A Brillouin scattering study of La_0.77Ca_0.23MnO₃ across the metal–insulator transition

Abstract

Temperature-dependent Brillouin scattering studies have been carried out on La_0.77Ca_0.23MnO₃ across the paramagnetic insulator–ferromagnetic metal (I–M) transition (T_C ∼ 230 K). The spectra show modes corresponding to a Surface Rayleigh Wave (SRW) and a High Velocity Pseudo-surface Wave (HVPSAW) along with bulk acoustic waves (B1 and B2). The Brillouin shifts associated with the SRW and HVPSAW increase, whereas the B1 and B2 frequencies decrease, below T_C. The temperature dependence of the SRW and HVPSAW modes is related to the increase in the elastic constant C₁₁ across the I–M transition. The decrease in frequency across the I–M transition of the bulk modes is understood to be due to enhanced self-energy corrections as a result of increased magnon–phonon interaction across the I–M transition. Correspondingly, these modes show a large increase in the Full Width at Half Maximum (FWHM) as the temperature decreases. We also observe a central peak whose width is maximum at T_C.