Brillouin spectroscopy on doped SmS

SmS becomes intermediate valent at an applied pressure of about 6.5 kbar. On the other hand, SmS doped with La or Tm is already intermediate valent at normal pressure and room temperature. The doping atoms (depending on their concentration) create new occupied states in the SmS gap which lead to the typical hybridised 4f(i)/4f(i-1)-5d(1)-states. The La-cation is always trivalent in LaS, whereas the Tm-cation has a valence between 2 and 3. Therefore, we expect to see a difference in the intermediate valent behaviour. A strong evidence of intermediate valence is a negative C-12 and a negative Poisson's ratio. Using high-resolution Brillouin-spectroscopy we measured the phase velocity of the surface acoustic waves in the (100)-plane of Sm1-xLaxS and Sm1-xTmxS. Applying a standard fit-algorithm we calculated all three elastic constants (C-11, C-12 and C-44) from the angular dispersion relation. To get more reliable results the compression-moduli also have been determined and linked to the elastic constants C-11 and C-12 in the fit model.