Low-Frequency Optical Studies of High Critical Transition Temperature Superconductors

The lattice dynamics and electronic responses of High T_{rm c} Superconductors (HTCSs), as revealed by various methods of optical studies, are presented, analyzed and interpreted. First, some background material is briefly developed. Following this are experimental results of reflection studies on aligned YBa_2Cu_3 O_{7-delta} and on oriented Tl_2Ba_2 CaCu_2O_8 : the former show that HTCSs containing CuO-sheets should be thought of as 2+1-dimensional; the latter indicate that the conductivity of the HTCSs can be modeled as having two-components: a narrow, Drude-like feature in the far -IR (that eventually collapses to a delta -function in a clean limit), and a Lorentz-like mid-IR feature. Using photo-induced absorption studies, this mid -IR feature is traced to polarons shaking off phonons. Our data are consistent with the polaron transport theory developed by Reik in the mid 1960's; fits indicate that the polarons consist of 5-7 phonons, each having a frequency around 200 cm^{-1}. Conventional application of the "classical Bardeen -Cooper-Schrieffer theories" as extended by Mattis-Bardeen --and with the explicit incorporation of the frequency dependence of the electron-phonon interaction due to Eliashberg--fail to predict high enough T_{rm c}s. At the same time, experimental evidence for an anharmonic potential well of the apex oxygen is accumulating. We thus propose that the harmonic phonons of the classical models be replaced with more generalized lattice excitations associated with this anharmonic potential; specifically, we think along the lines of a dynamical Jahn -Teller effect involving the apex oxygens. Various theoretical models that are currently being studied in the literature for this anharmonic potential will be briefly introduced and discussed. Lastly, the role of the intermediate "charge-reservoirs" is studied: by comparing two structures (one with, one without reservoir layers), we find experimentally that the layers not only act as reservoirs, but also seem to determine the transition temperature. The dissertation ends with a summary of the results.