We introduce experimental investigations on the high Tc superconductivity at 80 K in the bilayer nickelate La3Ni2O7 under a 14 GPa pressure. The superconductivity emerges coincidently with a structural transition from Amam to I4/mmm. Both zero resistance and diamagnetic response, which are essential for superconductivity, were observed. A multislice electron ptychography technique was employed to visualize the oxygen. The results confirm the superconducting phase is the bilayer phase of La3Ni2O7 with low oxygen vacancies.
In this video review article, I first introduce the background of the studies on superconducting materials. Superconductivity was discovered in 1911. Extensive materials that show superconductivity have been discovered. Most appear superconductivity with a transition temperature, Tc, lower than 40 K. The mechanism can be regarded as the electron-phonon coupling, the so-called BCS theory. However, the mechanism of unconventional superconducting materials is still a mystery. Looking for high-Tc superconductors is one of the most front topics in physics. Here we will introduce a new nickelate compound, La3Ni2O7, which shows superconductivity at 80 K above 14 GPa. The background of investigation of the high-Tc superconductivity in nickelates will be introduced.
In this part, I introduce the discovery of superconductivity at 80 K and 14 GPa in La3Ni2O7. The single crystals of La3Ni2O7 were grown by the optical floating zone furnace. At ambient pressure, the single crystals show two anomalies at 110 K and 153 K in resistance, which may be related to spin and charge density waves. Under pressures above 14 GPa, the resistance and inductive magnetic susceptibility show evidence for a superconducting transition at around 80 K. X-ray diffraction measurements reveal a structural transition. Density functional theory suggests two orbitals of Ni ions across the Fermi level under pressure in the superconducting state. Investigations on the microscopic structures reveal inhomogeneity of the oxygen vacancies, and the oxygen vacancies are mainly located on the inner apical oxygen site.
In this part, I summarize the important properties of the superconductivity discovered in La3Ni2O7 and those related to the materials.
Work at SYSU was supported by the National Natural Science Foundation of China (Grants No. 12425404 and 12174454), the National Key Research and Development Program of China (Grant No. 2023YFA1406500), the Guangdong Basic and Applied Basic Research Funds (Grant No. 2024B1515020040), Guangzhou Basic and Applied Basic Research Funds (Grant No. 2024A04J6417), and Guangdong Provincial Key Laboratory of Magnetoelectric Physics and Devices (Grant No. 2022B1212010008).
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