Articoli di riviste: "Infinite-layer nickelates"

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Nomura, Yusuke, e Ryotaro Arita. "Superconductivity in infinite-layer nickelates". Reports on Progress in Physics 85, n. 5 (28 marzo 2022): 052501. http://dx.doi.org/10.1088/1361-6633/ac5a60.

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Abstract The recent discovery of the superconductivity in the doped infinite layer nickelates RNiO2 (R = La, Pr, Nd) is of great interest since the nickelates are isostructural to doped (Ca, Sr)CuO2 having 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 high T 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 age’ of superconductivity.

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Lu, H., M. Rossi, A. Nag, M. Osada, D. F. Li, K. Lee, B. Y. Wang et al. "Magnetic excitations in infinite-layer nickelates". Science 373, n. 6551 (8 luglio 2021): 213–16. http://dx.doi.org/10.1126/science.abd7726.

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The discovery of superconductivity in infinite-layer nickelates brings us tantalizingly close to a material class that mirrors the cuprate superconductors. We measured the magnetic excitations in these nickelates using resonant inelastic x-ray scattering at the Ni L3-edge. Undoped NdNiO2 possesses a branch of dispersive excitations with a bandwidth of approximately 200 milli–electron volts, which is reminiscent of the spin wave of strongly coupled, antiferromagnetically aligned spins on a square lattice. The substantial damping of these modes indicates the importance of coupling to rare-earth itinerant electrons. Upon doping, the spectral weight and energy decrease slightly, whereas the modes become overdamped. Our results highlight the role of Mottness in infinite-layer nickelates.

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Ji, Yaoyao, Junhua Liu, Lin Li e Zhaoliang Liao. "Superconductivity in infinite layer nickelates". Journal of Applied Physics 130, n. 6 (14 agosto 2021): 060901. http://dx.doi.org/10.1063/5.0056328.

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Gabay, Marc, Stefano Gariglio e Jean-Marc Triscone. "Functionally doped infinite-layer nickelates". Nature Materials 21, n. 2 (febbraio 2022): 139–40. http://dx.doi.org/10.1038/s41563-021-01163-4.

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Lin, Hai, Dariusz Jakub Gawryluk, Yannick Maximilian Klein, Shangxiong Huangfu, Ekaterina Pomjakushina, Fabian von Rohr e Andreas Schilling. "Universal spin-glass behaviour in bulk LaNiO₂, PrNiO₂ and NdNiO₂". New Journal of Physics 24, n. 1 (1 gennaio 2022): 013022. http://dx.doi.org/10.1088/1367-2630/ac465e.

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Abstract Motivated by the recent discovery of superconductivity in infinite-layer nickelate thin films, we report on a synthesis and magnetization study on bulk samples of the parent compounds RNiO2 (R = La, Pr, Nd). The frequency-dependent peaks of the alternating current magnetic susceptibility, along with remarkable memory effects, characterize spin-glass states. Furthermore, various phenomenological parameters via different spin glass models show strong similarity within these three compounds as well as with other rare-earth metal nickelates. The universal spin-glass behaviour distinguishes the nickelates from the parent compound CaCuO2 of cuprate superconductors, which has the same crystal structure and d 9 electronic configuration but undergoes a long-range antiferromagnetic order. Our investigations may indicate a distinctly different nature of magnetism and superconductivity in the bulk nickelates than in the cuprates.

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Hirsch, J. E., e F. Marsiglio. "Hole superconductivity in infinite-layer nickelates". Physica C: Superconductivity and its Applications 566 (novembre 2019): 1353534. http://dx.doi.org/10.1016/j.physc.2019.1353534.

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Li, Yueying, Xiangbin Cai, Wenjie Sun, Jiangfeng Yang, Wei Guo, Zhengbin Gu, Ye Zhu e Yuefeng Nie. "Synthesis of Chemically Sharp Interface in NdNiO₃/SrTiO₃ Heterostructures". Chinese Physics Letters 40, n. 7 (1 giugno 2023): 076801. http://dx.doi.org/10.1088/0256-307x/40/7/076801.

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The nickel-based superconductivity provides a fascinating new platform to explore high-T c superconductivity. As the infinite-layer nickelates are obtained by removing the apical oxygens from the precursor perovskite phase, the crystalline quality of the perovskite phase is crucial in synthesizing high quality superconducting nickelates. Especially, cation-related defects, such as the Ruddlesden–Popper-type (RP-type) faults, are unlikely to disappear after the topotactic reduction process and should be avoided during the growth of the perovskite phase. Herein, using reactive molecular beam epitaxy, we report the atomic-scale engineering of the interface structure and demonstrate its impact in reducing crystalline defects in Nd-based nickelate/SrTiO3 heterostructures. A simultaneous deposition of stoichiometric Nd and Ni directly on SrTiO3 substrates results in prominent Nd vacancies and Ti diffusion at the interface and RP-type defects in nickelate films. In contrast, inserting an extra [NdO] monolayer before the simultaneous deposition of Nd and Ni forms a sharp interface and greatly eliminates RP-type defects in nickelate films. A possible explanation related to the polar discontinuity is also discussed. Our results provide an effective method to synthesize high-quality precursor perovskite phase for the investigation of the novel superconductivity in nickelates.

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Jin, Hyo-Sun, Warren E. Pickett e Kwan-Woo Lee. "A d ⁸ anti-Hund’s singlet insulator in an infinite-layer nickelate". Journal of Physics: Materials 5, n. 2 (1 aprile 2022): 024008. http://dx.doi.org/10.1088/2515-7639/ac6040.

