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Recently, a large family of superconductors emerged as mixed-anion layered phosphides, AP_2-xX_x (A = Zr, Hf; X = Se, S), which is based on nonsymmorphic phosphorus square-net lattice.

Here, we report our photoemission studies on the electronic structure of a typical phosphorus-based superconductor, ZrP_2-xSe_x, with a critical temperature of T_c = 6.3 K. Core-level spectra reveal that the atomic charges of P are distinctly different between in-plane and mixed-anion sites. Valence-band spectra exhibit an upward energy shift of 0.49 eV in going from x = 0.85 to x = 0.55, indicating that replacing Se with P indeed serves as hole doping. Angle-resolved photoemission spectra unveil the presence of a diamond-shaped loop of Dirac nodal-line at 1.2 eV below Fermi level in direct analogy with a well-known Dirac nodal-line semimetal, ZrSiS, based on silicon square-net lattice. A simple model for the fundamental electronic structure of ZrP_2-xSe_x is deduced from our experiments and calculations.

Our results indicate that the nonsymmorphic phosphorus square-net lattice provides a tunable platform for coexistence of the Dirac nodal line and the superconductivity.