A Superconducting Praseodymium Nickelate with Infinite Layer Structure

Electronic correlations and partial gap in the bilayer nickelate La3Ni2O7

The discovery of superconductivity with a critical temperature of about 80 K in La3Ni2O7 single crystals under pressure has received enormous attention. La3Ni2O7 is not superconducting under ambient pressure but exhibits a transition at T ∗ ≃ 115 K. Understanding the electronic correlations and charge dynamics is an important step towards the origin of superconductivity and other instabilities. Here, our optical study shows that La3Ni2O7 features strong electronic correlations which significantly reduce the electron's kinetic energy and place this system in the proximity of the Mott phase. The low-frequency optical conductivity reveals two Drude components arising from multiple bands at the Fermi level. The transition at T ∗ removes the Drude component exhibiting non-Fermi liquid behavior, whereas the one with Fermi-liquid behavior is barely affected. These observations in combination with theoretical results suggest that the Fermi surface dominated by the Ni-d 3 z 2 - r 2 orbital is removed due to the transition at T ∗. Our experimental results provide pivotal information for understanding the transition at T ∗ and superconductivity in La3Ni2O7.

Pivotal Role of Modifiable A-Site Doping in Enhancing Valence Stability and Excited State Dynamics of MnO6

The controlled regulation of A-site in rare earth manganate perovskites can orderly arrange the electronic states, leading to the emergence of unique transport properties. However, it is challenging to balance crystal structure stability and property variations during the multi-ion doping. In this study, a series of multivalent manganate perovskites are synthesized by hydrothermal method through the A-site multielement doping, which enables the manganese atoms with varying valence states to orderly arrange at the B site. Powder X-ray diffraction (PXRD) and X-ray absorption spectra (XAS) confirm that the splitting of the K─O hybrid orbitals in the crystal effectively prevents any distortion of the MnO6 octahedron, thereby facilitating the ordered arrangement of Mn (III) -Mn (IV) -Mn (V) at the B-site and promoting superstructure formation. The transient absorption spectra (TAS) reveals that the sequential arrangement of Mn (III) - Mn (IV) - Mn(V) better forms the charge transfer channels, and thereby makes the photodynamic properties of the sample composition-dependent. These photodynamic properties will facilitate the study of exciton-electron coupling behavior in LCKMO crystals during electrical transport.

Extensive hydrogen incorporation is not necessary for superconductivity in topotactically reduced nickelates

A key open question in the study of layered superconducting nickelate films is the role that hydrogen incorporation into the lattice plays in the appearance of the superconducting state. Due to the challenges of stabilizing highly crystalline square planar nickelate films, films are prepared by the deposition of a more stable parent compound which is then transformed into the target phase via a topotactic reaction with a strongly reducing agent such as CaH2. Recent studies, both experimental and theoretical, have introduced the possibility that the incorporation of hydrogen from the reducing agent into the nickelate lattice may be critical for the superconductivity. In this work, we use secondary ion mass spectrometry to examine superconducting La1-xXxNiO2 / SrTiO3 (X = Ca and Sr) and Nd6Ni5O12 / NdGaO3 films, along with non-superconducting NdNiO2 / SrTiO3 and (Nd,Sr)NiO2 / SrTiO3. We find no evidence for extensive hydrogen incorporation across a broad range of samples, including both superconducting and non-superconducting films. Theoretical calculations indicate that hydrogen incorporation is broadly energetically unfavorable in these systems, supporting our conclusion that extensive hydrogen incorporation is not generally required to achieve a superconducting state in layered square-planar nickelates.

Emergence of Competing Stripe Phases in Undoped Infinite-Layer Nickelates

Recent discovery of superconductivity in infinite-layer nickelates has ignited renewed theoretical and experimental interest in the role of electronic correlations in their properties. Here, using first-principles simulations, we show that the parent compound of the nickelate family, LaNiO_{2}, hosts competing low-energy stripe phases, similar to doped cuprates. The stripe states are shown to be driven by multiorbital electronic mechanisms and Peierls distortions. Our study indicates that both strong correlations and electron-phonon coupling effects play a key role in the physics of infinite-layer nickelates, and sheds light on the microscopic origin of electronic inhomogeneity and the lack of long-range order in the nickelates.

A new perspective on ductile high-Tcsuperconductors under ambient pressure: few-hydrogen metal-bonded hydrides

Superconducting materials have garnered widespread attention due to their zero-resistance characteristic and complete diamagnetism. After more than 100 years of exploration, various high-temperature superconducting materials including cuprates, nickelates, iron-based compounds, and ultra-high pressure multi-hydrides have been discovered. However, the practical application of these materials is severely hindered by their poor ductility and/or the need for high-pressure conditions to maintain structural stability. To address these challenges, we first provide a new thought to build high-temperature superconducting materials based on few-hydrogen metal-bonded hydrides under ambient pressure. We then review the related research efforts in this article. Moreover, based on the bonding type of atoms, we classify the existing important superconducting materials and propose the new concepts of pseudo-metal and quasi-metal superconductivity, which are expected to be helpful for the design of new high-temperature superconducting materials in the future.

Advancing Superconductivity with Interface Engineering

The development of superconducting materials has attracted significant attention not only for their improved performance, such as high transition temperature (TC), but also for the exploration of their underlying physical mechanisms. Recently, considerable efforts have been focused on interfaces of materials, a distinct category capable of inducing superconductivity at non-superconducting material interfaces or augmenting the TC at the interface between a superconducting material and a non-superconducting material. Here, two distinct types of interfaces along with their unique characteristics are reviewed: interfacial superconductivity and interface-enhanced superconductivity, with a focus on the crucial factors and potential mechanisms responsible for enhancing superconducting performance. A series of materials systems is discussed, encompassing both historical developments and recent progress from the perspectives of technical innovations and the exploration of new material classes. The overarching goal is to illuminate pathways toward achieving high TC, expanding the potential of superconducting parameters across interfaces, and propelling superconductivity research toward practical, high-temperature applications.

