Results of Critical Current Measurements

Film Critical Current Densities Critical current densities of thin films have been reported by several hundreds of papers a few representative but by no means inclusive are noted here in addition to those mentioned above. Desirable attributes of thin films for technology are high transition temperature to zero resistance, high critical current, low substrate temperature during deposition, no high temperature post anneal, and atomically smooth surface without pinholes. A thermal coevaporation of yttrium, barium, and copper in an oxygen atmosphere have been deposited by Berberich (34) on substrates at 650°C with Tc s of 91 K on MgO and 89 K on SrTiOs without post anneal. Although critical currents of 106 A/cm2 were obtained at 4 K, values of 104 A/cm2 were found at 77 K. However, 

Other deposition techniques have been used (35) and a promising technique of chemical vapor deposition has been reported and substantial critical current densities obtained by Berry et al. (39) and by Watanabe et al. (40). Values of Jc at 77 K and at 2 T and 27 T were 4 x 105 and 6.5 x 104 A/cm2 respectively (40). The surface of the film shows significant growth of c axis material in the plane as well as the desired normal to the plane growth. On the other hand this mixed phase growth may supply additional pinning centers for the improved critical current. 

Films of Bi-Sr-Ca-Cu-O have been produced by a wide variety of techniques, a recent effort by Steinbeck et al. (6) shows results for a film with Tc = 90 K and Jc = 8 x 104 A/cm2 at 77 K. This film was prepared on an MgO substrate and a packed micaceous-like structure is seen on the top surface of the film. 

Other substrates are being investigated for purposes such as obtaining reduced high frequency losses. The LaA103 material has been used (42) for frequencies of 10 GHz. Reasonable critical current densities are found, Jc 8 x 104 A/cm2 at 77 K. A bilayer structure of the film was found at the substrate surface the first layer had the c axis normal to the substrate and the second layer had the c axis in the plane. 

The limitation on critical currents has been discussed by J. Clem (49) as due to weak link or Josephson coupling between grains. 

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