or  low temperature  (generally if they need other techniques to be cooled under their critical temperature). By material : they can be  chemical elements  (as  mercury  or  lead ),  alloys  (as  niobium-titanium  or  germanium-niobium  or  niobium nitride ),  ceramics  (as  YBCO  or the  magnesium diboride ), or  organic superconductors  (as  fullerenes