An international scientific team has produced the first direct evidence of a Cooper pair density wave, a state of electronic matter first predicted by theorists in 1964. The discovery, described in a paper published in Nature, may provide key insights into the workings of high temperature superconductors.
Superconductivity was first discovered in metals cooled almost to absolute zero (-273.15 °C or -459.67 °F). Recently developed materials called cuprates—copper oxides laced with other atoms—superconduct at temperatures as “high” as 148 degrees above absolute zero (-125 °C). In superconductors, electrons join in pairs that are magnetically neutral so they do not interact with atoms and can move without resistance.
Researchers had predicted that Cooper pairs of electrons in a superconductor could exist in two possible states. In one, the pairs 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 is generally called a superconductor. Or the Cooper pairs could periodically vary in density across space in a so-called Cooper pair density wave. For decades, this novel state has been elusive, possibly because no instrument capable of observing it existed.
But a research team led by J.C. Séamus Davis, a physicist at Brookhaven Lab and Cornell University, and Andrew P. Mackenzie, director of the Max-Planck Institute CPMS in Dresden, Germany, has developed a new way to directly image Cooper pairs. The team observed the Cooper pairs in the superconductor Bi2Sr2CaCu2O8, a material belonging to the family of high-temperature superconductors bismuth strontium calcium copper oxide, using an incredibly sensitive scanning tunneling microscope on a sample 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 the world’s first scanning Josephson tunneling microscope, according to Davis.
Flow of current made of Cooper pairs between the sample and the tip revealed the density of Cooper pairs and it showed periodic variations across the sample, with a wavelength of four crystal unit cells. The team had thus found a Cooper pair density wave state in a high-temperature superconductor, confirming the 50-year-old prediction.
The researchers noted that the technique could be used to search for Cooper pair density waves in other cuprates as well as more recently discovered iron based superconductors.
This work was supported by a grant to Davis from the EPiQS Program of the Gordon and Betty Moore Foundation and by the US Department of Energy’s Office of Science. The collaboration also included scientists in Scotland, Germany, Japan and Korea.