For the first time ever, researchers have revealed the first direct evidence of a mysterious state of electronic superconducting matter - a "Cooper pair density wave" - that was predicted by theorists back in 1964. The findings stem from a study that was conducted by scientists from the U.S. Department of Energy's Brookhaven National Laboratory, Cornell University and various other collaborators.
"Cooper pairs" are pairs of electrons in a superconductor, and scientists predicted that these pairs could exist in two states. One is a "superfluid" - a standard superconductor in today's world - that consists of particles in the same quantum state, moving as a single entity and carrying zero resistance. The second is a state where the pairs periodically vary in their density across space, referred to as a Cooper pair density wave.
Although this novel state of matter has been predicted, no instrument has detected its existence, until now. The team used a scanning tunneling microscope (STM) to image the unique state of electrical matter directly.
Superconductivity was discovered in metals cooled to almost absolute zero - 273.15 degrees Celsius - although recent materials called cuprates have the ability to superconduct at temperatures as "high" as -125 degrees Celsius.
The team studied a cuprate composed of bismuth, strontium, and calcium and used a very sensitive STM in order to scan the surface with a sub-nanometer resolution. The sample studied was refrigerated at a temperature within a few thousandths of a degree above absolute zero.
At these temperatures, Cooper pairs can jump from one superconductor to another if the distance between them permits, which is referred to as Josephson tunneling. In order to observe the pairs, the team lowered the tip of their probe to touch the surface of the cuprate and pick up a flake of the material.
After the piece of material was removed, the Cooper pairs were able to make their way from the surface of the superconductor to the superconducting tip through Josephson tunneling, creating what the team called the first scanning Josephson tunneling microscope.
Examination of the current flow stemming from the Cooper pairs between the cuprate sample and the superconducting tip revealed that the density of the pairs showed periodic variations across various points on the sample, pointing to a Cooper pair density wave and marking the first time that the mysterious state of superconducting matter has been directly detected.
The findings were published in the April 13 issue of the journal Nature.
