  THz-range dielectric spectroscopy and first-principle-based
effective-Hamiltonian molecular dynamics simulations were employed to elucidate
the dielectric response in the paraelectric phase of (Ba,Sr)TiO3 solid
solutions. Analysis of the resulting dielectric spectra suggests the existence
of a crossover between two different regimes: a higher-temperature regime
governed by the soft mode only versus a lower-temperature regime exhibiting a
coupled soft mode/central mode dynamics. Interestingly, a single
phenomenological coupling model can be used to adjust the THz dielectric
response in the entire range of the paraelectric phase (i.e., even at high
temperature). We conclude that the central peak is associated with thermally
activated processes, and that it cannot be discerned anymore in the dielectric
spectra when the rate of these thermally activated processes exceeds certain
characteristic frequency of the system.
