Characterization of superconducting SmBa 2 Cu 3 O 7 films grown by pulsed laser deposition
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We have investigated epitaxial superconducting SmBa2Cu3O7 (Sm123) films grown by pulsed-laser deposition on single-crystal SrTiO3 substrates. The deposition temperature plays an important role in determining the superconducting properties of Sm123 films. The superconducting transition temperature increases with the deposition temperature whereas the transition width decreases at deposition temperatures in the range of 700–875 °C. A Sm123 film deposited at 850 °C exhibits a transition temperature above 93 K with a transition width less than 0.5 K. Even though Sm123 films exhibit a higher transition temperature than YBa2Cu3O7 (Y123), the Sm123 shows lower critical current density at liquid-nitrogen temperature. The nominal critical current density of Sm123 film is less than 1 MA/cm2 at 75.4 K. Nevertheless, the Sm123 films have less anisotropy and stronger pinning characteristics compared to Y123. They are also much smoother with fewer particulates, as revealed by scanning electron microscopy.
I. INTRODUCTION
Recently, tremendous efforts have been devoted to deposit high-performance superconducting YBa2Cu3O7 (Y123) films for second-generation coated conductors.1–3 The techniques widely used to grow Y123 films for coated conductors include pulsed laser deposition (PLD),2 e-beam deposition,4 metalorganic chemicalvapor deposition,5 liquid-phase epitaxy,6 and postdeposition processes such as metalorganic decomposition using trifluoroacetates,7 etc. In terms of the total critical current/unit width of the tape, PLD is one of the most successful techniques for producing high-performance thick (>1 m) Y123 films. For example, critical current values over 100 A at 75 K have been routinely demonstrated on centimeter-width Y123 films on polycrystalline Ni-based alloy.8 However, the use of Y123 films produced by PLD for coated conductors also has its limitations. For example, the generation of particulates and outgrowth nucleation make the surface very rough for film thickness more than 1 m. This surface roughening promotes the growth of a-axis-oriented grains. Even worse, the surface roughness or disordered morphology leads to microporosity in the growing Y123 film due to selective diffusion and
nucleation, atomic shadowing, and/or coalescenceinduced void formation. This porosity in turn blocks current paths and eventually becomes severe enough that there is no continuous current path through the upper layers of the film.9 On the other hand, thin films of some superconducting materials such as SmBa2Cu3O7 (Sm123) show a much better surface morphology than Y123.10–13 This is important for the fabrication of reproducible thin-film devices. By the same token, the smoother nature of Sm123 films is also important for the fabrication of coated conductors where a thick film is necessary. However, there are only limited investigations on the deposition and characterization of Sm123 films. This may in part be due to the relatively low critical transition temperature and critical current density of Sm123 films deposited by PLD.14 It
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