Intermetallic α-FeSi2: Realization of Type-I, Type-II, and Hybrid Nodal Line States in a Single Material via Tunable Valleys

Different types (type-I, hybrid, andtype-II) of nodal lines exhibit diverse electron behaviors and show rich potential applications in electronic devices. To date, it is still a great challenge to realize all three types of nodal lines in a single material. In this work, by using alpha-FeSi2 as an example, we show the feasibility of realizing type-I, type-II, and hybrid nodal line states in a single material via tunable valleys in the electronic structure. In the native state, alpha-FeSi2 shows two type-I nodal lines near the Fermi level in the k(z) = 0 and k(z) = pi planes, protected by the time-reversal, inversion, and mirror symmetries. Very interestingly, alpha-FeSi2 exhibits two valleys near the band crossings which form the nodal lines. We further find the positions of the valleys and band crossing are tunable under weak perturbations such as small (<= 2%) compressive strains. With the tunable valleys, alpha-FeSi2 can not only realize type-I but also type-II and hybrid nodal lines. Our results provide a feasible route to design different types of nodal lines in a single material.