Abstract
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$_2$.
Highlights
The primary feature is that a pair of active bands eliminate half-filled physics and, due to instabilities, preclude the AFM phase seen in CaCuO2
A disparate variety of interacting models [1,2,3,4,5,6,7,8,9,10,11,12,13,14] based on density functional theory (DFT) studies [15,16,17,18,19,20,21,22] have been proposed to illuminate the electronic and magnetic behavior underlying the discovery [23] and experimental study [24,25,26] of long-sought superconductivity in a layered nickelate, hole-doped NdNiO2 (NNO)
Undoped NNO is conducting; CCO is insulating; CCO orders antiferromagnetically, whereas no signature of order is seen in NNO (and its isoelectronic but nonsuperconducting sister LaNiO2 (LNO) [28,29,30,31]) and there is no sign of heavy-fermion screening of Ni moments
Summary
A disparate variety of interacting models [1,2,3,4,5,6,7,8,9,10,11,12,13,14] based on density functional theory (DFT) studies [15,16,17,18,19,20,21,22] have been proposed to illuminate the electronic and magnetic behavior underlying the discovery [23] and experimental study [24,25,26] of long-sought superconductivity in a layered nickelate, hole-doped NdNiO2 (NNO). The electronic structures of the d9 infinite-layer cuprates and nickelates were compared some time ago, with some similarities but substantial differences being noted [15,16] and quantified more recently by Wannier function analysis [18,19], and some impact of the Nd moments has been noted [17]. Seem as fundamental as the difference in ground states of the undoped materials: antiferromagnetic (AFM) insulator for CCO, and disordered moment conductor for NNO, the latter being a conducting quantum paramagnetic (MQPM) phase in the Sachdev-Read classification [32].
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