Electronic structure of the parent compound of superconducting infinite-layer nickelates

Abstract

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 [1][2][3][4][5][6][7][8][9][10]. The recent discovery of superconductivity in doped infinite-layer nickelates RNiO2 (R = rare-earth element) [11,12] further strengthens these efforts. With a crystal structure similar to the infinite-layer cupratestransition metal oxide layers separated by a rare-earth spacer layerformal valence counting suggests that these materials have monovalent Ni 1+ cations with the same 3d electron count as Cu 2+ 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 weaklyinteracting three-dimensional 5d metallic state. This three-dimensional metallic state hybridizes with a quasi-two-dimensional, strongly correlated state with 3dx 2 -y 2 symmetry in the NiO2 layers. Thus, the infinite-layer nickelate can be regarded as a sibling of the rare earth intermetallics [13-15], 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.While the mechanism of superconductivity in the cuprates remains a subject of intense research, early on it was suggested that the conditions required for realizing high Tc superconductivity are rooted in the physics of a two-dimensional electron system subject to strong local repulsion [16,17]. This describes the Mott (charge-transfer) insulators in the stoichiometric parent compounds, characterized by spin ½ Heisenberg antiferromagnetism, from which superconductivity emerges upon doping. A long-standing question regards whether these "cuprate-Mott" conditions can be realized in other oxides; and extensive efforts to synthesize and engineer nickel oxides (nickelates) have promised such a realization [1-10]. Infinite-layer NdNiO2 became the first such nickelate superconductor following the recent discovery of superconductivity in Srdoped samples [11]. The undoped parent compound, produced by removing the apical oxygen atoms from the perovskite nickelate NdNiO3 using a metal hydride-based soft chemistry reduction process [10,[18][19][20], appears to be a close sibling of the cuprates-it is isostructural to the infinitelayer cuprates with monovalent Ni 1+ cations and possesses the same 3d 9 electron count as that of Cu 2+ cations in undoped cuprates. Yet, as we will reveal, the electronic structure of the undoped RNiO2 (R = La and Nd) remains distinct from the Mott, or charge-transfer, compounds of undoped cuprates, and even...

Role of 4f states in infinite-layer NdNiO2

“…This undoped NdNiO 2 FS differs in significant ways from that of nonmagnetic CaCuO 2 but is similar to LaNiO 2 . Xray absorption spectra of the O K and Ni L 3 edges [20] do not reveal significant differences between LaNiO 2 and NdNiO 2 .…”

“…Note that this coupling is Hund's rule alignment of 4f and 5d states and therefore anti-Kondo coupling of the local moment to the conduction band, as opposed to some suggested Kondo modeling of Nd 0.8 Sr 0.2 NiO 2 . [4,20] Such anti-Kondo coupling has been studied in other lanthanide compounds. [33] We return below to the K ≈ 0.5 eV intra-atomic exchange coupling of Nd 5d states.…”

Absence of superconductivity in bulk Nd1−xSrxNiO2

“…Recently, superconductivity was observed at 9 -15 K in the strontium doped infinite-layer nickelate thin films of Nd0.8Sr0.2NiO2 on SrTiO3 substrate [18] . This work has stimulated enormous interests, and plenty of theoretical works have been carried out [19][20][21][22][23][24][25][26][27][28][29][30][31] . Among them, Zhang et al propose the parent compound of Nd0.8Sr0.2NiO2 as a self-doped Mott insulator [26] .…”

Effective Hamiltonian for nickelate oxides Nd1−xSrxNiO2

“…Exact diagonalization (ED) studies of the Ni 5 O 16 cluster within the three-band Hubbard model are used to select and map the low-energy spectra onto the effective oneband t-t -J model. According to ED results on finite clusters, in hole-doped nickelates, the doped holes primarily locate on Ni sites in a good agreement with the HSE results in Sec.II A and the experiment 28 , while for cuprate the holes mainly locate on oxygen site 22,23 . The physics of the NiO 2 is a doped Mott insulator described by the effective on-band t-t -J model with t = 265 meV, t = −21 meV, and J = 28.6 meV.…”