Vortex activation energy in high-Tc superconductors from transport measurements: new systematics*

From electrical transport measurements on thin films of YBCO with and without nano-precipitate ``columnar defects,'' we extract information about the activation energy for vortex creep. We also obtain new scaling between temperature, critical current density $J_C$, and the power law index $n$ describing the voltage-current relation $V\propto I^n$. This scaling occurs in the range of field and temperature where $J_C$ decays as a power-law of magnetic field. In this regime the $n$-value can unexpectedly increase as applied field increases. We will discuss implications of these observations and compare systems with and without such scaling.