This thesis studies the intrinsic disorder effects and the transport
and magnetic properties of ring-shaped epitaxial thin films and
superconducting tunnel junctions (STJs) of the high temperature
superconductor YBa$_2$Cu$_3$O$_{7-delta}$. We used an
unconventional contactless technique that allows us to directly
measure the persistent current of superconducting rings.
In order to study the disorder effects on the persistent current, we
slowly increased oxygen vacancies in YBa$_2$Cu$_3$O$_{7-delta}$ by
changing $delta$ from 0.03 to 0.55 in steps of $sim$0.021.
Monitoring the corresponding changes in the temperature dependence
of the persistent current revealed an anomaly in its flow within a
certain range of disorder. We found that this anomaly is directly
related to the occurrence of a spinodal decomposition of oxygen
vacancies in YBCO, which we explain as a competition between two
coexisting phases, oxygen rich and oxygen deficient. The analysis of
the time dependence of the persistent current revealed that
increasing oxygen vacancies transforms the vortex structure from
quasi-lattice into a glass and subsequently into a pinned liquid
phase. Our results also exhibited the first evidence of
self-organization of the vortex structure with increasing disorder.
We also performed the first direct measurement of the temperature
dependence of the $c$-axis persistent current ($J_c$) that is purely
due to tunnelling Cooper-pairs through intrinsic Josephson junctions
(IJJs) of YBCO. This is made possible by incorporating IJJs of YBCO
into ring-shaped films. Then, we studied the temperature dependence
of the persistent current of YBCO nanowires embedded in
SrTiO$_3$-barrier integrated between two semi-ring-shaped YBCO thin
films and systematically varied the nanowires length. Our
observations revealed that $J_c$ has two different temperature
dependences: a GL-dependence ($J_c propto (T_c - T)^{3/2}$) at low
temperatures which we found the same in all studied samples, and
another power law dependence ($J_c propto (T_c - T)^{alpha >
3/2}$) at high temperatures which turned out to depend on the length
of the nanowires. We attribute the cross-over between these two
temperature dependences to the depinning and the dissipative motion
of vortices.
These experimental approaches and findings not only provide new
information, but more importantly open new avenues of investigating
the transport and magnetic properties of superconducting films,
junctions, and nanowires.
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:AEU.10048/606 |
| Date | 11 1900 |
| Creators | Mansour, Ahmad Ibrahim |
| Contributors | Chow, Kim H. (Physics), Jung, Jan A. (Physics), Fenrich, Frances (Physics), Heimpel, Moritz (Physics), Etsell, Thomas H. (Chemical and Materials Engineering), Christen, David K. (Superconductive and Energy Efficient Materials, Materials Science and Technology Division,Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA) |
| Source Sets | Library and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada |
| Language | en_US |
| Detected Language | English |
| Type | Thesis |
| Format | 5127698 bytes, application/pdf |
| Relation | A.I. Mansour, R. Ma, M. Egilmez, M.M. Saber, I. Fan, K.H. Chow, and J. Jung: Physical Review B 79, 172504 (2009). Selected for the June 1, 2009 issue of Virtual Journal of Applications of Superconductivity., A.I. Mansour, M.M Saber, K.H. Chow, and J. Jung,: Applied Physics Letters 93, 142509 (2008). Selected for the October 15, 2008 issue of Virtual Journal of Applications of Superconductivity., A.I. Mansour, M. Egilmez, I. Fan, K.H. Chow, J. Jung, E.M. Choi, H.S. Lee, S.I. Lee, and H. Darhmaoui: Applied Physics Letters 90, 162511 (2007). |
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