Experimental observation of the electronic structure in infinite-layer nickelate superconductors: Similarities and differences with cuprate superconductors

The mechanism of high-temperature superconductivity remains one of the most important unresolved mysteries in condensed matter physics. Since the discovery of high-temperature superconductivity in cuprates in 1986, there has been a long-standing effort to achieve similar high-temperature superconductivity in the neighboring element nickel. In 2019, superconductivity was finally realized in nickelates with an infinite-layer structure, attracting great attention in the superconductivity field and providing a new platform for studying the long-standing problem of high-temperature superconductivity. However, synthesizing nickelate superconductors is complex, and only a few research groups worldwide can prepare high-quality samples. Moreover, these samples often have surfaces covered by amorphous layers, making it challenging to conduct experimental studies with surface-sensitive probes. In particular, angle-resolved photoemission spectroscopy (ARPES), which has played a crucial role in directly measuring the band structure in cuprate and iron-based high-temperature superconductors, has not been applicable to nickelate superconductors. Due to the lack of experimental electronic structure data since their discovery, many proposed theoretical models of infinite-layer nickelate superconductors cannot be scrutinized, and there are still controversies on many fundamental issues, including the Fermi surface structure and the orbital components contributing to the Fermi surface.

In the past five years, the group led by Profs. Rui Peng, Haichao Xu and Donglai Feng has optimized the synthesizing process and developed in-situ preparation technology, obtaining nickelate superconductors with unprecedented surface quality [Fig.1(a)]. Based on this, the research team conducted a comparative study between the parent compounds and the optimally-doped superconducting phases of nickelate superconductors [Figs.1(b)-1(c)], revealing for the first time the electronic structure of nickelate superconductors and their similarities and differences with cuprate superconductors. This achievement has been published in the National Science Review 2024, titled "Cuprate-like Electronic Structures in Infinite-layer Nickelates with Substantial Hole Dopings," with Xiang Ding and Yu Fan as co-first authors, and Rui Peng, Haichao Xu, and Donglai Feng as co-corresponding authors.

For the first time, the research team directly revealed the electronic structure of the parent and superconducting phases of the nickelate superconductor system experimentally and confirmed the orbital components of each Fermi surface. It was found that the experimentally observed Fermi surface structure is different from previous DFT calculations [Fig.2(a)], including the absence of an electron pocket at the Г point, a lower level of self-doping, a more two-dimensional behavior of the d_x2-y2 orbital component, and the absence of Lifshitz transition in the Z-A-R plane. These findings not only determine the electronic structure of nickelate superconductors and their evolution from the parent compound to the superconducting phase, but also provide an experimental benchmark for the development of nickelate superconductivity theory. Moreover, by resolving the evolution of the Fermi surface volume from the parent compounds to the superconducting phase, it was revealed that the superconducting dome in the nickelate phase diagram is located at a substantially higher doping range than the generic phase diagram of cuprates [Fig.2(b)]. This implies an intriguing difference between cuprate and nickelate superconductors, which is crucial for studying the general mechanism of unconventional superconductivity. Finally, the methods developed by the research team will also help in studying the nickelate superconductor family with various other surface-sensitive techniques. This work represents a milestone in the microscopic understanding of superconductivity in nickelates, which may also give retrospect on the current understanding of cuprates.

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