Localized and resonant guided elastic waves in an adsorbed layer on a semi-infinite superlattice

The pseudoguided acoustic modes associated with an adlayer deposited on a s ubstrate may become very weak resonances, difficult to detect experimentall y, due to their interaction with the substrate modes. In this paper, we sho w that the observation of these modes can be made possible, or at least fac ilitated, when the substrate is a superlattice instead of being a homogeneo us medium. This is essentially due to the existence of minigaps, and also o f two types of polarizations for the minibands, which may prohibit the prop agation of the guided modes of the adsorbed layer into the superlattice and therefore increase the degree of localization of these guided modes. These features are especially relevant at high acoustic velocities Cin particula r around and above the longitudinal velocity of sound in the adlayer) or in the case of a hard layer where the resonances may become very weak if the substrate is made of a homogeneous material. The purpose of this paper is t o demonstrate theoretically this new phenomenon with a few illustrations. T he localized and resonant modes associated with the adlayer are obtained fr om a calculation of the total and local density-of-states by means of a Gre en's function method. The existence and behavior of the guided modes are di scussed as a function of the nature and thickness of the adsorbate, the nat ure of the layer in the superlattice that is in contact with the adlayer, a nd the wave vector k(parallel to) (parallel to the layers).