From the first-principles calculations, we have studied the electronic and magnetic structures of compound Na2Ti2Pn2O (Pn = As or Sb). We find that in the ground state Na2Ti2As2O is a blocked checkerboard antiferromagnetic semiconductor with a small band gap of about 0.15 eV, in contrast, Na2Ti2Sb2O is a bi-collinear antiferromagnetic semimetal, both with a small moment of about 0.5μB around each Ti atom. We show that there is a strong Fermi surface nesting in Na2Ti2Pn2O. 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.
@article{arxiv.1210.3481,
title = {Layered pnictide-oxide Na$_2$Ti$_2$Pn$_2$O (Pn=As, Sb): a paradigm for spin density waves},
author = {Xun-Wang Yan and Zhong-Yi Lu},
journal= {arXiv preprint arXiv:1210.3481},
year = {2013}
}