Doctor of Philosophy (Ph.D.)
This thesis presents measurements of collective modes and ultrafast carrier relaxation dynamics in charge-density-wave (CDW) conductors and colossal magnetoresistance (CMR) manganites. A femtosecond laser pump pulse excites a broad frequency spectrum of low-energy collective modes and electron-hole pairs thereby changing its optical properties. The low-energy collective excitations and quasiparticle relaxation and recombination processes are monitored by measuring the resulting photoinduced absorption as a function of probe pulse wavelength and time delay.;A general model was developed for the photogeneration and detection mechanism of collective modes based on light absorption in two-color pump-probe experiments. A broad spectrum of collective modes (phasons and amplitudons) with frequencies down to a few GHz is excited and propagates normal to the surface into the material. The dispersion of the long-wavelength phason and amplitudon can be measured by changing the probe wavelength.;The first pump-probe spectroscopy was performed from the ultraviolet to mid-infrared wavelength range to study low-frequency collective excitations, including temperature evolution, dispersion, damping, and anisotropy of amplitude mode and transverse phason in quasi-one dimensional CDW conductors, K 0.3MoO3 and K0.33MoO3 on ultrafast time scale. The transverse phason exhibits an acoustic-like dispersion relation in the frequency range from 5--40 GHz. The phason velocity is strongly anisotropic with a very weak temperature dependence. In contrast, the amplitude mode exhibits a weak (optic-like) dispersion relation with a frequency of 1.66 THz at 30 K.;The studies were extended to doped perovskite manganite thin films and single crystals. A low-energy collective mode is observed and discussed in terms of the opening of a pseudogap resulting from charge/orbital ordering phases. The softening of the collective mode is necessary to explain by combining a cooperative Jahn-Teller type distortion of the MnO6 octahedra with the collective mode. The quasiparticle dynamics in the vicinity of the metal-insulator transition is strongly affected by the presence of a pseudogap, phase separation and percolation, which are strongly dependent on temperature. A very long-lived relaxation process is observed due to a slow spin relaxation process. The dynamics of the spin system is further investigated in strained and unstrained thin films, which show a strong strain effect.
© The Author
Ren, Yuhang, "Time-resolved optical studies of colossal magnetoresistance and charge -density wave materials" (2003). Dissertations, Theses, and Masters Projects. Paper 1539623421.