Spectroscopy of stripe order in La$_{1.8}$Sr$_{0.2}$NiO$_{4}$ using resonant soft x-ray diffraction

We studied the electronic structure and temperature dependence of the stripe phase in Sr-doped La$_{2}$NiO$_{4}$ using resonant diffraction at the Ni $L_{2,3}$ and La $M_{4,5}$ edge in combination with a realistic microscopic theory. Making use of the very high sensitivity of the method to ordered modulations of the electronic state, we found the doped holes to be mainly located on the oxygen ligands in the NiO$_{2}$ planes, each centered around one Ni ion. The Ni ions in the hole-rich stripes are essentially in a high-spin d$^{8}$ state and the hole spin is coupled antiparallel to the Ni spin in close analogy to the Zhang-Rice singlet state in cuprates. Such an understanding of the charge-ordered phase could not be obtained before, neither by conventional x-ray or neutron diffraction techniques, nor by x-ray absorption spectroscopy.