Coherent dynamics of macroscopic electronic order through a symmetry breaking transition

The study of the temporal evolution of systems undergoing symmetry breaking phase transitions-whether it is in condensed-matter physics, cosmology or finance(1-6)-is difficult because they are hard to repeat, or they occur very rapidly. Here we report a high-time-resolution study of the evolution of both bosonic and fermionic excitations through an electronic charge-ordering symmetry breaking phase transition. Periodically quenching our system with femtosecond optical pulses, we subsequently detect hitherto-unrecorded coherent aperiodic undulations of the order parameter, critical slowing down of the collective mode and evolution of the particle-hole gap as the system evolves through the transition. Modelling on the basis of Ginzburg-Landau theory is used to reproduce the observations without free parameters. Of particular interest is the observation of spectrotemporal distortions arising from spontaneous annihilation of topological defects, analogous to those discussed by the Kibble-Zurek cosmological model(2,3).