Nonsymmorphic symmetry, zone-boundary Dirac line node and magnetic instability in Sr₂IrO₄

Peierls distortion is a robust instability occurred in a partially-filled one-dimensional (1D) band with its susceptibility diverging, which is attributed to Fermi surface topology in 1D. Searching for two dimensional analog of Peierls instability is elusive since the topology of Fermi surface in higher than 1D experiences the deficit of nesting, in turn, and exhibits to weak susceptibility. In this talk, we present that such a difficulty can be overcome by non-symmorphic symmetry-protected Dirac line node (DLN) along Brillouin zone boundary in Sr₂IrO₄ whose space group is of this kind. Depending on the rotation angle of IrO₆ octahedron, the susceptibility grows fast and diverges at a critical angle where DLN becomes flat. The infinitesimal Hubbard U interaction is enough to develop the magnetism in Iridates 214 near the critical rotation angle. This provides a unified viewpoint on magnetic instability in Sr₂IrO₄, originating from the cooperation between non-symmorphic crystal symmetry and electron correlation.