FIZIKA A 8 (1999) 4 , 383-392

ACOUSTIC COLLECTIVE EXCITATIONS AND STATIC DIELECTRIC RESPONSE IN INCOMMENSURATE CRYSTALS WITH REAL ORDER PARAMETER

V. DANANIĆ^a, A. BJELIŠ^b and M. LATKOVIĆ^b

^aDepartment of Physics, Faculty of Chemical Engineering and Technology,
University of Zagreb, Marulićev trg 19, HR-10000 Zagreb, Croatia

^bDepartment of Theoretical Physics, Faculty of Science, University of Zagreb,
Bijenička 32, HR-10000 Zagreb, Croatia

Dedicated to Professor Boran Leontić on the occasion of his 70^th birthday

Received 25 February 2000; Accepted 10 April 2000

Starting from the basic Landau model for the incommensurate-commensurate materials of the class I I, we derive the spectrum of collective modes for all (meta)stable states from the corresponding phase diagram. It is shown that all incommensurate states posses Goldstone modes with acoustic dispersions. The representation in terms of collective modes is also used in the calculation and discussion of static dielectric response for systems with the commensurate wave number in the center of the Brillouin zone.

PACS numbers: 71.45.Gm, 72.15.Nj, 77.22.Ch
UDC 536.65, 530.152.2

Keywords: class I I incommensurate-commensurate materials, Landau model, spectrum of collective modes, Goldstone modes, dielectric response

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