Charge density wave ordering in NbSe3: possible models and the experimental evidence

TL;DR

This paper reevaluates charge density wave models in NbSe3, proposing an alternative explanation involving nano-domains and layered modulations that better fit experimental data from various microscopic techniques.

Contribution

It introduces a new model of CDW ordering in NbSe3 based on nano-domains and layered modulations, improving upon previous incommensurate modulation models.

Findings

The traditional CDW model does not fully explain microscopic observations.

A new model with modulated nano-domains aligns better with experimental evidence.

LT STM reveals long periodic modulations due to beating between two CDWs.

Abstract

Charge density wave (CDW) ordering in the prototypical low-dimensional compound NbSe3 is reconsidered. We show that the widely accepted CDW 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 (BCTP) columns, does not explain some details, revealed by various microscopic methods. The suggested alternative explanation is in a better accord with the entire experimental evidence, including low-temperature (LT) scanning tunneling microscopy (STM) results. It is based on the existence of modulated layered nano-domains formed below both CDW onset temperatures. According to this model, two of the three slightly different BCTP types of columns are modulated by the same wave vector, either q1 or q2, which can easily switch over in a domain as a whole. This approach explains the presence of the q2 modulation in the STM images recorded above the T2 CDW transition and the absence of the 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. These pairs of columns, both either of type-III or type-I, modulated by the two alternative CDWs, represent the basic modulation units, ordered into nano-domains.