Pressure-induced superconductivity in bi-layer nickelate La₃Ni₂O_7+δ

The recent report of pressure-induced superconductivity in the La₃Ni₂O₇ has garnered considerable attention, where the superconducting transition temperature (T_c) is reported to be as high as 80 K [1] in perovskite Ruddlesden-Popper crystal structure [2]. To explore oxygen content dependence of superconducting properties of La₃Ni₂O₇, we synthesized La₃Ni₂O_7+δ samples via different routes and performed electrical resistivity measurements under high pressure.

Parent single-phase La₃Ni₂O₇ were synthesized by solid state reaction and then annealed in air or in high-pressure oxygen. The oxygen contents of samples were evaluated by thermogravimetry. Electrical resistance measurements under high pressure were performed using PPMS and Diamond anvil cell with boron-doped diamond electrodes [3]. Cubic boron nitride was used as a pressure medium.

In the sample of annealed in high pressure oxygen, the electrical resistance decreased over the whole temperature range with increasing pressure, and superconducting transition appears at 26.1 GPa with T_c = 78 K. Compared to the previous work [1], appearance of superconductivity is about 10 GPa higher, and the decrease in T_c with increasing pressure is more pronounced. This result suggests that superconducting properties of La₃Ni₂O₇ may depend on oxygen content. In the presentation, we will also report the results of resistivity measurements on samples that were annealed in air.