Christopher Dana Keener

Vortex lattice melting and interlayer coupling in Bi2Sr2CaCu2O8+δ crystals

We find that resistive hysteresis accompanies vortex lattice melting in Bi2Sr2CaCu2O8+° single crystals, together with a sharp drop of the resistance with decreasing temperature. The melting transition temperatureTm was current dependent, indicating that current drives the vortex system out of equilibrium. AboveTm, we find a second sharp feature in the data which we interpret to signify the decoupling transition of the vortex line liquid. Our measurements show nonlocal conductivity in the vortex liquid phase, which correlates with the strength of interlayer vortex coupling. Finally, we review existing literature on vortex lattice melting in BSCCO. While the melting curve varies significantly for different samples, the measurements appear to probe the same physical process in all cases.

Vortex coupling in Bi2Sr2CaCu2O8+y, using controlled sample geometry

The flux transformer geometry has been useful for studying vortex coupling in Bi2Sr2CaCu2O8+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 Bi2Sr2CaCu2O8+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.