Séminaires IQ/RQMP: Yao Shen

Des biscuits seront offerts dans la cuisinette de l’IQ dès 10h30, heure à partir de laquelle le café sera gratuit jusqu’à 11h00. Le séminaire débutera à 11h00. Venez en grand nombre!

Cookies will be available in the IQ kitchenette starting at 10:30, at which time coffee will be free until 11:00. Talk starts at 11:00. A nice occasion to discuss!

Yao Shen 

Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory

Abstract:
The discovery of superconductivity in square planar low valence nickelates has ignited a vigorous debate regarding their essential electronic properties: Do these materials have appreciable oxygen charge-transfer character and superexchange akin to the cuprates or are they in a distinct Mott-Hubbard regime where oxygen plays a minimal role and superexchange is negligible? Do they host charge order as cuprates and what is the electronic character of this symmetry breaking? Here, we resolve these questions using O K-edge and Ni L-edge RIXS measurements of the low valance nickelate La4Ni3O8 and a prototypical cuprate La2−xSrxCuO4 (x=0.35), interpreting the results with exact diagonalization calculations [1,2]. As expected, the cuprate lies deep in the charge-transfer regime of the Zaanen-Sawatzky-Allen (ZSA) scheme. The nickelate, however, is not well-described by either limit of the ZSA scheme and is found to be of mixed charge-transfer / Mott-Hubbard character with the Coulomb repulsion U of similar size to the charge trans-fer energy Δ. Nevertheless, the transition-metal-oxygen hopping is larger in La4Ni3O8 than in La2−xSrxCuO4, which leads to a significant superexchange interaction of J~70 meV and an appreciable hole occupation of the ligand O orbitals in La4Ni3O8 despite its larger Δ. Moreover, we identify intertwined involvements of Ni 3𝑑𝑑𝑥𝑥2−𝑦𝑦2, 3𝑑𝑑3𝑧𝑧2−𝑟𝑟2, and O 2𝑝𝑝𝜎𝜎 orbitals in its formation of diagonal charge order. More specifically, the Ni 3𝑑𝑑𝑥𝑥2−𝑦𝑦2 orbitals, strongly hybridized with planar O 2𝑝𝑝𝜎𝜎, largely shape the spatial charge distribution and lead to Ni site-centered charge order. The 3𝑑𝑑3𝑧𝑧2−𝑟𝑟2 orbitals play a small, but non-negligible role in the charge order as they hybridize with the rare-earth 5d orbitals. Our results clarify the essential characteristics of low valence nickelates and put strong constraints on theoretical interpretations of superconductivity in these materials.
References
[1] J. Q. Lin et al., Phys. Rev. Lett. 126, 087001 (2021).
[2] Y. Shen et al., Phys. Rev. X 12, 011055 (2022)

https://umontreal.zoom.us/j/88647167648?pwd=RjQxTFIzcDA5bWQ1YkdvYkwvOXVYdz09
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