Study of Nickelates Could Help Guide the Hunt for High-Temperature Superconductors

Physicists at RIKEN have performed calculations and unraveled the behavior of electrons in a nickel oxide material, which could be helpful in the search for high-temperature superconductors.

Superconductors have the ability to transport electrical current without any resistance. They are used to develop powerful electromagnets or sensitive instruments to measure magnetic fields.

Traditional superconductivity is based on a kind of electron pairing that takes place only at very low temperatures. Therefore, it is essential to cool down superconducting devices using high-cost liquefied gases.

However, nearly three decades earlier, scientists found that certain cuprate materials exhibited the potential to turn into superconductors at comparatively warm temperatures, of up to −140 °C. The fundamental cause of this high-temperature superconductivity is still not clear.

In 2019, scientists discovered that a strontium-doped neodymium nickel oxide (Nd_0.8Sr_0.2NiO₂) had the ability to superconduct below the temperature of −258 °C. The finding became famous not due to the temperature but because the crystal structure of the nickelate material is quite similar to that of cuprates, and may be useful as a test-bed to gain better insights into how superconductivity works in such materials.

The nickelate material includes alternating layers of NiO₂ and Nd. Yusuke Nomura and his team from the RIKEN Center for Emergent Matter Science investigated the impact of the interactions between specific electrons in these two layers on superconductivity.

The calculations performed by the researchers revealed that the electrons in the NiO₂ layer interact strongly with one another. This phenomenon is analogous to cuprates in which the strong correlation in the CuO₂ layer is regarded to have a major role in their high-temperature superconductivity.

But the cuprates and nickelates are different in one way: electrons in the neodymium layer of the nickelates are partially occupied, thereby forming the Fermi pocket—a comparatively small region in the Brillouin zone around which the Fermi surface exists. Such pockets do not exist in cuprates, which may render the nickelate material an imperfect match for cuprates.

Computational models were used by Nomura and his colleagues to analyze whether the pockets could be avoided by adjusting the material’s chemical composition, thereby creating a nickelate that is a better analog for the cuprates.

The researchers discovered that two compounds could be a better fit: sodium calcium nickel oxide (NaCa₂NiO₃) and sodium neodymium nickel oxide (NaNd₂NiO₄).

If the proposed nickelates are synthesized, they will be real nickel analogues of cuprate superconductors. The next stage is to clarify the difference and similarity between the nickelates and cuprates in a more systematic way, and get deeper insight into the superconducting mechanism in both systems.

Yusuke Nomura, Researcher, RIKEN Center for Emergent Matter Science