Instabilities near the onset of spin density wave order in metals

Understand

We discuss the low energy theory of two-dimensional metals near the onset of spin density wave order.

• It is well known that such a metal has a superconducting instability induced by the formation of spin-singlet pairs of electrons, with the pairing amplitude changing sign between regions of the Fermi surface connected by the spin density wave ordering wavevector.
• Here we review recent arguments that there is an additional instability which is nearly as strong: towards the onset of a modulated bond order which is locally an Ising-nematic order.
• This new instability is a consequence of an emergent "pseudospin" symmetry of the low energy theory---the symmetry maps the sign-changing pairing amplitude to the bond order parameter.