Oxygen Intercalation And Related Properties Of La₂CuO₄ Bulk Samples And Thin Films

In this thesis, oxygen intercalation and related properties in bulk-and thinfilm-La2CuO4, the mother material of the first high temperature superconductor, havebeen studied. The studies focus on three aspects: intercalation of excess oxygen intoLa2CuO4 (LCO) powder samples by doping of CuO vapor during sintering,intercalation of excess oxygen into LCO thin films through the process ofpost-deposition, ex-situ chemical oxidation and the effects of Ag doping in LCOpellets and thin films. Using CuO vapor as doping resource, oxygen light doping can be achieved inLCO powder sample during sintering. The ultimate sintered sample shows granularsuperconductivity and dense grain boundaries which enhances the Josephsoncoupling between superconducting granules. In such oxygen lightly doped sample,the low temperature resistance versus temperature has a power linear relationship inthe case of low disorder, while in high disorder the relationship becomes the knownlogarithmic linear. We believe that these two relationships are extremes associatingwith the low and high disorder, respectively. In general case, both of the relationshipshave contribution to the observed resistance, and hence we propose a new expressionof resistance with which the previous experimental data can be fitted remarkably well.Magnetic measurement indicates that there are three magnetic transitions in the curveof magnetization-temperature in the oxygen lightly doped sample. The high and lowtemperature transitions correspond to the Néel and superconducting transitions,respectively. Besides, a weak transition occurs at a medium temperature of about 110K. We argue that this weak transition, as a consequence of phase separation,originates from the condensation of polarized clusters in CuO2 plane. Excess oxygen can be convenient inserted into LCO thin films through theprocess of post-deposition, ex-situ chemical oxidation. Various experiments evidencethat excess oxygen is indeed introduced into the films after chemical oxidationtreatments. The inserted oxygen can be classified into two types: the first type one - III -Abstractwith high mobility has high contribution to conductivity and exists as O- in thediffusion channels. Another one with relative high stability has contribution tosuperconductivity and locates in the interstitial positions. Chemical oxidation of LCOepitaxial thin films exhibits time effect. That is, resistances of the oxidized filmsshow semiconducting behavior initially and convert to superconducting behavior asoxidation time increases. With further increasing oxidation time, the resistancesreturn to semiconducting behavior again. This effect is found to originate from thelanthanum deficiency that causes an insulating layer in the topmost of the films due tothe chemical reaction between the surface of the films and oxidizing solution. Theinsulating layer acting as a protect layer limits the interior part of the film for furtherreaction with the agent and seriously affects the superconductivity. Time effect doesnot appear in chemical oxidation of c-axis textured LCO thin films, because theabsence of the insulating layer due to the presence of other oriented grains. The effects of Ag doping in LCO pellets and thin films are found to increase thecarriers' concentration resulting in low resistance and to introduce disorder whichsuppresses superconductivity to some extend.