Thermal transport and magnetic properties of superconductors.

We address several aspects of the thermal transport and magnetic properties of conventional and high-T$\sb{c}$ superconductors. We first derive an expression for the effects of boundaries on phonon transport in small samples of circular and rectangular cross section. A variation of this treatment is then applied to calculate the thermal conductivity and phonon mean-free path of $Y Ba\sb2Cu\sb3O\sb{7-\delta}$ thin films, revealing that the characteristic peak in the thermal conductivity may be entirely suppressed for sufficiently thin films. We also investigate the thermal transport of bulk $Y Ba\sb2Cu\sb3O\sb{7-\delta}$ in the presence of a magnetic field. Our studies indicate that the thermal conductivity may serve as a tool to investigate the static ordering of the vortex array. We also address the dynamic properties of magnetic vortices by examining the evidence that superconductors in the Bean critical state may exhibit self-organized criticality. We present our own results from computer simulations that support this claim. This simulation also provides a microscopic derivation of the Bean critical state model in one dimension and demonstrates that the only necessary ingredients for Bean behavior are repulsive interactions between vortices and attractive interactions between vortices and randomly distributed pinning sites. Our results indicate that the dependence of the critical current on magnetic field follows the Kim modification of the Bean model.PhDPhysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/103834/1/9409791.pdfDescription of 9409791.pdf : Restricted to UM users only