Giant Dynamical Paramagnetism in the driven pseudogap phase of YBCO

We will discuss recent experiments in the pseudogap phase of the high Tc cuprate YBCO that have been interpreted as the light induced Meissner effect . A special feature of these materials is a bilayer structure with a large difference of Josephson couplings within the bilayers and between them. Motivated by this hierarchy of scales, we introduce a model that consists of bilayers of copper-oxygen planes with a local superconducting phase that persists up to the pseudo-gap temperature at equilibrium. Under pumping, the time evolution of the relative phase in the bilayers is described by a driven sine-Gordon equation. We will argue that the experimentally observed phenomena can be explained by a new type of dynamical instability in the sine-Gordon model triggered by the strong terahertz pump pulse. This interpretation suggests that these experiments reveal strong superconducting correlations in the pseudogap state but do not require photoinduced superconductivity.