Nonlinear and frequency-dependent transport phenomena in low-dimensional conductors

Abstract

Nonlinear and frequency-dependent electrical conductivity is more a rule than an exception in materials with highly anisotropic electronic structure. Disorder leads to localization of the electronic wave functions, and the temperature-( T), electric field-( E), and frequency (ω)-dependent transport are due to random transfer rates between localized single particle states, a process fundamentally different from band transport. Interactions lead to collective modes, represented by a periodic modulation of the charge or spin density. The charge density wave (CDW) mode is pinned by impurities, but for small pinning forces, it can be depinned by moderate electric fields, leading to nonlinear conductivity due to a sliding CDW. Both classical and quantum models account for the field and frequency dependent response; they also describe current oscillation phenomena and effects which arise when both dc and ac excitations are applied. For strong pinning the collective mode cannot be depinned at small electric field strengths, but nonlinear (soliton) excitations of the collective modes may be responsible for the nonlinear conductivity observed. In all these cases field-and frequency-dependent transport is strongly related. This feature is reproduced by various models, and therefore a detailed study of σ( T, E,ω) is called for to distinguish between the various sources of novel transport phenomena in these new types of solids.