NONLINEAR RESONANCE IN CRYSTALS STRONTIUM-TITANATE AND BARIUM-TITANATE

Raman spectra of strontium titanate and barium titanate are examined. Raman modes of B-2g at 235 cm(-1) and E-g at 615 cm(-1), and A(1)(LO) modes at 196 and 474 cm(-1) are recorded in the tetragonal phases in c rystal SrTiO3 and the reduced BaTiO3, respectively. All the four Raman lines are asymmetric cut-off on low-energy sides, manifesting the non linear interference with the positive coupling coefficients in these t wo samples which are well compared with BiSrCaCuO with a positive coup ling coefficient and YBaCuO with a negative coupling coefficient. The nonlinear resonance including the signs of the Fano coefficient is dis cussed to attribute to the anharmonic interaction term in the potentia l function which distorts the lineshapes and provides a radiationless channel for the phonons.