Direct evidence of non-Zhang-Rice Cu3+centers in La2Li0.5Cu0.5O4

Abstract

A well-isolated Zhang-Rice (ZR) singlet as a ground state of the Cu${}^{3+}$ center in hole-doped cuprates is a leading paradigm in modern theories of high-temperature superconductivity. However, a dramatic temperature evolution of the ${}^{6,7}$Li NMR signal in La${}_{2}$Li${}_{0.5}$Cu${}_{0.5}$O${}_{4}$, a system with a regular lattice of well-isolated Cu${}^{3+}$ centers, reveals significant magnetic fluctuations and suggests a quasidegeneracy to be a generic property of their ground state at variance with the simple ZR model. We argue for a competition of the ZR state with nearby states formed by a ``doped'' hole occupying purely oxygen nonbonding ${a}_{2g}(\ensuremath{\pi})$ and ${e}_{u}(\ensuremath{\pi})$ orbitals rather than a conventional ${b}_{1g}({d}_{{x}^{2}\ensuremath{-}{y}^{2}})$ Cu 3$d$-O 2$p$ hybrid. The temperature variation of the ${}^{6,7}$Li NMR line shape and spin-lattice relaxation rate point to a gradual slowing down of some magnetic order parameter's fluctuations without distinct signatures of a phase transition down to $T=2$ K. This behavior agrees with a stripelike ferrodistortive fluctuating $Ammm$ order in a two-dimensional structure of the (CuLi)O${}_{2}$ planes accompanied by unconventional oxygen orbital antiferromagnetic fluctuations.