CONTINUOUS CONTROL OF THE SUPERCONDUCTING TRANSITION-TEMPERATURE FROMOVERDOPED TO UNDERDOPED REGIMES IN TETRAGONAL TL2BA2CUO6-FILMS(DELTA THIN)

Epitaxial superconducting thin films Tl2Ba2CuO6+delta (Tl-2201) with a continuous range of doping levels have been synthesized. The films we re made by rf magnetron sputtering and two-step postdeposition anneali ng, and their doping levels were controlled by varying the oxygen cont ent through annealing in flowing argon. By annealing in flowing argon, the as-grown Tl-2201 thin films were changed continuously from the in itially overdoped state, through the optimally doped state and then to the underdoped state. The normal state resistivity versus temperature curves of all the underdoped Tl-2201 thin films are convex, whereas t hose of the overdoped Tl-2201 thin films are all concave. Transmission electron microscopy bright-field images and selected area diffraction patterns of the 58 K underdoped Tl-2201 thin film showed neither twin ning bands nor spot. splitting associated with an orthorhombic crystal lattice. Therefore, it is concluded that underdoped Tl-2201 thin film s are also tetragonal just like all the overdoped and optimally doped Tl-2201 thin films. (C) 1997 American Institute of Physics.