We provide a set of computational experiments based on \textit{ab initio} calculations to elucidate whether a cuprate-like antiferromagnetic insulating state can be present in the phase diagram of the low-valence layered nickelate family (R$_{n+1}$Ni$_n$O$_{2n+2}$, R= rare-earth, $n=1-\infty$) in proximity to half-filling. It is well established that at $d^9$ filling the infinite-layer ($n=\infty$) nickelate is metallic, in contrast to cuprates wherein an antiferromagnetic insulator is expected. We show that for the Ruddlesden-Popper (RP) reduced phases of the series (finite $n$) an antiferromagnetic insulating ground state can naturally be obtained instead at $d^9$ filling, due to the spacer RO$_2$ fluorite slabs present in their structure that block the $c$-axis dispersion. In the $n=\infty$ nickelate, the same type of solution can be derived if the off-plane R-Ni coupling is suppressed. We show how this can be achieved if a structural element that cuts off the $c$-axis dispersion is introduced (i.e. vacuum in a monolayer of RNiO$_2$, or a blocking layer in multilayers formed by (RNiO$_2$)$_1$/(RNaO$_2$)$_1$).
Neutron and X-ray scattering studies have provided strong evidence for coupled spatial modulations of charge and spin densities in layered nickelates and cuprates. The accumulated results for La(2-x)Sr(x)NiO(4+d) are consistent with the strongly-modulated topological-stripe concept. Clues from Nd-doped La(2-x)Sr(x)CuO(4) suggest similar behavior for the cuprates. The experimental results are summarized, and features that conflict with an interpretation based on a Fermi-surface instability are emphasized. A rationalization for the differences in transport properties between the cuprates and nic…
Motivated by the recent discovery of superconductivity in the Sr-doped layered nickelate NdNiO$_2$, we perform a systematic computational materials design of layered nickelates that are dynamically stable and whose electronic structure better mimics the electronic structure of high-$T_c$ cuprates than NdNiO$_2$. While the Ni $3d$ orbitals are self-doped from the $d^9$ configuration in NdNiO$_2$ and the Nd-layer states form Fermi pockets, we find more than 10 promising compounds for which the self-doping is almost or even completely suppressed. We derive effective single-band models for those m…
The recent discovery of superconductivity in Sr-doped NdNiO$_2$, with a critical temperature of $10-15$ K suggests the possibility of a new family of nickel-based high-temperature superconductors (HTS). NdNiO$_{2}$ is the $n=\infty$ member of a larger series of layered nickelates with chemical formula R$_{n+1}$Ni$_{n}$O$_{2n+2}$ (R $=$ La, Nd, Pr; $n = 2, 3, \dots, \infty$). The $n=3$ member has been experimentally and theoretically shown to be cuprate-like and a promising HTS candidate if electron doping could be achieved. The higher-order $n=4,5,$ and $6$ members of the series fall directly …
The interplay between dimensionality and various phases of matter is a central inquiry in condensed matter physics. New phases are often discovered through spontaneously broken symmetry. Understanding the dimensionality of superconductivity in the high-temperature cuprate analogue $-$ layered nickelates and revealing a new symmetry-breaking state are the keys to deciphering the underlying pairing mechanism. Here, we demonstrate the highly-tunable dimensionality and a broken rotational symmetry state in the superconductivity of square-planar layered nickelates. The superconducting state, probed…
This article summarizes recent work on the many-body (beyond density functional theory) electronic structure of layered rare-earth nickelates, both in the context of the materials themselves and in comparison to the high-temperature superconducting (high-$T_c$) layered copper-oxide compounds. It aims to outline the current state of our understanding of layered nickelates and to show how the analysis of these fascinating materials can shed light on fundamental questions in modern electronic structure theory. A prime focus is determining how the interacting physics defined over a wide energy ran…
