We analyze the electronic structure of different surface terminations for infinite-layer nickelates. Surface NiO$_2$ layers are found to be buckled, in contrast to planar bulk layers. While the rare-earth terminated surface fermiology is similar to the bulk limit of the nickelates, the NiO$_2$ terminated surface band structure is significantly altered, originating from the effect of absence of rare-earth atoms on the crystal field splitting. Contrary to the bulk Fermi surfaces, there are two Ni-$3d$ Fermi pockets, giving rise to enhanced spectral weight around the $\bar{\text{M}}$ point in the surface Brillouin zone. From a strong-coupling analysis, we obtain dominant extended $s$-wave superconductivity for the surface layer, as opposed to $d$-wave for the bulk. This finding distinguishes the nickelates from isostructural cuprates, where the analogous surface pairing mechanism is less pronounced. Our results are consistent with region-dependent gap structures revealed in recent STM measurements and provide an ansatz to interpret experimental data of surface-sensitive measurements on the infinite-layer nickelates.
Superconducting nickelates appear to be difficult to synthesize. Since the chemical reduction of ABO3 (A: rare earth; B transition metal) with CaH2 may result in both, ABO2 and ABO2H, we calculate the topotactic H binding energy by density functional theory (DFT). We find intercalating H is energetically favorable for LaNiO2 but not for Sr-doped NdNiO2. This has dramatic consequences for the electronic structure as determined by DFT+dynamical mean field theory: that of 3d9 LaNiO2 is similar to (doped) cuprates, 3d8 LaNiO2H is a two-orbital Mott insulator. Topotactical H might hence explain why…
We perform core-level X-ray photoemission spectroscopy (XPS) and electronic Raman scattering studies of electronic structures and spin fluctuations in the bulk samples of the nickelate oxide NdNiO$_2$. According to Nd $3d$ and O $1s$ XPS spectra, we conclude that NdNiO$_2$ has a large transfer energy. From the analysis of the main line of the Ni $2p_{3/2}$ XPS, we confirm the NiO$_2$ planes in NdNiO$_2$ are of Mott-Hubbard type in the Zaanen-Sawatzky-Allen scheme. The two-magnon peak in the Raman scattering provides direct evidence for the strong spin-fluctuation in NdNiO$_2$. The peak positio…
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 …
Despite the structural resemblance of certain cuprate and nickelate parent compounds there is a striking spread of $T_c$ among such transition metal oxide superconductors. We adopt a minimal two-orbital $e_g$ model which covers cuprates and nickelate heterostructures in different parametric limits, and analyse its superconducting instabilities. The joint consideration of interactions, doping, Fermiology, and in particular the $e_g$ orbital splitting allows us to explain the strongly differing pairing propensities in cuprate and nickelate superconductors.
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…
The recent discovery of the superconductivity in the doped infinite layer nickelates $R$NiO$_2$ ($R$=La, Pr, Nd) is of great interest since the nickelates are isostructural to doped (Ca,Sr)CuO$_2$ having superconducting transition temperature ($T_{\rm c}$) of about 110 K. Verifying the commonalities and differences between these oxides will certainly give a new insight into the mechanism of high $T_{\rm c}$ superconductivity in correlated electron systems. In this paper, we review experimental and theoretical works on this new superconductor and discuss the future perspectives for the "nickel …
Recently Li et al (2019 Nature 572 624) discovered a new type of oxide superconductor Nd0.8Sr0.2NiO2 with T$_c$ = 14 K. To classify superconductivity in this infinite-layer nickelate experimental upper critical field, B$_{c2}$(T), and the self-field critical current densities, J$_c$(sf,T), reported by Li et al (2019 Nature 572 624), are analysed in assumption of s-, d-, and p-wave pairing symmetries and single- and multiple-band superconductivity. Based on deduced the ground-state superconducting energy gap, $Δ$(0), the London penetration depth, $λ$(0), the relative jump in electronic specific…
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）
We derive the effective single-band Hamiltonian in the flat NiO$_2$ planes for nickelate compounds Nd$_{1-x}$Sr$_x$NiO$_2$. We first implement the first-principles calculation to study electronic structures of nickelates using the Heyd-Scuseria-Ernzerhof hybrid density functional and derive a three-band Hubbard model for Ni-O $pdσ$ bands of Ni$^+$ $3d_{x^2-y^2}$ and O$^{2-}$ $2p_{x/y}$ orbitals in the NiO$_2$ planes. To obtain the effective one-band $t$-$t'$-$J$ model Hamiltonian, we perform the exact diagonalization of the three-band Hubbard model for the Ni$_5$O$_{16}$ cluster and map the lo…
The discovery of superconductivity in Sr-doped NdNiO$_{2}$ is a crucial breakthrough in the long pursuit for nickel oxide materials with electronic and magnetic properties similar to those of the cuprates. NdNiO$_2$ is the infinite-layer member of a family of square-planar nickelates with general chemical formula R$_{n+1}$Ni$_n$O$_{2n+2}$ (R = La, Pr, Nd, $n= 2, 3, ... \infty$). In this letter, we investigate superconductivity in the trilayer member of this series (R$_4$Ni$_3$O$_8$) using a combination of first-principles and $t-J$ model calculations. R$_4$Ni$_3$O$_8$ compounds resemble cuprat…
The recent observation of superconductivity in infinite-layer Nd$_{1-x}$Sr$_x$NiO$_2$ thin films has attracted a lot of attention, since this compound is electronically and structurally analogous to the superconducting cuprates. Due to the challenges in the phase stabilization upon chemical doping with Sr, we synthesized artificial superlattices of LaNiO$_3$ embedded in insulating LaGaO$_3$, and used layer-selective topotactic reactions to reduce the nickelate layers to LaNiO$_{2}$. Hole doping is achieved via interfacial oxygen atoms and tuned via the layer thickness. We used electrical trans…
