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For pt.I see ibid., vol.16, no.14, p.2641 (1983). External electric field influences the fluctuation spectrum of the phase variable in the incommensurate phase of K 2 SeO 4 - and (NH 4 ) 2 BeF 4 -like crystals. As well as the usual zero-field branches (phason-like, acoustic-like) a new branch appears for non-zero fields. Analytic description of all these branches is given within the harmonic approximation.
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O. Hudák (1983) studied this question.
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