Transport and coupling of phonons, electrons, and magnons in complex materials

In nanoscale systems, in which the relevant length scales can be comparable to the mean free paths and wavelengths of the energy, charge and spin carriers, it is necessary to examine the microscopic transport of heat, spin and charge at the atomic scale and the quantization of the associated quasiparticles. The intricacies of the transport dynamics can be even more complicated in materials with atomic scale complexities, such as incommensurate crystals, magnetic materials, and quasi-one-dimensional systems. Meanwhile, the transport properties and coupling between these quasiparticles is important in determining the strength of various thermoelectric and spincaloritronic phenomena, as well as the reliability of nanoscale electronics. This work seeks to further the understanding of the complicated transport dynamics in complex structured materials at nanometer and micrometer length scales, and to address some of the fundamental questions about the interactions between energy, charge and spin carriers in the conducting polymer poly(3,4- ethylenedioxythiophene) (PEDOT), the incommensurate higher manganese silicide (HMS) thermoelectric material, and the magnetic insulator yttrium iron garnet (YIG). These questions are addressed through a number of combined experimental approaches through the use of thermal conductance and thermoelectric property measurements of suspended nanostructures, inelastic neutron scattering, Brillouin light scattering, and electron microscopy. According to in-plane thermal and thermoelectric transport measurements of PEDOT thin films, the electronic thermal conductivity of this conducting polymer is found to be significant and exceeds that predicted by the Wiedemann-Franz law for metals. Furthermore, thermoelectric transport measurements of suspended HMS nanoribbons show a reduction in the lattice thermal conductivity by approximately a factor of two compared to bulk HMS, which is qualitatively consistent with that predicted from a diffuson model for thermal conductivity derived from the phonon dispersion of HMS. Lastly, pressure dependent Brillouin light scattering spectroscopy is used to determine the influence of hydrostatic stress on the dispersions of magnons and phonons in YIG, in order to determine the magnon and phonon peak frequency shift associated with localized laser heating induced strain.