Gap opening in the essential spectrum of the elasticity theory problem in a periodic half-layer

Abstract

Abstract. Rayleigh waves are studied in an elastic half-layer with a periodic end and rigidly clamped faces. It is established that the essential spectrum of the corresponding problem of elasticity theory has a band structure, and an example of a waveguide is presented in which a gap opens in the essential spectrum; i.e., an interval arises that contains points of an at most discrete spectrum. §1. Introduction 1. Preamble. In the case of a homogeneous isotropic elastic half-space, surface waves were discovered by Lord Rayleigh [1], and since then many investigations devoted to similar effects have appeared (see a survey of modern literature in [2], and also the paper [3], which is absent in [2]). A Rayleigh wave is a plane wave of the forma vector-valued factor U (z) decaying exponentially as z = x 3 → −∞. The arising of such waves explains specific wave processes in elastic bodies.The wave number k ∈ R + = [0, +∞) determines a frequency cutoff ω † (k) above which, i.e., for ω ≥ ω † (k), the wave (1.1) exists necessarily. In the present paper we deal with a problem related to a similar phenomenon. Namely, we study an elastic, but not necessarily homogeneous and isotropic cushion Ω 0 having the form of a half-layer with a periodic end and rigidly clamped side faces (see Figure 1, where the clamped surface is shadowed). Some Rayleigh waves decaying exponentially as z → −∞ can propagate along the end of the cushion, and if Ω 0 is a cylinder ∆ × R, then we have a single cutoff ω † > 0; i.e., the corresponding operator of the elasticity theory system acquires a continuous spectrum [ω † , +∞). Our main goal in this paper is to show that, in the periodic case, a gap can open in the essential spectrum; i.e., an interval can exist the ends of which belong to the continuous spectrum, but inside which only points of the discrete spectrum may occur. Some of the results were announced earlier in [4].

Bands in the spectrum of a periodic elastic waveguide

“…In literature spectral gaps in essential spectra for scalar equations and Maxwell's system in infinite periodic media have been considered in many papers, see for example [7], [8], [9], [12], [13], [34]. For results on gaps in (quasiperiodic, unbounded) waveguides we mention the papers [2], [6], [10], [24]; for an approach based on parameter-dependent Korn-type inequalities, see [5], [25], [26], [28], [27]. A comparison of the present work with the paper [31] was already presented above.…”