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Quantum critical response at the onset of spin density wave order in two-dimensional metals

We study the frequency dependence of the electron self energy and the optical conductivity in a recently developed field theory of the spin density wave quantum phase transition in two dimensional metals. We focus on the interplay between the Fermi surface `hot spots' and the remainder of the `cold' Fermi surface. Scattering of electrons off the fluctuations of the spin density order parameter, ϕ, is strongest at the hot spots; we compute the conductivity due to this scattering in a rainbow approximation. We point out the importance of composite operators, built out of products of the primary electron or $ϕ$ fields: these have important effects also away from the hot spots. The simplest composite operator, ϕ^2, leads to non-Fermi liquid behavior on the entire Fermi surface. We also find an intermediate frequency window in which the cold electrons loose their quasiparticle form due to effectively one-dimensional scattering processes. The latter processes are part of umklapp scattering which leads to singular contributions to the optical conductivity at the lowest frequencies at zero temperature.