Transport and infrared properties of ion irradiated yttrium barium copper oxide thin films

Abstract

<p>The discovery of high temperature superconductors prompted the reconsideration of numerous aspects of superconductivity. Among the most fundamental and controversial is the origin of dissipation in a current carrying superconductor. This is also very important from an applied perspective since the onset of dissipation, often described by the critical current density Jc, determines the current carrying capacity of a material. In this thesis, low energy, light ion irradiation has been implemented as a means of controlled sample modification. This type of irradiation has the advantage that it does not alter the stoichiometry of the sample but only results in disorder through the creation of point defects. This thesis explores the influence of ion damage on the normal and superconducting properties of YBa₂Cu₃O₇₋δ thin films. Of particular interest is the contrast between the strong sensitivity of Jc and the more modest sensitivity of the critical temperature, Tc, to ion damage. The evolution of the temperature dependent resistivity was measured systematically as a function of ion damage. The decrease in Tc with ion damage was compared to the predictions of several relevant models. A simple model based on Matthiessen's rule was used to describe the scattering rate and carrier density at low damage levels. At higher damage levels the system undergoes a superconductor-insulator transition which was found to be consistent with the Ioffe-Regel criterion. However, the resistivity in the insulating state is not described by any conventional models for strongly disordered materials. Measurements of the non-linear electric field-current density characteristics were performed for several levels of ion damage over a range of temperatures and compared with several models for dissipation. The data were found to be well described at all temperatures and damage levels by a model involving the quantum nucleation of vortex loops. The variation of the superconducting carrier density, ns, with ion damage was extracted and compared with independent measurements of the same quantity by infrared transmission in identically irradiated YBa₂Cu₃O₇₋δ thin films. This represents the first time a direct connection has been established between Jc and ns in high temperature superconductors.</p>