But pioneering research, led by physicists at the University of Bristol and published this week in the journal Nature Physics, has identified the vital mechanism which explains how electrons in these copper-oxide high-temperature superconductors combine to transmit power with no loss of energy – the ‘superconductive’ state of infinite electrical conduction.
Study co-lead author Nigel Hussey, Professor of Physics at the University of Bristol, said: “Superconductivity is a remarkable, almost magical, phenomenon which has long fascinated famous physicists and resulted in various breakthroughs recognised with Nobel Prizes over the decades.
“The processes which facilitate higher temperature superconductivity, however, have remained a source of mystery and fascination. Through measurements carried out at the European Magnetic Field Laboratory in the Netherlands and subsequent careful analysis, we have found the smoking gun which solves part of the equation and explains what causes this perplexing state.”
In conventional, lower temperature superconductors, the mechanism through which electrons undergo superconductivity involves the vibrations of the ions that make up the crystalline lattice of the material. For many years scientists have been searching for a new mechanism for high-temperature superconductivity, likely involving entities that vibrate at higher frequencies than the ions themselves. Electrons were the prime candidate as they possess both a spin – like a spinning top, producing a tiny magnetic field – and a charge producing a current that can fluctuate in space and time.
Although spin and charge fluctuations have been mapped out in exquisite detail in these copper-oxide materials, in large part thanks to the endeavours of Professor Stephen Hayden and his group at Bristol, evidence linking the strength of interaction between the conduction electrons and these fluctuations together with Tc – the critical temperature below which the material goes superconducting – has been missing. Like with conventional superconductors, however, this information is encoded in the resistance of the material and unlocking this information from the resistance itself was believed to hold the key to identifying the relevant interaction.
So the team of international scientists from Bristol, the Netherlands, China and the US, created a special thin-film cuprate sample, in this case, lanthanum cerium copper oxide, and deposited it layer-by-atomic-layer onto a substrate to create a smooth gradient of Tc – from its maximum value to zero – along its entire length.
Using lithography, the researchers then patterned a series of electrical contacts along the film to enable the resistance to be measured as a function of finely-spaced values of Tc while keeping the level of disorder across the film the same. A series of experiments were performed at the high magnetic field facility to extract the change in resistance as a function of temperature, magnetic field strength and Tc.
Study lead experimentalist Dr Caitlin Duffy, a postdoctoral researcher at the French National Centre for Scientific Research and former PhD student of Prof Hussey, explained: “Employing some analytical tricks on the data obtained in high magnetic fields, we were then able to identify the dominant source of resistance in this material as scattering off the fluctuating spins. The correlation between the strength of this interaction and Tc then confirmed that the origin for superconductivity in this material is indeed the interaction between the conduction electrons and spin fluctuations.”
While this in itself constitutes a major breakthrough, the researchers have only solved half of the superconductivity riddle, as there are two ways of inducing superconductivity in the parent copper-oxide. One is to introduce electrons, as they did here by substituting lanthanum with cerium; the other is to remove electrons, or introduce so-called ‘holes’ into the system, through other chemical substitutions.
“As it happens, the really high Tc values are found only on the hole-doped side. Given our study pinpoints how superconductivity emerges when electrons are doped into the system, it’s tempting to presume that in order to increase Tc on the hole-doped side, we simply need to increase the strength of coupling to these fluctuation spins, or have them fluctuate at a higher frequency,” Prof Hussey added.
“But things are not that simple. Although electron- and hole-doped cuprates are made from the same cloth, they are stitched together in different patterns. Indeed, comparing these results with our earlier high-field studies on the hole-doped counterparts, informs us that there must be a second, missing ingredient responsible for the higher Tc values on the hole-doped side. While we have found the first key ingredient, the task now is to go and find the second. Only then can theorists develop the full story for high-temperature superconductivity. In my opinion, whoever succeeds in that quest will surely be a strong candidate for a future Nobel Prize in Physics.”
Paper
‘Evidence for spin-fluctuation-mediated pairing in electron-doped cuprates’ by Caitlin Duffy et al. in Nature Physics