Precision Measurements on High T sub c Superconductors

To understand the superconductivity in copper oxide based high T sub c superconductors, accurate and precise measurements of their properties are needed. We shall discuss two measurements, i) an electron density study on Ba sub 2 YCu sub 3 O sub 7 and ii) the determination of the resistivity tensor in Bi sub 2 Sr sub 2 CaCu sub 2 O sub 8. Both measurements require high precision and the data has to be corrected for various effects. The electron density determination in Ba sub 2 YCu sub 3 O sub 7 allows the refinement of anisotropic temperature parameters for all atoms and anharmonic temperature parameters for the metal atoms. In particular, we find a local minimum in the potential well for barium, indicating the possibility of two site tunneling.