In 1986, Karl Müller and Johannes Bednorz discovered the first high temperature superconductor, La 2-x Ba x CuO 4, which is superconducting at temperatures as high as 39 kelvin. It was realized that the high temperature superconductors are examples of strongly correlated materials where the electron–electron interactions play an important role. A satisfactory theoretical description of high-temperature superconductors is still not known and the field of strongly correlated materials continues to be an active research topic.