A technique for measuring the 3-dimensional to 2-dimensional conductivity change of YBCO superconductors at the normal-to-superconducting phase change

It is well known that conduction in YBa2Cu3O7 (YBCO) is by means of copper "planes" and "chains," where planes and chains describe the degree of bondi ng between copper and oxygen. Changes of conductivity versus temperature ha ve been used to show that conduction in YBCO in the normal state is 3-dimen sional, while conduction approaching the onset of the superconducting state is first 2-dimensional and then 3-dimensional. We have found another metho d to monitor this 2-to-3 transition. Using square samples, and measuring th e voltage at each corner caused by a current applied to the opposite corner s, one can find the conductivities along the x-axis and the y-axis. The rat io of these conductivities is unity for homogeneous samples in the normal s tate. However, in transition to the superconducting state, the ratio of con ductivities changes. We examine this change as a function of sample purity, sample history, and exposure to an external magnetic field. Our data are c onsistent with data reported in the literature, and they suggest the existe nce of another state change deep in the superconducting state, which is onl y observable with the application of a magnetic field. Measurements were also carried out to correlate the anisotropy with sample porosity. Measurements of normal state resistivity, critical temperature, a nd critical current characterize the sample's porosity, and these data affe ct the anisotropy in the superconducting state in a manner directly proport ional to the porosity. (C) 1998 Kluwer Academic Publishers.