The observation of high-$T_c$ superconductivity (HTSC) in concomitant with pressure-induced orthorhombic-tetragonal structural transition in the bilayer La$_{3}$Ni$_2$O$_7$ has sparked hopes of achieving HTSC by stabilizing the tetragonal phase at ambient pressure. To mimic the effect of external physical pressures, the application of chemical pressure via replacing La$^3$$^+$ with smaller rare-earth R$^3$$^+$ has been considered as a potential route. Here we clarify the distinct effects of chemical and physical pressures on the structural transition of bilayer nickelates through a combined experimental and theoretical investigation. Contrary to general expectations, we find that substitutions of smaller R$^3$$^+$ for La$^3$$^+$ in La$_{3-x}$R$_x$Ni$_2$O$_{7-δ}$, despite of an overall lattice contraction, produce stronger orthorhombic structural distortions and thus require higher pressures to induce the structural transition. We established a quantitative relationship between the critical pressure $P_c$ for structural transition and the average size of $A$-site cations. A linear extrapolation of $P_c$ versus <$r_A$> yields a putative critical value of <$r_A$>$_c$ ~ 1.23 angstrom for $P_c$ ~ 1 bar. The negative correlation between $P_c$ and <$r_A$> indicates that it is unlikely to reduce $P_c$ to ambient by replacing La$^3$$^+$ with smaller R$^3$$^+$ ions. Instead, partial substitution of La$^3$$^+$ with larger cations such as alkaline-earth Sr$^2$$^+$ or Ba$^2$$^+$ might be a feasible approach. Our results provide valuable guidelines in the quest of ambient-pressure HTSC in bilayer nickelates.
Hierarchical structure of primary and hybridization-induced superconducting correlations in bilayer nickelates
College of Industrial Technology, Nihon University（日本大学工业技术学部）；Research Organization of Science and Technology, Ritsumeikan University（立命馆大学科学技术研究院）；Faculty of Engineering, Tottori University（鸟取大学工学部）；Department of Physics, The University of Osaka（大阪大学物理学系）
Squeezing dynamical singlets in bilayer nickelates
Center for Computational Quantum Physics, Flatiron Institute（Flatiron研究所计算量子物理中心）；Department of Physics, Columbia University（哥伦比亚大学物理系）；Université Paris-Saclay, CNRS, CEA, Institut de physique théorique（巴黎萨克雷大学、法国国家科学研究中心、法国原子能和替代能源委员会、理论物理研究所）
Polar, checkerboard charge order in bilayer nickelate La3Ni2O7
The University of Tokyo（东京大学）；Beijing National Laboratory for Condensed Matter Physics, and Institute of Physics, Chinese Academy of Sciences（中国科学院物理研究所（北京凝聚态物理国家实验室））；University of Chinese Academy of Sciences（中国科学院大学）；Japan Synchrotron Radiation Research Institute (JASRI)（日本同步辐射研究所）
Interlayer Five-Spin Polaron in Superconducting Bilayer Nickelates
Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory（斯坦福材料能源科学研究所，SLAC国家加速器实验室）；Department of Physics, Stanford University（斯坦福大学物理系）；National Synchrotron Light Source II, Brookhaven National Laboratory（美国国家同步辐射光源II，布鲁克海文国家实验室）；Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory（斯坦福同步辐射光源，SLAC国家加速器实验室）；Advanced Light Source, Lawrence Berkeley National Laboratory（先进光源，劳伦斯伯克利国家实验室）；Canadian Light Source（加拿大光源）；Department of Physics, University of Florida（佛罗里达大学物理系）；Department of Materials Science and Engineering, Stanford University（斯坦福大学材料科学与工程系）
We report nuclear magnetic resonance data in the bilayer nickelate La3Ni2O6. This material belongs to a family of low valence nickel oxides with square planar coordination of the Ni ions and is isoelectronic to the high temperature superconducting cuprates. Although the three layer nickelate compound exhibits a spin-state transition accompanied by antiferromagnetic order, the bilayer material shows no phase transition. The NMR data in the bilayer La3Ni2O6 reveal the presence of antiferromagnetic spin fluctuations down to 5K that are surprisingly similar to those present in the magnetically ord…
Single-crystal structural phase diagram of stoichiometric bilayer nickelate La3Ni2O7 under hydrostatic pressure
The University of Tokyo（东京大学）；RIKEN Center for Emergent Matter Science (CEMS)（理化学研究所 新兴物质科学中心）；Tohoku University（东北大学）；Japan Synchrotron Radiation Research Institute (JASRI)（日本同步辐射研究所）；Tokyo College, The University of Tokyo（东京大学东京学院）；Institute of Engineering Innovation, The University of Tokyo（东京大学工程创新研究所）；RIKEN Pioneering Research Institute (PRI)（理化学研究所 先驱研究 institute）
Structural symmetry effects on the competition of density waves and superconductivity in bilayer nickelates
Ruhr-Universität Bochum（波鸿鲁尔大学）；Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences（中国科学院物理研究所（北京凝聚态物理国家研究中心））；School of Physical Sciences, University of Chinese Academy of Sciences（中国科学院大学物理科学学院）
Triplon-mediated pairing and the superconducting gap structure in bilayer nickelates
