زادت البحث عن المواد الأكسيدية التي تتمتع بخصائص فيزيائية مشابهة للمواد السوبركوندكتور العالية Tc المبنية على المعادن الانتقالية البديلة مثل النيكل، وذلك على مر الزمن. وأثارت اكتشاف السوبركوندكتورية في النيكلات اللامعة المحدودة RNiO2 (R = عنصر الأرثية النادر) هذه الجهود بشكل أكثر. وبمصباح الجوهري المشابه للنيكلات اللامعة المحدودة - طبقات من المعادن الانتقالية الأكسيدية مفصولة بطبقة باسفر من عناصر الأرثية - يشير العد بالفالنس لهذه المواد إلى أن الكاتيونات Ni1 + تتمتع بنفس عدد الإلكترونات 3d الذي يتمتع به Cu2 + في النيكلات. هنا، نستخدم الاشعة السينية مع النظرية التفاضلية للكثافة لإظهار أن البنية الإلكترونية لRNiO2 (R = La، Nd) ، على الرغم من أنها مشابهة للنيكلات، تشمل فروقات كبيرة. على عكس النيكلات التي تتمتع بطبقات أكسيد معادن الانتقالية منفصلة بطبقات معادن الأرثية الإطارية، يدعم طبقة الباسفر الأرثية في النيكلات اللامعة المحدودة حالة معدنية ثلاثية الأبعاد ضعيفة التفاعل. ويخلط هذا الدوال المعدنية الثلاثية الأبعاد بحالة قصيرة الأجل ذات الثنائية الأبعاد بشكل قوي المرتبطة مع تركيب 3dx2-y2. لذلك، يمكن أن يعتبر النيكلات اللامعة المحدودة أخوة للمعادن الإنترمتالية الأرثية المعروفة بشكل واسع بسبب سلوكها الفرميون الثقيل، حيث تلعب طبقات NiO2 المرتبطة دورا مشابها للحالات 4f في مركبات الفرميون الثقيل الأرثية. ويحل هذا المعادن الأكسيد-الإنترمتالية الكوندو أو أندرسون الشبكية المميز بدلا من الموطن الموت الذي يظهر منه السوبركوندكتورية عند التحميل.
The search for oxide materials with physical properties similar to the cuprate high Tc superconductors, but based on alternative transition metals such as nickel, has grown and evolved over time. The recent discovery of superconductivity in doped infinite-layer nickelates RNiO2 (R = rare-earth element) further strengthens these efforts.With a crystal structure similar to the infinite-layer cuprates - transition metal oxide layers separated by a rare-earth spacer layer - formal valence counting suggests that these materials have monovalent Ni1+ cations with the same 3d electron count as Cu2+ in the cuprates. Here, we use x-ray spectroscopy in concert with density functional theory to show that the electronic structure of RNiO2 (R = La, Nd), while similar to the cuprates, includes significant distinctions. Unlike cuprates with insulating spacer layers between the CuO2 planes, the rare-earth spacer layer in the infinite-layer nickelate supports a weakly-interacting three-dimensional 5d metallic state. This three-dimensional metallic state hybridizes with a quasi-two-dimensional, strongly correlated state with 3dx2-y2 symmetry in the NiO2 layers. Thus, the infinite-layer nickelate can be regarded as a sibling of the rare earth intermetallics, well-known for heavy Fermion behavior, where the NiO2 correlated layers play an analogous role to the 4f states in rare-earth heavy Fermion compounds. This unique Kondo- or Anderson-lattice-like oxide-intermetallic replaces the Mott insulator as the reference state from which superconductivity emerges upon doping.
