Instabilities near the onset of spin density wave order in metals

Abstract

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 that is nearly as strong: towards the onset of a modulated bond order that 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.