In 1957 the Bardeen-Cooper-Schrieffer (BCS) theory emerged as the first quantum mechanical model of what would become known as conventional superconductors. Below a critical temperature, the highest-energy electrons in those materials form pairs with antiparallel spins. Pairing up allows the electrons to act like bosons rather than fermions and condense into a collective state that moves without resistance. (See the article by Howard Hart Jr and Roland Schmitt, Physics Today, February 1964, page 31.)
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 large magnetic field could induce a different type of superconducting state.1 Known as Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconductivity, the state’s parameters would vary periodically in space, unlike the homogeneous BCS state.
Direct evidence of FFLO superconductivity has long been elusive, however, in large part because the predicted state is unstable. A few materials, such as quasi-two-dimensional organics and the heavy-fermion system cerium cobalt indium-5, have shown some signatures of a potential FFLO state. Now Kenji Ishida of Kyoto University in Japan, his graduate student Katsuki Kinjo, and their colleagues have found the most direct evidence to date of the state.2 Their observation of modulations in strontium ruthenate’s spin density, illustrated in figure 1, points to inhomogeneous superconductivity.
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