Superconductivity in Freestanding Infinite-Layer Nickelate Membranes

The observation of superconductivity in infinite-layer nickelates has attracted significant attention due to its potential as a new platform for exploring high-Tc superconductivity. However, thus far, superconductivity has only been observed in epitaxial thin films, which limits the manipulation capabilities and modulation methods compared to two-dimensional exfoliated materials. Given the exceptionally giant strain tunability and stacking capability of freestanding membranes, separating superconducting nickelates from the as-grown substrate is a novel way to engineer the superconductivity and uncover the underlying physics. Herein, this work reports the synthesis of the superconducting freestanding La0.8Sr0.2NiO2 membranes (T c zero = 10.6 K ${T}_{\mathrm{c}}^{\mathrm{zero}}\ =\ 10.6\ \mathrm{K}$ ), emphasizing the crucial roles of the interface engineering in the precursor phase film growth and the quick transfer process in achieving superconductivity. This work offers a new versatile platform for investigating superconductivity in nickelates, such as the pairing symmetry via constructing Josephson tunneling junctions and higher Tc values via high-pressure experiments.

Cuprate-like electronic structures in infinite-layer nickelates with substantial hole dopings

Superconducting infinite-layer (IL) nickelates offer a new platform for investigating the long-standing problem of high-temperature superconductivity. Many models were proposed to understand the superconducting mechanism of nickelates based on the calculated electronic structure, and the multiple Fermi surfaces and multiple orbitals involved create complications and controversial conclusions. Over the past five years, the lack of direct measurements of the electronic structure has hindered the understanding of nickelate superconductors. Here we fill this gap by directly resolving the electronic structures of the parent compound LaNiO2 and superconducting La0.8Ca0.2NiO2 using angle-resolved photoemission spectroscopy. We find that their Fermi surfaces consist of a quasi-2D hole pocket and a 3D electron pocket at the Brillouin zone corner, whose volumes change upon Ca doping. The Fermi surface topology and band dispersion of the hole pocket closely resemble those observed in hole-doped cuprates. However, the cuprate-like band exhibits significantly higher hole doping in superconducting La0.8Ca0.2NiO2 compared to superconducting cuprates, highlighting the disparities in the electronic states of the superconducting phase. Our observations highlight the novel aspects of the IL nickelates, and pave the way toward the microscopic understanding of the IL nickelate family and its superconductivity.

Strong interlayer magnetic exchange coupling in La3Ni2O7-δ revealed by inelastic neutron scattering

After several decades of studies of high-temperature superconductivity, there is no compelling theory for the mechanism yet; however, the spin fluctuations have been widely believed to play a crucial role in forming the superconducting Cooper pairs. The recent discovery of high-temperature superconductivity near 80 K in the bilayer nickelate La3Ni2O7 under pressure provides a new platform to elucidate the origins of high-temperature superconductivity. We perform elastic and inelastic neutron scattering studies on a polycrystalline sample of La3Ni2O7-δ at ambient pressure. No magnetic order can be identified down to 10 K. The absence of long-range magnetic order in neutron diffraction measurements may be ascribed to the smallness of the magnetic moment. However, we observe a weak flat spin-fluctuation signal in the inelastic scattering spectra at ∼ 45 meV. The observed spin excitations could be interpreted as a result of strong interlayer and weak intralayer magnetic couplings for stripe-type antiferromagnetic orders. Our results provide crucial information on the spin dynamics and are thus important for understanding the superconductivity in La3Ni2O7.

Toward Reliable Synthesis of Superconducting Infinite Layer Nickelate Thin Films by Topochemical Reduction

Infinite layer (IL) nickelates provide a new route beyond copper oxides to address outstanding questions in the field of unconventional superconductivity. However, their synthesis poses considerable challenges, largely hindering experimental research on this new class of oxide superconductors. That synthesis is achieved in a two-step process that yields the most thermodynamically stable perovskite phase first, then the IL phase by topotactic reduction, the quality of the starting phase playing a crucial role. Here, a reliable synthesis of superconducting IL  nickelate films is reported after successive topochemical reductions of a parent perovskite phase with nearly optimal stoichiometry. Careful analysis of the transport properties of the incompletely reduced films reveals an improvement in the strange metal behavior of their normal state resistivity over subsequent topochemical reductions, offering insight into the reduction process.

Unconventional superconductivity without doping in infinite-layer nickelates under pressure

High-temperature unconventional superconductivity quite generically emerges from doping a strongly correlated parent compound, often (close to) an antiferromagnetic insulator. The recently developed dynamical vertex approximation is a state-of-the-art technique that has quantitatively predicted the superconducting dome of nickelates. Here, we apply it to study the effect of pressure in the infinite-layer nickelate SrxPr1-xNiO2. We reproduce the increase of the critical temperature (Tc) under pressure found in experiment up to 12 GPa. According to our results, Tc can be further increased with higher pressures. Even without Sr-doping the parent compound, PrNiO2, will become a high-temperature superconductor thanks to a strongly enhanced self-doping of the Nid x 2 - y 2 orbital under pressure. With a maximal Tc of 100 K around 100 GPa, nickelate superconductors can reach that of the best cuprates.

Long-Range Structural Order in a Hidden Phase of Ruddlesden-Popper Bilayer Nickelate La3Ni2O7

The recent discovery of superconductivity in the Ruddlesden-Popper bilayer nickelate, specifically La3Ni2O7, has generated significant interest in the exploration of high-temperature superconductivity within this material family. In this study, we present the crystallographic and electrical resistivity properties of two distinct Ruddlesden-Popper nickelates: the bilayer La3Ni2O7 (referred to as 2222-phase) and a previously uncharacterized phase, La3Ni2O7 (1313-phase). The 2222-phase is characterized by a pseudo F-centered orthorhombic lattice, featuring bilayer perovskite [LaNiO3] layers interspaced by rock salt [LaO] layers, forming a repeated ...2222... sequence. Intriguingly, the 1313-phase, which displays semiconducting properties, crystallizes in the Cmmm space group and exhibits a pronounced predilection for a C-centered orthorhombic lattice. Within this structure, the perovskite [LaNiO3] layers exhibit a distinctive long-range ordered arrangement, alternating between single- and trilayer configurations, resulting in a ...1313... sequence. This report contributes to novel insights into the crystallography and the structure-property relationship of Ruddlesden-Popper nickelates, paving the way for further investigations into their unique physical properties.

