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The interplay between topology and crystalline symmetries in materials canlead to a variety of topological crystalline insulator (TCI) states. Despitesignificant effort towards their experimental realization, so far onlyPb$_{1-x}$Sn$_x$Te has been confirmed as a mirror-symmetry protected TCI. Here,based on first-principles calculations combined with a symmetry analysis, weidentify a rotational-symmetry protected TCI state in the transition-metaldipnictide RX$_2$ family, where R = Ta or Nb and X = P, As, or Sb. TakingTaAs$_2$ as an exemplar system, we show that its low-energy band structureconsists of two types of bulk nodal lines in the absence of spin-orbit coupling(SOC) effects. Turning on the SOC opens a continuous bandgap in the energyspectrum and drives the system into a $C_2T$-symmetry-protected TCI state. Onthe (010) surface, we show the presence of rotational-symmetry-protectednontrivial Dirac cone states within a local bulk energy gap of $\sim$ 300 meV.Interestingly, the Dirac cones have tilted energy dispersion, realizing atype-II Dirac fermion state in a topological crystalline insulator. Our resultsthus indicate that the TaAs$_2$ materials family provides an ideal setting forexploring the unique physics associated with type-II Dirac fermions inrotational-symmetry-protected TCIs.

Topological crystalline insulator state with type-II Dirac fermions in transition metal dipnictides