טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentShkedy Lior
SubjectMagneto-Transport Investigation of Anisotropic Properties
in the ab plane of YBCO Films and Junctions
DepartmentDepartment of Physics
Supervisor Professor Emeritus Emil Polturak


Abstract

The anisotropy of the HTSC in the a-b plane is revealed in different properties of these superconductors, some of which are studied in this work. First, we measured the resistance of YBCO films below the superconducting temperature subjected to a magnetic field. The resistance of the film which is due to flux motion, was measured along different crystallographic directions and showed a definite dependence on direction which was not observed before. We interpreted this dependence on direction as a result of the anisotropy of the pinning potential in the superconductor or as a consequence of a non isotropic vortex core structure.


Next we prepared junctions made of two YBCO superconductors separated by Ga-doped YBCO barrier creating an SNS junction. We observed peaks in the dynamic resistance spectra of these junctions. The peaks are identified as McMillan-Rowell oscillations which originate from multiple reflections of quasiparticles in the barrier. From these peaks, measured in junctions oriented along the node direction we put an upper limit on the subdominant component of the superconducting energy gap.


The main part of this work is the measurement of the resistance of these junctions as a function of an applied magnetic field. We observed a positive linear magnetoresistance, which can be explained by a model based on the field dependence of the proximity effect in the barrier of the junction. The success of this model means that the proximity effect in underdoped HTSC materials can be described using the theory of the proximity effect used for low Tc superconductors. This is also the first indirect measurement of the dependence of the proximity effect on magnetic field in the HTSC.