A significant effort was invested into investigation of dynamical phase transitions in superconducting and CDW systems. The control of condensed matter systems out of equilibrium by laser pulses allows us to investigate the system trajectories through symmetrybreaking phase transitions. Thus the evolution of both collective modes and singleparticle excitations can be followed through diverse phase transitions with femtosecond resolution. We presented experimental observations of the order parameter trajectory in the normal superconductor transition and charge density wave ordering transitions. Of particular interest is the coherent evolution of topological defects forming during the transition via the Kibble–Zurek mechanism, which appears to be measurable in optical pump–probe experiments. Experiments on CDW systems reveal some new phenomena, such as coherent oscillations of the order parameter, the creation and emission of dispersive amplitude modes upon the annihilation of topological defects, and mixing with weakly coupled finite frequency (massive) bosons as described in J. Phys.: Condens. Matter 25, 404206 (2013). The paper was written at the invitation of Tom Kibble, and the entire journal issue was devoted to Cosmology in the laboratory experiments in his honour. The paper also reveals some new ideas concerning dark matter excitations within the phase transition picture in and discusses the observation of the annihilation of domain walls and the ensuing emission of Higgs bosons detected in femtosecond optical experiments.
COBISS.SI-ID: 27052327
We study the incoherent recombination of topological defects created during a rapid quench of a chargedensity wave system through the electronic ordering transition. Using a specially devised threepulse femtosecond optical spectroscopy technique we follow the evolution of the order parameter over a wide range of time scales. By careful consideration of thermal processes we can clearly identify intrinsic topological defect annihilation processes on a time scale ˜30ps and find a possible signature of extrinsic defect dominated relaxation dynamics occurring on longer time scales.
COBISS.SI-ID: 26700071
In classical superconductors an energy gap and phase coherence appear simultaneously with pairing at the transition to the superconducting state. In high-temperature superconductors, the possibility that pairing and phase coherence are distinct and independent processes has led to intense experimental search of their separate manifestations. We showed that it is possible to clearly separate fluctuation dynamics of the superconducting pairing amplitude from the phase relaxation above the critical transition temperature. Empirically establishing a close correspondence between the superfluid density measured by THz spectroscopy and superconducting optical pump-probe response over a wide region of temperature, we found that in differently doped Bi2Sr2CaCu2O8+d crystals the pairing gap amplitude monotonically extends well beyond the critical temperature, while the phase coherence shows a pronounced power-law divergence at the critical temperature, thus showing that phase coherence and gap formation are distinct processes which occur on different timescales.
COBISS.SI-ID: 27876647
We systematically investigate the photoexcited (PE) quasiparticle (QP) relaxation and lowenergy electronic structure in electron doped Ba (Fe1−x Cox )2As2 single crystals as a function of Co doping. The evolution of the photoinduced reflectivity transients with x proceeds with no abrupt changes. In the orthorhombic spindensitywave (SDW) state, a bottleneck associated with a partial chargegap opening is detected, similar to previous results in different SDW iron pnictides. The relative charge gap magnitude 2Δ(0)/kBTs decreases with increasing x. In the superconducting (SC) state, an additional relaxational component appears due to a partial (or complete) destruction of the SC state proceeding on a sub0.5picosecond timescale. The optical SCstate destruction energy, Up/kB = 0.3K/Fe, is determined near the optimal doping. The subsequent relatively slow recovery of the SC state indicates clean SC gaps. The T dependence of the transient reflectivity amplitude in the normal state is consistent with the presence of a pseudogap in the QP density of states. The polarization anisotropy of the transients suggests that the pseudogaplike behavior might be associated with a broken fourfold rotational symmetry resulting from nematic electronic fluctuations persisting up to T ~ 200 K at any x. The second moment of the Eliashberg function, obtained from the relaxation rate in the metallic state at higher temperatures, indicates a moderate electron phonon coupling, λ ~0.3.
COBISS.SI-ID: 25970215
The transient photoinduced reflectivity and the transient photoinduced magnetooptical Kerr angle were measured in a La0.875Sr0.125MnO3 single crystal as functions of temperature. A separate photoinduced transient ferromagnetic phase is found to form within the pseudocubic lowtemperature insulating ferromagnetic phase after photoexcitation. The characteristic time of transient phase formation is on the order of 10 ps. The similarity with Pr0.6Ca0.4MnO3 indicates that the photoinduced transient ferromagnetic phase is a general property of the insulating ferromagnetic state in colossalmagnetoresistive manganites.
COBISS.SI-ID: 24566055