Impact of Polarity Mismatch in Infinite-Layer Nickelates

The recent discovery of superconductivity in infinite-layer Sr-doped NdNiO2 grown on SrTiO3(001) provides a new platform to explore the conducting mechanism of unconventional superconductors. However, the electronic structure of infinite-layer nickelates remains controversial. In this paper, we systematically compare the structural and electronic properties of NdNiO2 films grown on SrTiO3 and LaAlO3 substrates using first-principles calculations. Our results show that the lattice reconstruction accompanied by electronic reconstruction occurs in nickelate films on both substrates. Although both heterostructures (HSs) are conducting at the interface, the SrTiO3-based HS shows distinct atomic displacement in the interfacial TiO2 layer and significant electron accumulation deep into three SrTiO3 layers below the interface, while the LaAlO3-based HS shows negligible atomic displacement and electron localization in the interfacial AlO2 layer, reflecting the impact of polarity mismatch on the electronic structure. Further, Wannier function calculations reveal that the interface stress has no obvious effect on the splitting energy and hopping integral between Ni 3d and Nd-layer orbitals. Although the hybridization between Ni 3dx2-y2 and Nd 5d orbitals is tiny, the hybridization between the Ni 3dx2-y2 orbital and an itinerant interstitial s (IIS) orbital is significantly strong in both cases, suggesting that the IIS orbital may play a critical role in the superconductivity of nickelates.

Direct observation of strong surface reconstruction in partially reduced nickelate films

The polarity of a surface can affect the electronic and structural properties of oxide thin films through electrostatic effects. Understanding the mechanism behind these effects requires knowledge of the atomic structure and electrostatic characteristics at the surface. In this study, we use annular bright-field imaging to investigate the surface structure of a Pr0.8Sr0.2NiO2+x (0 < x < 1) film. We observe a polar distortion coupled with octahedral rotations in a fully oxidized Pr0.8Sr0.2NiO3 sample, and a stronger polar distortion in a partially reduced sample. Its spatial depth extent is about three unit cells from the surface. Additionally, we use four-dimensional scanning transmission electron microscopy (4D-STEM) to directly image the local atomic electric field surrounding Ni atoms near the surface and discover distinct valence variations of Ni atoms, which are confirmed by atomic-resolution electron energy-loss spectroscopy (EELS). Our results suggest that the strong surface reconstruction in the reduced sample is closely related to the formation of oxygen vacancies from topochemical reduction. These findings provide insights into the understanding and evolution of surface polarity at the atomic level.

s^{±}-Wave Pairing and the Destructive Role of Apical-Oxygen Deficiencies in La_{3}Ni_{2}O_{7} under Pressure

Recently, the bilayer perovskite nickelate La_{3}Ni_{2}O_{7} has been reported to show evidence of high-temperature superconductivity (SC) under a moderate pressure of about 14 GPa. To investigate the superconducting mechanism, pairing symmetry, and the role of apical-oxygen deficiencies in this material, we perform a random-phase approximation based study on a bilayer model consisting of the d_{x^{2}-y^{2}} and d_{3z^{2}-r^{2}} orbitals of Ni atoms in both the pristine crystal and the crystal with apical-oxygen deficiencies. Our analysis reveals an s^{±}-wave pairing symmetry driven by spin fluctuations. The crucial role of pressure lies in that it induces the emergence of the γ pocket, which is involved in the strongest Fermi-surface nesting. We further found the emergence of local moments in the vicinity of apical-oxygen deficiencies, which significantly suppresses the T_{c}. Therefore, it is possible to significantly enhance the T_{c} by eliminating oxygen deficiencies during the synthesis of the samples.

Correlated Electronic Structure of La_{3}Ni_{2}O_{7} under Pressure

Recently, superconductivity with a T_{c} up to 78 K has been reported in bulk samples of the bilayer nickelate La_{3}Ni_{2}O_{7} at pressures above 14 GPa. Important theoretical tasks are the formulation of relevant low-energy models and the clarification of the normal state properties. Here, we study the correlated electronic structure of the high-pressure phase in a four-orbital low-energy subspace using different many-body approaches: GW, dynamical mean field theory (DMFT), extended DMFT (EDMFT) and GW+EDMFT, with realistic frequency-dependent interaction parameters. The nonlocal correlation and screening effects captured by GW+EDMFT result in an instability toward the formation of charge stripes, with the 3d_{z^{2}} as the main active orbital. We also comment on the potential relevance of the rare-earth self-doping pocket, since hole doping suppresses the ordering tendency.

Intermittent Defect Fluctuations in Oxide Heterostructures

The heterogeneous nature, local presence, and dynamic evolution of defects typically govern the ionic and electronic properties of a wide variety of functional materials. While the last 50 years have seen considerable efforts into development of new methods to identify the nature of defects in complex materials, such as the perovskite oxides, very little is known about defect dynamics and their influence on the functionality of a material. Here, the discovery of the intermittent behavior of point defects (oxygen vacancies) in oxide heterostructures employing X-ray photon correlation spectroscopy is reported. Local fluctuations between two ordered phases in strained SrCoOx with different degrees of stability of the oxygen vacancies are observed. Ab-initio-informed phase-field modeling reveals that fluctuations between the competing ordered phases are modulated by the oxygen ion/vacancy interaction energy and epitaxial strain. The results demonstrate how defect dynamics, evidenced by measurement and modeling of their temporal fluctuations, give rise to stochastic properties that now can be fully characterized using coherent X-rays, coupled for the first time to multiscale modeling in functional complex oxide heterostructures. The study and its findings open new avenues for engineering the dynamical response of functional materials used in neuromorphic and electrochemical applications.

Photoinduced Phase Transition in Infinite-Layer Nickelates

The quantum phase transition caused by regulating the electronic correlation in strongly correlated quantum materials has been a research hotspot in condensed matter science. Herein, a photon-induced quantum phase transition from the Kondo-Mott insulating state to the low temperature metallic one accompanying with the magnetoresistance changing from negative to positive in the infinite-layer NdNiO2 films is reported, where the antiferromagnetic coupling among the Ni1+ localized spins and the Kondo effect are effectively suppressed by manipulating the correlation of Ni-3d and Nd-5d electrons under the photoirradiation. Moreover, the critical temperature Tc of the superconducting-like transition exhibits a dome-shaped evolution with the maximum up to ≈42 K, and the electrons dominate the transport process proved by the Hall effect measurements. These findings not only make the photoinduction a promising way to control the quantum phase transition by manipulating the electronic correlation in Mott-like insulators, but also shed some light on the possibility of the superconducting in electron-doped nickelates.