We theoretically investigate the unconventional superconductivity in the newly discovered infinite-layer nickelates Nd$_{1-x}$Sr$_{x}$NiO$_{2}$ based on a two-band model. By analyzing the transport experiments, we propose that the doped holes dominantly enter the Ni $d_{xy}$ or/and $d_{3z^{2}-r^{2}}$ orbitals as charged carriers, and form a conducting band. Via the onsite Hund coupling, the doped holes are coupled to the Ni localized holes in the $d_{x^{2}-y^{2}}$ orbital band. We demonstrate that this two-band model could be further reduced to a Hund-Heisenberg model. Using the reduced model,…
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…
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…
A strong anisotropic distribution of the holes in Ni 3d and O 2p orbitals is observed in the polarization dependent O1s and Ni2p3/2 x-ray absorption spectroscopy of the linear-chain nickelate Y1-xCaxBaNiO5 (x = 0, 0.05, 0.1, 0.2), which demonstrates the one-dimensional nature of the electronic state in these compounds. Furthermore, the additional holes introduced by Ca-doping occupy both O 2p and Ni 3d orbitals along the NiO5 chains. By comparing the experimental Ni 2p3/2 absorption spectra of Y1-xCaxBaNiO5 to those from charge transfer multiplet calculations we can derive the orbital characte…
The recent observation of superconductivity in Nd$_{0.8}$Sr$_{0.2}$NiO$_2$ has raised fundamental questions about the hierarchy of the underlying electronic structure. Calculations suggest that this system falls in the Mott-Hubbard regime, rather than the charge-transfer configuration of other nickel oxides and the superconducting cuprates. Here, we use state-of-the-art, locally-resolved electron energy loss spectroscopy to directly probe the Mott-Hubbard character of Nd$_{1-x}$Sr$_x$NiO$_2$. Upon doping, we observe emergent hybridization reminiscent of the Zhang-Rice singlet via the oxygen-pr…
A variety of nickel oxide compounds have long been studied for their manifestation of various correlated electron phenomena. Recently, superconductivity was observed in nanoscale infinite layer nickelate thin films of Nd$_{0.8}$Sr$_{0.2}$NiO$_2$, epitaxially stabilized on SrTiO$_3$ substrates via topotactic reduction from the perovskite precursor phase. Here we present the synthesis and properties of PrNiO$_2$ thin films on SrTiO$_3$. Upon doping in Pr$_{0.8}$Sr$_{0.2}$NiO$_2$, we observe superconductivity with a transition temperature of 7-12 K, and robust critical current density at 2 K of 3…
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$_{…
Nickel-based complex oxides have served as a playground for decades in the quest for a copper-oxide analog of the high-temperature superconductivity. They may provide clues towards understanding the mechanism and an alternative route for high-temperature superconductors. The recent discovery of superconductivity in the infinite-layer nickelate thin films has fulfilled this pursuit. However, material synthesis remains challenging, direct demonstration of perfect diamagnetism is still missing, and understanding of the role of the interface and bulk to the superconducting properties is still lack…
The observation of superconductivity in infinite layer nickelate (Nd,Sr)NiO$_{2}$ thin films has led to rapid theoretical and experimental investigations of these copper-oxide-analogue systems [1-15]. Superconductivity has also been found in (Pr,Sr)NiO$_{2}$ [16,17], but not previously in (La,Sr)NiO$_{2}$ [2], raising a fundamental question whether superconductivity is associated with the presence of rare-earth moments [18,19]. Here we show that with significant materials optimization, substantial portions of the La$_{1-x}$Sr$_{x}$NiO$_{2}$ phase diagram can enter the regime of coherent low-te…
Field re-entrant superconductivity in Eu-doped infinite-layer nickelates
City University of Hong Kong（香港城市大学）；Southern University of Science and Technology（南方科技大学）；Shenzhen Research Institute of City University of Hong Kong（香港城市大学深圳研究院）；Chinese Academy of Sciences（中国科学院）；Huazhong University of Science and Technology（华中科技大学）；University of Science and Technology of China（中国科学技术大学）；Boston College（波士顿学院）；Tsinghua University（清华大学）
Combining density functional theory simulations and active learning of neural networks, we explore formation energies of oxygen vacancy layers, lattice parameters, and their correlations in infinite-layer versus perovskite oxides across the periodic table, and place the superconducting nickelate and cuprate families in a comprehensive statistical context. We show that neural networks predict these observables with high precision, using only 30-50% of the data for training. Element embedding autonomously identifies concepts of chemical similarity between the individual elements in line with hum…
The large antiferromagnetic exchange coupling in the parent high-$T_{\rm c}$ cuprate superconductors is believed to play a crucial role in pairing the superconducting carriers. The recent observation of superconductivity in hole-doped infinite-layer (IL-) NdNiO$_2$ brings to the fore the relevance of magnetic coupling in high-$T_{\rm c}$ superconductors, particularly because no magnetic ordering is observed in the undoped IL-NdNiO$_2$ unlike in parent copper oxides. Here, we investigate the electronic structure and the nature of magnetic exchange in IL-NdNiO$_2$ using state-of-the-art many-bod…
High quality, ultrathin, superconducting films are required for advanced devices such as hot-electron bolometers, superconducting nanowire single photon detectors, and quantum applications. Using Hybrid Physical-Chemical Vapor Deposition (HPCVD), we show that MgB2 films as thin as 4 nm can be fabricated on the carbon terminated 6H-SiC (0001) surface with a superconducting transition temperature above 33K and a rms roughness of 0.7 nm. Remarkably, the film quality is a function of the SiC surface termination, with the C-terminated surface preferred to the Si-terminated surface. To understand th…
