QUASI-ONE-DIMENSIONAL EXCITONIC INSULATOR

The density response function of a square array of infinite parallel metallic strands of finite radius has been studied. Exchange and correlation effects inside each strand were treated in an effective-field approximation. Interstrand Coulomb correlations were also included through the random-phase approximation, as described by Williams and Bloch (1974). The calculated spectrum exhibits a softening of the plasmon branch which, because of intrastrand exchange and correlation, becomes a low-frequency exciton at large momenta and goes unstable for k approximately=2kF. This excitonic instability is resolved, within mean-field theory, by the appearance of charge-density waves in the new insulating ground state of the strands. The present approach, while applicable only to systems with a sufficient degree of three-dimensionality (as fluctuations are neglected), elucidates the excitonic nature of both the charge-density-wave (CDW) band gap of this insulator and of the low-lying phase and amplitude-mode excitations. A comparison is made with other approaches.