Elusive state of superconducting matter discovered after 50 years

Scientists at the US Department of Energy's Brookhaven National Laboratory, Cornell University and other institutions have produced the first direct evidence of a state of electronic matter first predicted by theorists in 1964. The discovery, reported in a paper in Nature, 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 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 – producing the characteristic properties of a superconductor. Or the Cooper pairs could periodically vary in density across space, to produce a so-called ‘Cooper pair density wave’. For decades, this novel state has proved elusive, possibly because no instrument capable of observing it existed.

Now, a research team led by J.C. Séamus Davis, a physicist at Brookhaven Lab and a professor in the physical sciences at Cornell, and Andrew Mackenzie, director of the Max-Planck Institute for Chemical Physics of Solids in Dresden, Germany, has developed a new way to use a scanning tunneling microscope (STM) to image Cooper pairs directly.

The studies were carried out by research associate Mohammed Hamidian (now at Harvard University) and graduate student Stephen Edkins (now at St. Andrews University in the UK). They were working as members of Davis' research group at Cornell and with Kazuhiro Fujita, a physicist in Brookhaven Lab's Condensed Matter Physics and Materials Science Department.

Superconductivity was first discovered in metals cooled almost to absolute zero (-273°C). More recently, scientists discovered that materials called cuprates – copper oxides laced with other atoms – become superconducting at temperatures as ‘high’ as 148K (-125°C). 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 that incorporated bismuth, strontium and calcium (Bi₂Sr₂CaCu₂O₈) with an incredibly sensitive STM that can scan a surface with sub-nanometer resolution. The cuprate sample they studied was 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 surface of the superconductor and the superconducting flake on the tip. The instrument became "the world's first scanning Josephson tunneling microscope", said Davis.

A 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 team had found a Cooper pair density wave state in a high-temperature superconductor, confirming the 50-year-old 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.

This story is adapted from material from Brookhaven National Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.