National Laboratory of Solid State Microstructures and School of Physics, Nanjing University（南京大学固体微结构物理国家重点实验室和物理学院）；Collaborative Innovation Center of Advanced Microstructures, Nanjing University（南京大学先进微结构协同创新中心）；Max Planck Institute for Solid State Research（马克斯·普朗克固体研究所）
Entropy stabilization and effect of A-site ionic size in bilayer nickelates
School of Physics, Zhejiang University（浙江大学物理学院）；New Cornerstone Science Laboratory, Center for Correlated Matter and School of Physics, Zhejiang University（新 cornerstone 科学实验室，关联物质中心及浙江大学物理学院）；Institute of Fundamental and Transdisciplinary Research, and State Key Laboratory of Silicon and Advanced Semiconductor Materials, Zhejiang University（浙江大学基础与交叉研究院，硅材料与先进半导体材料国家重点实验室）；Collaborative Innovation Centre of Advanced Microstructures, Nanjing University（南京大学先进微结构协同创新中心）
Electronic structures across superconductor-insulator transition in Ruddlesden-Popper bilayer nickelate films
University of Science and Technology of China（中国科学技术大学）；Hefei National Laboratory, University of Science and Technology of China（合肥国家实验室，中国科学技术大学）；Hefei National Research Center for Physical Sciences at the Microscale, University of Science and Technology of China（微观尺度物理科学合肥国家研究中心，中国科学技术大学）；Southern University of Science and Technology（南方科技大学）；Quantum Science Center of Guangdong-Hong Kong-Macao Greater Bay Area（粤港澳大湾区量子科学中心）；Nanjing University（南京大学）；Shanghai Jiao Tong University（上海交通大学）；Tsinghua University（清华大学）
Three-Dimensional Electronic Structures in Superconducting Ruddlesden-Popper Bilayer Nickelate Films
Southern University of Science and Technology（南方科技大学）；Quantum Science Center of Guangdong-Hong Kong-Macao Greater Bay Area（粤港澳大湾区量子科学中心）；University of Science and Technology of China（中国科学技术大学）；Shanghai Jiao Tong University（上海交通大学）；Tsinghua University（清华大学）
Theoretical proposal of superconductivity in hole-doped reduced bilayer nickelate La3Ni2O6: a manifestation of orbital-space bilayer model with incipient bands
The University of Osaka（大阪大学）；Tottori University（鸟取大学）
Out-of-plane bond-order phase, superconductivity, and their competition in the $t$-$J_\parallel$-$J_\perp$ model: Possible implications for bilayer nickelates
Origin of Spin Stripes in Bilayer Nickelate La$_3$Ni$_2$O$_7$
The Chinese University of Hong Kong（香港中文大学）；Johns Hopkins University（约翰斯·霍普金斯大学）；Stanford University（斯坦福大学）；SLAC National Accelerator Laboratory（斯坦福直线加速器中心）
Orbital Order and Superconductivity in Bilayer Nickelate Compounds
Max Planck Institute for Solid State Research（马克斯·普朗克固体的研究所）；Department of Condensed Matter Physics, Faculty of Science, Masaryk University（马萨里克大学理学院凝聚态物理系）
Quantum phase transition driven by competing intralayer and interlayer hopping in bilayer nickelates
International Center for Quantum Design of Functional Materials (ICQD), Hefei National Research Center for Physical Sciences at the Microscale, University of Science and Technology of China（中国科学技术大学合肥微尺度物质科学国家研究中心量子功能材料国际合作中心）；Department of Physics, University of Science and Technology of China（中国科学技术大学物理系）；Hefei National Laboratory, University of Science and Technology of China（中国科学技术大学合肥国家实验室）
The correlated electronic structure of the infinite-layer compounds NdNiO$_2$ and SrCuO$_2$ at stoichiometry and with finite hole doping is compared. Key differences are elucidated from an advanced first-principles many-body perspective. Contrary to the charge-transfer insulating cuprate, the self-doped nickelate remains non-insulating even for large interaction strength, though the Ni-$d_{x^2-y^2}$ spectral weight is also gapped in that limit. Hybridization between Ni$(3d)$ and Nd$(5d)$ is crucial for the appearance of the self-doping band. Upon realistic hole doping, Sr$_{1-y}$CuO$_2$ shows …
Spin Fluctuations in the Rare-Earth Doped Bilayer Nickelates
Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences（中国科学院物理研究所凝聚态物理国家实验室）；School of Physical Sciences, University of Chinese Academy of Sciences（中国科学院大学物理科学学院）；Institute of Neutron Science and Technology, Guangdong Provincial Key Laboratory of Magnetoelectric Physics and Devices, School of Physics, Sun Yat-Sen University（中山大学物理学院中子科学与技术研究所广东省磁电物理与器件重点实验室）；ISIS Neutron and Muon Source, Rutherford Appleton Laboratory（卢瑟福·阿普尔顿实验室ISIS中子与μ子源）；Physics Department, University of Johannesburg（约翰内斯堡大学物理系）；Institut Laue-Langevin（朗之万研究所）；School of Chemical and Process Engineering, University of Leeds（利兹大学化学与工艺工程学院）
Collective spin excitations in trilayer nickelate La$_4$Ni$_3$O$_{10}$
The Chinese University of Hong Kong（香港中文大学）；State Key Laboratory of Quantum Information Technologies and Materials, The Chinese University of Hong Kong（香港中文大学量子信息技术与材料国家重点实验室）；Physik-Institut, Universität Zürich（苏黎世大学物理研究所）；European Synchrotron Radiation Facility（欧洲同步辐射装置）；Fudan University（复旦大学）