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Abstract The status of nickelate superconductors in relation to cuprate high temperature superconductors is one of the concepts being discussed in high temperature superconductivity in correlated transition metal oxides. New additions to the class of infinite layer nickelates can provide essential input relating to connections or distinctions. A recently synthesized compound Ba2NiO2(AgSe)2, which contains isolated ‘infinite layer’ NiO2 planes, may lead to new insights. Our investigations have discovered that, at density functional theory mean field level, the ground state consists of an unusual e g singlet on the Ni2+ ion arising from large but separate Mott insulating gaps in both e g orbitals, but with different, anti-Hund’s, spin directions of their moments. This textured singlet incorporates at the least new physics, and potentially a new platform for nickelate superconductivity, which might be of an unconventional form for transition metal oxides due to the unconventional undoped state. We include in this paper a comparison of electronic structure parameters of Ba2NiO2(AgSe)2 with a better characterized infinite layer nickelate LaNiO2. We provide more analysis of the d 8 anti-Hund’s singlet that emerges in this compound, and consider a minimally correlated wavefunction for this singlet in an itinerant background, and begin discussion of excitations—real or virtual—that may figure into new electronic phases.

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Zhang, Yajun, Xu He e Philippe Ghosez. "Magnetic excitations in infinite-layer LaNiO₂". Applied Physics Letters 122, n. 15 (10 aprile 2023): 152401. http://dx.doi.org/10.1063/5.0141039.

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The observation of superconductivity in infinite-layer nickelates provides an appealing new platform to explore a superconducting mechanism. Rationalizing the ground state magnetic order and spin dynamics in undoped compounds are the foundation for understanding the superconducting mechanism. Here, magnetic properties of infinite-layer LaNiO2 are investigated and compared with cuprate analog CaCuO2 by combining first-principles and spin-wave theory calculations. We reveal that LaNiO2 exhibits quasi-two-dimensional (2D) antiferromagnetic (AFM) order that mimics that of cuprate superconductors. Moreover, the electronic origin of the quasi-2D AFM state and the simulated dispersion of magnetic excitations in LaNiO2 show strong resemblance to that of NdNiO2. The establishment of a direct connection with the cuprates from the electron, orbital, and spin degrees of freedom provides solid theoretical basis to elucidate the origin of superconductivity in infinite-layer nickelates.

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Ptok, Andrzej, Surajit Basak, Przemysław Piekarz e Andrzej M. Oleś. "Influence of f Electrons on the Electronic Band Structure of Rare-Earth Nickelates". Condensed Matter 8, n. 1 (8 febbraio 2023): 19. http://dx.doi.org/10.3390/condmat8010019.

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Recently, superconductivity was discovered in the infinite layer of hole-doped nickelates NdNiO2. Contrary to this, superconductivity in LaNiO2 is still under debate. This indicates the crucial role played by the f electrons on the electronic structure and the pairing mechanism of infinite-layer nickelates. Here, we discuss the role of the electron correlations in the f electron states and their influence on the electronic structure. We show that the lattice parameters are in good agreement with the experimental values, independent of the chosen parameters within the DFT+U approach. Increasing Coulomb interaction U tends to shift the f states away from the Fermi level. Surprisingly, independently of the position of f states with respect to the Fermi energy, these states play an important role in the electronic band structure, which can be reflected in the modification of the NdNiO2 effective models.

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Bernardini, F., V. Olevano, X. Blase e A. Cano. "Infinite-layer fluoro-nickelates as d 9 model materials". Journal of Physics: Materials 3, n. 3 (18 giugno 2020): 035003. http://dx.doi.org/10.1088/2515-7639/ab885d.

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Lokshin, Konstantin A., Daniel Mitchell, Maxim V. Lobanov, Viktor Struzhkin e Takeshi Egami. "Synthesis and Characterization of Pure Infinite Layer Ni⁺ Nickelates: LnNiO₂ (Ln = La, Nd, Pr) and La₃Ni₂O₆". ECS Journal of Solid State Science and Technology 11, n. 4 (1 aprile 2022): 044008. http://dx.doi.org/10.1149/2162-8777/ac6623.

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A synthesis method of pure low valence nickelates using a custom built H2 circulation apparatus is described. Pure infinite layer LnNiO2 (Ln = La, Nd, Pr) and La3Ni2O6 nickelates have been successfully prepared using this method and characterized by x-ray diffraction. Resistivity of La3Ni2O6 was measured as a function of temperature and pressure up to ∼2 GPa and revealed significant pressure-induced changes in both magnitude and pressure dependence of resistivity. The existence of a hidden insulator-metal transition in La3Ni2O6 is proposed at pressures above 100 GPa.

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Plienbumrung, Tharathep, Maria Daghofer, Jean-Baptiste Morée e Andrzej M. Oleś. "Single-Band versus Two-Band Description of Magnetism in Infinite-Layer Nickelates". Condensed Matter 8, n. 4 (6 dicembre 2023): 107. http://dx.doi.org/10.3390/condmat8040107.

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We present a weak-coupling analysis of magnetism in infinite-layer nickelates, where we compare a single-band description with a two-band model. Both models predict that (i) hybridization due to hopping is negligible, and (ii) the magnetic properties are characterized by very similar dynamic structure factors, S(k→,ω), at the points (π,π,0) and (π,π,π). This gives effectively a two-dimensional description of the magnetic properties.

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Hepting, M., D. Li, C. J. Jia, H. Lu, E. Paris, Y. Tseng, X. Feng et al. "Electronic structure of the parent compound of superconducting infinite-layer nickelates". Nature Materials 19, n. 4 (20 gennaio 2020): 381–85. http://dx.doi.org/10.1038/s41563-019-0585-z.

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Goodge, Berit H., Danfeng Li, Kyuho Lee, Motoki Osada, Bai Yang Wang, George A. Sawatzky, Harold Y. Hwang e Lena F. Kourkoutis. "Doping evolution of the Mott–Hubbard landscape in infinite-layer nickelates". Proceedings of the National Academy of Sciences 118, n. 2 (4 gennaio 2021): e2007683118. http://dx.doi.org/10.1073/pnas.2007683118.

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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.

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Lee, Kyuho, Berit H. Goodge, Danfeng Li, Motoki Osada, Bai Yang Wang, Yi Cui, Lena F. Kourkoutis e Harold Y. Hwang. "Aspects of the synthesis of thin film superconducting infinite-layer nickelates". APL Materials 8, n. 4 (1 aprile 2020): 041107. http://dx.doi.org/10.1063/5.0005103.

