We study the incoherent recombination of topological defects created during a rapid quench of a charge-density-wave system through the electronic ordering transition. Using a specially devised 3-pulse femtosecond optical spectroscopy technique we fol
low the evolution of the order parameter over a wide range of timescales. By careful consideration of thermal processes we can clearly identify intrinsic topological defect annihilation processes on a timescale ~30 ps and find a signature of extrinsic defect-dominated relaxation dynamics is found to occurring on longer timescales.
Systems which rapidly evolve through symmetry-breaking transitions on timescales comparable to the fluctuation timescale of the single-particle excitations may behave very differently than under controlled near-ergodic conditions. A real-time investi
gation with high temporal resolution may reveal new insights into the ordering through the transition that are not available in static experiments. We present an investigation of the system trajectory through a normal-to-superconductor transition in a prototype high-temperature superconducting cuprate in which such a situation occurs. Using a multiple pulse femtosecond spectroscopy technique we measure the system trajectory and time-evolution of the single-particle excitations through the transition in La$_{1.9}$Sr$_{0.1}$CuO$_{4}$ and compare the data to a simulation based on time-dependent Ginzburg-Landau theory, using laser excitation fluence as an adjustable parameter controlling the quench conditions in both experiment and theory. The comparison reveals the presence of significant superconducting fluctuations which precede the transition on short timescales. By including superconducting fluctuations as a seed for the growth of superconducting order we can obtain a satisfactory agreement of the theory with the experiment. Remarkably, the pseudogap excitations apparently play no role in this process.
We investigate the quasiparticle relaxation and low-energy electronic structure in undoped SrFe_2As_2 exhibiting spin-density wave (SDW) ordering using optical pump-probe femtosecond spectroscopy. A remarkable critical slowing down of the quasipartic
le relaxation dynamics at the SDW transition temperature T_SDW = 200K is observed. From temperature dependence of the transient reflectivity amplitude we determine the SDW-state charge gap magnitude, 2Delta_SDW/k_BT_SDW=7.2+-1. The second moment of the Eliashberg function, lambda<(hbar omega)^2>=110+-10meV^2, determined from the relaxation time above T_SDW, is similar to SmFeAsO and BaFe_2As_2 indicating a rather small electron phonon coupling constant unless the electron-phonon spectral function (alpha^2F(omega) is strongly enhanced in the low-energy phonon region.
