Covalency makes a smaller form factor !

Theories involving highly energetic spin fluctuations are among the leading contenders for explaining high-temperature superconductivity in the cuprates¹. These theories could be tested by inelastic neutron scattering (INS), as a change in the magnetic scattering intensity that marks the entry into the superconducting state provides a precise quantitative measure of the spin-interaction energy involved in the superconductivity^{2, 3, 4, 5, 6, 7, 8, 9, 10, 11}. However, the absolute intensities of spin fluctuations measured in neutron scattering experiments vary widely, and are usually much smaller than expected from fundamental sum rules, resulting in 'missing' INS intensity^{2, 3, 4, 5, 12, 13}. Here, we solve this problem by studying magnetic excitations in the one-dimensional related compound, Sr₂CuO₃, for which an exact theory of the dynamical spin response has recently been developed. In this case, the missing INS intensity can be unambiguously identified and associated with the strongly covalent nature of magnetic orbitals. We find that whereas the energies of spin excitations in Sr₂CuO₃ are well described by the nearest-neighbour spin-1/2 Heisenberg Hamiltonian, the corresponding magnetic INS intensities are modified markedly by the strong 2p–3d hybridization of Cu and O states. Hence, the ionic picture of magnetism, where spins reside on the atomic-like 3d orbitals of Cu²⁺ ions, fails markedly in the cuprates.

A recent high Tc model seems promising in solving the underestimated INS intensity. This model explicitly covers the spin-spin interaction between the spin of O holes and the spin of Cu holes. Such interaction effectively makes a smaller scattering form factor, which may give a good fit into observations. Details to be worked out !

[1]Nature Physics 5, 867 - 872 (2009);
[2]J.Phys.:Condens.Matter, 21:075702(2009)