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Studies of a dilute Heisenberg antiferromagnet and the ferroelectric transition in CsH2AsO4

An alternative method of analysing coupled oscillator fits of Raman spectra is presented. It is compared with conventional methods of obtaining the temperature dependence of the ferroelectric mode in KDP type ferroelectrics. As continuous structural phase transitions are approached, an increase in both the Debye-Waller factor and the nuclear quadrupole coupling coefficient is predicted. The predicted form for uniaxial ferroelectrics is shown to differ from that at other transitions because of its macroscopic electric field. It is shown to be consistent with the temperature dependence of the quadrupole resonance in CsDA. The form at other transitions allows the critical exponent to be obtained. Previously reported structure in DCsDA is shown to result from multiple Bragg scattering in which two ferroelectric domains are involved. Variations in the hydrogen-deuterium concentration are believed to cause the 60K smearing of the transition. Measurements of the 12 acoustic mode are consistent with the elastic constant 066 being small. Measurements of the temperature dependence of the shear angle show that the transition in CsDA is first order. The neutron scattering from the magnetic excitations in the substitutionally disordered antiferromagnet Mn0_68Zn0 32F2 have been measured at 6K. The results show structure in the line shape which is similar to that predicted by some of the calculations based on the coherent potential approximation and to that predicted in recent computer simulations. Some of the critical exponents pertaining to the antiferromagnetic transition in the dilute disordered system, Mn 0.63Zn0 312, have been obtained by measuring the wave vector and temperature dependence of the neutron scattering. These are compared with the exponents found in the pure system, Mn?2, and also with some of the recent predictions of the renormalisation group.

Identiferoai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:657730
Date January 1976
CreatorsMeyer, Gerald M.
PublisherUniversity of Edinburgh
Source SetsEthos UK
Detected LanguageEnglish
TypeElectronic Thesis or Dissertation
Sourcehttp://hdl.handle.net/1842/15377

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