Thermal conductivity of high-temperature superconductors.

I report on the thermal conductivity of high temperature superconductors. The investigation is naturally categorized into two temperature ranges: very low temperatures, $\sim$0.06K $<$ T $<$ $\sim$6K, where sintered $\rm YBa\sb2Cu\sb3O\sb{7-\delta}$, $\rm Ba\sb{1-x}K\sb xBiO\sb3$, $\rm Tl\sb2Ba\sb2Ca\sb2Cu\sb3O\sb{10}$, $\rm Bi\sb2Sr\sb2Ca\sb2Cu\sb3O\sb{10}$, hot-pressed $\rm Bi\sb2Sr\sb2Ca\sb1Cu\sb2O\sb8$ and single crystal $\rm La\sb2Sr\sb xCu\sb3O\sb{7-\delta}$ samples are studied, and in the temperature range 6K $<$ T $<$ 250K, in which sintered $\rm Ba\sb{1-x}K\sb xBiO\sb3$, $\rm Tl\sb2Ba\sb2Ca\sb2Cu\sb3O\sb{10}$, $\rm Bi\sb2Sr\sb2Ca\sb2Cu\sb3O\sb{10}$ and single crystal $\rm YBa\sb2Cu\sb3O\sb{7-\delta}$ are investigated. In addition, measurements were performed in a magnetic field (up to 6 Tesla) for sintered $\rm Ba\sb{1-x}K\sb xBiO\sb3$ and single crystal $\rm YBa\sb2Cu\sb3O\sb{7-\delta}$. For very low temperatures a unified description of the thermal conductivity, $\kappa$, of high temperature superconductors is not yet established. An expected T$\sp3$ dependence is not obeyed, and deviations from this dependence can be divided into two categories: (1) A power law dependence weaker than cubic, and (2) An anomalous linear temperature dependence for T $<$ $\sim$0.5K. The possible origin of both phenomena is discussed. For temperatures near the superconducting transition temperature. T$\sb{\rm c}$, I find that the behavior of $\kappa$ for the Bi-O based superconductor, $\rm Ba\sb{1-x}K\sb xBiO\sb3$, is dramatically different than that of the Cu-O based superconductors. In the $\rm Ba\sb{1-x}K\sb xBiO\sb3$ system, a decrease in $\kappa$ is observed as the temperature is lowered below T$\sb{\rm c}$. In contrast, for Cu-O based superconductors, a marked increase in $\kappa$ is observed as the temperature is lowered below T$\sb{\rm c}$. This effect is particularly dramatic for the $\rm YBa\sb2Cu\sb3O\sb{7-\delta}$ single crystals, and I present a theoretical model to describe the data of this system. From the theoretical fit I extract an estimate of the magnitude of the phonon-electron coupling strength, $\lambda$, and the superconducting energy gap, $\Delta$. In addition, I show that boundary scattering is significant up to high temperatures, and that long wavelength phonons, $\sim$100A at 77K, dominate the lattice thermal conductivity. Finally, I show that the thermal conductivity of single crystal $\rm YBa\sb2Cu\sb3O\sb{7-\delta}$ is sensitive to a magnetic field. Abrikosov vortices are very effective phonon scatterers, and their presence leads to a significant degradation in the thermal conductivity. An anisotropic behavior in $\kappa$(H) is observed, and is in qualitative agreement with a geometrical argument based on the anisotropy of the superconducting coherence length. Strong hysteresis effects in the dependence of the thermal conductivity on the field are observed, indicative of the magnetic vortex pinning in this system.PhDPhysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/105590/1/9135674.pdfDescription of 9135674.pdf : Restricted to UM users only