Quantum-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 many important questions about its fundamental electronic and magnetic processes. One unexplained feature is that, for $x$=0 and as opposed to CaCuO$_2$, NdNiO$_2$ with the same formal $d^9$ configuration does not order antiferromagnetically (AFM). We study the experimentally inaccessible AFM phase, finding for the static lattice, a flat-band, one-dimensional van Hove singularity (vHs) pinned to the Fermi level is unstable to charge, spin, and lattice orders. These flat-band instabilities thereby limit incipient AFM order at low temperature and, being assisted by zero point fluctuations of the oxygen lattice, preclude the AFM phase seen in CaCuO$_2$. This strongly AFM correlated, conducting spin-liquid phase forms the platform for superconductivity in NdNiO$_2$.