The striped-type superstructure in the -brass phase has been investigated both experimentally and theoretically. Electron microscopy observation shows that, in addition to the existence of the striped pattern of the striped-type superstructure, the so-called jellyfish pattern consists of three inversion domains belonging to one of two types. The analysis of the electron-diffraction pattern further leads to an important conclusion that the striped-type superstructure can be interpreted as a structure of the normal -brass type modulated by two incommensurate waves. On the basis of the experimental results, the electronic origin and the discommensurate structure of the striped-type superstructure are discussed. First, the relation between the Fermi surface and the repeated Brillouin zone indicates that the two incommensurate waves have wave vectors determined as a spanning vector of the flat portion of the Fermi surface. Secondly, the Ginzburg-Landau-type model, whose order parameter is the charge density due to the charge density waves, is proposed, and reproduces the experimentally obtained change in the period of the striped-type superstructure. It is also shown that there is a phase slip of 2/3 across the discommensuration. These features derived based on the model are entirely consistent with those observed experimentally. Accordingly, it is concluded that the striped-type superstructure in the -brass phase can be interpreted as the charge-density-wave state in the three-dimensional system.
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