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This image shows two states of a cuprate high-temperature superconductor simultaneously: Each circle represents the two electrons of a Cooper pair, which exist at relatively low energy and can carry current with no resistance.
In this image, the superconducting Cooper-pair state is superimposed on a dashed pattern that indicates the static positions of electrons caught in a quantum "traffic jam" at higher energy - when the material acts as a Mott-insulator incapable of carrying current.
Original public domain image from Flickr
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