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

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.

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.

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.

Observation of perfect diamagnetism and interfacial effect on the electronic structures in infinite layer Nd0.8Sr0.2NiO2 superconductors

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 lacking. Here, we show high-quality Nd_0.8Sr_0.2NiO₂ thin films with different thicknesses and demonstrate the interface and strain effects on the electrical, magnetic and optical properties. Perfect diamagnetism is achieved, confirming the occurrence of superconductivity in the films. Unlike the thick films in which the normal-state Hall-coefficient changes signs as the temperature decreases, the Hall-coefficient of films thinner than 5.5 nm remains negative, suggesting a thickness-driven band structure modification. Moreover, X-ray absorption spectroscopy reveals the Ni-O hybridization nature in doped infinite-layer nickelates, and the hybridization is enhanced as the thickness decreases. Consistent with band structure calculations on the nickelate/SrTiO₃ heterostructure, the interface and strain effect induce a dominating electron-like band in the ultrathin film, thus causing the sign-change of the Hall-coefficient.

Nickelate superconductors attract enormous attention in the field of high-temperature superconductivity. Here the authors report observation of perfect diamagnetism and interfacial effect on the electronic structures in infinite layer Nd_0.8Sr_0.2NiO₂ superconductors.

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

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.