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Superconducting-like behaviors have been observed, in the past decade, in various materials under intense pump fields. In the cuprate $\rm Y Ba_2 Cu_2 O_{6+x}$ (YBCO), such remarkable effects have been reported up to the pseudogap temperature. Recently, a transient magnetic field proportional to and aligned with an external field was measured in a breakthrough experiment with pumped YBCO, suggesting flux expulsion via the Meissner effect. We discuss an alternative explanation, based on the idea that the pump-induced, far-from-equilibrium dynamics of YBCO reveals short range superconducting correlations in equilibrium much more dramatically than traditional perturbative probes. Under optical pumping, the system evolves according to a driven sine-Gordon equation, which exhibits a novel instability in the presence of an external magnetic field. This phenomenon leads to a giant paramagnetic magnetization in the same direction as the external field. Results of the theoretical model are in quantitative agreement with the experimental data. The driven YBCO experiment thus has the important consequence of revealing the presence of local pairing in the pseudogap phase. More broadly, the observed instability constitutes a new mechanism for amplifying external magnetic fields at ultrafast time scales.
Reference: M. H. Michael, D. De Santis, E. A. Demler, P. A. Lee, “Giant Dynamical Paramagnetism in the driven pseudogap phase of $\rm Y Ba_2 Cu_2 O_{6+x}$”, arXiv:2410.12919v1 (2024)