An electronic origin of charge order in infinite-layer nickelates

A charge order (CO) with a wavevector [Formula: see text] is observed in infinite-layer nickelates. Here we use first-principles calculations to demonstrate a charge-transfer-driven CO mechanism in infinite-layer nickelates, which leads to a characteristic Ni1+-Ni2+-Ni1+ stripe state. For every three Ni atoms, due to the presence of near-Fermi-level conduction bands, Hubbard interaction on Ni-d orbitals transfers electrons on one Ni atom to conduction bands and leaves electrons on the other two Ni atoms to become more localized. We further derive a low-energy effective model to elucidate that the CO state arises from a delicate competition between Hubbard interaction on Ni-d orbitals and charge transfer energy between Ni-d orbitals and conduction bands. With physically reasonable parameters, [Formula: see text] CO state is more stable than uniform paramagnetic state and usual checkerboard antiferromagnetic state. Our work highlights the multi-band nature of infinite-layer nickelates, which leads to some distinctive correlated properties that are not found in cuprates.

Superconducting Nd1-xEuxNiO2 thin films using in situ synthesis

We report on superconductivity in Nd_1-xEu_xNiO₂ using Eu as a 4f dopant of the parent NdNiO₂ infinite-layer compound. We use an all-in situ molecular beam epitaxy reduction process to achieve the superconducting phase, providing an alternate method to the ex situ CaH₂ reduction process to induce superconductivity in the infinite-layer nickelates. The Nd_1-xEu_xNiO₂ samples exhibit a step-terrace structure on their surfaces, have a T_c onset of 21 K at x = 0.25, and have a large upper critical field that may be related to Eu 4f doping.

Intrinsic Coherence Length Anisotropy in Nickelates and Some Iron-Based Superconductors

Nickelate superconductors, R_1-xA_xNiO₂ (where R is a rare earth metal and A = Sr, Ca), experimentally discovered in 2019, exhibit many unexplained mysteries, such as the existence of a superconducting state with T_c (up to 18 K) in thin films and yet absent in bulk materials. Another unexplained mystery of nickelates is their temperature-dependent upper critical field, Bc2(T), which can be nicely fitted to two-dimensional (2D) models; however, the deduced film thickness, dsc,GL, exceeds the physical film thickness, dsc, by a manifold. To address the latter, it should be noted that 2D models assume that dsc is less than the in-plane and out-of-plane ground-state coherence lengths, dsc<ξab(0) and dsc<ξc(0), respectively, and, in addition, that the inequality ξc(0)<ξab(0) satisfies. Analysis of the reported experimental Bc2(T) data showed that at least one of these conditions does not satisfy for R_1-xA_xNiO₂ films. This implies that nickelate films are not 2D superconductors, despite the superconducting state being observed only in thin films. Based on this, here we propose an analytical three-dimensional (3D) model for a global data fit of in-plane and out-of-plane Bc2(T) in nickelates. The model is based on a heuristic expression for temperature-dependent coherence length anisotropy: γξ(T)=γξ(0)1-1a×TTc, where a>1 is a unitless free-fitting parameter. The proposed expression for γξ(T), perhaps, has a much broader application because it has been successfully applied to bulk pnictide and chalcogenide superconductors.

Modulations in Superconductors: Probes of Underlying Physics

The importance of modulations is elevated to an unprecedented level, due to the delicate conditions required to bring out exotic phenomena in quantum materials, such as topological materials, magnetic materials, and superconductors. Recently, state-of-the-art modulation techniques in material science, such as electric-double-layer transistor, piezoelectric-based strain apparatus, angle twisting, and nanofabrication, have been utilized in superconductors. They not only efficiently increase the tuning capability to the broader ranges but also extend the tuning dimensionality to unprecedented degrees of freedom, including quantum fluctuations of competing phases, electronic correlation, and phase coherence essential to global superconductivity. Here, for a comprehensive review, these techniques together with the established modulation methods, such as elemental substitution, annealing, and polarization-induced gating, are contextualized. Depending on the mechanism of each method, the modulations are categorized into stoichiometric manipulation, electrostatic gating, mechanical modulation, and geometrical design. Their recent advances are highlighted by applications in newly discovered superconductors, e.g., nickelates, Kagome metals, and magic-angle graphene. Overall, the review is to provide systematic modulations in emergent superconductors and serve as the coordinate for future investigations, which can stimulate researchers in superconductivity and other fields to perform various modulations toward a thorough understanding of quantum materials.

Rare-earth control of phase transitions in infinite-layer nickelates

Perovskite nickelates RNiO3 (R = rare-earth ion) exhibit complex rare-earth ion dependent phase diagram and high tunability of various appealing properties. Here, combining first- and finite-temperature second-principles calculations, we explicitly demonstrate that the superior merits of the interplay among lattice, electron, and spin degrees of freedom can be passed to RNiO2, which recently gained significant interest as superconductors. We unveil that decreasing the rare-earth size directly modulates the structural, electronic, and magnetic properties and naturally groups infinite-layer nickelates into two categories in terms of the Fermi surface and magnetic dimensionality: compounds with large rare-earth sizes (La, Pr) closely resemble the key properties of CaCuO2, showing quasi-two-dimensional (2D) antiferromagnetic (AFM) correlations and strongly localized d x 2 - y 2 orbitals around the Fermi level; the compounds with small rare-earth sizes (Nd-Lu) are highly analogous to ferropnictides, showing three-dimensional (3D) magnetic dimensionality and strong k z dispersion of d 3 z 2 - r 2 electrons at the Fermi level. Additionally, we highlight that RNiO2 with R = Nd-Lu exhibit on cooling a structural transition with the appearance of oxygen rotation motion, which is softened by the reduction of rare-earth size and enhanced by spin-rotation couplings. The rare-earth control of k z dispersion and structural phase transition might be the key factors differentiating the distinct upper critical field and resistivity in different compounds. The established original phase diagram summarizing the temperature and rare-earth controlled structural, electronic, and magnetic transitions in RNiO2 compounds provides rich structural and chemical flexibility to tailor the superconducting property.

