Zigzag chain structure transition and orbital fluctuations in Ni-based superconductors

We investigate the electronic state and structure transition of BaNi ₂As₂, which shows a similar superconducting phase diagram as Fe-based superconductors. We construct the ten-orbital tight-binding model for BaNi₂As₂ by using the maximally localized Wannier function method. The Coulomb and quadrupole-quadrupole interactions are treated within the random-phase approximation. We obtain the strong developments of charge quadrupole susceptibilities driven by the in-plane and out-of-plane oscillations of Ni ions. The largest susceptibility is either Ox ²-y²-quadrupole susceptibility at q - (π0;π) or Oxz(yz)-quadrupole susceptibility at q - (πππ), depending on the level splitting between dX²-Y² and d_XZ(YZ). These antiferro-quadrupole fluctuations would then be the origin of the strong coupling superconductivity in Ni-based superconductors. Also, we propose that the antiferro-quadrupole O_X2-Y2 order with q - (π 0; π) is the origin of the zigzag chain structure reported in experiments. We identify similarities and differences between Ni- and Fe-based superconductors.