What Are Superconducting Materials?

Superconducting materials are substances that, when cooled below a characteristic critical temperature (Tc), exhibit zero DC electrical resistance and expel magnetic flux from their interiors. These two properties, the absence of resistive loss and the Meissner effect, are the defining signatures of the superconducting state and distinguish it from merely low-resistance conductors. The discovery of superconductivity by Heike Kamerlingh Onnes in mercury in 1911 opened a field of materials science concerned with understanding which materials superconduct, why, at what temperatures and magnetic fields, and how their properties can be optimized for engineering applications.

Superconducting materials span a wide range of chemical compositions and crystal structures, from elemental metals to complex cuprate oxides, and their properties vary widely. The selection of a superconducting material for a given application involves tradeoffs among critical temperature, critical magnetic field, critical current density, mechanical properties, and fabricability.

Type I and Type II Superconductors

The distinction between Type I and Type II superconductors is fundamental to materials classification. Type I superconductors, which include most elemental metals such as aluminum, tin, and lead, expel all magnetic flux below a single critical field Hc and transition abruptly to the normal state above it. Their critical fields are too low for most practical applications. Type II superconductors support a mixed state between two critical fields: below Hc1, they expel flux completely; between Hc1 and Hc2, quantized vortices of normal-state material penetrate the bulk while the surrounding material remains superconducting; above Hc2, superconductivity is destroyed. The upper critical field Hc2 of many Type II materials is far higher than the fields achievable by Type I superconductors, which is why Type II materials underpin practical magnet and power applications. NIST's materials data on superconductors catalogs critical parameters for a broad range of Type I and II materials.

High-Temperature Superconductors

High-temperature superconductors (HTS) are copper-oxide-based (cuprate) materials that superconduct at temperatures accessible with liquid nitrogen (77 kelvin) or higher. YBCO (YBa2Cu3O7-x, Tc ~92 K) and BSCCO (Bi-Sr-Ca-Cu-O, Tc up to ~110 K) are the most technologically important cuprates. Their high critical temperatures dramatically reduce the cost and complexity of cryogenic systems compared to low-temperature superconductors that require liquid helium. The physical mechanism of HTS, involving strongly correlated electron physics and d-wave pairing symmetry, differs fundamentally from the phonon-mediated pairing described by BCS theory. Nature Materials publications on high-temperature superconductors document both materials advances and ongoing theoretical debates about the pairing mechanism.

Granular and Multifilamentary Superconductors

Practical superconducting wire must carry large currents in high magnetic fields while remaining mechanically stable. Granular superconductors, composed of superconducting grains separated by insulating or weakly superconducting boundaries, are often not suitable for high-current applications because grain boundaries act as weak links that limit current flow. Multifilamentary conductors address this by embedding many fine superconducting filaments in a normal metal matrix. Niobium-titanium (NbTi) wire, the dominant conductor for magnet applications at fields up to about 10 tesla, consists of thousands of NbTi filaments in a copper matrix. Niobium-tin (Nb3Sn) supports higher fields at the cost of greater brittleness. IEEE Transactions on Applied Superconductivity is the primary venue for wire development and conductor characterization results.

Iron-Based and Other Unconventional Superconductors

The discovery of iron-based superconductors in 2008 opened a new materials class with critical temperatures up to about 55 kelvin in bulk and higher in thin films. These materials exhibit s-wave pairing with a sign-changing order parameter, distinguishing them from both conventional BCS superconductors and cuprates. Other unconventional families include heavy-fermion superconductors and organic charge-transfer salts. Each family presents distinct structural and electronic characteristics that test and extend theoretical models of superconductivity.

Applications

• NbTi and Nb3Sn multifilamentary conductors form the windings of superconducting magnets in MRI scanners, particle accelerators, and nuclear fusion devices.
• HTS YBCO and BSCCO tapes are used in power cables, fault current limiters, and motors that operate in liquid nitrogen rather than liquid helium.
• Elemental aluminum thin films (a Type I superconductor) are used in superconducting qubit circuits because of their native oxide tunnel barrier quality.
• Granular superconducting films are studied as bolometric detectors for astrophysical observations requiring extreme sensitivity.
• MgB2 (magnesium diboride, Tc ~39 K) is explored as a lower-cost alternative conductor for applications where its intermediate critical temperature is sufficient.