Thickness-Dependent Interface Polarity in Infinite-Layer Nickelate Superlattices

The interface polarity plays a vital role in the physical properties of oxide heterointerfaces because it can cause specific modifications of the electronic and atomic structure. Reconstruction due to the strong polarity of the NdNiO₂/SrTiO₃ interface in recently discovered superconducting nickelate films may play an important role, as no superconductivity has been observed in the bulk. By employing four-dimensional scanning transmission electron microscopy and electron energy-loss spectroscopy, we studied effects of oxygen distribution, polyhedral distortion, elemental intermixing, and dimensionality in NdNiO₂/SrTiO₃ superlattices grown on SrTiO₃ (001) substrates. Oxygen distribution maps show a gradual variation of the oxygen content in the nickelate layer. Remarkably, we demonstrate thickness-dependent interface reconstruction due to a polar discontinuity. An average cation displacement of ∼0.025 nm at interfaces in 8NdNiO₂/4SrTiO₃ superlattices is twice larger than that in 4NdNiO₂/2SrTiO₃ superlattices. Our results provide insights into the understanding of reconstructions at NdNiO₂/SrTiO₃ polar interfaces.

Resolving the polar interface of infinite-layer nickelate thin films

Nickel-based superconductors provide a long-awaited experimental platform to explore possible cuprate-like superconductivity. Despite similar crystal structure and d electron filling, however, superconductivity in nickelates has thus far only been stabilized in thin-film geometry, raising questions about the polar interface between substrate and thin film. Here we conduct a detailed experimental and theoretical study of the prototypical interface between Nd_1-xSr_xNiO₂ and SrTiO₃. Atomic-resolution electron energy loss spectroscopy in the scanning transmission electron microscope reveals the formation of a single intermediate Nd(Ti,Ni)O₃ layer. Density functional theory calculations with a Hubbard U term show how the observed structure alleviates the polar discontinuity. We explore the effects of oxygen occupancy, hole doping and cation structure to disentangle the contributions of each for reducing interface charge density. Resolving the non-trivial interface structure will be instructive for future synthesis of nickelate films on other substrates and in vertical heterostructures.

Resonant inelastic x-ray scattering data for Ruddlesden-Popper and reduced Ruddlesden-Popper nickelates

Ruddlesden-Popper and reduced Ruddlesden-Popper nickelates are intriguing candidates for mimicking the properties of high-temperature superconducting cuprates. The degree of similarity between these nickelates and cuprates has been the subject of considerable debate. Resonant inelastic x-ray scattering (RIXS) has played an important role in exploring their electronic and magnetic excitations, but these efforts have been stymied by inconsistencies between different samples and the lack of publicly available data for detailed comparison. To address this issue, we present open RIXS data on La₄Ni₃O₁₀ and La₄Ni₃O₈.

Limits to the strain engineering of layered square-planar nickelate thin films

The layered square-planar nickelates, Ndn+1NinO2n+2, are an appealing system to tune the electronic properties of square-planar nickelates via dimensionality; indeed, superconductivity was recently observed in Nd6Ni5O12 thin films. Here, we investigate the role of epitaxial strain in the competing requirements for the synthesis of the n = 3 Ruddlesden-Popper compound, Nd4Ni3O10, and subsequent reduction to the square-planar phase, Nd4Ni3O8. We synthesize our highest quality Nd4Ni3O10 films under compressive strain on LaAlO3 (001), while Nd4Ni3O10 on NdGaO3 (110) exhibits tensile strain-induced rock salt faults but retains bulk-like transport properties. A high density of extended defects forms in Nd4Ni3O10 on SrTiO3 (001). Films reduced on LaAlO3 become insulating and form compressive strain-induced c-axis canting defects, while Nd4Ni3O8 films on NdGaO3 are metallic. This work provides a pathway to the synthesis of Ndn+1NinO2n+2 thin films and sets limits on the ability to strain engineer these compounds via epitaxy.

Recent Advances for the Synthesis and Applications of 2-Dimensional Ternary Layered Materials

Layered materials with unique structures and symmetries have attracted tremendous interest for constructing 2-dimensional (2D) structures. The weak interlayer interaction renders them to be readily isolated into various ultrathin nanosheets with exotic properties and diverse applications. In order to enrich the library of 2D materials, extensive progress has been made in the field of ternary layered materials. Consequently, many brand-new materials are derived, which greatly extend the members of 2D realm. In this review, we emphasize the recent progress made in synthesis and exploration of ternary layered materials. We first classify them in terms of stoichiometric ratio and summarize their difference in interlayer interaction, which is of great importance to produce corresponding 2D materials. The compositional and structural characteristics of resultant 2D ternary materials are then discussed so as to realize desired structures and properties. As a new family of 2D materials, we overview the layer-dependent properties and related applications in the fields of electronics, optoelectronics, and energy storage and conversion. The review finally provides a perspective for this rapidly developing field.

Electronic structure and correlations in planar trilayer nickelate Pr4Ni3O8

The discovery of superconductivity in planar nickelates raises the question of how the electronic structure and correlations of Ni¹⁺ compounds compare to those of the Cu²⁺ cuprate superconductors. Here, we present an angle-resolved photoemission spectroscopy (ARPES) study of the trilayer nickelate Pr₄Ni₃O₈, revealing a Fermi surface resembling that of the hole-doped cuprates but with critical differences. Specifically, the main portions of the Fermi surface are extremely similar to that of the bilayer cuprates, with an additional piece that can accommodate additional hole doping. We find that the electronic correlations are about twice as strong in the nickelates and are almost k-independent, indicating that they originate from a local effect, likely the Mott interaction, whereas cuprate interactions are somewhat less local. Nevertheless, the nickelates still demonstrate the strange-metal behavior in the electron scattering rates. Understanding the similarities and differences between these two families of strongly correlated superconductors is an important challenge.

