ENHANCEMENT OF THE SHEAR ELASTIC-CONSTANT IN PERIODIC AND QUASI-PERIODIC TA/AL SUPERLATTICES

The Brillouin light-scattering technique has been exploited in order to reveal surface acoustic phonons in both periodic and Fibonacci quasiperiodic Ta/Al superlattices. In periodic specimens, about 0.5/mum thick, it is found that, as the superlattice period is reduced from 10 to 4 nm, the phase velocity of the Rayleigh acoustic mode exhibits an anomalous increase, corresponding to a 20% enhancement of the effective shear elastic constant c44. In 2k-component and 3k-component Fibonacci quasiperiodic superlattices, with total thickness of about 1.5 mum, an enhancement of c44 similar to that of periodic specimens has been evidenced, which is connected to the average density of the interfaces, rather than to the quasiperiodicity. This enhancement can be attributed to the presence of interfaces extended over a number of atomic planes, due to the mutual interdiffusion between Ta and Al, as inferred by x-ray and electron microscopy experiments.