Enhancement of the c_{11} elastic constant of Ag/Pd superlattice films as determined from longitudinal guided modes

In Brillouin-light-scattering studies of a series of Ag/Pd superlattice films we have observed scattering from longitudinal guided modes (LGMs) whose velocities are determined primarily by the ${\mathit{c}}_{11}$ elastic constant. From the measured velocity of the first-order LGM, we have obtained the first accurate determination of ${\mathit{c}}_{11}$ for metallic thin films. For the Ag/Pd superlattice films, large increases in both the longitudinal (${\mathit{c}}_{11}$) and shear (${\mathit{c}}_{55}$) elastic constants were observed as the superlattice modulation wavelength Λ was decreased below Λ=60 Å.