Layered pnictide-oxide Na$_2$Ti$_2$Pn$_2$O (Pn=As, Sb): a paradigm for spin density waves

TL;DR

This study uses first-principles calculations to explore the electronic and magnetic properties of layered Na$_2$Ti$_2$Pn$_2$O compounds, revealing their spin density wave states and structural distortions related to Fermi surface nesting.

Contribution

It demonstrates that Na$_2$Ti$_2$Pn$_2$O compounds exhibit spin density waves driven by Fermi surface nesting, with distinct magnetic ground states for Pn=As and Sb, and links structural distortions to magnetic ordering.

Findings

Na$_2$Ti$_2$As$_2$O is a blocked checkerboard antiferromagnetic semiconductor.

Na$_2$Ti$_2$Sb$_2$O is a bi-collinear antiferromagnetic semimetal.

Both compounds exhibit spin density waves induced by Fermi surface nesting.

Abstract

From the first-principles calculations, we have studied the electronic and magnetic structures of compound Na$_2$Ti$_2$Pn$_2$O (Pn = As or Sb). We find that in the ground state Na$_2$Ti$_2$As$_2$O is a blocked checkerboard antiferromagnetic semiconductor with a small band gap of about 0.15 eV, in contrast, Na$_2$Ti$_2$Sb$_2$O is a bi-collinear antiferromagnetic semimetal, both with a small moment of about 0.5$\mu_B$ around each Ti atom. We show that there is a strong Fermi surface nesting in Na$_2$Ti$_2$Pn$_2$O. And we verify that the blocked checkerboard and bi-collinear antiferromagnetic states both are the spin density waves induced by the Fermi surface nesting. A tetramer structural distortion is found in company with the formation of a blocked checkerboard antiferromagnetic order, in good agreement with the experimentally observed commensurate structural distortion but with space group symmetry retained after the anomaly happening. Further analysis and discussion in connection with experimental observations are given as well.