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 CDW 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.