Superconductivity
Below a critical temperature certain materials conduct electricity with exactly zero resistance and expel magnetic fields, a macroscopic quantum state explained by the pairing of electrons.
Definition
Superconductivity is a thermodynamic phase, entered below a critical temperature, in which electrons bind into Cooper pairs that condense into a single coherent quantum state, producing exactly zero electrical resistance and the expulsion of magnetic flux known as the Meissner effect.
Scope
This area covers the phenomenology and microscopic theory of superconductivity: zero resistance and the Meissner effect, the London and Ginzburg-Landau phenomenological theories, the BCS theory of Cooper pairing, type-I and type-II behavior with flux vortices, the Josephson effect, and the still-unexplained high-temperature cuprate and iron-based superconductors. It treats the superconducting state as a macroscopic quantum phenomenon and connects to magnetism, phonons, and strong electron correlation.
Sub-topics
Core questions
- Why does a superconductor expel magnetic field (the Meissner effect) rather than merely having zero resistance?
- How does the BCS mechanism let electrons, which repel one another, bind into Cooper pairs?
- What distinguishes type-I from type-II superconductors, and how do flux vortices arise?
- Why do the high-temperature cuprate superconductors remain unexplained by conventional BCS theory?
Key concepts
- Zero resistance and the Meissner effect
- Cooper pairs and the superconducting energy gap
- London and Ginzburg-Landau theories
- Type-I, type-II superconductors and flux vortices
- Josephson effect and macroscopic phase coherence
Key theories
- BCS theory
- Bardeen, Cooper, and Schrieffer showed that a weak phonon-mediated attraction binds electrons near the Fermi surface into Cooper pairs that condense into a coherent state with an energy gap, explaining zero resistance, the Meissner effect, and the isotope effect.
- Ginzburg-Landau theory
- A phenomenological order-parameter theory describes the superconducting transition and spatial variations of the condensate; its ratio of penetration depth to coherence length classifies superconductors as type-I or type-II and predicts the Abrikosov vortex lattice.
Clinical relevance
Superconductors enable lossless power transmission, the high-field magnets used in MRI scanners and particle accelerators, and ultrasensitive SQUID magnetometers and quantum-computing qubits based on the Josephson effect; high-temperature superconductivity remains one of the central open problems in physics.
History
Kamerlingh Onnes discovered superconductivity in mercury in 1911; the Meissner effect (1933) and the London and Ginzburg-Landau phenomenologies preceded the 1957 BCS microscopic theory, and the 1986 discovery of cuprate superconductivity by Bednorz and Müller opened the still-open chapter of high-temperature superconductivity.
Debates
- Mechanism of high-temperature superconductivity
- The pairing mechanism in cuprate and other unconventional superconductors is not settled; whether it is driven by spin fluctuations, other electronic correlations, or some phonon-assisted process remains an active and unresolved question.
Key figures
- John Bardeen
- Heike Kamerlingh Onnes
- Vitaly Ginzburg
Related topics
Seminal works
- bardeen1957
- bednorz1986
- tinkham2004
Frequently asked questions
- Is a superconductor just a perfect conductor?
- No. A perfect conductor would merely trap whatever field was present; a superconductor actively expels magnetic flux (the Meissner effect), which marks it as a distinct thermodynamic phase rather than simply a metal with no resistance.
- How can electrons that repel each other pair up?
- In conventional superconductors one electron distorts the positive ion lattice, and the resulting concentration of positive charge attracts a second electron; this phonon-mediated attraction can overcome the screened Coulomb repulsion and bind a Cooper pair.