An international team of scientists have demonstrates that, in a solid material, topological phenomena with maximal magnitude can appear, with their strength quantifiable by the so-called Chern number of quasiparticles. Their experiments have shown for the first time that the theoretically predicted maximum Chern number – an equivalent to the electrical charge for elementary particles – can be reached, and also controlled, in a real material.
For topological materials, the behaviour of electrons is very different from those in conventional matter, with the magnitude of many such phenomena being directly proportional to the Chern number. The field of topological materials are a hot topic in condensed matter physics, as shown by the award of the Nobel Prize in 2016, with hope that such materials could offer new kinds of electronic components and superconductors.
These are materials whose lattice structures have a well-defined handedness, as they cannot be transformed into their mirror image through rotations and translations. In some of the materials studied such coupling is comparatively low, making it difficult to resolve the splitting of interest. In addition, preparing clean and flat surfaces of relevant crystals has been very difficult, meaning that spectroscopic signatures tended to be washed out.
However, as reported in Science [Schröter et al. Science (2020) DOI: 10.1126/science.aaz3480], it was shown that, in a chiral material, a maximal Chern number can be measured, a finding that could lead to new research pathways in both fundamental science and applications, particularly as they also found the sign of the maximal Chern number can be controlled ‘by hand’ due to the relationship between the handedness of the crystal structure and that of the electronic wavefunction in chiral crystals.
It has been predicted theoretically that, for topological semimetals, the Chern number cannot be more than a magnitude of four, but this number should have an upper limit, something not so far achieved. However, it was here observed that, in the topological semimetal palladium gallium (PdGa), the Chern number can reach the maximum value that is possible in any metallic crystal. The team were able to overcome limitations by using PdGa crystals, with the material exhibiting strong spin–orbit coupling, demonstrating that the chiral nature of the PdGa crystals provides the potential for also controlling the sign of that number, while measurements showed signatures in the electronic structure of PdGa that demonstrated the maximal Chern number was realized.
This could lead to new factors at the interface between different enantiomers, one with a Chern number of plus four and another with minus four, and there are potential applications as chiral topological semimetals can have useful phenomena, including as quantized photocurrents.