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Hepting, M., D. Li, C. J. Jia, H. Lu, E. Paris, Y. Tseng, X. Feng et al. "Publisher Correction: Electronic structure of the parent compound of superconducting infinite-layer nickelates". Nature Materials 19, n. 9 (13 luglio 2020): 1036. http://dx.doi.org/10.1038/s41563-020-0761-1.

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Bernardini, F., L. Iglesias, M. Bibes e A. Cano. "Thin-Film Aspects of Superconducting Nickelates". Frontiers in Physics 10 (9 febbraio 2022). http://dx.doi.org/10.3389/fphy.2022.828007.

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The discovery of superconductivity in infinite-layer nickelates has attracted much attention due to their association to the high-Tc cuprates. Cuprate superconductivity was first demonstrated in bulk samples and subsequently in thin films. In the nickelates, however, the situation has been reversed: although surging as a bulk phenomenon, nickelate superconductivity has only been reported in thin films so far. At the same time, the specifics of infinite-layer nickelates yield distinct interface and surface effects that determine their bulk vs thin-film behavior. In this paper, we provide an overview on these important aspects.

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Hsu, Y. T., M. Osada, B. Y. Wang, M. Berben, C. Duffy, S. P. Harvey, K. Lee et al. "Correlated Insulating Behavior in Infinite-Layer Nickelates". Frontiers in Physics 10 (24 marzo 2022). http://dx.doi.org/10.3389/fphy.2022.846639.

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Unlike their cuprate counterparts, the undoped nickelates are weak insulators without long-range antiferromagnetic order. Identifying the origin of this insulating behavior, found on both sides of the superconducting dome, is potentially a crucial step in the development of a coherent understanding of nickelate superconductivity. In this work, we study the normal-state resistivity of infinite-layer nickelates using high magnetic fields to suppress the superconductivity and examine the impact of disorder and doping on its overall temperature (T) dependence. In superconducting samples, the resistivity of Nd- and La-based nickelates continues to exhibit weakly insulating behavior with a magnitude and functional form similar to that found in underdoped electron-doped cuprates. We find a systematic evolution of the insulating behavior as a function of nominal hole doping across different rare-earth families, suggesting a pivotal role for strong electron interactions, and uncover a correlation between the suppression of the resistivity upturn and the robustness of the superconductivity. By contrast, we find very little correlation between the level of disorder and the magnitude and onset temperature of the resistivity upturn. Combining these experimental observations with previous Hall effect measurements on these two nickelate families, we consider various possible origins for this correlated insulator behavior and its evolution across their respective phase diagrams.

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Hu, Kejun, Qing Li, Dongsheng Song, Yingze Jia, Zhiyao Liang, Shuai Wang, Haifeng Du, Hai-Hu Wen e Binghui Ge. "Atomic scale disorder and reconstruction in bulk infinite-layer nickelates lacking superconductivity". Nature Communications 15, n. 1 (14 giugno 2024). http://dx.doi.org/10.1038/s41467-024-49533-1.

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AbstractThe recent discovery of superconductivity in infinite-layer nickelate films has sparked significant interest and expanded the realm of superconductors, in which the infinite-layer structure and proper chemical doping are both of the essence. Nonetheless, the reasons for the absence of superconductivity in bulk infinite-layer nickelates remain puzzling. Herein, we investigate atomic defects and electronic structures in bulk infinite-layer Nd0.8Sr0.2NiO2 using scanning transmission electron microscopy. Our observations reveal the presence of three-dimensional (3D) block-like structural domains resulting from intersecting defect structures, disrupting the continuity within crystal grains, which could be a crucial factor in giving rise to the insulating character and inhibiting the emergence of superconductivity. Moreover, the infinite-layer structure, without complete topotactic reduction, retains interstitial oxygen atoms on the Nd atomic plane in bulk nickelates, possibly further aggravating the local distortions of NiO2 planes and hindering the superconductivity. These findings shed light on the existence of structural and atomic defects in bulk nickelates and provide valuable insights into the influence of proper topotactic reduction and structural orders on superconductivity.

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Krieger, Guillaume, Aravind Raji, Laurent Schlur, Gilles Versini, Corinne Bouillet, Marc Lenertz, Jerome Robert, Alexandre Gloter, Nathalie Viart e Daniele PREZIOSI. "Synthesis of infinite-layer nickelates and influence of the capping-layer on magnetotransport". Journal of Physics D: Applied Physics, 23 novembre 2022. http://dx.doi.org/10.1088/1361-6463/aca54a.

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Abstract The recent discovery of a zero-resistance state in nickel-based compounds has generated a re-excitement about the long-standing problem in condensed matter of high-critical-temperature superconductivity, in light of the analogies between infinite-layer nickelates and cuprates. However, despite some formal valence and crystal symmetry analogies, the electronic properties of infinite-layer nickelates are remarkably original accounting, among other properties, of a unique Nd5d-Ni3d hybridization. This designates infinite-layer nickelates as a new class of oxide superconductors which should be considered on their own. Here we show our approach to synthesize Nd1-xSrxNiO2 (x = 0, 0.05 and 0.2) thin films with and without a SrTiO3 capping-layer. Our infinite-layer nickelate thin films were characterized by very smooth and step-terraced surface morphologies. Angle-dependent anisotropic magnetoresistance measurements performed with a magnetic field rotating in-plane or out-of-plane with respect to the sample surface, rendered important information about the magnetic properties of undoped SrTiO3-capped and uncapped samples. Our results point at a key role of the capping-layer in controlling the magnitude and the anisotropy of the anisotropic magnetoresistance properties. We discuss this control in terms of a combined effect between the Nd-Ni hybridization and an intra-atomic exchange coupling between the Nd-4f and Nd-5d states, the latter essentially contributing to the (magneto)transport. Further studies foresee the influence of the capping layer on infinite-layer nickelates with no magnetic rare-earth.

