Search for new superconductors and novel physical properties in thin films and interfaces
- This dissertation describes our unique experimental attempts to search for new high temperature superconductors. In this thesis, we focus on three different materials systems (graphite-sulfur composites, copper (Cu) / copper monoxide (CuO) bilayers and doped tungsten oxides), all of which have been reported in the literature to exhibit trace very high temperature superconductivity. Without simply trying to reproduce the reported results by exactly following the synthesis procedure in the original reports, however, we first consider what theoretical insights might have relevance to possible superconductivity in these systems. Suitable model systems based on such theoretical considerations are then designed, synthesized, and examined in order to study the physical properties in a carefully controlled way. For the case of graphite-sulfur composites, based on experimental convenience (selenium is easier to deposit than sulfur) as well as theoretical considerations, the interface of selenium and epitaxial graphene thin films was selected as the model system. Clean graphene thin films were successfully synthesized, and then a Se layer was deposited on the graphene by pulsed laser deposition in a carefully controlled way. On the basis of photoemission spectroscopy and transport measurements, however, no superconductivity was found, and it is suggested that the Se overlayer merely increases disorder for the electrons in graphene. For the case of Cu/CuO, based on transport measurements no superconductivity was detected in our experiments using clean bilayer interfaces between copper (Cu) and copper monoxide (CuO). However, by careful magnetotransport measurements and theoretical considerations, this model system was found to exhibit a novel interface phenomenon (in which antiferromagnetic spin correlations are induced into the Cu layer due to the adjacent antiferromagnetic CuO) that has not been observed previously. We term this phenomenon an "antiferromagnetic proximity effect". For the case of doped tungsten oxides, we first synthesized single-layer potassium tungsten oxide thin films and characterized the physical properties as a function of the temperatures used in a vacuum annealing process, which presumably controls the carrier density. The films show a number of interesting superconducting properties, including a strange curvature of upper critical fields as a function of temperature and an anomalously high transition temperature in one sample. Motivated by these observations and theoretical considerations, we also synthesized bilayer structures in an attempt to enhance superconducting transition temperature. In some cases, it was found that such bilayers in fact have significantly higher transition temperatures than the monolayer films.
|Type of resource
|electronic; electronic resource; remote
|1 online resource.
|Stanford University, Department of Applied Physics
|Geballe, Theodore H
|Geballe, Theodore H
|Statement of responsibility
|Submitted to the Department of Applied Physics.
|Thesis (Ph.D.)--Stanford University, 2012.
- © 2012 by Ko Munakata
- This work is licensed under a Creative Commons Attribution Non Commercial 3.0 Unported license (CC BY-NC).
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