especially rare in nonmagnetic, three-dimensional bulk materials. Until now, experimental control of flat-band energies near the Fermi level was largely achieved only in layered systems that were twisted at special angles, such as magic-angle bilayer graphene.

Why this matters: bringing flat bands close to the Fermi level in a bulk Weyl system creates a platform where unusual electronic behavior from both flat-band physics and band topology might interact. Flat bands can amplify the effects of electron interactions because the electrons have little kinetic energy. Topology in Weyl materials can produce unusual surface states and transport behavior. Having both features in one material makes TaRhTe4 an interesting place to search for new phenomena affecting conductivity and thermodynamics.