Pressure contacts to Bi2212 whiskers

Eight electrical silver contacts were attached to very small and thin orthorhombic Bi2212 single-crystal whiskers by the application of pressure in a small cell developed especially for this purpose. No heating treatment of the sample and the contacts was necessary; thus, diffusion of the contact material into the sample was avoided. Four contacts were made to the top ab-face of the whisker, and four to the bottom ab-face, the crystallographic c-axis being along the thickness of the whisker. This arrangement allowed electrical measurements to be made in a variety of ways. These measurements revealed that the current distribution is not uniform through the highly anisotropic Bi2212 superconductor. A contact resistance of a few ohms was seen and it changed by less than 30% between the superconducting transition of the whiskers and 4.2 K. This contact resistance is due to the normal surface layers of the whiskers and shows a temperature dependence similar to that of metallic glasses. However, the contact resistance is only of the order of 10% of the resistance of the whiskers in the normal state and stays constant at any given temperature below the superconducting transition of the whiskers which makes this pressure contact technique valuable. (author)