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Spatial ordering of the charge density waves in NbSe$_3$

Ordering of the two incommensurate charge density waves (CDW), $\mathbf{q_1}$ = (0.0, 0.243, 0.0) and $\mathbf{q_2}$ = (0.5, 0.263,0.5) in the quasi-one-dimensional NbSe$_3$ structure is studied by means of low temperature scanning tunneling microscopy. Larger (100) van der Waals surfaces are analyzed using one-dimensional Fourier transforms along the three types of trigonal prismatic columns, running parallel to the monoclinic $\mathbf{b_0}$ direction. The procedure enables unambiguous differentiation between both CDWs modulating individual columns. In addition, it allows a quantitative comparison of modulation amplitudes along different columns of the same type and, in case of sufficiently large images, also along individual columns. It is confirmed that each CDW modulates both type-III and type-I columns with the narrowest isosceles bases. As a consequence beating is observed between the two slightly different modes. The bridging type-II columns appear not to be modulated. On a large scale the modulation amplitudes along individual columns vary, suggesting formation of CDW nanodomains. The possibility of interchanging both CDWs along columns forming symmetry-related pairs results in a charge difference, which might be the possible origin of CDW sliding.