What is wrong with paramagnons?

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Systems with itinerant fermions close to a zero temperature quantum phase transition like the high temperature superconductors exhibit unusual non-Fermi liquid properties. The interaction of the long-range and low-energy fluctuations of the incipient order with the fermions modify the dynamical properties of the fermions strongly by inducing effective long-range interactions. Close to the transition the interaction of the order parameter fluctuations becomes important. In this paper we discuss the physics of the non-Gaussian order parameter fluctuations on the electronic spectrum and illustrate their effect by considering the charge-density-wave transition and the phase fluctuations in a two-dimensional d-wave superconductor.