Effect of oxygen stoichiometry on the vortex-glass phase transition in yttrium barium copper oxide thin films
Abstract
The discovery of high temperature superconductors has deeply affected our view of the phenomenology of type-II superconductors. The high superconducting transition temperature together with the high anisotropy displayed by these compounds, greatly enhance the effects of the thermal fluctuations, thus leading to many interesting effects such as the melting of the vortex-lattice into a vortex-liquid. On the other hand, the introduction of strong quenched disorder into the system turns the Abrikosov vortex-lattice into a vortex-glass. The vortex-glass phase is separated from the vortex-liquid phase by a continuous phase transition melting boundary H$\sb{\rm g}$(T). In this thesis, the results of a systematic experimental analysis of the effects of anisotropy on the melting line H$\sb{\rm g}$(T), are presented. This analysis was performed on a series of YBa$\sb2$Cu$\sb3$O$\sb{7-\delta}$ epitaxial thin films with values of $\delta$ ranging from 0.05 to 0.65. Using magneto-transport measurements, the vortex-glass boundaries of YBa$\sb2$Cu$\sb3$O$\sb{7-\delta}$ thin films were constructed as a function of oxygen stoichiometry, $\delta$. The vortex-glass phase boundary was observed to broaden with $\delta$. A dimensional crossover field, H$\sb0$, separating three-dimensional from quasi-two-dimensional behavior, was observed for films with $\delta \ge$ 0.24. This crossover occurs at a temperature T$\sb0 \cong$ T$\sb{\rm c}$/2. The vortex-glass boundaries of all the films can be collapsed onto a single curve when plotted as reduced field H$\sb{\rm g}$/H$\sb{\rm o}$ versus reduced temperature T/T$\sb{\rm c}$. The specific relation H$\sb0 \approx$ 1.2$\Phi\sb0$/(s$\sp2\gamma\sp2)$ is determined for the YBa$\sb2$Cu$\sb3$O$\sb{7-\delta}$ thin films, where s is the multilayer spacing and $\gamma$ is the anisotropy. Thus, the relation H$\sb{\rm g}$(T) $\propto$ 1/$\gamma\sp2$ is established.
Degree
Ph.D.
Advisors
McElfresh, Purdue University.
Subject Area
Condensation
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