| superconductivity | |
|---|---|
| Name | Superconductivity |
| Field | Condensed matter physics |
| Discovered | 1911 |
| Discoverer | Heike Kamerlingh Onnes |
| First observed | Mercury at 4.2 K |
superconductivity
Superconductivity is a quantum mechanical state of certain materials characterized by exactly zero electrical resistance and the expulsion of magnetic flux below a critical temperature. It underpins key phenomena and technologies in quantum mechanics and condensed matter physics and provides paradigms for macroscopic quantum coherence relevant to quantum computing and research in many-body physics.
Superconductivity was discovered in 1911 by Heike Kamerlingh Onnes at the Leiden University laboratory while measuring the resistance of mercury as temperature approached absolute zero. Early theoretical milestones included the two-fluid model of Felix Bloch and Fritz London's electrodynamic description, followed by the phenomenological Ginzburg–Landau theory (1950) developed by Vladimir Ginzburg and Lev Landau. The microscopic explanation arrived with the 1957 BCS theory by John Bardeen, Leon Cooper, and Robert Schrieffer, which linked superconductivity to electron pairing mediated by lattice vibrations. Later discoveries—high-temperature superconductivity in cuprate superconductors by J. Georg Bednorz and K. Alex Müller (1986), iron-based superconductors (2008), and superconducting hydrides under pressure—have continually reshaped experimental and theoretical research agendas at institutions such as Bell Labs, CERN, MIT, and Max Planck Society laboratories.
BCS theory explains superconductivity via formation of Cooper pairs—bound pairs of electrons with opposite momentum and spin—caused by effective attraction mediated by phonon exchange, yielding an energy gap in the single-particle excitation spectrum. Key theoretical constructs include the BCS wavefunction, the superconducting energy gap Δ, and quasiparticles described by Bogoliubov–de Gennes equations. Beyond BCS, mechanisms invoked for unconventional superconductors include spin fluctuations, charge-density-wave interactions, and orbital-selective pairing; theoretical frameworks include Eliashberg theory for strong-coupling phonon-mediated pairing and resonating valence bond ideas associated with P. W. Anderson. Numerical and field-theoretic approaches such as Quantum Monte Carlo, DMFT, and renormalization group analyses are extensively used to study correlated-electron models like the Hubbard model and t-J model.
Superconductors are commonly classified as conventional (phonon-mediated) and unconventional (non-phononic pairing). Classical conventional examples include elemental metals (e.g., lead, niobium) and alloys used in niobium–tin and niobium–titanium wires for magnets. Unconventional families include cuprate superconductors (e.g., YBCO), iron-based superconductors (iron pnictides such as LaFeAsO), and heavy fermion compounds (e.g., CeCoIn5). Organic superconductors like the Bechgaard salts and molecular charge-transfer salts exhibit low-dimensional physics. Recently reported superconductivity in high-pressure hydrides (e.g., H3S, LaH10) has led to critical temperatures approaching room temperature under extreme pressure, explored at facilities such as Lawrence Livermore National Laboratory and Max Planck Institute for Chemistry.
The defining electromagnetic signature is the Meissner effect—perfect diamagnetism first demonstrated by Walther Meissner and Robert Ochsenfeld—distinct from ideal conductors. Magnetic flux through a superconducting ring is quantized in units of the flux quantum Φ0 = h/2e, observed in experiments using SQUIDs. Superconductors are characterized by critical temperature Tc, critical magnetic field Hc (or Hc1 and Hc2 for type II), and critical current density Jc. The London equations (Fritz and Heinz London) capture electromagnetic screening via the London penetration depth, while Ginzburg–Landau theory introduces an order parameter and coherence length ξ; the dimensionless Ginzburg–Landau parameter κ = λ/ξ distinguishes type I and type II behavior and the formation of Abrikosov vortex lattices.
Characterization employs electrical transport (four-probe resistance), magnetic susceptibility (SQUID magnetometry), heat capacity, and critical current measurements. Spectroscopic probes include STM and tunneling spectroscopy (Josephson junctions) to measure the superconducting gap and coherence peaks, and ARPES to map electronic band structure and gap anisotropy in materials like cuprates. Muon spin rotation/relaxation (μSR) provides internal field distributions in vortex states. Neutron scattering and nuclear magnetic resonance (NMR) probe spin dynamics and pairing symmetry; high-pressure diamond anvil cell experiments investigate hydride superconductors. Facilities and collaborations such as SNS (Spallation Neutron Source), European Synchrotron Radiation Facility, and national laboratories enable advanced measurements.
Superconductors enable high-field magnets for MRI scanners and particle accelerators like those at CERN and Fermilab using niobium-based superconducting radio-frequency cavities and magnets. SQUIDs (superconducting quantum interference devices) serve as ultra-sensitive magnetometers for geophysics and biomagnetism. Superconducting wires and tapes (e.g., REBCO coated conductors) enable power transmission cables, fault current limiters, and high-field laboratory magnets. In quantum computing, superconducting qubits (transmons, flux qubits) implemented by groups at IBM, Google Quantum AI, and university labs exploit Josephson junctions for scalable circuits. Superconducting detectors such as transition-edge sensors and kinetic inductance detectors are used in astrophysics and quantum sensing.
Outstanding questions include the microscopic mechanism of high-Tc superconductivity in cuprates and the pairing symmetry in many unconventional materials. The search for intrinsic topological superconductivity and emergent Majorana modes at boundaries and in proximitized heterostructures (e.g., semiconductor nanowires coupled to superconductors) links superconductivity to topological phases studied within topological quantum computing. Strongly correlated electron systems continue to challenge theory, motivating advances in quantum field theory techniques, tensor network methods, and experiments at facilities such as National High Magnetic Field Laboratory. Progress has implications for energy technology, quantum information, and the fundamental understanding of macroscopic quantum order.
Category:Condensed matter physics Category:Quantum mechanics