Progression of the vortex-solid to vortex-liquid phase boundary with oxygen doping in Y$_{0.8}$Ca$_{0.2}$Ba$_{2}$Cu$_{3}$O$_{x}$ Films

By extending magneto-transport measurements to magnetic fields of 35 tesla we have been able to examine the vortex-solid to vortex-liquid transition of thin film Y$_{0.8}$Ca$_{0.2}$Ba$_{2}$Cu$_{3}$O$_{x}$ samples (6.45~$\le $~x~$\le $ 7.0) over a field-temperature range larger than heretofore reported. It is found in this work that the shape of the phase boundary, H$_{g}$(T), evolves from a very shallow low-field temperature dependence to an extremely rapid high field temperature dependence in the highly underdoped regime (x~$\approx $ 6.45). However, in the lightly overdoped regime (x $\approx $ 6.9 - 7.0), H$_{g}$(T) displays an increasingly steep low-field temperature dependence followed by a lessening of the steepness of the high-field region as oxygen content increases. This trend suggests that the boundary of the dissipation-less superconducting region of this unconventional high-T$_{c}$ cuprate based compound is evolving in the overdoped state towards a form that is consistent with what is observed in conventional superconductors. This research was supported by U.S. DOE Grant No. DE-FG02-04ER46105. A portion of this work was performed at the National High Magnetic Field Laboratory, which is supported by NSF Cooperative Agreement No. DMR-0084173, by the State of Florida, and by the DOE.