Elusive state of superconducting matter finally discovered

13 Apr 2016

A schematic image representing a periodic variation in the density of Cooper pairs (pairs of blue arrows pointing in opposite directions) within a cuprate superconductor, via Brookhaven National Laboratory

A 50-year search for particular evidence of Cooper pairs – an integral part of the theory of superconductors – may now be over after a novel use of a microscope and a tunnel.

Cooper pairs were predicted more than 50 years ago, with superconducting electronic matter forming some of the most innovative research in the field of physics ever since. And now, thanks to scientists in the US and Scotland, direct evidence of how the matter looks has emerged.

Cooper pairs of electrons were predicted to exist in superconductors. The idea was the electrons, bound together at low temperatures, could exist in two possible states.

Cooper pairs under the microscope

Firstly, they could form a ‘superfluid’, which is basically a superconductor, each particle moving as a single entity with zero resistance. The other state would have varied density, which has never been truly found before – although the prediction was generally accepted.

Now, though, a research team in New York has developed a new way to use a scanning tunnelling microscope (STM) to image Cooper pairs directly, with the research published in Nature.

One of the reasons it has taken so long to find this evidence is the ultra-low temperatures needed to develop superconductors – close to absolute zero (-273ºC). However, recently developed materials called cuprates – copper oxides laced with other atoms – can become superconductors at higher temperatures (-125ºC).

A hop, skip and jump

Researchers Mohammed Hamidian from Harvard and Stephen Edkins from St Andrews studied a type of cuprate using an incredibly sensitive STM that scans a surface with sub-nanometer resolution.

At marginally above absolute zero, Cooper pairs can hop from one superconductor to another, something called Josephson tunnelling.

To observe Cooper pairs, the researchers dipped the tip of the STM into the surface and picked up a flake of the cuprate material – this created a tunnel for the microscope to scan through.

Lead researcher J.C. Séamus Davis, a physicist at Brookhaven Lab, called this “the world’s first scanning Josephson tunnelling microscope”. The scan revealed Cooper pairs’ density, which varied periodically across the sample. Over 50 years of waiting ended by going through a tunnel.

Gordon Hunt was a journalist with Silicon Republic