Dirac Semimetal and Connected Nodal Lines in IrGa, ScPd, and RuAs Compounds

The topological nontrivial material host exotic physical phenomena such as topo- logically protected surface states which cannot be found in a normal material. The search for a simple structure with various topological phases is an emerging topic. In addition, the strong spin-orbit coupling (SOC) drives the system into different topo- logical phases. In this dissertation, the topological properties of different compounds were investigated by using the first-principles calculations method. The band structures of ScPd and IrGa compounds have nodal lines located at kx=π and ky=π mirror planes. These nodal lines are connected along M-R direction. The investigation of different simple cubic structures indicated that they share a similar type of connected nodal lines, which can be studied in one dimension. The ScPd has a new type of Dirac node which is different from the type-I and type-II Dirac nodes. It is originated from the crossing of the quadratic and linear bands, and the corresponding Fermi sheet at the position of this Dirac node is formed by only one Fermi pocket. When the SOC is included there is a continuous energy gap across the Brillouin zone (BZ). The Z2 topological invariant is nontrivial, and the surface states are located in the gap region. The electronic band structure of the orthorhombic structure of RuAs shows cross- ings along Γ − Y that make a nodal line at kz=0 plane. The hourglass structure is formed by the four bands which stick together along R-S direction and make a double nodal line. This nodal line is protected by the nonsymmorphic, inversion, and time reversal symmetries of the crystal structure. When the SOC is included the system exhibits several Dirac nodes protected by the nonsymmorphic symmetry, and located at S, Y, and R points. The corresponding nontrivial topological surface states emerge from the Dirac node. The ScIrP shows a strong magnitude of SOC due to Ir-atoms, and the band structure calculated with SOC split at the Fermi level.Physics, Department o