Superconductors allow electrical current to travel without resistance, and therefore no voltage drop across them or energy loss within them. Superconductors must be cooled below their critical temperatures to have no electrical resistance. Some elements, compounds, and alloys that are superconductors are lead and niobium nitride, and a niobium-titanium alloy. All these require liquid helium to cool them to their critical temperatures. In the 1980s some ceramics were found to have much higher transition temperatures that could be reached using much cheaper liquid nitrogen. The first found was yttrium barium copper oxide. As of 2008 a family of materials including iron, such as lanthanum oxygen fluorine iron arsenide, was developed.