Adiabatic small polaron hopping and spin-polarized tunneling in perovskite oxides.

摘要

This thesis is comprised of two distinct parts, unified in their focus on the transport properties of perovskite oxides.; The occurrence of colossal magnetoresistance and metal-insulator transitions in the manganites has challenged physicists to understand the transport properties of these materials. Part I reports on the epitaxial growth and properties of La_1−xCa_xMnO₃ thin films deposited by pulsed laser ablation, and explains their high temperature conduction mechanism. Conductivity measurements from 300 K to 1200 K of La_1−xCa _xMnO₃ thin films, with doping from x = 0 to x = 1, show that the entire doping range fits the adiabatic small polaron model. Furthermore the x dependence of the conductivity prefactor explicitly shows the effects of on-site coulomb repulsion, i.e. a polaron can not hop into an occupied site. Instead of increasing monotonically as more carriers are introduced, the conductivity prefactor starts to decrease at x = 0.2 and, as expected for Hubbard band splitting, is reduced to almost zero when the lattice is full.; The failure of colossal magnetoresistance materials to produce large room temperature results has led to a search for highly spin-polarized materials to be used in spin dependent tunneling devices. Part II reports on the growth of reproducible La_2/3Sr_1/3MnO₃/SrTiO ₃/Al tunnel junctions with high quality gap characteristics for spin-polarization measurements. I analyzed the spin polarized tunneling data in terms of numerical solutions to Maki's equations which include the effects of orbital depairing, the Zeeman splitting of the spin states, and spin-orbit scattering. I show that there are two solutions to these equations, and identify the correct solution. High quality fits to the data with these solutions yield a La _2/3Sr_1/3MnO₃ spin-polarization of P = +71.5 ± 1.0%. I also discuss my spin-polarized tunneling measurements on SrRuO ₃. In contrast to all other materials measured with this technique, I find the spin-polarization to be negative, P = −9.5 ± 0.4%. This should make it possible to perform a test of the generalized Julliere model for a negatively spin-polarized electrode, by measuring the magnetoresistance of a La_2/3Sr_1/3MnO₃/SrTiO₃/SrRuO ₃ tunnel junction. The generalized Julliere theory predicts it should be inverse.