An international team of researchers, led by Prof. Séamus Davis of Cornell University and Brookhaven National Laboratory, and Dr. Andrew Mackenzie of the Max-Planck Institute in Dresden, Germany, has produced the first direct evidence of a ‘Cooper-pair density wave’ — a state of electronic matter first predicted by theorists in 1964.
The prediction was that so-called ‘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 scientists usually call a superconductor. Or the Cooper pairs could periodically vary in density across space – a Cooper pair density wave.
For more than five decades, this novel state has been elusive, possibly because no instrument capable of observing it existed.
Now Prof. Davis, Dr. Mackenzie and their colleagues 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 (minus 459.67 degrees Fahrenheit, or minus 273.15 degrees Celsius).
Recently developed materials called cuprates superconduct at temperatures as high as 148 degrees above absolute zero (minus 193 degrees Fahrenheit, or minus 125 degrees Celsius).
In superconductors, electrons join in pairs that are magnetically neutral so they do not interact with atoms and can move without resistance.
The scientists 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 team 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 the world’s first scanning Josephson tunneling microscope,” Prof. Davis said.
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 four crystal unit cells.
The discovery, described in a paper published today in the journal Nature (arXiv.org preprint), may provide key insights into the workings of high-temperature superconductors.
M.H. Hamidian et al. Detection of a Cooper-pair density wave in Bi2Sr2CaCu2O8+x. Nature, published online April 13, 2016; doi: 10.1038/nature17411