Scanning tunneling microscopy at the NbSe3 surface: Evidence for interaction between q1 and q2 charge density waves in the pinned regime

We have investigated both q(1) and q(2) charge density wave (CDW) states taking place in NbSe3 by means of low-temperature scanning tunneling microscopy (STM) under ultrahigh vacuum on the in situ cleaved (b, c) surface. High-resolution topographical images with atomic lattice resolution were obtained in the temperature range between 5 and 140 K. The careful and thorough analysis of the dependence of the STM images on bias polarity, energy, and temperature allowed us to identify unambiguously the three different types of chains composing the NbSe3 unit cell at all temperatures, resolving contradictions from previous STM results. From two-dimensional Fourier transform of the STM images, we show that at the surface plane both CDW's wave vectors are in very good agreement with bulk reported values projected on the (b, c) plane. The q(1) CDW has, for wave vector, q(1)=0.24b*. Spatially, the q(1) modulation is essentially developed on type III chains with a weak contribution on type II neighboring chains. The q(2) CDW has, for wave vector, the projected value q(2p)=0.26b*+0.5c*. This modulation is mainly developed on type I chains but surprisingly has an important contribution on type III chains with an amplitude similar to the q(1) contribution on these chains. This simultaneous double modulation on chain III leads to a beating phenomenon between the q(1) and q(2p) periodicities and gives rise to a new domain superstructure developed along the chain axis which is characterized by the vector u=2x(q(2p)-q(1))-c*=2x(0.26-0.24)b*. We believe that these new features give a clue of the coupling between the q(1) and q(2) CDWs in the pinned regime. Whereas most studies investigated the various characteristics of both CDWs by probing the Nb atoms properties, our results are consistent with the interpretation according to which the electronic local density of states probed by STM is mostly that of the surface Se atoms.