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Mitchell, J. F. "A Nickelate Renaissance". Frontiers in Physics 9 (23 dicembre 2021). http://dx.doi.org/10.3389/fphy.2021.813483.

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The 2019 discovery of high temperature superconductivity in layered nickelate films, Nd1-xSrNiO2, 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 this d9 configuration in Ni1+ has proven to be a synthetic challenge, only recently overcome. These crystalline specimens are opening windows that shed new light on the cuprate-nickelate analogy and reveal nuances that leave the relationship between cuprates and nickelates very much an area open to debate. This Perspective gives a qualitative, phenomenological account of these newly discovered superconductors and multilayer members of the infinite layer nickelate family. The focus is on our current understanding of electronic and magnetic properties of these materials as well as some future opportunities, explored from the viewpoint of synthetic challenges and some suggested developments in materials discovery and growth to make further progress in this rejuvenated field.

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Di Cataldo, Simone, Paul Worm, Jan M. Tomczak, Liang Si e Karsten Held. "Unconventional superconductivity without doping in infinite-layer nickelates under pressure". Nature Communications 15, n. 1 (10 maggio 2024). http://dx.doi.org/10.1038/s41467-024-48169-5.

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AbstractHigh-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 Ni $${d}_{{x}^{2}-{y}^{2}}$$ d 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.

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Xu, Minghui, Yan Zhao, Yu Chen, Xiang Ding, Huaqian Leng, Zheng Hu, Xiaoqiang Wu et al. "Robust Superconductivity in Infinite‐Layer Nickelates". Advanced Science, 29 aprile 2024. http://dx.doi.org/10.1002/advs.202305252.

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AbstractThe recent discovery of nickelate superconductivity represents an important step toward understanding the four‐decade mastery of unconventional high‐temperature superconductivity. However, the synthesis of the infinite‐layer nickelate superconductors shows great challenges. Particularly, surface capping layers are usually unitized to facilitate the sample synthesis. This leads to an important question whether nickelate superconductors with d9 configuration and ultralow valence of Ni1+ are in metastable state and whether nickelate superconductivity can be robust? In this work, a series of redox cycling experiments are performed across the phase transition between perovskite Nd0.8Sr0.2NiO3 and infinite‐layer Nd0.8Sr0.2NiO2. The infinite‐layer Nd0.8Sr0.2NiO2 is quite robust in the redox environment and can survive the cycling experiments with unchanged crystallographic quality. However, as the cycling number goes on, the perovskite Nd0.8Sr0.2NiO3 shows structural degradation, suggesting stability of nickelate superconductivity is not restricted by the ultralow valence of Ni1+, but by the quality of its perovskite precursor. The observed robustness of infinite‐layer Nd0.8Sr0.2NiO2 up to ten redox cycles further indicates that if an ideal high‐quality perovskite precursor can be obtained, infinite‐layer nickelate superconductivity can be very stable and sustainable under environmental conditions. This work provides important implications for potential device applications for nickelate superconductors.

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Ding, Xiang, Yu Fan, Xiaoxiao Wang, Chihao Li, Zhitong An, Jiahao Ye, Shenglin Tang et al. "Cuprate-like Electronic Structures in Infinite-Layer Nickelates with Substantial Hole Dopings". National Science Review, 4 giugno 2024. http://dx.doi.org/10.1093/nsr/nwae194.

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Abstract The 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 its superconducting mechanisms based on the calculated electronic structure, and the multiple Fermi surfaces and multiple orbitals involved create complications and controversial conclusions. Over the past 5 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 (ARPES). We find that their Fermi surfaces consist of a quasi-two-dimensional (quasi-2D) hole pocket and a three-dimensional (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.

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Chen, Dachuan, Paul Worm, Liang Si, Chunxiao Zhang, Fenglin Deng, Peiheng Jiang e Zhicheng Zhong. "Electronic structure of cuprate-nickelate infinite-layer heterostructure". Chinese Physics B, 9 maggio 2023. http://dx.doi.org/10.1088/1674-1056/acd368.

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Abstract The discovery superconductivity in Sr/Ca-doped infinite-layer nickelates Nd(La)NiO2 thin films inspired extensively experimental and theoretical research. However, the research on the possibilities of enhanced critical temperature by interface heterostructuring is still lacking. Due to the similarities of the crystal structure and band structure of infinite-layer nickelate LaNiO2 and cuprate CaCuO2, we investigate the crystal, electronic and magnetic properties of LaNiO2:CaCuO2 heterostructure using density functional theory and dynamical mean-field theory. Our theoretical results demonstrate that, even a very weak inter-layer z-direction bond is formed, an intrinsic charge transfer between Cu-3d x 2-y 2 and Ni-3d x 2-y 2 orbital is obtained. The weak interlayer hopping between Cu and Ni leaves a parallel band contributed by Ni/Cu-3d x 2-y 2 orbitals near the Fermi energy. Such an infinite-layer heterostructure with negligible interlayer interaction and robust charge transfer opens a new way for interface engineering and nickelate superconductors.

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Parzyck, C. T., N. K. Gupta, Y. Wu, V. Anil, L. Bhatt, M. Bouliane, R. Gong et al. "Absence of 3a0 charge density wave order in the infinite-layer nickelate NdNiO2". Nature Materials, 26 gennaio 2024. http://dx.doi.org/10.1038/s41563-024-01797-0.

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AbstractA hallmark of many unconventional superconductors is the presence of many-body interactions that give rise to broken-symmetry states intertwined with superconductivity. Recent resonant soft X-ray scattering experiments report commensurate 3a0 charge density wave order in infinite-layer nickelates, which has important implications regarding the universal interplay between charge order and superconductivity in both cuprates and nickelates. Here we present X-ray scattering and spectroscopy measurements on a series of NdNiO2+x samples, which reveal that the signatures of charge density wave order are absent in fully reduced, single-phase NdNiO2. The 3a0 superlattice peak instead originates from a partially reduced impurity phase where excess apical oxygens form ordered rows with three-unit-cell periodicity. The absence of any observable charge density wave order in NdNiO2 highlights a crucial difference between the phase diagrams of cuprate and nickelate superconductors.

