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A touch more unconventional

The date of: 2021-07-09
viewed: 13
Superconductivity is one of the most fascinating, macroscopically observable quantum phenomena. A notable aspect of its description is the existence of an attractive interaction, a “glue,” between electrons. This can bind them into Cooper pairs, which then condense into a common superconducting ground state. In contrast to conventional superconductors, which are described by the theory of Bardeen, Cooper, and Schrieffer, a primary phonon-mediated pairing mechanism is unlikely in the high-temperature (Tc) superconducting cuprates, and therefore they are referred to as unconventional. One characteristic of all high-Tc cuprates are the strong, quasi–two-dimensional antiferromagnetic correlations, with spin waves or magnons as possible alternative pairing glue. On page 213 of this issue, Lu et al. (1) report on antiferromagnetic magnon-like spin excitations in superconducting infinite-layer nickelate thin films.
The recent discovery of this new class of potentially unconventional superconductors, Nd1?xSrxNiO2 (2), has sparked great interest. Although the Tc of 15 K is lower than the Tc ≤ 130 K in the cuprates, their structural and electronic similarity immediately raised the possibility of magnetic correlations and a related pairing mechanism. To this point, the authors measured the spin wave dispersion of Nd1?xSrxNiO2 with resonant inelastic x-ray scattering. A nearest-neighbor superexchange coupling of J = 64 meV was determined, which quantifies the coupling of two neighboring magnetic nickel ions through the nonmagnetic oxygen ion in between. This large superexchange interaction is unexpected because it is not much smaller than the J found in cuprates of over 100 meV.
The outstanding question is whether the new nickelate superconductors are just like the high-Tc cuprates. At first glance, the isostructural transition-metal-oxide planes contain monovalent Ni1+ ions that are isoelectronic to Cu2+, with a single spin ? in the planar 3dx2?y2 orbital on a square lattice (see the figure). In both materials, strong electron-electron correlations cause the partially occupied transition-metal d-band to split up into so-called lower and upper Hubbard subbands. One major difference in the electronic structure is the relative energy of the oxygen-2p–derived bands with respect to the Hubbard subbands. Whereas in cuprates, the 2p bands lie in between, in nickelates, they are located below both subbands. In a local picture, this translates into a strongly reduced hybridization of nickel 3d- and oxygen 2p-orbitals with possibly important consequences for the magnetic superexchange interaction. Reduced hybridization reduces the effective hopping t between magnetic nickel ions by way of the oxygen ligand. Because the coupling constant J is proportional to t2/U, where U is the on-site Coulomb repulsion, values 10 times smaller than those in cuprates have been estimated (3, 4). Quantum chemical calculations (5), however, provide an alternative argument that both t and U are reduced in the nickelates. These calculations predict a superexchange coupling J = 77 meV for the bulk structure of NdNiO2, which is relatively close to the one reported by Lu et al.

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