We present a comparative density-functional theory plus dynamical mean-field theory (DFT+DMFT) study of the two known superconducting members of the rare-earth (R) layered nickelate family: hole-doped RNiO$_{2}$ ($n=\infty$) and R$_{6}$Ni$_{5}$O$_{12}$ ($n=5$). At the same nominal carrier concentration, these two materials exhibit nearly identical electronic structures and many-body correlations effects: mass enhancements, self-energies, and occupations. However, the fermiology of the quintuple-layer nickelate is more two-dimensional-like than its infinite-layer counterpart making this new sup…
The layered square-planar nickelates, Nd$_{n+1}$Ni$_{n}$O$_{2n+2}$, are an appealing system to tune the electronic properties of square-planar nickelates via dimensionality; indeed, superconductivity was recently observed in Nd$_{6}$Ni$_{5}$O$_{12}$ thin films. Here, we investigate the role of epitaxial strain in the competing requirements for the synthesis of the $n=3$ Ruddlesden-Popper compound, Nd$_{4}$Ni$_{3}$O$_{10}$, and subsequent reduction to the square-planar phase, Nd$_{4}$Ni$_{3}$O$_{8}$. We synthesize our highest quality Nd$_{4}$Ni$_{3}$O$_{10}$ films under compressive strain on La…
We explore the layered palladium oxides La$_2$PdO$_4$, LaPdO$_2$ and La$_4$Pd$_3$O$_8$ via ab initio calculations. La$_2$PdO$_4$, being low spin $d^8$, is quite different from its high spin nickel analog. Hypothetical LaPdO$_2$, despite its $d^9$ configuration, has a paramagnetic electronic structure very different from cuprates. On the other hand, the hypothetical trilayer compound La$_4$Pd$_3$O$_8$ ($d^{8.67}$) is more promising in that its paramagnetic electronic structure is very similar to that of overdoped cuprates. But even in the $d^9$ limit (achieved by partial substitution of La with…
Motivated by the recent discovery of superconductivity in doped NdNiO$_2$, we study the magnetic exchange interaction $J$ in layered $d^9$ nickelates from first principles. The mother compounds of the high-$T_{\rm c}$ cuprates belong to the charge-transfer regime in the Zaanen-Sawatzky-Allen diagram and have $J$ larger than 100 meV. While this feature makes the cuprates very different from other transition metal oxides, it is of great interest whether layered $d^9$ nickelates can also have such a large $J$. However, one complexity is that NdNiO$_2$ is not a Mott insulator due to carrier doping…
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 …
The 2019 discovery of high temperature superconductivity in layered nickelate films, Nd$_{1-x}$SrNiO$_2$, has galvanized a community that has been studying nickelates for more than 30 years both as cuprate analogs and in their own right. On the surface, infinite layer nickelates, and their multilayer analogs, should be promising candidates based on our understanding of cuprates: square planar coordination and a parent d9 configuration that places a single hole in a dx2-y2 planar orbital makes nickelates seem poised for superconductivity. But creating crystals and films of sufficient quality of…
We report the observation of superconductivity in infinite-layer Ca-doped LaNiO$_2$ (La$_{1-x}$Ca$_x$NiO$_2$) thin films and construct their phase diagram. Unlike the metal-insulator transition in Nd- and Pr-based nickelates, the undoped and underdoped La1-xCaxNiO2 thin films are entirely insulating from 300 K down to 2 K. A superconducting dome is observed at 0.15<x<0.3 with weakly insulating behavior at the overdoped regime. Moreover, the sign of the Hall coefficient $R_H$ changes at low temperature for samples with a higher doping level. However, distinct from the Nd- and Pr-based nickelate…
