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Fluctuation-frustrated flat band instabilities in NdNiO2

The discovery that Nd$_{1-x}$Sr$_x$NiO$_2$, with the CaCuO$_2$ infinite-layer structure, superconducts up to 15 K around the hole-doping level $x$=0.2 raises the crucial question of its fundamental electronic and magnetic processes. The unexplained basic feature that we address is that, for $x$=0 and as opposed to strongly antiferromagnetic (AFM) CaCuO$_2$, NdNiO$_2$ with the same structure and formal $d^9$ configuration does not undergo AFM order. We study this issue not in the conventional manner, as energetically unfavored or as frustrated magnetic order, but as an instability of the AFM phase itself. We are able to obtain the static AFM ordered state, but find that a flat-band, one-dimensional-like van Hove singularity (vHs) is pinned to the Fermi level. This situation is unusual in a non-half-filled, effectively two-band system. The vHs makes the AFM phase unstable to spin-density disproportionation, breathing and half-breathing lattice distortions, and (innate or parasitic) charge-density disproportionation. These flat-band instabilities, distant relatives of single band cuprate models, thereby inhibit but do not eliminate incipient AFM tendencies at low temperature. The primary feature is that a pair of active bands ($d_{x^2-y^2}$, $d_{z^2})$ eliminate half-filled physics and, due to instabilities, preclude the AFM phase seen in CaCuO$_2$. This strongly AFM correlated, conducting spin-liquid phase with strong participation of the Ni $d_{z^2}$ orbital, forms the platform for superconductivity in NdNiO$_2$.