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 symmetry-breaking phase transitions. Thus the evolution of both collective modes and single-particle 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 continued our research of relaxation of quasiparticles in iron-based pnictide superconductors. We systematically investigate the photoexcited (PE) quasi-particle (QP) relaxation and low-energy electronic structure in electron doped 1111-structure Sm(Fe0.93Co0.07)AsO single crystal. We find that the behaviour is qualitatively identical to the 122-structure Ba(Fe,Co)2As2 including the presence of a normal state pseudogap and a marked twofold symmetry breaking in the tetragonal phase that we relate to the electronic nematicity. The twofold symmetry breaking appears to be a general feature of the electron-doped iron pnictides. The results were published in Phys. Rev. B 87, 174525 (2013).
COBISS.SI-ID: 26768167
In CDW systems we studied the incoherent recombination of topological defects created during a rapid quench of a charge-density-wave system through the electronic ordering transition. Using the above mentioned 3-pulse femtosecond optical spectroscopy technique we follow the evolution of the order parameter over a wide range of timescales after the quench. By careful consideration of thermal processes we clearly identified intrinsic topological defect annihilation processes in TbTe3 on a timescale of ~30 ps and found a signature of extrinsic defect-dominated relaxation dynamics occurring on longer timescales. Similar effect was observed also in blue bronze and 2H-TaSe2. The results were published in Phys. Rev. Lett. 110, 156401 (2013).
COBISS.SI-ID: 26700071