A new breakthrough in the understanding of nickel oxide superconductors

A new study shows that nickel oxide superconductors, which conduct electricity losslessly at higher temperatures than conventional superconductors, contain a type of quantum matter called charge density waves, or CDWs, that can support superconductivity.

The presence of CDW shows that these newly discovered materials, also known as nickelates, are capable of forming correlated states – “electron soups” that can host a variety of quantum phases, including superconductivity, according to studies. researchers at the Department of Energy’s SLAC National Accelerator Laboratory. and Stanford University reported in Natural physics Today.

“Unlike any other superconductor we know of, CDWs appear even before we dope the material by replacing some atoms with others to change the number of free electrons to move around,” said scientist Wei-Sheng Lee. and SLAC Principal Investigator. with the Stanford Institute for Materials and Energy Science (SIMES), which led the study.

“This makes nickelates a very interesting new system – a new playground for studying unconventional superconductors. »

Nickelates and cuprates

In the 35 years since the discovery of the first unconventional “high-temperature” superconductors, researchers have struggled to find one capable of carrying electricity without loss at near room temperature. It would be a game-changing development, enabling things like perfectly efficient power lines, maglev trains, and a host of other futuristic, energy-efficient technologies.

But while a vigorous global research effort has identified many aspects of their nature and behavior, people still don’t know exactly how these materials become superconductors.

Thus, the discovery of nickelate’s superconducting powers by SIMES researchers three years ago was exciting because it gave researchers a new perspective on the problem.

Since then, SIMES researchers have been exploring the electronic structure of nickelates – essentially the behavior of their electrons – and their magnetic behaviour. These studies revealed important similarities and subtle differences between nickelates and copper oxides, or cuprates—the first high-temperature superconductors ever discovered and still world record holders for high-temperature operation at everyday pressures.

Since nickel and copper are found side by side in the periodic table of elements, the researchers were not surprised to see a kinship and had actually suspected that nickelates might make good superconductors. But it turned out to be extremely difficult to build materials with exactly the right properties.

“It’s still very new,” Lee said. “People are still struggling to synthesize thin films of these materials and understand how different conditions can affect the underlying microscopic mechanisms related to superconductivity. »

Ripples of frozen electrons

CDWs are just one of the strange states of matter that are finding their way to prominence in superconducting materials. You can think of them as a pattern of frozen electron waves superimposed on the atomic structure of the material, with higher electron density in the peaks of the waves and lower electron density in the valleys.

As researchers adjust the material’s temperature and doping level, different states appear and disappear. When conditions are ideal, the electrons in the material lose their individual identities and form an electron soup, and quantum states such as superconductivity and CDWs can emerge.

A previous study by the SIMES group did not find CDW in nickelates containing the rare earth element neodymium. However, in this latest study, the SIMES team created and investigated a different nickelate material in which neodymium was replaced by another rare earth element, lanthanum.

“The appearance of CDWs can be very sensitive to factors such as stress or disorder in their environment, which can be tuned by using different rare earth elements,” explained Matteo Rossi, who led the experiments while he was a postdoctoral researcher at SLAC.

The team conducted experiments at three X-ray light sources – the Diamond Light Source in the UK, the Stanford Synchrotron Radiation Lightsource at SLAC and the Advanced Light Source at DOE’s Lawrence Berkeley National Laboratory. Each of these facilities offered specialized tools for examining and understanding the material at a fundamental level. All experiments had to be performed externally due to pandemic restrictions.

“Essentially self-doped”

The experiments showed that this nickelate could accommodate both CDWs and superconducting states of matter – and that these states were already present before the material was doped. This was surprising since doping is usually an essential element in bringing materials to superconductivity.

Lee said the fact that this nickelate is essentially self-doping makes it significantly different from cuprates.

“This makes nickelates a very interesting new system for studying how these quantum phases compete or intertwine,” he said. “And this means that many of the tools used to study other unconventional superconductors may also be relevant to this one. »

The samples used in this study were synthesized in the laboratory of Stanford and SLAC professor and SIMES director Harold Hwang. Primary funding came from the DOE Office of Science. The Stanford Synchrotron Radiation Light Source and Advanced Light Source are DOE Office of Science user facilities.

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