The diatomic linear chain of masses coupled by harmonic springs is a textbo
ok model for vibrational normal modes (phonons) in crystals. In addition to
propagating acoustic and optic branches, this model is known to support a
''gap mode'' localized at the surface, provided the atom at the surface has
light rather than heavy mass. An elementary argument is given which explai
ns this mode and provides values for the frequency and localization length.
By reinterpreting this made in different ways, we obtain the frequency and
localization length for three other interesting modes: (1) the surface vib
rational mode of a light mass impurity at the surface of a monatomic chain;
(2) the localized vibrational mode of a stacking fault in a diatomic chain
; and (3) the localized vibrational mode of a light mass impurity in a mona
tomic chain. (C) 2000 American Association of Physics Teachers.