Effects of Rayleigh waves on the essential spectrum in perturbed doubly   periodic elliptic problems
  We give an example of a scalar second order differential operator in the plane with double periodic coefficients and describe its modification, which causes an additional spectral band in the essential spectrum. The modified operator is obtained by applying to the coefficients a mirror reflection with respect to a vertical or horizontal line. This change gives rise to Rayleigh type waves localized near the line. The results are proven using asymptotic analysis, and they are based on high contrast of the coefficient functions. 
F.L. Bakharev, G. Cardone, S.A. Nazarov, J. Taskinen
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