Exploring the 4d1analogue of cuprates: theoretical studies on bulk NbF4and NbF4monolayer stabilized on MgO (001) plane

The recent research in infinite-layer nickelates has inspired new effort in finding the cuprate analogs. Here we propose that NbF₄, which contains niobium-centered fluorine octahedra, is a promising 4d¹analogue of cuprates. Using the density functional theory, we first show that bulk NbF₄is in close proximity tod¹configuration, with Nb4dxyorbital nearly half-filled. A single band with dominating4dxycharacter crosses the Fermi level, forming a square-like Fermi surface. The intralayer G-type antiferromagnetic (AFM) order is energetically favored and the Coulomb interaction drives the system into an AFM insulator. Next we demonstrate that the NbF₄layer can be stabilized on MgO substrate with main electronic and magnetic features retained, offering an alternative route to realize the NbF₄-related high-T_csuperconductors. Furthermore, we derive effective single orbital models for both systems and investigate the electron correlation effects via functional renormalization group. We find that the G-type AFM dominates near half-filling butdx2-y2-wave superconductivity (SC) prevails upon suitable hole/electron doping. Based on the striking similarities between NbF₄and cuprates, we suggest that NbF₄-related compounds may be exotic candidates for searching new high-T_csuperconductors.

Antiferromagnetic insulating state in layered nickelates at half filling

We provide a set of computational experiments based on 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[Formula: see text]Ni[Formula: see text]O[Formula: see text], R= rare-earth, [Formula: see text]) in proximity to half-filling. It is well established that at [Formula: see text] filling the infinite-layer ([Formula: see text]) 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 [Formula: see text] filling, due to the spacer RO[Formula: see text] fluorite slabs present in their structure that block the c-axis dispersion. In the [Formula: see text] 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[Formula: see text], or a blocking layer in multilayers formed by (RNiO[Formula: see text])[Formula: see text]/(RNaO[Formula: see text])[Formula: see text]).

Superconducting Instabilities in Strongly Correlated Infinite-Layer Nickelates

The discovery of superconductivity in infinite-layer nickelates has added a new family of materials to the fascinating growing class of unconventional superconductors. By incorporating the strongly correlated multiorbital nature of the low-energy electronic degrees of freedom, we compute the leading superconducting instability from magnetic fluctuations relevant for infinite-layer nickelates. Specifically, by properly including the doping dependence of the Ni d_{x^{2}-y^{2}} and d_{z^{2}} orbitals as well as the self-doping band, we uncover a transition from d-wave pairing symmetry to nodal s_{±} superconductivity, driven by strong fluctuations in the d_{z^{2}}-dominated orbital states. We discuss the properties of the resulting superconducting condensates in light of recent tunneling and penetration depth experiments probing the detailed superconducting gap structure of these materials.

Revealing the role of interfacial heterogeneous nucleation in the metastable thin film growth of rare-earth nickelate electronic transition materials

Although rare-earth nickelates (ReNiO₃, Re ≠ La) exhibit abundant electronic phases and widely adjustable metal to insulator electronic transition properties, their practical electronic applications are largely impeded by their intrinsic meta-stability. Apart from elevating the oxygen reaction pressure, heterogeneous nucleation is expected to be an alternative strategy that enables the crystallization of ReNiO₃ at low meta-stability. In this work, the respective roles of high oxygen pressure and heterogeneous interface in triggering ReNiO₃ thin film growth in the metastable state are revealed. ReNiO₃ (Re = Nd, Sm, Eu, Gd and Dy) thin films grown on a LaAlO₃ single crystal substrate show effective crystallization at atmospheric pressure without the necessity to apply high oxygen pressure, suggesting that the interfacial bonding between the ReNiO₃ and substrates can sufficiently reduce the positive Gibbs formation energy of ReNiO₃, which is further verified by the first-principles calculations. Nevertheless, the abrupt electronic transitions only appear in ReNiO₃ thin films grown at high oxygen pressure, in which case the oxygen vacancies are effectively eliminated via high oxygen pressure reactions as indicated by near-edge X-ray absorption fine structure (NEXAFS) analysis. This work unveils the synergistic effects of heterogeneous nucleation and high oxygen pressure on the growth of high quality ReNiO₃ thin films.

Self-healing Growth of LaNiO3 on a Mixed-Terminated Perovskite Surface

Developing atomic-scale synthesis control is a prerequisite for understanding and engineering the exotic physics inherent to transition-metal oxide heterostructures. Thus, far, however, the number of materials systems explored has been extremely limited, particularly with regard to the crystalline substrate, which is routinely SrTiO₃. Here, we investigate the growth of a rare-earth nickelate─LaNiO₃─on (LaAlO₃)(Sr₂AlTaO₆) (LSAT) (001) by oxide molecular beam epitaxy (MBE). Whereas the LSAT substrates are smooth, they do not exhibit the single surface termination usually assumed necessary for control over the interface structure. Performing both nonresonant and resonant anomalous in situ synchrotron surface X-ray scattering during MBE growth, we show that reproducible heterostructures can be achieved regardless of both the mixed surface termination and the layer-by-layer deposition sequence. The rearrangement of the layers occurs dynamically during growth, resulting in the fabrication of high-quality LaNiO₃/LSAT heterostructures with a sharp and consistent interfacial structure. This is due to the thermodynamics of the deposition window as well as the nature of the chemical species at interfaces─here, the flexible charge state of nickel at the oxide surface. This has important implications regarding the use of a wider variety of substrates for fundamental studies on complex oxide synthesis.

Superconductivity in infinite-layer nickelates

The recent discovery of the superconductivity in the doped infinite layer nickelatesRNiO₂(R= La, Pr, Nd) is of great interest since the nickelates are isostructural to doped (Ca, Sr)CuO₂having superconducting transition temperature (T_c) of about 110 K. Verifying the commonalities and differences between these oxides will certainly give a new insight into the mechanism of highT_csuperconductivity 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 age' of superconductivity.

Quantifying transfer learning synergies in infinite-layer and perovskite nitrides, oxides, and fluorides

We combine density functional theory simulations and active learning (AL) of element-embedding neural networks (NNs) to explore the sample efficiency for the prediction of vacancy layer formation energies and lattice parameters inABX_ninfinite-layer (n= 2) versus perovskite (n= 3) nitrides, oxides, and fluorides in the spirit of transfer learning. Following a comprehensive data analysis from different thermodynamic, structural, and statistical perspectives, we show that NNs model these observables with high precision, using merely∼30%of the data for training and exclusively theA-,B-, andX-site element names as minimal input devoid of any physicala prioriinformation. Element embedding autonomously arranges the chemical elements with a characteristic recurrent topology, such that their relations are consistent with human knowledge. We compare two different embedding strategies and show that these techniques render additional input such as atomic properties negligible. Simultaneously, we demonstrate that AL is largely independent of the initial training set, and exemplify its superiority over randomly composed training sets. Despite their highly distinct chemistry, the present approach successfully identifies fundamental quantum-mechanical universalities between nitrides, oxides, and fluorides that enhance the combined prediction accuracy by up to 16% with respect to three specialized NNs at equivalent numerical effort. This quantification of synergistic effects provides an impression of the transfer learning improvements one may expect for similarly complex materials. Finally, by embedding the tensor product of theBandXsites and subsequent quantitative cluster analysis, we establish from an unbiased artificial-intelligence perspective that oxides and nitrides exhibit significant parallels, whereas fluorides constitute a rather distinct materials class.

