Why is high-temperature superconductivity considered unconventional?

The cuprates superconduct far above the temperatures BCS phonon pairing was thought to allow, emerge from insulating magnetic parents rather than good metals, and have d-wave rather than s-wave pairing, so their mechanism appears to be electronic rather than the conventional lattice-vibration one.

Has the mechanism of high-temperature superconductivity been solved?

No. Despite enormous effort the pairing mechanism of the cuprates remains unresolved; it is widely regarded as one of the most important open problems in condensed matter physics.

High-Temperature Superconductors

The discovery of superconductivity in copper-oxide ceramics above the boiling point of liquid nitrogen overturned expectations and revealed an unconventional pairing mechanism that BCS theory does not explain.

Definition

High-temperature superconductors are materials, principally the copper-oxide (cuprate) ceramics, that superconduct at temperatures far above the conventional limit; they emerge from doping antiferromagnetic Mott insulators, exhibit d-wave pairing, and are believed to be driven by an electronic, not simple phonon, mechanism that remains unexplained.

Scope

This topic covers the cuprate and related high-temperature superconductors: their layered copper-oxide structure, the antiferromagnetic Mott-insulating parent compounds, the phase diagram with doping including the pseudogap and the superconducting dome, the d-wave pairing symmetry, and the central unsolved problem of the pairing mechanism. It also touches on iron-based superconductors and high-pressure hydrides. It contrasts these unconventional superconductors with the conventional BCS picture of the sibling topics.

Core questions

What structural and electronic features distinguish cuprate superconductors from conventional metals?
How does the superconducting state emerge from doping an antiferromagnetic Mott insulator?
What is the pseudogap, and how does the phase diagram organize the cuprates?
Why does conventional BCS theory fail to explain high-temperature superconductivity?

Key concepts

Cuprate copper-oxide layers
Doped antiferromagnetic Mott insulator parent
Phase diagram, pseudogap, and superconducting dome
d-wave pairing symmetry
Iron-based and hydride superconductors

Clinical relevance

High-temperature superconductors can operate with inexpensive liquid-nitrogen cooling, enabling power cables, fault-current limiters, and high-field magnets; understanding their mechanism is also one of the deepest open problems in physics, central to the theory of strongly correlated electrons.

History

Bednorz and Müller discovered superconductivity near 35 K in a lanthanum cuprate in 1986, winning the Nobel Prize the next year; the 1987 discovery of YBa2Cu3O7 with a transition temperature of 93 K, above liquid-nitrogen temperature, triggered an explosion of research that continues.

Debates

Pairing mechanism of the cuprates: Decades after their discovery, there is no consensus on what binds the electrons in high-temperature superconductors; spin-fluctuation, resonating-valence-bond, and other strongly correlated electronic scenarios compete, and the role of the pseudogap remains contested.

Key figures

Johannes Georg Bednorz
Karl Alexander Müller
Philip Warren Anderson

Seminal works

bednorz1986
wu1987

Frequently asked questions

Why is high-temperature superconductivity considered unconventional?: The cuprates superconduct far above the temperatures BCS phonon pairing was thought to allow, emerge from insulating magnetic parents rather than good metals, and have d-wave rather than s-wave pairing, so their mechanism appears to be electronic rather than the conventional lattice-vibration one.
Has the mechanism of high-temperature superconductivity been solved?: No. Despite enormous effort the pairing mechanism of the cuprates remains unresolved; it is widely regarded as one of the most important open problems in condensed matter physics.