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

TL;DR

This paper explores the electronic structure and phase transitions in BaNi2As2, revealing strong quadrupole fluctuations that may drive superconductivity and structural changes, with implications for understanding Ni-based versus Fe-based superconductors.

Contribution

It constructs a ten-orbital tight-binding model and identifies antiferro-quadrupole fluctuations as key to superconductivity and structural transitions in BaNi2As2.

Findings

Charge quadrupole susceptibilities are strongly developed.

Antiferro-quadrupole fluctuations are linked to superconductivity.

Proposed origin of zigzag chain structure as quadrupole order.

Abstract

We investigate the electronic state and structure transition of BaNi2As2, 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 O_{X^2-Y^2}-quadrupole susceptibility at q = (pi, 0, pi) or O_{XZ(YZ)}-quadrupole susceptibility at q = (pi, pi, pi), depending on the level splitting between d_{X^2-Y^2} 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_{X^2-Y^2} order with q = (pi, 0, pi) is the origin of the zigzag chain structure reported in experiments. We identify similarities and differences between Ni- and Fe-based superconductors.