Spontaneous quantum Hall effect via thermally induced quadratic Fermi point

TL;DR

This paper demonstrates that a quadratic Fermi point with non-zero winding number can emerge in a nested Fermi surface with collinear spin-density-wave order, leading to a spontaneous quantum Hall effect at low temperatures.

Contribution

It reveals a new mechanism for topological Fermi points arising from magnetic order in trivial Fermi surfaces, and predicts a spontaneous quantum Hall insulator.

Findings

Quadratic Fermi point characterized by non-zero winding number identified.

Collinear orderings are unstable against scalar spin chirality.

Spontaneous quantum Hall insulator emerges at low temperature.

Abstract

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.