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Register a new userA superlattice approach to doping infinite-layer nickelates
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The recent observation of superconductivity in infinite-layer Nd$_{1-x}$Sr$_x$NiO$_2$ thin films has attracted a lot of attention, since this compound is electronically and structurally analogous to the superconducting cuprates. Due to the challenges in the phase stabilization upon chemical doping with Sr, we synthesized artificial superlattices of LaNiO$_3$ embedded in insulating LaGaO$_3$, and used layer-selective topotactic reactions to reduce the nickelate layers to LaNiO$_{2}$. Hole doping is achieved via interfacial oxygen atoms and tuned via the layer thickness. We used electrical transport measurements, transmission electron microscopy, and x-ray spectroscopy together with ab initio calculations to track changes in the local nickel electronic configuration upon reduction and found that these changes are reversible. Our experimental and theoretical data indicate that the doped holes are trapped at the interfacial quadratic pyramidal Ni sites. Calculations for electron-doped cases predict a different behavior, with evenly distributed electrons among the layers, thus opening up interesting perspectives for interfacial doping of transition metal oxides.
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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 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.
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.
We provide a set of computational experiments based on textit{ab initio} calculations to elucidate whether a cuprate-like antiferromagnetic insulating state can be present in the phase diagram of the infinite-layer nickelate family (RNiO$_2$, R= rare-earth). We show that metallicity in the parent phase is produced by an R-d band that requires hybridization with the Ni-d bands to become largely dispersive. If this off-plane R-Ni coupling is suppressed, the system is an antiferromagnetic insulator since that largely dispersive band is no longer able to cross the Fermi level. As such, the reduction of the strong out-of-plane Ni-d hopping leads to an electronic structure closer to the nominal Ni-d$^9$ occupation as the self-doping effect -- understood as charge transfer from the Ni-d to the R-d orbitals -- disappears. This can be achieved if a structural element that suppresses the c-axis dispersion is introduced (i.e. vacuum in a monolayer of NdNiO$_2$, or a blocking layer in multilayers formed by (NdNiO$_2$)$_1$/(NdNaO$_2$)$_1$). We also show how the reduced Ruddlesden-Popper counterparts (R$_4$Ni$_3$O$_8$) are able to produce the same effect due to the presence of fluorite RO$_2$ blocking slabs.
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 inf
inite-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.
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