Commensurate-incommensurate transitions in coupled chain systems at 0 K

Abstract

We study the theory for the structure of coupled incommensurate chains in the weak-coupling limit. Both chains are deformable. The emphasis is on the commensurate-incommensurate transition. In this limit, the structure is described in terms of commensurate domains separated by walls which have intrachain\char22{}and interchain\char22{}interactions. It is argued that the latter cause defects of one chain to be pinned on defects of the other, so that the commensurate-incommensurate transition is in general complicated and exhibits a behavior reminiscent of the devil's staircase. We show that crystals formed from interpenetrating chains may have different types of commensurate-incommensurate transitions, depending on whether one sublattice only forms defects (walls) at the transition, or both simultaneously. In the last section we discuss the fluctuations of defect planes at low temperatures in quasi-one-dimensional conductors.