Non-linear quantum critical dynamics and fluctuation-dissipation ratios far from equilibrium

• Critical Kondo destruction explains the unusual properties of quantum critical heavy fermion compounds.
• We review the concept of effective temperatures in models of critical Kondo destruction.
• We compare effective temperatures found near non-interacting and fully interacting fixed points.
• A comparison with non-interacting quantum impurity models is presented.

Non-thermal correlations of strongly correlated electron systems and the far-from-equilibrium properties of phases of condensed matter have become a topical research area. Here, an overview of the non-linear dynamics found near continuous zero-temperature phase transitions within the context of effective temperatures is presented. In particular, we focus on models of critical Kondo destruction. Such a quantum critical state, where Kondo screening is destroyed in a critical fashion, is realized in a number of rare earth intermetallics. This raises the possibility of experimentally testing for the existence of fluctuation-dissipation relations far from equilibrium in terms of effective temperatures. Finally, we present an analysis of a non-interacting, critical reference system, the pseudogap resonant level model, in terms of effective temperatures and contrast these results with those obtained near interacting quantum critical points.