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SUMMARY:Spin Correlations in the Paramagnetic Phase of Quantum Magnets - J
 on Goff\, Department of Physics\, Royal Holloway\, University of London
DTSTART:20080507T101500Z
DTEND:20080507T111500Z
UID:TALK11127@talks.cam.ac.uk
CONTACT:Michael Sutherland
DESCRIPTION:Frustrated quantum magnets exhibit a strongly correlated phase
  over a wide range of temperatures below their Curie-Weiss temperatures\, 
 before ordering long range. It is of great interest to study this spin flu
 id state\, but it is always difficult to separate the weak neutron scatter
 ing signal from the background. We solve this problem using XYZ polarizati
 on analysis\, allowing the purely magnetic neutron scattering signal to be
  isolated. I shall describe recent experiments to study square-lattice Hei
 senberg antiferromagnets\, emphasizing how the spin correlations in the pa
 ramagnetic phase give new information on the exchange mechanism. We find t
 hat the behaviour of La2CuO4 agrees with the predictions of the quantum no
 n-linear sigma model\, and that it is the first electronic material to exh
 ibit ring exchange. We observe quantum order from disorder for Pb2VO(PO4)2
 \, and we find ferromagnetic nearest-neighbour exchange J1 and antiferroma
 gnetic next-nearest-neighbour exchange J2 of comparable magnitude\, allowi
 ng the exploration of a new region of the J1-J2 phase diagram.
LOCATION:Mott Seminar Room\, Cavendish Laboratory\, Department of Physics
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