Broken Local Symmetry in Paraelectric BaTiO3 Proved by Second Harmonic Generation

Precursor dynamics of a cubic to tetragonal ferroelectric phase transition in BaTiO3 is studied by the accurate measurement of the second harmonic generation (SHG) integral intensities. A finite signal holds for the SHG integrated intensity above the ferroelectric Curie temperature Tc=403 K. Above the Burn’s temperature Td≈580 K, the power law with the exponent γ=1 shows normal SHG nature originating from the hyper-Raman scattering by dynamical polar excitations, while, below Td, a SHG signal from polar nanoregions becomes dominant with the larger exponent γ=2. Such a crossover of the power law exponent near Td is discussed on the basis of the effective Hamiltonian method and Monte Carlo simulation