Critical pairing fluctuations in the normal state of a superconductor: Pseudogap and quasiparticle damping
AbstractWe study the effect of critical pairing fluctuations on the electronic properties in the normal state of a clean superconductor in three dimensions. Using a functional renormalization group approach to take the non-Gaussian nature of critical fluctuations into account, we show microscopically that in the BCS regime, where the inverse coherence length is much smaller than the Fermi wave vector, critical pairing fluctuations give rise to a nonanalytic contribution to the quasiparticle damping of order TcGiln(80/Gi), where the Ginzburg-Levanyuk number Gi is a dimensionless measure for the width of the critical region. As a consequence, there is a temperature window above Tc where the quasiparticle damping due to critical pairing fluctuations can be larger than the usual T2 Fermi liquid damping due to noncritical scattering processes. On the other hand, in the strong coupling regime where Gi is of order unity, we find within the Gaussian approximation that the quasiparticle damping due to critical pairing fluctuations is proportional to the temperature. Moreover, we show that in the vicinity of the critical temperature Tc the electronic density of states exhibits a fluctuation-induced pseudogap. We also use functional renormalization group methods to derive and classify various types of higher-order scattering processes induced by the pairing interaction in Fermi systems close to the superconducting instability.