Auswahl der wissenschaftlichen Literatur zum Thema „Infinite-layer nickelates“

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.

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.

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.

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.

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.

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.

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.

Les systèmes de matériaux tels que les oxydes de métaux de transition (TMO) présentent des fonctionnalités robustes, fortement liées à leurs degrés de liberté électroniques et structurels. Il est possible de stabiliser de nouvelles structures TMO présentant de nouvelles propriétés en contrôlant ces degrés de liberté, comme c'est le cas des nickelates supraconducteurs à couche infinie (IL), des gaz d'électrons bidimensionnels (2DEG) dans le KTaO₃, etc. Dans cette thèse, une combinaison de techniques complémentaires a été employée, à savoir la microscopie électronique à transmission à balayage (STEM), la spectroscopie de perte d'énergie électronique (EELS), la microscopie électronique à quatre dimensions (4D), la spectroscopie de photoémission de rayons X durs (HAXPES), des calculs ab-initio complémentaires et des expériences de diffusion des rayons X pour élucider les origines de la physique complexe présentée par ces systèmes. Cette thèse commence par explorer les origines des ordres concurrents tels que l'ordre de charge périodique 3a₀ dans les IL-nickelates, observés dans les expériences de diffusion des rayons X. Ici, grâce à une analyse combinée avec STEM-EELS, 4D-STEM et HAXPES, cet ordre particulier s'est avéré provenir d'un ordre {303}pc particulier de vacances d'oxygène dans le film mince de nickelate. Une exploration plus poussée a permis de découvrir une nouvelle phase de nickelate coordonnée à trois composants et ordonnée en valence, de formule A₉B₉O₂₂, qui est un intermédiaire entre la pérovskite mère et le nickelate IL réduit. Compte tenu de la contribution possible de la nanostructure de l'interface de la couche mince du substrat à la supraconductivité, une étude combinée avec STEM-EELS, 4D-STEM, HAXPES et des calculs ab-initio de l'interface a été réalisée. Il a été constaté qu'il existe des interfaces de type n et de type p très différentes dans les échantillons supraconducteurs. Cette non-universalité de la nanostructure de l'interface dans les échantillons supraconducteurs de nickelate d'IL a permis de dissocier l'influence de l'interface et la supraconductivité dans les nickelates d'IL. Cela a suscité l'intérêt pour l'étude d'une interface d'oxyde, où l'interface est supraconductrice, et dans la partie suivante, les 2DEGs supraconducteurs dans AlOₓ/KTaO₃ ont été explorés. L'aspect électronique et structurel de l'interface AlOₓ/KTaO₃ contrôlant les 2DEG a été étudié en utilisant STEM-EELS et HAXPES. Une carte de l'espace réel du 2DEG a été obtenue, ainsi que des indications d'une expansion significative de la cellule unitaire dans cette région. La méthode des ondes stationnaires (SW) résolues en couches a également indiqué un déplacement polaire substantiel pour les atomes de Ta réduits à l'interface. Alors que cette thèse explore les aspects structurels et électroniques de systèmes spécifiques, l'approche combinée utilisant les électrons (STEM-EELS, 4D-STEM) et les rayons X (HAXPES) peut être appliquée à une large gamme de systèmes TMO. Elle permet d'élucider les origines des propriétés complexes qu'ils présentent
Material systems such as transition metal oxides (TMO) exhibits robust functionalities, strongly coupled with its electronic and structural degrees of freedom. One can stabilize novel TMO structures hosting novel properties by controlling these degrees of freedom, as is the case of superconducting infinite-layer (IL) nickelates, two-dimensional electron gases (2DEGs) in KTaO₃, etc. In this thesis, a combination of complementary techniques has been employed that is the scanning transmission electron microscopy (STEM)- electron energy loss spectroscopy (EELS), four dimensional (4D)-STEM, hard X-ray photoemission spectroscopy (HAXPES) and complementary ab-initio calculations and X-ray scattering experiments to elucidate the origins of the complex physics exhibited by these systems. This thesis begins by exploring the origins of competing orders such as the 3a₀ periodic charge order in IL-nickelates, observed in X-ray scattering experiments. Here, through a combined analysis with STEM-EELS, 4D-STEM and HAXPES, this particular ordering was found to be originating from a particular {303}pc ordering of oxygen vacancies in the nickelate thin-film. Further exploration resulted in the discovery of a new valence ordered and tri-component coordinated nickelate phase with the formula A₉B₉O₂₂, that is an intermediate between the parent perovskite and reduced IL-nickelate. Considering the possible contribution of the substrate thin film interface nanostructure to the superconductivity, a combined study with STEM-EELS, 4D-STEM, HAXPES and ab-initio calculations of the interface was done. It was found that there are highly different n-type and p-type interfaces exists in superconducting samples. This non-universality of interface nanostructure in superconducting IL-nickelate samples, decoupled the interface influence and superconductivity in IL-nickelates. This generated interest in studying an oxide interface, where the interface is superconducting, and in the followed part, the superconducting 2DEGs in AlOₓ/KTaO₃ was explored. The electronic and structural aspect of the AlOₓ/KTaO₃ interface controlling the 2DEG was studied using STEM-EELS and HAXPES. A real space map of the 2DEG was obtained, along with indications of a significant unit cell expansion in this region. Layer resolved standing wave (SW)-HAXPES also indicated a substantial polar like displacement for the reduced Ta atoms at the interface. While this thesis explores the structural and electronic aspects of specific systems, the combined approach using electrons (STEM-EELS, 4D-STEM) and X-rays (HAXPES) can be applied to a wide range of TMO systems. It can unravel the origins of complex properties exhibited by them

The phase diagrams of the heavy fermion, transition-metal (copper-, nickel-) oxides materials have a wide variety of different phases. It is believed that, the strong correlation among electrons governs most of the phases in these materials, and hence, they are called strongly correlated systems. The purpose of the thesis is to understand the microscopic origin of the superconductivity in the strongly correlated systems and subsequently compare/predict the experimental outcomes of the theory. It is well known that heavy fermion, transition-metal oxide systems are unconventional superconductors. However, contrary to the old results, new experiments performed on the heavy fermion systems point towards a fully gapped conventional superconductivity. Similarly, in the cuprate superconductors, the d-wave symmetry of the superconducting order parameter is well known in the copper-oxide layer. However, counter-evidence of nodeless superconductivity is observed in the underdoped region of cuprates. Recently superconductivity is observed in infinite-layer nickelates NdNiO2 and PrNiO2, a maximum Tc ~ 15 K. Based on the above-mentioned experimental motivations, we formulate a new mechanism of superconductivity in the heavy fermion system where attractive potential between impurity and conduction electrons are mediated by emergent boson fields in the slave-boson theory. We developed a self-consistent theory for the superconducting gap and found good agreement with experimental results. We found a s-wave like, fully gapped superconducting channel. For the cuprates and nickelates, we use spin-fluctuation mediated pairing potential, with multi-band random phase approximation to predict pairing symmetries of the gap function. In YBCO cuprate, we found that, if we dope the CuO chain state while keeping the CuO4 plane state’s doping fixed, the pairing symmetry change from the nodal d-wave to a nodal f-wave symmetry. We explore superconductivity in RNiO2 (R = Nd, La, Pr), based on two orbitals, Ni dx2−y2 , and R axial orbital. The axial orbital consists of Nd/La d, and Ni dz2 orbitals. We found that the superconductivity is orbital-selective in RNiO2. In this thesis, we use analytical and numerical methods to analyse the superconducting properties relevant from theoretical and experimental perspectives.