Since the discovery in 1986 that copper oxide materials, or cuprates, could carry electrical current without loss at surprisingly high temperatures, scientists have looked for other unconventional superconductors that could operate even closer to room temperature. . This would enable a multitude of everyday applications that could transform society by making energy transmission more efficient, for example.
Nickel oxides, or nickelates, seemed to be a promising candidate. They are made from nickel, which is found next to copper in the periodic table, and the two elements have common characteristics. It was not unreasonable to think that superconductivity would be one of them.
But it took years of testing before scientists at the Department of Energy and Stanford University’s SLAC National Accelerator Laboratory finally created the first nickelate that showed clear signs of superconductivity.
Researchers at SLAC, Stanford and Diamond Light Source made the first measurements of the magnetic excitations that propagated through the new material like ripples in a pond. The results reveal both important similarities and subtle differences between nickelates and cuprates. Scientists published their results in Science today.
“It’s exciting because it gives us a new angle to explore how unconventional superconductors work, which remains an open question after more than 30 years of research,” said
Haiyu Lu, a Stanford graduate student who did most of the research with Stanford postdoctoral researcher Matteo Rossi and SLAC scientist Wei-Sheng Lee.
âAmong other things,â he said, âwe want to understand the nature of the relationship between cuprates and nickelates: are they just neighbors, waving and doing their own thing, or rather like cousins? Who share family traits and ways of doing things? things?”
The results of that study, he said, add to a growing body of evidence that their relationship is close.
Turns in a checkerboard
Cuprates and nickelates have similar structures, with their atoms arranged in a rigid lattice. Both come as thin, two-dimensional sheets layered on top of other elements, such as rare earth ions. These thin sheets become superconducting when cooled below a certain temperature and the density of their free electrons is adjusted in a process known as doping.
The first superconducting nickelate was discovered in 2019 at SLAC and Stanford. Last year, the same SLAC / Stanford team that performed this latest experiment published the first detailed study of the electronic behavior of nickelate. This study established that in undoped nickelate, electrons flow freely in the nickel oxide layers, but the electrons in the middle layers also contribute to the flow electrons. This creates a quite different 3D metallic state than what we see in cuprates, which are insulators when not doped.
Magnetism is also important in superconductivity. It is created by the spins of electrons in a material. When they are all facing the same direction, whether up or down, the material is magnetic in the sense that it could stick to your refrigerator door.
Cuprates, on the other hand, are antiferromagnetic: their electronic spins form a checkerboard pattern, so that each descending spin is surrounded by ascending spins and vice versa. The alternating spins cancel each other out, so the material as a whole is not magnetic in the ordinary sense.
Would nickelate have the same characteristics? To find out, the researchers took samples from the Diamond Light Source synchrotron in the UK for examination with resonant inelastic x-ray scattering, or RIXS. In this technique, scientists scatter X-rays on a sample of material. This injection of energy creates magnetic excitations – ripples that run through the material and randomly reverse the spins of some of its electrons. RIXS allows scientists to measure very weak excitations that could not be observed otherwise.
Creation of new recipes
âWhat we find is quite interesting,â said Lee. âThe data shows that nickelate has the same type of antiferromagnetic interaction as cuprates. It also has similar magnetic energy, which reflects the strength of the interactions between neighboring spins that hold this magnetic order in place. type of physics is important in both.
But there are also differences, noted Rossi. The magnetic excitations propagate less in the nickelates and are extinguished more quickly. Doping also affects the two materials differently; the positively charged “holes” it creates are concentrated around nickel atoms in nickelates and around oxygen atoms in cuprates, affecting the behavior of their electrons.
As this work continues, Rossi said, the team will test how the doping of nickelate in various ways and the exchange of different rare earth elements in the layers between the nickel oxide sheets affect the superconductivity of the material – paving the way, they hope, for the discovery of better superconductors.