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The long-standing puzzle of missing inelastic neutron scattering intensity in the cuprates

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One dimensional S=1/2 antiferromagnets are of considerable interest, both as model systems heavily influenced by quantum fluctuations, and also for their relevance towards understanding high-temperature superconductors. It is well-established that in one-dimension the excitations of the electron are best described as independent excitations of spin and charge, known as spinons and holons. I will present measurements of the spinon excitations in Sr2CuO3 as measured using inelastic neutron scattering (INS). In most real analogs of 1D S=1/2 antiferromagnets the spin excitation energies are an order of magnitude lower than the charge excitations, but in Sr2CuO3 the bandwidth of the multi-spinon continuum is about 0.7 eV, a significant fraction of the charge gap of 1.4 eV. Our initial aim was to look for any influence on the magnetic correlations arising from proximity of the charge excitations, which had been suggested by calculations [1]. Contrary to our expectations, we found the measured INS intensity to be just 30% of that predicted, but otherwise the excitations were well described in the limit of large intrasite exchange to hopping ratio (U/t), corresponding to the Heisenberg limit [2].

Recent density functional theory calculations (LDA+U) for the magnetic form factor in Sr2CuO3 provide the solution for this inconsistency: the ‘missing’ INS intensity is due to the covalency of the magnetic Cu electrons, as they are heavily involved in bonding with the neighbouring oxygen atoms [3]. This result has significant implications for the study of cuprate systems, as it provides compelling evidence that the ionic picture of magnetism fails markedly in the cuprates.

[1] M.J. Bhaseen et al., Phys. Rev. B 71 020405 (R) (2005). [2] J.-S. Caux et al., J. Stat. Mech. P12013 (2006). [3] A. C. Walters et al., Nature Phys. 5 867 (2009).

This talk is part of the Quantum Matter Seminar series.

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