Abstract:
With Low-Temperature-Electron-Microscopy (LTSEM) it is possible to analyse the transport properties of solids at low temperatures. In particular it is possible to image the supercurrent density j_s in Josephson junctions. This was demonstrated by comparing TTREM-images with calculated values for j_s. In this thesis ramp-type Nd$_{2-x}$Ce$_{x}$CuO$_{4-y}$/Nb-Josephson-junctions (NCCO/Nb) and Josephson junctions with a ferromagnetic interlayer Nb/Al-Al$_2$O$_3$/NiCu/Nb, so-called SIFS (superconductor-insulator-ferromagnet-superconductor) Josephson junctions were studied.It was demonstrated that LTSEM provides direct imaging of the sign change of the order parameter in superconductors with $d_{x^{2}-y^{2}}$-symmetry. This was a controversial issue over the last decade.
A step like variation in the thickness of the F-layer allows the fabrication of linear and annular Josephson junctions with different numbers of 0 and pi facets. With the LTSEM $0$-, $\pi$-, $0$-$\pi$-, $0$-$\pi$-$0$-, $0/2$-$\pi$-$0/2$-, $20\times$($0$-$\pi$)- as well as square-shaped-, circular- and annular-Josephson-junctions were studied. It was demonstrated, that these junctions are of good quality and have critical current densities up to 42 A/cm2 at T=4.2 K, which is a record value for SIFS junctions with a NiCu F-layer so far. By comparing the measurements with simulations a first indication of a semifluxon at the $0$-$\pi$-boundary was found.