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, q1=(0,0.241,0) and q2=(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 q2 modulation in the STM images recorded above the T2 CDW transition and for the absence of q2 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.