The structural symmetry of epitaxial Tl2Ba2CuO6+δ thin films

Epitaxial films of Tl2Ba2CuO6+δ (Tl‐2201) have been prepared on SrTiO3 by sputtering and postdeposition annealing. The films were deposited onto (100)SrTiO3 single crystal substrates by rf magnetron sputtering and then annealed using a two‐step process. The annealed films typically exhibit transition temperatures of about 11 K. Subsequent reannealing to optimize the oxygen doping in the film results in an increase in the transition temperature, and optimally reannealed films exhibit transition temperatures of about 83 K. The crystal structure of these films has been studied by x‐ray diffraction and transmission electron microscopy. These results indicate that both the annealed (Tc=11 K) and optimally reannealed samples (Tc=83 K) have a tetragonal crystal structure. This structural information is crucial in evaluating recent measurements on these films to determine the electron pairing symmetry (d wave or s wave) in high‐Tc superconductors.