The conventional view of superconductivity is that it occurs when pairs of electrons of opposite momentum and spin bind to form a new zero-momentum quantum state called a Cooper pair. In theory, such pairs can exist with finite momentum, thereby generating states with spatially modulating density of Cooper pairs. Such a state has been created in ultracold lithium-6 gas, although it has hitherto not been seen in a superconductor. Here Séamus Davis and colleagues use Josephson tunnelling microscopy to image Cooper pair tunnelling from a superconducting microscope tip to the quantum condensate of Bi2Sr2CaCu2O8+x, thus demonstrating the spatially modulated density of Cooper pairs as predicted.
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