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Quantum phases of matter known as superconductors transmit electrical current with zero resistance. Microscopically, this phenomenon arises from the fact that it is energetically favourable for electrons to bind into two-electron states, dubbed Cooper pairs, that move collectively and cooperatively without energy loss. A Cooper pair is said to be spin-singlet when its two electron spins (intrinsic angular momenta) point in opposite directions and the pair has a total spin of zero, whereas spin-triplet Cooper pairs have a total spin of 1, and the two electron spins can be aligned in the same direction. Most experimentally known superconductors have spin-singlet Cooper pairs; these include metals (such as lead and niobium) that demonstrate conventional superconductivity, and cuprates (layered copper oxide compounds) that exhibit unconventional superconductivity. Writing in Nature, Cao et al.1 report evidence for unconventional superconductivity associated with spin-triplet Cooper pairs.

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