Unraveling a chemical-bond-driven root of topology in three-dimensional chiral crystals

Abstract: Chirality manifests across multiple scales, yielding unique phenomena that break mirror symmetry. In chiral materials, unexpectedly large spin-filtering or photogalvanic effects have been observed even in materials composed of light elements, implying crucial influence of their topological electronic states. However, an underlying framework that links chemical bonding and electronic topology remains elusive, preventing the rational design of quantum chiral properties. Here we identify the chiral bonding network responsible for multifold topological fermions by combining synchrotron X-ray diffraction and first-principles calculations on cubic chiral crystals, CoSi and FeSi. Based on the observations of asymmetric valence electron distributions around the transition metals, together with analyses of their bonding to sevenfold-coordinated silicon atoms, we develop a three-dimensional Su-Schrieffer-Heeger model, showing that inter-site hopping on this chiral network creates multifold fermions with doubled topological invariants. Topological features can be switched by reversing the crystalline chirality or tuning electron filling. Our results highlight that implementing strong spin-orbit coupling is not the sole route to realize robust topological phases at elevated temperatures and offer a practical design principle for exploiting chiral topology. Moreover, this real-space framework naturally extends to other elementary excitations or artificial metamaterials, enabling various quantum functionalities through an intuitive approach to chirality engineering.