There is a growing compelling experimental evidence that a quantum complex matter scenario made of multiple electronic components, and competing quantum phases is needed to grab the key physics of high critical temperature (Tc) superconductivity in layered cuprates. While it is known that defects self-organization controls Tc, the mechanism remains an open issue. Here we focus on the theoretical prediction of the multi-band electronic structure and the formation of broken Fermi surfaces generated by the self-organization of oxygen interstitials O-i atomic wires in the spacer layers in HgBa2CuO…
General trends of electronic structures, superconducting pairing, and magnetic correlations in the Ruddlesden-Popper nickelate $m$-layered superconductors La$_{m+1}$Ni$_{m}$O$_{3m+1}$
In-Plane Ni-O-Ni Bond Angles as Structural Fingerprints of Superconductivity in Layered Nickelates: Effects of Pressure, Strain, Layering, and Correlations
The recent observation of superconductivity in thin film infinite-layer nickelates$^{1-3}$ offers a different angle to investigate superconductivity in layered oxides$^{4}$. A wide range of candidate models have been proposed$^{5-10}$, emphasizing single- or multi-orbital electronic structure, Kondo or Hund's coupling, and analogies to cuprates. Clearly, further experimental characterization of the superconducting state is needed to develop a full understanding of the nickelates. Here we use magnetotransport measurements to probe the superconducting anisotropy in Nd$_{0.775}$Sr$_{0.225}$NiO$_{…
The experimental observations of stripes in superconducting cuprates and insulating nickelates clearly show the modulation in charge and spin density. However, these have proven to be rather insensitive to the harmonic structure and (site or bond) ordering. Using 139La NMR in La2NiO4.17, we show that in the 1/3 hole doped nickelate below the freezing temperature the stripes are strongly solitonic and site ordered with Ni3+ ions carrying S=1/2 in the domain walls and Ni2+ ions with S=1 in the domains.
Since the discovery of high-temperature superconductivity in the copper oxide materials, there have been sustained efforts to both understand the origins of this phase and discover new cuprate-like superconducting materials. One prime materials platform has been the rare-earth nickelates and indeed superconductivity was recently discovered in the doped compound Nd$_{0.8}$Sr$_{0.2}$NiO$_2$. Undoped NdNiO$_2$ belongs to a series of layered square-planar nickelates with chemical formula Nd$_{n+1}$Ni$_n$O$_{2n+2}$ and is known as the 'infinite-layer' ($n = \infty$) nickelate. Here, we report the s…
The search for oxide materials with physical properties similar to the cuprate high Tc superconductors, but based on alternative transition metals such as nickel, has grown and evolved over time. The recent discovery of superconductivity in doped infinite-layer nickelates RNiO2 (R = rare-earth element) further strengthens these efforts.With a crystal structure similar to the infinite-layer cuprates - transition metal oxide layers separated by a rare-earth spacer layer - formal valence counting suggests that these materials have monovalent Ni1+ cations with the same 3d electron count as Cu2+ in…
Superconductivity at the metal-insulator phase boundary in a bulk nickelate at ambient pressure
Argonne National Laboratory（阿贡国家实验室）；University of Illinois Chicago（伊利诺伊大学芝加哥分校）；School of Physics, Southeast University（东南大学物理学院）；Norfolk State University（诺福克州立大学）
Spin and orbital excitations in undoped infinite layers: a comparison between superconducting PrNiO2 and insulating CaCuO2
Politecnico di Milano（米兰理工大学）；University of Cologne（科隆大学）；Université de Strasbourg（斯特拉斯堡大学）；European XFEL（欧洲X射线自由电子激光装置）；Ca’ Foscari University of Venice（威尼斯卡福斯卡里大学）；Chalmers University of Technology（查尔姆斯理工大学）；ESRF, The European Synchrotron（欧洲同步辐射装置）；Università di Roma Tor Vergata（罗马托维爾加塔大学）
The discovery of superconductivity in thin films ($\sim$10 nm) of infinite-layer hole-doped NdNiO$_2$ has invigorated the field of high-temperature superconductivity research, reviving the debate over contrasting views that nickelates that are isostructural with cuprates are either (1) sisters of the high-temperature superconductors, or (2) that differences between nickel and copper at equal band filling should be the focus of attention. Each viewpoint has its merits, and each has its limitations, suggesting that such a simple picture must be superseded by a more holistic comparison of the two…
