Effect of covalent bonding on magnetism and the missing neutron intensity in copper oxide compoundsTheories involving highly energetic spin fluctuations are among the leading contenders for explaining high-temperature superconductivity in the cuprates. 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 superconductors. 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. Andrew Walters, a PhD student from the LCN has solved this problem in collaboration with colleagues from STFC and Brookhaven National Laboratory 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. Andrew discovered 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. Andrew’s achievement has been recognized by the award of the 2009 Marshall Stoneham Prize, a newly established prize to be awarded annually to an outstanding PhD thesis in the area of condensed matter and materials physics.
The results of Andrew’s study have been published in Nature Physics: |
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