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Kang, Byungkyun, Corey Melnick, Patrick Semon, Siheon Ryee, Myung Joon Han, Gabriel Kotliar e Sangkook Choi. "Infinite-layer nickelates as Ni-eg Hund’s metals". npj Quantum Materials 8, n. 1 (12 luglio 2023). http://dx.doi.org/10.1038/s41535-023-00568-5.

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AbstractThe recent and exciting discovery of superconductivity in the hole-doped infinite-layer nickelate Nd1−δSrδNiO2 draws strong attention to correlated quantum materials. From a theoretical view point, this class of unconventional superconducting materials provides an opportunity to unveil a physics hidden in correlated quantum materials. Here we study the temperature and doping dependence of the local spectrum as well as the charge, spin and orbital susceptibilities from first principles. By using ab initio LQSGW+DMFT methodology, we show that onsite Hund’s coupling in Ni-d orbitals gives rise to multiple signatures of Hund’s metallic phase in Ni-eg orbitals. The proposed picture of the nickelates as an eg (two orbital) Hund’s metal differs from the picture of the Fe-based superconductors as a five orbital Hund’s metal as well as the picture of the cuprates as doped charge transfer insulators. Our finding uncover a new class of the Hund’s metals and has potential implications for the broad range of correlated two orbital systems away from half-filling.

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Ji, Haoran, Yi Liu, Yanan Li, Xiang Ding, Zheyuan Xie, Chengcheng Ji, Shichao Qi et al. "Rotational symmetry breaking in superconducting nickelate Nd0.8Sr0.2NiO2 films". Nature Communications 14, n. 1 (7 novembre 2023). http://dx.doi.org/10.1038/s41467-023-42988-8.

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AbstractThe infinite-layer nickelates, isostructural to the high-Tc cuprate superconductors, have emerged as a promising platform to host unconventional superconductivity and stimulated growing interest in the condensed matter community. Despite considerable attention, the superconducting pairing symmetry of the nickelate superconductors, the fundamental characteristic of a superconducting state, is still under debate. Moreover, the strong electronic correlation in the nickelates may give rise to a rich phase diagram, where the underlying interplay between the superconductivity and other emerging quantum states with broken symmetry is awaiting exploration. Here, we study the angular dependence of the transport properties of the infinite-layer nickelate Nd0.8Sr0.2NiO2 superconducting films with Corbino-disk configuration. The azimuthal angular dependence of the magnetoresistance (R(φ)) manifests the rotational symmetry breaking from isotropy to four-fold (C4) anisotropy with increasing magnetic field, revealing a symmetry-breaking phase transition. Approaching the low-temperature and large-magnetic-field regime, an additional two-fold (C2) symmetric component in the R(φ) curves and an anomalous upturn of the temperature-dependent critical field are observed simultaneously, suggesting the emergence of an exotic electronic phase. Our work uncovers the evolution of the quantum states with different rotational symmetries in nickelate superconductors and provides deep insight into their global phase diagram.

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30

Yan, Shengjun, Wei Mao, Wenjie Sun, Yueying Li, Haoying Sun, Jiangfeng Yang, Bo Hao et al. "Superconductivity in Freestanding Infinite‐Layer Nickelate Membranes". Advanced Materials, 7 giugno 2024. http://dx.doi.org/10.1002/adma.202402916.

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AbstractThe 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, we report the synthesis of the superconducting freestanding La0.8Sr0.2NiO2 membranes (TCZero = 10.6 K), emphasizing the crucial roles of the interface engineering in the precursor phase film growth and the quick transfer process in achieving superconductivity. Our work offers a new versatile platform for investigating the superconductivity in nickelates, such as the pairing symmetry via constructing Josephson tunneling junctions and higher Tc values via high‐pressure experiments.This article is protected by copyright. All rights reserved

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Lei, Yihan, Yanghe Wang, Jiahao Song, Jinxin Ge, Dirui Wu, Yingli Zhang e Changjian Li. "Probing Nickelate Superconductors at the Atomic Scale: A STEM Review". Chinese Physics B, 1 agosto 2024. http://dx.doi.org/10.1088/1674-1056/ad6a0d.

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Abstract The discovery of nickelate superconductors, including doped infinite-layer (IL) nickelates RNiO2 (R= La, Pr, Nd), layered square-planar nickelate Nd6Ni5O12, and the Ruddlesden-Popper (RP) phase La3Ni2O7, has spurred immense interest in fundamental research and potential applications. Scanning transmission electron microscopy (STEM) has proven crucial for understanding structure-property correlations in these diverse nickelate superconducting systems. In this review, we summarize the key findings from various modes of STEM, elucidating the mechanism of different nickelate superconductors. We also discuss future perspectives on emerging STEM techniques for unraveling the pairing mechanism in the “nickel age” of superconductivity.

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Li, Yueying, Wenjie Sun, Jiangfeng Yang, Xiangbin Cai, Wei Guo, Zhengbin Gu, Ye Zhu e Yuefeng Nie. "Impact of Cation Stoichiometry on the Crystalline Structure and Superconductivity in Nickelates". Frontiers in Physics 9 (3 settembre 2021). http://dx.doi.org/10.3389/fphy.2021.719534.

