Researchers from the Chinese Academy of Sciences have made a breakthrough in understanding high-temperature superconductivity in a new family of materials – bilayer nickelates. Their findings, published in Nature, provide critical experimental evidence for bulk high-temperature superconductivity in pressurized La2PrNi2O7, a significant step forward in the quest for room-temperature superconductors.
Bilayer Nickelates: Challenges Along The Way
The road to uncovering the mysteries of high-temperature superconductivity in bilayer nickelates has been a long and difficult one. The n=2 Ruddlesden-Popper (R-P) nickelate oxides, La3Ni2O7, have been previously investigated and were shown to suffer from various sample-quality issues such as chemical inhomogeneity, oxygen vacancies along with the coexistence of different R-P phases. These complications resulted in large sample-dependent behaviors and no direct experimental support for bulk high-temperature superconductivity of this compound.
The Institute of Physics team from the Chinese Academy managed to overcome these problems in collaboration with other groups. They also used polycrystalline samples of the bilayer nickelate La3-xPrxNi2O7-δ, prepared by a wet chemistry sol-gel method to obtain better-controlled quality and reproducibility. This enabled them to systematically address the sample-quality problem by replacing La with smaller Pr ions, which ultimately resulted in successful synthesis of high-purity, ideal bilayer R-P structure La2Ni2O7 samples.
Revealed: Bulk High-Temperature Superconductivity
Evidence of real bulk high-temperature superconductivity in the pressurized La2PrNi2O7 samples”We were able to gather key evidence for bulk superconductivity in the resistivity and ac magnetic susceptibility measurements performed by our team. They reported zero resistance under hydrostatical pressures with Tconset ∼ 82.5K and(Tczero) ∼ 60K, as well as clear diamagnetic response (χ = -1) with a superconducting shielding volume fraction of 97%, which is highly unusual in any high TC superconductors at these temperatures.
These results confirmed the R-P-bilayered phase as the origin of high-Tc superconductivity and implied that the intergrowths of minor 327/4310 or 327/214 phases are harmful for bulk superconductivity in La3Ni2O7-δ. The team used a large suite of experimental methods to show how the structural disorders suppress high-temperature superconductivity in this material.
Conclusions and Future Directions
The high-purity bulk metallic sample of La2PrNi2O7 with bulk high-temperature superconductivity under pressure we obtained not only resolves the existing controversies but also provides valuable hints for optimization and synthesis of nickel-based high-temperature superconductors. The discovery opens the field for investigation of bulk high-temperature superconductivity in bilayer nickelates and may stimulate research into new families of high-temperature superconducting (HTS) materials based on transition metals.
Research such as this is key to furthering the development of room-temperature superconductors, and provides crucial insight into how structure, composition and superconducting properties interplay in these strange materials. Using the knowledge garnered from this study, researchers could then work on improving the synthesis and optimizing of conductivity in bilayer nickelates, which is a step closer to real world applications where high temperature superconductors find potential uses in many different industries.
