Imperfect Nesting and Impurity Effect in Quasi One-Dimensional Charge and Spin Density Waves.

A theory is systematically developed on the quasi one dimensional charge (CDW) and spin density waves (SDW). First a review is given of the Peierls theory of Fermi surface instability, which says that a pure one dimensional metal is not stable against the interaction with momentum Q = 2k_ f (where k_ f is the Fermi vector), thus leading to a CDW or SDW insulating phase transition. If only the thermodynamics is considered, this model is similar to the BCS theory of a conventional superconductor. Comparison with the experimental data of CDW in NbSe_3 and SDW in (TMTSF)-salts indicates that one has to modify the Peierls model. For example, the ratio of the order parameter at zero temperature and transition temperature in NBSe_3 is about 2Delta_0/T_ c ~eq 11 ~ 14, noticeably bigger than the BCS value of 3.52. Emphasizing the inter-chain coupling existing in these materials, the concept of imperfect nesting of Fermi surface (parameterized by varepsilon_0) is postulated. A mean field Green's function theory is developed to incorporate this idea. Computations are given of the transition temperature and order parameter. Suppression of both T_ c and Delta by varepsilon _0 are found. The calculation of the tunneling conductance reveals the experimental feature and give the estimates of the value of varepsilon_0 . The imperfect nesting effect is found to be more important in NbSe_3 than in (TMTSF) -salts. The threshold field is also computed by including varepsilon_0. The impurity effect is discussed separately to the extent of thermodynamics. The discussion is similar to the theory of magnetic impurity in a BCS superconductor. Both T_ c and Delta are also found to be reduced by impurity scattering. Future work is pointed out to be the incorporation of impurity effect into the imperfect nesting effect. (Copies available exclusively from Micrographics Department, Doheny Library, USC, Los Angeles, CA 90089-0182.).