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SUMMARY:The long-standing puzzle of missing inelastic neutron scattering i
 ntensity in the cuprates - Andrew C. Walters\, European Synchrotron Radiat
 ion Facility\, Grenoble\, France
DTSTART:20100115T140000Z
DTEND:20100115T150000Z
UID:TALK22389@talks.cam.ac.uk
CONTACT:Michael Sutherland
DESCRIPTION:One dimensional S=1/2 antiferromagnets are of considerable int
 erest\, both as model systems heavily influenced by quantum fluctuations\,
  and also for their relevance towards understanding high-temperature\nsupe
 rconductors. 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 s
 pinon excitations in Sr2CuO3 as\nmeasured using inelastic neutron scatteri
 ng (INS). In most real analogs of 1D S=1/2 antiferromagnets the spin excit
 ation 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 pr
 oximity of the\ncharge excitations\, which had been suggested by calculati
 ons [1]. Contrary to our expectations\, we found the measured INS intensit
 y to be just 30% of that predicted\, but otherwise the excitations were we
 ll described in the limit of large intrasite exchange\nto hopping ratio (U
 /t)\, corresponding to the Heisenberg limit [2].\n\nRecent density functio
 nal theory calculations (LDA+U) for the magnetic form factor in Sr2CuO3 pr
 ovide the solution for this inconsistency: the 'missing' INS intensity is 
 due to the covalency of the magnetic Cu\nelectrons\, as they are heavily i
 nvolved in bonding with the neighbouring oxygen atoms [3]. This result has
  significant implications for the study of cuprate systems\, as it provide
 s compelling evidence that the\nionic picture of magnetism fails markedly 
 in the cuprates.\n\n[1] M.J. Bhaseen et al.\, Phys. Rev. B 71 020405(R) (2
 005).\n[2] J.-S. Caux et al.\, J. Stat. Mech. P12013 (2006).\n[3] A. C. Wa
 lters et al.\, Nature Phys. 5 867 (2009).
LOCATION:Mott Seminar Room\, Cavendish Laboratory\, Department of Physics
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