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The recent discovery of superconductivity in infinite-layer nickelate films has aroused great interest since it provides a new platform to explore the mechanism of high-temperature superconductivity. However, superconductivity only appears in the thin film form and synthesizing superconducting nickelate films is extremely challenging, limiting the in-depth studies on this compound. Here, we explore the critical parameters in the growth of high-quality nickelate films using molecular beam epitaxy. We found that stoichiometry is crucial in optimizing the crystalline structure and realizing superconductivity in nickelate films. In precursor NdNiO3 films, optimal stoichiometry of cations yields the most compact lattice while off-stoichiometry of cations causes obvious lattice expansion, influencing the subsequent topotactic reduction and the emergence of superconductivity in infinite-layer nickelates. Surprisingly, in-situ reflection high energy electron diffraction indicates that some impurity phases always appear once Sr ions are doped into NdNiO3 although the X-ray diffraction data are of high quality. While these impurity phases do not seem to suppress the superconductivity, their impacts on the electronic and magnetic structure deserve further studies. Our work demonstrates and highlights the significance of cation stoichiometry in the superconducting nickelate family.

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Lane, Christopher, Ruiqi Zhang, Bernardo Barbiellini, Robert S. Markiewicz, Arun Bansil, Jianwei Sun e Jian-Xin Zhu. "Competing incommensurate spin fluctuations and magnetic excitations in infinite-layer nickelate superconductors". Communications Physics 6, n. 1 (3 maggio 2023). http://dx.doi.org/10.1038/s42005-023-01213-0.

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AbstractThe recently discovered infinite-layer nickelates show great promise in helping to disentangle the various cooperative mechanisms responsible for high-temperature superconductivity. However, lack of antiferromagnetic order in the pristine nickelates presents a challenge for connecting the physics of the cuprates and nickelates. Here, by using a quantum many-body Green’s function-based approach to treat the electronic and magnetic structures, we unveil the presence of many two- and three-dimensional magnetic stripe instabilities that are shown to persist across the phase diagram of LaNiO2. Our analysis indicates that the magnetic properties of the infinite-layer nickelates are closer to those of the doped cuprates, which host a stripe ground state, rather than the undoped cuprates. The computed longitudinal-spin, transverse-spin, and charge spectra of LaNiO2 are found to contain an admixture of contributions from localized and itinerant carriers. Theoretically obtained dispersion of magnetic excitations (spin-flip) is found to be in good accord with the results of recent resonant inelastic X-ray scattering experiments. Our study gives insight into the origin of strong magnetic competition in the infinite-layer nickelates and their relationship with the cuprates.

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Gao, Qiang, Shiyu Fan, Qisi Wang, Jiarui Li, Xiaolin Ren, Izabela Biało, Annabella Drewanowski et al. "Magnetic excitations in strained infinite-layer nickelate PrNiO2 films". Nature Communications 15, n. 1 (3 luglio 2024). http://dx.doi.org/10.1038/s41467-024-49940-4.

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AbstractStrongly correlated materials respond sensitively to external perturbations such as strain, pressure, and doping. In the recently discovered superconducting infinite-layer nickelates, the superconducting transition temperature can be enhanced via only ~ 1% compressive strain-tuning with the root of such enhancement still being elusive. Using resonant inelastic x-ray scattering (RIXS), we investigate the magnetic excitations in infinite-layer PrNiO2 thin films grown on two different substrates, namely SrTiO3 (STO) and (LaAlO3)0.3(Sr2TaAlO6)0.7 (LSAT) enforcing different strain on the nickelates films. The magnon bandwidth of PrNiO2 shows only marginal response to strain-tuning, in sharp contrast to the enhancement of the superconducting transition temperature Tc in the doped superconducting samples. These results suggest the bandwidth of spin excitations of the parent compounds is similar under strain while Tc in the doped ones is not, and thus provide important empirics for the understanding of superconductivity in infinite-layer nickelates.

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Been, Emily M., Kuan H. Hsu, Yi Hu, Brian Moritz, Yi Cui, Chunjing Jia e Thomas P. Devereaux. "On the Nature of Valence Charge and Spin Excitations via Multi-Orbital Hubbard Models for Infinite-Layer Nickelates". Frontiers in Physics 10 (28 febbraio 2022). http://dx.doi.org/10.3389/fphy.2022.836959.

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Building upon the recent progress on the intriguing underlying physics for the newly discovered infinite-layer nickelates, in this article we review an examination of valence charge and spin excitations via multi-orbital Hubbard models as way to determine the fundamental building blocks for Hamiltonians that can describe the low energy properties of infinite-layer nickelates. We summarize key results from density-functional approaches, and apply them to the study of x-ray absorption to determine the valence ground states of infinite-layer nickelates in their parent form, and show that a fundamental d9 configuration as in the cuprates is incompatible with a self-doped ground state having holes in both dx2−y2 and a rare-earth-derived axial orbital. When doped, we determine that the rare-earth-derived orbitals empty and additional holes form low spin (S = 0) d8 Ni states, which can be well-described as a doped single-band Hubbard model. Using exact diagonalization for a 2-orbital model involving Ni and rare-earth orbitals, we find clear magnons at 1/2 filling that persist when doped, albeit with larger damping, and with a dependence on the precise orbital energy separation between the Ni- and rare-earth-derived orbitals. Taken together, a full two-band model for infinite-layer nickelates can well describe the valence charge and spin excitations observed experimentally.

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Li, Yueying, Xiangbin Cai, Wenjie Sun, Jiangfeng Yang, Wei Guo, Zhengbin Gu, Ye Zhu e Yuefeng Nie. "Synthesis of Chemically Sharp Interface in NdNiO₃/SrTiO₃ Heterostructures". Chinese Physics Letters, 29 maggio 2023. http://dx.doi.org/10.1088/0256-307x/40/6/076801.

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Abstract The nickel-based superconductivity provides a fascinating new platform to explore the high-T c superconductivity. As the infinite-layer nickelates are obtained by removing the apical oxygens from the precursor perovskite phase, the crystalline quality of the perovskite phase is crucial in synthesizing high quality superconducting nickelates. Especially, cation-related defects, such as the Ruddlesden-Popper-type (RP-type) faults, are unlikely to disappear after the topotactic reduction process and should be avoided during the growth of the perovskite phase. Herein, using reactive molecular beam epitaxy, we report the atomic-scale engineering of the interface structure and demonstrate its impact in reducing crystalline defects in Nd-based nickelate/SrTiO3 heterostructures. A simultaneous deposition of stoichiometric Nd and Ni directly on SrTiO3 substrate results in prominent Nd vacancies and Ti diffusion at the interface and RP-type defects in nickelate films. In contrast, inserting an extra [NdO] monolayer before the simultaneous deposition of Nd and Ni forms a sharp interface and greatly eliminates RP-type defects in nickelate films. A possible explanation related to the polar discontinuity is also discussed. Our results provide an effective method to synthesize high-quality precursor perovskite phase for the investigation of the novel superconductivity in nickelates.

