Researchers from Fudan University in Shanghai, China, report that they have discovered high-temperature superconductivity in trilayer single crystals of nickel-oxide materials under high pressure. These materials appear to superconduct in a different way than the better-known copper-oxide superconductors, and the researchers say they could become a new platform for studying high-temperature superconductivity.
Superconductors are materials that conduct electricity without resistance when cooled to below a certain critical transition temperature Tc. The first superconductor to be discovered was solid mercury in 1911, but its transition temperature is only a few degrees above absolute zero, meaning that expensive liquid helium coolant is required to keep it in the superconducting phase. Several other “conventional” superconductors, as they are known, were discovered shortly afterwards, all with similarly low values of Tc.
In the late 1980s, however, physicists discovered a new class of “high-temperature” superconductors that have a Tc above the boiling point of liquid nitrogen (77 K). These “unconventional” superconductors are not metals. Instead, they are insulators containing copper oxides (cuprates). Their existence suggests that superconductivity could persist at even higher temperatures, and perhaps even at room temperature – with huge implications for technologies ranging from electricity transmission lines to magnetic resonance imaging.
Nickel oxides could be good high-temperature superconductors
More recently, researchers identified nickel oxide materials – nickelates – as additional high-temperature superconductors. In 2019, a team at Stanford University in the US observed superconductivity in materials containing an effectively infinite number of periodically repeating planes of nickel and oxygen atoms. Then, in 2023, a team led by Meng Wang of China’s Sun Yat-Sen University detected signs of superconductivity in bilayer lanthanum nickel oxide (La3Ni2O7) at 80 K under a pressure of 14 gigapascals.
In the latest work, researchers led by Jun Zhao say that they have found evidence for superconductivity in a nickelate with the chemical formula La4Ni 3O10−δ (where δ can range from 0 to 0.04). Zhao and colleagues obtained this result by placing crystals of the material into a diamond anvil cell, which is a device that can generate extreme pressures of more than 400 GPa (or 4 x 106 atmospheres) as it squeezes the sample between the flattened tip of two tiny, gem-grade diamond crystals.
Evidence of superconductivity
In a paper published in Nature, the researchers report two pieces of evidence for superconductivity in their sample. The first is zero electrical resistance – that is, a complete disappearance of electrical resistance at a Tc of around 30 K and a pressure of 69 GPa. The second is the Meissner effect, which is the expulsion of a magnetic field.
“Through direct current susceptibility measurements, we detected a significant diamagnetic response, indicating that the material expels magnetic fields,” Zhao tells Physics World. “These measurements also enabled us to determine the superconducting volume fraction (that is, how much of the material is superconducting and whether superconductivity prevails throughout the material or just a small area). We found that it exceeds 80%, which confirms the bulk nature of superconductivity in this compound.”
The behaviour of this nickelate compound differs from that of the cuprate superconductors. For cuprates, Tc depends on the number of copper oxide layers in the material and reaches a maximum for structures comprising three layers. For nickelates, however, Tc appears to decrease as more NiO2 layers are added. This suggests that their superconductivity stems from a different mechanism – perhaps even one that conforms to the standard theory of superconductivity, known as BCS theory after the initials of its discoverers.
According to this theory, mercury and most metallic elements superconduct below their Tc because their fermionic electrons pair up to create bosons called Cooper pairs. This pairing occurs due to interactions between the electrons and phonons, which are quasiparticles arising from vibrations of the material’s crystal lattice. However, this theory usually falls short for high-temperature superconductors, so it is intriguing that it might explain some aspects of nickelate behaviour, Zhao says.
Palladium oxides could make better superconductors
“That the layer-dependent Tc in nickelates is distinct from that observed in cuprates suggests unique interlayer coupling and charge transfer mechanism specific to the former,” says Zhao. “Such a unique trilayer structure provides a good platform to understand the role of this coupling in electron pairing and could allow us to better understand the mechanisms behind superconductivity in general and lead to the development of new superconducting materials and applications.”
A promising class of superconducting materials?
Weiwei Xie, a chemist at Michigan State University, US, who was not involved in this work, says that La4Ni 3O10−δ might indeed be a conventional superconductor and that the new study could help to establish nickel oxides as a promising class of superconducting materials. However, she notes that several recent papers claiming to have observed high temperature superconductivity in a different group of materials – hydrides – were later retracted because their findings could not be reproduced by independent research groups. “These papers are never far from our minds,” she tells Physics World.
In a News and Views article published in Nature, however, Xie strikes a hopeful note. “The (new) report has set the stage for a potentially fruitful path of research that could lead to an end to the controversy surrounding unreliable measurements,” she writes.
For their part, the Fudan University researchers say they now aim to identify other differences between the superconducting mechanisms in the nickelates and cuprates. “We will also be continuing to search for more superconducting nickelates,” Zhao reveals.
