Controlling hole concentration in superconducting YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ thin films for spectroscopic studies$^{\ast }$

One of the remaining controversial issues in the high-$T_{c}$ cuprate superconductors is whether the $d$-wave order parameter is robust over the whole doping range. As an essential first step to address this, we optimize a growth procedure for high-quality sputter-deposited YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ thin films with various hole concentrations by controlling the oxygen content (and by cation doping in the future). Two different approaches have been attempted successfully: annealing in an oxygen-controlled environment and ozonization. We have developed an \textit{in-situ} annealing procedure employing two-step post-deposition anneals in vacuum and O$_{2}$. The oxygen content is estimated from high-resolution x-ray diffraction data. This procedure produces high-quality thin films, optimally doped (e.g., 7-$\delta \approx $6.88, $T_{c,on}$=91.8 K, and $\Delta T_{c}<$0.9 K) and underdoped (e.g., 7-$\delta \approx $6.43, $T_{c,on}$=55.1 K, and $\Delta T_{c}<$2.9 K). Preliminary ozonization experiments show that varying the oxygen content in a controlled manner is feasible, especially in the overdoped regime. We will present detailed transport measurements, materials characterizations, and some initial conductance spectra from planar tunneling and Andreev reflection spectroscopies. *Supported by the U.S. DoE Award No. DEFG02-91ER45439 through the FSMRL and the CMM at UIUC.