Superconductivity in the rare-earth-free Tl–Ba–Cu–O system above liquid-nitrogen temperature

Abstract

The initial discovery by Bednorz and Muller¹ of 35-K superconduc-tivity in the La–Ba–Cu–O system has stimulated worldwide activity in searching for higher-temperature superconductors. Elemental substitution has proved to be most effective in raising transition temperature. Substitution of Sr for Ba has produced 40-K super-conductivity^2–5, and substitution of Y for La has produced a new high-temperature superconductor with transition temperature above liquid-nitrogen temperature⁶. A class of superconducting compounds of the form RBa₂Cu₃O_7−x has been explored by further substitutions of other rare earths (Y is considered in the rare-earth [R] category here) for Y^7–13. To date, a rare earth, an alkaline earth, copper and oxygen have been required for all high-temperature superconductors^14,15. (Zhang et al.¹⁴ reported 90-K superconductivity in the Th–Ba–Pb(Zr) –Cu–O system. Pan et al.¹⁵ reported 50-K superconductivity in the Y–Ba–Ag–O system. As Th is a member of the actinide series which belongs to the same Group 3B in the periodic table as the lanthanide series and Ag belongs to the same Group IB as Cu, high-temperature supercon-ductors are still thought to be closed in the Group 3B–Group 2A–Group 1B–oxygen system.) Only partial substitutions have led to superconductors, but with no significant rise of transition tem-perature (the only exception is 40-K superconductivity in La₂CuO_4−x, refs 16, 17). Here we report superconductivity in the rare-earth-free Tl–Ba–Cu–O system. We have observed sharp drops of resistance starting above 90 K with zero resistance at 81 K in this system. Magnetic measurements have confirmed that these sharp drops of resistance in the Tl–Ba–Cu-–O samples origi-nate from superconductivity. The samples are stable in air for at least two months, and their preparation is easily reproduced.