Charge density waves in NbSe3: The models and the experimental evidence

Charge density wave (CDW) ordering in the prototypical low-dimensional compound NbSe3 is reconsidered. It is shown that the model with two incommensurate modulations, q(1) = (0, 0.241, 0) and q(2) = (0.5, 0.260, 0.5), localized on type-III and type-I bi-capped trigonal prismatic columns, does not explain details, revealed by various microscopic methods. An alternative explanation, based on the existence of modulated layered nano-domains below both COW onset temperatures, is suggested. It accounts for the presence of the q(2) modulation in the STM images recorded above the T-2 CDW transition and for the absence of q(2) satellites in the corresponding diffraction patterns. The long periodic modulation, detected by LT STM, is attributed to a beating between the two CDWs, centered on adjacent columns of the same type. Such pairs of alternatively modulated columns represent the basic modulation units, ordered into nano-domains. (C) 2010 Elsevier Ltd. All rights reserved.