A schematic image representing a periodic variation in the density of Cooper pairs (pairs of blue arrows pointing in opposite directions) within a cuprate superconductor. Densely packed rows of Cooper pairs alternate with regions having lower pair density and no pairs at all. Such a “Cooper pair density wave” was predicted 50 years ago but was just discovered using a unique “scanning Josephson tunneling microscope. Credit: Brookhaven National Laboratory
Scientists have produced the first direct evidence of a state of electronic matter first predicted by theorists in 1964. The discovery may provide key insights into the workings of high-temperature superconductors. The prediction was that “Cooper pairs” of electrons in a superconductor could exist in 2 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 0 resistance -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.
Spatial variations and modulations in cuprate energy gaps.
Now a research team led by Prof. J.C. Séamus Davis and Andrew P. Mackenzie, Director of the Max-Planck Institute CPMS have developed a new way to use a scanning tunneling microscope (STM) to image Cooper pairs directly. Superconductivity was first discovered in metals cooled almost to absolute zero. Recently developed materials called cuprates – copper oxides laced with other atoms – superconduct at temperatures as “high” as 148 degrees above absolute zero (-125 Celsius). In superconductors, electrons join in pairs that are magnetically neutral so they do not interact with atoms and can move without resistance.
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 0K. At these temps, 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.”
Visualizing the Cooper-pair density wave in Bi2Sr2CaCu2O8+x.
Flow of current made of Cooper pairs between the sample and the tip reveals the density of Cooper pairs at any point, and it showed periodic variations across the sample, with a wavelength of 4 crystal unit cells. The team had found a Cooper pair density wave state in a high-temperature superconductor, confirming the 50yo prediction. A collateral finding was that Cooper pairs were not seen in the vicinity of a few zinc atoms that had been introduced as impurities, making the overall map of Cooper pairs into “Swiss cheese.”
The researchers noted that their technique could be used to search for Cooper-pair density waves in other cuprates as well as more recently discovered iron-based superconductors. https://www.bnl.gov/newsroom/news.php?a=11830
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