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Cornell University
LASSP -  Laboratory of Atomic and Solid State Physics

Cornell Laboratory for Atomic and Solid State Physics

Séamus Davis's Group detect Cooper pair density waves

The prediction was that “Cooper pairs” of electrons in a superconductor could exist in two possible states. They could form a “superfluid” where all the particles are in the same quantum state and all move as a single entity, carrying current with zero resistance – what we usually call a superconductor. Or the Cooper pairs could periodically vary in density across space, a so-called “Cooper pair density wave.” For decades, this novel state has been elusive, possibly because no instrument capable of observing it existed.

Now a research team led by J.C. Séamus Davis, the James Gilbert White Distinguished Professor in the Physical Sciences, and Andrew P. Mackenzie, director of the Max-Planck Institute CPMS in Dresden, Germany, has developed a new way to use a scanning tunneling microscope (STM) to image Cooper pairs directly.

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Hamidian and Edkins studied a cuprate incorporating bismuth, strontium and calcium (Bi2Sr2CaCu2O8) using an incredibly sensitive STM that scans a surface with sub-nanometer resolution, on a sample that is refrigerated to within a few thousandths of a degree above absolute zero.

At these temperatures Cooper pairs can hop across short distances from one superconductor to another, a phenomenon known as Josephson tunneling. To observe Cooper pairs, the researchers briefly lowered the tip of the probe to touch the surface and pick up a flake of the cuprate material. Cooper pairs could then tunnel between the superconductor surface and the superconducting tip. The instrument became, Davis said, “the world’s first scanning Josephson tunneling microscope.”

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 Graphic depiction of a periodic variation in the density of Cooper pairs (pairs of blue arrows pointing up and down) across the surface of a cuprate superconductor.