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In 1957 the Bardeen-Cooper-Schrieffer theory emerged as the first quantum mechanical model of the mechanism behind what would become known as conventional superconductors. Below a critical temperature, the highest-energy electrons in those materials form pairs with antiparallel spins. (See the article by Howard R. Hart Jr and Roland W. Schmitt, Physics Today, February 1964, page 31.) The pairs have no net momentum and thus create a homogeneous superconducting state. An applied magnetic field, if strong enough, twists the spins apart and destroys the material’s superconductivity.

But other models for superconductivity exist. In 1964, for example, Peter Fulde and Richard Ferrell and, independently, Anatoly Larkin and Yuri Ovchinnikov predicted that a strong enough magnetic field could create a different type of superconducting state. Known as Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconductivity, the state would feature spin-up electrons with different momenta from those spin down because of Zeeman splitting of the energy states. The pairs would thus have a net momentum, and the spin and electron-pair densities of the FFLO state would vary periodically in space.

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