An international team of researchers led by Professor JC Séamus Davis from the Department of Physics at the University of Oxford, has announced results that support one of the leading theories explaining the atomic-scale mechanism behind high-temperature superconductivity. The findings are published in PNAS.
For almost 40 years, one of the ‘Holy Grails’ in physics research has been to understand the phenomenon of high-temperature superconductivity: where certain materials can conduct electricity with zero resistance at much warmer temperatures than conventional, low-temperature superconductors. Developing these into room-temperature superconductors would ultimately enable 100% efficient transmission of electrical energy, and fascinating new application ranging from levitating trains to quantum computers. But until now, the atomic-scale mechanism behind high-temperature superconductivity has been unknown, with several possible theories put forward.
Superconducting materials were originally discovered in 1911 when Dutch physicist Heike Kamerlingh Onnes was investigating the electrical properties of mercury. He found that at around 4 degrees above the absolute zero of temperature (ie at 4 Kelvin, which is equivalent to – 269 Celsius), the electrical resistance of the supercooled mercury disappeared. When he applied an electrical current to the solution, it persisted even after the battery was disconnected.
It took until 1958 for the mechanism behind this ‘low-temperature superconductivity’ to be elucidated by Nobel Prize researchers, John Bardeen, Leon Cooper and Robert Schrieffer (named ‘the BCS theory’ after its discoverers). According to this theory, in low-temperature superconductors the electrical resistance is minimised because the electrons that carry the current are bound together in stable ‘Cooper pairs.’ This enables them to move efficiently through the material, rather than randomly bouncing around (which causes resistance). The binding of the electrons in Cooper pairs is caused by a coupling interaction between the electrons and the thermal vibrations of the metal atoms of the material (called an ‘electron-phonon’ interaction). However, this effect can only occur at very low temperatures, since higher temperatures cause the electrons to have more thermal energy, so that they break free of the bonds that bind the Cooper pairs together. Consequently low-temperature superconductors (such as those used in MRI scanners) have to be kept extremely cool, limiting their applications.
Unlocking the future
In 1987, however, ‘high-temperature’ (ie above 70 K) superconductivity was unexpectedly discovered in copper oxide materials. Although these still require cooling with liquid nitrogen, this is more available and practical than using liquid helium, which is needed to reach the lower temperatures required for conventional superconductors. Furthermore, understanding the mechanism which enabled these materials to have superconducting properties at higher temperatures could ultimately enable the development of superconductors that work at ambient-temperatures. Until now, however, this has remained unknown.
One of the most promising theories proposed that, instead of interacting with the thermal vibrations, the electrons are controlled by magnetic interactions through a quantum mechanical process called superexchange. According to this theory, each copper electron acts as a tiny bar magnet, with a north and south pole, giving it a property called ‘magnetic spin.’ The ‘charge-transfer superexchange’ theory of superconductivity posits that, in neighbouring copper atoms, these magnetic spins bind into electron pairs with opposite pointing spins, via a quantum mechanical communication through the intervening oxygen atom. In theory, the strength of this electron pairing is then controlled by the difference between the energy levels of the orbitals inside the copper and oxygen atoms, called the charge transfer energy. This is because the power of the magnetic superexchange interaction depends delicately on whether an electron can easily hop from copper to copper through the intervening oxygen orbital.
Pioneering new techniques
‘For the charge-transfer superexchange theory to be correct, we would expect the strength of the electron pairing to depend in a very specific way on the charge transfer energy difference between orbitals on every oxygen atom and every adjacent copper atom in the structure,’ explains Professor Davis. ‘But until now, we had no way to directly measure either the electron-pair wavefunctions or the charge transfer energies - and especially not at every atom in the material!’
To investigate this, the team, involving scientists in Ireland, the USA, Japan and Germany, developed two new microscopy techniques. The first of these, called spectroscopic imaging scanning tunnelling microscopy, measured the difference in energy between the copper and oxygen atom orbitals, as a function of their location. The second method, called scanned Josephson tunnelling microscopy, measured the amplitude of the electron-pair (superconductor) wave function at every oxygen atom and at every copper atom.
‘By visualising the superconductive electron pair density as a function of ‘charge-transfer’ differences between orbital energies, for the first time ever we were able to measure precisely the relationship required to validate or invalidate the theory,’ said Professor Davis.
As predicted by the theory, the results showed the quantitatively predicted anticorrelation between the charge-transfer energy difference between adjacent oxygen and copper orbitals and the amplitude of electron-pair wavefunctions.
‘This discovery could prove an historic step towards the development of room-temperature superconductors,’ continues Professor Davis. ‘Ultimately, these materials could have far-reaching applications ranging from maglev trains, nuclear fusion reactors, quantum computers, and high-energy particle accelerators, not to mention super-efficient energy transfer and storage.’
Wangping Ren, a PhD student at Oxford University, co-led with Shane O’Mahony of University College Cork, the visualisation of energy differences between adjacent oxygen and copper orbitals and their effects on the amplitude of electron-pair wavefunctions. He comments: ‘In confirming the charge-transfer superexchange hypothesis so clearly and directly at atomic scale we have redirected the field. Researchers can now concentrate on developing and using this theory to design new superconducting materials with even higher operating temperatures.’
Professor Davis adds: ‘This has been one of the Holy Grails of problems in physics research for nearly 40 years. Many people believe that cheap, readily available room-temperature superconductors would be as revolutionary for the human civilization as the introduction of electricity itself.’
On the electron pairing mechanism of copper-oxide high temperature superconductivity, S M O’Mahony et al, PNAS