Excitations in commensurate and incommensurate BaMnF4: a light scattering study

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Abstract
Brillouin and Raman spectroscopy have been used to characterise the zone-centre phonons of BaMnF4 and to elucidate the dynamics of the commensurate-incommensurate transition at Ti=247K. The experiments for Ti have revealed three low-frequency temperature-dependent modes and these are interpreted in terms of the phase and amplitude excitations of the ground state (phasons and amplitudons) predicted by theory. In particular, the c(aa)b Brillouin spectrum contains scattering localised at approximately 0.6 cm-1 that is attributed to the phason. There is also a critical central peak, which appears to be static in origin. Above Ti, all quasi-transverse acoustic phonons in 90 degrees scattering show critical behaviour in intensity with an exponent of approximately 0.55. All of the elastic and piezoelectric constants and most of the elasto-optic constants have been determined for commensurate BaMnF4 at 295K.