Superconducting Yttrium-Barium Thin Films: Preparation by Laser Deposition and Two-Dimensional Phase Transition Properties

Laser deposition of high-T_{ rm c} superconducting YBa_2 Cu_3O_{ rm 7-x} thin films was performed. By an ArF excimer laser, the films were deposited on several (001)-oriented substrates: SrTiO_3, yttria-stablized ZrO_2 and MgO. They exhibited critical temperatures above 86 K and critical current densities above 1 times 10 ^6 A/cm^2 at 77 K and 1 times 10^7 A/cm^2 at 4.2 K. X-ray diffraction and TEM analysis showed a high-degree orientation of the films: c-axis perpendicular to the substrate surface, and a- or b-axis aligned in the plane dominantly. The laser deposition process was investigated. From the optical emission spectroscopy, it was identified that Y, Ba and Cu atoms and their ions existed in the laser -created plume, as well as their oxides such as YO, BaO and CuO. The results of the in situ resistance measurement revealed that a certain oxygen partial pressure during the deposition was essential to the in situ YBa _2Cu_3O_ {rm 7-x} thin film deposition without high-temperature annealing. It provided deposition conditions under which tetragonal YBa_2Cu _3O_6 was thermodynamically stable. The tetragonal phase, formed in situ, could be transformed to the orthorhombic phase upon oxygen backfilling at the end of deposition. The possibility of in situ YBa _2Cu_3O _{rm 7-x} thin film deposition lies on this fact. The Kosterlitz-Thouless (KT) transition, which takes place in 2D superconductors, and the conductivity fluctuation, which is a probe of the dimensionality associated with the order parameter, were studied to examine the 2D nature of the laser-deposited YBa_2Cu _3O_{rm 7-x} thin films. The experimentally observed phenomena, i.e. the power-law dependence of the I-V characteristics with a universal jump of the exponent, the critical behavior, the Ginzberg-Landau asymptotic behavior, and the resistance as a function of temperature near superconducting transition, were all consistent with the theory of KT transition. The KT transition was believed from the quasi-2D vortex-antivortex pair unbinding in these films. The quasi-2D nature was related to the intrinsic anisotropy of the material. The observed conductivity fluctuation might be influenced by the sample quality, therefore, the sample preparation method.