Optical studies of high temperature superconductors and electronic dielectric materials

Infrared and optical spectroscopy has been applied to study both the normal state and superconducting state electronic properties of cuprate superconductors. Two important parameters used in our experiments are the applied field and substitutional doping.; The ab-plane optical responses of Ca-doped YBa 2Cu3O7-delta films were studied from optimally doped region to overdoped regimes. The temperature dependent reflectance spectra were measured from far infrared (20 cm-1) to ultraviolet (43,000 cm-1). The spectra were analyzed by a two component model and marginal Fermi liquid model. The result indicates a further increase of plasma frequency which is consistent with the study of BSCO samples by other groups. Another interesting result is the decreased superfluid density in the overdoped region. This result is consistent with decreased superconducting transition temperature with increasing the doping level within the overdoped region.; Magnetic field dependent, low temperature infrared transmittance was used to study the vortex dynamics in high temperature superconductors. Optimally doped YBa2Cu3O7-delta and YBa2Cu 3O6 samples were used for the measurements. We saw no significant field-sensitive features in the far infrared transmittance spectra at low temperature.; The temperature dependence of the reflectance of cubic bismuth pyrochlores Bi3/2ZnTa3/2O7 (BZT), Bi3/2MgNb 3/2O7 (BMN), Bi3/2MgTa3/2O7 (BMT) and Bi3/2Zn0.92Nb1.5O6.92 (BZN) has been investigated by infrared spectroscopy. Spectra were collected from 30 to 3300 cm-1 between 50 and 300 K, and the optical constants were estimated by Kramers-Kronig analysis and classical dispersion theory. In addition, BZN was studied by terahertz technique at lower frequencies (3 cm-1 to 60 cm-1) between 300 K and 50 K. Infrared-active phonon modes have been assigned to specific bending and stretching vibrational modes. A previously unassigned infrared mode at about 850 cm -1 is discussed. A splitting of the B-O stretching phonon modes and O-B-O bending modes is assigned to mixed cation occupancy. The temperature dependence of the phonon frequencies and the damping coefficients are consistent with a decrease of lattice constant and with orientational disorder at low temperatures.