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.