Understanding the electronic behavior of Ni$^{2+}$ in a square planar environment of oxygen is the key to unravel the origin of the recently discovered superconductivity in the hole doped nickelate Nd$_{0.8}$Sr$_{0.2}$NiO$_2$. To identify the major similarities/dissimilarities between nickelate and cuprate superconductivity, the study of the electronic structure of Ni$^{2+}$ and Cu$^{2+}$ in an identical square planar environment is essential. In order to address these questions, we investigate the electronic structure of Sr$_2$CuO$_3$ and Ni doped Sr$_2$CuO$_3$ single crystals containing (Cu/…
Superconducting phase diagram of multi-layer square-planar nickelates
Harvard University（哈佛大学）；John A. Paulson School of Engineering and Applied Sciences, Harvard University（哈佛大学生物工程与应用科学学院）；Cornell University（康奈尔大学）；Arizona State University（亚利桑那州立大学）；Diamond Light Source（钻石光源）；Brookhaven National Laboratory（布鲁克海文国家实验室）；Argonne National Laboratory（阿贡国家实验室）；Max Planck Institute for Chemical Physics of Solids（马克斯·普朗克固体化学物理研究所）
Recently, superconductivity at about 9-15K was discovered in Nd$_{1-x}$Sr$_x$NiO$_2$ infinite-layer thin films, which has stimulated enormous interests in related rare-earth nickelates. Usually, the first step to synthesize this 112 phase is to fabricate the RNiO$_3$ phase, however, it was reported that the 113 phase is very difficult to be synthesized successfully due to the formation of unusual Ni$^{3+}$ oxidation state. And the difficulty of preparation is enhanced as the ionic radius of rare-earth element decreases. In this work, we report the synthesis and investigation on multiple physic…
We solve the Sp(N) Heisenberg and SU(N) Hubbard-Heisenberg models on the anisotropic triangular lattice in the large-N limit. These two models may describe respectively the magnetic and electronic properties of the family of layered organic materials $κ$-(BEDT-TTF)$_2$X. The Heisenberg model is also relevant to the frustrated antiferromagnet, Cs$_2$CuCl$_4$. We find rich phase diagrams for each model. The Sp(N) antiferromagnet is shown to have five different phases as a function of the size of the spin and the degree of anisotropy of the triangular lattice. The effects of fluctuations at finit…
We report the phase diagram of infinite layer Pr$_{1-x}$Sr$_{x}$NiO$_2$ thin films synthesized via topotactic reduction from the perovskite precursor phase using CaH$_2$. Based on the electrical transport properties, we find a doping-dependent superconducting dome extending between $x$ = 0.12 and 0.28, with a maximum superconducting transition temperature $T_{\rm{c}}$ of 14 K at $x$ = 0.18, bounded by weakly insulating behavior on both sides. In contrast to the narrower dome observed in Nd$_{1-x}$Sr$_{x}$NiO$_2$, a local $T_{\rm{c}}$ suppression near $x$ = 0.2 was not observed for the Pr$_{1-x…
A1-xFe2-ySe2 (A = K, Cs, Rb, Tl) are recently discovered iron-based superconductors with critical temperatures (Tc) ranging up to 32 K. Their parent phases have unique properties when compared with other iron-based superconductors; e.g., their crystal structures include ordered Fe vacancies, their normal states are antiferromagnetic (AFM) insulating phases, and they have extremely high Néel transition temperatures. However, control of carrier doping into the parent AFM insulators has been difficult due to their intrinsic phase separation. Here, we fabricated an Fe-vacancy-ordered TlFe1.6Se2 in…
Expanding the trilayer Ruddlesden-Popper nickelate family: Synthesis and characterization of Sm$_4$Ni$_3$O$_{10-δ}$ single crystals
National Laboratory of Solid State Microstructures and Department of Physics, Center for Superconducting Physics and Materials, Collaborative Innovation Center of Advanced Microstructures, Nanjing University（南京大学固体微结构物理国家重点实验室、物理系、超导物理与材料中心、先进微结构协同创新中心）
