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 substrate may become very weak resonances, difficult to detect experimentally, due to their interaction with the substrate modes. In this paper, we show that the observation of these modes can be made possible, or at least facilitated, when the substrate is a superlattice instead of being a homogeneous medium. This is essentially due to the existence of minigaps, and also of two types of polarizations for the minibands, which may prohibit the propagation 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 particular 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 to demonstrate theoretically this new phenomenon with a few illustrations. The localized and resonant modes associated with the adlayer are obtained from a calculation of the total and local density-of-states by means of a Green's function method. The existence and behavior of the guided modes are discussed as a function of the nature and thickness of the adsorbate, the nature of the layer in the superlattice that is in contact with the adlayer, and the wave vector k(parallel to) (parallel to the layers).