Even after 25 years of research the pairing mechanism and - at least for electron doped compounds - also the order parameter symmetry of the high transition temperature (high-Tc) cuprate superconductors is still under debate. One of the reasons is th
e complex crystal structure of most of these materials. An exception are the infinite layer (IL) compounds consisting essentially of CuO2 planes. Unfortunately, these materials are difficult to grow and, thus, there are only few experimental investigations. Recently, we succeeded in depositing high quality films of the electron doped IL compound Sr1-xLaxCuO2 (SLCO), with x approximately 0.15, and on the fabrication of well-defined grain boundary Josephson junctions (GBJs) based on such SLCO films. Here we report on a phase sensitive study of the superconducting order parameter based on GBJ SQUIDs from a SLCO film grown on a tetracrystal substrate. Our results show that also the parent structure of the high-Tc cuprates has dx2-y2-wave symmetry, which thus seems to be inherent to cuprate superconductivity.
A chemical etching method was developed for (110) and (001) NdGaO3 single crystal substrates in order to obtain an atomically flat GaO2-x - terminated surface. Depending on the surface step density the substrates were etched in pH-controlled NH4F- or
NH4Cl-based solutions, followed by an annealing step at temperatures of 800-1000oC, in air or in oxygen flow, in order to recrystallize the surface. Atomic Force Microscopy (AFM) and high-pressure Reflection High Energy Electron Diffraction (RHEED) were used to analyse the surface morphology of the samples after every treatment. Studies on the chemistry and characteristics of the terminating layer showed that the chemically etched NdGaO3 substrate surface has a GaO2-x termination and that the (110) and (001) NdGaO3 surfaces are characterized by a different free surface energy, which is lower for latter.
Cerium-doped manganite thin films were grown epitaxially by pulsed laser deposition at $720 ^circ$C and oxygen pressure $p_{O_2}=1-25 $Pa and were subjected to different annealing steps. According to x-ray diffraction (XRD) data, the formation of CeO
$_2$ as a secondary phase could be avoided for $p_{O_2}ge 8 $Pa. However, transmission electron microscopy shows the presence of CeO$_2$ nanoclusters, even in those films which appear to be single phase in XRD. With O$_2$ annealing, the metal-to-insulator transition temperature increases, while the saturation magnetization decreases and stays well below the theoretical value for electron-doped La$_{0.7}$Ce$_{0.3}$MnO$_3$ with mixed Mn$^{3+}$/Mn$^{2+}$ valences. The same trend is observed with decreasing film thickness from 100 to 20 nm, indicating a higher oxygen content for thinner films. Hall measurements on a film which shows a metal-to-insulator transition clearly reveal holes as dominating charge carriers. Combining data from x-ray photoemission spectroscopy, for determination of the oxygen content, and x-ray absorption spectroscopy (XAS), for determination of the hole concentration and cation valences, we find that with increasing oxygen content the hole concentration increases and Mn valences are shifted from 2+ to 4+. The dominating Mn valences in the films are Mn$^{3+}$ and Mn$^{4+}$, and only a small amount of Mn$^{2+}$ ions can be observed by XAS. Mn$^{2+}$ and Ce$^{4+}$ XAS signals obtained in surface-sensitive total electron yield mode are strongly reduced in the bulk-sensitive fluorescence mode, which indicates hole-doping in the bulk for those films which do show a metal-to-insulator transition.
