K. Onnes discovered superconductivity in mercury but soon realized that other metals such as tin, lead or aluminium are also superconductors. In fact, more than half of the basic elements of the periodic table become superconducting if they are cooled to a sufficient temperature. In some cases, a pressure also must be applied on the material.
Throughout the history of superconductivity studies, chemists and physicists have worked on inventing and testing superconducting materials for improved performance: metals that become superconductors at a higher temperature or that could resist higher magnetic fields or stronger electric currents. These materials are generally synthesized in laboratories; they cannot be found in nature.
Sometimes, we make a distinction between "classical superconductors" and "new superconductors" depending on whether they superconduct at an intermediate low or a very low temperature. In fact, these boundaries do not really exist and this definition is vague. Another distinction relies on the origin of the superconductivity: "classical superconductors" follow the BCS theory, whereas the "new superconductors" either do not follow it or the origin is unknown. Sometimes, we call those "high critical temperature superconductors", meaning mostly cuprates, which are the "hottest" superconductors. However, what physicists call a "high temperature" is still lower than -135°C!
Among the classical superconductors, the ones that are most used today are A15 family alloys, especially NbTi (a niobium and titanium alloy) which superconducts below 9 kelvins (-264°C) and can resist up to 15 teslas.
Another one more efficient and more expensive superconductor is Nb3Sn (niobium and tin alloy) which superconducts below 18 kelvins (-255°C) and can resist up to 30 teslas.
Those alloys are used in MRIs, for instance..
