Getting to grips with superconductivity

The explanation of this schematic diagram may do your head in if you’re not a physicist, but the work is well worth noting.

For years researchers have been trying to decipher the electronic details of high-temperature superconductors, which have the potential to revolutionise energy transmission and electronics because of their ability to carry electric current with no energy loss when cooled below a certain temperature. 

Details of their microscopic electronic structure could reveal how different phases compete or interact with superconductivity – a state in which like-charged electrons somehow overcome their repulsion to pair up and flow freely. 

The ultimate goal is to understand how to make these materials act as superconductors without the need for supercooling.

Now a team from the US Department of Energy’s Brookhaven National Laboratory says it has taken an important step in that direction by providing definitive evidence for the existence of a state of matter known as a pair density wave, which was first predicted by theorists some 50 years ago. 

Their results, published in the journal Nature, show that this phase coexists with superconductivity in a well-known bismuth-based copper-oxide (cuprate) superconductor.

“This is the first direct spectroscopic evidence that the pair density wave exists at zero magnetic field,” says research leader Kazuhiro Fujita. “We’ve identified that the pair density wave plays an important role in this material. Our results show that these two states of matter – pair density wave and superconductivity – coexist and interact.”

And the diagram? It maps out the binding energy (or superconducting energy gap) of individual electrons in a cuprate superconductor as measured by a sensitive microscope scanning across the surface. 

The size of the blue and yellow blobs surrounding individual atoms (red rods with arrowheads indicating their spin orientations) indicates the size of the energy gap (the larger the blobs the bigger the gap and stronger the electron-pair binding at that location). 

When scanning across horizontal rows, the pattern increases to a maximum, then decreases to a minimum (no blobs), increases to another maximum with the opposite orientation (yellow and blue blobs switched) and then a minimum again, repeating this pattern every eight rows. 

These modulations are the first direct evidence of a “pair density wave,” a state of matter that coexists with superconductivity and may play a role in its emergence.

We’ll leave you with it.