A study of a one-dimensional system may have finally resolved the long-standing discrepancy between the expected and measured inelastic neutron scattering intensities in the high-temperature cuprate superconductors. 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 superconductivity2,3,4,5,6,7,8,9,10,11. 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 intensity2,3,4,5,12,13. 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.

We have investigated the spin dynamics of the nearly ideal one-dimensional S=1 antiferromagnet Ni(${\mathrm{C}}_2$ ${\mathrm{H}}_8$ ${\mathrm{N}}_2$ ${)}_2$ ${\mathrm{NO}}_2$ ${\mathrm{ClO}}_4$ (NENP) by inelastic neutron scattering. The measurements have been performed as a function of both the wave vector (in the vicinity of the antiferromagnetic point q=π) and the magnetic field, for field configurations parallel (up to 5 T) or perpendicular to the chain axis (up to 10 T). Our experimental results at low temperature are in good quantitative agreement with most theoretical predictions (both analytical and numerical) established for the spin-1 Haldane-gap system in presence of finite orthorhombic anisotropy: nonmagnetic singlet ground state, three gaps in the excitation spectrum corresponding to the excited triplet (${\mathrm{\Delta }}_{\mathit{y}}^0$≊1.05 meV, ${\mathrm{\Delta }}_{\mathit{x}}^0$≊1.23 meV, ${\mathrm{\Delta }}_{\mathit{z}}^0$≊2.5 meV), finite correlation lengths at T=0 (${\xi }_{\mathit{x}\mathit{y}}$/d≊8,${\xi }_{\mathit{z}}$/d≊4) and unconventional field dependencies. The evolution with field of the various modes and their respective polarization have been determined experimentally and are compared to recent field-theory treatments of the S=1 anisotropic antiferromagnetic chain together with numerical calculations on finite-size systems. We present results concerning the field dependencies of the correlation lengths, both in field parallel and perpendicular to the chain axis, which are not fully understood from the existing theories.

We have investigated the spin dynamics in the strongly correlated chain copper oxide ${\mathrm{S}\mathrm{r}\mathrm{C}\mathrm{u}\mathrm{O}}_2$ for energies up to $\gtrsim 0.6\mathrm{e}\mathrm{V}$ using inelastic neutron scattering. We observe a gapless continuum of magnetic excitations, which is well described by the “Müller ansatz” for the two-spinon continuum in the $S=1/2$ antiferromagnetic Heisenberg spin chain. The lower boundary of the continuum extends up to $\approx 360\mathrm{m}\mathrm{e}\mathrm{V}$, which corresponds to an exchange constant $J=226(12)\mathrm{m}\mathrm{e}\mathrm{V}$.

Orbitals and charge go their separate ways: In certain materials at very low temperatures, an electron's spin can separate from its charge, zooming through the crystal in the form of a “spinon.” Such materials are usually one-dimensional, and their atoms have spins of 1/2. Wu et al. observed related behavior in a three-dimensional metal, Yb ₂ Pt ₂ Pb, where the Yb ions have a large magnetic moment that has its origin in the electrons' orbital motion rather than their spin. Neutron-scattering measurements indicated that these large magnetic moments can flip their direction through an exchange process similar to the one that occurs in spin 1/2 systems. This process results in effective charge-orbital separation. Science , this issue p. 1206