Superconductivity in infinite-layer nickelate La1-xCaxNiO2 thin films

We report the observation of superconductivity in infinite-layer Ca-doped LaNiO₂ (La_1-xCa_xNiO₂) thin films and construct their phase diagram. Unlike the metal-insulator transition in Nd- and Pr-based nickelates, the undoped and underdoped La_1-xCa_xNiO₂ 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 nickelates, the R_H-sign-change temperature remains at around 35 K as the doping increases, which begs further theoretical and experimental investigation to reveal the role of the 4f orbital to the (multi)band nature of the superconducting nickelates. Our results also emphasize a notable role of lattice correlation on the multiband structures of the infinite-layer nickelates.

Superconductivity in a quintuple-layer square-planar nickelate

Since the discovery of high-temperature superconductivity in copper oxide materials¹, there have been sustained efforts to both understand the origins of this phase and discover new cuprate-like superconducting materials². One prime materials platform has been the rare-earth nickelates and, indeed, superconductivity was recently discovered in the doped compound Nd_0.8Sr_0.2NiO₂ (ref. ³). Undoped NdNiO₂ belongs to a series of layered square-planar nickelates with chemical formula Nd_n+1Ni_nO_2n+2 and is known as the 'infinite-layer' (n = ∞) nickelate. Here we report the synthesis of the quintuple-layer (n = 5) member of this series, Nd₆Ni₅O₁₂, in which optimal cuprate-like electron filling (d^8.8) is achieved without chemical doping. We observe a superconducting transition beginning at ~13 K. Electronic structure calculations, in tandem with magnetoresistive and spectroscopic measurements, suggest that Nd₆Ni₅O₁₂ interpolates between cuprate-like and infinite-layer nickelate-like behaviour. In engineering a distinct superconducting nickelate, we identify the square-planar nickelates as a new family of superconductors that can be tuned via both doping and dimensionality.

Superconductivity in nickel-based 112 systems

Superconductivity has been discovered recently in infinite-layer nickel-based 112 thin films R_1−xA_xNiO₂ (R = La, Nd, Pr and A = Sr, Ca). They are isostructural to the infinite-layer cuprate (Ca,Sr)CuO₂ and are supposed to have a formal Ni 3d⁹ valence, thus providing a new platform to study the unconventional pairing mechanism of high-temperature superconductors. This important discovery immediately triggers a huge amount of innovative scientific curiosity in the field. In this paper, we try to give an overview of the recent research progress on the newly found superconducting nickelate systems, both from experimental and theoretical aspects. We mainly focus on the electronic structures, magnetic excitations, phase diagrams and superconducting gaps, and finally make some open discussions for possible pairing symmetries in Ni-based 112 systems.

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The infinite-layer nickel-based 112 thin films R_1−xA_xNiO₂ can host superconductivity up to 15 K

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R_1−xA_xNiO₂ is a multiband system, in which the short-range antiferromagnetic fluctuations can be detected

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R_1−xA_xNiO₂ has an unconventional superconducting pairing sate with a robust d-wave gap and a full gap without unified understanding

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The nickelate system provides a new platform for researching unconventional superconductivity

Antiferromagnetism in Ni-Based Superconductors

Due to the lack of any magnetic order down to 1.7 K in the parent bulk compound NdNiO₂ , the recently discovered 9-15 K superconductivity in the infinite-layer Nd_0.8 Sr_0.2 NiO₂ thin films has provided an exciting playground for unearthing new superconductivity mechanisms. Herein, the successful synthesis of a series of superconducting Nd_0.8 Sr_0.2 NiO₂ thin films ranging from 8 to 40 nm is reported. The large exchange bias effect is observed between the superconducting Nd_0.8 Sr_0.2 NiO₂ films and a thin ferromagnetic layer, which suggests the existence of the antiferromagnetic order. Furthermore, the existence of the antiferromagnetic order is evidenced by X-ray magnetic linear dichroism measurements. These experimental results are fundamentally critical for the current field.

Observation of Josephson-like Tunneling Junction Characteristics and Positive Magnetoresistance in Oxygen Deficient Nickelate Films of Nd0.8Sr0.2NiO3-δ

Nickelate films have recently attracted broad attention due to the observation of superconductivity in the infinite layer phase of Nd0.8Sr0.2NiO2 (obtained by reducing Sr doped NdNiO3 films) and their similarity to the cuprates high temperature superconductors. Here, we report on the observation of a new type of transport in oxygen poor Nd0.8Sr0.2NiO3-δ films. At high temperatures, variable range hopping is observed while at low temperatures a novel tunneling behavior is found where a Josephson-like tunneling junction characteristic with serial resistance is revealed. We attribute this phenomenon to coupling between superconductive (S) surfaces of the grains in our Oxygen poor films via the insulating (I) grain boundaries, which yields SIS junctions in series with the normal (N) resistance of the grains themselves. The similarity of the observed conductance spectra to the tunneling junction characteristic with Josephson-like current is striking, and seems to support the existence of superconductivity in our samples.

Topotactic transformation of single crystals: From perovskite to infinite-layer nickelates

Topotactic transformations between related crystal structures are a powerful emerging route for the synthesis of novel quantum materials. Whereas most such “soft chemistry” experiments have been carried out on polycrystalline powders or thin films, the topotactic modification of single crystals, the gold standard for physical property measurements on quantum materials, has been studied only sparsely. Here, we report the topotactic reduction of La_1−xCa_xNiO₃ single crystals to La_1−xCa_xNiO_2+δ using CaH₂ as the reducing agent. The transformation from the three-dimensional perovskite to the quasi–two-dimensional infinite-layer phase was thoroughly characterized by x-ray diffraction, electron microscopy, Raman spectroscopy, magnetometry, and electrical transport measurements. Our work demonstrates that the infinite-layer structure can be realized as a bulk phase in crystals with micrometer-sized single domains. The electronic properties of these specimens resemble those of epitaxial thin films rather than powders with similar compositions.

