$ω/T$ scaling and IR/UV-mixing in Ising-nematic quantum critical metals

Abstract: The instability of a Fermi surface against Ising nematic order destroys the quasiparticle character of the low-energy degrees of freedom. Therefore, observables exhibit deviations from Fermi liquid behavior which gives rise to the term Ising nematic quantum critical metal. To obtain a theoretical description we use a finite-temperature version of Eliashberg theory which allows to treat the strong coupling between quantum and thermal fluctuations in the absence of well-defined quasiparticles. Here, we use this self-consistent, diagrammatic approach to compute, in particular, the nematic susceptibility and the non-Fermi liquid correlations. Upon decreasing the temperature, the susceptibility crosses over from a $(T \log T)^{-1}$ to $T^{-2/3}$ behavior, which is induced by the absence of quasiparticles and restores the Ising nematic critical scaling. Correspondingly, the fermions obey a simple $\omega/T$ scaling law at low enough temperatures. However, this regime is characterized by strong IR-UV mixing since the proportionality factors exhibit a dependence on the spectral width of the non-quasiparticle excitations and on the underlying lattice. Tuning the parameters of the model, therefore, gives rise to several scenarios for the breakdown of the scaling theory. We discuss them within the Eliashberg approach and estimate the related crossover scales. We also show that the leading order vertex corrections do not change the scaling with temperature or coupling constants.