Dynamical structural instability and its implications for the physical properties of infinite-layer nickelates

摘要

We use first-principles calculations to find that in infinite-layer nickelates RNiO2, the widely studied tetragonal P4/mmm structure is only dynamically stable for early lanthanide elements R = La-Sm. For late lanthanide elements R = Eu-Lu, an imaginary phonon frequency appears at A = (??, ??, ??) point. For those infinite-layer nickelates, condensation of this phonon mode into the P4/mmm structure leads to a more energetically favorable I4/mcm structure that is characterized by an out-of-phase rotation of ???NiO4 square.??? Special attention is given to two borderline cases: PmNiO2 and SmNiO2, in which both the P4/mmm structure and the I4/mcm structure are local minima, and the energy difference between the two structures can be fine tuned by epitaxial strain. Compared to the P4/mmm structure, RNiO2 in the I4/mcm structure has a substantially reduced Ni dx2???y2 bandwidth, a smaller Ni d occupancy, a ???cleaner??? Fermi surface with a lanthanide d-derived electron pocket suppressed at the I' point, and a decreased critical UNi to stabilize long-range antiferromagnetic ordering. All these features imply enhanced correlation effects and favor Mott physics. Our work reveals the importance of structure-property relation in infinite-layer nickelates, in particular, the spontaneous ???NiO4 square??? rotation provides a tuning knob to render RNiO2 in the I4/mcm structure a closer analogy to superconducting infinitelayer cuprates.

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