In April 1986, Georg Bednorz and Karl Müller, working at IBM in Zurich, discovered that certain semiconducting oxides became superconducting at relatively high temperature, in particular, a lanthanum barium copper oxide becomes superconducting at 35 K. This oxide was an oxygen-deficient perovskite-related material that proved promising and stimulated the search for related compounds with higher superconducting transition temperatures. In 1987, Bednorz and Müller were jointly awarded the Nobel Prize in Physics for this work.
Following Bednorz and Müllers discovery, a team led by Paul Ching Wu Chu at the University of Alabama in Huntsville and University of Houston discovered that YBCO has a superconducting transition critical temperature (T') of 93 K. The first samples were YBaCuO, but this was an average composition for two phases, a black and a green one. Workers at Bell Laboratories  identified the black phase as the superconductor, determined its composition YBaCuO and synthesized it in single phase  
YBCO was the first material found to become superconducting above 77 K, the boiling point of liquid nitrogen, whereas the majority of other superconductors require more expensive cryogens.  Nonetheless, YBCO and its many related materials have yet to displace superconductors requiring liquid helium for cooling.