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Puphal, P., V. Sundaramurthy, V. Zimmermann, K. Küster, U. Starke, M. Isobe, B. Keimer e M. Hepting. "Phase formation in hole- and electron-doped rare-earth nickelate single crystals". APL Materials 11, n. 8 (1 agosto 2023). http://dx.doi.org/10.1063/5.0160912.

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The recent discovery of superconductivity in hole-doped infinite-layer nickelates has triggered a great interest in the synthesis of novel nickelate phases, which have primarily been examined in thin film samples. Here, we report the high-pressure optical floating zone growth of various perovskite and perovskite-derived rare-earth nickelate single-crystals and investigate the effects of hole-, electron-, and self-doping. For hole-doping with Ca and Sr, we observe phase separations during the growth process when a substitution level of 8% is exceeded. A similar trend emerges for electron-doping with Ce and Zr. Employing lower doping levels allows us to grow sizable crystals in the perovskite phase, which exhibit significantly different electronic and magnetic properties than the undoped parent compounds, such as decreased resistivity and a suppressed magnetic response. Our insights into the doping-dependent phase formation and the resulting properties of the synthesized crystals reveal limitations and opportunities for the exploration and manipulation of electronic states in rare-earth nickelates.

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Talantsev, Evgeny F. "Quantifying interaction mechanism in infinite layer nickelate superconductors". Journal of Applied Physics 134, n. 11 (19 settembre 2023). http://dx.doi.org/10.1063/5.0166329.

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The connection between the long-range antiferromagnetic order in cuprates and the high-temperature superconductivity is a scientific problem that has yet to be solved after nearly four decades. The properties and difficulties of describing nickelate superconductors are similar to those of cuprates. Recently, Fowlie et al. [Nat. Phys. 18, 1043 (2022)] aimed to detect the antiferromagnetic order in R1−xSrxNiO2 (R = Nd, Pr, La; x ∼ 0, 0.2) films by using the muon spin rotation (μSR) technique. The research group reported the presence of short-range antiferromagnetic order in every nickelate studied. Here, our goal was to prove that this interaction is present in the nickelate films. We did this by analyzing the temperature dependent resistivity, ρ(T), data from the research group. Global ρ(T) data fits to the advanced Bloch–Grüneisen model showed that each of the R1−xSrxNiO2 compounds can be characterized by a unique power-law exponent, p (where p = 2 for the electron–electron scattering, p = 3 for the electron–magnon scattering, and p = 5 for the electron–phonon scattering), and global characteristic temperature, Tω (which has the meaning of the Debye temperature at p = 5). We found that p = 2.0 in Nd- and Pr-based compounds and p = 1.3 for La-based compounds. The latter value does not have any interpretation within established theoretical models. We also analyzed ρ(T) data for Nd1–xSrxNiO2(0.125≤x≤0.325) reported by Lee et al. [Nature 619, 288 (2023)]. Our analysis of nickelates led us to conclude that a new theoretical model is needed to describe ρ(T) in materials exhibiting a short-range antiferromagnetic order.

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Zeng, Shengwei, Changjian Li, Lin Er Chow, Yu Cao, Zhaoting Zhang, Chi Sin Tang, Xinmao Yin et al. "Superconductivity in infinite-layer nickelate La _1−x Ca _x NiO ₂ thin films". Science Advances 8, n. 7 (18 febbraio 2022). http://dx.doi.org/10.1126/sciadv.abl9927.

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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 La 1− x Ca x NiO 2 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.

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Bernardini, F., A. Bosin e A. Cano. "Geometric effects in the infinite-layer nickelates". Physical Review Materials 6, n. 4 (28 aprile 2022). http://dx.doi.org/10.1103/physrevmaterials.6.044807.

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Fowlie, Jennifer, Marios Hadjimichael, Maria M. Martins, Danfeng Li, Motoki Osada, Bai Yang Wang, Kyuho Lee et al. "Intrinsic magnetism in superconducting infinite-layer nickelates". Nature Physics, 1 agosto 2022. http://dx.doi.org/10.1038/s41567-022-01684-y.

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Ortiz, R. A., H. Menke, F. Misják, D. T. Mantadakis, K. Fürsich, E. Schierle, G. Logvenov et al. "Superlattice approach to doping infinite-layer nickelates". Physical Review B 104, n. 16 (22 ottobre 2021). http://dx.doi.org/10.1103/physrevb.104.165137.

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43

Yang, Zhen, Kui‐juan Jin, Yulin Gan, Cheng Ma, Zhicheng Zhong, Ye Yuan, Chen Ge et al. "Photoinduced Phase Transition in Infinite‐Layer Nickelates". Small, 25 giugno 2023. http://dx.doi.org/10.1002/smll.202304146.

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44

Parzyck, C. T., V. Anil, Y. Wu, B. H. Goodge, M. Roddy, L. F. Kourkoutis, D. G. Schlom e K. M. Shen. "Synthesis of thin film infinite-layer nickelates by atomic hydrogen reduction: Clarifying the role of the capping layer". APL Materials 12, n. 3 (1 marzo 2024). http://dx.doi.org/10.1063/5.0197304.