We theoretically investigate the unconventional superconductivity in the newly discovered infinite-layer nickelates Nd$_{1-x}$Sr$_{x}$NiO$_{2}$ based on a two-band model. By analyzing the transport experiments, we propose that the doped holes dominantly enter the Ni $d_{xy}$ or/and $d_{3z^{2}-r^{2}}$ orbitals as charged carriers, and form a conducting band. Via the onsite Hund coupling, the doped holes are coupled to the Ni localized holes in the $d_{x^{2}-y^{2}}$ orbital band. We demonstrate that this two-band model could be further reduced to a Hund-Heisenberg model. Using the reduced model, we show the non-Fermi liquid state above the critical $T_{c}$ could stem from the carriers coupled to the spin fluctuations of the localized holes. In the superconducting phase, the short-range spin fluctuations mediate the carriers into Cooper pairs and establish $d_{x^{2}-y^{2}}$-wave superconductivity. We further predict that the doped holes ferromagnetically coupled with the local magnetic moments remain itinerant even at very low temperature, and thus the pseudogap hardly emerges in nickelates. Our work provides a new superconductivity mechanism for strongly correlated multi-orbital systems and paves a distinct way to exploring new superconductors in transition or rare-earth metal oxides.
The recent discovery of the superconductivity in the doped infinite layer nickelates $R$NiO$_2$ ($R$=La, Pr, Nd) is of great interest since the nickelates are isostructural to doped (Ca,Sr)CuO$_2$ having superconducting transition temperature ($T_{rm c}$) of about 110 K. Verifying the commonalities and differences between these oxides will certainly give a new insight into the mechanism of high $T_{rm c}$ superconductivity in correlated electron systems. In this paper, we review experimental and theoretical works on this new superconductor and discuss the future perspectives for the nickel age of superconductivity.
The recent discovery of superconductivity in oxygen-reduced monovalent nickelates has raised a new platform for the study of unconventional superconductivity, with similarities and differences with the cuprate high temperature superconductors. In this paper we investigate the family of infinite-layer nickelates $R$NiO$_2$ with rare-earth $R$ spanning across the lanthanide series, introducing a new and non-trivial knob with which to tune nickelate superconductivity. When traversing from La to Lu, the out-of-plane lattice constant decreases dramatically with an accompanying increase of Ni $ d_{x^2-y^2}$ bandwidth; however, surprisingly, the role of oxygen charge transfer diminishes. In contrast, the magnetic exchange grows across the lanthanides which may be favorable to superconductivity. Moreover, compensation effects from the itinerant $5d$ electrons present a closer analogy to Kondo lattices, indicating a stronger interplay between charge transfer, bandwidth renormalization, compensation, and magnetic exchange. We also obtain the microscopic Hamiltonian using Wannier downfolding technique, which will provide the starting point for further many-body theoretical studies.
The discovery of superconductivity in infinite-layer nickelates brings us tantalizingly close to a new material class that mirrors the cuprate superconductors. Here, we report on magnetic excitations in these nickelates, measured using resonant inelastic x-ray scattering (RIXS) at the Ni L3-edge, to shed light on the material complexity and microscopic physics. Undoped NdNiO2 possesses a branch of dispersive excitations with a bandwidth of approximately 200 meV, reminiscent of strongly-coupled, antiferromagnetically aligned spins on a square lattice, despite a lack of evidence for long range magnetic order. The significant damping of these modes indicates the importance of coupling to rare-earth itinerant electrons. Upon doping, the spectral weight and energy decrease slightly, while the modes become overdamped. Our results highlight the role of Mottness in infinite-layer nickelates.
The discovery of infinite-layer nickelate superconductors has spurred enormous interest. While the Ni$^{1+}$ cations possess nominally the same 3d$^9$ configuration as Cu$^{2+}$ in high-$T_C$ cuprates, the electronic structure consistencies and variances remain elusive, due to the lack of direct experimental probes. Here, we present a soft x-ray photoemission spectroscopy study on both parent and doped infinite-layer Pr-nickelate thin films with a doped perovskite reference. By identifying the Ni character with resonant photoemission and comparison to density function theory + U calculations, we estimate U ~ 5 eV, smaller than the charge transfer energy $Delta$ ~ 8 eV, in contrast to the cuprates being charge transfer insulators. Near the Fermi level (EF), we observe a signature of rare-earth spectral intensity in the parent compound, which is depleted upon doping. The parent compound, self-doped from rare-earth electrons, exhibits higher density of states at EF but manifests weaker superconducting instability than the Sr-doped case, demonstrating a complex interplay between the strongly-correlated Ni 3d and the weakly-interacting rare-earth 5d states in these oxide-intermetallic nickelates.