טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentAronov Pavel
SubjectCrossed Andreev Reflection Effect in YBa2Cu3O7-Gama
Junctions with the Itinerant Ferromagnet SrRuO3
DepartmentDepartment of Physics
Supervisor Professor Emeritus Gad Koren


Abstract

In this study we looked for a crossed Andreev reflection effect (CARE) in super-conductor-ferromagnet-superconductor (SFS) ramp-type junctions. A regular Andreev reflection is suppressed in superconductor-ferromagnet junctions due to the finite polarization of the ferromagnetic material. It was proposed that nonlocal crossed Andreev reflection can occur in a structure consisting of two ferromagnetic electrodes in close contact with a superconductor. In this process an electron incident from one of the electrodes onto the FS interface can create a Cooper pair in S by picking up an electron with opposite spin from the other electrode while a hole is Andreev reflected back into the second electrode. For the CARE process to occur the distance between the two F electrodes must be smaller than the coherence length of the superconductor. It was conjectured that this effect can appear in SFS junctions when the ferromagnetic barrier has many magnetic domains, which play a role of F electrodes with different orientation of the magnetization. Then a crossed Andreev reflection can occur in the intersection of the domain walls and the interfaces with the S electrodes.


We prepared SFS ramp-type junctions using YBa2Cu3O7-δ (YBCO) as the super-conducting material and SrRuO3 (SRO) as the ferromagnetic barrier. These materials were chosen since they fulfil a necessary condition for CARE to occur, namely that the domain wall width of SRO is of the same order of magnitude as the coherence length of YBCO. In our junctions the barrier thickness varied in the range of 4 to 45 nm, while for the study of very thick barriers we used SF ramp-type junctions.


The differential conductance spectra of our junctions were measured under various temperatures and magnetic fields. We observed an asymmetry in the conductance spectra, and shifts of bound state peaks with field, which are typical of magnetic tunneling junctions. In many of our junctions a zero bias conductance peak (ZBCP) was observed. The height of this peak decreased linearly with increasing magnetic field in SFS junctions as expected for a CARE process. In SF junctions however, the decrease versus applied magnetic field was almost exponential thus pointing to a different transport mechanism in these junctions which seems to be of magnetic scattering origin.