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http://hdl.handle.net/11375/8631
Title: | The optical properties of high-T(c) superconductors grown by pulsed laser deposition |
Authors: | Hughes, Robert |
Advisor: | Timusk, T. |
Department: | Physics |
Keywords: | Physics;Physics |
Publication Date: | Sep-1992 |
Abstract: | <p>The discovery of superconductivity at temperatures in excess of 30 K has led to an unprecedented effort in both fundamental and materials research. Four separate projects will be presented here which encompass both types of research. The first two involve the deposition of superconducting thin films by the pulsed laser deposition technique. The final two projects use optical spectroscopy to probe the fundamental excitations which govern these exotic materials. All four projects are summarized below. Superconducting (Pb(0.75}Cu(0.25)Sr₂(Y1-yCay)Cu₂O₇ films were successfully deposited by pulsed laser deposition. The films are grown in situ on (100) LaAlO₃ at the relatively low substrate temperature of 620℃. The films are highly oriented with their c-axis perpendicular to the substrate and exhibit a surface morphology which is unique to the oxide superconductors. The growth procedure used yields relatively sharp superconducting transitions and a maximum Tc that exceeds all other reported values. By substituting Ca for Y it is possible to systematically vary the superconducting transition temperature from 10 K to a maximum of 86 K. The laser deposition system was also used to deposit superconducting Nd(1.85)Ce(0.15)CuO(4+δ) thin films. By carefully controlling the film's oxygen content through high temperature vacuum anneals it is possible to obtain a superconductor with a transition temperature which onsets at 21 K. The high temperature anneal can result in a film surface which is copper deficient. By optimizing the growth parameters we have been able to eliminate this layer resulting in films with superior homogeneity. The films are highly oriented with their c-axis perpendicular to the substrate and show a surface morphology of high quality. The far infrared optical properties of the Nd{1.85}Ce{0.15}CuO{4+δ} films were measured using Michelson interferometry. The reflectance spectra clearly show the presence of a ledge at 420 cm⁻¹ offering further evidence that this feature is common to all the cuprate superconductors. The optical conductivity obtained from Kramers-Kronig analysis can be divided into two components. The first component, which is strongly temperature dependent, is fit to a Drude model. This fit indicates that Nd{1.85}Ce{0.15}CuO{4+δ} has a constant plasma frequency and a temperature dependent scattering rate which closely follows the temperature dependence exhibited by the dc resistivity. The second component shows little temperature dependence and is described by a model which couples phonons to a midinfrared band. Comparisons between the optical conductivity and the structures seen by tunneling and inelastic neutron scattering measurements are made. In the final project, the far infrared reflectance of partially transparent (Pb{0.75}Cu{0.25})Sr₂(Y{0.4}Ca{0.6})Cu₂O₇ films were measured. As was the case for Nd{1.85}Ce{0.15}CuO{4+δ}, the normal state results were described in terms of the two component model. Once again, the Drude component showed a constant plasma frequency and a scattering rate which closely follows the temperature dependence of the dc resistivity while the second component showed only a small change as the temperature was varied. In the superconducting state the reflectance spectra do not exhibit the clean limit behaviour that has been observed in the other oxide superconductors. A measurable fraction of the Drude component's oscillator strength does not condense into the delta function which creates the possibility of observing structure associated with a superconducting energy gap.</p> |
URI: | http://hdl.handle.net/11375/8631 |
Identifier: | opendissertations/3820 4837 1723160 |
Appears in Collections: | Open Access Dissertations and Theses |
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