Direct observation of Dirac nodal-line fermions in a mixed-anion superconductor HfP_2-xSe_x

In recent years, there has been an active exploration of materials for superconductors with Dirac nodal-line in their electronic structure. MSiCh (M=Zr, Hf; Ch=S,Se) is known as an ideal Dirac nodal-line semimetal with a Si square-net as a glide plane. It is then expected that the crystal structure of MSiCh can give us a hint for material exploration of nodal-line superconductors. Here we focus on a superconductor MP_2-xCh_x, which is isostructural to MSiCh with a P-square net. Previously, we predicted and observed the nodal-line accompanied by fast Dirac particles in ZrP_1.24Se_0.57. In order to systematically understand the changes in electronic structure by different elements, we have performed an angle-resolved photoelectron spectroscopy of HfP_1.48Se_0.37 using synchrotron radiation. We have found the nodal-loop with the fastest Dirac velocity (1.3×10⁶ m/s). Its origin and the effect of spin-orbit coupling will be discussed by comparing with the results of MSiS and ZrP_1.24Se_0.57 in terms of lattice constants and the energy of the elemental orbitals.