Gapless electronic systems containing topologically nontrivial Fermi points are sources of various topological insulators. Whereas most of these special band-crossing points are built in the electronic structure of the non-interacting lattice models, we show that a quadratic Fermi point characterized by a non-zero winding number emerges with a collinear triple-Q spin-density-wave state that arises from a perfectly nested but topologically trivial Fermi surface. We obtain a universal low-energy Hamiltonian for the quadratic Fermi point and show that such collinear orderings are unstable against the onset of scalar spin chirality that opens a gap and induces a spontaneous quantum Hall insulator as the temperature tends to zero.
@article{arxiv.1204.5737,
title = {Spontaneous quantum Hall effect via thermally induced quadratic Fermi point},
author = {Gia-Wei Chern and C. D. Batista},
journal= {arXiv preprint arXiv:1204.5737},
year = {2012}
}