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We present an integrated procedure for the synthesis of infinite-layer nickelates using molecular-beam epitaxy with gas-phase reduction by atomic hydrogen. We first discuss challenges in the growth and characterization of perovskite NdNiO3/SrTiO3, arising from post growth crack formation in stoichiometric films. We then detail a procedure for fully reducing NdNiO3 films to the infinite-layer phase, NdNiO2, using atomic hydrogen; the resulting films display excellent structural quality, smooth surfaces, and lower residual resistivities than films reduced by other methods. We utilize the in situ nature of this technique to investigate the role that SrTiO3 capping layers play in the reduction process, illustrating their importance in preventing the formation of secondary phases at the exposed nickelate surface. A comparative bulk- and surface-sensitive study indicates that the formation of a polycrystalline crust on the film surface serves to limit the reduction process.

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Chen, Hanghui, Yi-feng Yang, Guang-Ming Zhang e Hongquan Liu. "An electronic origin of charge order in infinite-layer nickelates". Nature Communications 14, n. 1 (6 settembre 2023). http://dx.doi.org/10.1038/s41467-023-41236-3.

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AbstractA charge order (CO) with a wavevector $${{{{{{{\bf{q}}}}}}}}\simeq \left(\frac{1}{3},\, 0,\, 0\right)$$ q ≃ 1 3 , 0 , 0 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, $${{{{{{{\bf{q}}}}}}}}=\left(\frac{1}{3},\, 0,\, 0\right)$$ q = 1 3 , 0 , 0 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.

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Wu, Yu-Mi, Pascal Puphal, Masahiko Isobe, Bernhard Keimer, Matthias Hepting, Y. Eren Suyolcu e Peter A. van Aken. "Unraveling nano-scale effects of topotactic reduction in LaNiO2 crystals". APL Materials 12, n. 9 (1 settembre 2024). http://dx.doi.org/10.1063/5.0227732.

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Infinite-layer nickelates stand as a promising frontier in the exploration of unconventional superconductivity. Their synthesis through topotactic oxygen reduction from the parent perovskite phase remains a complex and elusive process. This study delves into the nano-scale effects of the topotactic lattice transformation within LaNiO2 crystals. Leveraging high-resolution scanning transmission electron microscopy and spectroscopy, our investigations uncover a panorama of structural alterations, including grain boundaries and coherent twin boundaries, triggered by reduction-induced transformations. In addition, our analyses unveil the formation of an oxygen-rich disordered transition phase encircling impurities and pervading crystalline domains and the internal strain is accommodated by grain boundary formation. By unraveling these nano-scale effects, our findings provide insights into the microscopic intricacies of the topotactic reduction process elucidating the transition from the perovskite to the infinite-layer phase within nickelate bulk crystals.

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Gutiérrez‐Llorente, Araceli, Aravind Raji, Dongxin Zhang, Laurent Divay, Alexandre Gloter, Fernando Gallego, Christophe Galindo, Manuel Bibes e Lucía Iglesias. "Toward Reliable Synthesis of Superconducting Infinite Layer Nickelate Thin Films by Topochemical Reduction". Advanced Science, 18 aprile 2024. http://dx.doi.org/10.1002/advs.202309092.

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AbstractInfinite 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.

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Ren, Xiaolin, Jiarui Li, Wei-Chih Chen, Qiang Gao, Joshua J. Sanchez, Jordyn Hales, Hailan Luo et al. "Possible strain-induced enhancement of the superconducting onset transition temperature in infinite-layer nickelates". Communications Physics 6, n. 1 (27 novembre 2023). http://dx.doi.org/10.1038/s42005-023-01464-x.

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AbstractThe mechanism of unconventional superconductivity in correlated materials remains a great challenge in condensed matter physics. The recent discovery of superconductivity in infinite-layer nickelates, as an analog to high-Tc cuprates, has opened a new route to tackle this challenge. By growing 8 nm Pr0.8Sr0.2NiO2 films on the (LaAlO3)0.3(Sr2AlTaO6)0.7 substrate, we successfully raise the superconducting onset transition temperature Tc in the widely studied SrTiO3-substrated nickelates from 9 K into 15 K, which indicates compressive strain is an efficient protocol to further enhance superconductivity in infinite-layer nickelates. Additionally, the x-ray absorption spectroscopy, combined with the first-principles and many-body simulations, suggest a crucial role of the hybridization between Ni and O orbitals in the unconventional pairing. These results also suggest the increase of Tc be driven by the change of charge-transfer nature that would narrow the origin of general unconventional superconductivity in correlated materials to the covalence of transition metals and ligands.

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Wang, Bai Yang, Tiffany C. Wang, Yu-Te Hsu, Motoki Osada, Kyuho Lee, Chunjing Jia, Caitlin Duffy et al. "Effects of rare-earth magnetism on the superconducting upper critical field in infinite-layer nickelates". Science Advances 9, n. 20 (19 maggio 2023). http://dx.doi.org/10.1126/sciadv.adf6655.

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The search for superconductivity in infinite-layer nickelates was motivated by analogy to the cuprates, and this perspective has framed much of the initial consideration of this material. However, a growing number of studies have highlighted the involvement of rare-earth orbitals; in that context, the consequences of varying the rare-earth element in the superconducting nickelates have been much debated. Here, we show notable differences in the magnitude and anisotropy of the superconducting upper critical field across the La-, Pr-, and Nd-nickelates. These distinctions originate from the 4 f electron characteristics of the rare-earth ions in the lattice: They are absent for La 3+ , nonmagnetic for the Pr 3+ singlet ground state, and magnetic for the Nd 3+ Kramer’s doublet. The unique polar and azimuthal angle-dependent magnetoresistance found in the Nd-nickelates can be understood to arise from the magnetic contribution of the Nd 3+ 4 f moments. Such robust and tunable superconductivity suggests potential in future high-field applications.

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Kreisel, Andreas, Brian M. Andersen, Astrid T. Rømer, Ilya M. Eremin e Frank Lechermann. "Superconducting Instabilities in Strongly Correlated Infinite-Layer Nickelates". Physical Review Letters 129, n. 7 (11 agosto 2022). http://dx.doi.org/10.1103/physrevlett.129.077002.

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