J. Toulouse, E. Iolin, B. Hennion, D. Petitgrand, and R. Erwin
The damping (Γa) of the transverse acoustic (TA) phonon in single crystals of the relaxor KTa1−xNbxO3 with x=0.15–0.17 was studied by means of high resolution inelastic cold neutron scattering near the (200) Brillouin Zone (BZ) point where diffuse scattering is absent, although it is present near (110). In a wide range of temperatures centered on the phase transition, T=195K÷108K , the TA phonon width (damping) exhibits a step increase around momentum q=0.07 , goes through a shallow maximum at q=0.09–0.12 , and remains high above and up to the highest momentum studied of q=0.16 . These experimental results are explained in terms of a resonant interaction between the TA phonon and the collective or correlated reorientation through tunneling of the off-center Nb+5 ions. The observed TA damping is successfully reproduced in a simple model that includes an interaction between the TA phonon and a dispersionless localized mode (LM) with frequency ωL and damping ΓL(ΓL<ωL) , itself coupled to the transverse optic (TO) mode. Maximum damping of the TA phonon occurs when its frequency is ωa≈ωL . The values of ωL and ΓL are moderately dependent on temperature, but the oscillator strength, M2 , of the resonant damping exhibits a strong maximum in the range T∼120K÷150K in which neutron diffuse scattering near the (110) BZ point is also maximum and the dielectric susceptibility exhibits the relaxor behavior. The maximum value of M appears to be due to the increasing number of polar nanodomains. In support of the proposed model, the observed value of ωL≈0.7THz is found to be similar to the estimate previously obtained by Girshberg and Yacoby [J. Phys.: Condens. Matter 24, 015901 (2012)]. Alternatively, the TA phonon damping can be successfully fitted in the framework of an empirical Havriliak-Negami (HN) relaxation model that includes a strong resonancelike transient contribution.
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