High‐Temperature Supercurrents May Not Be Forever

Synthesis of thin films of the new oxide superconductors that have large critical current densities has been an important development in taming the properties of those materials for useful applications. A superconductor becomes normal when a current stronger than the critical current density is passed through it. Many applications, however, require large values of the critical current density. “Good” films of the 90‐K superconductor $YBa2Cu3O7−y$ with critical current densities as large as $4×106 amps/cm2$ at 77 K have been reported, according to Praveen Chaudhari (IBM Yorktown Heights). These values of the critical current density, as well as the magnetic properties of the epitaxial films, compare quite favorably with those of the commercially used niobium‐titanium superconductor at 4.2 K.