Vortex coupling in Bi2Sr2CaCu2O8+y,using controlled sample geometry

The flux transformer geometry has been useful for studying vortex coupling in Bi₂Sr₂CaCu₂O_8+y crystals but is limited by the difficulty of controlling sample size. We have used an argon ion mill to etch two channels into the top face of a Bi₂Sr₂CaCu₂O_8+y crystal to reproduce the effects of the flux transformer geometry with more precise control over effective sample dimensions. The temperature dependence of the transformer voltage in such a crystal shows a pronounced peak between 87 K and 89 K which is linear with current. Application of a magnetic field (H c-axis) sharply suppresses the peak, broadens it, and shifts it to lower temperatures. The peak in the transformer voltage and its current and field dependence are consistent with interlayer Josephson coupling of thermally excited vortices.