Superconductivity

Superconductivity is a quantum mechanical phenomenon characterized by the complete absence of electrical resistance and the expulsion of magnetic fields occurring in certain materials when cooled below a characteristic critical temperature. It was first discovered in mercury by Dutch physicist Heike Kamerlingh Onnes at the University of Leiden in 1911. The theoretical understanding of this state of matter was advanced through the work of John Bardeen, Leon Cooper, and John Robert Schrieffer, whose BCS theory remains foundational, while the discovery of high-temperature superconductivity in copper oxides by Johannes Georg Bednorz and Karl Alexander Müller at IBM's Zurich Research Laboratory opened new frontiers.

History

The phenomenon was first observed by Heike Kamerlingh Onnes in 1911, shortly after his successful liquefaction of helium, which allowed him to reach temperatures near absolute zero. For decades, superconductivity was considered a low-temperature curiosity, with subsequent discoveries including niobium and niobium-tin alloys. A pivotal breakthrough came in 1933 with the discovery of the Meissner effect by Walther Meissner and Robert Ochsenfeld at the Physikalisch-Technische Bundesanstalt, confirming it as a distinct thermodynamic phase. The post-war era saw the development of the first successful microscopic theory, BCS theory, in 1957 by John Bardeen, Leon Cooper, and John Robert Schrieffer, for which they received the Nobel Prize in Physics. The field was revolutionized in 1986 by the work of Johannes Georg Bednorz and Karl Alexander Müller on lanthanum barium copper oxide, heralding the era of high-temperature superconductivity.

Properties and classification

The defining properties are zero DC electrical resistance and the perfect diamagnetism of the Meissner effect. Superconductors are classified primarily as Type-I, which exhibit a single critical field, and Type-II, which allow magnetic flux to penetrate in quantized vortices above a lower critical field. They are further categorized by their critical temperature, with "high-temperature" generally referring to materials like yttrium barium copper oxide that superconduct above the boiling point of liquid nitrogen. Other key properties include the existence of a superconducting gap and the macroscopic quantum phenomenon described by a single wave function.

Theory

The dominant microscopic theory is the BCS theory, which posits that electrons form Cooper pairs via interactions with the crystal lattice, or phonons, leading to a collective ground state. This theory successfully explains conventional superconductors. For unconventional superconductors, such as the cuprates and iron pnictides, alternative mechanisms involving antiferromagnetism, spin fluctuations, or other electronic interactions are actively investigated. The Ginzburg–Landau theory, developed by Vitaly Ginzburg and Lev Landau, provides a powerful phenomenological framework describing the phase transition, while the concept of spontaneous symmetry breaking is central to the modern understanding.

Materials

A vast array of materials exhibit this phenomenon. Elemental superconductors include niobium, lead, and mercury. Important intermetallic compounds and alloys are niobium-tin and niobium-titanium, which are workhorses for MRI scanner magnets. The cuprate family, such as yttrium barium copper oxide and bismuth strontium calcium copper oxide, are iconic high-temperature superconductors. Other significant classes include iron pnictides like lanthanum oxygen fluorine arsenide, organic charge-transfer salts, and, more recently, hydrogen-rich materials like lanthanum decahydride under high pressure, as studied by teams at the Max Planck Institute.

Applications

The most widespread large-scale application is in the production of high-field magnets for medical MRI, nuclear magnetic resonance spectroscopy, and particle accelerators like the Large Hadron Collider at CERN. Maglev train systems, such as the SCMaglev in Japan, utilize the levitation from the Meissner effect. In electronics, superconducting circuits are foundational for SQUID magnetometers, used in geophysics and magnetocardiography, and are the basis for qubits in quantum computers developed by companies like IBM and Google. Other applications include low-loss power cables and advanced concepts in fusion power like ITER.

Challenges and future directions

The primary challenge remains the discovery of a material that exhibits room-temperature superconductivity at ambient pressure, a goal pursued by researchers worldwide, including at institutions like the University of Rochester and the University of Chicago. Understanding the pairing mechanism in high-temperature and unconventional superconductors is a major open question in condensed matter physics. Practical challenges include manufacturing brittle ceramic cuprates into long, flexible wires for power grids and reducing the cost of cooling systems. Future directions involve the search for new material families through computational methods like those employed at the Materials Project, and the integration of superconducting electronics with conventional semiconductor technology.