Superconductivity is a state of matter where certain materials, when cooled below a specific temperature (the critical temperature, or \(T_c\)), exhibit two properties: the complete disappearance of electrical resistance and the expulsion of magnetic fields. The expulsion of magnetic fields is known as the Meissner effect, which is the principle behind magnetic levitation. Materials that achieve this state range from simple metallic elements to complex ceramic compounds. The composition of the material dictates its \(T_c\), determining whether it can be cooled with relatively inexpensive liquid nitrogen or requires much colder liquid helium.
Metallic and Alloy Superconductors
The earliest discovered superconductors were simple metallic elements, which typically require cooling to temperatures near absolute zero. Elemental metals like lead (Pb) and mercury (Hg) were among the first studied, with mercury achieving superconductivity at approximately 4.2 Kelvin (K). These materials are often classified as Type I superconductors and exhibit a sharp transition to the superconducting state. They are primarily described by the Bardeen-Cooper-Schrieffer (BCS) theory, which explains superconductivity as the pairing of electrons into “Cooper pairs.”
The most commercially relevant conventional superconductors are metallic alloys, classified as Type II. These alloys maintain their superconducting state even in strong magnetic fields, making them essential for high-field applications. The most widely used alloy is Niobium-Titanium (\(\text{NbTi}\)), which is used extensively in Magnetic Resonance Imaging (MRI) machines and particle accelerators, operating below its \(T_c\) of around 9.3 K. Niobium-Tin (\(\text{Nb}_3\text{Sn}\)) has a higher \(T_c\) of approximately 18.3 K and is favored for extremely high-field magnets where \(\text{NbTi}\) is insufficient.
Ceramic High-Temperature Superconductors
High-Temperature Superconductors (HTS) emerged in 1986, representing a major shift in superconducting materials. These materials are primarily ceramic compounds, fundamentally different from pure metals and metallic alloys. They are often referred to as cuprates because their structure is based on stacked layers of copper-oxide (\(\text{CuO}_2\)) planes.
The most famous example is Yttrium Barium Copper Oxide (\(\text{YBa}_2\text{Cu}_3\text{O}_{7-\text{x}}\)), abbreviated as YBCO. This compound was the first material discovered with a \(T_c\) high enough (around 93 K) to be cooled using liquid nitrogen, which boils at 77 K. Since liquid nitrogen is cheaper and easier to handle than liquid helium, HTS materials are more practical for widespread use. Structurally, these cuprates are highly anisotropic, meaning the superconducting current primarily flows along the copper-oxide planes.
Newer and Exotic Superconductor Compositions
The search for higher critical temperatures led to the discovery of new families of compounds outside the metallic or cuprate categories. One significant discovery was Magnesium Diboride (\(\text{MgB}_2\)), an intermetallic compound with a simple hexagonal layered structure. Although its \(T_c\) of approximately 39 K is lower than the cuprates, it is the highest known among conventional, BCS-type superconductors. The simple composition of \(\text{MgB}_2\) and its easy fabrication into wires make it practical for applications requiring temperatures between liquid helium and liquid nitrogen.
Another major development was the discovery of Iron-based Superconductors (FeSCs), which contain layers of iron atoms bonded to pnictogen (like Arsenic or Phosphorus) or chalcogen (like Selenium or Tellurium) elements. These iron pnictides and chalcogenides have a layered structure similar to cuprates but with iron-based planes. They can achieve \(T_c\) values up to 58 K, making them the second-highest class of high-temperature superconductors. Research has also explored non-metallic compositions, including organic superconductors (crystalline structures based on carbon-containing molecules) and heavy fermion compounds (incorporating rare-earth or actinide elements).