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

A well-isolated Zhang-Rice (ZR) singlet as a ground state of the Cu3 + center in hole-doped cuprates is a leading paradigm in modern theories of high-temperature superconductivity. However, a dramatic temperature evolution of the 67Li NMR signal in La2Li0.5Cu 0.5O4, a system with a regular lattice of well-isolated Cu3+ 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 a2g(π) and eu(π) orbitals rather than a conventional b1g(dx2-y2) Cu 3d-O 2p hybrid. The temperature variation of the 6,7Li 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)O2 planes accompanied by unconventional oxygen orbital antiferromagnetic fluctuations. © 2012 American Physical Society