The extremely-2D cuprate: the peculiar physics of the infinite layer CaCuO2

The electronic and magnetic structure of high Tc superconductors is well but not perfectly described by the Hubbard model applied to a single CuO$_2$ square lattice. The third dimension is however decisive in setting the energy of the main parameters, such as the ligand field spitting of $3d$ orbitals and the in-plane hopping integrals beyond the first neighbors. While the main super-exchange $J_1$ and the hopping integral t are grossly the same in all cuprates, the next terms in the series ($J_2$, $J_3$, $J_c$; $t’$, $t”$) are sensitive to deviation from 180° of the Cu-O-Cu bridges and to the presence and distance of apical oxygen, i.e. to the third dimension. The infinite layer CaCuO$_2$ is an almost ideal realization of the flat 2D CuO2 square lattice: very little buckling of the planes, no apical ligands, no space for extra oxygen in the Ca planes that would bring in doping but also disorder. Consequently, longer range hopping terms are particularly large in CaCuO$_2$, leading to a ring exchange comparable in energy to $J_1$ [1]. Using resonant inelastic x-ray scattering (RIXS) at the Cu L$_3$ edge we studied the peculiarities of the orbital and magnetic excitations of CaCuO$_2$. We found that dd excitations split into more terms than in the systems having localized electron states and the same local C$_4$ symmetry, and disperse in energy: they do not have a perfectly localized character because hopping depend on momentum, lifting degeneracy of dxz and dyz states. We also found that the collapse of the magnon excitation at the (1/2,0) X-point, common to all cuprates but stronger in CaCuO$_2$, is not due to the dominance of multi-magnons but rather to the spill-over into a continuum of partly deconfined spinon-pairs. These results establish CaCuO$_2$ as the best realization of the ideal 2D AF lattice with wide range hopping integrals, approaching the conditions for resonant bond solid or spin liquid realization while still showing a Néel AF ground state.