Nickelate Superconductivity without Rare-Earth Magnetism: (La,Sr)NiO2

The occurrence of unconventional superconductivity in cuprates has long motivated the search for manifestations in other layered transition metal oxides. Recently, superconductivity is found in infinite-layer nickelate (Nd,Sr)NiO₂ and (Pr,Sr)NiO₂ thin films, formed by topotactic reduction from the perovskite precursor phase. A topic of much current interest is whether rare-earth moments are essential for superconductivity in this system. In this study, it is found that with significant materials optimization, substantial portions of the La_{1- x} Sr_x NiO₂ phase diagram can enter the regime of coherent low-temperature transport (x = 0.14 - 0.20), with subsequent superconducting transitions and a maximum onset of ≈9 K at x = 0.20. Additionally, the unexpected indication of a superconducting ground state in undoped LaNiO₂ is observed, which likely reflects the self-doped nature of the electronic structure. Combining the results of (La/Pr/Nd)_{1- x} Sr_x NiO₂ reveals a generalized superconducting dome, characterized by systematic shifts in the unit cell volume and in the relative electron-hole populations across the lanthanides.

Synthesis and physical properties of perovskite Sm1-xSrxNiO3(x= 0, 0.2) and infinite-layer Sm0.8Sr0.2NiO2nickelates

Recently, superconductivity at about 9-15 K was discovered in Nd_1-xSr_xNiO₂(Nd-112,x≈ 0.125-0.25) 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 theRNiO₃(R-113,R: rare-earth element) phase, however, it was reported that the 113 phase is very difficult to be synthesized successfully due to the formation of unusual Ni³⁺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 physical properties of polycrystalline perovskites Sm_1-xSr_xNiO₃(x= 0, 0.2) in which the ionic radius of Sm³⁺is smaller than that of Pr³⁺and Nd³⁺in related superconducting thin films. The structural and compositional analyses conducted by x-ray diffraction and energy dispersive x-ray spectrum reveal that the samples mainly contain the perovskite phase of Sm_1-xSr_xNiO₃with small amount of NiO impurities. Magnetization and resistivity measurements indicate that the parent phase SmNiO₃undergoes a paramagnetic-antiferromagnetic transition at about 224 K on a global insulating background. In contrast, the Sr-doped sample Sm_0.8Sr_0.2NiO₃shows a metallic behavior from 300 K down to about 12 K, while below 12 K the resistivity exhibits a slight logarithmic increase. Meanwhile, from the magnetization curves, we can see that a possible spin-glass state occurs below 12 K in Sm_0.8Sr_0.2NiO₃. Using a soft chemical reduction method, we also obtain the infinite-layer phase Sm_0.8Sr_0.2NiO₂with square NiO₂planes. The compound shows an insulating behavior which can be described by the three-dimensional variable-range-hopping model. And superconductivity is still absent in the polycrystalline Sm_0.8Sr_0.2NiO₂.

Hund Physics Landscape of Two-Orbital Systems

Motivated by the recent discovery of superconductivity in infinite-layer nickelates RE_{1-δ}Sr_{δ}NiO_{2} (RE=Nd, Pr), we study the role of Hund coupling J in a quarter-filled two-orbital Hubbard model, which has been on the periphery of the attention. A region of negative effective Coulomb interaction of this model is revealed to be differentiated from three- and five-orbital models in their typical Hund metal active fillings. We identify distinctive regimes including four different correlated metals, one of which stems from the proximity to a Mott insulator, while the other three, which we call "intermediate" metal, weak Hund metal, and valence-skipping metal, from the effect of J being away from Mottness. Defining criteria characterizing these metals is suggested, establishing the existence of Hund metallicity in two-orbital systems.

Doping evolution of the Mott-Hubbard landscape in infinite-layer nickelates

Copper-based (cuprate) oxides are not only the original but also one of the best-studied families of “high-temperature” superconductors. With nominally identical crystal structure and electron count, nickel-based (nickelate) compounds have been widely pursued for decades as a possible analog to the cuprates. The recent demonstration of superconductivity in nickelate thin films has provided an experimental platform to explore the possible connections between the copper- and nickel-based superconductors. Here, we perform highly localized spectroscopic measurements to reveal a number of key differences between the two systems, particularly with regard to the hybridization between the O and metal (Cu or Ni) orbitals.

The recent observation of superconductivity in Nd0.8Sr0.2NiO2 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 Nd1−xSrxNiO2. Upon doping, we observe emergent hybridization reminiscent of the Zhang–Rice singlet via the oxygen-projected states, modification of the Nd 5d states, and the systematic evolution of Ni 3d hybridization and filling. These experimental data provide direct evidence for the multiband electronic structure of the superconducting infinite-layer nickelates, particularly via the effects of hole doping on not only the oxygen but also nickel and rare-earth bands.

Optical Properties of the Infinite-Layer La_{1-x}Sr_{x}NiO_{2} and Hidden Hund's Physics

We investigate the optical properties of the normal state of the infinite-layer La_{1-x}Sr_{x}NiO_{2} using density functional theory plus dynamical mean-field theory. We find a correlated metal which exhibits substantial transfer of spectral weight to high energies relative to the density functional theory. The correlations are not due to Mott physics, which would suppress the charge fluctuations and the integrated optical spectral weight as we approach a putative insulating state. Instead, we find the unusual situation, that the integrated optical spectral weight decreases with doping and increases with increasing temperature. We contrast this with the coherent component of the optical conductivity, which decreases with increasing temperature as a result of a coherence-incoherence crossover. Our studies reveal that the effective crystal field splitting is dynamical and increases strongly at low frequency. This leads to a picture of a Hund's metallic state, where dynamical orbital fluctuations are visible at intermediate energies, while at low energies a Fermi surface with primarily d_{x^{2}-y^{2}} character emerges. The infinite-layer nickelates are thus in an intermediate position between the iron based high temperature superconductors where multiorbital Hund's physics dominates and a one-band system such as the cuprates. To capture this physics we propose a low-energy two-band model with atom centered e_{g} states.