New research from a team of scientists led by Dr. Qiaoshi Zeng from the HPSTAR, Prof. Jun Zhao from Fudan University, and Dr. Jiangang Guo from the Institute of Physics of CAS, revealed "bulk superconductivity" in trilayer lanthanum nickelate (La₄Ni₃O_10-δ) single crystals. This discovery was confirmed by both pressure-induced zero resistance at low temperatures and the diamagnetism signal of the Meissner effect, facilitated by their recently developed in-situ high-pressure electrical transport and direct current magnetic susceptibility techniques under hydrostatic pressure conditions. Published in Nature, their work provides a robust model system and establishes a reliable technique to advance the study of nickelate-based superconductors and uncover the fundamental mechanisms behind high-temperature unconventional superconductivity.

Superconductivity, as an exotic macroscopic quantum phenomenon, has been a hot topic in condensed matter physics since its discovery early last century. Despite significant challenges, the quest for new high-temperature superconductors continues unabated due to their broad applications, particularly in the era of electrical technology. In 2023, the discovery of superconductivity in nickel oxide La₃Ni₂O₇ at a temperature of 80 kelvin opened a new field of superconductivity research and sparked a global surge in investigations on nickel-based superconductors. For cuprates and iron-based superconductors, superconductivity is generally considered to arise from the suppression of static long-range magnetic order in their parent phase. So, what leads to superconducting in nickelates, which share many similarities in crystal and band structure with cuprates? Will magnetic fluctuation play an equally important role in nickelates? How does the interlayer coupling influence the superconducting behavior of nickel-based superconductors? These important fundamental questions have aroused great interest to scientists and call for deeper exploration to advance our understanding of this intriguing class of materials.

Unlike many other nickelates that are insulators, the trilayer La₄Ni₃O₁₀ maintains its metallic character even at low temperatures under ambient pressure. Furthermore, the trilayer nickelate La₄Ni₃O₁₀ exhibits a static asymmetrical magnetic and charge sequence, reminiscent of the band characteristics found in copper-and iron-based superconductors. This unique band property, coupled with its trilayer structure, positions La₄Ni₃O₁₀ as an ideal candidate for exploring the interplay between magnetism, interlayer coupling, and potential superconductivity of nickel oxides, as well as superconductivity mechanisms.

At first, the team grew high-quality La₄Ni₃O_10-δ single crystal samples with a small amount of oxygen vacancy in an optical floating zone furnace under high oxygen pressure. And then, they carried out detailed crystal structure and electrical and magnetic transportation measurements on trilayer La₄Ni₃O_10-δsingle crystals. Employing in situ high-pressure single crystal and powder crystal X-ray diffraction, they observed that La₄Ni₃O_10-δ had a crystal structure transition at approximately 15 GPa. This transition led to an increase of the Ni-O-Ni bond angle of the adjacent NiO₂ layer by about 14°, signifying a profound enhancement of the interlayer coupling in the sample.

"In-situ high-pressure electrical and magnetic transportation measurements of single crystal samples are very challenging under extremely high-pressure conditions,” said Dr. Zeng. “After years of exploration, we have successfully developed a reliable technology platform at HPSTAR for conducting in-situ electrical transport and DC magnetic susceptibility measurements under hydrostatic high-pressure conditions."

In this work, helium was employed as the hydrostatic pressure transmission medium. Through in-situ high-pressure and low-temperature resistance measurements, the team observed zero resistance in La₄Ni₃O_10-δ at 43 GPa, signifying that the sample became superconducting at 20 K. As the pressure increased to 65 GPa, the superconducting transition temperature rose to 30 K, demonstrating enhanced superconductivity under further compression. Additionally, they observed gradual suppression of superconductivity under magnetic fields through temperature-dependent magnetoresistance measurements, an essential characteristic of superconductors. Moreover, above the superconducting transition temperature, La₄Ni₃O_10-δ exhibited "strange metal" behavior - linear dependence between resistance and temperature, which is similar to cuprates, signifying they share some similar superconducting mechanisms. Furthermore, in-situ ultrasensitive DC magnetic susceptibility measurements revealed a notable diamagnetic response in the sample, confirming the bulk nature of the superconducting transition with a superconducting volume fraction as high as ~86%.

"We observed definitive evidence for bulk superconductivity in La₄Ni₃O_10-δ, including zero resistance and diamagnetic response from the Meissner effect," stated Dr. Zeng. "This contrasts sharply with the contentious superconductivity observed in La₃Ni₂O₇."

Caption: Field dependences of electrical resistance for sample at 63 GPa and sample loading schematic.

Based on the data from multiple in-situ high-pressure and low-temperature electrical transport measurements, the team established a temperature-pressure superconducting phase diagram of La₄Ni₃O_10-δ, revealing the interplay among superconductivity, spin density wave order, and strange metal behavior in this nickelate. La₄Ni₃O_10-δ features some similarities to the phase diagrams of copper - and iron-based superconductors, of which the high-temperature superconductivity emerges upon suppression of static magnetic sequence. However, unlike cuprates, where the highest superconducting transition temperature occurs in the three-layer system, the superconducting transition temperature of the three-layer La₄Ni₃O_10-δ is lower than the 80 K transition temperature found in the two-layer La₄Ni₃O_10-δ. This distinction reveals different interlayer interactions between these two systems.

In addition, it is worth emphasizing that the advanced techniques of in situ high hydrostatic pressure electric transport and direct current magnetic susceptibility employed in this research provide new insights into the study of high-temperature superconductivity in similar materials, such as nickel oxides. The study used helium, nitrogen and KBr solids as pressure mediums. Notably, zero resistance was observed only in samples subjected to helium pressure media, whereas samples under KBr pressure, which was less hydrostatic, exhibited residual resistance, likely due to the shear stress imposed on the sample. These findings underscore the critical role of a highly hydrostatic pressure environment in investigating unconventional ceramic superconductors. Such conditions could help elucidate the intrinsic pressure behavior of materials and facilitate a deeper understanding of the complex mechanisms governing high-temperature superconductivity.

北京高压科学研究中心的曾桥石研究员等利用发展的等静水压环境的高压原位电输运和直流磁化率测量技术平台，和复旦大学物理系的赵俊教授以及中国科学院物理所的郭建刚研究员等团队合作，在三层镍酸镧(La₄Ni₃O_10-δ )单晶样品中首次通过实验确认压力诱导的 “体超导”转变。该研究为进一步深入研究镍基超导，揭示高温非常规超导现象的普遍机理提供了可靠的模型体系和研究技术。相关结果于近日以“Superconductivity in pressurized trilayer La₄Ni₃O_10-δ single crystals”为题发表于Nature。