Acoustic spectroscopy near phase transitions

Here we give an overview about out recent elastic susceptibility measurements in different single crystals and discuss the main results - In KSCN entropy fluctuations were found at low frequencies. - In C-60 a glass transition was detected and the freezing process was studied in detail. - In SrTiO3 and KMnF3 we have detected ferroelastic domain wall motion below T-c. - In SrTiO3 nonlinear elastic anomalies were found in the quantum paraelectric region. From these examples it becomes clear, that a variety of phenomena, i.e. low frequency dynamics due to entropy fluctuations, glass freezing, domain wall motion, soliton motion etc. can be well studied by acoustic spectroscopy, i.e. measurements of the complex dynamic elastic susceptibility in a broad temperature and frequency range.