Recently, the Research Group of Zhou Kejin, a scientist at the DIAMOND Light Source RIXS Line Station in the United Kingdom, and the Qiao Liang Research Group of the School of Physics of the University of Electronic Science and Technology of China published a paper on Nature Materials under the title of “Charge density waves in infinite-layer NdNiO2 nickelates”. Charles C. Tam, a doctoral student at Diamond Light Source in the United Kingdom, Jaewon Choi, a postdoctoral fellow, and Ding Xiang, a 2019 doctoral student at the University of Electronic Science and Technology of China, were co-first authors of the paper. The study’s main participants also included Diamond light source scientist Stefano Agrestini/Abhishek Nag, Wu Mei/Gao Peng of peking university electron microscopy center, Huang Bing of Beijing Computational Science Center, and Luo Huiqian of the Institute of Physics of the Chinese Academy of Sciences.
The research of high-temperature superconductivity has great scientific significance and broad application prospects. Since the discovery of copper-oxide-based high-temperature superconductivity in 1986, reproducing superconductivity in materials with cuprate-like structures has long been a goal scientists have sought. For nearly 30 years, nickelates with infinite layers of structure between iron (iron-based superconductivity) and copper (copper-based superconductivity) in the periodic table were thought to be most likely to be superconducting, but have not been able to achieve it. Until 2019, the research group of Professor Hwang of Stanford University in the United StatesIt is the first to find superconductivity (nickel-based superconductivity) in nickel oxide epitaxial films based on infinite layer structure (Nd0.8Sr0.2NiO2), which is of epoch-making significance. This is also an important progress in the field of superconductivity research in recent years, which immediately attracted a large number of follow-up theoretical and experimental studies. However, due to the extremely difficult sample preparation of infinite layer nickel-based superconducting films, only a few research groups at home and abroad can prepare thin film samples with superconducting states, which has become a challenge in the current nickel-based superconductivity research. Still, over the past three years, nickel-based superconductivity has seen many exciting new discoveries, including the electronic structure of the parent material (NdNiO2).[2,3], doping phase diagram[4,5], low-energy magnetic excitation, magnetic order[6,7], and nickel-oxide superconductors with infinite layer structures of the Ruddlesden-Popper phaseWait. However, the various ordered states that are closely related to superconductivity, such as charge density waves (CDW) and spin density waves (SDWs), which are common in copper-based high-temperature superconductivity, have not been experimentally confirmed in nickel-based superconductivity.
Given the importance of charge sequencing in the study of superconducting materials, the team observed charge density waves (CDW) in infinite layers of NdNiO2 films through synchrotron radiation-based resonant X-ray inelastic scattering spectroscopy (RIXS). Spectroscopic studies have shown a strong link between CDW and Nd5d-Ni 3d orbital hybridization. After entering the superconducting state under 20% Sr doping, the CDW disappears. This work demonstrates the presence of CDW in an infinite layer of nickelates, with a different multi-orbital, unique low-energy excitation physical properties than cuprates. To further explore the characteristics of CDW in infinite layer NdNiO2, the team obtained different NdNiO2 films (NNO2-1, NNO2-3, and NNO2-2) by varying the reducing annealing temperatures (200°C, 220°C, and 290°C). Figure 1 shows the RIXS test for NNO2-1 momentum, where the observed quasielastic scattering peaks are derived from the disruption of translational symmetry caused by charge density modulation.
Figure 1. CDW in NdNiO2 film NNO2-1.
The team further quantitatively fitted the XAS uptake spectrum of the Ni L3-side (Figure 2a-h) from which the Nd5d and Ni3d orbital occupancy could be extracted (Figure 2m-n). The results showed that the orbital contents of both Nd5d3z2-r2 and Ni3d3z2-r2 were gradually reduced, indicating that Nd-Ni hybridization decreased from NNO2-1 to NNO2-3. Figure 2i-l summarizes the integral quasielastic peak strength as a q-|| along the (H, 0) direction A function that clearly shows that CDW is present in all NdNiO2 samples. The results of the parent and superconducting samples showed that the Nd5d hybrid orbitals contributed positively to the CDW ordered state in infinite nickelates.
Figure 2. Nd 5d–Ni 3d orbital hybridization and CDW in NdNiO2 and superconducting Nd0.8Sr0.2NiO2.
The participation of Nd 5dxy, Nd 5d3z2-r2, and Ni 3dx2-y2 orbits in the formation of CDW suggests that a minimal multi-orbit model is needed to describe the low-energy physics of infinite nickelates. The study found no signs of CDW in superconducting Nd0.8Sr0.2NiO2, which means that the next step is to study Sr-doped Nd1-xSrxNiO3 with different carrier concentrations to understand the origin of CDW and superconductivity. The above results provide an important reference for further understanding the superconducting origin of nickel-based superconductors, constructing theoretical models and understanding their superconductivity mechanisms.
It is worth noting that the study of nickel-based superconducting CDW has received widespread attention from scholars in the field, in addition to the team, there are two other research teams that have also found the CDW phenomenon in similar nickel-based superconducting samples at the same time period. Interested readers can refer to the research published at Nature Physics by the Team of Dr. Wei-Sheng Lee from Professor Hwang of Stanford University/Stanford National Accelerator Center in the United States, and research from the team of Dr. Preziosi, Professor Ariando of the National University of Singapore/French National Centre for Scientific Research, published at Physiological Review Letters。
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3 Nat. Mater. 19, 381–385 (2020)
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10 Phys. Rev. Lett. 129, 027002 (2022)
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