Theory of impurity-induced lattice distortions in the normal phase of charge-density-wave systems

A phenomenological Ginzburg-Landau theory is applied to the normal phase of one-dimensional charge-density-wave systems with a finite concentration of impurities. It is found that the interaction between the impurities and the highly polarizable electron gas leads to a strong and oscillatory impurity-impurity interaction, which in turn leads to ordered impurity arrangements and to sizable periodic lattice distortions. The effect is very strongly dependent on the charge of the impurities, their concentration, and their (interstitial) location in the lattice unit cell. All these factors modify drastically the periodicity associated with the lattice distortion. The theory explains satisfactorily the changing modulation recently observed by atomic-force microscopy in the normal (room-temperature) phase of ${\mathrm{NbSe}}_3$ with a variety of impurities.